Steam gasification of char from wood chips fast pyrolysis: Development of a semi-empirical model for a fluidized bed reactor application

This study, performed in the context of GAYA project, focuses on the development of a simple predictive model about steam gasification of char from woodchips fast pyrolysis. A semi-empirical model was developed through experiments in a macro thermogravimetric analyzer which owns the peculiar ability of fast heating, as well as to deal with macro-size particles and higher mass loads compared to conventional TGA. The experimental results show that gasification is controlled by chemical kinetics and internal transfer phenomena. During gasification, char particles can be considered as isothermal in a given range of temperatures and particle sizes, more likely for low values. The gasification model was based on the effectiveness factor, which involves the chemical kinetics and diffusion rate. The chemical kinetics were expressed by a classical Arrhenius law, whereas empirical expressions from mathematical fitting of the experimental data were established for the diffusion coefficient and surface function. The diffusion coefficient from this work is suspected to probably include supplementary rate limiting phenomena, apart from steam porous diffusion, such as H₂ inhibition and/or the decrease of temperature within char particles because of the endothermic character of gasification. The model globally predicts with accuracy the gasification rate in typical operating conditions of a fluidized bed reactor. For its simplicity and reliability, this approach can be used for the modelling of char gasification in the conditions of interest.     

Energy value as a factor of agroforestry wood species selectivity in Akinyele and Ido local government areas of Oyo State, Nigeria

Wood usage for cooking and heating is still very relevant in most developing countries especially those of sub-Saharan Africa and many parts of Asia. Therefore, sustainable means of generating it for this and other purposes are necessary bearing in mind the influence of indigenous knowledge/users' perspective on any production method regarding success and sustenance. In conformity with this view, questionnaires were administered on 240 respondents in 8 rural communities of Akinyele and Ido Local Government Areas (LGAs) of Oyo State, Nigeria, to elicit information on species that can be used as fuelwood, preferred by the respondents for incorporation into and/or retention in agroforestry plots, out of which 179 (i.e. 75% of the total number of questionnaires administered) were successfully retrieved for statistical analyses. Twelve woody species namely: Annona senegalensis, Anogeissus leiocarpus, Bridelia ferruginea, Daniellia oliveri, Detarium microcarpum, Gardenia ternifolia, Hymenocardia acida, Lophira lanceolata, Parkia biglobosa, Terminalia avicennioides, Triplochiton scleroxylon and Vitellaria paradoxa were prioritized on the basis of respondents' preference using a ranking pattern. Friedman chi-square analysis showed that there was no significant difference (p < 0.05) in the ranking pattern of the respondents from the two LGAs. The mean net calorific values (NCV) of the 12 species were found to be 17.71, 18.63, 18.04, 16.03, 17.67, 18.46, 19.00, 21.68, 19.63, 18.25, 14.65, and 19.47 MJ kg⁻¹ respectively. The result of a two-way analysis of variance indicated a significant variation (p < 0.05) in NCV data for all the species pooled together but not for each species except for D. oliveri with a follow-up test using Fisher's Least Significant Difference. The Pearson's moment correlation analysis gave positive coefficient values (r = 0.868 and 0.874, p < 0.05) between NCV and the cumulative ranking values in Akinyele and Ido LGAs respectively. There was also a coefficient value of 0.873 (p < 0.05) between the cumulative ranking values in the two LGAs. In line with the outcome of this study, it was therefore recommended that native intelligence/indigenous knowledge and/or users' perspective should be part of the criteria for selecting potential fuelwood species for incorporation into and/or retention in agroforestry systems in this and other areas with similar characteristics.     

Selective non-catalytic reduction of flue gas in a circulating fluidized bed

Selective non-catalytic reduction can meet the requirements of the new National Emission Regulation due to the low NOx emission characteristic of circulating fluidized bed boilers. In this work, ammonia was injected into the simulated circulating fluidized bed flue gas as a reducing agent. Optimum reaction conditions were obtained as: temperature of 920°C, residence time of 0.6 s, and a normalized stoichiometric ratio (NSR₁) of 1.5. H₂, as an additive, made a shift of 170°C towards a lower temperature, while CH₄ made a shift of 80°C.     

