Innovative system for decentralized CHP on basis of biomass gasification with process optimized production of a low-tar producer gas
Motivation and Goals
Biomass gasification is a promising CHP technology in the lower and middle range of power, due to its high electrical efficiency (= 25%) compared to other CHP systems based on combustion. Tar contamination of the producer gas obtained by traditional shaft reactor systems (min. > 250 mg tar/m³n) with the demand for a costly gas cleaning and residuals treatment/handling has been a major obstacle for the successful introduction of the technology to the market. Additionally, shaft reactor systems have the disadvantage of being sensitive to wood chip size and humidity, as well as imposing technical problems to up-scaling. Staged gasification was designed to overcome these difficulties by primary action.
Within this project a test rig (50 kWel) for the staged gasification of biomass with separated reactors for pyrolysis, partial oxidation and reduction was extended and intensively investigated by experiment.
- Survey of existing gasification systems with a focus on primary measures on tar reduction,
- Development of a simulation model supporting the apparatus dimensioning,
- Final investigation and validation of the pyrolysis apparatus,
- Completion of the test rig: plant engineering and construction, integration of measurement and control devices, integration of process automation,
- Experimental study of process and gas quality parameters
All project goals could be successfully fulfilled. A new approach to biomass gasification was designed and experimentally investigated. With the staging of the process into separate reactors a heavy reduction of tar contaminants could be achieved by primary measures (between min. 96 and 99,9% in comparison to "old" systems). The remaining tar concentration is in the range of up to 10 mg/m³n, which is below the limits set by e.g. internal combustion (IC) engines 50 mg/m³ n. These results could be realized in all test runs (duration up to three days), independent from the investigated biomass composition (bark content up to 11%m) and biomass humidity (up to 100%m,dry). For this reason, it is possible to avoid an in general cost intensive, wet gas cleaning with regard to the condensable tar load - giving a big economic advantage to gasification plants in the lower and middle range of power. Additionally the risk of fouling with tar condensates at the heat exchanger and piping of the total plant is highly reduced, coming along with an increase of the plant availability. Hereby, the chances of a successful introduction of this technology to the market are improved significantly.
Based on the fact that the tar problem is solved by staged gasification as reported in this study, future research work will focus on the total process chain, with simplified efforts in gas cleaning. Critical pollutants that remain are ammonia, and metal aerosols (e.g. potassium), the first forming fuel-NOx in the IC engine, the latter imposing problems to the oxidation catalysts (poisoning) after the engine. In view of this, ammonia and metal concentrations in the raw producer gas as well as important dust parameters like particle size distribution and structure were additionally measured within this project.
Finally, an economic study was conducted based on the expenditures for the construction of the test rig, showing that operation of biomass gasification plants can already be economically for plants with an electric power of 50 kWel (+20 kWel) or above.
Dipl.-Ing. Friedrich Lettner
TU Graz, Institut für Wärmetechnik
Tel: +43 (0)316/873-7811
Fax: +43 (0)316/873-7305
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