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Ash chemistry and fuel design focusing on combustion of phosphorus-rich biomass
Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics. (Thermochemical Energy Conversion Laboratory)ORCID iD: 0000-0002-5777-9241
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Biomass is increasingly used as a feedstock in global energy production. This may present operational challenges in energy conversion processes which are related to the inorganic content of these biomasses. As a larger variety of biomass is used the need for a basic understanding of ash transformation reactions becomes increasingly important. This is not only to reduce operational problems but also to facilitate the use of ash as a nutrient source for new biomass production.

Ash transformation reactions were examined in the present work using the Lewis acid-base concept. The model presented in Paper I was further extended and discussed, including the definition of tertiary ash transformation reactions as reaction steps where negatively charged molecular ions, Lewis bases, other than hydroxides are present in the reactants. The effect of such reactions for bonding of various metal ions, Lewis acids, were discussed. It was found that the formation of various phosphates through secondary and tertiary ash transformation reactions is important for the behaviour of biomass ash in combustion. The suggested model was supported by findings in Papers II-VIII.

The experimental findings in Papers II-VIII were discussed in terms of ash transformation reactions. The fuel design choices made to investigate the effect of phosphorus in particular on ash transformation reactions were high-lighted. Addition of phosphoric acid to woody-type and agricultural biomasses showed that phosphate formation has a large influence on the speciation of Si, S, and Cl. Co-combustion of a problematic agricultural residue with other biomasses showed that the relation between phosphorus, alkali and alkaline earth metal content is important. Co-combustion of biosolids with wheat straw was shown to greatly improve the combustion properties of wheat straw.

It was suggested that fuel analyses should be presented using molar concentration (mole/kg) in diagrams based on ash transformation reactions and elements forming Lewis acids or bases. This may facilitate the assessment of the combustion behaviour of a fuel. Some comments were made on fuel design and additives, specifically pointing out that phosphorus content should always be carefully considered in relation to alkali and alkaline earth metals in fuels and fuel blends.

Place, publisher, year, edition, pages
Umeå: Umeå universitet , 2014. , 50 p.
Keyword [en]
phosphorus, biomass, combustion, ash chemistry, fuel design, ash transformation, phosphorus-rich, ash-forming elements, fuel fingerprint, ash transformation reactions, Lewis base, Lewis acid
National Category
Inorganic Chemistry Energy Engineering
Research subject
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:umu:diva-88505ISBN: 978-91-7601-070-9 (print)OAI: oai:DiVA.org:umu-88505DiVA: diva2:716973
Public defence
2014-06-05, N430, Naturvetarhuset, Umeå universitet, Umeå, 10:00 (English)
Opponent
Supervisors
Funder
Bio4Energy
Available from: 2014-05-15 Created: 2014-05-08 Last updated: 2014-05-15Bibliographically approved
List of papers
1. Ash Transformation Chemistry during Combustion of Biomass
Open this publication in new window or tab >>Ash Transformation Chemistry during Combustion of Biomass
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2012 (English)In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 26, no 1, 85-93 p.Article in journal (Refereed) Published
Abstract [en]

There is relatively extensive knowledge available concerning ash transformation reactions during combustion of woody biomass. In recent decades, the use of these energy carriers has increased, from a low-technology residential small-scale level to an industrial scale. Along this evolution, ash chemical-related phenomena for woody biomass have been observed and studied. Therefore, presently the understanding for these are, if not complete, fairly good. However, because the demand for CO2-neutral energy resources has increased recently and will continue to increase in the foreseeable future, other biomasses, such as, for instance, agricultural crops, have become highly interesting. The ash-forming matter in agricultural biomass is rather different in comparison to woody biomass, with a higher content of phosphorus as a distinctive feature. The knowledge about the ash transformation behavior in these systems is far from complete. Here, an attempt to give a schematic but general description of the ash transformation reactions of biomass fuels is presented in terms of a conceptual model, with the intention to provide guidance in the understanding of ash matter behavior in the use of any biomass fuel, primarily from the knowledge of the concentrations of ash-forming elements. The model was organized in primary and secondary reactions. Restrictions on the theoretical model in terms of reactivity limitations and physical conditions of the conversion process were discussed and exemplified, and some principal differences between biomass ashes dominated by Si and P, separately, were outlined and discussed.

