تحلیل ترمودینامیکی و بررسی اثرات متقابل پارامترها در سیستم ریفرمینگ بخارآب بیوگاز-آب شیرین‌کن رطوبت‌زن-رطوبت‌زدا

Abstract
In this paper, biogas steam reforming (BSR) coupled with a humidification dehumidification unit (HDH) was proposed and the novel thermodynamic analysis interaction effects on energy efficiency and exergy efficiency via the design of experiments was used. Comprehensive thermodynamic modelling has been performed using EES software. From the outlet results, the energy efficiency, exergy efficiency, hydrogen mass flow rate and freshwater mass flow rate of the system are obtained 82.39% and 72.65%, 0.1071 kg/s and 0.211 kg/s, respectively. Thus, by utilizing the thermodynamic analysis of the combined system by EES software and transferring the experiments based on the central composite design for the input parameters (the inlet temperature of the dehumidifier, humidifier and dehumidifier circulated mass flow rate and the desalination heater inlet temperature) extracted by the design expert software, the results show the impact of the interaction of the input parameters. In RSM model, the central composite design (CCD) is employed in the experimental design. values in energy and exergy efficiency responses were calculated 99.99% and 99.97%, respectively that shows the model has a good accuracy. The optimum points for parameters of A, B and C and also responses of energy efficiency and exergy efficiency are obtained 310 K, 8 kg/s, 450 K, 0.9051% and 0.7313%, respectively.
Introduction
Hydrogen, a very versatile fuel, can be produced from various materials and by several methods. Industrial-scale production of hydrogen has been operational in the oil and gas industry for more than a century and forms the base of the modern chemical industry. In centralized facilities and distributed generation, hydrogen has to be supplied to stationary fuel cell applications from nearby hydrogen mass-production processes. There are three established methods for reforming fuels: steam reforming (SR), partial oxidation (POX) and auto-thermal reforming (ATR). All of the mentioned methods produce a syngas mixture; however, the difference in reaction temperatures and oxidants yields different CO concentrations in the syngas mixture. The H2 production generally decreases in the order of steam reforming, auto-thermal reforming and partial oxidation [1]. Biogas is used as the most practical renewable energy source in place of fossil fuels for power and hydrogen production which has a main role in the minimization of global warming. Biogas can be achieved by biomass anaerobic fermentation and decomposition which itself is composed of different organic materials, namely, 60-70% methane, 30-40% carbon dioxide, and other negligible gases such as hydrogen, nitrogen, oxygen, mono oxide, and hydrogen sulphide. Therefore, due to the high contribution of carbon dioxide and methane (greenhouse gases) in biogas mixtures, one can effectively utilize them in various reforming processes to produce hydrogen [2].
Many countries have initiated their programs for large-scale exploitation of biogas resources. This requires many investigations and scholars’ efforts to model recovery processes from municipal and industrial waste for producing electricity or other forms of commodities [3]. In recent years, hydrogen as clean energy has been considered for energy production which is compatible with the environment and widely used in chemical and power plant industries [4, 5]. In addition, hydrogen can be effectively converted to electricity by fuel cell systems with negligible greenhouse effects, or vice versa [6, 7].
The scarcity of fresh water is one of the major problems that engulf challenges in human societies. This has a major impact on population growth and economics. It is predicted that by 2025, nearly 70% of the world's population will suffer from problems with freshwater shortage. One of the manners to compensate for this shortage is by improving desalination technologies at the industrial and domestic scales. HDH is one of the main techniques in water desalination processes. This method is more appropriate for household scales than other thermal desalination processes due to its benefits like less operating costs at low capacities, working in moderate operating conditions, and lack of sensitivity to the quality of inlet saline water in comparison with membrane desalination processes [20]. Different researchers in this field offered various designs of HDH processes to improve the performance of conventional HDH systems, including hybrid humidification dehumidification-heat pump (HDHHP) [21], vacuum humidification dehumidification (VHDH) processes [22], humidification compression (HC) [23], and injection-extraction technologies [24].
