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Oscillating transient flame propagation of biochar dust cloud considering thermal losses and particles porosity
Abstract
This research investigates the transient flame propagation and oscillation phenomenon in the flame speed of porous biochar dust cloud. Time-dependent mass, momentum and energy equations are solved in the spherical coordinate. The gas phase reaction includes the chemical reactions, thermodynamic properties, and multi-element transition properties. To account for the porosity effects of particles, the biochar dusts are modeled as spherical particles with unlimited number of pores (semi sphere) on the surface. The particle trajectory is governed by the equation of motion. The thermophoretic, gravitational, buoyancy and drag forces are employed in this model. In the energy equation, the absorption and radiation emissions by particles is considered. The results reveal that the inertia differences between the particles and gas causes a difference in the velocities of these two phases at the flame front—which is more evident at the early stages of flame propagation when there is a significant change in the density of dust particles. Moreover, the oscillation is further intensified by enhancing the oxygen concentration due to a higher reaction rate, and, as a result, higher velocity difference between the two phases.
... Methanol dehydration reaction occurs in the gas phase and is almost exothermic. The heat of the reaction is much lower than in a one-step process [19]. Of course, this method is fully proven in large-scale plants. ...
Today, dimethyl ether (DME) is changing to ordinarily worn as a superb aerosol propellant and refrigerant for its eco-friendly characteristics. Lately, with the development of novel chemical energy in the coal industries, it has become a fascinating field of research as an alternative green fuel for diesel machines due to the high cetane number. The DME synthesis processes include catalytic dehydrating methanol in an adiabatic fixed-bed reactor. In this study, to investigate the chemical conditions of the methanol dehydration reaction, CFD simulations of the adiabatic reactor have been assessed. The advantage of the work is a sensitivity analysis was run to find the effect of pressure, kinetics, and velocity on the reactor performance. The results showed that using a γ-Al2O3 catalyst with selective mechanical properties and unique surface properties is a convenient choice for DME synthesis. The CFD simulation results also show that the laboratory data such as pressure, energy, and velocity in the adiabatic reactor meet the reaction requirements well, and deliberated a major vision of what happened in the reactor. Also, the graphs of the temperature profile with changes in physical properties pomp that methanol dehydration reaction strongly depends on environmental factors and gives different results under the influence of other conditions.
Direct numerical simulation (DNS) is performed to characterize volatile combustion of isolated coal particles and closely spaced particle ensembles in laminar and turbulent flow. In part I of the paper the transient evolution of devolatilization and group flame effects were studied, Tufano et al. (2018). This analysis was limited to laminar flows and relatively low particle Reynolds numbers Rep. Here, we investigate the effect of large Rep and considerable levels of turbulence on the devolatilization and burning behavior of the particles. The complex physico-chemical interactions during PCC are characterized for laminar flow conditions first, before arrays of infinite particle layers are subjected to turbulence. Increasing Rep in laminar flow leads to delayed ignition of single particles with local extinction due to high upstream scalar dissipation rates and the formation of wake flame structures downstream of the particle. An attempt is made to recover the single particle results with a standard steady laminar flamelet approach, which is shown to work well at low Rep, but fails at high Reynolds numbers, where multi-dimensional effects occur and must be incorporated into flamelet modeling. It is found that applying standard film theory to model the effect of convection on devolatilization rates can lead to qualitatively wrong trends and up to 66% error in the peak devolatilization rate compared to the DNS results at Rep=8. The analysis of particle arrays in laminar flow shows a strong dependence of flame interactions on the values of Rep due to the different extent of the particle wakes. The occurrence of significant levels of turbulence introduces a wide range of additional chemical states due to the randomness of the turbulent fluctuations that can either act to increase or decrease the local strain, in turn weakening or enhancing particle interactions. For the studied conditions turbulence slightly promotes the mass release from the most upstream particle set, but considerably delays the volatile release from the downstream particles, which is explained by the different extents and degrees of interaction of the up- and downstream volatile flames. The present results are considered useful for the development of LES sub-grid scale combustion models for pulverized coal flames, such as flamelets and others.
This article aims to study suitability and merits of employing a biofuel in methane-air jet flames for energy security and environmental sustainability purposes. A special biofuel (methyl decanoate, methyl 9-decenoate, and n-heptane) oxidation with 118 species reduced/skeletal mechanism and 837 reactions is combusted instead of methane. The biofuel was fed in the main jet inlet of the well-known Sandia flame D (SFD) while the hot pilot jet is still responsible for ignition. The open-source software OpenFOAM was used for simulating turbulent biofuel-air combustion. To check the accuracy of computational results, the system was initially validated with SFD experimental results and good agreements were found. After ignition, mean temperature distribution and species mean mass fraction at different distances in axial and radial directions were investigated. Results showed that the biofuel can be effectively used as an alternative to SFD not only for generating a reasonable temperature as methane does, but also for significantly reduction in principal greenhouse gas emissions.
