What type of energy is released that is not useful when burning coal?

When burning coal, a significant amount of thermal energy is released, but a portion of this energy is not useful due to the production of waste heat and emissions, such as carbon dioxide and other pollutants. Additionally, energy is lost in the form of unburned carbon and ash. This energy, while produced during combustion, is not harnessed for practical use, contributing to inefficiencies in coal as an energy source.