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Role of Surface Morphology on Bed Material Activation during Indirect Gasification of Wood
Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Kemigården 4, 412 96 Gothenburg, Sweden.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.ORCID iD: 0000-0003-1203-0410
Chalmers University of Technology, Department of Physics, Kemigården 1, Gothenburg, SE-41296, Sweden.
Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Kemigården 4, 412 96 Gothenburg, Sweden.
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2023 (English)In: Fuel, ISSN 0016-2361, E-ISSN 1873-7153, Vol. 333, Part 1, article id 126387Article in journal (Refereed) Published
Abstract [en]

Olivine and alkali-feldspar were utilized in separate campaigns in an indirect dual fluidized bed gasification campaign with woody biomass as fuel. After three days, both bed materials were reported to be active towards tar removal and exhibited oxygen-carrying abilities and had formed an ash layer consisting of an outer ash deposition layer and an inner interaction layer.

X-ray microtomography analysis concluded that a preferred deposition of ash happens onto convex regions of the bed particles, which results in an increase in thickness of the ash layer over convex regions. This effect is most pronounced for the outer layer which is a product of ash deposition. The inner layer exhibits a homogeneous thickness and is probably formed by interaction of Ca from the outer layer with the particles. Transmission electron microscopy revealed the presence of Fe and Mn on the surface of the particles in a solid solution with Mg. The oxygen-carrying effect which is found for aged particles is therefore attributed to the presence of Fe and Mn on the surface of aged particles. Alkali were found on the surface of both particles which are likely contributing to the catalytic activity of the material towards tar removal.

Place, publisher, year, edition, pages
Elsevier, 2023. Vol. 333, Part 1, article id 126387
Keywords [en]
Fluidized bed, Bed material, Layer formation, Olivine, Feldspar, Material characterization
National Category
Energy Engineering Bioenergy
Research subject
Energy Engineering
Identifiers
URN: urn:nbn:se:ltu:diva-93675DOI: 10.1016/j.fuel.2022.126387ISI: 000880106400002Scopus ID: 2-s2.0-85140309835OAI: oai:DiVA.org:ltu-93675DiVA, id: diva2:1705172
Funder
Swedish Energy Agency, 50450-1, P46533-1
Note

Validerad;2022;Nivå 2;2022-11-02 (sofila);

This article has previously appeared as a manuscript in a thesis

Available from: 2022-10-21 Created: 2022-10-21 Last updated: 2024-04-19Bibliographically approved
In thesis
1. The effect of surface morphology on bed particle layer characteristics in fluidized bed combustion and gasification of woody biomass
Open this publication in new window or tab >>The effect of surface morphology on bed particle layer characteristics in fluidized bed combustion and gasification of woody biomass
2022 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

A critical process phenomenon in fluidized bed combustion and gasification is the bed particle layer formation which might be problematic in cases where it instigates bed agglomeration and bed material deposition or could improve the process performance in cases where it has a positive effect on tar reduction in biomass gasification. The interactions between ash forming matter for a wide variety of wood-derived fuels with different types of bed materials in fluidized bed combustion and gasification have been profusely studied, and the underlying mechanism of bed particle layer formation has been suggested. Howbeit, the influence of bed material’s surface properties on bed layer characteristics has not been elaborated in the literature.

In this thesis, the effect of surface morphology on the formation of bed particle layers and crack layers has been investigated for different bed material types in combustion and gasification of woody biomass. Samples were selected from different full-scale and bench-scale conversion units at different ages from the start-up to investigate the development of the bed layer at different surface morphologies over the bed particle. Natural sand samples (consisting mainly of quartz and K-feldspar) were taken from a 30 MWth bubbling fluidized bed (BFB30) combustion unit and a 90 MWth circulating fluidized bed (CFB90) combustion unit, K-feldspar and olivine bed samples were taken from a dual fluidized bed (DFB) gasification unit (consisting of a 2-4 MWth bubbling fluidized bed gasifier and a 12 MWth circulating fluidized bed combustor), and ilmenite bed samples were taken from a 5 kWth bench scale bubbling fluidized bed (BFB5)combustion unit. 

