Influence of Different Lignosulphonates on the Properties of Desulfurant Sorbents Prepared by Hydration of Ca (OH)2 and Fly Ash

Abstract

The influence of different lignosulphonates used as additives on the physicochemical properties and desulfurization behavior of mixtures of Ca(OH)2 and fly ash was investigated. Sorbents were prepared by the hydration of fly ash and Ca(OH)2 at 3:1 or 5:3 fly ash /Ca(OH)2 initial weight ratios for 7 or 15 h, at 90 °C and atmospheric pressure. The lignosulphonates tested are trade named: Borrebond 55-L, Borrebond 55-S, Borresperse CaI-50, and Borresperse NaI-50. The properties characterized in the sorbents include Ca(OH)2 conversion, BET surface area, and X-ray diffraction pattern. The sorbents were tested in a flue gas desulfurization reaction at low temperature. The effect of the additives on the desulfurant properties of the sorbents depends greatly on the amount of additive, with the type of lignosulphonate used being less relevant. A positive effect was found when the weight of the additive was 0.1 or 0.2% of the initial amount of solid raw materials. Higher percentages of additive produced the opposite effect, decreasing greatly the specific surface area and the SO2 capture of the sorbents. When additives are used in low amounts, a general low increase in the specific surface area of the sorbents is found; this structural property and the hydrophilic character of lignosulphonates present in the sorbents, that increase the retention of the water from the flue gas, can explain the higher amounts of SO2 retained per mol of calcium with these sorbents. When lignosulfonates are added at higher amounts to the slurry of fly ash and Ca(OH)2, their agglomerating properties prevail and the raw materials remain unreacted in the sorbents that exhibit low surface areas and are unable to capture SO2 on their surface.