Research on the

Mechanism and Propagation Characteristics of Combustion and Explosion of Natural Gas/Ammonia/Hydrogen Mixed Gas Fuel

Authors: Changqi Liu, Zhenyi Liu, Qingming Liu, Qiqi Liu, Chuang Liu, Zihao Xiu, Jun Yang, Zongling He, Zhisong Wang

Abstract

The addition of ammonia and hydrogen into natural gas fuel is an effective method to reduce carbon emissions. This study aims to investigate the effect of adding ammonia and hydrogen on the mechanism of natural gas combustion and emission characteristics. Based on a self-developed mixed gas deflagrate experimental platform, the deflagrate characteristics, emission characteristics, and chemical reaction kinetics mechanism of mixed gas fuels under different composition ratios (natural gas 0–100%, hydrogen 10–85%, and ammonia 0–100%) were studied. The results indicate that the propagation of the deflagration shock wave can be categorized into an initial stage (L < 3 m) and a development stage (L > 3 m) based on the observed trend of shock wave intensity variation with distance. The intensity of the deflagration shock wave for the mixed gases increases monotonically as the hydrogen content ratio rises. In contrast, the impact of the ammonia content ratio on the shock wave intensity exhibits a distinct pattern that varies with changes in the equivalence ratio and hydrogen content ratio. In terms of carbon emissions per unit of heat value produced by the fuel, adding hydrogen to natural gas proves to be more effective at reducing carbon emissions than adding ammonia. When the ammonia content ratio is 50% and the hydrogen content ratio is 40%, the combustion performance of the mixed gas fuel is similar to that of natural gas, but its carbon emissions are lower than 30% of natural gas, making it a new type of mixed fuel with potential application value; the interaction between reflected pressure waves and flames is the main reason for the fluctuation of deflagrate shock wave pressure; ammonia lowers the temperature of the reaction system by reducing the concentration of OH radicals.

Introduction

As traditional fossil fuel reserves dwindle and the greenhouse effect intensifies, using clean and renewable fuels to mitigate carbon emissions has emerged as a focal point of research within the energy and power sector.^1−3^ Ammonia and hydrogen, two gases that exhibit significant potential in energy conversion, are particularly relevant in the context of efforts to reduce greenhouse gas emissions and address climate change. First, both ammonia and hydrogen do not produce carbon dioxide (CO2) or other harmful greenhouse gases during combustion, which stands in stark contrast to conventional fossil fuels such as coal, oil, and natural gas, which release substantial amounts of CO2 and other pollutants, adversely impacting the environment. Second, the high energy density and rapid combustion characteristics of ammonia and hydrogen confer advantages in applications with high energy demands. For instance, hydrogen’s application in fuel cells can power vehicles, while ammonia can be utilized for electricity generation and heating. Ammonia and hydrogen, recognized as carbon-neutral fuels, have garnered extensive attention and research due to their environmental benefits, high energy density, diverse production pathways, and relative ease of storage and transportation. Their potential to serve as sustainable alternatives to traditional energy sources is a subject of significant scholarly interest.^4−6^

Recent research indicates that the combustion characteristics and detonation mechanisms of hydrogen, ammonia, and their mixed gases have garnered widespread attention and in-depth investigation. These research findings have not only provided a theoretical foundation for applying these two fuels but also offered significant references for technological development and safety control in related fields. Initially, Porowski et al.^7^ compared numerical models of premixed hydrogen/air laminar flame speeds to determine the predictive accuracy of each model. This work provides a reference for selecting appropriate numerical models, contributing to enhancing the combustion simulation precision. Bei et al.^8^ focused on the initiation process of supercritical water (H2–O2) reactions, identifying the initial concentration of hydrogen and the initial temperature of the system as critical factors affecting the reaction initiation. This discovery is of great importance for understanding and controlling supercritical water oxidation reactions. Subsequently, Wang et al.^9^ studied the combustion characteristics of ammonia/air-premixed jets through numerical simulation, particularly the optimal conditions for achieving the highest combustion efficiency under low NO emission conditions. This research is significant for reducing nitrogen oxide emissions during ammonia combustion, facilitating the realization of more environmentally friendly combustion technologies. Zhou et al.’s^10^ research focused on establishing a detailed kinetic model for ammonia/syngas combustion, which was validated against literature data. Through kinetic modeling analysis, they elucidated the impact of fuel composition, equivalence ratio, and initial temperature on laminar flame propagation characteristics, providing a theoretical basis for controlling and optimizing combustion processes. Zheng et al.^11^ explored the effects of radiation reabsorption on the laminar flame speed and NO emissions of ammonia/hydrogen/air through numerical simulation. The results of this study aid in a better understanding and prediction of the flame behavior and effective control of NO emissions in practical applications. Another study by Zhang et al.^12^ investigated the influence of initial pressure and temperature on the lower flammability limit of ammonia/hydrogen mixtures through experiments and simulations. The findings of this research have practical guidance value for the safe use and potential risk assessment of mixed gases. Zhang et al.’s^13^ research also explored the detonation characteristics of hydrogen/air mixtures at different initial pressures and the impact of the blast arrestor’s position on flame propagation through experimental and numerical methods. This study revealed essential patterns in how the position of the blast arrestor affects the effectiveness of detonation suppression, guiding the design and layout of blast arrestors in practical applications. In summary, these studies encompass various aspects, including combustion characteristics, detonation mechanisms, kinetic modeling, environmental impacts, and numerical simulation of hydrogen, ammonia, and their mixed gases.^14−17^ These research outcomes not only enrich our understanding of the combustion processes of these fuels but also provide essential theoretical and practical guidance for developing and applying related technologies. Through these studies, we can better harness hydrogen and ammonia as clean energy sources while ensuring their safety and environmental friendliness in use.^18−25^

However, the laminar flame velocity of ammonia combustion is low, the combustibility is low, and the required ignition energy is high,^26^ which limits its direct application due to fuel properties. Due to its inherent flammability and explosiveness, the direct use of hydrogen is limited by safety,^13^ so mixing ammonia and hydrogen into natural gas is also a current research hotspot.^27−29^

