流體化床燃燒爐中煙氣迴流對燃燒特性及氮氧化物排放效應之研究

The Effect of Flue Gas Recirculation on NOx Emission and Combustion Behavior of a Fluidized Bed Combustor

葉千豪(Chien-Hao Yeh)

指導教授 ： 錢建嵩

中原大學/工學院/化學工程研究所/碩士(2009年)

https://doi.org/10.6840/cycu200901328

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。迴流煙氣與階段燃燒已證實能有效降低氮氧化物之排放。為使燃燒室呈缺氧狀態(fuel-rich condition)，勢必迫使通入空氣量減少，造成流體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之先導型渦旋式流體化床焚化爐中進行，燃燒室底面積為0.8 m × 0.4 m，乾舷區內徑為0.75 m，二次空氣由乾舷區底部以切線方向進入爐內，注入點距分配板2.05 m。以1～8mm煤粒與0.394mm煤粉為燃料，矽砂為床質，利用回應曲面法分析實驗數據，探討在一次風加入迴流煙氣時，煤粉佔總進料量比例、床內計量比氧量、過量氧率對爐內溫度分佈、氮氧化物排放、燃燒效率及各區段燃燒份額之影響。
研究結果顯示，操作參數對於爐床溫度之影響，以煤粉佔總進料量比例與床內計量比氧量影響效應最大，過量氧率影響最小。操作參數對於乾舷區溫度之影響，以煤粉佔總進料量比例影響效應最大，其次是過量氧率，床內計量比氧量影響效應最小。操作參數對於爐出口NOx排放濃度之影響，以過量氧率影響效應最大，其次是煤粉佔總進料量比例，床內計量比氧量影響效應最小。迴流煙氣取代部分一次風，可使爐內產生階段燃燒效果並維持燃燒室氣固混合均勻，有效降低NOx之排放濃度，且NOx排放濃度隨著迴流煙氣比例的增加而降低。爐出口NOx排放濃度達到最低之操作條件為煤粉佔總進料量比例20%、床內計量比氧量80%與過量氧率40%。操作參數對於燃燒效率之影響，以床內計量比氧量影響效應最大，其次是煤粉佔總進料量比例，過量氧率影響效應最小。燃燒效率達到最佳之操作條件為煤粉佔總進料量比例20%、床內計量比氧量110%與過量氧率60%。爐內燃燒份額方面，當煤粉佔總進料量比例增加時，床區燃燒份額下降，飛濺區與二次風區燃燒份額增加，而乾舷區燃燒份額沒有變化。當床內計量比氧量較高時，增加床內煤粒與床上方煤粉的燃燒反應，造成床區燃燒份額與飛濺區燃燒份額較高，而二次風區與乾舷區燃燒份額較低。當床內計量比氧量較低時，迴流煙氣的比例增加，燃燒室形成富燃料區，產生較多揮發物上升至二次風區與乾舷區燃燒，造成二次風區與乾舷區燃燒份額增加。當過量氧增加時，床區與飛濺區的燃燒份額沒有變化，二次風區燃燒份額隨著過量氧的增加有上升的趨勢，而乾舷區燃燒份額沒有變化。

關鍵字

回應曲面法 ； 燃燒效率 ； 燃燒份額 ； 氮氧化物 ； 流體化床燃燒爐 ； 迴流煙氣

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 flue gas recirculation and staged combustion technique gave significant reductions in NOx 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 temperature distribution, NOx emissions, combustion efficiency, and combustion proportion 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. 1 to 8 mm coal and 0.394 mm coal were used as the fuels. Silica sand was employed as the bed material. The effects of various operating parameters, such as feeding ratio of 0.394 mm coal, stoichiometric oxygen in the combustion chamber, the excess oxygen ratio were investigated. These operating conditions were determined by means of response surface methodology.
The experimental results show that the feeding ratio of 0.394 mm coal and the stoichiometric oxygen in the combustion chamber have a more important effect on the bed temperature than the excess oxygen ratio. The feeding ratio of 0.394 mm coal has a more important effect on the freeboard temperature than the excess oxygen ratio and the stoichiometric oxygen in the combustion chamber. The feeding ratio of 0.394 mm coal has a more important effect on the freeboard temperature than the excess oxygen ratio and the stoichiometric oxygen in the combustion chamber. The excess oxygen ratio has a more important effect on the NOx emission than the feeding ratio of 0.394 mm coal and the stoichiometric oxygen in the combustion chamber. The recirculated flue gas is used to substitute a proportion of the primary air to form a fuel-rich condition in the combustion chamber and reduce NOx emission efficiently. Therefore, the effect of staged combustion can be achieved without decreasing the fluidization quality. NOx emission also decreases with increasing ratios of flue gas recirculation. The optimum operating condition for the minimun NOx emission is at feeding ratio of 0.394 mm coal=20%, stoichiometric oxygen in the combustion chamber =80%, and excess oxygen ratio=40%.
The stoichiometric oxygen in the combustion chamber has a more important effect on the combustion efficiency than the feeding ratio of 0.394 mm coal and the excess oxygen ratio. The optimum operating condition for the highest combustion efficiency is at feeding ratio of 0.394 mm coal=20%, stoichiometric oxygen in the combustion chamber =110%, and excess oxygen ratio=60%. As the feeding ratio of 0.394 mm coal is increasing, the combustion proportion in the bed zone is decreased, but in the splash zone and second air zone are increased; furthermore, the combustion proportion in the freeboard zone is not change. As the stoichiometric oxygen in the combustion chamber is increased, the combustion proportion in the bed zone and splash zone are increased because the combustion reaction is increased. As the stoichiometric oxygen in the combustion chamber is decreased, the combustion proportion in the second air zone and freeboard zone are increased because more volatile produces from combustor and combusts in those zones. As the excess oxygen ratio is increasing, the combustion proportion in the second air zone is increased, but in the bed zone, splash zone and freeboard zone is not change.

並列關鍵字

Response surface methodology ； combustion proportion ； combustion efficiency ； Nitrogen oxides emissions ； Flue gas recirculation ； Vortexing fluidized bed combustor

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