Dev. Chem. Eng. Mineral Process. 14(1/2), pp. 259-276, 2006. CFD Modeling of Biomass Combustion in Palm Oil Waste Boiler Hong-ming Yan* and Teck-poh Lai Chemical Engineering, School of Engineering and Science Curtin University of Technology, Sarawak Campus, Miri, Malaysia A computational fluid dynamics model (using CFX) has been developed to simulate the hydrodynamic behavior of the fluid and the combustion process in the boiler that is utilized in palm oil mills in Malaysia. The predicted gas temperature from the model is quite close to that measured in the boiler. The temperature of the inlet air has played an important role in the improvement of the boiler operation and in the control of the combustion process. The temperature profile in the combustion chamber is not uniform and a very high temperature zone has been found. The baffles inside the boiler have greatly enhanced the convection heat transfer. Further refinement of the model, to include the combustion kinetics for palm fiber fuel and modeling of the heat transfer inside the boiler, will provide more accurate predictions. Introduction Malaysia is a leading nation in oil palm production, being the largest producer and exporter of palm oil in the world. The total annual exports of palm oil and oil palm products are about RM 22 billion (AustS7.86 billion). Malaysia also generates about 9 million tonnes of empty fruit bunches (EFB), 8.0 million tonnes of fiber, and 3 million tonnes of palm shell every year as wastes [ 11. EFB is not considered for fuel uses because of its h g h moisture content (65%). Palm oil mills generally have excess fiber and shell, which are not used and have to be disposed off separately. There are more than 270 palm oil mills operating in Malaysia that utilize mainly palm fiber and partly palm shell in their boilers as fuel [ 1,2]. In most palm oil mills, the boiler used is the model “Vickers Babcock TW16” and its design is based on 1970s technology [3]. Its primary use is to generate steam for electricity generation from a steam turbine and also the sterilization of palm fruits in a sterilizer. The boiler operates at 21 bar gauge, and produces 21 tonnes of steam per hour at saturation temperature. The combustion system is the air swept spout-firing system and is a semi-fluidized system. In normal operation, palm fiber and palm shell fed into the boiler are 6.5 tonnes and 2.25 tonnes per hour respectively. Percentage of excess air used is about 57%. Feed water (about 99°C at 1.3 bar) is pumped into the steam drum by an electric pump, flows down via the downcomer pipes, and circulates via the risers back to the steam drum. During this process, steam is generated via heat ~~ * Author for correspondence @an.hong.ming@curtin.edu.my). 259 Hong-ming Yan and Teck-poh Lai exchanger inside the boiler. Before leaving the steam drum and hence the boiler, the generated steam at saturated temperature goes through the steam separator where liquid water is separated mechanically via baffles and perforated plates. Then, it goes to a non-condensing turbine for power generation, hence there is no condensate return to the boiler. Meanwhile, the flue gas passes through a cyclone before being released to atmosphere. No air preheater or steam superheater are installed in the process. In general, the boiler is divided into two parts; namely, combustion chamber and convection chamber. In the combustion chamber, there are three feed stages located in the front side. At the bottom part of the feed stages, there are six secondary air holes. Primary air is fed from the bottom of the combustion chamber. There are also six secondary air holes on the rear side of the combustion chamber. These secondary air holes will help not only to complete the combustion process, but also to create turbulence flow inside the chamber. There are tubes which contain water mounted on the wall of the combustion chamber. There is a baffle located between the combustion chamber and convection chamber. In the convection chamber, there are two baffles located on the upstream and downstream of the chamber. The heat transfer mainly occurs in this region by means of convection heat transfer. Tubes are connected from the top to the bottom of the water drum through the baffles. The present utilization of palm wastes (palm fiber and shell) as fuel in the boiler is creating serious emission problem in the industry. Kawser and Farid [ 11 showed that boilers, using palm waste as fuel at palm oil mills, appear to produce high levels of dust emissions of up to 11.6 g NmJ, compared to the allowable limit of 0.4 g Nm-3 thus