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Innovative components for decentralised combined heat and power generation from biomass gasification

Exploitable results

Based on the experiments described above, a set of parameters for stable operating conditions for both gasification technologies are delivered. The test results show that the highest hydrogen contents were obtained with steam gasification, but with very high tar amounts. The totality of the made gasification tests shows that it is very helpful to admix oxygen to allothermal steam gasification. This enhances the pure steam gasification process in two important ways. It lowers the tar content to values that make the tar cracking more feasible and the fuel throughput is increased. The lowest tar contents were measured in the air gasification tests. The low hydrogen yield of air gasification could be increased through admixing of steam and through the use of a tar cracker. The use of olvine-sand as bed material was possible without any restriction due to temperature and pressure. Magnesite was limited to high temperature and the use of gasification agent air. At low temperatures around 400-500 degrees Celsius it showed tendencies to build hydroxides which led to agglomeration of particles. Dolomite had similar problems at a pressure of 10 bar with the gasification agent steam in combination with temperatures above 700 degrees Celsius. Hydrogen was a strong catalyst poison over 650 degrees Celsius for dolomite. The work was successful and brought new approaches to Jenbacher AG which are of interest also to running series developments. The intensively cooled pre-chamber, which was developed especially for the direct gas in-blow, is already produced in series. The proceedings with the in-blow of natural gas into the inlet runner has been very promising due to the first tests on the test bench. Nevertheless it has to be further optimised to be produced in series and to take over the place of the recent gas mixing concept in operation. It seems as well, that, due to the knowledge up to now, with this concept a bmep of at least 8 bar is to be reached with biogas. These 8 bar bmep result to a performance of about 750 kW with a 12 cylinder engine, pursued was a performance of 700 kW at the project definition. In cooperation with Jenbacher AG, the Technical University Graz had developed a very promising method of mixture formation. In a concept study performed with CFD it was shown that gas feeding through a pipe in the intake port makes little sense with regard to mixture formation. The concept of blowing the gas in via two pipes through the open intake valves during the intake cycle proved to be even less favourable. Direct feeding of the combustion gas into the combustion chamber with a mixing chamber proved to be unsuitable as the fuel-air mixture at the spark plug position is far too rich to be ignited at ignition timing. The most suitable concept proved to be the method of blowing the gas in through two rings located immediately before the intake valves on the circumference of the intake port. This concept was implemented. Test bed tests have shown that the new concept is nearly equivalent to the ideal basic version in terms of efficiency, emission behaviour and engine stability. The gasification of biomass with steam offers advantages regarding the gas composition and the heating value. However, a high technical effort was necessary for the heat supply to the gasifier and for the heat recovery. To reduce the effort with regard to economic aspects the operating temperature for the heat exchanger equipment was lowered and consequently the number of heat exchangers decreased. The attained electrical efficiency was about 25%. The overall efficiency of the CHP unit was about 44%. This relatively low value was caused by the high amount of energy required for steam generation which could not be recovered to the process or be used for district heat generation. The district heat generation was done at a high temperature level. If the emphasis of such a plant would be the electricity generation than the district heat generation could be replaced by a small steam cycle. Due to the allothermal gasification the gasifier was supplied with heat through the combustion of the produced syngas. About 18% of the syngas was combusted for it. If an external high temperature heat source could be integrated for the heat supply to the gasifier the efficiency of this unit would increase significantly.