Alttan beslemeli sabit yatakta kömür yanması
Başlık çevirisi mevcut değil.
- Tez No: 39145
- Danışmanlar: PROF.DR. AHMET ARISOY
- Tez Türü: Doktora
- Konular: Enerji, Makine Mühendisliği, Energy, Mechanical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1993
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 125
Özet
ÖZET Kararlı rejimde sabit katı yakıt yatağında yanma hali için tek boyutlu matematiksel bir model geliştirilmiştir. Bu modelin fiziksel esasını, yakıt ve havanın ızgara üzerine aynı akış yönünde ve aşağıdan yukarıya doğru beslendiği yakma sistemi teşkil etmektedir. Bu sistemde katı yakıt yanması, tutuşma bölgesinden olan ısı geçişi sonucunda reaktif maddede kimyasal reaksiyonların açığa çıkışı ile gerçekleşir. Model, ısı geçiş katsayıları ve reaksiyon hızlarının hesaplandığı alt programlarla desteklenmiştir. Kömür taneciklerinin aynı büyüklükte ve küresel formda olduğu, yakıt yatağı porozitesinin yanma sırasında değişmediği kabul edilmiştir. Kömürün sabit karbon, uçucu madde, nem ve külden oluştuğu gözönüne alınmıştır. Uçucu maddeyi oluşturan bileşenlerin açığa çıkışı, birbirinden bağımsız birinci dereceden paralel reaksiyonlar ile modellenmiştir. Tanecik yanması için küçülen çekirdek modeli esas alınmıştır. Isının yanma bölgesinden kömüre, iletim, taşınım ve ışınımdan oluşan mekanizma ile geçtiği dikkate alınmıştır. Yanma odası duvarlarından olan ısı kayıpları ihmal edilmiştir. Yanma, ince bir yatak elemanı için yukarıda sıralanan kabulleri içeren enerji, kütle ve bileşenlerin korunum denklemleri ile matematiksel olarak tariflenmiştir. Denklemler sonlu farklar metodu uygulanarak iteratif olarak çözümlenmiştir. Model, sıcaklık ve yanma ürünleri ile karbonun yakıt yatağı boyunca değişimini verebilmektedir. Model sonuçları bu çalışmanın deneysel bölümünde elde edilen sonuçlarla karşılaştırılmıştır. Bu amaçla kurulan deney tesisatında sıcaklık ve yanma ürünlerinin değişimleri ölçülmüştür. Deney sonuçlarının ışığında kömür cinsinin, tanecik çapının ve hava besleme akısının yanma üzerindeki etkisi araştırılmıştır. Hava besleme akısına bağlı olarak kömür besleme akısının değişimi belirlenmiştir. Deney şartlarının veri olarak kullanıldığı model çalışmasından elde edilen sonuçların, deney sonuçları ile uyum içinde olduğu görülmüştür. xvıı
Özet (Çeviri)
UNDERFEED FIXED BED COAL COMBUSTION SUMMARY The object of this Ph.D. thesis is to develop a mathematical model of steady-state underfeed fixed bed coal combustion. In this combustion system the coal is fed from underneath the bed so that the reaction front is traveling down and the volatiles are carried up into the bed. Air is supplied through the tuyeres. As feeding continues, the burning pile rises in height until its profile reaches the natural angle of response and the fuel particles spill over down the grate and on to the ash bars. The fixed bed coal combustion is described mathematically for a thin coal bed layer by means of energy, mass and species conservation equations. The model is supported with the computer codes obtaining reaction rates, physical properties and thermal conductivity. Using this computer simulation, combustion product concentrations, temperature and fixed carbon gradients can be predicted. To test the performance of the model, experiments have been carried out. The physical system of underfeed fixed bed reactor is simulated with a combustion pot. By means of the experiments coal flux is determined as a function of air flux. The model results are compared with the experimental results. THE MODEL ASSUMPTIONS In the following, the assumptions and the conditions of macroscopic coal reaction are given. 1. Steady-state combustion is assumed. 2. One-dimensional combustion behavior is assumed in the direction of coal and air flow. 3. Heat losses through the reactor are ignored. 4. All the coal particles are assumed to be spherical in shape and dimensionally uniform. xix5. Heat transfer in fixed bed is described with an effective thermal conductivity including conduction, radiation and the effect of the gas flow on the heat exchange. 6. Laminar and turbulent flow of gas and the effect of the particles to one another are considered in calculating heat convection and mass transfer coefficient. 7. Specific heat of coal and gas mixture are obtained proportional to the mass fraction of their components as a function of temperature. 8. Diffusion and viscosity of gas mixture are given as a function of temperature. 9. Particles are composed of fixed carbon, volatile matter, moisture and ash. 10. Volatile matter is assumed to be consist of C02, CO, CH4, H2, H2S, N and the production kinetic of these species is modeled with parallel first order reactions. 11. Combustion kinetics of volatile matter species is considered separately for each of the volatile matters. 12. It is assumed that fixed carbon is distributed all over the ash framework. It is also considered as the combustion of carbon goes on the carbon sphere shrinks in size and the reaction occurs at the outer surface of this unreacted core. 13. During the combustion, porosity of fixed bed remains constant. 