Abstract
This study presents the numerical investigation of co-firing behaviour of pulverized straw particles with coal in air and O2/CO2 mixtures. Validation is conducted in a semi-technical once-through 30 kW swirl-stabilized furnace. Reference air-fuel (21% O2) and oxy-fuel (30% O2) cases were considered for four different coal/straw ratios (100% coal, 20% straw, 50% straw and 100% straw). AVL Fire is used as a CFD modelling tool. A comprehensive grid independency test was conducted considering three different grid sizes. Ignition performance, emission characteristics, heating profile, residence time were evaluated for different fuel ratios under air and oxy-fuel conditions. This work has shown that no significant changes occur to the fundamental combustion characteristics for straw compared to coal when burned in the O2/CO2 atmosphere to air firing case. It was found that with 20% straw sharing, sensible performances were observed similar to that of 100% coal combustion. Also a critical analytical analysis was conducted to investigate the heating performance for straw particle of different sizes (100 μm, 330 μm and 1000 μm). The heating profiles show significant differences between the three particle sizes assuming isothermal temperature gradient and heating by both radiation and convection. It is seen that the variation of the level of SO2 is not significant between air and oxy-firing cases. However, slightly less amount of SO2 is predicted in oxy-firing case. The reduction in NO is due to the effects of higher NO levels in the flue gas promoting gas-phase reduction, significantly higher levels of CO in the flame zone and the usage of a high-NOx burner in combination. The heating profiles for different size of straw particle suggest that there is a definite influence on the overall performance of the furnace. Larger particle do not ignite properly due to lower residence time leading to lower burnout. The possibility of burnout of the larger straw particle size is less due to less residence time in air-firing compared to oxy-firing case.
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