Xiaotao Bi

Torrefaction at 200–350 °C in an inert or oxygen-deficient environment is a promising pre-treatment to improve the properties of biomass for energy utilization. Fluidized beds with enhanced heat and mass transfer could be a candidate for biomass thermal treatment. Gas pulsation has been proven effective in fluidizing biomass particles with unconventional natures that caused problems in fluidization. A novel pulsed fluidized bed (PFB) reactor for continuous biomass torrefaction was designed and commissioned in this work. The bed has a length-to-width ratio of 10:3 to limit the solids residence time distribution (RTD), leading to uniform solids products. Biomass particles transport and backmixing were studied by solids RTD measurement. A 2D axial dispersion model with an exchange flow between the active and stagnant zones was proposed to quantify solids backmixing by fitting the model to RTD curves. Horizontal dispersion coefficient representing the backmixing degree were slightly higher in the deeper bed, increased with increasing particle velocity in the shallow bed, and increased greatly with increased gas velocity. Less backmixing was obtained when the bed was operated under a gas pulsation frequency close to the bed natural frequency. Horizontal solids dispersion coefficients obtained in horizontal PFB were significantly smaller than values calculated from literature. A correlation of horizontal dispersion coefficients for biomass particles in the horizontal PFB of shallow beds was established. Continuous biomass torrefaction was successfully performed in the PFB unit with a feed rate of up to 2 kg/h. Maximum weight loss was identified at the frequency range of 2–4 Hz for feed rates of 1 kg/h and 1.5 kg/h. The temperature near the reactor exit exhibited the most significant influence on the biomass weight loss and the product properties, i.e., higher heating value (HHV), contents of proximate analysis components, and elemental carbon content, whereas the feed rate had a slight effect. Torrefied biomass obtained in fluidizing gas with 3–6 vol.% oxygen concentrations showed similar HHV with that of non-oxidative torrefaction. Compared to other continuous torrefaction technologies, the PFB torrefier utilized small particles with low residence time.

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In this study, the availability of various biomass resources in British Columbia (BC) is estimated, including forestry resources, agricultural waste and municipal solid waste. Since the enormous potential for bioenergy production identified is insufficient to replace the entirety of fossil fuel consumption in BC, the exploitation of limited biomass resources must be optimized based on Greenhouse Gas (GHG) reduction and costs. Life Cycle Assessment is conducted to analyze the GHG reduction potential and other environmental impacts of various bioenergy options. Minimum selling prices and GHG reduction costs are calculated to indicate economic viability.For lignocellulosic feedstocks, the biomass-fired heat-only boiler (HB) has the highest GHG savings; however, it may impose health risks in densely populated urban areas due to flue gas emissions and should be limited to large-scale implementations with effective emission control. HB is also the most cost-effective in GHG mitigation. In comparison, liquid biofuels and renewable natural gas slightly less effective in GHG mitigation and substantially more costly.For animal manure, food waste, and crop residues, anaerobic digestion can be used to convert these biomass residues into biogas. The results show that the biogas-fired HB has higher GHG savings and lower GHG reduction costs than cogeneration or upgrading to RNG. Biogas-fired HB systems can be further integrated with other common agricultural practices in BC to generate additional environmental and economic benefits.At present, full-scale implementation of refined biofuel technologies will lead to prohibitive extra costs; hence, HB systems should be prioritized in the short-term due to the inherent advantage in conversion efficiencies. In the long term, technological breakthroughs in improving efficiency and extracting high-value byproducts will be the key to the prospect of refined biofuels.

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This thesis investigates several key aspects of the supply systems of torrefied and conventional wood pellet (TWP/CWP) from British Columbia (BC): what are the economic, environmental, and energetic (“3E”) performances of TWPs and CWPs supplied from BC into different markets? What is the best pathway for making TWPs? Can the TWPs production process be operated auto-thermally? If so, under what operating conditions? A simulation platform is developed, including models for rotary and fluidized bed dryers, directly and indirectly heated rotary and fluidized bed torrefiers, and integrating heat and mass transfer, kinetics, particle hydrodynamics, thermodynamics and element evolutions. The auto-thermal operation boundaries are identified for the torrefaction system. The boundaries are influenced by drying technology, N₂ flowrate, biomass properties and torrefaction conditions. A heat and mass integration scheme is proposed to avoid the use of N2 for torrefaction by recycling flue gases and to expand the auto-thermal operation boundaries. CWP and TWP production processes are analyzed, revealing that torrefying the biomass before grinding can reduce the “3E” impacts significantly. Due to auto-thermal operation, electricity is the main energy consumption and contributor to greenhouse gas (GHG) emissions. Capital costs contribute about 10% of the total production costs, with the remaining 90% being the operating cost, within which raw material, electricity, and labor are the major components. The minimum selling price at which BC TWPs is estimated as ~$6.7/GJ, equivalent to 140$/t. The “3E” performances of BC CWP/TWPs supply chains to the UK, Japan, Ontario and Alberta are quantified with uncertainties considered. TWPs can reduce “3E” impacts by about 25% in comparison with CWPs. Transportation is the main energy consumer and GHG emission contributor, while transportation and production are the major cost stages. There is significant potential to replace coal with BC TWPs domestically and overseas, particularly in the UK, EU and Pacific Asia, due to the comparative advantages of BC’s clean electricity system and rich biomass resources.

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Local impact assessment of biomass-based district energy systems (DES) is still in its infancy. There has been a lack of appropriate assessment methods for parameters with broad variability on local scale, and lack of DES impact assessments. This study investigates how would: 1) the inclusion of site-specific terrain, land use and microclimatic characteristics, variable population density and breathing rates affect accuracy of assessments on local air quality and health; 2) an incremental increase of PM₂.₅, NOx and CO concentrations from DES contribute to ambient air quality and population exposure, 3) life-cycle GHG emissions from DES contribute to global warming, and 4) the introduction of biomass affect economics of DES compared to the fossil fuel-based DES. Utilizing dispersion modeling the study established an assessment approach which confirmed the need for inclusion of population dynamics, site-specific microclimatic characteristics, and diurnal circulation patterns. Otherwise, health risks could potentially be underestimated by more than 20%. Applying this approach on a small-scale biomass gasification plant (BRDF), the study concluded that the health impact was the highest for NO₂ (677 DALY) when all energy was produced by biomass, and for PM₂.₅ (64 DALY) if all energy was produced by natural gas. Complete replacement of Power House (PH) by one biomass plant can result in almost 28% higher impact compared to 513 DALY when both BRDF and PH are operational. NO₂ emissions from the BRDF exceeded the air quality objectives (BCAQO) in all seasons except during summer. Although overall incremental contribution of PM₂.₅ is at least one order of magnitude lower than BCAQO, the maximum PM₂.₅ emissions from the PH could adversely add to the already high background concentrations. Meeting energy demand solely by an expanded full-scale BRDF from locally supplied biomass reduces GHG annually to 3.81E+06 kg CO₂eq from 7.08E+07 kg CO₂eq when energy was produced solely by the current PH. An introduction of biomass increased total costs by $19 M compared to existing PH, but saved $8.4 M in carbon tax over plants’ lifetime. $3.3 M of societal damages could be avoided over plants’ lifetime in case of combined use of natural gas and biomass.

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This thesis evaluates K₃PO₄, clinoptilolite, bentonite and their combinations as potential additives for enhancing microwave absorption, catalyzing pyrolysis of biomass and improving bio-oil and biochar qualities. Catalyst load ratio, pyrolysis temperature, liquid and solid product yields, bio-oil and biochar properties are examined to screen selected catalysts in terms of their effectiveness in increasing microwave absorption and improving bio-oil and biochar qualities. Thermogravimetric analysis (TGA) was also used to study the catalytic behaviour of those catalysts to interpret its performance in microwave-assisted catalytic pyrolysis and to study the catalytic pyrolysis kinetics for each of the three major biomass components, i.e., hemicellulose, cellulose and lignin, using the lumped three parallel reactions model. The performance of the produced biochars is evaluated in terms of their ability to improve soil water holding capacity (WHC), cation exchange capacity (CEC) and fertility of loamy sand soil. The capacity of those biochars in reducing bioavailability, phytotoxicity and uptake of heavy metals by wheat plants and the efficacy of those biochars in increasing soil fertility and plant growth in contaminated soil were also investigated.K₃PO₄, clinoptilolite and bentonite all showed good catalytic activities in microwave-assisted pyrolysis, resulting in reduced acidity, viscosity and water content of bio-oil product and catalyst loading and combination of different catalysts are controlling parameters on heating rate and product quality. The synergistic effects were observed in the combination of K₃PO₄ and clinoptilolite or bentonite, resulting in higher-than-expected microwave heating rate, in conjunction with improved bio-oil and biochar quality. Biochar produced from mixing K₃PO₄ and clinoptilolite or bentonite with biomass showed better performance in reducing toxicity and uptake of heavy metals than biochars produced from single catalyst. Catalytic microwave-assisted pyrolysis could be one potential approach for tailoring biochar quality to improve soil physiochemical properties. High microwave absorption, high water and nutrient affinity, desirable plant nutrients and high catalytic performance are the four key features of an effective additive for microwave-assisted biomass pyrolysis for making high quality bio-oil and biochars.

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A number of fluidized bed reactor processes operating at high temperature require that solid particles be circulated back and forth between two reactor vessels. Since the circulation rate strongly affects mass and energy balances, and therefore greatly influences hydrodynamics and performance of the system, a reliable technique for its accurate measurement would be helpful in monitoring and modeling the process. However, there are no reported techniques suitable for measuring this critical hydrodynamic parameter at elevated temperatures typical of gasification systems.A novel thermal-tracing technique was developed for measuring the solids circulation rate between two vessels. Packets of particles at lower temperatures are injected into a downward-moving packed bed of solids at elevated temperature, creating reduced-temperature zones inside the moving bed. The transit time of the cold-particle-clusters between pairs of thermocouples is determined by cross correlation, allowing the flux to be estimated. The technique was shown to provide sensitive and reproducible data for a cold model unit with injection of dry ice. The technique was then applied to determine solids circulation rates between the bubbling bed gasifier and the riser combustor of a pilot scale dual fluidized bed gasification system. A number of conditions are imposed on the data to eliminate unsatisfactory data at high temperatures. Data which satisfy the discrimination criteria led to measured solids circulation fluxes up to 133 kg/m²s at temperatures up to 856°C in the gasifier test section. A novel butterfly valve technique was developed to validate the thermal-tracing technique at high temperatures. Closing the valve causes solids to accumulate in the downcomer section of the pilot gasifier. The elevation of the top surface of these solids was tracked with high-temperature capacitance sensors, facilitating determination of the solids circulation flux between the two reactors of the pilot plant. The fluxes were also estimated using two indirect methods based on pressure balance and energy balance techniques. Agreement among the fluxes obtained from applying these four techniques are reasonable given the difficulty in measuring solids circulation rates.

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Biomass is a promising energy source that has been considered in a variety of thermal conversion processes where fluidized beds with their exceptional heat and mass transfer rates, are often considered as potential candidates. However, the fluidization of biomass is held back by its cohesive nature. This work has demonstrated that pulsed gas flow in fluidized bed is highly effective in overcoming channeling, partial and complete defluidization, without the need for inert bed particles. Both heat transfer and mass transfer were investigated in a pulsed fluidized bed with 0.15 m by 0.10 m rectangular cross-section area, and a fluidized bed with a tapered bottom to improve reactor performance. Biomass used in this work included Douglas fir, pine and switchgrass. Batch drying test was selected as an indirect indicator of gas–solid contact, heat and mass transfer. Mass transfer was evaluated through batch drying tests, where better gas–solid contact and mass transfer was assessed through the water removal efficiency. An optimum operating condition was identified after analyzing the intricate relationship between pulsation frequency, gas flow rate and the hydrodynamics. A two-phase drying model that linked single-particle mass transfer to macroscopic hydrodynamics in fluidized bed was implemented to verify the effect of flow rate, temperature and biomass properties on drying and mass transfer. Good agreement was observed between the modelled effective diffusivity and experimental results. Bed-to-surface heat transfer coefficients of all three biomass species in two reactor geometries were measured at various operating conditions. The heat transfer coefficient was influenced greatly by the intensity and frequency of gas pulsation, where both particle convection and gas convection existed. A new heat transfer model was proposed to address the influence of gas pulsation. Modelling results showed good agreement with experimental data.

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Lime-enhanced biomass gasification in a dual fluidized bed (DFB) reactor is a promising technology that allows enhanced hydrogen production from a renewable resource with simultaneous CO₂ capture via calcium looping.In this thesis, modeling Ca-looping in a DFB biomass gasifier is broken down into different steps. Firstly, a comprehensive single particle model is developed, based on conservation of mass, energy and momentum, with two different biomass pyrolysis kinetic schemes for particles of changing thermo-physical properties. Secondly, a coupled particle and reactor model of biomass drying and pyrolysis in a bubbling fluidized bed reactor is developed to predict the yields of pyrolysis products and composition as a function of process operating parameters. Thirdly, our coupled particle and reactor model is extended to steam gasification of biomass in a bubbling fluidized bed (BFB) gasifier, and its applicability is tested by comparing predictions with independent experimental data from the literature. For steam gasification of pine sawdust at a reactor temperature of 750°C, the H₂ mole fraction in the product gas increases with increasing steam-to-biomass ratio because of the water-gas, steam methane reforming and water-gas shift (WGS) reactions. Elevating the reactor temperature reverses the exothermic WGS reaction towards more CO production and CO₂ consumption. Fourthly, the BFB gasifier model is expanded into a generic two-phase fluidized bed reactor model to evaluate the performance of the UBC dual fluidized bed gasifier under steady-state operating conditions. Finally, integrated biomass gasification with cyclic CO₂ capture in a DFB reactor is simulated by developing a model which takes into account sorbent loss of reactivity due to sintering during cyclic operation.This comprehensive reactor model is developed and tested based on a stepwise approach. Unlike previous models, this is a predictive model that minimizes reliance on empirical correlations. By coupling single particle and reactor models, biomass drying, pyrolysis and gasification are studied as a continuous process. A gap of knowledge in predicting major compounds composition in pyrolysis gas is addressed. Furthermore, the kinetic model is capable of accommodating in situ CO₂ capture during cyclic operation.

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Size reduction is an essential operation for preparing biomass for the production of pellets, biofuels and bioproducts. Size reduction ranks second in terms of energy consumption after drying in a pelleting operation. The major challenge in sizing and operating a grinder is the difficulty in predicting the performance of a grinder and the quality of product due to the variability in structure and composition of the biomass. As a result, grinders are often over- designed to handle a wide range of biomass species, leading to disproportionate equipment size and operating costs. This research investigated factors influencing the power requirement for grinding biomass and developed mechanistic model equations to predict energy input to a grinder to achieve a targeted particle size. Two softwood species and three hardwood species were ground in a knife mill and/or a hammer mill. The experimental data consisted of power inputs, mass flow rates, and particle size reduction ratios. The well-known mechanistic model equations: Rittinger, Kick, and Bond, which relate energy input to particle size reduction, were evaluated and the Rittinger equation was found to give the best prediction of the experimental data. Douglas-fir consumed the least specific energy of grinding, 132-178 kJ kg‐¹, followed by aspen, 197-232 kJ kg‐¹, pine, 201-263 kJ kg‐¹, and poplar, 252-297 kJ kg‐¹. Specific surface area (m² kg‐¹) created was largest for aspen and smallest for Douglas-fir. Correspondingly, Douglas-fir consumed the least specific energy and aspen, with the largest specific surface area created, required the highest specific energy. These data suggest that the specific energy has a direct relation with the total surface area created as a result of size reduction, as captured by the Rittinger equation. Ground Douglas-fir and willow were also pelletized in a single pelletization unit. The combined grinding/densification energy input decreased with increasing particle size. The properties most significantly affecting the grinding energy consumption based on the comparison of the Rittinger’s constant, kR, were lignin content, particle density, and fibre length. Woody biomass of a higher lignin content, lower particle density, and longer fibre length requires more energy input to be ground to a targeted size.

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Novel electrostatic dual-tip probes, combined with suitable signal analysis methods, were developed for in-situ measurement and monitoring of particle charge density levels and bubble properties inside gas-fluidized beds. The probes were calibrated in several particulate flow devices : ejector-funnel, motor-pulley, vertical tube and vibration tray setups, as well as a two-dimensional fluidized bed. The effects of particle charge density, solid flux, particle velocity and angle of impact on the transferred current received by the probe from charged particles were quantified. For dual-tip (two-material) probes, substantial differences were observed in the signals from the two tips made of different materials, arising mainly from charge transfer and depending on the hydrodynamics and charge density inside the bed. The probes were deployed with glass beads and polyethylene particles for both single bubble injection and freely bubbling experiments in two- and three-dimensional fluidization columns of different scales. Statistical and Fast Fourier Transform analysis showed that current signals were strongly affected by the local hydrodynamics in the fluidized bed. The amplitudes of current signal peaks, peak frequencies, as well as mean and standard deviations of the current increased with increasing superficial gas velocity. Local particle charge density and bubble behaviour were estimated by a signal processing procedure with decoupling methods. The probes were tested in steady state experiments, as well as in dynamic experiments by abruptly changing the superficial gas velocity or adding antistatic agent. Both particle charge density and bubble rise velocity obtained from the probes were of the same order of magnitude and followed similar trends as those directly measured by a Faraday cup and video images, respectively. The electrostatic probe signal was found to not always be consistent with the charge polarity and charge density on the particles. The probe signals and particles charge densities may have different polarity and relative magnitudes for different operating conditions and particle properties : density, mean size and size range, dielectric constant, sphericity, roughness and hydrophobicity. Particles with narrow size distribution and larger mean size generated higher charge densities. The novel probe has potential for in-situ monitoring electrostatic charges and hydrodynamic behaviour in gas-solid fluidized beds.

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The effects of superfical gas velocity, pressure, and temperature on entrainment and electrostatic charge with glass beads and polyethylene as bed particles were investigated in a fluidization column of 0.15 m inner diameter and 2.0 m height. Four collision probes at different levels, a freeboard sampler, and a current detection pipe measured the electrostatics in the bed, freeboard, and column exit. The entrainment and electrostatic charge inside the bed and freeboard region increased as the superficial gas velocity or pressure increased. Temperature had negligible effect on the entrainment over the limited range studied. However, electrostatic charges decreased and the charge polarity reversed as the bed temperature increased from 20 to 75 °C. The calculated electrostatic forces resulting from fine-fine and fine-coarse particle interactions are comparable to the gravitational force on fine particles in the fluidized bed. Entrainment empirical correlations developed in this work showed much better performance after the effect of electrostatic forces was taken into account, with the entrainment flux deceasing as electrostatic forces increase. The Choi et al. (1999) entrainment correlation shows better prediction of the entrainment flux in our system after the effect of the electrostatic force is considered. The electrostatic charges in the bed decreased with increasing air relative humidity. The charge density of fines decreased and entrainment increased as the air relative humidity increased. The relative humdity had no effect on the charge density or entrainment of polyethylene particles, which can also probably be attributed to the hydrophobic nature of polyethylene.The magnitude of electrostatic charges generated inside the fluidized bed increased slightly as the size of the coarse particles decreased. The entrainment decreased as the coarse particle size decreased. The electrostatic charges increased and entrainment decreased as the coarse particle density increased. The magnitude of electrostatic charges generated inside the fluidized bed increased and entrainment decreased as the fine particle density increased. The electrostatic charges and entrainment also increased as the fine concentration increased. The fines concentration had little or no effect on fines charge densities. Bipolar charging was observed in all experiments with fine particles charged positively, whereas large particles were charged negatively.

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The generation of electrical charges, reported in gas-solid fluidized beds for over sixty years, can cause serious problems like wall sheeting in polyolefin reactors, leading to costly shutdown, electrical shock hazards and even explosions. Understanding the associated phenomena plays an important role to avoid these problems. In this study an attempt has been made to broaden the understanding of electrostatics in fluidized beds by adopting computational fluid dynamics (CFD), using the Two-Fluid-Model in MFIX (an open-source code originated by the U.S. Department of Energy). The Maxwell equations were incorporated in the MFIX code. The resulting model is then used to investigate how electrostatics modify bubble shape, size, velocity and interaction for three cases: (a) single bubbles, (b) bubble pairs in vertical and horizontal alignment, and (c) a freely-bubbling bed. In each of these cases, a two-dimensional column, partially filled with mono-sized particles, is simulated for both uncharged and charged particles.In case (a), it is predicted that single bubbles elongate and rise more quickly in charged particles than in uncharged ones. For case (b), electrostatics cause asymmetry of coalescence for a pair of vertically-aligned bubbles, while leading to the migration of a side bubble towards the axis of the column and changing the leading-trailing role for a pair of horizontally-aligned bubbles. Finally in case (c), the simulation predicts that electrostatics decrease bubble size and frequency in the free bubbling regime, accompanied by a change in the spatial distribution of bubbles, causing them to rise more towards the axis of the column.An attempt was also made to test experimentally the single bubble simulations. To reach this goal, a two-dimensional fluidization column was built with a central jet to inject single bubbles. The setup is equipped with a novel Faraday-cup device to measure the charge density accurately. The experimental results indicates a small decrease in bubble size and an increase in bubble height-to-width ratio with increasing charge density, accompanied by an increase in particles raining from the bubble roof. The assumption of uniform charge density on the particles is identified as a significant reason for differences between observed and predicted behaviour.

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Densification can resolve the logistical challenges encountered when large volumes of biomass are required for conversion processes to benefit from economies-of-scale. Despite the higher density of pellets, they easily disintegrate into fines due to impact or moisture sorption during handling and storage. Fines accumulation can lead to explosion, off-gassing and self-combustion, threatening the occupational health and safety of the workers. The current study investigates the use of several hydrothermal pretreatments to improve pellet quality in terms of mechanical strength and moisture sorption resistance, while lowering energy input during size reduction, drying and densification steps. Pretreatment of ground softwood particles (Pine, Spruce, Douglas fir whitewood and bark) with external saturated steam at 220°C for 5 min resulted in the higher calorific values, higher hydrophobicity and higher carbon percentage. These changes along with the dark brownish colour of steam treated material indicated a mild degree of torrefaction when compared to dry torrefaction at higher temperatures. Despite a slightly lower density, the mechanical strength of pellets made of steam treated particles increased considerably. Mechanical energy input for pelletization of treated material was higher than the untreated pellets when compressed under the same force for all species and bark samples.Hydrothermal pretreatment of wet Douglas fir wood particles, by steam generated from the moisture inside the material, resulted in the same characteristics as those obtained from pretreatments by external steam. Increased treatment temperature increased the hydrophobicity compared to untreated pellets.Sulfur-dioxide catalyzed steam pretreatment substantially reduced the particle size of Douglas fir woodchips, eliminating any further grinding requirement for pelletization. The SO₂-catalyzed steam treated pellets had a higher density and exhibited a two-time higher mechanical strength compared to untreated pellets. Despite a higher moisture adsorption capacity than the untreated, treated pellets remained intact under highly humid (30°C, 90% RH) conditions. The high heating values, low ash content and good overall carbohydrate recovery of SO₂-catalyzed steam treated pellets indicate their potential suitability for both biochemical and thermo-chemical applications.

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The internal circulating fluidized bed (i-CFB) reactor exhibits an ability to overcome the negative impact of excessive O₂ present in the flue gas on selective catalytic reduction of NOx with hydrocarbons as the reductant (HC-SCR) by decoupling NOx adsorption and reaction into two separate zones. A mathematical model has been developed in this study, which includes three sub-models: hydrodynamics, adsorption and reaction kinetics. Each sub-model was developed separately and validated by experimental data before they were integrated into the i-CFB model.For the hydrodynamics of i-CFB, solids circulation rates, which were later used for model parameter fitting, were measured using optical fibre probe. The hydrodynamics model was then developed based mass and pressure balance. Adsorption isotherm and deNOx reaction kinetics were developed based on a series of fixed bed experimental data: O₂ adsorption, NOx adsorption and NOx reaction. The kinetic model was further evaluated by fluidized bed adsorption and reaction experiments.The simulation results of the integrated i-CFB model showed good agreement with the experimental data. It is observed from the model that the performance of the current laboratory scale i-CFB reactor was dominated by the catalyst reactivity, rather than the catalyst adsorption rate, because of too short a solids residence time in the reduction zone for the deNOx reaction. Simulation results for i-CFBs with different cross sectional areas of the adsorption and reduction zones showed that a large reduction zone could significantly enhance the overall deNOx efficiency, and there existed an optimal reduction zone to adsorption zone area ratio at which NOx conversion is maximized at a given operating condition. It was also observed that the performance of i-CFB reactors with a larger reduction zone is less sensitive to gas bypass from reduction zone to adsorption zone.Overall, the i-CFB model developed in this study can be used as a tool to assist reactor design and scale up, and to provide guidance on how to further improve the NOx reduction efficiency. The simulation results showed that it is possible to achieve a higher deNOx efficiency higher while avoiding the negative effects of flue gas O₂.

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British Columbia (BC) has become a major producer and exporter of wood pellets in the world. But the low energy density, the low water resistivity, the short shelf life, and the transportation cost impede the market development. Torrefaction, a thermal treatment without air or oxygen at 200-300°C, may provide a solution. The present study developed the torrefaction kinetics of BC softwood residues in a thermogravimetric analyzer (TGA), studied the effect of the torrefaction reaction conditions on the properties of torrefied sawdust in a bench-scale fixed bed reactor and a bench-scale fluidized bed reactor, and identified the suitable conditions for making durable torrefied pellets in a press machine using torrefied samples. The weight loss of BC softwood residues significantly depended on the torrefaction temperature, the residence time, the particle size, and the oxygen concentration in the carrier gas. The weight loss could be approximately estimated from the weight loss of the chemical compositions. A two-component and one-step first order reaction kinetic model gave a good agreement with data over short residence time on the weight loss range of 0 to 40% at the temperature of 260-300°C. The heating value of torrefied pellets had a close relationship with the weight loss, increasing with increasing the severity of torrefaction. The torrefied samples were more difficult to be compressed into strong pellets under the same conditions as used for making the control (regular, untreated, conventional) pellets. More energy was needed for compacting torrefied samples into torrefied pellets. Increasing the die temperature and adding moisture into torrefied samples could improve the quality of torrefied pellets. The moisture content and density of torrefied pellets were lower than control pellets. Considering the quality of torrefied pellets, the optimal torrefaction conditions appeared to correspond to a weight loss of about 30%, which gave a 20% increase in pellet heating value and good hydrophobicity. The suitable densification conditions corresponded to a die temperature of 230°C, or over 110°C for torrefied samples conditioned to 10% moisture content.

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Water management in PEM fuel cells has received extensive attention for its key role in fuel cell operation. Several water management issues have been identified that needed further investigation, i.e., droplet behaviour on the GDL surface, two-phase flow patterns in gas flow channels, impact of two-phase flow on PEM fuel cell performance, impact of flow mal-distribution on PEM fuel cell performance, and mitigation of flow maldistribution. In this work, those issues were investigated based on simulations using computational fluid dynamics (CFD) method.Using the Volume of Fluid (VOF) two-phase flow model, droplet behaviour and two-phase flow patterns in mini-channels were identified consistently in both simulations and experimental visualizations. The microstructure of the GDL was found to play a significant role in the formation of local two-phase flow patterns, and the wettability of both GDL and channel wall materials greatly impacted on the two-phase flow patterns. A novel 1+3D two-phase flow and reaction model was developed to study the impact of two-phase flow on PEM fuel cell performances. The existence of two-phase flow, especially the slug flow, in gas flow channels was found to be detrimental to the fuel cell performance and stability. Uneven liquid flow distribution into two parallel gas channels significantly reduces the fuel cell output voltage because of the induced severe non-uniform gas distribution, which should be avoided in the operation due to its negative effect on the fuel cell performance and durability. Finally, several maldistribution mitigation methods were tested in the simulation. It was found that utilizing narrow communication channels or adding gas inlet resistances could effectively reduce the gas flow maldistribution.

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Wood pellet is regarded as a clean fuel for combustion with low ash content (less than 1% by weight) and a high heating value around 21500 MJ/m³ compared to a heating value of 5400 MJ/m³ for dry wood chips. However, pellet is easily disintegrated into fines due to impact or moisture sorption during handling and storage. Fines may promote dust explosion during handling or self-heating of pellets in storage. The present study investigates the use of steam explosion pretreatment to improve the pellet durability in terms of mechanical strength and moisture sorption resistance. In this research, a batch steam explosion unit consisting of a steam generator, a steam treatment reactor, and control devices was developed. Steam explosion experiments were carried out on Douglas Fir at 2 temperatures (200°C and 220°C), 2 treatment durations (5 min, 10 min), and 2 particle sizes (0.4 mm and 0.9 mm). It was found that the bulk density and tapped density of steam treated wood increased with the treatment severity. The pellets made with biomass treated at different combinations of temperature-time were 1.4 to 3.3 times stronger than untreated pellets. The steam treated biomass required 12% to 81% more energy to form durable pellets than the untreated biomass. Energy input to produce 45000 metric ton regular pellets and steam exploded pellets was estimated. The input energy ranged from 2.80 to 3.52 MJ/kg. Producing pellets from untreated biomass consumed the least energy while pellets made from biomass treated with saturated steam at 220°C for 10 minutes consumed the highest. A kinetic model for pseudolignin formation during steam explosion was developed. Based on the experimental data in this research and published literature, it was postulated that the creation of pseudolignin is responsible for improved durability of steam exploded pellets. A reaction model was developed to predict the formation of pseudolignin and evaluate the optimized treatment condition for making durable and water repellent wood pellets.

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The influences of operating pressure, temperature and gas velocity on electrostatics in a fluidized bed of glass beads and different grades of polyethylene resin were investigated in a fluidization column of 150 mm inner diameter and 2.0 m height. Eight collision probes at different levels and radial positions measured the electrostatics in the bed. The electrostatics increased as pressure increased, probably due to an increase in bubble rise velocity, frequency and volume fraction. As the pressure increased, particle-particle and particle-wall collisions near the distributor and wall contributed heavily to charge generation. Temperature also played a role. At higher temperatures (up to 90°C), the polarity of net cumulative charge in the bed reversed. As the superficial gas velocity increased, the electrostatics increased. However, at higher gas velocities, the polarity in the freeboard was opposite to that in the bed, indicating that fines entrained from the column carried charges, resulting in a net charge of opposite polarity to that inside the bed.For Geldart group B particles the degree of electrification in the bed slightly increased with decreasing particle size. Charging for group A particles was significantly greater than for group B particles. For binary mixtures of group A and B particles the electrostatics increased as the proportion of small particles increased. As the relative humidity (RH) of fluidizing air increased, the electrostatics decreased. For the RH range (5-30%) explored, the sensitivity of the charging to RH varied significantly depending on the location of the probes.As the proportion of fine glass beads (
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An integrated NOx adsorption-reduction process has been proposed in this study for the treatment of flue gases under lean-burn conditions by decoupling the adsorption and reduction into two different zones. The hypothesis has then been validated in a novel internal circulating fluidized bed.The adsorption and reaction performance of Fe/ZSM-5 for the selective catalytic reduction (SCR) of NOx with propylene was investigated in a fixed bed reactor. The fine Fe/ZSM-5(Albemarle) catalyst showed reasonable NOx adsorption capacity, and the adsorption performance of the catalyst was closely related to the particle size and other catalyst properties. Fe/ZSM-5 catalyst was sensitive to the reaction temperature and space velocity, and exhibited acceptable activity when O₂ concentration was controlled at a low level. Water in the flue gas was found to slightly enhance the reactivity of Fe/ZSM-5(Albemarle), while the presence of CO₂ showed little effect. SO₂ severely inhibited the reactivity of Fe/ZSM-5(Albemarle), and the deactivated catalyst could be only partially regenerated. Configurations of the reactor influenced the hydrodynamic performance significantly in a cold model internal circulating fluidized bed (ICFB) reactor. For all configurations investigated, the high gas bypass ratio from the annulus to draft tube (RAD) and low draft tube to annulus gas bypass ratio (RDA) were observed, with the highest RDA associated with the conical distributor which showed the flexible and stable operation over a wide range of gas velocities. Solids circulation rates increased with the increase of gas velocities both in the annulus and the draft tube. Gas bypass was also studied in a hot model ICFB reactor. The results showed that the orientation of perforated holes on the conical distributor could be adjusted to reduce RAD and/or enhance RDA. Coarse Fe/ZSM-5(PUC) and fine Fe/ZSM-5(Albemarle) catalysts were used in an ICFB and a conventional bubbling fluidized bed to test the NOx reduction performance. Coarse Fe/ZSM-5(PUC) catalyst showed poor catalytic activity, while fine Fe/ZSM-5(Albemarle) catalyst exhibited promising NOx reduction performance and strong inhibiting ability to the negative impact of excessive O₂ in the ICFB reactor, proving that the adsorption-reduction two-zone reactor is effective for the NOx removal from oxygen-rich combustion flue gases.

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