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  • 1.
    Moradian, Farzad
    University of Borås, Faculty of Textiles, Engineering and Business.
    Ash Behavior in Fluidized-Bed Combustion and Gasification of Biomass and Waste Fuels: Experimental and Modeling Approach2016Doctoral 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.

  • 2.
    Moradian, Farzad
    University of Borås, School of Engineering.
    Co-Combustion of Municipal Solid Waste and Animal Waste: Experiment and Simulation Studies2013Licentiate thesis, monograph (Other academic)
    Abstract [en]

    Co-combustion of animal carcasses and slaughterhouse waste products (animal waste), which are classed as high-risk infectious waste, has been considered as a “fuel opportunity” for waste-to-energy boilers. In this study, the impact of co-combustion of animal waste with municipal solid waste (MSW) on operational issues such as bed agglomeration, deposit formation and emission was investigated, employing experimental and theoretical methods. In the experimental section, a series of full-scale tests in a bubbling fluidised-bed boiler were carried out, to determine the effects of animal waste co-combustion on the issues addressed. Two combustion scenarios were considered, identified as the reference (Ref) case and the animal waste (AW) case. In the Ref case, a solid-waste fuel mix, consisting of sorted and pretreated industry and household waste was combusted. In the AW case, 20 wt% AW was added to the reference fuel mix. The collected samples, which included super-heater deposits, fuel mixes and bed and fly ashes, were analysed, using chemical fractionation, SEM-EDX and XRD. In addition, the flue gases´ emission rate were continuously analysed, using FTIR spectrometry. The results showed positive effects from co-combustion of AW, indicating decreased deposit formation and lower risk of bed agglomeration, as well as reduced emissions of NOx and SO2. Moreover, it was found that the concentrations of P, Ca, S and Cl were enriched in the bed materials. In the theoretical section, thermodynamic calculations, with respect to experimental data, were performed to provide greater understanding of the ash transformation behaviour and the related melting temperature. The calculations mainly focused on bed agglomeration, where addition of AW to the MSW considerably reduced the risk of agglomeration. The results of equilibrium products and phase diagram information for the bed ashes suggested melt-induced agglomeration as a possible cause of the formation of sticky layers on the bed particle in the Ref case. Moreover, it was concluded that higher amounts of calcium phosphate and sulfates increased the first melting temperature of the bed ashes in the AW case.

  • 3.
    Moradian, Farzad
    et al.
    University of Borås, Faculty of Textiles, Engineering and Business.
    A. Tchoffor, Placid
    SP Technical Research Institute of Sweden.
    O. Davidsson, Kent
    SP Technical Research Institute of Sweden.
    Pettersson, Anita
    University of Borås, Faculty of Textiles, Engineering and Business.
    Backman, Rainer
    Umeå University.
    Thermodynamic equilibrium prediction of bed agglomeration tendency in dual fluidized-bed gasification of forest residues2016In: Fuel processing technology, ISSN 0378-3820, E-ISSN 1873-7188, Vol. 154Article in journal (Refereed)
    Abstract [en]

    Dual fluidized-bed (DFB) gasification is one of the recently developed technologies for production of heat, power, transportation fuels and synthetic chemicals through steam gasification of biomass. Bed agglomeration is a serious ash-related problem that should be taken into account when biomass-based fuels are selected for fluidized-bed gasification and combustion. This study developed a thermodynamic equilibrium model to assess the risk of bed agglomeration in gasification and combustion reactors of a DFB gasifier using biomass (forest residues) as feedstock. The modelling approach combined thermodynamic equilibrium calculations with chemical fractionation technique to predict the composition and melting behaviour of the fuel-derived ash as well as bed particles coating layer in the gasification and combustion reactors. FactSage was employed for the thermodynamic equilibrium calculations. The modelling results were then compared with experimental data obtained from a full-scale DFB gasifier to estimate the reliability and validity of the predictive model. In general, a good agreement was found between the modelling results and experimental observations. For the forest residues as feedstock and olivine as bed material, the modelling results indicate a low risk of bed agglomeration in the DFB gasifier, as long as the dominant temperature in the combustion zone is below 1020 °C. In contrast, quartz as bed material in the DFB gasifier was shown to significantly increase the risk of bed agglomeration through coating-induced agglomeration mechanism. © 2016 Elsevier B.V.

