Influence of collaborative disposal of sewage sludge in the fly ash partitioning and PM10 emission from a subcritical coal-fired boiler

Abstract

Co-combustion of sewage sludge and coal in coal-fired power plants may be a reasonable alternative for intractable sewage sludge. The co-combustion experiments were conducted on a 600 MW subcritical coal-fired boiler to discuss the fly ash partitioning and PM10 emission characteristics. The sewage sludge was sent to the boiler through coal pulverizer with blending ratios of 5 and 10 wt% based on the coal mass. The results showed that the particle size distributions (PSDs) of PM10 present the typical two-peak distribution. The mass yields of PM1 dramatically decreased by 57.83 % and 61.67 % for the 5 % and 10 % blending ratios, respectively, compared with single coal combustion, corresponding to the increase in PM1-10 emissions. The reduced PM1 appears to be transferred toward PM1-10 during co-combustion. The Fe Al-silicate and the Fe-Ca-Si-Al species were formed by the co-combustion of sludge and coal, which were derived from the active Fe-rich fine particles in sludge and mainly mullite and Ca-Si-Al found in coal ash. These transformations can be intensified by the high temperature, intense turbulent flow fields and the consequent frequent contact and collision of minerals in the large capacity boiler to form molten aluminosilicates. Therefore, the condensable Ca and S vapor from coal combustion and fine Si and Al adhere to the surface of molten Fe Al-silicate to form PM1+, which is the main reason for PM1 reduction during co-combustion. In addition, Fe-rich and Ca-rich flocculants in sludge contribute to PM1-10 largely, while some of the fine Al-Si particles are partitioned from PM1 to PM1-10 as captured by the molten aluminosilicates, both contributing to the increased PM1-10 emission after co-combustion.