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Ash Behavior in Fluidized-Bed Combustion and Gasification of Biomass and Waste Fuels: Experimental and Modeling Approach
University of Borås, Faculty of Textiles, Engineering and Business.
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Over the past few decades, a growing interest in the thermal conversion of alternative fuels such as biomass and waste-derived fuels has been observed among the energy-producing companies. Apart from meeting the increasing demand for sustainable heat and power production, other advantages such as reducing global warming and ameliorating landfilling issues have been identified. Among the available thermal conversion technologies, combustion in grate-fired furnaces is by far the most common mode of fuel conversion. In recent years, Fluidized-Bed (FB) technologies have grown to become one of the most suitable technologies for combustion and gasification of biomass and waste-derived fuels.In spite of the benefits, however, some difficulties are attributed to the thermal conversion of the alternative fuels. Ash-related issues could be a potential problem, as low-grade fuels may include considerable concentrations of ash-forming elements such as K, Na, S, Ca, Mg, P, Si and Cl. These elements undergo many undesirable chemical and physical transformations during the thermal conversion, and often cause operational problems such as deposition-related issues, slag formation in furnaces, corrosion of the heat transfer surfaces, and bed agglomeration of the fluidized-beds. Ash-related problems in the utility boilers are a major concern that may result in decreased efficiency, unscheduled outages, equipment failures, increased cleaning and high maintenance costs.This thesis investigated the ash behavior and ash-related problems in two different FB conversion systems: a Bubbling Fluidized-Bed (BFB) boiler combusting solid waste, and a Dual Fluidized-Bed (DFB) gasifier using biomass as feedstock. Full-scale measurements, chemical analysis of fuel and ash, as well as thermodynamic equilibrium modeling have been carried out for the BFB boiler (Papers I-IV), to investigate the impact of reduced-bed temperature (RBT) and also co-combustion of animal waste (AW) on the ash transformation behavior and the extent of ash-related issues in the boiler. For the DFB gasifier (Paper V), a thermodynamic equilibrium model was developed to assess the risk of bed agglomeration when forest residues are used as feedstock.The experimental results showed that the RBT and AW co-combustion could decrease or even resolve the ash-related issues in the BFB boiler, resulting in a lower deposit-growth rate in the superheater region, eliminating agglomerates, and a less corrosive deposit (in RBT case). Thermodynamic equilibrium modeling of the BFB boiler gave a better understanding of the ash transformation behavior, and also proved to be a reliable tool for predicting the risk of bed agglomeration and fouling. The modeling of the DFB gasifier indicated a low risk of bed agglomeration using the forest residues as feedstock and olivine as bed material, which was in good agreement following the observations in a full-scale DFB gasifier.

Place, publisher, year, edition, pages
Borås: Högskolan i Borås, 2016.
Series
Skrifter från Högskolan i Borås, ISSN 0280-381X ; 78
Keyword [en]
Fluidized-bed, combustion, gasification, waste-derived fuels, biomass, ash-related problems, deposit, fouling, slagging, bed agglomeration, thermodynamic equilibrium modeling
National Category
Environmental Biotechnology
Identifiers
URN: urn:nbn:se:hb:diva-9563ISBN: 978-91-88269-14-0 (print)ISBN: 978-91-88269-15-7 (print)OAI: oai:DiVA.org:hb-9563DiVA: diva2:917912
Public defence
2016-06-09, E310, University of Borås, Allégatan 1, Borås, 10:00 (English)
Available from: 2016-05-11 Created: 2016-04-08 Last updated: 2017-05-02Bibliographically approved
List of papers
1. Reduced bed temperature in a commercial waste to energy boiler: Impact on ash and deposit formation
Open this publication in new window or tab >>Reduced bed temperature in a commercial waste to energy boiler: Impact on ash and deposit formation
2013 (English)In: Fuel processing technology, ISSN 0378-3820, E-ISSN 1873-7188, Vol. 105, 28-36 p.Article in journal (Refereed) Published
Abstract [en]

