MODELING OF BIOMASS GASIFICATION Venko Petkov, Emil Mihailov, Nadezhda Kazakova

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1 Journal Journal of Chemical of Technology and and Metallurgy, 9, 9, 1, 01, 1, MDELING F BIMASS GASIFICATIN enko etkov, Emil Mihailov, Nadezhda azakova Department of hysical Metallurgy and Thermal Equipment University of Chemical Technology and Metallurgy 8 l. hridski, 1756 Sofia, Bulgaria venko@uctm.edu Received 30 July 013 Accepted 5 November 013 ABSTRACT ptimal conversion of chemical energy of the biomass or other solid fuel into the desired gas depends on proper configuration, sizing, and choice of gasifier operating conditions. ptimum operating conditions are often derived through trials on the unit or by experiments on pilot plants. Simulation, or mathematical modeling, allows the designer or plant engineer to reasonably optimize the operation or design of the plant. The good mathematical model can: find optimum operating conditions or a design for the gasifier, provide information on extreme operating conditions (high temperature, high pressure) where experiments are difficult to perform, provide information over a much wider range of conditions than those obtained experimentally, better interpret experimental results and analyze abnormal behavior of a gasifier, if that occurs. The equilibrium model is independent of the gasifier design which can make them more suitable for a system study of the most important process parameters. The use of an equilibrium model assumes that the residence time of the reactants in the gasifier is high enough to reach chemical equilibrium. For established biomass ultimate analysis, temperature of gasification air and temperature of produced gas, combining the mass balance equations with the equations for the equilibrium constants and equation of energy balance, the equivalence ratio (ER) and composition of produced gas can be obtained. A mathematical model for investigation of the influence of temperature of the produced gas and temperature of gasification air on the process parameters was developed. It can be used for estimation and design of gasification equipment. eywords: gasification, biomass, mathematical modeling, energy balance. INTRDUCTIN ptimal conversion of chemical energy of the biomass or other solid fuel into the desired gas depends on proper configuration, sizing, and choice of gasifier operating conditions. ptimum operating conditions are often derived through trials on the unit or by experiments on pilot plants. There is, however, one major limitation with experimental data. If one of the variables of the original process changes, the optimum operating condition chosen from the specific experimental condition is no longer valid. The right choice between experiment and modeling, then, is necessary for a reliable design. Simulation, or mathematical modeling, allows the designer or plant engineer to optimize reasonably the operation or the design of the plant. The good mathematical model can [1]: Find optimum operating conditions or a design for the gasifier; rovide information on extreme operating conditions (high temperature, high pressure) where experiments are difficult to perform; rovide information over a much wider range of conditions than those obtained experimentally; Better interpret experimental results and analyze abnormal behavior of a gasifier, if that occurs. Gasifier simulation models may be classified into the following groups: 9

