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From torrefaction to gasification: Pilot scale studies for upgrading of biomass
Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics. (Anders Nordin)ORCID iD: 0000-0002-1874-6447
2015 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Från torrefiering till förgasning : Experiment i pilotskala för förädling av biomassa (Swedish)
Abstract [en]

Increasing the share of biomass, preferably by replacing fossil fuels, is one way to mitigate the present climate change. Fossil coal can be directly replaced by co-combustion of coal and biomass and fossil engine fuels (gasoline and diesel) could potentially partly be replaced by synthetic renewable fuels produced via entrained flow gasification of biomass. The use of biomass in these processes is so far limited, partly because of the fibrous and hygroscopic nature of biomass which leads to problem in storing, transportation, handling and feeding.

This thesis demonstrates how the challenging characteristics of raw biomass are mitigated by the pretreatment method torrefaction. Torrefaction is a process where biomass is heated in an oxygen deficient atmosphere to typically between 240 and 350°C for a time period of 2 minutes to 1 hour. Most of the torrefaction R&D in the literature have so far been performed with bench-scale batch reactors. For the purpose of carefully studying continuous torrefaction, a 20 kg/h torrefaction pilot plant was therefore designed, constructed and evaluated.

The overall conclusion from this thesis is that the many benefits of torrefied biomass are valid also when produced with a continuous pilot plant and for typically Swedish forest biomasses. Some of the documented improved biomass properties are increased heating value, increased energy density, higher friability (lower milling energy) and less hydrophilic biomass (less moisture uptake). Most of the improvements can be attributed to the decomposition of hemicellulose and cellulose during torrefaction.

The most common variables for describing the torrefaction degree are mass yield or anhydrous weight loss but both are challenging to determine for continuous processes. We therefore evaluated three different methods (one existing and two new suggestions) to determine degree of torrefaction that not require measurement of mass loss. The degree of torrefaction based on analyzed higher heating value of the raw and torrefied biomass (DTFHHV) predicted mass yield most accurate and had lowest combined uncertainty.

Pelletizing biomass enhance transportation and handling but results from pelletization of torrefied biomass is still very limited in the literature and mainly reported from single pellet presses. A pelletization study of torrefied spruce with a ring die in pilot scale was therefore performed. The bulk energy density was found to be 14.6 GJ/m3 for pelletized torrefied spruce (mass yield 75%), a 40% increase compared to regular white pellets and therefore are torrefied pellets more favorable for long distance transports. More optimization of the torrefied biomass and the pelletization process is though needed for acquiring industrial quality pellets with lower amount of fines and higher pellet durability than attained in the present study.

Powders from milled raw biomass are generally problematic for feeding and handling and torrefied biomass has been proposed to mitigate these issues. The influence of torrefaction and pelletization on powder and particle properties after milling was therefore studied. The results show that powder from torrefied biomass were enhanced with higher bulk densities, lower angle of repose as well as smaller less elongated particles with less surface roughness. Even higher powder qualities were achieved by pelletizing the torrefied biomass before milling, i.e. another reason for commercial torrefied biomass to be pelletized.

Entrained flow gasification (EFG) is a promising option for conversion of biomass to other more convenient renewable energy carriers such as electricity, liquid biofuels and green petrochemicals. Also for EFGs are torrefied fuels very limited studied. Raw and torrefied logging residues were successfully gasified in a pilot scale pressurized entrained flow biomass gasifier at 2 bar(a) with a fuel feed corresponding to 270 kWth. Significantly lower methane content (50% decrease) in the syngas was also demonstrated for the torrefied fuel with mass yield 49%. The low milling energy consumption for the torrefied fuels compared to the raw fuel was beneficial for the gasification plant efficiency.

Place, publisher, year, edition, pages
Umeå: Umeå universitet , 2015. , 58 p.
Keyword [en]
Torrefaction, biomass, pilot scale, continuous reactor, grindability, entrained flow gasification, degree of torrefaction, biomass powder
National Category
Chemical Process Engineering
Identifiers
URN: urn:nbn:se:umu:diva-103046ISBN: 978-91-7601-287-1 (print)OAI: oai:DiVA.org:umu-103046DiVA: diva2:812251
Public defence
2015-06-10, N450, Naturvetarhuset, Umeå universitet, Umeå, 13:00 (English)
Opponent
Supervisors
Available from: 2015-05-20 Created: 2015-05-18 Last updated: 2015-05-20Bibliographically approved
List of papers
1. Effects of temperature and residence time on continuous torrefaction of spruce wood
Open this publication in new window or tab >>Effects of temperature and residence time on continuous torrefaction of spruce wood
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2015 (English)In: Fuel processing technology, ISSN 0378-3820, E-ISSN 1873-7188, Vol. 134, 387-398 p.Article in journal (Refereed) Published
Abstract [en]

