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Biomass conversion through syngas-based biorefineries: thermochemical process integration opportunities
Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The replacement of fossil resources through renewable alternatives is one way to mitigate global climate change. Biomass is the only renewable source of carbon available for replacing oil as a refining feedstock. Therefore, it needs to be utilized not just as a fuel but for both biochemical and thermochemical conversion through biorefining. Optimizing and combining various conversion processes using a system perspective to maximize the valorization, biomass usage, and environmental benefits is of importance. This thesis work has evaluated the integration opportunities for various thermochemical conversion processes within a biorefinery system.

The aim for all evaluated concepts were syngas production through gasification or reforming. Two potential residue streams from an existing biorefinery were evaluated as gasification feedstocks, thereby combining biochemical and thermochemical conversion. Torrefaction as a biomass pretreatment for gasification end-use was evaluated based on improved feedstock characteristics, process benefits, and integration aspects. A system concept, “Bio2Fuels”, was suggested and evaluated for low-temperature slow pyrolysis as a way to achieve simultaneous biomass refinement and transport driven CO2 negativity.

Syngas was identified as a very suitable intermediate product for residue streams from biochemical conversion. Resulting syngas composition and quality showed hydrolysis residue as suitable gasification feedstock, providing some adjustments in the feedstock preparation. Gasification combined with torrefaction pretreatment demonstrated reduced syngas tar content. The co-gasification of biogas and wood in a FBG was successfully demonstrated with increased syngas H2/CO ratio compared to wood gasification, however high temperatures (≥1000°C) were required for efficient CH4 conversion. The demonstrated improved feedstock characteristics for torrefied biomass may facilitate gasification of biomass residue feedstocks in a biorefinery. Also, integration of a torrefaction unit on-site at the biorefinery or off-site with other industries could make use of excess low-value heat for the drying step with improved overall thermal efficiency. The Bio2Fuels concept provides a new application for slow pyrolysis. The experimental evaluation demonstrated significant hydrogen and carbon separation, and no significant volatilization of ash-forming elements (S and Cl excluded)  in low-temperature (<400°C) pyrolysis. The initial reforming test showed high syngas CH4 content, indicating the need for catalytic reforming.

The collective results from the present work indicate that the application of thermochemical conversion processes into a biorefinery system, making use of by-products from biochemical conversion and biomass residues as feedstocks, has significant potential for energy integration, increased product output, and climate change mitigation.

Place, publisher, year, edition, pages
Umeå: Umeå universitet , 2017. , 67 p.
Keyword [en]
Biomass, biorefinery, thermochemical conversion, torrefaction, slow pyrolysis, gasification, process integration, carbon negativity
National Category
Chemical Process Engineering
Identifiers
URN: urn:nbn:se:umu:diva-139839ISBN: 978-91-7601-427-1 (print)OAI: oai:DiVA.org:umu-139839DiVA: diva2:1143905
Public defence
2017-10-20, N430, Naturvetarhuset, Umeå, 13:00 (Swedish)
Opponent
Supervisors
Available from: 2017-09-29 Created: 2017-09-22 Last updated: 2017-09-29Bibliographically approved
List of papers
1. Syngas production by combined biomass gasification and in situ biogas reforming
Open this publication in new window or tab >>Syngas production by combined biomass gasification and in situ biogas reforming
2015 (English)In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 29, no 6, 3725-3731 p.Article in journal (Refereed) Published
Abstract [en]

For small- to medium-sized streams of biogas (methane) produced at a biorefinery site where cost-efficient reforming by traditional methods are unavailable, combined biomass gasification and methane reforming could facilitate co-conversion and increase the H-2/CO ratio in the syngas from the gasification plant. In the present work, co-gasification of biomass with CH4 was evaluated by means of a parametric chemical equilibrium study for both wood/CH4 and black liquor/CH4 feedstocks and bench-scale fluidized-bed gasification experiments for a wood/peat/CH4 fuel mixture. The parametric study indicated that high-temperature, and steam and oxygen addition all facilitate a high conversion rate, i.e., methane reforming. Evaluating the influence of the gasification temperature on CH4 reforming and increasing the H-2/CO ratio experimentally demonstrated that high temperatures are required for efficient co-conversion.

