A process for capturing undesirable combustion products produced in a high temperature combustion system. Very finely sized particles of alkaline earth carbonates or hydroxides, with or without added ground fly ash, are provided in slurry form and are injected into the high temperature zone of a furnace. The particles flash calcine to provide substantially unagglomerated particles having a particle size of less than about 0.5 microns to capture and neutralize SOx, NOx, and toxic chemicals.