As a solid energy carrier, biomass generally has a few disadvantages, which limits its use for coal replacement and as a feedstock for entrained flow gasification. The hydrophilic and fibrous nature, the low calorific value and low bulk energy content imply high accumulated costs in the whole supply chain and severe challenges in more advanced conversion systems. By thermally pretreating the biomass by torrefaction, these properties may be significantly improved. A continuous torrefaction rotary drum reactor was designed, constructed and evaluated to enable an accurate process control and allow a homogeneous well-defined high quality product to be produced. The combined effects of torrefaction temperature (260–310 °C) and residence time (8–25 min) on a large number of product properties (> 25) were determined for Norway spruce. The resulting mass and energy yields were 46–97% and 62–99%, respectively. Exothermic reactions were evident both at low (260 °C) and high temperatures (310 °C) but with no thermal runaway observed. Increased torrefaction severity resulted in decreased milling energy consumption, angle of repose, mass and energy yield, content of volatile matter, hydrogen, cellulose and hemicellulose. Hydrophobicity, heating value, carbon and fixed carbon contents increased. For all responses, the effect of torrefaction temperature was larger than the effect of residence time. Substantial interaction effects were present for mass and energy yields, volatile matter and hydrogen content. Another correlation found was the relationship of hemicellulose degradation and the brittleness of the torrefied product. Data also suggest secondary char forming reactions during the torrefaction process, resulting in higher fixed carbon content in the torrefied material than expected. The results also suggest torrefaction temperature and residence time not to be totally interchangeable.

Keyword
Torrefaction, Hydrophobicity, Grindability, Rotary drum, Continuous reactor
National Category
Chemical Process Engineering
Identifiers
urn:nbn:se:umu:diva-103041 (URN)10.1016/j.fuproc.2015.02.021 (DOI)000353739200047 ()
Funder
Bio4EnergySwedish Energy Agency, 31489-1
Available from: 2015-05-18 Created: 2015-05-18 Last updated: 2017-09-28Bibliographically approved
2. Evaluation of existing and new approaches to determine degree of torrefaction
Open this publication in new window or tab >>Evaluation of existing and new approaches to determine degree of torrefaction
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Torrefaction is a promising thermal pretreatment method for biomass in which many material properties are enhanced. The biomass components (hemicellulose, cellulose and lignin) degrade in this thermal process to different extents depending on type of process, treatment temperature, residence time and biomass type. Torrefaction severity is usually defined by biomass weight loss or mass yield, but other approaches to determine degree of torrefaction have also been suggested. For continuous and large scale facilities, mass yield can be challenging to determine and another approach to determine torrefaction severity is therefore desired. In this study, one existing and two new approaches for determining degree of torrefaction are presented, compared and evaluated including uncertainty analysis. The three approaches were based on analysis of; volatile matter, thermochemical properties (enthalpy of formation), and higher heating value. 

All three methods were highly correlated to mass yield and independent of torrefaction process. The degree of torrefaction based on higher heating value predicted mass yield most accurate, had lowest measurement uncertainty and the results were independent of biomass type. In the evaluation of the method based on formation enthalpy it was revealed that the increase in heating values for torrefied biomasses could be explained by the combination of increase in enthalpy of formation and decrease in oxygen content.

National Category
Chemical Process Engineering
Identifiers
urn:nbn:se:umu:diva-103043 (URN)
Funder
Bio4Energy
Available from: 2015-05-18 Created: 2015-05-18 Last updated: 2015-05-19
3. Oxygen enhanced torrefaction - An initial feasibility study
Open this publication in new window or tab >>Oxygen enhanced torrefaction - An initial feasibility study
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Upcoming world market prices on thermally treated and densified biomass to be feasible for coal replacement put a high demand on the process suppliers to deliver cost efficient and high energy yield conversion systems with a capacity to produce a high quality product. One of the more complex and vital parts in a torrefaction facility is the indirect or direct heat transfer system applied. This is a critical task that also may limit the scale-up potential and thus influence the economy of scale of a complete torrefaction system.

In the present study, it was demonstrated that the torrefaction reactions in a rotary drum pilot reactor (20 kgDS/h) potentially may be operated autothermally by a low level injection of air directly into the reactor for controlled in-situ partial combustion of the released torrefaction gases. Both concurrent and countercurrent gas flow patterns were evaluated for different process temperatures. At higher temperatures (338°C) in countercurrent gas flow mode, steady-state torrefaction was reached without external heat supply. The resulting torrefied biomass had higher heating value, higher carbon content and lower milling energy consumption, compared to non-oxidative torrefied biomass with same mass yield. Condensation of torrefaction gas compounds is a suggested reason.No significant decrease in the combustibility of the torrefaction gas was experienced.  The demonstrated Oxygen Enhanced Torrefaction (OET) mode thus has the potential to improve the torrefaction systems in terms of scale-up performance with reduced investment and operational costs but further validation work is needed to confirm the present findings and also to identify working conditions.

