Study of the characteristics of the ashing process of straw/coal combustion

► This paper presented an experimental study of the characteristics of the ashing process during straw/coal co-combustion. ► The proximate analysis showed that the blends with coal content between 5 wt.% and 15 wt.% had the effect of reducing the ash quantity. And the blends with coal content between 20 wt.% and 90 wt.% demonstrated effect of promoting ash and therefore increasing the ash quantity. The blend with 10 wt.% coal has a prominent ash inhibiting effect and the sample with 40 wt.% coal has the highest ash promotion. The ashing processes of those two blends were studied in detail in a Thermal Gravity Analyzer with the comparison to the ones of straw and coal. ► The ash fusion points were measured. The oxides contents of the ashes were measured by Energy Dispersive X-ray Fluorescence spectrometry (EDX). And the minerals composition were examined by X-ray powder diffractometry (XRD). ► The experiments had identified that with the blend of 5-15 wt.% coal, the potassium K was accelerated into gaseous products during the volatile release stage at the temperature of 250-380 °C and therefore created an ash inhibition effect. The K2O content in the ash of the blend with 10 wt.% coal is lowest and minimum minerals with K element was found in this sample. ► The mechanisms of ash promotion effect for blends with coal content greater than 20 wt.% was due to the slow thermal reaction with oxidant and the eliminated decomposition of CaCO3 and CaSO4 at pyrolysis stage since the coupling reactions of Ca and Al produced stable CaAl8Fe4O19 KSi3AlO8 during combustion stage and resulted in much less gaseous products than expected at high temperature. The evidences revealed the mechanisms of the coupling reactions of the coal and straw on the ashing stages.