Numerical Simulation and Performance Evaluation of Hydrogen-Enriched Natural Gas for an Industrial Burner in a Testing Chamber

Abstract

A two-step numerical concept was developed for modelling combustion and predicting nitrogen oxide emissions. The model was validated by the Sandia flame D experiment and with measurement data from burners on industrial furnaces. In this paper, the developed model was implemented to evaluate the influence of hydrogen blending with natural gas up to 40 vol.% on an industrial burner with oxidizer temperatures at 300 K and 813 K to assess the performance of the burner without altering the power output of the burner. An experimental test facility is under construction, and the feasibility of using this industrial burner on the test facility with different fuel mixtures was analyzed. Temperature, flow field, and emission characteristics were investigated. Using 40 vol.% hydrogen with natural gas resulted in a decrease of 14.82% in CO₂ emissions and an increase of in 16.1% NO emissions when combusted with air at 300 K. The temperature profile indicated that the burner produces a symmetrical flame profile with preheated air and an asymmetrical flame profile with ambient air.

1. Introduction

The industrial sector is pivotal in global energy consumption and greenhouse gas emissions. As industries strive to reduce their carbon footprint and transition to cleaner energy sources, hydrogen has emerged as a promising fuel for industrial furnaces [1]. Its ability to significantly lower emissions when compared to conventional fuels makes hydrogen an attractive option for decarbonizing industrial processes. Natural gas is considered a valuable energy resource for manufacturing industries due to its abundance. However, it contributes to the emission carbon dioxide, which is a major source of climate change. The rate of fossil fuel consumption and the destructive effect of greenhouse gas emissions on the environment has prompted researchers to develop alternative fuels. Hydrogen provides a pathway to decarbonization by producing only water vapour as a combustion byproduct, thereby reducing the release of CO₂ and CO in the atmosphere [2,3,4].

Blending hydrogen with natural gas has emerged as a transitional approach towards reducing carbon emissions in existing natural gas infrastructure, asusing pure hydrogen in natural gas pipelines without appropriate modifications can cause safety problems [5,6]. Studies have shown that up to 17 vol.% of hydrogen can be introduced in the natural gas pipelines without difficulties [7]. By introducing a certain percentage of hydrogen into the natural gas supply, the resulting blend can help decrease the carbon intensity of the gas used for heating, power generation, and industrial processes. This blended gas, commonly called “Hydrogen-Enriched Natural Gas (HENG)”, retains the advantages of natural gas infrastructure while offering environmental benefits. Gradual increases in hydrogen blending ratios can help pave the way towards a smoother transition to a hydrogen-based energy system, facilitating the decarbonization efforts required to combat climate change and achieve a more sustainable future.

The selection of appropriate burner technology is critical when considering the utilization of hydrogen in industrial furnaces. While certain burner types, such as atmospheric burners [8], can be modified to accommodate hydrogen-enriched natural gas with suitable adjustments, others, such as premix burners, may require more extensive modifications in order to achieve stabilised flames and prevent flashback at high volumetric flow rates [9]. Hydrogen has a wide flammability range and a high flame speed, making it highly combustible. It has lower ignition energy than natural gas, enabling faster combustion. Natural gas, on the other hand, has a narrower flammability range and a lower flame speed [10]. The compatibility of burner technologies with hydrogen-rich fuel blends should be carefully evaluated while considering factors such as flame stability, pollutant emissions, and safety considerations. By ensuring the compatibility of burners, industries can effectively harness the potential of hydrogen in their furnace operations while optimizing performance and minimizing any associated challenges.