National Category
Chemical Process Engineering
Identifiers
urn:nbn:se:umu:diva-103045 (URN)
Available from: 2015-05-18 Created: 2015-05-18 Last updated: 2017-11-15
4. Effects of moisture content, torrefaction temperature, and die temperature in pilot scale pelletizing of torrefied Norway spruce
Open this publication in new window or tab >>Effects of moisture content, torrefaction temperature, and die temperature in pilot scale pelletizing of torrefied Norway spruce
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2013 (English)In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 102, 827-832 p.Article in journal (Refereed) Published
Abstract [en]

Pilot scale pelletizing of torrefied Norway spruce was performed in a factorial design with controlled factors at two levels: material moisture content (11% and 15%) and torrefaction temperature (270 and 300 °C), and die temperature as an uncontrolled factor (60–105 °C). Compared to commercial wood pellets, produced pellets had comparable bulk densities (630–710 kg/m3) but lower pellet durability (80–90%). Energy consumption for pelletizing of torrefied materials was approximately 100% higher than for softwood pelletizing, despite using a much shorter die channel length (35 vs. 55 mm:s), and the amounts of fines were high (10–30%). Die temperature showed a strong positive correlation with pellet production rate. Material moisture content had little influence on pellet quality and production rate, but addition of water created handling problems due to bad flow behavior.

Keyword
Torrefied biomass, Pellet quality, Energy consumption, Bulk density, Bulk density, Compaction
National Category
Energy Engineering
Identifiers
urn:nbn:se:umu:diva-62289 (URN)10.1016/j.apenergy.2012.08.046 (DOI)
Available from: 2012-12-14 Created: 2012-12-14 Last updated: 2017-12-06Bibliographically approved
5. Powder characteristics of torrefied and pelletized biomass
Open this publication in new window or tab >>Powder characteristics of torrefied and pelletized biomass
2014 (English)Conference paper, Published paper (Other academic)
Abstract [en]

Co-firing biomass with coal is an efficient way of mitigating severe climate change by reducing the total atmospheric greenhouse gas burden. Although biomass is considered close to CO2-neutral, present biomass based energy carriers all however suffer from several logistical challenges. The high moisture content, hydrophobic and fibrous nature, low energy density and bulky characteristics are all challenges impeding large-scale industrial use. The thermal pre-treatment method torrefaction enhances the above biomass fuel qualities and significantly facilitate large scale industrial use of biomass raw materials with respect to, handling, storage, feeding as well as for final conversion. In the present work we evaluate the influence of torrefaction on some fuel powder quality aspects. Five different types of torrefied pellets was compared to a reference (regular white wood pellets) with respect to milling energy and different powder characteristics such as aerated and tapped bulk density, particle size distribution, angle of repose and image analysis. Milling energy decreased by between 77 and 93 % for torrefied pellets compared to wood pellets, and the resulting black powders consisted of considerably smaller particle sizes. Particles from torrefied fuels were also less elongated and more circular/less rough, properties which positively influence flowability. Powder bulk density increased by 90 % at most. Angle of repose, previously documented to be negatively correlated to flowability, decreased from 66° for the reference powder to 57° for the torrefied sample with the lowest angle. Torrefaction temperature affected the results considerably more than moisture content prior to pelletization. The total evidences found therefore suggest a positive effect of torrefaction on both powder production and characteristics.

Keyword
Powderized biomass fuel, torrefaction, pelletization, particles, flowability, morphology (particle shape), particle size distribution (PSD), tapped density
National Category
Chemical Process Engineering
Identifiers
urn:nbn:se:umu:diva-96675 (URN)
Conference
Impacts of Fuel Quality on Power Production October 26 –31, 2014, Snowbird, Utah, USA, 2014
Funder
Bio4EnergySwedish Energy Agency
Available from: 2014-11-25 Created: 2014-11-25 Last updated: 2015-05-19Bibliographically approved
6. Entrained flow gasification of torrefied wood residues
Open this publication in new window or tab >>Entrained flow gasification of torrefied wood residues
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2014 (English)In: Fuel processing technology, ISSN 0378-3820, E-ISSN 1873-7188, Vol. 125, 51-58 p.Article in journal (Refereed) Published
Abstract [en]

In this work, four different fuels were gasified in a pressurized entrained flow pilot plant gasifier at approximately 270 kW(th). The different fuels were; two torrefied wood residues, one raw wood residue and one torrefied stem wood. The system pressure and oxygen equivalence ratio (lambda) were held constant for all four gasification experiments. It was found that the torrefaction pretreatment significantly reduced the milling energy consumption for fuel size reduction, which in turn contributed to increased gasification plant efficiency. Furthermore, the results indicate that the carbon conversion efficiency may be enhanced by an intermediate torrefaction pretreatment, whereas both less severe torrefaction and more severe torrefaction resulted in reduced carbon conversions. The results also indicate that the CH4 yield was significantly reduced for the most severely torrefied fuel.

Keyword
Entrained-flow gasification, Biomass, Torrefaction, Wood, Syngas, Methane
National Category
Bioenergy
Identifiers
urn:nbn:se:umu:diva-91248 (URN)10.1016/j.fuproc.2014.03.026 (DOI)000337876300006 ()
Available from: 2014-07-31 Created: 2014-07-28 Last updated: 2017-12-05Bibliographically approved

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