NO_x Emission from a Circulating Fluidized Bed Boiler Cofiring Coal and Corn Stalk Pellets

Abstract

The NO x emission from a circulating fluidized bed (CFB) boiler with 50 MW rated thermal output (50 MW th ) during the cofiring of anthracite coal and pelletized corn stalk biomass was investigated. Cofiring could improve combustion efficiency and, thus, the thermal efficiency of the boiler. However, contrary to most results found in bubbling fluidized bed (BFB) combustors and the one found in a CFB boiler burning rice husk, NO x concentration in flue gas and fuel nitrogen conversion ratio increased with the biomass mass fraction. Several causes for such a contradiction were discussed and the configuration in combination with feedstock location of biomass was regarded as the most important one. In a CFB boiler, fuel is mostly burnt above the dense bed, opposite to that in a BFB boiler, and, thus, the average oxygen concentration above the dense bed, where biomass was fed, was higher. High bed temperature and high primary air ratio used for burning the anthracite coal increased NO x emission and enhanced the variation trend. In addition, the catalyst-contained ash also could promote NO x formation. The results indicated that biomass is not necessarily an effective cofiring fuel to reduce NO x emission in a CFB boiler and in order to reduce NO x emission, low primary air ratio, and locating the feedstock ports of biomass below the secondary air should be considered.

The Combustion Performance of Pelletized Jatropha Seed Residue in a Vortexing Fluidized-Bed Combustor

“…Additionally, because the primary gas enters the combustion chamber with S b lower than 100%, the global fuel-rich atmosphere may induce a high concentration of NH 3 , which prefers reacting with NO to form N 2 under such conditions. 35 However, a recovery of NO is observed between 1.26 and 2.26 m above the distributor. As mentioned above, there is still some unburned matter, including nitrogenous species, and they are prone to oxidize quickly in this zone due to the high freeboard temperature accompanied by excess oxygen.…”

“…A great amount of CO can still be found at 1.26 m above the distributor, implying that some unburned matter (char and volatile matter) is present in this region in spite of the secondary gas injection. The NO reduction is probably accomplished by the following equations: Additionally, because the primary gas enters the combustion chamber with S b lower than 100%, the global fuel-rich atmosphere may induce a high concentration of NH 3 , which prefers reacting with NO to form N 2 under such conditions …”

The theory and practice of NO emission control for circulating fluidized bed boilers based on the re-specification of the fluidization state

“…These small molecular precursors will be oxidized to N 2 , NO, N 2 O subsequently via NCO, NH i radicals etc. Meanwhile, the generated NO x can be reduced to N 2 by both homogeneous and heterogeneous pathways [6,7]. Charnitrogen can also be oxidized to NO x , but the conversion rate is usually lower due to the reduction effect from the pore structure of char.…”

Effects of Fuel Staging on the NO Emission Reduction during Biomass–Biomass Co-combustion in a Fluidized-Bed Combustor

“…Co-firing of coal with biomass has been extensively studied previously . Apart from the environmental benefits (a substantial reduction of NO x , SO 2 , and CO 2 emissions), coal–biomass co-firing offers some economic advantages, such as lowered fuel costs and improved efficiency of biomass conversion to electricity in power generation units. , The extent of the system improvement (in terms of environmental characteristics and thermal/combustion efficiency) is dependent on the properties of co-fired fuels and their mass/energy shares in total fuel supply, co-combustion method, and operating conditions. , …”

“…12,13 The extent of the system improvement (in terms of environmental characteristics and thermal/ combustion efficiency) is dependent on the properties of co-fired fuels and their mass/energy shares in total fuel supply, co-combustion method, and operating conditions. 13,14 However, there is a lack of information on the co-firing of two or more biomasses in fluidized-bed combustion systems. As reported by some pioneering studies, fluidized-bed combustion systems can be effectively used for co-firing problematic biomass fuels (e.g., with unacceptable emission characteristics and/or very low calorific value), where individual burning is accompanied by operational problems and/or leads to substantial environmental impacts.…”