Akışkan yataklı kömür yakıcısı modeli ve ikinci kanun analizi
A Comprehensive model and the second law analysis of a fluidized bed coal compustor
- Tez No: 14303
- Danışmanlar: Y.DOÇ.DR. ABDURAHMAN KILIÇ
- Tez Türü: Doktora
- Konular: Makine Mühendisliği, Mechanical Engineering
- Anahtar Kelimeler: Akışkan yatak, Akışkan yataklı yakıcılar, Kömür, Fluidized bed, Fluidized bed combustors, Coal
- Yıl: 1990
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: Belirtilmemiş.
Özet
ÖZET Bu çalışmanın amacı akışkan yataklı kömür yakıcısının çeşitli çalışma ve yükleme şartlarında verimini termodinamiğin ikinci kanunu ile analiz etmek ve verimi etkileyen parametreleri belirlemektir. Bu amaçla hazırlanan ve aktif yatak ve serbest bölgeyi kapsayan bir akışkan yatak modelinin ikinci kanun analizi yapılmıştır. önerilen model daha önce geliştirilmiş çeşitli modellerde kısmen ya da tamamen ele alınan işlemlerin yanı sıra, aktif yatak ve serbest bölgede kimyasal reaksiyonlar sonucu kok taneciklerinin çap ve yoğunlukların da meydana gelen değişimleri, serbest bölgede taneciklerin kütle ve gaz hızına bağlı hareketlerini, yatakta kabarcıkların birleşmelerini, yatak katı madde karışma sının kimyasal reaksiyonlar ve N0“ ve S0= tutulmasına etkisini de ilave olarak kapsamaktadır. Geliştirilen modelin sonuçları, çeşitli deneysel sonuçlarla karşılaştırılarak modelin farklı geometrik özelliklere ve çalışma şartlarına haiz akışkan yatak yakıcılar için uygunluğu incelenmiştir. Geliştirilen akışkan yatak modelinde, ikinci kanun analizi ile akışkan yatak kömür yakıcısının verimi ve kömürün kimyasal kullanılabilirliği hesaplanarak kullanılabilir enerji kaybı ve tersinmezlikler üzerin de durulmuş ve verimi arttırıcı öneriler verilmiştir. Modelin esas alındığı parametreler değiştirilerek akış kan yatak kömür yakıcısının sıcaklık dağılımına ve dolayısıyla performansına tesir eden işletme şartları belirlenmiştir. Geliştirilen akışkan yatak modeli ve çözüm yöntemi kullanılarak, bir akışkan yatak yakıcının işletme ve dizayn parametrelerinin optimum değerleri modelle hesaplanan kayıp kullanılabilir enerjilerin incelenmesi ile bulunabilir. Bunun yanı sıra, akışkan yataklı yakıcıların çalışma parametrelerinin değiştirilmesinin, verim, baca gazlarındaki mol oranları, yatak karbon oranı ve S0a ve ND”emisyonları üzerindeki etkisi de tayin edilebilir. -x-
Özet (Çeviri)
SUMMARY A COMPREHENSIVE MODEL AND THE SECOND LAW ANALYSIS OF A FLUID I ZED BED COAL COMBUSTOR The steep rise in the cost of fuels which has taken place in the last decade has led to a search for ways of upgrading and modifying fuels in an efficient manner to fit in with the pattern of demand in the na tional economies of different countries. Accordingly the Second Law analysis for means of improving the per formance of the production plants become a necessity. The thermodynamic analysis of an energy conversion system, if based solely on the First Law of Thermodynamics, is more or less a tabulation of energies received or rejected by the system. However, the ther - modynamics losses which occur within the system or its subsystems cannot be accurately evaluated on the base of First Law alone. The result of the analysis based on the First Law alone, are not only insufficient, in fact they can be misleading. If the limitations imposed by the Second Law of Thermodynamics are considered in con junction with the provisions of the First Law, a more comprehensive analysis can be made than is possible with the currently prevailing heat and energy account. Therefore, the main aim of this study is to evaluate the parameters to improve the effectiveness of the sys tem and to verify the causes of inefficiencies in a fluidized bed coal combustor through The Second Law Analysis. In this study, a theoretical model for a fluidized bed coal combustor <FBCC) has been developed and the availability analysis of the system is performed. The renewed interest in combustion of coal has resulted in the search for new process developments. Among these the fluidized beds has been shown to possess some technical and economical advantages over more conventional combustors. High heat transfer coef ficients allow for reduced heat transfer surfaces, hence more compact units. Also it would appear that fluidized bed combustion may have some pollution control potential. In fact, lower temperatures of operation may minimize N0“ formation and emission and SO;, can react