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Ash transformation during thermochemical conversion of agricultural biomass in entrained flow conditions
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.ORCID iD: 0009-0005-0010-2351
2024 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Agricultural biomass is considered a prospective renewable feedstock in the energy conversion units. Despite its widespread availability, agricultural biomass is often underutilized in the thermochemical processes due to its high ash content. These types of biomass usually contain a relatively high share of potassium (K), silicon (Si), and phosphorus (P). The presence of these elements in the fuel can cause ash-related issues like slagging, deposit formation, fine particle emissions, etc. However, extracting these elements during thermochemical conversion processes can increase the economic viability of using agricultural biomassas a feedstock. There is still a lack of detailed understanding regarding ash-transformation reactions in entrained flow conversion of such biomassas sortments. Therefore, detailed knowledge regarding ash-transformation pathways during the thermochemical conversion of agricultural biomass can target the aim of reducing or eliminating ash-related issues together with recovering valuable Si- and K-P-rich compounds.

The main objectives of this work were, therefore, to 1) investigate the ash transformation pathways of Si and P during entrained flow combustion of different types of Si- and P-rich agricultural biomass and 2) investigate the potential of extracting valuable Si- and K-P compounds with high purity from the gas phase and/or residual ashes formed during entrained flow conversion of different agricultural biomass. Experiments were conducted within a lab-scale laminar drop tube furnace (DTF) at 1200 and 1450 °C in combustion conditions (using air) and in pyrolysis conditions (using N2). Three different agricultural biomass types were used, namely, rice husks, grass, and draff (i.e., beer brewing residue). Rice husks represent Si-rich fuel with minor amounts of P, K, and Ca. The grass fuel represents K-Si-rich fuel with moderate amounts of P, Ca, and Mg. Draff, on the other hand, represents a P-rich fuel with a relatively high share of Si and a moderate to minor Ca, Mg, and K content. The produced residual materials, i.e., coarse ash (> 1 μm), fine particle (< 1μm) fractions, and chars after the experiments were morphologically and chemically characterized by SEM-EDS, XRD, ICP-AES, IC, and CHN-analysis. The interpretation of experimental findings and theoretical assessment of ash transformation were facilitated through TECs.

The combustion experiments using a laminar drop tube furnace revealed distinctive ash transformations across the studied agricultural biomass. For all fuels, Si was entirely retained in the coarse ash (> 1 μm) fractions, whereas P was found mainly in coarse ash fractions and in a minor to moderate amount in the fine particle (< 1 μm) ash fractions.

For the rice husks, most of the minor ash-forming elements (i.e., K, Ca, Mg, and P) present in the fuel were retained at the inner surface of the chars during the pyrolysis/devolatilization experiments conducted in the DTF, which were later incorporated into a melt found in the coarse ash fractions after combustion experiments. Consequently, Si-rich molten spheres were formed at the inner surface of the coarse ashes. The share of molten ash increased in the coarse ash fractions with the combustion temperature. Concerning grass, the retained P in the residual coarse ash fractions was found in a K-Ca-Mg-rich phosphosilicate melt and crystalline Ca9MgK(PO4)7(OH), Ca5(SiO4)0.3(PO4)2.7(OH)0.7, and Ca7(SiO4)2(PO4)2. The phosphosilicate melt was most likely formed through the initial formation of a K-rich silicate melt, with subsequent incorporation of Ca, Mg, and P. As for draff, the P retained in the coarse ash fractions was identified as Ca-Mg-rich phosphosilicate melt and crystalline Ca3Mg3(PO4)4. This phosphosilicate melt most likely originated from the phytate-derived Ca-Mg-phosphates that melt and interact with Si-rich particles/sites.

TECs predicted that extracting pure Si-containing compounds (i.e., SiC(s)) from the gas phase during the entrained flow conversion of rice husks would require very high temperatures in the flame (i.e., around 2000 °C) to volatilize a moderate amount of Si present in the rice husks. Additionally, it would require a pyrolysis cooling atmosphere and relatively high surface temperatures (i.e., around 1500 °C) to form potentially valuable Si-containing compounds. These conditions are challenging to achieve in practice. However, the experimental investigations showed the possibility of extracting relatively pure silica from the residual coarse ash fractions collected after the entrained flow combustion of rice husks. TECs did not predict the probability of forming pure Si and/or K-P-containing compounds from the gas phase for grass fuel. Regarding draff fuel, both TECs and experimental investigations indicated that a surplus of P to Si and cations in the fuel can facilitate the formation of valuable K-phosphates and/or H3PO4 from the gas phase at lower surface temperatures (i.e., <400 °C). Moreover, the coarse ash fractions obtained after the combustion of grass and draff primarily contained different phosphosilicate melts. The plant availability of P in such melts needs to be evaluated.