Elbaz et al.^30^ studied the flame stability, NO emission, and flame structure of NH3/CH4/air premixed flame and obtained the effects of equivalent ratio, ammonia fuel fraction, and other factors on flame stability, NO emission, and flame structure. An et al.^31^ studied the emission characteristics of CH4/NH3/air cofired premix cyclone flame with the mole fraction of NH3 up to 60% and found that nitrogen composition, residence time, and temperature were significant factors affecting the concentration distribution of NO. Ji et al.^32^ studied the flame structure and blowing characteristics of NH3/CH4 mixed fuel using a swirl combustion chamber. They found that adding methane to ammonia can increase the flame’s tensile strength and improve its stability. Okafor et al.^33,34^ conducted experimental and numerical studies on the laminar combustion velocity of methane/ammonia/air-premixed mixtures under high-pressure and atmospheric conditions. The unstretched laminar combustion velocity and Markstein length of CH4/NH3/air flames under high-pressure and atmospheric conditions were measured, and detailed reaction mechanisms under high-pressure and atmospheric conditions were established based on the mechanisms of GRI Mech 3.0 and Tian et al.^200^ Zhang et al.^35^ conducted a numerical study on the laminar premixed countercurrent flame of NH3/CH4/air premixed fuel at standard temperature and pressure and found that with the increase of CH4 molar fraction in NH3/CH4 blended fuel, the flame extinction elongation increased. The flame extinction limit reached the maximum under the rarefied combustion condition (φ = 0.9). Cai et al.^36^ studied the explosion risk of a hydrogen/methane mixture based on the pipeline. They found that the explosion risk of mixed gas increased monotonically with an increased hydrogen mixing ratio and flow field intensity. Wu et al.^37^ provide a comprehensive overview of combustible gas safety parameters and insight into the explosive hazards of mixed mixtures. Sun et al.^38^ studied the combustion characteristics (such as flame structure, temperature, and matter field) and velocity field of ammonia/methane in industrial DLN gas turbine burners with the ammonia mass fraction up to 70% through numerical simulation and obtained the influence of increasing ammonia content on combustion performance and emission of mixed fuel.

The above research has carried out a detailed study on the combustion characteristics and reaction mechanism of natural gas mixed with ammonia and hydrogen, respectively, providing theoretical support and reference for developing and applying natural gas/ammonia and natural gas/hydrogen blended fuels. According to the above research, hydrogen and ammonia are clean energy sources with their advantages and disadvantages. When these two gases are separately incorporated into natural gas, the combustion properties of the mixed gas change monotonically, which limits the practical application space of the mixed gas. Various combustion parameters of natural gas are between ammonia and hydrogen, so considering adding ammonia and hydrogen to natural gas simultaneously can reduce emissions while ensuring its original fuel properties.^39−42^ In addition, the deflagrate performance of a natural gas/ammonia/hydrogen mixture can also be adjusted to meet different needs.

However, most existing research focuses on aspects such as flame structure, combustion speed, and emissions with relatively little attention given to deflagration characteristics. Clarifying the various deflagration parameters of natural gas, ammonia, and hydrogen mixed fuels is essential for their promotion and application. Current experimental and simulation studies often use methane as a proxy for natural gas, although natural gas comprises not only methane but also ethane, propane, butane, and various impurities. The presence of these components can affect the accuracy of combustion characteristic measurements. To advance the application of natural gas/ammonia/hydrogen blended fuels and more effectively address potential explosion disasters, this study comprehensively investigates the chemical reaction kinetic mechanisms, deflagration characteristics, and carbon emission characteristics of natural gas/ammonia/hydrogen blended gases through a combination of experimental and numerical simulation approaches. Initially, a series of experiments were conducted on a self-developed mixed gas deflagration experimental platform to examine the deflagration characteristics of natural gas/ammonia/hydrogen blended fuels at different ammonia content ratios (0–100%), hydrogen content ratios (10–85%), and fuel concentrations (=0.8–1.2). Subsequently, carbon emissions from the mixed fuels were analyzed by using gas analysis instruments. Finally, the flame propagation characteristics and chemical reaction kinetics of the natural gas/ammonia/hydrogen mixed fuel were discussed, shedding light on the underlying mechanisms of various phenomena observed during deflagration.

Experiments and Methods

The multicomponent deflagration characteristic test platform built is shown in Figure 1. The horizontal gas deflagrate pipeline is equipped with a gas cycle premix system, ignition system, schlieren system, pressure data acquisition system, synchronous trigger system, and product acquisition and analysis system. The total length of the horizontal pipeline is 5.1 m, and the inner diameter is 0.1 m. After the gas is injected according to the designed partial pressure, it is circulated for 3 min with the circulating fan, and then it is left for 2 min. The circulating fan valve is closed, and the ignition is carried out through the synchronous trigger. The ignition system, the high-speed camera of the schlieren system, and the data acquisition instrument of the pressure data acquisition system relate to the synchronous trigger, and the high-speed camera and the pressure data acquisition instrument are simultaneously triggered to collect and record data during the ignition. A gas analyzer was used to analyze the product after ignition. Finally, the pipeline is vacuumed by a vacuum pump, and the following experiment is carried out. A total of 12 pressure sensors are installed in the pipeline, as shown in Figure 1. The diameter of the concave mirror of the schlieren system is 300 mm, and the height of the viewing window is 35 mm. The high-speed photography is shot at 90,000 fps, and the model is FASTCAM SA1.1. The gas collector collects CO and CO2 concentrations with a range of 100,000 ppm. In this experiment, the initial pressure in the pipeline during ignition is 101.325 kPa, the temperature is about 300 K, and the ignition voltage is set to 205 V. The position of the observation window is set in the fourth section of the pipeline, through which shock waves can be captured while also observing the process of accelerated flame development. If the position of the observation window is moved forward, the observed flame propagation process is easily affected by obstacles. If the position of the observation window is moved backward, then the observed flame propagation process is easily affected by the shock wave reflected by the blind plate. The ignition end of the pipeline’s first section is provided with obstacles with a total length of 0.6 m, composed of steel rings connected and fixed by four thin iron rods with a diameter of 3 mm. The distance between the first ring and the ignition position is 0.12 m. The outer diameter of the ring is marked as R1 of 50 mm, equal to the inner diameter of the pipeline, and the inner diameter of the ring is marked as R2 = 22.5 mm. The schematic diagram of obstacles is shown in Figure 2. The formula for calculating the block ratio (BR) is shown in eq 1.1

When studying the combustion and explosion mechanism and characteristics of the natural gas/ammonia/hydrogen mixture, ammonia is added to natural gas (ammonia content ratio is 0–100%). Different proportions of the natural gas/ammonia mixture are taken as a whole, and finally, hydrogen is added to the natural gas/ammonia mixture (hydrogen content ratio is 10–85%). Therefore, the ammonia content ratio is defined by the volume ratio of ammonia/ammonia plus natural gas, and the hydrogen content ratio is defined by the volume ratio of hydrogen/the total fuel. This definition can facilitate the longitudinal comparison of mixed gas ignition and explosion characteristics with the exact ammonia/natural gas ratios under different hydrogen content ratios. The component effects on the ignition and deflagrate characteristics at equivalent concentrations are studied experimentally. The physical and chemical parameters of natural gas, ammonia, and hydrogen used in the experiment are shown in Table 1.

The ammonia content ratio FNH3~~ is defined by formula 22

The hydrogen content ratio FH2~~ is defined by formula 33

The fuel fractions for each component are defined as CH2~~, CNH3~~, CNG.

The relationship between the fraction of mixed gas fuel and the dosage ratio is CNH3~~ + CH2~~ + CNG = 1

The equivalence ratio (φ) is defined as follows4For the convenience of description, L is defined as the distance from each position of the pipeline to the front of the ignition side.

The partial pressures of each component were defined as PH2~~, PNH3~~, and PNG, and the partial pressures of each component at different fuel content ratios of the gas mixture were calculated according to the reaction equation.

The detailed experimental plan has been uploaded as a Supporting Information file.

The term “ deflagrate shock wave” refers to the shock wave generated as the flame front propagates at subsonic speeds during the combustion of explosive gas mixtures. In this study, the propagation trajectory of the deflagrate shock wave was captured by using the schlieren system and high-speed photography within the experimental setup. Additionally, pressure transducers mounted along the pipeline were employed to monitor the deflagrate shock wave’s real-time propagation and intensity variations. The errors in the experimental results mainly come from the premixing degree of fuel and air, different ignition initial temperatures, smoothness of the pipe wall, sensor errors, and gas distribution errors. To ensure experimental accuracy, each experiment is repeated three times, and all experimental data are taken as the average of the three experiments. The shock wave velocity is the average velocity between the two points on both sides. The time when the pressure jumps is recorded and the average velocity of the shock wave between the two measuring points is calculated based on the distance between the measuring points and the time interval between the pressure jumps.

Results and Discussion

Effect of the Component Effect on the Pressure

of Deflagrate Shock Wave

Figure 3a shows the peak overpressure at different positions in the pipeline when φ = 0.8 and = 0. With the increase of the hydrogen content ratio and propagation distance, the propagation and development of deflagration shock waves show different laws. According to the variation law of peak overpressure with the increase of χH2~~, the propagation process is divided into the initial stage of deflagration shock wave propagation (L < 3 m) and the development stage of deflagration shock wave propagation (L > 3 m). The two regions are separated by blue lines in Figure 3.

First, the initial stage of deflagration shock wave propagation (L < 3 m) is

When the hydrogen content ratio (χH2~~) was 10 and 25%, the peak overpressure at the front end of the pipeline (L < 3 m) developed from low to high after ignition, and the initial overpressure of propagation was small (26.95 and 36.12 kPa, respectively). The overpressure increases slowly in the range of L = 1.35–1.65 m and rapidly in the range of L = 1.65–2.25 m. When L = 2.25 m and Po = 115.78 and 151.07 kPa, the overpressure increases at a triple rate, and the overpressure rises slowly in the range of L = 2.25–3 m.

When χH2~~ = 40%, in the initial stage of propagation (L < 3 m), the peak overpressure first increases, then decreases, and then the propagation stabilizes. The peak overpressure at L = 1.35 m is P = 127.55 kPa, and the value of the peak overpressure is more than twice that of χH2~~ = 10% and χH2~~ = 25%. When L = 1.65 m and P = 169.52 kPa, the peak overpressure increases at a double rate, and in the range of L = 1.65–2.25 m, the peak overpressure rises rapidly.

When χH2~~ = 55%, the overpressure value of the first measuring point is higher in the initial stage of deflagrator shock wave propagation (L < 3 m), and the overall peak overpressure first propagates at a sizable initial value, significantly decreases at L = 2.55 m, and then propagates steadily. In the range of L = 1.35–2.25 m, the peak overpressure increased first and then reduced, Po = 200.47, 225.23, 190.26 kPa, and the value of the peak overpressure changed little. When L = 2.55 and 2.85 m, the peak overpressure value is similar, Po = 151.86 and 154.28 kPa.

When χH2~~ = 70 and 85%, the initial overpressure was the maximum overpressure (PMAX) in the whole propagation process, which was 273.3 and 303.15 kPa, respectively; in the range of L = 1.35–2.85 m, the peak overpressure showed a downward trend. The overpressure decreases obviously in the r = 1.35–1.65 m and L = 2.25–2.55 m range, while the overpressure changes little in the r = 1.65–2.25 m and L = 2.55–2.85 m range. The initial overpressure (peak overpressure of L = 1.35 m) increased with the increase of hydrogen content ratio (χH2~~). The overpressure value increased slightly when χH2~~ = 10–25% and χH2~~ = 70–85%, and the increase value was 9.17 and 29.85 kPa, respectively. When χH2~~ = 25–70%, the initial overpressure increased significantly, and the increase values were 91.42, 72.93, and 72.82 kPa, respectively. With the hydrogen content ratio increase, the initial overpressure change is not linear.

The development stage of deflagration shock wave propagation (L > 3 m) is

When L > 3 m, the mixed fuel overpressure reaches the peak position of the peak overpressure curve when L = 3.15 m, the trough position of the peak overpressure curve when L = 3.45 m, the peak position again when L = 3.75–4.05 m, and the trough again when L = 4.35 m. When L = 4.95 m, the distance from the blind plate of the pipeline is 0.15 m, which is greatly affected by the reflected wave. When the overpressure of the measuring point here increases sharply, this overpressure is the reflected overpressure. The higher the hydrogen content ratio (χH2~~), the higher the overpressure. It shows that adding hydrogen increases the gas mixture’s reaction rate and explosion intensity. In the development stage of flagellation shock wave propagation, in Figure 3, it propagates forward in a state of fluctuation, that is, overpressure from weak to strong and then from strong to weak, and this process repeats.

