Oxy-fuel conversion of sub-bituminous coal particles in fluidized bed and pulverized combustors

Oxy-fuel combustion in pulverized coal (PC) and fluidized bed (FB) boilers is being increasingly investigated due to its potential use for carbon dioxide capture. The combustion conditions in the two types of unit differ significantly because of fuel size, furnace temperature, and fluid dynamics. These differences affect the change of combustion characteristics from air (O2/N2) to oxy-fuel (O2/CO2) conditions in PC and FB in different ways. In this paper, the oxy-fuel combustion behavior of a single sub-bituminous coal particle in PC is compared with that in FB conditions. The FB data were measured in our bench-scale FB test rig, whereas the PC data were collected from literature. The FB tests were performed at 1088 K with 0%vol < O2 < 40%vol, using sub-bituminous coal with a diameter of 6 mm. An extensive list of parameters is compared, including the ignition-delay time, volatiles' flame temperature, devolatilization time, burnout time and peak temperature of the coal particle. Results indicate that the impact of shifting from air-firing to oxy-firing affects the devolatilization, burnout times, and peak temperature in PC and FB differently: PC particles require higher concentration of oxygen than coarse particles for FB to attain similar results as in air-firing. However, the impact on the volatile flame temperature of shifting to oxy-fuel flame is similar in PC and FB. In general, the CO2 atmosphere delays ignition compared to air-firing, particularly for coarse particles at low O2 concentration.