A comparison study of the inhabitation effect of phosphate tailing and kaolin on the release of K in biomass straw

Deposition and corrosion in boilers caused by the release of alkali compounds in agricultural residua pose significant challenges to the widespread utilization of biomass energy. Phosphate tailings (PT), which is rich in Ca, Mg, and P, have the potential to mitigate the potassium (K) release of biomass straw. However, the impact of PT on K immigration has not been investigated. To elucidate the influence of PT on K immigration, thermal analysis, K concentration determination, and mineral evolution during heating of ash were conducted. These results were compared with those obtained from biomass mixed with kaolin, an effective additive known to alleviate K release in biomass. The findings revealed that PT resulted in lower K release at temperatures below 600 ℃ compared to kaolin. Because the reaction between PT and K-compounds to form K-phosphate occurs more readily than the formation of K-silicate at lower temperatures. However, the effectiveness of PT diminished as the temperature increased. One reason is the decomposition of K-phosphate, resulting in the volatilization of K at higher temperatures. Another reason is the competition among Ca, Mg, and K for the opportunity to react with Si and Al, leading to the formation of more stable Ca and Mg aluminosilicate and facilitating the release of K. It is noteworthy that the addition of kaolin to rice straw causes a violent release of K in 1000–1200 ℃. This can be attributed to the surplus of Si in the system, promoting the release after the eutectic melting between K-silicate and SiO₂.