The results of the numerical simulation of the flame front propagation in coal-dust- methane-air mixture in an enclosed volume with the ignition source in the center of the volume are presented. The mathematical model is based on a dual-velocity two-phase model of the reacting gas-dispersion medium. The system of equations includes the mass-conversation equation, the impulse-conversation equation, the total energy-conversation equation of the gas and particles taking into account the thermal conductivity and chemical reactions in the gas and on the particle surface, mass-conversation equation of the mixture gas components considering the diffusion and the burn-out and the particle burn-out equation. The influence of the coal particle mass on the pressure in the volume after the mixture burn out and on the burn-out time has been investigated. It has been shown that the burning rate of the coal-dust methane air mixtures depends on the coal particle size.
The main aim of this research is focused on determining the velocity and particle density profiles across
the flame propagation of micro-lycopodium dust particles. In this model, it is tried to incorporate the
forces acting on the particle such as thermophoretic, gravitational and buoyancy in the Lagrangian equation of motion. For this purpose, it is considered that the flame structure has four zones (i.e., preheat,
vaporization, reaction and post flame zones) and the temperature profile, as the unknown parameter in
the thermophoretic force, is extracted from this model. Consequently, employing the Lagrangian equation
with the known elements results in the velocity distribution versus the forefront of the combustion region.
Satisfactory agreement is achieved between the present model and previously published experiments. It is
concluded that the maximum particle concentration and velocity are gained on the flame front with the
gradual decrease in the distance away from this location.
The densification of bio-chars pyrolyzed at different temperatures were investigated to elucidate the effect of temperature on the properties of bio-char pellets and determine the bonding mechanism of pellets. Optimized process conditions were obtained with 128MPa compressive pressure and 35% water addition content. Results showed that both the volume density and compressive strength of bio-char pellets initially decreased and subsequently increased, while the energy consumption increased first and then decreased, with the increase of pyrolysis temperature. The moisture adsorption of bio-char pellets was noticeably lower than raw woody shavings but had elevated than the corresponding char particles. Hydrophilic functional groups, particle size and binder were the main factors that contributed to the cementation of bio-char particles at different temperatures. The result indicated that pyrolysis of woody shavings at 550-650°C and followed by densification was suitable to form bio-char pellets for application as renewable biofuels.
A theory is presented for the steady combustion of a spherical carbon particle in the slow viscous flow of an oxidizing ambient. Heterogeneous surface reactions produce radial convection and diffusion from the particle, which couples with the ambient convection to affect the rate of particle mass loss. In the theory, a small Reynolds number characterizes the ambient convection. However, the radial convection, associated with the particle mass loss, is not necessarily small. The gas stream function and chemical species are described. The emphasis is upon explicit analytical relationships for particle mass loss rates that can be used to interpret experimental observations.
This paper deals with the development and detailed assessment of a biomass-fuelled electrochemical power generation system employing biomass gasification, solid oxide fuel cell, gas turbine, and organic Rankine cycle. The performance of the plant has been monitored with the changes in important operating parameters with respect to energetic, exergetic, economic and environmental points of view. The proposed system yields the maximum 49.29% 1st law efficiency and 44.57% 2nd law efficiency. Exergetic analysis reveals that among all the elements of the system, biomass gasifier is the highest (41.22%) contributor in total exergy destruction, followed by the heat exchanger (14.01%), and the organic Rankine boiler (13.27%). Economic analysis reveals that the plant can generate electricity at a minimum rate of 0.0654 /year. This study also reveals that the present system is better compared to previously investigated other biomass-fuelled power generation plants considering the efficiency and economy together.
The combustion characteristics and kinetics of straw biochars thermally pretreated via torrefaction, slow pyrolysis, or hydrothermal carbonisation (HTC) and co-combusted with bituminous coal were examined in this study to explore the effects of various pretreatment methods and blending ratios on the combustion characteristics of biochars to assess potential practical applications. Non-isothermal experiments were performed using a thermogravimetry analyser. The results showed that the thermogravimetric curve of biochar obtained after HTC was similar to that of raw straw, thereby indicating similar combustion characteristics and reactivity. The average reactivity (Rm) index of hydrochar and straw was 1.648 and 2.082, respectively. By contrast, the biochars produced after torrefaction or pyrolysis were more similar to bituminous coal, with the Rm index of 0.696, 0.78 and 0.773, respectively. A positive interaction was observed during the char combustion stage of co-firing, with the activation energy of the blends tending towards the lower side. The catalytic effect of biomass ash was likely the predominant reason for the better interaction of pyrolytic biochar with coal.
In this study, non-premixed multi-zone combustion of porous lycopodium particles in a counter-flow configuration at steady state was modeled using a derived analytical model. To model the porosity, a reasonable series form is considered. In the derived analytical model, both convective and radiative heat losses were included. The overall combustion process was mathematically divided into pre-heating, drying, vaporization, flame, and post-flame zones and lycopodium particles and oxidizer are injected from opposite sides locating infinity away the stagnation plane. The particles are firstly dried and then vaporized in a limitedly finite region to produce a gaseous fuel of special chemical composition. Relevant conservation equations were formulated in both dimensional and dimensionless forms. Applying suitable jump and boundary conditions, the corresponding continuity and energy equations were solved using Mathematica and Matlab software. The derived analytical model was first validated through comparison against literature data and a satisfactory agreement was achieved. Then temperature distribution and mass fractions of fuel and oxidizer along the x direction were presented to describe the combustion characteristics. Finally, a series of parametric studies were conducted to study the effects of key operating conditions.
