There are few experimental data of a fundamental nature that clearly demonstrate the similarities and differences in burning rates between single phase and two phase combustion, either in laminar or turbulent conditions. Such data are essential towards a better understanding of the spray combustion phenomena as well as a whole system. In the present study, experimental investigations of combustion of droplet and vapour air mixtures under quiescent and turbulence conditions have been conducted in a fan stirred combustion vessel. Aerosols were generated by expansion of gaseous pre-mixture to produce a homogeneously distributed suspension of fuel droplets. Spherically expanding flames following central ignition were employed to quantify the flame structure and propagation rate. The effect of droplets on flame propagation was investigated by comparing the burning rate of gaseous mixtures at initial pressure and temperature close to those of aerosol mixtures. In quiescent conditions, aerosols of two different fuels, isooctane and ethanol, were investigated at near atmospheric conditions. The effect of fuel droplets, up to 31 J.1m diameter, on laminar flame propagation was examined at a wide range of equivalence ratios. In the early stages of flame development, inertia of fuel droplets leads to local enrichment in equivalence ratio which increases the initial burning rate of lean aerosols but decreases that of rich ones. For the later stages of flame propagation, the presence of liquid droplets causes earlier onset of instabilities and cellularity than for gaseous flames, particularly at rich conditions. This leads to an enhanced burning rate and is probably due to heat loss from the flame and local disturbances due to droplet evaporation and subsequent diffusion processes. In turbulent studies, the effect of isooctane droplets up to 14 J.1m in diameter on flame propagation was examined at various values of root mean square turbulence velocities between zero and 4.0 mls. It is suggested that during early flame development, the turbulence was found to induce droplet motion before flame initiation which dominated over those resulting from the flame, negating the effect of droplet inertia. In the later stages, the presence of droplets in a low turbulent flame resulted in a significant burning rate enhancement. However, this enhancement became progressively less important as turbulent wrinkling became dominant. Between low and high turbulence, there was a transition regime between instability dominated and turbulence dominated regimes. As a consequence, the burning rate enhancement due to droplets under this transition range was rather complex.

Explosion tests were carried out in four medium-scale test-vessels incorporating closed, vented, duct vented and interconnected vessels. A systematic investigation into the influence of homogeneous and stratified mixtures was undertaken by varying mixture reactivity, ignition position, injection position and mixture composition. A feature of this work has been the similarities in explosion phenomena between stratified and homogeneous explosions and between partially filled and fully filled geometries to the conclusion that the explosion severity recorded in stratified mixtures towards the lean flammability limit was in many cases much higher than the fuel concentration would normally suggest. Stratified mixtures with global equivalence ratio around stoichiometric produced significantly lower pressures than their homogeneous equivalents. However, stratified (globally) near-limit mixtures produced overpressures that were several hundred mbar higher than those of the equivalent homogeneous mixtures. Even beyond the flammable range (globally) the stratified mixtures produced significant overpressures. The phenomena discussed in this thesis illustrate the difficulty in designing adequate protection for such vented, duct vented and interconnected geometries, since even relatively small pocket of weak fuel-air mixtures produced relatively severe explosions. This can have implications for the safety design of inter-connected installations which are not intended to be subject to flammable mixtures. While it is an important conclusion from the work presented in this chapter that close to the flammability limits the stratified explosion severity was greater than its global concentration would normally indicate, it should be stressed that homogeneous stoichiometric tests still constitute the worst case tests. Therefore, it is not the suggestion of this work that the design of vented vessels should be modified to represent the maxima obtained in stratified work. However, the value of this research in the field of post-explosion investigation is clear.

