. "4 Alternative Technologies." Transitions to Alternative Transportation Technologies--A Focus on Hydrogen. Washington, DC: The National Academies Press, 2008.
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Transitions to Alternative Transportation Technologies — A Focus on Hydrogen
4 Alternative Technologies for Light-Duty Vehicles
In keeping with its statement of task, the committee considered “whether other technologies would be less expensive or could be more quickly implemented than fuel cell technologies to achieve significant reductions in carbon dioxide emissions and oil imports.” After considering a range of alternative technologies and the budget constraints of the study, the committee chose to evaluate in depth two options that received increased emphasis in the Energy Independence and Security Act of 2007 (EISA 2007): (1) evolutionary improvements in internal combustion engines (ICEs) and hybrid electric vehicles (HEVs) and (2) a biofuel option (i.e., fuel derived from biomass). Many of these vehicle technologies will be needed through 2020 to meet the significantly higher fuel economy standards required by EISA 2007. These are not the only technologies that could make a contribution to improved fuel economy and reduced carbon dioxide (CO2) emissions, but they are the ones for which the committee felt confidence in projecting technology availability, costs, and consumer acceptance. Others are discussed briefly below.
The ultimate goal of this chapter is to estimate the extent to which continuing evolution of light-duty vehicle technologies, increased use of improved hybrid electric vehicles, and the use of biofuels can reduce oil imports and greenhouse gas emissions through 2050.
EVOLUTIONARY VEHICLE TECHNOLOGIES
Over the past 25 years, oil consumption in the light-duty vehicle fleet has grown because of an increase in the number of vehicles in the fleet and the annual miles driven, along with a shift to light-duty trucks for personal use, including sport utility vehicles (SUVs). To meet today’s environmental and energy challenges, there is a need to markedly improve the fuel efficiency of the light-duty fleet in order to lower both CO2 emissions and oil imports from their current upward trajectories.
Recent History
Before discussing evolutionary vehicle technologies, it is useful to review the evolution of some key automotive technologies since the 1960s. These changes were driven in part by two major sets of regulations:
Corporate average fuel economy (CAFE)1standards. These provisions were established by Congress in 1975. The near-term goal was to double new car fuel economy by model year 1985.
The Clean Air Act of 1970. The act (as amended in 1990) for the first time set federal limits on vehicles’ emissions of so-called criteria pollutants (such as lead, particulates, carbon monoxide, volatile organic compounds, and nitrogen oxides [NOx]).2
Industry responded to this pair of regulatory challenges with a “total-systems” approach to optimize the sparkignition engine vehicle and its fuel. That approach yielded numerous important changes in major components, including switching to unleaded fuel, addition of the catalytic converter, engine computer control, port fuel injection, the four-speed automatic transmission with torque-converter lock-up, and approximately 1,000 pounds of weight reduction due to platform and material changes. Even though
1
The Energy Policy Conservation Act, enacted into law by Congress in 1975, added Title V, “Improving Automotive Efficiency,” to the Motor Vehicle Information and Cost Savings Act and established CAFE standards for passenger cars and light trucks. The act was passed in response to the 1973-1974 Arab oil embargo. An overview of these regulations is available at http://www.nhtsa.dot.gov/cars/rules/café/overview.htm.
2
Title II, Part A of the act covers motor vehicles. It gives the administrator of the Environmental Protection Agency the duty “to prescribe (and from time to time revise) in accordance with the provisions of this section, standards applicable to the emission of any air pollutant from any class or classes of new motor vehicles or new motor vehicle engines, which in his judgment cause, or contribute to, air pollution which may reasonably be anticipated to endanger public health or welfare.”
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4
Alternative Technologies for Light-Duty Vehicles
In keeping with its statement of task, the committee considered “whether other technologies would be less expensive or could be more quickly implemented than fuel cell technologies to achieve significant reductions in carbon dioxide emissions and oil imports.” After considering a range of alternative technologies and the budget constraints of the study, the committee chose to evaluate in depth two options that received increased emphasis in the Energy Independence and Security Act of 2007 (EISA 2007): (1) evolutionary improvements in internal combustion engines (ICEs) and hybrid electric vehicles (HEVs) and (2) a biofuel option (i.e., fuel derived from biomass). Many of these vehicle technologies will be needed through 2020 to meet the significantly higher fuel economy standards required by EISA 2007. These are not the only technologies that could make a contribution to improved fuel economy and reduced carbon dioxide (CO2) emissions, but they are the ones for which the committee felt confidence in projecting technology availability, costs, and consumer acceptance. Others are discussed briefly below.
The ultimate goal of this chapter is to estimate the extent to which continuing evolution of light-duty vehicle technologies, increased use of improved hybrid electric vehicles, and the use of biofuels can reduce oil imports and greenhouse gas emissions through 2050.
EVOLUTIONARY VEHICLE TECHNOLOGIES
Over the past 25 years, oil consumption in the light-duty vehicle fleet has grown because of an increase in the number of vehicles in the fleet and the annual miles driven, along with a shift to light-duty trucks for personal use, including sport utility vehicles (SUVs). To meet today’s environmental and energy challenges, there is a need to markedly improve the fuel efficiency of the light-duty fleet in order to lower both CO2 emissions and oil imports from their current upward trajectories.
Recent History
Before discussing evolutionary vehicle technologies, it is useful to review the evolution of some key automotive technologies since the 1960s. These changes were driven in part by two major sets of regulations:
Corporate average fuel economy (CAFE)1 standards. These provisions were established by Congress in 1975. The near-term goal was to double new car fuel economy by model year 1985.
The Clean Air Act of 1970. The act (as amended in 1990) for the first time set federal limits on vehicles’ emissions of so-called criteria pollutants (such as lead, particulates, carbon monoxide, volatile organic compounds, and nitrogen oxides [NOx]).2
Industry responded to this pair of regulatory challenges with a “total-systems” approach to optimize the sparkignition engine vehicle and its fuel. That approach yielded numerous important changes in major components, including switching to unleaded fuel, addition of the catalytic converter, engine computer control, port fuel injection, the four-speed automatic transmission with torque-converter lock-up, and approximately 1,000 pounds of weight reduction due to platform and material changes. Even though
1
The Energy Policy Conservation Act, enacted into law by Congress in 1975, added Title V, “Improving Automotive Efficiency,” to the Motor Vehicle Information and Cost Savings Act and established CAFE standards for passenger cars and light trucks. The act was passed in response to the 1973-1974 Arab oil embargo. An overview of these regulations is available at http://www.nhtsa.dot.gov/cars/rules/café/overview.htm.
2
Title II, Part A of the act covers motor vehicles. It gives the administrator of the Environmental Protection Agency the duty “to prescribe (and from time to time revise) in accordance with the provisions of this section, standards applicable to the emission of any air pollutant from any class or classes of new motor vehicles or new motor vehicle engines, which in his judgment cause, or contribute to, air pollution which may reasonably be anticipated to endanger public health or welfare.”
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FIGURE 4.1 U.S. light-duty vehicle fuel efficiency and performance trends from 1975 to 2005. NOTE: Three-year moving average, used to smooth curves, means that for each year, what is shown is the average of that year with the two previous years. SOURCE: EPA (2006).
there were major increases in the use of plastics and aluminum, the highest percentage new material introduction was high-strength steel. Since 1980, vehicle efficiency has continued to improve even as air pollution laws and regulations have tightened, forcing vehicle designers to accommodate a multiplicity of goals. Engines, transmissions, drivetrain components, and vehicle aerodynamics have all improved remarkably, with these improvements spread among emissions reduction, improved performance, greater weight, and more power-consuming accessories in the cabin (EPA, 2005).
Since 1987, only a small fraction of these improvements have been directed to fuel economy, as shown in Figure 4.1 (EPA, 2006). After an initial marked drop in average vehicle weight and a significant fuel economy increase, most of the continuing improvement in power train technology went to overcome a steady increase in vehicle weight and to provide enhanced performance, particularly faster acceleration. The baseline case projects this trend to continue into the future because it is driven by consumer choices similar to those that have been made over the past 20 years. Although some changes in motorist attitudes can be expected, driven by higher fuel prices and an enhanced environmental consciousness, they are unlikely to produce radical improvements in light-duty vehicle fuel efficiency. However, suitable policies promoting fuel efficiency could change vehicle design priorities and result in significantly improved vehicle fuel efficiency, leading to reductions in oil imports and CO2 emissions.
During the same period in Europe, major advances have been made in compression-ignition (diesel-fueled) engines; today, such engines form a major part of Europe’s CO2 reduction efforts. There is concern about NOx and particulate emissions from diesel engines, and the standards for these emissions have been tightened, as have the specifications on diesel fuel to enable effective emission control technologies. This activity has resulted in the development of new after-treatment devices for NOx and particulates for diesel engines. These technologies continue to improve.
In 1997, the hybrid electric vehicle was introduced in Japan and, in 2000, imported to the United States. In 2006, 364,845 HEVs (with 254,545 in the United States) were sold worldwide out of 68,727,429 total global vehicle sales. The National Research Council (NRC) report on the hydrogen economy and fuel cell vehicle, which was released in February 2004 but had access only to actual year-end 2002 HEV sales numbers, projected that 2006 HEV sales in the United States would represent 2 percent of the market and would increase 1 percent annually thereafter for the next 9 years and 5 percent per year for the next 10 years (NRC, 2004). The actual 2006 U.S. sales were 1.5 percent. As shown in Figure 4.2, even though gasoline prices have fluctuated dramatically from 2004 to the present, the actual increase in sales was less in 2006 than in 2005 (DOE, 2007).
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FIGURE 4.2 U.S. hybrid electric vehicle sales through 2006. Note that the order of vehicles in the key matches the order indicated in the bars of the graph from top to bottom. SOURCE: DOE (2007). Available at http://www1.eere.energy.gov/vehiclesandfuels/facts/2007_fcvt_fotw462.html.
Potential for Reducing Oil Use in Conventional Light-Duty Vehicles
This section examines options for improving fuel economy in conventional gasoline, diesel, and hybrid power train vehicles. Although not discussed explicitly in this chapter, CO2 reductions directly follow improvements in fuel efficiency in these types of vehicles. CO2 estimates are presented in Chapter 6. Various transportation demand reduction efforts are under way, but are not considered in this report.
Gasoline and diesel power train vehicle technologies are considered mature, but they will continue to advance and improve for the foreseeable future. The following material reviews the areas in which improvements can be expected to continue, evaluates the potential extent of these improvements, and projects their impact on fuel economy. Potential cost increases are also estimated, although these are more qualitative. For improved vehicle technology to have a significant effect on CO2 emissions of the light-duty fleet and on oil imports, it must be directed to fuel efficiency improvements and must be included in a large fraction of all vehicles manufactured and sold in a given year; then it, or further improved versions, must continue to be manufactured for the time required to turn over a majority of the vehicles in the existing fleet. This is a decadal process.