Computational Fluid Dynamics (CFD) is a valuable tool in studying and analyzing hydrogen blending with natural gas. It allows researchers to understand the flow patterns, turbulence characteristics, emissions, and mixing dynamics of hydrogen and natural gas blends. Mayrhofer et al. [11] employed a flamelet-based combustion model to investigate the combustion of pure hydrogen with preheated air. Pashchenko [12] conducted numerical simulations to study hydrogen-rich fuel combustion, analyzing NO_x emissions across various fuel mixtures. Rahman et al. [13] explored the blending of hydrogen with natural gas using a swirl burner and found that introducing 10 vol.% of hydrogen led to increased thermal NO_x emissions. Galletti et al. [14] performed computational investigations involving a mixture of 60 vol.% hydrogen and methane. Hua et al. [15] engaged in numerical simulations of premixed hydrogen–air mixtures within a microchamber and examined the impact of chamber size and wall heat conductivity on combustion behaviour. Xue et al. [16] developed a numerical model for analyzing underground gas storage and assessed the stability of the reservoir model. Guo et al. [17] utilized Large Eddy Simulation (LES) to comprehend flame stabilization characteristics related to hydrogen enrichment in a premixed flame.

The present paper investigates the feasibility of using hydrogen-enriched natural gas in an industrial burner using numerical models. A burner testing facility is being constructed at the Chair of Thermal Processing Technology, Montanuniversität Leoben; a numerical model was developed to analyse the combustion and emission characteristics of industrial burners. The developed numerical model was then used to investigate the industrial burner mounted on the testing facility. These preliminary investigations with numerical models helped to analyse the performance of the burner as well as of the test facility, which is currently under construction. The model was validated by experimental data [18] and data on industrial furnaces [19]. Hence, the developed model was applied to investigate the burner with a fuel mixture consisting of 0-40 vol.% of hydrogen and with two different air temperatures, i.e., 300 K and 813 K. The temperature, flow parameters, and emission characteristics were then compared and analysed.

2. Materials and Methods

This section explains the numerical model, the properties of the burner, and the testing facility used for modelling. The numerical model was developed in OpenFOAM 2.4.x [20] and validated with experimental data from [18]. The flamelet model was unable to predict the NO_x emissions accurately. Ihme and Pitsch [21] and Cutrone et al. [22] overpredicted NO_x emissions with an unsteady flamelet model. Due to the slow chemical reactions of the nitrogen oxide formation path, the flamelet model does not predict NO_x emissions precisely. Implementing a detailed chemistry approach such as the Eddy Dissipation concept requires extensive computational effort to solve the full chemistry with detailed reaction mechanisms in order to predict NO_x emissions. Hence, a two-step model was developed which uses a computationally cheap flamelet model to predict the flame parameters such as temperature and flow field, which are then transferred to the developed postprocessor containing a detailed chemistry mechanism, which solves only the species transport equation to predict nitrogen oxide emissions. Because the model has been well described in previous publications [19,23,24,25,26,27], only a brief overview is presented here.

2.1. Model Description

The steady-state flamelet model developed by Peters [23] and implemented for OpenFOAM by Cuoci et al. [24,25] was used to determine the flow parameters such as velocity and pressure and the combustion parameters such as temperature, mixture fraction, and enthalpy. The flamelet model is based on the assumption that a turbulent flame can be represented as a collection of laminar flamelets, with each having its own local characteristics. The flamelet model assumes that the local mixture composition predominantly determines the local flame structure and is not significantly influenced by turbulent fluctuations. This assumption allows the flamelet model to separate the chemical and turbulent effects, thereby simplifying the modelling of turbulent combustion. The flame structure is decomposed into the progress variable and the mixture fraction. The progress variable represents the reaction progress of the flame, indicating how much combustion has occurred, while the mixture fraction represents the local composition of the mixture between fuel and oxidizer. These variables define a two-dimensional manifold called the “flamelet surface” in the progress variable–mixture fraction space. The transport equations for the progress variable and the mixture fraction are solved along with the governing equations for mass, momentum, and energy conservation. The model solves the basic continuity and momentum equations using the RANS approach, and the turbulence is solved using the k–$ϵ$ turbulence model.

The progress variable equation describes the flame propagation, and is solved using a flamelet library that contains precomputed laminar flame solutions for a range of mixture compositions. The laminar flamelet profiles are generated for various scalar dissipation rates and enthalpy defects until the flame is extinguished using the libOpenSmoke [24,25] utility. The libOpenSmoke utility creates flamelet profiles considering the species composition and the temperature of the fuel and oxidizer. By combining the solutions of the progress variable and mixture fraction equations, the flamelet model provides information on the local flame structure, including the temperature, species concentrations, and heat release rate.