Future energy supplies, as a result of governmental policy and environmental legislation, will increasingly be borne from renewable sources. Use of biomass to provide fuels and chemicals to replace those derived from coal and oil will be crucial in providing diverse, sustainable and secure supplies for years to come. Production of the full range of solid, liquid and gaseous fuels from biomass to replace those from non-renewable sources is achievable, but not in the quantities currently consumed at present, hence demand will increase with an ever-expanding world population, coupled with competing markets for food production. Biomass used as fuel has environmental benefits, with CO2 emissions being reduced, however, because biomass energy density is much lower than coal or oil, more prone to microbial degradation and because many biomass sources have high water content, transportation is much more expensive, therefore energy densification techniques are required to overcome these hurdles. Various thermal technologies exist to upgrade biomass to fuels; these include gasification, pyrolysis, anaerobic digestion, hydrothermal processing and torrefaction. Hydrothermal processing (HTP) is an environmentally benign method of energy densification and can be used to produce directly liquid and solid fuels and indirectly gaseous fuels. The reactions are carried out in hot, compressed water in temperatures of 160 – 400°C for reaction times ranging from seconds to days. Benefits of HTP reactions include high energy density liquid and solid fuels, low gaseous emissions and high product yields are also claimed. For oil production, homogeneous alkali catalysts have been used with high returns with equally high temperatures. Biological components of energy crops biomass comprise largely of cellulose, hemicellulose and lignin, with differing ratios and types between plant species. Taking each individual lignocellulosic component and reacting in isolation presents expected data from each when reacting as a whole, allowing comparisons to be drawn with ‘raw’ biomass, Miscanthus and willow in this study. Oil formation under HTL conditions with alkali catalyst was deemed to be the best fuel product, but yields were lower than expected and the catalyst concentration was brought into question. Another surprising development was the formation of oil from lignin under these conditions, though the process of lignin removal may account for this phenomenon. LC-MS analysis of HTL derived aqueous phase lignin indicated the presence of high molecular weight polymers, heteroatomic and substituted polyaromatic compounds, with λmax outside the rage of 190 – 400nm scanned and notable by their absence. Results from the factorial study in Chapter 5 showed that of all the reaction conditions tested (temperature, reaction time and catalyst) the greatest effect on all lignocellulosic compounds was temperature. Though it was expected that cellulose and xylan would behave in a similar manner due to the likeness of their polymeric composition, this was not the case for many responses compared. At the conditions in this study, lignin was found to be the least affected by any of the variables. Overall, the use of catalyst, though beneficial for increasing the calorific content and yield of cellulose oil, had a detrimental effect on the other components. KOH catalyst reduced aqueous phase acidity through formation of buffers, but post calculation, xylan was found to have produced more acidic species. Biomass energy crops Miscanthus and willow were reacted in Chapter 6. Evident from this data are similarities with individual biomass components found earlier. Hemicellulose content of Miscanthus was the reason for greater solubility, though with increasing reaction times, rate limiting condensation polymerisation reaction took place to increase char yields. The most energy efficient hydrothermal conversion method was HTC, with catalyst use only beneficial for oil production. Analysis of chars by AES indicated lower alkali metals concentrations than raw analysis (K, Na, Mg, Ca). Aqueous phases contained high concentrations of these leached metals. Implications are reduction of inorganic matter improves fuel quality and less likelihood of combustion boiler problems. The final part of the study was comparison of HTC and torrefaction. Information from literature sources and pilot plant produced torrefied materials were compared and contrasted. Drying biomass is the largest processing requirement for this process. Suitability of feedstock is largely dependent therefore on water content. Similarities are evident between products and energy yields, but torrefaction is much closer to commercial realisation due to technological advancements needed for large scale HTP systems.

In this thesis, a model that couples a reduced alkali kinetic mechanism for alkali sulphate formation during biomass combustion with an ash deposition model using computational fluid dynamics (CFD) techniques has been presented. Starting with a detailed gas-phase kinetic mechanism for the alkali chemistry, a systematic reduction procedure has been performed using a sensitivity analysis to reduce the reaction mechanism to a level that can be implemented into a CFD calculation. An ash deposition model that takes into consideration the ash-sticking probability and the condensation of potassium salts has been developed. The reduced mechanism and the deposition model developed are implemented into a CFD model to predict ash depositions in a 10 MWth biomass grate furnace. Also, a CFD model to predict the deposition rates for the co-combustion of coal with biomass has been developed. This deposition model is based on the combined sticking probabilities of the ash particle viscosity and the melting behaviour of the ash particles. A Numerical Slagging Index (NSI) is also employed to estimate the degree of the sintering of the deposits. Experimental data from the Entrained Flow Reactor (EFR) at Imperial College, London, have been used to validate the models. The predicted results from both the ash deposition models agreed with the experimental measurements, and the NSI has successfully ranked the investigated coal-biomass mixtures according to their degree of sintering.

