Mechanics, Materials Science & Engineering, October 2015 – ISSN 2412-5954
Computer Aided Analysis and Prototype Testing of an Improved Biogas Reactor For Biomass System Jeremy (Zheng) Li 1a 1 – Ph.D., Associate Professor, School of Engineering, University of Bridgeport, USA a – zhengli@bridgeport.edu
Keywords: green resource, sustainable energy, biomass system, environmental protection, fuel efficacy, cost effective, biogas enrichment. ABSTRACT. The alternative fuel resources substituting for conventional fuels are required due to less availability of fuel resources than demand in the market. A large amount of crude oil and petroleum products are required to be imported in many countries over the world. Also the environmental pollution is another serious problem when use petroleum products. Biogas, with the composition of 54.5% CH4, 39.5% CO2, and 6% other elements (i.e., H2, N2, H2S, and O2), is a clear green fuel that can substitute the regular petroleum fuels to reduce the pollutant elements. Biogas can be produced by performing enriching, scrubbing, and bottling processes. The purification process can be further applied to take away the pollutants in biogas. The pure biogas process analyzed in this research is compressed to 2950 psi while being filled into gas cylinder. The daily produced biogas capacity is around 5480 ft3 and the processing efficacy is affected by surrounding environment and other factors. The design and development of this biogas system is assisted through mathematical analysis, 3D modeling, computational simulation, and prototype testing. Both computer aided analysis and prototype testing show close results which validate the feasibility of this biogas system in biomass applications.
Introduction. Biomass, the biological materials, can be produced by processing some surviving or lately deceased organisms including crops or materials relevant to crops [1]. Biomass is considered as one type of energy resource that can be applied to generate heat by direct combustion or can be changed to other types of bio-fuel by some technologies [2]. The biomass can be changed to bio-fuel by alternative ways including biochemical, chemical, and thermal methodologies. The current common fuels used in commercial transportation systems are petrol or diesel and its demand is increasing sharply due to modern industrialization. Because of the shortage in natural petroleum supply, a lot of petrol-related products have to be imported from outside of country. In addition, the tightened environmental pollution control on the emission of transportation systems requires finding alternative fuel resources [3, 4]. The green and sustainable energy resources, such as bio-fuel, biogas, solar energy, wind power, and geothermal, can be used as alternative energy resources in different applications. Although the natural gas has methane, ethane, propane, butane and other elements, biogas posses 68-78% enriched methane [5, 6]. Pure methane can be potentially produced from biogas by using scrubber. Because enriched methane can be easily bottle-compressed after being produced from biogas, it can be potentially used as gas fuel for many different applications. Some organic wastes including commercial/residential wastes, sewage waste, and community solid waste can be used as source stocks to make biogas [7]. Biogas is one of the green, sustainable and clean fuels and the wastes generated during biogas production can be utilized for making fertilizer products [8, 9]. The common source materials to produce biogas are usually biodegradable wastes existed in many commercial/residential areas including wastes from human, paper, food, and many other organic materials [10]. 1. Biogas enrichment process. To produce pure methane, the contained impurities in source materials must be removed by methods of filtration, such as membrane segregation, physical absorption, chemical separation, and water absorption [11]. The physical absorption process can be used to get rid of impurities including carbon-dioxide and hydrogen-sulfide by water scrubbing technique [12]. The pressured water from pump flows into scrubber at the top section and biogas from storage vessel enters scrubber at the bottom sections. Since the weight of biogas is lighter than the weight of water, biogas travels upward and water flows downward through scrubber baffle. As soon as biogas physically contacts water, the MMSE Journal. Open Access www.mmse.xyz
Mechanics, Materials Science & Engineering, October 2015 – ISSN 2412-5954
impurities including carbon-dioxide and hydrogen-sulfide will be absorbed by water and removed impurities can be collected in the filter at bottom of scrubber [13]. The purified biogas in biomass well system gets dry as it moves through the heat exchanger device and pressured up to 1250 psi by the firststage compressor. The flow of biogas is controlled via a rotameter to ensure no overflow inside scrubber. When enough biogas is monitored inside scrubber, extra biogas can be temporarily stored in the first storage container for later cycle. Biogas flows through the first storage container, where its pressure being reduced, into scrubber at the bottom. After the contact between upward biogas and downward water inside scrubber, the impurities in biogas can be removed by physical absorption from water and purified biogas can continuously flow through top of scrubber into second storage container. Biogas continuously flows through gas filter, where it gets more purified, into second compressor. Biogas is pressured to 2950 psi at second compressor before being filled in biogas container.
