OCR Text |
Show Diluent addition. The most effective control of thermal N0X is a reduction of the peak temperatures in the flame zone. One approach is the addition of an inert diluent to the fuel or air stream, thereby lowering the theoretical flame temperature at which combustion takes place. The two most common approaches are flue gas recirculation (i.e., the addition of relatively cool combustion gases recycled from the flue and mixed with the combustion air) and water injection (i.e., addition of water or steam to either the air or fuel stream). Of the two, water injection is the more effective on a mass basis due to the latent heat of vaporization effect; however, it imposes a stack heat loss that can be avoided with flue gas recirculation. These techniques are most effective for thermal N0X and appear to have little effect on fuel N0X. For example, 70 to 90 percent maximum reductions have been observed for natural gas and distillate oil in field and laboratory studies; for heavy oil, the range is 20 to 50 percent; and for coal, 10 to 30 percent. Flue gas recirculation alone or in conjunction with other techniques is used to achieve emission standards for gas and oil fired utility boilers. Its potential drawbacks are increased mechanical complexity and capital costs. Water injection is the state-of-the-art N0X control technique for gas turbine engines. Staged combustion. Staged combustion is based on operation with a rich primary combustion zone in the furnace to reduce oxygen availability and peak temperature, followed by secondary air injection to achieve carbon burnout. The reduced oxygen availability reduces the conversion of fuel nitrogen to NO and the reduced peak temperature and subsequent heat removal prior to secondary air addition reduces thermal N0X. One method of implementing this control is illustrated in Figure 1. The air supplied to the burners is less than the amount necessary to completely combust the fuel, which produces a fuel-rich primary zone. Although the effectiveness of staged combustion increases significantly as the primary stoichiometry is decreased toward 75 percent of theoretical air, actual coal furnace primary zone stoichiometry is limited to 95 to 100 percent theoretical air by operational considerations (e.g. , slagging, corrosion). The secondary air is added above the top row of burners and 3-7 7 |