Defluidization Prediction and Prevention during Cocombustion of ReEngineered Feedstock with Coal in a Bubbling Fluidized Bed Combustor

Despite the increase in applications of renewable energy sources, coal combustion for electricity production remains important. The use of alkali and alkaline earth sorbents to address the emissions or their introduction during cocombustion of biomass can cause defluidization of fluidized bed combustors. This article presents the results obtained from an extensive experimental campaign aimed at exploring the performance of different defluidization inhibition measures and defluidization outcomes of a pilot-scale atmospheric bubbling fluidized bed combustor cofired with ReEngineered Feedstock (ReEF) and coal. ReEF is a solid fuel made from the nonrecyclable fraction of municipal solid waste to which alkaline air emission control reagents are physically bound. The cocombustion of coal and ReEF combines fuel- and sorbent-blending techniques for improved combustion and higher efficiency emission reduction. An earlier study of the group proved these favorable aspects during the cocombustion of a granular bituminous coal and ReEF in a pilot-scale bubbling fluidized bed combustor of coarse silica sand particles. However, some formulations of ReEF led to the bed defluidization during the 800–1000 °C operation. In the first phase of the current study, the performance of different counteractive/pre-emptive measures to delay/prevent the defluidization incidents during the coal–ReEF cocombustion trials in the same combustor unit was examined. Although the application of counteractive measures, e.g., a decrease in the operating temperature and an increase in the superficial gas velocity, could delay the defluidization incident, replacing coarse silica with olivine sand significantly extended the temperature range of operation without defluidization. In the second phase of the current study, employing the more commonly used coarse silica sand, the cocombustion of bituminous coal with over 50 ReEF formulations was performed to explore defluidization outcomes in the 800–1000 °C range. The data collected from these trials illustrated trends that could be presented as a defluidization map. The map relates the sodium and calcium composition limits for the ReEF that would defluidize a bed of coarse silica at different temperature levels. The defluidization boundaries obtained at 900 and 1000 °C were described by adopting an empirical formulation. Extrapolation, following an Arrhenius type temperature dependency, over the 800–1100 °C range in 50 °C increments showed acceptable predictability of the approach, satisfying the industrial requirements. The applicability of the predictive tool, developed on the basis of 1 h of operation at each operating temperature, was subsequently tested over extended periods (>8 h) at 850 °C.

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