Kömürlerin kendiliğnden yanmasının teorik ve deneysel incelenmesi
Theoretical and experimental investigations of spontonequs combustion of coals
- Tez No: 39611
- Danışmanlar: PROF.DR. AHMET ARISOY
- Tez Türü: Doktora
- Konular: Makine Mühendisliği, Mechanical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1994
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 142
Özet
ÖZET Sunulan çalışmada, yığın halinde depo edilen kömürlerin kendiliğinden yanması olayı teorik ve deneysel olarak incelenmiştir. Olayın matematiksel modellenmesi, içerisinde, kimyasal reaksiyon meydana gelen gözenekli ortamlardaki ısı ve kütle transferi olaylarının formülasyonu esasına dayanmaktadır. Buna göre, zamana bağlı ve tek boyutlu olarak geliştirilen matematiksel model, katı ve gaz fazı için ayrı ayrı olmak üzere enerjinin, oksijenin ve nemin korunumu denklemlerinden oluşmuştur. Model ayrıca, kömürün ve gazın çeşitli fiziksel ve kimyasal özeliklerinin değişken olarak ele alındığı bir kısmı yardımcı denklemleri de kapsamaktadır. Tüm bu eşitlikler yardımı ile, yığın içerisinde oluşan konveksiyon, iletim ve difuzyon olayları, reaksiyon sonucu üretilen ısı ve tüketilen oksijen miktarları, buharlaşma ve yoğuşma olayları, ve ayrıca katı ve gaz arası taşınımla olan ısı geçişi olayı uygun bir şekilde formüle edilmiştir. Herhangi bir teorik model yardımı ile sayısal çözümleme yapabilmek için, öncelikle kömürün fiziksel ve kinetik özelikleri ile ilgili bir kısım deneysel veriye gereksinim duyulmaktadır. Bu verileri elde etmek, kömürleri kendiliğinden yanmaya olan yatkınlıklanna göre sınıflandırmak ve özellikle teorik modelin doğruluğunu araştırmak amacıyla deneysel bir çalışma yapılmıştır. Kullanılan deney düzeneği, esas olarak 3 m uzunluğunda ve 0.3 m çapında silindirik bir kanaldan ibarettir. Deneylerde, bu kanal içerisine yerleştirilen kömür örnekleri hava akısına maruz bırakılarak, zaman ve konuma bağlı sıcaklık artışları ve reaksiyon sonucu tüketilen oksijen miktarları belirlenmiştir. Çan, Tunçbilek, Seyitömer ve Gediz kömür örnekleri üzerinde yapılan deneyler sonucunda kömür cinsinin, kömür parçacık boyutunun ve hava debisinin kendiliğinden yanma olayı üzerindeki etkileri irdelenmiştir. Deney sonuçlan, kömür- oksijen reaksiyonu için kinetik parametrelerin belirlenmesi amacıyla ayrıca kullanılmıştır. Böylece, kinetik parametreler göz önüne alman değişkenlere bağlı olarak ayrı ayrı belirlenmiştir. Buharlaşma/yoğuşma olayları ile ilgili kinetik parametrelerin belirlenmesi amacıyla ayrı bir deneysel çalışma yapılmıştır. Bu çalışmada, sıcaklığı ve nemi kontrol edilebilen bir firm ortamında tutulan kömür örneklerindeki zamana bağlı kütle değişimleri tespit edilmiştir. Eîde edilen verilere göre, sözü edilen her kömür için buharlaşma ile ilgili izoterm eğrileri ve buharlaşma hız sabitleri belirlenmiştir. Model eşitlikleri sonlu farklar metodu uygulanarak iteratif olarak çözülmüştür. Elde edilen sonuçlar sıcaklığın, oksijenin, kömür neminin ve kömürün ısıl değer kaybının zaman ve/veya konuma bağlı değişimleri olarak verilmiştir. İncelemelerde gaz hızı, kömür cinsi, yığının sıkılık oranı, kömür başlangıç sıcaklığı ve parçacık boyutu değişken olarak ele alınmış ve böylece kendiliğinden tutuşma olayının oluşumu ve oluşum süreleri belirlenmiştir. Ayrıca, modele ilaveler yapılarak göz önüne alınan kömürlerin açık yığınları için kritik yığın yükseklikleri de belirlenmiştir. Bu yükseklikler, üç aylık güvenli bir depolama süresi esas alınarak, Çan kömürü için 0.85 m, Tunçbilek kömürü için 1.2 m ve Seyitömer ve Gediz kömürleri için ise 1.8 m olarak bulunmuştur. XVI
Özet (Çeviri)
THEORETICAL AND EXPERIMENTAL INVESTIGATIONS OF SPONTANEOUS COMBUSTION OF COALS SUMMARY The spontaneous combustion of coal stockpiles have been investigated theoretically and experimentally in this study. Mathematical modelling of spontaneous combustion is based on the formulation of heat and mass transfer within the porous media of chemically reactive materials. So, the one-dimensional non-steady-state model comprises conservation equations for oxygen and energy for both gaseous and solid phases, water vapour for gaseous phase and inherent moisture of the coal. These coupled equations include particular terms of the accumulation, the flux terms of convection and difiusion or conduction, source terms with respect to oxidation and evaporation (or condensation) and also heat exchange between the solid and gas phases. A first-order Arrhenius reaction rate for oxidation under the pore difiusionally and chemically controlled reaction regime is considered. Evaporation (or condensation) rate is defined by considering