causing serious environmental and health problems (black smoke). In addition, further studies showed that numerous problems have been encountered in the operation of the boilers, including salt formation in the tube interior and water carryover in the generated steam, and asWslag formation on the tube exterior [4]. Thus, improvement and optimization of the boiler operation has become a viable tool to overcome these problems. The aim is to present a simulation study for boilers using a computational fluid dynamics approach. Model Development (i) Transport Equations In CFX 5 [5], the transport equations solved are the Navier-Stokes equations with additional ancillary equations such as the Reynolds stress equation. In order to solve for these equations, the K-E turbulence model has to be introduced, which is the standard industry turbulence model. The following governing equations are used in CFXS. The continuity equation: -aP -+V.(pU) at 260 =0 CFD Modeling of Biomass Combustion in Palm Oil Waste Boiler The momentum equation: ...(2) The energy equation: --aph'o' 'a at at +V (pUhl(,,) = V (RV T ) + S, ... (3) where h,,, is given by: I h,,,,= h + -U 2 . .(4) * and h is a function of temperature and pressure. (ii) Reynolds Averaged Navier Stokes (RANS) Equation Turbulent flow occurs inside the boiler where it will enhance the heat transfer and mixing process, therefore, the transport equations are solved with the turbulence model, However, turbulent flow will produce a flow field, whch is chaotic, and a highly fluctuating instantaneous velocity. In fact, a three-dimensional flow field is generated and the flow is dependent on an unknown time function [6].Therefore, the transport equations above are not solved directly. In order to consider the turbulent behavior, the Reynolds Averaged Navier-Stokes (RANS) equation is introduced. RANS is applied to solve the model because the averaged and fluctuating components are used to solve the transport equations for turbulent flow. This has good agreement as suggested by Reynolds [ 6 ] .The average component is then given by: u- = I --/,I+& At Udt ...( 5 ) where At is a time scale that is large relative to turbulent fluctuation, but small relative to the time scale for which the equations are solved. Substituting the averaged component into the above transport equations, the Reynolds averaged equations are then given as: ap v .(pU) =0 at ,. apu f V . ( p u ~ u ) = V . ( o - p u ) + S , ~ .. f at 261 Hong-ming Yan and Teck-poh Lai where pu @ u is the Reynolds stress, which is the non-linear convective term in the non-averaged equations. aP -+ V *(UhIo,+ p u-h - A V T ) = at at ...(8) and h,,, is then given by: I h,,,,= h + - U ' + k 2 where k is the turbulence kinetic energy given by: As the model involves solving for multicomponent fluid flow (N2,CO, COz, O2 and CH4), the above equations are further expanded. Multicomponent fluid is a mixture of fluid where the mixture consists of different components (species) each with their own physical properties. When solving the equations, the average property of the fluid, whch is mainly governed by the composition and proportion of each component, is used. In CFXS, when calculating the property of a component which contains more than one material, an assumption is made so that the constituent materials can form an ideal mixture. The materials are assumed to be mixed at the molecular level. When solving the transport equations for a multicomponent fluid, the flow of the species is transported and difhsed through the fluid. Hence, in order to solve for multicomponent fluid flow, the density in the above equations has an important role. The variation of density greatly affects conservation of the mass balance. From knowledge of temperature and pressure for the mixture, the density of the mixture is given by: where Y, is the mass fraction of each component, and p, is the material density of each component. To solve for the conservation of energy for a multicomponent fluid, an additional diffusion term is added to Equation (1 1) as given by: 262 CFD Modeling of Biomass Combustion in Palm Oil Waste Boiler The conservation of energy equation for a multicomponent fluid is then expanded to: The above transport equations have introduced additional ancillary equations such as the Reynolds stress equation. In order to solve these equations, a turbulence model has to be introduced. (iii) Turbulence Model The K-E model [7, 81 is the standard industry turbulence model where K is the turbulence kinetic energy and E is the turbulence eddy dissipation. In this model, the transport equations consist of the continuity equation (1) and the momentum equation: 92 +v at ( p U 8U )- v ( p U , V U ) .. = V p ’i- V ~ ( , u ~ ~ V iUB) ’ where B is the sum of body forces; peg is the effective viscosity; and p ’ is the modified pressure given by: 2 and peg is given by: Pc,,= P +- P! .. .