Laminar mass burning and entrainment velocities and flame instabilities of i-octane,ethanol and hydrous ethanol/air aerosols

The paper reports experiments employing the cloud chamber technique for creating fuel aerosols, in studies of premixed laminar flames. Spherical explosion flames were initiated at different times after the start of expansion of the original gaseous mixture to lower pressure. Flame speeds were measured close to atmospheric pressure, over a range of equivalence ratios of iso-octane, ethanol and hydrous ethanol with air. A methodology was developed for deriving mass burning velocities and entrainment velocities, as well as mass burning fluxes, from the measurements of aerosol number densities, droplet sizes and flame speeds. It was vital to estimate whether droplet evaporation was completed in the flame preheat zone. This was done by calculating the spatial progress of droplet evaporation for the different aerosols from values of the evaporation rate constants of the different fuels.     

Parameterization of aerosols from burning biomass in the Brazil-SR radiative transfer model

A new aerosol parameterization was developed and implemented for operational use with the BRASIL-SR model. The goal is to improve the assessment of solar energy resources in Brazil. Optical properties of the aerosols from burning biomass were obtained using software package OPAC and are in good agreement with previous field measurements made in Brazil. Three different mixture ratios of black carbon were used to cover the full range of typical measured values. The atmospheric transport model SMOKE provided the aerosol profile. An evaluating period of 11 days in August/1995 and ground measurements from six sites situated in Amazon region was used to validate the results. The global solar irradiation estimates obtained with new aerosol parameterization, presented smaller mean bias error in all ground sites. The correlation among estimates and measured values for surface global solar irradiation improved about 2.5 times by adopting an aerosol composition with 5% of black carbon.     

Comparison of iso-octane burning rates between single-phase and two-phase combustion for small droplets

Two-phase combustion is a widespread mechanism of energy conversion that is of practical importance in gas turbines, diesel and spark ignition engines, furnaces, and hazardous environments. However, the exploration of important parameters in combustion systems of practical application is difficult, due to the multiplicity of dependent variables. In the present work, combustion rates of well-defined droplet suspensions of iso-octane have been measured using techniques employed for gaseous combustion. This required a full characterization of the aerosols produced in the combustion apparatus, which determined that the maximum droplet size produced was around 30 μm. Comparisons of two-phase with single-phase laminar mixtures suggest that there were negligible differences in the burning velocity of an aerosol and a gaseous mixture at the same overall equivalence ratio and similar conditions for iso-octane. At high stretch rates, flames remained smooth and droplet enhancement was negligible. However, at lower rates of stretch, both gaseous and aerosol flames became unstable and cellular, and this cellularity, in some cases, increased the burning rate. The values of Markstein length measured for aerosol flames had trends similar to those for gaseous-phase mixtures (Markstein length decreased with equivalence ratio), but were lower than in gaseous combustion. The values of Markstein length in aerosol flames also decreased with liquid equivalence ratio and/or Sauter mean diameter. All this indicates a higher tendency to instabilities in aerosol flames compared to gaseous combustion. A qualitative explanation for the lower values of Markstein length in aerosol combustion is given. It is suggested in the present work that aerosol flames became unstable, and hence had faster burning rates, under conditions that would not result in unstable gaseous flames. Comparisons, qualitative and in terms of dimensionless groups, of two-phase with single-phase turbulent combustion also suggest no enhancement.     

Nuclear power for the production of synthetic fuels and feedstocks

Due to depletion of national and foreign resources, development of alternative or synthetic carbonaceous fuels and feedstocks (SCFF) is recommended, using nuclear power not only to provide for the therma, electrical, and hydrogen economies, but also, ultimately, to provide a stable carbonaceous fuel economy. The value or carbonaceous raw material for conversion to SCFF is much higher than burning them for their BTU value alone, and the authors sets forth the process concept for using all forms of carbonaceous materials as sources of raw materials for SCFF. To reduce costs it is recommended to build nuclear plants to provide peaking power for the grids and to utilise the off-peak power for production of SCFF.     

NUMERICAL INVESTIGATION OF MASS AND HEAT TRANSFER IN SODIUM POOL COMBUSTION

Liquid sodium pool combustion is a coupled phenomenon of heat and mass transfer, chemical reaction, and aerosol dynamics. This article presents numerical investigation of the coupled phenomena using a new computational tool. Boundary layer equations are solved to obtain the thermalhydraulic field above the pool surface. The chemical reaction and heat and mass transfer are solved interactively considering radiation heat transfer and behavior of reaction product aerosols. Numerical results are compared with experiments, and agreement is excellent concerning burning rate, flame temperature, and height. It is concluded that the sodium pool combustion is self-limited and negative feedback is at work.     

