Ash deposition behaviours upon the combustion of low-rank coal blends in a 3 MWth pilot-scale pulverised coal-fired furnace

Abstract

This paper aims to clarify the characteristics of ash deposits formed upon the blending of two low-rank lignites that have distinctively different properties, i.e. one has a low ash melting point, rich in Na and S, while another has a higher ash melting point, rich in Ca and Cl. A 3 MWth pilot-scale furnace has been used for the combustion, with a variety of cooling oil-jacketed probes installed along the furnace for ash deposit sampling. As have been confirmed, there are strong synergetic interactions between the ash-forming elements of the two lignites. At the mass fractions of 50% and 70% for the high-Cl coal, the ash fusion temperatures are even higher than the two single coals. The synergetic interactions between two coals enhanced the formation of crystallised species such as hematite, sulfates and carbonates that are mixed with Ca-Si-O matrix to reduce its stickiness and slagging propensity in the burner vicinity. The blending of up to 50wt% high-Cl coal resulted in the enhanced emission of gaseous HCl and Cl2, whereas the high-Cl coal mass fraction above 50wt% enhanced the formation of NaCl in super-heater ash deposits, via the chlorination of sodium from low-Cl coal, which are detrimental to tube corrosion. The Cl chemistry is complicated for the lignite blending combustion. The emission of gaseous Cl2 is remarkable, accounting for 6–8% of the total chorine that is far higher than the predicted values based on the thermodynamic equilibrium. The Cl2 emitted at high temperatures is also stable, with almost all the consumption reactions being frozen upon the rapid quenching of flue gas. The blending of up to 50wt% high-Cl coal is viable in minimising ash slagging and fouling. However, special materials and/or coating technologies should be employed to minimise the Cl2-related corrosion for the water tubes.