Transformation of aluminum-rich minerals during combustion of a bauxite-bearing Chinese coal

Samples of two high-aluminum coals and an associated fly ash were collected from a coal-fired power plant and a coalfield in Inner Mongolia, China. The mineralogy and physicochemical characteristics of low-temperature ash (LTA), high-temperature ash (HTA), and fly ash from those coals were studied by X-ray diffraction (XRD), X-ray fluorescence (XRF), and field scanning electron microscopy with energy dispersive X-ray spectroscopy (FSEM-EDX). The transformation of typical aluminum-bearing minerals at high temperature was investigated by systematic drop tube furnace (DTF) experiments and thermogravimetric analysis. The results show that the aluminum-bearing minerals in the high-Al coal are mainly boehmite and kaolinite. High temperature treatment transforms the aluminum-rich minerals to gamma alumina (γ-Al2O3), corundum (α-Al2O3), and an amorphous phase. γ-Al2O3 is the main mineral in the HTA (17.4 wt.%), while α-Al2O3 and mullite are the main minerals in the fly ash. The high-aluminum fly ash particles are irregular and their shapes are related to their compositions. The degree of irregularity of the high-aluminum fly ash particles is proportional to their aluminum content. The phase transformation of boehmite in the coal during high temperature treatment appears to have involved four stages including: boehmite dehydroxylation, transitional θ-Al2O3 formation, crystal nucleation and α-Al2O3 formation, and growth of α-Al2O3 crystals. The DTF experimental results indicated that the growth of α-Al2O3 crystals has a significant impact on PM emissions. Understanding the mineral transformation mechanism is therefore helpful in reducing PM emissions.

Research highlights► Alumina with different crystal structures were found in high temperature ash. ► The occurrences of minerals have significant influence on mineral transformation. ► The growth of α-Al2O3 crystal has a significant impact on PM emission.