Combined nitrogen oxides/sulfur dioxide control in a spray- dryer/fabric-filter system

Abstract

Caustic soda was evaluated for its value as an additive in combined nitrogen oxides/sulfur oxides (NO{sub x}/SO{sub x}) removal from coal combustion gases in a four-week field test at a 22-MW (electric) industrial-scale spray-dryer facility at Argonne National Laboratory (ANL). A combination of modified process conditions and additions of caustic soda to the lime-based spary-dryer system yielded some removal of NO{sub X} from the flue gases, but an increased sorbent ratio was needed to maintain the SO{sub 2}-removal ability. The higher concentration of NO{sub 2} at the system outlet than at the inlet indicated that the oxidation of NO to NO{sub 2} was a key step in the overall removal process; a large increase in the concentration of nitrates was correlated with the increased oxidation of NO to NO{sub 2}. The test data also indicated that the outlet temperature from the spray dryer was a key factor in simultaneous NO{sub x}/SO{sub x} removal. Above 190{degree}F, a relationship for NO{sub x} removal was found between the equivalence ratio of the reactants and the pollutants, the thickness of the filter cake, and the ratio of SO{sub x} to NO{sub x} at the system entrance. The existence of temperature window'' ideal for NO{submore » x} removal could not be verified during the field tests, primarily due to an upper temperature limit of 210-215{degree}F. Also, as the filter-cake thickness increased, so did NO{sub x} removal. NO{sub x} removal (up to 45%) was enhanced by high ratios of SO{sub x}/NO{sub x} (>5:1) in the flue gas. At low SO{sub x}/NO{sub x} ratios (<2:1), virtually no NO{sub x} removal (<10%) took place, even when SO{sub x} removal was well over 90%. 15 refs., 29 figs., 16 tabs.« less