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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
Alternative TitleRelease of alkali metals and its influence on the pollutants formation during thermal utilization of high-alkali coals
赵京
Thesis Advisor魏小林 ; 李腾
2019-11-23
Degree Grantor中国科学院大学
Place of Conferral北京
Subtype博士
Degree Discipline流体力学
Keyword高碱煤,碱金属,赋存形式,释放迁移,nox,pm,固废焚烧
Abstract

我国是煤炭消耗大国,随着煤炭资源的逐渐枯竭,新疆准东煤田、哈密煤田等的开发利用将会是未来煤炭使用的主要来源之一。但是,新疆大部分煤均属于碱金属含量较高的煤,在燃烧过程碱金属将会以NaClNaOH等形式释放到烟气中去,导致高碱煤在利用过程不仅会对锅炉高温受热面造成严重的积灰、结渣及腐蚀现象,也会对燃烧过程的污染物(NOxPM)的生成产生影响。因此,为了推进高碱煤在未来的清洁、高效的利用,本文将通过实验研究、理论计算以及模拟分析针对高碱煤中碱金属的赋存形式、释放迁移以及在燃烧过程如何影响NO以及PM形成和释放进行了系统的研究。同时基于上述研究结论,揭示了碱/碱土金属在固废/固废-污泥焚烧过程的释放迁移行为以及如何在固废焚烧设备上引起积灰、结渣现象。所得到的主要结论如下:

(1) 高碱煤灰分析中碱金属含量由于在测量过程发生释放而导致结果偏低。根据化学萃取法可以精确获得碱金属的赋存形态以及含量,发现高碱煤中碱金属Na主要以水溶性和醋酸铵溶性为主,其次为酸溶性以及不溶性,而K主要以不溶性形式存在。根据离子电荷守恒计算分析了均匀分布条件下的水溶性Na具体存在形式,发现准东煤中水溶性Na主要以NaClNa2SO4以及NaHCO3三种形式存在,而哈密大南湖煤中Na主要以NaClNa2SO4以及NaNO3形式存在。此外,还发现高碱准东煤中水溶性Cl的摩尔含量与水溶性Na的含量存在线性关系。

(2) 高碱煤在热利用过程,水溶性Na在反应温度低于200时会随着水分的挥发而向煤颗粒表面富集并释放;当温度升高至300-700范围内时,高碱煤中挥发分的大量释放会促进煤中有机Na的释放,同时此阶段还发生了可溶性Na向不溶性Na间形态的转化;当反应温度超过800后,已经达到大部分Na盐的熔点,此时Na主要以NaClNaOH以及Na2O等形式释放。燃烧底灰以及飞灰中碱金属的赋存形式并不相同,底灰中碱金属Na主要以不溶性形式存在,而飞灰中碱金属主要以水溶性形式存在。此外,高岭土添加剂能够显著降低Na的释放,主要产物为硅铝酸盐。

(3) 高碱煤燃烧过程释放的碱金属Na以及Cl会通过影响HO以及OH等自由基的浓度而影响NO主要前驱体HCNNH3的氧化。NaCl能够直接消耗H自由基(NaCl + H = Na + HClH + O2 = O + OH)降低HO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 + H2ONa + 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。HClNO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HO2反应生成更多的OOH自由基。HCl促进NO生成的主要反应机理为HCl + O = Cl + OHHCl + OH = Cl + H2OCl + HNO = HCl + NO

(4) 高碱煤燃烧过程,超细模态颗粒物的形成主要受到碱金属和硫等矿物蒸气的均相成核和异相凝结的控制,中间模态以及粗模态的颗粒物主要受到焦炭粗矿物质颗粒燃烧过程的破碎以及细颗粒的熔融聚合的影响。随着反应温度的增加,亚微米颗粒物PM1的生成量降低,并且出峰位置向更小的粒径偏移。碱金属盐添加剂能够显著促进亚微米颗粒物的形成,在等质量分数的前提下,其对亚微米颗粒物生成的影响顺序由大到小分别为:NaCl>Na2SO4>NaAc。漂珠添加剂中具有较高含量的游离SiO2,能够通过化学反应捕获碱金属等矿物质来抑制超细模态颗粒物的生成,并能够在表面形成Na-Al-Si(T)的熔融软化层通过物理碰撞方式吸附中间模态颗粒物。

(5) 固废以及固废-污泥联合焚烧过程,碱/碱土金属会以NaClKCl以及CaCl2的形式进行释放,并在烟气中存在SO2时,能够被硫化形成碱/碱土金属硫酸盐。气相碱/碱土金属盐首先会在炉膛内形成透辉石以及黄长石等物质而造成炉膛结渣;然后进入余热锅炉后,会在换热器表面以CaSO4NaCl以及KCl等形式沉积而造成换热设备的结渣;最后进入反应塔后,能够通过“侯氏制碱法”原理形成CaCO3Na2CO3组合的混合物质而引起结渣现象。此外,在余热锅炉内随着烟气温度的降低,碱金属也将从烟气中析出并与飞灰结合,降低灰熔点,并增加其粘结性,其中飞灰中碱金属主要以水溶性的NaClKClK3Na(SO4)2形式存在。

Other Abstract

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.

5During 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.

Language中文
Document Type学位论文
Identifierhttp://dspace.imech.ac.cn/handle/311007/80722
Collection高温气体动力学国家重点实验室
Corresponding Author赵京
Recommended Citation
GB/T 7714
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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