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Alkali-enhanced gasification of biomass: laboratory-scale experimental studies
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.ORCID iD: 0000-0001-9074-7439
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Gasification seeks to break carbonaceous materials into synthetic gas (CO+H2) which can be subsequently upgraded into valuable products. Thus gasification can be utilized to convert low grade biomass stocks into carbon-neutral chemicals heat and power. Nonetheless, gasification produces tar and soot as a by-product, impurities which deposit on cold surfaces thereby risking operation downstream of the gasifier. Cleaning the syngas after the gasifier is a conventional way to attenuate the problem, yet a complex and expensive one. Thus, tar and soot should preferably be addressed already in the gasifier. Given that these impurities are non-equilibrium species they could be targeted by using some sort of catalytic material. Alkali elements have precisely shown to possess catalytic activity on char gasification, besides they have also been associated with a decrease in tar and soot. Yet, to design a functional alkali-catalysed gasification process we need to investigate in more detail on what exact products does alkali show an activity on, on what stage, under what circumstances and, on the measure that it is possible, the mechanism. This was investigated on the basis of experimental work that approached the topic from two opposite sides. On the one hand, we studied the effects of diluting the alkali content of a Na-rich black liquor (BL) by blending it with pyrolysis oil (PO), and on the other hand, we investigated adding various amounts of alkali on more conventional types of biomass fuels. Most of the experiments were conducted on a laminar drop tube furnace but the reactivity of BL chars was also studied through thermogravimetric analysis.

Alkali was found to catalyse heterogeneous gasification reactions (e.g. char) and to lead to much lower yields of C2 hydrocarbons, heavy tars and soot, favouring the presence of lighter species over large aromatic clusters. Alkali was hypothesized to reduce the quantity of soot by inhibiting the formation and growth of PAH, key intermediates on the road to soot. Besides, it was found that the initial contact between the alkali and the organic matrix was not critical, neither for gas impurities nor regarding char conversion, suggesting that the activity of alkali was a gas-induced phenomenon. The latter implied the existence of a vaporization-condensation cycle that could supply alkali into the char. Nonetheless, the beneficial effects by alkali were impaired by the affinity of Si to capture K and form potassium silicates which are inert. This interaction effect was particularly noticeable on char conversion as the silicates are not only inert but also liquid and viscous and prompt to encapsulate the char particles, thereby limiting mass transfer.

The experiments with blends of BL and PO showed that the concentration of alkali in BL could be decreased by 30% without any sign of a decrease in the catalytic activity on char gasification, thus indicating the existence a saturation threshold. Furthermore, adding PO into BL lead to a further reduction on the quantities of tar and soot, this finding was attributed to changes in the fuel composition unrelated to alkali. In any case the experiments with BL-based fuels showed lower amounts of tar and soot than those from alkali-impregnated biomass powder. The difference was partially attributed to the content of S in BL. The subsequent investigation targeting the role of S confirmed that S possessed a soot inhibiting role similar to that of alkali, yet unlike K, it did not show a catalytic effect on char gasification.

 

Place, publisher, year, edition, pages
Luleå: Luleå tekniska universitet, 2018.
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Keywords [en]
alkali, biomass, gasification, tar, soot, potassium, char, syngas, enhanced
National Category
Engineering and Technology Energy Engineering
Research subject
Energy Engineering
Identifiers
URN: urn:nbn:se:ltu:diva-71261ISBN: 978-91-7790-236-2 (print)ISBN: 978-91-7790-237-9 (electronic)OAI: oai:DiVA.org:ltu-71261DiVA, id: diva2:1256935
Public defence
2018-12-14, E632, Luleå, 10:00 (English)
Opponent
Supervisors
Available from: 2018-10-19 Created: 2018-10-18 Last updated: 2018-11-21Bibliographically approved
List of papers
1. Reduction of tar and soot formation from entrained-flow gasification of woody biomass by alkali impregnation
Open this publication in new window or tab >>Reduction of tar and soot formation from entrained-flow gasification of woody biomass by alkali impregnation
Show others...
2017 (English)In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 31, no 5, p. 5104-5110Article in journal (Refereed) Published
Abstract [en]

