Combustion performance and high temperature hydrodynamics in a spouted and spout-fluid bed

Combustion of Minto coal, a sub-bituminous eastern coal which is highly agglomerating and has a high sulphur content, was carried out in a 0.15 m internal diameter half-column spout-fluid bed combustor in inert beds of sand, with limestone addition for sulphur capture. The average bed temperature ranged from 800 to 900°C, flue gas oxygen level was 2.5 to 11.0%, auxiliary to total air was 0 to 0.50, and Ca/S molar ratio was 2.5. High vale coal was employed in hydrodynamic runs. Aspects studied included combustion efficiency, sulphur capture efficiency, axial and radial temperature profiles, axial O₂ and CO₂ concentration profiles, axial SO₂ concentration profiles, minimum spouting velocity, spouting stability, and maximum spoutable bed height. The principal problem encountered with Minto coal in this equipment was agglomeration during the heat-up period. A spout-fluid bed has proved to be great favourable for handling agglomerating coal relative to the standard spouted bed. When limestone was used as bed material, less sintering was encountered. However, limestone could not stand up to spouting for prolonged periods because of excessive attrition. Combustion efficiencies were found to be higher than 80% in the temperature range of 800 to 900°C without solid fines recycle. An increase of temperature between 800°C and 840°C was beneficial for combustion efficiency, while a further increase up to 885°C did not seem to have a significant effect on combustion efficiency. Increase of auxiliary/total air ratio was favourable to combustion efficiency at elevated temperatures. Sulphur capture efficiency passed through a maximum with increasing temperature between 800°C and 900°C The maximum value was obtained at around 830°C. NOx emission increased linearly with increasing flue gas oxygen level. No abrupt temperature increase above the bed surface was observed in both spouted and spout-fluid beds investigated in the present study. Temperature may increase above the bed surface for low excess oxygen runs in view of the substantial amount of combustion found to occur in the freeboard. Temperatures were more uniform after the introduction of auxiliary air. Most oxygen was consumed below the bed surface. Axial profiles showed a significant SO₂ jump in the spout over the bed height. Combustion and sulphation could be considered to occur in two main stages: (1) Combustion of carbon, at the same time as most of the sulphur is released. (2) Sulphation of the sorbent. The Mathur and Gishler (1955) and Wu et al. (1987) equations gave poor agreement with the minimum spouting velocity, Ums, over the entire range of temperature. For large particles Ums tended to increase with increasing temperature, while for small particles it decreased with increasing temperature. Gas viscosity should be taken into consideration for predicting Ums. A considerably greater effect of auxiliary to total air ratio, q/Qt, on total minimum spouting velocity was found at elevated temperatures than at room temperature. At the maximum spoutable bed height, the value of Um/Umf was found to decrease with increasing temperature and to be smaller than unity at elevated temperatures. The McNab and Bridgwater (1977) expression correctly predicted the observed trends of Hm and worked reasonably well at high temperatures, although it was found to over-predict Hm at lower temperatures. Hm decreased with increasing temperature for all particle sizes, with a faster decrease for smaller particles. Fluidization in the annulus was never observed as the termination mechanism of spouting at high temperatures.Applied Science, Faculty ofChemical and Biological Engineering, Department ofGraduat