Stages Of Combustion Process Research Articles

Modeling Conjugate Heat Transfer in an Anode Baking Furnace Using OpenFoam

The operation of large industrial furnaces will continue to rely on hydrocarbon fuels in the near foreseeable future. Mathematical modeling and numerical simulation is expected to deliver key insights to implement measures to further reduce pollutant emissions. These measures include the design optimization of the burners, the dilution of oxidizer with exhaust gasses, and the mixing of natural gas with hydrogen. In this paper, we target the numerical simulation of non-premixed turbulent combustion of natural gas in a single heating section of a ring pit anode baking furnace. In previous work, we performed combustion simulations using a commercial flow simulator combined with an open-source package for the three-dimensional mesh generation. This motivates switching to a fully open-source software stack. In this paper, we develop a Reynolds-Averaged Navier-Stokes model for the turbulent flow combined with an infinitely fast mixed-is-burnt model for the non-premixed combustion and a participating media model for the radiative heat transfer in OpenFoam. The heat transfer to the refractory brick lining is taken into account by a conjugate heat transfer model. Numerical simulations provide valuable insight into the heat release and chemical species distribution in the staged combustion process using two burners. Results show that at the operating conditions implemented, higher peak temperatures are formed at the burner closest to the air inlet. This results in a larger thermal nitric-oxide concentration. The inclusion of the heat absorption in the refractory bricks results in a more uniform temperature on the symmetry plane at the center of the section. The peak in thermal nitric-oxides is reduced by a factor of four compared to the model with adiabatic walls.

Study of pyrolysis and combustion kinetics of oil sludge

ABSTRACT In order to deal with the shortage of oil and natural gas, the supplement energies, such as oil sludge, waste plastics, waste rubber needed to be studied. The pyrolysis and combustion methods have advantages when they are chosen to treat oil sludge. The investigation of kinetics is significantly important to know the kinetic parameters and the reaction mechanisms and to provide important theoretical basis for the process design of pyrolysis and combustion to treat oil sludge. Most previous studies on pyrolysis and combustion kinetics of oil sludge regarded pyrolysis process or combustion process, and rarely split pyrolysis or combustion process to calculate kinetic parameters separately. In this work, the experiment of pyrolysis and combustion were carried out by thermal gravimetric analyzer. After splitting the pyrolysis and combustion processes into two stages, respectively, the methods of Coats-Redfern and distributed activation energy model (DAEM) were used to calculate the kinetic parameters. Based on the results of thermogravimetric analysis, the pyrolysis and combustion process can be divided into two stages. For the pyrolysis process, the first stage is mainly with the evolution of adsorbed volatile hydrocarbon and the rupture of weaker chemical bond and the second stage is the cracking of organic macromolecules. Meanwhile, in the first stage of combustion process, small molecular organic matter in dry basis oil sludge will volatilize and then burn on the surface of the crucible and the gas phase outside the crucible, and in the second stage, macromolecular organic matter will volatilize, pyrolyze and burn, and the diffusion of oxygen into the crucible will cause the fixed carbon to burn. According to the kinetic results of the pyrolysis process, the activation energy of the first stage calculated by the Coats-Redfern model are all less than 40 kJ/mol, and that of the second stage increases from 148.14 kJ/mol to 184.97 kJ/mol with the increase of the heating rate. The results of DAEM model show that with the increase of the conversion rate, the activation energy of the first stage increases from 60.82 kJ/mol to 81.05 kJ/mol, and that of the second stage increases from 120.60 kJ/mol to 237.07 kJ/mol. For the combustion process, the activation energy calculated using Coats-Redfern model decreases from 120.18 kJ/mol to 73.48 kJ/mol in the first stage, and that of the second stage is relatively low. The results of DAEM model show that with the increase of the conversion rate, the activation energy decreases from 129.53 kJ/mol to 62.36 kJ/mol in the first stage and that in the second stage is stable at about 70 kJ/mol.

Synergistic effect of hydrothermal co-carbonization of sewage sludge with fruit and agricultural wastes on hydrochar fuel quality and combustion behavior

In order to improve fuel quality of sewage sludge, fruit and agricultural wastes have been selected for hydrothermal co-carbonization. After hydrothermal co-carbonization, organics retention was facilitated, while O/C and H/C atomic ratios of hydrochars were substantially upgraded. Particularly, hydrochar from hydrothermal co-carbonization of sewage sludge with peanut shells at mass ratio of 1:3 (denoted as “SS:PS = 1:3”) showed the highest fuel ratio of 0.79 and its carbon content was increased to 50.0% with significantly decreased O/C and H/C atomic ratios. Furthermore, higher heating value of hydrochars from hydrothermal co-carbonization was increased by nearly 2.65-fold and reached 21.72 MJ/kg. Moreover, the most favorable aromatization occurred when sewage sludge and peanut shells blending ratio was 3:1 or 1:1, whereas hydrothermal co-carbonization induced more CO and OH than COOH in hydrochars due to synergistic decarboxylation. A relatively higher value of point of zero charge for hydrochars from hydrothermal co-carbonization implied improved hydrophobicity. Combustion kinetics results indicated that hydrothermal co-carbonization balanced activation energies of hydrochars in devolatilization/combustion stage and char combustion process, rendering a more stable and lasting combustion profile. Hydrochars “SS:PS = 1:3” demonstrated desirable combustion performance. Therefore, hydrothermal co-carbonization can realize sustainable utilization of organic solid wastes towards superior hydrochar solid biofuels.

Thermal and Mixing Efficiency Enhancement in Nonuniform-Compression Scramjets

Numerical simulations are presented to compare the heat addition, thermodynamic cycle, and mixing efficiency of generic uniform- and nonuniform-compression scramjets. The work builds on previous numerical studies that showed the coupling of nonuniform-inlet compression and thermal compression enhanced the performance of low-inlet compression scramjets. In line with previous work, both engines have the same inlet-fuelling method, inlet contraction ratio, and combustor geometry. The freestream corresponds to a Mach 10 flight condition with a dynamic pressure of 100 kPa. This study shows the mixing, heat addition, and thermodynamic cycle efficiency in the nonuniform-compression engine are higher than the uniform-compression engine. The mixing and heat release efficiency increase by 10 and 20% respectively. Enhancement of these performance parameters in the nonuniform-compression engine is caused by a staged-combustion process. This study offers new insight into how purposely induced nonuniform compression can be used to enhance scramjet engine performance.

N-heptane ignition delay time with temperature criterion for HCCI combustion

Ignition delay time is key to any hydrocarbon combustion process. In that sense, this parameter has to be known accurately, and especially for internal combustion engine applications. Combustion timing is one of the most important factors influencing overall engine performances like power output, combustion efficiency, emissions, in-cylinder peak pressure, etc. In the case of two stage combustion process (e.g. HCCI mode), this parameter is controlled by chemical kinetics. In this paper, an ignition delay time model including 7 direct reactions and 13 species coupled with a temperature criterion is described. This mechanism has been obtained from the previous 26-step n-heptane reduced mechanism, focusing on reactions group of the low temperature regime which is the most important phase during the two stage combustion process. The complete model works with 7 reactions until the critical temperature is reached, leading to the detection of the ignition delay time value. The resulting ignition delay times obtained with the 7-step model have been compared to those of a calculation using a full kinetics of n-heptane developed by Lawrence Livermore National Laboratory. This comparison has pointed out that the model reproduces the ignition delay time with a good accuracy with differences smaller than 2 CAD and a computational time around one millisecond.

Effects of catalysts on combustion characteristics and kinetics of coal-char blends

The effects of Fe2O3, CaO, and MnO2 on the combustion characteristics and kinetics of coal-char blends were investigated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The results indicated that catalysts exhibited positive effects on the combustion characteristics of coal-char blends, especially in the initial period of coal-char blends combustion. With catalysts addition (mass 1.5%), it could improves volatile matter release, and reduces ignition point, promotes char to begin burning under lower temperature. The ignition index (C) was increased, respectively, by 27% for Fe2O3, 6% for CaO, 11.3% for MnO2, and the combustion characteristic index (S) was increased respectively, by 29% for Fe2O3, 5% for CaO, 8.3% for MnO2. In addition, two kinetic models (R2 and F1) were adopted to calculate the kinetic parameters in different stage of combustion processes. The results showed that with Fe2O3 or CaO addition, the activation energy at second stage decreases from 86.0 KJ/mol to 76.92 KJ/mol and 75.12 KJ/mol, respectively. There are no obvious decreases at the third stage of samples combustion process.

