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Bio-coal for the sustainable industry: A scientific approach to optimizing production, storage, and usages
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science. (Energy Engineering)ORCID iD: 0000-0001-8372-4386
2021 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Bio-coal produced from biomass is a promising material to replace fossil coal in order to achieve net-zero greenhouse gas emission from the industrial sector. Bio-coal with quality comparable to that of fossil coal can be produced by high-temperature pyrolysis at ≥500 ºC, but the production efficiency is relatively low due to low bio-coal yield at high pyrolysis temperatures. This trade-off suffers the economic feasibility of bio-coal production. The overall objective of this doctoral thesis is to develop a pyrolysis process that can produce bio-coal for fossil coal replacement in the industrial sector, while maintaining a high process efficiency.   To increase bio-coal yield and process efficiency, secondary char formation during the pyrolysis of thick biomass, for example, woodchips, is the primary method considered in this work. Secondary char formation can be promoted by increasing volatile concentration during pyrolysis and/or extending residence time of volatiles inside the pore structure of wood particles. This study investigated how to increase secondary char formation using bio-oil recycling and CO2 purging. Bio-oil recycling increased bio-coal yield by not only increasing the reactants, but also through the synergetic effect between bio-oil and woodchips upon physical contact. Using CO2 as a purging gas reduced mass diffusion of volatiles inside the pore structure of woodchips, producing extra bio-coal. In addition, the effect of these techniques can be maximized by ensuring good contact between the volatiles and the solid surface using thick particles and slow heating. In parallel, a numerical model of pyrolysis in a rotary kiln reactor was developed to increase the understanding of parameter implementation in pyrolysis reactors. Two important parameters were studied: rotation speed and feeding rate. Rotation speed controlled the solid residence time, while the feeding rate influenced the heat capacity of holdup materials and product distribution.   Bio-coal is prone to self-heating and usually causes spontaneous ignition during production, storage, and transportation, which can lead to losses in the production and health of workers. In this study, self-heating at low temperatures was investigated by using numerical simulations describing the changes in local properties inside different bio-coal containers such as closed metal containers and woven plastic bags. The kinetic parameters of bio-coal were measured and implemented in the model. It was observed that the bio-coal temperature slowly increased from the initial temperature due to the heat released during O2 chemisorption. Thermal runaway occurred in some storage conditions, even at intial bio-coal temperatures of ca. 155 ºC. The simulation results suggest that self-heating can be mitigated by using small and wide particle distribution, limited storage volume, and low ambient temperature. This study also provides the criteria for estimating the cooling demands in bio-coal production processes.   Bio-coal properties are the main challenges for utilizing it as a substitute for fossil coal. Although the elemental composition and heating value of the bio-coal produced in this study are equivalent to those of fossil coal, the reactivity of bio-coal is relatively high. To replace fossil coal in existing industrial processes, bio-coal reactivity is preferred to be similar to that of fossil coal to avoid major process modifications. This thesis has concluded that pyrolysis temperature, heating rate, and biomass feedstock are the major parameters influencing the gasification rate under chemical reaction limitation. It was found that potassium in biomasses increased bio-coal reactivity even at low gasification temperatures such as 800 ºC, while calcium did not play a significant role at temperatures below 1600 ºC. Furthermore, bio-coal reactivity increased only slightly by promoting secondary char formation using the proposed methods. These findings suggest that we can achieve high bio-coal yield, both mass and energy, while maintaining similar fuel properties through pyrolysis with bio-oil recycling and CO2 purging.   In the most industrially relevant applications, the gasification rate is dominated by diffusion mass transfer. Therefore, it is necessary to reflect gasification behavior of bio-coal under these circumstances. At the particle scale, where intraparticle diffusion controls the overall reaction rate, bio-coal particle size was nearly constant until high conversion. This implies that particle size changes should be considered only at high conversion. Meanwhile, large particles exhibit low gasification rate at the particle scale following the Thiele modulus. The contrary result appears at the packed bed scale, where both intraparticle and interparticle diffusions play roles. Large particles increased the gasification rate in packed beds because of the large bed channel size, high void fraction, and low tortuosity. This observation led to an opportunity to minimize the apparent gasification rate in a packed bed by using polydisperse particles, which have a wide particle size distribution. Large particles maximize the intraparticle diffusivity of CO2, while small particles fill the gaps between large particles, thus increasing interparticle diffusivity, which reduces apparent reactivity. This outcome was confirmed experimentally.   By combining the knowledge obtained in this doctoral thesis, an efficient pyrolysis process is proposed to produce bio-coal for a sustainable industry.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2021.
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Energy Engineering
Research subject
Energy Engineering
Identifiers
URN: urn:nbn:se:ltu:diva-84564ISBN: 978-91-7790-862-3 (print)ISBN: 978-91-7790-863-0 (electronic)OAI: oai:DiVA.org:ltu-84564DiVA, id: diva2:1556171
Public defence
2021-09-24, E231, Luleå University of Technology, Luleå, 09:00 (English)
Opponent
Supervisors
Available from: 2021-05-20 Created: 2021-05-20 Last updated: 2021-09-02Bibliographically approved
List of papers
1. Increasing efficiency of charcoal production with bio-oil recycling
Open this publication in new window or tab >>Increasing efficiency of charcoal production with bio-oil recycling
2018 (English)In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 32, no 9, p. 9650-9658Article in journal (Refereed) Published
Abstract [en]

Charcoal from biomass is a promising alternative for fossil coal. Although its quality increases at high pyrolysis temperature, charcoal yield decreases, meaning lower economic performances of charcoal production processes. This work aims at demonstrating potential methods to increase charcoal yield while keeping its quality at satisfying levels. We suggested the recycling of bio-oil from pyrolysis process as a primary measure. In addition, we also investigated in detail the consequence of utilizing CO2 instead of N2 as reaction media under practical conditions (i.e. thick particles). An experimental investigation was carried out in a macro-thermogravimetric (macro-TG) reactor. Sample (woodchips, bio-oil, and woodchips embedded with bio-oil) was exposed to the reaction temperature either instantaneously (isothermal condition) or by slow heating (slow pyrolysis) in controlled gas flows of N2 and CO2. The results showed that char yield increases with the bio-oil recycling on wood chips at all pyrolysis temperatures (300–700 °C). By 20% of bio-oil embedding on wood chips, charcoal yield increased by 18.3% on average. The increase of charcoal yield was not only because of the increase in reactants, but also due to the synergetic effect between bio-oil and wood chips upon physical contact. Bio-oil recycling had negligible effects on the property of charcoal, such as carbon content and heating value. Although CO2 did not affect primary pyrolysis, it had effects on mass transfer processes. As a result, significantly higher char yield was obtained from pyrolysis in CO2 than in N2 by ensuring a good contact of volatiles and solid surface (i.e. usage of thick particles and slow heating). This study suggests that we can achieve high charcoal yield while maintaining the similar charcoal property by bio-oil recycling, CO2 purging, use of thick particles, and slow heating.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2018
National Category
Energy Engineering
Research subject
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-70583 (URN)10.1021/acs.energyfuels.8b02333 (DOI)000445711700071 ()2-s2.0-85052873835 (Scopus ID)
Note

Validerad;2018;Nivå 2;2018-10-15 (johcin) 

Available from: 2018-08-24 Created: 2018-08-24 Last updated: 2021-05-20Bibliographically approved
2. Effects of Pyrolysis Conditions and Feedstocks on the Properties and Gasification Reactivity of Charcoal from Woodchips
Open this publication in new window or tab >>Effects of Pyrolysis Conditions and Feedstocks on the Properties and Gasification Reactivity of Charcoal from Woodchips
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2020 (English)In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 34, no 7, p. 8353-8365Article in journal (Refereed) Published
Abstract [en]

Pyrolysis conditions in charcoal production affect yields, properties, and further use of charcoal. Reactivity is a critical property when using charcoal as an alternative to fossil coal and coke, as fuel or reductant, in different industrial processes. This work aimed to obtain a holistic understanding of the effects of pyrolysis conditions on the reactivity of charcoal. Notably, this study focuses on the complex effects that appear when producing charcoal from large biomass particles in comparison with the literature on pulverized biomass. Charcoals were produced from woodchips under a variety of pyrolysis conditions (heating rate, temperature, reaction gas, type of biomass, and bio-oil embedding). Gasification reactivity of produced charcoal was determined through thermogravimetric analysis under isothermal conditions of 850 degrees C and 20% of CO2. The charcoals were characterized for the elemental composition, specific surface area, pore volume and distribution, and carbon structure. The analysis results were used to elucidate the relationship between the pyrolysis conditions and the reactivity. Heating rate and temperature were the most influential pyrolysis parameters affecting charcoal reactivity, followed by the reaction gas and bio-oil embedding. The effects of these pyrolysis conditions on charcoal reactivity could primarily be explained by the difference in the meso- and macropore volume and the size and structural order of aromatic clusters. The lower reactivity of slow pyrolysis charcoals also coincided with their lower catalytic inorganic content. The reactivity difference between spruce and birch charcoals appears to be mainly caused by the difference in catalytically active inorganic elements. Contrary to pyrolysis of pulverized biomass, a low heating rate produced a higher specific surface area compared with a high heating rate. Furthermore, the porous structure and the reactivity of charcoal produced from woodchips were influenced when the secondary char formation was promoted, which cannot be observed in pyrolysis of pulverized biomass.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2020
National Category
Energy Engineering Applied Mechanics
Research subject
Experimental Mechanics; Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-80479 (URN)10.1021/acs.energyfuels.0c00592 (DOI)000551544900047 ()2-s2.0-85090237592 (Scopus ID)
Note