  • 4.
    Moradian, Farzad
    et al.
    University of Borås, School of Engineering.
    Pettersson, Anita
    University of Borås, School of Engineering.
    Herstad Svärd, Solvie
    Richards, Tobias
    Co-combustion of animal waste in a commercial waste-to-energy BFB boiler2013In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 6, no 12, p. 6170-6187Article in journal (Refereed)
    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.

  • 5.
    Moradian, Farzad
    et al.
    University of Borås, School of Engineering.
    Pettersson, Anita
    University of Borås, School of Engineering.
    Richards, Tobias
    University of Borås, School of Engineering.
    Bed Agglomeration Characteristics during Cocombustion of Animal Waste with Municipal Solid Waste in a Bubbling Fluidized-Bed Boiler: A Thermodynamic Modeling Approach2014In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 28, no 3, p. 2236-2247Article in journal (Refereed)
    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.

  • 6.
    Moradian, Farzad
    et al.
    University of Borås, Faculty of Textiles, Engineering and Business.
    Pettersson, Anita
    University of Borås, Faculty of Textiles, Engineering and Business.
    Richards, Tobias
    University of Borås, Faculty of Textiles, Engineering and Business.
    Thermodynamic Equilibrium Model Applied to Predict Fouling Tendency in a Commercial Fluidized-Bed Boiler, Combusting Solid Waste2015In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 29, no 5, p. 3483-3494Article in journal (Refereed)
    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.

  • 7.
    Pettersson, Anita
    et al.
    University of Borås, School of Engineering.
    Herstad Svärd, Solvie
    Moradian, Farzad
    University of Borås, School of Engineering.
    Slaktavfall och kadaver förbättrar förbränningsmiljön vid avfallsförbränning i BFB-pannor- ett Waste Refinery projekt2012In: Återvinnare för industrin 2012Article in journal (Other (popular science, discussion, etc.))
    Abstract [sv]

    Kadaver och visst slakteriavfall är klassat med smittorisk sedan utbrottet av ”galna ko-sjukan” på 1990-talet. Detta betyder att det måste behandlas termiskt t.ex. brännas. I början av 2000-talet togs det därför fram en metod i Sverige för att bearbeta de animaliska restprodukterna till ett biobränsle för energiproduktion. Men vilka krav ställer det på pannorna?

  • 8.
    Pettersson, Anita
    et al.
    University of Borås, School of Engineering.
    Moradian, Farzad
    University of Borås, School of Engineering.
    Niklasson, Fredrik
    Herstad Svärd, Solvie
    Richards, Tobias
    University of Borås, School of Engineering.
    Examples of Full scale tests on BFB Waste to Energy boilers (WtE) with direct impact on the future operation of the facility2012Conference paper (Other academic)
    Abstract [en]

    The growing problem with municipal solid waste has helped to accelerate de development of Waste to Energy plants (WtE). However, WtE-plants have problems with agglomeration, deposition and corrosion. And at the same time new waste streams are considered for combustion. Co-combustion has sometimes proven to have positive effects on the combustion environment. In this project full scale tests were performed on two twin 20 MWth WtE Bubbling Fluidized Bed (BFB) boilers in Borås, Sweden. The aim of the research was to investigate if a lowered bed temperature by means of flue gas recirculation or addition of animal waste to the normal waste mix (NWM) could improve the boiler performance. The bed temperature was decreased from 870°C, the boiler design temperature, to around 750°C. The animal waste is a pumpable slurry consisting of crushed carcasses and slaughterhouse waste classified with risk of infection because of BSE (Bovine spongiforme encephalopathy or the mad cow disease). The result shoved both decreased deposit formation rate and decreased agglomeration tendency of the bed. And in the case with animal waste addition the NOx emission was reduced with 50% compared to ordinary performance. Furthermore the ammonia addition for NOx reduction was also cut by half in this case.