X-ray microtomography analysis (XMT) was utilized to inspect the bed layer distribution and determine surface morphology in 3D, including measurements of the bed particle layer thickness over the particle surface. Distribution of the crack layers inside the bed core was also observed through the XMT images. Moreover, scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) along with transmission electron microscopy (TEM) were employed to investigate the elemental compositional distribution and to support interpretation of XMT results.  

The results revealed that for all types of bed materials, regardless of the conversion process, both inner and outer layers could be observed over the convex-shaped regions on the bed particle surfaces after some days, while the concave-shaped regions are mainly covered with a thin inner layer. Consequently, a thicker overall layer could be observed over the convex-shaped regions compared to the concave regions. Results from the combustion processes units show that K-feldspar bed particles retain a thinner overall layer compared to quartz when both are exposed to the same process condition and fuel ash composition. Moreover, almost no tendency for crack layer formation was observed in K-feldspar particles while up to 19% and 32% of the quartz bed particle could be engaged by the crack layers in bed samples taken from the BFB30 and CFB90, respectively. It was also observed that the crack layers are initiated from the concave-shaped regions on the bed surface where gaseous alkali can penetrate into the bed core through the inward cracks in the thin inner layer.

In the BFB5 combustor, it was observed for the samples taken 6 and 12 hours after the start-up that a thin Fe-rich layer ascends over the convex areas over the ilmenite surface due to the outward migration of Fe from the bed particle core. Over time, this iron-rich layer is covered with a Ca-rich outer layer that obstructs further migration of the Fe through the convex areas. For the older particles (i.e., 30 and 42 hours from the start-up) high concentration of Fe was observed in the concaves and more porous areas inside the bed particle core. 

Measurement of the average overall layer thickness for typical K-feldspar and olivine bed particles from the DFB gasifier showed that the latter exhibits thinner overall bed layer thickness. The elemental composition of the bed layer over the concave and convex regions was observed to be different for both bed materials. A slightly higher Fe concentration was observed over the convex areas on olivine. A similar trend was likewise noticed for Mn on both bed types.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2022
Series
Licentiate thesis / Luleå University of Technology, ISSN 1402-1757
National Category
Energy Engineering
Research subject
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-93678 (URN)978-91-8048-198-4 (ISBN)978-91-8048-199-1 (ISBN)
Presentation
2022-12-15, E231, Luleå tekniska univerisitet, Luleå, 13:30 (English)
Opponent
Supervisors
Available from: 2022-10-21 Created: 2022-10-21 Last updated: 2023-09-05Bibliographically approved
2. Role of Surface Morphology on Bed Particle Layer Formation During Thermal Conversion of Woody Biomass in Fluidized Beds
Open this publication in new window or tab >>Role of Surface Morphology on Bed Particle Layer Formation During Thermal Conversion of Woody Biomass in Fluidized Beds
2024 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Bed particle layer formation is a crucial aspect of fluidized bed conversion of woody biomass, which can positively and negatively impact the process performance. The layer formation may lead to issues such as bed agglomeration and bed material deposition or reduced oxygen transport capacity of oxygen carriers. However, bed layer formation may positively affect the product gas composition in gasification. While extensive research has been conducted on the interactions between the ash-forming constituents from various wood-derived fuels and different bed materials, the influence of the bed particle surface properties on the bed layer characteristics remains largely unexplored in the existing literature.