The flamelet profiles can be extended to describe the behaviour of the turbulent flame front. This is achieved by applying a statistical method called probability density functions (PDF). The PDF represents the probability distribution of a particular physical quantity, such as the mixture fraction Z, the scalar dissipation rate ${\chi }_{st}$, and the enthalpy defect ${\varphi }_H$. The tabulated flamelet profiles help solve only two additional Favre-averaged transport equations, one for mixture fraction Z and another for mixture fraction variance $Z{}^{\prime \prime }$ [23].

An in-house postprocessor was developed at the Chair of Thermal Processing Technology [19,26,27]. The NO_x postprocessor used the data from the flamelet model, in which all the species involved in the NO formation path were disregarded and only the transient species transport equation was solved with a detailed GRI 3.0 mechanism.

$$\frac{\partial \left(\rho Y_i\right)}{\partial t}+\nabla ·\left(\rho u{Y}_i\right)-\nabla ·{\mu }_{eff}(\nabla Y_i)=RR\left(Y_i\right)$$

(1)

The postprocessor was validated as shown in Figure 1 with experimental data from Barlow and Frank [18] and measurement data from an industrial furnace [19]. The model successfully estimated NO_x emissions at 63 ppm, closely aligning with the measured data of 65 ppm. For more information on the validation of the numerical model, the reader is referred to [19,26,27].

2.2. Burner and Testing Chamber

A state-of-the-art burner testing facility is currently under construction at the Chair of Thermal Processing Technology. This facility is specifically designed to conduct comprehensive investigations on various types of burners using different fuel mixtures, including natural gas. At stoichiometric conditions, the test facility can handle a maximum power output capacity of up to 1 MW for natural gas–air burners. The facility’s dimensions have been carefully determined to optimize experimental capabilities. The testing chamber features a box geometry of 1.7 × 2.25 × 4.7 m, as illustrated in Figure 2. These dimensions have been calculated using advanced simulation methods, ensuring an optimal test environment. To facilitate detailed measurements and observations during experiments, the testing chamber is equipped with 23 strategically positioned measuring ports. These ports are distributed throughout the chamber, with two ports located on the door and seven ports on each of the floor, ceiling, and sidewall. These ports are equipped with quartz viewing glass, allowing for visual monitoring of flame behaviour and other critical parameters. To ensure accurate and reliable experimental results, the testing chamber is extensively insulated with 250 mm thick ceramic fibres on the walls. This robust insulation provides exceptional thermal stability and minimizes heat losses, enabling the maintenance of an operating temperature of up to 1500 °C. For a comprehensive understanding of the testing facility, interested readers should refer to the detailed description provided by Spijker et al. [28,29].

2.3. Model Setup

In this numerical study, the focus lies on the analysis of blending hydrogen with natural gas in a burner which was originally designed for natural gas combustion. As depicted in Figure 3, the burner under consideration is mounted on the testing facility and modelled with a maximum power output of 396 kW. Fuel mixtures consisting of 0%, 10%, 20%, 30%, and 40% hydrogen by volume were considered. Throughout the simulations, the fuel temperature was maintained at a constant 300 K. The oxidizer used was air, and two different inlet temperatures, 300 K and 813 K, were investigated. To ensure consistent power output from the burner, the mass flow rates of the fuel and oxidizer were adjusted as outlined in

Table 1. Mass flows of fuel and air inlets for different fuel mixtures.

H2 vol.-%	0% H2	10% H2	20% H2	30% H2	40% H2
${\displaystyle \frac{{\dot{m}}_{Fuel}}{{\dot{m}}_{Fuel-for-0\%H_2}}}$	1	0.9947	0.9888	0.9816	0.9731
${\displaystyle \frac{{\dot{m}}_{Air}}{{\dot{m}}_{Air-for-0\%H_2}}}$	1	0.9924	0.9707	0.9452	0.9145

. The fuel composition for different fuel mixtures is presented in

Table 2. Fuel compositions of different fuel mixtures (in mole fractions).