National Category
Bioenergy
Identifiers
urn:nbn:se:umu:diva-106571 (URN)10.1021/acs.energyfuels.5b00405 (DOI)000356755000024 ()
Available from: 2015-07-20 Created: 2015-07-20 Last updated: 2017-09-27Bibliographically approved
2. Torrefaction and gasification of lignocellulosic hydrolysis residue from bio-ethanol production
Open this publication in new window or tab >>Torrefaction and gasification of lignocellulosic hydrolysis residue from bio-ethanol production
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Production of lignocellulosic ethanol through hydrolysis (acid or enzymatic) combined with fermentation generate a large amount of residue consisting of mainly lignin and un-ydrolyzed cellulose. The significant amount of energy retained in this residue require further conversion as a measure to ensure economic viability for the total process. Thermal conversion of the hydrolysis residue through gasification for syngas production would improve the fuel yield in addition to the overall plant efficiency. Also, torrefaction of various biomass feedstocks has been shown to significantly improve biomass fuel characteristics in addition to having substantial positive effect on the energy consumption of the particle size reduction. The present work was an evaluation of hydrolysis residue and torrefied hydrolysis residue as gasification feedstocks in a bench-scale fluidized bed gasifier, based on syngas composition, particle formation, tar production and volatilization behavior. In addition, the effects of torrefaction on hydrolysis residue material characteristics were separately evaluated, including the influence of the process parameters on milling energy consumption and morphology. All torrefaction data was fitted to multiple linear regression models with good reproducibility and fit. The results confirm the previously reported improved feedstock characteristics resulting from torrefaction of biomass, however residence time was proved the most influential process parameter on the torrefaction severity, most likely derived from the lack of hemicellulose in the residue. The resulting syngas composition and quality indicated that both non-torrefied and torrefied hydrolysis residue were suitable gasification feedstocks. The hydrolysis residue product gas had elevated tar concentration but the torrefied residue demonstrated a significant reduction in the tar content (particularly the heavy tar components), compared to both raw hydrolyis residue and the wood reference feedstock. Hence, torrefaction may significantly reduce tar related problems in downstream equipment/processes.

Keyword
torrefaction, gasification, enzymatic hydrolysis, bio-ethanol, lignocellulosic biomass, hydrolysis residue
National Category
Chemical Process Engineering
Identifiers
urn:nbn:se:umu:diva-139834 (URN)
Available from: 2017-09-22 Created: 2017-09-22 Last updated: 2017-09-28Bibliographically approved
3. 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 (&gt; 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
4. Process and system integration aspects of biomass torrefaction
Open this publication in new window or tab >>Process and system integration aspects of biomass torrefaction
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2010 (English)In: 18th European Biomass Conference and Exhibition: Proceedings, 2010Conference paper, Published paper (Other academic)
Abstract [en]

The pre-treatment method torrefaction has been shown to significantly improve biomass fuel characteristics such as energy density, moisture content, milling energy, feeding and hydrophobic properties. These improvements establish torrefaction as a key process in facilitating an expanding market for biomass raw materials. Most of the previous work has focused on evaluating and optimizing the torrefaction process alone. However, to fully explore the maximum energy/exergy and cost efficiency of biomass torrefaction, the entire fuel supply chain and site specific systems must be considered; including logistics, scale and integration with other processes. The present work in progress aims to develop a model that incorporates optimization of the biomass supply chain and process integration systems together with the torrefaction process in order to avoid sub-optimization.

Keyword
analysis, conversion systems, integration, model, supply chain, torrefaction
National Category
Chemical Process Engineering
Identifiers
urn:nbn:se:umu:diva-92862 (URN)10.5071/18thEUBCE2010-OB3.4 (DOI)
Conference
18th European Biomass Conference and Exhibition, Lyon, France, May 3-7, 2010
Available from: 2014-09-08 Created: 2014-09-08 Last updated: 2017-09-28Bibliographically approved
5. Low-temperature slow pyrolysis of biomass for H2-enriched syngas production and carbon negativity
Open this publication in new window or tab >>Low-temperature slow pyrolysis of biomass for H2-enriched syngas production and carbon negativity
(English)Manuscript (preprint) (Other academic)
Abstract [en]

To optimally utilize biomass resources as feedstock for fuels and chemicals production as well as for a potential substantial carbon sink, a dedicated process and system concept is suggested. The desired outcome of the process is a hydrogen-enriched pyrolysis gas and a carbon-enriched char, also retaining the ash-forming elements. To obtain a transport-driven large-scale CO2 negative system, the char is suggested as co-firing fuel in a facility with carbon capture and storage technology. In the present work, the basis for this Bio2Fuels separation concept was evaluated by 1) analysis of previously published empirical data for pyrolysis, and 2) chemical equilibrium calculations. The former analysis indicated on the potential for a significant separation of H and C to the pyrolysis gas and char respectively, with ~80% of the hydrogen and 40-60% of the carbon from the raw feedstock present in the pyrolysis gas product. Based on analyzed thermochemical driving forces, most of the ash-forming elements can be expected to be retained in the char, and an ash and alkali-free gas may be achieved at temperatures below 500°C. In addition, chemical equilibrium modelling of the pyrolysis gas reforming demonstrated a significantly increased H2/CO ratio in the syngas compared to gasification of the raw biomass.