Particle combustion rates for mechanically alloyed Al-Ti and aluminum powders burning in air

Mechanically alloyed aluminum-rich powders of Al-Ti (10, 15, 20, and 25 atom% of Ti) were produced and their combustion was compared to that of aluminum and titanium powders of comparable sizes. A laminar lifted-flame aerosol burner developed recently was used in this research. The aerosols were produced and burned in air. Measured flame speeds were higher for the aerosols of Al-Ti mechanical alloys than for the aerosols of pure Al or Ti. The particle combustion rates were evaluated based on the comparison of the measured and calculated profiles of the flame radiation. To calculate radiation profiles a simple model of particle combustion was used, in which both the radiation intensity and particle burn time were expressed as power functions of the initial particle size. For all the powders, the burning particle radiation intensity was observed to be best described by a function proportional to the cube of the initial particle size. For aluminum aerosol, the best match between the experimental and calculated flame radiation profiles was observed for the linear particle combustion law, when the particle combustion time, t, was expressed as a function of the initial particle size, d, as t. To match the experimental and calculated flame radiation profiles for the Al-Ti mechanical alloys, the combustion times of individual particles could be described by either d¹ or d² expressions. The burning time of mechanically alloyed particles increased with the increase of titanium concentration. The overall combustion times for aluminum particles are significantly longer than those for mechanically alloyed particles of Al-Ti of the same size.     

Catalytic decomposition of liquid hydrocarbons in an aerosol reactor: A potential solar route to hydrogen production

Catalytic decomposition of liquid fuels (n-octane, iso-octane, 1-octene, toluene and methylcyclohexane) is achieved in a continuous tubular aerosol reactor as a model for the solar initiated production of hydrogen, and easily separable CO free carbonaceous aerosol product. The effects of fuel molecular structure and catalyst concentration on the overall hydrogen yield were studied. Iron aerosol particles used as the catalysts, were produced on-the-fly by thermal decomposition of iron pentacarbonyl. The addition of iron catalyst significantly decreases the onset temperature of hydrogen generation as well as improves the reaction kinetics by lowering the reaction activation energy. The activation energy without and with iron addition was 260 and 100 kJ/mol, respectively representing a decrease of over 60%. We find that with the addition of iron, toluene exhibits the highest hydrogen yield enhancement at 900 °C, with a 6 times yield increase over thermal decomposition. The highest H₂ yield obtained was 81% of the theoretical possible, for n-octane at 1050 °C. The general trend in hydrogen yield enhancement is that the higher the non-catalytic thermal decomposition yield, the weaker the catalytic enhancement. The gaseous decomposition products were characterized using a mass spectrometer. An XRD analysis was conducted on the wall deposit to determine the product composition and samples for electron-microscopic analysis were collected exiting the furnace by electrostatically precipitating the aerosol onto a TEM grid.     

Combustion Characteristics of 24 Lignite Samples

Abstract

In this study, the combustion characteristics such as thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTGA), burning profile, ignition temperature, and peak temperature were analyzed for 24 lignite samples from different areas of Turkey. The samples were heated up to 900°C at a constant rate of 10°C/min in a 5 mL/min flow of dry air. The burning profiles of the samples studied, combined with proximate, sulfur analysis and calorimetry results, contribute to a clearer identification of lignite samples' structure and a better understanding of the coalification process. The lignite samples have been tested with particle size of 0–0.05 mm. Ignition temperatures of the samples have been determined from their burning profiles.     

Modeling char oxidation behavior under Zone II burning conditions at elevated pressures

For accurate modeling of the coal combustion process at elevated pressures, account must be made for variations in char-particle structure. As pressure is increased, particle swelling increases during the devolatilization of certain bituminous coals, yielding a variety of char-particle structures, from uniform high-density particles to thin-walled non-uniform low-density particles having large internal void volumes. Since under Zone II burning conditions the char conversion rate depends upon the accessibility of the internal surfaces, the char structure plays a key role in determining particle burnout times. In our approach to characterize the impact of char structure on particle burning rates, effectiveness factors appropriate for thin-walled cenospherical particles and thick-walled particles having a few large cavities are defined and related to the effectiveness factor for uniform high-density particles that have no large voids, only a random distribution of pores having a mean pore size in the sub-micron range. For the uniform case, the Thiele modulus approach is used to account for Zone II type burning in which internal burning is limited by the combined effects of pore diffusion and the intrinsic chemical reactivity of the carbonaceous material. In the paper, the impact of having a variety of char structures in a mix of particles burning under Zone II burning conditions is demonstrated.     

Impact of Pinatubo aerosols on the seasonal trends of global, direct and diffuse irradiance in two northern mid-latitude sites

In June 1991, Mt Pinatubo's eruption in the Philippines ejected a staggering 20 million metric tons of SO₂ into the stratosphere which resulted in an aerosol cloud covering most of the Earth within a few months after the eruption. In this article we illustrate how the seasonal trends of global, direct and diffuse solar radiation were modified by the eruption in two mid-latitude sites in Europe and the U.S., totalling about 12 yr of hourly data. A spectacular increase in the diffuse fraction of solar radiation as opposed to a decrease in direct radiation which extended from late 1991 to mid 1993 was observed in both sites and for clear sky conditions. Global radiation was not altered significantly from these data sets. Sunphotometer measurements in one of the sites show the volcanic aerosols tripled the total atmospheric aerosol optical depth at 1 μm and approximately doubled the aerosol optical depth at 0.5 μm.     

Simulation of a solar energy system by means of an electrical resistance network

A reduction in global surface irradiance occurs with increasing aerosol loadings when the aerosols are absorbing. For scattering aerosols, a reduction is pronounced for isotropic scattering (characteristic of small particles) but reduction is not as significant for scattering with a high anisotropy of a large forward peak (characteristic of large particles). This distinction between isotropic and anisotropic scattering becomes small or null over highly reflecting terrain; and actually for reflectivities higher than 0.5 and solar elevation angles close to the zenith, the global irradiance can be slightly higher for isotropic scattering than in the case of an anisotropy of a forward peak. Under such conditions, which can be encountered in reflective infrared bands over dense vegetation or over sandy deserts (close to noon, in low latitudes) the surface irradiance becomes nearly independent of the aerosol optical thickness.The above conclusions are derived based on analytical treatment of simplified single scattering approximation to the radiative transfer through a turbid atmosphere, which is valid only for a low or a moderate optical thickness.Computations of the spectral irradiance using the explicit expression are presented in the form of tables and graphs, for two anisotropy parameters of aerosols and three surface reflectivities.     

Dependence of the spectral surface irradiance on aerosol properties and surface reflectivity

A reduction in global surface irradiance occurs with increasing aerosol loadings when the aerosols are absorbing. For scattering aerosols, a reduction is pronounced for isotropic scattering (characteristic of small particles) but reduction is not as significant for scattering with a high anisotropy of a large forward peak (characteristic of large particles). This distinction between isotropic and anisotropic scattering becomes small or null over highly reflecting terrain; and actually for reflectivities higher than 0.5 and solar elevation angles close to the zenith, the global irradiance can be slightly higher for isotropic scattering than in the case of an anisotropy of a forward peak. Under such conditions, which can be encountered in reflective infrared bands over dense vegetation or over sandy deserts (close to noon, in low latitudes) the surface irradiance becomes nearly independent of the aerosol optical thickness.The above conclusions are derived based on analytical treatment of simplified single scattering approximation to the radiative transfer through a turbid atmosphere, which is valid only for a low or a moderate optical thickness.Computations of the spectral irradiance using the explicit expression are presented in the form of tables and graphs, for two anisotropy parameters of aerosols and three surface reflectivities.     

Numerical investigation of coke oven gas (COG) injection into an ironmaking blast furnace (BF)

Blast furnace (BF) ironmaking is the predominating process for producing hot metal (HM). It consumes huge quantities of carbonaceous fuel materials and leads to massive CO₂ emissions. The injection of coke oven gas (COG) into the BF is considered a promising solution. It recovers the COG that is a kind of off-gas in the steelwork, and reuses the COG as an H₂-intensive fuel in the BF to partially replace the use of carbonaceous fuel materials. However, thus far, the technology is not widely adopted, mainly due to the lack of understanding regarding the effects of key operational parameters of COG injection on BF performance. In addition, the coupling effect of COG injection and BF operation particularly the control at furnace top is not clear, leading to the low utilization efficiency. In this work, a continuum-based BF process model is developed and validated to consider the injection of COG into a commercial scale BF through the tuyere. The model is validated by comparing the calculated key performance indicators with those measured in production. The impact of COG injection rate is studied and its coupling effects with top burden distribution have also been clarified. The simulation results show that an increased COG injection rate leads to improved BF performance, in terms of increased productivity and decreased consumption of carbonaceous fuel materials. However, the utilization efficiency of COG and the replacement ratio of carbonaceous fuel materials by COG is decreased. An optimum top burden distribution can be identified, which can improve the utilization efficiency of injected COG and achieve a relatively high replacement ratio. The findings from this work should be useful to guide production of BF with H₂-intensive fuel injection, which helps to save the use of carbonaceous fuel materials and reduce CO₂ emission.     