( 16) where p, is the turbulence viscosity. In the K - E turbulence model, the turbulent viscosity is a function of K and E. Thus, p, is then given by: K- P, = C,P- . .( 17) & where C, is a constant with a value of 0.09. 2 63 Hang-ming Yan and Teck-poh Lai The turbulent kinetic energy and turbulence dissipation rate are then defined as: where CEI,Cc2,6,and 0, are constants. (iv) Combustion Model To model the combustion process inside the boiler, an eddy dissipation model is used for the combustion model due to its simplicity and robustness [8,91. Moreover, the eddy dissipation model is suitable for a wide range of turbulent reacting flows. The transport equation used in CFX 5 to model the combustion process is given by: where the term S,is due to the chemical reaction rate involving component i. The chemical reaction can be expressed as follows: ?=Ail r=A.il ..(21) where k is the elementary reactions involving component i, and Vki is the stoichiometric coefficient for a component in the elementary reaction k. As Si is the rate of production of component i, thus, Si can be expressed as: where R k is the elementary reaction rate of progress for reaction k, and is calculated in the eddy dissipation model in CFX5. The eddy dissipation model utilizes the concept that the reactants are mixed at the molecular level and react instantly to produce the product, where the chemical reaction is relatively fast compared to the transport process in the flow. That is “when it is mixed, then it is burnt”. In this model, the reaction rate is assumed proportional to the mixing time, where: & rate a k and k is the turbulence kinetic energy and E is the dissipation. 264 . . .(23) CFD Modeling of Biomass Combustion in Palm Oil Waste Boiler Model Assumption (0 Geometry of the Model I Main Air Figure I . Geometry of the model. A detailed geometry of the model was not created due to limitations of the available computing hardware. Thus, all tubes in the boiler have been omitted while retaining the basic structure of the boiler (including baffles). The simplified geometry for the model is shown in Figure 1. The boiler consists of two chambers, namely the convection chamber and combustion chamber. The fuel (palm fiber) is fed into the boiler through three feed ducts connected to the side of the combustion chamber. The feed ducts are named as the first, the middle and the third, respectively, according to the sequence from the rear to the front. The air is fed into the boiler via three sets of inlets. The primary air inlet is located at the bottom of the combustion chamber, but the secondary air is fed into the boiler through both the inlets above the feed ducts and on the rear side of the combustion chamber. More detailed information concerning the three-dimensional geometry of the boiler model is available [ 101. (ii) Primary Air Inlet Duct Size The primary air inlet duct size is designed with a 5 mm hole diameter, and a distance of 20 mm between the centers of each hole. With an overall boiler size of 4600 mm x 3074 mm, the total number of primary air holes is 35,35 1. To include this number of 265 Hong-ming Yan and Teck-poh Lai holes in the modeling is extremely difficult. Hence, the dimension of the primary air hole is changed while the total area is retained. The diameter of the primary air hole is then changed to 100 mm with a distance of 300 mm from each other. The air hole size has a significant effect on the mixing of fuel and air and, therefore, this will be further compensated for by the ease of mixing of methane gas and air in their molecular state. The momentum of CH4 can also hrther enhance the mixing between fuel and air. (iii) Fuel Palm fiber and shell are wastes produced from the palm oil mills. The proximate and ultimate analyses of the palm fiber are given in Table 1. This version of CFX-5 available cannot simulate the combustion process of solid fuels, hence methane gas (CH,) is chosen in the simulation to infer palm fiber. The amount of CH4 used is based on the heating value of fiber as given by: Mass of CH4 = ...