Fluidized bed co-combustion of rice husk pellets and moisturized rice husk: The effects of co-combustion methods on gaseous emissions

This work explores the potential of three co-combustion methods for reducing NOx in a fluidized-bed combustor. Pelletized rice husk (base fuel) was co-fired with moisturized rice husk (secondary fuel) in this reactor using silica sand as the bed material. Four groups of experiments for (1) conventional combustion of rice husk pellets, (2) co-firing pre-mixed fuels, (3) co-firing using fuel staging with bottom air injection, and (4) co-firing using a reburning method combining fuel staging and air staging, were performed at a 200 kW heat input to the reactor. In the test series, the energy fraction of the secondary fuel in the total fuel supply (EF2) was within 0-0.25, with excess air (EA) varying from 20% to 80% at given EF2. During the reburning tests, the secondary-to-total air ratio (SA/TA) ranged from 0.1 to 0.4, at each EA. The findings revealed that the effects of EF2, EA, and SA/TA on the combustion and emission characteristics of the reactor were substantial. An optimization analysis was performed to determine the optimal EF2, EA, and SA/TA, leading to minimal emission costs of the applied co-firing techniques. Under optimal operating conditions, the combustor ensures high (∼99%) combustion efficiency with minimum emission costs and reduced NO emission: by about 13% when co-firing pre-mixed fuels, by 37% for the fuel-staged co-combustion, and by 53% when using reburning, as compared to 167-176 cm3 m−1 from burning the base fuel alone. However, some increase in the CO and CxHy emissions was observed when using the proposed co-firing techniques.