H2 vol.-%	0% H2	10% H2	20% H2	30% H2	40% H2
CH4	0.9797	0.88173	0.78376	0.68579	0.58782
C2H6	0.0105	0.00945	0.0084	0.00735	0.0063
H2	0	0.1	0.2	0.3	0.4
CO2	0.0011	0.00099	0.00088	0.00077	0.00066
N2	0.0087	0.00783	0.00696	0.00609	0.00522

. For the simulation with 0 vol.% of hydrogen, the mass flow of fuel was initialised at 0.00733 kg/s with 1.5 vol.% of excess oxygen. The geometry was modelled with Solidworks 2020 modelling software and was meshed with the ANSYS ICEM meshing tool [30]. The computational mesh was carefully chosen to avoid the influence of the number of computational cells. Two meshes with 5.7 million (coarse) and 7.2 million (fine) computational cells were analyzed. The temperature at the exhaust was determined to be 1747 K and 1733 K for coarse and fine meshes, respectively, resulting in a difference of 0.8%. The orthogonal quality of the mesh was checked with ICEM and maintained a value of more than 0.3. The mesh had a very fine refinement near the wall region, and the values of y+ were found to be less than 3. Hence, all the computations were performed with a mesh size of 5.7 million cells. By comprehensively analyzing the performance parameters and emissions characteristics of the burner under various hydrogen blending conditions, in this study we aimed to provide valuable insights into the feasibility and potential benefits of utilizing hydrogen-enriched natural gas in industrial combustion systems.

The main objective of the study was to maintain the same power output for different fuel mixtures to ensure that the hydrogen-enriched natural gas did not influence the combustion products in the industrial furnace, i.e., to maintain similar temperature profiles for different fuel mixtures by altering the mass flow of the fuel and oxidizer in order to analyze the behaviour of CO₂ and NO emissions. In addition, because the test facility is currently under construction, these numerical simulations can provide insights into using a variety of burners in connection with the burning chamber.

3. Results and Discussion

This study investigated the temperature, velocity, and emission characteristics of different hydrogen–natural gas blends for the studied industrial burner. The results obtained from our computational simulations revealed valuable insights into the combustion behaviour and pollutant emissions of different hydrogen blending ratios. The flame stability, temperature profiles, and flow parameters were analyzed under various operating conditions. Furthermore, nitrogen oxide (NO${}_{\mathrm{X}}$) and carbon monoxide (CO) emissions were quantified to assess the environmental impact. The findings from this research provide essential data for optimizing the use of hydrogen–natural gas blends in industrial burners, contributing to the development of cleaner and more sustainable combustion technologies.

In analysing the temperature profiles of gas mixtures with 0 vol.% H₂ and 40 vol.% H₂, i.e., the minimum and maximum hydrogen content, we found that the use of hydrogen increases the maximum observed temperature with only a minimal increase in the length of the temperature iso-contours. As illustrated in Figure 4a, the maximum temperature observed for the natural gas mixture along the axis of the burner was 1853 K. In contrast, the fuel mixture with 40 vol.% hydrogen was able to reach a maximum temperature of 1873 K. When preheated air was used (Figure 4b), the natural gas mixture reached a maximum temperature of 2219 K, whereas 2272 K was observed with a 40 vol.% hydrogen in the mixture. Hence the influence of hydrogen blending with natural gas on the behaviour of temperature profiles is significantly less, as the difference in observed maximum temperatures between natural gas and fuel mixture with 40 vol.% hydrogen was less than 20 K with ambient air and less than 53 K with preheated air. In addition, it should be noted that the theoretical increase in the adiabatic flame temperature was 26 K when comparing the natural gas and fuel mixture with 40 vol.% hydrogen, which is in agreement with the measured simulation data.

Analyzing the iso-contours of temperature on the yz plane shown in Figure 5 and Figure 6, it can be seen that the temperature difference of the flame on the left and right sides is distinct when using ambient air at 300 K. The temperature on the left side of the flame is lower, at approximately 1600 K, while on the right side it reaches more than 1800 K. This behaviour is mainly due to the arrangement of the nozzles and air inlets present in the burner, and indicates that proper mixing of air and fuel in the burning zone did not take place due to insufficient velocity magnitudes. When using preheated air at 813 K, the high-velocity magnitudes result in better mixing and both sides of the flame are able to reach the same temperatures of 2000 K and 2200 K for the natural gas and fuel mixture with 40 vol.% hydrogen.

