Removal of hydrogen chloride from flue gas by dry sorbents prepared from the hydrothermal reaction of silica and calcium hydroxide

Experimental work was performed to determine the cause(s) of reactivity enhancement exhibited by solids produced by hydrothermal reaction of silica and calcium hydroxide (CH) in tests in which the solids were reacted with flue gas containing hydrogen chloride (HCl). Three types of sorbents were prepared by reacting calcium hydroxide with amorphous silica, crystalline silica, and a low-calcium fly ash. In addition to reactivity tests, sorbents underwent powder x-ray diffraction analysis, surface area analysis, and scanning electron microscopy analysis. Sorbents prepared using amorphous silica resulted in enhanced utilization over that of CH alone for all silica: CH mass ratios at all slurry times. The crystalline silica/CH sorbents resulted in no reactivity enhancement over that of CH, except in the case of high silicaiCH ratio and long slurry time. Sorbents prepared from fly ash exhibited enhanced reactivity to an extent that was between that of the amorphous silica sorbents and the crystalline silica sorbents. X-ray diffraction indicated that poorly crystallized calcium silicate hydrate formed from the amorphous silica/CH reactants, and that more calcium silicate hydrate formed with increasing silica: CH ratio and increasing slurry time. Little to no calcium silicate hydrate was seen in the x-ray patterns of the crystalline silica/CH sorbents. Scanning electron microscopy of the amorphous silica/CH sorbents pointed to the formation of the calcium silicate hydrate called C-S-H(I). Within the limits of the experiments, utilization of the amorphous silica/CH sorbents was found to have a linear relationship with the molar SiO₂:CaO ratio of the starting components as long as there was CH remaining in the solids (as demonstrated by x-ray diffraction). Behavior of sorbents in which no CH remained differed from that of sorbents with a CH excess. The degree of enhanced reactivity was related to the ability of the silica to dissolve in the high pH solutions. Greater availability of dissolved silica translated into proportionally more calcium silicate hydrate and less CH in the resulting sorbent. The cause of the reactivity enhancement was the increase in production of calcium silicate hydrate.