A general review of the literature pertaining to the combustion of coal in an atmospheric fluidized bed of ,inert particles is presented. In particular, the phenomena of fluidization and combustion have been investigated and the status of research and development in various parts of the world is considered. A 300 mm diameter refractory lined open top atmospheric fluidized bed combustor has been built to study the combustion efficiencies and entrainment rates of the fluidized-bed combustion process in shallow fluidized beds, with static bed heights ranging from about ISO mm to 23J mm. A low pressure drop the of distributed was used for all of the tests so as to test a system compatible with most industrial requirements. As the combustor vessel is refractory lined, cooling is provided by supplying air to the rig well in excess of that required for stoichiometric combustion. As a result, no oxygen deficient regions occur within the fluidized bed, ensuring complete combustion of both the fixed carbon component of the coal to carbon dioxide and the volatile component within the bed section. Experimental results have been obtained from the combustion of a coal with a high fines content of which there is at present a supply which exceeds the demand. The coal has been burned in an inert bed comprising a closely graded silica sand. It has been found possible to correlate the combustion efficiencies in terms of the bed temperature, superficial gas velocity and the static bed height within the following ranges of these parameters: Bed Temperature Gas Velocity Static Bed Height 700 to 10000C 0,9 to 1,5 m/s 150 to 230 mm by using a bed material substantially different from the coal feed, it has been found possible to separate the (i) ABSTRACT A

