The Study Of Occurrence And Transformation Of Chromium During Coal Combustion

Coal has occupied an important place in energy structure of China for a long time,while coal combustion is the main way of coal utilization.Hazardous trace elements which are attached to the particulate matters,can exist in the environment for a long-term.As a result,they are threatening the surrounding environment and human bodies.The chromium is considered to be one of the hazardous trace elements.However,due to the low concentration and release of chromium,few researches about the release mechanisms and the migration characteristics of chromium during coal combustion were reported.Migration and volatilization characterizations of chromium during combustion of a Linhuan coal sample were studied in a muffle roaster at various temperatures.This paper analyzed the characteristics of migration and volatilization and the environmental effects of chromium during coal combustion.The conclusions showed as follows:(1)The coal in Linhuan belongs to low-volatile,high-ash and low-sulfur bituminous coal,and composed of mineral elements like SiO2、Al2O3、Fe2O3.The mineral phases were clay minerals(kaolinite,dickite,illite)and quartz in coal,while the mineral phases were quartz,mullite and hematite as combustion temperature reached to 1000 ℃.The coal had sticky surface gradually and a mass loss which was attributed to the decomposition of organic matter and kaolinite during coal combustion.(2)The concentration of chromium in Linhuan coal is 31.77 μg g-1,which is higher than the Chinese coal by a factor of 2.1.The volatilization proportion of chromium also increased with the increase of retention time,which is basically released before 60 min of coal combustion.The volatilization proportion of chromium increased as the temperature got up,especially the temperature was higher than 900 ℃,and the releasing intensity value of chromium varied at different temperature regions.Chromium has a similar release behavior with vanadium,cobalt and nickel,which due to their similar chemical properties.(3)The modes of occurrence of chromium in Linhuan coal were mainly bound in residual,followed by Fe-Mn oxides,organic matter,ion exchangeable,carbonates.The decomposition of carbonates,Fe-Mn oxides and organic matter caused the release of chromium during coal combustion.For the ash obtained at 1000 ℃,the proportions of chromium in five forms were all reduced,suggesting that these forms of chromium were transformed from the solid phase to the volatile phase,especially the decomposition/oxidation of chromium in Fe-Mn oxides minerals is the main cause.Chromium may reacted with the mineral elements to produce chromate,which affecting the distribution of chromium in gas/solid phase during coal combustion.(4)It was calculated that about 15.08%of chromium emitted into the atmosphere,and the annual emission is about 15.63 t,which has a little volatility.The enrichment factors and pollution factors of chromium were used for evaluating the influence of the atmospheric environment.The results showed that the enrichment factor of chromium was larger in the fly ash in power plant,and decreased with the increase of temperature,while the pollution factors of chromium increased with the increase of temperature.(5)It was calculated that the emissions of chromium in coal combustion experiment and power plant were between 0.30—2.05 mg·m-3,which were much higher than the air pollution emission standard GB16297—1996(0.07 mg m-3).It provided a theoretical basis for controlling the emission of chromium during coal combustion in power plant and established the standard of pollutant emission based on the chromium.