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Show This is a key finding because it indicates that an increase in temperature leads to a larger fraction of coal nitrogen evolving with a relatively constant portion of volatile matter. Therefore, high temperatures in the near burner zone during staged combustion will force a larger portion of the fuel nitrogen to be released with the volatile matter in a low stoichiometric zone and result in a lower NO production. 8) Application In general, operational and wall furnace conditions affect the flame temperature because they influence both the radiative loss from the flame and the temperature of the bulk gases entrained by the secondary air stream. However, the combustion parameters that affect the thermal environment are not readily separated from other controlling parameters. For example, increasing temperature should increase NO emissions, but the effect may also be dependent on the availability of oxygen. Thus, it may be difficult to separate the effects of temperature and mixing. For example, in practical burner systems, the pre-ignition region is most important because it has a major impact on oxygen availability in the volatile heat release region. If the primary air to coal ratio is constant, then the location of the ignition zone will dictate the amount of oxygen from the secondary air stream that mixes with the coal jet. If the ignition front is located at or very close to the burner, most of the oxygen will be consumed by the volatile diffusion flame that surrounds the coal jet, and therefore will not be available to mix with subsequent devolatilizing coal particles. Furthermore, in practical burners, the location of the ignition zone is dependent on a number of factors, including: • Burner aerodynamic design, including the relative velocity of the primary and secondary jets, and/or the amount of swirl. • Coal particle size - an increase in the amount of small particles will promote early ignition. • Temperature - increasing the preheat temperature will promote early ignition because of the impact on volatile release. • Coal volatile content - all conditions being equal, the ignition distance will increase with decreasing volatile content. Pilot-scale and subsequent full-scale combustion tests have demonstrated that NOx emissions can be significantly reduced by controlling the coal/air mixing processes to minimize the fuel bound nitrogen conversion to NO. An understanding of coal devolatilization and combustion processes combined with adequate control of the air/fuel mixing patterns, provides the foundation for low NO firing applications and the basis for the ultimate prediction of NO formation. Figure 2 illustrates the processes in a typical tangential fired burner occurring during the combustion of pulverized coal particles and the processes that influence the fate of fuel nitrogen. For simplicity, the pulverized coal flame has been divided into three regions: 1) pre-ignition (convective and radiative heat up), 2) volatile energy release, and 3) char burnout. The pre-ignition region covers the time the coal particles leave the burner until there is sufficient volatile products present for ignition to occur. In this region the coal particles are heated primarily by convection as they mix with the secondary combustion air (auxiliary air) and recirculated furnace gases. As the particle temperature rises, volatile combustibles are released and mixed with available 6 |