Keyword
biomass, slow pyrolysis, carbon negativity, hydrogen, pyrolysis gas reforming, syngas
National Category
Chemical Process Engineering
Identifiers
urn:nbn:se:umu:diva-97525 (URN)
Available from: 2014-12-19 Created: 2014-12-19 Last updated: 2017-09-28Bibliographically approved
6. Hydrogen and carbon separation by low-temperature slow pyrolysis of biomass: experimental validation
Open this publication in new window or tab >>Hydrogen and carbon separation by low-temperature slow pyrolysis of biomass: experimental validation
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Previous work have indicated that slow pyrolysis may be used to separate hydrogen and carbon in a biomass feedstock into different product fractions. The hydrogen predominantly ends up in the pyrolysis gas fraction, whereas the carbon is mainly retained in the char. A system concept was suggested using low-temperature slow pyrolysis to achieve; a) transportation fuel/chemical production from the volatilized fraction, and b) potential carbon negativity by sequestering the carbon from the biochar fraction after use for electricity and/or heat production. The present work aimed to identify important process parameters, validate the hydrogen and carbon separation potential, and identify a potential process optimum for spruce wood slow pyrolysis. The process temperature was shown as the most important factor influencing the hydrogen and carbon pyrolysis gas yields, whereas the residence time factor only showed significant influence on the product yields for the shorter residence times. All experiments demonstrated significant hydrogen and carbon separation to gas and char respectively, particularly for lower process temperatures. An optimum process operation temperature was not found but from an industrial perspective, the suggested preferable temperature interval lies within the lowtemperature pyrolysis range (350-400°C), just above high temperature torrefaction (~300°C).

Keyword
biomass, pyrolysis, torrefaction, Norway spruce, syngas, hydrogen
National Category
Chemical Process Engineering
Identifiers
urn:nbn:se:umu:diva-139836 (URN)
Available from: 2017-09-22 Created: 2017-09-22 Last updated: 2017-09-28Bibliographically approved
7. Pilot scale experimental validation of the Bio2fuels low-temperature slow pyrolysis system concept
Open this publication in new window or tab >>Pilot scale experimental validation of the Bio2fuels low-temperature slow pyrolysis system concept
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The “Bio2Fuels” concept previously suggested may potentially achieve a transport driven carbon negativity by use of a combination of; low-temperature slow pyrolysis/high-temperature torrefaction; gas reforming; fuel synthesis; coal replacement by the solid bio-coal stream; and CCS of the resulting flue gases. The initial pre-treatment process suggested may potentially exhibit several advantages and may well facilitate an appealing and cost-efficient conversion system. The present work was comprised of pilot-scale pyrolysis experiments on softwood pellets using a continuous auger screw torrefaction/pyrolysis reactor for validation of the process in the temperature range of 300-425°C. All products were analyzed for composition and the pyrolysis gas (permanent gases + bio-oil) was sampled for particulate matter, permanent gas and bio-oil composition. The volatilization propensity of ash-forming elements was analyzed based on alkali deposits on impactor plates with SEM analysis and ICP-AES analysis of the bio-oil. The volatilization of sulfur and chlorine was also evaluated via char retainment. In addition, an initial test run of thermal pyrolysis gas reforming was performed by operating the thermal oxidation burner in gasification/reforming mode. The results showed that the hydrogen and oxygen in the biomass feedstock were volatilized at lower temperatures than the feedstock carbon, with the desired resulting hydrogen/carbon separation into pyrolysis gas and biochar, but also enrichment of oxygen in the pyrolysis gas. The hydrogen pyrolysis gas yield was >75% for pyrolysis temperatures ≥375°C and the corresponding carbon gas yield ranged from 50% to 63%. Most of the hydrogen in the pyrolysis gas was bound in the bio-oil as water and various hydrocarbons. No significant volatilization of alkali elements was observed through either analysis method. The most abundant permanent gas formed was CO2 and with a CH4 concentration of about 9%vol. The thermal reforming experiments also demonstrated a high CH4 syngas concentration, strongly indicating the need for a catalytic reforming process.

Keyword
biomass, CO2 negativity, slow pyrolysis, gas reforming, syngas, hydrogen, H2/CO ratio
National Category
Chemical Process Engineering
Identifiers
urn:nbn:se:umu:diva-139837 (URN)
Available from: 2017-09-22 Created: 2017-09-22 Last updated: 2017-09-28Bibliographically approved

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