injected to adjust the ratio of hydrogen to carbon monoxide in the syngas and/or as a temperature moderator. As produced, this gas contains impurities, which can be stripped out using well-developed refinery gas cleanup having very high demonstrated removal rates (Meyers, 1997). Shifting from direct coal combustion in air to gasification in oxygen can become more attractive and more cost-effective as emissions regulations are further tightened. After cleaning to meet the requirements for subsequent processing, the syngas can be converted into electricity by combined cycle technology (gas turbine plus steam turbine), fuel cells, or gas turbine plus fuel cell hybrid power plants at high energy conversion efficiencies. These are the most likely combinations of coal-conversion technology and energy-conversion technology with the potential to achieve the 60 percent higher heating value (HHV) efficiency target of the Vision 21 Program. By reaction with additional steam downstream of the gasification reactor, syngas can also be converted into a mixture of hydrogen and CO2. This mixture can then be separated into essentially pure streams of hydrogen for fuel or chemical use and CO2 that can be sequestered (NRC, 2000).
Other approaches to coal gasification have been developed that utilize air instead of high-purity oxygen. The potential reward for using air is avoidance of the cost of an air separation plant to produce oxygen and the energy consumed in the plant’s operation. These cost-saving factors are offset by the large amount of nitrogen introduced into the system, which increases the size and energy costs associated with cleanup of the relatively dilute syngas stream. The presence of nitrogen also increases the cost of separating CO2 from the syngas as part of a sequestration scheme. As a result, air-blown gasification is not considered to be compatible with sequestration systems. One of the most important advantages of oxygen-blown coal gasification technology relative to coal combustion technologies that use air, as well as coal gasification technologies that use air, is that it is compatible with the need for relatively low-cost CO2 separation required for CO2 sequestration.
Gasification plants that process feed materials with very low or negative cost, such as petroleum coke and residual oil, can be commercially justified today for various combinations of hydrogen, by-product steam, and power production. Coal gasification for hydrogen production for chemical manufacturing is also widely practiced. More than 160 gasification plants worldwide are in operation producing the equivalent of 50,000 megawatts (thermal) (MWt) of syngas (Simbeck, 2002).
Four coal-fueled integrated gasification combined cycle (IGCC) single-train demonstration power plants with outputs greater than 250 MW have been built since 1995, two in Europe and two in the United States. Each of these plants was built with a significant subsidy as part of a government-sponsored program. As expected, each of the plants has taken 3 to 5 years to approach the upper range of availability, 70-80 percent, that was predicted when they were designed.