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Oxygen-enhanced combustion: Theory and applications.

機譯：氧氣增強燃燒：??理論與應用。

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In Part I, coflow flame experiments were performed to compare and evaluate the influence of flame structure on soot formation when operating under normal and inverse flame conditions. Flame structure was shown to influence soot formation in a similar fashion for normal and inverse flames when the effects of residence time were removed. The simple model previously discussed was modified to account for finite-rate chemistry and residence time effects, and was correlated with experimental data leading to the determination of the critical local temperature and critical local C/O ratio for soot inception in ethylene flames. The presence of appreciable oxygen at the location of maximum temperature was investigated using a flame code with detailed chemistry. The mechanisms responsible for O2 at xTmax in high Zst flames were determined and explained. This phenomenon was attributed to a shifting of the location of maximum temperature relative to the location of oxygen depletion, and the temperature shift was explained by considering the variations in the heat release profile at high Zst .;A second numerical investigation was also conducted to evaluate the significance of the local critical C/O ratio as a parameter describing soot-free conditions, the role of oxidizing species at this location, and changes that occur in the chemical pathway to the formation of soot precursors at high Zst . The critical local C/O ratio was shown to correspond to the edge of the radical pool for flames of any Zst, and oxidizing species did not appear to accelerate soot precursor oxidation at high Zst as previously thought. A reverse pathway analysis was used to determine the dominant chemical pathway leading to the formation of soot precursors. At high Z st, a key soot precursor formation step was observed to reverse leading to the destruction of propargyl (C3H3) to form acetylene (C2H2) as opposed to benzene (C6H6) and phenyl (C6H5). The existence of soot-free flames at long residence times was attributed to this phenomenon.;In Part II of this work, a form of OEC currently being considered as an enabling technology for carbon dioxide capture from pulverized coal (PC) utility plants, termed oxy-fuel combustion, was considered. Oxy-fuel combustion utilizes oxygen and recycled flue gases (RFG) as the oxidizer instead of air, therefore the concentration of oxygen in the coal carrier stream, as well as any other concentric stream or quiescent environment, is a variable. The viability of oxy-fuel combustion can be enhanced by its ability to reduce capital and operational costs by, for example, lowering the emissions of nitrogen oxide species (NOx) in situ. Studies have demonstrated that oxy-fuel combustion can lower NOx emissions by as much as 70% when compared to conventional coal/air combustion, largely due to the reduction of recycled NOx to molecular nitrogen when interacting with hydrocarbon species in the flame.;This work investigates the potential for reduced NOx emissions under oxy-fuel conditions through variations in the gas composition of the fuel carrier and concentric oxidizer streams. Nitric oxide (NO) emissions were measured during the combustion of PC and PC/sawdust mixtures under air-fired and oxy-fuel conditions. The effects of excess oxygen, secondary oxidizer swirl, carrier gas flow rate, and sawdust cofiring on NO emissions were investigated. Under oxy-fuel conditions, the effect of varying the compositions of the carrier gas and concentric oxidizer streams on NO emissions was also investigated. Under the optimal oxy-fuel conditions, NO emissions were reduced by 20% when compared to air-firing. Cofiring coal with sawdust that contained less fuel bound nitrogen did not reduce the NO emissions under air-fired or oxy-fuel conditions. Changing the adiabatic flame temperature by varying the oxygen concentration in the concentric oxidizer stream did not significantly influence NO emissions until the temperature was too low and flame instabilities were observed. When increasing the oxygen concentration in the coal carrier gas a critical local stoichiometric ratio was observed that led to increased NO emissions. (Abstract shortened by UMI.)
機譯：在第一部分中，進行了同流火焰實驗，以比較和評估在正常和逆向火焰條件下運行時火焰結構對煙灰形成的影響。當去除停留時間的影響時，火焰結構顯示出對正向和反向火焰以類似的方式影響煙灰形成。修改了先前討論的簡單模型，以解決有限速率化學和停留時間的影響，并將其與導致確定乙烯火焰中煙灰開始的臨界局部溫度和臨界局部C / O比的實驗數(shù)據(jù)相關聯(lián)。使用具有詳細化學信息的火焰代碼研究了在最高溫度位置存在的明顯氧氣。確定并解釋了高Zst火焰中xTmax處的O2的機理。這種現(xiàn)象歸因于最高溫度位置相對于氧耗竭位置的偏移，并通過考慮高Zst時放熱曲線的變化來解釋溫度偏移。；還進行了第二次數(shù)值研究以評估局部臨界C / O比作為描述無煙煤條件的參數(shù)的重要性，在該位置的氧化物質(zhì)的作用以及在高Zst形成煙塵前體的化學途徑中發(fā)生的變化。臨界局部C / O比顯示為對應于任何Zst火焰的自由基庫的邊緣，并且氧化物種似乎沒有像以前認為的那樣在高Zst時加速煙灰前體氧化。反向途徑分析用于確定導致煙灰前體形成的主要化學途徑。在較高的Z st下，觀察到關鍵的煙灰前體形成步驟逆轉，導致炔丙基（C3H3）破壞，形成乙炔（C2H2），而苯（C6H6）和苯基（C6H5）則相反。這種現(xiàn)象歸因于長時間停留時無煙灰火焰的存在。在這項工作的第二部分中，一種OEC形式目前被認為是一種從粉煤（PC）公用設施中捕獲二氧化碳的促進技術?？紤]了氧燃料燃燒。含氧燃料燃燒利用氧氣和循環(huán)煙氣（RFG）代替空氣作為氧化劑，因此，煤載氣流以及任何其他同心流或靜態(tài)環(huán)境中氧氣的濃度是可變的。氧燃料燃燒的可行性可以通過降低資本成本和運營成本（例如通過降低原位氮氧化物（NOx）的排放）來提高。研究表明，與傳統(tǒng)的煤/空氣燃燒相比，含氧燃料燃燒可以將NOx排放降低多達70％，這主要是由于與火焰中的碳氫化合物相互作用時，再循環(huán)的NOx還原為分子氮。通過改變?nèi)剂陷d體和同心氧化劑流的氣體組成，研究了在含氧燃料條件下減少NOx排放的潛力。在空氣和含氧燃料條件下，燃燒PC和PC /木屑混合物時，測量了一氧化氮（NO）的排放量。研究了過量氧氣，二次氧化劑旋流，載氣流速和鋸末共燒對NO排放的影響。在含氧燃料條件下，還研究了改變載氣和同心氧化劑流的成分對NO排放的影響。在最佳的含氧燃料條件下，與空燃相比，NO排放減少了20％。在空氣燃燒或含氧燃料條件下，將木屑與木屑共燒，其中木屑的含氮量較少，但不能減少NO排放。通過改變同心氧化劑流中的氧氣濃度來改變絕熱火焰溫度，不會顯著影響NO的排放，除非溫度太低并且觀察到火焰不穩(wěn)定。當增加煤載氣中的氧氣濃度時，觀察到臨界的局部化學計量比，導致增加的NO排放。 （摘要由UMI縮短。）

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