Conference Dates

March 8-13, 2009

Abstract

Solid particles and tars are among the non-desirable products of synthesis gas produced during biomass gasification. Removal of fly ashes is necessary in order to comply with emission limits as well as avoid their deposition in downstream units. Condensation of tars, on the other side, can cause clogging. A catalytic hot gas filter can remove both solids and tars, when operating at temperatures as high as 850oC. Catalytic hot gas filter elements are under development in order to solve this issue.

A lab-scale filtration unit has been designed and constructed at Delft University of Technology. The unit contains one ceramic hot gas filter candle which is made of a SiC porous structure coated with a mullite membrane. The integration of a Nickel-based catalyst layer allows the dual function of particle filtration and tar cracking. The filter vessel is part of a set-up that is equipped with a tar evaporator and a pre-heater, both located upstream of the filter unit.

This paper presents the results of the first set of experimental tests that have been performed with this unit. A dust-free model gas was used and consisted of a mixture of CO (14%), CO2 (14%), H2 (7%), CH4 (5%), and varying concentrations of N2 (30, 40, 50%) and H2O (30, 20, 10%). Naphthalene (2g/Nm3 and 5g/Nm3) was adopted as model compound in order to study the catalytic conversion of heavier hydrocarbon species to H2. A gas face velocity of 2.5cm/s was selected for tests performed at atmospheric conditions and at operating temperatures varying between 700oC and 850oC. The pressure drop through the filter candle was continuously monitored during the process. The gas composition was measured upstream and downstream of the filter unit by means of an on-line micro-GC, while naphthalene concentration was attained with the SPA method.

The following findings were obtained: higher naphthalene conversion with increasing temperatures and better conversion at any temperature when 2g/Nm3 were used compared to 5g/Nm3. Tests at 850oC and 30 vol% H2O produced a conversion of 99.4% with 2g/Nm3 while 98.5% with 5g/Nm3. Experiments with higher steam content showed higher conversion values. Methane concentration was also affected thus indicating that reforming reactions took place as well. Pressure drop measurements showed that no carbon deposition occurred on the candle during the tests, even at lower temperatures. At this stage new experiments are being performed with different face velocities and tar compounds in order to provide enough data for future tests with real gasification gas.

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