Effects of silicon-aluminum additives on ash mineralogy, morphology, and transformation of sodium, calcium, and iron during oxy-fuel combustion of zhundong high-alkali coal

Zhundong coalfield with super-huge reserve is the largest integrated coalfield in China, while its high content of alkali and alkaline earth metals (AAEMs) gives rise to severe slagging and fouling problems during combustion. Previous studies have been conducted on the release and transformation behaviors of the AAEMs during air combustion of Zhundong coal but with little concern on iron. However, the effects of silicon-aluminum additives on oxy-fuel combustion of high-alkali coal have been rarely evaluated yet. The present study focused on the effects of four silicon-aluminum additives (Al2O3, SiO2, kaolin and diatomite) on the chemical form transformation of sodium (Na), calcium (Ca) and iron (Fe), ash morphology and mineralogy of Zhundong high-alkali coal during oxy-fuel combustion. Experimental results showed that the silicon-aluminum additives could increase the ash yield of Zhundong coal and hinder the release of Na, Ca and Fe, while various additives had different effects on the element transformation. Almost all the additives promoted the transformation of water-soluble sodium to insoluble form during oxy-fuel combustion and the amount of insoluble sodium was increased correspondingly. The variations of sodium present in forms of hydrochloric acid-soluble and ammonium acetate-soluble were highly related to coal type. Various silicon-aluminum additives exhibited different microstructures and mineral types, and the ways of retaining AAEMs were also different. With an increase in the proportion of diatomite additive, the sodium contents of water-soluble and ammonium acetate-soluble forms declined; however, the content of sodium in form of hydrochloric acid-soluble increased at first and then decreased. This work can provide a beneficial information to promote the clean, effective and safe utilization of Zhundong coals in large-scale.