Digitala Vetenskapliga Arkivet

Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Mitigating Off-Gassing and Self-Heating in Fuel Wood Pellets Storage: A Raw Materials Selection and Pre-Treatment Centred Approach
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Chemical Sciences (from 2013).ORCID iD: 0000-0002-5842-2404
2024 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Wood pellets have become a preferred solid biomass fuel for heat and power generation due to their standardized nature, known properties, and consistent quality. However, pellets produced from fresh sawdust sometimes undergo self-heating and off-gassing during storage. This poses a challenge for their transportation and storage. The aim of this thesis was to increase the body of knowledge towards understanding the mechanisms underlying the self-heating and off-gassing of wood pellets, and offer solutions for producing wood pellets from freshly generated sawdust, with reduced self-heating and off-gassing tendencies.

The effects of total wood extractive content and types of extractives in the raw material on off-gassing of wood pellets were investigated through two separate studies (papers I and II). The results from paper I showed that the total amount of extractives in the raw material has little effect on off-gassing. While gas emissions were reduced for pellets produced from Scots pine sawdust that had low amounts of extractives (stored and acetone extracted), the coefficients of determination (R2) from the linear correlation analysis between off-gassing and the total extractive content of the raw materials were below 0.5 for all the three off-gasses indicating low or no correlation. The results of cellulose pellets with added additive oils (paper II) showed that the off-gassing is highly dependent on the type of extractives in the raw material. The highest mean concentrations of the carbon oxides and methane were recorded from cellulose pellets with added linseed oil. Pellets with added linseed oil had higher off-gas emissions due to the high content of unsaturated fatty acids of 73.9% linolenic and 7.6% linoleic.

The effects of raw material type and pre-treatments on self-heating and off-gassing of wood pellets were also investigated through two separate studies (papers III and IV). The results from both studies indicated significant influences of both raw material type and drying temperature. There was a strong linear correlation between off-gassing and sapwood content, with correlation coefficient (R) values greater than 0.9 at p < 0.001 for all the off-gases (paper III). An increase in sapwood content of the raw material (fresh Scots pine sawdust) led to a significant increase in off-gassing of CO2, CO and CH4, and O2 consumption. Storing of sawdust for over six months prior to pellet production, and increasing the temperature of drying the sawdust led to significant reduction of off-gassing for sapwood pellets. For heartwood pellets, increasing the drying temperature resulted in increased off-gassing and raw material storage had no effect. In the other study (paper IV), the pellets produced from Scots pine mature wood sawdust were more prone to self-heating and off-gassing compared to those produced from juvenile wood sawdust. Steam drying the sawdust at high temperature and pressure led to a significant reduction in heat and gas generation for both materials. Furthermore, the study established a notable connection between self-heating and off-gassing, the storage piles with high temperature increase also exhibited high concentrations of off-gases.

The overall results indicated that a biological process, in combination with the chemical oxidation of unsaturated fatty acids lay behind the self-heating and off-gassing of wood pellets. The other notable effect was that methane formation is dependent on anaerobic conditions, whereas formation of carbon oxides can occur both under aerobic and anaerobic conditions. While storing fresh sawdust for a period of time prior to pellet production remains the most effective method for mitigating self-heating and off-gassing during pellet storage, sorting and separating the raw materials at source can facilitate the development of storage schedules tailored to specific raw materials, thereby reducing on the raw material storage time.

Abstract [en]

The global wood pellets production increased from about 18 million tonnes in 2012 to about 46 million tonnes in 2022. Wood pellets have become a preferred solid biomass fuel for heat and power generation due to their standardized nature, known properties, and consistent quality. However, pellets produced from fresh sawdust sometimes undergo self-heating and off-gassing and this poses a challenge for their transportation and storage. The aim of this thesis was to increase the body of knowledge towards understanding the mechanisms underlying the self-heating and off-gassing of wood pellets, and offer solutions for producing wood pellets with reduced tendencies for self-heating and off-gassing using freshly generated sawdust.

The results showed that a biological process, in combination with the chemical oxidation of unsaturated fatty acids lay behind the self-heating and off-gassing of wood pellets. While storing of fresh sawdust for a period of time prior to pellet production remains the most effective method for mitigating self-heating and off-gassing, sorting and separating the raw materials at source can facilitate the development of storage schedules tailored to specific raw materials, thereby reducing on the raw material storage time.

Abstract [en]

Wood pellets have become a preferred solid biomass fuel for heat and power generation due to their standardized nature, known properties, and consistent quality. However, pellets produced from fresh sawdust sometimes undergo self-heating and off-gassing and this poses a challenge for their transportation and storage. The aim of this thesis was to increase the body of knowledge towards understanding the mechanisms underlying the self-heating and off-gassing of wood pellets.

