再燃燒技術對於一氧化碳鍋爐脫硝效率影響之研究

Numerical Studay of the Reburning Technology for NOx Reduction in a Carbon Monoxide Boiler

梁競文(Ching-Wen Liang)

指導教授 ： 葉俊郎

國立虎尾科技大學/工程學院/航空與電子科技研究所/碩士(2013年)

https://doi.org/10.6827/NFU.2013.00002

摘要

一氧化碳鍋爐在煉油製程中佔有非常重要的角色，它可以將再生器中的一氧化碳(CO)熱能回收再利用，所產生的蒸汽可提供給其他設備使用，其操作溫度相當高（可達1200℃），因此，會產生一氧化碳鍋爐問題，其停車檢查維修所造成之生產損失每日以千萬元起計，其潛藏的工安與環保問題更是不容忽視，因此，本研究即在此背景下針對一氧化碳鍋爐的三維燃燒流場進行詳細探討；再燃燒技術是燃燒系統降低NOx排放的最有效方法之一，本研究以數值方法探討再燃燒技術運應於一氧化碳鍋爐三維燃燒流場之NOx去除效果，研究中針對二次噴孔大小、位置與數量、主燃料與二次燃料比例、入口空氣與二次空氣比例、以及入口空氣流量與二次空氣流量等之影響進行深入探討。
由研究結果發現NOx去除主要發生於脫硝段後方迴流區較強的區域。二次孔徑較小時，二次燃料與二次空氣噴入速度較快、噴入深度較深，有助於再燃反應與NOx去除效果，且CO生成量也較低；在另一方面，當二次孔徑較大時，二次燃料與二次空氣噴入速度較慢、噴入深度較淺，比較無助於再燃反應及NOx去除效果，且燃料較集中於噴孔附近而形成局部高溫區，CO生成量也比較高。二次孔徑較大時，當燃料與空氣的組合趨向於『貧燃料』操作(亦即，入口燃料降低而二次燃料提高，或入口空氣提高而二次空氣降低)時，NOx的去除效果較好；當二次孔徑較小時，雖然NOx的去除效果仍以『貧燃料』的操作條件較好，但二次燃料質量分率對於NOx的去除效果影響較不明顯。當二次孔徑較小時，二次噴孔位置對於NOx生成量的影響較不明顯；相反地，當二次孔徑較大時，二次噴孔位置對於NOx生成量的影響比較明顯，當二次燃料由上游迴流較強處的噴孔噴入時NOx還原效果較好，此外，當二次空氣噴孔較靠近二次燃料噴孔時，NOx的還原效果亦較好。當二次孔徑較大時，在二次燃料噴入位置附近的溫度會上升，靠近迴流較強處的二次燃料孔附近的溫度上升最多；此外，在二次空氣噴入的位置附近溫度呈現先降低後上升的趨勢。當二次燃料噴孔越靠近上游迴流較強處、二次空氣噴孔越靠近二次燃料噴孔時，由於再燃反應較完全，因此，CO生成量越低。在相同二次燃料與二次空氣噴入量的情況下，單孔的排列方式比三孔的排列方式NOx還原效果好，且平均溫度較低。CO生成量隨著入口空氣流量的增加而降低，當入口空氣流量達到原始爐型入口空氣流量的四倍時，CO生成量可降至250ppm以下。入口空氣流量增加時，流場截面平均溫度降低，但NO生成量提高。最後，增加二次空氣流量對於降低CO含量、溫度與NO含量的效果不如增加入口空氣流量。

關鍵詞：一氧化碳鍋爐、再燃燒、氮氧化物生成

關鍵字

一氧化碳鍋爐 ； 再燃燒 ； 氮氧化物生成

並列摘要

Carbon Monoxide（CO）boiler plays an important role in the oil refinery process. It can retrieve the thermal energy of CO from the regenerator for application. CO boiler utilizes the burning gas（CO）from the regenerator as fuel. Then CO reacts to form CO2 and release large amount of heat. The hot flue gas then flows through the superheat section and exchanges heat with water in the cooling tubes to produce superheated steam which can be used by other equipments. The operating temperature in a CO boiler can be as high as 1200℃. This can lead to the interior or exterior problems of a CO boiler. The loss caused by product loss of a CO boiler can be as high as one million US$ per day. Furthermore, the potential threats of safety and environment protection cannot be ignored. This study adopts a CO boiler from the Formosa Petrochemical Corporation (FPC) in Taiwan as the model for numerical investigation. The combustion and fluid flow in the FPC CO boiler is examined with emphasis on the effect of reburning upon NOx reduction. Reburning technology is one of the most promising and cost-effective NOx reduction strategies for combustion systems. The influences of some important parameters related to reburning are inspected, including the size, position and number of reburn fuel or air holes, the primary/reburn fuel ratio, the inlet/reburn air ratio, as well as the inlet air flow rate and reburn air flow rate.
From the simulation results, it is found that NOx reduction occurs mainly behind the DeNOx section where recirculation is strong. For a smaller reburn hole, the injection velocity and the penetration depth are larger. This leads to a better NOx reduction and a lower CO concentration. On the other hand, for a larger reburn hole, the injection velocity and the penetration depth are smaller. This leads to a local high temperature region near the reburn hole and a worse NOx reduction as well as a higher CO concentration. NOx reduction is better with a smaller primary/reburn fuel ratio and a larger inlet/reburn air ratio. However, the influence of primary/reburn fuel ratio and inlet/reburn air ratio on NOx reduction is less pronounced for a smaller reburn hole. For a larger reburn hole, NOx reduction is better when the reburn fuel is injected more upstream where recirculation is stronger and when the reburn air hole is closer to the reburn fuel hole. On the other hand, the influence of reburn hole location on NOx reduction is less pronounced for a smaller reburn hole. For a larger reburn hole, the cross-sectional average temperature arises near the reburn fuel hole and the temperature rise is higher when the reburn fuel is injected more upstream where recirculation is stronger. In addition, the cross-sectional average temperature decreases first and then arises near the reburn air injection location. The CO concentration is lower when the reburn fuel is injected more upstream where recirculation is stronger and when the reburn air hole is closer to the reburn fuel hole. Based on the same reburn fuel and air flow rates, single-reburn-hole arrangement yields lower NOx concentration and temperature than the three-reburn-holes arrangement does. The CO concentration and the cross-sectional average temperature decrease with the inlet air flow rate and the reburn air flow rate while the NO concentration increases with the inlet air flow rate and the reburn air flow rate. Reducing CO by increasing inlet air flow rate is better than by increasing reburn air flow rate.
Keyword：CO boiler, Reburning, NOx Reduction

並列關鍵字

CO boiler ； Reburning ； NOx Reduction

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