A process for capturing undesirable combustion products produced in a high temperature combustion system in which a carbonaceous fuel is utilized. Very finely sized particles of alkaline earth carbonates or hydroxides, with or without added ground ash, are provided in slurry form, are dried and milled to provide unagglomerated, sub-micron-sized particles that are injected along with pulverized coal into the high temperature combustion zone of a furnace. The particles capture and neutralize the gases that result in condensable acids, including SOx, NOx, HCL, and HF, as well as capturing toxic metals that are present in the combustion products, they mitigate ash fouling and slagging, and they facilitate economic heat exchange that permits fuel savings and recovery of water for use in other processes.