Intense Low-oxygen Dilution Combustion Research Articles

Numerical Study on the Autoignition of Biogas in Moderate or Intense Low Oxygen Dilution Nonpremixed Combustion Systems.

The ignition delay of biogas in mixing layers is investigated using a one-dimensional combustion model, with its application in Moderate or Intense Low oxygen Dilution (MILD) combustion being the focus. The current study reveals the key aspects of the ignition of biogas in a nonpremixed, igniting mixing layer with a hot oxidizer of low oxygen content. The observed characteristics are contrasted against the existing studies on ignition in homogeneous mixtures under similar conditions. Biogas is considered here as a mixture of CH4 with variable amounts CO2. The influence of reactive, thermal, and transport properties of CO2 on the ignition is evaluated using artificial species to mimic the respective characteristics of CO2. While the ignition delay in homogeneous mixtures shows a strong dependence on CO2 content in the fuel, the ignition delay predictions from one-dimensional mixing layers show no significant influence of CO2 levels in biogas. In addition, the influence of oxidizer composition and temperature on ignition delay is determined for CO2 levels ranging from 0% to 90%. A sensitivity analysis of chemical reactions on the ignition delay shows a negligible effect of CO2 concentration in biogas. The current study emphasizes the role of oxidizer composition and temperature on the ignition characteristics of a MILD biogas flame.

Assessment of steady and unsteady flamelet models for MILD combustion modeling

Moderate or intense low-oxygen dilution (MILD) combustion is a novel combustion technology with the comparable chemical and turbulent mixing timescales. In the most of literatures, relatively expensive volume-based models are recommended for this combustion regime while this regime is not completely idealized homogenized reactor and nor strong flamelet like. This paper is focused on the assessment of the lower cost, flamelet approach for MILD condition. In this way, simplifying JHC burner of Dally et al. is considered to model using RANS approach. The effects of inclusion of unity versus non-unity Lewis numbers, radiation heat transfer, and various scalar dissipation rates are evaluated. Results show that the flamelet model may not be totally rejected for the MILD condition because it could still capture flame characteristics relatively acceptable. Choosing an appropriate scalar dissipation rate value is discussed for single flamelet and higher values are recommended by O2 increment. Moreover, considering non-unity Lewis numbers improved species concentration at the jet centerline for low O2 levels. Furthermore, the unsteady flamelet model with radiation had a better prediction for NOx especially for higher O2 Levels.

Numerical Investigation of Moderate or Intense Low-Oxygen Dilution Combustion in a Cyclonic Burner Using a Flamelet-Generated Manifold Approach

Innovative solutions in terms of energy efficiency and pollutant emission abatement require the development of new combustion technologies. In particular, several solutions imply a process based on...

Transported PDF simulation of turbulent CH4/H2 flames under MILD conditions with particle-level sensitivity analysis

Transported probability density function (TPDF) simulation with sensitivity analysis has been conducted for turbulent non-premixed CH4/H2 flames of the jet-into-hot-coflow (JHC) burner, which is a typical model to emulate moderate or intense low oxygen dilution combustion (MILD). Specifically, two cases with different levels of oxygen in the coflow stream, namely HM1 and HM3, are simulated to reveal the differences between MILD and hot-temperature combustion. The TPDF simulation well predicts the temperature and species distributions including those of OH, CO and NO for both cases with a 25-species mechanism. The reduced reaction activity in HM1 as reflected in the peak OH concentration is well correlated to the reduced oxygen in the coflow stream. The particle-level local sensitivities with respect to mixing and chemical reaction further show dramatic differences in the flame characteristics. HM1 is less sensitive to mixing and reaction parameters than HM3 due to the suppressed combustion process. Specifically, for HM1 the sensitivities to mixing and chemical reactions have comparable magnitude, indicating that the combustion progress is controlled by both mixing and reaction in MILD combustion. For HM3, there is however a change in the combustion mode: during the flame initialization, the combustion progress is more sensitive to chemical reactions, indicating that finite-rate chemistry is the controlling process during the autoignition process for flame stabilization; at further downstream where the flame has established, the combustion progress is controlled by mixing, which is characteristic of nonpremixed flames. An examination of the particles with the largest sensitivities reveals the difference in the controlling mixtures for flame stabilization, namely, the stoichiometric mixtures are important for HM1, whereas, fuel-lean mixtures are controlling for HM3. The study demonstrates the potential of TPDF simulations with sensitivity analysis to investigate the effects of finite-rate chemistry on the flame characteristics and emissions, and reveal the controlling physio-chemical processes in MILD combustion.