Figure 3b shows the peak overpressure at different positions in the pipeline when φ = 0.8 and = 10%. The overall law of deflagration shock wave propagation is similar to that of = 0. Compared with = 0, when = 10%, the overall peak overpressure decreased, and when χH2~~ = 10%, the change law of the whole development process changed, and the overpressure development was slower. In the range of L < 3 m, when χH2~~ = 40–55%, the overpressure curve changes more gently. The results showed that adding 10% ammonia significantly reduced the deflagration shock wave intensity and reaction rate of the mixed fuel; that is, adding 10% ammonia inhibited the deflagration shock wave intensity of the natural gas/hydrogen mixture.

Figure 3c–g shows the peak overpressure at different positions in the pipeline when φ = 0.8 and = 25–100%. The overall law of deflagration shock wave propagation is similar to that when = 0 and = 10%. With the increase of , when χH2~~ < 55%, the explosion intensity of the gas mixture gradually decreased, and the maximum overpressure gradually decreased. When ≥ 50% and χH2~~ < 40, the proportion of ammonia in the mixed fuel is the largest, and the peak overpressure is dominated by ammonia. At this time, the overpressure development is relatively slow. In the range of L < 4.2 m, the overpressure is in a slow development process, and the overpressure increases gradually, and the overpressure reaches the maximum at 4.05 m. When χH2~~ ≥ 55%, hydrogen accounted for the most significant proportion of the mixed fuel, and the peak overpressure was dominated by hydrogen. No matter how much ammonia was accounted for, the deflagration shock wave propagation of the mixed fuel was almost unaffected. When > 50%, χH2~~ < 25, = 100%, χH2~~ < 55, the measured pressure curve has been a slow rising process, and no obvious pressure jump can be detected. Only when the pressure propagates to the tail of the pipeline can the reflected pressure jump be measured from the last measuring point. Currently, the propagation pressure of the mixed gas in the pipeline is low. The speed is slow, indicating that under this fuel ratio, the primary reaction state in the pipeline is combustion rather than deflagration, suggesting that with the increase of ammonia content ratio, the reaction rate of the mixed gas and the intensity of deflagration shock wave continue to decrease.

In summary, an increase in the hydrogen content ratio enhances the reactivity, combustion rate, and energy release of the mixed gas, thereby intensifying the strength of the deflagration shock wave. On the other hand, the addition of ammonia acts as a suppressant, reducing the combustion reaction rate and the intensity of the deflagration shock wave. When the ammonia content ratio = 0–100%, during the initial phase of the deflagration shock wave propagation (L < 3 m), the initial overpressure (peak overpressure at L = 1.35 m) increases monotonically with the increase of the hydrogen content ratio (χH2~~), but the change in initial overpressure is not linear with the increase of the hydrogen content ratio. When χH2~~ < 40% and the combined content of natural gas and ammonia in the mixed gas exceeds 50%, the relatively long ignition delay time of ammonia and natural gas, along with the lower reaction rate and lower concentrations of OH and H radicals, results in poor flame stability and slower flame development. This leads to an amplification of the negative feedback mechanism between the flame and obstacles and a weakening of the positive feedback, causing the flame acceleration to be less pronounced after passing through obstacles. The velocity of the flame after passing through obstacles is less than the self-sustained velocity under pipeline conditions, and the flame continues to accelerate and propagate.

As χH2~~ increases, the concentrations of critical radicals OH and H in the reaction process of the mixed gas fuel significantly increase, the reaction rate grows, the ignition delay time shortens, and the flame develops rapidly after ignition. The promoting effect of obstacles on flame acceleration is enhanced, and the propagation speed of the flame after acceleration through obstacles far exceeds the speed that could be achieved through free development in the pipeline environment. The flame propagation speed rapidly decreases due to the inability to self-sustain, leading to a drop in shock wave overpressure.

Overall, the peak overpressure of the mixed fuel tends to follow the same developmental trend after experiencing different initial stages. Beyond L > 3 m, the general propagation trend is similar, with stable propagation observed in the L = 2.55–2.85 m range and entry into a fluctuating state beyond L > 3 m. This fluctuation state is primarily due to two factors: on the one hand, the flame, during its propagation, is subject to wall friction, heat loss, and acceleration mechanisms following flame surface deformation, leading to continuous changes in flame morphology and velocity, resulting in fluctuations in the velocity and pressure of the shock wave. On the other hand, it is caused by the oscillation of the shock wave induced by the flame surface between the shock wave and the flame surface. After the shock wave induced by the flame surface catches up to the leading shock wave, it provides energy to the leading shock wave and reflects upon encountering it, forming a reflected pressure wave. Multiple reflected waves move toward the flame surface, impeding the flame propagation upon meeting it. At this point, the flame surface decouples from the shock wave, leading to shock wave attenuation and a decrease in pressure. After a brief deceleration, the flame accelerates and continues to propagate forward because the reflected wave alters the original shape of the flame and increases flame turbulence, and then the flame surface recouples with the shock wave, increasing the shock wave velocity and pressure. This process repeats, causing continuous fluctuations in the shock wave overpressure curve.

Figure 4 shows the maximum overpressure (PMAX) under different component effects when φ = 0.8. For the same ammonia content ratio, the maximum overpressure increases monotonically with the increase of hydrogen content ratio and the maximum value is obtained when χH2~~ = 85%. The deflagration overpressure of mixed fuel shows a different trend with the increase of the ammonia content ratio under different hydrogen content ratios.

When χH2~~ = 10–55%, the maximum overpressure decreases monotonically with the increase of ammonia content ratio, the maximum value is obtained when = 0, and the minimum value is obtained when = 100%, and the decreasing range decreases gradually with the increase of hydrogen content ratio.

When χH2~~ = 55%, the maximum overpressure curve decreased slightly with the increase of ammonia content ratio, and the curve was flat as a whole, with a difference of 30 kPa between the maximum value and the minimum value. The addition of ammonia inhibited the explosive shock wave intensity of the mixed gas.

When χH2~~ = 70–85%, the maximum overpressure decreases first and then increases, which indicates that the blending of ammonia gas and hydrogen gas in the mixed fuel to a certain extent produces a mixture combustion phenomenon, which increases the ignition and explosion performance of the mixed fuel. However, due to ammonia’s weak ignition and explosion performance when the ammonia component dominates the low level in the mixed fuel, its inhibition effect on the mixed fuel explosion is greater than the promotion effect. Therefore, when the ammonia content ratio is increased, the maximum overpressure decreases. When χH2~~ = 70–85%, hydrogen dominated the fuel mixture, and ammonia showed the effect of promoting explosion.

Figure 5 shows the peak overpressure at different positions in the pipeline when φ = 1.0, = 0–100%, and = 0–100% for each hydrogen content ratio. Compared with φ = 0.8, when φ = 1.0 and = 0–90% (Figure 5a–f), the peak overpressure of deflagrate shock wave increases, but the development law is similar. In the initial stage, with the rise in hydrogen content ratio, the overpressure of the deflagrate shock wave changes from small to large to enormous. The peak overpressure curve fluctuates when L > 3 m. Compared with φ = 0.8, when φ = 1.0, = 100% (Figure 5g), and χH2~~ = 25%, the effective peak overpressure cannot be measured. When χH2~~ = 40 and 55%, the deflagging shock wave intensity of the gas mixture is significantly weakened, and the distance to reach the first wave peak increases. It shows that with the rise of the fuel concentration, the inhibition effect of the high ammonia ratio on the ignition and deflagrate performance of the mixed gas is enhanced. In the L > 3 m region, the peak overpressure increases when χH2~~ = 55–85%, indicating that the deflagration shock wave strength of ammonia gas and hydrogen gas increases with the increase of equivalent ratio. In contrast, the deflagration shock wave strength of natural gas/hydrogen gas increases relatively little.

Figure 6 shows the maximum overpressure of mixed gases with different component ratios when φ = 1.0. With the increase of hydrogen content ratio, the maximum overpressure curve changes from continuous decrease to first decrease and then increase. When χH2~~ = 10–40%, the maximum overpressure monotonically reduced with the increase of ammonia content ratio. When χH2~~ = 55%, the maximum overpressure decreases first and then flattens with the increase of ammonia content ratio. When χH2~~ = 70–85%, the maximum overpressure decreases first and then increases with the increase of ammonia content ratio. The maximum overpressure of the ammonia/hydrogen binary mixture is slightly lower than that of the natural gas/hydrogen binary mixture and is larger than that of other components. When φ = 0.8, the maximum overpressure of the ammonia/hydrogen binary gas mixture increases when χH2~~ = 70–85%, and the maximum overpressure is the smallest when = 50%. With the increase of the equivalent ratio, the inflection points of the maximum overpressure curve falling first and then rising are advanced, indicating that with the increase of the equivalent ratio, the maximum overpressure curve will increase rapidly. When hydrogen is used as the gas mixture’s main fuel, the gas mixture’s deflagration shock wave strength increases, and the promoting effect of ammonia gas on the gas mixture’s deflagration performance is enhanced.

Figure 7 shows the peak overpressure at different positions in the pipeline when φ = 1.2 and χH2~~ = 10–85% for each ammonia content ratio. The evolution of deflagration shock wave propagation is like then φ = 0.8 and φ = 1.0. As can be seen from Figure 7a, when = 0, the development trend of deflagrate shock wave changes from slow growth to gradual attenuation with the increase of hydrogen content ratio in the range of L < 3 m. When χH2~~ = 10%, the peak overpressure increases steadily but slowly with the increase of distance. When χH2~~ = 25%, the overpressure increases rapidly in the range of L = 1.62–2.25 m; compared with the overpressure change of the distribution ratio of the same group at the low equivalent ratio, the overpressure increases more obviously with the increase of hydrogen content ratio. When χH2~~ = 40–55%, the overpressure increases first and then decreases, and the overpressure curve is relatively flat. When χH2~~ = 70–85%, the overpressure showed a decreasing trend. In the L > 3 m range, the overpressure curve passes through peaks and troughs with the increase of distance and is in a state of wave propagation. The propagation law is similar, and the overpressure value when φ = 1.2 is between φ = 0.8 and φ = 1.0. The maximum overpressure occurs when φ = 1.0. As shown in Figure 7b–d, as the ammonia content ratio increases from 10 to 50%, the deflagrate shock wave strength of the mixed gas decreases monotonically. Compared with the overpressure curves of other equivalent ratios, under the same ammonia content ratio, the overpressure increases more significantly with an increased hydrogen content ratio, and the space between the curves is more obvious. As can be seen from Figure 7e,f, in the range of L < 3 m, the peak overpressure increases with the increase of ammonia content ratio. With the increase in ammonia content ratio, the peak overpressure of the mixed gas also presents a trend of first decreasing and then increasing in the initial propagation stage.

Figure 8 shows the maximum overpressure of the gas mixture at different component ratios when φ = 1.2. When χH2~~ = 10%, the maximum overpressure can be measured at = 0–50%, and PMAX decreases with the increase of ammonia content ratio. When χH2~~ = 25%, the maximum overpressure can be measured at = 0–90%. PMAX decreases with the ammonia content ratio, and the maximum overpressure decrease increases when the ammonia content ratio exceeds 50%. When χH2~~ = 40%, PMAX first decreases and then increases with the increase of ammonia content ratio, and the maximum overpressure is the smallest when = 90%. When χH2~~ = 55–85%, PMAX first decreased and then increased with the increase of ammonia content ratio, and the maximum overpressure was the smallest at = 50%. Compared with χH2~~ = 40%, the minimum ammonia content ratio of the curve was smaller, and the maximum of the curve was 100% at = 100%. Compared with the maximum overpressure in Figures 5 and 6 (the equivalent ratio is 0.8 and 1.0, respectively), when φ = 1.2, the main difference is that the changing trend of the maximum overpressure is different. Under the same hydrogen content ratio, as the ammonia content ratio increases, the inflection points of the maximum overpressure decrease and then increase gradually, approaching the lower hydrogen content ratio and the lower ammonia content ratio. With the increase of the equivalent ratio, the range of the ratio of ammonia to promote the deflagration shock wave strength of the mixed gas increases. In other words, with the increase of the equivalent ratio, the increase of the ammonia content ratio on the intensity of the mixed gas deflagration shock wave gradually becomes a promotion effect.