Optical and electrical characterization of carbon nanoparticles produced in laminar premixed flames

Optical and electrical properties of carbonaceous particles produced in laboratory scale, premixed ethylene/air flames are obtained. Light absorption and Raman spectroscopy show that the change in particle nanostructure follows a graphitization trajectory as the flame richness increases. The optical band gap decreases and the size of the aromatic network in the particle increases, while the interlayer spacing between parallel layers decreases. The electrical conductivity of the materials increases by increasing flame richness in agreement to the graphitization trajectory. A non-ohmic behavior has been found and explained in terms of electron tunneling in a percolative network. Our results show that the electrical properties of flame formed carbon nanoparticles are strongly dependent on their nanostructure, and hence they have to be used carefully for the determination of particle concentration with conductometric sensors. Moreover, the dependence of the electrical properties of combustion formed particles might be useful for the development of cheap sensors for the selective detection of different classes of combustion aerosols.     

Evaluation on carbon nanocapsules for supercapacitors using a titanium cavity electrode

We synthesize carbon nanocapsules (CNCs) by a flame combustion method and evaluate their potential as the electrode material for electrochemical double layer capacitor using a titanium cavity electrode (TCE). Identical process is conducted on commercially available carbonaceous materials such as Vulcan XC72R, Black Pearl 2000 (BP2000), multi-walled carbon nanotubes (MWCNTs), and active carbon (AC1100) for comparison purposes. Images from Scanning electron microscope and Transmission electron microscope on the CNCs demonstrate irregular-shaped particles in average size of 10-20 nm with graphene layers on perimeter compassing a hollow core. Electrochemical characterizations including cyclic voltammetry (CV), current reversal chronopotentiometry (CRC), and impedance spectroscopy are carried out in 1N H₂SO₄ to determine the specific capacitance and cycle life time. Among these samples, the BP2000 still delivers the highest specific capacitance in F g⁻¹ but the CNCs demonstrate the largest value in μF cm². In addition, the CNCs exhibit impressive life time for 5000 cycles without notable degradation. Consistent results are obtained by CV, CRC, and impedance measurements, validating the TCE as a facile tool to perform reliable electrochemical evaluations.     

A model for calculating hourly global solar radiation from satellite data in the tropics

A model for calculating global solar radiation from geostationary satellite data is presented. The model is designed to calculate the monthly average hourly global radiation in the tropics with high aerosol load. This model represents a physical relation between the earth-atmospheric albedo derived from GMS5 satellite data and the absorption and scattering coefficients of various atmospheric constituents. The absorption of solar radiation by water vapour which is important for the tropics, was calculated from ambient temperature and relative humidity. The relationship between the visibility and solar radiation depletion due to aerosols was developed for a high aerosol load environment. This relationship was used to calculate solar radiation depletion by aerosols in the model. The total column ozone from TOMS/EP satellite was employed for the determination of solar radiation absorbed by ozone. Solar radiation from four pyranometer stations was used to formulate the relationship between the satellite band earth-atmospheric albedo and broadband earth-atmospheric albedo required by the model. To test its performance, the model was used to compute the monthly average hourly global radiation at 25 solar radiation monitoring stations in tropical areas in Thailand. It was found that the values of monthly average of hourly global radiations calculated from the model were in good agreement with those obtained from the measurements, with the root mean square difference of 10%. After the validation the model was employed to generate hourly solar radiation maps of Thailand. These maps reveal the diurnal and season variation of solar radiation over the country.     

