Combustion Zone Thickness Research Articles

Effects of methane concentration on flame propagation mechanisms and dynamic characteristics of methane/coal dust explosions

The study explores the flame propagation and explosion dynamic behavior of gas/powder two-phase compound explosions, taking methane and coal dust mixture as the research object. The influence of different methane concentrations on the compound explosion characteristics was researched. The vertical pipeline, high-speed camera, ion current probe, and pressure sensor be used to test the propagation behaviors, microstructure characteristics, and explosion dynamics of compound explosion flames. The function of methane concentration on the propagation behaviors, combustion reaction zone thickness, flame temperature, propagation velocity, and explosion pressure of compound flame was revealed, and the propagation mechanisms of gas/powder compound explosion flames were explained. The experimental research discovered that the compound flame of methane/coal dust explosion presents a white luminous zone with uniform distribution and appears to have an approximate homogeneous combustion state. The explosion dynamics parameters all showed a trend of first increased and then decreased as the methane concentration increased, but the reaction zone thickness of the combustion first reduced and then increased. In addition, the dominant position of homogeneous or heterogeneous combustion in the combustion reaction zone will also switch with the change in methane concentration. The compound flame is similar to premixed gas combustion under optimal methane concentration, the combustion zone thickness (δ) and preheating zone thickness (β) are the smallest, and the compound explosion shows powerful performance. In addition, the flame propagation mechanisms of methane/coal dust compound explosion were elucidated based on the theory of heat.

Modelling the effect of sinter machine speed on bed temperature and coke combustion characteristics in iron ore sintering process

ABSTRACT In the present study, the influence of industrial sinter machine speed on the temperature distribution of the bed is evaluated using a one-dimensional transient mathematical model. The model is developed considering the law of conservation of mass, momentum, and energy of solid and gas phases. The prediction of bed temperature from the model is compared with the measured lab scale pot test and the exit gas temperature of an industrial sinter machine wind boxes shows a reliable agreement. The effect of machine speed on the temperature of the bed is quantified using Maximum Temperature, and the coke combustion characteristics are expressed in terms of Duration Time of Coke Combustion and Combustion Zone Thickness. Eventually, the Melting Zone Thickness and melt fraction of the bed is evaluated. This model result demonstrates that the increasing industrial sinter machine speed significantly affects the temperature distribution of the top layer of the sinter bed.

Study of stand-support sintering to achieve high oxygen potential in iron ore sintering to enhance productivity and reduce CO content in exhaust gas

Abstract Coarse coke particles, thick combustion zone, and poor thermal permeability during sintering have been widely reported to lead to incomplete solid fuel combustion and low oxygen potential, which induces the formation of harmful gas and decreases the energy efficiency. To solve these problems, stand-support sintering with an ultrathick 1000 mm bed was first proposed in this paper. The influences of the stand height and coke size on the process parameters and sinter quality were studied through a series of experiments. The results showed that the average gas flow velocity increased from 0.24 m/s to 0.32 m/s and the maximum combustion zone thickness decreased from 202 mm to 140 mm, which indicated that stand-support sintering could substantially improve the thermal permeability and enhance the oxidizing atmosphere. The fuel consumption was reduced by 1.28 kg/t (2.25%), the generated CO in exhaust gas was reduced, and the production efficiency increased by 17.4% when the stand height increased from 0 to 500 mm. Moreover, a 3D high-energy X-ray computerized tomography (CT) scanner was first used to quantitatively confirm the effect of the stand on the distribution and size of the pores in the whole sinter cake. Research findings showed that the porosity increased by 7.1% when the height of the stand increased from 0 to 500 mm, which further illustrated the effect of stand-support sintering. The present study provided an effective way to optimize the energy efficiency and reduce the CO content in exhaust gas.

