Investigation of mixed metal sorbent/catalysts for the simultaneous removal of sulfur and nitrogen oxides

Abstract

EXECUTIVE SUMMARYSimultaneous removal of SO 2 and NO x using a regenerable solid sorbent will constitute an important improvement over the use of separate processes for the removal of these two pollutants from stack gases and possibly eliminate several shortcomings of the individual SO 2 and NO x removal operations. The work done at PETC and the DOE-funded investigation of the investigators on the sulfation and regeneration of alumina-supported cerium oxide sorbents have shown that they can perform well at relatively high temperatures (823-900 K) as regenerable desulfurization sorbents. Survey of the recent literature shows that addition of copper oxide to ceria lowers the sulfation temperature of ceria down to 773 K, sulfated ceria-based sorbents can function as selective SCR catalysts even at elevated temperatures, SO 2 can be directly reduced to sulfur by CO on CuO-ceria catalysts, and ceria-based catalysts may have a potential for selective catalytic reduction of NO x by methane. These observations indicate a possibility of developing a ceria-based sorbent/catalyst which can remove both SO 2 and NO x from flue gases within a relatively wide temperature window, produce significant amounts of elemental sulfur during regeneration, and use methane for the selective catalytic reduction of NO x .The original objective of this research was to conduct kinetic and parametric studies of the selective catalytic reduction of NO x with NH 3 and CH 4 over alumina-supported cerium oxide and copper oxidecerium oxide sorbent/catalysts; investigate SO 2 removal at lower temperatures by supported copper oxidecerium oxide sorbents; and investigate the possibility of elemental sulfur production during regeneration with CO or with CH 4 -air mixtures.The first two objectives were realized, SCR with NH 3 was limited to a few number of experiments; iv instead propylene was investigated as a reductant in SCR due to the several shortcomings of NH 3 as a reductant. The last objective, elemental sulfur production, was not accomplished because it was found that CO formation from partial oxidation of CH 4 was not observed under all reaction conditions studied in this research and thus, no elemental sulfur formation could be obtained with CH 4 and O 2 . Runs with CO as reductant produced almost 100% conversion of NO in gases containing stoichiometric amount of oxygen at 573 and 673 K, but considering the feasibility of using CO in commercial power plants, it was decided not to pursue this objective any further.The sorbents consisting of cerium oxide and copper oxide impregnated on alumina have been prepared and characterized. Their sulfation performance has been investigated in a TGA setup, studying mainly the effects of temperature, sorbent composition, metal loading and support type. As a result of the sulfation experiments, a relatively wide temperature window was established for the use of aluminasupported cerium oxide-copper oxide as regenerable sorbents for sulfur dioxide removal. In the 723-823 K temperature range, cerium oxide-copp...