Increased fuel flexibility and modulation capability of fixed-bed biomass gasifiers by means of model-based control

Being almost carbon-neutral, the energetic utilization of biomass became more important within the last decades. Especially in the combustion of biomass significant development steps have been achieved and biomass combustion systems are widely spread from small-scale applications typically used for residential heating up to medium and large-scale applications for combined production of heat and power. Parallel to the increase of practically implemented biomass combustion systems, a strong focus of research and development was given to biomass gasification. The main difference between biomass combustion and gasification is the incomplete burnout in gasification leading to a combustible product gas. This resulting product gas gives very high flexibility in the production of heat and power, transportation fuels, chemicals or valuable gases. By using biomass gasification for combined heat and power production, typically autothermal, atmospheric and air-blown gasification systems are considered. Besides fluidized-bed gasifiers or entrained flow reactors, fixed-bed gasifiers have been successfully commercialized for small-scale gasification systems, especially for combined heat and power production. These state-of-the-art smallscale biomass gasification systems can be considered to be robust enough for practical application but their limitations to very specific fuel properties, load modulation capabilities and steady state operation are a remaining barrier on the way towards a wider market distribution. In order to increase the gasifiers fuel flexibility and load modulation capability, an appropriate improvement of their automation and control should be carried out. Currently, the automation and control applied in the different small-scale gasification systems is rather simple since the main external disturbances are avoided a priori by holding the fuel properties as well as the load demand as constant as possible. Consequently, the control just has to keep the system in a steady state what can be achieved by a comparatively small degree of automation and instrumentation sufficiently well. Up to now the research in small-scale biomass gasification focused on procedural and mechanical issues in order to bring a significant amount of plants into economically feasible, practical operation what can be considered as achieved by several technologies. However, in terms of automation and control no significant research has been conducted up to now in the field of small-scale biomass gasification. The most promising approach for the control of such complex systems are model-based control strategies which enable an explicit consideration of the couplings and nonlinear correlations between the different process variables. The system is described by a comparatively simple but nonlinear, mathematical model used as a basis for the controller design. The advantages of such model-based control strategies mainly result from the explicit consideration of all couplings and nonlinear correlations between the different variables leading to increased fuel flexibility, increased load modulation capability, decreased maintenance effort for the operators, increased efficiency and decreased pollutant emissions. This project aims for developing model-based control strategies for selected processes of a fixed-bed biomass gasification system combined with an internal combustion engine for combined production of heat and power. The control should increase the systems fuel flexibility as well as the load modulation capability. The development of the model-based control strategy will be done both experimentally at a real-scale gasification system operated by Urbas Maschinenfabrik Ges.m.b.H and theoretically on the basis of control theoretical, thermochemical or thermotechnical considerations. Finally, the modelbased control strategies for selected processes to be developed will be implemented and experimentally verified by means of the real-scale gasification system.