(24) Y(MW,,) HC". where Hfis the heating value of fiber (Jkg); F is the feed rate of fiber (kg/hr); H C H is~ the heating value of methane ( J h o l ) ; and M W C H ~is the molecular weight of methane (kg/kmol). However, a direct use of Equation (24) may produce some significant discrepancies in the final result, as methane gas and fiber are not physically identical. Therefore, some fine-tuning was implemented so that the combustion mechanism can take into account this physical effect. The ash inside the fiber may absorb a fraction of heat, and thus may reduce the total energy released. With this information, the amount of CH4 is further reduced (corresponding to the heat absorbed) so that the temperature rise in the boiler is reasonable with reference to the experimental data. Ash and residue contents of the fiber are also substituted by inert materials such as COz or Nt when modeling the combustion process with CFX-5. Thus, the reduction in CH4, and the addition of COz or N1,are used to compensate for the physical effects of CHI and fiber. The fuel composition used in the simulation is CH4 20%, C02 40%, and H 2 040%. Tablel. Proximate and ultimate analysis ofpalm fiber. Ultimate analysis Hydrogen Carbon Surfur Nitrogen Oxygen (by d$) Ash Total 266 (wt%) 6.00 47.20 0.30 1.40 36.70 8.40 100.00 Proximate analysis (dry) Volatile matter Fixed carbon Ash Total: Heating value (MJkg) (Wt%) 72.80 18.80 8.40 100.00 20.35 CFD Modeling of Biomass Combustion in Palm Oil Waste Boiler Table 2. Feed conditions Feed Fuel Primary air Secondary air Mass jlow rate (kg/hr) 2,414 48,279 20,69 1 Temperature (K) 298 298 298 Pressure (bar g) 1 1 1 (iv) Feed Conditions The total mass flow rate of CHI at the feed inlet is 2414 k g h . The ratio of the excess air is 1.5, the total primary air feed rate and secondary air feed rate are 48279 kglhr and 20691 k g h , respectively. Both fuel and air are fed into the boiler at atmospheric pressure. At the boiler outlet, the pressure approaches zero gauge pressure. The feed conditions are given in Table 2. (v) Meshing CFX-5 provides automatic mesh generation, which includes unstructured triangular surface mesh generation and tetrahedraYprismatic volume mesh generation. There are three modes of volume meshing in CFX-5, including Advancing Front and Inflation (AFI), Paving and Isomeshing, and Volume Mesh Import (VMI) [ 5 ] . Mesh generation is the process by which spatial discretisation of the model is accomplished. By default, CFX-Build meshing is based on trianguladtetrahedral element discretisation, which is used in the current modeling work. In CFX-5, the mesh is prepared in two stages [ 5 ] : Interactive facilities within CFX-Build are used to generate the surface mesh of triangular elements. 0 The volume mesh is generated from the surface mesh during the creation of the GTM file. The most common method for meshing is the AFI method in CFX-5. It is suitable for most geometries. The AFI method is a region-based mesh generation application. This method uses all the current mesh control settings to determine the appropriate size of the mesh in a particular region. In general, the element size is determined by the minimum length scale from all mesh controls, the local length scale from surface mesh parameters and global length scale [ 5 ] . To determine mesh parameters correctly, the maximum edge length for volume mesh elements is specified as its default value of 14. Generally, the maximum edge length is used to provide a maximum background volume mesh scale. The default is a value between 1-5%of the maximum model dimension. Special consideration should be given to the fuel feed region, where mesh control is required for better meshing around that region. When creating mesh control, the constant surface mesh spacing is determined so that it will set the mesh control edge length to a fixed value of 0.6. The expansion factor is defined as 1.2, which determines how fast the mesh edge length scale is expanded from the edge of the radius of influence into the background volume mesh. 267 Hong-ming Yan and Teck-poh Lai (vi) Solution Convergence In CFXS-, the Equations (1)-(23) are solved by the finite volume method. Starting everything from zero, trial and error is required in order to obtain a satisfactory result. It is often difficult to model the reacting flow when compared to the non-reacting flow, Typically, the time required for non-reacting flow is small and usually the number of iterations should be less than 100. However, when the reacting flow is modeled, the complexity not only increases with time but also is very dependent upon the number of iterations. In order to obtain reasonable and accurate results, the root mean square (RMS) value calculated is controlled to less than The smaller the value of RMS, the greater the time required to perform the calculation. However, the physical time step has to be defined so that a robust calculation can be provided. In this case, the time step is 2 seconds as a base value to allow better convergence of the reaction rate. This value can be fixther reduced to achieve better resolution of the simulation conditions. Results and Discussion (0 Temperature Profile Figure 2 shows that after the fuel enters the boiler, the mixing and preheating occurs first at the bottom section. The combustion process then progresses instantly when the mixing process occurs and the gas mixture temperature is increased. This phenomenon can be seen from the temperature profile in Figure 2, that is, the region 1 (approx. 