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Simulation of Anaerobic Digestion for Biogas Production from Food Waste Using SuperPro Designer
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 19 (2019) 1315–1320
www.materialstoday.com/proceedings
ICCSE 2018
Simulation of Anaerobic Digestion for Biogas Production from Food Waste Using SuperPro Designer N. Haruna*, N. A. Othmana, N. A. Zakia, N. A. Mat Rasula, R. A. Samaha, H. Hashimb a
Faculty of Chemical and Natural Resources Engineering, Universiti Malaysia Pahang, Lebuhraya Tun Razak, 23600 Gambang, Kuantan, Pahang b School of Chemical and Energy Engineering, Universiti Teknologi Malaysia (UTM), 81310 Skudai, Johor, Malaysia
Abstract Anaerobic digestion (AD) process is a form of waste-to-energy technology commonly used to produce biogas from organic matter. The purpose of this study was to develop a simulation model of anaerobic digestion process in treating food waste to produce biogas using SuperPro Designer software. The component registration, reaction list, reactor model, and process conditions were specified in the software. The Anaerobic Digestion Model No. 1 (ADM1) was applied in this simulation. The main processes of AD, which are hydrolysis, acidogenic, acetogenic and methanogenesis, were divided into two stages. The first stage, which is hydrolysis, was modeled using a stoichiometry reactor. While the second stage, which consists of acidogenic, acetogenic, and methanogenesis, were represented by an anaerobic digester. The simulation yielded 57% of methane and 25% of carbon dioxide. Sensitivity analysis was performed to study the effect of hydraulic retention time (HRT) and food waste-towater ratio on the methane production. The results of simulation show that an increase in HRT increased the methane composition in biogas due to longer time provided for microorganism activities to yield biogas. The results also show an increase in the methane composition when the water ratio in feed was increased because the moisture content escalated the hydrolysis and methanogenesis processes by increasing the microbes and enzymes attachment areas on the surface of the substrate. The purification of methane was performed using water scrubbing technology and the methane purity achieved was about 98.7%. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Chemical Sciences and Engineering: Advance and New Materials, ICCSE 2018. Keywords: anaerobic digestion, superpro, biogas, food waste, waste to energy
* Corresponding author. Tel.: +6-09-5492885 ; Fax: +6-09-5492889 E-mail address: [email protected]
2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Chemical Sciences and Engineering: Advance and New Materials, ICCSE 2018.
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Nomenclature AD ADM1 GHG HRT HTU MSW POME VFA WTE
anaerobic digestion anaerobic digestion model 1 greenhouse gas hydraulic retention time height of transfer unit municipal solid waste palm oil mill effluent volatile fatty acid waste to energy
1. Introduction Nowadays, waste-to-energy (WTE) transformation has been recognized as an important approach in waste management and economic development. Generally, WTE is utilized to harvest biomass from agricultural and forestry wastes (e.g., paddy straw, palm oil biomass, and logging residues). WTE practices are applicable to different waste categories such as liquid, solid (e.g., domestic sewage), and gaseous (e.g., refinery flue gas). WTE technologies can be divided into three types: biological treatment, thermal treatment, and landfill. Biological treatment includes anaerobic digestion process with the production of biogas [1]. The major sources for biogas in Malaysia are palm oil mill effluent (POME), livestock manure, and municipal solid waste (MSW) [2, 3] . Municipal solid waste (MSW) is produced from the daily basic items such as plastics, glasses, newspaper, and food scraps. MSW is mostly discarded from residential, commercial, and institutional areas. According to Gerlat [4], the predicted amount of the generated MSW throughout the world could be doubled by 2025. The traditional approaches for MSW disposal are mainly landfill, incineration, and aerobic composting [5]. In Malaysia, only 2% of MSW is recycled while 42% is incinerated or chemically treated, and the rest (56%) is dumped into landfill to decompose [6]. Most of the current practices in disposing MSW exploit landfilling in an open dumping area where it raises the community awareness on the environmental, health, and safety issues. Landfill is also one of the largest anthropogenic greenhouse gas (GHG) sources where the methane gas (CH4) emitted that is caused by the natural anaerobic decomposition of the organic fraction of MSW [7]. The transformation of organic fraction from MSW trough anaerobic digestion will produce methane gas which can be utilized as a renewable energy to supply heat and electricity [8]. In addition, methane is a clean energy that has a great potential