Energy and Economic Analyses of Models Developed for Sustainable Hydrogen Production from Biogas-Based Electricity and Sewage Sludge

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1 Energy and Economic Analyses of Models Developed for Sustainable Hydrogen Production from Biogas-Based Electricity and Sewage Sludge AYŞEGÜL ABUŞOĞLU, SİNAN DEMİR, ALPEREN TOZLU Department of Mechanical Engineering University of Gaziantep Gaziantep TURKEY Abstract: - Three models were developed for the use of biogas-based electricity and sewage sludge obtained from a municipal wastewater treatment plant for hydrogen production. These models included alkaline, PEM and high temperature water electrolysis. Energy and economic analyses were performed on the models by applying thermodynamic procedures and the results were compared. The daily hydrogen production rates of the models were calculated as 594, and kg for models 1, 2 and 3, respectively. The electricity costs of the models were calculated as 3.60, 3.43 and 2.47 $/kg-h 2, for models 1, 2 and 3, respectively. In terms of the hydrogen production rate, the high temperature electrolysis process was found to be superior to the other models, followed by the PEM electrolysis process, whereas, in terms of the hydrogen production cost, alkaline electrolysis was found to be superior to the other models. This paper aimed to determine the most appropriate model for a wastewater treatment plant among the considered models in terms of both hydrogen production and hydrogen production cost. Key-Words: - Hydrogen production, Electrolysis, Biohydrogen, Wastewater treatment plant, Biogas. 1 Introduction Hydrogen is an energy carrier and it is proposed as an alternative energy source at present and in the future. Several hydrogen production processes exist, including steam methane reforming, partial oxidation of methane, autothermal reforming of methane, coal gasification, biomass pyrolysis and gasification, electrolysis, the sulfur-iodine cycle, photo-synthetic/photo-biological water splitting, and direct photocatalytic water splitting [1]. Each technology is at a different stage of development, and each offers unique opportunities, benefits and challenges [2]. As of today, most produced hydrogen (80-85%) is obtained by the steam reforming and gasification processes [3]. Although these preferred hydrogen production methods are considered more economical compared to the others, they are non-sustainable and not ecofriendly. In the long run, hydrogen should preferably be produced from renewable sources such as the electrolysis of water with renewable electricity or by means of biological hydrogen production [4]. The main task of a wastewater treatment plant (WWTP) is to treat wastewater to an extent that it can be discharged into a natural water body after minimizing the harmful impact on natural water quality. The wastewater treatment process requires large amounts of energy. However, wastewater sludge can be used as a renewable energy source. As a result of the anaerobic digestion process, sludge is degraded to produce biogas which consists of 60-70% CH 4, 30-35% CO 2, 1-2% H 2 S, and 0.3-3% N 2 with various minor impurities, notably NH 3 and halides. This biogas can be used to produce heat and electricity onsite [5]. Electricity produced by the cogeneration facilities of a WWTP can be used as work input to the installed water electrolysis unit for hydrogen production. This hydrogen production method, in which biogas is used as a renewable energy source, can be considered as an eco-friendly or green process. The importance of electricity obtained from renewable energy sources is increased for the production of hydrogen [6]. In the electrolysis process, electricity is used to decompose water into its elemental components: hydrogen and oxygen. Electrolysis is often seen as the preferred method of hydrogen production, as it is the only process which does not rely on fossil fuels. It also provides high product purity, and is feasible for both small and large scale hydrogen production. Many different types of electrolysis cells have been proposed and constructed such as alkaline, PEM and high temperature steam electrolysis processes [7]. Another option is biohydrogen production from bio-renewables. ISBN:

2 Buy SmartDraw!- purchased copies print this document without a watermark. Visit or call Recent Advances in Energy, Environment and Financial Planning Biohydrogen is a renewable biofuel produced from bio-renewable feedstocks by chemical, thermochemical, biological, biochemical, and biophotolytic methods. Processes for biological hydrogen production mostly operate at ambient temperature and pressure, and are expected to be less energy intensive than thermochemical hydrogen production methods. These processes can use a variety of feed stocks as carbon sources, such as organic waste materials, which facilitates waste recycling. Since activated sludge waste obtained from municipal WWTPs contains high levels of organic matter, it can be used as a carbon source and is considered a potential substrate for hydrogen production. Direct and indirect bio-photolysis, photo-fermentation, dark fermentation and integrated dark and photo-fermentation can be given as some examples of biohydrogen production processes [8-10]. Numerous studies have been undertaken to conduct water electrolysis and biohydrogen production [11-17]. However, only a few of these studies have been directly related to biogas-based hydrogen production and biohydrogen production from sewage sludge. Coskun et al. [6] performed an energy analysis of hydrogen production with biogasbased electricity. In their study, a facility generating its own electricity from biogas obtained from a WWTP was considered for investigation. The hydrogen production process conducted using biogas-based electricity was examined by three methods of electrolysis and PEM electrolyzing system was found as having higher overall system efficiencies of the other two electrolyzing systems. Pandu and Joseph [18] reviewed biohydrogen production processes and stated that the major biological processes discussed for hydrogen production were bio-photolysis of water by algae, dark fermentation, photo-fermentation of organic materials and sequential dark and photofermentation processes. Genc [19] reviewed biohydrogen production from wastewater sludge and emphasized the importance of sludge pretreatment due to the low sludge yield of fermentative hydrogen production methods. Wang et al. [10] presented a study on biohydrogen production from wastewater sludge by Clostridium bifermentans. This work examined the anaerobic digestion of wastewater sludge using a Clostridium strain isolated from the sludge as the inoculum. Despite the existence of numerous studies on hydrogen production from renewable energy sources, hydrogen production from biogas-based electricity and sewage sludge are very limited in the open literature, to the best of our knowledge. In this study, five different hydrogen production models were developed based on the outputs of an actual municipal WWTP. These five models include alkaline, PEM, high temperature water electrolysis, hydrogen sulfur alkaline electrolysis and dark fermentation hydrogen production processes. The energy relations and economic analyses of the models were performed using the data provided from the plant management and previously published works [5, 20] of the authors. The significance of the models presented in this paper is that they can be used to predict the hydrogen production potential of any municipal wastewater treatment plant since these plants have more or less similar processes and outputs such as biogas, sewage sludge, electricity, etc. 2 Model Descriptions The GASKI WWTP is a municipal wastewater treatment plant located in the city of Gaziantep, Turkey, and the flow schematic of the facility is given in Fig. 1. The plant treats nearly 222,000 m 3 /day of domestic wastewater using primary, secondary (biological) and tertiary (anaerobic sludge digestion) treatments. The daily biogas production is nearly m 3, as a result of the sludge stabilization process which takes place in the anaerobic digesters of the plant. 61% (9275 m 3 /day) of the total biogas produced in the anaerobic digestion system is used as a fuel for the installed gas engine powered cogeneration facility on the campus of the WWTP. The remaining part (5925 m 3 /day) is reserved in the biogas storage tank. The electricity production of the cogeneration plant is 1000 kwh. The digested sludge is sent to the dewatering unit of the WWTP to increase the dry matter content to 22%. The mass flow rate of the digested sludge is reduced to 2.48 kg/s as it exists the de-watering stage. In this study, five models were developed for the use of actual WWTP outputs (biogas, electricity and sewage sludge) for hydrogen production. The models are described in detail below. Fig. 1- Flow schematic of GASKI WWTP ISBN:

3 In model 1 (see Fig. 2), an alkaline electrolysis process was considered for hydrogen production. In this model, the work output of the biogas engine powered cogeneration plant, 1000 kwh, is used as the work input to the electrolysis process. Water is heated before entering the electrolysis stage. For the process of heating water, there are two different options at the plant. The first involves the use of a very small amount of reserved biogas (about kg/s) in a boiler (see Fig. 1a); the second option is to use the waste heat of the exhaust gas from the cogeneration plant by means of an exhaust gas heat exchanger (see Fig. 1b). The mass flow rate of the water entering the electrolysis process is taken as kg/s. The temperature and pressure of preheated water by the boiler are 80 C and 1 bar, respectively. In model 2 (see Fig. 3), a PEM electrolysis process was considered. In this model, work input to the electrolysis process and biogas consumed through the boiler to preheat the water are the same as in model 1. As mentioned above, exhaust gas from the cogeneration unit can be used if available. In this model, the mass flow rate of the water entering the electrolysis process is taken as kg/s. The temperature and pressure of the water preheated by the boiler are 80 C and 1 bar, respectively. In model 3 (see Fig. 4), a high temperature electrolysis process was considered for hydrogen production. In this model, the work demand of the electrolysis system is also provided by the cogeneration system. A small amount of the reserved biogas (0.025 kg/s) from the WWTP can be used to produce high temperature steam in the boiler. The mass flow rate of the water entering the electrolysis process is taken as 0.09 kg/s. The temperature and pressure of the steam produced in the boiler are 800 C and 5 bar, respectively. Fig. 2- Hydrogen production model 1 with an alkaline electrolysis process using a (a) biogas boiler or, (b) exhaust gas heat exchanger Fig. 3- Hydrogen production model 2 with a PEM electrolysis process using a (a) biogas boiler or, (b) exhaust gas heat exchanger Fig. 4- Hydrogen production model 3 with a high temperature electrolysis process 3 Energy and Economic Analyses The minimum work needed for 1 kg of hydrogen production by the water and hydrogen sulfide electrolysis processes can be calculated by the following equations: Gelect,H 2O wrev, elect M ( kj / kg) (1) H 2 Gelect,H 2S wrev, elect ( kj / kg) (2) M H 2 where G elect, H2O and G elect, H 2S are the Gibbs free energy (kj/kmol) of water and hydrogen sulfide, respectively, and M H is the molar mass of 2 hydrogen (kg/kmol). Thus, the actual work demand can be calculated as: wrev,elect wact, elect ( kj / kg) (3) th The thermal efficiency of the electrolysis unit can be calculated as: H E ΔH th (4) G Losses Ecell where H is the enthalpy change of the water decomposition reaction as energy input. E ΔH is the equilibrium voltage and E cell is the cell voltage which is always between V at a current ISBN:

4 density of Am -2 for industrial water electrolysis [21]. For alkaline and PEM electrolysis, Eq. (4) can be rewritten in a simple form as: 1.48 th (5) Ecell According to the price of electricity, the unit cost of hydrogen can be calculated with the following equation: Cost C W (6) H 2 electricit y demand where C electricit y is the unit cost of electricity produced by the cogeneration unit of the WWTP and is taken as $/kwh [20]. W demand is the electricity work needed for hydrogen production using these models in kwh/kg H 2. The total cost of an electrolyzer unit consist of capital costs (55%), electricity costs (35%) and operating and maintenance costs (10%) [22]. 4 Results and Discussion In this study, we considered an actual municipal WWTP and elaborated five hydrogen production models. Hydrogen production rates and hydrogen production costs were calculated using thermodynamic calculations and a simple economic analysis procedure. For model 1, water is heated up to 80 C; the value of the Gibbs free energy of water at this temperature is 228,378 kj/kmol. The heat requirements of the water heating process was calculated to be kw. The daily hydrogen produced by model 1 was calculated as 594 kg and the actual electricity cost of hydrogen production was found to be 3.60 $/kg-h 2. Using Eq. (5), the thermal efficiency of the electrolysis process for model 1 was found to be 78%. The thermodynamic data and the results of the energy analyses for model 1, according to the nomenclature provided in Fig. 2, are given in Table 1. For model 2, the heat requirements of the heating process of water were calculated to be 15.0 kw. Using