Von Rittinger theory adapted to wood chip and pellet milling,in a laboratory scale hammermill

The study draws upon the milling theories developed for the ore processing industry (Von Rittinger, Kick and Bond theories) in order to define a method for characterising wood chip and pellet energy consumption during milling.Energy consumption during wood milling depends on three main factors: the material moisture content, the particle size difference between the feed and the milled product, and the material itself. The latter may be characterised by a single parameter based on an adaptation of Von Rittinger's constant.A relation characterising wood pellet energy consumption as a function of the particle size distribution of the pellet ingredients and the milled pellets is proposed. This is characteristic of each type of pellet for each moisture content value considered.     

Environmental and utility planning implications of electricity loss reduction in a developing country: a comparative study of technical options

This paper discusses the environmental and utility planning implications of load (i.e. ‘variable’) and non-load (i.e. ‘core’) loss reduction options in the case of a mixed hydro-thermal power system from a long-term electricity generation expansion planning perspective. A key finding of the study is that a reduction of electricity losses in transmission and distribution need not always reduce emissions of air pollutants; and that neither loss reduction option is consistently superior over time from the environmental perspective. For a given level of loss reduction, long-run average cost of electricity generation with the variable loss reduction option generation was found to be less than that with the core loss reduction option. © 1998 John Wiley & Sons, Ltd.     

Open-air storage of fine and coarse wood chips of poplar from short rotation coppice in covered piles

The cultivation of short rotation coppices (SRC) on agricultural land represents an economically and environmentally promising option for sustainable provision of bioenergy. Not only the further development of efficient harvesting machinery, but also the development of harvest-optimised storage systems are necessary to implement cost-efficient cultivation and use strategies for SRC in practice. The storage of fine wood chips from poplar harvest with a forage harvester results in high dry matter losses of up to 25%. Tractor-mounted mower-chippers can harvest coarse wood chips that might possess more favourable storage and drying properties. The main objective of the current research project was to develop and perform a storage experiment in which the storage behaviour of fine and coarse wood chips could be examined and compared in detail over a period of nine months. In this experiment two covered storage piles (height 3.5 m), with over 500 m³ fine and coarse wood chips respectively, were examined under practice scale conditions in Germany. After nine months of storage the fine chips in the core of the storage pile had dried to a moisture content of 34% with dry matter losses of 22%. Coarse chips, on the other hand, achieved a moisture content of 29% and dry matter losses of 21% in the same period. The maximum moisture content of 40% required by heating plants in practice is achieved by fine chips after 6.5 months and by the coarse chips already after 3.5 months.     

Co-benefits of CO2 emission reduction in a developing country

In this paper, we examine the co-benefits of reducing CO₂ emissions in Thailand during 2005–2050 in terms of local pollutant emissions as well as the role of renewable-, biomass- and nuclear-energy. It also examines the implications of CO₂ emission reduction policy on energy security of the country. The analyses are based on a long term energy system model of Thailand using the MARKAL framework. The study shows that the power sector would account for the largest share (over 60%) in total CO₂ emission reduction followed by the industrial and transport sectors. Under the CO₂ emission reduction target of 30%, there would be a reduction in SO₂ emission by 43% from the base case level. With the CO₂ emission reduction target of 10–30%, the cumulative net energy imports in the country during 2005–2050 would be reduced in the range of over 16 thousand PJ to 26 thousand PJ from the base case emission level. Under the CO₂ emission reduction targets, the primary energy supply system would be diversified towards lower use of coal and higher use of natural gas, biomass and nuclear fuels.     

Demand reduction in the UK—with a focus on the non-domestic sector

A demand reduction strategy is considered in the context of the UK and in the light of the UK Government's 2006 Energy Review. This paper discusses how a mechanism—a Demand Reduction Obligation (DRO)—can be established to achieve radical energy demand reduction targets in electricity and gas use in the industrial, commercial and public administration sectors. A DRO would require energy suppliers to invest in energy-saving measures so as to reduce energy demand in these sectors. The investment for this activity would be funded by energy suppliers who would increase prices in order to cover the cost of achieving the carbon reductions. Public opinion surveys suggest that a large proportion of the public would prefer to support demand reduction measures compared to other energy options. It may be practical to deliver carbon emission reductions equivalent to around 30% of emissions from the UK electricity sector over a 15-year period through a broad-based demand reduction strategy. Demand reduction is considered in the context of an assessment of costs and resources available from other low carbon options including renewable energy and nuclear power.     

A new method for simulating the combustion of a large biomass particle—A combination of a volume reaction model and front reaction approximation

Combining a volume reaction model and front reaction approximation is proposed to simulate the combustion of a large biomass particle. Two intraparticle processes—drying and char oxidation—are simplified as front reaction because they are transport controlled. The other intraparticle process—pyrolysis—is described as the volume reaction because it is controlled by both heat transfer and kinetics. A new numerical method based on the basic mechanism of the process is applied to mitigate oscillations of the solution of the front reactions. To compare the calculation results with the experimental results presented in the literature, combustion of cubic wood particles between 5 and 25 mm is chosen to test the new method. Drying, pyrolysis, char oxidation, vapor condensation, shrinkage of the process, heat transfer via conduction, diffusion, convection, radiation and mass transfer via diffusion, and convection inside particle are taken into account. Finite volumes attached to solid materials are used to discretize the domain and explicit method with variable time step is used to calculate the process. A program was written and the calculation showed that the conversion of a particle is almost independent of computational mesh from 10 cells on. However there is significant instability in the mass loss rate curve when the number of cells is less than 20. Predictions for different particle sizes, furnace temperatures and moisture contents were compared with measurements and they agree reasonably well. The results highlight the significance of pyrolysis kinetics on prediction. Thus, the front reaction model of pyrolysis assuming a constant reaction temperature of 773 K is sometimes inadequate. The proposed method also showed that moisture content and pyrolysis reactivity significantly affect the thickness of devolatilizing fuel.     

Drag reduction and heat transfer characteristics of water solutions with surfactants in a straight pipe

The drag reduction and heat transfer characteristics of water solutions with two kinds of surfactants (cetyltrimethylammonium bromide and dodecyltrimethylammonium chloride) in a straight pipe were investigated experimentally. The flow resistance and heat transfer of water solution flow with the two kinds of surfactants were markedly reduced as compared with those of pure water flow. Useful nondimensional correlative equations for flow resistance and heat transfer were derived in terms of various nondimensional parameters. © 1998 Scripta Technica. Heat Trans Jpn Res, 27(1): 1–15, 1998     

Three-dimension simulation of two-phase flows in a thin gas flow channel of PEM fuel cell using a volume of fluid method

This study investigates the two-phase flow in a thin gas flow channel of PEM fuel cells and wall contact angle's impact using the volume of fluid (VOF) method with tracked two-phase interface. The VOF results are compared with experimental data, theoretical solution and analytical data in terms of flow pattern, pressure drop and water fraction. Stable film flow is predicted, as observed experimentally, for the contact angle ranging from 5° to 40° including varying contact angles at different walls of a channel. The contact angle is found to have small impact on the gas pressure drop for the stratified flow regime, but it determines the meniscus of the two-phase interface, which affects the optical detection of the liquid thickness in experiment. The work is important to study of two-phase flow dynamics, multichannel design, experimental design and control of two-phase flows in thin gas flow channels for PEM fuel cells.     

Target-aimed versus wishful-thinking in designing efficient GHG reduction strategies for a metropolitan city: Taipei

In recent years, many national and local governments claim for a specific GHG (greenhouse gas) reduction goal targeted for many years later. In 2005, the Taipei City government announced that Taipei's total GHG emission in 2015 will reach the same level as that in 2005 and then down to 75% of that level at year 2030. However, based on the estimated energy consumption and GHG emission and the proposed emission reduction plans from the local government, it is clear that these goals are not going to be accomplished.     

Optimization of RuxSey electrocatalyst loading for oxygen reduction in a PEMFC

The synthesis, characterization and optimization of Ru_xSe_y catalyst loading as a cathode electrode for a single polymer electrolyte membrane fuel cell, PEMFC were investigated. Ru_xSe_y catalyst was synthesized via a decarbonylation of Ru₃(CO)₁₂ and elemental selenium in 1,6-hexanediol under refluxing conditions for 2 h. The powder electrocatalyst was characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), and examined for the oxygen reduction reaction (ORR) in 0.5M H₂SO₄ by rotating disk electrode (RDE) and in membrane-electrode assemblies, MEAs for a single PEMFC. Results indicate the formation of agglomerates of crystalline particles with nanometric size embedded in an amorphous phase. The catalyst exhibited high current density and lower overpotential for the ORR compared to that of Ru_x cluster catalyst. Dispersed Ru_xSe_y catalyst loading on Vulcan carbon was optimized as a cathode electrode by performance testing in a single H₂–O₂ fuel cell.     

Promoting the reduction reactivity of magnetite by introducing trace-K-ions in hydrogen direct reduction

Hydrogen metallurgy exhibits considerable potential for application in the steel industry. However, it is restricted by the low utilization efficiency of hydrogen owing to the poor kinetic conditions and low reduction efficiency, which limit the development of ironmaking via hydrogen direct reduction. Therefore, improving the reduction efficiency of hydrogen direct reduction is critical. In this study, the reactivity of magnetite was considered. The impregnation of trace K⁺ (0.8 wt%) was found to significantly promote the reduction efficiency, and a possible mechanism was proposed. The increase in the metallization ratio from 88.51% to 99.43% indicated that the 0.8 wt% K⁺-promoted magnetite exhibited an improved reaction efficiency due to its higher reactivity. Furthermore, modified magnetite reacted under a broader range of gas conditions, e.g., the partial pressure of hydrogen could be reduced to 57%. According to the refined X-ray diffraction patterns, the promotion of the reduction reactivity could mainly be attributed to the increased Fe–O bond polarities, in addition to the solid-state diffusion of O²⁻, due to the formation of more oxygen vacancies. An enhanced reduction efficiency during hydrogen direct reduction was realized, which could provide promising guidance for the green, sustainable development of hydrogen direct reduction.     

Simulation and experimental study of the NOx reduction by unburned H2 in TWC for a hydrogen engine

The unburned H₂ can be used to reduce NO emission in conventional TWC (three-way catalyst) for a hydrogen internal combustion engine when it works at equivalence ratio marginally higher than the stoichiometric ratio. To explore the effects and feasibility of this reaction, a Perfectly Stirred Reactor simulation model of TWC has been built with simplified mechanisms. Experiments on a 2.3 L turbocharged hydrogen engine are used to verify the conclusion. It shows that rising initial temperature accelerates the reduction of NO and the maximum reaction rate occurs at 400 °C temperature. The conversion efficiency of NO remains approximately 0 when temperatures below 300 °C. The efficiency reaches a peak value of approximately 98% with 400 °C and declines gradually. The unburned H₂ to NO mixing ratio greater than 1.5 in TWC guarantees 100% NO conversion efficiency. The experiments indicate that the NOx concentration decreases from 2056 ppm to 41 ppm at the stoichiometric ratio after the treatment of TWC and NOx reaches 0 ppm with a rich ratio. Results also demonstrate that the suitable reaction temperatures for TWC locate in the range of 400 °C–500 °C. Therefore, if the temperature and the mixing ratio are appropriate, it can achieve zero emissions with NOx reduction by unburned H₂ in conventional TWC for a hydrogen engine.     

Mathematical model of direct reduction in multilayer pellets made of metallurgical dust in a rotary hearth furnace

A mathematical model of direct reduction of metallurgical dust pellet layers has been established in a rotary hearth furnace. The effects of radiation shielding of the upper layer to lower layer and the heating effects of the hearth were considered in the model. The control equations of the model were discretized into algebraic equations based on the control volume integration method and numerically solved by the alternating direction implicit method. The effects of pellet arrangement, furnace temperature, and the height of the pellet layer were investigated by the model. The results show that staggered arrangement of metallurgical dust pellets can improve average temperature of pellet layer. Thus, the degree of metallization and zinc removal rate of the pellet layer are improved. With the increase of furnace temperature, degree of metallization increases rapidly. Compared with iron oxide reductions, furnace temperature has less effect on zinc removal rate. With the height of pellet bed increasing to four layers, both degree of metallization and zinc removal rate of the fourth layer are slightly higher than that of the third layer. After heating for 15 min, extending heating time has little influence on zinc removal rate of each layer.     

Simultaneous hydrogen generation and COD reduction in a photoanode-based microbial electrolysis cell

A phenolic precursor-based carbon film dispersed with cerium oxide (CeO₂) and reduced graphene oxide (rGO) is prepared as the electrodes of a microbial electrolysis cell (MEC) for simultaneous hydrogen (H₂) production and reduction of chemical oxygen demand (COD) in wastewater under visible light irradiation. The oxides are in situ dispersed in the polymerization reaction mixture during the synthesis stage of the phenolic film. The maximum Coulombic energy efficiency, overall H₂ recovery and cathodic recovery of the CeO₂-rGO/carbon film-based MEC are measured to be approximately 95, 93 and 98%, respectively at 1 V (Ag/AgCl), whereas COD reduction in the wastewater is measured to be approximately 95%. The improved hydrogen evolution and organic degradation are attributed to the electroconductive graphitic rGO and photocatalytic CeO₂, respectively. The metal oxide-rGO-dispersed carbon nanocomposite film-based MEC provides an efficient means for simultaneously producing H₂ and treating the organics-laden wastewater.     

Greenhouse gases (GHG) emissions reduction in a power system predominantly based on lignite

In this paper the GHG mitigation potential of a power system with prevailing use of lignite is assessed through the example of the Macedonian power system. The analysis is conducted using the WASP model in order to develop three different scenarios (business as usual - BAU and two mitigation scenarios) for the power system expansion over the period 2008-2025. In the first mitigation scenario two gas power plants with combined cycle are planned to replace some of the lignite-based capacities. The second mitigation scenario, besides the gas power plants, assumes electricity consumption reduction related to the large industrial consumers and an increased share of new renewable energy sources. Detailed calculations of the GHG emissions are made for all scenarios. The comparison of emissions in 2025 and in 2008 shows that the increase of 78% in the case of predominantly lignite BAU scenario is reduced to 41% by the first mitigation scenario, and to 14% by the second mitigation scenario. The mitigation costs appeared to be less then 10 $/t CO₂-eq for the first mitigation scenario, and even negative for the second one.     

Design a promising non-precious electro-catalyst for oxygen reduction reaction in fuel cells

We utilized quantum-chemical computations for computing the thermodynamic variations in the Gibbs free energy of the potential reaction steps in the oxygen reduction reaction (ORR) of the nickel- and nitrogen-doped graphene (NiN₃-Gr) catalyst's active center. We observed the consistency of the energies of adsorption for all O-containing intermediates. The great thermodynamic driving forces (or motives) for reducing OOH into 2OH1 or O1 and the minor motives for generating H₂O₂ show the advantageousness of following a 4-electron pathway compared to a 2-electron one. This reaction's rate-determining step has 0.94 eV energy barrier that is associated with the first H₂O molecule formation. According to the thermodynamics results, at lower than 0.51 V electrode potential by the 4e^? pathway, the elementary steps of ORR are downhill. The last step, which is the OH reduction into H₂O with the largest value of ΔG, acts as the 4e^? pathway's thermodynamic RDS. Experimental scientists can use the theoretical results achieved in the current study in order to synthesize and select appropriate combinations of NiN₃-Gr for applications in fuel cells.     

Estimation of particle volume fraction, mass fraction and number density in thermophoretic deposition systems

A method is presented to predict the soot volume fraction in soot-laden gas streams in systems where thermophoresis is the dominant mechanism of particle deposition onto adjoining surfaces. In particular, we considered deposition of silica particles on a circular cylinder in cross-flow to a premix CH₄/O₂ flame, a setup similar to the one used in the outside vapor deposition process used for making optical fibers. Silica particles were produced by introducing SiCl₄ along with the premix gases to the burner and were collected on a cylinder. Heat flux and mass deposition rate measurements on the cylinder were performed and recorded as a function of time. Considering thermophoresis to be the dominant mechanism of particle deposition, a simple theory was developed to establish the relationship between the measured quantities. The theory predicted that the thickness at any given time t was expected to increase linearly with the integral of (integrated from t=0 to t=t), where q^′′(t) is the heat flux. Such a linear relationship was observed for five different reactant flow rates confirming thermophoresis to be the dominant mechanism of particle deposition. Soot volume fraction and soot mass fraction were calculated from the slope of these linear fits and were seen to be in good agreement with the estimates of the soot fraction from light scattering measurements. Based on the light scattering estimates of particle diameter, particle number densities were also estimated.