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Doğal gaz yanmasında NOx emisyonu

NOx emissions in natural gas combustion

  1. Tez No: 21981
  2. Yazar: ÜMİT ZAFER ÇELİK
  3. Danışmanlar: PROF. DR. AHMET ARISOY
  4. Tez Türü: Yüksek Lisans
  5. Konular: Enerji, Energy
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1992
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 82

Özet

ÖZET Doğal gaz yanmasında metan gazı, yanma işlemine et ki eden esas bileşik olduğundan doğal gaz yanmasında azot kimyasının incelenmesi bakımından büyük önem kazanmaktadır. Metan-hava karışımının premiks, laminer. adyabatik yanma şartları bugüne kadar birçok çalışmada incelenmiş ve incelenmeye devam etmektedir. Doğal gaz yanması özellikle katı yakıtlara nazaran çevreyi kirletici etkisi az olan bir enerji kaynağı olması ve diğer enerji kaynaklarının çoğundan daha ucuza sağlanabilmesi bakımından önem kazanmıştır. Bu çalışmada doğal gaz ve hava karışımının direkt olarak yanması değil daha ziya de doğal gazı oluşturan C başta metan olmak üzere) gaz bileşenlerin NO kimyası elealınmıştır. Yanma işlemine etki eden önemli parametrelerin Cekivalans oran, sıcaklık, akış hızı, radikal konsantrasyonları, yakıt parçacık boyutu, vb. ) NOx oluşum ve yıkım mekanizmalarına olan etkileri araştırılmıştır. NOx oluşturan mekaniz malar bölümler halinde incelenmiş, metan- hava yanma işleminde ürünlerin mol oranlarını, sıcaklığı, yanma ener jisi gibi bilgileri hesaplayan CREK programı bu bilgiler ışığında düzenlenerek denge ve değişen kinetik şartlar için çıkışlar alınmıştır. NO mol oranlarının ekivalans oran ve sıcaklığa göre değişimleri diyagrama dökülmüş ve önceden yapılmış çalışmalarla karşılaştırılmıştır. NO2 bileşiğinin çok hızlı güç reaktörleri hariç hemen hemen her prosesde ihmal edildiği görülmüş ve bu- yüzden NO mekanizmalarına daha fazla önem verilmiştir. NO oluşumu üç ana bölümde toplanmıştır. Isıl-NO oluşu mu Zeldovich mekanizmasıyla oluşmaktadır ve NO oluşumuna sıcaklığın yükselen değerlerine karşılık çok kuvvetli olarak etkilemektedir. Ani-NO oluşumu CH bileşiğinin reaksiyonlarıyla başlamakta, düşük sıcaklıklarda ve yüksek akış hızlarında önem kazanmaktadır. Yakıt-NO mekanizmaları yanma ortamında veya yakıt içinde azot bileşiklerinin bulunmasıyla önem kazanmakta ve yüksek hızlarda NO oluşumunu etkilemektedir. Model kıyaslamasında ısıl mekanizmanın doğal gaz yanmasında NO oluşumuna en etkin faktör olduğu saptanmıştır. Akış hızları arttıkça eksik yanma meydana gelmekte ve ısıl mekanizmanın ilerleyememesi sonucu NO konsantrasyonlarında büyük düşüşler görülmektedir.

Özet (Çeviri)

