A comprehensive three-dimensional mathematical model is developed for studying the microwave-assisted pyrolysis of biomass. Kraft Lignin is considered as biomass feedstock in the model, and a mixture of lignin and char, is used as the sample for pyrolysis. A lumped kinetic model which considers three lumped pyrolysis products (gas, liquid and remaining solid fractions) is coupled with the governing equations for the microwave field, heat transfer, mass transfer, Darcy fluid flow and a transient numerical analysis is performed. The distribution of electric field in the microwave cavity, and the distribution of electric field, temperature, and pyrolysis products within the lignin sample are presented. The lignin sample is predicted to undergo volumetric heating when subjected to microwave heating. Accordingly the reaction zone extends from the center of the biomass sample bed towards the outer surface. Preliminary evaluation of the applicability of the model for assessing the effect of different parameters on the microwave pyrolysis of lignin is also carried out.
In this work, a one-dimensional numerical uid model is developed for co-axial dielec- tric barrier discharge (DBD) in pure helium and a parametric study is performed to systematically study the in uence of relative permittivity of the dielectric barrier and the applied voltage amplitude and frequency on the discharge performance. Discharge current, gap voltage and spatially averaged electron density pro les are presented as a function of relative permittivity and voltage parameters. For the geometry un- der consideration, both the applied voltage parameters are shown to increase the maximum amplitude of the discharge current peak up to a certain threshold value, above which it stabilized or decreased slowly. The spatially averaged electron density pro les follow a similar trend as the discharge current. Relative permittivity of the dielectric barrier is predicted to have a positive in uence on the discharge current. At lower frequency it is also shown to lead a transition from Townsend to glow dis- charge mode. Spatially and time averaged power density is also calculated and is shown to increase with increasing relative permittivity, applied voltage amplitude and frequency.
A comprehensive 3D coupled mathematical model is developed to study the microwave assisted thermocatalytic decomposition of methane with activated carbon as the catalyst. A simple reaction kinetic model for methane conversion (accounting for catalyst deactivation) is developed from previously published experimental data and coupled with the governing equations for the microwaves, heat transfer, mass transfer and fluid flow physics. Temperature distribution and concentration profiles of CH4 & H2 in the catalyst bed are presented. The temperature profiles at di erent input power values predict a non-uniform temperature distribution with hot-spots near the top and bottom of the catalyst. The concentration profiles predict a linear variation of CH4 and H2 concentration along the length of the reactor and show a good agreement with experimental conversion values. The influence of volumetric hourly space velocity on methane conversion is also investigated.
In this study, plasma-catalytic steam reforming of toluene as a biomass tar model compound was carried out in a coaxial dielectric barrier discharge (DBD) plasma reactor. The effect of Ni/Al2O3 catalysts with different nickel loadings (5?20 wt%) on the plasma-catalytic gas cleaning process was evaluated in terms of toluene conversion, gas yield, by-products formation and energy efficiency of the plasma-catalytic process. Compared to the plasma reaction without a catalyst, the combination of DBD with the Ni/Al2O3 catalysts significantly enhanced the toluene conversion, hydrogen yield and energy efficiency of the hybrid plasma process, while significantly reduced the production of organic by-products. Increasing Ni loading of the catalyst improved the performance of the plasma-catalytic processing of toluene, with the highest toluene conversion of 52% and energy efficiency of 2.6 g/kWh when placing the 20 wt% Ni/Al2O3 catalyst in the plasma. The possible reaction pathways in the hybrid plasma-catalytic process were proposed through the combined analysis of both gas and liquid products.
In this work, a two-dimensional numerical
fluid model is developed for a partially
packed dielectric barrier discharge (DBD) in pure helium. In
fluence of packing on
the discharge characteristics is studied by comparing the results of DBD with partial
packing with those obtained for DBD with no packing. In the axial partial packing
configuration studied in this work, the electric field strength was shown to be en
hanced at the top surface of the spherical packing material and at the contact points
between the packing and the dielectric layer. For each value of applied potential,
DBD with partial packing showed an increase in the number of pulses in the current
profile in the positive half cycle of the applied voltage, as compared to DBD with
no packing. Addition of partial packing to the plasma-alone DBD also led to an
increase in the electron and ion number densities at the moment of breakdown. The
time averaged electron energy profiles showed that a much higher range of electron
energy can be achieved with the use of partial packing as compared to no packing
in a DBD, at the same applied power. The spatially and time averaged values over
one voltage cycle also showed an increase in power density and electron energy on
inclusion of partial packing in the DBD. For the applied voltage parameters studied
in this work, the discharge was found to be consistently homogeneous and showed
the characteristics of atmospheric pressure glow discharge.
This review presents the developments in the mathematical models for various
bioelectrochemical systems. A number of modeling approaches starting
with the simple description of biological and electrochemical processes in
terms of ordinary differential equations to very detailed 2D and 3D models
that study the spatial distribution of substrates and biomass, have been
developed to study BES performance. Additionally, mathematical models
focused on studying a particular process such as ion diffusion through membrane
and new modeling approaches such as artifcial intelligence methods,
cellular network models, etc., have also been described. While most mathematical
models are still focused on performance studies and optimization of
microbial fuel cells, new models to study other BESs such as microbial electrolysis
cell, microbial electrosynthesis and microbial desalination cell have
also been reported and discussed in this review.
A novel framework integrating dynamic simulation (DS), life cycle assessment (LCA) and techno-economic assessment (TEA) of bioelectrochemical system (BES) has been developed to study for the first time wastewater treatment by removal of chemical oxygen demand (COD) by oxidation in anode and thereby harvesting electron and proton for carbon dioxide reduction reaction or reuse to produce products in cathode. Increases in initial COD and applied potential increase COD removal and production (in this case formic acid) rates. DS correlations are used in LCA and TEA for holistic performance analyses. The cost of production of HCOOH is ¬0.015?0.005g?1 for its production rate of 0.094?0.26kgyr?1 and a COD removal rate of 0.038?0.106kgyr?1. The life cycle (LC) benefits by avoiding fossil-based formic acid production (93%) and electricity for wastewater treatment (12%) outweigh LC costs of operation and assemblage of BES (?5%), giving a net 61MJkg-1HCOOH saving.
A two-dimensional numerical
fluid model is developed for studying the influence of
packing configurations on dielectric barrier discharge (DBD) characteristics. Dis-
charge current profiles, and time averaged electric field strength, electron number
density and electron temperature distributions are compared for the three DBD configurations, plain DBD with no packing, partially packed DBD and fully packed DBD.
The results show a strong change in discharge behaviour occurs when a DBD is fully
packed as compared to partial packing or no packing. While the average electric
field strength and electron temperature of a fully packed DBD are higher relative
to the other DBD configurations, the average electron density is substantially lower
and may impede the DBD reactor performance under certain operating conditions.
Possible scenarios of the synergistic effect of the combination of plasma with catalysis
are also discussed.
Bioelectrochemical systems (BESs) have been catalogued as a technological solution to three pressing global challenges: environmental pollution, resource scarcity, and freshwater scarcity. This study explores the social risks along the supply chain of requisite components of BESs for two functionalities: (i) copper recovery from spent lees and (ii) formic acid production via CO2 reduction, based on the UK?s trade policy. The methodology employed in this study is based on the UNEP/SETAC guidelines for social life-cycle assessment (S-LCA) of products. Relevant trade data from UN COMTRADE database and generic social data from New Earth?s social hotspot database were compiled for the S-LCA. The results revealed that about 75% of the components are imported from the European Union. However, the social risks were found to vary regardless of the magnitude or country of imports. ?Labour and Decent Work? was identified as the most critical impact category across all countries of imports, while the import of copper showed relatively higher risk than other components. The study concludes that BESs are a promising sustainable technology for resource recovery from wastewater. Nevertheless, it is recommended that further research efforts should concentrate on stakeholder engagement in order to fully grasp the potential social risks.
This study describes and evaluates a dynamic computational model for two chamber microbial
electrosynthesis (MES) system. The analysis is based on redox mediators and a two population
model, describing bioelectrochemical kinetics at both anode and cathode. Mass transfer rates
of substrate and bacteria in the two chambers are combined with the kinetics and Ohm?s law to
derive an expression for the cell current density. Effect of operational parameters such as initial
substrate concentration at anode & cathode and the operation cycle time, on MES performance
are evaluated in terms of product formation rate, substrate consumption and Coulombic efficiency
(CE). For fixed operation cycle time of 3 or 4 days, the anode and cathode initial substrate concentrations
show linear relationship with product formation rate; however MES operation with 2
day cycle time shows a more complex behaviour, with acetic acid production rates reaching a
plateau and even a slight decrease at higher concentrations of the two substrates. It is also
shown that there is a trade-off between product formation rate and substrate consumption & CE.
MES performance for operation with cycle time being controlled by substrate consumption is also
described. Results from the analysis demonstrate the interdependence of the system parameters
and highlight the importance of multi-objective system optimization based on targeted end-use.
In this work, a dynamic computational model is developed for a single
chamber microbial fuel cell (MFC), consisting of a bio-catalyzed anode and
an air-cathode. Electron transfer from the biomass to the anode is assumed
to take place via intracellular mediators as they undergo transformation between
reduced and oxidized forms. A two-population model is used to describe
the biofilm at the anode and the MFC current is calculated based on
charge transfer and Ohm's law, while assuming a non-limiting cathode reaction
rate. The open circuit voltage and the internal resistance of the cell are
expressed as a function of substrate concentration. The effect of operating
parameters such as the initial substrate (COD) concentration and external
resistance, on the Coulombic efficiency, COD removal rate and power density
of the MFC system is studied. Even with the simple formulation, model
predictions were found to be in agreement with observed trends in experimental studies. This model can be used as a convenient tool for performing
detailed parametric analysis of a range of parameters and assist in process
Sadhukhan Jhuma, Gadkari Siddharth, Martinez-Hernandez Elias, Ng Kok Siew, Shemfe Mobolaji, Torres-Garcia Enelio, Lynch Jim (2019) Novel macroalgae (seaweed) biorefinery systems for integrated chemical, protein, salt, nutrient and mineral extractions and environmental protection by green synthesis and life cycle sustainability assessments,Green Chemistry
Royal Society of Chemistry
Highly efficient macroalgae based chemical factories and environmental protection have been comprehensively studied for the first time to displace fossil resources to mitigate climate change impact. Wild macroalgae by (bio)phytoremediation and residual macroalgae by biosorption can be used to treat wastewaters, marine environment, soil and sludge. Cultured macroalgae can be processed through drying, milling, grinding, suspension in deionised water and filtration extracting sap of heavy metals; centrifugation of solids recovering nutrients; ion exchange resins of supernatants separating protein and polysaccharides; dialysis purifying protein from salts and pretreatment of polysaccharides producing a sugar platform. Protein profiling shows the presence of the essential amino acids as well as others as food additive, flavour enhancer and pharmaceutical ingredient. Sugars can be converted into a chemical: levulinic acid by controlled acid hydrolysis; 2,5-furandicarboxylic acid by heterogeneous catalytic reaction; succinic acid by tricarboxylic acid cycle; lactic acid by fermentation, with
3-5 times market value than bioethanol. Protein, sugar based chemical and inorganics give the highest to the lowest climate change impact savings of 12, 3 and 1 kg CO2 equivalent kg-1 product. Their cost of production is estimated at $2010 t-1, significantly lower than their market prices, making the integrated marine biorefinery system economically more attractive than lignocellulosic terrestrial biorefinery systems. Social life cycle assessment indicates that the highest to the lowest avoided social impacts will be from the displacements of animal based protein, sugars and minerals, in Indonesia, China and Philippines (producing 27 million tonnes per annum, 93% of global production), respectively.
A two-dimensional mathematical model has been developed for characterizing and predicting the dynamic performance of an air-cathode MFC with graphite fiber brush used as anode. The charge transfer kinetics are coupled to the mass balance at both electrodes considering the brush anode as a porous matrix. The model has been used to study the effect of design (electrode spacing and anode size) as well as operational (substrate concentration) parameters on the MFC performance. Two-dimensional dynamic simulation allows visual representation of the local overpotential, current density and reaction rates in the brush anode and helps in understanding how these factors impact the overall MFC performance. The numerical results show that while decreasing electrode spacing and increasing initial substrate concentration both have a positive influence on power density of the MFC, reducing anode size does not affect MFC performance till almost 60 brush material has been removed. The proposed mathematical model can help guide experimental/pilot/industrial scale protocols for optimal performance.
This study presents a steady state, two dimensional mathematical model of microbial fuel cells (MFCs) developed by coupling mass, charge and energy balance with the bioelectrochemical reactions. The model parameters are estimated and validated using experimental results obtained from ýve aircathode MFCs operated at different temperatures. Model analysis correctly
predicts the nonlinear performance trend of MFCs with temperatures ranging between 20 oC - 40 oC. The two dimensional distribution allows the computation
of local current density and reaction rates in the bioýlm, helping to correctly capture the interdependence of system variables and predict the
drop in power density at higher temperatures. Model applicability for parametric
analysis and process optimization is further highlighted by studying the effect of electrode spacing and ionic strength on MFC performance.
We present a correlation for determining the power density of microbial fuel cells based on dimensional analysis. Important operational, design and biological parameters are non-dimensionalized using a selection of scaling variables. Experimental data from various microbial fuel cell studies operating over a wide range of system parameters are analyzed to attest accuracy of the model in predicting power output. The correlation predicts nonlinear dependencies between power density, substrate concentration, solution conductivity, external resistance, and electrode spacing. The straightforward applicability without the need for any significant computational resources, while preserving good level of accuracy; makes this correlation useful in focusing the experimental effort for the design and optimization of microbial fuel cells.