碩士生: 戴明偉
畢業年分: 2024年8月
論文名稱: 碳氫火焰輔助氨/空氣燃燒過程中熵生成率和NOx排放機制之樹質研究(中文) / Numerical study of NOx emission and entropy generation in ammonia/air combustion with assistance of hydrocarbon/air flames (英文)
中文摘要:
氨氣燃燒系統因涉及 MILD 燃燒與富-熄-貧燃燒等複雜現象，需採用特別方法以穩定燃燒。本研究使用簡化三口燃燒器，透過碳氫燃料母火提供熱源以輔助氨氣燃燒，設計三種條件：Case 1 為 CH₄/air 預混火焰，Case 2 為 (90%CH₄+10%H₂)/air，Case 3 為 C₂H₆/air。並結合 Star CCM+ 模擬與 Glarborg 詳細反應機制，探討 NOx 的生成路徑。模擬結果顯示，NO 生成主要經由六種路徑，包括 prompt NO、fuel NO，以及 thermal NO 下分出的 NNH-route、HNO-route 與 N₂O-route。研究中發現 prompt NO 指標物 NCN 與 fuel NO 指標物 HCN 會透過 R.1004 (NCN+H↔HCN+N) 反應互相轉換；此外，在氨氣出口亦偵測到 CHxNHy 反應生成 HCN 與 NCO，而 NNH 與 N₂O 則透過 R.677 (NNH+O↔N₂O+O) 互換。
實驗比較三種條件後發現，在母火中添加氫氣或改為乙烷皆可顯著抑制 NO 的生成，其中 fuel NO 減少幅度最大，在 Case 3 中降低達 75%，prompt NO 亦減少 12.5%。整體 NO 質量分數在 Case 2 與 Case 3 分別降低 10% 與 31%。最後，可用能分析顯示，雖然 Case 3 的總 exergy 較高，但歸一化後比例與其他條件相近，證明添加氫氣或使用高碳燃料不僅可減少 NO 生成，亦能維持燃燒系統之可用能穩定性。
英文摘要:
Ammonia combustion systems involve complex phenomena such as MILD combustion and rich–quench–lean (RQL) modes, requiring specialized approaches for stable operation. In this study, a simplified three-port burner was employed to investigate ammonia combustion assisted by hydrocarbon pilot flames. Three cases were considered: Case 1 with CH₄/air premixed flame, Case 2 with (90%CH₄+10%H₂)/air, and Case 3 with C₂H₆/air. Numerical simulations were conducted using Star CCM+ with the detailed Glarborg mechanism to analyze NOx formation pathways. Six distinct routes were identified: prompt NO, fuel NO, and three thermal NO sub-routes, namely the NNH-intermediate route, HNO-intermediate route, and the N₂O route. Results showed that the prompt NO indicator NCN and the fuel NO indicator HCN interconvert via reaction R.1004 (NCN+H↔HCN+N). Additionally, CHxNHy reactions involving methyl and amino groups were observed at the ammonia outlet, producing HCN and NCO species, while NNH and N₂O interconverted through R.677 (NNH+O↔N₂O+O).
The results further indicate that adding hydrogen to the pilot flame or replacing methane with ethane significantly suppressed NO formation. Among the six routes, the fuel NO pathway exhibited the largest reduction, decreasing by 75% in Case 3, while prompt NO decreased by 12.5%. The overall NO mass fraction was reduced by 10% and 31% in Case 2 and Case 3, respectively.
Exergy analysis revealed that although Case 3 exhibited a relatively higher total exergy, normalization showed similar ratios across all cases. This suggests that even with a lower overall temperature distribution, Case 3 maintained comparable exergy levels. In conclusion, introducing small amounts of hydrogen or using higher-carbon hydrocarbons in pilot flames can effectively reduce NO formation while maintaining combustion exergy, offering valuable insights for the design of low-emission ammonia-based combustion systems.
