Experimental Study On Transformation Of Mercury With Calcium-based Sorbents In Air-and-fuel-staged System

The main reason of atmospheric mercury emissions is the mercury that contained incoal. The mercury that contained in coal does great harm to the health of human beingsand the ecological environment. The emissions and control of it concern all countries inthe world.In this paper, there is detail introduction about China’s energy structure, as well asthe emissions and harm of mercury. The mercury generating mechanism and the presentstudy of pollutants’ combined removal are also expounded. With air staged and fuelstaged technology of Harbin Institute of Technology, we studied impacts to mercuryemission in the condition of no classification and no adsorbent, compound classificationand no adsorbent, compound classification and CaO/CaCl2adsorbent by the use ofEntrained Flow Combustor with Multiple Reaction Segment.Studies had shown that gaseous mercury from burning coal was the mainly form inthe condition of no classification and no adsorbent, accounting for about90%, theelemental mercury was about40%of the total mercury in the flue gas. The distributionand shapes of mercury were almost unchanged in air-and-fuel-staged and no adsorbentcompared with no classification and no adsorbent. As CaO additive was mixed with thereburning fuel and the Ca/S ratio increasing, there was a greater reduction of divalentmercury and increase of particulate mercury in air-and–fuel-staged. When Ca/S=1,2,3,the ratio of Hg2+to total mercury decreased from51.0%to20.4%,13.0%,9.2%, theproportion of particulate mercury increased from8.7%to21.1%,44.2%,53.9%. Hg0changed little to the original conditions, the removal efficiency of elemental mercury wasonly8%when Ca/S>2. When CaCl2additive was mixed to keep Ca/S=2with thereburning fuel under the compound classification, the percentage concentration of Hg2+to the total emissions of mercury decreased from51.0%to7.1%, the Hg0decreased from40.3%to8.5%, while particulate mercury has a significant increased from the8.7%to84.4%t. The CaCl2additive has good removal effect on mercury oxidation. Then CaCl2additive and CaO additive were all mixed with the reburning fuel under the compoundclassification, kept Ca/S=2and changed the amount of CaCl2to100μg/g,200μg/g,400μg/g,800μg/g (coal basis). Studies had shown that with the increasing of chlorine,Hg0decreased and particulate mercury increased. Considering the oxidation effect,economic benefit and boiler corrosion,400μg/g of chlorine is the best. In this contition,the ratio of Hg0to total mercury decreased was14.3%, the proportion of particulatemercury was72.6%and the Hg2+was13.1%.The author did experiment on a220t/h pulverized coal-fired power plant which isequipped with low NOx combusor for mercury transformation. When the calcium sorbent was not sprayed, mercury was mainly present in the flue gas up to91%, theconcentration of total gaseous mercury was13.2μg/m3, while the bottom ash content wasonly3%,6%was in the surface of fly ash particle. After calcium sorbent was injectedinto the furnace, the gaseous mercury reduced to62%, mercury contained in the bottomash was2%, mercury contained in fly ash increased to36%. For the distribution ofmercury in the flue gas, the elemental mercury was almost unchanged after spraying thecalcium sorbent, content of oxidized mercury decreased from71%to40%.