Integrated bio-refineries offer a potential for a more sustainable production of fuels and chemicals. However, the sustainability implications of integrated bio-refineries are still poorly understood. Therefore, this work aims to contribute towards a better understanding of the sustainability of these systems. For these purposes, a methodological framework has been developed to assess the sustainability of different 2nd generation feedstocks to produce bio-ethanol, energy, and platform chemicals using bio-chemical or thermo-chemical routes in an integrated bio-refinery.The methodology involves environmental, techno-economic, and social assessment of the bio-refinery supply chain. Life cycle assessment (LCA) is used for the environmental assessment. The economic assessment is carried out using life cycle costing (LCC) along side traditional economic indicators such as net present value and payback period. Social issues such as employment provision and health and safety are considered within the social sustainability assessment. The methodology has been applied to two case studies using the bio-chemical and the thermo-chemical conversion routes and four feedstocks: wheat straw, poplar, miscanthus and forest residue.For the conditions assumed in this work and per litre of ethanol produced, the LCA results indicate that the thermo-chemical conversion is more environmentally sustainable than the bio-chemical route for eight out of 11 environmental impacts considered. The LCA results also indicate that the main hot spot in the supply chain for both conversion routes is feedstock cultivation. The thermo-chemical route is economically more sustainable than the bio-chemical because of the lower capital and operating costs. From the social sustainability point of view, the results suggest that provision of employment would be higher in the bio-chemical route but so would the health and safety risks.

In this research we are following the thermo-chemical degradation of wood in the absence of oxygen. The objectives are to evaluate the influence of heating rates on pyrolysis products obtained from wood pyrolysis and to evaluate the influence of acid pre-treatment on pyrolysis products. Depending on the wood heating rates, pyrolysis can be categorized as Flash pyrolysis, Fast pyrolysis, and Slow pyrolysis. We have evaluated the volatile products obtained at different heating rates and the volatile products obtained from sulfuric acid pre-treatment by using gas chromatography- mass spectrometry (GC-MS). We have also performed thermo-gravimetric analysis (TGA) of raw wood samples and sulfuric acid pre-treated wood samples of Yellow Pine to determine the changes in weight in relation to change in temperature. Our results indicated that by using the Flash, Fast, and Slow heating rates, the overall volatile products obtained from wood pyrolysis (i.e. the overall list of all the compounds obtained from different temperature ranges in wood pyrolysis by using different heating rates) were the same, but the volatile products obtained at different temperature ranges like Room temperature-300°C, 300°C - 400°C, and 400°C -500°C in Flash, Fast, and Slow pyrolysis were different. Most of the volatile products obtained from the pyrolysis of untreated wood were phenols. Our results also indicated that the pretreatment of wood with sulfuric acid alters the charcoal properties and releases gaseous products including furan derivatives that are useful as fuels or fuel additives. The sulfuric acid (10%) pretreatment of wood followed by slow pyrolysis produced maximum yield of charcoal, indicated by the lowest mass % decrease of 58.234. The production of furan derivatives increased by using sulfuric acid pre-treatment, which is a good improvement for the production of Furanics, the furan based biofuels. The furan based biofuels are of increasing research interest because of their significant advantages over the first generation biofuels. The thermogravimetric analysis (TGA) results indicated that the acid pre-treatment altered the decomposition rate of pyrolysis and lowered the onset of temperature for decomposition. The use of thermal degradation of plants for creating chemicals and fuels is seeing renewed interest across the globe as it is considered carbon-neutral and it uses a renewable feedstock. The information obtained from this research work will also be valued by industries, such as charcoal and activated carbon producers, which currently perform biomass pyrolysis, by allowing them to form approaches that optimize their energy use and minimize waste.