with limestone added to the bed. Although a steady growth of technical knowledge has taken place largely in the design area, and in the pilot plants and large scale experiments, little has been done in the field of -xi-mathematical modeling of combustion in fluidized beds. Some mathematical models have been developed for pre dicting the performance of FBCC under various conditions Almost all of these models are based on the two-phase theory of fluidization and address to specific phases of FBC operation. This is understandable since the process involves complex coupled phenomena occurring in the bed with the chemical reactions, heat and mass transfer, particle size reduction, gas and solid flow divisions, etc. In this study an attempt has been made to develop such a model that can be employed to simulate the performance of FBC under a wide range of operating conditions. The model takes into account the following processes occurring in the FBCC: 1. Devolati 1 ization of coal, subsequent combus tion of volatiles followed by residual char. 2. Sulfur dioxide capture by limestone par ticles 3. NOVc release and reduction of NO by the char 4. Entraintment of char and limestone particles, solid mixing. 5. Bubble hydrodynamics 6. Heat transfer between gas and solids and heat transfer surfaces and bed material. 7. Freeboard reactions The following assumptions are made during the modeling of FBCCs 1. Single-phase backflow cell model is used in representing the solid mixing in fluidized beds. 2. Two - phase bubble assemblage model is adapted for gas phase material balances. 3. Solids exchange between the bubble phase and emulsion phase is assumed to be rapid. 4. Gas phase exchange coefficient between the bubble phase and emulsion phase is a function of bubble diameter and is distributed axially. 5. Bubble size is a function of bed diameter and height above the distributor. When cooling tubes are present bubble size in the tubes region of the bed is based on the horizontal pitch distance between the tubes. 6. Volatiles are released in the emulsion phase and its rate is proportional to the solid mixing rate. 7. The flow rate through the emulsion phase corresponds to minimum fluidization velocity. B. Volatile nitrogen and sulfur increase as a function of bed temperature. Sulfur and nitrogen in the residual char are assumed to be released as sulfur dioxide and N0”during the combustion of char. -xi i-The FBCC model as shown in Fig. (4.1) forms the basis of the algorithm. The fluidized bed is split up vertically into individual cells, and the temperature and concentration in each cell are homogeneous. Solid particles motion is considered according to the models developed separately in the bed and in the freeboard. Assuming that no reducing zones occur in the fluidized bed, one gets the following balances for each cell. 1. Seven component balances for 02, V.P., CO, C03, SO3., NO, H=0 in the emulsion phase, 2. Four component balances for 0S, C0=, S0=, NO in the bubble phase, 3. One total mass balance of the fluidized bed material, 4. One carbon balance of the fluidized bed material, 5. Three energy balances for the fluidized bed, one for bed,one for freeboard and one for cooling water. These balance equations are obtained by using the following general cases for emulsion and bubble phases respectful 1 y. For emulsion phase; lncoaing component flow free (i-1) cell outgoing coaponent flow fro* i cell coaponent flow exchange with bubble phase coaponent produced by cbea.reaction coaponent consuaed by ches. reaction for the component oxygen example; 17.5 y“,02.i -S3 ye,Cp.i< )/2 - S2 yc,r,=,t/§.,t l+24.7y”.D2.i <n“,ı-ı y”.u,i-i +. nu,t) »<*= The similar way, the material balances are obtained by using the general form for bubble phase ıncoaıng coaponent flow fro. (i-1) cell outgoing coaponent flow froa i cell coaponent Han exchange with eaulsion phase coaponent produced by chea.reaction coaponent consuaed by chea. reaction for cell i, for the component oxygen example;“b,l Yb.rra.l = Ob, 1-1 yto,a=,t-l ”Sı !yb,o3,l“”y>,a2,t) -Sa yb,o2,l - Si y«,r<-i.l /2 The overall material balance for the solids in -x 1 1 1 -the itn cell, in terms o-f the backmix flow in emulsion and bubble phases, m“,i and mw.i. is given by 8»w,t -»«,1+1 =®«,t-l -ffl.,1 +ffib. l+ffiteal, t -«yan, t +»ft«l,t The material balance for the carbon in the i*-** cell is given as follows ffl«,t«-l Hc,l*l _ Btw.t X=.t _ ffl«,lXc.t +B». 