Place, publisher, year, edition, pages
Washington DC: American Chemical Society, 2012
National Category
Energy Engineering
Identifiers
urn:nbn:se:umu:diva-52666 (URN)10.1021/ef201205b (DOI)000299583400010 ()
Available from: 2012-03-05 Created: 2012-02-28 Last updated: 2017-12-07Bibliographically approved
2. Influence of phosphorus on alkali distribution during combustion of logging residues and wheat straw in a bench-scale fluidized bed
Open this publication in new window or tab >>Influence of phosphorus on alkali distribution during combustion of logging residues and wheat straw in a bench-scale fluidized bed
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2012 (English)In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 26, no 5, 3012-3023 p.Article in journal (Refereed) Published
Abstract [en]

The influence of phosphorus on the alkali distribution in fluidized (quartz) bed combustion using two different typical biomasses (logging residues and wheat straw) was studied. Phosphoric acid (H3PO4) was used as an additive. The produced ash fractions were analyzed for morphology and elemental composition by scanning electron microscopy-energy-dispersive spectroscopy (SEM-EDS), and crystalline phases by powder X-ray diffraction (P-XRD). For both fuel assortments tested, a reduction of volatilized deposit and fine particle-forming matter, containing mainly KCl, was achieved by adding phosphorus. For the wheat straw, this effect was considerable at medium and high phosphorus addition. As a consequence, an increased amount of potassium was found in the coarse ash particle fractions, principally as CaKPO4, KMgPO4, and CaK2P2O7, at the same time that the levels of HCl and SO2 in the flue gases increased. Generally, the addition of phosphorus to the studied biomasses changed the alkali distribution from being dominated by amorphous K-silicate coarse ash fractions and fine particulate KCl, to a system dominated by crystalline coarse ash of K-Ca/Mg-phosphates and fine particulate K2SO4. This implies that the fouling and high-temperature corrosion observed in industrial-scale combustion of problematic biofuels can possibly be reduced by employing additives rich in reactive phosphorus, on the condition that the higher concentrations of acidic gases can be tolerated. In order to achieve these effects, the relationship between alkali and alkaline-earth metals (i.e., (K + Na)/(Ca + Mg)) in the overall fuel ash must be considered. With respect to this, the formation of low-temperature-melting alkali-rich phosphates should not be promoted, to avoid potential increases in bed agglomeration tendencies and phosphorus release from the bed.

National Category
Chemical Engineering
Identifiers
urn:nbn:se:umu:diva-56220 (URN)10.1021/ef300275e (DOI)000304073000055 ()
Available from: 2012-06-12 Created: 2012-06-12 Last updated: 2017-12-07Bibliographically approved
3. Bed agglomeration characteristics in fluidized quartz bed combustion of phosphorus-rich biomass fuels
Open this publication in new window or tab >>Bed agglomeration characteristics in fluidized quartz bed combustion of phosphorus-rich biomass fuels
2011 (English)In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 25, no 3, 937-947 p.Article in journal (Refereed) Published
Abstract [en]