A biogas steam reforming system is proposed by Cipiti et al. [8] in a temperature range of 700 -900 °C, where theoretical and empirical studies are performed. Based on their results, the increment of the temperature and steam-to-carbon molar ratio can improve the hydrogen generation rate. Gargari et al. [9] proposed a new hybrid system for power and hydrogen generation purposes using a combination of a Gas Turbine-Modular Helium Reactor (GT-MHR) as a topping system for power production and biogas steam reforming as the bottoming cycle for hydrogen production. A comprehensive thermodynamic analysis, as well as the parametric study, is performed to investigate the feasibility of the proposed system. Based on their obtained results, the power generation and hydrogen production capacity of their hybrid GT-MHR/BSR plant were calculated 260.13 MW and 0.217 kg/s, respectively.
Abbasi and Pourrahmani [27] proposed a novel geothermal integrated system with two different configurations to produce freshwater and hydrogen. The results of the exergy destruction and exergy destruction cost rates for both configurations indicated that TEG has the highest values, among other components. Kalina cycle and the HDH unit have the least exergy destructions in the current cycle. Also, optimization studies revealed that the optimal mode is superior in terms of exergy efficiency, freshwater cost, and hydrogen cost with the values of 22.49%, 2.94 $/m3 and 7.37 $/kg, respectively. Ghaebi and Ahmadi [28] introduced an innovative hybrid system coupled with HRSG and HDH desalination units. The outcomes exhibit that the introduced trigeneration system generates heating load, net electricity, and distilled water of 370.2 kW, 1605 kW, and 345.708 kg/h, respectively. Based on this scenario, the trigeneration energetic and exergetic efficiencies are computed 85.56% and 63.04%, respectively. Additionally, among all elements, the SOFC stack and afterburner are recognized as the most destructive components by 233.3 kW and 173.3 kW, respectively. An exhaustive parametric evaluation is carried out through the study and it is figured out that the main factors of the system can have a maximum point in terms of the fuel utilization factor, desalination flow ratio, and desalination maximum temperature.
The full trial design is not cost and time effective. The design of experiments (DOE) method is a useful approach to minimize time, cost, and the number of trials. DOE methods are utilized to optimize response variables in the presence of various factors with different levels. DOE is the application of geometric principles to statistical sampling to obtain desired results. Achieving the desired response with the lowest number of trials is the most important objective in DOE [22, 23]. DOE enables the simultaneous study of several factors and assessment of their statistical significance, as well as the evaluation of interaction effects. Response surface methodology (RSM) is a powerful tool for experimental design, analyzing, modeling, and optimization of any multivariable system and also is one of the most applicable DOE methods [32]. One of the most valuable outputs of RSM is to present a regression model to predict the response variable based on the considered input parameters. Multi-objective optimization performance is another important tool of RSM that is used in many engineering applications [33]. For instance, Mostafa Pourali et al. [34] performed a comprehensive investigation to study the effects of various design parameters on CH4 conversion in a catalytic microchannel for hydrogen production. the RSM is employed to study the effects of channel height, inlet velocity and temperature, wall thickness and conductivity, and external heat flux on CH4 conversion. It is found that the inlet gas temperature, among different parameters, has the most influence on the overall performance of the microchannel hydrogen production. Also, the maximum necessary heat of reforming reaction increases by 84% and 26% if the CH4 conversion changes from 50% to 60% and 60% to 70%, respectively.
Rahimi-Ahar and Hatamipour. [36] compared the freshwater productivity of a three-stage vacuum humidification-dehumidification (VHDH) desalination system to a single-stage VHDH system using the response surface methodology (RSM). The optimal values of saline water to air mass flow rate ratio (sw/a) and humidifier pressure (PH) of 1.77 and 33 kPa led to the maximum freshwater productivity. In all operating conditions, through converting the single-stage humidification to two-stage and three-stage humidification processes, the humidity ratio was enhanced by about 51% and 19%, respectively.
In the present study, a comprehensive energy and exergy analysis of a biogas steam reforming system combined with a HDH for hydrogen and fresh water coproduction is performed. Although several studies were reported using RSM in different engineering problems, the application of RSM in performance evaluation of the co-generation system is scarce, which offers another scope for the present study. Regression models using RSM are presented to estimate the output parameters and the effect of considered parameters and their interactions are studied on the output parameters. The motivation and objective of the current study are:
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