to be an alternative fuel of biogas [9]. Food waste is a typical form of organic matter with a high potential for energy production through anaerobic degradation. Food waste is mainly composed of carbohydrate polymers (starch, cellulose, and hemicelluloses), lignin, proteins, lipids, organic acids, and a remaining smaller inorganic part [10]. According to Zhang et al. [11], due to a relatively high moisture content in food waste, bioconversion technologies, such as anaerobic digestion (AD), are more suitable compared to thermochemical conversion technologies, such as combustion and gasification. AD is a biological process in which the microorganisms break down the material with the absence of oxygen. The AD of food waste is a biological process that involves biodegradation of organic substrate into biogas through four steps which are hydrolysis, acidogenesis, acetogenesis, and methanogenesis [5]. In the hydrolysis stage, the macromolecule organic matters in solids are firstly broken into easily dissolved monomers including the transformation from carbohydrates, protein, and fat to sugar, amino acid, and long-chain fatty acid, respectively. Acidogenesis is the stage where the monomers are further decomposed into short-chain fatty acid with the presence of many kinds of bacteria [12]. Acetogenesis stage is a biological reaction where the organic substrates such as volatile fatty acid and long chain fatty acids are anaerobically oxidized by hydrogen-producing acetogenic bacteria and converted into hydrogen, acetate, and carbon dioxide. Methanogenesis is the last stage in anaerobic digestion process for the production of biogas. In this stage, methane and carbon dioxide are produced. In anaerobic digestion, numerous of reactions are involved and many possible intermediate compounds will be produced. Therefore, process model and simulation are developed to predict and optimize the overall performance of anaerobic systems as well as for design and control purposes. The optimum levels of affecting factors are to be investigated such as hydraulic retention time (HRT) and food waste-to-water ratio. According to Liu et al. [12],
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HRT affects the production of methane in the early stage where the microorganisms consume the substrate to yield more biogas. However, the prolonged HRT could lead to lower microorganism concentration and consequently will decrease the production of biogas. Water content is also a significant parameter which affects the AD of solid wastes. A study reveals that at lower solid content (high quantity of water), the percentage of volatile solid destruction is more and the quantity of biogas (mixture of methane and carbon dioxide) produced is also high but actually produces less quantity of methane gas [13]. 2. Methodology The simulation was performed by using SuperPro Designer software. The biochemical reaction kinetics involved in AD is constructed based on ADM1 model obtained from Rajendran et al. [14]. Some of the unavailable components in the software databank were registered and defined manually. The composition of raw materials was defined based on a research by Mel et al. [15] which covered the review of solid waste composition in Malaysia. The unit processes were chosen and placed in sequence and connected by stream lines. There are two stages of reactions in this process; the first one is the hydrolysis reaction that is based on the extent of reaction and the second is the other reactions that depend on the reaction kinetic. A stoichiometric reactor was used for hydrolysis while the remaining stage was conducted in an anaerobic digester. The complete AD process flow diagram is shown in Fig.1. The input stream contains the raw materials of food waste which are carbohydrates, proteins, and fat with a weight composition of 55%, 26%, and 19%, respectively. The input stream is mixed with water stream at ratio of 1:2. The mixture of food waste and water enters the stoichiometry reactor at 55 °C (thermophilic) and 1 atm. There are 13 reactions involved in this reactor. The monomers produced next enter the anaerobic digester to proceed with the second stage of anaerobic digestion which involves 33 reactions. In this study, two anaerobic digesters were applied. The first digester represents the degradation of amino acid while the second digester represents acetogenesis, acidogenesis, and methanogenesis. Both digesters discharge two streams where the upper stream discharges gas (biogas) and the bottom stream discharges leachate. Both digesters were set at 55 °C and 1 atm. The products from the upper stream of the second digester enter an absorption column to separate the main component of biogas which is methane. Water scrubbing technology was applied to remove CO2 and the traces element in biogas to get a high purity of methane. The absorption column was set at 40 °C and 1 atm.
Fig. 1. SuperPro Designer model for anaerobic digestion process.