Technical Improvements in Gasoline and Diesel-Powered Light-duty Vehicles
The approaches to improving the fuel economy for gasoline and diesel vehicles are well understood. Each of these areas offers considerable potential fuel efficiency benefits. The methods considered here include the following:
Efficiency improvements
Transmission evolution
Vehicle weight reduction
Aerodynamic improvements, reduced rolling resistance
Efficiency Improvements—Spark-ignition Engine. Technical improvements that can be applied to spark-ignited internal combustion engines include the following:
Variable valve timing (VVT) and variable valve lift offer improvements in part-load engine efficiency. Engines only operate at full load during hard acceleration and hill climbing; the remainder of the time, engine operation is at part load, when much less power or a smaller engine is all that is needed. VVT and lift allow the engine to supply the reduced power requirement with improved fuel efficiency. The next major change is camless valve actuation (CVA), which can vary lift and timing and also allow strategies such as cylinder deactivation at light loads while simultaneously reducing valve-train friction.
Cylinder deactivation (also called cylinder cut-out) can also reduce fuel consumption under part-load vehicle operation. Efficiency is improved by having fewer cylinders working at higher load. This technology is already being implemented, especially in V8 engines, but constitutes a small fraction of the market to date. Application to four- and six-cylinder engines is more difficult because of noise and vibration problems.
Gasoline direct injection (GDI) and GDI combined with turbocharging allow higher compression ratio operation because of the cooling effect of in-cylinder fuel evaporation, which protects against knock. Raising the compression ratio increases efficiency (which is why diesels are relatively efficient), but conventional spark-ignition engines are susceptible to pre-ignition (knock) if the compression is too high. With GDI, the cooling effect counters the heat from the high compression. This technology benefits from variable-geometry turbines for turbo boosting and variable compression ratio. This produces more horsepower from a smaller engine, allowing weight reduction and fuel savings. This technology option is in limited production in Europe and Japan and is on a few models in the United States.
Homogeneous-charge compression ignition (HCCI) enabled by CVA might decrease fuel consumption more than GDI with turbocharging. HCCI involves the introduction of a homogeneous air-fuel mixture, where the fuel is a gasoline-range hydrocarbon, into the cylinder. HCCI uses the same kind of “charge” as a spark-ignition engine but with higher compression, whereas a classical compression-ignition engine uses a stratified charge with a higher-boiling fuel that is injected directly into highly compressed air in the cylinder. Although HCCI has typically been considered a separate technology, its components will most likely be implemented as engine technology advances providing additional reductions in fuel consumption.
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Other changes that would result in improved fuel consumption include reduced friction in the engine and drivetrain via new materials or better lubricants; intelligent start-stop with enhanced starter and battery, which involves engine shutoff when the vehicle stops (e.g., at a red light); variable compression ratio technology; improved lean-NOx catalyst technology; and improved engine controls and integration.
Normally aspirated spark-ignition engine efficiency is expected to improve significantly as these technologies are successfully applied. Collectively, their application can be expected to increase peak engine efficiency by about 7 to 9 percent by 2030. For turbocharged spark-ignition engines, the efficiency gain could be better than that projected for the normally aspirated spark-ignition engine by 2030 (Kasseris and Heywood, 2007). Turbocharged gasoline direct-injection engines with variable valve lift and timing could approach the efficiency of today’s diesel engines (see discussion below), which will also see efficiency improvements during this time. Turbocharging is used commercially in a small fraction of light-duty vehicles today, mainly to enhance performance. These engine efficiency gains can be translated into reductions in fuel consumption and CO2 emissions.
Efficiency Improvements—Compression-ignition (Diesel) Engines. Significant improvements to diesel engines have been made over the past 20 years, but further gains can be expected. The several technologies that will add to this include improved fuel injection, higher boost levels, series turbocharging, more advanced engine control technology, and variable valve train control. Although the major issue with diesel—emissions—has largely been resolved, current emission control technology to meet U.S. emission laws is complex and costly. Several currently produced European diesels have met the Environmental Protection Agency’s (EPA’s) Tier 2 and California’s LEV (low emission vehicle) 2 emissions standards, although doing so subtracts from their overall efficiency. The diesel’s fuel consumption is 18 to 25 percent better (lower) than that of a normally aspirated sparkignition engine, and it is robust across all driving conditions. Diesel engines offer tremendous low-end torque and are well suited to towing and cargo hauling. With the application of the technologies noted above, diesel engine fuel consumption can be expected to be reduced further by about 30 percent over typical diesel fuel consumption today after subtracting for emissions control (Kasseris and Heywood, 2007).
Transmission Evolution. Transmission type and operation also have a significant effect on fuel consumption. U.S. drivers have favored automatic transmissions, which have been less efficient than manual transmissions. New transmission technologies provide the user-friendliness of automatic transmissions with the efficiency of a manual transmission. Developments, such as the conventional 6/7/8 speed automatic transmission and the automated manual transmission, which will soon be offered on some European models, will be almost as efficient as manual transmissions. With these technologies, transmission efficiency should be about 93 percent, a significant gain over typical automatic transmissions today (Kasseris and Heywood, 2007). In addition, transmissions with more speeds allow the engine to operate closer to its optimum efficiency, reducing fuel consumption. Continuously variable transmissions (CVTs) allow the vehicle to vary the gear ratio continuously as it goes through its driving cycle for even greater power train efficiency (Nishigaya et al., 2001; Burke et al., 2003).
Vehicle Weight Reduction. Reducing vehicle weight is a significant factor in improving vehicle fuel economy. Weight reduction reduces the energy required to move the vehicle. In doing so, the power plant may be reduced in size and weight itself, further improving fuel efficiency. Weight can be reduced by using advanced materials such as advanced, high-strength steel, aluminum, magnesium, carbon fiber composites, and plastics or by redesign to reduce the weight without using advanced materials. Estimates of potential weight reductions range from 10 to 33 percent (Weiss et al., 2000; An et al., 2001). Although weight reductions in the upper end of this range are technically possible, reductions in the range of 5 to 10 percent by 2025 are probably achievable at a reasonable cost (Duleep, 2007), although some automakers are already planning to go further.
Other Improvements. Vehicle aerodynamics, as measured by a reduction in the coefficient of drag (Cd), has improved recently by at least 1 percent per year (Hucho, 1998; Weiss et al., 2000; An et al., 2001). If this trend continued for the next 25 years, it would provide a significant reduction in fuel consumption if car size remains the same. Reduction in car size (frontal area) would result in added fuel consumption benefits because total resistance would be reduced. Vehicle rolling resistance has historically undergone an estimated reduction of 1.1 to 1.6 percent annually. An NRC committee recently concluded (NRC, 2006) that a 10 percent reduction in rolling friction is possible for today’s replacement tires, and further improvements are possible.
Summary of Evolutionary Conventional Vehicle Improvements
A summary of the reductions in fuel consumption for spark-ignition powered vehicles from the technology advances reviewed above for the periods 2006-2015 and 2016-2025 is given in Table 4.1, based on Duleep’s work (Duleep, 2007). These technology-driven fuel-consumption improvements must be focused on delivering efficiency gains (versus providing more performance or motorist comforts) and have to be broadly incorporated into the light-duty fleet if they are to have the desired impact on reducing fuel con-
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TABLE 4.1 Potential Percentage Reductions in Fuel Consumption (gallons per mile) for Spark-ignition Vehicles Expected from Advances in Conventional Vehicle Technology by Category, Projected to 2025
2006-2015
2016-2025
Engine and transmission
12-16%
18-22%
Weight, drag, and tire loss reduction
6-9%
10-13%
Accessories
2-3%
3-4%
Idle stop
3-4%
3-4%
NOTE: Values for 2016-2025 include those of 2006-2015. SOURCE: Duleep (2007).
sumption and CO2 emissions. Combining the projections for improvements in engine, transmission, weight, parasitic loss (including friction losses, rolling resistance, and air drag), accessories, and idle-stop components suggests reductions in fuel consumption, relative to today’s vehicle and performance, of 21 to 29 percent (average = 25 percent) by 2015 and 31 to 37 percent (average = 34 percent) by 2025.
Heywood and colleagues at Massachusetts Institute of Technology (MIT) have carried out a series of studies focused on light-duty vehicle fuel efficiency (Heywood, 2007; Kasseris and Heywood, 2007; Kromer and Heywood, 2007). Their studies rely on many of the technologies discussed above but include more aggressive reductions in weight and rolling resistance and aerodynamic drag. Figure 4.3 summarizes the projected fuel consumption improvements for combinations of technology for gasoline- and diesel-powered light-duty vehicles and hybrid electric vehicles
FIGURE 4.3 Fuel consumption of light-duty vehicles with different power trains using projected 2030 technology compared to a typical 2005 gasoline-powered vehicle. NOTE: To convert to gallons per 100 miles, multiply liters per 100 km by 0.426. SOURCE: Heywood (2007).
in 2030 compared to a conventional spark-ignition gasoline engine in a 2005 midsized sedan from Heywood’s work.
The Heywood fuel consumption improvements result from changes in the engine and transmission and include appropriate vehicle weight reductions as well. It is assumed that the improvements are entirely dedicated to reduced fuel consumption. The 2030 spark-ignition gasoline-powered vehicle, with automatic manual transmission and weight reduction included, is projected to have a 38 percent reduction in fuel consumption from the 2005 version of the gasoline vehicle technology. Spark-ignition gasoline-powered vehicles with turbocharger, automatic manual transmission, weight reduction, and parasitic-loss reductions are projected to be capable of a 44 percent reduction in fuel consumption versus today’s spark-ignition gasoline vehicle and an 11 percent improvement over the 2030 spark-ignition gasoline vehicle. Heywood also concludes that CVT technology could reduce fuel consumption further, reaching a 48 percent reduction in fuel consumption compared to today. The turbocharged version of the gasoline vehicle would have an estimated $500 cost premium compared to the 2030 gasoline vehicle.