850°C) around the region 2 (approx. 25°C). At the interface between the regions 1 and 2 is where the molecular level of each reactant species interacts with each other and where the combustion process occurs. Note that the region 3 (over 1500°C) is the highest temperature zone. Other regions at different temperatures are due to the cooling process of the feed air and feed fuel, which are not preheated. A significant difference between the temperature profiles at the first and third feed ducts is shown in Figures 2a and 2c, where they should be the same. This is probably due to the inappropriate location of the boiler feed ducts, or because of the flame behavior where the shape and direction are unpredictable. A large region 1 is seen in Figure 2b in the area around the second feed duct because primary feed air is concentrated at the center of the boiler. A relatively high temperature region occurs at the rear side of the combustion chamber near the secondary process air inlet. It is proposed that this effect is caused by the high momentum of the hel. Methane gas then reaches the rear side of the combustion chamber, mixes with air, and combusts without further cooling by the incoming air. However, thls does not apply to biomass where it will be further retarded by the gravitational force before it reaches the rear of the combustion chamber. Combustion gases produced at high temperatures have a lower density compared to the air flowing into the boiler. Due to the variation in density, buoyancy forces result in gases flowing upward [8]. Hot gases that are flowing upward in the boiler will eventually create an induction flow and entrain the surrounding fluids. From these results, the temperature is found to be uniform at the convection chamber (see region 4, approx. 1100 - 1300°C) when compared with the combustion 268 CFD Modeling of Biomass Combustion in Palm Oil Waste Boiler Figure 2 a Firstfeed duct temperature profile. Figure t b . Second feed duct tenipernture projle. [Note: A reprint of this paper with coloured regions shown on Figures 2, 3 and 4 is available directly froin rlie author, HMY, by etnail as a PDFfile.1 269 Hong-ming Yan and Teck-poh Lai Figure 2c. Thirdfeed duct temperature profie. chamber. This is significant, as it ensures uniform heat transfer in the convection chamber. The combustion gas temperature has also dropped from a high temperature (near flame temperature, the region 1) to a temperature of around 120OOC (region 4). This is due to heat exchange between the hot gas and cold feed air and methane gas as previously discussed. After this point, the temperature of the inlet air will have an important influence on the temperature profile in the boiler. Thus, the amount of inlet air is an essential parameter for boiler operation and control of the combustion process. The predicted combustion gas temperature is quite closed to the design temperature (12OOOC). The uniform temperature profile in the convection chamber is because heat transfer between hot gases and the heat exchanger inside the convection chamber is not considered in the model, as mentioned previously. (lo Velocity Profle The velocity profile of the combustion gases in the boiler is shown in Figure 3a. The first secondary air, which is fed into the boiler just above the feed duct, does not contribute significantly to the mixing process. However, the other secondary air fed into the combustor from the opposite side to the feed duct at the lower position has made direct contact with the fuel flowing down from the feed duct, and both are diverted upward by the primary air. Therefore, the presence of the secondary air at the 2 70 CFD Modeling of Biomass Combustion in Palm Oil Waste Boiler lower position does not assist in completion of the combustion process, but rather assists in the direct mixing process together with the primary air. Figure 3b shows the gas streamlines, it can be seen that swirl