# Experimental study of CO2 diluted oxycombustion for a decarbonized industry

Laura Pirateque Henao, Bertrand Lecordier, Corine Lacour, Armelle Cessou, David Honoré

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Laura Pirateque Henao, Bertrand Lecordier, Corine Lacour, Armelle Cessou, David Honoré. Experimental study of CO2 diluted oxycombustion for a decarbonized industry. European Combustion Meeting 2025, Apr 2025, Edimbourg, United Kingdom. 2025. hal-05070729

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Distributed under a Creative Commons Attribution 4.0 International License
Experimental study of CO₂ diluted oxycombustion for
a decarbonized industry

Laura PIRATEQUE HENAO<sup>a</sup>, Bertrand LECORDIER<sup>a</sup>, Corine LACOUR<sup>a</sup>, Armelle CESSOU<sup>a</sup>, David HONORE<sup>a*</sup>

<sup>a</sup>INSA Rouen Normandie, Univ Rouen Normandie, CNRS, Normandie Univ, CORIA UMR 6614, F-76000 Rouen, France

**Abstract**

In recent years, many environmental policies have been implemented aiming to a net zero emission energy cycle. However, given the impossibility of completely eliminating CO₂ emissions from each industrial sector, methods of CO₂ capture and sequestration from the flue gases (CCUS) have been proposed. One of these methods is oxycombustion, which removes the N₂ in the oxidizer, allowing us to obtain flue gases without N₂, which will facilitate CO₂ capture. Without the ballast of N₂, oxyflame temperatures are higher than conventional combustion. Unfortunately, this is not suitable for many current industrial processes. Therefore, to increase the applicability of this CCS method, a Recirculating Flue Gas (RFG) is implemented, inducing CO₂ diluted oxycombustion operating conditions.

This paper presents the experimental study of methane/oxygen flames under different CO₂ dilution rates. This dilution is made both in the fuel and in the oxidizer. The (CH₄ – CO₂) mixture is homologous to a biogas, which is composed primarily of these two species. The O₂ is diluted by a range of CO₂ volumetric rates to study the effects of RFG in the flame. Long jet flames are obtained with a coaxial burner and are studied in a laboratory-scale combustion chamber that isolates the oxyflame from the ambient air. The stability limits for each dilution case are determined through measurements of the flue gas composition, particularly CO and CH₄. The effects of different dilution rates are also observed through flame visualization and mean tomographic OH* chemiluminescence imaging. Additionally, the temperature profiles throughout the chamber are presented.

*Keywords:* Oxyfuel combustion; carbon capture and sequestration; Recirculating Flue Gas; OH* Chemiluminescence

<footnote>*Corresponding author</footnote>
# 1. Introduction

In order to achieve the levels of decarbonization proposed by several international projects, such as Net Zero [1], different strategies are being developed and implemented in all economic sectors. Particularly, in the industrial sector there are some hard-to-abate industries, where it is impossible to employ some of the most used decarbonization strategies like electrification. This is where carbon capture, utilization and storage (CCUS) techniques take an important role to reduce the CO₂ emitted to the atmosphere. This strategy proposes to capture the CO₂ produced by industrial processes directly from the flue gas. Once captured, several options exist for the disposition of this greenhouse gas. One option is permanent underground storage in geological reservoirs or through carbon mineralization by injection into reactive rocks [2]. Other option is the utilisation of the captured carbon dioxide as a feedstock for industrial processes, for instance, the production of alternative fuels through water electrolysis, such as SAFs (Synthetic Aeronautic Fuels) [3] to further reduce the global carbon footprint.

One of the three methods for carbon capture is oxycombustion, which removes the N₂ in the oxidizer, allowing us to obtain flue gases without N₂, which will facilitate CO₂ capture. Without the ballast of N₂, oxyflame temperatures are higher than conventional combustion. Unfortunately, this is not suitable for revamping many current industrial processes. Therefore, to increase the applicability of this CCUS method, a Recycled Flue Gas (RFG) is implemented, inducing CO₂ diluted oxycombustion operating conditions.

