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Benzene-char conversion and particle-vapor characterization in biomass gasification
Linnaeus University, Faculty of Technology, Department of Built Environment and Energy Technology.ORCID iD: 0000-0002-8086-1518
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Biomass is increasingly attracting attention as an alternative to fossil fuels. Gasification permits the thermochemical conversion of biomass into a product gas rich in carbon monoxide and hydrogen. The product gas can be further processed to generate heat, electricity, synthesis gas, chemicals, and biofuels. Particulate matter (PM), inorganic and organic (tars) impurities are formed as contaminants in the gasification process. In this thesis PM and tars formed during atmospheric fluidized bed biomass gasification are characterized and the conversion of a model tar compound (benzene) using a biomass based char aerosol in high temperature (HT) applications is investigated.

PM in the product gas of a steam-blown atmospheric bubbling fluidized bed gasifier was characterized for mass size distribution and concentration, morphology, and elemental composition. The hot product gas was extracted using a HT- dilution probe combined with a primary and a secondary thermodenuder to adsorb tars and investigate the volatility/thermal stability of the remaining aerosol, respectively. Size distributions with three distinct modes were established. The fine and intermediate modes were mainly formed by tar and alkali vapors that had condensed in the sampling and conditioning systems. The coarse mode mainly consisted of the original particles, which are char, fly ash, and fragmented bed material. The presented PM sampling and conditioning system also showed the potential for online monitoring of heavy tars.

The tar conversion performance of finely dispersed char particles within a HT-filter and an Al2O3 bed were tested experimentally using benzene as the model-tar. Benzene plus steam (or CO2) were simultaneously supplied to a tubular ceramic reactor that was heated electrically. Fragmented char particles were suspended and continuously supplied via a separate supply line. A HT-filter or a packed bed of crushed Al2O3 balls was positioned in the reactor to retain the char particles. The benzene conversion in the so formed hot char bed was investigated by varying the temperature of the filter or bed, gas flow rates, benzene concentrations, gasification media, char type, char mass and char concentration.

Increasing the ratio of the char mass and gas flow rate (also referred to as char weight time) enhanced the benzene conversion. This was accomplished by increasing the supplied char concentrations, reducing the gas flow rates or slowing the char gasification reactions. The latter was achieved by lowering the steam concentrations or changing the gasification medium from steam to CO2. Increasing the temperature of the Al2O3 bed did not only raise the char gasification rate and thus reduce the char weight time but also showed to enhance the specific benzene conversion activity of the woody char samples. However, in the 900−1100 °C temperature range, the combined effect was to lower benzene conversions at higher temperatures. The apparent rate constant of the benzene conversion was slightly higher when CO2 rather than steam was used as the gasification medium. Increasing the benzene concentration slightly reduced the benzene conversion. Activated carbon pellets showed higher benzene conversions compared to a pine wood char which was related to the higher specific surface area of the activated carbon pellets. In contrast to a commercially available barbeque charcoal made from broadleaf wood, steam-activated woody charcoal converted benzene even in the absence of steam. This was probably due to the earlier steam activation of the woody charcoal and thus higher microporous surface area compared with that of the barbeque charcoal. Doping the woody barbeque charcoal with approximately 0.7 wt. % iron or 2 wt. % potassium did not improve the specific benzene conversion of the char. For a certain char concentration, however, the doping increased the char gasification rate, leaving less char in the packed alumina bed, thus leading to overall lower benzene conversions.

Place, publisher, year, edition, pages
Växjö: Linnaeus University Press, 2018.
Series
Linnaeus University Dissertations ; 321/2018
Keywords [en]
biomass gasification, fluidized bed, particulate matter, aerosol, char, tars, benzene conversion.
National Category
Engineering and Technology
Research subject
Technology (byts ev till Engineering), Bioenergy Technology
Identifiers
URN: urn:nbn:se:lnu:diva-73636ISBN: 978-91-88761-60-6 (print)ISBN: 978-91-88761-61-3 (electronic)OAI: oai:DiVA.org:lnu-73636DiVA, id: diva2:1201804
Public defence
2018-05-31, Södra-salen, Hus M, Växjö, 10:00 (English)
Opponent
Supervisors
Available from: 2018-05-03 Created: 2018-04-26 Last updated: 2018-09-20Bibliographically approved
List of papers
1. Benzene conversion in a packed bed loaded with biomass char particles
Open this publication in new window or tab >>Benzene conversion in a packed bed loaded with biomass char particles
2018 (English)In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 32, no 1, p. 554-560Article in journal (Refereed) Published
Abstract [en]

