Absorption of HCl and SO2 from humidified flue gas with calcium silicate solids

The absorption of HCl and SO2 with calcium silicate was studied in a bench- scale, fixed-bed reactor at 120 degrees C. From 0 to 3.5% relative humidity (RH), an increase in relative humidity increased sorbent utilization by re action with HCl. From 3.5 to 19% RH, final sorbent loading by HCl was const ant at 1.4 mol/mol of Ca2+. The absorption rate of HCl was first-order in H Cl concentration from 250 to 3250 ppm. When calcium silicate was exposed to HCl and SO2 simultaneously in the absence of O-2 or NO2, no SO2 remained l oaded with the solids at the end of an experiment. Any SO2 that was absorbe d was eventually emitted from the solids in favor of increased HCl absorpti on. The addition of O-2 to the simulated flue gas caused improved SO2 absor ption but had little effect on HCl absorption. A dramatic increase in final SO2 loading and a decrease in final HCl loading was observed when NO2 was added to the gas stream. Adding 50 ppm NO2 increased SO2 loading from 0 to 0.73 mol/mol of Ca2+ and decreased HCl loading from 1.4 to 0.61 mol/mol of Ca2+. In the presence of NO2, increasing the SO2/HCl inlet ratio increased final SO2 loading and decreased final HCl loading. In experiments without H Cl from 90 to 150 degrees C, it was found that a low concentration of NO2 i ncreased final SO2 loading more at higher temperatures. The experimental da ta from the fixed bed were modeled using a modification of the shrinking co re model. Flux equations and estimated parameters were then used to predict the performance of HCl and SO2 absorption by calcium silicate on the surfa ce of a bag filter. The predictions suggested that, at reasonable gas condi tions in the absence of SO2, HCl penetration through the bag filter can be reduced below 20%. With simulated municipal waste combustion flue gas (low SO2/HCl ratio) with 50 ppm NO2, HCl and SO2 penetration can be reduced to l ess than 5%. At coal-fired boiler conditions (high SO2/HCl ratio) with 50 p pm NO2, HCl penetration can be reduced to 2% while SO2 penetration was pred icted to be 40%.