Influence of coke content on sintering process of chromium-containing vanadium-titanium magnetite

The sintering of chromium-containing vanadium-titanium magnetite using different coke contents was studied through the sintering pot tests, X-ray diffraction analysis and mineralogical phase analysis. Results showed that, as the coke content increased from 3.2% to 4.4%, the liquid phase and combustion zone thickness increased while the vertical sintering rate and ratio of sintered product decreased. In addition, the combustion ratio of exhaust gas also increased with increasing the coke content, indicating that combustion zone temperature also increased, and the excessive the coke content in the sintering process of vanadiumtitanium magnetite is harmful. As the coke content increased, the magnetite, silicates, and perovskite contents of the sintered ore increased while the contents of hematite and calcium ferrite of sintered ore decreased; drum strength decreased, and reduction degradation properties increased while reduction ability decreased. We found that the appropriate coke content for the sintering process is 3.6 wt%.

The Relationship between Sinter Mix Composition and Flame Front Properties by a Novel Experimental Approach

ABSTRACTThis study explores a new technique for iron ore sintering to assess some flame front properties in a laboratory test, such as the flame front speed, combustion zone duration time, combustion zone thickness, combustion rate, and study the relationship between sinter mix composition and flame front properties. Results agree with those obtained by other researchers using embedded thermocouples and also predictions from a numerical sintering model well. When flame front moves forward, combustion zone duration time (DTCZ) and thickness increases. DTCZ increases from 1.65 min at the top interface to 1.80 min at the bottom interface. Combustion zone thickness is closely related with flame front speed. Melt in the iron ore sintering process has a large influence on flame front parameters, which affects oxygen diffusion on the coke surface, sinter bed permeability, and the heat transfer. Coke combustion rate is closely related with flame front speed and single coke particle burn-out time.

Emission and laser absorption spectroscopy of flat flames in aluminum suspensions

Imaging emission spectroscopy, spatially resolved laser-absorption spectroscopy, and particle image velocimetry (PIV) are applied to a flat flame stabilized in a suspension of micron-sized aluminum. The results from the combination of diagnostics are used to infer the combustion regime of the particles and to estimate the characteristic combustion time of the suspension. It is observed that the reaction zone of the flame in stoichiometric aluminum–air suspensions exhibits strong self-reversal of the atomic aluminum emission lines. These lines also exhibit high optical depths in both emission and absorption spectroscopy. The strong self-reversal and high optical depths indicate high concentrations of aluminum vapor within the reaction zone of the flame at multiple temperatures. These features provide evidence of the formation of vapor-phase micro-diffusion flames around the individual particles in the suspension. In aluminum–methane–air flames, the lack of self-reversal and lower optical depths of the aluminum atomic lines indicate the absence of vapor-phase micro-diffusion flames, and point to a more heterogeneous, and likely kinetically-controlled, particle combustion regime. The reaction zone thickness is estimated from the spatially resolved profiles of aluminum resonance lines in both absorption and emission through the flame. The emission measurements yield a reaction zone thickness on the order of 1.7±0.3 mm in aluminum–air flames, and the absorption measurements yield a thickness on the order of 2.3±0.5. It is demonstrated that the combination of the combustion zone thickness measurement, flame temperatures determined from molecular AlO emission spectra, and particle velocity measurements from the PIV diagnostic permits an estimation of the burning time in the suspension. The burning time in stoichiometric aluminum–air suspensions using the suite of diagnostics is estimated to be on the order of 0.7 ms.

Mathematical modeling and combustion characteristic evaluation of a flue gas recirculation iron ore sintering process

Flue gas recirculation sintering (FGRS) technology has been applied for two decades with the aim of reducing pollutant emissions. Compared with the conventional sintering (CS), the changes of input gas conditions may influence the bed combustion process greatly. Mathematical models have been developed to predict sintering behavior quantitatively, but few of the previous work focused on FGRS process. In this study, a multiphase theory-based mathematical model is established. This model considers nine kinds of major physicochemical reactions, in which six modes of gaseous reactions make it more comprehensive and accurate to model FGRS process. Heat transfer within/between different solid and gas phases are modeled in better manners. Geometric changes caused by reactive-factors are modeled in simple terms. Sub-models are available to simulate the effects of the temperature, gas supply, composition and content of recirculated gas on combustion characteristics in the sintering bed. Good agreements between simulated and measured results have been obtained from contrasting to six sinter pot tests based on FGRS technology. Four combustion parameters are selected to evaluate quantitatively the advantages and potential problems of FGRS technology. Results show that the flatter maximum temperature (MaxT) profile for FGRS compared with that for CS implies a stronger tumble strength of the sintered ore. The broader MaxT and combustion zone thickness (CZT) curve indicate a higher degree of melt fraction, together with a lower FFS and productivity. To better investigation, further parameter simulation and process optimization of FGRS technology is necessary.

Heat transfer of spent ion exchange resin in iron ore sintering process

Spent ion exchange resin (SIER) is a kind of solid waste which derived from water treatment process in iron and steel industry. Utilization of SIER as a replacement of fuel in sintering process is a promising method of SIER treatment. In order to investigate the feasibility of this method, heat transfer effects of SIER in iron ore sintering process were studied in this paper via numerical simulation. A 3D unsteady numerical reference model was developed on the basis of the porous media model and local non-equilibrium thermodynamics model and verified by the data form measurement. Heat transfer effects of different SIER mass fractions (0–8%) in sintering material during the sintering process were studied on the numerical model established in this paper. The results showed that with the increasing mass fraction of SIER, the maximum combustion zone thickness and flame front speed are both increased, the heating-up point, the moment when solid temperature reached the top and the maximum temperatures were all became earlier in different location of sintering bed. When SIER content is 8%, the maximum sintering temperature exceeds the maximum limit of best sintering temperature.

Experimental and numerical studies of the effects of hydrogen addition on the structure of a laminar methane–nitrogen jet in hot coflow under MILD conditions

Abstract In this work we investigate the effects of hydrogen addition on the flame structure of MILD combustion both experimentally and numerically using a laminar-jet-in-hot-coflow (LJHC) geometry. The addition of hydrogen appreciably decreases the flame height (∼25%), however only modestly affects the maximal flame temperature and the thickness of combustion zone. The NO distribution is dominated by mixing of the NO formed in the coflow with the reaction products of the diluted fuel, with negligible NO formation from the fuel in all flames studied. The numerical data are in reasonably good agreement with the measurements.

Hydrodynamic instability of the coflow filtration combustion: Numerical simulation

The stability of the front to infinitely small distor� tions was analyzed under the assumption that the reac� tor channel width is infinitely large in comparison with the combustion zone thickness (5). It was concluded that the plane filtration combustion wave may be unstable. The experimental investigation of the filtra� tion combustion wave stability confirmed (6) that the propagation of a plane filtration combustion front may be unstable and may be accompanied by the formation of structures similar to Saffman-Taylor fingers, which were observed as a highviscosity liquid was displaced by a lowviscosity liquid (7). In this work, for the first time, we made mathemat� ical modeling of twodimensional unsteady filtration combustion during the forced oxidant filtration in the combustion wave direction. The purposes of the study was to determine the conditions for the occurrence of filtration combustion with a stable plane front in chan� nels of finite width and investigate the filtration com� bustion wave propagation modes in the range of insta� bility of the plane reaction front. The physical formulation of the problem is the fol� lowing. A gas mixture with oxidant concentration a0 is fed to a reactor filled with a porous mixture containing a fuel component. The gas flow rate at the inlet (x = 0) is maintained constant (G0). Reaction initiation with a hightemperature source leads to reaction front prop� agation in the х direction at rate u. As the fuel is burned out, the structure of the porous medium changes, because of which the porosity, thermal diffusivity, and permeability of the combustion products may differ from the respective characteristics of the initial reac� tion mixture. The problem is considered in the plane geometry under the assumption that the reactor is shaped as a thin slitlike channel. If the reaction front propagation velocities are low, which is characteristic of filtration combustion waves, the difference between the temperatures of the parti� cles and the gas can be ignored and the porous medium can be considered locally uniform in temper� ature and concentration. Under this assumption, the dimensionless system of equations describing filtra� tion combustion has the form (1, 2)

Saffman–Taylor problem in filtration combustion

In the hydrodynamic approximation, postulating the combustion zone thickness to be much smaller than the channel width, the problem of filtration combustion (FC) has the same spectrum of solutions as the well-known Saffaman-Taylor (ST) problem of the displacement of a high-viscosity liquid by a low-viscosity one. In both cases, the spectrum of solutions includes families of symmetrical and asymmetrical fingers. In the ST problem, asymmetrical fingers are considered unrealizable because of the effect of surface tension at the interface between the liquids, which suppresses the formation of asymmetrical fingers. For FC, there is no exclusion of asymmetrical solutions, since the interphase boundary is a reaction front with zero surface tension. In the present work, it is experimentally demonstrated that an increase in the channel width above a critical value, a planar FC wave loses its stability and the combustion front propagates along one of the channel walls in the form of an asymmetrical finger.

