The Experimental Study Of Fly Ash Properties And Its Effect On Flue Gas Mercury Removal

In this paper the related background were summarized. In order to studythe influence of characteristics of fly ash on flue gas mercury, the mercuryemission level of a coal-fired unit#1of eastern China was tested, and thecharacteristics of fly ash of unit#1were analyzed. The results show that theelectrostatic precipitator （ESP） could remove27.9%and17.7%of flue gasmercury, which were absorbed by fly ash. The unburned carbon （UBC） andmercury content of fly ash of different size were measured, the results showedthat these characteristics had no necessary connection. The experiment resultsshowed that fine particles （25μm⁶0μm） absorbed more mercury （above260μg/kg）, and large particles （60μm¹00μm） absorbed less mercury （about90μg/kg）. The residence time of smaller fly ash particle was relatively long,with more response time to promote the oxidation of flue gas mercury, thuscaptured more flue gas mercury.In order to study the factors affecting fly ash absorbing mercury, differentfly ash collected from three power plants in eastern China （Referred to as forpower plant A, B and C） were studied in the self-developed lab-scale fixed bed system. The experiment results showed that fly ash did not only absorbmercury, but also oxidized it. Fly ash samples saturated as the reactionprogress, but the oxidation was still happening. Fly ash had initial mercuryadsorption efficiency about20%, and reached saturation less than half an hour.Power plant C fly ash had the most powerful mercury oxidation capacity, itcould oxidize20%of the element mercury, and the average oxidation capacityof power plant A and power plant B were about12%and8%. It was foundthat the fly ash surface area and high ignition loss had certain positive effect toflue gas mercury adsorption and oxidation.The flotation separation method and magnetic separation method wereused to separate unburned carbon and magnetic beads from three kinds of flyash in order to search the key content of fly ash affecting on mercuryadsorption and oxidation. The results showed that the mercury adsorption ofUBC in fly ash was stronger than fly ash, but the oxidation ability was weaker.The commercial activated carbon absorbed much more mercury than UBC,but the effect on mercury was nearly zero.The results of magnetic beads showed that the mercury adsorption abilitywas poor, but the effect on flue gas mercury speciation was very strong. Themercury oxidation ratio of the beginning of reaction and two hours later,power plant A were30%and15%, power plant B were20%and10%andpower plant C were45%and25%. The chemical reagent SiO₂, Al₂O₃, MgO,CaO, andα-Fe₂O₃and γ-Fe₂O₃were used to study the effect of single composition, and the experiment results showed that SiO₂, Al₂O₃, CaO, MgOhad little effect, which can be neglected. The γ-Fe₂O₃had much strongereffect on flue gas mercury speciation than α-Fe₂O₃, the mercury oxidationratio of the beginning and two hours later ofγ-Fe₂O₃were70%and45%,and mercury oxidation ratio of α-Fe₂O₃first increased from20%to40%,and then gradually reduced. Combined with the XRD results, it showed thatthe main form of iron oxide in power plant A and B wasα-Fe₂O₃, while themain form of iron oxide in power plant C wasγ-Fe₂O₃. It can be seen thatγ-Fe₂O₃was the most active component in fly ash.The mercury speciation experiment of power plant C magnetic beadsunder different components of flue gas had been carried out, and the resultsshow that among these flue gas composition, HCl had the biggest impact onflue gas mercury speciation, O2could improve the fly ash for flue gas mercuryoxidation with the existence of HCl; NO and SO2had small effect on flue gasspeciation, there was no obvious promoting or inhibition been found. SO₂could weaken fly ash adsorption and oxidation ability while HCl existed, it isbelieved that SO2had competition reaction with mercury in fly ash surface.