DEVELOPMENT OF DISPOSABLE SORBENTS FOR CHLORIDE REMOVAL FROM HIGH TEMPERATURE COAL-DERIVED GASES

Abstract

Advanced integrated-gasification combined-cycle (IGCC) and integrated-gasification fuel cell (IGFC) systems require the development of high temperature sorbents for the removal of hydrogen chloride (HCl) vapor to less than 1 parts-per-million (ppm) levels. HCl is a highly reactive, corrosive, and toxic gas which must be removed to meet environmental regulations, to protect power generation equipment, and to minimize deterioration of hot gas desulfurization sorbents. The objective of this program was to develop disposable, alkali-based sorbents capable of reducing HCl vapor levels to less than 1 ppm in the temperature range from 400 to 750 C and pressures in the range from 1 to 20 atm. The primary areas of focus of this program were to investigate different methods of sorbent fabrication, testing their suitability for different reactor configurations, obtaining reaction kinetics data, and conducting a preliminary economic feasibility assessment. This program was a joint effort between SRI International (SRI), Research Triangle Institute (RTI), and General Electric Corporate Research and Development (GE-CRD). SRI, the prime contractor and RTI, a major subcontractor, performed most of the work in this program. Thermochemical calculations indicated that sodium-based sorbents were capable of reducing HCl vapor levels to less than 1 ppm at temperatures up to 650 C, but the regeneration of spent sorbents would require complex process steps. Nahcolite (NaHCO{sub 3}), a naturally-occurring mineral, could be used as an inexpensive sorbent to remove HCl vapor in hot coal gas streams. In the current program, nahcolite powder was used to fabricate pellets suitable for fixed-bed reactors and granules suitable for fluidized-bed reactors. Pilot-scale equipment were used to prepare sorbents in large batches: pellets by disk pelletization and extrusion techniques, and granules by granulation and spray-drying techniques. Bench-scale fixed- and fluidized-bed reactors were assembled at SRI and RTI to conduct tests at high-temperature, high-pressure conditions (HTHP). The HTHP tests confirmed the ability of nahcolite pellets and granules to reduce the HCl vapor levels to less than 1 ppm levels with a very high sorbent utilization for chloride capture. The effect of several operating variables such as temperature, pressure, presence of hydrogen sulfide, and sorbent preparation methods was studied on the efficacy of HCl removal by the sorbent. Pilot-scale tests were performed in the fluidized-bed mode at the gasifier facility at the GE-CRD. Sorbent exposure tests were also conducted using a hot coal gas stream from the DOE/FETC's fluidized-bed gasifier at Morgantown, WV. These tests confirmed the results obtained at SRI and RTI. A preliminary economic assessment showed that the cost of HCl removal in a commercial IGCC system will be about $0.001/kWh (1 mills/kWh).