Biomass combustion with carbon capture and storage (Bio-CCS) has been identified as a key contributing technology to long-term carbon emissions reductions in many global and UK scenarios, but the development and implementation of this technology would require significant technical and non-technical barriers to be overcome. This thesis describes research undertaken to address these barriers, to contribute to the role that Bio-CCS could play in reducing carbon emissions for the UK electricity sector. Technical studies investigate the characteristics of coal and biomass combustion in atmospheres relevant to CCS at bench-scale and in a 20kW furnace. Analysis of bench-scale results, using a modified Coats-Redfern procedure, suggests oxygen-enrichment increases reactivity during the breakdown of cellulosic material and char oxidation. At 20kW scale, experiments that investigate biomass blending ratio and extent of oxidant staging conclude that, compared to air-firing of coal, cofiring in oxygen-enriched, oxidant-staged conditions results in enhanced combustion, reduced NO emissions and a flue gas richer in CO2. Cofiring of coal with 15% biomass is also carried out in partial-oxyfuel combustion atmospheres. The results suggest no major technical issues, showing biomass cofiring and oxygen-enrichment counter reductions in reactivity due to higher CO2 concentrations. However, further tests show that dedicated firing of biomass in such conditions would likely require modifications to the combustion set up. The development of an industry depends on more than its level of technological readiness. Modelled as a technical innovation system, development of Bio-CCS is gauged from the results of an expert survey on the wider UK CCS industry and analysis of relevant publications. Findings show that, as well as biomass sustainability criteria and a need to reward negative emission processes, the development of Bio-CCS is dependent on the wider CCS industry in the UK which, driven by uncertainties, has suffered due to a lack of market creation and entrepreneurship.

Measurements are reported of premixed hydrogen-air turbulent burning velocities, made by the double kernel method during explosions. Turbulence was created by four high speed fans driven by electric motors within the explosion vessel. This arrangement created a central region of uniform, isotropic turbulence in which all measurements were made. The ratio of turbulent to laminar burning velocity correlates well with both the turbulent Reynolds number of the reactants and the ratio of laminar burning velocity to r. m. s. turbulent velocity. The use of hydrogen-air mixtures has extended the data on premixed turbulent combustion to regimes with higher values of the last dimensionless ratio. At high values of the ratio there is evidence of a wrinkled laminar flame structure, but at lower values a small scale eddy structure seems to be dominant. A two eddy theory of turbulent combustion is presented. This rests-upon the assumption, supported by a good deal of experimental evidence, that two scales of eddy are particularly important. One is associated with the integral scale of turbulence, the other with the Kolmogorov microscale. It is assumed that all the material in the large eddies is used in the formation of the smaller dissipative eddies. It is assumed that laminar flame propagation occurs through the large eddies, whilst two approaches are considered in the case of dissipative eddies. In the first approach, laminar flame propagation across a vortex tube is employed, whilst in the second the concept of reaction time in the vortex tube is used. It is shown that the rate of burning in small eddies can be many times greater than that in large eddies. Theoretical values are obtained for the ratio of turbulent to laminar burning velocity, in terms of turbulent Reynolds number and the ratio of laminar burning velocity to r. m. s. turbulent velocity. These are in fair agreement with experimental values, but more data are required on the intermittency and chemical lifetimes of small eddies. Experiments are reported on the effect of turbulence upon flammability limits. These are narrowed as turbulence increases, but counter-action may be taken by increasing the spark iginition energy and by establishing the initial flame in a shielded region where the turbulence is reduced. The relevance of the theory to these results is discussed. Finally, the application of these findings in practical combustion chambers is discussed

