(137c) Novel Warm Gas Multi-Contaminant Removal System

Gasification technologies convert coal and biomass feedstocks into synthesis gas feed streams that can either be used as a fuel for highly efficient power generation cycles or converted into value-added chemicals and transportation fuels.  However, the coal/biomass-derived synthesis gas contains a myriad of trace contaminants (e.g. mercury, arsenic, selenium) and ammonia (NH₃) that must be removed to eliminate power plant emissions; NH₃ will convert into NO_xin the gas turbine combustor (an acid rain precursor), while Hg is designated as an hazardous air pollutant and being regulated by the U.S. EPA.  These contaminants also poison the catalysts used in the manufacturing processes that converts synthesis gas into fuels or chemicals. 

Although conventional clean-up technologies can effectively remove these contaminants, they typically require aqueous quenching and cooling of the syngas to around 100°F, followed by scrubbing with chemical or physical solvents, and finally absorption/adsorption of trace contaminants on solid sorbents.  However, cooling the syngas and condensation of steam significantly penalizes the cycle efficiency.  The power plant efficiency can be improved 3 to 4% if the synthesis gas is not cooled below its dew point because the energy in high pressure steam can be recovered by expanding it steam in the gas turbine and eliminating the energy penalty associated with the re-heating of the synthesis gas.  Warm gas clean-up also eliminates the need for the expensive heat exchangers (to cool the synthesis gas to the operating temperature of the clean-up system and then to re-heat it back to before the combined cycle) and the gray water treatment problems associated with processing the large amounts of condensate. 

In a DOE funded SBIR project, TDA Research, Inc. (TDA) is developing a chemical absorbent-based gas clean-up technology to remove the NH₃ and hydrogen cyanide (HCN) as well as all trace metal contaminants (e.g., Hg, As, Se and Cd) from the synthesis gas at high temperatures (500^oF) in a single process step.  The sorbent can be operated in a regenerable manner to remove NH₃ and Hg, while irreversibly absorbing all other contaminants.  The regenerable operation for the Hg (which has very low affinity to any surface at high temperature) and NH₃ (which is present in very high concentration in the gas) ensures high utilization of the sorbent, while its high capacity to other contaminants and low cost ensures its economical use as an expendable sorbent. We will present the results from this sorbent development effort at the meeting.