流體化床燃燒中氮氧化物排放之研究

A Comprehensive Study on NOx Emissions in Fluidized-bed Combustion

李品慰(Pin-Wei Li)

指導教授 ： 錢建嵩

中原大學/工學院/化學工程研究所/博士(2019年)

https://doi.org/10.6840/cycu201900218

摘要

為全面性了解NOx (NO + NO2)在流體化床燃燒中的排放特性，本研究統整NOx之生成機制、還原機制與減量技術，並探討燃料特性對NOx生成與轉化之影響。此外，儘管流體化床之NOx排放相較於其他燃燒設備來得低，但未來可能無法符合更為嚴厲之排放標準。本研究統整歐盟、美國與中國大陸之現行NOx排放標準，並與我國標準比較；以新建之大型燃煤發電廠而言，NOx排放標準係以我國之30 ppm最為嚴格，其次分別為美國(44 ppm)、中國大陸(49 ppm)和歐盟(73 ppm)。
根據生成機制可知，熱式一氧化氮於流體化床燃燒條件下之生成量小於0.01 ppm，故可以忽略不計。燃料氮可分為char-N與volatile-N，char-N之非均相氧化為NO主要生成路徑，而volatile-N大多先轉化為HCN或NH3再進一步氧化，其轉化行為取決於溫度、燃料結構、加熱速率…等。
儘管無法避免NOx在燃燒過程中生成，但可藉由還原反應降低NOx排放濃度，其中包含NO與char、CO和氨之反應。Char與NO之非均相還原係影響NOx最終排放之關鍵因素，且係由C(N)與NO反應形成N2。CO雖可與NO進行催化還原，但其濃度需達1%以上才有明顯效果。氨與NO之反應可分為選擇性催化還原(selective catalytic reduction, SCR)與選擇性非催化還原(selective non-catalytic reduction, SNCR)，SCR雖脫硝效率高，但成本與空間需求對既存燃燒設備而言較不實際；SNCR係於適當之高溫區間注入氨或尿素以還原NO且不需觸媒床，脫硝效率可達60 – 80%。
相較於末端的煙氣處理，以燃燒控制技術來降低NOx排放更具經濟效益，故本研究係研究溫度、過量氧、分級燃燒與煙氣迴流(flue gas recirculation, FGR)對NOx排放減量之影響。結果顯示，相較於熱傳管，以灑水控制床溫更能有效降低NOx排放，但須考慮末端管線腐蝕之可能性。過量氧每減少10%，NOx排放便降低8 – 29 ppm，但在低溫下其影響較弱。在流體化床中，分級燃燒可使NOx排放降低20 – 60%，而造成NOx排放減量係因一次風流速減少導致停滯時間增加，而非氧濃度降低造成的還原氣氛。儘管文獻指出FGR可令NOx排放減少約30%，但其效果在本研究中並不明確，此可能係燃料特性或操作條件所導致。
燃料特性被視為影響NOx排放之關鍵因素，本研究探討氮含量、燃料元素比、char-N與volatile-N之分佈特性、以及fuel-N之結構對NOx排放與轉化之影響，並分析其關聯性。結果顯示，fuel-N轉化率會隨氮含量增加呈指數衰退，但排放濃度卻無明顯趨勢。在800 °C時，CH/N比與fuel-N轉化率之迴歸趨勢優於O/N比與H/N比。Volatile-N之NOx轉化率明顯高於char-N，顯示揮發份中的氮較易排放NOx。燃料氮之結構對NOx排放具有顯著影響，而此乃與芳香環之熱穩定性及支鏈結構有關。
綜上所述，採用燃燒控制技術固然可將大多數燃料之NOx排放降至100 ppm左右，但對於排放量較高之燃料而言其效果有限，此結果意味著燃料本身特性為關鍵影響因子，若需降至更低時則需採用SNCR或SCR以達排放標準。

