Role of char in NOx formation during coal combustion at a regenerator temperature of calcium looping process

• Coal was burned at Calcium Looping process regenerator temperature.
• A dual-fluidized bed system and a circulating fluidized bed combustor were used.
• NOx emissions from two types of reactors were compared for three kinds of coal.
• For dual-fluidized bed, a part of char was consumed in the carbonator.
• An empirical relationship between NOx conversion and char combustion was obtained.

Coal of three kinds was burned in an oxygen-enriched atmosphere using two types of reactor both of which had the same fast fluidized bed for coal combustion. One was a dual-fluidized bed system (dual-FB), simulating Calcium Looping process comprised a fast fluidized bed regenerator and a bubbling bed carbonator. The other was a conventional single circulating fluidized bed combustor (single-CFBC). In both systems, coal combustion in oxygen-enriched atmosphere was carried out under regenerator temperature condition of Calcium Looping process. Inert quartz sand was used as the bed material to evaluate carbon consumption in the carbonator of dual-FB. Formation of NOx in the fast fluidized beds was measured for both reactors. For dual-FB, formation of CO and CO2 in the carbonator was also measured. High-volatile matter coal was found to be favorable to reduce CO and CO2 formation in the carbonator, but conversion of the fuel-N to NOx of high-volatile matter coal was higher than that from low-volatile coal. The emissions of NOx from single-CFBC were less than those from the regenerator of dual-FB under the same combustion condition. From the emissions of CO and CO2 from the carbonator, the decrease in char combustion in the regenerator of dual-FB was calculated. An empirical relationship between the conversion of fuel-N to NOx in the fast fluidized bed and the ratio of fixed carbon to volatile matter of the fuel was obtained.

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