Eq. (5), the thermal efficiency of the electrolysis process for model 2 was found to be 82%. The daily hydrogen produced by model 2 was calculated as kg and the actual electricity cost of hydrogen production was found to be 3.43 $/kg- H 2. The thermodynamic data and the results of the energy analyses for model 2, according to the nomenclature provided in Fig. 3, are given in Table 2. For model 3, the value of the Gibbs free energy of steam is 18,519 kj/kmol at 800 C. The heat requirements of the boiling process for steam production were calculated to be 363 kw. The daily hydrogen produced by model 3 was calculated as kg and the actual electricity cost of hydrogen was found to be 2.47 $/kg-h 2. Using Eq. (4), the thermal efficiency of the electrolysis process for model 3 was found to be 94%. The thermodynamic data and the results of the energy for model 3, according to the nomenclature provided in Fig. 4 are given in Table 3. Table 1- Thermodynamic data and results of the energy analyses of model 1 with respect to the points indicated in Fig. 2 State No Fluid Values 1 Biogas inlet (kg/s) 2 Work 1000kWh 3 Water C and 1bar 4 Pure water C and 1bar 5 Heated water C and 1bar Biogas (with boiler) (kg/s) (Fig.1 (a)) 6 Heat (with EGHE) kw (Fig.1 (b)) 7 Hydrogen (g) 594 kg/day 8 Oxygen (g) 4,762.8 kg/day Type of electrolysis: Alkaline Efficiency of electrolysis: 78% Minimum power consumption of electrolysis: 113,282.7 kj/kg (31.47 kwh/kg H 2) Actual power consumption of electrolysis: 145, kj/kg (40.34 kwh/kg H 2) Cost of electricity: $/kwh Minimum electricity cost of hydrogen: 2.81 $/kg H 2 Actual electricity cost of hydrogen: 3.60 $/kg H 2 Table 2- Thermodynamic data and results of the energy analyses of model 2 with respect to the indicated points in Fig. 3 State No Fluid Values 1 Biogas inlet (kg/s) 2 Work 1000kWh 3 Water C and 1bar 4 Pure water C and 1bar 5 Heated water C and 1bar 6 Biogas (with boiler) (kg/s) (Fig.2 (a)) Heat (with EGHE) 15.0 kw (Fig.2 (b)) 7 Hydrogen gas kg/day 8 Oxygen gas kg/day Type of electrolysis: PEM Efficiency of electrolysis: 82% Minimum power consumption of electrolysis: 113,282.7 kj/kg (31.47 kwh/kg H 2) Actual power consumption of electrolysis: 138,149.6 kj/kg (38.37 kwh/kg H 2) Cost of electricity: $/kwh Minimum electricity cost of hydrogen: 2.81 $/kg H 2 Actual electricity cost of hydrogen: 3.43 $/kg H 2 ISBN:

5 Table 3- Thermodynamic data and results of the energy and analyses of model 3 with respect to the indicated points in Fig. 4 State No Fluid Values 1 Biogas inlet (kg/s) 2 Work 1000 kwh 3 Water 0.09 C and 1bar 4 Pure water 0.09 C and 1bar 5 Steam 0.09 C and 5bar 6 Biogas (with boiler) (kg/s) Heat (with EGHE) 363 kw 7 Hydrogen gas kg/day 8 Oxygen gas 6,948.8 kg/day Type of electrolysis: High temperature electrolysis Efficiency of electrolysis: 94% Minimum power consumption of electrolysis: 93,511.4 kj/kg (25.98 kwh/kg H 2) Actual power consumption of electrolysis: 99,480.2 kj/kg (27.63 kwh/kg H 2) Cost of electricity: $/kwh Minimum electricity cost of hydrogen: 2.32 $/kg H 2 Actual electricity cost of hydrogen: 2.47 $/kg H 2 The hydrogen production rate is most strongly affected by the electrolysis temperature of the water electrolysis process. Fig. 7 shows how the daily hydrogen production rate changes with respect to the operation temperature of the electrolysis process for models 1, 2 and 3. According to Fig. 7, it can be seen that as the operation temperature of the electrolysis process increases, the hydrogen production rate increases. H2 (kg/day) H2 (kg/day) (a) T (K) T (K) (b) H2 (kg/day) T (K) (c) Fig. 7- The daily hydrogen production rate with respect to the electrolysis operation temperature for (a) model 1 (b) model 2 (c) model 3 The hydrogen production rate of the models developed for the GASKI WWTP are compared to each other. As can be seen, model 3 had the highest hydrogen production rate compared to the other four models. Among the three models, model 3 had the lowest hydrogen production rate. The electricity produced by the biogas powered cogeneration system of the WWTP was totally consumed by the electrolysis processes for hydrogen production in the first three models. According to the economic analysis based on the electricity costs of the models, model 3 had the lowest hydrogen production cost. This was due to the low hydrogen production rate of the model as a result of the low amount of hydrogen sulfide in the biogas. However, considering the remarkable economic and environmental advantages of the high temperature electrolysis