The bed agglomeration characteristics during combustion of phosphorus-rich biomass fuels and fuel mixtures were determined in a fluidized (quartz) bed reactor (5 kW). The fuels studied (separately and in mixtures) included logging residues, bark, willow, wheat straw, and phosphorus-rich fuels, like rapeseed meal (RM) and wheat distillers dried grain with solubles (DDGS). Phosphoric acid was used as a fuel additive. Bed material samples and agglomerates were studied by means of scanning electron microscopy (SEM) in combination with energy-dispersive X-ray spectroscopy (EDX), in order to analyze the morphological and compositional changes of coating/reaction layers and necks between agglomerated bed particles. Furthermore, bed ash particles were separated by sieving from the bed material samples and analyzed with SEM/EDS and powder X-ray diffraction (XRD). For logging residues, bark, and willow, with fuel ash rich in Ca and K but with low contents of P and organically bound Si, the bed layer formation is initiated by reactions of gaseous or liquid K compounds with the surface of the bed material grains, resulting in the formation of a potassium silicate melt. The last process is accompanied by the diffusion/dissolving of Ca into the melt and consequent viscous flow sintering and agglomeration. The addition of high enough phosphorus content to convert the available fuel ash basic oxides into phosphates reduced the amount of K available for the reaction with the quartz bed material grains, thus preventing the formation of an inner bed particle layer in the combustion of logging residues, bark, and willow. Some of the phosphate-rich ash particles, formed during the fuel conversion, adhered and reacted with the bed material grains to form noncontinuous phosphate−silicate coating layers, which were found responsible for the agglomeration process. Adding phosphorus-rich fuels/additives to fuels rich in K and Si (e.g., wheat straw) leads to the formation of alkali-rich phosphate−silicate ash particles that also adhered to the bed particles and caused agglomeration. The melting behavior of the bed particle layers/coatings formed during combustion of phosphorus-rich fuels and fuel mixtures is an important controlling factor behind the agglomeration tendency of the fuel and is heavily dependent on the content of alkaline earth metals in the fuel. A general observation is that phosphorus is the controlling element in ash transformation reactions during biomass combustion in fluidized quartz beds because of the high stability of phosphate compounds.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2011
National Category
Natural Sciences Chemical Engineering Inorganic Chemistry
Research subject
Inorganic Chemistry
Identifiers
urn:nbn:se:umu:diva-60947 (URN)10.1021/ef101451e (DOI)000289613300010 ()
Available from: 2012-11-05 Created: 2012-11-05 Last updated: 2017-12-07Bibliographically approved
4. Combustion and fuel characterisation of wheat distillers dried grain with solubles (DDGS) and possible combustion applications
Open this publication in new window or tab >>Combustion and fuel characterisation of wheat distillers dried grain with solubles (DDGS) and possible combustion applications
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2012 (English)In: Fuel, ISSN 0016-2361, E-ISSN 1873-7153, Vol. 102, 208-220 p.Article in journal (Refereed) Published
Abstract [en]

The present transition to a sustainable global energy system requires that biomass is increasingly combusted for heat and power production. Agricultural fuels considered include alkali-rich fuels with high phosphorus content. One such fuel is wheat distiller’s dried grain with solubles (wheat DDGS) from wheat-based ethanol production. Further increases in ethanol production may saturate the current market for wheat DDGS as livestock feed, and fuel uses are therefore considered. Fuel properties of wheat DDGS have been determined. The ash content (5.4 ± 1.6 %wt d.s.) is similar to many agricultural fuels. In comparison to most other biomass fuels the sulphur content is high (0.538 ± 0.232 %wt d.s.), and so are the contents of nitrogen (5.1 ± 0.6 %wt d.s.), phosphorus (0.960. ± 0.073 %wt d.s.) and potassium (1.30 ± 0.35 %wt d.s.). To determine fuel-specific combustion properties, wheat DDGS and mixes between wheat DDGS and logging residues (LR 60 %wt d.s. and DDGS 40 %wt d.s.), and wheat straw (wheat straw 50 %wt d.s., DDGS 50 %wt d.s.) were pelletized and combusted in a bubbling fluidised bed combustor (5 kW) and in a pellets burner combustor (20 kW). Pure wheat DDGS powder was also combusted in a powder burner (150 kW). Wheat DDGS had a high bed agglomeration and slagging tendency compared to other biomass fuels, although these tendencies were significantly lower for the mixture with the Ca-rich LR, probably reflecting the higher first melting temperatures of K–Ca/Mg-phosphates compared to K-phosphates. Combustion and co-combustion of wheat DDGS resulted in relatively large emissions of fine particles (<1 μm) for all combustion appliances. For powder combustion PMtot was sixteen times higher than from softwood stem wood. While the Cl concentrations of the fine particles from the the mixture of LR and wheat DDGS in fluidised bed combustion were lower than from combustion of pure LR, the Cl- and P-concentrations were considerably higher from the wheat DDGS mixtures combusted in the other appliances at higher fuel particle temperature. The particles from powder combustion of wheat DDGS contained mainly K, P, Cl, Na and S, and as KPO3 (i.e. the main phase identified with XRD) is known to have a low melting temperature, this suggests that powder combustion of wheat DDGS should be used with caution. The high slagging and bed agglomeration tendency of wheat DDGS, and the high emissions of fine particles rich in K, P and Cl from combustion at high temperature, mean that it is best used mixed with other fuels, preferably with high Ca and Mg contents, and in equipment where fuel particle temperatures during combustion are moderate, i.e. fluidised beds and possibly grate combustors rather than powder combustors.

