The utilization of low calorific value gases (LCVG) in combustion devices presents particular challenges in terms of ignition and sustained combustion stability due to the presence of non-combustible components. Moderate or intense low-oxygen dilution (MILD) combustion has emerged as a promising technology for LCVG combustion, offering numerous advantages such as high combustion efficiency, reduced pollutant emissions, and increased fuel flexibility. However, the current body of research in this area is fragmented, making it challenging to draw meaningful comparisons between studies and hindering its practical application. This paper provides a comprehensive review of conventional and MILD combustion of LCVG. To understand the impact of composition on combustion, the fuels are classified based on their composition of hydrogen, carbon monoxide, methane, carbon dioxide, nitrogen, and water. We also delve into the chemical and physical effects of composition, including reaction kinetics and turbulence mixing, and provide an overview of the burners and methods used in establishing MILD combustion. Furthermore, computational fluid dynamics (CFD) models and chemical kinetics in MILD combustion are also thoroughly discussed. The presence of a large amount of dilution gas in LCVG increases the self-ignition temperature and ignition delay time of the mixture, making preheating the reactants a critical consideration. In MILD combustion, it is crucial to have an inlet reactant temperature higher than the self-ignition temperature (Tin>Tsi) to mitigate the difficulties associated with ignition and unstable combustion. The heat release in MILD combustion should be moderate to ensure that the combustion temperature does not become too high. The non-combustible components of LCVG are beneficial in this regard, as they allow for a temperature increase of less than the self-ignition temperature (ΔT<Tsi). Hydrogen is the most reactive component in LCVG, and its content directly impacts the establishment, efficiency, and pollutant emissions of MILD combustion. Carbon dioxide, nitrogen, and water act as diluents, helping to reduce NOx emissions in MILD combustion. Although a burner may have the potential to be used for MILD combustion, it must be optimised for LCVG with variable composition in order to achieve the lowest pollutant emissions. Further research is necessary to verify and improve simulation models and chemical kinetics. This article provides theoretical support for the practical application of MILD combustion of LCVG with variable composition. © 2024 Elsevier Ltd

The present study is an extension of our earlier mixing field investigation in partially premixed flames. In the current work, the link between the mixing field and flame structure, and hence flame curvature and surface density are experimentally studied. The flame structure is captured using a high-speed OH-PLIF technique, whereas the crosswise mixing fields were previously presented in our earlier study. A concentric flow slot burner is employed for producing turbulent planar partially premixed natural gas flames at different levels of mixture inhomogeneity. The effects of the level mixture inhomogeneity, the equivalence ratio, the air to fuel stream velocity ratio, and Reynolds number “Re” on the flame characteristics are investigated. The link between the previously reported data on the mixing field and the flame structure are examined. The correlations between the mixing field and the flame structure were clearly observed in the current study. That is, as the mixture becomes highly inhomogeneous, in rich input jet conditions cases, altering Reynolds number and the mixing length led to a pronounced change in both flame structure and flame curvature while altering L/D influences the flame structure. On the other hand, the effects of the mixing length and Reynolds number on the flame structure and curvature in a more homogeneous mixture were not significant. The data showed that the flame surface density was inversely proportional to the level of the mixture inhomogeneity and directly proportional to the Reynolds number. The significant effect of the jet equivalence ratio on the level of mixture inhomogeneity leads to a significant change in the flame structure and flame curvature. In addition, if the mixture inhomogeneity is less pronounced the variation s of the mixing length and Reynolds number lead to minimal effects on the flame structure and flame curvature. © 2023 Elsevier Ltd

