Effects of Bio-coal Briquette Shape on Transport Phenomena in a Curing Unit by CFD Technique M. H. Narasingha*, K. Pana-Suppamassadu Department of Chemical Engineering, King Mongkut s University of Technology North Bangkok, 1518 Piboonsongkhram Rd., Bangsu, Bangkok 10800, Thailand * Corresponding author. Tel: +6685 111 1433, Fax: +662 587 0024, E-mail: mhc@kmutnb.ac.th, monpilai@gmail.com Abstract: Cofiring biomass with coal has gained attention in recent years with an attempt to reduce net CO 2 and SO 2 emissions from coal-based power plants. Bio-coal briquetting technology provides among the technologies associated with co-combustion of biomass and coal, and this technology shows a low-cost and environmentally friendly option since the raw materials are normally coal fines (waste) and agricultural wastes. Curing is an essential step of briquetting, in which the significant mechanical and physico-chemical properties of the products are improved. To achieve an appropriate design of the curing unit, an understanding in transport phenomena occurred inside the unit is needed and the shape of briquette is concerned as an important effect on the transport phenomena. The briquette shapes of interest in this study were spherical, cylindrical, cubical, and soap-like shapes. The involved transport phenomena of the briquettes and near-surrounding within the designed-curing unit were investigated through simulation method using a Finite Element approach (COMSOL Multiphysics 3.5a). From the simulation results, it is clear that the shape of briquette played a vital role in moisture removal during the curing process. The heat transfer is found to rely strongly on conditions at the briquette surface, while the mass transfer is found to be affected by the physical properties of the briquette, which in turn depending on shape. Moreover, the heat and mass transfers as occurred in the bio-coal briquette have remarkably similar characteristics to those of the coal briquette but the rates, due to their differences in apparent physical and transport properties. These findings can be used as crucial information for a proper design of a curing unit that suitable and flexible enough to the briquettes with different shapes and/or a variety of biomass raw materials. Key-Words: - Bio-coal briquette; curing; biomass; briquette shape 1 Introduction The utilization of coal in the area of energy production is still large in some developing countries due to their rapid growth on industrialization and economic development, resulting in a serious concern on global environment problems. Moreover, a great amount of coal fines is also being discarded from coal preparation processes that subsequently causing the pollution, apart from the direct combustion. Cofiring biomass with coal has gained attention in recent years with an attempt to reduce net CO 2 and SO 2 emissions from coal consumption since biomass is a renewable fuel and also being discarded as agricultural and forestry wastes in a large amount throughout the years. It is estimated that the biomass, including municipal solid waste, can contribute nearly 43% of the total energy required for the developing countries and 26% for some developed countries [1]. Bio-coal briquetting technology provides among the technologies associated with cocombustion of biomass and coal, and this technology shows a low-cost and environmentally friendly option [2, 3]. The curing process is an essential step of briquetting, in which the significant mechanical and physico-chemical properties of the products are improved. To design an appropriate curing unit for the briquetting process, raw-materials and product characteristics and optimum curing conditions, including transport phenomena within the unit, are all taken into consideration. From the previous work [4], a lab-scale curing unit was designed based on information obtained for coal briquette in a soap-like shape. However, information on transport phenomena that occurred inside the unit still needs an investigation. In this study, shape of briquette is concerned as an important effect on the transport phenomena occurred inside the unit. The briquette shapes of interest in the study are spherical, cylindrical, cubical, and soap-like shapes. By using CFD technique, the involved transport phenomena of the briquettes and near-surrounding within the curing unit can be explored, and the comparative study for bio-coal and coal briquettes are reported. 2 Methodology To gain an insight of the involved transport phenomena of the curing process, a computational approach based on Finite-Element (FE) was adopted ISBN: 978-1-61804-026-8 294