Waste combustion for power production is associated with many problems due to the composition and inhomogeneity of the fuel stream. A reduction of alkaline and chlorine products in the superheater region should ease these problems significantly. Ashes and deposits from different combustion tests in a commercial 20 MWth bubbling fluidised bed (BFB) boiler were characterised by XRD and SEM-EDX. The fuel combusted was a mix of sorted municipal solid waste (MSW) and industrial waste, often referred to as RDF (refuse derived duel). These waste fuels often contain more alkali and chlorine than does biomass and are therefore considered risky fuels prone to causing bed agglomeration, deposit formation, and corrosion. The aim of this study was to investigate whether a lowered bed temperature could change alkali and chlorine distribution in the boiler to reduce corrosion and deposit formation. The boiler used was designed for a bed temperature in the range of 850–900 °C, which in this investigation was decreased by approximately 150 °C. Data were collected through deposit measurements and solid sampling. The lowered bed temperature resulted in reduced demand for fresh sand, decreased agglomeration, and reduced rates of deposit formation.

Place, publisher, year, edition, pages
Elsevier BV, 2013
Keyword
Reduced bed temperature, RDF, Alkali, agglomeration, Deposit formation, BFB, Resursåtervinning
National Category
Energy Engineering
Research subject
Resource Recovery
Identifiers
urn:nbn:se:hb:diva-1382 (URN)10.1016/j.fuproc.2011.09.001 (DOI)000312414400005 ()2320/11633 (Local ID)2320/11633 (Archive number)2320/11633 (OAI)
Available from: 2015-11-13 Created: 2015-11-13 Last updated: 2017-05-02Bibliographically approved
2. Co-combustion of animal waste in a commercial waste-to-energy BFB boiler
Open this publication in new window or tab >>Co-combustion of animal waste in a commercial waste-to-energy BFB boiler
2013 (English)In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 6, no 12, 6170-6187 p.Article in journal (Refereed) Published
Abstract [en]

Co-combustion of animal waste, in waste-to-energy boilers, is considered a method to produce both heat and power and to dispose of possibly infected animal wastes. This research conducted full-scale combustion tests to identify the impact of changed fuel composition on a fluidized-bed boiler. The impact was characterized by analyzing the deposit formation rate, deposit composition, ash composition, and emissions. Two combustion tests, denoted the reference case and animal waste case, were performed based on different fuel mixes. In the reference case, a normal solid waste fuel mix was combusted in the boiler, containing sorted industry and household waste. In the animal waste case, 20 wt% animal waste was added to the reference fuel mix. The collected samples, comprising sampling probe deposits, fuel mixes, bed ash, return sand, boiler ash, cyclone ash and filter ash, were analyzed using chemical fractionation, SEM-EDX and XRD. The results indicate decreased deposit formation due to animal waste co-combustion. SEM-EDX and chemical fractionation identified higher concentrations of P, Ca, S, and Cl in the bed materials in the animal waste case. Moreover, the risk of bed agglomeration was lower in the animal waste case and also a decreased rate of NOx and SO2 emissions were observed.

Place, publisher, year, edition, pages
M D P I AG, 2013
Keyword
bubbling fluidized bed (BFB) boiler, animal waste, MSW, deposit, ash, Resursåtervinning
National Category
Engineering and Technology Chemical Engineering
Research subject
Resource Recovery
Identifiers
urn:nbn:se:hb:diva-1792 (URN)10.3390/en6126170 (DOI)000330290600003 ()2320/13279 (Local ID)2320/13279 (Archive number)2320/13279 (OAI)
Available from: 2015-11-13 Created: 2015-11-13 Last updated: 2017-05-02Bibliographically approved
3. Bed Agglomeration Characteristics during Cocombustion of Animal Waste with Municipal Solid Waste in a Bubbling Fluidized-Bed Boiler: A Thermodynamic Modeling Approach
Open this publication in new window or tab >>Bed Agglomeration Characteristics during Cocombustion of Animal Waste with Municipal Solid Waste in a Bubbling Fluidized-Bed Boiler: A Thermodynamic Modeling Approach
2014 (English)In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 28, no 3, 2236-2247 p.Article in journal (Refereed) Published
Abstract [en]