In order to mitigate inherent intermittency of solar energy and satisfy the energy demands, a novel steam/air biomass gasification combined cooling, heating and power system with solar energy is proposed in this work. In the system concentrated solar energy collected with dish optical configuration is used to generate high temperature steam, which acts as a gasification agent to drive the biomass gasification. When solar radiation intensity is lower than the design point (the maximum in whole year), the hybrid steam/air biomass gasification model is conducted for continuous syngas production. Syngas based chemical energy is released for trigeneration in a distributed system. The reaction kinetics models of steam-biomass gasification using solar energy are developed and the thermodynamics performances of the system are numerical investigated. Under the designate conditions, the primary energy efficiency reaches 51.34%. Based on the local weather data, the off-design performances of the system and the annual energy-saving potential are evaluated. Compared with the reference system that combines a conventional air biomass gasification trigeneration system and a solar dish/Stirling engine system, the new system has remarkable advantage in thermal performances. An economic analysis is conducted to evaluate the technical feasibility of the proposed system. This research provides a promising method for the efficient utilization of biomass and solar energy.
China's coal-fired power industry contributes about half of the country's total emissions and is undergoing active transformation in response to internal improvements related to efficiency and external forces related to the increasing penetration of variable renewable energy into China's grids. Together, these changes are slowing the rate of growth in emissions, but the effects of individual policy measures are difficult to discern. In this study, we develop a high-resolution inventory of emissions factors for the coal-fired power industry in China, apply it to developing province-level emission factors, and demonstrate the relative effectiveness of four policy measures internal to the coal-fired power industry (namely, raising the entry threshold, eliminating outdated capacity, improving existing plants and enhancing operation loads) relative to external contributions from renewable power development in different provinces across China. We provide data for 49 power plant configurations covering 99.7% of the existing coal-fired power fleet in China during the 13th Five Year Plan period (2016–2020). We find that the provincial carbon emission factors of the coal-fired power industry are expected to decrease by 40–60 g CO2eq/kWh in 2020 compared with 2016, corresponding to a total carbon reduction potential of 265 Mt CO2eq across the industry. The four driving factors contribute various reductions in the carbon intensity by province and result in decreases in total emissions in Shanghai, Zhejiang and Beijing. For comparison, we calculate that the decarbonization potential from non-coal power deployment is 593 Mt CO2eq in 2020 compared with 2016, with the province-to-province variability mostly depending on the endowment of the non-coal resources. The variation across the country suggests that strategies for continued decarbonization beyond 2020 should be tailored to the specific situation in each province.
Due to perspective of biomass usage as a viable source of energy, this paper suggests a potential theoretical approach for studying multiregion nonadiabatic premixed flames with counterflow design crossing through the mixture of air (oxidizer) and lycopodium particles (biofuel). In this research, convective and radiative heat losses are analytically described. Due to the properties of lycopodium, roles of drying and vaporization are included so that the flame structure is created from preheating, drying, vaporization, reaction, and postflame regions. To follow temperature profile and mass fraction of the biofuel in solid and gaseous phases, dimensionalized and nondimensionalized forms of mass and energy balances are expressed. To ensure the continuity and calculate the positions of drying, vaporization, and flame fronts, interface matching conditions are derived employing matlab and mathematica software. For validation purpose, results for temperature profile is compared with those provided in a previous research study and an appropriate is observed under the same conditions. Finally, changes in flame velocity, flame temperature, solid and gaseous fuel mass fractions, and particle size with position measured from the position of stagnation plane, strain rate, and heat transfer coefficient in the presence/absence of losses are evaluated.
The quest for producing cost-effective and efficient adsorbents for CO 2 capture has received enormous attention in recent times. Biomass-derived porous carbons are considered to be the most preferred adsorbent materials for CO 2 capture owing to their excellent textural properties, tunable porosity and low cost. Different type of activation processes including solid-state activation possess generate appropriate morphology and other physico-chemical properties in these materials which enable them to act as effective adsorbents for CO 2 capture. In this review, the key scientific results from published literature have been consolidated and critical commentary has been provided to give a broad insight into the production of biochar and activated porous carbons and their application in CO 2 capture. A thorough review of the mechanism of pyrolysis for cellulose, hemicellulose and lignin has been presented in detail. The ability of different activating agents to produce activated porous carbons has been discussed. A summary of the application of biochar and activated porous carbons for CO 2 capture has been included. The review concludes with an overview of future outlook and potential research direction that could be undertaken for advancing the utilization of biomass-derived porous carbon materials for applications including CO 2 capture and energy storage.
