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Char conversion kinetics and aerosol characterization in biomass gasification
Linnaeus University, Faculty of Technology, Department of Building and Energy Technology. (Bioenergy)
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Biomass gasification is a thermochemical conversion by partial oxidation at elevated temperature of solid biomass into a gaseous energy carrier. The product gas contains the major components CO, H2, CO2, CH4, as well as some tar and inorganic impurities and solid particles such as ash, bed material, soot and char. The aim of this work is to develop an aerosol-based method to investigate on-line the reactivity of the suspended biomass char particles at high temperatures, and to apply aerosol measurement systems for sampling and characterizing particulate matter in the hot product gas from gasifiers.

An aerosol-based method including the steps for generating, transporting, and oxidizing suspended char particles (0.5–10 µm) was proposed and developed for investigation of char reactivity at high temperatures. An aerodynamic particle sizer (APS) spectrometer was used to measure the particle size distributions. A tapered element oscillating microbalance (TEOM) was used to measure the change of mass concentrations of particles in the carrier gas, before and after conversion. The intrinsic kinetics of various biomass (wood, straw, miscanthus) char particles have been experimentally established in a wide temperature range for both combustion (in air/oxygen) and gasification (in 33 vol% CO2 or 33 vol% steam), up to 800°C and 1300°C, respectively, by combining the aerosol method with thermogravimetric analysis (TGA). The general CO2 or steam gasification reactivity of chars from different biomass could be ranked as wood > miscanthus > straw. In CO2 or steam gasification, the reactivity of char samples measured by the aerosol method at 1300°C would vary by a factor of 4-9 comparing with the extrapolated estimation from the TGA results at the low temperatures. This indicates that high-temperature reactivity estimation by extrapolation should be used with care. Variations of the morphology and the effective density of char particles during conversion indicated that in the initial stage of char conversion (either combustion or gasification), pore growth was dominant up to a certain conversion, and shrinkage or fusing would occur in the later stage. The aerosol-based method presents a set of benefits which are advantageous compared to previously established techniques: no mass transfer limitation at high temperatures; the flexibility to switch to different gas agent combined with continuous feeding of char sample; and the on-line measurement of particle mass and size. The aerosol method is not applicable under the conditions where the reaction rate is slow, since longer residence time will increase the probability of particle losses. In addition to laboratory applications, the aerosol method has potentials for on-line investigation of concentration and reactivity of suspended char fragments sampled directly from the product gas in different types of gasifiers.

Particulate matter (10 nm–10 µm) in the product gas was characterized for the size distribution, morphology and elemental composition by both on-line and off-line techniques. An aerosol particle measurement system including a dilution probe connected in series with a packed activated carbon bed was applied to extract aerosol from the hot product gas produced in the gasifiers using wood as feedstock: an indirect bubbling fluidized bed gasifier and a circulating fluidized bed (CFB) gasifier. The fine and coarse particles from the CFB gasifier contained calcium and magnesium, indicating the contributions from the ash and the magnesite bed material. From the indirect gasifier, the fine-mode (<0.5 µm) particles were dominated by potassium and chlorine whereas the coarse-mode (>0.5 µm) particles were dominated by calcium and silicon, probably from the ash and the bed material. Char fragments were identified in the hot product gas and contribute to the coarse-mode particles in both gasifiers.

 

Place, publisher, year, edition, pages
Växjö: Linnaeus University Press, 2013.
Series
Linnaeus University Dissertations ; 150/2013
Keywords [en]
Biomass gasification, fluidized bed, char, kinetics, aerosol, APS, TEOM
National Category
Chemical Engineering Bioenergy
Research subject
Technology (byts ev till Engineering), Bioenergy Technology
Identifiers
URN: urn:nbn:se:lnu:diva-31347ISBN: 978-91-87427-56-5 (print)OAI: oai:DiVA.org:lnu-31347DiVA, id: diva2:682166
Public defence
2013-10-18, M1083, Hus M, Växjö, 10:00 (English)
Opponent
Supervisors
Available from: 2014-02-25 Created: 2013-12-23 Last updated: 2014-02-25Bibliographically approved
List of papers
1. Online investigation of steam gasification kinetics of biomass chars up to high temperatures
Open this publication in new window or tab >>Online investigation of steam gasification kinetics of biomass chars up to high temperatures
2014 (English)In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 28, no 1, p. 607-613Article in journal (Refereed) Published
Abstract [en]

In this study, a novel aerosol-based method has been further developed and applied for on-line investigating of steam gasification kinetics of suspended biomass char particles. By combining the aerosol method with thermogravimetric analysis, the gasification kinetics were established in steam (33 vol%)-N2 atmosphere from 800°C to 1300°C for char samples produced from pelletized wood, straw, and miscanthus. The aerosol method includes steps for generating, suspending, and gasifying char particles. The conversion of the char particles was established by measuring the change in particle size distributions and mass concentrations using an aerodynamic particle sizer (APS) spectrometer and a tapered element oscillating microbalance (TEOM), respectively. The reactivity of three char samples could be ranked as wood > miscanthus > straw. The activation energy was 155 kJ·mol-1 for wood char, 199 kJ·mol-1 for miscanthus char, and 222 kJ·mol-1 for straw char. Results interpreted from TEOM and APS measurements indicated that the effective density of char particles initially decreased until a certain level of conversion was reached, and then remained constant. 

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2014
Keywords
biomass, steam gasification, char, kinetics, aerosol, APS, TEOM
National Category
Chemical Engineering Bioenergy
Research subject
Technology (byts ev till Engineering), Bioenergy Technology
Identifiers
urn:nbn:se:lnu:diva-31344 (URN)10.1021/ef402343x (DOI)000330018200068 ()2-s2.0-84892729774 (Scopus ID)
Available from: 2013-12-21 Created: 2013-12-21 Last updated: 2017-12-06Bibliographically approved
2. Investigation of the intrinsic CO2 gasification kinetics of biomass char at medium to high temperatures
Open this publication in new window or tab >>Investigation of the intrinsic CO2 gasification kinetics of biomass char at medium to high temperatures
2013 (English)In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 109, no SI, p. 220-228Article in journal (Refereed) Published
Abstract [en]

In total eight char samples from pelletized wood, miscanthus, and straw were prepared under various pyrolysis conditions. The CO2 gasification kinetics for each sample was established in the temperature range from 800 °C to 1300 °C by the combination of thermogravimetric analysis (TGA) and a novel aerosol-based method. The aerosol-based method was used for the high temperature range between 1100 °C and 1300 °C, by gasifying suspended char particles (0.5–10 μm) in an oxidizing carrier gas. A tapered element oscillating microbalance (TEOM) was used to measure the change of mass concentrations of particles in the carrier gas, before and after gasification. The results showed that the aerosol-based method could be used to investigate the intrinsic gasification kinetics of biomass char, at least up to 1300 °C. All char samples showed similar reactivity in the low temperature range. However, above 1000 °C there were significant differences in reactivity, and at 1300 °C the conversion of the wood was in the order of 10 times faster than that of straw. The general char reactivity order in this study was wood > miscanthus > straw.

Place, publisher, year, edition, pages
Elsevier, 2013
Keywords
Biomass; Char; Gasification kinetics; Aerosol; TEOM
National Category
Chemical Engineering Bioenergy
Research subject
Technology (byts ev till Engineering), Bioenergy Technology
Identifiers
urn:nbn:se:lnu:diva-25295 (URN)10.1016/j.apenergy.2013.04.027 (DOI)000321724000025 ()2-s2.0-84877335598 (Scopus ID)
Available from: 2013-05-06 Created: 2013-04-11 Last updated: 2017-12-06Bibliographically approved
3. Aerosol-based method for investigating biomass char reactivity at high temperatures
Open this publication in new window or tab >>Aerosol-based method for investigating biomass char reactivity at high temperatures
2011 (English)In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 158, no 7, p. 1426-1437Article in journal (Refereed) Published
Abstract [en]

An aerosol-based method was proposed and developed to characterize particles fragmented from biomass chars during oxidation. The chars were prepared from both wood and miscanthus pellets under various pyrolysis conditions. Char fragments with aerodynamic diameters in the range of 0.5–10 μm were suspended and transported in a reactive gas through an aerosol reactor, which was heated by an electric oven. The oxidation of char particles in the reactor was investigated by determining on-line the particle size distributions before and after passage through the reactor using an aerodynamic particle sizer (APS) spectrometer. The interpretation of APS data was evaluated by both experiments and models in which the fine char particles were assumed to keep either constant density or constant diameter during the oxidation process. The results indicate that the aerosol-based method can be used to determine reaction kinetics of char particles in the high temperature range, where oxidation is normally controlled by diffusion limitation if measuring with the conventional techniques. The application of the aerosol method indicated that high pyrolysis temperature and prolonged retention time will reduce the char reactivity.

