Experiments on pulverised coal combustion in air and 02/C02 mixtures of various molar ratios, were conducted in a 20 kW-rated, down-fired furnace equipped with a single pulverised fuel (pt) burner, which was designed for the laboratory-scale experimental studies on coal combustion in air. In coal-02/C02 combustion tests, all the oxidants and fuel were delivered into the furnace with the same configurations as those in the coal-air combustion firing tests. In each test, the coal firing rate was fixed, and the furnace stoichiometric ratio was fixed at SR=1.20. Seven bituminous coals with fuel ratio ranging from 1.50 to 2.33 were used in the study. The effectiveness of air/oxidant staging on reducing NOx emissions was investigated for combustion in air and 02/C02 mixtures. The fate of recycle NO in combustion with different oxidants and combustion conditions was also investigated. Continuous furnace operations with stable flames and a comparable operating temperature to that in air were established for 02/C02 combustion, without major operational problems related to burner ignition, flame stability, coal firing and the effect of oxidants switching, both in unstaged and staged conditions. The results show that temperature and emission profiles are highly influenced by the oxidant compositions. A continuous flame could not be sustained by the direct replacement of combustion air with 02/C02 mixture with the same O2 concentration as air (21 :79-02/C02). In 02/C02 atmospheres, NOx Conversion Ratio (CRNO'..) decreased with the increasing concentration of the CO2 in the oxidant and combustion with 21 :79-02/C02 produced NOx of about one-fourth to that in air. With a same firing rate and combustion stoichiometry. coal combustion in 30:70-02/C02 produced a similar flame temperature profile to that in air combustion.while producing a significantly lower furnace NO, emISSIOn and a higher char burnout. The NO" Conversion ratio (CRNox) ranged from 27.7 - 39.70/0 in air and 18.4 - 35.5% in 30:70-02:C02. The Burnout Efficiency (1180) in air and in 30:70- 02/C02 ranged from 92.5-98.50/0 and 95.0-99.3 % respectively. Compared to that in air combustion, NOx conversion was more sensitive to coal prope11ies in 30:70- 02/C02. The CO concentration in the combustion zone of the 30:70-02/C02 mixture was more than 50% higher than that of air but the Je'eJ decreased to an insignificant level at the exhaust. With the presence of air in the oxidant from atmospheric leakages, a high CO2 concentration of more than 80% of the flue gas was attained in 30:70-02:C02 combustion, compared to around 15% in air firing. The CO2 concentration in the flue gas could be increased further to more than 90 % by reducing air infiltration into the combustor. The staged combustion tests result show that oxidant staging is a very effective method in reducing NOx emissions for coal combustion in 30:70-02IC02. and can be more effective than in staged air combustion. For coal combustion in air, staging with SR1=0.80 reduced NOx emission by 54 - 650/0, while combustion in 30:70-02/C02. reduced NO" by 44 - 73 %. Compared to normal air combustion, staged combustion in 30:70-02:C02 reduced the overall furnace NOx by 67-77 0/0. The recycled NO tests results show that the NO Reduction Efficiency (l1No) depends on the combustion media. combustion conditions and NO recycling injection locations, and is influenced by the coal properties but not by the recycled NO concentrations. Compared to that in air, NO Reduction Efficiency in 30:70- 02:C02 is more sensiti'e to coal properties, particularly coal Fuel Ratio (FR).
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