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Iron-based materials as tar cracking catalyst in waste gasification
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Technology.
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The treatment of municipal solid waste (MSW) in Sweden has changed during the past decades due to national legislation and European Union directives. The former landfills have more or less been abandoned in favour of material recycling and waste incineration. On a yearly basis approximately 2.2 million tonnes waste are incinerated in Sweden with heat recovery and to some extent also with electricity generation, though at a low efficiency. It is desirable to alter this utilisation and instead employ MSW as fuel in a fluid bed gasification process. Then electrical energy may be produced at a much higher efficiency. However, MSW contain about 1 % chlorine in the form of ordinary table salt (NaCl) from food scraps. This implies that the tar cracking catalyst, dolomite, which is normally employed in gasification, will suffer from poisoning if applied under such conditions. Then the tar cracking capacity will be reduced or vanish completely with time. Consequently, an alternative catalyst, more resistant to chlorine, is needed.

Preliminary research at KTH has indicated that iron in its metallic state may possess tar cracking ability. With this information at hand and participating in the project “Energy from Waste” an experimental campaign was launched. Numerous experiments were conducted using iron as tar cracking catalyst. First iron sinter pellets from LKAB were employed. They were reduced in situ with a stream of hydrogen before they were applied. Later iron-based granules from Höganäs AB were tested. These materials were delivered in the metallic state. In all tests the KTH atmospheric fluidised bed gasifier with a secondary catalytic reactor housing the catalytic material was deployed. Mostly, the applied fuel was birch. The results show that metallic iron possesses an intrinsic ability, almost in the range of dolomite, to crack tars. Calculations indicate that iron may be more resistant to chlorine than dolomite. The exploration of metallic iron’s excellent tar cracking capacity led to the innovative manufacture of an iron catalytic tar cracking filter as well as a general knowledge of its tar cracking capacity. This filter with dual functionality would be a general improvement of the gasification process since it among other things would make the process denser.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology , 2011. , 73 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2011:27
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-33043ISBN: 978-91-7415-941-7OAI: oai:DiVA.org:kth-33043DiVA: diva2:413107
Public defence
2011-05-19, F3, Lindstedsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20110428Available from: 2011-04-28 Created: 2011-04-27 Last updated: 2011-04-28Bibliographically approved
List of papers
1. Metallic iron as a tar breakdown catalyst related to atmospheric, fluidised bed gasification of biomass
Open this publication in new window or tab >>Metallic iron as a tar breakdown catalyst related to atmospheric, fluidised bed gasification of biomass
2006 (English)In: Fuel, ISSN 0016-2361, Vol. 85, no 06-maj, 689-694 p.Article in journal (Refereed) Published
Abstract [en]

Tar formation is a major drawback when biomass is converted in a gasifier to obtain gas aimed for utilisation in power production plants or for production of chemicals. Catalytic cracking is an efficient method to diminish the tar content in the gas mixture. In this study, the capability of metallic iron and iron oxides to catalytically crack tars has been experimentally examined. To obtain metallic iron, small grains of hematite (Fe2O3) were placed in a secondary reactor downstream the gasifier and reduced in situ prior to catalytic operation. The fuel used in the atmospheric fluidised bed gasifier was Swedish birch with a moisture content of approximately 7 wt%. The influence of temperature in the range 700-900 degrees C and), values (i.e. equivalence ratio, ER) between 0 and 0.20 have been investigated. In essence, the results show that raising the temperature in the catalytic bed to approximately 900 degrees C yields almost 100% tar breakdown. Moreover, increasing the). value also improves the overall tar cracking activity. The iron oxides did not demonstrate any catalytic activity.

Keyword
biomass gasification, catalytic tar reduction, metallic iron
National Category
Chemical Process Engineering
Identifiers
urn:nbn:se:kth:diva-15383 (URN)10.1016/j.fuel.2005.08.026 (DOI)000234764900013 ()2-s2.0-28244433478 (ScopusID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2011-04-28Bibliographically approved
2. Elemental iron as a tar breakdown catalyst in conjunction with atmospheric fluidized bed gasification of biomass: A thermodynamic study
Open this publication in new window or tab >>Elemental iron as a tar breakdown catalyst in conjunction with atmospheric fluidized bed gasification of biomass: A thermodynamic study
2006 (English)In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 20, no 3, 890-895 p.Article in journal (Refereed) Published
Abstract [en]

