Characterization, pelletization, and incineration behaviour of coal sludge–hay fuel blends with emphasis on emissions and environmental impacts

• Coal sludge–hay blends were successfully pelletized and combusted in a fluidized bed • Stable combustion was achieved with minimal variation in CO and NOx emissions • Pellet mechanical strength increased with higher hay content due to improved compressibility • SO2 and HCl emissions strongly depended on coal sludge share in the fuel blend • Results confirm technical feasibility of sludge–biomass pellets for waste-to-energy This study investigates the pelletization and incineration behaviour of coal sludge blended with hay as a potential waste-to-energy fuel. The objective was to assess fuel performance, emission characteristics, and environmental implications using blend ratios from 100% hay to 100% coal sludge. The produced pellets were combusted under controlled conditions, and resulting flue gas components were analysed, including CO 2 , CO, NO x , SO 2 , HCl, HF, and trace hydrocarbons. The results demonstrate stable incineration across all fuel mixtures. Carbon monoxide concentrations remained within a narrow range (68 – 77 mg∙m –3 n ), and nitrogen oxides levels showed minimal variation (604 – 610 mg∙m –3 n ), indicating that changing fuel composition did not significantly influence oxidation efficiency. Flue gas moisture decreased with increasing sludge share, consistent with its lower volatile content. In contrast, sulfur- and chlorine-derived emissions showed a strong dependence on sludge content. SO₂ increased from 64 mg∙m –3 n for pure hay to 738 mg∙m –3 n for pure sludge, while HCl rose from 17 mg∙m –3 n to 81 mg∙m –3 n . Smaller increases were also observed for CH 4 and HF. Specifically, for CH₄, the increase was from 0.3 mg∙m –3 n to 2.3 mg∙m –3 n , and for HF from 2.2 mg∙m –3 n to 2.9 mg∙m –3 n , with an increasing proportion of coal sludge. These findings align with published studies showing that acid gas emissions are governed primarily by fuel chemistry rather than incineration tuning. Overall, the results indicate that coal-sludge–biomass pellets are technically feasible as an alternative fuel but require suitable flue-gas cleaning and optimisation to manage elevated SO 2 and HCl emissions. The study contributes to ongoing efforts to transform industrial residues into circular and lower-impact energy resources.