This study investigates the conversion of benzene in a packed bed containing fine char particles. Benzene and steam were simultaneously supplied to a tubular ceramic reactor that was heated electrically. Fragmented char particles were suspended and continuously supplied via a separate supply line. A packed bed of crushed alumina balls was positioned in the reactor to retain the char particles. The benzene conversion in the hot char bed was investigated by varying the bed temperature (900–1100 °C), steam concentration (0–27 vol %), and char concentration (5–50 g Nm–3). The highest conversions achieved in the experiments were approximately 75%. At comparable char concentrations, similar benzene conversions occurred at 900 and 1000 °C. Increasing the temperature to 1100 °C or increasing the steam concentration reduced the benzene conversion. The results indicate that the reduced conversion was due to enhanced char gasification reactions at elevated temperatures and steam concentrations and thus to reduced char mass in the packed bed.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2018
Keywords
Tar, char, biomass
National Category
Bioenergy
Research subject
Technology (byts ev till Engineering), Bioenergy Technology
Identifiers
urn:nbn:se:lnu:diva-69105 (URN)10.1021/acs.energyfuels.7b03236 (DOI)000423253200059 ()
Available from: 2017-12-06 Created: 2017-12-06 Last updated: 2018-04-26Bibliographically approved
2. Decomposition of benzene using char aerosol particles dispersed in a high-temperature filter
Open this publication in new window or tab >>Decomposition of benzene using char aerosol particles dispersed in a high-temperature filter
2017 (English)In: Energy, ISSN 0360-5442, E-ISSN 1873-6785, Vol. 118, p. 1345-1352Article in journal (Refereed) Published
Abstract [en]

In this study the tar-removal suitability of char particles finely dispersed in a high-temperature filter was investigated. Benzene was selected as the model tar. An aerosol-based method was designed and used to investigate the benzene decomposition behaviour. Two types of char were used: commercially available activated charcoal and pine char prepared in the laboratory. The conversion behaviour of both chars was investigated in the temperature range between 750 and 900 °C using steam as the gasification medium. During the experiments, different benzene concentrations, amounts of deposited char and gas residence times were tested. The results indicate that both activated carbon and pine char reduced the benzene concentration. Activated carbon generally produced higher and more stable benzene conversions compared to the pine char particles. Decreasing the benzene concentration or increasing the gas residence time or char mass improved the benzene conversion. It was concluded that the char gasification rate became slower while benzene was simultaneously converted. The aerosol-based method was also used to investigate benzene decomposition behaviour while continuously supplying fresh char particles together with steam at 1000 °C. In that way, the deactivated and gasified char particles were steadily replaced, preventing the benzene conversion from decreasing over time.

Place, publisher, year, edition, pages
Elsevier, 2017
Keywords
Biomass, Gasification, Char, Tar, Particles
National Category
Bioenergy
Research subject
Technology (byts ev till Engineering), Bioenergy Technology
Identifiers
urn:nbn:se:lnu:diva-58100 (URN)10.1016/j.energy.2016.11.016 (DOI)000395048900116 ()
Funder
Swedish Energy Agency
Available from: 2016-11-11 Created: 2016-11-11 Last updated: 2018-04-26Bibliographically approved
3. Characterization of particulate matter formed during wood pellet gasification in an indirect bubbling fluidized bed gasifier using aerosol measurement techniques
Open this publication in new window or tab >>Characterization of particulate matter formed during wood pellet gasification in an indirect bubbling fluidized bed gasifier using aerosol measurement techniques
2015 (English)In: Fuel processing technology, ISSN 0378-3820, E-ISSN 1873-7188, Vol. 138, p. 578-587Article in journal (Refereed) Published
Abstract [en]

This study characterizes particulate matter, organic compounds, and inorganic compounds formed in an atmospheric indirect bubbling fluidized bed gasifier at two different steam-to-fuel ratios using wood pellets as fuel. The sampling and conditioning system consisted of a high-temperature dilution probe to quench aerosol dynamics and condense inorganic vapors, a primary thermodenuder to adsorb tar components, and a secondary thermodenuder to investigate the volatility/thermal stability of the remaining aerosol. Both online and offline instruments were used to characterize the aerosol in terms of number size distribution, mass size distribution, particle mass concentration, particle number concentration, morphology, and elemental analysis. Size distributions with three distinct modes were established. The fine and intermediate modes were mainly formed by tar and alkali vapors that had condensed in the sampling and conditioning systems. The coarse mode mainly consisted of the original particles, which are char, fly ash, and fragmented bed material. At the higher steam-to-fuel ratio, tar components seem to be reduced and more coarse-mode particles emitted compared to the low steam case. Furthermore, a possibility for online monitoring of heavy tar is suggested. (C) 2015 Elsevier B.V. All rights reserved.

Keywords
Gasification, Biofuels, Particle sampling, Tar
National Category
Bioenergy
Research subject
Technology (byts ev till Engineering), Bioenergy Technology
Identifiers
urn:nbn:se:lnu:diva-47074 (URN)10.1016/j.fuproc.2015.06.041 (DOI)000362920200066 ()2-s2.0-84954196789 (Scopus ID)
Available from: 2015-11-06 Created: 2015-11-06 Last updated: 2018-04-26Bibliographically approved

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