3. Results The simulation results obtained from SuperPro Designer were recorded and compared with the typical biogas composition obtained from Rouse [16] as depicted in Table 1. The comparison was done based on the volume percentage of the product produced. The simulations results obtained from this study show a reasonable agreement with the typical composition of biogas. Methane and carbon dioxide were the major components in the biogas stream which accounted for 56.78% and 24.51%, respectively. Also, around 7.6% of traces were present which consisted of hydrogen sulfide, ammonia, hydrogen, nitrogen, and oxygen.
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Table 1. Comparison of biogas composition Component
Rouse (2013)
This study
(vol%)
(vol%)
Methane (CH4)
50–75
56.78
Carbon dioxide (CO2)
25–50
24.51
Nitrogen (N2)
0–10
1.77
Hydrogen (H2)
0–1
0
Hydrogen sulfide (H2S)
0–3
5.64
0
0.47
Oxygen (O2)
Water scrubbing is the most commonly used technology for biogas purification to obtain high purity methane because CO2 and H2S are more soluble in water compared to methane [17]. The height of transfer unit (HTU) was determined by the amount of water introduced to the absorption column. When the amount of water was increased, the value of HTU slightly decreased. In this simulation, 400 kg/batch of water was used which gave 98.7% of methane and HTU of 9 m. 3.1 Effect of hydraulic retention time (HRT) on methane production
Methane composition (vol%)
The first sensitivity analysis was conducted for different HRTs used in the anaerobic digestion process. The HRT was varied from 15 to 30 days. Fig. 2 shows the changes of methane composition according to the changes in the HRT.
65 60 55 50 45 40 35 30 15
20
25
30
Hydraulic retention time (day) Fig. 2. The effect of hydraulic retention time (HRT) on methane composition
Methane composition linearly increased when the HRT was increased. Methane compositions at various HRTs of 15, 20, 25 and 30 days were 48.09%, 52.05%, 56.00%, and 59.95%, respectively. Generally, the methanogens have a long regeneration time compared with the hydrolysis acidogenesis bacteria. To avoid being washed out from the reactor, the HRT must be long enough to retain the methanogens. For example, Ma et al. [18] indicated that the anaerobic sequential batch reactor treating a dilute waste stream was failed when the HRT was shorter than 2 days, for the HRT was too short to exceed the microorganism growth limits. In addition, Liu et al. [12] also stated that the longer HRT could promote the microorganism activities in the system and yield more biogas. In line with this study, the longer HRT produced more methane.
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3.2 Effect of moisture content on methane production The food waste with a rate of 200 kg/batch was mixed with water at different ratios. The amount of food waste was set constant while the amount of water was increased 100 kg/batch to investigate the effect of moisture content on methane production. Fig. 3 shows that the methane production rate declined when the moisture content in the feed was increased. Inversely, carbon dioxide production slightly dropped when more water was fed in the feed stream. A mass amount of methane was produced only when the moisture content of typical food waste was higher than 80%, while higher content of moisture was needed when the content of putrescible waste was higher in MSW [19]. However, carbohydrate requires more water to increase the contact area of microbes on the surface of solid food waste that will enhance the hydrolysis reaction. Veluchamy and Kalamdhad [20] reported that the production of methane achieved the maximum rate with a moisture content between 80% and 85% for the AD of lignocellulose organic compound.
Methane composition (vol%)
70 60 50 40 30 20 10 0 1:1
1:1.5 1:2 1:2.5 Food waste-to-water ratio Methane
1:3
Carbon dioxide
Fig. 3. The effect of moisture content on methane composition
4. Conclusions In this study, a simulation of anaerobic digestion to produce biogas from municipal solid waste by using SuperPro Designer software was conducted. Stoichiometric reactor and anaerobic digester reactor were used in this simulation where hydrolysis, acetogenesis, acidogenesis, and methanogenesis reactions were considered. The temperature used was 55 °C and the pressure was 1 atm. The simulation focused on food waste with the composition of 55% of carbohydrate, 26% of protein, and 19% of fat. The result obtained from the simulation shows that the percentage of methane and carbon dioxide were 56.78% and 24.51%, respectively. This result is comparable with the typical biogas composition where the percentages of carbon dioxide and methane were 25%–50% and 50%– 75%, respectively. In addition, the increase in hydraulic retention time (HRT) and moisture content increased the methane composition in biogas. The clean biogas after purification process contained 98.7% of methane.
