Tests on the vapor-phase mercury removal by chemically modified heavy-oil fly ashes in a fixed-bed and an entrained-flow reactor

摘要

Activated carbon has been usually considered for the sorbent to capture vapor -phase mercury from coalfired flue gases. However, the amount of carbon injection and operation cost is still high in order for the required level of mercury removal efficiency to be achieved. Activated carbons impregnated with chemicals and several novel mercury sorbents are under development to reduce the amount of sorbent usage and operational cost of sorbent injection technology. Carbon-based industrial waste such as waste tires or petroleum coke is one of the raw materials for the synthesis of novel mercury sorbents. In this work, unburned carbon derived from the combustion of heavy -oil, heavy-oil fly ash, was modified by sulfur impregnation following carbon dioxide activation. Vapor-phase mercury uptake by chemically modified heavy-oil fly ash was tested in a fixed-bed reactor and an entrained flow reactor of bench-scale apparatus. Mercury was supplied into the fixed-bed reactor in elemental form. The result was compared to the mercury uptake of commercially available activated carbons. The maximum mercury removal efficiency of sulfur-impregnated heavy-oil fly ash was 98 100 percent in the beginning stage of the adsorption reaction in the fixed bed reactor. In the entrained flow reactor, the mercury removal efficiency of modified heavy-oil fly ash was above 70 percent at the sorbent to mercury ratio of about 17,300. The mercury removal efficiency of chemically modified heavy-oil fly ash was comparable to those of commercial activated carbons in spite of its much smaller surface area and lower sulfur content. The result of this study suggested that chemically modified heavy -oil fly ash could be one of the candidates for a novel mercury sorbent to remove vapor -phase mercury from coal -fired flue gases. The benefit of the use of HOFAs as the source material to develop novel mercury sorbent is that we can cut down sorbent price, enhance the economic efficiency of sorbent injection technology, reduce industrial waste and waste disposal cost, and recycle HOFA to a more valuable material.