Also shown in Figure 4.3 is the projected fuel consumption of a 2030 compression-ignition diesel-powered vehicle. The 2030 diesel-powered vehicle, with weight reduction, is projected to have 47 to 50 percent lower fuel consumption than the 2005 spark-ignition gasoline vehicle on a gasoline energy-equivalent basis. It also has a projected 15 percent lower fuel consumption than the 2030 spark-ignition gasoline vehicle on a gasoline energy-equivalent basis. On a fuel volume basis (kilometers per liter of diesel fuel), the 2030 diesel engine shows an additional 15 percent benefit over the 2030 gasoline engine because of the greater energy density of diesel fuel. The diesel-powered vehicle is projected to have a cost premium of about $1,200 over the 2030 gasoline-powered vehicle. If diesels achieved significant market penetration, they could increase the potential reduction in fuel consumption to 3 percent per year. However, the committee was too uncertain of the cost penalty for diesels that meet emission regulations to be willing to include increased penetration in the scenarios of Chapter 6.
Based on the work reviewed above, and assuming a baseline of 25 mpg (miles per gallon) and fuel increases starting in 2010, Duleep’s projections correspond to a 2.3 to 2.9 percent compounding annual reduction in fuel consumption through 2025, or an average of 2.6 percent per year. This results in the average value from Duleep’s analysis, a 34 percent fuel consumption reduction, being applied by 2025. This estimate assumes the diesel improvements discussed above, but the diesel percentage of the fleet remains constant over time. While the data from Heywood suggest that this pace could continue through 2030, the committee judges, based on the discussion in the recent NRC review of the FreedomCar Fuel Partnership (NRC, 2008), that continued weight reductions beyond 2025 would be slower to enter the
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fleet, delaying Heywood’s potential for gasoline vehicles by about 5 years and slowing the pace of vehicle fuel consumption reduction to about 1.7 percent per year between 2025 and 2035. This leads to a 44 percent reduction in fuel consumption compared to 2009, which is Heywood’s projection for the turbocharged spark-ignition gasoline vehicle with automatic manual transmission.
Further, a 0.5 percent per year reduction in fuel consumption was assumed from 2035 to 2050. At this pace, a 48 percent reduction in fuel consumption would be achieved by 2050. This improvement could be reached if Heywood’s more aggressive projections for turbocharged spark-ignition gasoline vehicles with continuously variable transmissions are realized. Alternatively, the fuel consumption reductions shown in Figure 4.3 do not define the limit to how far conventional fuel economy can go. Heywood’s analysis looks at a 20 percent weight reduction, relying mainly on higher-strength metals, but future carbon composites could lead to weight reductions of one-third or more. Even limited use of carbon composites to achieve a 25 percent total weight reduction would boost Heywood’s spark-ignition vehicles to a 46-48 percent reduction in fuel consumption, up from 44 percent. Weight reductions through changes in the fleet mix, rather than changes in materials would have similar effects. A 10 percentage point shift from trucks to cars (i.e., increasing car share to 60 percent) would increase Heywood’s spark-ignition vehicles to a 45-48 percent reduction in fuel consumption. In conclusion, the spark-ignition internal combustion engine vehicle can produce marked reductions in fuel consumption and CO2 emissions in the intermediate and long term at little additional cost. Policy initiatives may be necessary to help speed additional weight reduction and associated fuel consumption reductions.
Thus, the committee judges that evolutionary vehicle technologies could, if focused on vehicle efficiency, reduce fuel consumption by 2.6 percent per year through 2025, 1.7 percent per year in the 2025-2035 time frame, and 0.5 percent per year between 2035 and 2050.
Increased penetration of diesel engines into the light-duty vehicle fleet was limited in the projections made here (and in Case 2 in Chapter 6) because of the poor public perception of diesels in the United States and concerns over meeting future tailpipe emission specifications. However, advanced diesel power trains could offer an additional 15 percent reduction in fuel consumption and a similar level of CO2 emissions reductions over advanced conventional spark-ignition power trains without turbochargers and have cost advantages over hybrid electric vehicles (Kromer and Heywood, 2007). In a high-fuel-cost environment, diesels could become a growing fraction of the light-duty vehicle fleet promoted by some shifts in government positions on diesels and a positive public relations program.
Potential for Reducing Oil Use in Hybrid Electric Light-duty Vehicles
Hybrid electric vehicles and plug-in hybrid electric vehicles (PHEVs) provide increased energy efficiency by mating a battery-powered electric motor with a conventional gasoline engine. The engine can be smaller than in conventional vehicles, and therefore more economical, but acceleration can be just as great because additional power is supplied by the electric motor. The batteries can be charged in part by regenerative braking, which captures the energy that otherwise would be lost in braking. In addition, the electric motor allows the engine to be operated at or near its optimum efficiency a greater fraction of the time.
The two hybrids are similar, except that future PHEVs would have much larger battery packs that are anticipated to be able to operate the vehicle for as much as 40 miles without the gasoline engine. When the battery charge is low, the engine kicks in and operates normally, recharging the batteries and providing power. The batteries for PHEVs can also be charged from the electric grid. Thus, for some driving patterns, no gasoline at all would be needed. PHEVs are not available commercially yet because the advanced batteries that would make them viable are not ready. As battery technology improves and costs decrease, both of these hybrid power trains will become more effective and have a smaller cost differential relative to conventional engine vehicles.
All-electric vehicles would use no gasoline, but they are even more dependent on improved battery technology than PHEVs. All-electric vehicles will have CO2 emissions that are determined by the emitted CO2 content of the electricity on the power grid.
Hybrid Electric Vehicles
When HEVs were first developed, the technology was dedicated almost exclusively to improving fuel economy. However, as with other power trains, as HEV technology has begun to mature, the benefits of hybrid electric vehicles have shifted to now represent a blend of fuel efficiency, performance, and larger vehicles. Still, HEVs presently are the most efficient light-duty vehicles. Continued improvement relative to conventional vehicle technology is likely to occur. These improvements are expected to arise largely from improved vehicle integration, allowing tighter, more optimized control of the engine operating points. Due in part to scale economies and in part to significant reductions in the cost of high-power batteries, the incremental cost of HEVs relative to conventional technology vehicles is expected to decrease. Furthermore, HEVs will benefit from continued technological development in terms of vehicle weight and accessory loads, as well as improvements in engine technology. CO2 emissions from hybrid electric vehicles are reduced as fuel consumption decreases, and current models, such as the Ford Escape Hybrid, the Honda Civic Hybrid, Nissan
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TABLE 4.2 Comparison of Projected Improvements in Vehicle Fuel Consumption from Advances in Conventional Vehicle Technology
Fuel Consumption (liters/100 km)
From Kromer and Heywood (2007)
From Weiss et al. (2000)
Relative to 2005 Gasoline
Relative to 2005 Gasoline
Relative to 2030 Gasoline
Relative to 2030 Gasoline
2005 Gasoline ICE
8.8
1
2005 Diesel
7.4
0.84
2005 Turbo
7.9
0.9
2005 Hybrid
5.7
0.65
2030 Gasoline
5.5
0.63
1.00
2030 Diesel
4.7
0.53
0.85
0.61
1.00
2030 Turbo
4.9
0.56
0.89
0.45
0.77
2030 Hybrid
3.1
0.35
0.56
0.54
0.88
2030 Plug-In
1.9
0.21
0.34
0.38
0.615
Altima Hybrid, the two-wheel drive Chevrolet Tahoe Hybrid, and the Toyota Camry and Highlander hybrids, achieve fuel consumption reductions of 26 to 34 percent, with a simple average of about 29 percent.
As indicated in Figure 4.3, analysis from MIT indicates that CO2 emissions and fuel consumption could be as much as 37 percent lower than those from a competitive 2030 spark-ignition gasoline vehicle (Kromer and Heywood, 2007). Table 4.2 compares the results of Kromer and Heywood (2007) and those of an earlier study, “On The Road in 2020,” by Weiss et al. (2000).3 The Kromer and Heywood (2007) study projects that HEVs will improve at a rate significantly faster than conventional gasoline vehicles. This may be expected in the earlier years because they are relatively new and should enjoy a steeper learning curve, but hybrids should be a mainstream technology before 2020.
While the committee acknowledges the significant potential for hybrids outlined in Kromer and Heywood, it concluded that most advances in hybrid technology are likely to lower the cost of battery packs (which will increase their appeal to consumers relative to conventional vehicles, and thus their market share) rather than increase fuel economy. To simplify the analysis in this report, the committee assumed that hybrids achieve a constant 29 percent fuel consumption reduction compared to conventional vehicles (which also improve each year) in each year. While this is conservative compared to Kromer and Heywood, it still leads to a 60 mpg average for new spark-ignition hybrids by 2050.
Thus, the committee judges that hybrid electric vehicles could, if focused on vehicle efficiency, consistently provide a 29 percent reduction in fuel consumption relative to comparable evolutionary ICEVs. This assumes that hybrids will have reached their peak in terms of fuel consumption reductions relative to conventional vehicles by 2009 and future improvements in hybrid fuel economy will be due, primarily, to the same technologies used to improve conventional vehicles. As a result this analysis assumes that hybrid vehicles reduce fuel consumption by 2.6 percent per year from 2010 through 2025, 1.7 percent per year in 2025-2035, and 0.5 percent per year between 2035 and 2050, the same as for evolutionary ICEVs.
Plug-in Hybrids
PHEVs are designed to be charged from the power grid and to have a significant range without requiring operation of the internal combustion engine or only using the engine under higher-power driving conditions. This range is dependent on the availability of economic, high-energy battery technology. Cost-effective battery technology for even a 20-mile trip is not available today. PHEVs could become commercially available within a few years if battery performance and costs continue to improve rapidly.
PHEVs will be significantly more expensive than HEVs. For a 30-mile-range plug-in hybrid, which is not considered viable with existing battery technology, the incremental cost is estimated to be between $3,800 and $4,300 over an HEV, the largest component of which is the battery, estimated to be about $2,500 (Kromer and Heywood, 2007).
PHEVs could save more oil than HEVs. Table 4.2 shows an improvement of 1.2 liters per 100 km (about one-half gallon per 100 miles), but the actual savings would be highly dependent on driving patterns and the frequency with which
3
Weiss et al. (2000) did not consider plug-in hybrids. Otherwise, the results of these two studies agree reasonably well.
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the vehicle is plugged in.4 However, the plug-in hybrid’s CO2 emissions reduction will not be nearly as large as suggested by their lower fuel consumption because of the CO2 emissions associated with power generation. Today about two-thirds of our electricity is generated from fossil fuels (mainly coal and natural gas, none of which employs carbon capture and sequestering because of the expense). Still, PHEVs offer a potential path for incremental CO2 emissions savings through increased decarbonization of the power sector—an opportunity that does not exist for HEVs.
The committee chose not to include PHEVs in the alternative vehicle case even though they have significant long-term potential. The main issue for the committee was predicting the rate of battery advancement to achieve significant driving distances. This, in the committee’s view, and as presented in the recent FreedomCar Fuel Partnership review, is more than evolutionary technology (NRC, 2008). In addition, consumers’ acceptance of the additional cost relative to HEVs and their willingness and ability to plug the vehicle in essentially every day are uncertain.