1 SUMMARY NO EMISSIONS IN NATURAL GAS COMBUSTION X The chemical reactions of nitrogen compounds that oc cur in combustion processes have been the subject for more than fifty years. Much of this research has been motiva ted by the impact of the nitrogen compounds emmi t ted from combustion sources on the enviroment and by the role that these species play in combustion of energetic materials. At the present time many of the complex reaction paths for the formation and destruction of nitrogen species in combustion have been established and rate parameters for many of the elementary reactions have been determined. Available information on the rate parameters for these important reactions has been surveyed» and recommendations for the rate coefficients for these reactions have been provided. In the present study our current understanding of the mechanism and rate parameters for the gas phase reactions of nitrogen compounds that are relevant to air pollution Ccombustion generated) is illustrated by comparison of the results from detailed kinetics calculations with experi mental and kinetics calculations that have been performed previously. Sensitivity and rate of production analyses have been shown to determine which elementary reactions have great importance in the nitrogen conversion process. In the following sections, the gas phase reaction mechanisms and rate parameters for thermal, prompt and fuel NO and nitrogen-dioxide formation and removal processes are discussed. By the help of general behaviour of nitrogen species and standard path of methane in gas phase combustion process, a new reaction mechanism for methane has been adapted to the previous methane mechanism. The computational calculations by CREK program have been performed to find the new diagrams of nitrogen profiles for concentration, equivalence ratio of excess air coefficient and temperature. Here we are interested in premixed, laminar, one dimensional methane/air flames.Thermal-NO Mechanism Nitric oxide is an important nitrogen oxide compound emitted from combustion sources. In the combustion of clean fuels C fuels not containing nitrogen compounds), oxidation of atmospheric nitrogen by the thermal mechanism is a major source of nitrogen compounds emissions. The three principal reactionsthat comprise the thermal-NO for mation mechanism are: NO + N CAD N + O -«- * NO + O CA2) N + OH v- * NO + H (A3) There have been direct and indirect measurements of the rate coefficients of these three reactions, and these datas have been critically evaluated. The expressions for for the rate coefficients for reactions Al and A3 are in very near values in different studies. Hence, in general it is necessary to couple the thermal-NO reactions to the reaction sequence describing oxidation of the fuel. Since -the Overall rate of NO formation by the thermal mechanism- is generally slow comparing to the fuel oxidation reaction. Following the suggestion of Zeldovich, it is always assumed that the thermal-NO formation reactions can be decoupled from the fuel oxidation process. In this situation, NO formation rates are calculated assuming equilibrium values of temperature and concentrations of oxygen, nitrogen and nitrogen atoms. There is a noticeable acceleration of the maximum NO formation rate above that calculated using Zeldovich model results decreasing with increasing temperature while non- equilibrium effects on the NO formation rate are evident over a wide temperature range, the accelerated rates are sufficiently low so that very little NO is formed by the accelerated reactions. viPrompt NO Mechanism Nitric oxide formation rates in combustion of hydrocarbon fuels can exceed those.attributable to direct oxidation of molecular nitrogen by the thermal mechanism discussed before. Numerous studies have shown that prompt NO in hydrocarbon flames is formed primarily by a reaction sequence that is initiated by the rapid reaction of hydrocarbon radicals with molecular nitrogen leading to formation of amines or cyano compounds that subsequently react to form NO. CH and CH are the major contributors. HCN + N CA4) HCN +NH CAS) H CN +N CA6) 2 CN + N CA7) Fig 3-1 (page 73 is a reaction coordinate diagram on estimates of the two CH«.+ Nz rate coefficients can be based. Consider first the reaction HCN + NH. The other reaction channel involving methylene and molecular nitrogen; CH + N «.- J» HCN +NH CAS) 2 2 is endo thermic by 79 kcal/mol and is isoelectronic with the thermal-NO reaction, O + N « * NO + N. On this 2 basis this reaction also is an insignificant contributor to prompt-NO in calculations. Another reaction that does lead to prompt NO is the reaction CH + N «. ~* HCN + N. Here the first step in the process is the formation of metastable HCN but only for pressure dependancy other case (also generally i t runs as a pressure independant reaction) reaction AS is the first step and dominant. Prompt NO formation involves three separate kinetic issue: CD CH concentration C2) the rate of molecular nitrogen fixation C3) the rates of inter conversion among fixed nit- r ogen f r agmen ts. vii[hÖçn]- +H CH + N PN l J {ÎİMÇÖ[h + M + OH + H HCN ? O NCO f * ? H >? NH +Hı - >-l N - >. N ? OH +o 2 CNİ ı- OH ? O +H + C HCNO u. +CH, +HCCO 2 m the conversion of HCN to NO by t,he above mechanism is no longer rapid; Cb> the recycle of NO to HCN by the mechanism below begins to inhibit NO production; Cc) the reaction A2 shifts directions from reverse to forward. In this manner the mechanism below becomes very effective. C + NO CH + NO CH, NO CN + O HCN + O HCNO + H CA21) CA22) CA23) Beyond Wa 1.4, the peaks in the CH and TFN concentrations occur simultaneously. Chain branching has been shown by the reactions below. H + OH + CH OH 2 O 3 CH 8 M. H CH OH 2 CHO + M 2 CA59) CA765 CA77> NO production from fuel nitrogen A principal source of nitrogen oxide emissions in the combustion of fossil fuels is the nitrogen chemically bound in the fuel. This fuel nitrogen is a particularly important source of nitrogen oxide emissions for coal and coal-derived fuels, which typically contain O.S-2.0 % nit rogen by weight. The conversion of fuel nitrogen to NO is nearly independent of the identity of the model compound, but strongly dependent on the local combustion environment C temperature and stoichiometry) and on the initial level of nitrogen compound in the fuel-air mixture. Hidrogen cyanide appears to be the principal product when the fuel nitrogen is bound in an aromatic ring, ammonia when the fuel nitrogen is in the form of amines. Hence, a reaction mechanism for NO production from fuel nitrogen involves reactions important in the oxidation of HCN and NH, as in the case of prompt NO mechanism. ixThe oxidation of HCN An important aspect of both prompt-NO and NO formation fuel nitrogen is the conversion of hydrogen cya nide to NO. The HCN removal is controlled by the reaction of HCN with oxygen atoms. HCN + O HCN + O NCO + H NH + CO CA18) CA26) The distribution of NO and N in the flame is gover ned by the N-atom reactions. 2 N + OH N + NO NO + H N + O CA3) CAD +H + OH ? O CN HCN ? O tNO, +o NH ?vN KH N >.“2 ? OH l+H ? O, +OH 2 >? NO ? N ? O NCO Fig. 2 The reaction path diagram illustrating the reaction mechanism by which HCN is converted to NO and N. 2Comparable solutions have been given on diagrams of propane, coal and methane combustion processes with air lhat vere obtained from theoretical and experimental studi ea. Fig. S below shows the comparable solutions of ^C”»b» is ?he solution of the new model,“a”is the «option of the previous CREK model,“c”is an experimental study Numbered 2 is the NO mol fraction of. 2.7* NH added methane anSTf or only propane). NO mole fractions of coal have been shown in fig.6-d for isothermal processes. 1200 1 1 1 1 1 1 0 O.E 0.4 0.8 0.B 1 1.2 1.4 Cd> Fig. 5 Comparable solutions xivNO HCN N mechanism 2 As noted by many investigators» in rich combustion systems there is the possibility of reaction of NO with hydrocarbon free radicals, leading to the formation of HCN and, eventually, of molecular nitrogen. They have suggested that processes in which CH + NO reactions are significant and can be utilized as effective NO control strategies. x C H 2 2 -t-O ->\ HCCO +o + H -4- CH, CH +M, +H | 2 ? NO 1“ HCNO + H CH + NO HCN +o NCO /V- +K + NO Fig. 3 The reaction path diagram of the conversion NO to N2 by HCN. Fig. 3 above is showing the reaction path diagram illustrating the NO HCN -» N conversion mechanism. The bold lines indicate the most important reactions. As can be seen here HCN is directly produced from CH and NO can produce directly CN radicals. xiNitrogen Dioxide Mechanism Experimental studies have shown that a significant fraction of the nitrogen oxide emmi ss i ons from some combustion sources, such as gas turbines and gas appliances, can be NO and in measurements of NO there are relatively large 2N0 /NO ratios near the flame zone. 2 X NO formation and destruction in flames can occur by the following reaction sequence, NO a NO NO - OH OH O CA56) CAS7) CAS8) At flame temperatures sient species NO can 2 exist only at tran- If NO persists in the combustion product then there must be quenching of the N02 formed in the flame. This quenching might occur in turbulent flames by rapid mixing of hot and cold fluid elements which serve to quench the NO removal process by reduction in radical concentrations. Modelling HCN and Prompt-NO Mechanisms and Comparing Solutions A fortran-4 program has been used to solve the equilibrium and kinetic solutions of metane-air combustion for premixed, laminar and adiabatic situations. CREK program has been written in ANS C American National Standards) subsequently proved on an IBM 360 computer at the Technical University of Istanbul. The mentioned mechanisms of very basic prompt-NO and HCN conversion at the previous sections have modified. The solutions of NO mole fractions due to the rate of excess air coefficient have been compared to the prior mechanisms and three different studies. xiiiNO HCN N mechanism 2 As noted by many investigators» in rich combustion systems there is the possibility of reaction of NO with hydrocarbon free radicals, leading to the formation of HCN and, eventually, of molecular nitrogen. They have suggested that processes in which CH + NO reactions are significant and can be utilized as effective NO control strategies. x C H 2 2 -t-O ->\ HCCO +o + H -4- CH, CH +M, +H | 2 ? NO 1”HCNO + H CH + NO HCN +o NCO /V- +K + NO Fig. 3 The reaction path diagram of the conversion NO to N2 by HCN. Fig. 3 above is showing the reaction path diagram illustrating the NO HCN -» N conversion mechanism. The bold lines indicate the most important reactions. As can be seen here HCN is directly produced from CH and NO can produce directly CN radicals. xi

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