2 enko etkov, Emil Mihailov, Nadezhda azakova Thermodynamic equilibrium models; inetic models. Thermodynamic Equilibrium Models Biomass contains a large number of complex organic compounds, moisture (W), and a small amount of inorganic impurities known as ash (ASH). A typical ultimate analysis is C + H + + N + S + ASH + W = % (1) Here, C, H,, N and S are the mass percentages of carbon, hydrogen, oxygen, nitrogen, and sulfur, respectively, in the fuel. Not all fuels contain all of these elements. For example, the vast majority of biomass may not contain any sulfur. The moisture or water in the fuel is expressed separately as W. Thus, hydrogen or oxygen in the ultimate analysis does not include the hydrogen and oxygen in the moisture, but only the hydrogen and oxygen present in the organic components of the fuel. The equilibrium model is independent of the gasifier design which can make them more suitable for a system study of the most important process parameters []. The use of an equilibrium model assumes that the residence time of the reactants in the gasifier is high enough to reach chemical equilibrium [3]. The calculations assume that the main product gases,, H, H, CH and char are at equilibrium. Nitrogen is considered inert. Formation of tars is neglected although it has to be taken into account during plant operation []. All gases are assumed to behave as ideal gases. Chemical equilibrium is determined by either of the following: The equilibrium constant; Minimization of the Gibbs free energy. In a combustor, the amount of air supplied is basically determined by the stoichiometry, which depends on the fuel composition, and excess air requirement. In a gasifier on the other hand, the air supply is only a fraction of the stoichiometric rate. The term equivalence ratio (ER) is often used in connection with gasifier air supply. Equivalence ratio is defined as the ratio of actual air fuel L ratio to the stoichiometric air fuel ratio L o. The quality of gas obtained from a gasifier depends strongly on the value of (ER) employed, which must be significantly below 1.0 to ensure a condition far from complete combustion. An excessively low value of (ER) (0.) results in several problems including incomplete gasification, excessive char formation and low heating value of the product gas. n the other hand, too high a value of (ER) (0.) results in excessive formation of products of complete combustion, such as and H at the expense of desirable products like and H. This causes a decrease in the heating value of the gas. In practical gasification systems, the value of (ER) is normally maintained at 0,0 to 0,30 [5]. Noting that dry air contains 1 % oxygen, 79 % nitrogen, by volume, the dry air required for complete combustion of a unit mass L o, is given by: L o = (1+ 3,76).(0,01867.C + 0,056.H-0,007.) () Gasification involves a series of endothermic reactions supported by the heat produced from the combustion reaction described above. Gasification yields combustible gases such as hydrogen, carbon monoxide, methane and char through a series of reactions. The following are four major gasification reactions [5]: 1. Water gas reaction.. Boudouard reaction. 3. Methanation. Brief descriptions of these reactions are given below. Water gas reaction is the partial oxidation of carbon by steam, which could come from a host of different sources, such as water vapor associated with the incoming air, vapor produced from the evaporation of water, and pyrolysis of the solid fuel. Steam reacts with the hot carbon according to the heterogeneous water gas reaction: C + H = H , 38 kj.(mol carbon) -1 (3) In some gasifiers, steam is supplied as the gasification medium with or without air or oxygen. The carbon dioxide present in the gasifier reacts with char to produce according to the following endothermic reaction, which is known as the Boudouard reaction: + C = -17,58 kj.(mol carbon) -1 () Methane could also form in the gasifier through the following overall reaction: 95

3 Journal of Chemical Technology and Metallurgy, 9, 1, 01 C + H = CH + 7, 90 kj.(mol carbon) -1 (5) The composition of the gas obtained from a gasifier depends on a number of parameters such as: fuel composition; gasifying medium; operating pressure; temperature; moisture content of the fuels; mode of bringing the reactants into contact inside the gasifier, etc. It is very difficult to predict the exact composition of the gas from a gasifier. Consideration of chemical equilibrium of the components of the gas often provides a useful insight into understanding of the performance of a gasifier through experimental research. However, it is found [6] that the gasifier products do vary from their equilibrium values. In any case, the chemical equilibrium gives good starting values. Chemical equilibrium requires that at each temperature, there is an equilibrium constant for each reaction. Given sufficient time the concentration of these gases will reach their equilibrium concentrations. From a thermodynamic point of view, the equilibrium state gives the maximum conversion for a given reaction condition. An equilibrium model is effective at higher temperatures, where it can show useful trends in operating parameter variations [7]. For equilibrium modeling, one may use stoichiometric or nonstoichiometric methods [8]. Estimating equilibrium gas composition The product gas of gasification is generally a mixture of several gases, including moisture or steam. Given sufficient time the concentration of these gases will reach their equilibrium concentrations expressed in respective partial pressures, i i = g where: is pressure of the reactor; i - H H,,,, CH, N volume of product gases, m 3 (kg biomass) -1 ; g = + H m 3 kg -1. H CH N In the stoichiometric method, the model incorporates the chemical reactions and species involved. It usually starts by selecting all species containing C, H, and, or any other dominant elements. If other elements form a minor part of the product gas, they are often neglected [5]. Let L = (ER)L0 actual air fuel denote the air supply in (m 3 dry air).(kg dry fuel) -1. (C) is the carbon content of the fuel, (kg carbon).(kg dry fuel) -1. Carbon balance Carbon is split between,, CH and char C 1,867C = + + CH + (6) where i is represents the volume of a constituent of the gas; C - (kg char).(kg dry fuel) -1 [1]. Hydrogen balance Let W represents the moisture content of fuel, (kg water).(kg dry fuel) H + 1.W + (ER)Lo. 0,001gH = = H + + (7) H CH where: H - hydrogen content of the fuel (kg hydrogen). (kg dry fuel) -1 g - humidity associated with air, g.(m dry air) -3. H balance If is the oxygen content of the fuel (kg oxygen). (kg dry fuel) -1, then we write the molar balance of as follows: 0,7. + 0,63.W + (ER)L ( /18.0,001g o H ) = = 0,5( + H ) + (8) N balance If N is the nitrogen content of the fuel (kg nitrogen). (kg dry fuel) -1, the molar balance of N gives: 0,8. N ( ER). L N + = (9) For a gasifier pressure,, the equilibrium constants 96

