An atmospheric pressure, fluidized bed combustion system burning high-chlorine coals in the convection section

The possibility of fireside corrosion in power plant boiler components is always a major concern when the fuels include high-sulfur and high-chlorine coals (or refuse waste). Sulfur and chloride products may play important roles especially in fireside corrosion in atmospheric pressure, fluidized bed combustion (AFBC) systems, caused by the capture of sulfur and chlorine by limestone used as bed material in the combustor, and the resulting deposition of sulfur- or chlorine-rich compounds onto metallic surfaces. Results were reported from tests in a 0.1-MWth AFBC system where 1,000-h test burns were conducted using two coals with widely differing chlorine levels, and limestone was used as the sulfur sorbent. Coupons of three stainless steels (Types 304 [UNS S30400], 309 [UNS S30900], 347 [UNS S34700]) were exposed to the hot flue gases in the freeboard (approximate to 10 cm below the location of the convection pass tubes). Deposits formed on the alloys contained high sulfur concentrations in their outer parts, as well as sodium, potassium, magnesium. and calcium. Sulfur appeared to be associated with calcium and magnesium suggesting that the fly ash may have reacted further after being deposited on the surface of the coupon. Areas of high sulfur concentration also correlated well with areas of high chromium content of the inner layers of the scales. Cross sections of samples indicated that sulfur had penetrated into the alloy and reacted to form sulfide corrosion products. There was no direct evidence to show that alkali chlorides were involved in the corrosion process. No chloride was identified in the alloy samples. There was slight oxide spoliation observed on all three alloys, with the degree of spallation in the following order: Type 304 > Type 347 > Type 309.