the moisture adsorption/desorption properties of coals, involving both equilibrium and kinetic parameters. Preliminary steps in quantitative analysis of spontaneous combustion require a knowledge of the kinetic parameters for the coal-oxygen reaction and also adsorption and desorption of moisture. To obtain the necessary input data for the calculations with the model, and to investigate experimentally the process of spontaneous combustion laboratory measurements were conducted using two different experimental setups. First experimental setup was mainly a cylinder of 3 m length and 0.3 m diameter. This was used as a reactor in which the coal samples were oxidized to follow the occurrence of the process of spontaneous combustion. The results obtained in this work were used to determine the kinetic parameters for coal-oxygen reaction and to analyze the validity of the mathematical model. Second setup was an electrical oven which was used to investigate the drying characteristics of coals. 1. Introduction Spontaneous combustion of coal is an important problem in mining, long distance transportation and storage. This is because coal reacts with the oxygen in the air and exothermic reaction occurs even at initially ambient conditions. The heat of reaction accumulates and the reaction becomes progressively faster, the quality of the coal deteriorates and thermal runaway may take place. So the process of spontaneous combustion results not only in a financial loss but also in leading to flaming combustion presenting hazardous situations. xvuIt is for these reasons, the phenomenon of spontaneous combustion of coal has been of fundamental and practical importance to scientists. Several theoretical and experimental studies have been carried on the spontaneous combustion. But, experimental studies compose the largest segments. The main purposes of these studies were to develop methods for determining the conditions at which the coal pile could undergo spontaneous combustion, to predict the safe storage time at those conditions, and to determine the influences of factors contributing to the spontaneous heating. In the spontaneous combustion of coal stockpiles the following types of processes play a significant role; 1- Processes causing a heat effect: The most important of these processes are the oxidation of coal by oxygen from the air and the possible catalytic effects on it by water vapour and pyrite; and adsorption/desorption of water due to the difference between real and equilibrium concentrations of coal and air. Both heat of condensation (evaporation) and heat of wetting are involved. 2- Reactant transfer: The most important reactants are oxygen and water. They are transferred by diffusion and convection processes. 3- Heat transfer: Heat produced by the reaction is transferred due to the temperature gradient. Conduction and convection are the responsible mechanisms. Spontaneous combustion of coal in a pile related to the above processes depends on many factors, such as coal rank, temperature, air flow rate, the porosity of the coal pile, moisture content of coal and its changes, particle size of coal. etc. Preliminary steps in making quantitative analysis of spontaneous combustion with a computer model require a knowledge of a large number of coal and gas properties, and kinetic parameters appear in the model equations. These may be divided into there main groups as follows. 1- Thermo-physical properties, such as thermal conductivity, densities, specific heat capacities, diffusivities, etc. 2- Kinetic parameters for coal-oxygen reaction, mainly associated reaction rate and its heat effects. 3- Moisture adsorption/desorption properties, involving both equilibrium and kinetic parameters. These parameters must be obtained by methods which ensure realistic and representative data for full-scale storage conditions. In fact various experimental techniques have been used for the determination of oxidation rates of coal from the analysis of oxygen consumption or heat release rates at low temperatures and investigation of the liability of coals to spontaneous heating. But, studies have been mainly performed with small-scale test apparatus. So, the results obtained should be xvmextrapolated to the actual coal pile before use because the process depends on oxidation conditions. There have been several theoretical studies of spontaneous combustion. Among these, only a few studies have been made, dealing with quantitative analysis of moisture and particle size effects although it is well known that these parameters play very significant role on the