National Category
Industrial Biotechnology
Research subject
Technology (byts ev till Engineering), Bioenergy Technology
Identifiers
urn:nbn:se:lnu:diva-7044 (URN)10.1016/j.combustflame.2010.11.015 (DOI)2-s2.0-79955881888 (Scopus ID)
Available from: 2010-08-12 Created: 2010-08-09 Last updated: 2017-12-12Bibliographically approved
4. Characterization of particulate matter in the hot product gas from atmospheric fluidized bed biomass gasifiers
Open this publication in new window or tab >>Characterization of particulate matter in the hot product gas from atmospheric fluidized bed biomass gasifiers
2011 (English)In: Biomass and Bioenergy, ISSN 0961-9534, E-ISSN 1873-2909, Vol. 35, no Supplement 1, p. 71-78Article in journal (Refereed) Published
Abstract [en]

This study compares the characteristics of particulate matter (PM) in the hot product gas from three different atmospheric fluidized bed biomass gasifiers: a bubbling fluidized bed (BFB) gasifier, a circulating fluidized bed (CFB) gasifier, and an indirect BFB gasifier (the latter integrated with a CFB boiler). All gasifiers displayed a bimodal particle mass size distribution with a fine mode in the <0.5 μm size range and a coarse mode in the >0.5 μm size range. Compared with the mass concentration of the coarse mode the mass concentration of the fine mode was low in all gasifiers. For both the BFB and CFB gasifiers the fine-mode PM had a similar inorganic composition, indicating an origin from the ash and the magnesite bed material used in both gasifiers. In the indirect BFB gasifier the fine-mode PM was instead dominated by potassium and chlorine, and the tar fraction properties evoked tar condensation in the sampling system that affected mainly the fine-mode PM. The coarse-mode PM in the BFB gasifier was dominated by char fragments abraded from the pyrolyzed wood pellets. In the CFB gasifier the coarse-mode PM was mainly ash and magnesite bed material that passed through the process cyclone. In the indirect BFB gasifier the coarse-mode PM was mainly ash, probably originating both from the BFB gasifier and the CFB boiler.

Keywords
Biomass gasification, particulate matter
National Category
Bioenergy
Research subject
Technology (byts ev till Engineering), Bioenergy Technology
Identifiers
urn:nbn:se:lnu:diva-11263 (URN)10.1016/j.biombioe.2011.02.053 (DOI)2-s2.0-80054840488 (Scopus ID)
Available from: 2011-03-29 Created: 2011-03-29 Last updated: 2017-12-11Bibliographically approved
5. Characterization of Particulate Matter in the Hot Product Gas from Indirect Steam Bubbling Fluidized Bed Gasification of Wood Pellets
Open this publication in new window or tab >>Characterization of Particulate Matter in the Hot Product Gas from Indirect Steam Bubbling Fluidized Bed Gasification of Wood Pellets
Show others...
2011 (English)In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 25, no 4, p. 1781-1789Article in journal (Refereed) Published
Abstract [en]

This study characterized the particulate matter (PM) formed during the indirect steam bubbling fluidized bed gasification of wood pellets at atmospheric pressure. A system including a dilution probe, a bed of granular activated carbon, and a thermodenuder was used to sample the PM at high temperature with the aim of separating it from condensing inorganic vapors and tars. The particle mass size distribution was bimodal with a fine mode in the <0.5-μm size range and a dominating coarse mode in the >0.5-μm size range. The coarse mode was representatively characterized while condensing inorganic vapors and tars complicated the evaluation of the results for the fine-mode PM. Morphological analysis of the PM indicated that the char content was low. The inorganic fraction was dominated by potassium and chlorine for fine-mode PM and calcium and silicon for coarse-mode PM.

Keywords
Biomass gasification, particulate matter
National Category
Industrial Biotechnology
Research subject
Technology (byts ev till Engineering), Bioenergy Technology
Identifiers
urn:nbn:se:lnu:diva-11261 (URN)10.1021/ef101710u (DOI)2-s2.0-79955373271 (Scopus ID)
Available from: 2011-03-29 Created: 2011-03-29 Last updated: 2017-12-11Bibliographically approved

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