Progress report on the catalyst layers for hydrocarbon-fueled SOFCs

Solid oxide fuel cell (SOFC) is an energy conversion device that can directly convert the chemical energy of carbonaceous fuels into electricity. Solving the problem of carbon deposition in the conventional nickel-based anode is essential to improving the performance of SOFC when operating on carbonaceous fuels. Although impressive progress has been made in the development of alternative anode materials, nickel-based anodes with superior catalytic activity for carbonaceous fuels are still the most promising anode for the commercialization of SOFCs. The deposition of a catalyst layer with high catalytic activity for carbonaceous fuels over the nickel-based anode has been demonstrated as an effective way to enhance the performance and long-term stability of hydrocarbon-based SOFC. This review introduces the working principles of the catalyst layers, discusses the recent progress of the catalyst layer materials for hydrocarbon-fueled SOFC and issues of the different catalyst layer materials. Finally, some of the future prospects and challenges of the catalyst layers are summarized in this review article.     

A process for producing carbonaceous matter from tar sand,oil shale and olive cake

Many countries which do not produce oil are rich with other sources of energy that are not fully utilized, such as tar sand, oil shale and olive cake. Limited previous work was done on producing carbonaceous matter and separating volatile matter from combinations of tar sand, oil shale and olive cake. In this study, a process is designed and tested to produce carbonaceous matter from combinations of the three materials mentioned above. Results indicate that the process is successful in producing carbonaceous matter. The minimum temperature to achieve complete carbonization was found to be 500°C for a minimum heating time of 1.5 h. Carbonized materials were tested for their calorific values, which indicated a successful carbonization process. The proposed process can be scaled up and automated, and is expected to be economically feasible. Moreover, the process allows for control of unwanted exiting polluting gases and volatile matter and therefore is environmentally safe.     

Review on the interface engineering in the carbonaceous titania for the improved photocatalytic hydrogen production

Hydrogen is the prime source of energy with enormous attention in the current research development process as it is safe, clean, eco-friendly, and can be produced from renewable resources through simple catalytic reactions. Scalable production of hydrogen through photocatalysis has been achieved using carbon-modified semiconductors since 2009. In this direction, this review delivers comprehensive understandings into the interface and structural interactions between TiO₂ and carbonaceous materials such as carbon, carbon nanotubes, graphene, activated carbon, graphitic carbon nitride, carbon quantum dots, etc., and their influences toward improving the hydrogen generation activity of these systems. Besides, recently developed carbonaceous materials such as 3-D graphene, carbon nanohorns, and carbon nanocones have also been discussed on their character in the photocatalytic water splitting procedure. In general, the observed improvements in this carbon-modified TiO₂ attributed to the synergetic effects, which offer the active migration of charge carriers and reduced recombination rates in the photocatalyst. Finally, highlighting the future perspectives of the carbonaceous materials in photocatalytic applications are concluded.     

On the burning behavior of pulverized coal chars

A model that predicts the physical changes that pulverized coal char particles undergo during combustion has been developed. In the model, a burning particle is divided into a number of concentric annular volume elements. The mass loss rate, specific surface area, and apparent density in each volume element depend upon the local particle conditions, which vary as a consequence of the adsorbed oxygen and gas-phase oxygen concentration gradients inside the particle. The model predicts the particle's burning rate, temperature, diameter, apparent density, and specific surface area as combustion proceeds, given ambient conditions and initial char properties. A six-step heterogeneous reaction mechanism is used to describe carbon reactivity to oxygen. A distributed activation energy approach is used to account for the variation in desorption energies of adsorbed O-atoms on the carbonaceous surface. Model calculations support the three burning zones established for the oxidation of pulverized coal chars. The model indicates two types of zone II behavior, however. Under weak zone II burning conditions, constant-diameter burning occurs up to 30% to 50% conversion before burning commences with reductions in both size and apparent density. Under strong zone II conditions, particles burn with reductions in both size and apparent density after an initial short period (<2% conversion) of constant-diameter burning. Model predictions reveal that early in the oxidation process, there is mass loss at constant diameter under all zone II burning conditions. Such weak and strong burning behavior cannot be predicted with the commonly used power-law model for the mode of burning employing a single value for the burning mode parameter. Model calculations also reveal how specific surface area evolves when oxidation occurs in the zone II burning regime. Based on the calculated results, a surface area submodel that accounts for the effects of pore growth and coalescence during combustion under zone I conditions was modified to permit the characterization of the variations in specific surface area that occur during char conversion under zones II conditions. The modified surface area model is applicable to all burning regimes. Calculations also indicate that the particle's effectiveness factor varies during conversion under zone II burning conditions. With the adsorption/desorption mechanism employed, a near first-order Thiele modulus-effectiveness factor relationship is obeyed over the particle's lifetime.