Conclusion on Evolutionary Technologies
CONCLUSION: Continued advancements in conventional vehicles offer significant potential to reduce oil use and CO2 emissions through improved fuel economy, but policy measures and/or significant long-term increases in fuel costs probably will be required to realize these potential fuel economy gains in a significant number of on-road vehicles.
IMPACT OF BIOFUELS
Overview
By definition, a biofuel is any fuel that is produced from plant- or animal-based materials, generically referred to as biomass. The biofuel that is receiving the most attention today is ethanol, which can be produced from the starch and sugar in grains using fermentation and is referred to as grain ethanol. More than 6 billion gallons of grain ethanol were produced for vehicle fuel in the United States, primarily from corn, in 2007 (RFA, 2008). Ethanol can also be produced from the cellulosic material in plants using biotechnology and is referred to as cellulosic ethanol. Cellulosic ethanol production technology is evolving today and is not yet fully commercial. Higher alcohols, such as butanol, which can use the gasoline infrastructure directly, can also be produced from biomass by biological routes. Fuels that can substitute for diesel fuel, referred to as biodiesel, can be produced from plant oils and animal fats by esterification. A range of fuels can also be made from biomass using thermochemical conversion of the biomass to syngas followed by catalytic synthesis of a range of products, including gasoline and diesel. These are also biofuels because they are derived from biomass.
Biofuels provide the opportunity to sustainably produce liquid fuels for the U.S. transportation system, reduce oil imports, and reduce carbon dioxide emissions from the transportation sector because the CO2 emitted from their combustion is captured in the next plant growth cycle. The true impact of biofuels on CO2 emissions and oil use requires a full life-cycle analysis because fossil fuels, including oil, are used in growing and collecting biomass and, in the case of grain-based ethanol, in processing biomass into biofuels. Also, CO2 and other greenhouse gases can be released if land use and land management practices change in conjunction with the production of biomass for energy. When land use changes result in a reduction in soil carbon, that carbon is released and emitted as CO2. This is a major concern. Proper land use changes could result in storage of more carbon in the soil, decreasing net CO2 emissions. Biomass-based biofuels are an emerging area that is expected to see much change in the next several decades.
This section first discusses biomass availability and some of the associated issues. It then reviews the production technology for those biofuels that are commercial or close to commercialization. It also briefly discusses biofuel technologies that have longer-term potential but are today in the research stage. The primary biofuels for transportation considered here include ethanol from corn grain, ethanol from cellulose, biobutanol from starch and sugars, and biodiesel from plant oils. At the end of the biofuels section, gasification of biomass to produce synthesis gas is discussed briefly; this can be converted into diesel and gasoline. It represents a parallel route to biofuel production. Today, it is not clear which route to biofuels (biochemical or thermochemical) will be the most economic.
An important biofuel driver in the United States is political. The Energy Policy Act of 2005 mandates 7.5 billion gallons per year of ethanol and/or biodiesel in the nation’s fuel supply by 2012. The President’s State of the Union Address in January 2007 increased the target volume of renewable and alternative fuels to 35 billion gallons per year by 2017. (Then in December 2007, EISA 2007 was enacted with a goal of 36 billion gallons per year by 2020.) These initiatives are acting to drive production and demand for biofuels, particularly ethanol, in the United States. Note that the energy content of ethanol is approximately two-thirds that of gasoline—thus, 35 billion gallons replaces about 24 billion gallons of gasoline. However, if engines can be optimized to operate on ethanol instead of gasoline (e.g., a higher compression is possible with ethanol), they will be more efficient, somewhat compensating for the lower energy content of the fuel.
4
A 40-mile PHEV driven 40 miles every day and plugged in every night would use essentially no gasoline at all, saving as much as 365 gallons per year relative to a 40 mpg HEV. The same vehicle driven 20 miles every day or 80 miles every other day would save exactly half that.
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Biomass Availability
Biomass is anything that grows, and since it is renewable if grown responsibly, it represents a sustainable source of feedstocks for energy, fuels, and petrochemical production. For the present time, easily processed agricultural crops, such as corn, will likely be the major biomass feedstock to the emerging biofuels industry. Low- or even negative-cost materials such as waste greases and cooking oils will also be used to the extent available. In the mid term, agricultural crop residues, forestry residues, and short-rotation trees will likely provide the feedstocks to an expanding biofuels industry. However, in the longer term, energy crops including perennial grasses and algae will probably have to be added to the mix to supply the amount of feedstock needed to meet national bioenergy goals.
Current annual U.S. grain production includes about 11 billion bushels of corn, 2.9 billion bushels of soybeans, 2.0 billion bushels of wheat, and significantly lesser quantities of sorghum, rapeseed, oats, and barley (Perlack et al., 2005). Higher corn prices and increased interest in ethanol resulted in an increase in corn acreage from 78 million acres in 2006 to 93 million acres in 2007 (Achenbach, 2007). Much of this increased corn acreage came at the expense of other grains, mainly soybeans. However, most of the grain grown in the United States has traditionally been dedicated to food and livestock feed supply and as such has high societal economic value. The result is that a maximum of about 25 percent of any grain is likely to be available for production of transportation fuel. This observation is based on recent experience in which corn prices rose sharply when ~20 percent of the grain was diverted to ethanol production. The U.S. Department of Agriculture (USDA) estimates that in the mid term, the United States can meet food, feed, and export demand for corn and not seriously disrupt markets with less than 20 percent of the corn crop being diverted to ethanol (USDA-OCE, 2005).
In comparison, the amount of non-grain biomass in the United States is potentially significantly larger. Walsh et al. (2000) estimated the amount of biomass currently available in the United States at less than $50 per dry ton (in 1995 dollars) to include 135 million dry tons per year of forest and mill residues and 150 million dry tons per year of agricultural crop residues. However, most of the mill residues are already being used for heat and power in the lumber, pulp, and paper industry. The total estimated biomass available at this price in the United States increases to 510 million dry tons per year when municipal wood wastes (37 million dry tons) and dedicated energy crops (188 million dry tons) are included. This is an estimate of the biomass that was projected to be economically available in 2000 for less than $50 per dry ton in 1995 dollars (Walsh et al., 2000). This would be about $68 per dry ton in 2007 dollars. The cost of biomass supplied to an ethanol production plant would have to be less than about $65 per ton to be economical with future production technologies. However, there are significant uncertainties as to how much biomass would actually be available at this price and whether it could be converted into fuel at a close-to-acceptable cost.
More recently, Milbrandt (2005) estimated the current amount of biomass that is technically available (meaning without considering costs) in the United States to be 466 dry tons (423 million dry tonnes) per year using a state-by-state geographic information system (GIS) buildup. The major components are 184 million dry tons (132 million dry tonnes) per year of forest, mill, and urban wood residues; 173 million dry tons (157 million dry tonnes) per year of crop residues (estimated using a 35 percent recovery of total crop biomass residue); and 99 million dry tons (84 million dry tonnes) per year of switch grass on Conservation Reserve Program (CRP) lands. Perlack et al. (2005) estimated that the technically available sustainable annual forest resource was about 368 million dry tons per year, of which 142 million dry tons was currently being used, leaving about 226 million available for conversion to fuels or energy per year. They estimated that about 194 million dry tons of biomass per year could be sustainably removed from agricultural lands today. This number includes 15 million dry tons of grains to biofuels. They do not include energy crops in the currently available number.
Table 4.3 summarizes the committee’s estimate of the main components that make up the non-feed and food crop biomass available in the United States in the near term (today) at less than about $68 (2007 dollars) per ton, delivered to the processing plant. Also included in the table are other estimates for comparison. Crop residues are readily estimated (~60 percent recovery) and are available annually. Energy crops are estimated at 3 dry tons per acre per year on ~30 million acres of CRP land (which could be available in the near term). Forest residue is estimated at less than half of that projected to be available from logging residues and fire suppression (Perlack et al., 2005). Wood waste estimates are from Walsh et al. (2000) and Perlack et al. (2005). More than 95 percent of mill wastes are used internally already. Biomass costs, including transportation distance and cost, remain major uncertainties.
With future technology, Perlack et al. (2005) projected that today’s technically available biomass amount could be increased to more than a billion dry tons per year within 35 to 40 years through a combination of technology changes (e.g., higher crop yields and improved residue collection technology), full adoption of no-till cultivation, and changes in land use to accommodate large-scale production of perennial crops. Forest residues were projected to increase by more than 225 million dry tons per year from today’s use level, and by 88 million dry tons per year from today’s estimated forest biomass availability to 368 million dry tons per year, through a combination of recovering wood wastes produced through traditional and improved logging operations, forest thinning for fire suppression, and increased productivity and
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TABLE 4.3 Estimated Primary Solid Biomass Components Available in the United States in the Near Term and 2030 for Less Than About $65 per Ton (2007 dollars)
Source
Biomass Amount (million tons per year)
NRC Estimatea
Walsh et al. (2000)
Milbrandt (2005)
Perlack et al. (2005)
NRC 2030
Crop residues
160
50
173
179
315
Forest residues
55
44
62
136
55
Mill wastesa
5
90
88
106
Urban wood waste
30
37
34
37
30
Energy crops
85
188
99
—
100
Total
335
509
456
458
490
aNRC estimate includes only that fraction that is estimated as not already being used.
use of forest growth beyond lumber requirements. Agricultural biomass availability increased even more than forest residue availability. Under their moderate-yield growth case, crop residue yields were assumed to increase up to 50 percent for corn and 100 percent for soybeans, and the efficiency of recovery increases from 60 to 75 percent over the next 40 years. Thus, recoverable corn stover today is estimated at 75 million dry tons per year at 50 percent recovery. Total stover biomass is therefore 150 million dry tons per year. This increases by 50 percent to 225 million dry tons of total stover produced, and when 75 percent of this is recovered, corn stover available for biofuel production is 170 million dry tons per year. Estimates for small grains and soybeans increase from 6 million dry tons per year (many current residue levels are too low to recover economically) to 26 dry tons per year. Recoverable wheat straw was estimated to increase from 11 million dry tons per year to 35 million dry tons. Grain quantities for biofuel production increase about 70 percent. Perlack et al. (2005) project even more aggressive growth numbers in their high-yield version of this case. For example, corn stover increases to 256 million dry tons (versus 172 million) per year, and most other agricultural biomass sources increase similarly. Without perennial crops, the projected amount of sustainable agricultural-based biomass production ranges from 425 million to almost 600 million dry tons per year (including 56 to 97 million tons each year of grains to biofuels) by 2050, assuming the high-technology changes summarized above (Perlack et al., 2005). Perennial energy crop production is assumed on 60 million acres and woody energy crop production is assumed on 5 million acres, with significant yield increases for each included. These perennial energy crops contribute between 156 million and 377 million dry tons of biomass per year. Their high-technology case involves significant land use changes raising questions of storage or loss of carbon from the soil, which could affect the CO2 impacts of biofuels (see below). Figure 4.4 shows the projected maximum technically available biomass in the United States in 2050 under the high-technology scenario
FIGURE 4.4 Projected sustainable biomass technically available in the United States by 2050, with aggressive energy crops. SOURCE: Perlack et al. (2005).