A Theoretical and Experimental Study of Combustion Tubes

When the air injection technique is used in a reservoir, the usual way to determine the fuel that would be burned and the temperature profiles, among other parameters, is by experimentation in a combustion tube. In this work, using medium oil (27 API) from the Gulf of Mexico, combustion tube experiments were carried out. A mathematical model, which takes into account the heat transfer ahead and behind from the combustion front, the combustion zone thickness, and the heat generated and loss in the combustion front, was solved numerically and used to predict the experimental temperature profiles. After validation of the mathematical model, further investigations of the effect of heat losses (along, at the top, and at the bottom of the combustion tube) on temperature profiles were done. The calculated combustion zone temperature and temperature profiles are in good agreement with our experimental data. It was found that not considering an isolated boundary in the mathematical model allows obtaining better predictions for the temperature profiles behind and ahead from the combustion front. On the other hand, the combustion front temperature increases as the heat losses decrease and the combustion front temperature attains its maximum value when there is no heat loss.

How the In-Situ Combustion Process Works in a Fractured System: 2D Core- and Block-Scale Simulation

Summary Simulation of an in-situ combustion (ISC) process was performed for a fractured system at core and matrix-block scales. The aim of this work was: (1) To predict the ISC extinction/propagation condition(s), (2) understand the mechanism of oil recovery, and (3) provide some guidelines for ISC upscaling for a fractured system. The study was based on a fine-grid, single-porosity, multiphase, and multicomponent simulation using a thermal reservoir simulator. First, the simulator was validated for 1D combustion using the corresponding analytical solutions. 2D combustion was validated using experimental results available in the literature. It was found that the grid size should not be larger than the combustion-zone thickness in order for the results to be independent of grid size. ISC in the fractured system was strongly dependent on the oxygen diffusion coefficient, while the matrix permeability played an important role in oil production. The effect of each production mechanism was studied separately whenever it was possible. Oil production is governed mainly by oil drainage because of gravity force, which is enhanced by viscosity reduction; possible pressure-gradient generation in the ISC process seems to have a minor effect. The nature (oil-production rate, saturations distribution, shape of the combustion front) of ISC at core scale was different from that in a single block with surrounding fracture. The important characteristics of different zones (i.e., combustion, coke, and oil zones) at block scale were studied, and some preliminary guidelines for upscaling are presented.

Prediction of Thermal Conductivity Effects on in-situ Combustion Experiments

In this work a mathematical model for a combustion tube was developed. The model takes into account: (a) the heat transfer ahead and behind the combustion front, (b) the combustion zone thickness, (c) the heat generated and lost in the combustion front, and (d) the variation of thermal conductivity with temperature. The predicted combustion front temperature and temperature profiles are in good agreement with experimental data for heavy oils reported in literature. After validation of the mathematical model, further investigations of the effects of thermal conductivity on the temperature profiles ahead and behind the combustion front were studied. Expressions proposed in literature for effective thermal conductivity were used. It was found that: (a) when the functionality of thermal conductivity with temperature is taken into account, the predicted lost heat is different from that using constant thermal conductivity and (b) temperature profiles are affected principally at the beginning and at the end of the tube. The porosity effects on temperature profiles were also studied. In this case, the temperature profiles ahead and behind the combustion front and the combustion front temperature are smaller as the porosity becomes smaller.

Combustion characteristics in an iron ore sintering bed—evaluation of fuel substitution

In an iron ore sintering bed, combustion of solid fuel supplies heat needed for sintering of fine particles and determines the quality of the sintered ores and the productivity of the process. Coke has been widely used as an ideal fuel for this process, but recent attempts to partially replace coke with a less expensive fuel have been considered effective in the real applications. This paper reports simulation results of mathematical modeling of bed combustion as well as experimental observation from scale-downed pot tests. Combustion characteristics of the solid fuel bed are also described and fuel substitution from coke to anthracite coal is evaluated. A numerical model of the 1-D unsteady level considers the processes in the bed of particles: drying, devolatilization, char reactions, gaseous reactions, heat transfer, and geometrical changes of the bed material. The model can treat the solid material as multiple solid phases whose contents include fine particles of iron ore, limestone, coke, and coal. Quantitative parameters are newly defined for characterization of the bed combustion. These include flame front speed, sintering time, duration time in the combustion zone, combustion zone thickness, melting zone thickness, and maximum temperature. Relationships between these parameters and the mechanism of combustion propagation are also investigated. The results show quantitatively that temperature profiles, combustion propagation, and thickness of the combustion zone in the bed are dominated by combustion-related operating parameters. This description is applied to case studies for various coke contents, air supply rates, and fuel characteristics. It is shown that the different reactivity of anthracite in comparison with coke can significantly affect the bed combustion in terms of combustion propagation and the quality of the sintered ores in the upper part of the bed.

Estimation of Combustion Zone Thickness during In Situ Combustion Processes

Dry and wet combustion runs were conducted using three different crude oils having 12.4, 19.8 and 28.4 °API gravity. Combustion zone thickness measurements were made with four stationary in-situ sampling gas probes installed on the combustion tube at 36, 48, 60, and 72 cm from the top flange. Experiments were carried out in a thin-walled vertical tube having a 12.4 cm diameter and 124.5 cm length. Adiabatic control of the combustion tube was achieved during dry and wet combustion tests using external band heaters and an adiabatic control system. The average air flux was 23.0 m 3 (St)/(m 2 h), and injected WAR changed from 0.209 to 0.607 m 3 / Mm 3 (St). Experimental results showed that the average zone thickness was 80 mm for a dry combustion process while it was 50 mm for a wet combustion process. The combustion zone thickness decreased with increasing °API gravity of crude oils. With water injection, the combustion zone thickness decreased, as expected in a wet combustion temperature profile. Temperature and oxygen concentration data were used to evaluate the commonly used combustion reaction rate expression. The experimentally determined relationship between oxygen, combustion temperature, and reaction rate could not be represented by the commonly used first-order, Arrhenius-type reaction rate expression. Relative reaction rate data showed that dry combustion was kinetically controlled, whereas the wet combustion was oxygen-diffusion limited.

Predictive Tool To Aid Design and Operations of Pressurized Fixed Bed Coal Gasifiers

A mathematical model is presented to study the combustion and gasification of char with air and steam under pressure. A series of curves was obtained from the solutions of energy and mass balances for the probable range of operation in the gasification of char. Certain theoretical conclusions are drawn with respect to gasification of char in a fixed bed and found to be in accord with the results of the literature. A mathematical correlation is presented for conveniently calculating the product gas composition, the burning rate, the combustion zone thickness, the solids' maximum temperature, and the outlet gas temperature. This correlation is of general applicability, covers a wide range of operating conditions through use of multiple variance analysis, and may be used for pressures as high as 4200 kPa. Examples are given to illustrate the use and versatility of the correlation.

An Experimental Investigation of the Fireflooding Combustion Zone

Summary In a laboratory study, an automated, linear-traversing, sampling probe enabled real-time gas analysis and temperature measurements within the combustion zone. Results showed that the combustion-zone thickness was on the order of 1 in. [25 mm] for a nominal injected air flux of 20 scf/hr-ft2 [1.7 × 10-3 std m3/s·m2]. The combustion-zone thickness was found to be kinetically controlled. The combustion reaction could not be represented by the commonly used first-order, Arrhenius kinetics; however, the actual kinetics of combustion is unimportant for performance prediction because the combustion-front velocity is oxygen-flow-rate-controlled.

Studies in char gasification—III: The combustion zone

The fluidized bed steam-oxygen-char gasifier is modelled by treating the bed as one in which the solids are well mixed and the gases are in plug flow. This model permits a detailed analysis of the combustion zone. The entire reactor is assumed to be at a uniform temperature. From the results of the model calculations, it was concluded that the assumption of negligible combustion zone thickness made in the earlier two papers is acceptable This model predicts higher total carbon conversion and methane mole fraction in the product gas than the lumped model analysed in an earlier paper.