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Reducing VOCs off-gassing during production of pelletized steam exploded bark: impact of storage time and controlled ventilation
Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics. (Thermochemical Energy Conversion Laboratory (TEC-Lab))ORCID iD: 0000-0003-1095-9154
(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: urn:nbn:se:umu:diva-141893OAI: oai:DiVA.org:umu-141893DiVA, id: diva2:1157114
Available from: 2017-11-15 Created: 2017-11-15 Last updated: 2018-06-09
In thesis
1. Off-gassing from thermally treated lignocellulosic biomass
Open this publication in new window or tab >>Off-gassing from thermally treated lignocellulosic biomass
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
Keywords
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:nbn:se:umu:diva-141921 (URN)978-91-7601-809-5 (ISBN)
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: 2018-06-09Bibliographically approved

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Borén, EleonoraBroström, Markus

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