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with aggressive production of energy crops (Perlack et al., 2005). Although these numbers are for the technical amount available from the listed sources, the cost can be assumed to be as high as $100 per dry tonne (2007 dollars) for some portion of this supply. Most studies, including that of Perlack et al., estimate technically available biomass as a function of cost per tonne, typically to about or exceeding $100 per dry tonne, and different biomass types have different cost profiles. At these prices, particularly by 2050, both algae and alternative crops for biodiesel, which can be grown on land unsuitable for food crops, may add to available sources. However, at $100 per dry ton for feedstocks, biofuels are unlikely to be competitive with other options as long as carbon emissions are cost-free.
Environmental issues are also associated with massive, intensive production of biomass, particularly via more traditional agriculture. These are not well quantified today but could involve increased depletion of soil nutrients and micronutrients and potential reduction of soil carbon, resulting in increased CO2 emissions, significantly increased soil erosion, reduction in habitat and biodiversity, and increased stress on available water resources. Today, agriculture accounts for about 13 percent of greenhouse gas emissions, and land use changes account for about 19 percent (WRI, 2008). Land use changes frequently result in a loss of soil carbon; agricultural practices on cropland can either increase or decrease the amount of soil carbon (Tilman et al., 2007; Fargione et al., 2008). Thus, significant soil carbon loss can result in biofuels being a significant CO2 emissions source. The above issues will become increasingly important as the amount of biomass directed to energy and biofuel production increases.
After reviewing significant research on the major issues with biomass supply for biofuels including water availability, cost, and avoiding significant land use changes, the committee estimates that about 500 million dry tonnes per year of biomass could be sustainably produced in 20 years at reasonable cost. This involves using crop residues at twice current levels (see Table 4.3); using switchgrass, mixed prairie grasses, and other energy crops at what is considered a conservative yield on CRP lands (about 3 tons per acre per year on 30 million CRP acres); and using forest or wood wastes at a moderate level at a level projected to be available today (Table 4.3).
More specifically on the basis for the increases in Table 4.3, corn yields, both grain and stover, have been increasing at nearly 1.2 percent a year recently (Arkebauer et al., 2004), and if this continues at 1 percent per year, with increased recovery efficiency, stover recovery will double. Less aggressive increases for the remainder of the crop residues, increased recovery efficiency, a <10 percent increase in acreage planted, and other agricultural residues produce the projected biomass amount. The energy crop estimate is considered conservative in light of recent data showing an average of 5.5 to 8 dry tons of biomass pre acre per year for the majority of 23 locations across the United States (McLaughlin and Kszos, 2005; Perlack et al., 2005). The committee judges that the combination of changes to achieve these production levels appears achievable.
Beyond 2030, if the sustainable amount of biomass obtainable from forest lands, estimated at 200 million dry tons per year, is added to the estimated sustainable biomass available from agricultural lands (~500 million dry tons per year), the total annually available biomass for biofuels by 2050 is estimated to be about 700 million dry tons per year. This is less than the 1.3 billion dry tons estimated by Perlack et al. (2005) because of other energy uses for biomass such as power generation and expected cost and water limitations. As noted earlier, the quantities of biomass available as a function of price remain unclear.
Production: Grain- and Sugar-based Ethanol
Most of the ethanol produced in the United States today is produced from corn via either dry milling or wet milling production processes. In dry milling, corn is ground into a fine powder, and the resulting meal is mixed with water and the enzyme alpha-amylase, which breaks down the starch into its individual sugar (glucose) components. The mixture is then heated to kill undesirable bacteria, the mash is cooled, and a second enzyme, glucoamylase, is added to convert the glucose into dextrose. Yeast is added and ferments the dextrose to ethanol and CO2. The resulting mixture has a relatively low ethanol concentration in water because the yeast is inhibited when the ethanol content reaches a certain level (~13 percent ethanol). Ethanol is separated from water by distillation, which consumes a large amounts fossil fuel, typically natural gas, to distill off the water. The unconverted solid material consists of the corn’s protein and nutrients, which upon drying makes good animal feed. Wet milling is similar to dry milling except that the corn kernels are soaked in weak sulfurous acid for about 48 hours before grinding. The ground mixture is then separated into solid and aqueous components. The starch-containing liquor is hydrolyzed enzymatically, then fermented, and finally distilled as in dry milling. Wet milling is somewhat more expensive because of the additional processing but it is amenable to larger-scale plants. Typical fermentation times are about 48 hours; fermentation times beyond 72 hours typically experience an increased failure rate because of contamination with bacteria, acetogens, or other contaminants.
Ethanol production in Brazil starts with sugar cane. The sugar is pressed out of the cane and fermented. The crushed cane left over from the sugar removal (called bagasse) provides heat for the process and the distillation, effectively eliminating the need for fossil fuels in the manufacturing process and the resultant net CO2 emissions. Sugar cane can be grown much less expensively in Brazil and other tropical countries than in the United States, and Brazilian sugar-based ethanol has the lowest production cost of any biofuel, with
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costs running from $0.80 to $1.40 per gallon on a gasoline-equivalent basis (Paustain et al., 2006).
One bushel of corn can yield about 2.8 gallons of ethanol. To produce 3 billion gallons of ethanol per year requires about 1.1 billion bushels of corn, which in turn requires about 8 million acres. The United States currently produces about 11 billion bushels of corn annually on about 85 million acres. Thus, production of 3 billion gallons of ethanol per year consumes about 10 percent of our current corn crop. Increasing ethanol production to 6 billion, 9 billion, or 12 billion gallons per year would consume 20, 30, or 40 percent of our current corn crop, respectively. In view of the fact that we currently consume about 150 billion gallons of gasoline per year in our light-duty vehicle fleet, it is clear that corn ethanol cannot significantly impact oil imports for gasoline production, especially since ethanol contains less energy per unit volume, and much of that energy is in fact from fossil fuel. It seems unlikely that we could devote more than about 25 percent of our corn crop to ethanol because of the substantial cost increases in corn prices that would result and the impact of these higher prices across our economy and economies worldwide. Current experience is that the price of corn doubled when ~20 percent of the crop was directed to ethanol production. The U.S. Department of Agriculture (USDA) states that with projected increases in yield and corn acreage the United States should be able to produce about 11 billion gallons of ethanol per year while meeting food, feed, and export demands first (USDA-OCE, 2005). Thus, even with a 60 percent increase in corn production (increased yield and increased acreage), to move beyond 10 billion to 12 billion gallons of ethanol per year would require substantial ethanol imports, which could come from Brazil currently and in coming years potentially from a wide range of countries in Africa, Asia, and South America. Ethanol imports are currently discouraged by an import duty of $0.54 per gallon on ethanol for fuel use. This would most likely have to change if imports are to make substantial contributions to our biofuel supply. Furthermore, today there are only limited volumes of ethanol available globally, and many countries are bidding for it. Planning on large volumes of imports raises the issue of large land use changes in other countries, which carries with it other environmental issues, including the potential for increased CO2 emissions.
In 2002, with corn at $2.36 per bushel, natural gas at $3.00 per million British thermal units (Btu), and electricity at $0.04/kWeh, the cost of corn ethanol production was estimated at about $1.20 per gallon of ethanol, including return on capital (Whims, 2002). Unleaded gasoline on the Nymex in that time frame was about $0.80 per gallon. In 2007, due in large part to demand for ethanol, corn prices rose above $4.00 per bushel. Natural gas prices have also risen and are currently over $8.50 per million Btu. At $4.00 per bushel of corn and $8.50 per million Btu of natural gas, the cost of producing ethanol is about $1.70 per gallon of ethanol. If the total transportation cost to the blending terminal is $0.25 per gallon, the cost of ethanol ready to blend is $1.95 per gallon. If the $0.51 per gallon excise tax credit is applied, the apparent cost of ethanol at the blending terminal is $1.44 per gallon. Since ethanol contains 80,000 Btu per gallon, versus about 119,000 Btu per gallon for unleaded gasoline, the cost of ethanol on a gasoline energy-equivalent basis is about $2.90 per gallon. If the excise tax credit is applied, the apparent cost of ethanol per gallon at the blending terminal on a gasoline energy-equivalent basis is about $2.15. In April 2007, unleaded gasoline on the Nymex was about $2.00 per gallon (crude price was about $65 per barrel). As a consequence, due in part to the rise in corn prices attendant on higher demand, higher oil prices have not made ethanol competitive economically. Thus in 2007, the Nymex selling price of ethanol was about $2.05 per gallon. The higher cost can be justified on the basis of the high blending octane value of ethanol, but there are volume limits on this. In other words, once the required octane rating of the blended gasoline is reached, it is not justified economically to blend more of the higher-cost material into the pool because it only increases the cost of the fuel.
Cellulosic Ethanol
To address the ethanol volume issue as well as greenhouse gas emissions, biomass cellulose is a more attractive route because of its larger volume potential and because most of the feedstock is not in the food or feed chain. The stems, stocks, and leaves of plants and the trunks of trees (wood) are all composed of hemicellulose, cellulose, and lignin. Hemicellulose is a polymer of several sugars. Cellulose is a polymer of the six-carbon sugar glucose. These sugars can be fermented if the individual sugar units can be broken out of the hemicellulose and cellulose polymer chains. This is the challenge that must be overcome for cellulose- and hemicellulose-based biofuels to reach commercial viability via the biochemical route. Lignin is a highly cross-linked polymer that does not contain any sugar components but can be used as a renewable fuel for energy needed in processes used to convert the biomass to a liquid fuel.
Relevant biomass that could be a feedstock for cellulosic ethanol production can be essentially anything that grows. However, every biomass type is different and would be expected to offer different challenges for conversion. For illustrative purposes, consider the biomass associated with corn, with the exception of the grain. This biomass includes corn stocks, leaves, husks, and cobs and is referred to as corn stover. The corn stover is available only about one month per year and must be collected, bailed, and delivered to a central processing site. It must undergo washing and grinding for size reduction before it is introduced into the process.