關鍵字

流體化床燃燒 ； 氮氧化物排放

並列摘要

To understand NOx emission characteristics in fluidized-bed combustion (FBC) comprehensively, formation mechanism, reduction mechanism, and reduction technologies of NOx are integrated in this study. The effect of fuel characteristics on NOx formation and conversion is also discussed. Furthermore, despite the relatively low NOx emissions in FBC, it may not be able to meet the stricter emission standards in the near future. The current NOx emission standards in America, European Union (EU) and China are organized in this study and compared with those in Taiwan, and it can be found that for large coal-fired power plants, the strictest emission limit is 30 ppm legislated by Taiwan, followed by America (44 ppm), China (122 ppm), and lastly by EU (73 ppm).
According to the formation mechanism, the production of thermal NO in FBC conditions is less than 0.01 ppm, which is negligible compared with other sources. Fuel-N can be divided into char-N and volatile-N. The heterogeneous oxidation of char-N is the main formation path of NO, while most of volatile-N is converted to HCN and NH3 first before oxidation, and its conversion depends on temperature, fuel structure, heating rate, etc.
Although NOx formation in combustion process is inevitable, NOx concentrations are able to be decreased by the reduction reactions, including the reactions of NO with char, CO, and NH3. The heterogeneous reduction of NO and char is the key factor influencing the final emissions, and N2 is formed by the reaction between C(N) and NO. CO may reduce NO catalytically, but its concentration should be higher than 1% to reach significant effects. NO reduction by NH3 can be divided into selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR). Even though the de-NOx efficiency of SCR is high, the costs and the need for space are not practical for existing combustion units. In the SNCR process, NH3 or urea is injected into the zone with an appropriately high temperature in the absence of a catalyst bed, and the de-NOx efficiency is claimed to be 60 – 80%.
In comparison to the end-of-pipe treatment of flue gas, Reducing NOx by combustion control is much cost-effective, so the effects of temperature, excess oxygen, staged combustion, and flue gas recirculation (FGR) on NOx reduction are investigated in this study. The result indicates that controlling bed temperature by water spray is more effective in NOx reduction than by heat exchanger tubes, but the possibility of back-end corrosion should be taken into consideration. NOx emission decreases 8 – 29 ppm per 10% decrease of the excess oxygen, and the impact would be weakened at lower temperatures. In FBC, the application of staged combustion can reduce NOx emissions by 20 – 60%, and the reduction is mainly from the increased residence time by lowering the flow rate of primary gas instead of the reducing atmosphere generated by the decrease of oxygen concentration. Although it is suggested that approximate 30% of NOx emission can be reduced by means of FGR, its effect in this study, however, is unclear, which may be attributed to the fuel characteristics and operating conditions.
Fuel characteristic is regarded as the key factor for NOx emissions. The effects of nitrogen content, elemental ratios of fuels, the distribution between char-N and volatile-N as well as the fuel-N structure on the emission and conversion of NOx are investigated in this study, and the correlation between them is also analyzed. The result indicates that the fuel-N conversion decreases exponentially with nitrogen content, but there is no clear tendency for emission concentrations. The correlation between CH/N ratio and fuel-N conversion at 800 °C is superior to that of O/N ratio and H/N ratio. The conversion of volatile-N to NOx is evidently higher than that of char-N, indicating that nitrogen in volatiles is prone to emit NOx. The effect of fuel-N structure on NOx emission is significant, and it may be related to the thermal stability of aromatic rings and the structure of branch trains.
In summary, the application of combustion control is able to reduce NOx emission to 100 ppm approximately for most of fuels, but its efficacy is rather limited for those with higher emission values, implying that the inherent characteristics in fuel are key impact factors. SNCR or SCR should be adopted in case of lower emission limits.

並列關鍵字

fluidized-bed combustion ； NOx emissions

參考文獻

Aarna, I., Suuberg, E.M., 1997. A review of the kinetics of the nitric oxide-carbon reaction. Fuel 76, 475-491.

Aarna, I., Suuberg, E.M., 1998. A study of the reaction order of the NO-carbon gasification reaction. Symposium (International) on Combustion 27, 3061-3068.

Aarna, I., Suuberg, E.M., 1999. The Role of Carbon Monoxide in the NO−Carbon Reaction. Energy & Fuels 13, 1145-1153.

Abd-Alla, G.H., 2002. Using exhaust gas recirculation in internal combustion engines: a review. Energy Conversion and Management 43, 1027-1042.

Abraham, B.M., Asbury, J.G., Lynch, E.P., Teotia, A.P.S., 1982. Coal-oxygen process provides CO2 for enhanced recovery. Oil Gas J.; (United States), Medium: X; Size: Pages: 68-70, 75.