Turbulent flames with compositionally inhomogeneous mixtures are commonly used in many combustion systems. In this work, turbulent planar jet flames issued from a concentric flow slot burner, CFSB, were used to study the impact of mixture inhomogeneity near the flame sheet. The CFSB burner can control the mixing inhomogeneity by changing the mixing length “L” between the concentric fuel and air slot nozzles. At various levels of mixture inhomogeneity, the mixing field, presented by mixture fraction, Z, distribution, and the flame structure was investigated via simultaneous Rayleigh and OH-PLIF imaging technique. Our previous study investigated the mixing field immediately downstream of the burner exit in non-reacting conditions. The PDFs of Z showed that the mixing field covered a wide range of mixture fractions, where the high flame stabilization occurred when a large portion of the PDF(Z) was located within the fuel flammability limits. This work showed that further downstream, the highly stabilized flames were also obtained when the range of fluctuations in the mixture fraction was close to the stoichiometric mixture fraction. Moreover, plotting the mixing field using the scatter plot within the mixing regime diagram for various flame conditions showed that the mixing field downstream of the burner exit consistently follows the mixing diagram classification. Moreover, a close inspection of the flame structure showed that the flame sheet varies from thick, and corrugated at a low and high levels of mixing to thin flame sheet but less corrugated and that at a particular mixing normalized length L/D = 7. The flame corrugation data showed that the flame stability occurs at a minimum corrugation factor. © 2023

Partially premixed combustion is characterized by mixture fraction inhomogeneity upstream of the reaction zone and occurs in many applied combustion systems. The temporal and spatial fluctuations of the mixture fraction have tremendous impact on the combustion characteristics, emission formation, and flame stability. In this study, turbulent partially premixed flames are experimentally studied in a slot burner configuration. The local temperature and gas composition is determined by means of one-dimensional, simultaneous detection of Rayleigh and Raman scattering. The statistics of the mixture fraction are utilized to characterize the impact of the Reynolds number, the global equivalence ratio, the progress of mixing within the flame, as well as the mixing length on the mixing field. Furthermore, these effects are evaluated by means of a regime diagram for partially premixed flames. In this study, it is shown that the increase of the mixing length results in a significantly more stable flame. The impact of the Reynolds number on flame stability is found to be minor. © 2018 Taylor & Francis.

This study is geared toward generating highly stabilized partially premixed flames at various levels of turbulence and partially premixing. Therefore, with the help of the laser-induced breakdown spectroscopy (LIBS) technique, a new burner was constructed and employed to quantitatively estimate the mixture equivalence ratio (Φ) within the flame. Two turbulence generator disks, five degrees of partial premixing, and two fuels were used to assess the flame stability. Natural gas (NG) and liquefied petroleum gas (LPG) were used as fuels. The LIBS spectrum's most common atomic emission lines which include hydrogen, nitrogen, oxygen, and carbon, were chosen to establish the correlation between emission lines' intensity and the flame's mixture equivalence ratio. The results showed that the stability of NG flame was less sensitive to the variation of the partially premixing levels. In contrast, the LPG flames were more susceptible to the variation of the mixing degree. At a lower level of partially premixing, NG flames were more stable, and as the mixing degree increased, the stability of NG flames was reduced compared to LPG flames. In addition, the results showed that the equivalence ratio radial profiles are more homogeneous and have lower RMS fluctuation for the wider slot of the turbulence generator disc. Furthermore, the larger turbulence generator disk's higher turbulent intensity contributed in posting the mixing process and enhancing mixture homogeneity over even shorter recess distances than the smaller disk generator. © 2022 Elsevier Ltd

In most practical combustion devices, the actual combustion process occurs within different mixture inhomogeneity levels. Investigating the mixture fraction field upstream of the reaction zones of these flames is an essential step toward understanding their structure, stability, and emission formation. In this study, the mixture fraction fields were measured for turbulent non-reacting inhomogeneous mixtures immediately downstream from the slot burner exit, using Rayleigh scattering imaging. The slot burner had two concentric slots. The inner air slot can be recessed at distances upstream from the exit of the outer fuel slot, allowing various degrees of mixture inhomogeneity. Mixture fraction field statistics and the two-dimensional gradient were utilized to characterize the impact of the air-to-fuel velocity ratio, global equivalence ratio, fuel composition, Reynolds number, and the premixing length on the mixture mixing field, and thus flame stability. These impacts were evaluated by tracking the normalized mean mixture fraction and mixture fraction fluctuation transition across the regime diagram for partially premixed flames. The results showed that the air-to-fuel velocity ratio was the critical parameter affecting the mixture fraction field for the short premixing length. Stability results showed that the level of mixture inhomogeneity mainly influenced the flame stability. High flame stability is achieved if a large portion of the inhomogeneous mixture fraction is within the fuel flammability limits. © 2021 Elsevier Ltd