Figure 1. Biogas enrichment system The biogas enrichment system, shown in Fig. 1, consists of biomass well system (#1), heating device (# 2), first compressor (# 3), valve (#4), first storage container (#5), rotameter (# 6), scrubber (#7), second storage container (#8), second compressor (#9), and biogas container (# 10). 2. Computational simulation on biogas reactor in biomass system. The computational simulation has been performed to study and modify the performance of biogas reactor in biomass system. In biogas reactor unit, the external force is used to drive the rotation of biomass mixer to produce the biogas. The heating energy needed for digester and heating energy lost in system require to be considered. The normal temperature in digester for heating biogas plant changes from 24.5˚C to 38.5˚C and heating energy required in the digester depends on environmental conditions including surrounding temperature. The heating energy needed in digester can be specified by following equation (1) [14].
QREQ (C P M TREQ t ) QLOSS , where QREQ – heat energy required for starting process in digester (kWh);
C P – heat capacity of feeding material stocks (J/[kg*K°]); M – mass of feeding material stocks (kg); TREQ – temperature change of feeding material stocks between entering and leaving digester (K°); t – time (hours); MMSE Journal. Open Access www.mmse.xyz
(1)
Mechanics, Materials Science & Engineering, October 2015 – ISSN 2412-5954
QLOSS – heat loss through digester surface (kWh); The heat loss through digester surface QLOSS can be found from equation (2) [14].
QLOSS S C H TLOSS t ,
(2)
where, S – digester surface (m2); CH – heat transfer coefficient (W/[m2 K°]); TLOSS – temperature change between inside and outside digester surfaces. The raw feeding materials, driven by reactor blades, rotate rapidly to continuously produce biogas inside biogas reactor. To improve biogas reactor design, three different blade structures shown in Figs. 2 - 4 are analyzed.
Figure 2. Bioreactor blade 1
Figure 3. Bioreactor blade 2
Figure 4. Bioreactor blade 3 The Figs. 2 - 4 display that the bioreactors 1, 2, and 3 consist of vertical straight blades, inclined straight blades, and inclined curved blades respectively. The 3-D modeling of three different reactor blades and its relevant computational simulations have been performed to potentially improve functionality of biogas reactor system. The simulation results of stress profiles and deformation profiles of these three bioreactors are presented in Figs. 5 – 10.
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Mechanics, Materials Science & Engineering, October 2015 – ISSN 2412-5954
Figure 5. Stress profile in blade 1
Figure 6. Deformation profile in blade 1
Fig. 7 Stress profile in blade 2
Fig. 8 Deformation profile in blade 2
Fig. 9 Stress profile in blade 3
Fig. 10 Deformation profile in blade 3
Based on simulation results in Figs. 5 – 10, the maximum stress of 15867.97 psi in blade 1 is less than 21754.17 psi in blade 2 and 31431.21 psi in blade 2. The results show that the vertical straight blades has less stress produced in blade root areas compared to other two blades due to its geometrical shape and less force required to rotate reactor blades. Since the yield strength of mild steel is 36300 psi, the blade 1 design in biogas reactor can function appropriately with safety factor more than 2 and the maximum deformation in blade 1 is within the material allowable limit. Since the blade 2 and 3 designs have safety factors 1.15 and 1.67 respectively, these two blades require further modification and improvement. MMSE Journal. Open Access www.mmse.xyz
Mechanics, Materials Science & Engineering, October 2015 – ISSN 2412-5954
3. Prototype experiment. The biogas reactor has been prototyped and experiment has been performed to justify the design concept and confirm computational simulation. Tables 1, 2, and 3 display the experimental results of stress and deformation profiles in three different blades.