This study focuses in long flames which are widely used in industrial facilities such as furnaces or boilers. These CH₄/O₂ flames are experimentally studied for a series of CO₂ dilutions to emulate the utilization of a biogas or RFG conditions. Biogas composition differs according to its production method, it can be composed of 45% to 65% methane and 35% to 55% carbon dioxide [4]. During this work, the experimental analysis is done for flames from pure methane to a mixture of 50% CH₄ and 50% CO₂. Similarly, the oxidizer used is a mixture of O₂ and CO₂. This approach is intended to provide a better understanding of how the oxy-flame behaves under different ratios of recycled flue gas and its effect on flame stability.

# 2. Burner configuration and operating conditions

To obtain long turbulent flames a coaxial burner is used, with the fuel injection in the inner tube and the oxidizer in the external one. The amount of CO₂ present in the fuel and the oxidizer are defined in volume by:

$$ \alpha = \frac{Q_{CO_{2,o}}}{Q_{CO_{2,o}} + Q_{O_2}}, \qquad (1) $$

$$ \beta = \frac{Q_{CO_{2,f}}}{Q_{CO_{2,f}} + Q_{CH_4}}, \qquad (2) $$

where α and β represent the amount of CO₂ in the oxidizer and fuel respectively.

To improve flame stabilization and allow for a wide range of study cases, pure oxygen is injected between the fuel and oxidizer jets to act as a pilot flame by interacting directly with the methane. The ratio of this primary pure oxygen in the amount of total oxygen, R₀, is defined by:

$$ R_o = \frac{\dot{m}_{O_{2,i}}}{\dot{m}_{O_{2,i}} + \dot{m}_{O_{2,e}}}. \qquad (3) $$

where ḍ represents the mass flow rate, and the subscripts i and e the injection of primary and external oxygen respectively.

The burner configuration is defined by a parametric study of gas velocities, Reynolds numbers and velocities ratios for a constant nominal thermal power, Pₙ = 23 kW and an equivalence ratio, φ = 0.95. The selected burner configuration is represented in Fig. 1. The fuel injection in the central tube has an inner diameter, D₁ = 4 mm. The primary O₂ is injected in the middle annular jet with an external diameter, D₂,e = 6 mm. Both tubes have a wall thickness of 50 µm. The outward annular section, with an external diameter, D₃,e = 21.2 mm, corresponds to the (O₂ + CO₂) blend injection. The plenum chamber and convergent shapes in the burner are designed to ensure homogeneity and flow control at the exit of the burner [5].

<figure><img src="image_2.png" /><figcaption>Fig. 1: Cross-section of coaxial burner configuration.</figcaption></figure>
Fig. 2 shows the aerodynamic injection parameters of the three coaxial jets in function of the dilution rates. Fuel mixtures for different $\beta$ values are presented in dashed lines and both, primary oxygen and main oxidizer, with continuous lines. Plot (a) shows the volumetric flow rates for each mixture, for constant $P_N = 23$ kW, $\phi = 0.95$ and $R_o = 10\%$ and under different $CO_2$ ratios. Plot (b) represents the mean axial velocities, $w$, obtained for each jet cross section. The highest velocities are those of the fuel mixtures due to smaller injector dimensions. The increment in mixture volume, as seen in plot (a), has a bigger impact from $\alpha$ variations than from $\beta$ variations. Conversely, the gas velocity increment is wider for $\beta$ than for $\alpha$, as shown in plot (b). Plot (c) represents the mixtures Reynolds number, which is higher than 5000 for the fuel and oxidizer mixtures. This value was established to ensure a turbulent state of the flame. The hydraulic diameter used for the $Re$ calculations is $D_H = D_1$ for the inner tube and $D_H = D_e - D_i$ for the annular gas injections.

<figure><img src="image_3.png" /><figcaption>Fig. 2: Flow rates, axial velocities and Reynolds numbers for different dilution rates in fuel and oxidizer.</figcaption></figure>

### 3. Experimental setup

The flame studies are carried out in a laboratory-scale combustion chamber that isolates the oxyflame

from the ambient air, thus allowing the analysis of the flame under conditions closer to industrial furnaces. The cylindrical chamber, represented in Fig. 3, is 1200 mm in height and 400 mm in diameter. Thermococuments are placed throughout chamber height to obtain a longitudinal thermal profile. In the lower section of the chamber there are quartz optical windows at different angles that enables the utilization of several laser diagnostics techniques and flame imaging. The mean dry flue gas concentrations are measured for the species $CO$, $NO_x$, $CO_2$, $CH_4$ and $O_2$ with a Siemens analysis unit composed by Ultramat 23 and Oxymat 61 analyzers. The gas sample is obtained with a heated probe mounted at the chamber exhaust and carried to the analyzer by a traced line heated at 120° C to prevent vapor condensation.