flows are generated inside the boiler, and circulation is taking place at the bottom of the combustion chamber and convection chamber as well as between the baffles. The vortex (see Figures 3b and 3c) created inside the convection chamber of the boiler is due to the turbulent flow caused by the presence of baffles. Intuitively the circulation generated at the bottom of combustion chamber will improve the mixing of he1 and air. In addition, the flow is entrained inside the combustion chamber rather than causing vigorous mixing [8]. As discussed, the buoyancy generated by the hot combustion gas causes the gas to flow upward to the convection chamber. The gas flows from the combustion chamber to the next chamber by passing through the baffles, and the presence of baffles has enhanced turbulent flow of the hot gases. It is interesting to note that whenever the fluid flows through the edge of the baffles, a circulation region is generated which enhances the heat transfer process. Circulation is caused by the low-pressure generated by the turbulence flow. The vector profile shown in Figure 3c allows a better analysis of the hydrodynamic behavior inside the boiler. It shows that there are four circulation flows generated in the combustion chamber. Based on this flow behavior, once CH4 has entered the combustion chamber, then mixing starts at the lower right comer. The combustion process begins after this mixing, and the flame then moves upward by the buoyancy force of the hot gases. In fact, the flow pattern in the combustion chamber is not a major issue as radiation heat transfer is the dominate heat transfer mechanism. In the convection chamber, the circulation generated at the lower right comer does not contribute significantly to the heat transfer and the air accumulating in this region may act as a cooling medium. In the case of any entrained flow for the biomasses, accumulation of the solid particles may also occur and the heat transfer will be retarded by conduction rather than convection. This may subsequently affect the heat transfer performance in the convection chamber. (iii) CH,Mass Fraction Projile The mass fraction profiles of methane at different feed ducts are shown in Figures 4a and 4b. Overall, the simulated combustion of methane gas in the boiler has not reached completion where the mass fraction of methane gas remaining is about 20%. It appears that the overall performance of the boiler is poor. However, it should be noted that the design of this boiler is not intended for gas combustion but for solid combustion. The incomplete combustion process of methane gas is due to an insufficient amount of oxygen supplied, which is based on the real air flow rates to the boiler as shown in Table 2 . The high fraction of methane gas at the top comer of the combustion chamber is caused by the circulation flow where methane gas is accumulated. This occurs in the first and second feed ducts as shown in Figures 4a and 4b respectively. The primary air at the sidewall causes a large region 1 at the bottom of the feed duct. The region 2 between the regions 1 and 3 is where the fuel and the process air are mixed and the combustion process starts. The region 3 is slowly transformed into the region 4 as more methane gas is combusted. 271 Hong-ming Yan and Teck-poh Lai Figure 3a. Boiler velocity profile. Figure 3b. Boiler streamline plot. 2 72 CFD Modeling of Biomass Combustion in Palm Oil Waste Boiler Figure 3c. Boiler vector plot. Figure 4a. CH,mass fraction profile at first feed duct. 2 73 Hong-ming Yan and Teck-poh Lai Figure 46. CH4mass fraction profile at second feed duct. . 7- Figure 5. CH4 inass fraction versus vertical distance (m) in the combustion chamber. 2 74 CFD Modeling of Biomass Combustion in Palm Oil Waste Boiler There are considerable similarities between Figures 4a and 4b, the second feed duct is not restricted by any space confinement and the region 1 is smaller. Once again, the secondary air above the feed duct has aided the combustion process as methane gas is flowing upward. In Figure 4b, as there is no circulation, the momentum of methane gas whch exceeds the momentum of primary air has created the region 3 at the surface of the primary process air holes. The primary air comes out of the holes, and the mixing process commences instantly. To further analyze the combustion process of methane gas in the boiler, Figure 5 shows the methane mass fraction from the bottom of the combustion chamber to the top, at a position close to the secondary feed stage. Figure 5 shows that at the bottom of the combustion chamber (distance is negative