process, model 3 can be considered as the most appropriate model for municipal WWTPs in terms of both the hydrogen production rate and the hydrogen production cost. 5 Conclusion In this study, three hydrogen production models were developed, and energy and economic analyses were performed considering the actual operation data from an existing municipal WWTP. The following conclusions can be drawn based on the analyses and results: The daily hydrogen production rates of the models were calculated as 594,625.4 and kg, for models 1, 2 and 3, respectively. Of the models considered in this study, model 3 had the highest hydrogen production rate. The electricity costs of the models were calculated as 3.60, 3.43 and 2.47 $/kg-h 2, for models 1, 2 and 3, respectively. According to these ISBN:

6 results, model 1 had the highest electricity costs while model 3 has the lowest electricity costs. The total hydrogen production costs (capital, operating and maintenance, electricity) of the models were 10.29, 9.8 and 4.18 $/kg-h 2, for models 1, 2 and 3, respectively. According to the energy and economic analyses of these models, model 3 can be seen as the most appropriate one. In this model, as well as in the other electrolysis models, cogeneration waste heat (exhaust) can be used if available as a second option to heat water. References: [1] AP Simpsona, AE Lutzb, Exergy Analysis of Hydrogen Production via Steam Methane Reforming, International Journal of Hydrogen Energy, Vol. 32(18), 2007, pp [2] URL: [3] A. Konieczny, K. Mondal, T. Wiltowski, P. Dydo, Catalyst Development for Thermocatalytic Decomposition of Methane to Hydrogen, International Journal of Hydrogen Energy, Vol. 33(1), 2008, pp [4] T. de Vrije, PAM Claassen, Dark Hydrogen Fermentations. in: Reith JH, Wijffels RH, Barten H Editors. in Bio-Methane & Bio-Hydrogen, Netherlands: NL Agency for Energy and Environment, 2003, pp [5] A. Abusoglu, S. Demir, M. Kanoglu, Thermoeconomic Assessment of a Sustainable Municipal Wastewater Treatment System, Renewable Energy, Vol. 48, 2012, pp [6] C. Coskun, E. Akyuz, Z. Oktay, I. Dincer, Energy Analysis of Hydrogen Production Using Biogas-Based Electricity, International Journal of Hydrogen Energy, Vol. 36, 2011, pp [7] URL: tech_validation/pdfs/fcm02r0.pdf [8] S. Manish, R. Banerjee, Comparison of Biohydrogen Production Processes, International Journal of Hydrogen Energy, Vol. 33(1), 2008, pp [9] A. Demirbas, Biohydrogen for Future Engine Fuel Demands, London: Springer, [10] CC Wang, CW Chang, CP Chu, DJ Lee, BV Chang, CS Liao, Producing Hydrogen from Wastewater Sludge by Clostridium Bifermentants, Journal of Biotechnology, Vol. 102, 2003 pp [11] W. Kreuter, H. Hoffman, Electrolysis: the Important Energy Transformer in a World of Sustainable Energy, International Journal of Hydrogen Energy, Vol. 23(8), 1998 pp [12] JC Ganley, High Temperature and Pressure Alkaline Electrolysis, International Journal of Hydrogen Energy, Vol. 34(9), 2009, pp [13] M. Shen, N. Bennett, Y. Ding, K. Scott, A Concise Model for Evaluating Water Electrolysis, International Journal of Hydrogen Energy, Vol. 36(22), 2011, pp [14] I. Dincer, Green Methods for Hydrogen Production, International Journal of Hydrogen Energy, Vol. 37, 2012, pp [15] IK Kapdan, F. Kargi, Bio-Hydrogen Production from Waste Materials, Enzyme and Microbial Technology, Vol. 38, 2006, pp [16] KY Show, DY Lee, JH Tay, CY Lin, JS Chang, Biohydrogen Production: Current Perspectives and the Way Forward. International Journal of Hydrogen Energy, Vol. 37(20), 2012, pp [17] PL Chang, CW Hsu, CY Lin, CM Hsiung, Constructing a New Business Model for Fermentative Hydrogen Production from Wastewater Treatment, International Journal of Hydrogen Energy, Vol. 36, 2011, pp [18] K. Pandu, S. Joseph, Comparisons and Limitations of Biohydrogen Production Processes: A Review, International Journal of Advances in Engineering & Technology, Vol. 2, 2012, pp [19] N. Genc, Biohydrogen Production from Waste Sludge, Journal of Engineering and Natural Sciences, Vol. 28, 2010, pp [20] A. Abusoglu, S. Demir, M. Kanoglu, Thermoeconomic Analysis of a Biogas Engine Powered Cogeneration System, J. of Thermal Science and Technology, Vol. 33(2), 2013, pp [21] K. Zeng, D. Zhang, Recent Progress in Alkaline Water Electrolysis for Hydrogen Production and Applications, Progress in Energy and Combustion Science, Vol. 36, 2010, pp [22] J. Ivy, Summary Of Electrolytic Hydrogen Production, Milestone Completion Report. National Renewable Energy Laboratory, September NREL/MP-560e ISBN: