Flue Gas Recirculation Research Articles

Numerical simulation of CO emission in a sintering pot under flue gas recirculation

A transient two-dimensional sintering model coupling porous medium flow, interphase heat mass transfer and reactions was established through computational fluid dynamics (CFD) method by Comsol Multiphysics 6.0 for a sintering pot under sintering flue gas recirculation (SFGR). The ignition was considered and CO was the carbon combustion intermediate and experienced catalytic oxidation in the ferric oxide bed. The model was verified by sintering pot experimental data of flue gas temperature and components fractions with maximum deviation less than 10 %. The bed temperature, CO emission and solid fuel consumption were focused under different SFGR parameters by the verified model. Ignition was important for the beginning flue gas component fractions, especially when the bed height was less than 700 mm. The effect of SFGR parameters on the maximum bed temperature (MBT) was sequenced: inlet gas velocity > inlet CO fraction > inlet O2 fraction > inlet gas temperature. MBT grew up when each of the four parameters increased. The impact on the CO emission was different: inlet CO fraction > inlet O2 fraction > inlet gas velocity > inlet gas temperature. CO emission declined when each of the parameters increased. The total reaction rate of CO was studied for explanation. The reduction of solid fuel consumption was studied by energy conservation to evaluate the influence of inlet CO, inlet gas temperature and flue gas recirculation fraction. The three factors had positive linear correlation with solid fuel consumption reduction. The reduction maximally reached 16.6 % at flue gas recirculation fraction of 50 %, inlet 2 % CO and 473.15 K temperature, providing theoretical support for SFGR application.

Bluff-body MILD combustion regime for semicoke and bituminous coal mixtures with effect of oxidizer O2 concentration in O2/N2 and oxy-fuel (O2/CO2) atmospheres

The present study numerically investigates the effects of the O2 content (XO2) in the oxidizer on the MILD combustion behaviours of large-proportion semicoke mixtures, such as those involving temperature, MILD regime recognition, char surface consumption, homo/heterogeneous Damkhöler number, and NOx emission, under O2/N2 and O2/CO2 atmospheres. The results indicate that by increasing XO2 from 15 % to 30 %, the peak temperature increases significantly due to the increased char consumption rate, the high temperature region shrinks, and the axial length of the MILD regime region slightly increases from ∼1 to ∼1.4 m because of enhanced flue gas recirculation under O2/N2 or O2/CO2 atmosphere. The reduction effects of the char-O2 reaction proportions and the promoted effects of the char-CO2 reaction proportions due to the increased XO2 are both obviously weakened when N2 is substituted for CO2. A high O2 content and an O2/N2 atmosphere reduce the char burnout time. With increasing XO2, the maximum value of the turbulent Damkhöler number Dat increases from 6.79 to 10.16 under the O2/N2 atmosphere and from 3.99 to 10.6 under the O2/CO2 atmosphere, implying that a low O2 dilution and a CO2 environment promote the establishment of the homo-MILD regime. The maximum values of the surface Damkhöler number (Das-CO2 and Das-H2O) of the two gasification reactions (Char-CO2 and Char-H2O) have the same order of magnitude and are much lower than those of the surface Damkhöler number (Das-O2) of the Char-O2 oxidation reaction, i.e., Das-H2O < Das-CO2 ≪ Das-O2 < 1. The maximum values of the three surface Damkhöler numbers all increase with increasing XO2, while they decrease with the replacement of N2 by CO2, indicating that a high O2 content and an O2/N2 atmosphere could facilitate the establishment of an ideal hetero-MILD combustion regime to some extent. Low NOx emissions can be realized under moderate O2 and CO2 dilution conditions by regulating the oxy-MILD combustion regime of semicoke blends.