Validerad;2020;Nivå 2;2020-08-20 (johcin)

Available from: 2020-08-20 Created: 2020-08-20 Last updated: 2021-05-20Bibliographically approved
3. Modeling and pilot plant runs of slow biomass pyrolysis in a rotary kiln
Open this publication in new window or tab >>Modeling and pilot plant runs of slow biomass pyrolysis in a rotary kiln
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2017 (English)In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 207, p. 123-133Article in journal (Refereed) Published
Abstract [en]

Pyrolysis of biomass in a rotary kiln finds application both as an intermediate step in multistage gasification as well as a process on its own for the production of biochar. In this work, a numerical model for pyrolysis of lignocellulosic biomass in a rotary kiln is developed. The model is based on a set of conservation equations for mass and energy, combined with independent submodels for the pyrolysis reaction, heat transfer, and granular flow inside the kiln. The pyrolysis reaction is described by a two-step mechanism where biomass decays into gas, char, and tar that subsequently undergo further reactions; the heat transfer model accounts for conduction, convection and radiation inside the kiln; and the granular flow model is described by the well known Saeman model. The model is compared to experimental data obtained from a pilot scale rotary kiln pyrolyzer. In total 9 pilot plant trials at different feed flow rate and different heat supply were run. For moderate heat supplies we found good agreement between the model and the experiments while deviations were seen at high heat supply. Using the model to simulate various operation conditions reveals a strong interplay between heat transfer and granular flow which both are controlled by the kiln rotation speed. Also, the model indicates the importance of heat losses and lays the foundation for scale up calculations and process optimization.

Place, publisher, year, edition, pages
Elsevier, 2017
National Category
Energy Engineering
Research subject
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-64494 (URN)10.1016/j.apenergy.2017.06.034 (DOI)000417229300012 ()2-s2.0-85021244091 (Scopus ID)
Conference
8th International Conference on Applied Energy (ICAE2016), Bejing, Oct 8-11, 2016
Note

Konferensartikel i tidskrift

Available from: 2017-06-26 Created: 2017-06-26 Last updated: 2023-09-04Bibliographically approved
4. A Shortcut Method to Predict Particle Size Changes during Char Combustion and Gasification under regime II Conditions
Open this publication in new window or tab >>A Shortcut Method to Predict Particle Size Changes during Char Combustion and Gasification under regime II Conditions
Show others...
2022 (English)In: Combustion Science and Technology, ISSN 0010-2202, E-ISSN 1563-521X, Vol. 194, no 2, p. 272-291Article in journal (Refereed) Published
Abstract [en]

In most industrial applications, combustion and gasification of char progresses under regime II conditions. Unlike in other regimes, both particle size and density change simultaneously in regime II due to non-uniform consumption of carbon inside the particles. In this work, mathematical predictions of diameter changes in regime II were made by a one-dimensional simulation tool, where transient species balances are resolved locally inside the particle. This simulation is computationally expensive and usually not appropriate for the implementation in comprehensive CFD simulations of combustion or gasification processes. To overcome this restraint, an alternative shortcut method with affordable computation time has been developed and validated against the detailed model. This method allows the calculation of diameter changes during combustion and gasification from precalculated effectiveness factors. Additionally, the change of particle size has been investigated experimentally in a single particle converter setup. Therein, particles are fixed on a sample holder placed in the hot flue gas of a flat flame burner. Size and temperature trends are optically assessed by a 3CCD camera.

Place, publisher, year, edition, pages
Taylor & Francis, 2022
Keywords
combustion, gasification, char conversion, biomass, particle size change
National Category
Energy Engineering
Research subject
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-76792 (URN)10.1080/00102202.2019.1678919 (DOI)000492714600001 ()2-s2.0-85074512914 (Scopus ID)
Funder
Bio4EnergySwedish Research CouncilThe Kempe FoundationsThe Research Council of Norway, 267916
Note

Validerad;2022;Nivå 2;2022-03-01 (sofila);

Funder: German Research Foundation (215035359); Swedish Center for Biomass Gasification

Available from: 2019-11-20 Created: 2019-11-20 Last updated: 2022-07-04Bibliographically approved

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