By comparing the evolution process of deflagrate shock wave of mixed gas with the change of component ratio and the maximum overpressure under each distribution ratio when the equivalent ratio is 0.8, 1.0, and 1.2, the following results are

Overall Law of Deflagration Shock Wave Propagation of Mixed

Gas

In the initial stage of deflagration shock wave propagation (L < 3 m), the initial overpressure (peak overpressure of L = 1.35 m) increases monotonically with the increase of hydrogen content ratio (χH2~~). When χH2~~ < 70%, the initial overpressure is dominated by all components of the mixed fuel, and when χH2~~ > 55%, the initial overpressure is dominated by hydrogen. With the increase of χH2~~, the overpressure changes in the initial stage gradually changed from small to large, which means that the propagation law of deflagrate shock wave in the initial stage changed from slow development to gradual attenuation. This corresponds to the propagation characteristics of the mixture of natural gas, natural gas/hydrogen binary mixture, natural gas/ammonia/hydrogen material mixture, ammonia/hydrogen binary mixture, ammonia, and hydrogen as the main fuel in the initial stage of deflagration shock wave propagation with the change of deflagration intensity.

Overall, the peak overpressure of the mixed fuel tends to have the same development trend after different initial stages. After L > 3 m, its general propagation trend is the same, and it propagates steadily in the range of L = 2.55–2.85 m and enters a fluctuating state after L > 3 m.

Variation Law of Deflagration Shock Wave Intensity with a Composition

Ratio of Mixed Gas

Under all equivalent ratios, the intensity of the deflagration shock wave increases monotonically with an increase of the hydrogen content ratio. The influence of the ammonia content ratio on the deflagrate shock wave intensity of mixed gas has a different trend with the change of the equivalent ratio and hydrogen content ratio. With the same composition ratio, the deflagrate shock wave intensity of mixed gas increases first and then decreases with the increase of equivalent ratio and reaches the maximum when φ = 1.0 and the minimum when φ = 0.8. The deflagration shock wave intensity of gas mixtures with different composition ratios shows different variation trends with an increase of the equivalent ratio. With the increase of the equivalent ratio, the range of the ammonia content ratio, which promotes the deflagration shock wave strength of mixed gas, increases. That is to say, when χH2~~ = 70–85%, with the increase of equivalent ratio, the increase of ammonia content ratio gradually changed the inhibition effect on the intensity of deflagration shock wave of mixed gas into the promotion effect.

Analysis of Emission Characteristics of the

Natural Gas/Ammonia/Hydrogen Mixture

Figure 9 displays the concentrations of CO and CO2 in the combustion products of natural gas/ammonia/hydrogen mixtures at equivalence ratios (φ) of 0.8, 1, and 1.2. At φ = 0.8, the premixed gas contains more oxidants than necessary for the complete combustion of the fuel, leading to sufficient fuel combustion with excess oxidant. Consequently, the CO concentration in the combustion products is below 600 ppm across all fuel compositions. The concentration of CO2 in the combustion products monotonically decreases with an increase in the ammonia-to-hydrogen ratio, which correlates positively with a reduction in the natural gas fuel fraction. The most significant reduction in the CO2 concentration is observed when the ammonia content ratio rises from 50 to 75%. At φ = 1, the CO concentration in the products is less than 10,000 ppm, which is approximately a 1500% increase compared to φ = 0.8, representing about 10% of the CO2 concentration.

At an equivalence ratio of φ = 1.2, the concentration of CO in the combustion products significantly increases. When the ammonia ratio is 0 and the hydrogen content ratio is 10%, the CO concentration reaches 45,350 ppm. As the ammonia content increases, the concentration curves of CO and CO2 converge under the same fuel composition, suggesting that increasing the ammonia content ratio has a more pronounced effect on reducing CO2 emissions than on reducing the CO concentration. When the ammonia content ratio exceeds 50%, the CO concentration gradually exceeds that of CO2 as the hydrogen content ratio increases. This indicates that during the combustion process of the mixed gas, hydrogen reacts with oxygen first, consuming it. Consequently, the reaction CO + O2 = CO2 is impeded due to oxygen depletion, leading to a higher CO concentration than CO2 in the combustion products. As the hydrogen-to-ammonia ratio increases, carbon emissions decrease monotonically. Adding hydrogen is less effective in reducing the CO concentration in natural gas combustion products than in reducing the CO2 concentration. As the equivalence ratio (φ) increases, fuel consumption leads to less complete combustion, resulting in higher emissions.

Studying carbon emission characteristics should not only consider emission concentration but also comprehensively consider the fuel cost and safety of fuel applications. Analyzing and studying carbon emission characteristics based on the same amount of heat released by fuel is relatively more practical. Therefore, the emission characteristics of fuel combustion under a unit heat release were analyzed and studied.

The total heat production of combustion of mixed gases with different component concentrations (Et) is as follows5

In the formula, PH2~~, PNH3~~, and PNG are the partial pressures of hydrogen, ammonia, and natural gas in the experiment, and the unit is kPa. QH2~~, QNH3~~, and QNG are the low calorific values of hydrogen, ammonia, and natural gas, respectively. The low calorific value of natural gas is 35.2 MJ/m^3^, that of ammonia is 13.4 MJ/m^3^, and that of hydrogen is 120.9 MJ/m^3^; Vt is the experimental pipeline volume (0.04 m^3^). PS is the standard atmospheric pressure (101.325 kPa).

The carbon emission per unit heat production of the mixed gas (Ec) is as follows6

In the formula, Vce is the carbon emission volume of the mixed gas (converted from the measured concentration), and the unit is m^3^. The greater the Ec, the greater the carbon emission when generating the same heat, and the smaller the vice versa.

As shown in Figure 10, with the increase of ammonia and hydrogen content ratios, the mixed gas’s carbon emissions per unit of heat production monotonically decrease. The increase in the hydrogen content ratio has a more significant impact on the carbon emissions per unit of heat production than the ammonia content ratio because the calorific value of hydrogen per unit volume is much higher than that of ammonia per unit volume. Compared to the carbon emission concentration curve, the trend and magnitude of the increase in the hydrogen content ratio increase. Considering the carbon emissions per unit heat value generated by fuel, adding hydrogen is more efficient in reducing carbon emissions than adding ammonia.

