Gas cleaning for low tar producer gases from staged biomass gasification

Abstract

Biomass gasification is a promising CHP technology, due to its high electrical efficiency compared to other CHP systems in the lower and middle range of power. This power class has high potential with respect to the heat demand, and hence, biomass gasification is predestined for decentralized energy systems. Tar contamination of the producer gas obtained by traditional shaft reactor systems with the demand for a costly gas cleaning has been the main obstacle for the successful introduction of the technology to the market.

This project continues the R&D at the Institute of Thermal Engineering at Graz University of Technology (IWT, TU Graz) on the staged gasification, in which the whole process is improved by primary action, where tar load in the raw gas could be significantly reduced to 10 mg/m³n, which is below the limit of the IC engine (50 mg/m³n). This is essential for a successful introduction to the market, when operating costs are decreased (less effort in gas cleaning) and the operational availability of the gasification plant is enhanced.

The functionality of the IWT staged gasification process was proven in the first EdZ project. This present research project deals with the open questions for the complete process chain (gas cooling and cleaning), so that the staged gasification technology can be realized in a demonstration plant consequently. This questions are basically related to gas cooling and heat management of the total process (heat reflux to the staged gas production) as well as to gas cleaning of inorganic pollutants (N-, S-, Cl-, and alkali metal compounds) in a process chain that works without scrubbing for wet gas cleaning. If not removed, ammonia leads to high NOx emissions with the engine exhaust gas, potassium (K) causes e.g. damage to the catalyst after the gas engine, which is used to reduce the CO emissions in the engine exhaust.

The present research project comprises the following investigations:

1. Fundamental research and in the area of gas cooling and dry gas cleaning with regard to
   • an efficient aerosol precipitation for the prevention of catalyst deactivation used for CO emission control,
   • the study of corrosion and deposition caused by inorganic pollutants / aerosols,
   • the producer gas purification from ammonia for compliance with NOx emission limits,
   • the condensate for the compliance with the requirements of the public sewer system,
   • the treatment of possible impurities in the condensate of in the staged process,
   • testing and proving of the hereby designed gas cleaning system
2. Fundamental research and adoption/optimization of the test rig for staged gasification (50 kWel) with regard to
   • the reduction of inorganic pollutants by primary measures,
   • the possibility of ammonia treatment of condensate streams within staged gasification,
   • the improvement of the process simulation models.

With this proposed R&D work the staged biomass gasification can be brought closer to demonstration, being an efficient CHP technology for the decentralized energy production, safe in operation and economically advantageous. Solving the problems of inorganic producer gas pollutants ensures the long time functionality and availability of the total gasification plant as well as the compliance with exhaust gas emission limits of CO and NOx due to the lower risk of damaging in related engine- and exhaust gas treatment apparatus.

Project leader

Univ.-Ass. Dipl.-Ing. Dr.techn. Friedrich Lettner
Institut für Wärmetechnik, TU Graz
Inffeldgasse 25B, 8010 Graz
Tel.: +43 (316) 8737811
E-Mail: Friedrich.Lettner@TUGraz.at
Internet: www.iwt.tugraz.at

Project partners

• KWB - Kraft und Wärme aus Biomasse GmbH., St. Margarethen