Utilization of pig and cattle manure as solid fuels: energy potential and environmental considerations

This study assessed the feasibility and environmental implications of utilizing pig and cattle manure as solid fuels, with a focus on flue-gas emissions and ash recyclability. Cattle manure - derived solid fuel, produced by reducing moisture content to approximately 4%, achieved a lower heating value (LHV) of 14.6 MJ kg^-¹ and an ash content of 21.5%, meeting Republic of Korea solid-fuel quality standards (LHV ≥ 12.6 MJ kg^-¹, ash ≤ 30%, moisture < 20%). In contrast, pig manure - derived solid fuel exhibited a moisture content (12.5%) that satisfied the standard, but a lower LHV (8.5 MJ kg^-¹) and higher ash content (34.6%), failing to meet fuel quality criteria. Stack measurements revealed distinct emission characteristics. For cattle manure, selective non-catalytic reduction (SNCR) enabled stable emission control, with dust (1.3 mg m^-³), CO (105.7 ppm), NO_x (46.8 ppm), and SO_x (17.2 ppm) within regulatory limits. Conversely, pig manure combustion produced excessive SO_x emissions (286 ppm vs. 60 ppm limit) even with SNCR, indicating need for additional desulfurization or alternative utilization pathways. Residue analysis showed that over 90% of ash was recovered as bottom ash, with phosphorus concentrated in this fraction, indicating reuse potential. However, pig manure ash contained elevated Cu and Zn levels, exceeding fertilizer standards and requiring stabilization before recycling. Overall, cattle manure demonstrated favorable environmental performance, whereas pig manure posed challenges related to sulfur-related air pollution and heavy-metal enrichment. These results highlight the need to jointly consider environmental risk management and resource recovery when promoting livestock manure as a renewable energy source.