The concentration of nitrogen oxides in the combustion products of mixed gas fuels was analyzed using Chemkin-Pro software. As depicted in Figure 11, the mole fraction of NO initially increases and then decreases with an increase in the equivalence ratio. At φ = 0.8, NO formation reaches its peak because the combustion temperature is higher, which is favorable for NO formation. Excess oxygen also provides favorable conditions for generating both fuel-type NO and thermal-type NO, resulting in the highest total concentration at this equivalence ratio. Beyond φ = 0.8, the production of fuel-type NO decreases due to the consumption of oxygen. Below φ = 0.8, the combustion temperature drops due to insufficient fuel, reducing thermal NO. The maximum concentration of NO2 in the products is less than 20 ppm, which can be considered negligible.

As indicated in Figure 11, the concentration of NO2 in combustion products is low and can be disregarded; therefore, the study focused only on the NO concentration. As shown in Figure 12, the NO concentration in combustion products increases initially and then decreases with an increase in the ammonia content ratio. This trend is attributed to the addition of ammonia, which elevates the concentration of fuel-type NO. However, when the ammonia content ratio surpasses 50%, the combustion temperature of the mixed gas fuel drops, reducing the concentration of thermal-type NO. Conversely, as the hydrogen content ratio increases, NO concentration in the combustion products increases first and then decreases. The reason for this change is the opposite of that observed with the ammonia content ratio. An increase in the hydrogen content ratio raises the combustion temperature, significantly increasing the production of thermal-type NO. Nevertheless, when the hydrogen content ratio exceeds 85%, the concentration of fuel-type NO decreases due to the diminishing ammonia content in the fuel.

The overall concentration of NO is related to the combined production of fuel-type and thermal-type NO. When ammonia is added to the mixed gas fuel, the NO concentration in the combustion products significantly increases. The application of such fuel should consider both the reduction of COX emissions and the potential increase in NOX emissions comprehensively.

The formation of nitrogen oxides in combustion products primarily involves two fuel-type nitrogen oxides and thermal-type nitrogen oxides. The ammonia content ratio influences the formation of fuel-type nitrogen oxides in the fuel; the higher the nitrogen content, the greater the formation of fuel-type nitrogen oxides. Additionally, the temperature and oxidation conditions during combustion can affect the generation of fuel-type nitrogen oxides. Ammonia decomposes and oxidizes at high temperatures during combustion, producing NO and NO2.

The formation of thermal nitrogen oxides is predominantly influenced by the temperature. A larger hydrogen content ratio leads to higher temperatures, faster reaction rates, and increased NOx production. Moreover, the concentration of oxygen and the degree of local excess oxygen during combustion can also impact NOx generation.

Several measures can be implemented to mitigate NOX emissions, such as reducing the combustion temperature, optimizing the combustion air supply, and installing exhaust gas post-treatment equipment. Through these approaches, the emission of nitrogen oxides can be effectively controlled and minimized.

Combustion Characteristics and Chemical Reaction

Kinetics Analysis of Natural Gas/Ammonia/Hydrogen Mixture Gas

The proportions of the constituent gases determine the combustion characteristics of natural gas/ammonia/hydrogen mixtures. Hydrogen, due to its high reactivity and high combustion heat value, can significantly improve the combustion rate of mixed fuels. It rapidly reacts with oxygen during combustion to produce water, releasing a large amount of heat and, thus, accelerating the entire combustion process. However, when the hydrogen content reaches 40%, the combustion rate of the mixed gas fuel can far exceed that of pure natural gas, increasing the heat load on the combustion system. Ammonia, while having a combustion rate slower than that of hydrogen, can still effectively participate in combustion reactions at high temperatures. When the ammonia content is at 50%, it balances the excessively rapid combustion rate caused by hydrogen, allowing for stable combustion of the natural gas/ammonia/hydrogen mixture. Adding hydrogen can reduce the ignition delay of mixed fuels because it can react quickly, even at lower temperatures, thereby promoting ignition. This is essential for enhancing the starting performance and stability of the combustion system. In contrast, adding ammonia may increase the ignition delay, as its combustion reaction rate is relatively slow and the nitrogen in its combustion products is not readily involved in further reactions. While hydrogen’s high reactivity and combustion heat value contribute to increased combustion rates and reduced ignition delays, adding ammonia can help balance the combustion rate and reduce greenhouse gas emissions.

When = 50% and χH2~~ = 40%, the maximum overpressure of deflagration flame of mixed gas fuel is like that of natural gas, and the carbon emission is 30% lower than that of natural gas. Therefore, = 50% and χH2~~ = 40% mixed gas fuel is a kind of hybrid fuel with application potential; its advantage is that the combustion performance is similar to natural gas. Still, its carbon emission is 30% lower than that of natural gas. The combustion characteristics and reaction mechanism of the gas mixtures were analyzed when χH2~~ = 40% and = 0–100%.

Figure 13 shows the propagation evolution process of deflagration flame and shock wave of mixed gas with a hydrogen content ratio of 40% and an ammonia content ratio of 0–75%. It can be seen from Figure 13a that an obvious leading shock wave and reflected pressure wave are observed through the observation window, and the flame surface is a typical flame turbulence brush before being compressed by the reflected pressure wave. After being compressed by reflected pressure, the density and turbulence increase. As shown in Figure 13b–f, the propagation laws of the flagellation flame of mixed gas with different gas components in the pipeline are generally similar. The leading shock wave is in the front, and the flame is in the back. Increase the flame turbulence, and the flame accelerates after a brief deceleration. According to the action mechanism of flame and shock wave, the shock wave induced by flame catches up with the leading shock wave, and the leading shock wave has a larger density. When the shock wave catches up with the leading shock wave, it reflects in the direction of time and forms a pressure wave in reverse motion. After the opposite moving pressure wave meets the shock wave and transmits each other, the shock wave continues to catch up with the leading shock wave in front of the trunk to form a new reverse pressure wave. Like the formation process of the leading shock wave, the opposing moving pressure wave superimposes on each other to form a density wave that the schlieren system can observe. Because of the reaction of the reflected wave on the flame, the shock wave’s peak overpressure fluctuates up and down. With the increase in ammonia content ratio, the flame velocity and shock wave velocity show a decreasing trend, and the addition of ammonia weakens the flame intensity of the mixed gas.

Figure 14 shows the main reaction pathways of mixed gas fuel with an equivalence ratio of 1.0 and a hydrogen content ratio of 40%, and ammonia content ratios of 0, 50%, and 100%, respectively. When = 0, 50 and 100%, the corresponding font colors in the figure were black, blue, and red, and the corresponding system temperatures were 689.23, 645.74, and 328.83 K, respectively. With the increase in the ammonia concentration, the system’s temperature decreases monotonously when the same proportion of fuel is consumed. When = 100%, the system temperature decreased significantly compared with that when = 50%. When = 100%, the dependence of the reaction path on OH reached the maximum, and all the major reactions required the participation of OH, or the reaction with the participation of OH was the leading reaction. When = 0, the primary product CH3 of methane can produce a large amount of OH through the reaction of HO2+CH3 = OH + CH3O (1.64 × 10^–4^), which is much higher than the OH yield when = 100%. Therefore, the higher the ammonia, the stronger the reaction dependence on R39, which means that this reaction has the highest temperature sensitivity. At the same time, the temperature of the reaction system is reduced due to the limitation of the R39 yield.

Figure 15 shows the top ten elementary reactions corresponding to the highest absolute values of temperature sensitivity in each system when χH2~~ = 40%, φ = 1.0, and = 0, 25, 50, 75, and 100%. When = 0, 25, 50, 75, and 100%, the corresponding system temperatures were 2254.4, 2219.7, 2196.6, 2174.4, and 2137.5 K, respectively. The system’s temperature decreases monotonically with the increase in ammonia content ratio. R39 is the reaction with the highest sensitivity coefficient of all components, and R39 generates OH free radicals, which provide OH free radicals for almost all major exothermic reactions in the reaction system. With the increase of ammonia content ratio, the sensitivity coefficient of R39 increases monotonically, which is because, in the reaction process of natural gas, it can be seen from Figure 14 that the primary product CH3 of methane can produce OH through the reaction of HO2 + CH3 = OH + CH3O. In contrast, the primary product NH2 of ammonia cannot directly produce OH through the reaction. Therefore, with the increase in the ammonia content ratio, the absolute yield of R39 decreases, resulting in an insufficient supply of OH in the reaction system. The effect of R39 on the temperature of the reaction system is more obvious. R39 is not the most important thermogenic reaction in the system. Still, it affects the temperature of the reaction system by providing an OH radical for the main thermogenic reaction, and its yield or reaction rate determines the temperature of the reaction system, indicating that the temperature of the whole reaction system can be increased by increasing the yield of this reaction. This means that the yield of R39 limits the reaction system’s temperature rise, like the barrel’s short plate effect, where R39 is the short plate that determines the upper-temperature limit of the reaction system. The reaction with the greatest temperature sensitivity is generally the pre- or postreaction of the most important thermogenic reaction in the reaction system, whose main function is to provide free radicals for the main reaction or consume the products of the main reaction to promote the forward progress of the main reaction. Therefore, it can be considered that the change in the temperature sensitivity coefficient reflects the limiting effect of each elementary reaction on the main thermogenic or endothermic elementary reaction to a certain extent. Increasing or decreasing the yield of its main elementary reaction can change the temperature of the reaction system. With the increase in ammonia content ratio, the reactions containing carbon elements (R102, R122, R56, R53, R100, R169, and R11), which are the most sensitive, gradually decrease due to the decrease of the concentration of natural gas components. The sensitivity coefficients of nitrogen-containing elements (R257, R271, R260, R245, R265, R273, and R246) increased gradually. This also explains the inhibition effect of ammonia incorporation on the deflagration strength of the mixed gas from the mechanism of the chemical reaction kinetics.

Conclusions

The deflagrate propagation characteristics of a natural gas/ammonia/hydrogen mixed fuel in horizontal pipelines were studied through experiments. The carbon emissions in natural gas/ammonia/hydrogen mixed fuel products were analyzed and studied by using gas collection and analysis instruments. The deflagrate flame propagation characteristics of the mixed fuel were analyzed using a schlieren system; finally, the deflagrate reaction mechanism of natural gas/ammonia/hydrogen mixed fuel was studied at the microelemental level using Chemkin-Pro software. The main conclusions are as (1)According to the variation trend of deflagrate shock wave intensity with distance, its propagation process is divided into the initial stage (L < 3 m) and the development stage (L > 3 m). In the initial stage of deflagrate shock wave propagation (L < 3 m), as the deflagrate intensity of the mixed gas increases, the initial overpressure (peak overpressure at L = 1.35 m) monotonically increases. With the increase of χH2~~, the overpressure trend in the initial stage changed from a slow increase to a gradual decline. In the development stage of deflagrate shock wave propagation (L > 3 m), the peak overpressure of mixed gases with different deflagrate intensities shows the same fluctuation state with distance variation.(2)The intensity of the deflagration shock wave increases monotonically with the increase of χH2~~. It is worth mentioning that the influence of on the deflagration shock wave intensity of the gas mixture is related to φ and χH2~~. When χH2~~ < 70%, PMAX decreases monotonically with the increase of . When χH2~~ = 70–85%, PMAX decreases first and then increases with the increase of , and the inflection point of decreases with the increase of φ. With the same composition ratio, the deflagrate shock wave intensity of the gas mixture increases first and then decreases with the increase and reaches the maximum when φ = 1.0 and the minimum when φ = 0.8.(3)With the increase of and χH2~~, the carbon emission per unit of heat production decreased monotonically. Considering the carbon emission when the fuel produces a unit calorific value, incorporating hydrogen is more efficient than incorporating ammonia in reducing carbon emission.(4) = 50% and χH2~~ = 40%. The combustion performance of mixed gas fuel is similar to that of natural gas. Still, its carbon emission is 30% lower than that of natural gas, so it is a new type of mixed fuel with potential application value. The interaction between the reflected pressure wave and the flame is the main reason for the pressure fluctuation of the deflagration shock wave. The concentration of the OH radical in the reaction system decreased with the increase of , so the temperature of the reaction system decreased with the increase of .