Zhundong coal has attracted an increasing concern due to its super huge reserve but high content of alkali metals. Oxy-fuel combustion of Zhundong coal benefits the near-zero emission of pollutants in coal-fired power plants and promotes the large-scale utilization of high-alkali coal. However, few efforts if any have been conducted on oxy-fuel combustion of Zhundong coal. The previous studies related to Zhundong coal were mainly focused on sodium behaviors but little work has been performed on calcium and iron, while calcium and iron are very likely to generate significant influences on fouling problems in combustion of Zhundong coal. The present study aimed to elucidate the release and transformation behaviors of sodium, calcium, and iron in oxy-fuel combustion of Zhundong coal using a fixed-bed reactor. Experimental results indicated that calcium in Zhundong coal was mainly present as the ammonium acetate-soluble form, while the iron existed in forms of hydrochloric acid-soluble and insoluble. With the increasing combustion temperature, the ash yields of Zhundong coals decreased and the volatilization ratio of sodium increased, while the temperature had a weak influence on ash yield and the release of water-soluble sodium between 800 and 1000 °C. The variations of total calcium with combustion temperature were not significant, but transformations among various chemical forms occurred. The decreased iron of the hydrochloric acid-soluble form was transformed into the insoluble form and discharged into gaseous phase. Compared to the air case, oxy-fuel combustion with 21% oxygen led to more sodium and iron retained in residual ash, while it promoted the release of calcium. The mineral transformation and ash formation were susceptible to the high content of carbon dioxide under oxy-fuel condition and were strongly associated with the chemical forms of sodium, calcium, and iron within Zhundong coals. The crystalline mineral species in Zhundong ash were obviously influenced by the combustion temperature and partly affected by the atmosphere. The differences of mineral species of Zhundong ash between air and oxy-fuel cases were mainly present in the range of 800–1000 °C, which was closely related to the decomposition of calcite and transformation of calcium. The oxygen content dependency of transformation behaviors of sodium, calcium, and iron was greatly different during oxy-fuel combustion. This work possibly offered an improved understanding of the functional mechanisms of sodium, calcium, and iron on fouling issues.

中文翻译：

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准东煤的含氧燃烧过程中钠，钙和铁的释放和转化行为

准东煤由于储量巨大，碱金属含量高而引起了越来越多的关注。准东煤的富氧燃烧有利于燃煤电厂污染物的近零排放，并促进了高碱煤的大规模利用。但是，对于准东煤的氧燃料燃烧，几乎没有做出任何努力。之前有关准东煤的研究主要集中在钠的行为上，但对钙和铁的研究很少，而钙和铁极有可能对准东煤燃烧中的结垢问题产生重大影响。本研究旨在阐明使用固定床反应器在准东煤的氧燃料燃烧中钠，钙和铁的释放和转化行为。实验结果表明，准东煤中的钙主要以乙酸铵可溶形式存在，而铁以盐酸可溶和不溶形式存在。随着燃烧温度的升高，准东煤的灰分产率降低，钠的挥发率增加，而温度对灰分产率和水溶性钠的释放在800〜1000℃之间的影响较弱。总钙随燃烧温度的变化不显着，但发生了各种化学形式之间的转化。盐酸可溶形式的还原铁被转化为不溶形式，并排放到气相中。与空气情况相比，用21％的氧气进行含氧燃料燃烧会导致残留的灰分中残留更多的钠和铁，同时促进钙的释放。在含氧燃料条件下，矿物的转化和灰分的形成易受高含量二氧化碳的影响，并且与准东煤中钠，钙和铁的化学形式密切相关。准东灰分中的结晶矿物种类明显受燃烧温度的影响，部分受大气的影响。在空气和含氧燃料情况下，准东灰分的矿物种类差异主要存在于800–1000°C的范围内，这与方解石的分解和钙的转化密切相关。在氧燃料燃烧过程中，钠，钙和铁的转化行为对氧含量的依赖性差异很大。