Ce mémoire de maitrise de recherche en génie mécanique relate les essais menés au laboratoire de combustion de l’Université Laval entre mai 2016 et septembre 2018 sur l’assistance électrique à la combustion du point de vue des émissions gazeuses. La combustion a été effectuée avec deux types de combustible : du propane gazeux et de la biomasse solide. Pour l’étude sur le propane, deux brûleurs ont été utilisés. Le premier est un brûleur simple générant une flamme de diffusion d’entraînement laminaire où seul le débit de propane est contrôlé. Le deuxième est un brûleur annulaire « coflow » où le débit d’air circulant dans l’anneau extérieur et le débit de propane dans le cylindre intérieur peuvent tous les deux être réglés séparément. Dans le cas de la combustion de biomasse, un appareil de chauffage a été fourni par le partenaire industriel du projet. L’assistance électrique a été implémentée de deux façons différentes : • A travers un champ électrique généré par l’application d’une différence de potentiel entre plusieurs jeux d’électrodes aux formes variées. Cette différence de potentiel est établie entre deux bornes avec une amplitude allant de 5 kV à 20 kV et un courant alternatif sinusoïdale allant de10 kHz à 20 kHz. • En faisant circuler l’air d’alimentation de certaines zones de combustion dans un générateur de décharge à barrière diélectrique (DBD) cylindrique d’une puissance maximale de 150 W à une fréquence de 8.2 kHz. Pour l’échantillonnage des émissions, une suite de trois appareils de mesure a été utilisée à des fins d’analyse des gaz d’échappement. Un spectromètre Infrarouge, couplé à un détecteur d’ionisation de flamme et un détecteur d’oxygène, permet d’obtenir de manière relativement précise des informations sur les principales molécules gazeuses que composent les gaz d’échappement émanant de nos différents brûleurs avec ou sans activation de l’assistance électrique. Les protocoles d’essais sont réalisés surmesure pour chaque brûleur et type d’assistance électrique étudiée, afin de s’adapter aisément aux contraintes techniques propres à chacun. Une attention particulière a été portée aux émissions de monoxyde de carbone (CO), d’oxydes d’azote (NOx) et autres polluants atmosphériques ou gaz nocifs pour l’être humain dans nos essais, à une époque où les considérations environnementales sont au cœur de nombreux projets. Au travers des deux types d’assistance électrique et des différents brûleurs, les émissions de CO ont systématiquement été réduites lors de son activation, indépendamment de la fréquence de la décharge électrique responsable. Jusqu’à 70% de réduction fut observé sur les essais au propane, et jusqu’à 30% sur les essais concernant la biomasse. En outre, sur les essais au propane, une explication a été avancée pour faire la lumière sur le phénomène. Il semblerait que l’énergie apporté par le champ électrique soit utilisée en partie pour franchir la barrière d’énergie d’activation de certaines réactions limitantes de combustion, entrainant la diminution du CO dans les gaz résiduels observée. On a pu mettre en évidence que ce CO a été transformé en CO2 et que, le cas échéant, la modification de l’équilibre de réaction a permis d’augmenter l’efficacité de la combustion en réduisant la concentration en composés organiques volatiles (y compris le propane imbrûlé) dans les gaz d’échappement et en y augmentant la teneur en vapeur d’eau : l’autre principal produit de la combustion avec le CO2. Les émissions de NOx sont également réduites par l’activation du champ électrique sur la combustion de propane. Par ailleurs, un découplage des émissions CO/NOx semble avoir été mis en évidence dans l’un de nos scénarios d’essais, offrant une approche prometteuse pour des applications industrielles où les compromis entre CO et NOx sont très présents et contraignants. La structure du front de flamme a également montré être sensible aux variations de l’assistance électrique, particulièrement dans le cas des champs électriques. Le front de flamme est affecté par les directions principales des lignes de courant. Cet effet s’apparente beaucoup au « vent ionique » traditionnel; nos essais ont montré que la plage de fréquences originelles de ce mécanisme (<100 Hz) peut être élargie à des fréquences bien supérieures (de l’ordre de la dizaine de kilohertz). Un mécanisme a été proposé pour expliquer cet effet dans nos essais en se basant sur l’utilisation d’une partie de l’énergie apportée par le champ électrique pour entrainer les ions de la flamme dans la direction du gradient croissant d’intensité de la différence de potentiel électrique aux bornes des électrodes, et ce, même en présence d’un champ oscillant. / This Master’s degree thesis reports the tests that were conducted at Laval University combustion laboratory between May 2016 and September 2018 on electrically assisted combustion from gas emissions standpoint. Combustion was performed with two types of fuel: gas propane and solid biomass. For the study related to propane, two burners were used. The first is a simple burner producing a laminar entrained diffusion flame for which only the propane flowrate is controlled. The second one is an annular coflow burner where the airflow in the outer cylinder and the propane flow in the inner one can both be controlled separately. For the part on biomass combustion, a home-heating appliance was provided by the industrial partner for this project. The electrical enhancement of the flame was implemented in two different ways: - Through an electric field generated by an electric potential difference applied between pairs of electrodes of various geometries. The sine wave function electric potential difference is set between a pair of electrodes with an amplitude ranging from 5 kV to 20 kV and 10 kHz to 20 kHz: and - By circulating the combustion air of the heating appliance through a 150 W - 8.2 kHz dielectric barrier discharge generator prior to its injection in the stove. For gas sampling purposes, three in-line analysers were used to monitor flue gas. An InfraRed spectrometer coupled to a flame ionisation detector and an oximeter allows the obtention of relatively precise information on the major gas molecules emitted during combustion and their evolutions with and without electrical enhancement. The test protocols were tailored for each experiment in order to adapt to the technical challenges raised by each. Focus was brought to carbon monoxide (CO), nitrogen oxides (NOx) and other major atmospheric pollutants or harmful gas to humans in our tests, in a time where environmental considerations are at the heart of many projects. For both types of electrical assistance and the various burners, CO emissions were systematically reduced when activated, regardless of the discharge frequency. Up to 70% reductions were registered on propane tests, and up to 30% reduction on biomass tests. Additionally, for the tests on propane, an explanation was provided to shine some light on the phenomenon. It seems that the energy supplied by the electric field is used in parts to overcome the activation energy barrier for some limiting reactions in the combustion mechanism, leading to a diminution of CO in the exhaust gas. Measurements provided evidence that this CO was transformed into CO2, and that, when relevant, the modification of this chemical equilibrium lead to a diminution in organic compounds (including unburned propane) and an elevation in the water vapor concentration (the other main product of combustion reactions) in the exhaust gas. NOx emissions were also reduced by the activation of the electric field on propane combustion. Moreover, a decoupling of CO/NOx emissions seemed to be identified in one of the cases studied: hinting a promising approach for industrial applications where CO and NOx emissions trade-offs are numerous and constraining. The flame front structure also showed signs of being sensitive to the variations of the electric assistance, particularly in the case of applied electric fields. The flame front is shifted in the dominant direction of the electric current streamlines. This effect is very similar to the traditional understanding of the “ionic wind”; our study has shown that the common frequency range of this effect (<100 Hz) can be widened to higher frequencies (few kHz). A mechanism was proposed to explain this phenomenon in our tests, based on using parts of the energy provided by the electric field to transfer momentum to the ions in the flame in the direction of the increasing electric potential difference gradient, even with an oscillating field.

A mechanism for the oxidation and fragmentation of a char particle was developed. Qualitative agreement between the model simulations and experimental data observed in the literature, is found for the higher gas temperatures (1700K). However fundamental differences are found in the particle temperature histories and burnout times at low temperature (1250K). The role that fragmentation plays on the char particle history is incorporated into the model and the possible production of fine particulate through fragmentation is examined. A relatively large fraction of the mass of char available for fragmentation is produced early in the combustion history of the particle. Therefore, if this mechanism is important in the generation of fine particulate matter during char combustion, the simulations indicate that it would occur early in the combustion process. Due to the limited experimental data in the literature on the time resolved particle size distribution in the early stages of combustion, corroboration between model and experiment was not possible.

