Ash formation during pulverized fuel combustion: 3. The structure and strength of boiler deposits

Abstract

The chemical and physical structure, and the strength of ash deposits that were formed from a number of pulverized coals in a pilot scale combustor have been compared under conditions similar to those in a utility boiler furnace. Samples were collected on cooled targets in the combustor furnace, and their in situ strength was measured by ‘sootblowing’ with an air lance. Deposits were formed in two distinct layers. The initial deposit was of small (< 5 μm) discrete particles, with a higher proportion of iron to aluminosilicate particles than in the coal ash. This formed a thin (< 1 mm) layer on cooled substrates. Subsequent deposition of larger particles formed a thicker sintered layer in which calcium had combined with aluminosilicates to form a crystalline phase, and the iron and phosphorus were concentrated into a separate vitreous phase. With some coals, partial melting occurred at the outer surface and a dense slag deposit was formed. Iron, phosphorus and calcium all contribute to the tendency of the outer surface of a deposit to form slag, but it is the bonding strength of the inner layer which determines the difficulty of cleaning deposits by sootblowing. This is related mainly to the concentration of the vitreous phase, which is rich in iron and phosphorus.