The findings derived from this work offer valuable insights into the ash transformation of phosphorus (P) and silicon (Si) during the thermochemical conversion of agricultural biomass with varying ash compositions under entrained flow conditions. The obtained knowledge could be used to propose efficient measures to mitigate the associated ash-related problems and to propose interesting pathways to extract valuable Si- and K-P-rich components during the thermal conversion of agricultural biomass in entrained flow reactors.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2024.
Series
Licentiate thesis / Luleå University of Technology, ISSN 1402-1757
Keywords [en]
Agricultural biomass, entrained flow conditions, combustion, ash transformation, Si recovery, P recovery
National Category
Energy Engineering
Research subject
Energy Engineering
Identifiers
URN: urn:nbn:se:ltu:diva-102990ISBN: 978-91-8048-444-2 (print)ISBN: 978-91-8048-445-9 (electronic)OAI: oai:DiVA.org:ltu-102990DiVA, id: diva2:1814407
Presentation
2024-02-09, E632, Luleå Tekniska Universitet, Luleå, 13:00 (English)
Opponent
Supervisors
Available from: 2023-11-27 Created: 2023-11-24 Last updated: 2025-01-08Bibliographically approved
List of papers
1. Thermochemical equilibrium study of ash transformation during entrained flow combustion and gasification of agricultural residues focusing on the potential extraction of valuable Si, K-P compounds
Open this publication in new window or tab >>Thermochemical equilibrium study of ash transformation during entrained flow combustion and gasification of agricultural residues focusing on the potential extraction of valuable Si, K-P compounds
2022 (English)Conference paper, Oral presentation with published abstract (Refereed)
National Category
Energy Engineering
Research subject
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-102989 (URN)
Conference
28th International Conference on the Impact of Fuel Quality on Power Production and the Environment, Åre, Sweden, September 19-23, 2022
Funder
Swedish Energy Agency, 46443-2
Available from: 2023-11-24 Created: 2023-11-24 Last updated: 2025-01-08Bibliographically approved
2. Ash transformation during combustion of agricultural residues in entrained flow conditions with a focus on phosphorus
Open this publication in new window or tab >>Ash transformation during combustion of agricultural residues in entrained flow conditions with a focus on phosphorus
(English)Manuscript (preprint) (Other academic)
National Category
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-102987 (URN)
Available from: 2023-11-24 Created: 2023-11-24 Last updated: 2025-01-08
3. Ash Formation during Combustion of Rice Husks in Entrained Flow Conversion Conditions
Open this publication in new window or tab >>Ash Formation during Combustion of Rice Husks in Entrained Flow Conversion Conditions
2024 (English)In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 38, no 14, p. 13278-13294Article in journal (Refereed) Published
Abstract [en]

This study investigates the detailed ash transformation process during the combustion of rice husks in entrained flow conditions. The experiments were conducted in a lab-scale drop tube furnace at 1200 and 1450 °C in pyrolysis/devolatilization (using N2) and combustion (using air) conditions. The detailed ash transformation process during the different fuel conversion stages in combustion (i.e., devolatilization and char combustion) was investigated by comparing the results obtained in the pyrolysis/devolatilization experiments with the combustion experiments. The resulting residual chars, ashes, and particulate matter (PM) were collected and characterized by scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM–EDS), X-ray diffraction (XRD), inductively coupled plasma atomic emission spectroscopy (ICP-AES), ion chromatography (IC), and CHN analyses. Furthermore, the obtained results were interpreted via thermodynamic equilibrium calculations (TECs). For all investigated conditions, Si, Ca, and Mg were retained entirely in the coarse ash and char fractions (>1 μm). Meanwhile, K and P were found in coarse ash/char fractions and fine particulate fractions (<1 μm). A moderate, at 1200 °C, to high share, at 1450 °C, of the detected K and P was found in the fine particle fractions after combustion. The majority (>95%) of the detected S and Cl were volatilized during the experiments. The study showed an accumulation of minor ash-forming elements (i.e., K, Ca, Mg, P) on the inner part of rice husk chars, initiating melt formation during the char combustion stage. The identified melt at 1200 °C after combustion was rich in Si with minor amounts of K, Ca, Mg, and P. The share of molten ashes was increased at 1450 °C compared to that at 1200 °C. Overall, the results presented in this work reveal detailed insights into the ash transformation processes taking place in different parts of the fuel during the combustion of rice husks in entrained flow conditions.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2024
National Category
Energy Engineering
Research subject
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-102986 (URN)10.1021/acs.energyfuels.4c01413 (DOI)001259893800001 ()2-s2.0-85197092298 (Scopus ID)
Funder
Swedish Energy Agency, 46443-2
Note

Validerad;2024;Nivå 2;2024-08-15 (hanlid);

Full text license: CC BY;

This article has previously appeared as a manuscript in a thesis

Available from: 2023-11-24 Created: 2023-11-24 Last updated: 2024-08-15Bibliographically approved

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Pachchigar, Samarthkumar

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