Influence of Primary and Secondary Air Supply on Gaseous Emissions from a Small-Scale Staged Solid Biomass Fuel Combustor

The emissions from traditional biomass combustion systems for cooking and heating are, globally, the main cause for premature mortality as a result of air pollution. A staged combustion process that separates the thermochemical conversion of the solid fuel and the combustion of the released products offers potential to reduce harmful emissions for solid fuel combustion and could, therefore, help mitigate the issue. In the present study, the fundamental combustion behavior of a small-scale staged combustor was investigated, with a focus on an independent systematic analysis of relevant parameters. Natural and forced draft conditions as well as a combination of both were tested. The relative location of primary to secondary air was also varied. When lighting the fuel, higher air flows lead to faster ignition and lower emissions. A steady-state combustion phase is achieved when gasification products are burned with secondary air, which occurs mainly while the solid fuel is being pyrolyzed. After the steady-s...

A reduced order model for prediction of the burning rates of multicomponent pyrotechnic propellants

This study reports a reduced order model for the prediction of the burning rate of pyrotechnic compositions. The combustion process of most pyrotechnics is primarily driven by condensed phase reactions. A priory estimation of the burning rate of pyrotechnics with multiple components may not be possible using the established methods. The study provides a simplified approach based on integral analysis of a proposed combustion wave structure for estimating the burning rate when the pyrotechnic composition, pure component thermo-physical properties, and thermo-kinetics parameter are known. The proposed combustion wave assumes a staged combustion process where the oxidizer undergoes decomposition in a broad reactive zone while fuel combustion occurs in a thin surface region. This approach takes account of the effective thermal conductivity as well as porosity of the pyrotechnic matrix. The pyrotechnic compositions studied here are expected to burn conductively at atmospheric pressure with little or no overpressure. The phenomenology of the combustion process of energetic materials is elucidated, and the reduced order model is validated through a case study.

Development of a two-part n-heptane oxidation mechanism for two stage combustion process in internal combustion engines

This paper presents an attempt to build a very reduced kinetics mechanism of n-heptane to simulate the two stage ignition process in terms of ignition delay time and in-cylinder pressure profiles over the whole range of engine operations. Starting from the previous 26 reactions and 25 species mechanism, two reduced schemes have been developed, one with 18 reactions and 19 species and the other with 13 reactions and 14 species. The reduction step shows that when the reactions describing the first stage are reduced as in the 18-step model, the accuracy is poor. The second 13-step model, where the reaction path describing the low temperature period has been kept, is more reliable when the window of engine operations is restricted. From this reduction step, a two-part reaction mechanism linked with a temperature criterion has been developed, while maintaining a wide range of engine operating conditions. This mechanism includes a low temperature reaction group and a high temperature reaction group, linked with a transition temperature correlation. Ignition delay times calculated with the two-part model are compared to those from the detailed mechanism. In addition, the comparison of the Indicated Mean Effective Pressure (IMEP) with the results of the previous 26-step mechanism has been done. The results obtained with the present model are in good agreement with the 26-step one. Moreover, this model has a very short computational time and thus could be used in CFD simulations as well as single zone or multi-zone engine models, and also model-based design.

Multiple optical diagnostics on effect of fuel stratification degree on reactivity controlled compression ignition

Reactivity controlled compression ignition (RCCI) was investigated on a light-duty optical engine under different fuel stratification degrees, using multiple laser diagnostic techniques. The engine was run at a speed of 1200rpm and under a load of about 7bar gross IMEP. To form different fuel stratification degrees, the direct-injection timings of n-heptane were changed, while the port-injection timings of iso-octane was kept constant. The fuel/air equivalence ratio and primary reference fuel (PRF) number were quantified by the fuel-tracer planar laser-induced fluorescence (PLIF) under non-combusting condition. The results indicated that with retarding n-heptane injection timing from −90° CA ATDC (RCCI-90 case) to −10°CA ATDC (RCCI-10 case), regions of higher fuel concentration and reactivity moved downstream to the edge of combustion chamber before high-temperature heat release (HTHR) phase. Time-resolved natural combustion luminosity imaging and single-shot OH PLIF imaging indicated that RCCI-10 case presented a staged combustion process that an auto-ignition first happened in the region of high reactivity around the combustion chamber and then another auto-ignition process took place in the region of low reactivity in the central part of the combustion chamber. The staged combustion feature involved in RCCI combustion could result in lower combustion pressure-rise rate. PLIF images of formaldehyde showed that formaldehyde first formed during low-temperature heat release (LTHR) phase in the regions where n-heptane resided. With retarding n-heptane injection timings, both formaldehyde and OH PLIF images presented more stratified distribution, and the consumption of formaldehyde and formation of OH processes got slower. OH PLIF images indicated that HTHR phase of RCCI could extend to the central part of combustion chamber. In the low-load LTC conceptual model proposed by Musculus et al. (2013), no HTHR happened and UHC formed in the central part of combustion chamber. This meant that RCCI could have less UHC emission than LTC in theory.

