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Aerosol-based method for investigating biomass char reactivity at high temperatures
Linnaeus University, Faculty of Science and Engineering, School of Engineering. (Bioenergy)
Linnaeus University, Faculty of Science and Engineering, School of Engineering. (Bioenergy)
Linnaeus University, Faculty of Science and Engineering, School of Engineering. (Bioenergy)
2011 (English)In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 158, no 7, 1426-1437 p.Article 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.

Place, publisher, year, edition, pages
2011. Vol. 158, no 7, 1426-1437 p.
National Category
Industrial Biotechnology
Research subject
Technology (byts ev till Engineering), Bioenergy Technology
Identifiers
URN: urn:nbn:se:lnu:diva-7044DOI: 10.1016/j.combustflame.2010.11.015OAI: oai:DiVA.org:lnu-7044DiVA: diva2:337851
Available from: 2010-08-12 Created: 2010-08-09 Last updated: 2014-02-25Bibliographically approved
In thesis
1. Char conversion kinetics and aerosol characterization in biomass gasification
Open this publication in new window or tab >>Char conversion kinetics and aerosol characterization in biomass gasification
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
Keyword
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:nbn:se:lnu:diva-31347 (URN)978-91-87427-56-5 (ISBN)
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

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CiteExportLink to record
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