Tar and soot in product gas have been a major technical challenge toward the large-scale industrial installation of biomass gasification. This study aims at demonstrating that the formation of tar and soot can be reduced simultaneously using the catalytic activity of alkali metal species. Pine sawdust was impregnated with aqueous K2CO3 solution by wet impregnation methods prior to the gasification experiments. Raw and alkali-impregnated sawdust were gasified in a laminar drop-tube furnace at 900–1400 °C in a N2–CO2 mixture, because that creates conditions representative for an entrained-flow gasification process. At 900–1100 °C, char, soot and tar decreased with the temperature rise for both raw and alkali-impregnated sawdust. The change in tar and soot yields indicated that potassium inhibited the growth of polycyclic aromatic hydrocarbons and promoted the decomposition of light tar (with 1–2 aromatic rings). The results also indicated that the catalytic activity of potassium on tar decomposition exists in both solid and gas phases. Because alkali salts can be recovered from product gas as an aqueous solution, alkali-catalyzed gasification of woody biomass can be a promising process to produce clean product gas from the entrained-flow gasification process at a relatively low temperature.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2017
National Category
Energy Engineering
Research subject
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-63149 (URN)10.1021/acs.energyfuels.6b03480 (DOI)000402023600055 ()2-s2.0-85020552211 (Scopus ID)
Note

Validerad;2017;Nivå 2;2017-05-30 (rokbeg)

Available from: 2017-04-25 Created: 2017-04-25 Last updated: 2018-10-18Bibliographically approved
2. Fuel conversion characteristics of black liquor and pyrolysis oil mixtures: Efficient gasification with inherent catalyst
Open this publication in new window or tab >>Fuel conversion characteristics of black liquor and pyrolysis oil mixtures: Efficient gasification with inherent catalyst
2015 (English)In: Biomass and Bioenergy, ISSN 0961-9534, E-ISSN 1873-2909, Vol. 79, p. 155-165Article in journal (Refereed) Published
Abstract [en]

Alkali metals inherent in black liquor (BL) have strong catalytic activity during gasification. A catalytic co-gasification process based on BL with pyrolysis oil (PO) has the potential to be a part of efficient and fuel-flexible biofuel production systems. The objective of the paper is to investigate how adding PO into BL alters fuel conversion under gasification conditions. First, the conversion times of single fuel droplet were observed in a flat flame burner under different conditions. Fuel conversion times of PO/BL mixtures were significantly lower than PO and comparable to BL. Initial droplet size (300–1500 μm) was the main variable affecting devolatilization, indicating control by external heat transfer. Char oxidation was affected by droplet size and the surrounding gas composition. Then, the intrinsic reactivity of char gasification was measured in an isothermal thermogravimetric analyser at T = 993–1133 K under the flow of CO2–N2 mixtures. All the BL-based samples (100% BL, 20% PO/80% BL, and 30% PO/70% BL on mass basis) showed very high char conversion. Conversion rate of char gasification for PO/BL mixtures was comparable to that of pure BL although the fraction of alkali metal in char decreased because of mixing. The reactivities of BL and BL/PO chars were higher than the literature values for solid biomass and coal chars by several orders of magnitude. The combined results suggest that fuel mixtures containing up to 30% of PO on mass basis may be feasible in existing BL gasification technology.

National Category
Energy Engineering
Research subject
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-31991 (URN)10.1016/j.biombioe.2015.04.008 (DOI)000358973700014 ()2-s2.0-84951798142 (Scopus ID)652f18a7-33ad-4283-9637-61c3fd496633 (Local ID)652f18a7-33ad-4283-9637-61c3fd496633 (Archive number)652f18a7-33ad-4283-9637-61c3fd496633 (OAI)
Conference
European Biomass Conference and Exhibition : 23/06/2014 - 26/06/2014
Note
Validerad; 2015; Nivå 1; 20150504 (andbra); Konferensartikel i tidskriftAvailable from: 2016-09-30 Created: 2016-09-30 Last updated: 2018-10-18Bibliographically approved
3. Reduction of tar and soot in biomass gasification with potassium: Effect of impregnation method and inherent inorganic species
Open this publication in new window or tab >>Reduction of tar and soot in biomass gasification with potassium: Effect of impregnation method and inherent inorganic species
(English)Manuscript (preprint) (Other academic)
National Category
Engineering and Technology
Research subject
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-71260 (URN)
Available from: 2018-10-18 Created: 2018-10-18 Last updated: 2018-10-18
4. Co-gasification of black liquor and pyrolysis oil at high temperature: Part 2. Fuel conversion
Open this publication in new window or tab >>Co-gasification of black liquor and pyrolysis oil at high temperature: Part 2. Fuel conversion
2017 (English)In: Fuel, ISSN 0016-2361, E-ISSN 1873-7153, Vol. 197, p. 240-247Article in journal (Refereed) Published
Abstract [en]