Enhanced NOx reduction performance in the I-type radiant tube by combining a novel triple air-staging with flue gas recirculation

In this study, a novel triple air-staged combustion (ASC) with internal flue gas recirculation (FGR) was proposed to further achieve low NOx emissions and high combustion efficiency in the I-type radiant tube (RT). The effects of air vent structural parameters, including secondary air vent gap d (0, 1, 2, 3 mm) and diameter D (2, 4, 6, 8 mm) of tertiary air vents, on NOx emission and thermal efficiency were analyzed by numerical simulation. The results showed that NOx emissions increased from 33.7 to 66.0 mg/m3 as the secondary air vent gap changed from 0 to 3 mm. The thermal efficiency was higher than the original structure only at d=3 mm. When the tertiary air vent diameter increased from 2 to 8 mm, NOx increased from 49.3 to 180.4 mg/m3 and the thermal efficiency changed with a slight variation. On the premise of higher thermal efficiency than the original structure, the NOx at D=2 mm reached the minimum value of 49.3 mg/m3 under all optimized schemes, with a 64.0% reduction.

The roadmap towards the efficiency limit for supercritical carbon dioxide coal fired power plant

The Chinese Government has issued a series of documents to explain China’s control over the carbon dioxide (CO2) emission. One of the major tasks is to develop higher efficiency coal fired power plant, compared with current running power plant. In this paper, we aim to explore the roadmap to reach the efficiency limit for coal fired power plant using supercritical carbon dioxide cycle (sCO2 cycle). Referenced to Carnot cycle, the proposed roadmap is to increase the cycle efficiency by elevating average heat absorption temperature (Tave,h) and lowering average heat release temperature (Tave,l). In contrast to recompression cycle (RC), tri-compression cycle (TC) is introduced. Due to the increased Tave,h, TC achieves the second largest contribution for efficiency increment, followed by the reheating technique. Then, TC, double reheating (DRH) and intercooling (IC) are integrated as TC + DRH + IC in the power plant. To completely absorb flue gas energy over entire temperature range of (1500 ∼ 120) °C. A top cycle and a bottom cycle are connected for cascade utilization of flue gas energy. Overlap energy utilization is further utilized to fill the efficiency gap between top and bottom cycles. The proposed cycle also integrates the module boiler design to suppress the pressure drop penalty, and the flue gas recirculation to keep the heater surface temperature in an accepted level. A numerical model is developed for the comprehensive sCO2 cycle. At the main vapor parameters of 35 MPa/630 °C, the sCO2 coal fired power plant reaches the net power generation efficiency of 51.03%, which is higher than 48.12% for a supercritical water-steam power plant at the same capacity. Such efficiency improvement saves 175.2 kilotons of coal and reduces 396.4 kilotons of CO2 emission for 1000 MW capacity in a fascial year. Our work provides the guideline for the design and operation of large scale sCO2 coal fired power plant.

Calcination and desulfurization characteristics of calcium carbonate in pressurized oxy-combustion

Pressurized oxy-combustion is one of the most promising and efficient CO2 capture technologies for coal-fired power plants, which could reduce energy consumption for carbon capture. However, due to the effects of pressurization and flue gas recirculation, the partial pressure of sulfur oxides (SOx) in pressurized oxy-combustion is significantly higher than that of air-combustion, which leads to the corrosion risks and environment hazards. It is essential to investigate the SOx control in pressurized oxy-combustion. In this paper, the in-furnace calcination and desulfurization characteristics of calcium carbonate in pressurized oxy-combustion were carried out in a pressurized drop tube furnace. The findings show that the sulfation reaction in pressurized oxy-combustion is direct sulfation and the sulfation rate increases with the increase of temperature and pressure. The desulfurization efficiency in pressurized oxy-combustion is lower than that in atmospheric air atmosphere below 950 °C, but it reverses above 950 °C, indicating that the minimum desulfurization temperature in the furnace of the pressurized oxy-combustion is higher than that of atmospheric air atmosphere. Therefore, limestone can still be used as a sorbent in the pressurized oxy-combustion furnace. This study could contribute to the design of dry desulfurization systems in pressurized oxy-combustion combustors that use fossil fuel energies with high efficiency and low SOx emissions.

Numerical study of internal flue gas recirculation system applied to methane-hydrogen powered gas microturbine combustor

Sources of renewable energy have been increasingly used all over the world. This kind of energy is highly desirable because of its unlimited availability. Unfortunately, renewable energy production very much depends on weather conditions. Consequently, it is necessary to store the produced excess energy in order to use it when needed. There is a technology able to produce a hydrogen/methane fuel from excess renewable energy, which may be stored. This technology is called the Power-to-Gas technology (P2G). Since the efficiency of this technological process depends on the hydrogen fraction in the renewable energy fuel, there is a need to increase this fraction. Concurrently, the gas microturbine technology is increasingly widely used in various industries (aviation, energy, automotive, military, etc). The P2G technology and the gas microturbine technology are likely to be integrated in the near future and, as mentioned above, the hydrogen fraction in the methane-hydrogen fuel will tend to increase. In order to power a gas microturbine with the methane-hydrogen fuel, it will be necessary to modify the combustor to avoid an excessive temperature increase and flashbacks. In this paper it is proposed to apply an autonomous internal exhaust gas recirculation system to resolve the hydrogen combustion problems indicated above. The operating principle and the proposed design of the recirculation system and the latter’s impact on the combustor’s operating parameters and emissivity (NOx and CO) are presented.

Investigation of a novel combustion stabilization mechanism and combustion characteristics of a multi-nozzle array model combustor

Short and uniformly dispersed flames in multi-nozzle array combustors allow for shorter flue gas residence time, resulting in less NOx emissions at high combustor outlet temperature. A new combustion stabilization mechanism based on high-speed jet induced flue gas recirculation has been utilized in an array model combustor. The stabilization mechanism allows for stable combustion at higher nozzle outlet velocity. The flame stabilization mechanism and combustion characteristics for the model combustor fueled methane have been investigated by using the combination of experiments and numerical simulations. The parameters studied include nozzle outlet velocity, inlet air temperature, and heat loss ratio. The combustion characteristics include NOx/CO emissions and reaction zone distribution. The results show that the flame stabilization mechanism is a result of the combined effects of the inner and outer recirculation zone. Under atmospheric conditions within the adiabatic temperature range from 1500 K to 2100 K, the NOx emissions are less than 20 ppm@15%O2 and the CO emissions are less than 30 ppm@15%O2. As the heat loss ratio decreases, the NOx emissions for the model combustor increase. When the nozzle outlet velocity increases, the lean blowout (LBO) for the model combustor increases. With increasing inlet air temperature, the equivalent ratio range of combustion stability expands. The downstream of the reaction zone is mostly affected by the nozzle outlet velocity and inlet air temperature, while the root of the reaction zone is essentially unaffected due to the role of inner recirculation zone. The unique stabilization mechanism of the multi-nozzle array combustion is beneficial for fuel flexibility.

Investigation and Evaluation of Flue Gas Pollutants Emission in Waste-to-Energy Plant with Flue Gas Recirculation

The emissions of pollutants by waste-to-energy power plants, which contain more toxic substances owing to the complicated composition of municipal solid waste (MSW), such as NOx, SO2, HCl, HF, particulate matter, and heavy metals, has attracted increasing attention worldwide. To effectively control the pollutants, a flue gas cleaning system is indispensable in the operation of MSW incineration power plants. In this study, the flue gas cleaning system in a waste-to-energy power plant with flue gas recirculation (FGR) was evaluated. The concentrations of various pollutants were measured and compared with the standards at home and abroad. The results indicated that NOx emission can be effectively reduced by FGR, and that the emission concentration of NOx may meet the national emission standards only by adopting FGR. However, the emission levels of HCl and PM exceeded the limits in legislative standards; therefore, operation optimization or retrofit of a deacidification system and bag filter were proposed to comply with the international standards and near-zero-emissions goal.