Generalisation of the eddy-dissipation concept for jet flames with low turbulence and low Damköhler number

Moderate or intense low oxygen dilution (MILD) combustion has been the focus of a range of fundamental experimental and numerical studies. Reasonable agreement between experimental and numerical investigations, however, requires finite-rate chemistry models and, often, ad hoc model adjustment. To remedy this, an adaptive eddy dissipation concept (EDC) combustion model has previously been developed to target conditions encountered in MILD combustion; however, this model relies on a simplified, pre-defined assumption about the combustion chemistry. The present paper reports a generalised version of the modified EDC model without the need for an assumed, single-step chemical reaction or ad hoc coefficient tuning. The results show good agreement with experimental measurements of two CH4/H2 flames in hot coflows, showing improvements over the standard EDC model as well as the previously published modified EDC model. The updated version of the EDC model also demonstrates the capacity to reproduce the downstream transition in flame structure of a MILD jet flame seen experimentally, but which has previously proven challenging to capture computationally. Analyses of the previously identified dominant heat-release reactions provide insight into the structural differences between a conventional autoignitive flame and a flame in the MILD combustion regime, whilst highlighting the requirement for a generalised EDC combustion model.

An experimental study of the stability and performance characteristics of a Hybrid Solar Receiver Combustor operated in the MILD combustion regime

This study describes the performance and stability characteristics of a Hybrid Solar Receiver Combustor operated in the Moderate or Intense Low oxygen Dilution (MILD) combustion regime, in which the functions of a solar receiver and a combustor are integrated into a single device. The device was built and tested at a nominal capacity of 20 kWth for both the combustion-only (MILD and conventional combustion) and mixed-mode (a combination of both solar and combustion). Here, a 5 kWel xenon-arc solar simulator and natural gas were used as the energy sources, while the combustion mode was operated in the MILD combustion regime. The thermal efficiency, wall cavity temperature, heat flux distribution within the cavity and pollutant emissions are reported for the two modes of operation for a range of energy input, equivalence ratio, heat extraction, air preheat and solar-to-fuel energy input ratio. The stability limits for stable operations are also identified for each mode of operation. It was found that MILD combustion can be successfully stabilised within the HSRC in a wide range of operating conditions with and without air preheating, and in the mixed-mode of operation, providing ultra-low NOx and CO emissions. Also, the stability limits were found to increase by adding concentrated solar radiation to the combustion process. The thermal performance was found to be similar in both combustion-only (conventional combustion and MILD) and mixed-mode (up to ≈ 88% assuming reasonable heat recovery from the exhaust gas), confirming that an overall benefit can be derived from the device.

Chemical Explosive Mode Analysis for a Jet-in-Hot-Coflow burner operating in MILD combustion