This thesis thoroughly studied the role of bed particle surface morphology on the bed layer formation and its characteristics across different bed particle types within various fluidized bed conversion processes of woody biomass. The bed particle samples were collected from different bench-scale, semi-industrialscale, and industrial-scale conversion units at different time intervals from the start-up to assess the bed particle layer formation process throughout the bed particle age in different conversion processes of woody biomass. Natural sand bed samples (consisting mainly of quartz and K-feldspar) were taken from a 30 MWth bubbling fluidized bed (BFB30) combustion unit and a 90 MWth circulating fluidized bed (CFB90) combustion unit. K-feldspar and olivine were obtained from a dual fluidized bed (DFB12-4) indirect gasification unit comprising a 12 MWth circulating fluidized bed combustor and a 2-4 MWth bubbling fluidized bed gasifier. Different types of ilmenite samples were taken from a 5 kWth bench-scale bubbling fluidized bed (BFB5) and a 12 MWth circulating fluidized bed (CFB12) combustion unit. Additionally, quartz and natural sand samples (comprising mainly quartz, K-feldspar, and Na-feldspar) were gathered from three different industrial fast pyrolysis plants. Bed particles found in the samples were subjected to different analysis methods. Scanning electron microscopy (SEM), coupled with energy dispersive spectroscopy (EDS), served as a primary tool for obtaining information regarding the morphology and elemental composition of different layers present on the surface of bed particles and inside the bed particle core. In instances where layers were too thin to be adequately examined via SEM/EDS, transmission electron microscopy (TEM) was employed as a complementary analysis method. X-ray microtomography (XMT) was also utilized to explore the distribution of bed particle layers on the particle surfaces. This method facilitated both qualitative and quantitative assessments, including observation of the surface morphology of the bed particles, analysis of the bed layer distribution on the bed particle surface, along with measurements of bed particle layer thickness and the volume fraction of various features throughout the bed particles.

Comprehensive analysis of the results from different characterization techniques showed that for all studied bed particles, regardless of their chemical composition, the inner layer (i.e., the Ca-reaction layer) was thicker on convex areas and thinner or entirely missing in the concave regions. The outer layer, mainly consisting of Ca compounds (i.e., the ash-deposition layer), was more likely to be found on the convex areas of the bed particle surface. Overall, the total bed particle layer thickness (inner and outer layer) was larger on the convex areas compared to the concaves. Consequently, the concave regions can facilitate mass transfer to and from the bed particle core even after the full development of the Ca-rich inner layer. Therefore, in the case of using quartz and Na-feldspar particles where there was a high chemical potential to react with fuel-derived gaseous alkali, the inner-inner layer and the crack layers (together referred to as the K-reaction layers) were connected to concaves on the bed particle surface. For the studied oxygen carrier (i.e., ilmenite), where most of the convex regions at the bed particle surface were covered with the Ca-rich layer, Fe could still migrate to the bed particle surface through concaves.

Bed particle layer characteristics observed in the fast pyrolysis plants were, to some extent, different compared to those in the combustion and gasification of woody biomass. In general, the layers were considerably thinner in the fast pyrolysis process, with a similar exposure time compared to combustion and gasification. Crack layers were not detected in quartz and Na-feldspar bed particles in any of the studied fast pyrolysis plants, and the inner layer had a lower Ca concentration than that in the combustion or gasification. Further, only the Ca-reaction layer was identified on feldspar bed particles. However, the distribution pattern of the bed particle layers at different morphologies on the bed particle surface resembled that in combustion and gasification.

The findings indicated that apart from the chemical composition, the surface morphology of the bed particles plays a vital role in determining their performance throughout the fuel conversion process in the fluidized bed. Specifically, as the crack layer formation in quartz bed particles is linked to the concave-shaped areas on the bed particle surface, quartz bed particles characterized by fewer concaves experience less fragmentation during the conversion process. Conversely, when employing ilmenite as an oxygen carrier, the presence of concaves on the particle surface facilitates the outward migration of Fe over prolonged exposures, albeit potentially compromising the structural integrity of the bed materials. Thus, a trade-off exists between achieving a desired oxygen-carrying capacity and maintaining structural integrity over extended durations. Previous studies have suggested that the formation of a Ca-rich layer on bed particles can positively influence the composition of the product gas in the gasification process. Results from this work showed that the Ca-rich layer on bed particles intended for gasification, such as K-feldspar and olivine, is thicker and more evenly distributed on particles possessing a surface morphology featuring more frequent convex shapes.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2024
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Energy Engineering
Research subject
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-105164 (URN)978-91-8048-546-3 (ISBN)978-91-8048-547-0 (ISBN)
Public defence
2024-06-11, E632, Luleå University of Technology, Luleå, 09:00 (English)
Opponent
Supervisors
Available from: 2024-04-19 Created: 2024-04-19 Last updated: 2024-05-21Bibliographically approved

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Valizadeh, AliÖhman, Marcus

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