Soot Formation In Flames Research Articles

An easy but quantitative assessment of soot production rate and its dependence on temperature and pressure

The challenge of soot emission persists in combustion research due to the complexities of tracking the crucial stages of growth from fuel to soot nuclei and ultimately mature particles. Studying soot formation in flames often requires a sophisticated approach, involving detailed measurements of gaseous soot precursors and soot particles using multiple complementary diagnostics. On the other end of the spectrum of studies are simpler methods that capture the sooting tendency using a single index, akin to the cetane number in compression ignition engines and the octane number in spark ignition engines. This article seeks a middle ground, aiming to quantify the soot production rate while maintaining the simplicity of single-index characterizations. The approach involves establishing counterflow diffusion flames, measuring soot volume fraction through pyrometry, and accurately computing velocity and temperature profiles using a commercial code. These data allow for the quantification of the production rate from the soot governing equation. The methodology is applied to counterflow ethylene diffusion flames to examine the temperature dependence of the soot production rate across peak temperatures varying by several hundred degrees and pressures in the 1–32 atm range. The soot production rate per unit flame area falls within the range of 10−7–10−3 g/(cm2s) range and, when normalized with respect to the carbon flux, it ranges between 10−6 and nearly 10−2. On a logarithmic scale, it linearly correlates with the peak temperature at a fixed pressure. Although this study deals only with flames of ethylene, the approach can be generalized to any fuel. The resulting database should be valuable not only for industry practitioners but also to the scientific community for the global validation of detailed soot models.

Study on the application of laser diagnosis technology in the rapid real time measurement of soot

Based on previous soot studies, this paper proposes a new method to quantify the evolution of the soot generation mechanism in flames. The soot distribution in inverse diffusion flame at different fuel gas velocities was quantitatively measured by laser-induced incandescent (LII) technique, and the soot structure evolution was analyzed by Raman spectroscopy and field emission transmission electron microscopy (FETEM). Laser-induced breakdown spectroscopy (LIBS) and numerical simulation were used to investigate the association between the intensity ratio of hydrocarbon atoms of at different positions of the flame and soot formation. Meanwhile, a detailed chemical kinetic simulation of the formation process of soot was carried out, and the flame structure was divided in more detail. The flame state in different regions and the key reactions of soot formation were analyzed. The results presented that the volume fraction of soot measured by LII is consistent with the height-dependent graphitization of the flame and is identical to the yellow definition of the flame. The variation of the C/H atomic intensity ratio reflected the soot formation in flame. The CH intensity ratio in the flame axial direction showed a bimodal distribution, corresponding to the blue phase of soot precursor formation and the yellow phase of soot formation, respectively. In addition, the CH atomic intensity ratio and the CH atomic molar ratio showed a parabolic transition from incipient to mature soot in the soot production region. The CH intensity ratios at different heights are linear in the radial direction, and the main core region of soot production can be obtained when the CH intensity ratio is 0.10 ∼ –0.13. With radial distance increases, it is clear results that the most sensitive reaction of (precursor) A1 changes from C6H5OH + H = A1 + OH to C2H3 + HCO=C3H3 + OH. It reveals that with the increase of yellow light, the soot structure along the flame center line is gradually graphitized, and the graphitization degree is the highest at the axial height of 18.0De ∼ 22.4De. This is consistent with the distribution of CH atomic intensity ratio. The present results demonstrate that LIBS can quickly achieve the initial measurement of soot and provide a new method for the study of flame soot during its evolution.

Modeling soot formation in flames and reactors: Recent progress and current challenges

The study of soot has long been motivated by its adverse impacts on health and the environment. However, this combustion knowledge is also relevant to the production of carbon black and hydrogen via methane pyrolysis which are important commodities. Over the last decade, steady progress has been made in the development of detailed continuum models of soot formation in flames and reactors. Developing more comprehensive models has often been motivated by the need for predicting soot formation over a wider range of conditions (e.g., temperature, pressure, fuels). Measurements with novel experimental techniques have given us new insights into the chemistry, particle dynamics and optical properties of soot particles and even molecules and radicals forming them. Also, multi-scale modeling has enabled us to translate the detailed mechanisms of soot processes based on first principles into computationally efficient but accurate continuum models of soot formation in flames and reactors. However, important questions remain including (1) what is the mechanism of soot inception and surface growth, (2) which gas-phase species are involved in soot inception and surface growth (3) how surface growth and oxidation are affected by soot surface properties. Proposed models need to be evaluated against experimental data over a wide range of conditions to determine their predictive strength. These questions are critical for the accurate prediction of soot formation in flames and its emissions from engines. However, this knowledge can also be used to develop predictive process design and optimization tools for carbon black and other nanocarbon formation in reactors.

