تجزیه و تحلیل عملکرد سلول سوختی اکسید جامد ترکیبی و چرخه توربین گاز: استفاده از سوخت های جایگزین

In this paper, the hybrid solid oxide fuel cell (SOFC) and gas turbine (GT) model was applied to investigate the effects of the inlet fuel type and composition on the performance of the cycle. This type of analysis is vital for the real world utilization of manufactured fuels in the hybrid SOFC–GT system due to the fact that these fuel compositions depends on the type of material that is processed, the fuel production process, and process control parameters. In the first part of this paper, it is shown that the results of a limited number of studies on the utilization of non-conventional fuels have been published in the open literature. However, further studies are required in this area to investigate all aspects of the issue for different configurations and assumptions. Then, the results of the simulation of the syngas-fueled hybrid SOFC–GT cycle are employed to explain the variation of the stream properties throughout the cycle. This analysis can be very helpful in understanding cycle internal working and can provide some interesting insights to the system operation. Then, the detailed information of the operation of the methane-fueled SOFC–GT cycle is presented. For both syngas- and methane-fueled cycles, the operating conditions of the equipment are presented and compared. Moreover, the comparison of the characteristics of the system when it is operated with two different schemes to provide the required steam for the cycle, with anode recirculation and with an external source of water, provides some interesting insights to the system operation. For instance, it was shown that although the physical configuration of the cycle in two systems is the same, the actual configuration (the equipment actually taking part in the process) can be different. Finally, the results of the simulation for different types of the inlet fuel show that system outputs and operational parameters are greatly influenced by changes in the fuel type. Therefore, the possibility of variation of the inlet fuel type should be considered, and its impacts should be investigated before utilization of biogas, gasified biomass, and syngas as fuel in hybrid SOFC–GT cycles.

Although the utilization of renewable and alternative fuels has been very popular recently due to their environmental benefits and/or their renewable nature, their applications have been associated with some drawbacks. These drawbacks, especially when they are used to fuel fuel cell-based systems, should be studied and addressed before their widespread applications can be secured. One of the main problems that requires special attention is the variation of inlet fuel composition. Unlike gaseous fossil fuels such as natural gas, alternative fuels are produced by processing of feed stocks; therefore, their composition and quality vary with operating parameters of the process as well as the type of feed stock. For instance, in the cases of biogas and syngas, the type of biomass or coal that is processed can change the composition and quality of the produced fuels. Also, the fuel production process and process control parameters can affect the produced fuels. For example, both biomass and coal syngas are traditionally produced by the gasification process, where the fuel composition heavily depends on the gasifying agent, namely, steam, air, or oxygen. The fuel produced by this process mainly contains methane, hydrogen, carbon dioxide, water, nitrogen, and carbon monoxide; however, the concentration of each component depends on the gasifying agent and process control parameters. For example, in the oxygen, air, and steam-blown gasification processes, carbon monoxide/carbon dioxide, nitrogen, and steam can be found high in concentration, respectively [6], [12] and [15]. Thus, the composition of alternative fuels can be altered and this problem should be addressed to identify the effects of variation in fuel composition on the system’s overall performance before these fuels can be used in hybrid solid oxide fuel cell (SOFC) cycles. In most published papers on hybrid SOFC system modeling in the open literature, methane or natural gas has been used to fuel the system. However, there are limited numerical studies that investigated utilization of non-conventional fuels. Lobachyov and Richter [5], in one of the earliest numerical works in the field, developed a model that incorporated a coal gasification process into a hybrid SOFC–gas turbine (GT) cycle. Similarly, Kivisaari et al. [3] conducted a feasibility study to integrate a molten carbonate fuel cells (MCFC) or SOFC to a coal gasification plant in a combined heat and power (CHP) plant. Kuchonthara et al. [4] modeled the integrated power generation cycle combining thermochemical recuperation, brown coal gasification, and an SOFC. Rao et al. [8] compared an integrated gasification SOFC and an integrated gasification combined cycle (IGCC) in their thermoeconomic analysis. Van herle et al. [16] conducted an energy balance analysis on an existing biogas production unit integrated into an SOFC cycle in a small CHP system. Sucipta et al. [10] further investigated the gasification system integrated to an SOFC–micro gas turbine (MGT) cycle by adding and comparing different biomass gasification processes, namely, air, oxygen, and steam-blown. Using the same model, Sucipta et al. [11] studied the efficiency and temperature distributions in cases where natural gas, the normal fuel of the hybrid system, was mixed or completely replaced by biofuel. They investigated effects of composition changes on the performance of the SOFC–MGT hybrid system. They found that the efficiencies of the SOFC module and of the hybrid system noticeably decreased when natural gas was completely replaced by biofuel but the SOFC–MGT hybrid system could still be operated with reasonable performance. A more detailed review of these papers can be found elsewhere [18]. In this study, the model described in Suther et al. [13], and Suther et al. [14] was applied to observe the performance parameters of the hybrid SOFC–GT cycle with two configurations, with and without anode recirculation, when the system was fueled by syngas, methane, and other alternative fuels, including various forms of biogas and gasified biomass. The methodology, mathematical formulation, assumptions and simplifications, model constants and parameters, and model validation for the SOFC part of the model and hybrid SOFC–GT were presented elsewhere [13] and [14]. The models were developed using Aspen Plus® platform. In order to monitor the performance of the system, stream parameters such as temperature, pressure, flow rate (mass, volume, and molar), and composition of flows were considered. For the equipment, their characteristics, such as power, specific work, efficiency, and heat duty were investigated. This paper starts with the analysis of the syngas fueled hybrid SOFC–GT cycle, including thermodynamic properties of all major streams in the cycle and operational conditions of all main equipment. Then, the same analysis is presented for methane-fueled system and the results are compared with the results of the system fueled with syngas. Finally, the system performance is evaluated when it is fueled by natural gas and various forms of biogas and gasified biomass with different compositions.

The analysis of hybrid SOFC–GT cycles’ operation when the steam required for methane reforming and water–gas shift reactions is provided by the anode exhaust stream recirculation and via external source of water showed a significant difference (regardless of type of inlet fuel). Although the efficiency and output power of the two cycles were similar, the specific work of the cycle with anode recirculation was much higher than that in the cycle without anode recirculation, which means lower capital costs for former configuration for the same output power. Also, the output power of the SOFC in the cycle with anode recirculation was higher, whereas, the output power of the gas turbine was higher in the cycle without anode recirculation. The comparison of the system performance when it was fueled with syngas and methane (for both configurations) indicated that output power, specific work, and efficiency of the system increased when syngas was replaced with methane. The analysis of the operation of the system when fueled with a wide range of fuel types demonstrated that the hybrid SOFC–GT cycle efficiency can be between 59% and 75%, depending on the inlet fuel type. All other characteristics and operational parameters of the cycles were also remarkably influenced by the inlet fuel type, which highlights the importance of the evaluation of possible variations of inlet fuel compositions and its impacts on the performance of the hybrid SOFC–GT cycles, especially if the cycle is fueled with manufactured fuels.