Effect of Sludge Pellets Addition on Combustion Characteristics and Ash Behaviour of Municipal Solid Waste

Abstract

Co-combustion of sewage sludge and municipal solid waste (MSW) in grate furnaces (GF) is a promising method to meet strict environmental requirements and reduce construction costs given its high market share in China. However, considering the high ash content and low heat value of sludge, co-combustion of sludge and MSW in GF may lead to combustion instability and an increase of fly ash (FA) production, which further aggravates ash deposition and corrosion of heating surfaces. Besides, semi-dried sludge powder with tiny particle size may cause dust explosion and increase the clinker ignition loss of slag ash due to the leakage in grates. In this study, semi-dried sludge powder is compressed into pellets through a rotary ring die granulator to investigate the effect of sludge pellets addition on gaseous pollutant emissions, combustion characteristics and ash behavior (e.g., ash fusion temperature and the clinker ignition loss of slag ash). The results show that the ratio of FA produced from sludge pellets incineration is lower than that of sludge powder. The clinker ignition loss of slag ash of sludge pellets increases with the increase of its compressive strength. The highest clinker ignition loss under the low combustion temperature is 11.9% which is far beyond the limit (5%) for GF. The addition of desulfurizer (CaO, mass ratio: 1%) in sludge pellets is found useful to reduce sulfur dioxide emission with the maximum 84% desulphurization rate at the combustion section. The decrease of ash flow temperature from co-combustion of sludge and MSW is less than 10 °C compared to that from mono-combustion of MSW, which suggests that co-combustion of sludge with the mixing ratio of 10% in municipal solid waste incinerators (MSWI) has little impact on fly ash slagging characteristics. The ash characteristics and gaseous pollutant emissions during co-combustion of sludge pellets and municipal solid waste are investigated.