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Acknowledgements The authors would like to thank Universiti Malaysia Pahang (UMP) for the financial support through the research grant RDU1703238. References [1] Fazeli A., Bakhtvar F., Jahanshaloo L., Sidik N. A. C. & Bayat A. E. (2016). Malaysia׳s stand on municipal solid waste conversion to energy. Renewable and Sustainable Energy, 58, 1007-1016. [2] Mekhilef S., Barimania M., Safari A. & Salam Z. (2014). Malaysia’s renewable energy policies and programs with green aspects. Renewable and Sustainable Energy Reviews, 40, 497-504. [3] Ali R., Daut I. & Taib S. (2012). A review on existing and future energy sources for electrical power generation in Malaysia. Renewable and Sustainable Energy Reviews, 16(6), 4047-4055. [4] Gerlat A. (2012). Study: Municipal Solid Waste Generation Could Double by 2025. Retrieved from Waste360: http://www.waste360.com/research-and-statistics/study-municipal-solid-waste-generation-could-double2025 [5] Zhang, C., Sua, H., Baeyens, J. & Tan, T. (2014). Reviewing the anaerobic digestion of food waste for biogas production. Renewable and Sustainable Energy Reviews, 38, 383-392. [6] Anwar Z., Syukri, W. M., Rahman A. & Bakar A. N. (2013). Solid Waste Management in Malaysia: Status Challenges and Future Strategies. JUIC International Symposium [7] Tan S. T., Ho W. S, Hashim H., Lee C. T., Taib M. R. & Ho C. S. (2015). Energy, economic and environmental (3E) analysis of waste-to-energy (WTE) strategies for municipal solid waste (MSW) management in Malaysia. Energy Conversion and Management, 102, 111 – 120. [8] Malakahmad A., Basri N.E.A. & Zain S.M. (2012). Biomethanation of kitchen waste and sewage sludge in anaerobic baffled reactor. IEEE Symposium on Humanities,Science and Engineering Research (SHUSER). [9] Mohan S. & Jagadeesan K., (2013). Production of Biogas by Using Food Waste. International Journal of Engineering Research and Applications (IJERA), 3(4), 390-394. [10] Kiran E. U., Trzcinski A P., Ng W. J. & Liu Y. (2014). Bioconversion of food waste to energy: A review. Fuel. 134, 389-399. [11] Zhang R, El-Mashad H. M. Hartman K., Wang F., Liu G., Choate C. & Gamble P. (2007). Characterization of food waste as feedstock for anaerobic digestion. Bioresource technology. 98, 929-935. [12] Liu Y. ,Yu, M. , Wu C. , Wang Q. ,Gao M., Huang Q. and Ren, Y. (2018). A comprehensive review on food waste anaerobic digestion: Research updates and tendencies. Bioresource Technology, 1069-1076. [13] Sahito A. R., Mahar R. B. & Ahmed F. (2014). Effect of buffalo dung to the water ratio on production of methane through anaerobic digestion. Research Journal of Engineering & Technology, 33(2), (237-244). Mehran University, Jamshoro. [14] Rajendran K., Kankanala H. R., Lundin M. & Taherzadeh M. J. (2014). A novel process simulation model for anaerobic digestion using Aspen Plus. Bioresource Technology , 168, 7–13. [15] Mel, M., Ihsan, S., & Setyobudi, R. (2015). Process improvement of biogas production from anaerobic codigestion of cow dung and corn husk. Procedia Chemistry, 91-100. [16] Rouse S. (2013). Precise biogas flow measurement: overcoming the challenges of changing gas composition. 1–7. [17] Yang, L., Ge, X., Wan, C., Yu, F., and Li, Y. (2014). Progress and perspectives in converting biogas to transportation. Renewable and Sustainable Energy Reviews, 40, 1133–1152. [18] Ma J., Zhao B. & Frear C. (2013). Methanosarcina domination in anaerobic sequencing batch reactor at short hydraulic retention time. Bioresource Technology. 137, 41–50. [19] Qu X., He P. J., Shao L. M. & Bouchez T. (2009). Effect of Moisture Content on Anaerobic Methanization of Municipal Solid Waste. Pub Med, 30 (3), 918-23. [20] Veluchamy, C. and Kalamdhad A. S. (2017). A mass diffusion model on the effect of moisture content for solid-state anaerobic digestion. Journal of Cleaner Production, 162, 371-379.
ScienceDirect Materials Today: Proceedings 19 (2019) 1315–1320
www.materialstoday.com/proceedings
ICCSE 2018
Simulation of Anaerobic Digestion for Biogas Production from Food Waste Using SuperPro Designer N. Haruna*, N. A. Othmana, N. A. Zakia, N. A. Mat Rasula, R. A. Samaha, H. Hashimb a
Faculty of Chemical and Natural Resources Engineering, Universiti Malaysia Pahang, Lebuhraya Tun Razak, 23600 Gambang, Kuantan, Pahang b School of Chemical and Energy Engineering, Universiti Teknologi Malaysia (UTM), 81310 Skudai, Johor, Malaysia
Abstract Anaerobic digestion (AD) process is a form of waste-to-energy technology commonly used to produce biogas from organic matter. The purpose of this study was to develop a simulation model of anaerobic digestion process in treating food waste to produce biogas using SuperPro Designer software. The component registration, reaction list, reactor model, and process conditions were specified in the software. The Anaerobic Digestion Model No. 1 (ADM1) was applied in this simulation. The main processes of AD, which are hydrolysis, acidogenic, acetogenic and methanogenesis, were divided into two stages. The first stage, which is hydrolysis, was modeled using a stoichiometry reactor. While the second stage, which consists of acidogenic, acetogenic, and methanogenesis, were represented by an anaerobic digester. The simulation yielded 57% of methane and 25% of carbon dioxide. Sensitivity analysis was performed to study the effect of hydraulic retention time (HRT) and food waste-towater ratio on the methane production. The results of simulation show that an increase in HRT increased the methane composition in biogas due to longer time provided for microorganism activities to yield biogas. The results also show an increase in the methane composition when the water ratio in feed was increased because the moisture content escalated the hydrolysis and methanogenesis processes by increasing the microbes and enzymes attachment areas on the surface of the substrate. The purification of methane was performed using water scrubbing technology and the methane purity achieved was about 98.7%. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Chemical Sciences and Engineering: Advance and New Materials, ICCSE 2018. Keywords: anaerobic digestion, superpro, biogas, food waste, waste to energy
* Corresponding author. Tel.: +6-09-5492885 ; Fax: +6-09-5492889 E-mail address: [email protected]
2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Chemical Sciences and Engineering: Advance and New Materials, ICCSE 2018.
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Nomenclature AD ADM1 GHG HRT HTU MSW POME VFA WTE
anaerobic digestion anaerobic digestion model 1 greenhouse gas hydraulic retention time height of transfer unit municipal solid waste palm oil mill effluent volatile fatty acid waste to energy
1. Introduction Nowadays, waste-to-energy (WTE) transformation has been recognized as an important approach in waste management and economic development. Generally, WTE is utilized to harvest biomass from agricultural and forestry wastes (e.g., paddy straw, palm oil biomass, and logging residues). WTE practices are applicable to different waste categories such as liquid, solid (e.g., domestic sewage), and gaseous (e.g., refinery flue gas). WTE technologies can be divided into three types: biological treatment, thermal treatment, and landfill. Biological treatment includes anaerobic digestion process with the production of biogas [1]. The major sources for biogas in Malaysia are palm oil mill effluent (POME), livestock manure, and municipal solid waste (MSW) [2, 3] . Municipal solid waste (MSW) is produced from the daily basic items such as plastics, glasses, newspaper, and food scraps. MSW is mostly discarded from residential, commercial, and institutional areas. According to Gerlat [4], the predicted amount of the generated MSW throughout the world could be doubled by 2025. The traditional approaches for MSW disposal are mainly landfill, incineration, and aerobic composting [5]. In Malaysia, only 2% of MSW is recycled while 42% is incinerated or chemically treated, and the rest (56%) is dumped into landfill to decompose [6]. Most of the current practices in disposing MSW exploit landfilling in an open dumping area where it raises the community awareness on the environmental, health, and safety issues. Landfill is also one of the largest anthropogenic greenhouse gas (GHG) sources where the methane gas (CH4) emitted that is caused by the natural anaerobic decomposition of the organic fraction of MSW [7]. The transformation of organic fraction from MSW trough anaerobic digestion will produce methane gas which can be utilized as a renewable energy to supply heat and electricity [8]. In addition, methane is a clean energy that has a great potential to be an alternative fuel of biogas [9]. Food waste is a typical form of