This thesis project studied a number of aspects relating to the fluid, mechanics of circulating fluidised beds. Studies of the macrostructure of a 9.3 m high x .15 m dia. riser showed a strong dependence of one important macroscopic descriptor, the density profile, upon the geometry of the gas/solids exit and the location of the solids return. It was found that abrupt exits promoted inertial solids separation from the conveying gas which generated strong internal circulation patterns and high slip velocities. Microstructural studies, in support of the macrostructural investigation, and using a needle capacitance probe, showed how the radial density profile develops with height causing a gradual density decay. The structure, characterised by an "intermittency index," was strongly radially non-uniform at all locations in the lower regions of the column with pronounced aggregation or clustering at the highest densities. However, the cluster-like structures present at the base rapidly gave way to a more dilute core-annular type flow slightly further up the column. This radially non-uniform structure was used to explain a number of macroscopic phenomena. These included the effects of exit type, solids return location, secondary air addition and gas mixing. The results of the various studies, drawn together, allow fast fluidisation to be defined tentatively with respect to its relationships to choking, pneumatic transport, and other fluidisation regimes. Separate studies were performed to examine gas mixing and the transition to turbulent fluidisation. The gas residence time distribution was found to be substantially different from plug flow and could be characterised crudely by a two-zone model. The turbulent transition was found to be gradual, but nonetheless a transition, although a developed turbulent zone did not exist until well beyond transport conditions. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate

A thesis submitted to the Faculty of Engineering and Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy, Johannesburg 2017 / South Africa (SA) has an energy-intensive coal mining industry, where coal accounts for approximately 72% of total primary energy consumption in the country, particularly in the electricity sector, where 95% of total electricity generated is derived from coal. Pulverised coal combustion has been the preferred technology adopted for power generation in South Africa for many decades. These coal-fired power plants have no flue gas desulphurisation (FGD) equipment fitted at present. Therefore, these plants account for the majority of annual SO2, CO2, and NOx emissions, making them environmentally unsustainable for power generation. Such environmental issues add to the challenges for the power producer, who is required to meet not only energy demand, but also to compete with the export market for quality coals, and to ensure that electricity generation complies with ever-changing air quality standards. Circulating fluidised-bed combustion (CFBC), a technology for the combustion of coal, biomass, waste, has not been adequately explored or tested in South Africa previously. CFB combustion is currently under intense scrutiny amongst researchers evaluating its potential as an economic and environmentally acceptable technology, in particular for the burning of lowgrade coals. The main objective of this study is to undertake a case study using CFBC technology and to establish its potential for use in South Africa as a clean and cost-effective method in power generating for high-ash, low-grade coals. Experimental tests were conducted in a CFBC pilot plant in Finland, using two high ash coals, discarded coal from South Africa (SA) and a better quality coal from Russia for comparative purposes. A review was conducted of discard coals in South Africa in order to establish an inventory in support of their potential utilisation for power generation in circulating fluidised bed boilers. A further study established a comparison between pulverised coal (PC), and fluidised bed (FBC) technologies as a future benefit analysis. All four coals proved to have very high combustion efficiencies, despite significant quality differences in terms of petrographic composition and ash content. More specifically, the SA coals achieved combustion efficiencies of 99.6 %, 99.7 % and 99.8 %, where the Russian coal achieved 98.7 percent. The Russian coal was characterised as being low in ash and high in the reactive maceral vitrinite, the two South African coals possessed high ash content (35 to 45%), one with relatively high vitrinite, and the other very low vitrinite, whilst the South African discard possessed an ash content of 65-70% and extremely low reactive vitrinite content. All these factors lean towards the suitability of SA coals to the CFB technology. In terms of NOx emissions, all coals tested showed that their NOx and N2O emission meet the minimum requirements for small plants as set out by the European and SA standards, i.e. <300 ppm for a plant with generating capacity below 100 MW. This result is in agreement with data from the literature. The emission of SO2 depends on the sulphur content in the initial coal, which also has an impact on the Ca/S Ratio. SO2 emitted from the South African coals was higher than the national permitted standard, due to the low Ca/S ratio used. This was especially the case for South African discard. Vast reserves of discard coal containing from 2MJ/kg to 14 MJ/kg in calorific value have accumulated in South Africa since the last inventory of 2001, i.e. close to 1.5 billion tonnes are in existence. It is apparent that one of the looming challenges regarding discard coal is putting this ever-accumulating material to use. From the combustion results obtained in this research, it is proposed that such materials can be combusted in a CFBC boiler, and that it produces the same efficiency as other coals from South Africa and a clean coal from Europe. Ash distribution within the boiler was found to change in proportion of bed ash to fly ash, subject to the quality of the coal used. This is also likely to change the proportions of sulphur-absorbing sorbents in future. CO2 emissions from the coals under review were found to be very close, in the region of 12.8 to 13.8 percent. / XL2018

No description available.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1982. / Includes bibliographical references. / by Mark Thomas Nicastro. / Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1982.

The thermodynamic behaviours in a PFBC combustor were simulated for the ash from all of the six coals with sand and limestone as bed material. Ash components determined the ash thermodynamic behaviour at high temperature, and each component had different effects. For assessment of the potential for bed material agglomeration, the temperature at which 15% of the ash would become liquid (T15) was calculated with the coal ash, the cyclone ash and the cyclone ash mixed with varying amounts of limestone. Both the bed ash and fly ash, collected from an industrial PFBC plant, consisted of limestone/lime particles with different extent of sulphation, and coal ash particles. The calcium aluminosilicate material formed on the coal ash particles but not on the limestone particles. The aluminosilicate materials appeared to be formed from fine ash and lime particles at some local hot zones in the boiler. The melted materials may glue ash and bed material particle into large particles leading to bed agglomeration and defluidization. Four mechanisms were proposed for the formation of bed material agglomeration in PFBC, which may occur under different conditions. One mechanism explains the bed material agglomeration with the high localized high temperature zone due to the improper design or operation, while the bed agglomeration through the other three mechanisms results from the unsuitable coals burnt in the PFBC combustor. The maximum char temperature and the minimum T15 were used simultaneously to predict the tendency towards bed material agglomeration in PFBC burning different coals. Both char properties and ash properties should be considered during coal selection process for PFBC, to ameliorate the potential problem of bed agglomeration.

This thesis examines transport phenomena of cupric chloride (CuCI2) hydrolysis within a fluidized bed. Conversions of CuCi2 and steam as a fluidizing gas are numerically investigated using a new non-catalytic gas-solid reaction model, proposed in the literature but here updated for the purposes of the present study. The results are illustrated considering two cases of kinetics for the consumption of particles: Volumetric Model and Shrinking Core Model. Consistent results in terms of the conversion of reactants versus superficial velocity, bed inventory and bed temperature are obtained by developing new solution algorithms abased on each of the above kinetic models. / UOIT

Hydrodynamic effects play important roles in fluidized bed combustion processes. Since the motion of "bubbles" is an important influence on fluidized bed heat transfer, a better understanding of their behavior is necessary for improving the design of fluidized bed boilers. Using a two-dimensional bed, silica sand particles were fluidized with air at room conditions. The bubbling bed was videotaped, and both qualitative and quantitative information were gathered. Bubble characteristics such as size, rise velocity and frequency were studied while particle size and superficial gas velocity were varied. Results were compared with some existing theories and other similar research. The effect of internal surfaces at several heights in the bed was also studied. General bubble behavior agreed well with descriptions from previous research, and the expected spherical-cap bubble shape was observed. Both bubble size and rise velocity increased with particle size and with fluid velocity. Bubble frequency increased with fluid velocity, but decreased with increasing particle size and height in the bed. These results agree with previous work done using optical probes to measure bubble characteristics. Comparisons of data with empirical models showed general agreement. The presence of internal surfaces had the effect of reducing the bubble size, rise velocity, and frequency, and also of reducing the influence of changing particle size and superficial velocity on the bed behavior. / Graduation date: 1992

Thesis (Ph. D.)--Lehigh University, 2003. / Includes vita. Includes bibliographical references (leaves 104-107).

The iron and steel industry emitted 8 % of all CO2 emissions in Sweden, 2011. Investigating alternative energy carriers is the purpose of this thesis. By pyrolyzing biomass, an energetic solid, gaseous and liquid (bio oil) fraction is obtained. If pyrolyzing biomass in a fluidized-bed reactor, the highest value may be added to the combined products. Additional understanding of pyrolysis in fluidized beds is pursued, using Computational Fluid Dynamics (CFD) and comprehensive kinetic schemes. The obtained solid product is investigated as a bio-injectant in blast furnaces for ironmaking. A new approach of separately modeling, the primary and secondary pyrolysis, is developed in this thesis. A biomass particle devolatilizes during pyrolysis. Primary pyrolysis is the solid decomposition which results in the volatiles that can leave the particle. Secondary pyrolysis is the decompositions of these volatiles, primarily in the gas phase. The primary pyrolysis (35 species, 15 reactions) mainly occurs in the bed-zone and as such, the model needs to take into account the complex physical interaction of biomass-particles with the fluidizing media (sand) and the fluidizing agent (gas). This is accomplished by representing the components by Eulerian phases and implementing interaction terms, as well as using a Stiff Chemistry Solver for the implemented reactions.  The secondary pyrolysis (not considering heterogeneous reactions), mainly occurs outside the bed zone in one phase. The fluid flow is simpler but the chemistry is more complex, with a larger variety of molecules emerging. Carrying out the simulations time-effectively, for the secondary pyrolysis (134 species, 4169 reactions) is accomplished by using Dimension Reduction, Chemistry Agglomeration and In-situ Tabulation (ISAT); in a Probability Density Functional (PDF) framework. An analysis of the numerical results suggest that they can be matched adequately with experimental measurements, considering pressure profiles, temperature profiles and the overall yield of gas, solid and liquid products. Also, with some exceptions, the yield of major and minor gaseous species can be matched to some extent. Hence, the complex physics and chemistry of the integrated process can be considered fairly well-considered but improvements are possible. A parametric study of reaction atmospheres (or fluidizing agents), is pursued as means of understanding the process better. The models revealed significant effects of the atmosphere, both physically (during the primary and secondary pyrolysis) and chemically (during secondary pyrolysis). During primary pyrolysis, the physical influence of reaction atmospheres (N2, H2O) is investigated. When comparing steam to nitrogen, heat flux to the biomass particles, using steam, is better distributed on a bed level and on a particle level. During secondary pyrolysis, results suggest that turbulence interaction plays an important role in accelerating unwanted decomposition of the liquid-forming volatiles. Steam, which is one of the investigated atmospheres (N2, H2O, H2, CO, CO2), resulted in a lower extent of unwanted secondary pyrolysis. Altough, steam neither resulted in the shortest vapor residence time, nor the lowest peak temperature, nor the lowest peak radical concentration; all factors known to disfavor secondary pyrolysis. A repeated case, using a high degree of turbulence at the inlet, resulted in extensive decompositions. The attractiveness of the approach is apparent but more testing and development is required; also with regards to the kinetic schemes, which have been called for by several other researchers. The solid fraction after pyrolysis is known as charcoal. Regarding its use in blast furnaces; modelling results indicate that full substitution of fossil coal is possible. Substantial reductions in CO2 emissions are hence possible. Energy savings are furthermore possible due to the higher oxygen content of charcoal (and bio-injectants in general), which leads to larger volumes of blast furnace gas containing more latent energy (and less non-recoverable sensible energy). Energy savings are possible, even considering additional electricity consumption for oxygen enrichment and a higher injection-rate on energy basis. A survey of biomass availability and existing technology suppliers in Sweden, suggest that all injection into Blast furnace M3 in Luleå, can be covered by biomass. Based on statistics from 2008, replacement of coal-by-charcoal from pyrolysis could reduce the on-site carbon dioxide emissions by 28.1 % (or 17.3 % of the emissions from the whole industry). For reference, torrefied material and raw biomass can reduce the on-site emissions by 6.4 % and 5.7 % respectively. / Järn och stålindustrin stod för 8 % av alla koldioxidutsläpp i Sverige, 2011. Alternativa energibärare undersöks i denna avhandling. Genom pyrolys av biomassa, fås en energirik fast produkt, och samtidigt en gasformig och en vätskeformig produkt (bio-olja). Om en fluidbäddsreaktor används kan största möjliga mervärde tillföras de kombinerade produkterna. Djupare förståelse för pyrolys i fluidbäddar har eftersträvats med hjälp fluiddynamikberäkningar (CFD) och detaljerade kinetikscheman. Den fasta produkten har undersökts som bio-injektion i masugnar. En ny approach för modellering av primär och sekundär pyrolys separat, har utvecklats i denna avhandling. En biomassapartikel avflyktigas under pyrolys. Primär pyrolys är nedrytningen av den fasta biomassan till intermediärer (flyktiga ämnen) som kan lämna partikeln. Sekundärpyrolys är nedbrytning av dessa flyktiga ämnen, som primärt sker i gasfas. Primärpyrolysen (i detta arbete, 35 ämnen och 15 reaktioner) sker mestadels i bäddzonen och därmed behöver modellen ta hänsyn till den komplexa fysiska interaktionen av biomassapartiklarna med fluidbäddsmediet (sand) och fluidiseringsgasen. Detta åstadkoms med hjälp av Euleriska faser och interaktionstermer, samt en lösare för hantering av styva reaktionssystem. Sekundärpyrolysen sker huvudsakligen utanför bäddzonen. Fluiddynamiken är enklare men kemin är mer komplex, med fler ämnen närvarande. Att tidseffektivt köra beräkningarna, för sekundärpyrolysen (134 ämnen, 4169 reaktioner) åstadkoms med hjälp Dimensionsreducering, Kemiagglomerering och In-situtabulering (ISAT); som implementerats i en sannolikhetstäthetsfunktion (PDF). En analys av de numeriska beräkningarna antyder att de kan matchas med experimentella resultat, med avseende på tryckprofil, temperaturprofil, utbyte av gasformiga, fasta och vätskeformiga produkter. Dessutom, med några undantag, kan beräkningarna matchas ganska väl med de viktigaste gasformiga produkterna. Därmed kan de huvudsakliga fysiska och kemikaliska mekanismerna representeras av modellen men förbättringar är givetvis möjliga. En parameterstudie av reaktionsatmosfärer (dvs fluidiseringsgaser) genomfördes, för att förstå processen bättre. Modellen visade på betydande effekter av atmosfären, fysisk (både under primär och sekundärpyrolys), och kemiskt (under sekundärpyrolysen).   Under primärpyrolysen undersöktes den fysiska inverkan av reaktionsatmosfärer (N2, H2O). När ånga jämfördes med kvävgas, visade det sig att värmeflödet sker mer homogent på både bäddnivå och på partikelnivå, med ångatmosfär. Under sekundärpyrolysen, så antyder resultaten på att turbulensinteraktion spelar en viktig roll för accelererad oönskad sekundärpyrolys av de vätskebildande ämnena. Ånga som är en av de undersökta atmosfärerna (N2, H2O, H2, CO, CO2), resulterade i den lägsta omfattningen av sekundärpyrolys. Dock så ledde en ångatmosfär varken till den lägsta residenstiden, den lägsta peaktemperaturen eller den lägsta radikalkoncentrationen; som alla normalt motverkar sekundärpyrolysen. Ett repeterat case, med hög turbulens i inloppet, gav betydande sekundärpyrolys av de vätskebildande ämnena. Attraktiviteten av approachen är given men mer testning och utveckling behövs, som också påkallats av andra forskare. Den fasta produkten efter pyrolys kallas träkol. Angående dess applicering i masugnar, så visar modelleringsresultaten att full substitution av fossilt kol går att göra. Betydande minskningar i koldioxidutsläpp är därmed möjliga. Energibesparingar är dessutom möjligt, tack vare det höga syreinnehållet i träkol (och biobränslen generellt), vilket ger större volymer av masugnsgas med högre värmevärde (och mindre sensibel värme som inte är utvinnbar). Energibesparingar är möjliga även om hänsyn tas till högre eleffekt för syrgasanrikning i blästerluften och en högre injektionsåtgång på energibasis. En översikt över biomassatillgången och existerande teknikleverantörer i Sverige, indikerar att all injektion i Masugn 3 (i Luleå) kan ersättas med biomassa. Baserat på statistik från 2008, så kan ersatt kol med träkol, minska de platsspecifika koldioxidutsläppen med 28.1 % (eller 17.3 % av alla utsläpp från stålindustrin). Som jämförelse kan torrifierad biomassa and obehandlad biomassa reducera utsläppen med 6.4 % respektive 5.7 %. / <p>QC 20150827</p>

A three-stage Pressurized Fluidized Bed Combustor (PFBC) of principal dimensions, O,4Sm internal diameter by4m high was designed and fabricated to burn South African coals, with particular reference to coals unsuitable for burning in conventional boilers. The combustor which is the first of its kind and probably one of very few operational PFBCs in the world, was made of three jacketed sections positioned vertically one above the other and bolted together at the flanges. Distributor plates were located at the flanges which gave the combustor a multistage capability. A three, two, or one deep Fluidized Bed (FB) configurations were possible by removing the interstage distributors. Interstage solids circulation was made possible by the use of downcomers transporting solids downwards between the FBs. The solids were returned to the top FB using a pneumatic conveyor. The design of the PFBC was a sequence to a series of experimental and theoretical investigations which were carried out in order to provide us with the necessary PFBC design parameters. These investigations dealt with the following areas of research: (a) the development of a new type of cyclonic tuyere capable of transmitting through it high quantities of solids with the fluidizing gas, without choking, (b) the transfer and control of the downward flow of solids through downcomer pipes, (c) the control of the circulation of solids in a Circulatory system using a non-mechanical solids flow control valve, (d) the development of a new type of start up burner which could operate immersed under the solids, and (e) the combustion of coal in a small FB under batch conditions and the study of reaction kinetics of South African coals. On the basis of the results of the investigation in these research areas and the findings of research of individuals and of .organizations working in the field of fluidization technology the PFBC was designed, built, and successfully commissioned. A series of 12 runs, with each run lasting between 2 and 8 days, totalling more than 1500 hours, were carried out on the PFBC. Char and coal with ash content between 30 and 70 per cent were burnt in the combustor using various combinations of feeding ports and number of FBs. System pressures ranged between atmospheric and 6 bar(abs). For some of the runs the reactor was operated in a counter-current mode with solids and combustibles descending against the upflowing fluidizing air in order to study the effect that counter-current flow had on the efficiency of combustion. The combustion trials showed that the two-FB combustor, operated preferably without solids circulation, with the bottom FB acting as the main combustion cell and the top FB as a smuts burn-out cell, proved to be the most practical and most suitable combustor for burning South African high ash coals and fines or, in general, any low-grade carbonaceous materials of any size. With this configuration combustion efficiencies of up to 99 per cent, based on the combustibles in the feed and the ash, were achieved. The department computer (COC1700) was successfully linked with the PFBC for real time data logging and data processing. A mathematical model which was based on our research findings and the work of T.P. Chen and S.C. Saxena, C. Fryer and O.E. Potter, and D. Levenspiel was successfully developed and applied to the twoFB PFBC. The model describes the devolatilization and combustion of coal particles in the FB in accordance with a shrinking core type model and uses a population balance over all particles for the overall mass balance. The results from this model, which was put onto the computer, compared favourably with the experimental results and the model can be confidently used to predict the behaviour of the PFBC. It can also be easily adapted for use on any other single or multifluidized bed reactors provided that the assumptions made for the derivation of this mathematical model still hold. A mathematical model based on the work of H.C. Hottel and A.F. Sarofim, and L. Wender and G.T. Copper was also developed. This model describes the transfer of heat from the FB to the cooling coils using a stepwise heat and mass balance along the length of the cooling coil. Although this mathematical model was developed specifically for the cooling coils of our combustor it is strongly believed that it can also form the basis of a general purpose model. / Thesis (Ph.D.)--University of Natal, 1981.