Relevant to the activation of solid fuel particle, critical condition for the combustion rate has been obtained, above which particle combustion can successfully be accomplished. Use has been made of the asymptotics, with focusing on the temporal variation of the particle mass in the plateau stage, as well as that of the particle temperature. It has been confirmed that this critical condition is closely related to the heat loss, and that there exists a comprehensive parameter, consisting of the combustion rate, oxygen mass-fraction, and pressure ratio, which only depends on the ambient temperature. Appropriateness and/or usefulness of this critical condition has further been examined, by use of such experimental data in the literature as are reported to burn in the quiescent environment. Arrhenius plot of the comprehensive parameter has been used in experimental comparisons, in which it is indicated that the upper half region separated by the critical condition corresponds to that for the particle combustion with surface reactions activated. Experimental data used are those from semi-anthracite to low-rank coal char, as well as petroleum coke, including soot. A fair degree of agreement has been demonstrated, indicating that the larger particles that can burn in the diffusion controlled regime locate in the upper half region, and that the smaller ones that have failed to be activated in the surface reactions locate below the critical condition. By virtue of this critical condition, it is even suggested that prediction for the particle activation in the course of combustion can easily be made, by examining combustion rate measured in the quasi-steady state. Finally, it is confirmed again that the reduction in particle size, which also exerts influences on particle deactivation, does not necessarily favor the particle combustion.
Utilizing flexible and portable device to generate electricity via a natural process has aroused extensive and intensive interest. Herein, a low-cost graphene/carbon cloth (G/CC) is prepared to generate electricity induced by water evaporation that is driven from solar photothermal conversion. The G/CC device can constantly generate electricity approximately up to 0.37 V in 0.5 M NaCl solution through solar-driven water evaporation. High photothermal efficiency up to 83% is achieved when the device is applied to inorganic salt solutions or organic solvents under solar illumination with a power density of 1 kW m⁻². This work provides a simple method that can convert the environmental energy into electricity, and develops a new energy harvester as a green power supply system that is feasible in a use of water evaporation, pollutant treatment and medicine disinfection.
Non-premixed flames are extensively used in different industrial combustion systems such as gas turbines and coal furnaces. Therefore, reliable theoretical modelling of these flames is indispensable. The purpose of this study is to develop a comprehensive analytical model for counter-flow non-premixed flames employing volatile biomass particles as the fuel. In order to present a promising and accurate analysis for the flame structure and propagation, preheat, drying, vaporization, reaction and oxidizer zones are presumed. In the current analytical modelling, a non-asymptotic approach is used. In this regard, finite thicknesses are considered for the drying and vaporization zones. Lycopodium particles and air are taken as the biofuel and oxidizer, respectively. It is assumed that the gaseous fuel yielded from lycopodium particles is methane. Dimensionless parameters are defined and then non-dimensionalized forms of governing equations including mass and energy conservation equations are derived taking into account appropriate boundary and jump conditions for each zone. The equations are solved by Mathematica and Matlab software. Eventually, the impacts of fuel and oxidizer Lewis numbers, mass particle concentration parameter, equivalence ratio, lycopodium initial temperature on the flow strain rate, flame temperature, flame position and gaseous fuel and oxidizer mass fractions are elaborately examined.
6 a b s t r a c t 7 a r t i c l e i n f o 8 Available online xxxx 13 14 15 16 17 18 This article presents MHD heat and mass transfer flow of nanofluid over linear and non-linear stretching sheets 19 embedded in porous media under the influence of Brownian motion, thermophoresis, thermal radiation and 20 chemical reaction. Appropriate transformations reduce the non-linear partial differential systems into ordinary 21 differential equations. Galerkin Finite element method is employed to solve these momentum, temperature 22 and concentration equations numerically subject to the boundary conditions. The influence of various pertinent 23 parameters on velocity, temperature and concentration profiles of the fluid is discussed and the results are plot-24 ted through graphs. Furthermore, skin-friction coefficient, Nusselt number and Sherwood number are investigat-25 ed in detail and results are shown in tabular form. It is concluded that the velocity and temperature profiles 26 escalate, whereas concentration profile depreciates when Brownian motion parameter rises. 27 MHD 30 Nanofluid 31 Linear and non-linear stretching sheets 32 Thermophoresis 33 Brownian motion 34 35 36 37 38
This study focuses on the energy-efficient utilization of both solid and liquid wastes from palm oil mills, particularly their use for power generation. It includes the integration of a power generation system using empty fruit bunch (EFB) and palm oil mill effluent (POME). The proposed system mainly consists of three modules: EFB gasification, POME digestion, and additional organic Rankine cycle (ORC). EFBs are dried and converted into a syngas fuel with high calorific value through integrated drying and gasification processes. In addition, POME is converted into a biogas fuel for power generation. Biogas engine-based cogenerators are used for generating both electricity and heat. The remaining unused heat is recovered by ORC module to generate electricity. The influences of three EFB gasification temperatures (800, 900 and 1000 ºC) in EFB gasification module; and working fluids and pressure in ORC module are evaluated. Higher EFB gasification leads to higher generated electricity and remaining heat for ORC module. Power generation efficiency increases from 11.2 to 24.6 % in case of gasification temperature is increased from 800 to 1000 ºC. In addition, cyclohexane shows highest energy efficiency compared to toluene and n-heptane in ORC module. Higher pressure in ORC module also leads to higher energy efficiency. Finally, the highest total generated power and power generation efficiency obtained by the system are 8.3 MW and 30.4 %, respectively.
In the present article an analytical approach is employed to study the counterflow non-premixed dust flame structure in air. Lyco-podium is assumed as the organic fuel in in this paper. First, it is presumed that the fuel particles vaporize in a thin zone to form a gaseous fuel to react with the oxidizer. The reaction rate is presumed to be of the Arrhenius type in first order respect to oxidizer and fuel. Mass conservation equations of dust particles, oxidizer, gaseous fuel and the energy conservation equation for non-unity Lewis numbers of oxidizer and fuel are presented as the governing equations. Boundary conditions are applied for each zone for the purpose of solving the governing equations analytically. The flame temperature in terms of oxidizer and fuel’s Lewis numbers is calculated. Furthermore, the variation of flame position based on the fuel and oxidizer’s Lewis numbers is evaluated .Also, mass fraction and temperature profiles of oxidizer and fuel are presented. In addition, the variation of the ratio of critical extinction values of strain rates as a function of non-unity Lewis numbers of oxidizer and fuel to the unity Lewis numbers of them is investigated.
A bio-solution of natural organic enzyme-7F (NOE-7F) is used to pretreat bamboo, with emphasis on the influence the pretreatment upon the structure, reactivity, and torrefaction of the biomass. Two different operating modes accompanied by five different soaking durations are considered. In Mode 1 the bamboo is ground followed by pretreated by the bio-solution, and an inverse procedure is used in Mode 2. The results indicate that, with the operation of Mode 1, NOE-7F removes hemicellulose in the bamboo significantly , thereby improving the homogeneity of the biomass. This pretreated bamboo may be feasible for enzymatic hydrolysis to produce bioethanol. The penetration of the bio-solution into block bamboo becomes the controlling mechanism under Mode 2 operation, and therefore relatively less hemicellulose is consumed from Mode 2. The ignition and burnout temperatures of the pretreated bamboo are higher than those of the raw bamboo, revealing the lower reactivity and higher storage safety of the former. The atomic H/C and O/C ratios as well as the calorific value of the bamboo are insensitive to the pretreat-ments, whereas the crystalline structure of cellulose is affected by the bio-solution to a certain extent, regardless of Mode 1 or Mode 2 operation. This suggests that torrefaction is required if the pretreated bamboo is employed as a fuel. The pretreated bamboo with Mode 2 is more suitable for torrefaction because of higher torrefaction severity.
The present work was addressed to understand the contributions of the heat-treatment temperature and inorganic catalysis to the combustion characteristics of biochar. A cornstalk sample was leached by water and HCl to partially and fully remove active inorganic elements, respectively. The raw and leached samples were heat-treated at the temperatures of 200–900 °C. The resulting biochars were subjected to X-ray diffractometry and Raman spectroscopy analyses for structural characterization and thermogravimetric analysis for combustion characteristics, particularly, for char reactivity. It was found that the heat-treatment temperature has a significant effect on the evolution of the biochar structure, while inorganic matter has little influence. For low-temperature (200–400 °C) biochars, the heat-treatment temperature has a considerable influence on the devolatilizaiton and a marginal effect on char combustion behaviors, while the inorganic species, particularly, water-soluble species, have considerable influence on the behaviors of both stages. For high-temperature (500–900 °C) biochars, the heat-treatment temperature and inorganic matter are both important to char reactivity. The high contents of inorganic species of cornstalk resulted in the catalytic effect of inorganic species dominating over the influence of the carbonaceous structure ordering on char reactivity. With active inorganic species removed by leaching, char reactivity of biochars turned to decreasing monotonically with an increase of the treatment temperature.
Oxyfuel combustion technology is one of the near-zero emission clean coal technologies, which can realise the large-scale CO2 capture, low NOx emission, and easier removing of SO2. This paper describes the characteristics of pyrolysis, gasification and combustion of coals and chars in oxyfuel combustion applying the non-isothermal thermogravimetric analysis. Experimental results show that the pyrolysis reactivity is highly similar in N2 and CO2 at low temperature. The mass loss rate of experimental coals in CO2 increases significantly at around 810 °C. In comparison to the conventional combustion, thermogravimetric curves of coal and char combustion in O2/CO2 tend towards higher temperature zone. Sample mass, particle size and heating rate all have considerable effects on the heat and mass transport process, which influences the whole combustion process of coal particles in oxyfuel conditions. Char reactivity prepared in CO2 is almost equivalent to that prepared in N2. The results of kinetics analysis indicate that compensation effect exists between apparent activation energy and pre-exponential factor in different oxygen concentrations during both coal and char combustion.
This study discusses the design of a new experimental platform, the Hybrid Flame Analyzer (HFA) to measure burning velocity of gas, dust, and hybrid (gas and dust) premixed flames. The HFA is used to analyze a particle–gas–air system of coal dust particles (75–90 μm and 106–120 μm) in a premixed CH4–air (ϕg = 0.8, 1.0 and 1.2) flame. Experimental results show that particles usually increase the turbulent burning velocity. Smaller particle sizes and larger concentrations (>50 g/m3) increase turbulent burning velocity compared with larger particle sizes and lower concentration ranges. The experimental data is used to develop a correlation similar to turbulent gas flames to help modeling of the complex behavior.
The logistics of the fuel supply have a large impact on the economy of a biomass power generation facility, especially for low density biomass fuels like straw. A detailed cost analysis of a typical rice straw logistics process for two baling options in three regions of Thailand shows that the costs for all logistics operations vary from a minimum of 18.75USD/t for small rectangular bales in the Northern region of Thailand to maximum 19.89USD/t for large rectangular bales in the North-eastern region. The difference in costs is not very significant due to the higher ownership and operating costs of the equipment for using large rectangular bales; however, the specific fuel consumption cost is substantially lower by around 17.5% and a total transport cost reduction is about 31.5%. Analysis of the logistics economies of scale for projected power plant capacities of 2–35MWe showed that each doubling the capacity of the energy facility increases the specific costs of the logistics operations only by around 4% in all regions.
The asymptotic structure of counterflow and stagnant diffusion flames are analyzed in the limit for large values of the overall, nondimensional activation energy, Ta, characterizing the rate of the reaction and results are given for small values of the stoichiometric fuel to oxygen mass ratio. The chemical reaction between the fuel and the oxidizer is represented by a one-step, irreversible process. A new approach is developed to characterize the influence of the Lewis number of the fuel, LF and the Lewis number of the oxidizer, L0, on the outer and inner structure of near equilibrium diffusion flames. Explicit algebraic formulas to predict the critical conditions of flame extinction are also given.For counterflow diffusion flames at fixed values of L0, the flame moves significantly toward the oxidizer stream and the heat losses toward the oxidizer region of the flame increase significantly with decreasing values of LF. The value of the maximum flame temperature is relatively insensitive to the variations in LF although the value of the rate of strain at extinction, A, increases significantly with decreasing values of LF and increasing values of Ta. At fixed values of LF and decreasing values of L0, the flame moves slightly toward the fuel stream; the heat losses toward the fuel stream increase slightly and there is a moderate increase in the value of the maximum flame temperature. The value of A increases with decreasing values of L0 for large values of Ta and is relatively insensitive to variations in L0 for moderate values of Ta.The inner and outer structure for stagnant diffusion flames where convection is absent are qualitatively similar to those for counterflow diffusion flames. However, the value of the maximum flame temperature increases significantly with decreasing values of L0 and fixed values of LF.The results developed here are used to obtain overall chemical kinetic rate parameters characterizing the gas phase oxidation of methane using previously measured values of the critical conditions of flame extinction.
Coal mine explosions have been a problem for hundreds of years. The projected need for coal as an energy source will demand increased subsurface mining, in turn requiring improved techniques for the prevention of dust explosions. Preventive procedures, however, must rely on fundamental knowledge concerning the behavior of coal dust combustion. To this end, a review of coal dust combustion as reported in the literature is presented, and a mathematical model of flame propagation through coal dust-air mixtures is developed in detail. The model is basically used to predict burning velocities and to study the structure of coal dust-air flames.
A brief review of existing ideas on the nature of the thermophoresis of solids in gases is given. A method for calculating the rate of thermophoresis of coarse solids is described. A procedure for analysing experimental data and for choosing a high-quality material on the basis of this is proposed.
The adequacy of direct one-step chemical kinetics for describing ignition and extinction in initially unmixed gases is studied through the particular case of inviscid axisymmetric stagnation-point flow. Oxidant is assumed to blow from upstream infinity at a non-gaseous reservoir of pure fuel at its boiling (or sublimating) temperature. Before reaching the reservoir the oxidant reacts with gaseous fuel flowing in the opposite direction to form product and release heat. This heat is in part conducted and diffused to the reservoir interface to transform more fuel into the gaseous state and continue the steady-state burning. Second-order Arrhenius kinetics for Lewis-number unity is examined. A critical parameter characterizing the phenomenon is shown to be the first Damkohler similarity group D1, the ratio of a time characterizing the flow to a time characterizing the chemical activity.
A laser-Doppler velocimeter (LDV) study of velocity profiles in the laminar boundary layer adjacent to a heated flat plate revealed that the seed particles used for the LDV measurements were driven away from the plate surface by thermophoretic forces, causing a particle-free region within the boundary layer of approximately one half the boundary-layer thickness. Measurements of the thickness of this region were compared with particle trajectories calculated according to several theories for the thermophoretic force. It was found that the theory of Brock, with an improved value for the thermal slip coefficient, gave the best agreement with experiment for low Knudsen numbers, λ/ R = O (10 ⁻¹ ), where λ is the mean free path and R the particle radius.
Data obtained by other experimenters over a wider range of Knudsen numbers are compared, and a fitting formula for the thermophoretic force useful over the entire range 0 [les ] λ/ R [les ] ∞ is proposed which agrees within 20% or less with the majority of the available data.
Biomass as a fuel suffers from its bulky, fibrous, high moisture content and low-energy-density nature, leading to key issues including high transport cost and poor biomass grindability. This study investigates the possibility to pretreat biomass to produce biochar as a solid biofuel to address these issues. Biochars were produced from the pyrolysis of centimeter-sized particles of Western Australia (WA) mallee wood in a fixed-bed reactor at 300 to 500 °C and a heating rate of 10 °C/min. The data show that, at pyrolysis temperatures ≥320 °C, biochar as a fuel has similar fuel H/C and O/C ratios compared to Collie coal that is the only coal being mined in WA. Converting biomass to biochar leads to a substantial increase in fuel mass energy density from 10 GJ/ton of green biomass to 28 GJ/ton of biochars prepared from pyrolysis at 320 °C, in comparison to 26 GJ/ton for Collie coal. However, there is little improvement in fuel volumetric energy density, which is around 7−9 GJ/m3 in comparison to 17 GJ/m3 of Collie coal. Biochars are still bulky and grinding is required for volumetric energy densification. Biochar grindability experiments show that the fuel grindability increases drastically even at pyrolysis temperature as low as 300 °C. Further increase in pyrolysis temperature to 500 °C leads to only a small increase in biochar grindability. Under the grinding conditions, a significant size reduction (34−66% cumulative volumetric size below 75 μm) for biochars can be achieved after 4 minutes grinding (in comparison to only 19% for biomass after 15 minutes grinding), leading to a significant increase in volumetric energy density (e.g., from 8 to 19 GJ/m3 for biochar prepared from pyrolysis at 400 °C). Whereas grinding raw biomass typically results in large and fibrous particles, grinding biochars produces short and round particles. The results in this article indicate that biochar has desired fuel properties and potentially a good solution to address the key issues including high transport cost and poor grindability associated with the direct use of biomass as a fuel.
A digital particle image velocimetry technique that is appropriate for the experimental derivation offundamental flame properties was implemented. The technique allows for the determination of the instantaneous flowfield and is essential for fluid mechanics measurements in reduced gravity environments. Measurements of laminar flame speeds were conducted in the stagnation flow configuration just before a flame undergoes a transition from planar to Bunsen flame. Results obtained for lean CH4/air and C2H2/air flames were found to be in close agreement with prefious laser Doppler velocimetry data. Subsequently, measurements were conducted for the CH4 and C2H2 flames by independently varying the equivalence ratio and flame temperature to distinguish between temperature and concentration effects. The laminar flame speeds were also calculated using the GRI-Mech 3.0 mechanism. It was convincingly shown that under high-O2 and low-temperature conditions, the experimental laminar speeds are over predicted by the simulations especially for C2H6 flames. Additional experiments were conducted by adding H2 to lean C2H6/air flames and by diluting those mixtures by either He or N2 to vary the flame temperature. While for the He dilution case, the predictions noticeably overpredict the experiments, for N2 dilution, close aggrement was observed. Analyses of the flame structures revealed that for those fuel-lean flames, the burning rate largely depends on the competition of the two-body branching and three-body termination reaction between H and O2. It was not possible to point to possible kinetic deficiencies other than referring to uncertainties associated with the rates and collision efficiencies of three-body reactions. The high-O2 low-temperature region is of interrest not ouly to lean-premixed combustion, but also to flame ignition, and requires further exploration.
Carbon/carbon and zeolite/carbon composites have been prepared by pyrolytic carbon infiltration of organic and inorganic substrates with different porous structures. The chemical vapour infiltration kinetics of these substrates has been studied in a thermogravimetric system at atmospheric pressure, using benzene as pyrolytic carbon precursor. The rate of pyrolytic carbon infiltration seems to depend on the porosity of the substrate available to the pyrolytic carbon precursor, irrespective of the nature of the substrate studied. Activation energy values of about 180 kJ/mol were found for the different substrates used in the temperature range of 700–800 °C, where the cracking reaction of benzene takes place, predominantly, in a heterogeneous form. At higher temperatures homogeneous reactions compete with heterogeneous ones and higher values of activation energies (280–380 kJ/mol) were obtained. The oxidation of the pyrolytic carbon deposited on the different substrates studied takes place in the same range of temperature, which suggests the presence of a similar pyrolytic carbon structure on substrates of different nature or a similar accessibility to the deposited layer.
The structure of premixed flames propagating in combustible systems, containing uniformly distributed volatile fuel particles, in an oxidizing gas mixture, is analyzed. It is presumed that the fuel particles vaporize first to yield a gaseous fuel of known chemical structure, which is subsequently oxidized in the gas phase. The analysis is performed in the asymptotic limit, where the value of the characteristic Zeldovich number, based on the gas-phase oxidation of the gaseous fuel is large, and for values of φu ≥ 1.0, where φu is the equivalence ratio based on the fuel available in the fuel particles. The structure of the flame is presumed to consist of a preheat vaporization zone where the rate of the gas-phase chemical reaction is small, a reaction zone where convection and the rate of vaporization of the fuel particles are small and a convection zone where diffusive terms in the conservation equations are small. For given values φu the analysis yields results for the burning velocity and φg, where φg is the effective equivalence ratio in the reaction zone. The analysis shows that even though φu ≥ 1.0, for certain cases the calculated value of φg is less than unity. This prediction is in agreement with experimental observations.
