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Chemical Looping Combustion (CLC) is a technique which utilises lattice oxygen contained in solid particles of an inorganic oxide (the ‘oxygen carrier’) to combust fuels, enabling the collection of products of combustion (CO₂ and H₂O) undiluted with nitrogen. It offers, therefore, an attractive method for capturing CO₂ for use or for sequestration in the Earth. This Dissertation concerns the combustion of biomass chars in a fluidised bed using Chemical Looping Combustion with Oxygen Uncoupling (CLOU), a variant of CLC in which the oxygen carriers can release gaseous oxygen. In CLOU, the gaseous O₂ released from the oxygen carriers reacts heterogeneously with the particles of char, producing a mixture of combustion products, CO and CO₂. The CO, in turn, can react either with the gaseous O₂ or heterogeneously with lattice oxygen in the carrier particles. The overall objective of the research described in this Dissertation has been to investigate how the presence of the CLOU particles affects the rate of combustion, and hence, the burnout time of biomass char, an important fuel for future power generation, but which has received scant attention in the literature. To scrutinise the influence of the CLOU particles, the ratio of CO to CO₂ when combusting biomass char had first to be established experimentally, because that ratio affects fundamentally the transport processes occurring in CLOU combustion, e.g. the external rate of transfer of O₂ to the surface of the char particle. However, information about CO to CO₂ ratios when combusting biomass chars is lacking in the published literature, with previous investigators assuming that ratios were similar to those produced by coal chars. Experimental measurements of the ratio of CO to CO₂ from various types of char were undertaken using a thermogravimetric method. The experiments were conducted at known temperatures between 973 and 1173 K, with a range of partial pressures of O₂ (pO₂) from 0.0057 and 0.023 bar. The CO to CO₂ ratios for chars derived from biomass were significantly different to those for chars derived from coal. Thus, models that utilise the values of CO to CO₂ ratio measured from coal chars will be erroneous if applied to predict the combustion performance of a biomass char. Further investigations substantiated this conclusion insofar as the burnout time predicted in particle-scale models using the ratio of CO to CO₂ of a coal char under-predicted the results from independent burnout experiments with single particles birch char (~ 4 mm dia.) when combusted in a laminar flow of oxidising gas. To evaluate the influence of the CLOU materials on the rate of combustion, particle-scale models have been developed, accounting for the reactions and transport phenomena occurring within, and around, the particle of char in a CLOU arrangement. The char studied here was of a size typically used for fluidised combustion such that external mass transfer could be a significant controlling factor (~ 1 mm dia.). Results from the models were compared with experiments performed by combusting biomass char in a bubbling fluidised bed (i.d. 30 mm) of a CLOU material (a Cu-based oxygen carrier) or inert silica sand. The experiments were undertaken with a partial pressure of oxygen, pO₂, close to the equilibrium pressure of O₂ of the Cu-based oxygen carrier. The results from the model agreed with the experimental observations and revealed that, when a CLOU material is present, there is a significant increase in the combustion rate of char compared to that in silica sand. The increase stems from the release of oxygen by the carrier within the external mass transfer film surrounding the char and was predicted by the theoretical work. The models were also used for investigating the performance of CLOU when the particles of char were combusted under the typical operating conditions used in industrial applications, e.g. where concentrations of CO₂ might be substantially higher, giving contributions from gasification. Finally, the combustion of biomass char in a non-stoichiometric perovskite CLOU material, SrFeO3-δ, was also investigated, both experimentally and theoretically. The performance was compared with combustion in Cu-based CLOU materials and with combustion in an oxygen carrier without CLOU properties, namely Fe₂O₃. Experiments were undertaken at the same operating temperature and pO₂ in the fluidising gas. Significant differences in the burnout time when using the different bed materials were observed and plausible explanations, including accounting for the difference in the oxygen availability of the oxygen carriers, were discussed., Cambridge Trust, Trinity Henry Barlow Scholarship