The thesis comprises a fundamental study of spherical premixed flame propagation,originating at a point under both laminar and turbulent propagation. Schlieren cine photography has been employed to study laminar flame propagation, while planar mie scattering (PMS) has elucidated important aspects of turbulent flame propagation. Thrbulent flame curvature has also been studied using planar laser induced fluorescence (PLIF) images. Spherically expanding flames propagating at constant pressure have been employed to determine the unstretched laminar burning velocity and the effect of flame stretch, quantified by the associated Markstein lengths. Methane-air mixtures at initial temperatures between 300 and 400 K, and pressures between 0.1 and 1.0 MPa have been studied at equivalence ratios of 0.8, 1.0 and 1.2. Values of unstretched laminar burning velocity are correlated as functions of pressure, temperature and equivalence ratio. Two definitions of laminar burning velocity and their response to stretch due to curvature and flow strain are explored. Experimental results are compared with two sets of modeled predictions; one model considers the propagation of a spherically expanding flame using a reduced mechanism and the second considers a one dimensional flame using a full kinetic scheme. Data from the present experiments and computations are compared with those reported elsewhere. Comparisons are made with iso-octane-air mixtures and the contrast between fuels lighter and heavier than air is emphasized. Flame instability in laminar flame propagation become more pronounced at higher pressures, especially for lean and stoichiometric methane-air mixtures. Critical Peclet numbers for the onset of cellularity have been measured and related to the appropriate Markstein number. Analyses using flame photography clearly show the flame to accelerate as the instability develops, giving rise to a cellular flame structure. The underlying laws controlling the flame speed as cellularity develops have been explored. PMS images have been analysed to obtain the distributions of burned and unburned gas in turbulent flames. These have enabled turbulent burning velocities to be derived for stoichiometric methane-air at different turbulent r.m.s. velocities and initial pressures of 0.1 MPa and 0.5 MPa. A variety of ways of defining the turbulent burning velocity have been fruitfully explored. Relationships between these different burning velocities are deduced and their relationship with the turbulent flame speed derived. The deduced relationships have also been verified experimentally. Finally, distributions of flame curvature in turbulent flames have been measured experimentally using PMS and PLIF. The variance of the distribution increases with increase in the r.m.s. turbulent velocity and decrease in the Markstein number. Reasons for these effects are suggested.

Sustainability, security of supply, and diversity, as well as economic competitiveness are key components of energy policy. There is increasingly stringent legislation on the environmental impact of energy production, and there is growing pressure to reduce not just NOx and SOx emissions, but also C02 emissions. For both heating and electricity production it is likely that the plants will need to be fuel-flexible and could use one or more of several different feedstocks, for example coal and biomass. When coal is co-utilized with biomass there is added attractiveness because the biomass is C02 neutral, and there is interest in using wood waste, short rotation woody crops (e. g. willow coppice), or herbaceous crops (e. g. Miscanthus), refuse and waste derived fuels, or wastes such as sewage sludge or chicken litter. The co-utilisation of coal and biomass for heat and/or energy production results in pollutant reduction. Most notable is the impact on the emission of NOx, SOx, volatile organic compounds and polyaromatic hydrocarbons. These latter compounds largely arise from their formation and release during incomplete combustion/gasification. There is evidence that co-firing or co-gasifying coal and biomass results in a significant decrease in the emission of these compared to coal alone. The synergistic activity observed for toxic organic emissions is not well understood and is thought to involve chemical interaction between the volatiles from each fuel coupled with possible catalytic activity from the inorganic constituents of the fuels. Laboratory scale data on synergies in co-pyrolysis is conflicting. Characterisation of co-pyrolysis products from coal and biomass pyrolysis has received limited attention and the data is conflicting. Therefore this thesis seeks to understand possible interactions occurring during co-combustion and co-pyrolysis of fuels and looks at a number of variables, including coal rank, biomass type (with different amounts of catalytic components), heating rate, residence time and the physical form of the fuels. A better understanding of the factors influencing non-additive interactions may lead to optimization of the blending process and minimisation of toxic organic emissions. This work is of particular relevance to fixed bed and fluidised bed processes where the bed temperature is ca. 1000 'C (or there is a temperature profile through the bed). In these cases particle heating and pyrolysis occurs relatively slowly and interactions between the volatiles can take place. While studying the co-pyrolysis, thermogravimetry, batch pyrolysis and pyroprobe-GC/(MS or FID) were used. In addition, apart from the traditionaltechniques, this study aimed to develop a new technique - heated wire mesh pyrolysis coupled to a GUMS via a probe, which can sample at varying heights from the pyrolysing fuel, and these findings were complemented by the pyrolysis-GC/MS studies of the fuels. These studies suggest that biomass type can lead to a small change of the rate of the coal pyrolysis. Thus, slight synergistic effects were seen for the TGA study, where co-pyrolysed coals in blends often had lower peak temperatures compared to the coal alone, and higher volatile matter yields were produced. Analysis of the gases evolved were consistent with higher gas yields. This effect was present for certain biomass (e. g. oat straw) even after minerals were removed, and so this is not purely the result of catalytic ash components. For combustion studies two techniques were applied. Low heating rate was obtained in a TGA analyser. The high heating rate experiments were performed on pellets exposed to the flame of Meker-type burner. This combustion process was recorded with a high speed frame video recording system. These studies showed that strong synergy can be observed. The TGA combustion revealed the importance of the catalytic elements, particularly potassium, and showed that, ignition of biomass char in the blend aids the ignition of the coal char. As a result, mixtures reach maximum temperatures faster, than seen for the separate fuels. In many cases though, the char burn-out of the blends lasted a similar time to the coals alone. The combustion tests of stationary pellets revealed no pattern for the ignition delay, but exposed strong synergy in volatile combustion, indicating that for pellets of untreated fuel blends the combustion events are dominated by the coal behaviour i. e. the addition of demineralised biomass to the pellet, made it burn in a very similar way to coal alone. The synergy observed in the organic emissions during the combustion of coal and biomass in small appliances is not simply due to interactions of hot volatiles from coal and biomass above the combustion bed. Co-pyrolysis studies suggest that biomass type can lead to a small effect on the rate of the coal pyrolysis, and on the total volatile matter released, but that there are no major changes in the nature of the volatiles. Combustion studies indicate that synergy stronger than seen for pyrolysis tests can be observed, and the coal ignites and burns at lower temperature as a result of the earlier ignition and combustion of the biomass. The overall combustion time is still dominated by the coal char burn-out. Thus, synergy in emission reduction in the co-utilisation of coal and biomass is not simply due to interactions of volatiles in the vapour phase, rather, the processes of pyrolysis and combustion are linked and as such need to be studied together.

