English

Moderate or intense low-oxygen dilution (MILD) combustion is regarded as one of the most effective technologies to achieve extremely low NOx emissions of combustion. MILD combustion diluted by CO2 and by N2 are herein termed “MILD oxy-combustion” and “MILD air combustion”, respectively. The present study is to investigate the difference of the two by experimental observation in a furnace of 20 kW and chemical kinetics calculation of a well-stirred reactor (WSR). Also, to identify their difference in mechanism, reaction paths of combustion diluted with N2 and CO2 are examined. It is revealed that the region of MILD oxy-combustion is notably larger than that of MILD air combustion for gaseous fuels, which suggests that the requirement for establishing MILD combustion is less stringent with dilution by CO2 than by N2. The key reason is that the CO2 dilution substantially lowers the temperature rise because of combustion, delays the ignition, and slows the overall reaction rate, thus facilitating the occurren...

Expansion of stable operational range in MILD hydrogen combustion based on catalytic techniques

This work reports a newly preheated system with pulverized coal oxy-fuel moderate or intense low-oxygen dilution (MILD) combustion for low NOx emissions. During the experiment, high-temperature fuel preheated by a circulating fluidized bed unit would burn out in a down-fired combustor. This preheating process of pulverized coal tended to form a strong reducing atmosphere and promoted the fuel temperature above the ignition temperature, which contributed to NOx reduction and MILD combustion formation. During the oxy-fuel MILD combustion process, the CO2 concentration of flue gas reached approximately 95%, the NO2 and N2O emissions were nearly zero, and the NO emissions were 107.99 mg/MJ. The combustion efficiency was 98.21%, which indicates that the combustion efficiency would not decrease in MILD combustion. Also, NO emissions were further reduced to 37.25 mg/MJ through a rational arrangement of tertiary gas positions. For pulverized coal combustion, the accurate definition of MILD combustion should consider the disappearance of flame front besides the feature of temperature distribution.

Measurements of the velocity field in laminar ethylene inverse jet diffusion flames

Numerical simulation of propane MILD combustion in a lab-scale cylindrical furnace

A novel method to improve stability of MILD combustion in a highly heat-extracted furnace

The relative enhancements of volume-integrated burning rate and flame surface area under turbulence and heat release rate contributions due to different combustion and flame propagation modes have been compared between conventional and Moderate or Intense Low Oxygen Dilution (MILD) combustion of stratified mixtures (for a global mean equivalence ratio of 0.8) for the first time using three-dimensional Direct Numerical Simulation (DNS) data. The burning rate enhancement under turbulence is found to be comparable to the enhancement of flame surface area for the conventional stratified flames considered here, whereas the burning rate enhancement is found to be considerably greater than flame area augmentation in the MILD cases analyzed here. It has been found that almost all the heat release rate arises due to the lean-premixed mode of combustion for the conventional stratified flame cases considered here. In contrast, the lean premixed mode of combustion accounts for about 50% of the total heat release rate for the MILD cases where rich-premixed and non-premixed modes of combustion contribute significantly to the overall heat release rate. Under the conditions analyzed here, more than 75% of heat release rate arises from the propagation-dominated regions for both MILD combustion and stratified flame cases, but the trend is stronger in the conventional stratified mixture combustion. More than 80% of heat release rate in the globally lean conventional stratified flame cases considered here originates from back-supported flame propagation, whereas both front and back-supported flame propagation modes have comparable contributions to heat release in the MILD combustion cases. The non-premixed mode of combustion has been found to play a more important role in the MILD combustion cases than in the case of conventional stratified flames. This implies that the closures, which work reasonably well for MILD combustion of homogeneous mixtures, may not be sufficient for modeling of MILD combustion of stratified/inhomogeneous mixtures.

Simulation of coal combustion remains challenging due to the many physical processes involved which span a large range of length and time scales. Although detailed models exist for devolatilization, char oxidation and gas-phase kinetics, most simulation efforts simplify these models considerably to reduce the high cost of simulation. Three mixture fraction-based chemistry models are evaluated, namely, the steady laminar flamelet, equilibrium, and Burke-Schumann models in the scope of coal volatiles combustion. Coal volatiles are assumed to be composed of “light gasses” (CH\(_4\), CO, etc.) as well as tar, which refers to the various large aromatic compounds released during the devolatilization process. Here, tar is treated as a single empirical species. Each mixture fraction-based model is evaluated by comparing predicted gas phase properties to computations using finite-rate chemistry with a detailed reaction model. The results indicate that the reconstructions for gas phase temperature and composition from steady flamelet model is the most accurate. The Burke-Schumann chemistry model performed very poorly for predicting the gas phase temperature and composition under stoichiometric conditions. We apply the steady laminar flamelet model to Moderate or Intense Low Oxygen Dilution (MILD) combustion. A key requirement for MILD combustion is that mixing rates are sufficiently fast that gas-phase chemistry occurs nearly volumetrically, eliminating visible flame structures. A Well-stirred reactor assumption is applied to MILD combustion due to its characteristic of volumetric reactions. The necessary conditions to achieve MILD combustion, including recirculation rate of flue gas and heat loss, are determined under various mixture fractions and mass fractions of light gas in the fuel stream. We conclude that the increasing the recirculation rate and heat loss are helpful for achieving MILD regime. Additionally, we observe that the recirculation rate and heat loss values required to achieve MILD combustion increase as the fuel stream is enriched in light gases. Steady flamelet computations reveal that MILD combustion can be achieved when reactants are not well-mixed as long as the scalar dissipation rate is sufficiently large. Our considerations indicate that the steady laminar flamelet model provides a reliable method to model MILD combustion in the absence of well-mixed reactants.

Optimization of the Eddy Dissipation Concept (EDC) model for turbulence-chemistry interactions under hot diluted combustion of CH4/H2

3D DNS of MILD combustion: A detailed analysis of heat loss effects, preferential diffusion, and flame formation mechanisms

An a priori assessment of the Partially Stirred Reactor (PaSR) model for MILD combustion

ABSTRACTVarious commonly used markers for heat release are assessed using direct numerical simulation (DNS) data for Moderate or Intense Low-oxygen Dilution (MILD) combustion to find their suitability for non-premixed MILD combustion. The laser-induced fluorescence (LIF) signals of various markers are synthesized from the DNS data to construct their planar (PLIF) images which are compared to the heat release rate images obtained directly from the DNS data. The local OH values in heat releasing regions are observed to be very small compared to the background level coming from unreacted mixture diluted with exhaust gases. Furthermore, these values are very much smaller compared to those in burnt regions. This observation rises questions on the use of OH-PLIF for MILD combustion. However, the chemiluminescent image obtained using is shown to correlate well with the heat release. Two scalar-based PLIF markers, and , correlate well with the heat release. Flame index (FI) and chemical explosive mode analyses (CEMA) are used to identify premixed and non-premixed regions in MILD combustion. Although there is some agreement between the CEMA and FI results, large discrepancies are still observed. The schlieren images deduced from the DNS data showed that this technique can be used for a quick and qualitative identification of MILD combustion before applying expensive laser diagnostics.

Moderate or Intense Low Oxygen Dilution (MILD) combustion is a regime in which fuels burn in a distributed reaction zone generating ultra-low emissions and no visible flame front. Also known as flameless combustion, it can be achieved by recirculating flue gases into the reaction zone. A recirculation factor based on the mass recirculated is used to characterize a flameless regime. However, the original definition of MILD is based on the temperature levels of a reactor. This work connects both criteria by introducing a parameter called recirculation ratio derived theoretically from a mass balance. The ratio was numerically calculated by simulating a network of perfectly stirred reactors and performing an energy balance using open-source software. This methodology was validated against experimental and simulated data from the literature. Simulations were carried out for methane, ethane, propane, and hydrogen under adiabatic conditions and equivalence ratios from 0.6 to 1. Results indicate that a critical recirculation ratio is required for establishing unconditional MILD combustion, which changes with fuel type and equivalence ratio. In all cases studied, the critical ratio diminishes as the equivalence ratio is reduced. Hydrogen and methane require the highest and lowest critical ratio, respectively. Results suggest that externally diluting hydrogen with carbon dioxide could potentially reduce recirculation requirements for MILD combustion.

Numerical study on a novel burner designed to improve MILD combustion behaviors at the oxygen enriched condition

Effect of H2O/CO2 mixture on heat transfer characteristics of pulverized coal MILD-oxy combustion