This dissertation concerns solid spheres, with diameters ~ 6 - 10 mm, densities between ~700 - 1500 kg m-3 and emitting gas at various peripheral velocities, Ud, and their tendency to float or sink when introduced into gas-fluidised beds of Geldart Group B particles. This is relevant, for example, to the fluidised bed combustion of biomass, and the apparent tendency of the fuel to devolatilise predominantly near the upper surface of the bed with the attendant undesirable complications of unconverted volatile matter (VM) entering the freeboard. Inert spheres (viz. where Ud = 0) in a bubbling fluidised bed can sink, even if less dense than the fluidised medium, owing to the additional weight of bed particles which tend to settle on top of them forming a defluidised hood. A 2-D fluidised bed, at room temperature, was used to investigate the structure of the fluidised bed in the vicinity of a cylinder emitting gas, as a mimic of a 3-D system. It was found that if Ud is more than 0.7, then the gas emitted can fluidise the bed particles in the entire defluidised hood. Consequently, it was inferred for a 3-D system that gas emitting spheres are not burdened by a defluidised hood and will rise to the surface more rapidly than inert spheres, which are burdened. The hypothesis that a gas-emitting sphere forms a pocket of high pressure around its underside sufficient to enable it to hover above the surface of the fluidised bed, was investigated, in a mechanism akin to the Leidenfrost effect exhibited by liquid drops on a hot plate. Experimentation showed that this hypothesis could be rejected. In fact, by observing the structure of the bed and measuring the pressure around a gas-emitting cylinder close to the surface of a 2-D fluidised bed, it was found that the emission of gas from a freely-floating sphere decreases the net upthrust of the bed on its underside thereby causing the sphere to sink lower into the bed than buoyancy alone would suggest. However, it was also discovered that the emission of gas from a sphere sunk deep within a fluidised bed caused the net upthrust from the bed to increase, causing the sphere to rise more rapidly to the surface than an inert sphere. This suggests that there exists a stable depth at which gas-emitting spheres reach dynamic equilibrium just beneath the surface of the bed where the bed���s upthrust matches the weight of the sphere. An interesting aside of investigating the Leidenfrost mechanism was that, as far as Geldart Group B solids are concerned, experiments showed the two-phase theory of fluidisation holds exactly. To simulate spheres of devolatilising biomass, spheres of dry ice, sublimating in a hot fluidised bed were used, because dry ice emits a single, readily detectable gas. The spheres of dry ice were, however, much denser than any biomass fuel and so only segregated once the rate of sublimation was very high. The external heat transfer coefficient for the spheres of dry ice was measured at a variety of bed temperatures and bed particle sizes. Unlike inert particles, gas emitted by the dry ice particles caused the heat transfer coefficient to a) decrease as the bed material size was decreased and b) decrease as the bed temperature increased. For the first time, a heat transfer model, which accounted for the change in structure of the bed material near the gas-emitting particle, was developed to predict the rate of gas emitted from the dry ice particles and gave good agreement with the experimental results. A novel method for finding the peripheral velocity of VM, emitted by spheres of biomass during devolatilisation in a fluidised bed, was developed and validated experimentally. The mean molar mass and composition of the VM was measured, with the result that measuring the concentration of the combustion products of the VM alone could be used to find the molar flowrate of the VM. Using this method, values of Ud, for spheres of beech, devolatilising in a hot fluidised bed, were measured and, simultaneously, the depth of the spheres in the bed was determined using X-radiography. The simultaneous measurements of gas velocity and depth allowed the behaviour of freely floating, devolatilising spheres to be compared with the calculations obtained with the 2-D fluidised bed. The spheres of beech remained just beneath the surface of the bed throughout devolatilisation and were less influenced by the mixing motions of the bed than inert spheres, even when the fluidisation velocity was increased. The devolatilising beech behaved much as anticipated by the results of the 2-D bed experiments. Tentatively, a dimensionless plot was made which, brings the variables Ud, the incipient fluidisation velocity Umf, the densities of the gas-emitting particles and the bed material, and the depth at which a particle will neither rise nor sink in the bed, together. The plot shows under what conditions a gas-emitting particle is likely to have a sinking or rising tendency in a fluidised bed. The plot is a tool for predicting if segregation of a particular fuel particle is likely to occur in any bubbling fluidised bed. Overall, this dissertation concludes that the emission of VM from a devolatilising particle of biomass not only draws the particle to the surface of the bed but acts to keep it there, even at low rates of gas emission. To eliminate the segregation of biomass during combustion in a bubbling fluidised bed, the biomass must be denser than the emulsion phase of the fluidised bed and the velocity of VM leaving the biomass must be as low as possible. An impracticable degree of pre-processing of the biomass would be required to achieve these conditions., EPSRC

