Modern, combined-cycle electric power generation systems using gas turbines as the primary generators offer higher thermal efficiencies than conventional steam-turbine systems. Many of the commercial plants in operation today use natural gas-fired, combined-cycle systems in which the hot exhaust from the gas turbines is processed in heat recovery steam generators to afford steam for injection into steam turbines for additional power generation and improved efficiency. Steam injection into the gas turbines along with combustion gases adds further efficiency improvements. Overall thermal efficiencies to electric power are up to twice those of conventional fuel-fired steam turbine systems. Availabilities can be high, the environmental characteristics are excellent, and capital costs are considerably less per unit of electric power capacity compared to the costs of conventional coal-fired plants. One of the largest combined cycle, natural gas-fired plants in the world—a 2000-MW central station plant in Japan—operates at 95% availabilities (adjusted for mandated inspections).

Integration of coal gasification processes with combined-cycle technologies has opened the way for coal to fuel similar power generation plants at high efficiencies. Integrated gasification-combined cycle (IGCC) systems are in oper­ation throughout the world and have made it possible to resurrect the use of low-cost, high-sulfur fossil fuels for power generation because the gasification process is, in effect, a desulfurization process. Oxygen-blown gasification plants have dominated both commercial and demonstration coal gasification units since as far back as the 1920s. Seventeen commercial plants, having a total of 153 coal gasifiers, are reported to be in commercial operation worldwide (с/. Simbeck and Karp in Swanekamp, 1996). Oxygen is used rather than air in these plants because they produce synthesis gas-based chemicals and premium fuels. In the United States, modern air — and oxygen-blown, fluid-bed gasifica­tion processes equipped with hot-gas cleanup systems are being perfected for use with coal feedstocks in IGCC plants. These plants are expected to have good emissions characteristics with one exception—carbon dioxide emissions per unit of fuel will be about the same as those of conventional fossil-fueled power plants. Biomass fuels, because of their relatively short recycling time, would avoid this problem.

It is apparent from the discussion of biomass gasification in this chapter that innovative processes for producing low — and medium-energy fuel gases have been developed for virgin and waste biomass feedstocks and are either about to be or have already been commercialized. These technologies are much improved over conventional, air-blown gasification processes. The availability of suitable fuel gases from modem biomass gasification processes facilitates their coupling with combined cycle power plants in the same manner as fossil — fueled IGCC plants. Biomass-fueled IGCC plants (BIGCC), particularly those having smaller capacities and those used for combined waste disposal and energy recovery, are expected to contribute to the expected 600 GW of new electric generating capacity needed worldwide over the next several years. IGCC plants fueled with both coal and biomass as sequential or combined feedstocks would appear to be a viable alternative because, as already pointed out, some gasification processes are capable of converting both feedstocks. The heat load for conventional Rankine steam-cycle power production using boilers and steam turbines is about 14.8 to 16.9 MJ/kWh; BIGCC technology should have about 25% less heat load and therefore considerably improved economics. The economics of BIGCC systems even as small as 1 to 10 MW in capacity can be very site-specific, but appear to be capable of reasonable rates of return (Craig and Purvis, 1995). Larger biomass integrated-gasification/ steam-injected gas-turbine (BIG/STIG) cogeneration plants are projected to be

attractive investments for sugar producers, for example, who can use sugarcane bagasse as fuel (Larson et al, 1991).

A 30-MW power plant fueled with eucalyptus wood from short-rotation energy plantations is planned in Brazil to demonstrate BIGCC technology (Carpentieri, 1993; de Queiroz and do Nascimento, 1993). This plant is pro­jected to operate at an availability of 80% and an overall thermal efficiency of 43% to produce 210 GWh/year from 205,835 m3/year of wood chips contain­ing 35 wt % moisture. The energy cost is estimated to be $0,045 to $0.065/kWh. The first plant is estimated to have a capital cost of $60 million to $75 million (U. S.); subsequent plants are estimated to cost $39 million to $45 million (U. S.). For sugarcane bagasse, which will be tested as a potential feedstock, the heat rate is estimated to be 8.368 GJ/MWh with fuel consumption at 50 wt % moisture content of 1.021 kg/kWh. It is estimated that the cost of electric power production in 53-MW BIG/STIG plants in Brazil using briquetted sugarcane bagasse is $0,032 to $0.058/kWh (Larson et al., 1991).

Other advanced technologies that are receiving considerable attention in­clude improved designs for combining small biomass gasifiers with motor — generator sets or gas turbines in the multiple-kilowatt range and in the 1 to 5 MW range. Numerous configurations are being developed, although some assessments have ruled out conventional steam turbines because of their rela­tively low efficiency and high cost at small sizes. Examples of small systems under development include a 1-MW system consisting of a fixed-bed, down­draft gasifier, a gas cleaning system, and a spark-ignited gas engine-generator set; and a 1-MW system consisting of a pressurized fluid-bed gasifier, a hot- gas cleanup system, and a gas turbine (Purvis et al, 1996).

Another advanced technology that can use biomass gasification for power generation employs fuel cells. Fuel cells are devices that electrochemically convert the chemical energy contained in the fuel into direct current electricity and the oxidation products of the fuel. The fuels can be natural gas and the product gases from the gasification of solid fuels, including biomass and de­rived fuels such as hydrogen, and intermediate liquid fuels such as hydrocarbons and ethanol. In one sense, fuel cells are similar to electric batter­ies, but the tuei and oxidant are continuously supplied Irom external sources. So, unlike batteries, fuel cells are not consumed or depleted in the process. Also, because fuel cells are not heat engines, they are not Carnot limited and can achieve high fuel-energy-to-electric power conversion efficiencies that can be above 60% based on the energy content of the fuel supplied to the fuel cell. Among the fuel cell configurations, three different types are being developed for power generation by units 100 kW to 25 MW in capacity. They are differenti­ated by the electrolytes used within the cell—phosphoric acid, molten carbon­ate, and solid oxide. Some designs such as those that use molten carbonate and solid oxide electrolytes are operated at sufficiently elevated temperatures to be suitable for use in cogeneration applications. A few of these designs are believed to be operable at overall efficiencies as high as 85% based on the energy content of the fuel supplied to the fuel cell. A few small-scale power units using biomass fuels for specialty applications may become available in the next few years, but large-scale fuel-cell power plants are not expected to be available for generating central station power until well into the twenty- first century.