| It is known that the main energy consumption in china is coal. As this reason, the air pollution in china is coal-burning air pollution. NO_X emission by Coal fired boiler is one of these main air pollutants. With the development of economy, energy consumption grew so fast. The environmental problem is becoming more and more seriously, the NO_X emission is increasing year by year. Biomass is considered to be clean energy and renewable. Biomass co-firing with coal is considered to be one kind of effect method to unverilize biomass. Co-firing can be divided into two type, staged combustion and unstaged combustion. Both them can reduce the emission of NO_X, espicially staged combustion with biomass as the second fuel. But at present there is many different points on the reason why biomass co-firing in unstage combustion could reduce the NO_X emission. And the biomass reburning also has its shortage. So the NO_X emission character of the biomass is worth further research. And if biomass blend with coal could be an effective reburning fuel should be discussed.The NO emission character of biomass co-firing with coal is investigated in a tube heating furnace. The reason for the reduction of NO by biomass co-firing is analysed. The influence of flame temperature and biomass corporation rate on the NO emission is reviewed. And Experiments of reburning with pulverized solid fuels like coal, biomass, and blends of coal and biomass as second fuel are did in the reburning experimental facility. The problem that if coal blend with biomass could be an effective second fuel for NO reduce and if there exist synergism effects between coal and biomass as second fuel is studied. The influence of influencing factors such as flame temperature and excess air ratio on the NO emission has been reviewed.The NO emission character of biomass blend with coal is investigation in a tube heating furnace. The emission characteristic curve show different trend as the temperature change. For the biomass, the NO_X emission concentration shows single-peak. While, the NO_X emission concentration of coal shows single-peak at 800-1000℃and at 1100-1200℃, shows two -peak. The NO_X emission concentration of biomass blend with coal shows three-peak curve at 800-900℃, while at 1000-1100℃shows two-peak, when the temperature reach at 1200℃, the NO_X emission concentration shows single-peak . When the temperature lies between 800℃and 1200℃, The NO conversion rate of coal \biomass \coal blend with biomass and volatile N first increased and then decreased with the increase of temperature. The NO conversion rate of char N decreased with the increase of temperature. The contribution of volatile-N to NO formation is greater than that of char, and increase with the increase of temperature. The biomass NO conversation rate is greater than that of coal. The NO conversation rate of biomass blend with coal lies between that of the coal and biomass used to blend, and decrease with the increase of biomass mixed ratio. Biomass has diluting effect on NO emission when biomass co-fire with coal, there also exist synergistic interactions between coal and biomass, can reduce the NO emission. The effect of synergistic interactions is obvious at 800℃, and decrease with the increase of temperature.Pulverized solid fuels like coal, biomass, and blends of coal and biomass were used as second fuel in the re-burning experimental facility. The result shows that biomass and lignites are effective reburning fuels, the reduction efficiency of NO is low for high rank coal such as lean coal. The reduction efficiency of NO biomass blend with coal lies between that of the coal and biomass used to blend. There exist rule that the reduction efficency of the reburning fuel increase as the volatile increase. When coal combined with biomass there do have synergism effects. When the biomass mix rate is low, it was observed that the behavior of 50-50 blend was similar to coal. While, when the biomass mix rate is high the performance of 90-10 blend was similar to that of coal. As the biomass mix rate increase, the reduction efficiency of NO increases too. When mixing proportion of biomass lower than 20%, the NO reduction efficiency increase slowly. When the biomass mix rate is higher than 20%, reduction efficiency increase rapidlyUsually, the reduction efficiency decrease with the increase of excess air ratio. When use Cotton stalk as reburn fuel, the reduction efficiency reach maximum at excess air ratio of 0.7. While, when use wheat straw equi as reburn fuel, the reduction efficiency reach maximum at excess air ratio of 0.6. The temperature of the reburn zone has obviously effect on NO reduction. The reduction increases with increasing temperature, but when the temperature is over 1000℃, the reduction efficiency increase slowly as the temperature increase. Uasually the finer the fuel particle is, the higher the reduction efficiency could be reached. The particle size has little influence on the reduction efficiency of biomass reburn, but has great influence on the reduction of coal reburn. The reduction efficiency decrease with the increase of reburn fuel fraction. As the reburn fuel fraction raise from 10% to 20%, the reduction efficiency increased dramatically, but slower from 20% to 30%. The reduction efficiency decrease with the increase of residence time at reburn zone. When the residence time is longer than 0.8s the efficiency increase slowly.
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