The chamber is constantly cooled down with water flowing between the interior and exterior metal walls. The cooling water inlet and outlet temperature can be monitored, as well as its flowrate, to estimate the thermal balance of the combustion process. The inner chamber walls are insulated with refractive wool to ensure a high temperature environment.

$OH^*$ chemiluminescence imaging is collected with a PI-MAX 4 emICCD camera (1024 × 1024 pixels – 16 bits) equipped with f/2.8, 105 mm UV lens and a UG11 filter. For each study case, 1000 frames with an exposure time of 20 µs are collected to obtain a statistically detailed mean flame image. The latter are then treated with an Abel's Transformation to extract a mean tomographic of flame chemiluminescence image along the burner central plane.

### 4. Results and discussion

To characterize the flames obtained under different $CO_2$ dilution rates a parametric study was done. For the oxidizer, $\alpha$ was varied from 25% until the limit of stability. Measurements are taken with a step of 10% from 30% to 60%. After this point the step was refined to 1% to precisely find the $\alpha$ value of flame instabilities occurrence. This process was repeated for each $\beta$ value from 0 to 50% with a step of 10%. Thanks to the pilot flame between the coaxial jets, neither effects of lift-off nor blow-off were observed. Thus, to determine whether the flame is stable and the combustion is complete or not, a limit of 400 ppm is considered on the CO emissions.

Fig. 4 shows the turbulent long flames obtained with the coaxial burner for different $CO_2$ dilutions. The $\alpha$ dilution is increased horizontally and the $\beta$ dilution vertically. For all the flames, the camera parameters remain constant. However, the intensity and “color” of the flame vary with the amount on carbon dioxide. The flames at lower dilution rates present colors closer to orange and yellow due to elevated soot production. Indeed, the presence of carbon dioxide acts as an inhibitor of soot formation, caused by chemical and thermal effects of this inert molecule in the flame reactions [6] [7].
<figure><img src="image_4.png" /><figcaption>Fig. 3: Cross-section of combustion chamber and positions of optical quartz windows, thermocouples, fluxmeters and flue gas sampling point.</figcaption></figure>

The photographs on the last column show the unstable case of each β condition. Where it is possible to observe local extinctions of the flame represented by large instantaneous discontinuity in the flame emission.

For each case, several techniques were used to study the flame behavior, such as, flue gas pollutants concentration measurements, $OH^*$ chemiluminescence imaging and longitudinal chamber temperature measurement. This section presents the results obtained in this study. It should be noted that only a few illustrative study cases are going to be included in order to represent the tendencies that were observed during the entire experimental campaign.

## 4.1. Flue gas measurements

Table 1 presents the averaged flue gas composition of three β study cases, 0, 20% and 50%. For each case, the species concentration is presented for a low and high dilution case. The last column corresponds to the α case that produced an unstable flame, and the previous column to the last stable case.

Throughout every β case study, while varying α, the CO concentration is almost constant for a large range of dilution rates. However, near the unstable flame, there is a sudden increase in CO concentration while varying α by only 1%, as can be observed for α = 64%, 62% and 54% for each β set. For this

points the amount of CO is however under the chosen threshold of 400 ppm. A further increase of 1% leads to a drastic increment on CO to over 1000 ppm. For this case, the pollutant measurements present an oscillatory behavior without reaching a constant value even after a long period of time. During this cases the pressure inside the chamber also presents important variations of 1 to 2 mmH₂O. The increase in ppm of CH₄, for the limit case, is also a signal of an unstable and incomplete combustion.

The NOₓ concentrations for each flame are indicated in Table 1, for all the cases they can be considered as ultra-low emissions. This minor value corresponds to infiltration of ambient air due to low pressure inside the chamber, particularly for the low diluted flames, where a lower volume of CO₂ is injected. This minimal air flow combined with the high flame temperature, characteristic of low diluted oxy-flames, can favor the production of thermal NO, which is always low.

Fig. 5, represents the total amount of CO₂ present in the reactive mixtures, corresponding to both fuel and oxidizer dilutions. For each β set, the maximum oxidizer dilution is represented with a cross, the flame is stable for all the other cases at lower α values. The stability limits are found within a small range of total CO₂, between 53.5% and 55.7%. This zone could represent the flammability limit for confined methane oxy-flames diluted with carbon dioxide. However the experiments conducted in this study were made at φ = 0.95, which can impact the CO emissions and thus the stability limit. This was verified for the case of β = 0 where, for the unstable flame at α = 65%, the CO emissions decreased from an average of 1800 ppm to 70 ppm while reducing φ to 0.92.

Despite the total CO₂ values being near to each other, lower values are observed for the cases of high fuel CO₂ content of β = 40% and 50%. For these two cases, as shown previously on Fig. 2, the axial velocity is significantly higher, impacting the mixing mechanisms between of the coaxial jets. This effect will be better analyzed during the future high speed PIV and PLIF test campaign.

## 4.2. $OH^*$ Chemiluminescence imaging

Fig. 7 shows the Abel's inversion of the mean $OH^*$ chemiluminescence images. Each row presents a β study case, for a low, intermediate and high oxidizer dilution. The last column correspond to the unstable flame of each β series. Through the different cases it is noticeable that the intensity of the heat release zone decreases with the increment of CO₂ content in the fuel or in the oxidizer, although with some different effects between both of them.

For the oxidizer dilution, if we observe the β = 0 line, the reaction zone is formed by a very intense region of high heat release near the burner, corresponding to the pilot flame, followed by a second
<figure><img src="image_5.png" /><figcaption>Fig. 4: Flame photographs for different α and β dilutions. The green rectangle denotes the zone of the *OH*\* chemiluminescence imaging zone.</figcaption></figure>
<table><caption>Table 1: Dry flue gas measured composition</caption><thead><tr><th rowspan="2">β</th><th colspan="4">0%</th><th colspan="4">20%</th><th colspan="3">50%</th></tr><tr><th>25%</th><th>60%</th><th>64%</th><th>65%</th><th>25%</th><th>60%</th><th>62%</th><th>63%</th><th>25%</th><th>54%</th><th>55%</th></tr></thead><tbody><tr><td>α</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>O<sub>2</sub> [% vol]</td><td>9</td><td>3</td><td>3</td><td>3</td><td>7</td><td>3</td><td>3</td><td>3</td><td>6</td><td>4</td><td>4</td></tr><tr><td>CO<sub>2</sub> [% vol]</td><td>89</td><td>95</td><td>95</td><td>95</td><td>90</td><td>95</td><td>95</td><td>94</td><td>92</td><td>94</td><td>94</td></tr><tr><td>CO [ppm]</td><td>14</td><td>13</td><td>90</td><td>1800</td><td>13</td><td>14</td><td>238</td><td>1980</td><td>6</td><td>199</td><td>819</td></tr><tr><td>CH<sub>4</sub> [ppm]</td><td>2</td><td>2</td><td>7</td><td>385</td><td>0</td><td>0</td><td>20</td><td>462</td><td>0</td><td>25</td><td>162</td></tr><tr><td>NO<sub>x</sub> [ppm]</td><td>15</td><td>7</td><td>7</td><td>8</td><td>9</td><td>7</td><td>7</td><td>7</td><td>4</td><td>4</td><td>5</td></tr></tbody></table>

<figure><img src="image_6.png" /><figcaption>Fig. 5: Total theoretical CO₂ mixture composition for different values of α and β and flame stability limits (black crosses).</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 6: Zoom section of Abel's transformed for β = 0 and α = 65%.</figcaption></figure>

one of lower intensity, corresponding to the main diffusion flame. Both regions shorten with the increment on CO₂. In the further region, the substantial amount of carbon dioxide limits the interaction between methane and oxygen. For α > 60%, the heat release above 40 mm from the burner, is not strong enough to produce sufficient OH* emissions to be captured by the camera. In the region close to the burner an intense region of approximately 10 mm high remains visible for all cases, as seen in the zoom image of the unstable flame for β = 0 in Fig. 6, which corresponds to the pilot flame.

For the increment in CO₂ content on the fuel, when comparing the same α value, the further heat release region conserves its height, but the intensity of the zone closer to the burner decreases. This is because, the additional CO₂ is now creating an inert dilution between the methane and the pure oxygen. Furthermore, an increase in the β ratio results in a decrease in the distance from the central axis to each border of the flame. This decrease results in a reduction of the radial development of the flame in comparison to the cases of low CO₂ content on the fuel. This can be explained by the substantial increase in the fuel velocity as shown in Fig. 2. This variation on the inner jet velocity and momentum has an influence on the mixing layer characteristics of the coaxial jets [8].

### 4.3. Temperature profile in the chamber

The temperature along the longitudinal axis of the chamber was measured with thermocouples inserted in the chamber by 2.5 mm from the wall. The last thermocouple is placed in the exhaust of the chamber to measure the flue gas temperature as shown in Fig. 3. Fig. 8(a) represents the evolution of temperature through the chamber in function of α for β = 0, and Fig. 8(b) in function of β for α = 25%. The tendencies observed in these two limit cases are maintained for all the intermediate ones.

For all cases, the minimum temperature is found in the point closest to the burner between 450° C and 500° C, and it increases with the flame as the combustion gases get hotter until arriving to a maximum value at Z = 750 mm, and decreasing again until the exhaust convergent of the chamber.

When the variation of temperature is discussed in relation to the CO₂ amount, either in the fuel or the oxidizer, there are two opposite tendencies that can be noted. The first is present in the zone where the temperatures are globally higher, corresponding to the middle section of the chamber, for 250 < Z < 1000 mm. In this section the temperature decreases while the CO₂ content increases. This evolution of the temperature distribution can be correlated with the changes in flame structure observed in the OH* chemiluminescence images in Fig. 7, since the inten-
<figure><img src="image_8.png" /><figcaption>Fig. 7: Mean *OH*\* chemiluminescence images for flames under different *β* and *α* rates.</figcaption></figure>
sity of heat release decreases with increasing CO₂ concentration.

The second tendency is in the zone of lower temperatures, specifically for the first thermocouple positioned at Z < 60 mm. In this region, contrary to the previous one, the gas temperature increases when there is more CO₂ injected in the chamber.

However, it should be noted that all of the CO₂ dilution cases have similar temperature profiles. This behavior may favor the application of CO₂ diluted oxycombustion in industrial furnaces since it allows working at different RFG conditions and obtaining a consistent temperature profile.

<figure><img src="image_9.png" /><figcaption>Fig. 8: Temperature profile inside the chamber in function of α and β.</figcaption></figure>

## 5. Conclusion

Carbon capture and sequestration arises as an important strategy to reduce the CO₂ emissions to the atmosphere, especially on the hard-to-abate industries. Oxycombustion favor the generation of flue gas with a high concentration of CO₂, thereby enabling the capture of the greenhouse gas. For revamping of current industrial process, CO₂ diluted oxyflames need to be studied to improve the understanding of the RFG effects. Throughout this work, oxyflames were examined through a variety of experimental techniques under an enclosed environment. The following conclusions can be drawn from this study:

* The presence of CO₂ in the reactive mixture has been shown to decrease soot production.

* The CO emissions can be significant for high CO₂ diluted flames.

* The NOₓ emissions measured are minimal and correspond to small air infiltration in the chamber.

* The increase of carbon dioxide on the oxidizer reduces the heat release intensity of the main diffusion flame, while the increase of CO₂ in the fuel decreases the intensity of the pilot flame. This last variation also tightens the angle of the flame development.

* The temperature profile tendencies are consistent across different dilution cases, both for the fuel as well as the oxidizer. A low temperature is observed near the burner, which increases to a maximum temperature in Z = 750 mm and then decreases until the chamber exhaust.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

This work was supported by a French government grant managed by the Agence Nationale de la Recherche under the OXY3C project of the France 2030-PEPR SPLEEN program, reference ANR-22-PESP-0009.

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