煙氣迴流對流體化床燃燒爐中 氮氧化物排放之效應

The Effect of Flue Gas Recirculation on NOx and N2O from a Vortexing Fluidized Bed Combustor

江孟育(Meng-Yu Chiang)

指導教授 ： 錢建嵩

中原大學/工學院/化學工程學系/碩士(2007年)

https://doi.org/10.6840/cycu200700729

摘要

流體化床燃燒爐之操作溫度較低(700-900℃)，故爐內生成之氮氧化物係以燃料式氮氧化物(fuel-NOx)為主。階段燃燒已證實能有效抑制氮氧化物之排放；為使燃燒室呈缺氧狀態(fuel-rich condition)，勢必迫使通入空氣量減少，因而流化床內氣固混合品質下降。本研究利用迴流煙氣取代部分ㄧ次風，以達階段燃燒效果，並使床內氣固混合均勻。
本研究係於一總高4.6 m之渦旋式流體化床燃燒爐中進行，乾舷區內徑為0.75 m，燃燒室底面積為0.8 m × 0.4 m，二次空氣由乾舷區底部以切線方向進入爐內，距分配板2.05 m。以玉米、煤炭及小米做為燃料，矽砂為床質，探討不同操作條件對燃燒爐中各區段氮氧化物分布及出口排放濃度之影響。主要操作參數為：燃料種類、迴流煙氣與ㄧ次風風量比例、過量氧量及配風比。
研究結果顯示固定二次風風量下，隨迴流煙氣比例之增加，燃燒室內計量氧隨之下降，且NOx排放及爐內N2O濃度分佈下降。爐內總過量氧的增加，將使NOx排放量增加。玉米於相同空床氣速不同二次風配比下，二次風為55%計量空氣時，NOx有最低排放量。煤炭及玉米CO排放低於20ppm，但小米CO排放高於我國環保法規，嘗試以較高過量空氣及增加停滯時間，使其燃燒完全來降低CO，結果均無法符合法規標準；係因小米燃料性質特殊，以及停滯時間不足以致燃燒不完全；迴流煙氣的加入將造成燃燒更不完全並縮短停滯時間，勢必產生更高的CO排放量。燃燒玉米或煤炭僅少量或無N2O排放，只於爐內有較高濃度生成。

關鍵字

迴流煙氣 ； 氮氧化物 ； 流體化床燃燒爐

並列摘要

In the fluidized bed combustor, the oxidation of fuel-N is the major source of NOx formation. It can be attributed to the lower operating temperature to prevent the formation of the thermal NOx. The staged combustion technique gave significant reductions in both NOx and N2O emission. In this study the recirculated flue gas is used to substitute a proportion of the primary air to form a fuel-rich condition in the combustion chamber. Therefore, the effect of staged combustion can be achieved without decreasing the fluidization quality.
The effects of operating conditions on NOx and N2O emissions were investigated experimentally in a 0.75 m I.D. and 4.6 m in height pilot scale vortexing fluidized bed combustor (VFBC). The secondary air injected nozzles were installed tangentially at the bottom of freeboard. Corn, millet and coal were used as the fuels. Silica sand was employed as the bed material. The effects of various operating parameters, such as the ratio of flue gas re-circulated (FGR) flow rate to the primary air flow rate, the excess oxygen, and the types of fuels were investigated.
The experiment data show that the NOx emissions increased with excess oxygen in the combustor. NOx emission and N2O distribution can be decreased by increasing flue gas recirculation flow rate. As 55% stoichiometric air is injected by the secondary air, there is the lowest NOx emission during the combustion of corn. The concentration of N2O in the exit flue gas is very low, and it can be neglected. The CO emission by corn and coal were both lower than 20ppm. But millet was unable to meet the emission level of CO. This is due to the special properties of millet and not enough residual time.

並列關鍵字

NOx ； N2O ； Fluidized bed combustor ； Flue gas re-circulation

參考文獻

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