The cellulose-based process is more complex than that for ethanol production from grain, particularly because of the need for acid hydrolysis (or another processing step) to break up the complex cellulose-hemicellulose-lignin struc-
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ture before enzymes can free sugar molecules from the cellulose for fermentation, which is effectively the third major step. Each of these steps typically involves separate vessels. Further, the total residence time in the process is about twice as long as for grain ethanol production. These add capital and operating cost to the process. The increased hydrolysis and fermentation times can also lead to increased contamination and fermentation problems and to increased cost. Although the costs of the hydrolytic enzymes have been reduced tenfold or more over the last decade, this remains the key area in need of R&D attention. Cellulase is currently estimated to cost between $0.10 and $0.30 per gallon of ethanol produced (D’Aquino, 2007). A major development required for cellulosic ethanol to become competitive with grain ethanol and potentially with oil-based fuels is the development of new, more robust organisms that can hydrolyze cellulose and ferment the freed sugars in the same vessel at acceptable rates. This would reduce capital and operating costs. At this point there is no clear evidence that such robust organisms have been developed. Organisms that could operate at higher concentrations of ethanol would also reduce cost. Furthermore, different improved, more robust hydrolysis and fermentation cultures also require development.
Costs
Although the plant equipment can currently be specified and designed, a relatively large uncertainty in the production cost and the technical readiness will remain until the technology has been demonstrated on a commercial scale. Furthermore, the increased complexity, greater number of vessels, and longer residence times associated with cellulosic ethanol, compared to grain ethanol, production means that the cost of ethanol via the cellulosic route would significantly exceed that for the grain-based route. Using the above parameters, the estimated ethanol production cost is around $2.50 per gallon. Paustain et al. (2006) estimate the cost of a gallon of ethanol to be $1.95 ± 0.65 for biomass costing $35 per dry ton. While DOE targets for “mature” cellulosic biomass conversion technology project parity with fossil fuels for transportation, achieving such parity represents a large stretch for the technology, which is yet to be demonstrated commercially.
Production Volumes
The potential ethanol production from corn stover can be estimated from the expected yield per ton of biomass of a cellulosic ethanol plant. Using the Perlack et al. (2005) estimate of 75 million dry tons of corn stover currently available per year and 60 gallons of ethanol per dry tonne leads to 4.5 billion gallons of ethanol from corn stover per year. For a typical plant size of 2,000 dry tons per day, 110 plants would be required to produce this amount of cellulosic ethanol from corn stover. This is a similar number of plants to that in place for grain ethanol.
From the total estimated currently available crop residue of 160 million dry tons per year (see Table 4.3), including corn stover, and a yield of 60 gallons of ethanol per dry ton, it would be possible to produce about 10 billion gallons of ethanol per year. If process improvements increase the conversion efficiency to 90 gallons of ethanol per dry ton, 14 billion gallons of ethanol could be produced per year. If by 2030, crop residues total 315 million dry tons per year and sufficient forest residues and perennial crops are added to reach a total sustainable annual biomass production of 500 million dry tons, the ethanol production potential from cellulosic materials could reach about 40 billion gallons per year at an average yield of 80 gallons per dry ton. Producing 35 billion gallons of biofuels by 2017, the current administration and legislative target, is a large stretch that is probably not achievable (see Case 3 in Chapter 6) without major biofuel imports. Further, without the development of new organisms that can simplify the cellulose conversion process, reducing capital costs and residence time, cellulosic ethanol will remain significantly more expensive to produce than grain ethanol, requiring significant subsidies to be economically attractive.
Biobutanol
Biobutanol is another potential entrant into the automotive biofuel market. Biobutanol is a four-carbon alcohol (versus the two-carbon alcohol ethanol). Several technologies to produce it are in the R&D phase. The one receiving the most attention is the acetone-butanol-ethanol (ABE) process. As currently envisioned, this process involves the bioconversion of sugars or starches from sugar beets, sugar cane, corn, wheat, or cassava into biobutanol using a genetically engineered microorganism, Clostridium beijernickii BA101. The midterm goal is to start with cellulose, but this awaits the success of the economic conversion of cellulose and hemicelluloses into sugars. Biobutanol has many attractive features as a fuel. Its energy content is close to that of gasoline; it has a low vapor pressure; it is not sensitive to water; it is less hazardous to handle and less flammable than gasoline; and it has a slightly higher octane than gasoline. Thus, it can go directly into the existing distribution system and substitute directly for gasoline. Its main drawback to date is its cost. To attack the cost and initiate market entry, DuPont and BP have joined forces to retrofit an existing bioethanol plant to produce biobutanol using DuPont-modified biotechnology (Chase, 2006). An improved next-generation bioengineered organism is projected to be available within a few years. The promise of biobutanol is to start with cellulosic biomass as a feedstock. The cellulose approach is being studied but is far from commercial.
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Biodiesel
Biodiesel is a renewable fuel produced from vegetable or animal oils and fats. It is made by the transesterification of these feedstocks, typically with methanol. The reaction is catalyzed by a base such as sodium hydroxide (NaOH). The main products are a methyl ester, which is referred to as biodiesel, and glycerol, which has a variety of other uses (see Figure 4.5). If ethanol is used instead of methanol, the product is an ethyl ester; methanol has been preferred because it is cheaper than ethanol. Biodiesel has about 93 percent of the energy content per gallon of oil-based diesel fuel and has a cetane number between 50 and 60, with 55 being typical.
Cost of Biodiesel
Driven by the feedstock cost, which is about 80 percent of a plant’s operating cost, the cost of biodiesel has been significantly higher than the cost of conventional diesel. Operating costs (excluding feedstock costs) are $0.40 to $0.45 per gallon. The variable cost (operating + feedstock, excluding capital) of producing biodiesel has been in the range of $2.50-$2.80 per gallon for the last decade, with soybean oil around $2.00 per gallon (Hofman, 2003). This cost is for the use of methanol for esterification; if bio-based ethanol were used, the cost would be higher, but net CO2 emissions would be reduced. The largest cost component in this is for the feedstock, which for soybean oil has been around $2.00 per gallon of biodiesel. Increasing grain prices have increased the cost of soybean oil also. Yellow grease (used cooking oil) is about $1.00 per gallon but is limited in volume. Outlays to cover capital and other fixed costs and returns to investors are likely to be more than $0.20 per gallon, bringing the total cost of biodiesel to around $3.00 per gallon for traditionally priced soybean oil (Hofman, 2003). Wholesale diesel fuel during this period was about $1.80 per gallon, which means that biodiesel was not competitive. However, biodiesel from yellow grease was cost competitive. In 2007, soybean oil rose to $3.00 per gallon, and the cost of biodiesel was about $3.50 per gallon, whereas the cost of conventional diesel was about
FIGURE 4.5 Transesterification of vegetable oils.
$2.10 per gallon. In spring 2008, soybean oil rose to about $4.20 per gallon (Brasher, 2008). The current viability of the industry depends on government incentives, which are about $1.00 per gallon (Brasher, 2008), and programs.
Production of Biodiesel
U.S. production grew from about 500,000 gallons per year (33 barrels per day [bbl/d]) in 1999, to 2 million gallons per year in 2000, to 250 million gallons per year in 2006, and 450 million gallons per year in 2007 (Brasher, 2008). This compares with 690 million gallons per year in Germany and about 200 million in France in 2006. For most of the period up to 2006 in the United States, the utilization rate of installed capacity was less than 25 percent. In 2006, 250 million gallons were produced from an installed capacity of about 580 million gallons per year (about 45 percent capacity utilization [Hofman, 2003]). In Europe, the industry has grown rapidly, with capacity expanding in step with demand. The United States is also in the process of building massive additional capacity, with 57 plants under construction or in the planning stages as of January 2008 (Brasher, 2008). Further, South America and Asia are building excess capacity in expectation of large export markets (Weirauch, 2006). Countries in these regions are studying new plants such as jatropha, which can be grown in areas unsuitable to traditional crops.
To produce 1 billion gallons of biodiesel from soybean oil would require about 690 million bushels of soybeans, 22 percent of our recent annual soybean crop of about 3.1 billion bushels per year. As with corn ethanol, an expansion in soybean oil use for biodiesel beyond perhaps 35 percent of our soybean production is likely to cause significant economic ripples through food and agricultural markets. If we are to go beyond the level of 1.5 billion gallons of biodiesel per year, we will most likely need to depend substantially on imports. Yellow grease availability will limit biodiesel production from this material to about 100 million gallons per year (Radich, 2004). In the case of biodiesel, current technology is relatively straightforward and well proven. Because of the simplicity of the process, there is not much to be gained in terms of economies of scale or process optimization. Forward projections are fairly linear and depend on the availability of animal and vegetable oils. The major issue is cost. The other option involves producing diesel fuel via biomass gasification, pyrolysis, or Fischer-Tropsch synthesis. This is considered in “Gasification Routes to Biofuels” below in this chapter.
Biodiesel has the advantages that it has good requisite diesel fuel properties (excluding low-temperature properties) and can be blended into the fuel supply, where desired, to utilize the existing infrastructure. Because production is simpler and less energy intensive than for corn ethanol, the use of biodiesel, on an energy-equivalent basis, can reduce CO2 emissions per gallon by about 10 to 50 percent, depending
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on the plant oil source and the amount of fossil fuels used in growing, harvesting, and producing the biodiesel compared to a gallon of conventional diesel on a life-cycle basis (UK Department for Transport, 2008). For soybean oil the CO2 reduction is about 30 percent. Biodiesel can reduce oil consumption by 90 to 95 percent relative to oil-based diesel usage (Radich, 2004).
Future Biosynthetic Biofuels
Over the last 25 years significant research efforts have focused on algae for biofuel production. Algae can be grown in both salt- and freshwater environments, in shallow ponds, tubes, or raceways utilizing waste nutrients. One area of research is the development of algae that have high lipid productivity (Briggs, 2004; Pacheco, 2006). The oil would be extracted from the collected algae. Progress was made early in the period, but efforts have slowed greatly. Recent reevaluation suggests that current costs are well over $4.00 per gallon, and much more progress is needed if this technology is to have an impact in the foreseeable future (Pacheco, 2006). Alternatively, algae can be grown as a source of cellulose for biofuels.
With the rapid growth of synthetic biology and the increasing ability to engineer organisms to produce specific chemical or fuel products, the area of genetic engineering of biomass has witnessed significant renewed interest (Savage, 2007). Microbes that produce and excrete specific hydrocarbons minimize the energy-consuming separation costs that are the key issue with ethanol formation. Furthermore, properly designed hydrocarbon products in either the diesel range or the gasoline range could fit into the existing infrastructure without requiring new infrastructure as needed for ethanol at larger scales. Although no specific processes can be considered for commercial evaluation at this point, the rate of change and potential specificity that can be expected could produce rapid changes in the not-too-distant future. It is difficult to project future developments; thus, this technology is not considered further here, but it deserves careful tracking in the future.
