Co-pelletization of sewage sludge and biomass: Thermogravimetric analysis and ash deposits

• Co-pelletization of biomass with sludge could reduce the mean activation energy.
• The reactivity of sludge–biomass pellets was improved with increasing biomass.
• The slagging hazard of sludge–straw pellet was more serious than sludge–fir pellet.
• Fir would be preferable rather than straw for sludge–biomass pellet.

In this study, the combustion behaviors of sludge–biomass mixed pellets with different sludge ratios in air atmosphere were investigated by thermogravimetric analysis. The pellets consisted of sludge and Chinese fir or rice straw with sludge ratios of 0%, 25%, 50%, 75% and 100%, respectively. The thermogravimetric data were applied for kinetic analysis using the Coats and Redfern (CR) equation. Results showed that there were four main stages such as dehydration, volatile oxidation, char burning and burnout during thermal decomposition of pellets. The reactivities of sludge–biomass pellets were improved by the increment of the mixing ratio of biomass. In addition, the potential slagging behaviors of the resultant ashes were evaluated by X-ray fluorescence (XRF). The suitable sludge to biomass ratio should consider the combustion properties and ash slagging deposition of pellets. It is inappropriate for the production of sludge–straw pellet in the present state due to the high ash slagging potential.

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