Keyword
Biofuel, Combustion, Ash transformations, Alkali, Phosphorus
National Category
Natural Sciences Chemical Engineering
Identifiers
urn:nbn:se:umu:diva-60959 (URN)10.1016/j.fuel.2012.05.019 (DOI)000308804500025 ()
Available from: 2012-11-05 Created: 2012-11-05 Last updated: 2017-12-07Bibliographically approved
5. Fluidized-Bed Combustion of Mixtures of Rapeseed Cake and Bark: The Resulting Bed Agglomeration Characteristics
Open this publication in new window or tab >>Fluidized-Bed Combustion of Mixtures of Rapeseed Cake and Bark: The Resulting Bed Agglomeration Characteristics
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2012 (English)In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 26, no 4, 2028-2037 p.Article in journal (Refereed) Published
Abstract [en]

The bed agglomeration characteristics resulting from the combustion of 11 mixtures of rapeseed cake and spruce bark were studied in a bench-scale bubbling fluidized-bed reactor (5 kW). The objective was to determine the defluidization temperatures and the prevailing bed agglomeration mechanism as functions of the fuel mixture. Controlled fluidized-bed agglomeration tests were performed for each mixture with quartz sand as the bed material. The total defluidization temperatures and the initial defluidization temperatures were determined based on the measured pressure and temperature profiles in the bed. After combustion, bottom ash samples, agglomerates, and fly ash samples were analyzed by means of scanning electron microscope combined with energy dispersive X-ray detector (SEM-EDX). The composition of the ash-forming matter produced by the combustion of rapeseed cake is significantly different from that produced by the combustion of bark, resulting in different bed agglomeration tendencies. Bark contains ash-forming matter dominated by calcium, with some silicon and potassium, whereas rapeseed cake is rich in phosphorus, potassium, and sodium. The total defluidization temperature for pure bark was above 1045 degrees C, whereas, for rapeseed cake, defluidization occurred during combustion (800 degrees C). During the combustion of bark, the formation of a potassium-rich layer on the silica-bed grains was found to be a crucial for the formation of agglomerates. The low defluidization temperature for the rapeseed cake can be attributed to the formation of sticky ash, which is dominated by phosphates. Two main phosphate forms were observed in the neck between the silica grains: calcium-potassium/sodium phosphates, and magnesium potassium phosphates. As the proportion of bark increased, the Ca/P ratio increased in the fuel mixture, and the formation of high-temperature melting phosphates in the ash was favored. However, the addition of bark also favored the formation of a potassium-rich layer on the silica bed material, leading to the coexistence of both bed agglomeration mechanisms. In the present work, mixtures with a minimum of 60 wt % bark resulted in significantly increased defluidization temperatures and reduced bed agglomeration tendencies, compared to what occurs in rapeseed cake monocombustion.

Place, publisher, year, edition, pages
Washington, DC: American Chemical Society (ACS), 2012
National Category
Engineering and Technology
Identifiers
urn:nbn:se:umu:diva-55521 (URN)10.1021/ef300130e (DOI)000302924400005 ()
Available from: 2012-05-28 Created: 2012-05-21 Last updated: 2017-12-07Bibliographically approved
6. Combustion of biosolids in a bubbling fluidized bed part 1: main ash forming elements and ash distribution with a focus on phosphorus
Open this publication in new window or tab >>Combustion of biosolids in a bubbling fluidized bed part 1: main ash forming elements and ash distribution with a focus on phosphorus
2014 (English)In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 28, no 2, 1183-1190 p.Article in journal (Refereed) Published
Abstract [en]