about 1.2 m), the mass fraction of CH4 is zero since there is pure primary process air. As the vertical distance increases, the CH4 mass fraction also increases because pure methane gas is feeding into the chamber. The CH4 mass fraction will never become 1.0 as some CH4 is mixed with the surrounding process air and combusted before reaching the bottom of the boiler. The mass fraction then decreases as methane gas is burned, and eventually the combustion process stops due to: Insufficient molecular mixing between methane gas and process air; Insufficient ignition energy to reach the ignition temperature for methane gas. Conclusions A computational fluid dynamics model (using CFX-5) has been developed to simulate the hydrodynamic flow fields inside the boiler in palm oil mills by including the combustion reaction. The predicted combustion gas temperature from the model is quite close to the design operating temperature of the boiler. The temperature of the inlet air has plays an important role in the operational improvement of the boiler and assists in controlling the combustion process. There is a considerable difference between the temperature profiles at the first and third feed ducts of the boiler. The temperature profile in the combustion chamber is not uniform, there is htgh radiation intensity and radiation is the dominant heat transfer mechanism. However, the temperature profile is uniform in the convection chamber and there is stable heat transfer. The presence of baffles inside the boiler results in the higher velocity of the combustion gases in the convection chamber. The hydrodynamic behavior in the convection chamber may not be satisfactory because tortuous flows, which would enhance heat transfer, are not formed. The CH4 mass fraction profiles showed that the combustion process could not go to completion and at the boiler exit there is still 20% CH4 remaining. Hence, improvements are required to enhance the combustion process, e.g. the boiler should be modeled using solid fuel and including water tubes. Acknowledgments The authors acknowledge Mr Wong Lin Siong, mill manager, at Bintulu Lumber Development Palm Oil Mill, Sarawak, Malaysia, for technical information related to the boiler. Curtin University of Technology (Sarawak) research grant scheme is also acknowledged. 2 75 Hong-ming Yan and Teck-poh Lai Nomenclature k-E turbulence model constant (1 -44) k-E turbulence model constant (1.92) k-E turbulence model constant (0.09) Specific heat capacity at constant pressure (Jkg K) Reynolds stress model constant (1.45) Reynolds stress model constant (1.9) Specific static (thermodynamic) enthalpy (Jkg) Specific total enthalpy (Jkg) Turbulence kinetic energy per unit mass (Jkg) Static thermodynamic pressure (Pa) Modified pressure (Pa) Reynolds number Energy source (Jkg) Momentum source (kg d s ) Static temperature (K) Velocity of vector Uxyz ( d s ) Fluctuating velocity component in turbulent flow ( d s ) Molecular weight, ideal gas fluid mixture (kg/mol) Molecular diffbsion coefficient of component Mass fraction of component i in the fluid Turbulence dissipation rate (m2/s) Molecular (dynamic) viscosity (Pas) Turbulence viscosity (Pa.s) Effective viscosity (Pa.s) Density (kg/m3) Prandtl number k-E turbulence model constant (1 .O) k-E turbulence model constant (1.3) References Kawser, J.M., and Farid Nash, A. 2000. Oil Palm Shell as a Source of Phenol, J. Oil Palm Research, 12(1), 86-94. 2. Ministry of Primary Industries Malaysia, hflpr//www.kpu.gov.my/commo~j~/commo~i~~. htm 3. Vickers Hoskins (M) Sdn Bhd, 2001. htfpt//winv.vickershoskins.com.ni~//Hi./ifnr/ 4. hi, T.P., and Yan, H.M. 2003. Internal Industrial Survey Report on Palm Oil Mills, Chemical Engineering, Curtin University, Sarawak Campus, Malaysia. 5 . CFX-5. CFX International, AEA Technology, Harwell Laboratory, Didcot, Oxfordshire, UK. 6. Baukal, C.E. 2000. The John Zink Combustion Handbook, CRC Press, Florida, USA. 7. Turns, S.R. 2000. An Introduction to Combustion - Concepts and Applications, 2nd Edn, McGrawHill, Singapore. 8. Bonnan, G.L., and Ragland, K. W.I998 Combustion Engineering, McGraw-Hill, Singapore. 9. Fletcher, D.F., Haynes, B.S. Christo, F.C., and Joseph, S.D. 1999. A CFD-based Combustion Model of an Entrained Flow Biomass Gasifier, Applied Mathematical Modeling 24, Australia, pp.165-182. 10. Goh, Y.B., and Yan, H.M. 2003. Creation of Three-Dimensional Geometry Model for the Steam Boiler in Palm Oil Mills, Internal Research Report, Chemical Engineering, Curtin University Sarawak Campus, Malaysia. 1. 2 76

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