The efficiency and flexibility of the BL gasification process may improve by mixing BL with more energy-rich fuels such as pyrolysis oil (PO). To improve understanding of the fuel conversion process, blends of BL and PO were studied in an atmospheric drop tube furnace. Experiments were performed in varying atmosphere (5% and 0% CO2, balanced by N2), temperature (800–1400 °C), particle size (90–200 μm and 500–630 μm) and blending ratio (0%, 20% and 40% of PO in BL on weight basis). Additionally, pine wood was used as a reference fuel containing little alkali. The addition of PO to BL significantly increased the combined yield of CO and H2 and that of CH4. BL/based fuels showed much lower concentration of tar in syngas than pine wood. Remarkably, the addition of PO in BL further promoted tar reforming in presence of CO2. Unconverted carbon in the gasification residue decreased with increasing fractions of PO. Small fuel particles showed complete conversion at 1000 °C but larger particles did not reach complete conversion even at T = 1400 °C.

Place, publisher, year, edition, pages
Elsevier, 2017
National Category
Energy Engineering
Research subject
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-62177 (URN)10.1016/j.fuel.2017.01.108 (DOI)000398669900026 ()2-s2.0-85013444316 (Scopus ID)
Note

Validerad; 2017; Nivå 2; 2017-02-27 (andbra)

Available from: 2017-02-27 Created: 2017-02-27 Last updated: 2018-10-18Bibliographically approved
5. Co-gasification of black liquor and pyrolysis oil at high temperature: Part 1. Fate of alkali elements
Open this publication in new window or tab >>Co-gasification of black liquor and pyrolysis oil at high temperature: Part 1. Fate of alkali elements
2017 (English)In: Fuel, ISSN 0016-2361, E-ISSN 1873-7153, Vol. 202, p. 46-55Article in journal (Refereed) Published
Abstract [en]

The catalytic activity of alkali compounds in black liquor (BL) enables gasification at low temperatures with high carbon conversion and low tar and soot formation. The efficiency and flexibility of the BL gasification process may be improved by mixing BL with fuels with higher energy content such as pyrolysis oil (PO). The fate of alkali elements in blends of BL and PO was investigated, paying special attention to the amount of alkali remaining in the particles after experiments at high temperatures. Experiments were conducted in a drop tube furnace under different environments (5% and 0% vol. CO2 balanced with N2), varying temperature (800–1400 °C), particle size (90–200 µm, 500–630 µm) and blending ratio (0%, 20% and 40% of pyrolysis oil in black liquor). Thermodynamic analysis of the experimental cases was also performed.

The thermodynamic results qualitatively agreed with experimental measurements but in absolute values equilibrium under predicted alkali release. Alkali release to the gas phase was more severe under inert conditions than in the presence of CO2, but also in 5% CO2 most of the alkali was found in the gas phase at T = 1200 °C and above. However, the concentration of alkali in the gasification residue remained above 30% wt. and was insensitive to temperature variations and the amount of PO in the blend. Thermodynamic analysis and experimental mass balances indicated that elemental alkali strongly interacted with the reactor’s walls (Al2O3) by forming alkali aluminates. The experience indicated that adding PO into BL does not lead to alkali depletion during high temperature gasification.

Place, publisher, year, edition, pages
Elsevier, 2017
National Category
Energy Engineering
Research subject
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-63020 (URN)10.1016/j.fuel.2017.04.013 (DOI)000404078500006 ()2-s2.0-85017397407 (Scopus ID)
Note

Validerad; 2017; Nivå 2; 2017-04-12 (andbra)

Available from: 2017-04-12 Created: 2017-04-12 Last updated: 2018-10-18Bibliographically approved

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