|Alternative Title||Release of alkali metals and its influence on the pollutants formation during thermal utilization of high-alkali coals|
|Thesis Advisor||魏小林 ; 李腾|
|Place of Conferral||北京|
(3) 高碱煤燃烧过程释放的碱金属Na以及Cl会通过影响H、O以及OH等自由基的浓度而影响NO主要前驱体HCN和NH3的氧化。NaCl能够直接消耗H自由基(NaCl + H = Na + HCl，H + O2 = O + OH)降低H、O以及OH自由基浓度，抑制NO的生成，还能够与H2O反应生成NaOH通过NaOH + H = Na + H2O和Na + OH + M = NaOH + M形成的循环反应降低自由基的浓度。NaAc在燃烧过程会首先转换形成NaOH，再通过上述循环反应抑制NO生成，而Na2SO4由于化学性质相对稳定，对NO形成影响并不显著。HCl对NO形成的影响随着反应温度的升高呈现出先抑制后促进的现象。HCl抑制NO生成主要是通过HCl + H = Cl + H2来抑制H自由基浓度，进而抑制H与O2反应生成更多的O和OH自由基。HCl促进NO生成的主要反应机理为HCl + O = Cl + OH、HCl + OH = Cl + H2O和Cl + HNO = HCl + NO。
China is a country with large coal consumption. With the gradual depletion of coal resources, the development and utilization of Zhundong and Hami coalfields, which is produced in Xinjiang province, will be one of the main coal sources in the future. However, most of coal belongs to high-alkali coals in Xinjiang province. During combustion, the alkali metals will be released into flue gas in the form of NaCl, NaOH, etc., which will not only result in serious fouling, slagging and corrosion on the high-temperature heating surface of the boiler, but also influence the formation and emission of combustion pollutants (NOx, PM, etc.). Therefore, in order to promote the clean and efficient utilization of high-alkali coals in the future, this paper will study the occurrence and migration of alkali metals in high-alkali coals and the effect of alkali metals on NO and PM emission through experimental research, theoretical calculation and simulation analysis. At the same time, based on the above research conclusions, the release and migration behavior of alkali/alkaline earth metals is revealed during municipal solid waste incineration or waste and sewage sludge co-incineration. And the fouling and slagging caused by alkali/alkaline earth metals are also analyzed. The main conclusions are shown as follows:
(1) Due to the release of alkali metals in the measurement process, the content of alkali metals is low than real value in ash analyses of high-alkali coals. According to the chemical extraction method, the occurrence and content of alkali metals can be accurately obtained. It is found that Na in the high-alkali metals is mainly existed in the form of water-soluble and ammonium acetate-soluble, followed by acid-soluble and acid-insoluble and K is mainly existed in the form of acid-insoluble. According to the charge conservation calculation of ions, the water-soluble Na in Zhundong coal is considered to exist mainly in the form of NaCl, Na2SO4 and NaHCO3 under uniform distribution conditions, while water-soluble Na in Dananhu coal is mainly existed in the form of NaCl, Na2SO4 and NaNO3. In addition, it is found that the the nmolar content of water-soluble Cl and Na is linear in Zhundong coal.
(2) During heat utilization, the water-soluble Na can migrate to the surface with the water and then release with the evaporating water when the reaction temperature is lower than 200℃; When the temperature increases to the range of 300-700℃, the release process of organic Na in high-alkali coals will be promoted with the release of volatiles. At the same time, the conversion of soluble-Na into insoluble-Na is also occurred; When the reaction temperature exceeds 800℃, Na is mainly released in the form of NaCl, NaOH, and Na2O due to reaching to the melting point of most inorganic Na salts. The occurrence of alkali metals in botton ash and fly ash is different. The Na in bottom ash is mainly existed in the form of insoluble, while the Na in fly ash is is mainly existed in the form of water-soluble. In ddition, kaolin additives can reduce the release of Na, the main product is aluminosilicate.
(3) The release of Na and Cl during coal combustion can inhibit the oxidation of HCN and NH3 to form NO via decreasing the concentration of H, O, and OH radicals. NaCl can directly consume H radical (NaCl + H = Na + HCl, H + O2 = O + OH) to reduce the concentration of radicals and then inhibit the NO formation. NaCl can also react with H2O to form NaOH to decrease the concentration of radicals through the simple circular reactions between “NaOH + H = Na + H2O” and “Na + OH + M = NaOH + M”. NaAc is first converted into NaOH during coal combustion, and then inhibits the NO formation via the above circular reactions. The chemical properties of Na2SO4 are relatively stable, and the effect of Na2SO4 on NO emission is not significant. The HCl addition tends to inhibit NO formation at low temperature and exhibits promotion effect when the temperature is increasing. The inhibition of HCl on NO formation is mainly by reducing the concentration of H radical via “HCl + H = Cl + H2”, thereby inhibiting the reaction between H and O2 to form more O and OH radicals. The promotion of HCl on NO formation is via “HCl + O = Cl + OH”, “HCl + OH = Cl + H2O”and “Cl + HNO = NO + HCl”.
(4) During high-alkali coals combustion, the formation of ultrafine mode particles is mainly influenced by the homogeneous nucleation and heterogeneous condensation of minerals such as alkali metals and sulfur. The formation of central mode and coarse mode particles is mainly caused by char fragmentation, coarse mineral fragmentation, and aggregation of fine molten particles. The amounts of PM1 produced by the submicron particles gradually decrease with increasing the reaction temperature and the size of PM1 shifts to a smaller size. Na-additives can significantly promote the formation of submicron particles. On the premise of equal mass fraction, the order of its influence on the formation of submicron particles can be shown as: NaCl>Na2SO4>NaAc. Floating beads have large amounts of free silicon dioxides and can capture mineral vapors such as alkali metals through chemical reaction to inhibit the formation of ultrafine mode particles. Due to molten softening layer of Na-Al-Si (T) formation on the surface of floaring beads, the central mode particles can be adsorbed via physical collide and coalesce.
（5）During waste incineration or waste-sludge co-incineration, alkali/alkaline earth metals (AAEMs) are usually released in the form of NaCl, KCl and CaCl2, and the sulfates can be produced in the presence of SO2 in efflulent gas. Firstly, AAEMs vapors can form diopside and gehlenite to cause slagging in the furnace; secondly, after entering the waste heat boiler, CaSO4, NaCl and KCl can condense onto the surface of the superheater, which will cause slagging; finally, after entering the reaction tower, the mixture of CaCO3 and Na2CO3 formed via “Hou's process for soda production” can cause slagging. In addition, as the gas temperature decreased in the waste heat boiler, the alkali metals can condense onto the fly ashm thereby decreasing the fly ash metling point and enhancing the slagging. The alkali metals in fly ash are mainly in the form of water-soluble NaCl, KCl and K3Na(SO4)2.
|赵京. 高碱煤热利用过程中碱金属释放特性及其对污染物生成的影响[D]. 北京. 中国科学院大学,2019.|
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