Absorption of HCl and SO2 from Humidified Flue Gas with Calcium Silicate Solids

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The absorption of HCl and SO2 with calcium silicate was studied in a bench-scale, fixed-bed reactor at 120 °C. From 0 to 3.5% relative humidity (RH), an increase in relative humidity increased sorbent utilization by reaction with HCl. From 3.5 to 19% RH, final sorbent loading by HCl was constant at 1.4 mol/mol of Ca2+. The absorption rate of HCl was first-order in HCl concentration from 250 to 3250 ppm. When calcium silicate was exposed to HCl and SO2 simultaneously in the absence of O2 or NO2, no SO2 remained loaded with the solids at the end of an experiment. Any SO2 that was absorbed was eventually emitted from the solids in favor of increased HCl absorption. The addition of O2 to the simulated flue gas caused improved SO2 absorption 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 HCl from 90 to 150 °C, it was found that a low concentration of NO2 increased final SO2 loading more at higher temperatures. The experimental data from the fixed bed were modeled using a modification of the shrinking core model. Flux equations and estimated parameters were then used to predict the performance of HCl and SO2 absorption by calcium silicate on the surface of a bag filter. The predictions suggested that, at reasonable gas conditions 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 less than 5%. At coal-fired boiler conditions (high SO2/HCl ratio) with 50 ppm NO2, HCl penetration can be reduced to 2% while SO2 penetration was predicted to be 40%.