The dataset presented in this article is the supplementary data for the research article titled “The pyrolysis and combustion of polyoxymethylene in a fluidised bed with the possibility of incorporating CO2” [1], in which possible paths of polyoxymethylene conversion in the fluidised bed made from cenospheres and by means of various fluidising gases (air, N2, CO2) were tested. The use of CO2 as fluidising gas was particularly interesting because above 600°C its incorporation into process products (i.e. CO-rich flue gas) was observed. The gaseous products were detected using Fourier Transform Infrared Spectroscopy (FTIR, Gasmet DX-4000) at intervals of a few seconds. The data on the concentration changes over time will allow to evaluate and verificate of new kinetic models of polyoxymethylene degradation with the possibility of incorporating CO2.

The dataset presented in this article is the supplementary data for the research article by Żukowski and Berkowicz (doi:10.1016/j.combustflame.2019.05.024) [1], in which for the first time the inert and catalytic cenospheres were used as the fluidised bed material, giving the possibility to burn liquids inside the fluidised bed without the need for using specialised dosing systems. The instantaneous concentrations of the gaseous products during the combustion of glycerol samples were detected using Fourier Transform Infrared Spectroscopy (FTIR, Gasmet DX-4000). The accurate composition at the outlet of the reactor makes it possible to evaluate and verify new kinetic models of the glycerol combustion in the fluidised bed. It also will be helpful in creating new simplified models. The data presented here is essential for the evaluation of CFD combustion models which have to include accurate kinetic data.

This study investigates the formation of SO3 under air and oxy-fuel combustion environments in a bench-scale bubbling fluidized bed. Flue gas compositions typical of air and oxy-fuel environments were simulated by bottle gases. Parameters such as bed temperature, SO2, O2 and H2O concentrations were varied to determine their effect on the formation of SO3 in both tests with fly ash acting as a catalyst catalyzed and uncatalyzed tests. Catalytic tests were conducted by adding three different fly ashes generated from three different coals. Experimental results support the expectation of show increased SO3 concentration for oxy-fuel combustion environments compared to typical air-fired environments. SO3 concentrations increased with combustion temperature, O2 and H2O concentrations. Catalytic tests have shownshowed that the formation of SO3 depends on the chemical composition of the fly ash, while unburnt carbons in the ash tend to reduce SO3 concentrations.

Electricity production from biomass has the potential to significantly contribute to the power mix in Portugal with lesser environmental impact than non-renewable resources. This study focuses on electricity production from the combustion of residual forest biomass from eucalypt logging activities in Portugal using life cycle assessment. In Portugal, several power plants fuelled by residual forest biomass have been commissioned in the last few years. Most of the installations use fluidised bed furnaces, and the others use grate furnaces. This study aims to compare the environmental impacts associated with these two alternative combustion technologies. System boundaries include the stages of forest management, collection processing and transportation and energy conversion. The default impact assessment method used is that suggested in the International Reference Life Cycle Data System. In a sensitivity analysis, calculations are performed using the ReCiPe method. For all of the impact categories analysed, the fluidised bed presents the smallest environmental impact. Even when the grate furnace efficiency increases and the fluidised bed efficiency decreases in the sensitivity analysis, the fluidised bed has lower impacts than the grate technology and can be the best alternative in the implementation of new power plants.