Effects of ammonia addition on soot formation in ethylene laminar diffusion flames. Part 2. Further insights into soot inception, growth and oxidation

Ammonia, as an alternative fuel, is attracting unprecedented attention due to the absence of CO2 emission during its combustion. Effects of ammonia co-firing on soot formation in ethylene laminar diffusion flames have been studied in the first part of this series of papers, addressing the soot volume fraction change and reaction kinetics of gaseous soot precursors. This paper further investigates the effect of ammonia on evolution behaviors of soot particle, namely soot inception, growth and oxidation in the ethylene co-flow diffusion flame. The soot volume fraction (SVF) and spatial distribution in flames without/with ammonia addition (5 and 20 vol%) were measured using laser-induced incandescence (LII) method. A newly constructed C2H4/NH3/PAHs kinetic model consisting of 163 components and 1018 reactions and the CoFlame codes dedicated to simulating laminar co-flow diffusion flame coupled with fixed sectional soot particle model were used to model the flames and soot inception, surface growth, and oxidation processes in them. Numerical results well captured the experimental observation on both peak SVF and flame height, and confirmed that ammonia inhibited the soot formation in the flame, showing a peak SVF decrease of 2–4 % per 1 % ammonia addition. The numerical results revealed that the ammonia addition inhibited all soot inception, surface growth via HACA (hydrogen-abstraction-acetylene-addition) and PAH condensation, and oxidation via OH/O2 processes. Most importantly, inhibition of HACA reactions induced by ammonia was identified as the main reason for reducing soot formation. Kinetic analysis revealed that adding ammonia created a new pathway to consume H radical while decreasing the rate of H radical-generating reactions, leading to a decrease in H radical and ultimately a reduction in the HACA reaction rate.

Measurement of soot primary particle size in co-flow laminar diffusion flames of n-heptane at elevated pressures

To comprehensively understand soot formation in flames of liquid fuel, the soot primary particle size (PPS) in n-heptane\\air diffusion flame at pressures up to 5 bar was investigated in this work. A series of experiments were conducted in a high-pressure laminar flame burner system, and the spatial distributions of soot primary particle size in the flames at atmospheric and elevated pressures were evaluated by two-dimensional Time-Resolved Laser-Induced Incandescence (TiRe-LII). The results indicated a tendency that the conunt mean diameter (Dp,mean) of soot primary particle increased with enhancing laser fluence, which might be attributed to inaccuracy of the thermal accommodation coefficient at the increasing laser fluences. The spatial distribution of Dp,mean varied with pressures. Further, the evolution of Dp,mean along the centerline and the radial distribution of Dp,mean at the pressures of 1.0–3.0 bar were also presented. The zone of the maximum Dp,mean moved from the flame centerline to the flame edge as the pressure increases. The spatial distribution of Dp,mean at 3.5–5.0 bar showed the area of maximum Dp,mean located at the annular wing region in flames and being floated as the pressure increased. This study complemented the existing theoretical knowledge of spatial primary particle size distribution (PPSD) in n-heptane laminar diffusion flames.

In situ diagnostic investigation on the structure distribution of inverse diffusion flames based on laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) was applied to the measurement of methane-oxygen inverse diffusion flames. The correlation between the LIBS emission spectra intensity of different atoms and the overall flame equivalence ratio was investigated by varying the oxygen-fuel equivalence ratio ([O/C]e). Moreover, the applicability of LIBS for local chemical composition measurement in flames was analyzed. The results indicate that the dependence of O(777.2 nm)/H(656.3 nm) spectral intensity ratio on laser energy is negligible. The spectral intensities of C, H and O atoms first increase and then decrease with the increase of axial height, and the radial variations are symmetrically distributed. In addition, the variation of C(247.8 nm)/H(656.3 nm) and O(777.2 nm)/H(656.3 nm) atomic spectral intensity reflects the characteristics of soot formation in flame. The local equivalence ratios at different radial positions at different heights of lean and rich flames were characterized by establishing the linear relationship between the oxygen-hydrogen intensity ratio and the flame equivalence ratio, and the information on the flame structure and dynamics was provided. Thus, it is of great significance for the analysis of flame to reasonably clarify the stability of the determination of atomic composition by LIBS.

Impact of the maturation process on soot particle aggregation kinetics and morphology

Mature soot aggregates exhibit a morphology which is currently mimicked by diffusion-limited aggregation (DLCA) codes, i.e. with a sticking probability = 1 when two aggregates collide. Nevertheless, nascent soot particles may grow in the reaction-limited aggregation regime (RLCA), i.e. with a sticking probability ≪ 1. Yet, it remains to be seen how fast the transition from RLCA to DLCA occurs for soot particles and what is the impact on aggregation kinetics, particle size distribution and morphology. This work intends to fill this gap by exploring the aggregation of soot particles formed in a laminar premixed flame through numerical simulations. Results show that the transition from RLCA to DLCA is very fast and produces a moderate impact on soot formation dynamics and morphology. However, soot particles mass bulk density is found to play an important role and should be considered in future simulations of soot formation in flames.

Optical properties and structure of acetylene flame soot

Acetylene black is a special type of carbon black, obtained from the thermal decomposition of acetylene. Acetylene flames enable the investigation of the soot optical properties. A reliable knowledge of the optical properties of acetylene soot is necessary for interpreting the optical measurements in flames and the development of acetylene black applications in industry. However, a wide variation in the optical properties of soot is observed in different formation conditions. The main factors affecting soot formation in flames are fuel composition, equivalence ratio, and reaction time connected with the height above a burner. In this study, we measured the absolute value of the refractive index function of acetylene soot E(m,1064) at a wavelength of 1064 nm, as well as the ratio of the refractive index functions E(m,1064)/E(m,532) at laser wavelengths of 532 nm and 1064 nm in a premixed acetylene/air flame using laser-induced incandescence. The soot structure was studied using high-resolution electron microscopy. The obtained results showed that E(m,1064) increased from 0.19 to 0.53 and that the ratio of E(m,1064)/E(m,532) increased from 1.12 to 1.55 according to the height above a burner. These changes in the optical properties are attributed to differences in soot structure, such as different distances between graphene planes inside soot crystallites, which decreased from 0.43 to 0.36 nm as the height above the burner increased from 5 to 20 mm. It was concluded that the optical properties and structure of acetylene soot differ from those formed in other hydrocarbon flames.

Study of soot formations in co-flow laminar diffusion flames of n-heptane and oxygenated aromatic biofuels from atmospheric condition to 2.3 bar

Anisole and benzyl alcohol, both are lignin based oxygenated aromatic biofuels as a pair of isomers with benzene rings possessing different oxygen functional groups. This work investigated the characteristics of soot formation in laminar co-flow diffusion flames with n-heptane and its blends with anisole and benzyl alcohol from atmospheric to elevated pressures. With the high-pressure burner and pressurized Twin-Lines liquid fuel evaporization (TLLFE) system, a series of experiments were carried out to measure the sooting tendency by utilizing the Laser induced incandescence (LII) combining with Line-of-sight attenuation (LOSA) technique. The experimental results show that with the addition of anisole and benzyl alcohol, the soot formation in flames is enhanced, especially for the addition of benzyl alcohol. Moreover, the elevated pressure can enhance the soot formation in n-heptane and n-heptane/anisole flames, and the integrated soot volume fractions (SVFs) follow power law relationships with pressure. The power dependence on pressure for n-heptane/anisole flame is larger in comparison with that of n-heptane flame. These findings provide basic understanding of the soot formation in co-flow laminar diffusion flames of n-heptane and oxygenated aromatic biofuels.

A Long Short-Term Memory Neural Network for the Low-Cost Prediction of Soot Concentration in a Time-Dependent Flame

Particulate matter (soot) emissions from combustion processes have damaging health and environmental effects. Numerical techniques with varying levels of accuracy and computational time have been developed to model soot formation in flames. High-fidelity soot models come with a significant computational cost and as a result, accurate soot modelling becomes numerically prohibitive for simulations of industrial combustion devices. In the present study, an accurate and computationally inexpensive soot-estimating tool has been developed using a long short-term memory (LSTM) neural network. The LSTM network is used to estimate the soot volume fraction (fv) in a time-varying, laminar, ethylene/air coflow diffusion flame with 20 Hz periodic fluctuation on the fuel velocity and a 50% amplitude of modulation. The LSTM neural network is trained using data from CFD, where the network inputs are gas properties that are known to impact soot formation (such as temperature) and the network output is fv. The LSTM is shown to give accurate estimations of fv, achieving an average error (relative to CFD) in the peak fv of approximately 30% for the training data and 22% for the test data, all in a computational time that is orders-of-magnitude less than that of high-fidelity CFD modelling. The neural network approach shows great potential to be applied in industrial applications because it can accurately estimate the soot characteristics without the need to solve the soot-related terms and equations.

Effects of pressure on soot formation in laminar coflow methane/air diffusion flames doped with n-heptane and toluene between 2 and 8 atm

There is currently a lack of adequate understanding of soot formation in flames fueled with liquid hydrocarbons at elevated pressures. In this study, laminar coflow n-heptane and toluene doped CH4/air diffusion flames were numerically investigated under a constant carbon mass flow rate at pressures between 2 and 8 atm to understand how pressure affects the sooting propensity of these two main components of surrogate fuels mimicking gasoline. Numerical simulations were performed using a detailed reaction mechanism containing 175 species and 1175 reactions and a sectional soot model. Soot inception is modeled by collisions among pyrene, BAPYR and BGHIF. Soot surface growth and oxidation are modeled using the hydrogen abstraction acetylene addition (HACA) mechanism as well as PAH surface condensation. The predicted sensitivity of soot production to pressure is in better agreement with the measurements of Daca and Gülder (2017) when the soot aging effect is considered in HACA surface growth. The predicted soot concentrations are in overall good agreement with measurements. Propargyl recombination and propargyl reaction with propyne are important pathways for the formation of benzene. In methane+toluene flames, attack of toluene by H radical is an effective benzene formation pathway low in the flame, but the relative importance of benzene formation from toluene is reduced with increasing pressure. Although all the soot formation processes are enhanced with increasing pressure, PAH condensation is enhanced the most, followed by HACA and inception. At 6 and 8 atm, PAH condensation becomes comparable to HACA. The pressure dependence of the sooting propensity follows the order of methane+toluene < methane+n-heptane < methane, consistent with measurements. This result can be explained by the pressure dependence of benzene formation pathways, the kinetic effect of pressure, and the scrubbing effect of soot production on the gas-phase species involved in soot formation.

Effects of radiation modeling on non-premixed sooting flames simulations under oxyfuel conditions

Radiative heat modeling has a central role for the simulation of soot formation in flames. Radiation heat transfer, gas-phase chemistry and soot formation are strongly coupled and highly dependent on temperature. Thus, it is of primary importance that simplified radiative models are able to reproduce detailed simulations at conditions found in flames of interest. In this work, the capabilities of the superposition WSGG radiation model is compared to the LBL spectral integration for a set of laminar non-premixed counterflow flames of ethylene burning with oxygen-rich oxidizer. The numerical solutions for both thermal radiation approaches are fully coupled with detailed chemistry and a discrete sectional model for soot formation. The role of thermal radiation and the importance of radiation absorption on soot formation were explored through systematic replacement of N2 by CO2 on either the fuel or on the oxidizer. In addition, the validity of the simplified optically thin approximation for modeling such flames was also discussed. The spectral superposition WSGG model is able to accurately describe general flame structure and soot predictions respective to the LBL integration for both the reference flame and for the cases of CO2 addition in the fuel and in the oxidizer mixtures. For the CO2 addition flames, the WSGG model tends to overestimate the radiation emission on high temperature regions and to overestimate the radiation absorption on low temperatures towards the reactant with CO2 addition. However, these discrepancies were not sufficient to significantly influence the flame structure. Some non-negligible discrepancies are observed only at very small strain rates. Finally, the evaluation of the soot spectral radiation coefficient (Cκ,s) reveled an important influence on the soot volume volume fraction for low levels of CO2 addition.

A conservative method for numerical solution of the population balance equation, and application to soot formation

The objective of this paper is to present a finite volume method for the discretisation of the population balance equation with coagulation, growth and nucleation that combines: (a) accurate prediction of the distribution with a small number of sections, (b) conservation of the first moment (or any other single moment) in a coagulation process, (c) applicability to an arbitrary non-uniform grid, and (d) speed and robustness that make it suitable for combining with a CFD code for solving problems such as soot formation in flames. The conservation of the first moment of a distribution with respect to particle volume is of particular importance for two reasons: it is an invariant during a coagulation process and it represents conservation of mass. The method is based on a geometric evaluation of the double integrals arising from the finite volume discretisation of the coagulation terms and an exact balance of coagulation source and sink terms to ensure moment conservation. Extensive testing is performed by comparison with analytical solutions and direct numerical solutions of the discrete PBE for both theoretical and physically important coagulation kernels. Finally, the method is applied to the simulation of a laminar co-flow diffusion sooting flame, in order to assess its potential for coupling with CFD, chemical kinetics, transport and radiation models. The results show that accurate solutions can be obtained with a small number of sections, and that the PBE solution requires less than one fourth of the time of the complete simulation, only half of which is spent on the discretisation (the remaining being for the evaluation of the temperature dependence of the coagulation kernel).

Complete Scheme for Fullerene, Graphene, and Soot Formation in Flame

Soot formation processes have been studied for more than 100 years, they include empirical and phenomenological description of conversion of various fuels to soot particles. This article provides an overview of current state of soot formation foundations, including the chemistry of soot nucleation, nucleation, mass growth as well as size of soot particles. Consideration of this issue shows that a great progress has been made, but there is still a lot of ambiguity in many areas of our knowledge. This concerns the role of aromatic molecules and radicals in nucleation and subsequent growth mass in laminar pre-mixed flames. Along with environmental problems, in recent time there are considered soot particles as an environment for nanomaterial synthesis, such as fullerenes, carbon nanotubes and graphenes. In this regard, a complete scheme of nanomaterials formation in mode of soot formation has been developed.

Unveiling trends in soot nucleation and growth: When secondary ion mass spectrometry meets statistical analysis

At the present day, ultrafine soot particles have become the object of increasing attention due to their well-documented adverse effects on human health and climate. In particular, understanding soot nucleation is one of the most challenging problems toward a more controlled and cleaner combustion. Detailed information on the chemistry of nascent soot particles (NSPs) is expected to provide clues on the soot formation and growth reaction pathways. Herein, the early steps of soot formation in flames are addressed by investigating the chemical composition of NSPs and their molecular precursors by secondary ion mass spectrometry. The originality of this work lies on the combination of several factors that include: two different approaches to gain access to soot nucleation in premixed flames (analysis of a nucleation flame in which NSPs nucleate all along the reaction coordinate without growing in size) and in diffusion flames (separation of the precursors and soot regions); a sampling procedure that allows a rough separation of the condensable gas phase from soot particles; a large database of collected samples and high mass resolution that enable the use of an improved data reduction protocol based on hierarchical cluster analysis and principal component analysis for the data mining and interpretation.

Revealing the molecular structure of soot precursors

The earliest stages of soot formation in flames are believed to involve the formation of small, nanoscale clusters of polycyclic aromatic hydrocarbon molecules. The structure of these clusters is still highly uncertain, however, impeding the construction of quantitative models of soot inception and growth. To provide insight into the structure of incipient soot, we produced nanoclusters of hydrocarbon molecules by annealing coronene films deposited on Pt(111), and examined them with scanning tunneling microcopy. We find that clusters containing ∼20–100 molecules, are disordered agglomerations of stacks that are ∼5–6 molecules tall. These structures are quite distinct from crystalline coronene, but bear a striking resemblance to recently proposed models for the equilibrium structure of similarly-sized clusters that are assumed to initiate soot formation. In contrast to mature soot, the surfaces of these clusters contain very few molecules with graphitic planes oriented parallel to the surface.

Outlier analysis for a silicon nanoparticle population balance model

We assess the impact of individual experimental observations on a multivariate population balance model for the formation of silicon nanoparticles from the thermal decomposition of silane by means of basic regression influence diagnostics. The nanoparticle model is closely related to one which has been used to simulate soot formation in flames and includes morphological and compositional details which allow representation of primary particles within aggregates, and of coagulation, surface growth, and sintering processes. Predicted particle size distributions are optimised against 19 experiments across ranges of initial temperature, pressure, residence time, and initial silane mass fraction. The influence of each experimental observation on the model parameter estimates is then quantified using the Cook distance and DFBETA measures. Seven model parameters are included in the analysis, with five Arrhenius pre-exponential factors in the gas-phase kinetic rate expressions, and two kinetic rate constants in the population balance model. The analysis highlights certain experimental conditions and kinetic parameters which warrant closer inspection due to large influence, thus providing clues as to which aspects of the model require improvement. We find the insights provided can be useful for future model development and planning of experiments.

Experimental characterization of the different nitrogen dilution effects on soot formation in ethylene diffusion flames

This paper addresses the discrimination of dilution and thermal effects on soot production when nitrogen is added to the fuel stream of a steady laminar ethylene diffusion flame. In this context, the mixture-strength flame height Hf and visible flame height Hv are unambiguously documented and exhibit opposite trends within experimental diluted range (N2 volume fraction from 0 to 0.56). Simultaneous soot volume fraction and temperature fields are mapped for different N2 volume fractions by the Modulated Absorption/Emission (MAE) technique. Two characteristic flame heights and the temperature at the peak soot volume fraction are shown to be crucial parameters to assess the extent of the dilution and thermal effects together with their impact on soot formation in the flame. As a result, the mixture-strength flame height is proposed to characterize the soot formation rate within soot growth region, while the visible flame height is more appropriate to describe the overall sooting propensity in the flame. The probed soot temperature is recommended as the characteristic temperature to assess N2 dilution and thermal effects since the thermal effect is overestimated when using the adiabatic temperature.

Structural effects of biodiesel on soot formation in a laminar coflow diffusion flame

Structural effects of biodiesel in terms of unsaturation, i.e. C=C double bonds, its position and the presence of the ester moiety on soot formation are studied in this paper. Laminar coflow diffusion flames of 5-decene, 1-decene, n-decane, and a biodiesel surrogate consisting of a 50%/50% molar blend of n-decane and methyl-octanoate are selected for the investigation. It is observed that the presence of unsaturation promotes soot formation in the flames by increasing soot chemical surface growth species and soot inception species (aromatics) as it is evident from the larger primary particle size and number density of soot particles in the 1-decene, and 5-decene flames compared to the n-decane flame, respectively. More centrally located C=C double bond further speeds up soot inception as it promotes the formation of aromatic species. This is also evident from earlier detection of soot on the centerline and higher soot primary particle number density of the 5-decene flame compared to 1-decene and n-decane flames measured by the laser extinction and thermophoretic sampling methods, respectively. Effects of the ester moiety on soot formation are more on the concentrations of soot chemical surface growth species because the biodiesel surrogate flame has similar number density of primary particles compared to the n-decane flame but soot primary particles have smaller diameters.

Design of LPG burner for hot air puffing machine

In puffing machines the hot air is produced either by burning liquefied petroleum gas (LPG) or electric heaters to develop puffed ready-to-eat (RTE) product. In puffing machine the product is puffed in LPG flue gas mixed hot air based on the whirling bed principle. Therefore, to produce hot air by burning LPG, three non-premixed diffusion type burner configurations were designed to produce a stable blue flame with minimum soot length. In Type I burner of 20 cm length two concentric galvanized iron pipes-inner gas pipe of 2.7 cm (OD) and outer air pipe of 5 cm (OD) were used. The gas was introduced in the inner pipe of burner through copper pipe of 1.3 cm (OD) from outside. It works and produces the flame with an obstruction plate of diameter equal to inner burner pipe which was positioned in front of inner pipe for stability of flame. In experimental tests it was observed that flame do not catches when blower was started and forms a single jet unstable luminous (soot) flame at high air velocity. In Type II burner two steam pipes of diameter 4.7 and 2.7 cm (OD) were used for fabrication. In order to protect the flame from high velocity air, a truncated conical metal (cast iron) shield of 4.7 cm diameter was welded to the rim of air pipe. The Type II burner produced characteristic long blue flame and less soot length but there was soot formation in flame at relatively low air flow rates. To overcome the problem of flame instability and soot formation a third burner configuration was used. Three concentric steam pipes were used for Type III burner. It was observed that the secondary air from central pipe results in proper combustion, complete blue flame formation at the burner tip and better flame stability under variable air flow rates. In Type II and Type III burner, the flue gases of temperature ranging from 90 o – 300 o can be produced at gas flow rates from 7 – 22 lpm.