1 -l Xc. 1 -l=Bly*n. I -®b. 1 where X=.j. is the weight fraction of carbon in the cell. The following equation is given for the energy balance in the i*1”1 cell in the bed Cfe.n-ı ffiB.t+x Tin ~ CCn.i (st“,i + ffl«.i)+Ca.i nt3 Tt + tCfc.t-i Bw.t-1 *-Cg,t-l AmlTl-l + 81y«n.i flc + H»,u,l Hu.ca'*”Hb.u.l £|u + nca,t <Jeo- ffiksl.t Qkal.t“'' 'Ckb Bib, t.*? Afcet.l Cktb 1 Tg s Qau.t +0duv,l The energy balance for the freeboard can be obtained in the same manner, fCk.l-l Siafc,t-X + Ca,t-»nt-il Ti-i + ffiyan.t q* + n«,t tju.co + n”,i qca+[ca,i ni+ fflat.t Ci,,t 1 Ti = Q«u,i +Qduv,t The cooling water energy balance is given as follows: IDau Csu tTanjt.l. ~ Tesu».*- 1 ) = ~ UlD,t Aid.1 ATm,au,t For the developed balance and energy equations, the following boundary conditions are applied: yt..o = 0.21 no yt,.-o=,o=0 y-.o = yt..o yb.no.o =0 y-.=«,o = 0 y.,co2,o = 0 y-..oS.o = o y-.«0.o = o yb.cos.o = o m..i =0 These balance and energy equations with the applied boundary conditions are given in detail in Chapter IV. The simulated results of fluidized bed coal com- bustor is obtained by using a combined Relaxation Newton-Raphson method. The iteration rule for the defined variable vectors and function vectors is as follows: -xi v-3F -ı 3x 3F/ 3X is the matrix of the partial derivatives of the vector F with regard to the variables vector X. Con sidering the reactor model (Fig.4.1), this results in a quasi -block tr i diagonal structure for the Jacobian matrix which is given in Appendix B. General validity ?for the simulation is attained using an address field, so that any desired plant variants can be calculated. The quality and validity of the proposed fluidized bed model was tested for steady-state opera tion based on the experimental data reported by H.Atakül C963, M. Martens E 65 3 and results of experiments per - formed in the Exxon and NCB fluidized bed coal combus- tors. The calculated results corresponding well with the results of the experiments. Deviation from the ex perimental results are in the range of 5-iO V. both in the pilot scale and Exxon and NCB FBC combustors in figures <4.5-13). The availability analysis of the developed model has been done by making the available energy balances for the system. The general available energy balance for the steady flow process of a FBCC is as follows: available energy in to system available energy out of system available energy] lost Since the available energy input to the system largely depends on the available energy of coal, the chemical energy of the coal has been evaluated according to the following equation K<Ta,P0> = H«<Ta,Pa>+Ta A“s<T0,P=> + ARe(TD,P”) and the coal chemical availability has been found for different type of coals. With the aid of the mathematical combust or model the steady state characteristics of the coal burning fluidized bed combustor were investigated. Also the second law efficiency of the FBCC has been evaluated un der the different operation conditions considering the effect of different bed parameters such as coal particle diameter, tube arrangement, bed velocities and different coal and air flow rates. Proposed model can easily apply to a FBCC in dif ferent geometric sizes and thermal capacity.lt will also aid in the understanding of the performance of FBCC under the range of operating conditions. The optimum operating temperature, tube arrangement and spacing and gas residence time in the bed which would give maximum -xv-combustion efficiency and lower S02 and ND« emissions can be estimated. The following results are obtained from the analysis of the second law efficiency of the FBCC. The bed temperature profile can easily be ad- Justed by moving the cooling tubes positions as shown in the Fig. (4.21). The effectiveness of the FBCC increases as the cooling tubes are located at the 40-50 7. of the bed height Fig. (5.3). At higher specific loading rates, the second law efficiency of the FBCC increases (Fig. 5. 6>. However higher input flow velocities of the combustion air, the temperature profile of the bed is getting more uniform than before and also the bed average temperature is get ting lower due to higher heat flow from the bed with stack gases (Fig 5.5). As the more coarsely grained types of coal is used, FBCC efficiency decreases due to lower reaction rate at the coal particle surface (Fig. 5. 4) -xvi-
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