Gasification and Pyrolysis Routes to Biofuels
Biomass gasification involves the conversion of biomass to a mixture of carbon monoxide, hydrogen, carbon dioxide, methane, and other organics including bio-oils and tars, ash, and small char particles. The concentration of these gases and other materials depends on the process design and operating conditions of the gasifier. Gasification has the advantage that it can convert essentially any biomass to clean synthesis gas from which a broad range of products can be derived. Biomass gasification exhibits many similarities to coal gasification including a significant number of gasifier types and different approaches to gasification technology. Fuels produced via gasification of biomass should have low net CO2 emissions, and if gasification is combined with capture and sequestration of CO2 emissions, such processes should have a negative CO2 emission footprint. They should also result in an equivalent reduction in oil use.
Gasification is carried out under a variety of pressure and temperature conditions. When relatively low pressures are used, the process is basically rapid pyrolysis. Under low-pressure rapid pyrolysis conditions, primary products are a mix of hydrogen, CO, light hydrocarbons, bio-oil, and char. The heating is usually indirect and operation is at lower pressure, avoiding the need for an expensive air separation unit. The mix of primary products can be separated into several fractions for upgrading. Bio-oil, which is a complex mixture of organic compounds, can theoretically be refined further under process conditions that resemble oil refining. If this can be done commercially, it would provide an avenue to biomass-based liquid transportation fuels that would substitute directly for their oil-based counterparts. However, the requisite refining technologies need to be developed and demonstrated. In addition to deriving products from the biooil, the full set of gaseous and liquid pyrolysis products can be gasified, the gas stream can be compressed, the CO shifted to the appropriate CO-to-hydrogen molar ratio, and the CO2 removed. The resulting synthesis gas can be converted to transportation fuel (e.g., diesel fuel via Fischer-Tropsch synthesis). Another option is to produce methanol from CO and hydrogen. Methanol can then be converted to dimethyl ether (DME) or to gasoline using methanol-to-gasoline (MTG) technology, which has been commercially demonstrated in a 14,000 bbl/d plant in New Zealand that operated successfully from 1985 to 1996. MTG appears to be the most likely route because MTG gasoline blends directly into the gasoline pool and fully uses the existing infrastructure. DME would require a separate or greatly expanded infrastructure.
Gasification using direct firing with oxygen at higher pressures produces a relatively pure stream of CO and hydrogen, with some CO2 and other gases. This syngas mixture is easily shifted to the desired CO-to-hydrogen molar ratio and, after CO2 removal, can be converted to diesel fuel by the Fischer-Tropsch synthesis or to methanol and then to gasoline by MTG. This approach would have the highest capital cost but, because of the simplicity of fuel production and a high yield, could be competitive.
Several U.S. and European groups are developing advanced biomass gasification technologies, and there are about 10 different biomass gasifiers with a capacity greater than 100 tonnes per day operating in the United States and worldwide. There are more than 90 installations (most are small) and 60 manufacturers of gasification technologies (BTG, 2004). For example, at the McNeil Generating Station in Vermont, a low-pressure wood gasifier, which started operation in August 2000, is converting 200 tons of wood chips per day into fuel gas for electricity generation. Many of the gasification technologies have technical or operational challenges associated with them, but most of these problems
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are probably resolvable or manageable with commercial experience. The most persistent problem area appears to be biomass feed processing and handling, particularly if a gasifier operates at high pressure and/or must contend with different biomass feeds. DOE has funded five other advanced biomass R&D projects (DOE-EIA, 2007).
Although several of the available gasification technologies have been demonstrated commercially, the technology is not yet commercially proven for biomass gasification and the production of biofuels. The technology is still on a relatively steep learning curve, and the integration of gasification, gas cleanup, and biofuel synthesis or bio-oil refining is yet to be commercially established. The same can be said for biomass gasification for hydrogen production. A major difference with biomass gasification is that it will involve smaller units than coal gasification and will not see the economies of scale of coal gasification. This will increase the cost per unit product unless major process simplification and capital cost reduction can be achieved. The primary approach to this is to eliminate the air separation unit, which is typically required with most high-severity gasification technologies, and its cost. This leads to gasification by indirect heating, which could more appropriately be considered biomass pyrolysis, as discussed above and typically produces bio-oil, tar, and gases.
Several economic studies of biomass gasification have been reported, at times combined with demonstration projects. A Finnish company has estimated that its biomass gasification technology, which is operating at the 150 kWth feed scale, can produce diesel fuel for $2.50 to $3.10 per gallon (VTT, 2006). Jensen (2004) from Shell estimated the cost of Fischer-Tropsch diesel from biomass gasification to be about $1.80 per gallon. Using a consistent basis for comparison, rapeseed methyl ester biodiesel was estimated at about $4.50 per gallon. These two sets of estimates are for large-scale plants. Choren Industries is in the early operating phases of a gasification process coupled with Fischer-Tropsch technology from Shell to produce high-quality diesel from a 50-50 mixture of wood chips and wood waste in Germany (Choren Group, 2008). This facility will demonstrate the gasification, syngas cleanup, and synthesis technologies integrated together.
The World Bank (2007) made a series of estimates for biomass-based integrated gasification combined cycle (IGCC) plants that were about one-fifth the size of typical coal-based IGCC plants. The size was limited by the maximum distance at which it was economic to harvest and transport biomass fuel. The estimated total plant cost was $2,150/kWe (1997 dollars) for a 100-MWe wood gasification-combined cycle power plant. For a 32 MWe wood gasification-IGCC demonstration plant in Brazil the total plant cost was about $2,900/kWe (1997 dollars). This compares to an estimated total plant cost of $1,300/kWe (1997 dollars) for a 500 MWe coal-based IGCC. These results demonstrate that biomass gasification suffers from diseconomies of scale because of feedstock limitations. The higher capital cost associated with biomass gasification, combined with the higher cost of biomass feedstock supply, will make it difficult to compete with coal if carbon capture and storage (CSS) is not required.
Impact of Biofuels on Oil Imports and Greenhouse Gas Emissions
In part due to competition with the food and feed needs of the United States and the world, corn ethanol has severe limitations with respect to its potential volume and impact on oil imports and on reductions of CO2 emissions from the transportation sector. Corn production in the field is energy intensive, using fossil fuels for fertilizer production, cultivation, and harvesting. The production of ethanol also requires large amounts of energy, which typically comes from natural gas, with the result that net energy and net CO2 reductions are limited. Life-cycle estimates of the net energy of corn ethanol range from a loss to a more than 30 percent energy gain over the energy in the fossil fuels used in its production, depending on the assumptions made in the studies and on the system boundaries (i.e., what is included in and not included in the analysis). Figure 4.6 shows the range of results found in the literature. The specifics of the studies leading to these results are referenced and discussed by Farrell et al. (2006). A reasonable conclusion for grain ethanol is 18 to 25 percent energy gain over fossil inputs (Wang, 2005) and a similarly sized net CO2 emissions reduction relative to gasoline.
Figure 4.7 shows the primary energy inputs of fossil and other energy sources in megajoules of primary energy and the estimated greenhouse gas emissions in kilograms CO2 equivalent per megajoule of fuel for gasoline production and per megajoule of product energy for ethanol production, also from the work of Farrell et al. (2006). This shows that the use of dry-mill ethanol reduces CO2 emissions by about 25 percent over the use of gasoline on an energy-equivalent basis.
Total fossil fuel use is also reduced by 20 to 25 percent (for dry-milled ethanol), but since most of the fuel used in growing, harvesting, and producing ethanol is natural gas and coal, the reduction in oil use that ethanol can produce is much greater (see Figure 4.7). Thus, dry mill ethanol use could reduce the petroleum requirement by about 0.95 gallon per energy-equivalent gallon of ethanol used.
Net CO2 reductions improve when biomass, such as bagasse or lignin, supplies the energy needed for the conversion and separation. Thus, ethanol from cellulosic feedstocks can produce a roughly 88 percent reduction in net CO2 emissions on a fuel energy-equivalent basis (see Figure 4.5) (Wang, 2005; Farrell et al., 2006). Estimated emissions from today’s conventional vehicles are shown in Table 4.4. Because the cellulosic ethanol process is self-sufficient in terms of energy, the oil reduction achieved when cellulosic ethanol is used to displace a gallon of gasoline is about 0.93 gallon of oil on an energy-equivalent basis.
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FIGURE 4.6 Published estimates of range of impacts on net greenhouse gas (GHG) emissions (A) and oil inputs (B) for grain-based ethanol. For additional discussion and information, see http:www.sciencemag.org/cgi/content/full/sci;312/5781/1748b. SOURCE: Farrell et al. (2006). Reprinted with permission of the American Association for the Advancement of Science.
FIGURE 4.7 Primary energy inputs and net greenhouse gas (GHG) emissions for gasoline and ethanol (primary energy inputs are in megajoules per megajoule of product energy; GHG emissions are in kilograms of CO2 equivalent per megajoule of fuel). SOURCE: Farrell et al. (2006). Reprinted with permission of the American Association for the Advancement of Science.
TABLE 4.4 CO2 Emissions from Today’s Conventional Light-duty Gasoline and Diesel Engines in a Typical Family Sedan and from Fuels from Less Conventional Sources
Vehicle Technology or Fuel Source
Well-to-Wheels CO2 Emissions (g CO2/km)
Conventional gasoline 2005
165
Conventional diesel 2005
143
Ethanol from corn
131
Ethanol from cellulose
15
SOURCE: Heywood (2007).
Production Potential of Biofuels
Production of ethanol from grain is fully commercial. Figure 4.8 shows the corn-ethanol production capacity growth from 1990 to 2007. U.S. production capacity grew from 4.3 billion gallons annually at year-end 2004, to 5.9 billion gallons annually by year-end 2006, and about 7 billion gallons annually by year-end 2007. Considering only current plant construction under way, ethanol capacity will be at least 8 billion gallons per year by year-end 2008 and could be as much as 10 billion gallons per year if all proposed projects are completed. This is a doubling of capacity in 4 years and
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FIGURE 4.8 Growth in production of corn-based ethanol in the United States. SOURCE: Courtesy of Renewable Fuels Association.
is well ahead of mandated biofuel levels. However, the 10 billion to 12 billion gallons per year probably represents the limit with respect to corn availability, assuming that corn yields and acreage increase modestly. Production of ethanol from grain is not likely to see significant additional process cost reduction. Further, costs are not subject to economies of scale because current plant size is about at the limit and water use limitations will probably increase costs in future plants. Based on the previous analysis in this chapter, a forward projection of grain ethanol production is as follows:
8 billion gallons in 2008,
10 billion gallons in 2010, and
12 billion gallons in 2015.