Combustion in fluidized beds has several benefits, but a potential problem is bed agglomeration causing defluidisation. The most used counter measure is to regularly renew the bed material, inferring costs for new sand and deposition of spent material. For an adaptive optimization there is a need of a method which indicates when bed agglomeration is initializing, before it is too late to counteract. In this project, the conductivity of fluidized beds has been measured by a novel in-situ probe. The probe has been tested in a fluidized bed of sand and ashes at temperatures up to 1000°C. In addition, the probe has been tested in a fluidized bed while burning different fuels. The results show that the conductivity of the bed increases with temperature and concentration of ash. The conductivity varies strongly between different fuels. The signal from the probe reacts strongly to the onset of severe bed agglomeration, but it is hard to find any consistent tendencies that can be applied to predict it.

The aim of this study is to investigate the effect of the concentration of sulphur dioxide and steam, in the flue gas subjected to CO2 capture by calcium looping, on the performance of two limestone-based sorbents. The experimental calcium looping tests were carried out in a purposely-designed Twin Bed lab-scale apparatus (consisting of two identical interconnected bubbling fluidised bed reactors), able to reproduce a realistic particle thermal/chemical/mechanical history. The two natural limestones (particle size range 0.4–0.6 mm) were selected in view of their remarkably “high” and “poor” reactivity to CO2, respectively. Calcium looping tests consisted of ten calcination/carbonation cycles (plus an eleventh calcination). Carbonation was carried out at 650 °C in an atmosphere containing 15% CO2 to simulate a typical combustion flue gas, while calcination was operated at 940 °C at 70% CO2 to simulate oxy-combustion conditions. Six different operating conditions for carbonation were tested to study the effect of SO2 and/or H2O, where steam (when present) was fed at 10%, and SO2 (when present) either at 75 ppm (a typical concentration in a pre-desulphurised combustion flue gas) or 1500 ppm (raw flue gas). The CO2 capture capacity was calculated for each carbonation stage, and the attrition rate was obtained by collection of elutriated fines from both reactors. At the end of the test, sorbent particles were further analysed for the determination of: degree of calcium sulphation, particle size distribution, porosimetric and microscopic properties. Relationships among sorbent type, operating conditions and experimental results are here discussed in detail, with the aim of originally outlining general trends arising from the comparison of the performance of the two limestones, which are characterised by very different reactivity.

Combustion of residual forest biomass (RFB) derived from eucalypt (Eucalyptus globulus), pine (Pinus pinaster) and golden wattle (Acacia longifolia) was evaluated in a pilot-scale bubbling fluidised bed reactor (BFBR). During the combustion experiments, monitoring of temperature, pressure and exhaust gas composition has been made. Ash samples were collected at several locations along the furnace and flue gas treatment devices (cyclone and bag filter) after each combustion experiment and were analysed for their unburnt carbon content and chemical composition. Total suspended particles (TSP) in the combustion flue gas were evaluated at the inlet and outlet of cyclone and baghouse filter and further analysed for organic and elemental carbon, carbonates and 57 chemical elements. High particulate matter collection efficiencies in the range of 94-99% were observed for the baghouse, while removal rates of only 1.4-17% were registered for the cyclone. Due to the sand bed, Si was the major element in bottom ashes. Fly ashes, in particular those from eucalypt combustion, were especially rich in CaO, followed by relevant amounts of SiO2, MgO and K2O. Ash characteristics varied among experiments, showing that their inorganic composition strongly depends on both the biomass composition and combustion conditions. Inorganic constituents accounted for TSP mass fractions up to 40 wt%. Elemental carbon, organic matter and carbonates contributed to TSP mass fractions in the ranges 0.58-44%, 0.79-78% and 0.01-1.7%, respectively. published

