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

Abstract

An experimental study was performed to examine the ashing process during straw/coal co-combustion to determine the effect of the blending ratio on ash products. A total of eleven blending samples with coal contents varying from 5wt.% to 90wt.% along with pure wheat straw and pure coal samples were tested to determine the ash fusion points, oxide contents and mineral contents. Blends with coal contents between 5wt.% and 15wt.% were able to inhibit ash and reduced ash quantity. Blends with coal contents greater than 20wt.% promoted ash and increased ash quantity. Thermal decomposition processes during the combustion and ash pyrolysis of the blends with 10wt.% and 40wt.% and the samples of pure coal and pure wheat straw were simulated using a Thermal Gravity Analyser. The results indicated that the ashing processes of the blends were influenced by the coupling reactions of the minerals in the straw and coal. When using a blend of 10wt.% coal, more potassium (K) was accelerated into gaseous products during the volatile releasing and firing stage, which caused an ash quantity reduction effect. K2O content was lowest in this sample, and a minimum amount of K compounds was detected. With a blend of 40wt.% coal, because the coupling reactions of Ca and Al produced stable minerals of CaAl8Fe4O19 and KSi3AlO8, less CaCO3 and CaSO4 were produced. Thermal decomposition at the ash pyrolysis stage was very weak and resulted in much less gaseous products than what would be expected at high temperatures; therefore, more ash residues remained.