Ash fusion characteristics of biomass pellets during combustion

Abstract

Combustion of biomass pellets is one of the essential approaches to biomass energy utilization. However, the ash produced from the combustion of pellets may melt and affect combustion efficiency. Therefore, this study evaluated the ash formation and melting evolution characteristics, as well as their causes, of several typical biomass fuels, such as peanut shell pellets, wheat straw pellets, and corn straw pellets, during high-temperature combustion. The ash fusion temperatures of the biomass ash ranged from 1100 °C to 1300 °C. Combined with scanning electron microscopy–energy-dispersive spectrometry and X-ray diffraction analysis, the irregular aggregation of KCl, K2SO4, and other fusible matters was identified as the main factor affecting the ash fusion of wheat straw and corn straw pellets. Ash melting inhibited the combustion reaction of the wheat straw and corn straw pellets, which have the highest amount of silicon content in their molten ash at 1100 °C. However, the ash completely melted at 1300 °C, and the Si, MgO, and CaO contents increased and formed low-melting-point eutectics that were widely distributed in the molten ash. Higher temperatures that exceeded the ash flow temperature promoted the formation of various structures and eutectic materials of molten ash and accelerated carbon conversion. At 1300 °C, the ash melting process improved the combustion reactivity of the three biomass pellets, among which the combustion reaction of the corn straw pellets was the most obvious. At 1100 °C, the ash melting process only improved the combustion reactivity of peanut shell pellets and inhibited the combustion reactivity of wheat straw pellets and corn straw pellets. This study has provided a basis for solving the problem of fuel ash melting affecting combustion in the combustion equipment of biomass pellets.