Though combined cycle plants based on natural gas (Fig. 1.9a) are widely used, with the increasing cost of natural gas several coal gasification technologies are gaining acceptance. The technology is proven, and there are several plants in operation throughout the world. The advantages of integrated coal gasification combined cycle (IGCC) are
Ability to use of low grade fuels such as coal and biomass.
High efficiency, about 7-8% higher than conventional coal-based plants. A net efficiency of 45% is not impossible. With improvements in gasification and gas turbine technologies, the efficiency can reach 50% by 2010.
Fuel flexibility. The combined cycle portion of the plant can be fueled by natural gas, oil, or coal. A plant can switch from gas to coal as gas becomes unavailable or very expensive. Most gasifiers can handle different grades of coal. Gas turbine combustors can also handle different fuels with different heating values and gas analysis from low to high Btu.
Low SO2, NOx, and CO2 emissions. In an IGCC, 90% of the coal’s sulfur is removed before combustion. NOx is reduced by 90%, as is also the CO2 on lb/kWh basis. The coal gas is purified before combustion, unlike in a conventional coal-fired plant, where the flue gases are cleaned. Hence the quantity of effluent to be handled is much smaller. The composition of the fuel gas also allows for better chemistry while cleaning.
Low water consumption due to higher efficiency and lower heat losses.
Marketable by-products such as sulfur, sulfuric acid, and carbon dioxide.
Awide range of technologies such as fixed bed, fluidized bed, and entrained bed gasification.
Ability to make use of advances in gas turbine technology.
Availability of IGCC plants, which has been in excess of 90% and is improving.
Higher gas turbine power output possible due to about 14% larger mass flow of flue gases at the same combustion temperature compared to natural gas.
Decreasing installation costs due to advances in technology. $1000/kW will be achievable in the near future. Unit sizes range from 100 to 500 MW.
In an IGCC, coal is gasified in a gasifier by using steam and either air or oxygen to generate a low or high Btu gas, which is cleaned and fired in a gas turbine combustor. There are three processes for gasifying coal: fixed bed, fluidized bed, and entrained bed. Figure 1.3 Shows an IGCC plant. Typically, coal is gasified in the gasifier at pressure using steam, oxygen or air, and coal. The coal gas is cooled in a synthesis gas cooler, which also generates steam or superheats the steam generated elsewhere. It is then cleaned in a gas cleaning system, where the particulates and sulfur are removed. Hot gas cleaning methods are also being developed, which can improve the efficiency of the system even more. The clean coal gas is fired in the gas turbine combustor. The exhaust gases generate high pressure steam for the steam turbine and also for gasification. A portion of the air from the gas turbine compressor is also sent to the gasifier. There are several plants in operation throughout the world. In the United States the Wabash River plant, which began operation in 1995 (Fig. 1.3) generates 262 MW using the Destec process for gasification, which uses an entrained flow oxygen-blown gasifier. Coal is slurried, combined with 95% pure oxygen, and injected into the first stage of the gasifier, which operates at 400 psig and 2600°F. The coal slurry undergoes a partial oxidation reaction at temperatures that bring the coal’s ash above its melting point. From the gasifier the fluidized ash falls through a tap hole at the bottom. The synthesis gas flows into the second stage, where additional coal slurry is injected. At this stage the coal is pyrolyzed in an endothermic reaction with the hot synthesis gas. This enhances the heating value of the synthesis gas.
After leaving the second stage the synthesis gas flows into a gas cooler, which is a waste heat boiler that generates high pressure saturated steam at 1600 psia. After cooling, any remaining particulates are removed in a hot/dry filter. Further cooling of gas takes place in a series of exchangers. It is scrubbed to remove chlorides and passed through a catalyst that hydrolyzes the carbonyl sulfide into hydrogen sulfide. The H2S is removed by an acid gas removal system. A marketable elemental sulfur is produced as a by-product. Finally the sweet gas is moisturized and preheated before being sent to the gas turbine. The power block consists of a GE 192 MW MS7001A gas turbine, The exhaust gases generate steam in the HRSG, which generates power via a steam turbine. This is presently the largest gasification repowering project. The heat rate is around 8910 Btu/kWh (HHV) with SO2 emissions around 0.1lb/MM Btu, NOx 0.15, and particulates below detectable limits .
Coal will remain a major fuel, more so with the significant run-up in the price of gas, and IGCC plants have earned a permanent place in power generation technology. The heat exchanger and the HRSG are designed to meet the special requirements of this process. Oxygen-blown gasification has dominated commercial gasification processes, because these plants produce chemicals based on synthesis gas (H2 and CO) and premium fuels. Air-blown gasifiers, which generate low Btu gas, are also widely used in the industry. Air-blown gasification produces a gas in which the desirable chemical reactants are diluted by massive amounts of nitrogen. The gasifier capacity is cut in half when it is air-blown. The efficiency of conversion of feed to fuel gas is higher with oxygen-blown gasification. The air-blown gasification produces over twice as much gas as is generated by oxygen-blown operation; hence investment costs for air-blown systems and cleanup systems are higher. Cleanup costs are also higher because the partial pressures of the pollutants are higher in air-blown system raw gas. Compression costs are lower because the mass flow of an oxygen-blown system is smaller by 20-40%.
The Sierra Pacific Power Company’s Pinon-Pine project employs an air — blown system and a fluidized bed gasification process that uses low sulfur coal, most of which is captured in the bed itself by the use of limestone injection methods. A low Btu gas is generated, on the order of 130Btu/scf.