The scope of this study was twofold: to define the most suitable Egypt's biodiesel source, and to assess the impact of this biodiesel source with the addition of carbon nano additives on diesel engine performance and emissions. Environmental Key Performance Indicators were used as metrics to identify the appropriate biodiesel source with respect to several criteria including environmental, economic, ecological, and demographic situation. The most suitable Egypt's biodiesel source was identified from different sources including algae, Jatropha, coffee waste, rice straw, sugarcane, and switchgrass. Then, the production process of biodiesel from this renewable source by transesterification was carried out using chemical catalyst or biocatalyst. Following this, this source's impact with carbon nano additives on the performance and emissions of a 0.825-L single-cylinder diesel engine was further examined at engine loads varying between 0 and 4 kW and 1500 rpm constant engine speed. The carbon nano additives were added at a different mass fractions of 25, 50 and 100 ppm to Jatropha biodiesel and diesel blend (20 % jatropha biodiesel+ 80 % diesel) by volume. The results showed Jatropha as the most efficient biodiesel source among all the tested fuels to be produced in Egypt. Furthermore, the experimental engine test results revealed that Jatropha biodiesel without and with carbon nano additives consistently has a higher brake thermal efficiency and lower brake specific fuel consumption in comparison to that of diesel fuel. Likewise, Jatropha biodiesel without and with carbon nano additives enhanced the engine performance and consequently reduced the exhaust emissions including CO, CO2, HC, NOx, and smoke in comparison to that of diesel fuel. © 2021 Elsevier Ltd

Enhancement of biogas combustion characteristics will increase the possibility of the direct utilization of such an eco-friendly fuel in practical combustors. We report a full study of biogas combustion under partially premixed mode to investigate the stability of biogas free-jet flames using a concentric flow slot burner was used to evaluate the biogas combustion characteristics. Five mixtures of biogas ranging from 0%CO2 up to 40%CO2 were investigated to study the effect of CO2 proportion on the stability of biogas flames. A new well-defined stability procedure was followed by studying the four major observed phenomena in biogas flames: Stable Flame, Partially-lifted Flame, Fully-lifted Flame, and Extinction. Each phenomenon was clearly defined in addition to describing the operating conditions in terms of Reynolds Number (Re) and Equivalence ratio (ϕ). The effect of the premixing ratio (LD) on the combustion stability of biogas flames was also studied for five premixing ratios LD3, LD5, LD7, LD10, and LD16 to optimize the flame stability. The temperature of stable biogas flames was also measured at different CO2 percentages. The results showed that LD10 is the optimum premixing ratio to generate a stable biogas flame at all tested CO2 proportions. Partially premixed combustion in slot burner manages to sustain a stable biogas flame up to 30%CO2. However, increasing the CO2 to 40% eventually led to a weak and unstable flame regardless of the premixing ratio. © 2020 Elsevier Ltd

The co-combustion of sewage sludge (SS) and biomass is promising for achieving energy recovery and compensating for individual feedstock disadvantages. However, the emission of nitric oxide (NOx) during this process is of concern, but no relevant study has focused on this. In this study, the release characteristics of NOx during co-combustion of SS and corn straw (CS) were investigated under different mixing ratios, heating rates, and SS moisture contents. The synergetic effect of SS and CS on NOx reduction was determined and quantified. An independently designed one-dimensional unsteady model with a fully implicit scheme was used. The results showed that with the increase in the SS mixing ratio to 20, 40, 60, and 80 wt%, the NOx emission peak values were reduced by 26%, 38%, 35%, and 40%, respectively. The case with a 60 wt% water content of SS was the best choice for NOx reduction. Furthermore, the model could describe the NOx emission trend accurately, especially for CS combustion and SS/CS co-combustion. This study showed effective NOx reduction during SS/CS co-combustion, thereby indicating that co-combustion is beneficial for the disposal of SS. In the future, the mechanism of the synergetic effect of SS and CS will be investigated from a micro point of view by more comprehensive experiments coupled with process parameter simulation. © 2020 Elsevier Ltd

Turbulent flames with compositionally inhomogeneous mixtures are common in many combustion systems. Turbulent jet flames with a circular nozzle burner were used earlier to study the impact of inhomogeneous mixtures, and these studies showed that the nozzle radius affects the flame stability. Accordingly, planar turbulent flames with inhomogeneous turbulent jet are created in a concentric flow slot burner (CFSB) to avoid this effect in the present study. The stability characteristics, the mixing field structure, and the flame front structure were measured, and the correlations between stability and the mixing field structure were investigated. The mixture fraction field was measured in non-reacting jets at the nozzle exit using highly resolved Rayleigh scattering technique, and the flame front was measured in some selected turbulent flames using high-speed Planar Laser-Induced Fluorescence (PLIF) of OH technique. The data show strong correlations between flame stability and the range of mixture fraction fluctuations. The flames are highly stabilized within a mixing field environment with the range of fluctuation in mixture fraction close to the range of the flammability limits. The mixing field structure is also illustrated and discussed using a mixing regime diagram and showed that the scatter of the data of the different cases is consistent with the classified mixing regimes. Lean flames are stabilized in the current slot burner. The flame front structure topology varies consistently from thin, small curvature at the low level of turbulence and higher equivalence ratio to more wrinkled, larger curvature, but a thicker structure at a higher level of turbulence and lower equivalence ratio. © 2020 Elsevier Ltd

Tar pollution is the most serious problem hampering the application of biomass gasification. Microwave exhibits unique effects for treating biomass tar and thus have attracted considerable attention. However, as an energy-intensive technology, the optimal operating parameters of microwave have not been developed, and the energy feasibility remains unknown. In this study, a bench-scale microwave catalytic reforming (MCR) was built and tested for the thorough conversion of biomass model tar under ex-situ hot gas conditions. The effects of tar concentration levels and reaction temperatures on product distributions and reaction routes were carefully investigated, and the optimal MCR parameters were determined based on the net energy analyses of three corresponding gasification scenarios. The results showed that net energy efficiencies of 29.02%, 76.01%, and 93.04% can be obtained in scenarios with 100 g Nm−3, 10 g Nm−3, and 1 g Nm−3 of inlet tar, respectively. The results clearly proved that the MCR can be self-powered in the gasification processes regardless of the tar level. Moreover, this conclusion was tested in an operational biomass gasification power plant, and a comprehensive comparison was conducted between the MCR and conventional tar treatment. It was demonstrated that net energy efficiencies of 85.60% and 73.03% can be achieved by coupling the MCR with a water-cooling unit and with a single MCR, respectively. Additionally, the catalyst lifetime was tested in successive eight-hour experiments, and the carbon consumption of the catalyst was thoroughly investigated using a thermogravimetric analysis with multiple atmospheres. This study provided a foundation and new insights for treating biomass tar with microwaves, and it may be a milestone for its commercialisation. © 2020 Elsevier Ltd

Biogas combustion is a very essential topic for the development of many industrial combustion systems and engines. This fuel can replace current fossil fuels used in burners, engines, and many other applications. Understanding the combustion characteristics of this fuel and its stability in highly turbulent flames of practical interest is the aim of this work. The percentage of CO2 in Biogas varies between 25% and 45%, which affects the combustion stability and flame structure. The present work shows that the generation of Biogas is improved by adding Ni-Co-Ferrite or Ni-ferrite nano-additives. In this work, we selected 25 flames of mixtures of natural gas and CO2, where the ratio of CO2 varies from 0% to 40%. The flames are generated in a concentric flow slot burner that produces planar two-dimensional flames. The stability characteristics and the flame structure were investigated. The flame structure is presented in the form of temperature profiles in some selected flames using fine wire thermocouple measurements. The stability characteristics are illustrated for two limits of lifted flames and blow out. The production rate of Biogas can be increased by almost 30% using nano-additives of Ni-Co-Ferrite or Ni-ferrite. The data show that the stability of the flames is affected significantly for the 40% CO2 mixture. Therefore, it is recommended to keep CO2 percentage up to 30% for stable turbulent Biogas flames. On the other hand, partially premixed flames are highly stable for a certain level of mixture inhomogeneity at a mixing length ratio of L/D = 16. At this level, the mixture fraction fluctuations are expected to be within the flammability limits range based on previous investigations in round jet configuration. © 2019 Elsevier Inc.

Many Nanoparticles (NPs) were reported to significantly enhance the production of biogas from the anaerobic digestion of organic wastes. Herein, two types of NPs (Ni-Ferrite and Ni-Co-Ferrite) were synthesized using the co-precipitation method and characterized using X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM) with energy dispersive X-ray spectroscopy (EDS) techniques. Three different concentrations (20, 70 and 130 mg/l) of each type were added to separate biogas reactors to study their effect on the biogas production compared to blank reactor. A pressure-based biogas reactor was designed and manufactured specifically to test the effect of the NPs on the anaerobic digestion process. The pressure of the daily produced biogas was monitored using a pressure transducer and the volume of the biogas was calculated at standard conditions of pressure (1.013 bar) and temperature (15 °C). The results showed an increase in the biogas production of 6.6%, 5.9% and 32.9% upon the use of Ni-Co-Ferrite NPS at concentrations of 20, 70, and 130 mg/l compared to blank reactor, respectively. In addition, the Ni-Ferrite NPs achieved biogas enhancements of 30.8%, 28.5%, and 17.9% at concentrations of 20, 70 and 130 mg/l, respectively. © 2019 Elsevier Ltd

The creation of a more stable flame along with the extension of flammability limits under lean mixture combustion was the main motivation to develop a new burner design, which has been investigated in this research. The current burner configuration was utilized to create a wide range of higher turbulent intensities and to produce different degrees of mixture inhomogeneity, which acted to promote minimum pollution, highest performance and higher flame stability. The burner stability assessment was investigated using two types of fuel: natural gas (NG) and liquefied petroleum gas (LPG). They were tested under different degrees of partial premixing, and two turbulence generator disks for lean mixture at an equivalence ratio of φ = 0.8 were used. Following this, the Laser Induced Breakdown Spectroscopy (LIBS) technique was utilized to characterize and quantify the impact of changing the disk slit diameter on the distributions profiles of equivalence ratio or mixture fraction for a NG/air partially premixed flame. A series of homogeneous NG/air mixtures with different equivalence ratios were used to obtain the correlations between the measured emission lines of LIBS spectra and the global flame equivalence ratio. Consequently, the emission spectral lines ratios of H/N, H/O and C/N + O were utilized to predict the equivalence ratio distributions. The results demonstrated that for all of the mixing lengths, NG/air mixture with larger disk generator diameter yielded the maximum burner stability, whilst the LPG/air mixture with a larger disk generator diameter resulted in the minimum burner stability. Furthermore, the flame associated with the larger disk slit diameter had a uniform local equivalence ratio distribution and lower RMS fluctuation profiles of equivalence ratio in comparison to the lower disk slit diameter. © 2018 Elsevier Ltd