Abstract
•Sludge mixed pellet had reduced energy input compared with pure biomass pellet.•Improved hardness and combustion characteristics were obtained after adding sludge.•Biomass-sludge mixed pellet showed lower moisture uptake than pure biomass pellet.•UV-AF images indicated that the natural binders were primarily lignin and protein.•Sludge-biomass mixed pellets could be a suitable solid fuel.
Pelletization has been widely used for mass and energy densification of biomass. As the demand for pellets increases, the conventional raw materials will be insufficient. In this study, the feasibility of sludge mixed with biomass pellets as a solid fuel was researched. Sludge mixed pellets and pure biomass pellets were investigated to evaluate the energy consumption and pellet properties, including proximate analysis, moisture adsorption, pellet density, Meyer hardness, volume expansion, higher heating value and combustion characteristics. The results showed that sludge mixed pellets had reduced energy input, increased hardness, decreased moisture absorption rate and improved combustion characteristics compared with pure biomass pellets. However, the increased ash content of sludge mixed pellets should be noted. These results indicate that there is a balance between an improvement in fuel quality and an increase in ash content. The sludge mixed pellets could be a suitable solid fuel.