ABSTRACT: Full-scale waste combustion tests showed that adding animal waste (AW) to municipal solid waste (MSW) prevented bed agglomeration, and the reason for this fi nding was not fully understood. This study uses thermodynamic modeling to investigate the composition of equilibrium products for two combustion scenarios: monocombustion of MSW (the reference case) and cocombustion of AW with MSW (the AW case). The modeling was performed using FactSage, and experimental data obtained during the full-scale combustion tests were used as input data for the calculations. The results of equilibrium modeling, together with information extracted from ternary phase diagrams, suggest higher bed temperature as the primary cause for formation of bed agglomerates in the reference case. In addition, melt-induced agglomeration is suggested as the bed agglomeration mechanism in this case. In the AW case, however, reduced bed temperature, as well as enriched calcium phosphate and sulfate in the bottom ashes are considered to signi fi cantly decrease the slagging tendency.

Place, publisher, year, edition, pages
American Chemical Society, 2014
Keyword
Resursåtervinning
National Category
Chemical Sciences
Research subject
Resource Recovery
Identifiers
urn:nbn:se:hb:diva-1864 (URN)10.1021/ef402455h (DOI)000333381200072 ()2320/13716 (Local ID)2320/13716 (Archive number)2320/13716 (OAI)
Available from: 2015-11-13 Created: 2015-11-13 Last updated: 2017-05-02Bibliographically approved
4. Thermodynamic Equilibrium Model Applied to Predict Fouling Tendency in a Commercial Fluidized-Bed Boiler, Combusting Solid Waste
Open this publication in new window or tab >>Thermodynamic Equilibrium Model Applied to Predict Fouling Tendency in a Commercial Fluidized-Bed Boiler, Combusting Solid Waste
2015 (English)In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 29, no 5, 3483-3494 p.Article in journal (Refereed) Published
Abstract [en]

A thermodynamic equilibrium model, combined with an advanced fuel analysis, was applied to predict the fouling tendency in a commercial bubbling fluidized-bed (BFB) boiler, combusting a mixture of solid waste. In order to enhance the performance of the model, further modifications were made, considering the combustion pattern in the fluidized-bed system and also the temperature profile in the combustion zone. The modeling was performed using Factsage, and experimental data obtained during the full-scale measurements were used as input for the model, simulating the deposit formation in the real boiler. The simulation results were then compared with the results obtained during the full-scale combustion tests to estimate the accuracy and validity of the applied model. The thermodynamic equilibrium modeling proved to be a reliable tool for predicting the fouling in the BFB boiler, thus determining the fraction of the melt in the deposited salts formed on the heat transfer surfaces during the flue gas condensation. The calculations showed that the ratio of the SO2 to alkali chloride concentration in the flue gas was the decisive factor that affected the rate of the deposit formation in the boiler. Both the simulation and the experimental results indicated that lower bed temperatures and cocombustion of P-rich fuels decrease the deposition buildup in the boiler. © 2015 American Chemical Society.

Keyword
fluidized-bed combustion, fouling, waste-derived fuels, thermodynamic equilibrium modeling
National Category
Energy Engineering
Research subject
Resource Recovery
Identifiers
urn:nbn:se:hb:diva-8526 (URN)10.1021/acs.energyfuels.5b00346 (DOI)000355158200084 ()2-s2.0-84930227296 (Scopus ID)
Available from: 2016-01-14 Created: 2016-01-14 Last updated: 2017-05-02Bibliographically approved

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