Combustion co-emits NOx, SOx (fuel-dependent), particulates and CO; coal adds ash and trace metals. Fuel not burned through efficiency removes the whole basket proportionally.
From our guide: Environmental Impact of Industrial Heat & Emissions — full context, charts and sources there.
Boilers, kilns, heat exchangers, valves and steam lines lose energy continuously. Inzonex makes patented (UK GB2508992.1) removable modular insulation — snap-fastened covers engineered per temperature tier, not generic off-the-shelf jackets: