Intelligent biogas fuelled energy conversion technologies

The main idea of this research work has been evaluation of possibilities to use biogas in small scale energy conversion technologies developed for natural gas, without hardware modifications. Three technologies, i.e. micro gas turbine, gas engine and high temperature fuel cell have been studied, both experimentally and theoretically, to find out the stable operational window for each of them when the natural gas fuel is gradually replaced by biogas. The reference biogas assumed in this study consists of 6 0% methane and 40% CO2, which provides a fuel heating value of about half of the heating value of the natural gas. Therefore doubled fuel flow is required to maintain same heat release when using biogas. Different energy conversion technologies can handle various amount of biogas mixed with natural gas before operational instabilities occur. To simulate mixture of biogas and natural gas in this study, natural gas was mixed with gradually increasing amount of CO2 until engine stop. The results showed that internal combustion engines can cope with smallest amount of biogas without hardware modifications and the solid oxide fuel cell can operate on rather weak mixture of combustible and CO2 (in this case hydrogen from methane reformation). The micro gas turb ine could handle almost 40% biogas mixed with the natural gas at half load and ca. 26% of biogas could be mixed in without engine instability at full load, which resulted in 20% reduction of CO2 emission from the engine at full load. Given the fact that f uel composition from digestion processes varies as function of time, an intelligent controller and on-line fuel analyzer would be needed to operate the engine reliably at all operational conditions. However, utilizing distributed power generation requires intelligent monitoring tools for efficient operation. Therefore data based models for engine monitoring was also developed in this study, using artificial neural networks (ANN) and experimental data from the test rigs. The results showed that accurate an d user friendly monitoring tools can be developed using ANN to support the end-users during operation and for planning of condition based maintenance of the engine. The approach selected in this project to simulate biogas/NG mixture by mixing CO2 into natural gas stream provided additional synergy effects that were utilized in other ongoing projects. Given the fact that both new NG resources and NG produced from aging fields contain increasing amount of CO2, it was interesting to develop knowledge concerning level of CO2 content in the fuel flow that could be tolerated by small scale energy conversion technologies developed for standardized NG. Another synergetic effect of this project was knowledge development about the impact of higher CO2 content in the exhaust gas, due to higher CO2 concentration in the fuel, on post-combustion CO2 capture. The research group are currently collaborating with a UK based project (GAS-Facts) concerning evaluation of performance improvement for post combustion CCS from natural gas fired power plants. The validated gas turbine model developed in this project has been used for evaluation of novel cycles such as Humid Air Turbine and Exhaust gas recirculation plant.

Biogas is a collective term for gases formed by decomposition of various biological materials without the supply of air. Consisting mainly of methane and CO2, biogas offers the possibility as a renewable and environmentally sound fuel for distributed ener gy utilization. The project is targeted at biogas fuelled energy systems for combined heat and power production, investigating three different technologies. These are energy conversion through the use of gas engines, gas turbines and fuel cells. Using ex perience and competence on natural gas based systems, focus will be on performance, emissions, maintenance costs, availability etc. for the different technologies when using various mixtures of biogas. The objective is to optimize the utilization of bioga s through the use of advanced monitoring tools and by continuous monitoring of each energy conversion technology. Deviation from expected performance will be detected and necessary measures will be taken to run the system at its optima. The R&D tasks wil l have both an experimental and modelling side. Through long-term testing and monitoring, tools for surveillance and techno/economic optimization of biogas based energy systems will be developed. This will involve new control systems, intelligent monitori ng tools and surveillance technology. Measured performance, condition based maintenance, experienced and documented operation envelop will form the basis for development and validation of the models and analysis tools. The project's relevance with regard s to application potential is through the development of knowledge needed for efficient implementation of CO2 free energy production based on local resources. It is the ambition of the project partners to develop and provide knowledge, crucial for integra tion of biogas in the energy system.