The vast majority of the scientific and political communities agree that the release of greenhouse gases from anthropogenic sources is leading to an increase of global average temperature. One of the main gases involved in this effect is carbon dioxide (CO2). International scientific and governmental bodies such as the IPCC agree that CO2 release must be reduced and targets for the reduction of the amounts of gas that are released have been ratified by national governments across the world. One of the largest sources of anthropogenic CO2 is the combustion of fossil fuels for electricity production. Several methods of capturing the CO2 produced by combustion have been developed, with chemical looping combustion (CLC) being suggested by the IPCC as a promising method to achieve the targets in reducing the release of this gas. The combustion of solid fuel in a CLC reactor is not reasonable without modification of the process and chemical looping with oxygen uncoupling (CLOU) is the most promising method of achieving this goal. The CLC process use oxygen carriers to transport oxygen from the air to the fuel in a separate chamber for combustion. CLOU is a form of CLC where the oxygen carrier releases gaseous oxygen. The development of suitable oxygen carrier materials is important in order to progress CLC and CLOU to usable industrial scale technologies and much research has been done into developing them. One of the most widely studied oxygen carrier materials for CLOU is copper oxide supported on magnesium aluminate, however it has been found that this material degrades over many redox cycles, giving the particles a short usable lifetime. This study aims to determine the potential of a different material, with a long usable lifetime, for use in CLOU systems. This material again uses copper oxide, but now supported on bentonite and produced by mechanical mixing and pelletisation. This material is tested in terms of its initial characteristics consisting of crystalline phases determined by XRD, crushing strength and SFEG imaging. The ability of the material to be reduced in an inert environment, and oxidised in air has also been tested in a fluidised bed. The material was subjected to multiple reduction and oxidation cycles and the characteristics will be tested again. The study has endeavoured to determine if the material meets the requirements of oxygen carriers in terms of reducibility and oxidisibility, and whether the material maintains these characteristics over many redox cycles. The results of this study indicate that bentonite acts as a tough support for CuO particles, able to withstand use in a fluidised bed without significant attrition. XRD results show that the particles designated Cu60B40 actually contained the highest amount of CuO of all particles produced, despite having the smallest proportion of CuO added during production. Fluid bed testing shows that the Cu60B40 particles were also able to give off the most oxygen during CLOU testing. These results combine to indicate that sintering occurred in the particles containing the least amount of support, and that the support phase is necessary to maintain oxygen carrier performance, which is in line with previous research. TGA testing with reducing gas showed a much higher degree of conversion than the fluid bed testing in all samples. This indicates that these copper oxide- bentonite particles are better suited to use with gaseous fuels than to solid fuel. This study has followed on from work such as Tian et al(2008) and Arjmand, M. et al (2013) in looking at copper based particles, but takes a more fully rounded approach looking at both reduction in reducing gas, and in inert atmosphere, as well as performance while fluidised. This has shown unexpected pitfalls in oxygen carrier selection. In order to find copper based particles that are better suited to CLOU activity, it is suggested that further work be carried out into the use of cement as a support phase. It is important to retain the full range of tests carried out here to rigorously test these particles.

Over the last decades, several indices based on ash chemistry and ash fusibility have been used to predict the ash behaviour during coal combustion, namely, its tendency for slagging and fouling. However, due to the physical–chemical differences between coals and biomass, in this work only the applicability of an ash fusibility index (AFI) to the combustion and co-combustion of three types of biomass (straw pellets, olive cake and wood pellets) with coals was evaluated. The AFI values were compared with the behaviour of ash during combustion in a pilot fluidized bed and a close agreement was observed between them. For a better understanding of the mechanisms associated with bed ash sintering, they were evaluated by SEM/EDS and the elements present on the melted ash were identified. Evidences of different sintering mechanisms were found out for the fruit biomass and herbaceous biomass tested, depending on the relative proportions of problematic elements. The particles deposited on a fouling probe inserted in the FBC were analyzed by XRD and the differences between the compounds identified allowed concluding that the studied biomasses present different tendencies for fouling. Identification of KCl and K 2 SO 4 in the deposits confirmed the higher tendency for fouling of fruit biomass tested rather than wood pellets.

