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Off-gassing from thermally treated lignocellulosic biomass
Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics. (Thermochemical Energy Conversion Laboratory)
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Off-gassing of hazardous compounds is, together with self-heating and dust explosions, the main safety hazards within large-scale biomass storage and handling. Formation of CO, CO2, and VOCs with concurrent O2 depletion can occur to hazardous levels in enclosed stored forest products. Several incidents of CO poisoning and suffocation of oxygen depletion have resulted in fatalities and injuries during cargo vessel discharge of forest products and in conjunction with wood pellet storage rooms and silos. Technologies for torrefaction and steam explosion for thermal treatment of biomass are under development and approaching commercialization, but their off-gassing behavior is essentially unknown.

The overall objective of this thesis was to provide answers to one main question: “What is the off-gassing behaviour of thermally treated lignocellulosic biomass during storage?”. This was achieved by experimental studies and detailed analysis of off-gassing compounds sampled under realistic conditions, with special emphasis on the VOCs.

Presented results show that off-gassing behavior is influenced by numerous factors, in the following ways. CO, CO2 and CH4 off-gassing levels from torrefied and stream-exploded biomass and pellets, and accompanying O2 depletion, are comparable to or lower than corresponding from untreated biomass. The treatments also cause major compositional shifts in VOCs; emissions of terpenes and native aldehydes decline, but levels of volatile cell wall degradation products (notably furans and aromatics) increase. The severity of the thermal treatment is also important; increases in torrefaction severity increase CO off-gassing from torrefied pine to levels comparable to emissions from conventional pellets, and increase O2 depletion for both torrefied chips and pellets. Both treatment temperature and duration also influence degradation rates and VOC composition. The product cooling technique is influential too; water spraying in addition to heat exchange increased CO2 and VOCs off-gassing from torrefied pine chips, as well as O2 depletion. Moreover, the composition of emitted gases co-varied with pellets’ moisture content; pellets of more severely treated material retained less moisture, regardless of their pre-conditioning moisture content. However, no co-variance was found between off-gassing and pelletization settings, the resulting pellet quality, or storage time of torrefied chips before pelletization. Pelletization of steam-exploded bark increased subsequent VOC off-gassing, and induced compositional shifts relative to emissions from unpelletized steam-exploded material. In addition, CO, CO2 and CH4 off-gassing, and O2 depletion, were positively correlated with the storage temperature of torrefied softwood. Similarly, CO and CH4 emissions from steam-exploded softwood increased with increases in storage temperature, and VOC off-gassing from both torrefied and steam-exploded softwood was more affected by storage temperature than by treatment severity. Levels of CO, CO2 and CH4 increased, while levels of O2 and most VOCs decreased, during storage of both torrefied and steam-exploded softwood.CO, CO2 and O2 levels were more affected by storage time than by treatment severity. Levels of VOCs were not significantly decreased or altered by nitrogen purging of storage spaces of steam-exploded or torrefied softwood, or controlled headspace gas exchange (intermittent ventilation) during storage of steam-exploded bark.

In conclusion, rates of off-gassing of CO and CO2 from thermally treated biomass, and associated O2 depletion, are comparable to or lower than corresponding rates for untreated biomass. Thermal treatment induces shifts in both concentrations and profiles of VOCs. It is believed that the knowledge and insights gained provide refined foundations for future research and safe implementation of thermally treated fuels as energy carriers in renewable energy process chains.

Place, publisher, year, edition, pages
Umeå: Umeå universitet , 2017. , p. 94
Keyword [en]
Torrefaction, steam explosion, enclosed storage, CO, CO2, O2 depletion, VOCs, Tenax-TA, SPME, process settings, storage temperature, storage time
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:umu:diva-141921ISBN: 978-91-7601-809-5 (print)OAI: oai:DiVA.org:umu-141921DiVA, id: diva2:1157529
Public defence
2017-12-08, KB.E3.01, KBC-huset, Umeå, 09:00 (English)
Opponent
Supervisors
Funder
Bio4EnergyJ. Gust. Richert stiftelseSwedish Energy Agency
Available from: 2017-11-17 Created: 2017-11-16 Last updated: 2017-11-23Bibliographically approved
List of papers
1. Off-gassing of VOCs and permanent gases during storage of torrefied and steam exploded wood
Open this publication in new window or tab >>Off-gassing of VOCs and permanent gases during storage of torrefied and steam exploded wood
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2017 (English)In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 31, no 10, p. 10954-10965Article in journal (Refereed) Published
Abstract [en]

