OCR Text |
Show nitrogen and NOx emissions, however this correlation is significantly less than deterministic. It indicates that other factors influence NOx emissions when cofiring and trifiring wood waste and TDF with coal in cyclone boilers. Figure 3 also demonstrates that there are diminishing returns from reducing fuel nitrogen content in terms of reducing NOx emissions. The correlation between Table 2 and Figure 2 demonstrates that increasing the wood percentage, and supporting that wood with TDF, has some potential to provide the most significant means for reducing fuel nitrogen expressed in lb. N/I06 Btu. Fuel volatility, calculated as the volatile/fixed carbon ratio derived from the proximate analysis, also has a significant influence on NOx emissions. The equation defining this relationship is as follows: N0x.lblMMBtu = 7.84(V!FC? - 16. 15(V!FC) + 9.47 [2] Where V!FC is the volatile/fixed carbon ratio calculated from the proximate analysis. Note that the more volatile fuel mixtures have lower NOx emissions, and that there are diminishing returns for this parameter as well as for the fuel nitrogen content. Increased fuel volatility supports the reactions of staged combustion. The staged combustion pathway can follow the following general reaction sequences: Nitrogen containing fuel particle + heat ~ fuel volatiles [3] (e.g., NH3, NH2, CHN, CHNO) + char Then, in the absence of sufficient oxygen, for example [4] Alternatively, in the presence of significant excess 02, for example [5] Nitrogen bound in the char oxidizes directly in a gas-solids reaction; and it may react with CO exiting the surface of the fuel particle to form N, and ultimately N2. Alternatively, it may exit the particle as NO. Limited staging of combustion can occur, somewhat naturally, between the burner section of the cyclone and the barrel; the distribution of primary and secondary air, along with the heat transfer to the incoming fuel particles, provides for such limited staging of combustion in cyclones. Note that the value of increasing fuel volatility is virtually all achieved when the volatile/fixed carbon ratio approaches 1.0. Fuel oxygen content, and the consequent oxygen requirement from combustion air is the final fuel influence. The eastern coal requires 5.41 lb-m O2 from combustion air/l06 Btu, the western coal requires 5.38 Ib-m O2 from combustion air/l06 Btu, the wood waste requires 5.16 lb-m 0 21106 Btu and the TDF requires 5.54 Ib-m 0 21106 Btu. Clearly, as wood waste is added to the fuel blend, the requirement for outside air is diminished, and the consequent lb-m of both oxygen and 12 |