spontaneous combustion. In fact, the influences of moisture and particle size on the process are very complex. It is generally accepted that a difference between the humidity of surrounding air and the moisture content of coal results in further temperature changes depending on the dynamic and equilibrium moisture characteristics. To evaluate the general effect of particle size on combustion, the reaction of coal particle with oxygen can be divided into three regimes depending on the coal reactivity, reaction temperature, particle size and porosity of the coal. These comprise the chemically controlled, chemically and pore diffusionally controlled and external diffusionally controlled regimes. In many modelling attempts in literature, the chemically controlled reaction regime has been considered in which the reaction rate does not depend on the particle size. In this Ph.D. thesis, theoretical and experimental studies have been performed. The main differences of mathematical model comparing with those in literature are: oxidation rate was defined by considering chemically and pore diffusionally controlled regime; evaporation/condensation rate depends on coal and gas phase moisture contents and temperature. This model can be used for different purposes which are to make a parametric investigation, to determine the conditions of the flaming combustion, and also to predict the safe storage time of coal piles. All the kinetic parameters were determined experimentally. Another purpose of the experimental studies was to demonstrate the spontaneous heating behaviour of the coals in a large- scale experimental apparatus. The mathematical model was also verified by the experiments. 2. Experimental Studies Oxidation test: The aims of this study are: To demonstrate the spontaneous heating behaviour of coals To determine kinetic parameters for the coal-oxygen reaction To verify the one-dimensional mathematical model A cylindrical container of 3 m long, 0.3 m in diameter was used as the reactor. It was held at a slope of 0.5. To reduce the heat loss, the external surfaces were insulated with two layers of glass wool, each of 10 cm thickness. A heater was wound around the aluminum plate placed cylindrically between the glass wool layers, to minimize heat loss to the atmosphere. This heater operated by a differential temperature controller. This controller controls the temperature differences on the XIXsurfaces of the cylinder and the aluminum plate. In this way, it was possible to approach adiabatic conditions within the first insulation layer. The temperature was uniform along the length of the cylinder. This was because of the higher thermal conductivity of the 3 mm thick steel cylinder and the slow rise in temperature of the coal. For this reason, only one heater was necessary to minimize the heat loss from the cylinder surface to the atmosphere. Another heater was used at the inlet of the reactor to shorten the experiment time. Controllable air flow passed through the three-meter long coal bed. Air was provided by a fan. Coal samples were obtained freshly from coal mines of Çan, Tunçbilek, Seyitömer and Gediz. The samples were moved to the laboratory in well-sealed nylon sacks. In the laboratory, various particle size fractions were obtained by sieving in the ranges of 2-5, 5-10, 15-20 and 20-50 mm diameter for Çan samples and 5-10 mm diameter range for the others. The sieved samples were stored in well-sealed sacks until use in the experimental apparatus. During the experiments, inlet air temperature and air flow rate were held constant. Inlet air temperature was 75 °C. Considered air flow rates were 50, 100 and 200 1/h. After the coal samples were filled in the apparatus, the temperature in the coal bed were recorded at 1 1 different positions along the cylinder center line. The outlet gas was sampled intermittently by a gas analyzer. Drying test: This study has been made to determine the kinetic parameters for evaporation. For this purpose, an electrical oven was used as the test apparatus. The temperature and relative humidity of the oven atmosphere were controllable and held constant during the experiments. Çan, Tunçbilek, Seyitömer and Gediz coal samples were tested in the experiments. In each test, 10 gr of coal samples at the particle range of 5- 10 mm diameter was used. During the experiments, weight loss of the samples versus time was measured. The measurements were stopped when the samples were attained to the equilibrium condition in the oven atmosphere. 