Thermal treatment for upgrading of low-value feedstocks to improve fuel properties has gained large industrial interest in recent years. From a storage and transport perspective, hazardous off-gassing could be expected to decrease through the degradation of reactive biomass components. However, thermal treatment could also shift chemical compositions of volatile organic components, VOCs. While technologies are approaching commercialization, off-gassing behavior of the products, especially in terms of VOCs, is still unknown. In the present study, we measured off-gassing of VOCs together with CO, CO2, CH4, and O2 depletion from torrefied and steam exploded softwood during closed storage. The storage temperature, head space gas (air and N2), and storage time were varied. VOCs were monitored with a newly developed protocol based on active sampling with Tenax TA absorbent analyzed by thermal desorption-GC/MS. High VOC levels were found for both untreated and steam exploded softwood, but with a complete shift in composition from terpenes dominating the storage gas for untreated wood samples to an abundance of furfural in the headspace of steam exploded wood. Torrefied material emitted low levels of VOCs. By using multivariate statistics, it was shown that for both treatment methods and within the ranges tested, VOC off-gassing was affected first by the storage temperature and second by increasing treatment severity. Both steam exploded and torrefied biomass formed lower levels of CO than the reference biomass, but steam explosion caused a more severe O2 depletion.

Place, publisher, year, edition, pages
Washington: American Chemical Society (ACS), 2017
Keyword
volatile organic compounds, carbon monoxide, physicochemical characteristics, thermal degradation, physical properties, pellet types, solid wood, emissions, biomass, explosion
National Category
Other Chemical Engineering
Identifiers
urn:nbn:se:umu:diva-141086 (URN)10.1021/acs.energyfuels.7b01959 (DOI)000413710300066 ()
Available from: 2017-10-24 Created: 2017-10-24 Last updated: 2017-11-24Bibliographically approved
2. VOC off-gassing from pelletized steam exploded softwood bark: emissions at different industrial process steps
Open this publication in new window or tab >>VOC off-gassing from pelletized steam exploded softwood bark: emissions at different industrial process steps
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(English)In: Fuel processing technology, ISSN 0378-3820, E-ISSN 1873-7188Article in journal (Refereed) Accepted
Abstract [en]

Formation of hazardous gases during transport and storage of biomass for large-scale bioenergy production is an important safety concern. While off-gassing has been addressed in numerous studies for raw woody biomass, very few describe it in the context of biomass for bioenergy production pre-treated by thermal technologies such as steam explosion. Volatile Organic Components (VOCs) are expected to be altered by the treatment, but until now there is no research published on VOC profiles of steam exploded materials in industrial scale. In the present study, VOCs emitted from the products were evaluated by sampling from different production steps from steam explosion of softwood bark, and following the production chain including also pelletization. Off-gasses were actively sampled using Tenax TA absorbent and analyzed by GC-MS. The VOC formation dependency of operation and storage conditions at different process steps was evaluated by multivariate statistical analysis. We showed that the different process steps along the production line was the main influencing factor for VOC off-gassing amounts, with highest VOC levels directly after the steam explosion process. Treatment severity mainly altered the relative composition of VOC profiles with more terpenes emitted from milder treatment, whereas more severe treatment shifted VOCs composition to contain more furans, e.g. furfural. In summary, treatment by steam explosion leads to potentially problematic VOC off-gassing profiles from the material, and levels vary considerable along the production line. The findings are important from a fuel handling and working environment perspective.

National Category
Chemical Engineering
Identifiers
urn:nbn:se:umu:diva-141891 (URN)
Available from: 2017-11-15 Created: 2017-11-15 Last updated: 2017-11-16
3. Reducing VOCs off-gassing during production of pelletized steam exploded bark: impact of storage time and controlled ventilation
Open this publication in new window or tab >>Reducing VOCs off-gassing during production of pelletized steam exploded bark: impact of storage time and controlled ventilation
(English)Manuscript (preprint) (Other academic)
Abstract [en]

VOC off-gassing behavior of thermally treated biomass intended for bioenergy production has recently been shown to be vastly different to that of untreated biomass. Simple measures to reduce emissions, such as controlled ventilation and prolonged storage time, has been suggested but not previously studied in detail. In the present study, we monitored how VOC off-gassing was reduced over time (24–144h) in closed storage with and without ventilation. Steam exploded bark was collected directly from a pilot scale steam explosion plant, and before and after subsequent pelletizing. Storage and active sampling of VOCs in the headspace was done in a bench-scale set-up using Tenax-TA absorbent. The impact of storage time and ventilation to reduce VOCs was evaluated through multivariate statistical analysis. The results showed that VOC concentrations in the headspace were reduced by increased storage time, and that heavier VOCs reduced faster. No impact on either reducing or shifting VOC composition could be achieved by controlled ventilation during storage; instead, VOCs emitted to the same concentrations anew, independent of process step, storage time, or number of ventilations.