The heat flux demands for electronics cooling applications are quickly approaching the limits of conventional thermal management systems. To meet the demand of next generation electronics, a means for rejecting high heat fluxes at low temperatures in a compact system is an urgent need. To answer this challenge, in this work a gasketed chevron plate heat exchanger in conjunction with a slurry consisting of highly endothermic solid ammonium carbamate and a heat transfer fluid. A reduced-order 1-dimensional model was developed and used to solve the coupled equations for heat, mass, and momentum transfer. The feasibility of this chosen design for satisfying the heat rejection load of 2kW was also explored in this study. Also, a decomposition reaction using acetic acid and sodium bicarbonate was conducted in a plate heat exchanger (to simulate a configuration similar to the ammonium carbamate reactions). This enabled the experimental validation of the numerical predictions for the momentum transfer correlations used in this study (which in turn, are closely tied to both the heat transfer correlations and chemical kinetics models). These experiments also reveal important parameters of interest that are required for the reactor design. A numerical model was developed in this study and applied for estimating the reactor size required for achieving a power rating of 2 kW. It was found that this goal could be achieved with a plate heat exchanger weighing less than 70 kg (~100 lbs) and occupying a volume of 29 L (which is roughly the size of a typical desktop printer). Investigation of the hydrodynamic phenomena using flow visualization studies showed that the flow patterns were similar to those described in previous studies. This justified the adaptation of empirical correlations involving two-phase multipliers that were developed for air-water two-phase flows. High-speed video confirmed the absence of heterogeneous flow patterns and the prevalence of bubbly flow with bubble sizes typically less than 0.5 mm, which justifies the use of homogenous flow based correlations for vigorous gas-producing reactions inside a plate heat exchanger. Absolute pressure measurements - performed for experimental validation studies - indicate a significant rise in back pressure that are observed to be several times greater than the theoretically estimated values of frictional and gravitational pressure losses. The predictions from the numerical model were found to be consistent with the experimental measurements, with an average absolute error of ~26%

This work aims on the investigation of factors influencing the discharge characteristicsof a heat storage system, which is based on the reversible reaction system of Ca(OH)2and CaO. As storage, a packed bed reactor with embedded plate heat exchanger forindirect heat transfer is considered. The storage system was studied theoretically bymeans of finite element analysis of a corresponding mathematical model. Parametricstudies were carried out to determine the influence of reactor design and operationalmode on storage discharge. Analysis showed that heat and gas transport throughthe reaction bed as well as the heat capacity rate of the heat transfer fluid affect thedischarge characteristics to a great extent. To obtain favourable characteristics interms of the fraction of energy which can be extracted at rated power, a reaction frontperpendicular to the flow direction of the heat transfer fluid has to develop. Such afront arises for small bed dimensions in the main direction of heat transport withinthe bed and for low heat capacity rates of the heat transfer fluid. Depending on thedesign parameters, volumetric energy densities of up to 309 kWh/m3 were calculatedfor a storage system with 10 kW rated power output and a temperature increase ofthe heat transfer fluid of 100 K. Given these findings, this study is the basis for thedimensioning and design of a pilot scale heat exchanger reactor and will help toevaluate the technical feasibility of thermo-chemical heat storage systems.

Dans le futur, différentes sources d'énergies renouvelables comme les énergies de seconde génération produites à partir de déchets lignocellulosiques seront nécessaires pour palier à l'épuisement des énergies fossiles. Parmi ces énergies de seconde génération, le biohydrogène, le méthane et l'hythane produits à partir de procédés fermentaires anaérobies représentent des alternatives prometteuses. Cependant la production de biohydrogène et de méthane à partir de résidus lignocellulosiques est limitée par leurs structures récalcitrantes et une étape de prétraitement en amont des procédés fermentaires est souvent nécessaire. Ce travail a pour but d'étudier l'impact des facteurs biochimiques et structurels des résidus lignocellulosiques sur les performances de production d'hydrogène et de méthane, pour pouvoir par la suite développer des stratégies de prétaitements adaptées. Tout d'abord, sur un panel de vingt substrats lignocellulosiques, les potentiels hydrogène et méthane ont été corrélés aux paramètres biochimiques et structurels. Les résultats ont mis en évidence que le potentiel hydrogène est uniquement corrélé positivement à la teneur en sucres solubles. La production de méthane quant à elle est négativement corrélée à la teneur en lignine et, à un moindre degré, à la cristallinité de la cellulose, mais positivement à la teneur en sucres solubles, holocelluloses amorphes et protéines. Par la suite, des stratégies de prétraitements ont été établies pour améliorer la production d'hydrogène et de méthane. Le couplage prétaitements alcalins/enzymatique ainsi que les prétraitements à l'acide dilué, efficaces pour solubiliser les holocelluloses en sucres solubles ont été appliqués en amont de la production d'hydrogène. En combinant le pretraitement alcalin avec une hydrolyse enzymatique, le potentiel hydrogène des tiges de tournesol fut multiplié par quinze. En revanche, suite aux prétraitements acides, la production d'hydrogène fut inhibée à cause de la libération de sous-produits (furfural, 5-HMF et composés phénoliques) engendrant un changement d'espèces bactériennes vers des espèces non productrices d'hydrogène. Pour la production de méthane, cinq prétraitements thermo-chimiques (NaOH, H2O2, Ca(OH)2, HCl and FeCl3) efficaces pour délignifier ou solubiliser les holocelluloses ont été étudiés. Parmi ces prétraitements, la meilleure condition fut 55°C à une concentration de 4% NaOH pendant 24 h, résulant en une augmentation du potentiel méthane variant de 29 à 44 % en fonction des tiges de tournesol. Cette condition fut par la suite validée en réacteurs anaérobies continusavec une augmentation de 26.5% de la production de méthane. Un procédé à deux étages couplant la production d'hydrogène en batch suivi de la production de méthane en continu fut aussi étudié. Néanmoins, aucune différence significative en termes d'énergie produite ne fut observée entre les procédés à deux étages (H2/CH4) et à un étage (CH4). / In the future, various forms of renewable energy, such as second generation biofuels from lignocellulosic residues, will be required to replace fossil fuels. Among these, biohydrogen and methane produced through fermentative processes appear as interesting candidates. However, biohydrogen and/or methane production of lignocellulosic residues is often limited by the recalcitrant structure and a pretreatment step prior to fermentative processes is often required. Up to date, informations on lignocellulosic characteristics limiting both hydrogen and methane production are limited.Therefore, this work aims to investigate the effect of compositional and structural features of lignocellulosic residues on biohydrogen and methane performances for further developping appropriate pretreatments strategies. Firstly, a panel of twenty lignocellulosic residues was used to correlate both hydrogen and methane potentials with the compositional and structural characteristics. The results showed that hydrogen potential positively correlated with soluble carbohydrates only. Secondly, methane potential correlated negatively with lignin content and, in a lesser extent, with crystalline cellulose, but positively with the soluble carbohydrates, amorphous holocelluloses and protein contents. Pretreatments strategies were further developed to enhance both hydrogen and methane production of sunflower stalks. Dilute-acid and combined alkaline-enzymatic pretreatments, which were found efficient in solubilizing holocelluloses into soluble carbohydrates, were applied prior to biohydrogen potential tests. By combined alkaline-enzymatic pretreatment, hydrogen potential was fifteen times more than that of untreated samples. On the contrary, hydrogen production was inhibited after dilute-acid pretreatments due to the release of byproducts (furfural, 5-HMF and phenolic compounds) that led to microbial communities shift toward no hydrogen producing bacteria. Similarly, methane production, five thermo-chemical pretreatments (NaOH, H2O2, Ca(OH)2, HCl and FeCl3) found efficient in delignification or solubilization of holocelluloses, were considered. Among these pretreatments, the best conditions were 55°C with 4% NaOH for 24 h and led to an increase of 29-44 % in methane potential of sunflower stalks. This pretreatment condition was validated in one stage anaerobic mesophilic continuous digester for methane production and was found efficient to enhance from 26.5% the total energy produced compared to one stage-CH4 alone. Two-stage H2 (batch) / CH4 (continuous) process was also investigated. Nevertheless, in term of energy produced, no significant differences were observed between one-stage CH4 and two-stage H2 /CH4.

In this dissertation the heat flux prediction capabilities of Residual Distribution (RD) schemes for hypersonic flow fields are investigated. Two canonical configurations are considered: the flat plate and the blunt body (cylinder) problems, with a preference for the last one. Both simple perfect gas and more complex thermo-chemical non-equilibrium (TCNEQ) thermodynamic models have been considered.<p><p>The unexpected results identified early in the investigation lead to a thorough analysis to identify the causes of the unphysical hypersonic heating.<p><p>The first step taken is the assessment of the quality of flow field and heat transfer predictions obtained with RD methods for subsonic configurations. The result is positive, both for flat plate and cylinder configurations, as RD schemes produce accurate flow solutions and heat flux predictions whenever no shock waves are present, irrespective of the gas model employed.<p><p>Subsonic results prove that hypersonic heating anomalies are a consequence of the presence of a shock wave in the domain and/or the way it is handled numerically.<p><p>Regarding hypersonic flows, the carbuncle instability is discarded first as the cause of the erroneous stagnation heating. The anomalies are shown next to be insensitive to the kind and level of dissipation introduced via the (quasi-)positive contribution P to blended B schemes. Additionally, insufficient mesh resolution locally over the region where the shock wave is captured numerically is found to be irrelevant.<p><p>Capturing the bow shock in a manner that total enthalpy is preserved immediately before and after the numerical shock wave is, on the contrary, important for correct heating prediction.<p><p>However, a carefully conceived shock capturing term is, by itself, not sufficient to guarantee correct heating predictions, since the LP scheme employed (be it stand-alone in a shock fitting context or combined into a blended scheme for a shock capturing computation) needs to be immune to spurious recirculations in the stagnation point. <p><p>Once the causes inducing the heating anomalies identified, hypersonic shocked flows in TCNEQ conditions are studied.<p><p>In order to alleviate the computational effort necessary to handle many species non-equilibrium (NEQ) models, the extension of an entropic (or symmetrizing) variables formulation RD to the nS species, two temperature TCNEQ model is accomplished, and the savings in computational time it allows are demonstrated.<p><p>The multi-dimensional generalization of Roe-like linearizations for the TCNEQ model is addressed next: a study on the existence conditions of the linearized state guaranteeing discrete conservation is conducted.<p><p>Finally, the new dissipative terms derived for perfect gas are adapted to work under TCNEQ conditions; the resulting numerical schemes are free of the temperature undershoot and Mach number overshoot problem afflicting standard CRD schemes. / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

The present work focuses on the study of biomass gasification process for generating hydrogen rich synthetic gas with oxy-steam as reactants using experiments and modeling studies. Utilization of the syngas as a fuel in general applications like fuel cells, Fischer-Tropsch FT) process and production of various chemicals like DME, etc. are being considered to meet the demand for clean energy. This study comprises of experiments using an open top down draft reactor with oxygen and steam as reactants in the co-current configuration. Apart from the standard gasification performance evaluation; parametric study using equivalence ratio, steam-to-biomass ratio as major variables towards generation of syngas is addressed towards controlling H2/CO ratio. The gasification process is modeled as a packed bed reactor to predict the exit gas composition, propagation rate, bed temperature as a function of input reactants, temperature and mass flux with variation in thermo-physical properties of biomass. These results are compared with the present experiments as well as those in literature. Experiments are conducted using modified open top downdraft configuration reactor with lock hoppers and provision for oxy-steam injection, and the exit gas is connected to the cooling and cleaning system. The fully instrumented system is used to measure bed temperatures, steam and exit gas temperature, pressures at various locations, flow rates of fuel, reactants and product gas along with the gas composition. Preliminary investigations focused on using air as the reactant and towards establishing the packed bed performance by comparing with the experimental results from the literature and extended the study to O2-N2 mixtures. The study focuses on determining the propagation rate of the flame front in the packed bed reactor for various operating conditions. O2 is varied between 20-100% (vol.) in a mixture of O2-N2 to study the effect of O2 fraction on flame propagation rate and biomass conversion. With the increase in O2 fraction, the propagation rates are found to be very high and reaching over 10 mm/s, resulting in incomplete pyrolysis and poor biomass conversion. The flame propagation rate is found to vary with oxygen volume fraction as XO22.5, and stable operation is achieved with O2 fraction below 30%. Towards introducing H2O as a reactant for enhancing the hydrogen content in the syngas and also to reduce the propagation rates at higher ER, wet biomass is used. Stable operating conditions are achieved using wet biomass with moisture-to-biomass (H2O:Biomass) ratio between 0.6 to 1.1 (mass basis) and H2 yield up to 63 g/kg of dry biomass amounting to 33% volume fraction in the syngas. Identifying the limitation on the hydrogen yield and the criticality of achieving high quality gas; oxy-steam mixture is introduced as reactants with dry biomass as fuel. An electric boiler along with a superheater is used to generate superheated steam upto 700 K and pressure in the range of 0.4 MPa. Steam-to-biomass ratio (SBR) and ER is varied with towards generating hydrogen rich syngas with sustained continuous operation of oxy-steam gasification of dry biomass. The results are analysed with the variation of SBR for flame propagation rates, calorific value of product syngas, energy efficiency, H2 yield per kg of biomass and H2/CO ratio. Hydrogen yield of 104 g per kg of dry casuarina wood is achieved amounting to 50.5% volume fraction in dry syngas through oxy-steam gasification process compared to air gasification hydrogen yield of about 40 g per kg of fuel and 20% volume fraction. First and second law analysis for energy and exergy efficiency evaluation has been performed on the experimental results and compared with air gasification. Individual components of the energy input and output are analysed and discussed. H2 yield is found to increase with SBR with the reduction in energy density of syngas and also energy efficiency. Highest energy efficiency of 80.3% has been achieved at SBR of 0.75 (on molar basis) with H2 yield of 66 g/kg of biomass and LHV of 8.9 MJ/Nm3; whereas H2 yield of 104 g/kg of biomass is achieved at SBR of 2.7 with the lower efficiency of 65.6% and LHV of 7.4 MJ/Nm3. The energy density of the syngas achieved in the present study is roughly double compared to the LHV of typical product gas with air gasification. Elemental mass balance technique has been employed to identify carbon boundary at an SBR of 1.5. Controlling parameters for arriving at the desired H2/CO ratio in the product syngas have been identified. Optimum process parameters (ER and SBR) has been identified through experimental studies for sustained continuous oxy-steam gasification process, maximizing H2 yield, controlling the H2/CO ratio, high energy efficiency and high energy density in the product syngas. Increase in ER with SBR is required to compensate the reduction in O2 fraction in oxy-steam mixture and to maintain the desired bed temperature in the combustion zone. In the range of SBR of 0.75 to 2.7, ER requirement increases from 0.18 to 0.3. The sustained continuous operation is possible upto SBR of 1.5, till the carbon boundary is reached. Operating at high SBR is required for high H2 yield but sustained highest H2 yield is obtained as SBR of 1.5. H2/CO ratio in the syngas increases from 1.5 to 4 with the SBR and depending on the requirement of the downstream process (eg., FT synthesis), suitable SBR and ER combination is suggested. To obtain high energy density in syngas and high energy efficiency, operations at lower SBR is recommended. The modeling study is the extension of the work carried by Dasappa (1999) by incorporating wood pyrolysis model into the single particle and volatile combustion for the packed bed of particles. The packed bed reactor model comprises of array of single particles stacked in a vertical bed that deals with the detailed reaction rates along with the porous char spheres and thermo-physical phenomenon governed by the mass, species and energy conservation equations. Towards validating the pyrolysis and single particle conversion process, separate analysis and parametric study addressing the effects of thermo-physical parameters like particle size, density and thermal conductivity under varying conditions have been studied and compared with the available results from literature. It has been found that the devolatilisation time of particle (tc) follows closely the relationship with the particle diameter (d), thermal conductivity (k), density () and temperature (T) as: The complete combustion of a single particle flaming pyrolysis and char combustion has been studied and validated with the experimental results. For the reactor modeling, energy, mass and species conservation equations in the axial flow direction formulate the governing equations coupled to the detailed single particle analysis. Gas phase reactions involving combustion of volatiles and water gas shift reaction are solved in the packed bed. The model results are compared with the experimental results from wood gasification system with respect to the propagation rate, conversion times, exit gas composition and other bed parameters like conversion, peak bed temperatures, etc. The propagation rates compare well with experimental data over a range of oxygen concentration in the O2- N2 mixture, with a peak at 10 mm/s for 100 % O2. In the case of oxy-steam gasification of dry biomass, the results clearly suggest that the char conversion is an important component contributing to the bed movement and hence the overall effective propagation rate is an important parameter for co-current reactors. This is further analyzed using the carbon boundary points based on elemental balance technique. The model predictions for the exit gas composition from the oxy-steam gasification matches well with the experimental results over a wide range of equivalence ratio and steam to biomass ratio. The output gas composition and propagation rates are found to be a direct consequence of input mass flux and O2 fraction in oxy-steam mixture. The present study comprehensively addresses the oxy-steam gasification towards generating hydrogen rich syngas using experimental and model studies. The study also arrives at the parameters for design consideration towards operating an oxy-steam biomass gasification system. The following flow chart provides the overall aspects that are covered in the thesis chapter wise.