organic matter with a high potential for energy production through anaerobic degradation. Food waste is mainly composed of carbohydrate polymers (starch, cellulose, and hemicelluloses), lignin, proteins, lipids, organic acids, and a remaining smaller inorganic part [10]. According to Zhang et al. [11], due to a relatively high moisture content in food waste, bioconversion technologies, such as anaerobic digestion (AD), are more suitable compared to thermochemical conversion technologies, such as combustion and gasification. AD is a biological process in which the microorganisms break down the material with the absence of oxygen. The AD of food waste is a biological process that involves biodegradation of organic substrate into biogas through four steps which are hydrolysis, acidogenesis, acetogenesis, and methanogenesis [5]. In the hydrolysis stage, the macromolecule organic matters in solids are firstly broken into easily dissolved monomers including the transformation from carbohydrates, protein, and fat to sugar, amino acid, and long-chain fatty acid, respectively. Acidogenesis is the stage where the monomers are further decomposed into short-chain fatty acid with the presence of many kinds of bacteria [12]. Acetogenesis stage is a biological reaction where the organic substrates such as volatile fatty acid and long chain fatty acids are anaerobically oxidized by hydrogen-producing acetogenic bacteria and converted into hydrogen, acetate, and carbon dioxide. Methanogenesis is the last stage in anaerobic digestion process for the production of biogas. In this stage, methane and carbon dioxide are produced. In anaerobic digestion, numerous of reactions are involved and many possible intermediate compounds will be produced. Therefore, process model and simulation are developed to predict and optimize the overall performance of anaerobic systems as well as for design and control purposes. The optimum levels of affecting factors are to be investigated such as hydraulic retention time (HRT) and food waste-to-water ratio. According to Liu et al. [12],
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HRT affects the production of methane in the early stage where the microorganisms consume the substrate to yield more biogas. However, the prolonged HRT could lead to lower microorganism concentration and consequently will decrease the production of biogas. Water content is also a significant parameter which affects the AD of solid wastes. A study reveals that at lower solid content (high quantity of water), the percentage of volatile solid destruction is more and the quantity of biogas (mixture of methane and carbon dioxide) produced is also high but actually produces less quantity of methane gas [13]. 2. Methodology The simulation was performed by using SuperPro Designer software. The biochemical reaction kinetics involved in AD is constructed based on ADM1 model obtained from Rajendran et al. [14]. Some of the unavailable components in the software databank were registered and defined manually. The composition of raw materials was defined based on a research by Mel et al. [15] which covered the review of solid waste composition in Malaysia. The unit processes were chosen and placed in sequence and connected by stream lines. There are two stages of reactions in this process; the first one is the hydrolysis reaction that is based on the extent of reaction and the second is the other reactions that depend on the reaction kinetic. A stoichiometric reactor was used for hydrolysis while the remaining stage was conducted in an anaerobic digester. The complete AD process flow diagram is shown in Fig.1. The input stream contains the raw materials of food waste which are carbohydrates, proteins, and fat with a weight composition of 55%, 26%, and 19%, respectively. The input stream is mixed with water stream at ratio of 1:2. The mixture of food waste and water enters the stoichiometry reactor at 55 °C (thermophilic) and 1 atm. There are 13 reactions involved in this reactor. The monomers produced next enter the anaerobic digester to proceed with the second stage of anaerobic digestion which involves 33 reactions. In this study, two anaerobic digesters were applied. The first digester represents the degradation of amino acid while the second digester represents acetogenesis, acidogenesis, and methanogenesis. Both digesters discharge two streams where the upper stream discharges gas (biogas) and the bottom stream discharges leachate. Both digesters were set at 55 °C and 1 atm. The products from the upper stream of the second digester enter an absorption column to separate the main component of biogas which is methane. Water scrubbing technology was applied to remove CO2 and the traces element in biogas to get a high purity of methane. The absorption column was set at 40 °C and 1 atm.
Fig. 1. SuperPro Designer model for anaerobic digestion process.