Firing or co-firing of biomass in efficient power plants can lead to high-temperature corrosion of superheaters due to condensation of alkali chlorides into superheater deposits. Corrosion can be prevented if a significant portion of the alkali chlorides present in the flue gases is destroyed before reaching the superheaters. The alkali capturing power of aluminium and ferric sulphates was determined in a pilot-scale fluidised bed (FB) reactor. The reagents were added in solution, through a spraying nozzle, to the upper part of the freeboard. Both reagents, at economical dosages, fast and effectively destroyed the alkali chlorides by producing sufficient SO3 for the sulphation. Both the mass flow rate and type of sulphate affected the sulphation ability. Thus, the cation, too, plays a role in the reaction. The required chemical dosage is not directly proportional to the Sreagent/Cl2fuel ratio because alkali chlorides must compete with calcium and magnesium oxides and probably also with alkali oxides for the available SO3.

Characteristics of biomass used as fuel and distinct ash flows in a thermal power plant with BFBC (bubbling fluidised bed combustion) technology were studied. An integrated approach involving chemical composition, microscopy analysis, mineralogy, and thermal behaviour were used to characterize the ash. The ashes have a low unburned content, typically below 3 wt.% (dry basis), with higher values in ash from the electrostatic precipitator. The chemical element present in higher concentration in the several ash flows is Si (>20 wt.%, dry basis). The fly ash from the electrostatic precipitator are enriched in heavy metals when compared to other ash, and Zn is the heavy metal found in higher concentration (but

Atmospheric bubbling fluidized bed coal combustion of a bituminous coal and anthracite with particle diameters in the range 500−4000 μm was investigated in a pilot-plant facility. The experiments were conducted at steady-state conditions using three excess air levels (10, 25, and 50%) and bed temperatures in the 750−900 °C range. Combustion air was staged, with primary air accounting for 100, 80, and 60% of total combustion air. For both types of coal, high NO concentrations were found inside the bed. In general, the NO concentration decreased monotonically along the freeboard and toward the exit flue; however, during combustion with high air staging and low to moderate excess air, a significant additional NO formation occurred near the secondary air injection point. The results show that the bed temperature increase does not affect the NO flue gas concentration significantly. There is a positive correlation between excess air and the NO flue gas concentration. The air staging operation is very effective ...

The paper is an overview of the results obtained up to date from the combustion and co-combustion activities with Greek brown coal in different installations, both in semi-industrial and laboratory scale. Combustion tests with Greek lignite were realized in three different Circulating Fluidized Bed Combustion (CFBC) facilities. Low rank lignite was burned in a pilot scale facility of approx. 100kW thermal capacity, located in Athens (NTUA) and a semi-industrial scale of 1.2 MW thermal capacity, located at RWE's power station Niederaussem in Germany. Co-combustion tests with Greek xylitic lignite and waste wood were carried out in the 1 MWth CFBC installation of AE&E, in Austria. Lab-scale co-combustion tests of Greek pre-dried lignite with biomass were accomplished in a bubbling fluidized bed in order to investigate ash melting problems. The obtained results of all aforementioned activities showed that fluidized bed is the appropriate combustion technology to efficiently exploit the low quality Greek brown coal either alone or in conjunction with biomass species.

As general restrictions for the design of Oxyfuel pulverised coal-fired (PC) steam generators are commonly known, the purpose of this work is to show general restrictions for the design of an Oxyfuel coal-fired steam generator using a circulating fluidised bed combustor (CFBC) with external heat exchangers (EHEs). For the CFBC restrictions result on the one hand out of the used fuels composition. On the other hand certain restrictions out of state-of-the-art and steam generator geometries have to be considered within the design. © 2013 The Author.

