Experimental Study On Pulverized Coal Combustion And NO_x Emissions In High Temperature Air With Low Oxygen Concentration

The technology of high temperature air combustion has been successfully developed mainly to gaseous fuels with advantages of energy saving about 30%～70% and NO_x emissions reduced to about 40～70 mg/m~3. It is expected that the technology can also be used to pulverized coal combustion, especially for countries with coal as major energy resource.The high temperature air with temperature higher than 1073 K and oxygen concentration higher than 8% was achieved by combustion in high excess air ratio in a circulating fluidized bed, totally avoiding the plugging of particles to honeycomb heat exchangers in the traditional process.The test rig was designed and built in 2006. Pulverized coal was supplied to the down-fied combustor with a diameter of 220 mm and a height of 3000 mm. A series experiments were carried out on pulverized coal combustion and NO_x emissions in high temperature air. Numerical simulation were also made.High temperature air with temperature higher than 1073 K can be achieved steadily and continuously by circulating fluidized bed combustion in a high excess air ratio. There is good flame stability and a uniform temperature profile along the axis of the down-fired combustor. With the residence time of pulverized coal in the reducing zone shorter than 0.3 s, about 92.4% of the volatile and 78.7% of the fixed carbon have been conversed. At the condition of high temperature air, coal type and particle size have weak effects on the combustion characteristics, it is effective to stabilize the flame of anthracite. With the equivalence ratio of 0.7 in the reducing zone and the excess air ratio of 1.2, the NO_x emissions of brown coal and bituminous coal are 440 mg/m~3 and 390 mg/m~3, respectively. Furtherly, the NO_x emission of anthracite is only 332 mg/m~3, 69.8% lower than the NO_x emissions regulation for power plants in China burning coal with voaltile lower than 10%. Increasing the residence time of pulverized coal in the reducing zone, decreasing the oxygen concentration, decreasing the excess air ratio, decreasing the equivalence ratio, decreasing the furnace temperatures, the NO_x emissions decrease.The numerical simulation predictions, with FLUENT, are compared with the experimental data, giving a good agreement. The combustion model used in this calculation are suitable to pulverized coal combustion in the high temperature air with low oxygen concentration from a circulating fluidized bed. The simulation results also show that there is a uniform temperature profile and the maximum temperature decreases with a lower oxgen concentration.