| Coal occupies an important place in energy structure of China for a long time.Meanwhile coal is also an unclean energy, and increasing heavy metal pollution whencombustion. What’s more, they can exist in the atmosphere for a long-term and aredifficult to degrade. Besides, they can accumulate for high concentrations in organism.Facing these pollution problem, the modes of occurrence, migration andtransformation，and control methods of heavy metals from selected coal sample weresystematically researched in this article. The conclusions are as follows:①Basic characteristics of heavy metals in coal was studied. The result of the totalcontent analysis of heavy metals showed they were in small amounts and reachedgramme degree. Most of Zn, Cd and As were nearly all exceeding the average of coal inChina.It was a small percentage of Zn, Cd and As which were bound-organic bysequential extraction experiments, and the largest percent value was no more than20%.Zn and Cd existed mainly in the form of exchangeable, bound-sulfide and residue. Itwas found that the main speciation of As was bound-sulfide, which exceeded50%intotal content. We also discovered that the content of the heavy metal was proportional tothe ash by using the means of mathematical statistics which showed heavy metalsexisted mainly in the form of inorganic. The size of correlation of heavy metals in coal:Zn> Cd> As.②The migration and transformation performance of heavy metals wereinvestigated. During the experiment of coal static combustion, we found most of Zn,Cd and As had has evaporated, which followed the order of As>Zn> Cd. The heavymetal was more likely to volatilize which existed in the form of exchangeable,bound-sulfide and bound-organic. The minerals changed a lot during coal combustionand the heavy metals were more stabilized in ash than coal by X-ray diffraction.TheGibbs free energy of heavy metals was calculated in fuel reactor. Results showed thatsome low boiling point compounds were created and more heavy metals werevolatilized. In addition, the environmental effects evaluation was carried out during thecoal combustion. The subsequent extract experiment of ash showed that the content ofZn、Cd and As in the last step were high, and the percent value achieved50%.Meanwhile, the leaching quality of Zn, Cd, and As were lower than the standard valueof the GB3838-2002-Ⅳ, which was too low to damage environment. ③The accession of calcium carbonate applying in control the heavy metalpollutants was studied by two-factor variance analysis. It was observed that the used ofcalcium carbonate and raw coal at a mass ratio of1:100gave much better results, andburning temperature trend to great heat of1000℃and1100℃. The highest fixed ratioof Zn, Cd and As were38.41%,45.82%and23.93%respectively. However, thevolatilizing rates of them were all exceeding50%. After calcinations, calcium carbonateconverts into calcium oxide which has good ability to the heavy metal absorbent. At thesame time, calcium oxide can react with arsenic and give off calcium arsenate.④The accession of modified calcium carbonate applying in control the heavymetal pollutants was studied by orthogonal test．The results showed that the highestfixed ratio of Zn, Cd and As were72.77%（6﹟）,73.47%（5﹟） and58.44%（6﹟）respectively, which was calculated based on the calcium carbonate. Meanwhile，thevolatilizing rates of them were18.81%，15.40%and26.13%respectively. Whenconsidering the fixed rate of Zn, Cd and As, the optimal combination of parameterswere as follows: burning temperature1000℃, modifying agent Al2(SO4)3andmodulation ratio15. The method of modifying calcium carbonate resulted in thevariation of microphysical characteristic. The surface area，pore diameter and porosityof calcium carbonate modified by Al2(SO4)3were improved，and the best modulationratio was15. Furthermore, there was some γ-Al2O3produced on the particle surface. Inshort, it was conducive to control the heavy metals. DSC analysis showed that modifiedcalcium carbonate didn’t make the slagging characteristics of coal ash worse. |
PDF Full Text Request