Improvements in methods by which new or used coal fired boilers whether designed for coal or oil or natural gas firing can substantially improve their technical operation and reduce their capital and operating costs by implementing process steps that (a) minimize the adverse impacts of coal ash and slag on boiler surfaces and particulate emissions, which will improve coal combustion efficiency and facilitate the use of oil or gas designed boilers for coal firing, (b) drastically reduce the loss of water used to transport coal in slurry form to power plants, (c) minimize the combined total nitrogen oxides (NO.sub.x), sulfur dioxide (SO.sub.2), mercury (Hg), and carbon dioxide (CO.sub.2) emissions, (d) separate and permanently sequester carbon dioxide and (e) improve the coal and solid fuel combustion efficiency. In the method includes whereby slag formed from solid fuel ashes during combustion in boilers or furnaces is suppressed by introducing additional air in a post-primary combustion zone to lower the combustion gas temperatures below temperatures at which the ash softens or liquefies and adheres to boiler or furnace surfaces.