There is provided a process for operating a coal-fired furnace to generate heat. The process has the steps of a) providing the coal to the furnace and b) combusting the coal in the presence of a first slag-reducing ingredient and a second slag-reducing ingredient in amounts effective to reduce slag formation in the furnace. The first slag-reducing ingredient and the second slag-reducing ingredient are different substances. The first slag-reducing ingredient is selected from the group consisting of magnesium carbonate, magnesium hydroxide, magnesium oxide, magnesium sulfate, and combinations thereof. The second slag-reducing ingredient is selected from the group consisting of copper acetate, copper nitrate, aluminum nitrate, aluminum oxide, aluminum hydroxide, and ammonium phosphate. There is also provided a method for reducing slag formation in a coal-fired furnace.

A co-firing process is described using coal and processed biomass to reduce adverse by-products in a coal combusting apparatus. The coal feedstock is selected from coal, a coal substitute processed biomass, or an aggregate blend of coal and processed biomass. The biomass feedstock comprises processed biomass pellets. The total energy density is predetermined and can be similar to the coal component or higher than the coal component. The intracellular salt in the processed biomass is at least 60 wt % less for the processed organic-carbon-containing feedstock used to make the processed biomass pellets than that of the starting un-processed processed organic-carbon-containing feedstock.

A pellet burner includes: a burner body having a combustion chamber and an open top; a feed opening at a sidewall of the burner body; a through port under the burner body; and a grate structure above the through port and including two tiers of supports both secured to the burner body. Supports in the first tier are lower than the adjacent supports in the second tier. The second tier of supports are staggered with the first tier. A passage is between the two tiers of supports. The passage is narrower than a diameter D of the pellet fuel. A spacing between two adjacent supports in the second tier is greater than D but smaller than 2D. The feed opening is above the grate structure by no less than 3D. The pellet fuel drops from the feed opening. Air entering via the through port disperses the burnt pellet fuel ash.