Large Eddy Simulations (LES) of Moderate and Intense Low oxygen Dilution (MILD) combustion of a Jet-in-Hot-Coflow (JHC) burner were performed using detailed chemistry. On the contrary to traditional flames, where heat release is occurring in very thin fronts, MILD combustion occurs in the distributed reaction regime where the reaction zone is broad, thus, this paper applies a direct Arrhenius closure with detailed chemistry to resolve important details of the fuel oxidation reactions. Comparisons of LES results are in good agreement with experiments, demonstrating that the simulations capture the intermediate species and finite reaction rate effects. A Chemical Explosive Mode Analysis (CEMA) was used to determine the flame structure and to detect the pre- and post-ignition regions, including the contributions to the CEMs analyzing the Explosion Index (EI) and Participation Index (PI). To the best of our knowledge, a detailed study of CEMA on MILD or flameless regime has never been reported. The flame structure was clearly visualized with CEMA, as well as the lean and the rich flame fronts. Different flame zones close to the anchoring points of these turbulent lifted flames were selected and the analysis demonstrates the contributions of dominant chemical species, such as HO2 and O. The reactions related to the dominant local CEM were obtained to highlight the nature of the stabilization in these highly diluted operating conditions.

Optimization of Chemical Kinetics for Methane and Biomass Pyrolysis Products in Moderate or Intense Low-Oxygen Dilution Combustion

The performance of existing detailed chemical mechanisms with respect to moderate or intense low-oxygen dilution (MILD) combustion is not optimal. The use of optimization procedures can therefore be used to quantify and minimize the uncertainties in chemical mechanisms with respect to available experimental targets in these conditions. This work puts forth a methodology that improves the performance of chemical kinetics with respect to MILD combustion. The experimental data used in this paper are from a plug flow reactor, where the ignition delay time for methane and biomass pyrolysis products in MILD conditions was analyzed. The initial mechanism was then evaluated, and the reactions with the highest impact factors were used in the optimization process. The combination of parameters that gave the lowest error with respect to the experimental data was then determined, and the proposed mechanism performance was improved with respect to the experimental targets.

Liftoff behavior and combustion characteristic of jet flame in a coflow burner

ABSTRACTStabilization and autoignition mechanisms of lifted flames have been widely investigated to improve combustion efficiency and safety of combustion equipment. This paper focuses on liftoff behavior and combustion characteristic of methane and propane flames under various coflow conditions in a coflow burner. Unlike the case of free jet flame in ambient air, the different tendencies of liftoff height changes with jet velocity for both methane and propane flames in vitiated coflow illustrate a transition from conventional combustion to Moderate & Intense Low Oxygen Dilution (MILD) combustion. Flame temperature difference with radial position measured by primary spectrum pyrometry proves the transition regime.

Finite-rate chemistry modelling of non-conventional combustion regimes using a Partially-Stirred Reactor closure: Combustion model formulation and implementation details

The present work focuses on the numerical simulation of Moderate or Intense Low oxygen Dilution combustion condition, using the Partially-Stirred Reactor model for turbulence-chemistry interactions. The Partially-Stirred Reactor model assumes that reactions are confined in a specific region of the computational cell, whose mass fraction depends both on the mixing and the chemical time scales. Therefore, the appropriate choice of mixing and chemical time scales becomes crucial to ensure the accuracy of the numerical simulation prediction. Results show that the most appropriate choice for mixing time scale in Moderate or Intense Low oxygen Dilution combustion regime is to use a dynamic evaluation, in which the ratio between the variance of mixture fraction and its dissipation rate is adopted, rather than global estimations based on Kolmogorov or integral mixing scales. This is supported by the validation of the numerical results against experimental profiles of temperature and species mass fractions, available from measurements on the Adelaide Jet in Hot Co-flow burner. Different approaches for chemical time scale evaluation are also compared, using the species formation rates, the reaction rates and the eigenvalues of the formation rate Jacobian matrix. Different co-flow oxygen dilution levels and Reynolds numbers are considered in the validation work, to evaluate the applicability of Partially-Stirred Reactor approach over a wide range of operating conditions. Moreover, the influence of specifying uniform and non-uniform boundary conditions for the chemical scalars is assessed. The present work sheds light on the key mechanisms of turbulence-chemistry interactions in advanced combustion regimes. At the same time, it provides essential information to advance the predictive nature of computational tools used by scientists and engineers, to support the development of new technologies.

Assessment of On-the-Fly Chemistry Reduction and Tabulation Approaches for the Simulation of Moderate or Intense Low-Oxygen Dilution Combustion.

The current paper focuses on the numerical simulation of the Delft jet in hot co-flow (DJHC) burner, fed with natural gas and biogas, using the eddy dissipation concept (EDC) model with dynamic chemistry reduction and tabulation, i.e., tabulated dynamic adaptive chemistry (TDAC). The central processing unit (CPU) time saving provided by TDAC is evaluated for various EDC model constants and chemical mechanisms of increasing complexity, using a number of chemistry reduction approaches. Results show that the TDAC method provides speed-up factors of 1.4–2.0 and more than 10 when using a skeletal mechanism (DRM19) and a comprehensive kinetic mechanism (POLIMIC1C3HT), respectively. The directed relation graph with error propagation (DRGEP), dynamic adaptive chemistry (DAC), and elementary flux analysis (EFA) reduction models show superior performances when compared to other approaches, such as directed relation graph (DRG) and path flux analysis (PFA). All of the reduction models have been adapted for run-time reduction. Furthermore, the contribution of tabulation is more important with small mechanisms, while reduction plays a major role with large ones.

Experimental demonstration of the hybrid solar receiver combustor

We report the first-of-a-kind experimental demonstration of a direct hybrid between a solar cavity receiver and combustor, in which the functions of a solar receiver and a combustor are integrated into a single device. The device was built and tested at a nominal capacity of 20-kWth for all the three modes of operation, namely solar-only, combustion-only and mixed-mode (a combination of both solar and combustion). Here, a 5-kWel xenon-arc solar simulator and natural gas were used as the energy sources, while the combustion mode was operated in the Moderate or Intense Low oxygen Dilution (MILD) combustion regime to offer low NOx emissions and good heat transfer. The thermal efficiency, heat losses, heat flux distribution within the cavity and pollutant emissions are reported for the solar-only, combustion-only and mixed-modes of operation. The thermal performance was found to be similar in all modes of operation, assuming reasonable heat recovery from the exhaust gas. Since system losses are reduced by integration (e.g. by avoiding start-up and shut-down losses), this confirms that an overall benefit can be derived from the device. Nevertheless, there is a need to manage the significantly different heat flux distribution for the three modes of operation.

Thermochemical oscillation of methane MILD combustion diluted with N2/CO2/H2O

ABSTRACTStrict environmental rules endorse moderate or intense low oxygen dilution (MILD) combustion as a promising technology to increase efficiency while reducing pollutants emission. The experimental and theoretical investigation of oscillatory behaviours in methane MILD combustion is of interest to prevent undesired unstable combustion regimes. In this study new speciation measurements were obtained in a jet-stirred flow reactor (JSR) for stoichiometric mixtures of CH4 and O2, diluted in N2, CO2 and N2-H2O, at p = 1.1 atm and T = 720–1200 K. Oscillations were experimentally detected under specific temperature ranges, where system reactivity is sufficient to promote ignition, but not high enough to sustain complete methane conversion. A thorough kinetic discussion highlights reasons for the observed phenomena, mostly focusing on the effects of different dilutions.

Thermal performance analysis of a syngas-fuelled hybrid solar receiver combustor operated in the MILD combustion regime

ABSTRACTThis article examines the overall performance of a hybrid solar receiver combustor when operated under the moderate or intense low oxygen dilution (MILD) combustion mode and contrast it with performance under the solar-only mode. A three-dimensional computational fluid dynamics model, coupled with analytical model data, is utilised to investigate the influence of fuel type (syngas, natural gas and tail gas) on the thermal efficiency, heat transfer mechanisms and heat flux distribution within the cavity. It was found that irrespective of the fuel type, similar thermal performance characteristics can be achieved under the MILD combustion mode and the solar only mode of operation (given a cavity of sufficient length and an appropriate arrangement of the heat transfer fluid (HTF) pipes). Nonetheless, it was found that the type of fuel influences significantly the rate of radiative heat transfer and the ratio of radiative to convective heat transfer rates, and so the configuration must be optimised for each type of fuel. The total heat absorbed by the HTF pipes and the thermal efficiency are predicted to be higher for the fuelled-syngas MILD hybrid solar receiver combustor case than for the CH4-fuelled and a simulated tail gas-fuelled MILD cases (where the tail gas is the by-product from a Fischer–Tropsch process) owing to a greater rate of radiative heat transfer (the latter increases with the H2 concentration in the syngas). Also, the level of dilution of the fuel stream was found to significantly influence the thermal performance of the device. In addition, the calculated distribution of heat flux to the receiver pipes was found to be significantly different under the solar and combustion modes of operation, which needs to be considered in the selection of design strategies and materials.

Analysis of the Eddy Dissipation Concept formulation for MILD combustion modelling

Performance of the Eddy Dissipation Concept (EDC) in the regime of Moderate and Intense Low-oxygen Dilution (MILD) combustion is investigated. The special MILD features, where chemical and turbulence time scales are comparable (Damköhler number close to unity), have led several researchers to suggest modifications of EDC, mainly by changing model constants. EDC with standard and modified constants are compared, and the importance of each effect is outlined. Different fine-structure reactor models and their inflow/initial conditions are discussed and found to play a significant role. The reacting fraction of fine structures, which in virtually all other numerical studies is set to unity, is also discussed and found to be important. We observe better agreement with experiment when the reacting fraction is reduced below unity, which is also described by the original EDC. The results obtained with the variable reacting fraction are found to improve both the temperature distributions and the lift-off height predictions. The calculations are carried out with the use of open source software OpenFOAM. The main test case was the Delft Jet-in-Hot-Coflow burner emulating MILD regime at three different flow conditions (jet Reynolds numbers of 2500, 4100 and 8800).

Comparative Performance Assessment of a U-Shaped Recuperative Radiant Tube under Conventional and MILD Combustion Modes

AbstractModerate and intense low-oxygen dilution (MILD) combustion, characterized by low reaction rate, uniform heat flux, and significantly reduced pollutants, is a promising technology which is w...

Experimental study on NO emissions from pulverized char under MILD combustion in an O2/CO2 atmosphere preheated by a circulating fluidized bed

This study examined the combustion of pulverized char under an O2/CO2 atmosphere to (1) attempt to achieve a moderate or intense low-oxygen dilution (MILD) combustion process and (2) investigate the effects of gas distribution modes on NO emissions. First, fuel was preheated in a circulating fluidized bed (CFB) and then high-temperature preheated fuel from the CFB was burned in a down-fired combustor (DFC). The study was conducted with two secondary gas nozzle positions, three tertiary gas position arrangements, and four secondary oxygen ratios in the DFC. During the test process, the combustion temperature was uniform in the CFB and the CO2 concentration in the flue gas reached approximately 90%. MILD combustion was achieved when the secondary gas nozzle position was center and the char combustion reaction dispersed into the upper low-oxygen space in the DFC. The obvious flame front disappeared and the temperature profile in the DFC was more uniform which is also one of the important features of MILD combustion. This reduced NO emissions by around half while maintaining high combustion efficiency. NO emissions were further reduced by a particular arrangement of tertiary gas positions, but at the cost of reducing combustion efficiency.

What Differences Does Large Eddy Simulation Find among Traditional, High-Temperature, and Moderate or Intense Low Oxygen Dilution Combustion Processes of a CH4/H2 Jet Flame in Hot Oxidizer Coflow?

Large eddy simulation (LES) of a CH4/H2 diffusion jet flame in hot coflow (JHC) is undertaken to find distinct behaviors of moderate or intense low oxygen dilution (MILD) combustion (MILDC), high temperature combustion (HTC), and traditional combustion (TC). These three JHC flames are realized by using different coflow temperatures and oxygen mass fractions: (1) 1300 K and 9% for MILDC; (2) 1300 K and 30% for HTC; (3) 600 K and 30% for TC. The modeling of LES combining the eddy dissipation concept (EDC) with a global four-step reaction mechanism is validated by the JHC measurements of Dally et al. (Proc. Combust. Inst. 2002, 29, 1147–1154). The instantaneous and time-averaged velocities, temperatures, and species concentrations such as carbon monoxide (CO) are presented and compared for the three cases. It is demonstrated that the JHC flames of MILDC and HTC both develop from autoignition nearly immediately downstream of the nozzle exit, while the lift-off JHC flame of TC evolves from an induced-ignition ...

Optimal Equivalence Ratio to Minimize NO Emission during Moderate or Intense Low-Oxygen Dilution Combustion

The influence of injection conditions on NO emission during moderate or intense low-oxygen dilution (MILD) combustion was investigated through experiments and numerical simulations. The detailed GRI-Mech 2.11 mechanism and finite-rate chemistry effects were considered in simulations. The model was systematically validated using detailed in-furnace and exhaust emission measurements. The initial conditions investigated included the equivalence ratio (Φ), oxidant preheating temperature (TO), and thermal input (P). Notable modeling results were found: under MILD combustion, with Φ increasing from 0.5 to 1.2, NO production first decreases and then increases, with the minimum NO emission obtained at Φ ≈ 0.8. Moreover, this trend is obtained at various P and TO, although NO emission increases monotonically with increasing P or TO. Furthermore, we found that CO emission is generally extremely low and can be ignored when Φ ≤ 0.85 but is much higher when Φ > 0.9. Consequently, for the furnace considered here, an op...

Thermo-ecological evaluation of an integrated MILD oxy-fuel combustion power plant with CO2 capture, utilisation, and storage – A case study in Poland

This study investigated the environmental benefits of a new boiler design for a fossil fuel-based power plant with CO2 capture, utilisation, and storage (CCUS) through thermo-ecological cost (TEC) analysis. MILD oxy-fuel combustion (MOFC) combines the advantages of the moderate and intense low-oxygen dilution (MILD) combustion and oxy-fuel combustion (OFC) to achieve efficient and environmentally justified CO2 capture from fossil fuel-based power generation. The advantages of MOFC application are: (i) it increases the efficiency of the coal-fired boiler, (ii) it increases the purity of the CO2 in the flue gases, (iii) it reduces the oxygen consumption of the boiler by using lower oxidiser excess, and (iv) it reduces the energy consumption associated with CO2 recirculation. Therefore, using MOFC decreases the penalty of the overall net energy efficiency associated with the CO2 capture from coal-fired power plants.The environmental analysis in this study considered the TEC, which measures the depletion of non-renewable natural resources by estimating the cumulative exergy consumed by the production processes. Moreover, the additional exergy consumption that compensates for the negative impact of harmful emissions has also been considered. The data for the new boiler design were obtained by CFD modelling, while the other technological modules of the integrated MOFC power plant were modelled by process modelling software. The data concerning the CO2 transport, utilisation, and storage were obtained from the available databases and literature.Three configurations of the power plant were modelled and analysed from the environmental point of view. The first two were the reference coal-fired power plants (the conventional power plant and the oxy-fuel combustion power plant with CO2 capture), while the third case used the same power plant configuration but with an MOFC boiler. The results were compared to other CO2 capture options, such as post-combustion, using the electro-energy system in Poland as the reference. The results emphasised the importance of the emissions and energy consumption occurring upstream (e.g. coal extraction and transport) and downstream (e.g. CO2 transport and storage) of the project when assessing the environmental impact factors. Additionally, two industrial options for CO2 utilisation were proposed, because CO2 storage is generally opposed by the public.