This is the first in a series of three papers describing combustion of biosolids in a 5-kW bubbling fluidized bed, the ash chemistry, and possible application of the ash produced as a fertilizing agent. This part of the study aims to clarify whether the distribution of main ash forming elements from biosolids can be changed by modifying the fuel matrix, the crystalline compounds of which can be identified in the raw materials and what role the total composition may play for which compounds are formed during combustion. The biosolids were subjected to low-temperature ashing to investigate which crystalline compounds that were present in the raw materials. Combustion experiments of two different types of biosolids were conducted in a 5-kW benchscale bubbling fluidized bed at two different bed temperatures and with two different additives. The additives were chosen to investigate whether the addition of alkali (K2CO3) and alkaline-earth metal (CaCO3) would affect the speciation of phosphorus, so the molar ratios targeted in modified fuels were P:K = 1:1 and P:K:Ca = 1:1:1, respectively. After combustion the ash fractions were collected, the ash distribution was determined and the ash fractions were analyzed with regards to elemental composition (ICP-AES and SEM-EDS) and part of the bed ash was also analyzed qualitatively using XRD. There was no evidence of zeolites in the unmodified fuels, based on low-temperature ashing. During combustion, the biosolid pellets formed large bed ash particles, ash pellets, which contained most of the total ash content (54%–95% (w/w)). This ash fraction contained most of the phosphorus found in the ash and the only phosphate that was identified was a whitlockite, Ca9(K,Mg,Fe)(PO4)7, for all fuels and fuel mixtures. With the addition of potassium, cristobalite (SiO2) could no longer be identified via X-ray diffraction (XRD) in the bed ash particles and leucite (KAlSi2O6) was formed. Most of the alkaline-earth metals calcium and magnesium were also found in the bed ash. Both the formation of aluminum-containing alkali silicates and inclusion of calcium and magnesium in bed ash could assist in preventing bed agglomeration during co-combustion of biosolids with other renewable fuels in a full-scale bubbling fluidized bed.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2014
Keyword
sewage-sludge; chemical fractionation; organic contaminants; crystal-structures; rapeseed cake; biomass ashes; wheat-straw; fly ashes; cocombustion; metals
National Category
Inorganic Chemistry Chemical Engineering Bioenergy
Identifiers
urn:nbn:se:umu:diva-84975 (URN)10.1021/ef402320q (DOI)000331861800052 ()
Funder
Bio4EnergySwedish Research CouncilSwedish Energy Agency
Available from: 2014-01-24 Created: 2014-01-24 Last updated: 2017-12-06Bibliographically approved
7. Combustion of Biosolids in a Bubbling Fluidized Bed Part 2: Environmental Aspects of Ash from Combustion of Biosolids for Application as Fertilizer
Open this publication in new window or tab >>Combustion of Biosolids in a Bubbling Fluidized Bed Part 2: Environmental Aspects of Ash from Combustion of Biosolids for Application as Fertilizer
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(English)Manuscript (preprint) (Other academic)
National Category
Inorganic Chemistry Energy Engineering
Research subject
Inorganic Chemistry
Identifiers
urn:nbn:se:umu:diva-88504 (URN)
Available from: 2014-05-08 Created: 2014-05-08 Last updated: 2014-05-14Bibliographically approved
8. Effects on Ash Chemistry when Co-firing Municipal Sewage Sludge and Wheat Straw in a Fluidized Bed: Influence on the Ash Chemistry by Fuel Mixing
Open this publication in new window or tab >>Effects on Ash Chemistry when Co-firing Municipal Sewage Sludge and Wheat Straw in a Fluidized Bed: Influence on the Ash Chemistry by Fuel Mixing
2013 (English)In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 27, no 10, 5725-5732 p.Article in journal (Refereed) Published
Abstract [en]

Municipal sewage sludge (MSS) is of interest for co-combustion with problematic fuels, such as agricultural residues, because of its high content of inorganic elements, which may improve combustion properties of such problematic fuels. Ash transformation when co-combusting MSS with the agricultural residue wheat straw was examined using a bench-scale bubbling fluidized bed (5 kW). Wheat straw pellets were combusted with MSS in both a co-pelletized form and co-firing of separate fuel particles. This was performed to examine whether there is any advantage to either approach of introducing MSS together with a problematic fuel. Co-combusting wheat straw with MSS changed the bed agglomeration characteristics from being caused by the formation of low-temperature melting potassium silicates in the fuel ash to being caused by a higher temperature melting bed ash. This shift in ash chemistry had a significant positive effect on the initial defluidization temperature. The cyclone ash and fine particulate matter changed from being dominated by alkali in general and alkali chlorides in specific to an increased phosphate and sulfate formation, which reduces the risk of alkali-related fouling and corrosion. The influence of aluminosilicates may also play a role in the improvement of fuel ash behavior.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2013
National Category
Engineering and Technology
Identifiers
urn:nbn:se:umu:diva-83911 (URN)10.1021/ef401197q (DOI)000326126700015 ()
Funder
Swedish Research CouncilBio4Energy
Available from: 2013-12-11 Created: 2013-12-10 Last updated: 2017-12-06Bibliographically approved

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