In the present study, particulate emissions from the co-combustion of forest biomass residues with sewage sludge in a pilot-scale bubbling fluidised bed combustor were characterised. The combustion flue gas was exhausted to the atmosphere after passing through a cyclone separator. Physical-chemical characteristics of the particles were studied: i) morphology and aerosol size, surface and volume distributions before the cyclone and ii) chemical composition (carbonates, water soluble-inorganic ions, organic and elemental carbon) before and after the cyclone. Chemical composition data were used to calculate aerosol density and refractive index. Aerosols showed a unimodal size distribution with a geometric mean diameter of 2.25 ± 0.02 μm and a geometric standard deviation of 1.27 ± 0.01. The surface and volume mean diameters were 2.64 ± 0.02 μm and 2.91 ± 0.05 μm, respectively. Water-soluble inorganic ions were predominant in the fine particle fraction (PM2.5). The filters were loaded of crystallised mineral particles. The analysis revealed a dominance of calcium carbonate/oxide and halide (NaCl or KCl), sulphate and aluminosilicate nanocrystals forming larger mixed aggregates. This work was supported by Portuguese Science Foundation (FCT) through the projects PTDC/AAC-AMB/098112/2008 Bias-to-soil — Biomass ashes: Characteristics in relation to its origin, treatment and application on soil and PTDC/AMB/65706/2006 (BIOEMI). The electron microscopy analyses were conducted at CNME-UCM through the AEROCLIMA project (Fundación Ramón Areces). Ana I. Calvo acknowledges the pos-doc grant SFRH/BPD/64810/2009 from the FCT. We would also like to thank Diana Patoilo (University of Aveiro, Portugal) for supporting the chromatographic analyses, and Véronique Pont (University of Toulouse, France) and Darrel Baumgardner (Droplet Measurement Technologies, Boulder) for their valuable comments. published

In the important efforts to decrease the net CO2 emissions to the atmosphere, new, alternative fuels are being included in the fuel mixes used in utility boilers. However, these fuels have ash properties that are different from those of the traditionally used fuels and in some cases technical problems, such as ash fouling and corrosion occur due to this. Therefore, diagnostic and predictive methods are developed and used to avoid such problems. Determination of the chemical association forms of important elements, such as potassium and sodium, in the fuel by chemical fractionation is a method well defined for coal and biofuels, such as wood pellets, bark and forest residues. Chemical fractionation is a step by step leaching method extracting water soluble salts in the first step, ion exchangeable elements, such as organically associated sodium, calcium and magnesium in the second step and acid soluble compounds such as carbonates and sulfates in the third step. The solid residue fraction consists of silicates, oxides, sulfides and other minerals. The compound extracted in the two first steps is considered reactive in the combustion with a few exceptions. In this work, it has been applied to some waste fuels, i.e. sewage sludge, straw and refuse derived fuel (RDF), as well as to coal and wood. The present work also includes results from combustion tests in a fluidised bed boiler where three blends of the investigated fuels were used. The fractionation results for the fuel blends are weighted results of the fractionations of the pure fuels discussed above which are compared with fractionations of their corresponding fly ashes. The co-combustion strategy gave very good results in reducing ash problems. Possible chemical mechanisms involved are discussed in the article. © 2008 Elsevier Ltd. All rights reserved.

Dissertação para obtenção do Grau de Mestre em Ciência e Engenharia de Materiais do Instituto Superior Técnico No presente trabalho é apresentada a caracterização físico-química de Resíduos de Fragmentação Automóvel bem como das cinzas obtidas através da sua combustão num sistema piloto de leito fluidizado. A composição e a lixiviação do combustível foram comparadas com as cinzas utilizando as três metodologias de lixiviação mais usadas: o método europeu EN 12457-2, o método americano US TCLP 1311 e o método de disponibilidade holandês EA NEN 7371. Os resultados obtidos revelaram que, além da lixiviação de matéria orgânica, sob condições ácidas a libertação de metais pesados da FCRFA pode constituir um sério problema pelo que a sua deposição em aterro deverá ser abandonada. As cinzas resultantes do processo de combustão de FCRFA apresentaram alguns níveis de lixiviação de metais, como por exemplo Cr, todavia a destruição do conteúdo orgânico, associada à diminuição das quantidades de resíduo a depositar em aterro, fazem do tratamento térmico uma opcção a considerar na gestão deste tipo de resíduos.