in an attempt to simulate the hot-air flow structure, including heat and mass (moisture) transfers between the hot surrounding air and the briquettes formed in different shapes. A preliminary simulation of a lab-scale curing unit, as schematically shown in Fig. 1, along with the curing conditions was conducted in priority to obtain the numerical boundary conditions for the enclosed sub-domains around the briquettes. The curing unit comprised of a mesh-belt conveyer which was divided into 5 stages, and equipped with the axial fans and heating units that produced hightemperature air stream at 2 to 4 m/s. According to the previous study [4], the curing temperature and time were maintained throughout this study at 60 o C and 40 min, respectively. The fuel briquettes selected for this study are listed as following: i) the coal briquette consisting of bituminous coal at 80% wt and binder at 15% wt, and ii) the bio-coal briquette consisting of bituminous coal at 70% wt, biomass (saw dust) at 15% wt, and binder at 15% wt. The temperature distribution of hot air and the briquettes within the curing unit obtained from the preliminary simulation is shown in Fig. 2. To clarify the details in such transports, the sub-domains surrounding the briquettes formed in different shapes, also indicated in Fig. 2, were then applied in the simulations. The computational mesh was set at normal fineness, resulting in an approximate of 89,000 elements. For comparison, all shapes of the briquette were managed to have approximately the same surface-to-volume ratio; i.e. 0.0036 m -1 that falling in the practical range of coal curing. Since the briquette curing process is inherently dynamics, the temperature and concentration distributions within the briquette and the fluidstructure interaction between the briquette and hot air were therefore simulated using the time-dependent or transient solver mode. The development of those parameters at any instants and locations was then obtained and analyzed. The involved physics and governing equations are listed below; Eqs. 1 to 3. The sub-domain set up, and the applied boundary/initial conditions were summarized in the following tables; Table 1 to 6. The physical and transport properties and the applied boundary conditions are demonstrated in Table 7. 2.1 Incompressible Navier-Stokes Fig. 1. Schematic diagram of the curing unit for coal briquetting process. when ρ is the density (kg/m 3 ), u is the velocity vector (m/s), p is pressure (Pa), T is absolute temperature (K) F is the body force vector (N/m 3 ), η is dynamic viscosity (Pa s) Table 1. The sub-domain set up for incompressible flow. (1) Coal Bio-Coal Boundary Air Density ( kg / m 3 ) - - 1.051 Air Dynamic Viscosity (Pa.s) - - 199.88 10-7 Porosity 0.4 0.4 1 Permeability (m 2 ) 1 10-18 6 10-15 1 Fig. 2. Computational sub-domain surrounding each shape of coal briquette. ISBN: 978-1-61804-026-8 295
Table 2. The boundary set up for incompressible flow. Boundary Condition Velocity (m/s) Right 0.04 Left 0.05 Boundary condition Pressure (Pa) Top 0.028 Bottom 0.0305 Front 0.03 Back 0.03 Particle 2.2 Convection and Conduction Continuity when ρ is the density (kg/m 3 ), C p is the specific heat capacity at constant pressure (J/(kg K)), T is absolute temperature (K), u is the velocity vector (m/s), k is the thermal conductivity, Q contains the heat sources (W/m 3 ) Table 3. The sub-domain set up for convection and conduction. Coal Bio-Coal Air Thermal conductivity (W/m. K) 1.455 1.281 0.029 Density (kg/m 3 ) 1270 1050 1.05 Heat capacity (kj/kg. K) 1.80 1.75 1.01 Initial Temperature (K) 300 300 333 Table 4. The boundary set up for convection and conduction. Boundary condition Temperature (K) Right 333 Left 333 Top Convective flux Bottom 333 Front 333 Back 333 Particle Continuity 2.3 Convective and Diffusion (2) (3) when c i is the concentration of species i (mol/m 3 ), D i denotes its diffusion coefficient (mol/(m 3 s)), R i is the reaction rate for species i (W/m 3 ), u is the velocity vector (m/s), δ ts is a time-scaling coefficient. This coefficient is normally 1. If desired, you can change the time scale, for example, from seconds to minutes by setting it to 1/60. Table 5. The sub-domain set up for convective and diffusion. Diffusion Coefficient (m 2 /s) Initial Concentration (mol/m 3 ) Coal Bio-Coal Air 5.83 10-8 5.83 10-8 1.87 10-8 4240 5156 2.72 Table 6. The boundary set up for convective and diffusion. Boundary Condition Concentration (mol/m 3 ) Right 2.72 Left 2.72 Top Convective flux Bottom 2.72 Front 2.72 Back 2.72 Particle Continuity Table 7. The physical and transport properties and the applied boundary conditions. Coal Bio-Coal Surrounding Air Surface -to-volume (m -1 ) 4 10-3 4 10-3 - Density (kg/m 3 ) 1270 1050 1.05 Specific Heat Capacity (J/kg K) 1380 1322 1008 Initial Moisture Concentration (mol/m 3 ) 38,888 38,888 2.7 Initial