Table 1. Experimental results of stress and deformation profiles in blade 1 Blade 1 Number Stress Displace of Test (N/mm2) (mm) 1 15867.92 0.00699 2 15867.99 0.00698 3 15867.84 0.00688 4 15867.78 0.00665 5 15867.98 0.00659 6 15867.48 0.00648 7 15867.68 0.00638 8 15867.75 0.00657 9 15867.65 0.00654 10 15867.84 0.00666 Average 15867.79 0.00667
Table 2. Experimental results of stress and deformation profiles in blade 2 Number Blade 2 of Test Stress Displace (N/mm2) (mm) 1 31431.54 0.00799 2 31431.29 0.00788 3 31431.18 0.00778 4 31431.24 0.00754 5 31431.38 0.00766 6 31431.48 0.00778 7 31431.15 0.00799 8 31431.48 0.00786 9 31431.54 0.00788 10 31431.39 0.00785 Average 31431.37 0.00782 Table 3. Experimental results of stress and deformation profiles in blade 3
Number of Test 1 2 3 4 5 6 7 8 9 10 Average
Blade 3 Displace Stress (mm) (N/mm2) 21754.25 0.00529 21754.15 0.00548 21754.24 0.00538 21754.35 0.00524 21754.48 0.00518 21754.38 0.00522 21754.54 0.00512 21754.12 0.00518 21754.18 0.00524 21754.24 0.00525 21754.29 0.00526
The average maximum stress and deformation in Table 1 are 15867.79 psi and 0.00667 inches for blade 1 that are almost equal to 15867.97 psi and 0.00697 inches determined by computational simulation. The average maximum stress and deformation in Table 2 are 31431.37 psi and 0.00782 inches for blade 2 that are approximately same as 31754.17 psi and 0.00794 inches specified by computational simulation. The average maximum stress and deformation in Table 3 are 21754.29 psi and 0.00526 inches for blade 3 that are very close to 21754.17 psi and 0.00541inches found by computational simulation. Both computational simulation and prototype testing show proper function of biogas reactor and validate the feasibility of analytic methodology applied in this research. MMSE Journal. Open Access www.mmse.xyz
Mechanics, Materials Science & Engineering, October 2015 – ISSN 2412-5954
Summary. Biogas is a green/clean fuel resource that can be potentially used in many different applications. This sustainable energy resource, an alternative to the current conventional energy market, can be used to protect environment. The byproduct from biogas production can be potentially used for agricultural fertilizer and some production processes. Both computational simulation and prototype experiment on this biogas reactor are introduced to study and analyze this biogas reactor design for turther improvement. Both computational simulation and prototype experiment on three different reactor blades affirm that the vertical straight blade geometry in biogas reactor provides better performance than other two blade geometries. References [1] Binh, M., Phan, L., Duong, V., Nguyen, T., Tran, M., Nguyen, L., Nguyen, D. and Nguyen, L., 2014, “Evaluation of the production potential of bio-oil from Vietnamese biomass resources by fast pyrolysis”, Journal of Biomass and Bioenergy, Vol. 62, pp.74-81. [2] Dukes, C., Baker, S. and Greene, W., 2013, “In-wood grinding and screening of forest residues for biomass feedstock applications”, Journal of Biomass and Bioenergy, Vol. 54, pp.18-26. [3] Wilk, V., Schmid, J. and Hofbauer, H., 2013, “Influence of fuel feeding positions on gasification in dual fluidized bed gasifiers”, Journal of Biomass and Bioenergy, Vol. 54, pp.46-58. [4] Li, J., 2009, “Study and Development of an Energy-Saving Mechanical System”, International Journal of Recent Trends in Engineering, Vol.1, pp.51-54. [5] Bartley, M., Boeing, M., Corcoran, A., Holguin, F. and Schaub, T., 2013, “Effects of salinity on growth and lipid accumulation of biofuel microalga Nannochloropsis salina and invading organisms”, Journal of Biomass and Bioenergy, Vol. 54, pp.83-88. [6] Li, J., 2011, “Computer-Aided Design, Modeling and Simulation of A New Solar Still System Design”, Journal of Modeling and Simulation in Engineering, Vol. 2, pp. 1-5. [7] Starr, k., Gabarrell, X., Villalba, G., Talens, L. and Lombardi, L., 2014, “Potential CO2 savings through biomethane generation from municipal waste biogas”, Journal of Biomass and Bioenergy, Vol. 62, pp.8-16. [8] Savy, D. and Piccolo, A., 2014, “Physical–chemical characteristics of lignins separated from biomasses for second-generation ethanol”, Journal of Biomass and Bioenergy, Vol. 62, pp.58-67. [9] Li, J., 2012, “Computer-Aided Modeling and Analysis of an Energy-Saving Refrigerating System”, Journal of Mechanical Engineering and Automation, Vol. 2, pp. 9-12. [10] Melikoglu, M., 2013, “Solid-State Fermentation of Wheat Pieces by Aspergillus oryzae: Effects of Microwave Pretreatment on Enzyme Production in a Biorefinery”, International Journal of Green Energy, Vol. 10, pp.529-539. [11] Bridgwater, A., 2012, “Review of fast pyrolysis of biomass and product upgrading”, Journal of Biomass and Bioenergy, Vol. 38, pp. 68-94. [12] Baral, S., Pudasaini, S., Khanal, S. and Gurung, D., 2013, “Mathematical Modelling, Finite Element Simulation and Experimental Validation of Biogas-digester Slurry Temperature”, International Journal of Energy and Power Engineering, Vol. 2, pp.128-135. [13] Slade, R & Bauen, A., 2013, “Micro- algae cultivation for biofuels: Cost, energy balance, environmental impacts and future prospects”, Journal of Biomass and Bioenergy, Vol. 53, pp. 29-38. [14] Sukhatme, S., 2005, “A Textbook on Heat Transfer”, Universities Press, ISBN 8173715440.
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