3. Modelling of Spontaneous Combustion Modelling approach: The complete model involves considerable interaction among the physiochemical properties of coal, heat transfer, oxidant, water vapour and moisture content of coal, the manner in which the coal is stored, and the external environment that interacts with the coal. The present model is based on the following approaches and assumptions. 1- The model is one-dimensional, so heat and mass transfer perpendicular to the direction of the air flow are neglected. However, a heat conduction term in the second XXdirection is added to the model to determine the critical high of coal pile for a certain safe storage time. 2- The coal pile is homogeneous and isotropic with uniform spherical coal particles. 3- It is assumed that the coal density remains constant during the processes of oxidation, evaporation and condensation. 4- Thermal expansion of the heated gas in the bed is neglected. Only forced convection of air is considered. One-dimensional constant-velocity plug flow of air is assumed. 5- The variation of gas flow rate along the coal bed due to oxygen consumption, CO and CO2 production, evaporation and condensation is neglected. 6- Convective heat transfer occurs between the gas and the external surface of the particle. 7- If the coal has enough moisture (in the range of 0.5-8% for maximum rate of overall oxidation), exothermic reaction will exist, owing to the catalytic effect of moisture in the formation of peroxy complexes. The heat due to these reaction is assumed to be added to the heat of oxidation. 8- It is assumed that the heat of wetting comparing with that of condensation is negligible. 9- Oxygen consumption rate, which is described by the Arrhenius equation, is assumed to be first-order with respect to the oxygen concentration. Governing equations The model comprises six governing equations. These are the conservation equations of oxygen, vapour and energy for both the gaseous and solid phases, including particular terms of accumulation, the flux terms of convections and diffusion or conduction, source terms with respect to oxidation and evaporation (or condensation) and also heat exchange between the solid and gas phase. These equation are given below. Oxygen mass conservation in the gas phase: Oxygen mass balance in the coal particle: Moisture conservation in the gas phase: U-cO- Z-+V-T- =Diw., +arw (3) a ax lw ax XXIMoisture conservation in the solid phase: (4) d\ 3 d-^lpgCpg )-ıf+v(pgcP8 )^=4e) xr-Rah(T8 -T°) Energy conservation for the solid phase: apsCps-^-=^- f+-ah(Tg-Ts)-aAHwrw+asAHoPlok (6) Determination of Kinetic Parameters for Oxidation: The experimental measurements provide an approximate kinetic interpretation for the coal-oxygen reaction which is based upon the oxygen consumption rate. For this purpose, the oxygen conservation equation along the coal bed in the three-meter experimental apparatus is written neglecting the local accumulation and diflusion of oxygen as follows. Q*g-| +.*-. CO The first term represents the convection of oxygen and the second term accounts for the rate of oxygen consumption. The rate, ro, is first order in oxygen concentration of the bulk gas and is proportional to the n* power of the particle size. It obeys the Arrhenius equation: Mr f E^ Mc v RTj ro = Pg TT~ y Ao d° exP -^ (8) with the following parameter and substitution of Eq. (8) into Eq. (7) Y = i, x * A = A"d\ B-ij£, T-| ya L v «. the oxygen conservation equation is ^ + BYexp(-l)=0 (9) XX11Experimental measurements show that the temperature along the coal bed is approximately uniform so the weighted average temperature in the bed allows of integration of the above Eq. (9). By integrating with the boundary conditions (X=0; Y=l), the dimensionless oxygen concentration can be obtain as follows: Y(X) = exp (10) By taking twice the logarithm of Eq.(10) at X=l, the following equation is obtained: ln(a) = ln(B) - 1 (11) where, a = -ln[Y(l)] By fitting the experimental data to Eq.(ll), apparent activation energies and pre- exponential factors have been calculated. Using these kinetic parameters, reaction rate constant is defined traditionally as follows: (12) Determination of effectiveness factor for oxidation: In many attempts to model spontaneous heating, it has been assumed that oxidation occurs in the extreme regimes where the rate is either independent on particle size, in which case there is chemical control without pore diffusion restriction, or inversely proportional to the particle size, which assumes a non-porous particle. It should be noted that the oxidation rate for very porous coal particles with smaller sizes may be chemically controlled at relatively low