Atmospheric bubbling fluidised bed coal combustion (ABFBCC) of a bituminous coal and an anthracite with particle diameters in the range 500–4000 μm was investigated in a pilot-plant facility, with and without limestone addition. The experiments were conducted at steady-state conditions using three excess air levels (10, 25 and 50%) and bed temperatures in the range 750–900 °C. Combustion air was staged, with primary air accounting for 100, 80 and 60% of total combustion air. During limestone addition, in general, the NO emission decreases with the decrease in excess air and the increase in air staging, for both coals (as also observed without limestone). The bed temperature does not influence the NO emission significantly (as also observed without limestone); however, it was observed that during bituminous coal combustion there is a slight trend for a decrease on the NO emission with temperature increase in the range 825–900 °C, whereas for anthracite coal the trend is the opposite. On the other hand, the N 2 O emission increases with: the decrease in excess air, the increase in air staging (as opposed to what was observed without limestone), and the decrease in bed temperature (as also observed without limestone). Taking the coal combustion without limestone as reference, it was observed that the effect of limestone addition on the NO and N 2 O emission depends on the first stage stoichiometry: (1) under first stage fuel lean conditions the NO emission increases, while that of N 2 O decreases, (2) under first stage fuel rich conditions (for example, high air staging) the opposite trend is observed.

The catalytic combustion of methane has been investigated in a 0.10 m bubbling fluidized bed reactor with nonpremixed feedings of reactants. Copper dispersed on porous ç-Al2O3 spheres (1 mm diameter) characterized by high mechanical strength has been used as catalyst. The effect of design variables such as methane inlet concentration, bed temperature, and superficial gas velocity on methane conversion has been quantified in the ranges 4-10 vol %, 650-750 °C, and 0.40-1.30 m s-1, respectively. The propensity to attrition of the catalyst has been separately investigated under the experimental conditions tested. Results have been interpreted in the light of a simple reactor model which assumes a plug-flow pattern for the gas through the bed and mth-order catalytic kinetics with respect to fuel concentration.

Atmospheric Bubbling Fluidised Bed Combustion (ABFBC) of a bituminous coal and anthracite with particle size in the range 500–4000 μm was investigated in a pilot-plant facility, with and without limestone addition (particle size in range 710–1000 μm). The experiments were conducted at steady-state conditions using three excess air levels (10%, 25% and 50%) and bed temperatures in the 750–900 °C range. Combustion air was staged, with primary air accounting for 100%, 80% and 60% of total combustion air. Limestone addition is suitable for in-situ SO2 removal during ABFBC, with removal efficiencies in the range 25–85%. In general, the efficiency of SO2 removal by limestone decreases with an increase in air staging and a decrease in excess air. The increase in the bed temperature range 825–900 °C decreases the efficiency of SO2 removal by limestone, and in some situations a maximum can occur around 825 °C bed temperature. The reducing conditions at the first stage level, the secondary combustion, and the reductive decomposition of CaSO4 could be responsible for the low efficiency of SO2 removal by limestone during both high air staging and high bed temperatures.

Atmospheric Bubbling Fluidised Bed Combustion (ABFBC) of a bituminous coal and anthracite with particle diameters in the range 500–4000 μm was investigated in a pilot-plant facility (circular section with 0.25 m internal diameter and 3 m height). The experiments were conducted at steady-state conditions using three excess air levels (10, 25 and 50%) and bed temperatures in the 750–900 °C range. Combustion air was staged, with primary air accounting for 100, 80 and 60% of total combustion air. The effect of limestone addition was also tested. Large CO concentrations were observed inside the bed, up to 8 and 6% (v/v) in the cases of anthracite and bituminous coals, respectively. These concentrations decreased sharply as the gases emerged from the bed, and the CO flue gas concentration observed was in general less than 2000 and 4000 ppm, respectively. The CO flue gas concentration increased with air staging and with limestone addition, but decreased with either excess air or temperature increase. The observed results confirm the influence of sand particles (and probably of SO 2 ) in the ‘quenching’ of the oxygenated free radicals (HO and HO 2 ) reactions responsible for the CO oxidation inside the bed.