4 enko etkov, Emil Mihailov, Nadezhda azakova for reactions (3), (), and (5) are given by: R1 R = (10). = (11) CH R3 H H H = (1) (ER) ER Fig. 1. Influence of the temperature of the produced gas and temperature of gasification air on the equivalence ratio To estimate the values of the seven unknowns: H, H,,, CH N and C we need a total of seven equations from (6) to (1). Energy Balance An energy balance, based on the 1 kg dry fuel, for the process can be described by the following equation: LHg, kj/m 3 LHg LH + c ( ER) L T = LH + c T + Q (13) B A 0 A g g g g g where: c A is specific head capacity of air, J.m ; c g - specific head capacity of product gas, J.m -3-1 ; LH B - biomass low heating value in kj.kg -1 ; T A - temperature of gasification air o C; T g - temperature of produced gas o C; LH g - produced gas low heating value in kj.n -1.m -3. LH B = 39.1C H +.5S ,-15,1N-1,1Ash -,6(9H + W) kj kg -1,[9] (1) LHg = ( CH ) H kj m -3, [9] (15) n the left hand side of the equation, the two terms describes the total heat of formation and the enthalpy for air stream. The two first terms on the right hand side describes the total heat of formation and the total enthalpy for all product species. The heat loss of the system to the surroundings is denoted by Q. For established biomass ultimate analysis, temperature of gasification air and temperature of produced gas, combining the mass balance equations (6), (7), (8) and (9) with the equations for the equilibrium constants, (10), (11), (1), and equation of energy balance (13), the equivalence ratio (ER) and composition of produced gas can be obtained. Fig.. Influence of the temperature of the produced gas and temperature of gasification air on the low heat value of the gas (LHg). H, % H Fig. 3. Influence of the temperature of the produced gas of hydrogen. CH, % Fig.. Influence of the temperature of the produced gas of methane. CH

5 Journal of Chemical Technology and Metallurgy, 9, 1, 01, % Fig. 5. Influence of the temperature of the produced gas of carbon oxide. RESULTS AND DISCUSSIN The model of the gasification has been studied with different parameter values to evaluate its performance. This has been done by using rogram NAD. Two parameters have been studied; T A - temperature of gasification air and T g - temperature of produced gas to equivalence ratio (ER) and composition of produced gas. The temperatures of gasification air were, 00,, and C. The temperature of produced gas were 650, 750, 850, 950 and 1050 o C The composition for the fuel used in the study was (component value in % mass): C (, %), H (5,1 %), (35,7 %), N (0,3 %), Ash (1,35 %), W (15 %) LH B = 1537, kj m L = 3,9, m 3 (kg dry fuel) -1 Figs. 1 to 5 show the influence of the temperature of the produced gas and temperature of gasification air on the equivalence ratio, low heat value of the gas (LHg), volume percents of carbon oxide, hydrogen and methane NCLUSINS A mathematical model for investigation of the influence of temperature of the produced gas and temperature of gasification air on the process parameters was developed. It can be used for estimation and design of gasification equipment. REFERENCES 1.. Basu, Biomass Gasification and yrolysis ractical Design and Theory, Elsevier, G. Schuster, G. Loffler,. Weigl, H. Hofbauer, Biomass steam gasification - an extensive parametric modeling study, Bioresource technology, 77, 1, 001, A. Marbe, New pportunities and System Consequences for Biomass Integrated Gasification Technology in CH, hd Thesis, Chalmers University of Technology, A. Melgar, J. érez, H. Laget, A. Horillo, Thermochemical equilibrium modelling of a gasifying process, Energy Conversion and Management, 8, 1, 007, B. rabir, Combustion and gasification fluidized beds, Taylor & Francis Group, LLC, X.T. Li, J.R. Grace, A.. Watkinson, C.J Lim, A. Ergdenler, Equilibrium modeling of gasification: a free energy minimization approach and its application to a circulating fluidized bed coal gasifier, Fuel, 80,, 001, C.R. Altafini,.R. Wander, R.M. Barreto, rediction of the working parameters of a wood waste gasifier through an equilibrium model, Energy Conversion and Management,, 17, 003, Basu, Combustion and Gasification in Fluidized Beds. Taylor & Francis, 006, S.A. Channiwala,.. arikh, A unified correlation for estimating HH of solid, liquid and gaseous fuels, Fuel, 81, 00,