Boilers coal gasification waste heat

Because fuel costs are high, the search is on for processes with higher thermal efficiency and for ways to improve efficiencies of existing processes. One process being emphasized for its high efficiency is the gas turbine combined cycle. The gas turbine exhaust heat makes steam in a waste heat boiler. The steam drives turbines, often used as lielper turbines. References 1, 2, and 3 treat this subject and mention alternate equipment hookups, some in conjunction with coal gasification plants. 

The heat required for gasification is essentially supplied by the partial oxidation of the coal. Overall, the gasification reactions are exothermic, so waste heat boilers often are used at the gasifier effluent. The temperature, and therefore composition, of the product gas is dependent upon the amount of oxidant and steam, as well as the design of the reactor that each gasification process utilizes. 

Figure 5-6 is a simplified block flow diagram of the KoBra project. Raw brown coal is dried to 12% moisture by the newly developed WTA process which is a fluidized bed with steam and integrated waste heat recovery. Air for the gasification is extracted from the combustion turbine. The gasifier is sized at about 3,600 t/d of dry brown coal to match the 200 MW Siemens V 94.3 combustion turbine. The bottom ash and fly ash from the gasifier is combusted in a separate atmospheric pressure circulating fluidized-bed combustion (CFBC) boiler. 

WEW Process (24,25). Vereingte Elektrizitatswerke Westfalen is a German electric utility that has developed a unique dry feed, entrained-flow gasification process. The VEW concept relies on partial gasification with preheated air to produce low-Btu-coal-derived gas and char. After waste heat recovery, steam generation, and sulfur removal, the coal gas is fired in a combustion turbine. The char is fired in a pulverized coal boiler with the hot flue gas from the combustion turbine serving as the preheated combustion oxidant. This design requires additional cleanup of the pulverized boiler flue gas to meet emission requirements. 

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