3. Results The simulation results obtained from SuperPro Designer were recorded and compared with the typical biogas composition obtained from Rouse [16] as depicted in Table 1. The comparison was done based on the volume percentage of the product produced. The simulations results obtained from this study show a reasonable agreement with the typical composition of biogas. Methane and carbon dioxide were the major components in the biogas stream which accounted for 56.78% and 24.51%, respectively. Also, around 7.6% of traces were present which consisted of hydrogen sulfide, ammonia, hydrogen, nitrogen, and oxygen.
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Table 1. Comparison of biogas composition Component
Rouse (2013)
This study
(vol%)
(vol%)
Methane (CH4)
50–75
56.78
Carbon dioxide (CO2)
25–50
24.51
Nitrogen (N2)
0–10
1.77
Hydrogen (H2)
0–1
0
Hydrogen sulfide (H2S)
0–3
5.64
0
0.47
Oxygen (O2)
Water scrubbing is the most commonly used technology for biogas purification to obtain high purity methane because CO2 and H2S are more soluble in water compared to methane [17]. The height of transfer unit (HTU) was determined by the amount of water introduced to the absorption column. When the amount of water was increased, the value of HTU slightly decreased. In this simulation, 400 kg/batch of water was used which gave 98.7% of methane and HTU of 9 m. 3.1 Effect of hydraulic retention time (HRT) on methane production
Methane composition (vol%)
The first sensitivity analysis was conducted for different HRTs used in the anaerobic digestion process. The HRT was varied from 15 to 30 days. Fig. 2 shows the changes of methane composition according to the changes in the HRT.
65 60 55 50 45 40 35 30 15
20
25
30
Hydraulic retention time (day) Fig. 2. The effect of hydraulic retention time (HRT) on methane composition
Methane composition linearly increased when the HRT was increased. Methane compositions at various HRTs of 15, 20, 25 and 30 days were 48.09%, 52.05%, 56.00%, and 59.95%, respectively. Generally, the methanogens have a long regeneration time compared with the hydrolysis acidogenesis bacteria. To avoid being washed out from the reactor, the HRT must be long enough to retain the methanogens. For example, Ma et al. [18] indicated that the anaerobic sequential batch reactor treating a dilute waste stream was failed when the HRT was shorter than 2 days, for the HRT was too short to exceed the microorganism growth limits. In addition, Liu et al. [12] also stated that the longer HRT could promote the microorganism activities in the system and yield more biogas. In line with this study, the longer HRT produced more methane.
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3.2 Effect of moisture content on methane production The food waste with a rate of 200 kg/batch was mixed with water at different ratios. The amount of food waste was set constant while the amount of water was increased 100 kg/batch to investigate the effect of moisture content on methane production. Fig. 3 shows that the methane production rate declined when the moisture content in the feed was increased. Inversely, carbon dioxide production slightly dropped when more water was fed in the feed stream. A mass amount of methane was produced only when the moisture content of typical food waste was higher than 80%, while higher content of moisture was needed when the content of putrescible waste was higher in MSW [19]. However, carbohydrate requires more water to increase the contact area of microbes on the surface of solid food waste that will enhance the hydrolysis reaction. Veluchamy and Kalamdhad [20] reported that the production of methane achieved the maximum rate with a moisture content between 80% and 85% for the AD of lignocellulose organic compound.
Methane composition (vol%)
70 60 50 40 30 20 10 0 1:1
1:1.5 1:2 1:2.5 Food waste-to-water ratio Methane
1:3
Carbon dioxide
Fig. 3. The effect of moisture content on methane composition
4. Conclusions In this study, a simulation of anaerobic digestion to produce biogas from municipal solid waste by using SuperPro Designer software was conducted. Stoichiometric reactor and anaerobic digester reactor were used in this simulation where hydrolysis, acetogenesis, acidogenesis, and methanogenesis reactions were considered. The temperature used was 55 °C and the pressure was 1 atm. The simulation focused on food waste with the composition of 55% of carbohydrate, 26% of protein, and 19% of fat. The result obtained from the simulation shows that the percentage of methane and carbon dioxide were 56.78% and 24.51%, respectively. This result is comparable with the typical biogas composition where the percentages of carbon dioxide and methane were 25%–50% and 50%– 75%, respectively. In addition, the increase in hydraulic retention time (HRT) and moisture content increased the methane composition in biogas. The clean biogas after purification process contained 98.7% of methane.
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