Effect of phosphorus-based additives on the sintering characteristics of cornstalk ash

Combustion of agricultural residues for heat and electricity production is an efficient way to reduce amount of such waste source and emissions due to inappropriate treatment of them. However, unlike other biomass materials, agricultural residues often have high content of ash and certain ash forming elements with low meting temperatures, resulting in sintering and slagging during combustion processes . It complicates combustion of agricultural residues and restricts further utilization of this renewable energy source . In this work, sintering of a typical agricultural biomass cornstalk and effect of two phosphate additives, Ca(H 2 PO 4 ) 2 ·H 2 O and NH 4 H 2 PO 4 , were investigated by a combination of various techniques. Results indicated that the addition of Ca(H 2 PO 4 ) 2 ·H 2 O increased the initial melting temperature of cornstalk ash by 64–209 °C. The ash sintering behavior was greatly improved as a result of formation of high melting temperature K–Ca phosphates, Ca 9 MgK(PO 4 ) 7 and Ca 2 KP 3 O 10 upon addition of the Ca(H 2 PO 4 ) 2 ·H 2 O. It is accompanied with reduced generation of low melting temperature K-rich silicates and the formation of large ash aggregates. Addition of NH 4 H 2 PO 4 increased the initial ash melting temperature by 28–116 °C, but it had slight influence on the cornstalk ash sintering characteristics. The analysis revealed that, although the addition of NH 4 H 2 PO 4 resulted in the formation of high melting temperature K 2 CaP 2 O 7 , certain amount of K phosphates and silicates eutectics were still formed, initiating and promoting sintering of ash from mixture of cornstalk and NH 4 H 2 PO 4 . Reactions between Ca(H 2 PO 4 ) 2 ·H 2 O and potassium salts under different conditions were also studied. It was found that reactions between Ca(H 2 PO 4 ) 2 ·H 2 O and potassium salts might already start as the temperature higher than 300 °C. K 2 CaP 2 O 7 , Ca 2 P 2 O 7 , Ca(PO 3 ) 2 and KCa(PO 3 ) 3 were identified as main products from these reactions and amounts of them considerably depends on the heat treatment temperatures and mixing ratios between Ca(H 2 PO 4 ) 2 ·H 2 O and potassium salts. Compared to KCl, K 2 CO 3 had a higher reactivity toward the Ca(H 2 PO 4 ) 2 ·H 2 O under same study conditions. The current work shows the addition of Ca(H 2 PO 4 ) 2 ·H 2 O is a promising approach to alleviate the ash sintering issues and assure efficient and smooth combustion of cornstalk.