A pure two-fluid model for turbulent reacting gas-particle flow of coal particles is developed using a unified Eulerian treatment of both the gas and particle phases. The particles' history caused by mass transfer due to moisture evaporation, devolatilization and char reaction is described. Both velocity and temperature of the coal particles and the gas phase are predicted by solving the momentum and energy equations of the gas and particle phases, respectively. A k–ε–kk two-phase turbulence model, EBU–Arrhenius turbulent combustion model and four-flux radiation heat transfer model are incorporated into a comprehensive model. The above comprehensive mathematical model is used to simulate two-dimensional gas-particle flows and pulverized coal combustion in a newly designed tubular oxygen–coal combustor of blast furnace. Predicted results of isothermal gas-particle flows are in good agreement with those obtained by measurements. The results also show that the proposed tubular oxygen–coal combustor prolongs the coal particle residence time and enhances the mixing of coal and oxygen. Results indicate that smaller coal particles of 10 μm diameter are heated and devolatilized rapidly and have volatile combustion in the combustor, while larger coal particles of 40 and 70 μm in diameter are heated but not devolatilized, and combustion of such particles does not occur in the tubular combustor.
The overarching goal of this study is to improve our understanding of the extinction characteristics of spherical diffusion flames in microgravity. In particular, one of the key objectives is to assess the effects of gas radiation as a means to promote flame extinction. To investigate these phenomena, a one-dimensional computational model was developed to simulate the evolution of a spherical diffusion flame with consideration of detailed chemistry and transport properties. The model formulation was described along with the detailed numerical method. Radiation model was discussed with two aspects: radiation property model and radiative transfer model. Various levels of radiation models were implemented and the results were compared with experimental measurements of flame radius and temperature profiles. It was shown that the statistical narrow band model (SNB) combined with the discrete ordinate method (DOM) reproduced the experimental results with highest accuracy, and this combination of the radiation models were adopted in the subsequent parametric studies in Part II. Computational issues to optimize numerical accuracy and efficiency are also discussed.
The current status of understanding of flame structure and mechanisms of propagation in dust-air mixtures is reviewed. The equilibrium properties, which are those that depend on the energetics of the medium, are well described by existing computer codes. However, measurements of the dynamic parameters, which are those that depend on the rate of reaction (e.g. flame thickness, minimum ignition energy, burning velocity, etc.) are virtually non-existent for dust combustion. The limited measurements of flame structure that exist suggest that dust flames are thin and propagate predominantly by thermal and molecular diffusion rather than by radiative preheating. However, the available results are too few and far between to be able to draw firm conclusions. The more convenient experimental measurement appears to be the quenching distance. The present paper reviews existing measurements of the quenching distance and describes a novel experimental approach to achieving a homogeneously suspended dust-air medium for laminar dust combustion studies.
An adequate treatment of thermal radiation heat transfer is essential to a mathematical model of the combustion process or to a design of a combustion system. This paper reviews the fundamentals of radiation heat transfer and some recent progress in its modeling in combustion systems. Topics covered include radiative properties of combustion products and their modeling and methods of solving the radiative transfer equations. Examples of sample combustion systems in which radiation has been accounted for in the analysis are presented. In several technologically important, practical combustion systems coupling of radiation to other modes of heat transfer is discussed. Research needs are identified and potentially promising research topics are also suggested.
The structure of steady state diffusion flames is investigated by analyzing the mixing and chemical reaction of two opposed jets of fuel and oxidizer as a particular example. An Arrhenius one-step irreversible reaction has been considered in the realistic limit of large activation energies. The entire range of Damköhler numbers, or ratio of characteristic diffusion and chemical times, has been covered. When the resulting maximum temperature is plotted in terms of the Damköhler number (which is inversely proportional to the flow velocity) the characteristic S curve emerges from the analysis, with segments from the curve resulting from: 1.(a) A nearly frozen ignition regime where the temperature and concentrations deviations from its frozen flow values are small. The lower branch and bend of the S curve is covered by this regime.2.(b) A partial burning regime, where both reactants cross the reaction zone toward regions of frozen flow. This regime is unstable.3.(c) A premixed flame regime where only one of the reactants leaks through the reaction zone, which then separates a region of frozen flow from a region of near-equilibrium.4.(d) A near-equilibrium diffusion controlled regime, covering the upper branch of the S curve, with a thin reaction zone separating two regions of equilibrium flow.Analytical expressions are obtained, in particular, for the ignition and extinction conditions.
A model of combustion of a high-porosity carbon particle in oxygen is considered, which takes into account heterogeneous and
homogeneous chemical reactions inside the particles and radiative heat transfer. The boundaries of the domain where the burning
rate depends on the particle temperature are determined. The possibility of two combustion regimes is demonstrated: regime
with a high burning rate, where the carbon-oxygen reaction proceeds in a layer adjacent to the particle surface, and regime
with a low burning rate, where the reaction proceeds in the entire particle volume. In the regime with a high burning rate,
the main product of the reaction between carbon and oxygen is carbon monoxide, whereas both carbon monoxide and carbon dioxide
can be formed in the regime with a low burning rate. The kinetic equations of heterogeneous reactions C + O2 = CO2 and 2C + O2 = 2CO are determined, which reveal the retarding effect of carbon monoxide and dioxide on the rates of these reactions.