Investigating the iso-contours of the temperature profiles in Figure 5 shows that increasing the amount of hydrogen in the fuel mixture results in a minimal increase in the length of the iso-contours at 1700 K, 1900 K, and 2000 K. A significant increase in the length of the iso-contour at 1800 K with 40 vol.% hydrogen in the fuel mixture can be observed. Furthermore, a change in curvature is observed for the iso-contour at 1800 K. This can be attributed to the recirculated excess air in the burning chamber. The same can be seen in Figure 4 along the axis of the burner between 1.5 and 2 m, where the decreasing temperature suddenly increases due to air being pushed into the flame from both sides, as seen in Figure 7.

Until 0.5 m along the Z direction, where the combustion is not influenced by recirculation zones, both Figure 4a,b show that the temperature of the flame increases as the concentration of hydrogen in the fuel mixture increases. As soon as the flame starts interacting with the recirculated air, the proportional variation in the temperature to the amount of hydrogen in the fuel mixture is no longer observed. After 0.5 m, the flame’s temperature depends on the amount of excess air in the combustion zone. The higher temperatures for the fuel mixture with 40 vol.% hydrogen (Figure 4b) can be attributed to excess oxygen consumption, as seen in Figure 8b, in which a decrease of 20.26% oxygen at the exhaust is observed compared to natural gas. This indicates that the excess air in the combusting chamber was used up, resulting in higher temperatures in the case of the fuel mixture with 40 vol.% hydrogen (Figure 4b). This behaviour was only observed for the fuel mixture with 40 vol.% hydrogen when combusted with preheated air, and needs to be further investigated for mixtures with more than 40 vol.% hydrogen. This sudden increase in temperature along the axis of the burner was not observed with other fuel mixtures. Furthermore, the maximum decrease in the amount of oxygen at the exhaust was less than 4.27% with ambient air, as seen in Figure 8a.

The velocity vectors for the two air temperatures of 300 K and 813 K, are depicted in Figure 7. It can be observed that despite the consistent mass flow rate of air inlets, the density variation caused by the temperature difference leads to contrasting velocities. Specifically, the air at 813 K exhibits higher velocity, with a magnitude of 136 m/s, whereas the air at 300 K demonstrates a velocity of 92 m/s. The illustration reveals the presence of recirculation zones on both sides of the flame. It should be noted that the length of the test facility is 4.7 m and the geometry converges to a narrow exhaust pipe, which tends to divert the flow back into the testing chamber. From Figure 7a,b, it can be seen that the flow on the left side of the flame is moving backwards and rejoining near the flame region.

As the visual change between species mole fractions is not distinct for different fuel mixtures, only the species mole fractions for the fuel mixture with 40 vol.% hydrogen combusted with air at 300 K is depicted in Figure 9. From Figure 9a,b, methane and hydrogen have high mole fractions at the inlet and upon combustion, while the combusting chamber has significantly less methane and hydrogen. Because the combustion is not stoichiometric and has excess oxidizer inflow, Figure 9c shows the remaining unburnt oxygen in the combustion chamber. Comparing these results with the temperature profile from Figure 5b, the temperature of the flame on the left side is lower than the right side, which is related to the excess unburnt oxygen on the left side of the flame and lesser oxygen on the right side of the flame in Figure 9c. The same behaviour can be interpreted using CO₂ and H₂O in Figure 9d,e, where the mole fraction of both species are higher on the right side of the flame and lower on the left side of the flame.

The different species mole fractions are plotted along the axis of the burner in Figure 10. Figure 10a,b shows the mole fractions of methane and hydrogen. As the fuel inlet is along the axis of the burner, it can clearly be seen that the mole fractions of methane and hydrogen in the inlet at z = −0.8 m matches the data presented in Table 2, and that after combustion there is less methane and hydrogen. As the oxidizer is injected into the side of the burner and not along the axis, there is no oxygen at z = −0.8 m (Figure 10c). There is an increase in oxygen mole fraction until z = 0 m, and after combustion the amount of oxygen is reduced further. The influence of different fuel mixtures on the amount of oxygen is less, as the mass flow of the oxidizer was adjusted to maintain the same power output of the burner and the maximum difference in mole fraction of oxygen measured at the exhaust was only 4.27%, as stated in Figure 8a. The species mole fractions of OH, CO₂ and H₂O clearly show the influence of the amount of hydrogen present in the fuel mixture. From Figure 10d, it can be noted that increasing the amount of hydrogen in the fuel mixture increases the mole fraction of OH along the axis of the burner after z = 0 m. In the case of CO₂, as in Figure 10e, a clear decrease of CO₂ mole fractions can be observed with the increase in the percentage of hydrogen in the fuel mixture throughout the axis of the burner. In addition, comparing the H₂O mole fractions indicates a steady increase of H₂O after z = −0.4 m with the increase in the amount of hydrogen in the fuel.

The species mole fractions at the exhaust of the testing chamber were determined from the flamelet model, and are plotted in Figure 8 to analyse the influence of different fuel mixtures. The increase in hydrogen content in the fuel mixture results in a decrease of CO₂ and an increase in H₂O. In the case of 40 vol.% hydrogen in the fuel mixture, CO₂ decreases by 14.82% and 15.56% for air temperatures of 300 K and 813 K, respectively, whereas H₂O increases by 13.11% and 14.34%, respectively. Even though the mass flow of the oxidizer at the inlet was adjusted for different fuel mixtures, a minimal decrease in oxygen mole fractions was observed. When combusted with air at 300 K, a 4.27% decrease in oxygen was observed for 40 vol.% hydrogen in the fuel mixture compared to natural gas. Due to high-velocity magnitudes for the air temperature at 813 K, the recirculation zones influence the emissions at the exhaust. A steady increase or decrease in the emissions could not be observed with the air temperature at 813 K.

Previous studies have indicated that the flamelet model may overpredict NO emissions [21,22]; therefore, our in-house developed postprocessor was used to estimate the NO emissions. Due to the extensive computational effort required for the model, the NO emissions were determined only for natural gas and the fuel mixture with 40 vol.% hydrogen with air temperatures at 300 K. The reaction rates of NO for natural gas and the fuel mixture containing 40 vol.% hydrogen are shown in Figure 11, where the positive values indicate NO formation zones and the negative values indicate reduction zones. Comparing Figure 11a and Figure 11b, the area of NO formation zones is higher with 40 vol.% of hydrogen. To estimate and detect the percentage of NO emissions increase, the values at the exhaust were integrated from the postprocessor, and it was determined that the NO emission for fuel mixture with 40 vol% hydrogen was 16.1% higher than when using natural gas as a fuel. Wu et al. [31] carried out a similar study on laminar diffusion flames and observed an increase in NO_x emissions when blending hydrogen and methane.

4. Conclusions

The developed numerical model was implemented to investigate the feasibility of using hydrogen-enriched natural gas on an industrial burner mounted on the testing chamber. Fuel mixtures containing 0%, 10%, 20%, 30%, and 40% hydrogen by volume were investigated with oxidizer temperatures of 300 K and 813 K without altering the burner’s power output. The resulting temperature profile significantly differed between the left and right sides of the flame with air at ambient temperature. The burner produced a symmetrical flame profile for combustion when using preheated air. Increasing the amount of hydrogen in the fuel mixture resulted in a maximum temperature increase of 20 K when combusted with air at 300 K and and 53 K when combusted with air at 813 K.

Due to the narrow exhaust and the length of the burning chamber, the recirculation zones in the burning chamber influenced the flame profiles and emissions. This requires further investigation with experimental measurements and analysis of different burner types on the burning chamber. These recirculation zones resulted in longer computation time to achieve convergence. When 40 vol.% hydrogen was added to the fuel mixture, CO₂ mole fractions at the exhaust decreased by 14.82% and 15.56% for air temperatures of 300 K and 813 K, respectively, and NO emissions increased by 16.1% compared to natural gas with the air temperature at 300 K.