Heat extraction from a utility-scale oxy-fuel-fired CFB boiler

The temperature and heat extraction parameters of a utility-scale, oxy-fuel-fired boiler are investigated using a mathematical model that was originally developed and validated with data from 100-kWth to 4-MWth oxy-fuel-fired circulating fluidized bed units. This work involves an existing furnace, which was developed for air-firing, and evaluates its operational potentials under oxy-fuel conditions, allowing additional heat extraction through an external heat exchanger (EHE). The modeling shows that even though the heat extraction levels from the entire furnace, flue gas pass, and EHE increase with increases in the inlet O2 concentration, the heat extraction from the EHE dominates the heat extraction for high inlet O2 concentrations and, consequently, requires an increase in the circulating solids flow, which transfers heat from the furnace to the seal. While maintaining the same dense bed temperature as in the air-fired case, an increased inlet O2 concentration in the oxy-fired case leads to dramatic increases in the maximum in-furnace temperature and maximum heat extraction flux rate. Thus, to control the maximum furnace temperature, the circulating solids flux must be increased beyond what is required to close the heat balance across the CFB loop. For the conditions investigated, limitation of the maximum furnace temperature to 1273 K yields that 48%, 56%, and 70% are the highest possible inlet oxygen concentrations if the external circulation flux rate is limited to 10, 20, and 30 kg/m2/s, respectively.