Rapid microwave drying of non-food agricultural feedstock for improved biofuel production

Due to limited supply of traditional fossil based fuels, and increased interest in air and water quality along with other environmental concerns, there has been a rise in the utilization of biomass based energy sources. Many agricultural materials can be used for the production of biofuels, including materials that are typically underutilized such as sweet sorghum bagasse and otherwise nuisance species such as Chinese tallow tree seeds. The goal of this project was to examine the relationship between the dielectric properties of sweet sorghum bagasse and Chinese tallow tree (CTT) seeds, respectively, and frequency and moisture content; to determine pertinent thermal properties of these materials, and to optimize process parameters of a continuous belt microwave drying system for improved biofuel production. Prior to microwave drying, the elemental composition, fatty acid composition, oil content, and various thermal properties for each of the component layers of CTT seeds were investigated. These tests revealed dramatic differences between each of the component layers of CTT seeds. For both sorghum bagasse and CTT, the dielectric properties across a range of moisture contents and frequencies were measured. The values obtained here were applied to the calculation of the penetration depth of microwaves through the materials in order to illustrate how these materials would behave when exposed to microwave energy. The dielectric properties for each material were found to be dependent on both frequency and moisture content. For microwave drying tests, the parameters investigated include microwave power levels (300W, 750W, and 1.2kW) and ambient air temperatures (room temperature and 55°C) with total residence time of 5 minutes. Data collected included humidity, temperature, sample surface temperature, moisture content, and absorbed microwave power. The moisture removed when microwaves are used is greatly in excess of the internal air moisture holding capacity, due to forced removal of water from the samples via pressure gradients generated by direct interaction with the water molecules in the matrix. Results indicate that microwave drying achieves results better than the control with respect to moisture removed per unit energy input

An intelligent humidity control system for mushroom growing house by using beam-switching antennas with artificial neural networks

An automatic humidity control system for mushroom growing house based on the free-space technique is presented. The novelty of this work is the modified free-space technique by measuring the amplitude only of transmission coefficient |S21| that reflected from mushroom by using beam-switching antenna with artificial neural networks (ANNs) as a humidity sensor to control quantity and time of water misting nozzle. In the proposed system, the antenna is designed to act as the transmitting antenna at the frequency of 2.45 GHz. Its radiation patterns can be switched to 4 directions covering all corners of mushroom growing house. The measured |S21| from each direction are converted to direct current (DC) voltage by a radio frequency (RF) detector; then are trained with ANNs in the humidity range of 60-85%. The optimized ANNs structure consists of 4 input nodes, two layers of 5 hidden nodes, and 3 output nodes. To verify the proposed system, experiments were set up in controlled humidity mushroom growing house at the humidity level of 75-80% for 120 hours. The results showed that there was slightly average standard deviation (S.D.) of humidity level 1.36. Consequently, the performance of sensor system assures that it is able to apply for humidity control in large growing house

Production of Chemicals by Microwave Thermal Treatment of Lignin

RÉSUMÉ Ce travail a pour but d’étudier le potentiel de convertir un des composants de la biomasse lignocellulosique, la lignine, en produits à valeur ajoutée en utilisant la pyrolyse assistée par microondes (MWP). Pour atteindre cet objectif, plusieurs étapes ont été franchies. Nous avons tout d’abord réussi à prédire les profils de température au sein d’un matériau exposé aux ondes électromagnétiques (EMW) à l’aide d’un modèle mathématique tridimensionnel. Ensuite, un TGA-microondes (MW-TGA) original a été développé et mis en œuvre pour l’étude cinétique. Subséquemment, une comparaison entre la pyrolyse assistée par microondes et conventionnelle a été réalisée. L’étude structurelle détaillée de la bio-huile produite via MWP de la lignine kraft a été discutée en troisième étape. Finalement, un modèle cinétique des produits de la MWP ainsi que des produits chimiques extraits de la lignine kraft a été mis en place. Tout d’abord, un modèle mathématique tridimensionnel a été présenté pour simuler le profil de température à l’intérieur d’un matériau exposé aux ondes électromagnétiques à 2.45 GHz. Les applications de COMSOL-Multiphysics ont permis de simuler le profil de température transitoire pour la pinède, le carbone, le Pyrex et des combinaisons de ces matériaux sous différentes conditions. Les résultats prédits ont été comparés aux données expérimentales pour la validation du modèle. Cette étude nous a permis de conclure que le chauffage microondes (MWH) induit à une distribution non-uniforme de la température dûment à la longueur de pénétration (Dp) et à la perte surfacique de chaleur. Toutefois, le gradient de température peut être minimisé significativement si l’on réduit les dimensions du matériau exposé à deux fois la Dp et l’on place un bon isolant thermique à sa surface. Le positionnement des matériaux avec forte/faible capacité de convertir la radiation microonde en chaleur pourrait favoriser des zones chaudes/froides désirées à l’intérieur du matériau chauffé, ce qui permet un profil spécifique de température. En outre, l’addition de matériaux de forte capacité de convertir les microondes en chaleur à la charge permet d’atteindre des températures beaucoup plus importantes comparées au cas du matériau seul exposé à la même puissance et temps de chauffage. Les discussions présentées dans cette étude visent à améliorer l’état de l’art par rapport aux profils de température dans un matériau composite soumis au chauffage microondes ainsi qu’à développer une approche pour influencer/contrôler ces profils de température selon la sélection des matériaux. L’objectif principal de la deuxième étape est d’étudier la cinétique de la MWP versus la pyrolyse conventionnelle (CP). Pour ce faire, un MW-TGA original a été construit et équipé d’un thermomètre novateur. Ce thermomètre est exempt des désavantages des thermomètres traditionnels dans le cas du MWH. Ainsi, le travail expérimental impliquant la MWP et la CP de la sciure de bois a été accompli. Des programmes MATLAB® ont été développés pour estimer les paramètres cinétiques, à savoir l’énergie d’activation, le facteur pré-exponentiel ainsi que l’ordre de la réaction (Ea, ko, et n, respectivement). Nous avons essentiellement conclu de ce travail que la MWP a une vitesse de réaction plus importante que celle de la CP. Ceci peut s’expliquer par le fait que les EMW oscillantes ont engendré un mouvement chaotique plus aigu des molécules ce qui influence le paramètre ko. Malgré cet effet remarquable sur ce ko, l’énergie d’activation demeure presque constante dans les deux cas. La possibilité de l’influence directe des ondes sur les liaisons intermoléculaires semble être ténue vu que la longueur des ondes est beaucoup plus grande que la distance intramoléculaire. Ce résultat est aussi puissant qu’il permettrait d’interpréter une grande majorité des effets du MWH reportés dans différentes réactions. La troisième étape présente une analyse détaillée de la structure des huiles produites par MWP de la lignine kraft. L’effet de deux paramètres a été évalué : (1) l’ajout d’un bon convertisseur de microondes-en-chaleur (noir de carbone) entre 20 et 40 wt%, et (2) la puissance nominale des microondes entre 1.5 et 2.7 kW. Cinq combinaisons pour ces deux variables ont été choisies pour lesquelles la radiation microondes a été gardée pendant 800 s. Les températures finales atteintes, mesurées en tant que valeur moyenne spatiale, étaient 900, 980, 1065, 1150, et 1240 K. Les rendements en produits de pyrolyse, solides, gaz condensables, et gaz non-condensables ont été comparés pour les conditions opératoires étudiées. Les gaz condensables collectés ont été séparés selon une phase-huile, prédominée de produits chimiques, et une phase aqueuse contenant surtout de l’eau et ayant une densité moindre que la phase-huile. Les résultats obtenus montrent que l’augmentation de la vitesse de chauffe et de la température finale induit une augmentation du rendement en produits liquides. Les produits identifiés dans les huiles par GC-MS étaient majoritairement aromatiques : gaïacols, phénols, and catéchols. Toutefois, autour de 60 wt% n’a pas pu être identifié par GC-MS d’où le recours à la spectroscopie RMN 31P et 13C offrant plus de détails sur la composition structurelle des huiles. Selon l’analyse RMN, 80% du carbone détecté dans la phase-huile était un carbone aromatique. Les groupes hydroxyliques aliphatiques perçus dans la matière première ont été éliminés significativement dans l’huile; ceci est attribué à la formation provisoire de la molécule d’eau pendant la MWP. La concentration en groupes hydroxyliques phénoliques C5 substitués/condensés a baissée en faveur des groupes gaïacol, p-hydroxyphenyl, et catéchol hydroxyle. Un cheminement de dégradation détaillé pour chacune de ces conversions a été suggéré. Une telle étude est essentielle à la compréhension du cheminement de dégradation ainsi qu’à la composition structurelle des huiles de pyrolyse. La quatrième étape fait l’objet d’une étude cinétique pour la MWP de la lignine kraft en appliquant des modèles tridimensionnels. Pour atteindre cet objectif, le MW-TGA utilisé pour la deuxième étape a été modifié et utilisé. Les modifications apportées ont permis de séparer les gaz produits (condensables et non-condensables) en sept parties. Le matériau convertisseur de microondes-en-chaleur a été ajouté à 30 wt% de la masse totale et la puissance nominale était de 2.1 kW. Le premier modèle considère la conversion de la matière première en solide, gaz condensable et gaz non-condensable en considérant que chaque produit est un bloc individuel. Dans le second modèle, le liquide est séparé en huile, contenant que des produits chimiques et 0% d’eau, et en eau ne contenant aucun produit chimique. Les produits sont ainsi l’huile, l’eau, les gaz non-condensables et le solide. De plus amples recherches ont été réalisées dans le troisième modèle en analysant l’huile produite par GC-MS. L’huile est donc subdivisée en quatre catégories : (1) phénoliques, contenant tous les composés phénoliques identifiés, (2) aromatiques à haute masse moléculaire, comportant toutes les molécules lourdes et les produits non identifiés par GC-MS, (3) aromatiques monocyclique non-phénoliques et (4) aliphatiques. Par conséquent, le troisième modèle considère la pyrolyse de la lignine en sept produits : ceux cités précédemment plus l’eau, les gaz non-condensables et le solide. Les paramètres cinétiques de chaque modèle ont été estimés et appliqués pour prédire la distribution des produits pour chaque modèle. Finalement, les résultats prédits ont été comparés aux données expérimentales aux fins de validation. -----------ABSTRACT This work investigates the potential of converting one of the lignocellulosic biomass components, lignin, into value-added bio-products using microwave pyrolysis (MWP). To achieve this objective, a multi-step process was devised and accomplished. First, temperature profiles within a material exposed to electromagnetic waves (EMW) were predicted using a three dimensional mathematical model. Second, an original microwave-thermo gravimetric analyzer (MW-TGA) was designed and built for kinetic purposes, and the kinetics of MWP were investigated in contrast to conventional pyrolysis (CP). Third, a detailed structural investigation of a bio-oil produced from of kraft lignin using MWP was discussed at various conditions. Finally, a kinetic modeling of the MWP products from kraft lignin was achieved quantitatively, as well as qualitatively. In the first step, a three-dimensional mathematical model was created to simulate temperature profiles inside a material exposed to EMW at 2.45 GHz. COMSOL-Multiphysics applications were used to simulate transient temperature profiles of pinewood, carbon, Pyrex, and combinations of these materials under different conditions. The predicted results were compared against the experimental data in order to validate the presented model. The key conclusions of this study show that microwave heating (MWH) leads to non-uniform distribution of temperature due to material penetration depth (Dp) and surface heat loss. However, limiting the dimensions of the exposed material to twice the Dp and placing strong thermal insulation on the surface significantly minimize temperature gradients. The locations of materials which are strong or weak microwave-to-heat convertors can be manipulated to create desired hot or cold zones inside the heated material, which leads to specific temperature profiles. In addition, the homogenous mixing of a material strong microwave-to-heat converter with the payload exhibits a significant increase in temperature, compared to the virgin material exposed to the same power and heating time. This study aims at improving the understanding of temperature profiles within composite materials subjected to MWH, as well as developing approaches to influence/control temperature profiles through material selection. The main objective of the second step was to investigate the kinetics of MWP in contrast to CP. To achieve this objective, an original MW-TGA was built and equipped with an innovative thermometer, which does not suffer from the traditional drawbacks, particularly in case of MWH. Subsequently, experimental work on MWP and CP of sawdust was conducted. MATLAB® program codes were employed to estimate the kinetic parameters, activation energy, pre-exponential factors, and reaction orders (Ea, ko, and n, respectively). The key conclusions of this investigation indicate that MWP has a faster reaction rate than CP. This is a consequence of enhancing the molecular chaotic motion resulting from the oscillating EMW: the molecular mobility, which is represented by ko. Even though this noticeable effect on ko, the estimated value of Ea was almost the same in both cases, this might be a consequence of the tenuous possibility of direct hacking the molecule-bonds by applied EMW, since the wavelength of EMW is much longer than the intermolecular distance of the target material. This result is so significant that it can account for most of the effects observed in different reactions when MWH is applied. The third step investigated a detailed structural and compositional analysis of a bio-oil produced from kraft lignin using MWP. The effects of two parameters were considered: (1) loading of a strong microwave-to-heat convertor (char), 20-40 wt%, and (2) microwave nominal setting power, 1.5-2.7 kW. Five combinations of these two variables were chosen and applied for 800s of MWH. The reached final temperatures, measured as mean values, were 900, 980, 1065, 1150, and 1240 K. The yields of the pyrolysis products, solid, condensable gas, and non-condensable gas were compared at the conditions under investigation. The collected condensable gas was separated into oil phase, which is mostly chemicals, and aqueous phase, which is mostly water and lower density than the oil phase. The obtained results showed that increases the heating rate leads to an increase in the yield of the liquid product. The identified chemical compounds in the oil phase using GC-MS were mostly aromatics: guaiacols, phenols, and catechols. Nonetheless, at 60 wt%, the oil phases could not be identified using GC-MS. Therefore, 31P and 13C NMR spectroscopy were used to provide further detailed structural information. Based on the NMR analyses, up to 80% of the detected carbon atoms in the oil phase were aromatic carbons. The detected aliphatic hydroxyl groups in the virgin material were significantly eliminated in the oil phase, and this was attributed to water forming in the interim of MWP. The decreased concentrations of C5 substituted/condensed phenolic hydroxyl groups after MWP were attributed to an increase in the concentrations of guaiacyl, p-hydroxyphenyl, and catechol hydroxyl groups. Detailed degradation pathways for each of those conversions were suggested. Such an investigation is significant because it aims at improving the understanding of the degradation pathways of a lignin network, as well as the structure of the obtained bio-oil. In the final step, a kinetic investigation of kraft lignin products made from MWP was accomplished by applying three different models. To achieve this objective, the MW-TGA that was built in the second step was modified and used in this step. The modifications done on the MW-TGA enable the distribution of vapour products (condensable and non-condensable) up to 7 parts in the interim of MWP. The applied conditions were 30 wt% of char and a microwave nominal power setting of 2.1 kW. The first model considered the virgin material converted into condensable gas, non-condensable gas, and remaining solid, taking into consideration each product as an individual lump. In the second model, the liquid product was separated into oil, which is entirely chemical and contains 0 wt% water, and water, which contains 0 wt% chemicals. Therefore, the lumps of the second model were oil, water, non-condensable gas, and solid. Further investigations were achieved in the third model by analysing the oil product using GC-MS. The oil product was partitioned into four groups: (1) phenolics group, which contains all the identified phenolic components, (2) heavy molecular weight components group, which contents all the heavy molecular weight and the undefined components using a GC-MS analyzer, (3) aromatics with a single ring (non-phenolics) group, and (4) aliphatics group. Hence, the third model considered lignin converted into seven products, the above four groups, plus water, non-condensable gas, and solid. The kinetic parameters of each model were estimated, and then applied to predict the yield of each product at the selected temperatures. Finally, the predicted results were compared against the experimental data, which showed a high capacity of the presented models to estimate product yields

Understanding microwave pyrolysis of biomass materials

Global challenges related to energy security, resource sustainability and the environmental impacts of burning fossil fuels have led to an increasing need for switching to the use of clean and sustainable resources. Bio-oil produced through pyrolysis has been suggested as one of the sustainable alternatives to fossil resources for power generation as well as chemicals and biofuels production. Pyrolysis is a thermochemical process during which the biomass feedstock is heated in an inert atmosphere to produce gas, liquid (bio-oil) and solid (char) products. Microwave heating has been considered a promising technique for providing the energy required for biomass pyrolysis due to its volumetric and selective heating nature which allows for rapid heating in a cold environment. This helps to preserve the product quality by limiting secondary reactions. The aim of this research was to study the interactions between biomass materials and microwave energy during pyrolysis, and to develop a reliable and scalable microwave pyrolysis process. The dielectric properties of selected biomass materials were studied and found to vary significantly with temperature due to the physical and structural changes happening during pyrolysis. The loss factor of the biomass materials was found to reach a minimum value in the range between 300 oC and 400 oC followed by a sharp increase caused by the char formation. A microwave fluidised bed process was introduced as an attempt to overcome the challenges facing the scaling-up of microwave pyrolysis. The concept of microwave pyrolysis in a fluidised bed process was examined for the first time in this thesis. A systematic approach was followed for the process design taking into account the pyrolysis reaction requirements, the microwave-material interactions and the fluidisation behaviour of the biomass particles. The steps of the process design involved studying the fluidisation behaviour of selected biomass materials, theoretical analysis of the heat transfer in the fluidised bed, and electromagnetic simulations to support the cavity design. The developed process was built, and batch pyrolysis experiments were carried out to assess the yield and quality of the product as well as the energy requirement. Around 60 % to 70 % solid pyrolysed was achieved with 3.5 kJ·g-1 to 4.2 kJ·g-1 energy input. The developed microwave fluidised bed process has shown an ability to overcome many of the challenges associated with microwave pyrolysis of biomass including improvement in heating uniformity and ability to control the solid deposition in the process, placing it as a viable candidate for scaling-up. However, it was found to have some weaknesses including its limitations with regards to the size and shape of the biomass feed. Microwave pyrolysis of biomass submerged in a hydrocarbon liquid was introduced for the first time in this thesis as a potential alternative to overcome some of the limitations of the gas-based fluidised bed process. Batch pyrolysis experiments of wood blocks submerged in different hydrocarbon liquids showed that up 50 % solid pyrolysis could be achieved with only 1.9 kJ·g-1 energy input. It was found that the overall degree of pyrolysis obtained in the liquid system is lower than that obtained from the fluidised bed system. This was attributed to the large temperature gradient between the centre of the biomass particle/block and its surface in the liquid system leaving a considerable fraction of the outer layer of the block unpyrolysed. It was shown that the proposed liquid system was able to overcome many of the limitations of the gas-based systems

Thermo-Catalytic Upgrading of Pyrolysis Vapors Using Electromagnetic Heating

Electromagnetic heating offers several advantages such as rapid heating rates, accurate temperature control and energy efficiency over conventional reactors. The goal of this study was to design an effective and energy efficient catalytic reactor for pyrolysis vapor upgrading. An induction based catalytic reactor was designed for upgrading of pyrolysis vapors. The effect of catalyst bed temperatures (290°, 330° and 370°C) and biomass to catalyst ratios of 1, 1.5 and 2 was studied. The results were compared to conventional heating reactor. Induction heating reactor performance exceeded that of conventional heater. The biomass to catalyst ratio of 2 in combination with the temperature of 370°C gave the highest aromatics yield. A microwave based catalytic reactor was designed for pyrolysis vapor upgrading. Microwave heating had higher product selectivity and energy efficiency compared to conventional and induction heating reactors. Rate of deterioration of catalyst mainly due to coking was lower for microwave heated catalyst. Higher aromatic hydrocarbon yield, lower oxygen content and high heating value value of bio-oil was obtained by microwave heating of catalyst. A numerical model studying the microwave heating of porous catalyst bed was developed using COMSOL Multiphysics 5.1. The model was validated against the experimental data. The temperature profiles obtained from microwave heating were compared to those obtained from conventional heating. The model was in good agreement with the experimental results. The sample shape, size and position was found to have significant effect on microwave heating of porous catalyst bed

The application of microwave heating in bioenergy: A review on the microwave pre-treatment and upgrading technologies for biomass

Bioenergy, derived from biomass and/or biological (or biomass-derived) waste residues, has been acknowledged as a sustainable and clean burning source of renewable energy with the potential to reduce our reliance on fossil fuels (such as oil and natural gas). However, many bioenergy processes require some form of pre-treatment and/or upgrading procedure for biomass to generate a modified residue with more suitable properties and render it more compatible with the specific energy conversion route chosen. Many of these pre-treatments (or upgrading procedures) involve some form of substantive heating of the biomass to achieve this modification. Microwave (MW) heating has attracted much attention in recent years due to the advantages associated with dielectric heating effects. These advantages include rapid and efficient heating in a controlled environment, increasing processing rates and substantially shortening reaction times by up to 80%. However, despite this interest, the growth of industrial MW heating applications for bioenergy production has been hindered by a lack of understanding of the fundamentals of the MW heating mechanism when applied to biomass and waste residues. This article presents a review of the current scientific literature associated with the application of microwave heating for both the pre-treatment and upgrading of various biomass feedstocks across different bioenergy conversion pathways including thermal and biochemical processes. The fundamentals behind microwave heating will be explained, as well as discussion of the imperative areas which require further research and development to bridge the gap between fundamental science in the laboratory and the successful application of this technology at a commercial scale

Formation of Metallurgical Coke within Minutes through Coal Densification and Microwave Energy

This paper shows how feedstock densification gives rise to a step change in the time required to create a metallurgical grade coke using microwave energy. Five densified coking and non-coking coals were heated in a multi-mode microwave 2450 MHz cavity for varying treatment times (2-20 minutes) with a fixed power input (6 kW). Proximate analysis, intrinsic reactivity, coke reactivity, dielectric properties, and petrographic analysis of the coals and microwave produced lump cokes were compared to a commercial lump coke. Densifying the sample prior to microwave treatment enabled a dramatic acceleration of the coking process when combined with targeted high microwave energy densities. It was possible to form fused coke lump structures with only 2 minutes of microwave heating compared to 16-24 hours via conventional coking. Anisotropic coke morphologies (lenticular and circular) were formed from non-coking coal that are not possible with conventional coking and increasing treatment time improved overall coke reflectance. Three of the coals produced coke with equivalent coke reactivity index values of 20-30, which are in the acceptable range for blast furnaces. The study demonstrated that via this process, non-coking coals could potentially be used to produce high quality cokes, potentially expanding the raw material options for metallurgical coke production

Experimental investigations of bio-syngas Production using microwave pyrolysis of UAE’s Palm date seeds

From the start of the industrial revolution, the continued need for energy has been the most crucial issue in human history. An energy crisis started at the beginning of the 1970s when the number of machines, which became an essential part of our life, increased rapidly. Scientists made a huge effort to discover new sources of energy, with ‘biomass’ being one of the main focuses of researchers as a new renewable source of energy. In this thesis, a non conventional method of heating, using microwave power in a pyrolysis process of biomass waste from palm trees (Allig’s date seeds) for the production of bio-syngas to use in practical and industrial applications, is the main focus. Microwave heating has many advantages over conventional heating methods. In this method, the biomass heating occurs from the inside to the outside uniformly instead of heating the environment, as in the case of conventional heating. In designing the experimental work, a full factorial approach is utilized, using three parameter factors: particle sizes of (1790 μm, 783 μm, and 467 μm), microwave powers of (1,000 W, 700 W, and 300 W) and sample moisture contents of (0, 0.2, and 0.4). The yield of bio-syngas and temperature samples are monitored and measured throughout the tests using an “ETG MCA 100 Syn BIOGAS MULTIGAS ANALYZER” and an Omega Thermocouple respectively. In the last part of this work, a statistical analysis is conducted to nonlinearly model the gas yield average concentration percentages for CH4 and CO, as a function of all dependent parameters. The outcome of this study produces promising results, especially for CH4 and CO gas yields, which shows an average of 21% and 15% volume bases respectively. The yield of H2 gases is the lowest amongst all gas yields. The highest percentages of bio-syngas yield occurred at the highest microwave power, the smallest size of particles, and the driest samples. Allig date seeds as a biomass source in the microwave pyrolysis process demonstrate to be a promising source of renewable energy to be used in commercial and practical applications

A review of chemicals to produce activated carbon from agricultural waste biomass

The choice of activating agent for the thermochemical production of high-grade activated carbon (AC) from agricultural residues and wastes, such as feedstock, requires innovative methods. Overcoming energy losses, and using the best techniques to minimise secondary contamination and improve adsorptivity, are critical. Here, we review the importance and influence of activating agents on agricultural waste: how they react and compare conventional and microwave processes. In particular, adsorbent pore characteristics, surface chemistry interactions and production modes were compared with traditional methods. It was concluded that there are no best activating agents; rather, each agent reacts uniquely with a precursor, and the optimum choice depends on the target adsorbent. Natural chemicals can also be as effective as inorganic activating agents, and offer the advantages that they are usually safe, and readily available. The use of a microwave, as an innovative pyrolysis approach, can enhance the activation process within a duration of 1–4 h and temperature of 500–1200 °C, after which the yield and efficiency decline rapidly due to molecular breakdown. This study also examines the biomass milling process requirements; the influence of the dielectric properties, along with the effect of washing; and experimental setup challenges. The microwave setup system, biomass feed rate, product delivery, inert gas flow rate, reactor design and recovery lines are all important factors in the microwave activation process, and contribute to the overall efficiency of AC preparation. However, a major issue is a lack of large-scale industrial demonstration units for microwave technology

Understanding microwave pyrolysis of biomass materials

Global challenges related to energy security, resource sustainability and the environmental impacts of burning fossil fuels have led to an increasing need for switching to the use of clean and sustainable resources. Bio-oil produced through pyrolysis has been suggested as one of the sustainable alternatives to fossil resources for power generation as well as chemicals and biofuels production. Pyrolysis is a thermochemical process during which the biomass feedstock is heated in an inert atmosphere to produce gas, liquid (bio-oil) and solid (char) products. Microwave heating has been considered a promising technique for providing the energy required for biomass pyrolysis due to its volumetric and selective heating nature which allows for rapid heating in a cold environment. This helps to preserve the product quality by limiting secondary reactions. The aim of this research was to study the interactions between biomass materials and microwave energy during pyrolysis, and to develop a reliable and scalable microwave pyrolysis process. The dielectric properties of selected biomass materials were studied and found to vary significantly with temperature due to the physical and structural changes happening during pyrolysis. The loss factor of the biomass materials was found to reach a minimum value in the range between 300 oC and 400 oC followed by a sharp increase caused by the char formation. A microwave fluidised bed process was introduced as an attempt to overcome the challenges facing the scaling-up of microwave pyrolysis. The concept of microwave pyrolysis in a fluidised bed process was examined for the first time in this thesis. A systematic approach was followed for the process design taking into account the pyrolysis reaction requirements, the microwave-material interactions and the fluidisation behaviour of the biomass particles. The steps of the process design involved studying the fluidisation behaviour of selected biomass materials, theoretical analysis of the heat transfer in the fluidised bed, and electromagnetic simulations to support the cavity design. The developed process was built, and batch pyrolysis experiments were carried out to assess the yield and quality of the product as well as the energy requirement. Around 60 % to 70 % solid pyrolysed was achieved with 3.5 kJ·g-1 to 4.2 kJ·g-1 energy input. The developed microwave fluidised bed process has shown an ability to overcome many of the challenges associated with microwave pyrolysis of biomass including improvement in heating uniformity and ability to control the solid deposition in the process, placing it as a viable candidate for scaling-up. However, it was found to have some weaknesses including its limitations with regards to the size and shape of the biomass feed. Microwave pyrolysis of biomass submerged in a hydrocarbon liquid was introduced for the first time in this thesis as a potential alternative to overcome some of the limitations of the gas-based fluidised bed process. Batch pyrolysis experiments of wood blocks submerged in different hydrocarbon liquids showed that up 50 % solid pyrolysis could be achieved with only 1.9 kJ·g-1 energy input. It was found that the overall degree of pyrolysis obtained in the liquid system is lower than that obtained from the fluidised bed system. This was attributed to the large temperature gradient between the centre of the biomass particle/block and its surface in the liquid system leaving a considerable fraction of the outer layer of the block unpyrolysed. It was shown that the proposed liquid system was able to overcome many of the limitations of the gas-based systems