National Category
Chemical Engineering
Identifiers
urn:nbn:se:umu:diva-141893 (URN)
Available from: 2017-11-15 Created: 2017-11-15 Last updated: 2017-11-16
4. Combined effects of torrefaction and pelletization parameters on the quality of pellets produced from torrefied biomass
Open this publication in new window or tab >>Combined effects of torrefaction and pelletization parameters on the quality of pellets produced from torrefied biomass
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2017 (English)In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 191, p. 414-424Article in journal (Refereed) Published
Abstract [en]

A combined torrefaction and pelletization study was performed at industrially relevant settings using a factorial design. First, wood chips of Scots pine were torrefied at high temperatures (291-315 degrees C) and short residence times (6-12 min), facilitating high throughput in a continuous pilot-scale torrefaction process. Then the torrefied materials were pelletized, also in pilot-scale, using varying moisture contents (MCs) (10-14%), sieve sizes (4-6 mm), and press channel lengths (PCLs) (25 and 30 mm), in all 19 batches, each of 400 kg. The resulting so called black pellets exhibited bulk densities of 558-725 kg m(-3), durabilities of 46.3-86.5%, and fines contents of 3.8-85.8%. Through multiple linear regression modelling of all 11 responses, it was found that the parameter with the greatest influence on the responses was the torrefaction temperature, followed by torrefaction time, MC, and PCL. Longer PCL and higher MC resulted in higher pellet quality, with less fines and greater bulk density and durability. Furthermore, a low torrefaction degree decreased the amount of power required for pelletization. The energy required to grind pellets into a powder (<0.5 mm) decreased with increasing torrefaction degree as expected, but also with decreasing MC before pelletizing. Pyrolysis-GC/MS analysis of thermal degradation products from the pellets revealed correlations with the torrefaction temperature and time, but no correlations with the pelletization process. These results are useful for mapping chemical changes in torrefied materials and identifying complementary torrefaction and pelletization settings. Specifically of interest is adjustment of PCLs at low intervals to better match friction properties of torrefied materials.

Place, publisher, year, edition, pages
Elsevier, 2017
Keyword
Pine, Moisture, Press channel lengths, Durability, Fines, Degradation products
National Category
Chemical Process Engineering Bioenergy
Identifiers
urn:nbn:se:umu:diva-134212 (URN)10.1016/j.apenergy.2017.01.035 (DOI)000395963500033 ()
Available from: 2017-06-16 Created: 2017-06-16 Last updated: 2017-11-16Bibliographically approved
5. Off-gassing from pilot-scale torrefied pine wood chips: – impact of torrefaction severity, cooling technology, and storage time
Open this publication in new window or tab >>Off-gassing from pilot-scale torrefied pine wood chips: – impact of torrefaction severity, cooling technology, and storage time
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

During handling and storage of conventional wood-based energy carriers, O2 depletion as well as CO and CO2 off-gassing can reach hazardous levels, and certain irritating VOCs trespass exposure levels. When new thermally pre-treated biomass commodities are entering consumer markets, knowledge on these assortments’ off-gassing behaviour is needed. In this study, relative concentrations of VOCs, CO, CO2, and O2 in off-gases of five different pilot-scale torrefied pine wood chip assortments was monitored over 12 days. VOC composition shifted with increased torrefaction treatment; terpene concentrations decreased while furan and lignin derivates increased. Generally, VOC amounts decreased with storage time, but for the least severely torrefied chips (291°C, 6 min), certain VOCs increased; e.g. hexanal, acetone, and 2-pentylfuran. Torrefied chips was subject to two different cooling technologies: i) heat exchanging and ii) additional water spraying. Water spraying resulted in higher VOC concentrations, stronger O2 depletion, and higher CO2 by a factor four. 

National Category
Chemical Engineering
Identifiers
urn:nbn:se:umu:diva-141894 (URN)
Available from: 2017-11-15 Created: 2017-11-15 Last updated: 2017-11-16
6. Off-gassing from 16 pilot-scale produced pellets assortments of torrefied pine: impact of torrefaction severity, storage time, pelletization parameters, and pellet quality
Open this publication in new window or tab >>Off-gassing from 16 pilot-scale produced pellets assortments of torrefied pine: impact of torrefaction severity, storage time, pelletization parameters, and pellet quality
Show others...
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Off-gassing from wood pellets poses risks in large scale handling chains - yet little is known on off-gassing from pellets of torrefied wood. This study reports CO, CO2, and O2 concentrations in off-gases during storage of 16 torrefied and two untreated pellets assortments. According to an experimental design, pellets were produced in pilot scale from pine chips torrefied at five different set points. Off-gassing was assessed in relation to storage conditions, torrefaction and pelletization parameters, and pellet quality. Pellets from the most severely torrefied pine formed CO, CO2, and consumed O2 similarly to untreated pellets. Off-gassing was positively correlated to pellet moisture content; however, the most severely torrefied also retained the least moisture. Open air storage (20–270 days) of torrefied chips prior to pelletization did not affect off-gassing levels. Results are important for safe handling; torrefied pellets can cause comparable levels as untreated pellets of CO, CO2, and O2.

National Category
Chemical Engineering
Identifiers
urn:nbn:se:umu:diva-141899 (URN)
Available from: 2017-11-15 Created: 2017-11-15 Last updated: 2017-11-16

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