The chemistries of aviation fuels are invariably complex due to large hydrocarbon molecules. There are also large variations for a given fuel type. Furthermore, flow timescales encountered in high performance propulsion devices increasingly lead to difficulties associated with kinetically controlled or influenced phenomena such as flame stability, extinction and re-light. Current indications also suggest that fuel sources will become significantly more diverse in the future and may, for example, encompass Fischer-Tropsch and/or bio-derived components. The combustion properties of such fuels can vary significantly from those in current use and this work outlines a route towards surrogate fuel mechanisms of sufficient accuracy and generality to support the development of practical devices. A reaction class based route to the derivation of detailed chemical kinetic mechanisms for alkyl-substituted aromatics is outlined and applied to the cyclopentadiene/indene, benzene/naphthalene, toluene/1-methyl naphthalene systems. Work has also been extended to the n-propyl benzene system as well. These reaction classes were applied to model the oxidation of the above fuels with encouraging results. Important reaction channels during oxidation were identified and specifically, the methyl groups on aromatic rings have been identified as important in the context of radical scavenging. Furthermore, 1-methyl naphthalene may also be used to modulate sooting tendencies in aviation and Diesel surrogates. Results obtained from chemical kinetic modelling of cyclopentadiene, toluene, npropyl benzene, naphthalene and 1-methyl naphthalene oxidation in shock tubes, jet-stirred and plug-flow reactors at various sets of representative stoichiometries and temperatures are reported.

The current study focuses on the modelling of turbulent premixed or partially premixed flames over a wide range of combustion regimes using various fuels. Opposed flows featuring fractal-generated turbulence are examined using different classes of models. The reacting case is in the flamelet regime of combustion and two-scalar joint β -bimodal presumed PDF and transported PDF approaches are applied for scalar statistics. In the isothermal case the k − ε model works comparatively well, in contrast to previous studies, while in the reacting case the second moment closures are outperforming the eddy viscosity based closures. The transported PDF approach indicates an under-prediction of the turbulent burning velocity in this flow. The latter approach is therefore applied to compute freely propagating turbulent premixed flames using comprehensive chemistry. Turbulent burning velocities are extracted and compared with experimental data. The computed cases are covering the laminar flamelet to the distributed reaction zone regime. The mixture reactivity is also varied through different fuel/air mixtures and explored in terms of the Zeldovich number. The fuel/air composition studied include fuel-lean CH4, stoichiometric CH4 and C2H6 and fuel-rich H2 mixtures. The impact of molecular transport is investigated through the inclusion of an explicit analytical formulation. A multi-scale scalar dissipation rate closure that accounts for the influence of the Da number is extended in a simple manner to include Le number effects. An industrial swirl-stabilised partially premixed fuel-rich CH4 flame is simulated at realistic gas turbine conditions using the node-based Eulerian transported PDF approach coupled with a second moment closure for the velocity field. The case is in the well stirred reactor regime and the chemical kinetics is modelled using a global reaction scheme for hydrocarbon combustion. The flow field is initialised and compared with the predictions of the two-scalar joint β-bimodal presumed PDF approach.