Metallic iron as a catalyst for tar cracking in biomass gasification has been investigated. Based on previous studies showing that iron must be in its elemental form to catalyze the tar breakdown reactions, thermodynamic calculations suggest the existence of an operating window where iron is neither oxidized nor contaminated by carbon deposits. A straightforward biomass gasification model has been derived and used in conjunction with thermodynamics for making plots that illustrate the mentioned operating window, which is achievable under real conditions. Experiments made under these specific calculated conditions confirm that elemental iron effectively acts as a tar breakdown catalyst, resulting in an improved gas yield and a decrease in tar concentration. The desired operating window is governed mainly by adjusting the oxygen input (i.e., the equivalence ratio) and the temperature.

National Category
Chemical Process Engineering
Identifiers
urn:nbn:se:kth:diva-15674 (URN)10.1021/ef0502195 (DOI)000237576500003 ()2-s2.0-33745612953 (ScopusID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2011-04-28Bibliographically approved
3. Biomass gasification in an atmospheric fluidised bed: Tar reduction with experimental iron-based granules from Höganäs AB, Sweden
Open this publication in new window or tab >>Biomass gasification in an atmospheric fluidised bed: Tar reduction with experimental iron-based granules from Höganäs AB, Sweden
2011 (English)In: Catalysis Today, ISSN 0920-5861, E-ISSN 1873-4308, Vol. 176, no 1, 253-257 p.Article in journal (Refereed) Published
Abstract [en]

The present study investigates the effect of several experimental iron-based granules on biomass tar decomposition. The iron-based materials were provided by Höganäs AB and were all in their metallic state when they were applied in a secondary catalytic reactor. Bark-free birch was employed as fuel in an atmospheric fluidised bed reactor, and the tar concentration and gas composition in the producer gas were measured before and after the catalytic bed. The results demonstrate a clear tar reduction capacity for all the tested iron-based materials.

Keyword
Biomass gasification, Tar cracking, Iron-based materials, Catalyst characterisation Article Outline
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-33050 (URN)10.1016/j.cattod.2010.12.019 (DOI)000296527900049 ()2-s2.0-80054049467 (ScopusID)
Note
2nd International Symposium on Air Pollution Abatement Catalysis (APAC). Cracow, POLAND. SEP 08-11, 2010. QC 20120313Available from: 2011-04-28 Created: 2011-04-27 Last updated: 2014-10-22Bibliographically approved
4. Iron-based materials as tar depletion catalysts in biomass gasification: Dependency on oxygen potential
Open this publication in new window or tab >>Iron-based materials as tar depletion catalysts in biomass gasification: Dependency on oxygen potential
2012 (English)In: Fuel, ISSN 0016-2361, Vol. 95, no 1, 71-78 p.Article in journal (Refereed) Published
Abstract [en]

A study has been performed using experimental iron based granules as a tar breakdown catalyst in a biomass gasification gas. Previous examinations established that metallic iron located in a separate catalytic bed reactor has a stronger influence on the tar content and composition in the product gas than their corresponding iron oxides. The results from the present study show that tar diminution in the product gas is dependent on temperature, catalyst material and oxygen potential. Typically, values of 50-75% tar reduction were achieved when varying the catalytic bed temperature between 750 and 850 degrees C. Also, the oxidation state of the catalyst material has an influence on the tar content and gas composition in the gas. When changing the gasification temperature from 800 degrees C to 850 degrees C the oxygen potential in the producer gas also changes, resulting in a transition from oxidative to reductive conditions in the gas. This implies that when the gasification temperature is 800 degrees C, the catalyst is transformed from its metallic state to the iron oxide, wustite. Consequently, the tar reduction capacity of the catalyst is reduced by approximately 20%. In view of the overall results it can be concluded that the catalysts in their metallic states in general exhibits a better tar cracking capacity than their corresponding oxides. The iron material used is sintered iron powders manufactured at Hoganas AB, Sweden. The iron materials were dispensed in the metallic state.

Keyword
Biomass gasification, Oxygen potential catalytic tar reduction, Metallic iron
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-33072 (URN)10.1016/j.fuel.2011.06.002 (DOI)000300615900008 ()2-s2.0-84857040381 (ScopusID)
Note
QC 20120326Available from: 2011-04-28 Created: 2011-04-28 Last updated: 2014-10-22Bibliographically approved

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