Production of ethanol from cellulose is yet to be demonstrated at a commercial scale, and significant questions remain about the economic and commercial viability of the technology. Within the next 3 to 5 years, four to five commercial demonstration plants are expected. These will provide valuable information on cost, technology robustness, and particularly, commercial viability at the scale required to achieve large-scale cellulosic ethanol production. This information should be available by 2010. The committee expects the commercial and economic viability of cellulosic ethanol to remain a key issue for some time.
Cellulosic ethanol or other alternatives (e.g., biomass gasification, liquid-fuel synthesis) will be required to meet biofuel demand beyond levels achievable with grain ethanol. If commercially successful, cellulosic ethanol production could approach the growth experienced for grain ethanol. Cellulosic ethanol plants are similar to grain ethanol plants although somewhat more complex, and because of the dispersed nature of biomass, they will be limited in size to
TABLE 4.5 Key Assumptions and Parameters Used in Biomass-to-Biofuels Scenarios
Probable Case
Upper-bound Case
Biomass potentially available in the near-term (dry tons/yr)a
335b
335b
Biomass available in 2050 (dry tons/yr)
500
700
Average ethanol yield on biomass, 2008 to 2030 (gallons/dry ton)
60
60
Average ethanol yield on biomass, 2030 to 2050 (gallons/dry ton)
90
90
aBiomass use starts with crop residues (corn stover) and then adds other sources including energy crops as they become available, driven by cost.
bSee Table 4.3 for primary biomass components that make up this number.
about two to four times that of ethanol plants. For the rest of this discussion, the committee assumes that cellulosic ethanol is commercially demonstrated by 2010 and capacity begins to grow.5 Two cases are considered; both assume that cellulosic ethanol is economically competitive or there are sufficient fuel subsidies to make it competitive with oil-based fuels so that production capacity is built with private capital. One case is considered to be a measured response to the need to replace oil-derived liquid transportation fuels and is called the probable case. The other case involves a more aggressive application of the technology to generate liquid transportation fuels and is called the upper-bound case. Table 4.5 summarizes the key assumptions and parameters used for the two cases.
The probable case assumes a more measured pace of application of the technology in view of all the issues including process cost, water availability, biomass cost, other competitive uses of biomass and the ability to build plants and to increase biomass availability. For this case, the capacity build followed the grain ethanol capacity build experience, in which over a several-year period about 1 billion gallons of capacity was added per year. For this case (see Table 4.5 for details), the key assumptions are that the technology is commercially ready and there are 335 million dry tons of biomass available in the near term, increasing to 500 million dry tons available per year for conversion to biofuels by 2050. The upper-bound case also starts with 335 million dry tons of biomass available per year and assumes that by 2050,
5
The DOE roadmap on cellulosic ethanol is “to accelerate cellulosic ethanol research, helping to make biofuels practical and cost-competitive by 2012.” The three to five demonstration plants that DOE is funding should have achieved that goal by the end of 2010 or have identified the key remaining issues. If an economic business case can be made by that time, there is capital ready to build forward.
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700 million dry tons of biomass are available per year and converted to biofuels (ethanol). It assumes that many of the constraints are resolved, that sufficient additional biomass is available, and that once sufficient experience is gained, plant capacity build exceeds the grain ethanol experience. It is consistent with the aggressiveness of the Hydrogen Success scenario (Case 1) discussed in Chapters 3 and 6 and, thus, is the most logical case for comparison with the hydrogen cases. It also shows the maximum impact that biofuels can have on oil import reduction and on greenhouse gas emission reductions from light-duty vehicle transportation.
Biomass for biofuel production starts with agricultural crop residues, which are readily available today and then double over time. Grown-for-purpose energy crops and forest residues contribute later in the period, driven by price, location, and local availability. Because of the issues of gathering and transporting biomass, conversion plants will be limited in size, potentially two to four times as large as corn-based ethanol facilities and will not be able to gain further economies of scale. Ethanol yield is assumed to improve from 60 gallons per dry ton early in the build to 90 gallons per dry ton about mid period and beyond. Water will become a major issue as the production grows. For forest residues and energy crops grown on less desirable land, the areas where plants will be built will likely be dryer, and water is expected frequently to become a major issue. This and other factors, including the cost of biomass per dry ton, will limit full utilization of the technically available biomass. The upper-bound case removes or limits many of these restrictions
The infrastructure build for rapid growth in cellulosic ethanol production should be similar to that experienced for corn ethanol. The primary difference is that cellulosic ethanol plants will cost two to three times as much as corn ethanol plants due to greater complexity and increased material and construction costs. The infrastructure for delivery of the product to blend plants would be expected to remain largely the same as for grain ethanol, but it would have to be expanded to meet the greater production. Since the plants are relatively small and widely scattered, truck and rail would probably pick up most of the growth. Dedicated pipelines connecting areas of substantial production with large blend plants would be expected to develop in specific instances. Costs can be expected to decrease but will remain a significant issue. The estimated build-out and projected annual cellulosic ethanol production for each case are given below:
Probable Case
Upper-bound Case
1 billion gallons in 2011
1 billion gallons in 2011
5 billion gallons in 2015
6 billion gallons in 2015
12 billion gallons in 2020
16 billion gallons in 2020
18 billion gallons in 2025
28 billion gallons in 2025
32 billion gallons in 2035
44 billion gallons in 2035
45 billion gallons in 2050
63 billion gallons in 2050
Biobutanol should be commercially demonstrated in 2 to 3 years with the BP-DuPont plant noted above, but the main issue will be cost. It will probably require about a decade to define the extent to which costs can be reduced, for next-generation microbe technology to be developed and applied, and for technology developed to convert cellulosic biomass to biobutanol. This is when commercial viability will be clearer. Biobutanol is included in the scenario by assuming that it is produced at 10 percent of the cellulosic ethanol production level and is offset by 5 years. Thus, 0.1 billion gallons per year could be available in 2015, 0.5 billion gallons per year in 2020, and so forth.
Biodiesel production from plant and animal oils is fully commercial, and the technology is considered mature. The production cost is mainly in the plant oil cost, and the yield of oil per acre is low for crops suitable for the United States. This suggests that it will not be a major player in the U.S. light-duty vehicle fuel market. U.S. production costs are about $3.00 per gallon. The technology is simple and mature, and there has been no problem building plant capacity to date, with recent utilization of installed capacity at less than 50 percent.
Projected annual production volumes are as follows:
250 million gallons per year today,
600 million gallons in 2012, and
1.5 billion gallons in 2020 (maximum production).
The 1.5 billion gallons of production per year would consume about 30 percent of the soybean crop and probably cannot be increased. Corn and soybeans compete for the same land, limiting soybean production.
Because the manufacture of biodiesel is much less energy intensive, the estimated impact on CO2 emissions is a 30 to 35 percent reduction per gallon of biodiesel used, and the impact on oil consumption is about a 90 to 95 percent reduction relative to use of a gallon of regular diesel fuel. CO2 emissions include those associated with the methanol used in esterification. The energy contents of biodiesel and regular diesel are roughly the same.
Conclusions on Biofuels
CONCLUSION: Although use of corn- and oil-based biofuels can provide some benefits in reducing U.S. oil use and CO2 emissions, cellulosic biofuels will be required for such benefits to be significant. Lower-cost biofuel production methods and conversion processes will have to be developed for large-scale commercialization, but the initial high costs of biofuels, together with other barriers, may limit their market potential, absent policy interventions or significant oil price increases or supply disruptions.
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Biofuels offer the potential to reduce oil imports because they can replace a fraction of the liquid fuels needed for U.S. light-duty vehicle transportation. They can also reduce CO2 emissions because they use carbon that was captured by plants in their last growth cycle, not carbon stored during previous millennia, and the repeated growth cycles recapture the CO2 emitted during combustion of the fuel. Biofuels from different sources will have a different impact on oil imports and on net CO2 emissions.
Grain ethanol has a 20 to 25 percent energy gain over the fossil fuel inputs used for its production and, on average, reduces CO2 emissions by 18 to 25 percent over the use of gasoline on an energy-equivalent basis. Grain ethanol production is fully commercial but is constrained by grain availability because it competes with the use of grains for food and animal feed.
Much more biomass is available from non-grain sources. The technology for cellulosic ethanol has not yet been demonstrated for commercial production. It should be significantly better than grain ethanol with respect to CO2 emission reductions because plant lignin and other plant residues can be used to supply the needed manufacturing process heat, reducing the use of fossil fuels. The key issues for cellulosic ethanol are commercial readiness, economics, and sustainability of biomass production including maintenance or improvement of soil productivity.
Biomass gasification is technically feasible, and many components have been commercially demonstrated. If the CO2 produced in gasification were captured and sequestered, biomass gasification would have a negative CO2 emissions balance. Economics will be the primary issue with biomass gasification, as it is with cellulosic ethanol, but it currently appears to be more competitive and closer to commercial reality. For a given amount of biomass, the thermochemical routes will produce roughly the same amount of biofuels on an energy-equivalent basis as the biochemical routes. A potential route to biofuels is through the use of algae, either as a source of cellulose or as a means to produce hydrocarbons. These routes have yet to be demonstrated above the pilot scale.
Biodiesel production from plant and animal oils is fully commercial, and the technology is considered mature. The production cost is mainly in the plant oil cost and remains uncompetitively high.
Therefore, biofuels offer significant potential to reduce CO2 emissions from oil use by the U.S. light-duty vehicle fleet. The extent of these reductions is highly dependent on the biofuel source. Grain-based ethanol and biodiesel are severely limited by grain availability and cost. Non-grain biomass offers a large new source of biofuels, but cellulosic ethanol technology is not commercially ready and gasification routes to biofuels require commercial demonstration. Either or both of these routes could greatly expand biofuels production. However, their availability and commercial tim States will be able to meet the biofuels target of 35 billion gallons per year by 2017.
OVERALL CONCLUSION
CONCLUSION: The committee’s analysis indicates that at least two alternatives to HFCVs—advanced conventional vehicles and biofuels—have the potential to provide significant reductions in projected oil imports and CO2 emissions. However, the rate of growth of benefits from each of these two measures slows after two or three decades, while the growth rate of projected benefits from fuel cell vehicles is still increasing. The deepest cuts in oil use and CO2 emissions after about 2040 would come from hydrogen. See Chapter 6.
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