  • 9.
    Pettersson, Anita
    et al.
    University of Borås, School of Engineering.
    Niklasson, Fredrik
    Moradian, Farzad
    University of Borås, School of Engineering.
    Comparison of ashes and deposits obtained by RDF combustion in a BR-boiler applying different bed temperatures2010Conference paper (Refereed)
    Abstract [en]

    Chemical fractionation and SEM-EDX was used for characterisation of ashes and deposits from different combustion tests in a commercial 20 MW bubbling fluidized bed (BFB) boiler. The fuel combusted was a mix of sorted MSW (Municipal Solid Waste) and industrial waste often referred to as RDF (Refuse Derived Fuel) mostly containing combustible material as paper, plastics and wood. This fuel type often contains a lot of alkali and chlorine and is therefore considered as a risk fuel prone to cause bed agglomeration, deposit formation and corrosion. In order to investigate the impact of the bed temperature on the alkali and chlorine distribution in the boiler combustion tests were performed. The bed temperature for this boiler is designed to be in the range 850-900°C. In this investigation however the bed temperature was reduced to 700-750°C. Two deposit probes, each carrying two rings made of high alloy steel, were used for collection of deposits during combustion. In addition, samples taken on the bed ash, return sand, return shaft ash, cyclone ash and textile filter ash were analysed. By reducing the bed temperature the need for fresh bed sand was reduced and the fly ash flow decreased. In addition, the agglomerates found in the tests with the normal bed temperature disappeared totally when the bed temperature was reduced. The deposits formed on the bed ash and on the return sand particles were found to consist of compounds with melting temperatures between 675 and 801C, which could explain the difference in agglomeration tendency.

  • 10.
    Pettersson, Anita
    et al.
    University of Borås, School of Engineering.
    Niklasson, Fredrik
    Moradian, Farzad
    University of Borås, School of Engineering.
    Reduced bed temperature in a commercial waste to energy boiler: Impact on ash and deposit formation2013In: Fuel processing technology, ISSN 0378-3820, E-ISSN 1873-7188, Vol. 105, p. 28-36Article in journal (Refereed)
    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.

  • 11.
    Tchoffor, Placid
    et al.
    SP Technical Research Institute of Sweden, SE-501 15 Borås, Sweden.
    Moradian, Farzad
    University of Borås, Faculty of Textiles, Engineering and Business.
    Pettersson, Anita
    University of Borås, Faculty of Textiles, Engineering and Business.
    Kent, Davidsson
    SP Technical Research Institute of Sweden, SE-501 15 Borås, Sweden.
    Thunman, Henrik
    Chalmers University of Technology, 412 58 Gothenburg, Sweden.
    Influence of Fuel Ash Characteristics on the Release of Potassium, Chlorine, and Sulfur from Biomass Fuels under Steam-Fluidized Bed Gasification Conditions2016In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 30, p. 10435-10442Article in journal (Refereed)
    Abstract [en]

    Steam-fluidized bed gasification of biomass, which produces combustible gases from which transportation fuels can be synthesized, is a promising option for replacing the use of fossil fuels in the transportation sector. Similar to other thermal conversion processes, the release of potassium (K), chlorine (Cl), and sulfur (S) from biomass fuels to the gas phase during this process may be conducive to ash-related problems. Catalytic tar and char conversion by K has also been observed. In addition to operational conditions, the extent to which these elements are released to the gas phase may be affected by fuel ash characteristics such as the ash composition and the speciation (or association) of ash-forming elements in the fuel matrix. In the present work, the influence of these fuel ash characteristics on the extent to which K, Cl, and S are released from biomass fuels to the gas phase was studied under steam-fluidized bed gasification. The aim was to assess whether these fuel ash characteristics provide information that could be useful in making a quick judgment as to what extent K, Cl, and S would be released to the gas phase. To this end, the release of K, Cl, and S from forest residues and wheat straw during devolatilization and steam gasification of the char was quantified in a laboratory-scale bubbling fluidized bed reactor. The speciation of these elements in the virgin fuels was studied with chemical fractionation. The results reveal that the extent to which S is released from biomass fuels to the gas phase mainly depends on its speciation in the fuel matrix. While both the ash composition (mainly the Cl/K molar ratio) of the fuel and the speciation of K in the fuel matrix are relevant for the release of K, they appear to be unimportant with respect to the release of Cl.

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