NOVEL TECHNOLOGIES FOR GASEOUS CONTAMINANTS CONTROL (2001)

The overall objective of this project is to develop technologies for cleaning/conditioning the syngas from an integrated gasification combined cycle (IGCC) system to meet the tolerance limits for contaminants such as H{sub 2}S, COS, NH{sub 3}, HCN, HCl, and alkali for fuel cell and chemical production applications. RTI's approach is to develop a modular system that (1) removes reduced sulfur species to sub-ppm levels using a hybrid process consisting of a polymer membrane and a regenerable ZnO-coated monolith or a mixed metal oxide sorbent; (2) removes hydrogen chloride vapors to sub-ppm levels using an inexpensive, high-surface area material; and (3) removes NH{sub 3} with acidic adsorbents. RTI is working with MEDAL, Inc., and North Carolina State University (NCSU) to develop polymer membrane technology for bulk removal of H{sub 2}S from syngas. These membranes are being engineered to remove the acid gas components (H{sub 2}S, CO{sub 2}, NH{sub 3}, and H{sub 2}O) from syngas by focusing on the ''solubility selectivity'' of the novel polymer compositions. The desirable components of the syngas (H{sub 2} and CO) are maintained at high-pressure conditions as a non-permeate stream while the impurities are transported across the membrane to the low pressure side. RTI tested commercially available and novel materials from MEDAL using a high-temperature, high-pressure (HTHP) permeation apparatus. H{sub 2}S/H{sub 2} selectivities >30 were achieved, although there was a strong negative dependence with temperature. MEDAL believes that all the polymer compositions tested so far can be prepared as hollow fiber membrane modules using the existing manufacturing technology. For fuel cell and chemical applications, additional sulfur removal (beyond that achievable with the membranes) is required. To overcome limitations of conventional ZnO pellets, RTI is testing a monolith with a thin coating of high surface area zinc-oxide based materials. Alternatively, a regenerable sorbent developed by DOE/NETL (RVS-1) is being evaluated for this application. A multi-cycle test of 2-in. (5-cm) diameter monolith samples demonstrated that <0.5 ppm sulfur can be achieved. Removal of HCl vapors is being accomplished by low-cost materials that combine the known effectiveness of sodium carbonate as an active matrix used with enhanced surface area supports for greater reactivity and capacity at the required operating temperatures. RTI is working with SRI International on this task. Sorbents prepared using diatomaceous earth and sepiolite, impregnated with sodium carbonate achieved steady-state HCl level <100 ppb (target is 10 ppb). Research is continuing to optimize the impregnation and calcination procedures to provide an optimum pore size distribution and other properties. RTI and SRI International have established the feasibility of a process to selectively chemisorb NH3 from syngas on high surface area molecular sieve adsorbents at high temperatures by conducting a series of temperature-programmed reactions at 225 C (437 F). Significant levels of NH{sub 3} were adsorbed on highly acidic adsorbents; the adsorbed NH{sub 3} was subsequently recovered by heating the adsorbent and the regenerated adsorbent was reused. A comprehensive technical and economic evaluation of this modular gas cleaning process was conducted by Nexant to compare capital and operating cost with existing amine based processes. Nexant estimated a total installed cost of $42 million for the RTI process for a 500 MWe IGCC plant based on its current state of development. By comparison, Nexant estimated the installed cost for an equivalent sized plant based on the Rectisol process (which would achieve the same sulfur removal specification) to be $75 million. Thus the RTI process is economically competitive with a state-of-the-art process for syngas cleanup.