Effects of the modified kaolin sorbents on the reduction of ultrafine particulate matter (PM0.2) emissions during pulverized coal combustion

It is a good way to control the formation of particulate matter by adding sorbents during coal combustion. Kaolin is usually considered as the most effective sorbent. Modified kaolin was firstly used to reduce the formation of ultrafine particulate matter (PM0.2, particles with the aerodynamic diameter less than 0.2 μm) in this work. Two kinds of typical modification methods were selected, namely, the acid modification and the sulfate modification. Then, pulverized coal was burned with raw and modified kaolin separately in an electrical heated drop-tube furnace, and the mass size distribution of the fly ash particulate was obtained. What's more, the different kaolin sorbents were heat-treated with alkali metal vapor in a quartz tube reactor and the products at different residence time were sampled and characterized. The results indicate that compared with raw kaolin, all the modified kaolin can significantly reduce the formation of PM0.2. The yield of PM0.2 decreased by 16.83% with the addition of raw kaolin, which was further reduced by 41.44% and 19.84% respectively with the addition of the acid and sulfate modified kaolin. It is found that the kaolin modified by hydrochloric acid owns more active sites and more free silicon dioxides, hence enhancing the reaction between kaolin and alkali metal and yielding less PM0.2. The modification by aluminum sulfate not only obviously improved the surface characteristics, but also broke some functional groups of raw kaolin. Therefore more alkali metal were reacted by the kaolin and the formation of PM0.2 were effectively reduced.