Biomass waste has significant potential for cofiring with coal due to its abundant availability. However, the ash-related problems arising from biomass-based fuel must be addressed, which can lead to slagging-fouling problems in boilers and further disrupt their performance and efficiency. This chapter comprehensively discusses a new approach for ash evaluation during coal-biomass cofiring by combining theoretical calculations and experimental methods. The selected feedstocks consist of a mixture of palm oil empty fruit bunches and fronds (EFFR), agricultural waste such as rice husks (RH), municipal waste known as solid recovered fuel (SRF), and forestry waste in the form of mixed hardwood chips (WC). Initially, a thorough examination is usually conducted to investigate the slagging-fouling tendencies by employing ash indices. Subsequently, lab-scale combustion tests are performed, followed by a series of ash investigations, such as probe observation, scanning electron microscopy–energy dispersive X-ray spectroscopy, and X-ray diffraction. Additives will also be further studied to improve the ash properties with a high risk of slagging and fouling. These findings are important for clarifying several types of biomass wastes as potential cofiring fuels in existing pulverized coal-fired power plants.