تبدیل بخار زیست توده به گاز برای تولید SNG - طراحی فرآیند و تجزیه و تحلیل حساسیت

A process design for small-scale production of Substitute Natural Gas (SNG) by steam gasification of woody biomass is performed. In the course of this work, thermodynamic models for the novel process steps are developed and implemented into an already existing model library of commercial process simulation software IPSEpro. Mathematical models for allothermal steam gasification of biomass as well as for cleaning and methanation of product gas are provided by applying mass balances, energy balances and thermodynamic equilibrium equations. Using these models the whole process is integrated into the simulation software, a flowsheet for an optimum thermal integration of the single process steps is determined and energy savings are identified. Additionally, a sensitivity study is carried out in order to analyze the influence of various operation parameters. Their effects on amount and composition of the product gas and process efficiency are evaluated and discussed within this article.

In increasingly industrialized societies the energy consumption has risen steadily in the last decades. Besides, the dependence on fossil primary energy sources is very strong. Because of limited fossil fuel resources and environmental aspects the share of renewable and sustainable energy of the overall energy demand must be increased. Therefore political strategies have been developed and targets have been defined by the European Commission [1]. It is commonly agreed, that one promising option to reach these targets is the use of biomass as a renewable energy carrier. Biomass is carbon neutral, which means that during growth process as much CO2 is captured via photosynthesis as is released during thermal utilization and consequently net carbon emissions equal zero. Furthermore, biomass is independent from short-term availability fluctuations, unlike hydro, wind and solar power and is therefore suitable for stabilization of renewable energy systems [2]. In recent years, research and development strongly focuses on gasification of solid biomass for production of so called secondary generation fuels (SGF), like Substitute Natural Gas (SNG), Fischer–Tropsch fuels (BtL) or bio hydrogen (BioH2). Especially the production of SNG seems to be a reasonable way because the possibility to feed the SNG into an already existing natural gas infrastructure is given. Further the SNG process is characterized by high conversion efficiency [3]. The production of SNG via gasification of solid biomass and subsequent methanation of the synthesis gas requires a gasifier generating a producer gas with a suitable composition (low nitrogen, high hydrogen). Therefore, mostly allothermal steam gasification technologies are used. Due to the spatial separation of endothermic gasification and exothermic combustion there will be no mixing of the combustible producer gas with the flue gas respectively atmospheric nitrogen. Thus, a virtually nitrogen free, medium calorific value producer (respectively synthesis gas) with high hydrogen content, well suited for SNG production, can be generated using a cheap and technically simple gasification agent without the need for an air separation unit. In principle, also autothermal gasification using pure oxygen as gasifying agent would be suitable. However, air separation for generation of oxygen is expensive and therefore in general economically viable only for large-scale applications. Allothermal steam gasification therefore constitutes the technology of choice for small to middle scale applications, which are mainly considered for biomass due to its local distribution and availability limitations. Several projects deal intensively with the production of SNG, e.g. the EU-project “BioSNG” [4]. In the combined heat and power (CHP) plant in Güssing (Austria) [5] and [6] a 1 MW pilot and demonstration unit for the production of BioSNG was installed and successfully proven. The process of the Güssing CHP plant, as well as other present concepts, is based on atmospheric gasification and commercial cold gas cleaning. Thus, it has to be operated in the power range of 10–25 MW to become economically profitable. The combination of pressurized gasification and hot gas cleaning entails a clear process simplification and significant efficiency increase by which a decentralized SNG production is allowed in small arrangements [7]. Furthermore, the pressurized operation is favorable for subsequent methanation. Pressurized gasification is enabled by the application of the Biomass Heatpipe Reformer (HPR), an innovative allothermal steam gasification technology [8] and [9]. Main parts of the HPR are a combustion chamber and a reformer, both designed as bubbling fluidized bed reactors. Heat is transferred from the combustor into the allothermal reformer using so called Heatpipes. Heatpipes are enclosed metal pipes containing an alkali metal working fluid (e.g. Na, K). This working fluid is evaporated in the area of the exothermic combustion chamber fluidized bed and then condensed in the area of the endothermic gasification fluidized bed. The resulting elevated heat transport density enables complete decoupling of combustion and gasification and therefore pressurized operation of the reformer. This allows compact reactor design and integration, what leads to economic and energetic efficiency comparable to the performance of large systems [10]. A 500 kWth demonstration plant of the Biomass Heatpipe Reformer has been in operation in Pfaffenhofen (Germany) over the past two years showing stable operation [10]. Further, a commercial sized CHP application with a thermal power input of 1.3 MWth is currently under construction in Grassau (Germany). The objective of the work presented in this article is the performance of a process design for production of SNG using a combination of pressurized gasification with the HPR, hot gas cleaning and methanation. Therefore, mathematical models for the single process steps are required. Mathematical models can be divided into thermodynamic equilibrium and kinetic rate models. Kinetic rate models provide detailed information on the analyzed process and can give accurate results. But however, they are computationally intensive and always contain parameters which make them applicable only for one particular reactor design. In contrast, thermodynamic equilibrium models are computationally less intensive, independent of reactor design and therefore more convenient for process studies. Moreover, equilibrium models predict thermodynamic limits of performance under different conditions. This makes these models a useful tool for process design and optimization. Hence, many applications of equilibrium models are reported in literature [11], [12], [13], [14] and [15]. A detailed review and analysis of biomass gasification models is given in [16]. The presented article is organized in three parts. In the first part, thermodynamic equilibrium models developed for the SNG process will be described. A validation of the newly developed HPR model is performed within the second part. In the third part, results of process simulation are shown. Energy savings have been identified and an optimum thermal integration for the overall process has been determined. Furthermore, a comprehensive sensitivity analysis has been carried out to show the influence of the most important operation parameters.

A simulation model for the conversion of solid biomass into raw-SNG was developed using commercial process simulation software IPSEpro. Based on this model, mass balances, energy balances, product gas compositions and process efficiencies were determined. A process flowsheet for an optimum thermal integration was provided and the influence of different operation conditions of the used Biomass Heatpipe Reformer was analyzed. A promising cold gas efficiency of approximately 79% for the HPR and 68% for the entire process can be expected for the design operation case. This clearly demonstrates the potential of decentralized small-scale SNG production by the combination of pressurized gasification, hot gas cleaning and methanation. Results from the sensitivity analysis show an increase of the cold gas efficiency of the HPR with decreasing steam excess ratio, gasification temperature and biomass water content. An increase of fixed char conversion and air preheating temperature, by contrast, has a positive effect on the HPR cold gas efficiency. Among the analyzed parameters, the strongest influence on the cold gas efficiency of the HPR and the entire SNG process can be attributed to the gasification temperature and the air preheating temperature. The study confirms that thermodynamic equilibrium models form a useful tool for process design and optimization as well as for sensitivity studies of various operating parameters. However, pure equilibrium models have their limitations, as total equilibrium conditions may not be achieved in a real gasifier. Especially the heterogeneous char conversion is usually controlled by non-equilibrium factors, but also homogeneous reactions like the methane steam reforming do not reach thermodynamic equilibrium. This has to be considered by empirical data, if available, as done in the present work, or by implementation of kinetic rate expressions. A reasonable modification of the HPR model for future work could be the inclusion of a kinetic rate approach for various heterogeneous gasification reactions in order to predict the non-equilibrium fixed char conversion. Additionally, the implementation of the logarithmic deviation from thermodynamic equilibrium for the methane steam reforming reaction as a function of temperature, instead of a constant value, could possibly further improve the quality of the model results.