temperatures. This is attributed to a deeper penetration of oxygen to the particle interior with a constant concentration. But the oxidation regime changes from chemical control to chemical and pore diffusion control at increasingly higher temperatures, in which an oxygen concentration profile occurs within the particle. In such a case, the effect of particle size on the overall reaction rate becomes significant. The dependence of the overall oxidation rate on particle size has been described by defining an effectiveness factor which is the ratio of the actual to the maximum volumetric oxidation rate without pore diffusion restriction as follows: U2o 'dP2o dr R p^k 8 = ^- ' r=R (13) xxuxDetermination of evaporation/ condensation rate: Evaporation (or condensation) rate depends on the dynamic and the equilibrium moisture characteristics of coal. It is assumed that the rate conforms to the following equation: rw = ps|*-Kw(w*-w) (14) This implies determination of both kinetic rate coefficient and the equilibrium relationships between the moisture contents of coal and gas phase. Experimental measurements in drying tests have been used to determine these parameters. The procedure has been summarized below. By taking into account the Eq.(4) and Eq.(14), the variation of moisture content with time can be written as follows. aw di '' :-Kw(w*-W) (15) By integrating this equation with the initial conditions, W(o)=Wo, the following equation can be obtained. ln(f) = Kwt (16) W* -W where; f = W*-W0 The functional dependence of the equilibrium moisture content of the tested coals on the relative humidity of air has been obtained as the following equation: W^q-^-y (17) C2-q> By fitting the experimental data to the Eq.(16), Kw values for each coals have been calculated. Solution procedure: The equation system includes the terms of time derivatives, and first-order and second-order of position derivatives. For this reason, a numerical method is necessary to solve the differential equations of the model. In this study, the model equation were written in the form of finite difference. These were solved with a computer program written in FORTRAN language. In the solution, Newton-Raphson iterative technique has been used. XXIV4. Conclusion A mathematical model was developed describing the spontaneous combustion of coal stockpiles. A comparison of the results of the calculations with those of experimental measurements shows that this model in its present form provides a good description of the process. It can therefore be used for different purposes. These are: to make a parametric analysis for the determination of dominant parameters contributing to the spontaneous heating, to determine the conditions of the flaming combustion and to predict the safe storage time and conditions. Theoretical and/or experimental results show that the gas flow rate, moisture content of coal, coal reactivity, particle size of coal, porosity and height of the coal pile are very significant parameters in determining whether or not thermal runaway will occur in a certain time. It has been found that there are three different stages during the spontaneous heating of moist coals. In the first stage, the heat generated by the oxidation brings the temperature to about 50 °C before moisture evaporation starts to affect the heating significantly. In the second stage, a relatively large quantity of water is removed by air stream while the temperature rises from the 50 °C to 85 °C. During this period, the temperature may gradually level off at a certain temperature level depending on coal reactivity, gas flow rate and inherent moisture content of coal. Thus, drying process may delay significantly the thermal runaway because the temperature of coal in this stage has not gone above 85 °C until the most of the water has been removed. In the third stage, the temperature begins to increase rapidly due to the coal becoming dry. If the coal is relatively dry, the temperature increases exponentially starting from the ambient conditions without any delaying stages. The results show that the losses of calorific value of coals during the spontaneous combustion may reach to 30% depending on coal reactivity and oxygen supply. It was also found that the most important parameter to prevent the occurrence of flaming combustion at a certain time is height of the coal pile. For this purpose, critical heights for a three-months safe storage time have been determined which are 0.85 m for Çan coal, 1.2 m for Tunçbilek coal, 1.8 m for both Seyitömer and Gediz coals. XXV
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