South Africa has large coal reserves and produces approximately 74% of its primary energy from coal. Coal gasification using moving bed gasifiers is one of the most important coal utilisation technologies, consuming ± 17.5% of locally produced coal. This study was motivated by the need to investigate alternative coal gasification technologies for the utilisation of fine, high-ash and caking coals for future Integrated Gasification Combined Cycle (IGCC) and coal to liquids (CTL) plants. These coals are estimated to form a large percentage of the remaining coal reserves in South Africa and could be difficult to utilise efficiently in moving bed gasifiers. Fluidised bed gasification was identified as a technology that could potentially utilise these coals. Coals from the New Vaal and Grootegeluk collieries were selected as being suitable for this investigation. The coals were subjected to detailed characterisation, bench-scale and pilot-scale fluidised bed gasification tests. The results of the pilot-scale atmospheric bubbling fluidised bed gasification tests show that stable gasification is possible at temperatures between 880 °C and 980 °C. The maximum fixed carbon conversion achievable in the pilot plant is, however, limited to ± 88% due to the low reactivity of the coals tested and to thermal fragmentation and attrition of the coal in the gasifier. It was found that oxygen enrichment of the gasification air from 21% to 36% by means of oxygen addition produces a significant increase in the calorific value of the gas (3.0 MJ/Nm3 to 5.5 MJ/Nm3). This observation has not previously been reported at pilot-plant scale. A mathematical model for a bubbling fluidised bed coal gasifier was developed based on sub-models for fluidised bed hydrodynamics, coal devolatilisation, chemical reactions, transfer processes and fines generation. A coal devolatilisation sub-model to predict the products of coal devolatilisation in a fluidised bed gasifier was developed and incorporated into the model. Parameters associated with the rates of the gasification reactions and the devoltilisation process were obtained by means of bench-scale tests. The heat loss parameter (Q) in the model was estimated by means of a heat loss calculation. The results from the pilot-scale gasification tests were used to evaluate the predictive capability of the model. It was found that for temperature, fixed carbon conversion and calorific value of the gas the difference between measured and predicted values was less than 10%. Recommendations are made for further refinement of the model to improve its predictive capability and range of application. The model was used to study the effect of major operating variables on gasifier performance. It was found that increasing the reactant gas (air, oxygen and steam) temperature from 250 °C to 550 °C increases the calorific value of the gas by ± 9.3% and the gasification efficiency by ± 6.0%. Increasing the fluidised bed height has a positive effect on fixed carbon conversion; however, at higher bed heights the benefit of increasing the bed height is less due to the inhibiting effects of H2 and CO on the rates of char gasification. / PhD (Chemical Engineering), North-West University, Potchefstroom Campus, 2014

Fluidised bed drying is currently receiving much attention as a dewatering option after the beneficiation of fine coal (defined in this study as between 1mm and 2mm particles). The aim of this study was to investigate the removal of moisture from fine coal by using air at relatively low temperatures of between 25°C and 60°C within a controlled environment by lowering of the relative humidity of air. The first part of the experimental work was completed in a controlled climate chamber with the coal samples in a static non-fluidised state. Drying in the second part was carried out using a fluidised bed with conditioned air as the fluidising medium. Introduction of airflow to the system led to a lower moisture content in the coal samples and it also proved to have the ability to increase the drying rate. It was determined that the airflow had the ability to remove more free moisture from the filter cake. In addition more inherent moisture could also be removed by using upward flowing air, resulting in a lower equilibrium moisture content. It was proven that the airflow rate and relative humidity of the drying air contributed to faster drying rates. The effect of temperature was not as significant as expected, but higher temperatures did increase the drying rate at higher airflow and lower humidity conditions. The larger surface areas of particles create surface and capillary forces that prevent the moisture from leaving the finer coal particles. It was found that the rate of drying is independent of the moisture content in the coal sample. Just in terms of the fastest drying time and drying rate in the fluidised bed, it was concluded that the most efficient conditions is airflow above minimum fluidisation point causing vigorous mixing and maximum contact with the drying air. In addition to the high airflow it was concluded that 30% relative humidity and 55°C resulted in the fastest drying time. All the drying processes at all the airflow rates, temperature and relative humidity conditions were energy efficient. This process was shown to be energy positive, resulting in an overall energy gain. The overall energy consumption for the fluidised bed is lower than for all the dryer systems compared to and it compared favourably with other thermal drying technologies. It was therefore shown that this is a viable technology for the dewatering of fine coal. / MIng (Chemical Engineering), North-West University, Potchefstroom Campus, 2014