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Thesis (Ph. D.)--School of Mechanical Engineering, Georgia Institute of Technology, 2004. Directed by Ben T. Zinn. / Vita. Includes bibliographical references (leaves 136-141).

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Thesis (M.S.)--Worcester Polytechnic Institute. / Keywords: self-heating; smoldering ignition; plywood; bowes. Includes bibliographical references.

School of Chemical and Metallurgical Engineering, Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, South Africa, in fulfillment of the requirements for the degree of Master of Science in Engineering, November 2019 / This research focuses on the co-firing of low-quality coal with refuse derived fuel (RDF) as a means to utilise some of the abundant high-ash coal available in South Africa as a fuel co-fired with RDF in existing pulverised fuel boilers. The use of RDF is also a means to reduce the volume of waste dumped in landfill sites. The physicochemical characteristics of the RDF, run of mine coal (ROM) and discard coal were investigated, along with the co-combustion behaviour and kinetics of the RDFs, coal and their blends at different weight ratios. The blends tested contained 85%, 70%, 50% and 25% coal with the remaining proportion made up of RDF. The gaseous emissions and ash residue from the combustion of coal, RDF and coal/RDF blends were also analysed to determine the environmental impact of co-firing with RDF. The physicochemical analysis revealed that the run-of-mine and discard coal have relatively low calorific values of 21.7 MJ/kg and 16.7 MJ/kg, respectively. The RDF samples were comprised of plastic and paper, as well as smaller amounts of other materials. The RDF sample containing mostly plastic (PL) and the other containing mostly paper (PB) were found to have higher energy contents of 31.2 MJ/kg and 22.4 MJ/kg, respectively. The thermogravimetric analysis was performed in an atmosphere of air, over a temperature range of 25 – 850°C, and the results showed that the RDF samples had lower ignition, devolatilisation, and burnout temperatures compared to the coals. The ignition temperatures for the blended fuel occurs in the lower temperature region when RDF is added to the blend, likewise the peak temperatures and burnout temperatures shifted to a lower temperature zone. The activation energies (Ea) were determined using the Coats-Redfern method. The Ea for the ROM coal of 104.4 kJ/mol, was found to reduce to 31.4 kJ/mol for 75% PB + 25% coal and 35 kJ/mol for 75% PL + 25% coal blends, respectively. The discard coal which had an Ea of 109.9 kJ/mol was reduced to 30.9 kJ/mol with the (paper blend) and 33.5 kJ/mol with the (plastic blend) for the 75% RDF + 25% coal discard blends. The analysis of the ash for the chloride and alkali metal content in the RDFs, coal samples and their blends were determined with the use of ion chromatography and X-ray fluorescence (XRF) techniques. The co-combustion ash of discard coal and RDF showed a decrease in chloride and alkali metal content as the ratio of coal was increased in the blend. The calculated slagging and fouling indices showed that as the coal ratio in the blend increases, the propensity of the fuel to slag and foul the boiler surfaces decreases. The propensity to slag was found to be low for the ash obtained from the co-fired blends, while the propensity to foul decreased from high to medium range for all the blends with less than 75% of the RDF PB. The concentration of gases emitted from the combustion and co-combustion test was determined with the aid of an MGA 11 mobile gas analyzer connected online at 1 scan per second. The co-combustion of RDF with coal showed a decrease in SO2 emissions from (387 ppm) for the discard coal to within the legislated maximum emission for South African new coal fired plants. This was attained with samples containing ˃ 15% PL and ˃ 30% PB RDF. The lowest SO2 emission of 50 ppm was achieved for the blend of 25% discard coal (C2) + 75% PL. The RDF sample (PL) emitted the highest NOx emission of 143 ppm. The peak concentration of NOx emitted was increased with the addition of RDF during co-combustion, however, the duration of the emission was greatly reduced and all samples were within the South African standard limits. There was also an increase in the emissions of CO and CO2 which could be due to the high volatile matter content of the RDF. The lowest CO2 emissions was 6000 ppm and this was achieved with the blend of 85% C2 + 15% PB. It was established in this study that the most favourable fuel blend that could be used for power generation is that of discard coal (70%) and PL (30%). This was based on the activation energy obtained from this blend, with the lowest apparent activation energies of 55.8 kJ/mol and 54.2 kJ/mol for the volatile and char combustion, respectively. This makes this blend the preferred alternative fuel to be fired in the existing pulverised fuel boilers, or other type of industrial boilers, in South Africa. / PH2020