Flameless “Cool” Combustion in Multi-phase Configuration

Cool flames are commonly associated with engine-knock phenomenon in spark ignition engines and autoignition in diesel engines, and results from low temperature partial oxidation of the fuel air mixture that eventually leads to hot flame ignition. However, the possibility of a cool flame supporting quasi-steady combustion of a fuel droplet has never been speculated, let alone been experimentally observed. Recent microgravity droplet combustion onboard the International Space Station (ISS) under the Flame Extinguishment (FLEX) Experiment Program showed anomalous combustion of n-heptane droplets; high temperature combustion followed by radiative visible extinction and a transition to a second stage burn characterized by loss of visible flame emission. In the second stage the droplet regression continues eventually resulting in extinction diameters characteristic of diffusive extinction. Experimental examples of the two stage burning and extinction characteristics of isolated n-heptane droplets under microgravity conditions are presented and analyzed numerically. Predictions show that the second stage combustion occurs as a result of chemical kinetics associated with classical premixed “Cool Flame” phenomena. In contrast to the kinetic interactions responsible for premixed cool flame properties, those important to cool flame droplet burning are characteristically associated with the temperature range between the turnover temperature and the hot ignition. Initiation of and continuing second stage combustion involves a dynamic balance of heat generation from diffusively controlled chemical reaction and heat loss from radiation and diffusion. Within the noted temperature range, increasing reaction temperature leads to decreased chemical reaction rate and vice versa. As a result, changes of heat loss rate are dynamically balanced by heat release from chemical reaction rate as the droplet continue to burn and regress in size. Factors leading to initiation of the second stage burning phenomena are also investigated. The chemical kinetics dictating the second stage combustion and extinction process is also discussed.

Isolated n-heptane droplet combustion in microgravity: “Cool Flames” – Two-stage combustion

Recent experimentally observed two stage combustion of n-heptane droplets in microgravity is numerically studied. The simulations are conducted with detailed chemistry and transport in order to obtain insight into the features controlling the low temperature second stage burn. Predictions show that the second stage combustion occurs as a result of chemical kinetics associated with classical premixed “Cool Flame” phenomena. In contrast to the kinetic interactions responsible for premixed cool flame properties, those important to cool flame droplet burning are characteristically associated with the temperature range between the turnover temperature and the hot ignition. Initiation of and continuing second stage combustion involves a dynamic balance of heat generation from diffusively controlled chemical reaction and heat loss from radiation and diffusion. Within the noted temperature range, increasing reaction temperature leads to decreased chemical reaction rate and vice versa. As a result, changes of heat loss rate are dynamically balanced by heat release from chemical reaction rate as the droplet continues to burn and regress in size. At reaction temperatures below the turnover, heat loss over takes the heat release rate and extinction occurs. Should heat release exceed heat loss as the temperature increases to that for hot ignition, initiation of a high temperature burning phase may be possible. Parametric study on factors leading to initiation of the second stage burning phenomena are studied. Results show that both carbon dioxide and helium diluents can promote initiation of low temperature burning at smaller initial drop diameters than found with nitrogen as diluent. Small amounts of carbon dioxide and helium in the ambient is sufficient to activate the phenomena. The chemical kinetics dictating the second stage combustion and extinction process is also discussed.

CFD investigation into diesel fuel injection schemes with aid of Homogeneity Factor

A modified parameter named “Homogeneity Factor (HF) of in-cylinder charge” has been introduced as a new measure for supporting the understanding of the air–fuel mixing and combustion process in diesel engines. The effects of this parameter on engine performance and pollutant emissions have been studied using various EGR and split injection schemes on a DI diesel engine. In this study, twelve different injection strategies for which two injection pulses with different fuel amount for each pulse (up to 30% for the second pulse) and different separation between two pulses (up to 30° CA) were evaluated using CFD modeling. Results show the higher Homogeneity Factor will result in higher rate of air–fuel mixing and more complete combustion process. It was found when the dwell delay between injection pulses becomes longer, as it leaves more time for the air–fuel mixing during the late compression stroke, the maximum amount of Homogeneity Factor will be obtained at a later stage of combustion process and it can caused a reduction of NOx formation. This suggests, by optimizing split injection configurations accompanied with appropriate EGR rate, pollutant emissions can be decreased and controlled more accurately.

A weighted average global process model based on two−stage kinetic scheme for biomass combustion

This research study combustion kinetics of four biomass samples in China, red pine (Pinus tabulaeformis), corn straw (hybrid corn Zheng Dan-958), Bermuda grass and bamboo (Phyllostachys heterocycla var), using thermogravimetric analysis (TGA). Three stages of combustion process are identified as water evaporation, removal and combustion of volatile matters and combustion of char. Thermal kinetic parameters of each sample are calculated by using 1st order Coats–Redfern method based on the TGA data. It is found that the activation energy of the global process is in the range of 53.6–65.2 kJ mol−1 with a poor linear correlation. The experimental data are then used to develop a two−stage reaction kinetic scheme with low temperature region (2nd stage) and high temperature region (3rd stage). The activation energy of the second stage is in the range of 123.5–140.5 kJ mol−1, and that of the third stage was in the range of 59.4–93.4 kJ mol−1, both of which were based on the 1st order Coats–Redfern method. Because the global process of actual combustion is different from the TGA, a modified weighted average model is proposed based on the two−stage reaction kinetic scheme. According to the modified model, the kinetic parameters of the global process for actual combustion are calculated and are all found a little smaller than that of the 2nd stages. That will benefit for the combustion simulation and the design of facility of biomass fuel.

Measurement of nitrogen evolution in a staged oxy-combustion coal flame

Oxy-combustion is a technology attracting a significant amount of attention because it enables CO2 capture from the products of coal combustion. Understanding the potential for NOX reduction is a key factor in the economic analysis and design of oxy-combustion systems. In order to better understand NOX formation and destruction in oxy-combustion, measurements of major gas species and nitrogen containing species, NH3 and HCN, were obtained in a down-fired, premixed, staged, laboratory reactor. The near-burner region was fuel-rich and reducing followed by the addition of burnout oxidizer in order to simulate the staged combustion process of a full-scale boiler. In the vicinity of the burner, gas species measurements were obtained at a resolution of 25mm. The data show that nitrogen from the coal is rapidly converted to NOX in the first 50–100mm of the reactor followed by a reduction of NOX in the fuel-rich, near-burner region. NOX increases at the point where burnout oxidizer is added and then remains constant. A slight increase in NOX which is potentially the result of the thermal mechanism in air combustion does not occur in oxy-combustion. In the fuel-rich, near-burner region, stoichiometric ratio (S.R.)=0.75, oxy-fuel combustion produces more hydrocarbons, NH3, and HCN suggesting reburning reactions are more prevalent. Air combustion can be made to produce a similar NOX profile to oxy-combustion by lowering the S.R. (deeper staging) of the fuel-rich region. The minimum NOX achievable is approximately the same for air and oxy-combustion, but the oxy-combustion minimum NOX occurs at a higher S.R. in the fuel-rich zone which allows oxy-combustion to achieve higher carbon conversion for the same NOX reduction. The higher concentrations of NH3 and HCN seen at a given fuel-rich zone equivalence ratio also suggest recycled NOX may be reduced more efficiently in oxy-combustion than would be expected for air combustion.

Numerical Modeling of an Industrial Aluminum Melting Furnace

For the present work, a numerical simulation of the 100% oxy-firing combustion process inside an industrial aluminum remelting reverb furnace is presented. Three different configurations were analyzed: (i) a staged combustion process with parallel injection jets for oxygen and natural gas, (ii) a staged combustion process with a divergent jet for the oxygen, and (iii) a non-staged combustion process, with parallel jets. In all the cases, the injections were directed towards the aluminum bath, which was maintained at constant temperature. The numerical procedure was based on the finite volume formulation. The κ-ε model of turbulence was selected for simulating the turbulent flow field. The combustion process was calculated based on the finite rate models of Arrhenius and Magnussen, and the Discrete Transfer Radiation model was employed for predicting the radiation heat transfer. The numerical predictions allowed the determination of the flame patterns, species concentration distribution, temperature and velocity fields. This kind of analysis can be a powerful tool for evaluating design options such as the type, number and positioning of the burners. The present work illustrates a preliminary comparison of three types of burners. From the results obtained, the staged combustion process with a divergent jet presented the best configuration, since the flame length was not too long as to damage the refractory wall. Further it presented the largest region with low water vapor concentration close to the aluminum surface.

Ammonia chemistry below 1400 K under fuel-rich conditions in a flow reactor

The oxidation of NH 3 under fuel-rich conditions and moderate temperatures has been studied in terms of a chemical kinetic model over a wide range of conditions, based on the measurements of Hasegawa and Sato. Their experiments covered the fuels hydrogen (0 to 80 vol%), carbon monoxide (0 to 95 vol%), and methane (0 to 1.5 vol%), stoichiometries ranging from slightly lean to very fuel rich, temperatures from 300 to 1330 K, and NO levels from 0 to 2500 ppm. A detailed reaction mechanism has been established, based on earlier work on ammonia oxidation in flames and on selective noncatalytic reduction of NO by NH 3. The kinetic model reproduces the experimental trends qualitatively over the full range of conditions covered, and often the predictions are in quantitative agreement with the observations. Using reaction path analysis and sensitivity studies, the major reaction paths have been identified. The comparatively low temperatures in the present study, as well as the presence of NO, promote the reaction path NH 3→NH 2→N 2 (directly or via NNH), rather than the sequence NH 3→NH 2→NH→N important in flames. The major conversion of fuel-N species to N 2 occurs by reaction of amine radicals with NO, in particular NH 2+NO. In the presence of CH 4, NO is partly converted to cyanides by reaction with CH 3. The mechanism is recommended for modeling the reduction of NO by primary measures in the combustion of biomass, since it has been validated under conditions resembling the conversion of early nitrogenous volatile species in a staged combustion process. It is also appropriate for studies of NO formation in the combustion of gas from gasifying coal.

Modeling on Combustion Characteristics of Biocoalbriquettes

A biocoalbriquette, a new artificial solid fuel, is manufactured by a mixture of coal and biomass under a high-compression pressure. The combustion characteristics of biocoalbriquettes were investigated in this study experimentally and numerically. The combustion process of biocoalbriquettes appears in two stages: the volatile combustion stage followed by the char combustion stage. It was found that the volatile combustion happens over the whole pellet of biocoalbriquette, whereas the char combustion proceeds in a shrinking-core mode. A volume model and a shrinking-core reaction model were introduced and modified here to simulate the two stages of combustion process. The simulation results are found to be consistent with the experimental results.

Enhanced SO 3 emissions from staged combustion

Staged combustion is a recognized, effective means for lowering NO x emission. Examination of the staged combustion process suggests however that the high CO levels produced in the first stage may pump a sufficient level of oxygen atoms into the second stage to result in an increased (enhanced) SO 3 formation. Experiments were carried out in a small two-stage combustor which allowed for an examination of SO 3 formation under similar single- and two-stage conditions. The experiments show that staging does cause enhanced SO 3 formation, the enhancement is of short duration, and is dependent on the air/fuel ratio of each stage and the delay interval between the first and second stages. Kinetic analysis yields a value of k 1 =7.4×10 14 cm 6 mole −2 sec −1 for the reaction SO 2 +O+M=SO 3 +M and k 2 =1.5×10 11 cm 3 mole −1 sec −1 for the reaction SO 3 +O=SO 2 +O 2 (T=1685 K). The kinetic analysis also shows that enhancement of SO 3 formation in staged combustion can occur. However, the experimental results suggest that the enhancement may only be a transient phenomenon dependent on several combustion variables.