Temperature (K) 298 298 333 Diffusion Coefficient (m 2 /s) 5.83 10-8 5.83 10-8 5.93 10-8 3 Results and Discussion 3.1 Influence of Briquette Shape on Heat Transfer Fig. 4 illustrates the transient temperature distributions of the soap-like briquette from the beginning to beyond 0.3 sec of a lump capacity time constant. From the snap shot at 0 sec, the coal briquette had rather uniform temperature distribution all over body, while the temperature distribution of surrounding air exhibited the same contour as the shape of briquette. The higher temperature gradient can be observed on the bottom side of coal briquette that rested on the mesh belt. The asymmetrical temperature distribution of the surrounding air caused by the belt and the flow pattern ISBN: 978-1-61804-026-8 296
occurred around the coal briquette were found to affect the development of temperature distribution within the coal briquette, which in turn can influence the mass transfer mechanism. At later time, e.g. 600, 1500 and 2400 sec, the asymmetrical temperature distribution within the coal briquette became more apparent; i.e., the temperature detected at the bottom half of the coal briquette had become higher and closer to the hot air temperature while the temperature of the top-half was found to gradually increase from the initial coal temperature. Due to a significant temperature gradient within the coal, the lump capacitance analysis was in appropriate and called for the computational simulations to investigate the heat transfer behavior. The streamlines of air flows around the soap-likeshape coal briquette at each instant indicated the coupling between the fluid-structure interaction and the heat-transfer phenomenon. After the heat transferred from air to the coal briquette, the density gradient of air occurred and the hot air would be replaced by the colder air, causing an air plume as shown in Fig.4b - 4d. The natural-convectioninduced flow or air plume promoted more of asymmetrical temperature distributions. Fig. 4 Transient temperature distributions within the soap-like coal briquette and the surrounding air within sub-domain: (a) 0 sec, (b) 600 sec, (c) 1500 sec, and (d) 2400 sec. Fig. 5 Transient temperature distributions within the spherical coal briquette and the surrounding air within sub-domain: (a) 0 sec, (b) 600 sec, (c) 1500 sec, and (d) 2400 sec. In general, similar trends could be observed from Fig. 5 in case of the spherical coal. However, the temperature distribution within the spherical coal briquette differed from that in case of the soap-like briquette in such a way that the non-uniformity of temperature distribution within the spherical one was much lower. The lower non-uniformity of temperature distribution in the case of the spherical coal briquette can be attributed to the smaller area available for heat conduction when compared with the soap-like shape briquette. The heat conduction through the belt dominated the heat transfer and caused the temperature in the vicinity of the bottom of the soap-like coal briquette to become higher at a faster rate than that of the spherical coal briquette. 3.2 Influence of Briquette Shape on Moisture Removal In the curing unit, the temperature gradient is a driving force for an occurrence of heat conduction within the coal briquette and the heat convection at the coal-briquette surface, the concentration gradient is a driving force for a mass transfer to occur likewise. Mass diffusion within the coal briquette should be affected by the physical properties, e.g. porosity or permeability of coal, whereas the mass transfer at the interface between coal surface and hot air should be affected by the flow pattern and heat convection. Since the drying process for coal such in this case did not lie upon neither evaporation nor chemical reaction, but through the thermal coupling. The concentration profiles took a similar trend. The coal temperature and its gradient should also play a ISBN: 978-1-61804-026-8 297
role in drying as well as the diffusion coefficient of the moisture removal process. The behavior of moisture removal through the mass and coupling heat transfer was observed in the transient moisture concentration contours. The effect of briquette shape on the concentration distribution was demonstrated in Fig. 6. 