The results from paper I showed that the total amount of extractives in the raw material has little effect on off-gassing. Although pellets produced from Scots pine sawdust with lower extractive contents exhibited reduced concentrations of off-gasses; CO, CO2 and CH4, increasing the extractive content of the sawdust through the addition of additive oils did not result in increased off-gas emissions. The results of pure cellulose pellets with added additive oils showed that off-gassing is influenced by the type of extractives in the raw material (paper II). The highest concentrations of off-gasses were recorded from pure cellulose pellets with added linseed oil due to their high content of unsaturated fatty acids of 73.9% linolenic and 7.6% linoleic.

The results from papers III and IV indicated significant effects of both raw material and drying temperature on off-gassing and self-heating of wood pellets. There was a strong linear correlation between off-gassing and the sapwood content of the raw material, with correlation coefficient (R) values greater than 0.9 at p < 0.001 for all the off-gases (paper III). An increase in sapwood content led to a significant increase in off-gassing of CO2, CO and CH4, and O2 consumption. Storing of sawdust for over six months prior to pellet production, and increasing the drying temperature led to a significant reduction in off-gassing for sapwood pellets. For heartwood pellets, increasing the drying temperature resulted in increased off-gassing while raw material storage had no effect. In the other study (paper IV), the pellets produced from Scots pine mature wood sawdust were more prone to self-heating and off-gassing compared to those produced from juvenile wood sawdust. Steam drying the sawdust at high temperature led to a significant reduction in heat and gas generation for both materials.

The overall results indicated that a biological process, in combination with the chemical oxidation of unsaturated fatty acids lay behind the self-heating and off-gassing of wood pellets.

Place, publisher, year, edition, pages
Karlstad: Karlstads universitet, 2024. , p. 75
Series
Karlstad University Studies, ISSN 1403-8099 ; 2024:11
Keywords [en]
solid biofuels, densified biomass, heat generation, off-gas emissions, carbon monoxide, carbon dioxide, methane, fatty and resin acids
National Category
Energy Engineering
Research subject
Environmental and Energy Systems
Identifiers
URN: urn:nbn:se:kau:diva-99112DOI: 10.59217/hzkh1134ISBN: 978-91-7867-450-3 (print)ISBN: 978-91-7867-451-0 (electronic)OAI: oai:DiVA.org:kau-99112DiVA, id: diva2:1848032
Public defence
2024-05-07, Rejmersalen, 9C 204, Karlstad University, Karlstad, 08:30 (English)
Opponent
Supervisors
Funder
Swedish Energy Agency, 47997–1Vinnova, 2021-03727
Note

This study was part of the collaborative research projects between Karlstad University and the Swedish University of Agricultural Sciences, SLU. 

Available from: 2024-04-12 Created: 2024-04-02 Last updated: 2024-04-11Bibliographically approved
List of papers
1. Influence on off-gassing during storage of Scots pine wood pellets produced from sawdust with different extractive contents
Open this publication in new window or tab >>Influence on off-gassing during storage of Scots pine wood pellets produced from sawdust with different extractive contents
Show others...
2022 (English)In: Biomass and Bioenergy, ISSN 0961-9534, E-ISSN 1873-2909, Vol. 156, article id 106325Article in journal (Refereed) Published
Abstract [en]

Off-gassing and self-heating are the major challenges when it comes to transportation and storage of wood pellets. The heat generated due to self-heating poses a fire risk while off-gassing of toxic gasses such as carbon monoxide (CO) and some volatile organic compounds (VOCs) is an environmental and human health risk. With the increase in production volumes of wood pellets which has subsequently increased the amounts of wood pellets in transportation and storage, there is need to find lasting solutions to off-gassing and self-heating of wood pellets. The objective of this study was to test the off-gassing abilities of Scots pine wood pellets produced from sawdust with varying amounts of extractives. The aim is to come up with raw material pre-treatment measures so as to produce wood pellets that are not liable to off-gassing. Six (6) types of sawdust raw materials namely; fresh pine sawdust (FPS), stored pine sawdust (SPS), sawdust plus pine rosin (PRS), sawdust plus linseed oil (LOS), sawdust plus tall oil (TOS) and acetone extracted sawdust (AES) were used to produce the pellets. The produced pellets were then subjected to off-gassing tests under controlled conditions using the ECOM J2KN analyser. The concentrations of carbon monoxide, carbon dioxide and methane increased with storage time but slowed down towards the end of the nine days test period. The formation of these gasses were largely dependent on the type of extractives present in the raw material and not the total extractive content. The formation of methane started later than the other gases and coincided with the time when residual oxygen was depleted.