Dissimilatory sulphate reduction, brought about by the action of sulphate reducing bacteria (SRB) was used in the treatment of acid mine drainage (AMD) in a fluidised bed bioreactor. Biologically produced hydrogen sulphide and bicarbonate ions, by SRB, facilitated the precipitation of heavy metals and the generation of alkalinity in the synthetic acid mine water, respectively. The SRB that had been selected were able to utilize acetate as the sole carbon source and were capable of growing in the bioreactors at low pHs, facilitating an increase in the influent pH from 2.75-7.0 to 5.4-7.8, after a 24-hour hydraulic retention time (HRT). The precipitation efficiencies for Fe, Mn, Zn, Cu, Cr and Al after a HRT of 24 h as metal sulphides ranged between 84- 99% for influent pH values of between 4 and 7, and above 54% for influent pH values between 2.75 and 4. Microbial metabolic activity decreased with decreasing influent pH. This was inferred from the decreasing differences in chemical oxygen demand (COD) depletion rate over a 24 h HRT, as influent acidity levels approached pH 2.75. Molecular studies, using PCR-DGGE analysis on the microbial consortium in the bioreactor, revealed the presence of at least 8 different bacterial species in the consortium. Attempts at sequencing these bands yielded inconclusive results, with the bands showing sequence homology to a large number of previously uncultured and undescribed bacteria. Scanning electron microscopy confirmed the presence of bacteria of different morphology, as well as the presence of biofilms, which account for the heavy metal and low pH tolerances that the bacteria sustained.

In 2004 the Department of Chemical and Process Engineering (CAPE) at University of Canterbury began a programme to investigate using biomass gasification integrated combined cycle (BIGCC) technology to convert waste products and residues to useful energy for the wood processing sector. This research was conducted as a part of Objective Two of the programme to develop gasification and gas cleaning technology. This project involved commissioning and characterising the operation of the Fast Internal Circulating Fluidised Bed (FICFB) gasifier and comparing its operation with a more conventional up-draught process owned and operated by Page Macrae in Mount Manganui. The wood derived gas composition of each gasifier was measured using gas chromatography and these compositions were used to calculate lower heating values (LHV). The CAPE FICFB gasifier has proven to produce successfully a gas with a lower heating value of 10400-12500 kJ/Nm³. The Page Macrae gasification process produces a low quality gas with a lower heating value of 4100-5100 kJ/Nm³. This is much lower than the CAPE gasifier since the oxidant used in the up-draught gasification process is air and the product gas is diluted by nitrogen. The Page Macrae gasification system combusts wood derived gas to produce steam for a laminar veneer lumber (LVL) processing plant so gas quality and heating value are less important than in electrical production applications. Reducing the nitrogen content of the CAPE product gas will increase the heating value of the gas. Improvements to the boiler system will reduce the amount of air required for gasification and hence reduce the nitrogen content. Further improvements to gas quality can be gained from a change in the fuel feed point from on top of the gasification column's bubbling fluidised bed to the side of the bubbling fluidised bed. The CAPE gasifier is much more complicated and requires specialised operators but produces a gas suitable for gas engine and gas turbine technology. Overall the CAPE gasification system is more suited to BIGCC applications than the Page Macrae process.

Chemical looping combustion (CLC) is a promising fossil fuel combustion technology, which is able to separate CO2 from the flue gases without a large consumption of energy. In this thesis, the study was extended to look at the use of chemical looping materials within traditional fluidised bed combustion and investigation of the interaction between the fuel, the supplied air and the chemical looping agent. Three topics of chemical looping combustion are discussed, including 1) the Sherwood number in the fluidised bed; 2) properties of different oxygen carriers, Fe2O3 and CuO (with supporting materials), were tested in the fluidised bed reactor; 3) the simulation of a steady state and a dynamic model of a coal-fired CLC power plant using Fe2O3 as oxygen carriers. The Sherwood number, which represents the mass transfer rate, is important in the calculation of CLC process. With Sherwood number, the mass transfer rate kg around the acting particle can be calculated using correlation Sh=kg∙d/D, where d is the diameter of acting particle, and D is the diffusivity around the acting particle. Hayhurst and Parmar (Hayhurst and Parmar 2002) calculated the Sherwood number in the fluidised bed by using the CO/CO2 ratio, which was measured by the temperature difference between the carbon particle and the bulk phase (Hayhurst and Parmar 1998). However, the temperature of the particle could be overestimated, so the CO/CO2 ratio could be underestimated. In this thesis, a universal exhaust gas oxygen (UEGO) sensor was employed, which could measure the actual carbon consumption rate in the fluidised bed by oxidizing CO in the sample gas into CO2 and. Fe particles of the same size of the char particle is used to measure the O2 consumption rate, and thus eliminate uncertainty in the Sherwood number. The CO/CO2 ratio was calculated by using the carbon consumption rate and the O2 consumption rate. In contrast to Hayhurst and Parmar (Hayhurst and Parmar 2002) who assumed CO2 was the main product, for this char the actual ratio of CO/CO2 was almost zero. The measurement here is in agreement with Arthur. This more accurate determination of CO/CO2 allows a better estimate of the mass transfer coefficient and leads to a correction of the Hayhurst and Parmar’s (Hayhurst and Parmar 2002) correlation by a factor of 1⁄2. Interestingly, very small fluidised beds have mass transfer coefficients which are about twice that expected in a large bed (owing to the very different flow and indeterminate flow pattern). This means the correlation of Hayhurst and Parmar (Hayhurst and Parmar 2002), by fortuitous coincidence works wells for beds with diameters < 30 mm., without the correction factor, should be ignored. In the fluidised bed in a typical CLC process, different fluidising material could have different influence on the reactions. Thus, it is worth discussing different kinds of fluidising materials. The char combustion in the fluidised bed was simulated by using inert (sand) and active (Fe2O3 or CuO) fluidising materials, and air as fluidising gas. The results indicated that 1) CO combustion in the boundary layer leads to smaller carbon consumption rate and larger oxygen consumption rate; 2) Using Fe2O3 particles as fluidising materials slows down the carbon consumption rate, since the diffusivity of CO2 is smaller than CO; 3) CuO particles slow down the carbon consumption rate at large Sherwood number (Sh=2 or 2.5). The influence of using CuO as fluidising material is further discussed experimentally by using low O2 fluidising gas. The results indicated that since the amount of CuO used in the experiment is small, when the O2 concentration in the bulk phase is lower than the equilibrium concentration, the O2 concentration in the bulk phase gradually decreases, and the O2 concentration in the bulk phase has large influence on the char particle combustion. A steady state model of a coal-fired CLC power plant was simulated. The aim of the model was to test the suitable operating conditions of the power plant, such as recycle rate of oxygen carriers, for the power plant design. In the steady state model, the power plant consists of a combustor and a steam cycle. Hambach lignite coal, Polish bituminous coal and natural gas were tested as fuels. The results indicated that: (1) The effect of the fuel is largely due to the amount of oxygen required per GJ released; (2) Preheating is important, but seems to have a minor effect since the most of the heat is released at temperatures well above the pinch point; (3) since the temperatures of heat source in this research is well above the pinch point, all heat are usable for the steam cycle. In this case, the steam cycle and the chemical looping plant could be optimised separately; (4) As long as the preheat temperature of the air flow into the air reactor is higher than the temperature of turbines, in most of cases the power output is unaffected by the choice of variables, leaving the designer free to choose the most convenient. With the conclusions above, a dynamic model of a coal-fired CLC power plant using Fe2O3 as oxygen carrier is then simulated. The aims of this simulation include: 1) explaining the kinetics of Fe2O3 oxygen carriers at high temperature (1223K) in a fluidised bed reactor using Brown’s data (Brown 2010); 2) a 1GWth dynamic power plant was simulated to test different cases including changing power supply and power storage. In the dynamic model, a chemical looping power plant using Hambach lignite char is tested, and the parameters of the system are adjusted so as to simulate the operations of a real chemical looping power plant. The two-phase model is employed for the fluidised bed reactors. Experimental data from Brown (Brown 2010) was simulated using this model first to test its validity. Then the model is scaled up to simulate a 1GWth dynamic power plant. The ideal operation conditions are found, and a char stripper is found helpful for carbon capture.

Orientadores: Maria Regina Wolf Maciel, Henry França Meier, Antonio Andre Chivanga Barros / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Quimica / Made available in DSpace on 2018-08-03T15:25:11Z (GMT). No. of bitstreams: 1 Wiggers_VinicyusRodolfo_M.pdf: 4312543 bytes, checksum: c9c8ccdabb1190ba49ffe38d966b7226 (MD5) Previous issue date: 2003 / Resumo: A pirólise, uma das muitas alternativas de processos de conversão química de resíduos sólidos, tem recebido uma atenção especial de ambientalistas, engenheiros e da comunidade científica. Este processo tem sido testado em um número incontável de plantas piloto, e muitos sistemas em escala industrial já são operados com sucesso. Sofrendo aquecimento em uma atmosfera livre de oxigênio, muitas substâncias orgânicas de cadeia longa, podem ser fracionadas via craqueamento térmico e reações de condensação, em frações gasosas, líquidas e sólidas de menor peso molecular com maior valor econômico. Ao contrário da incineração que é altamente exotérmica, a pirólise é endotérmica, ocorre numa faixa de temperatura da ordem de 300 a 600°C e apresenta vantagens como a menor formação de dioxinas e a possibilidade de processo autotérmico. Este trabalho tem o propósito de simular, projetar e construir uma planta piloto de pirólise em reator de leito móvel através de solução de modelos matemáticos e métodos numéricos. Além disso, pretende-se desenvolver uma estratégia do tipo problema inverso para ajuste numérico de parâmetros térmicos e cinéticos do processo. Para tanto, projetou-se e montou-se um aparato experimental em escala piloto, com finalidade multi-propósito, para aquisição de dados do processo com e sem reação, mediante o desenvolvimento e uso do modelo microscópico unidimensional e permanente de conservação da massa e da energia / Abstract: Pyrolysis, one of many solid waste chemical conversion processes, has been receiving a special atenttion from engineers, researches and environment specialists. The pyrolysis has been tested in many pilot plants, and some industrial plants are operated with success. Heating in a controled atmosphere absenced of oxygen, an organic portion of waste materials can be converted into mixture gases, oils with lower molecular weight and others products with higher economic value. This trasnformation occurs through thermal cracking and condensation reaction. Whereas incineration is exothermic, pyrolysis is endothermic and runs in range temperatures from 300 to 600°C. This process still presents some advantages like smaller dioxines formation than the incineration and also the possibility of autothermal operation of the process This work has the propose to simulate, project and build a pilot plant of pyrolysis with a fluidised bed reactor, by using mathematical modelling and numerical methods for simulation of the process It also intends to develop strategies to solve an inverse problem to predict thermal and kinetics parameters of the model from experimental data obtaneid in a pilot plant and a metodology to determine a thermic and kinetic parameters of solid waste. To get this objective, a microscopic model l-D and steady state, of mass and energy conservation was developed, the simulation for design and the construction of an experimental aparatus in a pilot plant scale was realized, to get a process data with and without reaction, to intend a scale-up from process / Mestrado / Desenvolvimento de Processos Químicos / Mestre em Engenharia Química

Please read the abstract in the section, 00front of this document / Dissertation (MSc (Metallurgy))--University of Pretoria, 2004. / Materials Science and Metallurgical Engineering / MSc / unrestricted

No / Oxidative coupling of methane (OCM) is suggested to be a promising process for the conversion of the abundant natural gas into useful chemicals. However, this reaction faces many drawbacks such as low yields for higher hydrocarbons, fast catalyst deactivation, and huge heat effects of the reaction. Only a well-designed fluidized bed reactor is able to overcome effectively those disadvantages and to provide a satisfactory continuous operation. However, design approaches for fluidized bed reactors are still based on models developed during 70s and 80s, which cannot take into account various hydrodynamic effects on the reactor performance. Thus, a reactor designer has usually to rely on extensive experiments in order to improve the classical fluidized bed reactor design. In this work, the relevance of hydrodynamics, reactor geometry, and feeding policy on the performance of a fluidized bed reactor for the OCM is shown. For this purpose, several case studies of fluidized bed reactors are simulated in full 3D geometry under the same reaction conditions, but with different reactor geometries and feeding policy. These studies show the significance of hydrodynamic parameters for the reactor performance, and moreover, how fluidized bed reactor performance can be improved by a careful study of coupled momentum-mass transport-reaction phenomena. Furthermore, it can be demonstrated that a suitable distributed feeding policy of oxygen provides an improved yield while a traditional fluidized bed reactor design results in an inferior performance among all investigated cases.

Fluidized bed technology is used for thermal conversion of solid fuels (combustion and gasification) and is especially suitable for conversion of low-rank fuels such as biomass and waste. The performance of fluidized bed units depends on the fuel mixing and fuel-gas contact. Thus, it is important to understand these two phenomena in order to develop models for reliable design and scale up of fluidized bed units. This work investigates, under conditions representative for industrial fluidized bed units, the lateral fuel mixing (in a unit with a cross section of 1.44 m2 both at hot and cold conditions) and the bubble flow distribution (in a 1.2 m-wide 2-dimensional unit). The work confirms previous findings on the formation of preferred bubble paths and shows that these bubble paths are enhanced by lowering the fluidization velocity, increasing the dense bed height and reducing the pressure drop across the gas distributor. From the fuel mixing experiments, an estimation of the lateral effective dispersion coefficient to values in the order of 10-3 m2/s is obtained under both hot and cold conditions. The experiments under cold conditions give additional qualitative information on the fuel mixing patterns such as flotsam/jetsam tendencies. The camera probe developed for fuel tracking under hot conditions enables to study the fuel dispersion under real operation at relevant industrial scales. Based on the characteristics of the bubble path flow, a model for the horizontal fuel dispersion on a macroscopic scale is formulated and shown to be able to give a good description of the experimental data. As opposed to the commonly applied diffusion-type modeling of the lateral solids dispersion, the proposed model facilitates integration with models of the bubble flow. Thus, the present modeling work is a first step to provide a modeling of the fuel dispersion, which uses as inputs only the main operational parameters of the fluidized bed.

In the efforts to create sustainable production of heat and power and to reduce the net CO2 emissions to the atmosphere, alternative fuels are today being utilised. These fuels are, for example, biofuels and waste derived fuels such as different residues from the agricultural sector and the pulp and paper industry, municipal sewage sludge and municipal sorted solid waste. These fuels put new demands on the combustion facilities due to their chemical composition and this in turn calls for methods of prediction for the evaluation of their combustion behaviour. Most significant for the majority of these fuels are the high alkali and chlorine concentrations which cause bed agglomeration, deposit formation and corrosion on heat transfer surfaces. These problems can be solved if sufficient knowledge is obtained of the specific fuel or fuel mix. In this work, chemical fractionation, a step by step leaching method, was used on fuels, fuel mixes and fly ashes from co-combustion in a fluidised bed combustor. In addition, XRD and SEM-EDX were used for the fuel and fly ash characterisation. Different alkali chloride reducing additives i.e. kaolin, zeolites and sulphur were investigated as was the influence of various bed materials: silica sand, olivine sand and blast furnace slag (BFS). Some of the new, alternative fuels, such as municipal sewage sludge and meat and bone meal (MBM) contain high concentrations of phosphorus which is a very important nutrient essential in many biological processes. Phosphorus rock used as raw material in the phosphate industry is a depleting natural resource estimated to last for only 30-200 years according to different sources. The combustion of municipal sewage sludge enriches the phosphorus in the ashes while hazardous components such as pathogens and organic pollutants are rendered harmless after combustion. However, toxic heavy metals are also enriched in the ashes. One aim of the work was to find a sufficiently effective and low cost method for phosphorus extraction from fly ashes derived from municipal sewage sludge combustion. Two types of municipal sewage sludges were investigated using different chemicals for the phosphorus cleaning step in the waste water treatment plants. The first sewage sludge derived from a plant using iron sulphate as flocculant to precipitate phosphorus as iron phosphate. The second sludge meanwhile came from a plant using aluminium sulphate as flocculant to precipitate phosphorus as aluminium phosphate. Both sewage sludges were dewatered prior to combustion and co-combusted with wood pellets. At pH 1 nearly all the phosphorus was released from the fly ash derived from the sewage sludge where aluminium sulphate was used as a phosphorus precipitation agent. Iron sulphate as precipitant inhibited the phosphorus extraction from the ashes, resulting in only 50-80% of the phosphorus being released. Furthermore, the mobility of heavy metals to the leachates was investigated to establish whether the leachates were suitable as fertilisers. Only minor fractions of Pd, Hg, Cr, Cu, Mn, Co, Ni, As, Sb, V and Zn were found in the leachates, all well within the legislated limitations for fertilisers. However, one exception was Cd that was nearly totally dissolved in the leachate. Thus a decadmiation of the leachate is necessary prior to any utilisation of the ashes and reuse of phosphorus as fertiliser. / <p>Akademisk avhandling för avläggande av teknologie doktorsexamen vid Chalmers tekniska högskola försvaras vid offentlig disputation den 15 oktober 2008</p>