Numerical Research for MILD Burners with Different configurations

ABSTRACT In the experiment or simulation study of Moderate or Intense Low-Oxygen Dilution (MILD) combustion, scholars used different burner structures. In order to evaluate the effects of MILD burner configurations on combustion, five different burners are numerically studied. In addition, the effects of furnace outlet position are also investigated. The results show that the structure of the burner has a great influence on MILD combustion. The burners with high momentum nozzles deviating from the outlet has a more uniform temperature distribution, a lower Damköhler number (Da) and a higher flue gas recirculation rate , so it has a higher MILD combustion performance. When the oxidant inlet is at the center, more fuel inlets disperse the initial momentum of the fuel, which is not conducive to MILD combustion. When the fuel inlet is at the center, the oxidant inlet should be reasonably arranged to avoid hindering the dilution performance of the circulating flue gas. In addition, the outlet position also has a great influence on the flow in the furnace, and a reasonable outlet also has the effect of optimizing the MILD combustion.

Enhanced removal of nitric oxide and carbon dioxide from flue gas in a biofilter: Effect of flue gas recirculation and Fe(II)EDTA addition

Flue gas emission from combustion of fossil fuel is a growing environmental concern. In this study the effect of flue gas recirculation and Fe(II)EDTA addition on CO2 and NO removal from flue gas was explored in a biofilter. The effect of SO2 on CO2 and NOx removal efficiencies were also studied herein. The optimal removal efficiencies of CO2 (88.1%) and NO (85.8%) were achieved with the flue gas recirculation of 2:1. The presence of SO2 showed little impact on CO2 or NO removal while induced the decline of NO removal. The addition of Fe(II)EDTA clearly enhanced NO removal (~85%). The Illumina sequencing results showed that abundant sulfur-associated bacteria (Desulfobulbus, Desulfococcus, Sulfurovum, Chlorobium), nitrogen-associated bacteria (Thauera), fermentative bacteria (Longilinea and Anaerovorax), and various iron-reducing bacteria (Geobacter) were detected in the biofilter with Fe(II)EDTA addition. The synergistic effect of denitrifying/iron-reducing bacteria dedicated to the sustainable NO removal.

Mathematical investigation of syngas fired burner with various configurations in the BERL combustor

This paper discusses the effect of burning syngas in a configuration similar to that of the BERL (Burner Engineering Research Laboratory) 300 KW swirl-stabilized combustor which has previously been used with methane as a fuel. Due to the axisymmetric configuration, only 15° from the burner angle is studied with periodic boundary. The model adopts RANS (Reynolds Averaged Navier–Stokes) technique including a realizable k-ε turbulent scheme. The non-premixed combustion model used is based on applying flamelet concept. The study shows that the structure of the flame is affected with changing the burner quarl angle and the inlet air swirl number. The detailed parametric study of this synthetic fuel in the BERL paradigm relates easily to the accumulated experimental and numerical studies available in this configuration and then acquiring add noval value to the present work. The study also shows the presence of different recirculation zones, one of which is central recirculation zone and an external recirculation zone. When increasing quarl angle, the central recirculation zone is shifted outward and the turbulent interaction between the central fuel jet and the recirculation zone induces small vortices between these two flow patterns. On the other hand, the effect of flue gas recirculation and air staging into the burner on the reduction of NOx formation is discussed. The flue gas data show that FGR reduces NOx emissions significantly while having no influence on flame stability, total combustion efficiency, or CO emissions. Changes in fuel composition are also taken into account. The results reveal that when the hydrogen blending in the fuel increases, the temperature rises.

The comprehensive solution to decrease cooling wall temperatures of sCO2 boiler for coal fired power plant

Compared with water-steam cycle, supercritical carbon dioxide (sCO2) cycle has higher efficiency when applied in coal fired power plant. However, it also introduces challenges in boiler: because of the higher working fluid temperature and the lower convective heat transfer coefficient in boiler tubes, the cooling wall is more prone to overheating and bursting due to higher wall temperatures. Here, based on fundamental consideration of the thermal coupling between 3D radiation heat flux in furnace side and CO2 fluid in cooling wall tubes, we propose a comprehensive solution to decrease cooling wall temperatures. The solution includes four consecutive techniques: improved coupling in furnace width direction (CWD), flue gas recirculation for heat flux reduction (FGR), improved coupling in furnace height direction (CHD), and enhanced heat transfer in cooling wall tubes (EHT). A comprehensive thermal-hydraulic model is developed for a 1000 MWe power plant. It is found that the new solution can reduce the cooling wall temperatures from 670.5 °C to 635.0 °C, among which CWD, FGR, CHD and EHT contribute to the decrement of cooling wall temperatures by 13.3 °C, 4.4 °C, 6.8 °C and 11.0 °C, respectively, concluding that CWD and EHT are more effective than other techniques.

On optimization of the coke oven twin-heating flue design providing a substantial reduction of NOx emissions Part I: General description, validation of the models and interpretation of the results

The paper presents an altered design of the coke oven twin-heating flues which is optimized in terms of NOx emission. The heating flue modifications relate to the position and size of the air inlets as well as an improved internal exhaust gas re-circulation. The optimization process was carried out using CFD-based numerical modeling. To ensure better accuracy, the formation of NOx within the heating flues was predicted using full GRI-Mech 3.0 mechanism.Improvement of the flues construction significantly reduced nitrogen oxides emission. When burning coke oven gas (COG) the NOx concentration in the flue gas drops from 182 ppm (7% O2) to the value below 100 ppm (7% O2). In the case of mixed gas (MG), the content of nitrogen oxides (in exhaust gases) decreases from about 56 ppm (7% O2) to 42 ppm (7% O2). The predicted results indicate a satisfactory agreement with the measured values. The correlation coefficients for measured and predicted NOx concentrations are equal to 0.99 (COG) and 0.81 (MG). The standard deviations are at the level of ±23.5 ppm (COG) and ±17.1 ppm (MG).This paper is Part I and contains: selected 1D and 3D simulations results, predictions interpretation, investigation of the internal flue gas re-circulation as well as validation of the model developed for the optimization procedure. In the associated publication (Part II) the optimized designs of the unit under consideration (for both MG and COG) have been described. Moreover, for the baseline and optimized cases, a number of the operating characteristics have been presented.

Nonlinear modeling of industrial boiler NOx emissions

ABSTRACT Pollutant emissions into the atmosphere are recognized as a significant problem in fossil fuel combustion. The pollution emission measurement in industrial boilers is difficult and expensive but fundamental for monitoring and controlling. Frequently continuous emissions monitoring (CEM) system is out of service or useless due to obsolescence, high maintenance cost, and so on or simply is not installed. When a system for measuring pollutant emissions is not available, an alternative method must be employed to get the pollutant emission value. According to the black-box model approach, this article describes the nonlinear modeling of NOx emissions from a utility boiler. Bayesian-Gaussian (BG), multilayer perceptron (MLP), and Volterra polynomial basis functions (VPBF) neural networks are developed for model benchmarking. Experimental data from a utility boiler was acquired in order to model definition and evaluation. The models process three boiler variables oxygen excess, fuel mass flow and flue gas recirculation gates for NOx emission estimation. Models with BG show better performance than models with MLP and VPBF for NOx prediction. Implications: The technology to control NOx emissions generated by combustion operates under strict regulations. In order to reduce NOx emissions, theoretical models of NOx generation have been studied extensively, including nitrogen chemistry and the dynamic flow of gas particles which is very complex. The new technology trends would require the continuous measurement of high precision NOx emissions to achieve further reductions in NOx emissions. Currently, NOx emissions are measured by a Continuous Emission Monitoring system, which turns out to be extremely expensive and difficult to maintain, so alternative low-cost solutions are desirable. Our contribution shows how algorithms based on different artificial intelligence techniques are viable and quality alternatives for the measurement of continuous NOx emissions. The NOx emissions models based on IA algorithms are viable alternatives that have versatility and self-tuning capacity due to the fact that they are based on boiler operation parameters which have valuable information few explored nowadays.

On optimization of the coke oven twin-heating flue design providing a substantial reduction of NOx emissions. Part II: Optimized designs for COG- and MG- fired units as well as operating characteristics of the new heating flues

This article relates to the optimization of the twin heating flue construction in terms of NOx emission (see also Part I). The novel designs for both mixed-gas (MG) and coke oven gas (COG) have been developed using CFD-based numerical modelling and GRI-Mech 3.0 detailed mechanism (for NOx predictions). By analyzing the results of investigations presented in Part I, the construction modifications of the twin-heating flues have been found, to reduce NOx emission from about 180 ppm (7% O2) and 56 ppm (7% O2) to the values below 100 ppm and 42 ppm for both COG and MG, respectively. These improvements have been achieved by modifying the location\\size of the air and fuel inlets as well as by improving the internal flue gas re-circulation.To test the operation of the optimized heating flues, the operating characteristics for the heating flue temperature range from 1220 °C to 1360 °C have been prepared. The unit operation curves, predicted for the baseline case and modified cases, show that the proposed design modifications decrease NOx emission on average by 25% (MG) and 60% (COG). The flue gas re-circulation for COG has been increased from about 25% to over 70%. For MG, the exhaust gas re-circulation has been improved from about 30% to over 80%. Modifications aimed at intensifying the flue gas re-circulation increase pressure losses inside the heating flues from 10 Pa to 40 Pa for MG and from 5 Pa to 20 Pa for COG.

Economic, environmental and exergy analysis of the decarbonisation of cement production cycle

The aim of this study is to analyze the exergetic, environmental, and economic impact of the use of alternative fuels (Plasmix), instead of coal on the clinker production to valorise a non-recyclable waste and to limit global CO2 emissions. The cement production process was implemented in AspenPlus® environment, simulating different scenarios with the possibility to conduct the non recyclable plastic waste incineration in oxy-combustion conditions. Three scenarios were considered: without and with flue gas recirculation from the pre-heating tower and the oxy-combustion layout. Sensitive analyses were carried out to optimize cyclonic pre-heater tower working. Exergetic and economic analyses were performed, and the relevant impacts on economic profitability were analysed. The study reports the advantages and disadvantages of the different scenarios in terms of exergy efficiency, energy requirements, economic impact and pollutants emissions savings. Oxy-combustion layout resulted in 18.4% savings in terms of CO2 emissions but, at the same time, a lower Return of Investment (11.79%) with respect to the scenario with flue gas recirculation (11.82%). The exergy analysis of third scenario re returned the maximum obtainable value of the exergy efficiency (58.4%).

The impact of oxygen content in the primary air supply on fuel burning rate and pollutant emissions in a forced-draft biomass stove

Forced-draft biomass stoves have been proposed as a substitute for traditional solid fuel stoves in rural areas. The change in the O2 content of the supply air via flue gas recirculation (FGR) significantly affects the combustion and pollutant emission performances of this household stove type, which has not been thoroughly investigated. In this study, O2 content control system was designed to accurately adjust the O2 content of the primary air supply for a typical household forced-draft biomass stove. Then, six conditions with an O2 content of 16%, 17%, 18%, 19%, 20%, and 21% were tested. The dynamic burning rate and emission rates of carbon monoxide (CO), nitrous oxides (NOx), and particulate matter with an aerodynamic diameter ≤ 2.5 μm (PM2.5) were measured for each O2 content. The results showed that the average burning rate of the six O2 contents ranged from 9.6 to 16.9 g/min. There was a positive linear relationship between the average burning rate and the O2 content in the primary air supply. The CO and PM2.5 emission amounts exhibited the concave and convex quadratic relationships with O2 content, respectively, while trends in NOx emission amount had a negative linear relationship with the O2 content. The real-time burning rate and pollutant emission rates exhibited different trends that influenced the amount of emissions. When FGR was utilized with a constant primary airflow rate, an O2 content of 19.7% may be optimal for household biomass stoves. This study provides basic information on air supply design of household biomass stoves, which is essential for future clean stove design and improvement.

Effects of flue gas recirculation on combustion and heat flux distribution in 660 MW double-reheat tower-type boiler

The ultra-supercritical double-reheat boiler has attracted more attention because of high parameters (steam with high temperature and pressure), low pollution and large capacity. However, there are some difficulties in the development of this technology, such as adjusting the reheat steam temperature. In this study, computational fluid dynamics simulation is used to analyze a 660 MW double-reheat tower-type pulverized coal-fired boiler. The influence of flue gas recirculation (FGR) on heat transfer characteristics and combustion process in the furnace (including heating surfaces in the upper parts of the furnace) are evaluated. The user-defined function approach predicates the CO reduction effect on NOx. The results show that the flow at the horizontal section through the centerline of recirculating flus gas nozzles is rotating. The velocity distribution changes into an elliptical rotating flow when FGR ratio is 20%. At higher FGR ratios, the high-temperature area (1565–1700 K) shrinks and both the NOx concentration at the low-temperature superheater outlet and the O2 concentration in the burner zone descend. The O2 concentration at the low-temperature superheater outlet first increases and then decreases. In the main combustion zone, the heat flux peak of water-cooled wall is about 330 kW m−2. As FGR ratio increases from 0% to 20%, the rate of heat absorption of water-cooled wall to that of total boiler decreases by 3.50%. These rates for reheater and superheater increase by 2.53% and 2.13%, respectively.

Numerical modeling of distributed combustion without air dilution in a novel ultra-low emission turbulent swirl burner

Distributed combustion, often associated with the low-oxygen condition, offers ultra-low NOx emission. However, it was recently achieved without combustion air dilution or internal flue gas recirculation, using a distinct approach called mixture temperature-controlled combustion. Here, the fuel–air stream is cooled at the inlet to delay ignition and, hence, foster homogeneous mixture formation. This numerical study aims to understand its operation better and present a robust framework for distributed combustion modeling in a parameter range where such operation was not predicted before by any existing theory. Further, liquid fuel combustion was evaluated, which brings additional complexity. Four operating conditions were presented at which distributed combustion was observed. The reacting flow was modeled by flamelet-generated manifold, based on a detailed n-dodecane mechanism. The Zimont turbulent flame speed model was used with significantly reduced coefficients to achieve distributed combustion. The droplets of airblast atomization were tracked in a Lagrangian frame. The numerical results were validated by Schlieren images and acoustic spectra. It was concluded that the reactant dilution ratio remained below 0.25 through the combustion chamber, revealing that the homogeneous fuel–air mixture is the principal reason for excellent flame stability and ultra-low NOx emission without significant internal recirculation. The potential applications of these results are boilers, furnaces, and gas turbines.

Bluff-body MILD combustion of large-proportion semicoke-blend under various secondary air velocities

Secondary air jet velocity is strongly related to flue gas entrainment in semicoke-blend MILD combustion, which exerts a critical influence on the turbulence–chemistry interaction and combustion characteristics. To extend the application of moderate or intense low oxygen dilution (MILD) combustion to tangential fired boilers, this paper numerically investigates the effect of secondary air velocity (Vsec) on the turbulent flow and chemical reaction interaction behaviors of large-proportion semicoke and bituminous coal co-firing to establish the bluff-body MILD combustion regime. The results show that increasing Vsec abates the peak temperature by enlarging radiative heat transfer and enhances flue gas recirculation. The MILD regime first occurs at Vsec = 51.8 m/s and then extends quickly from 0.4 m to 1.2 m with further increasing Vsec. Increasing Vsec decreases the maximum of the turbulent Damköhler number Dat and increases the minimum of Karlovitz number Ka, implying that the predominant role of turbulent mixing gradually is transferred into the comparable role. The local-MILD regime (1 < Dat < 10) is enhanced significantly and an ideal MILD combustion regime occurs at a velocity of 158.6 m/s, i.e., Dat≈1, and Ka≫1, denoting a slow-chemistry regime. Meanwhile, the maximum heterogeneous Damköhler number Da,C-O2 decreases from 1.09 to 0.86, while the maximum Da,C-CO2 and Da,C-H2O decrease from 0.013 to 0.007 and from 0.003 to 0.0018, respectively, hinting that the char-O2 reaction is controlled by diffusion/kinetics, char-CO2 and char-H2O reactions are determined by kinetics, and they all proceed toward the kinetics-controlled regime with increasing Vsec. Low NOx emissions can be captured and coupled with small Dat by increasing Vsec, and weakening the transitional MILD regime (10 < Dat < 30) is of great importance when establishing the MILD regime.