Place, publisher, year, edition, pages
Elsevier, 2022
Keywords
Carbon dioxide, Carbon monoxide, Methane, Off-gassing, Wood extractives, Wood pellets, Acetone, Health risks, Heating, Pelletizing, Storage (materials), Volatile organic compounds, Wood, Environmental health risks, Extractives content, Fire risks, Offgassing, Pine sawdust, Scots pine wood, Self-heating, Toxic gas, Wood pellet, carbon, pine, Pelleting, Voc
National Category
Energy Engineering
Research subject
Environmental and Energy Systems
Identifiers
urn:nbn:se:kau:diva-88950 (URN)10.1016/j.biombioe.2021.106325 (DOI)2-s2.0-85121003662 (Scopus ID)
Available from: 2022-03-03 Created: 2022-03-03 Last updated: 2024-04-02Bibliographically approved
2. Understanding Off-Gassing of Biofuel Wood Pellets Using Pellets Produced from Pure Microcrystalline Cellulose with Different Additive Oils
Open this publication in new window or tab >>Understanding Off-Gassing of Biofuel Wood Pellets Using Pellets Produced from Pure Microcrystalline Cellulose with Different Additive Oils
Show others...
2022 (English)In: Energies, E-ISSN 1996-1073, Vol. 15, no 6, article id 2281Article in journal (Refereed) Published
Abstract [en]

Fuel wood pellets have the tendency of undergoing self-heating and off-gassing during storage and transportation. Self-heating can lead to spontaneous combustion and cause fires while toxic gasses such as carbon monoxide and some volatile organic compounds released due to off-gassing are a human health and environmental hazard. Previous research suggests that the self-heating and off-gassing of wood pellets are as a result of the oxidation of wood extractives. The aim of this study was to identify the extractives, i.e., fatty and resin acids that are responsible for the emissions of carbon monoxide, carbon dioxide and methane from wood pellets by testing the off-gassing tendencies of pellets produced from synthetic microcrystalline cellulose and different additive oils. The additive oils were intentionally selected to represent different types of wood extractives (mainly fatty and resin acids) and they included: tall oil, pine rosin, linseed oil and coconut oil. The highest mean concentrations of carbon monoxide, carbon dioxide and methane were recorded from cellulose pellets with added linseed oil. The concentrations of carbon monoxide and methane for the other four pellet types were negligible and there was no carbon dioxide emission. Pellets with added linseed oil had high off-gas emissions due to the high content of unsaturated fatty acids compared to other pellet types.

Place, publisher, year, edition, pages
MDPI, 2022
Keywords
off-gassing, cellulose pellets, additive oils, fatty and resin acids, solid biofuels, wood pellets, self-heating
National Category
Energy Engineering
Research subject
Chemistry; Energy Technology
Identifiers
urn:nbn:se:kau:diva-89516 (URN)10.3390/en15062281 (DOI)000775539800001 ()2-s2.0-85127671377 (Scopus ID)
Available from: 2022-04-14 Created: 2022-04-14 Last updated: 2024-04-02Bibliographically approved
3. Influence of Sapwood/Heartwood and Drying Temperature on Off-Gassing of Scots Pine Wood Pellets
Open this publication in new window or tab >>Influence of Sapwood/Heartwood and Drying Temperature on Off-Gassing of Scots Pine Wood Pellets
Show others...
2024 (English)In: Bioenergy Research, ISSN 1939-1234, E-ISSN 1939-1242, Vol. 17, no 1, p. 479-490Article in journal (Refereed) Published
Abstract [en]

Wood pellets produced from fresh sawdust can form and release uncontrolled gases during bulk storage, a tendency referred to as off-gassing. This study investigated the off-gassing tendencies of Scots pine wood pellets made from separated sapwood and heartwood sawdust. The effects of drying temperature, raw material storage, as well as varying proportions of sapwood and heartwood were also investigated. There was a strong linear correlation between off-gassing and sapwood content, with correlation coefficient (R) values greater than 0.9 at p < 0.001 for all the off-gases. An increase in sapwood content of the feedstock led to a significant increase in off-gassing of CO2, CO, and CH4, and O2 consumption. The drying temperature of the raw material had a significant effect on off-gassing of both sapwood (F (8, 26) = 51.32, p < 0.05) and heartwood (F (8, 26) = 334.1, p < 0.05) pellets. Increasing the drying temperature for heartwood resulted in increased off-gassing, while for sapwood, the off-gassing reduced. Storage of sapwood raw material before pelletization reduced the off-gassing of wood pellets, whereas for heartwood, it had no significant impact. Based on the results, it is suggested that a biological process, in combination with the chemical oxidation of fatty acids, lay behind the off-gassing of wood pellets. 

