Xiaotao Bi

Professor

Research Interests

Biomass and Bioenergy
Multiphase Chemical Reactors
Fluidization
Particle technology
Electrostatics of Powders
Life Cycle Analysis
Green Engineering
Industrial Symbiosis
Fuel Cells Water Management

Relevant Degree Programs

 

Research Methodology

Pilot fluidized bed reactors
Electrostatics measurement and characterization
Life Cycle Analysis Software and Database

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Master's students
Doctoral students
Postdoctoral Fellows
2019
2020

Biomass catalytic pyrolysis and torrefaction in pilot fluidized bed reactors.

Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - May 2019)
Wood fuel from British Columbia : multi-scale assessment of the economic, energetic and environmental efficiencies of the supply chains of conventional and torrefied wood pellets (2019)

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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Forest residues to energy : local air quality, health risks and greenhouse gas emissions (2018)

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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Microwave-assisted catalytic pyrolysis of biomass for improving bio-oil and biochar properties (2018)

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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Novel measurement of solids circulation rate in pilot-scale dual fluidized bed gasifier at high temperature (2018)

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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Heat and mass transfer in pulsed fluidized bed of biomass (2017)

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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Modeling of biomass steam gasification in a dual fluidized bed reactor with/without lime-based CO₂ capture (2017)

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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A study of cellulosic biomass size reduction (2016)

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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Probing the electrostatics and hydrodynamics in gas-solid fluidized beds (2015)

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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Effect of operating conditions and particle properties on electrostatics and entrainment in gas-solid fluidized beds (2014)

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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Electro-hydrodynamics of gas-solid fluidized beds (2013)

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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Hydrothermal pretreatment of softwood biomass and bark for pelletization (2013)

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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Modeling and simulation of a novel internal circulating fluidized bed reactor for selective catalytic reduction of nitrogen oxides (2013)

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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A study of softwood torrefaction and densification for the production of high quality wood pellets (2012)

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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Numerical simulations of gas-liquid two-phase flow in Polymer Electrolyte Membrane fuel cells (2012)

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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Steam explosion of biomass to produce durable wood pellets (2011)

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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Effect of operating parameters and particle properties on electrostatics in gas-solid fluidized beds (2009)

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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A novel fluidized bed reactor for integrated NOx adsorption-reduction with hydrocarbons (2008)

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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Master's Student Supervision (2010 - 2018)
Life cycle and techno-economic assessment of transportation biofuels from hydrothermal liquefaction of forest residues in British Columbia (2018)

Biofuels from hydrothermal liquefaction (HTL) of abundantly available forest residues in British Columbia (BC) can potentially make great contributions to reduce the greenhouse gas (GHG) emissions from the transportation sector. Life cycle and techno-economic assessment are conducted to evaluate the environmental and economic performance of a hypothetic 100 million liters per year (MLPY) HTL biofuel system in the Coast Region of BC based on three different supply chain designs.The life cycle GHG emission of HTL biofuels ranges from 17.0-20.5 g CO₂-eq/MJ, corresponding to 78%-82% reduction compared with petroleum fuels. A further reduction of 6.8 g CO₂-eq/MJ can be achieved when by-product biochar is applied for soil amendment. The conversion stage dominates the total GHG emissions, making up more than 50%. The process emitting most GHGs over the life cycle of HTL biofuels is HTL buffer production. Transportation emissions can be lowered by 83% if forest residues are converted to bio-oil before transportation. Process performance parameters (e.g., HTL energy requirement and biofuel yield) and the location specific parameter (e.g., electricity mix) have significant influence on the GHG emissions of HTL biofuels. The economic analysis shows that the minimum selling price (MSP) of HTL biofuels ranges from $0.82-$0.90 per liter of gasoline equivalent, which is about 63%-80% higher than that of petroleum fuels. Converting forest residues to bio-oil and wood pellets before transportation can significantly lower the variable operating cost but not the MSP of HTL biofuels, due to the considerable increase in capital investment. Bio-oil and biofuel yield can significantly influence the MSP of HTL biofuels. Therefore, technology advancement is needed to bring down the production cost of HTL biofuels, otherwise, a high carbon tax can be applied to make HTL biofuels competitive with petroleum fuels.

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Flow visualization in 3D printed PEM fuel cell bipolar plates (2017)

In recent years, due to the public concern on global warming, both increasing energy efficiency and developing green energy become crucially important. Fuel cells can be one of the most suitable clean energy solutions for the environment because of its high energy conversion efficiency and near zero emissions of criteria air pollutants at the use stage. To increase the energy efficiency of fuel cells, effectively utilize the Pt catalyst and increase the fuel cell durability, the uniform distribution of the reactants over the fuel cell active area is of great importance. Over the last decade, many researchers have focused on developing flow field design to homogenously distribute the reactant and to decrease the pressure drop in the bipolar plates. However, most of the previous studies are in the stage of numerical simulation, and the few experimental studies have used very simple flow field geometries. Not to mention that complex transport phenomena inside a fuel cell make even the numerical simulation challenging and time consuming, which hinders the quick screening of proposed modifications and new designs.While the conventional fabrication techniques are expensive and time consuming, 3D printing is a very good rapid prototyping method that can be used both to validate the simulation results and to supplement the tedious simulation work. The question is whether the results from 3D printed flow fields could be as accurate and reliable as flow fields fabricated with conventional methods.In the present research, we investigated the applicability of 3D printing in validating the simulation results and as a fast screening method. State of the art designs for anode, cathode and water cooling BPPs proposed and fabricated using Polyjet 3D printing, SLA 3D printing and laser-cutter technologies and the pressure drop and velocity profiles were measured for each plate. The results demonstrated that SLA 3D printing has great promises to serve as a screening tool in modifying the flow field design, as well as in validating the simulation results.

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Hydrodynamics and solids mixing behaviour of fluidized beds with inclined-hole distributor (2017)

Most research on the hydrodynamics and solids mixing of swirling fluidized beds has targeted applications relate to drying and combustion processes, with large mean particlediameters. A potential use of such reactors is in the area of catalyst regeneration to improve mixing. In the present study, the hydrodynamics and solids mixing behaviour of swirling fluidized beds were investigated for particles in Groups A and B of the Geldart classification. Three distributors were designed and fabricated in-house. They shared the same specifications, but differed in the orifice inclination angle (30, 45 and 90 to the horizontal). The effect of orifice angle on the hydrodynamics of a fluidized bed of glass beads was investigated. The study showed that, in an empty bed, the distributor pressure drop waslower for the inclined-hole distributors compared to the 90-hole distributor by a factor of 10%. In addition, bed pressure drop increased with the inclined-hole distributors as well with static bed height. Bed expansion was also investigated and found that in a shallow bed, the inclined-hole distributor led to less expansion compared to the 90-hole distributor. However, in a deep bed, the orifice angle had negligible influence on bed expansion. The minimum fluidization velocity was found to change with static bed height for the inclined-hole distributors, and it was also higher for steeper angles. Solids mixing was also explored, axial mixing for the 90-hole distributor and tangential mixing for all three distributors. Residence time distribution studies were conducted using phosphorescent tracer particles belonging to Group A, activated by ultraviolet light. The turnover time was estimated using the bubbling bed model and found to match the experimental results well. It was found that the probes installed at the walls of the fluidization column reduced the dense phase downward velocity. The tangential particle velocity was also estimated and was found to be highest for the 30-hole distributor, decreasing with increasing orifice angle. A dispersion model was used to describe tangential mixing for all three distributors which showed that the dispersion coefficient for the inclined-hole distributors was twice that for the 90-hole in a shallow bed.

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CO2 enrichment in ambient air by temperature swing adsorption and its applications for stimulating plant growth in greenhouses (2014)

Adsorption on proper adsorbents is one of the commonly used technologies to capture carbon dioxide. Zeolites, such as 13X, exhibit good adsorption capacity and selectivity towards CO₂. Compared with CO₂ capture from large point sources with high concentration of CO₂, direct capture from the ambient air plays a better role in the reduction of greenhouse gases. On the other hand, greenhouse crops can be benefited from CO₂ enrichment, typically around 1000 ppm. By applying temperature swing adsorption to ambient air, CO₂ concentration can be enriched from 400 ppm to about 1000 ppm, which can then be directly used for greenhouse CO₂ enrichment. The proposed method not only helps the capture of CO₂ from air but also provides an enriched CO₂ stream to greenhouses.In this study, the performance of zeolite 13X was evaluated in a fixed bed reactor for enriching ambient CO₂ concentration from 400 ppm to 1000 ppm by temperature swing adsorption under different operating conditions such as ambient temperature and moisture content. Results showed that 13X performed well for both CO₂ adsorption and desorption, and an enrichment factor of 3 can be reached, demonstrating the feasibility of the proposed TSA method. A lower adsorption temperature and a higher desorption temperature would result in a higher enriched CO₂ concentration.Finally, economic analyses have been carried out to compare the unit cost of proposed method for capturing one tonne CO₂ with the cost of other air capture technologies and the cost of CO₂ supply in current greenhouse operations. The unit cost of CO₂ enrichment by temperature swing adsorption seems to be quite competitive if the adsorption and desorption capacity of the currently tested adsorbent could be increased by six times to the level as reported in the literature.

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Growth optimization of Synechococcus sp. PCC7002 in laboratory photobioreactors (2014)

Microalgae have the potential to be a significant source of renewable energy. Microalgae reduce CO₂ emissions by consuming it via photosynthesis, and provide a cheap option to produce high value biological products. Synechococcus sp. PCC7002 is a microalga strain that possesses all of these potentials, and can be easily genetically modified. To utilize these potentials of Synechococcus sp. PCC7002, a method to optimize its growth in terms of a high biomass concentration and a high growth rate needs to be implemented. To achieve this objective, shake flask scale experiments, as well as reactor scale experiments were designed and conducted. 250 mL shake flasks with 100 mL of medium were used for the flask experiments. In the first experiment, the A+ medium was investigated. The optimal concentrations of the three important nutrient components, NaNO₃, FeCl₃, and KH₂PO₄ to achieve highest Xmax were determined to be 23.5 mM, 0.028 mM, and 0.72 mM respectively. The optimal concentrations to achieve highest µmax were 5.88 mM, 0.007 mM, and 0.18 mM respectively. Another factorial experiment regarding the effects of temperature and light intensity was carried out next. The optimal conditions within the tested range were determined to be 35˚C, 250µE/m²/s for maximum biomass concentration, and 35˚C, 150 µE/m²/s for maximum specific growth rate. The effects of inlet CO₂ concentrations were studied in the large scale continuously aerated reactor. The optimal concentration of CO₂ was found to be 8% by volume, and 3.1 g/L biomass concentration and 0.0186 hr-¹ maximum specific growth rate were achieved under this condition. Further increase in inlet CO₂ concentration led to a decrease in biomass concentration due to the lower pH associated with the higher carbonic acid concentration in the medium. Lastly, an experiment was completed using the recombinant strain, with a very good growth rate obtained at 33˚C, 300 µE/m²/s, and 0.5 L/min inlet gas with 10% CO₂. Under this condition, a maximum biomass concentration of 3.1 g/L, and a maximum specific growth rate of 0.0180 hr-¹ were achieved.

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Growth optimization of Synechococcus elongatus PCC 7942 in lab flask and 2D photobioreactor (2013)

One of the most promising mechanisms for the production of high value biologically active products is through the cultivation of microalgae. In addition to serving as a carbon capture system, this photosynthetic microorganism has demonstrated potential for recombinant protein expression as an approach towards sustainable development in biotechnology. Extensive studies on cyanobacterium Synechococcus elongatus PCC 7942 have assessed the dual function of carbon capture with product generation such as biodiesel and recombinant protein. In order to maximize CO₂ fixation and production rates of valuable product, a high cell growth rate needs to be achieved. Consequently, challenges in photobioreactor operation and cultivation need to be addressed, such as CO₂ mass transfer limitations, light availability, and minimizing energy consumption. Thus, the effects of the major growth factors need to be studied.In this research, the objectives were to optimize the specific growth rate and biomass concentration of S. elongatus by investigating the effects of medium composition, light intensity, temperature, and CO₂ concentration. Preliminary studies at the shake flask scale revealed that an optimization of components in BG-11 medium resulted in no significant improvements for the specific growth rate and biomass concentration. However, a maximum specific growth rate of 0.0519 1/h and a maximum biomass concentration of 0.496 g/L were achieved at 33⁰C and 120 μE/m²/s. A 1 L airlift photobioreactor was used to investigate the effects of light intensity, CO₂ concentration, and gas flow rate on the specific growth rate and biomass concentration. Additional experiments carried out in this photobioreactor revealed that air enriched with 5% CO₂ at 1 L/min, 33⁰C, and 120 μE/m²/s achieved a maximum biomass concentration of 1.006 g/L at a reduced specific growth rate of 0.0234 1/h. Further increases in CO₂ % and light intensity, as well as light/dark cycles, reduced the growth rate and biomass concentration. Mass transfer experiments also revealed that 5% CO₂ provided the best growth conditions, as growth was significantly limited by CO₂ when supplied with air, whereas 10% CO₂ reduced the pH and consequently reduced the specific growth rate.

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Life cycle analysis of an integrated biogas-based agriculture energy system (2013)

Large quantity of manure is generated in the livestock industry in British Columbia (BC) and naturalgas is being consumed intensively in BC’s agriculture sector. We proposed to integrate the livestockfarms and the greenhouses to promote waste-to-energy and waste-to-material exchanges followingthe principles of Industrial Ecology (IE). Natural gas consumptions on farms are replaced byrenewable biogas generated from anaerobic digestion (AD) of farm wastes (mainly livestock manure).CO₂ for plant enrichment in greenhouses is supplied by biogas combustion flue gases and theresidues (digestate) from digesters are used as animal bedding materials, plant growing media, andliquid fertilizers. An integrated dairy farm and greenhouse was first modeled. Co-digestion of manurewith a variety of organic farm wastes was further evaluated with an aim to enhance the biogasproduction. To address the problems of too much digestate surplus and high CO₂ demand forgreenhouse CO2 enrichment, the mushroom farm was further introduced into the integrated system.In this way, the digestate surplus can be used as a growing media for growing mushrooms and theCO₂–rich ventilation air from the mushroom can be directed to the greenhouse for CO₂ enrichment.A Life Cycle Analysis (LCA) was conducted to quantify the environmental impacts of each of theproposed cases in comparison to the conventional agriculture practices.The LCA results showed that the integrated dairy farm-greenhouse system reduces non-renewableenergy consumption, climate change, acidification, respiratory effects from organic emissions, andhuman toxicity by more than 50% compared to conventional operations; among which the reductionsin non-renewable energy consumption, climate change, and human toxicity are the most significant.If the digestate surplus is treated as a waste, the integrated system has a ~20% increase ineutrophication and respiratory effects from inorganic emissions. When other organic wastes are codigestedwith dairy manure, all the impacts can be further reduced in all cases. If a mushroom farm isintroduced to form an integrated dairy farm-greenhouse-mushroom farm system, a large greenhousecan be facilitated and the digestate can be largely reused; thus all the analyzed impacts aresignificantly reduced compared to the base scenario.

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A study of water crossover in polymer electrolyte membrane fuel cells (2011)

Water crossover between anode and cathode of polymer electrolyte membrane fuel cells has been studied together with fuel cell performance at steady state. The parameters considered included temperature, pressure, inlet humidity and the presence of a cathode microporous layer. In general water crossover was found to be increasingly toward the anode side with increasing current density up to a certain point beyond which a plateau was observed. Larger cathode-to-anode inlet humidity gradient, lower temperature and higher cathode pressure enhanced water crossover to the anode, due to a higher downstream humidity at the cathode catalyst layer.The presence of a cathode microporous layer enhanced water crossover to the anode only when the cathode inlet humidity was low. It was proposed that this layer imposed a larger diffusion barrier between the cathode channel and the membrane interface whose effects diminished at high relative humidity. The zero crossover rate under zero humidity gradients with no load regardless of the presence of the cathode microporous layer suggested that capillary action was not a contributing factor for the action of the layer.In addition, the quantitative data obtained by the water crossover measurement equipment were found to be useful in model validation and parameter estimations. The data could pinpoint inadequacies in models, as well as providing estimated parameters that were more consistent with changes in the oxygen concentration and fitted better to both the current density and water crossover data given a certain voltage.

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Development of British Columbia wood pellet life cycle inventory and its utilization in the evaluation of domestic pellet applications (2011)

An in-house life cycle inventory (LCI) database for British Columbia (BC) wood pellets is established. The LCI database is used to compare the performance of BC pellets exported to Rotterdam and BC pellets staying within BC in terms of energy penalty, percent of fossil fuel content in pellets arriving destination, and impacts (human health, ecosystem quality and climate change) by performing life cycle impact assessment (LCIA) in a commercial LCA software. The database is also utilized to assess two domestic applications of BC wood pellets: replacing natural gas combustion in UBC district heating facility with wood waste or wood pellet gasification, and replacing firewood in BC residential heating with wood pellets. Overall, the analysis indicated that marine transportation is responsible for over 40% of the life cycle energy consumption and more than 50% of each of the impact categories investigated. The energy penalty and fossil fuel content of exported pellets are roughly 50% and 90% higher than that of the non-exported pellets. For the district heating case study, the base scenario performs much better than all biomass gasification systems in all impact categories other than climate change. The saving in GHG emission is approximately 81% if woody biomasses are utilized. Over the entire life cycle, controlled wood waste gasification system performs better than controlled wood pellet gasification system due to the extra processing required for wood pellets. However, when looking at the health impact associated with stack emissions only, controlled wood pellet gasification would raise the health impact by 12% from the base case while controlled wood waste gasification would raise the impact by 133%.By switching from firewood to wood pellets for BC residential heating, the primary energy consumption and impacts on human health, ecosystem quality and climate change can be reduced by 34%, 95%, 27% and 17%, respectively. Over 90% reduction in external costs can also be achieved. In terms of economic viability, when bulk pellets are to be utilized, switching from firewood to pellet units would be reasonable as long as the unit to be replaced is not a fireplace insert.

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