3.3 Effect of Biomass Fig. 7 depicts the average temperature of fuel briquette as a function of curing time. Although the estimated time constant of each configuration of the coal briquette was nearly the same, but since the lump capacitance was not even in this case, the average temperature of the soap-like coal briquette was thus found to approach the steady state earlier as compared to those of the other shapes. The history of the average temperature for each shape of the coal briquette was similar to that of the corresponding shape of bio-coal one. The variations of the average moisture concentration within the fuel briquettes were illustrated in Fig. 8. It is found that the moisture concentrations reduced as time increased, regardless of the briquette shape and fuel type. However, the fuel briquettes with the soap-like shape exhibited the fastest declining rate of moisture content. This finding is in agreement with the results shown in Fig. 7, in which the soap-like fuel briquettes spent a shorter time to reach the temperature of hot air. The influence of biomass mixed in the briquette to the temperature and moisture distributions could also be noticed from the results obtained in Fig. 7 and 8, though not apparent since only 15% wt of biomass was added in the coal briquette for the simulation. It can be observed that the declining rate of moisture content of the bio-coal briquette is slightly slower than that of the coal briquette due to the physical and thermal properties of the biomass. For a further investigation on the effect of biomass with the soap-like shape, the detailed superimposed concentration profiles for both coal and bio-coal briquettes were presented in Fig. 9. The general time and spatial variations of moisture concentration were found similar, i.e., both Fig. 6 Steady state moisture concentration profiles at 2400 sec within coal briquettes and surrounding air within sub-domain: (a) soaplike (b) sphere (c) cubic, and (d) cylindrical. Fig. 7 The average temperature of the fuel briquettes: (a) coal, and (b) bio-coal. Fig. 8 The average moisture content of the coal briquettes: (a) coal, and (b) bio-coal. figures (Fig. 9a and 9b) had uniform concentration at the beginning and asymptotically declined with increasing time until it approached the relative moisture concentration of the curing hot air. The steep slope in the vicinity of boundary of briquette suggested of the film resistance for mass transfer (as well as for heat transfer), which was evident for both coal and bio-coal briquettes. Nevertheless, the biocomposition existing in the bio-coal briquette was expected to play a role in the thermal and mass diffusivities within the briquette. Therefore, a slight difference in the concentration and temperature profiles within both briquettes was observed. It is of interest to perform a further study on the effect of the amount of biomass added to the bio-coal briquette. ISBN: 978-1-61804-026-8 298
Fig. 9 Comparison of the concentration profiles between a) the coal briquette (left column) and b) the bio-coal (right column) briquette for the soap-like shape at various times (sec). 4 Conclusion From the simulation results, it is clear that the shape of briquette played a vital role in moisture removal during the curing process. Although the moisture concentrations was found to reduce as time increased regardless of the briquette shape and fuel type, but the fuel briquettes with the soap-like shape exhibited the fastest declining rate of moisture content. This is consistent with the finding that the average temperature of the soap-like briquette shape approached the steady state and reached the temperature of hot air in a shortest time as compared to those of the other shapes. Moreover, the heat and mass transfers as occurred in the bio-coal briquette have remarkably similar characteristics to those of the coal briquette but the rates, due to their differences in apparent physical and transport properties. This can suggest that the same curing conditions could be applied for the curing process of both coal and bio-coal briquettes. However, more work is still needed to clarify the effect of type and proportion of biomass added in the bio-coal briquette on the transport phenomena occurred in the curing unit, including the verification by experiments. These findings are expected to be used as crucial information for a proper design of a curing unit that suitable and flexible enough to the biofuel briquettes with different shapes and/or a variety of biomass raw materials [3] Grover, P.D., and Mishra, S.K., Biomass Briquetting: Technology and Practices, Regional Wood Energy Development Programme in Asia, Food and Agriculture Organization of the United Nations, Bangkok, April 1996. [4] Preangprom, S., Prototype of a Curing Unit for Coal Briquetting Process, Master Thesis, King Mongkut s University of Technology North Bangkok, 2008. References [1] Lu, G. et.al., Experimental Study on Combustion and Pollutant Control of Biobriquette, Energy & Fuels (2000), 14, 1133-1138. [2] Ryu, C. et.al., Pelletised Fuel Production from Coal Tailings and Spent Mushroom Compost, Fuel Processing Technology (2008), 89, 269-275. ISBN: 978-1-61804-026-8 299