Place, publisher, year, edition, pages
Springer, 2024
Keywords
Biofuel pellets, Bulk storage, Gas emissions, Carbon oxides, Methane, Feedstock composition
National Category
Energy Systems
Research subject
Environmental and Energy Systems
Identifiers
urn:nbn:se:kau:diva-96667 (URN)10.1007/s12155-023-10668-6 (DOI)001064448400001 ()2-s2.0-85170075647 (Scopus ID)
Funder
Swedish Energy Agency, 47997-1Vinnova, 2021-03727Karlstad University
Available from: 2023-09-11 Created: 2023-09-11 Last updated: 2024-07-23Bibliographically approved
4. Fuel Wood Pellets Produced from Sawdust of Scots Pine Mature and Juvenile Wood: Self-Heating and Off-Gassing Tests at Industrial Scale
Open this publication in new window or tab >>Fuel Wood Pellets Produced from Sawdust of Scots Pine Mature and Juvenile Wood: Self-Heating and Off-Gassing Tests at Industrial Scale
Show others...
2024 (English)In: Bioenergy Research, ISSN 1939-1234, E-ISSN 1939-1242Article in journal (Refereed) Epub ahead of print
Abstract [en]

This study investigated self-heating and off-gassing of Scots pine (Pinus sylvestris) wood pellets made from sawdust generated from separated mature and juvenile wood. The pellets were produced at an industrial scale and stored in large piles of about 7.2 tonnes. The production process involved drying the sawdust using three different methods and to varying moisture contents. The results indicated significant influences of both raw material type (F(6) = 61.97, p < 0.05) and drying method (F(2) = 65.38, p < 0.05) on the self-heating of the pellets. The results from the multiple regression analysis further showed that both the raw material type and pellet moisture content significantly influenced the temperature increase, with strong correlations observed for pellets produced using low-temperature drying (F(3, 14) = 83.52, multiple R2 = 0.95, p < 0.05), and medium temperature drying (F(3, 13) = 62.05, multiple R2 = 0.93, p < 0.05). The pellets produced from fresh mature wood sawdust were found to be more prone to self-heating and off-gassing while steam drying the sawdust at high temperature and pressure led to a significant reduction in heat and gas generation across all materials. The heightened self-heating and off-gassing in mature wood pellet can be attributed to a higher proportion of sapwood in the raw material. The probable explanations to the observed differences are in line with biological mechanisms for self-heating and off-gassing, as well as the chemical oxidation of fatty and resin acids.

Place, publisher, year, edition, pages
Springer, 2024
Keywords
Heat generation, Gas emissions, Feedstock composition, Sawdust drying, Pelletization process, Pellets storage
National Category
Energy Engineering Energy Systems
Research subject
Environmental and Energy Systems
Identifiers
urn:nbn:se:kau:diva-99023 (URN)10.1007/s12155-024-10736-5 (DOI)001180323400001 ()2-s2.0-85187461982 (Scopus ID)
Funder
Vinnova, 2021-03727Karlstad University
Available from: 2024-03-26 Created: 2024-03-26 Last updated: 2024-07-09Bibliographically approved
5. [Manuscript] Effects of Mature/Juvenile Wood Composition and Drying Temperature on Scots Pine Pellet Quality Properties
Open this publication in new window or tab >>[Manuscript] Effects of Mature/Juvenile Wood Composition and Drying Temperature on Scots Pine Pellet Quality Properties
(English)Manuscript (preprint) (Other academic)
National Category
Environmental Engineering
Research subject
Environmental and Energy Systems
Identifiers
urn:nbn:se:kau:diva-99111 (URN)
Available from: 2024-04-02 Created: 2024-04-02 Last updated: 2024-04-25Bibliographically approved

Open Access in DiVA

fulltext-KAPPAN(2386 kB)83 downloads
File information
File name FULLTEXT02.pdfFile size 2386 kBChecksum SHA-512
272e9d0be229330e83c5bb4f8eab452b805a67c8f341251af0d2c9c5ece789086d44da0c98873f3ffe90e3d8db2b8d46b08d8358fd0cc536a4f97f983ca2fc94
Type fulltextMimetype application/pdf

Other links

Publisher's full text

Search in DiVA

By author/editor
Siwale, Workson
By organisation
Department of Engineering and Chemical Sciences (from 2013)
Energy Engineering

Search outside of DiVA

GoogleGoogle Scholar
Total: 83 downloads
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

doi
isbn
urn-nbn

Altmetric score

doi
isbn
urn-nbn
Total: 822 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf