Dry absorption of hydrogen chloride and sulfur dioxide by calcium-based sorbents from humidified flue gas

A fixed-bed experimental apparatus was constructed to simulate acid gas absorption by powdered sorbents on a bag filter. The sorbents studied were hydrated lime and calcium silicate. Key variables included relative humidity, temperature, and the concentrations of HCl, SO₂, NO, NO₂, and O₂. The concentrations of HCl, SO₂, and NO ₂ at the outlet of the fixed bed were monitored by an FT-IR with a gas cell.; At a high relative humidity (19%), the HCl absorption rate and sorbent utilization were approximately the same for both hydrated lime and calcium silicate. At low relative humidity (3.5%), calcium silicate was more reactive than hydrated lime. At a concentration from 250 to 1000 ppm HCl the rate of absorption was first order in HCl concentration with both sorbents.; When HCl and SO₂ were present simultaneously in the absence of O₂ or NO₂, the reactivity of HCl dominated that of SO₂. At the end of an experiment with calcium silicate, no SO ₂ remained bound in the solids. With hydrated lime, a small amount of SO₂ would remain with the solids. With both hydrated lime and calcium silicate, the reactivity of SO₂ was increased in the presence of O₂ or NO₂. Even with increased SO₂ reactivity, the absorption of the acid gases by hydrated lime was still dominated by HCl reactivity. On the other hand, if sufficient NO₂ were in the gas stream (≥50 ppm) and the SO₂/HCl ratio was high, the reacted solids contained more sulfur than chloride.; The fixed-bed experiments were modeled using a semiempirical approach based on shrinking core theory. Parameters estimated from the experiments were then used to predict sorbent performance on a bag filter. With calcium silicate at conditions of a municipal waste incinerator, ≥90% removal of both HCl and SO₂ is possible at reasonable humidity and NO ₂ concentration. At conditions of a coal-fired boiler with calcium silicate, ≥90% HCl removal and approximately 60% SO₂ removal is possible at reasonable gas conditions. For both combustors and boilers with hydrated lime, high HCl removal levels are predicted but SO₂ removal is low (≤30%).