14. Temperature gradient all over the particle is assumed to be uniform. CONSERVATION EQUATIONS The conservation equations for energy, mass and species under steady-state condition provide the necessary basis for the mathematical description of the bed behavior. i. Energy conservation equation, for the solid and gas phases respectively cpk^(mkTk) = |-^z^+Ash(Tg-Tk) + 2;rj(-AHRj) (1) cPg^(AgTg) = ^|^^|-ASh(Tg-Tk) + Zri(-AHRi) (2> dxv B e/ dx V dxJ where m is mass flux, cp specific heat capacity, Xz effective thermal conductivity, r reaction rate, AHR heat of reaction, i and j gas and solid phase reactions respectively. ii. Conservation equation of mass mk + m = constant (3 ) xxiii. Conservation equation of species dx±i=ri (4) where ihj is mass flux and r reaction rate of species i. These differential equations require a large number of auxiliary algebraic equations which describe; a. physical properties including heat capacity, diffusivity and viscosity. b. effective thermal conductivity which includes radiative, convective and conductive heat exchange between gas and the particle c. rates of pyrolysis and oxidation of coal. VOLATILE MATTER RELEASE It is assumed that coal decomposed thermally as if it is a mixture of many species, each of which decomposed via an independent first order reaction and with a characteristic energy, dV{ = k(\f-vj (5) dt and rate constant is assumed to be ki=AiTbeaq)(-Ei/RT) (6) where Aj is the pre exponential factor, Ej is the activation energy of reaction i, Vj is the amount of product i, Vj is the amount of product i that could potentially be produced. VOLATILE MATTER COMBUSTION In the model the effects of both volatile matter release rate and volatile matter combustion rate are considered. It is assumed that all hydrocarbons in the volatile matter are CH4. CH4 oxidizes to CO and H2 by a fast reaction. Later H2 reacts with oxygen to H20. Similarly CO in the volatile matter react to form C02 by a homogeneous reaction. CH4 +l/202 ->CO + 2H2 (I) (CO)T > +l/202-»C02 (II) (CO)! XXI0^2 )uM > +l/202->H20 (III) (H2)i REACTION MECHANISM AND RATE OF CHAR COMBUSTION The following reaction mechanism is assumed for particle combustion. C + 02 -* C02 (IV) C + C02 -> 2CO (V) An overall reaction rate for direct heterogeneous reaction (IV) is defined as the rate of removal of carbon per unit external surface area per unit atmosphere partial pressure of oxygen in the gas. rs = ksp02 (7) where rs denotes rate of removal of carbon per unit external surface and po2 is the partial pressure of oxygen in the gas in the free stream. It is considered that the overall reaction rate constant depends both on the rate of transport of oxygen by diffusion to the surface and on the rate of reaction of oxygen with the particle. Hence the overall reaction rate constant is expressed as follows 11! Ks KD Kk where kD is mass transfer coefficient, kk reaction rate constant on the particle surface. THE SOLUTION STRATEGY The problem type for the species conservation equations is considered as initial value problem. These equations were solved analytically. The energy equations are considered as boundary layer problem. Ambient temperature is given as inlet conditions for coal and gas temperature. Outlet boundary condition is taken as a Cauchy type boundary condition. -Xz£+hAs(Tg-TN) = ssa0Tâ (9) where TN is the temperature above the fixed bed. Inlet and outlet boundary conditions for mass conservation are the total of the gas and xxiicoal mass fluxes at inlet. The coal bed is simulated by a uniform one- dimensional mesh in which energy equations are approximated by finite difference equations. To solve the energy equations, the first approximated values of the gas and coal temperatures are needed. After solving the energy equations, the species gradient in the fixed bed was obtained. The iteration is terminated when the difference between the temperature calculated at the last iteration and the temperature obtained before the last iteration is less than 2 K. By solving above equations, the variation of the temperature and product gases concentrations in 20 cm high fixed bed are obtained. EXPERIMENTAL STUDY The experiments were carried out in a deep shaft bed. The test equipment consists of two refractory lined cylinders. The interior cylinder is the combustion chamber. The hot gases run around the outer surface of the combustion chamber between the interior and the outer cylinder so that the heat loss from the combustion chamber was reduced. The diameter of the combustion chamber İs 300 mm and with height of 1 m. Air is supplied from the bottom through the air plenum by means of a fan. The combustion chamber is equipped with thermoelements and a gas sampling probe. The temperature is measured in the middle of the combustion chamber at four points. The combustion products (C02, CO, 02) are measured by Maihak Unor 6N Infrared Gas Analyzer. Coal is charged and ignited from the top of the combustion chamber. During the experiments the coal layer has been kept stationary and propagation of the combustion front is observed. By this testing configuration, the actual underfeed fixed bed combustion chamber is simulated. The experiments is carried out for two kinds of coals with two different particle size ranges of 30-40 mm and 10-30 mm at different air fluxes. COMPARISON OF MODEL PREDICTIONS WITH EXPERIMENTS Results of the model have been compared with the experimental data of the presented study. Temperature and the concentration gradients at both studies are seen to be in agreement. For the both coal types, the temperature takes its maximum value in 10 cm of the fixed bed. The temperature variation in the first 5 cm of the fixed bed is independent of the coal type and particle size. xxuiThe maximum value of the coal temperature both for the model and experimental results gets closer with the increasing excess air ratio. At both results gas temperature is greater than the coal temperature. The bed height for the completely burning of the coal varies between 10 and 12 cm according to the excess air ratio. At the model results carbon monoxide is piled up at the beginning of the concentration variation. This is due to the volatile matter oxidizing to the product carbon monoxide. This originality of carbon monoxide variation has been also observed at the experimental results The variation of the maximum coal feed flux with respect to the air feed flux has been obtained. The maximum coal feed flux for the coal type with more volatile matter at 30-40 mm particle size takes its maximum value by the excess air ratio 2,3 and at 10-30 mm particle size by excess air ratio 1,8. For the coal type with less volatile matter at 30-40 mm particle size the maximum coal feed flux is obtained by excess air ratio 1,3 and at 10-30 mm particle size by excess air ratio 1,4. XXIV
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