Influence of H_{2}O on Oxygen Enriched Diffusion Combustion of Natural Gas
OPEN ACCESS
O_{2}/H_{2}O combustion technology, as the next generation of oxyfuel combustion technology with great potential, can greatly increase the utilization rate of clean energy CH_{4}. In this paper, the natural gas combustion process under 6 operating conditions of O_{2}/H_{2}O atmosphere and O_{2}/FH_{2}O atmosphere is numerically simulated. The horizontal analysis is carried out on the characteristics of H_{2}O fraction, CO_{2} volume fraction and the amount of pollutants (NOx, carbon black), and indepth exploration of the content of additive H_{2}O and the influence of chemical action on the above characteristics. The research results show that the chemical effects of H_{2}O have a negative effect on combustion temperature, and the physical effects are dominant. The chemical effects of H_{2}O have a great impact on CO production and little effect on the production of CO when the proportion of H_{2}O is 6579%. The chemical effects of H_{2}O inhibit the formation of NOx and carbon black when the proportion of H_{2}O is within the range of 5570%. The chemical effect has the greatest impact on the formation of dyes (NOx, carbon black) when the proportion of H_{2}O is within the range of 6570%.
O_{2}/H_{2}O combustion, numerical simulation, oxygen enrichment, temperature, pollutants
As one of the most widely used technologies in modern industrial combustion, oxyfuel combustion technology has low risk and high feasibility and high efficiency. The new O_{2}/H_{2}O oxyfuel combustion technology can greatly increase the utilization rate of clean energy CH_{4 }[13], and its combination with the production process of natural gas hydrate can effectively promote the development of natural gas industry, and alleviate global warming and environmental pollution from the source.
As the next generation of oxyfuel combustion technology with great potential, O_{2}/H_{2}O combustion technology does not need to recycle the flue gas, and the combustion temperature adjustment is achieved by using water vapor to participate in the combustion process. This new type of oxyfuel combustion technology can make the entire combustion system more concise and compact, and the startstop process is easier. It not only reduces the system cost [46], but also makes full use of the waste heat of the flue gas, thus reducing the loss of waste heat, and avoiding the problem of impurity gas enrichment in O_{2}/CO_{2} flue gas cycle more effectively [710]. Scholars at home and abroad have conducted extensive theoretical and experimental research on oxyfuel combustion technology, most of which are related to the combustion characteristics, pollutants (NOX, carbon black) of oxyfuel combustion in O_{2}/N_{2}, O_{2}/CO_{2} atmospheres and the basic research work on the physicochemical effects of additives, but lack of application research of O_{2}/H_{2}O combustion technology. O_{2}/H_{2}O combustion technology has its unique advantages in terms of combustion characteristics and pollutant emissions. Therefore, a full understanding of the effect of additive H_{2}O on oxygenenriched diffusion combustion is of great significance to the development of O_{2}/H_{2}O oxyfuel combustion technology [11].
Therefore, it is necessary to analyze the different working conditions under O_{2}/H_{2}O atmosphere to have a clearer understanding of O_{2}/H_{2}O combustion technology and the influence of H_{2}O on oxygen enriched diffusion combustion of natural gas, and to provide more sufficient basic theoretical support for the industrial application of O_{2}/H_{2}O combustion technology in the future. Thus, this paper analyzes the distribution of combustion temperature and pollutant (NOx, carbon black) generation under the same O_{2} concentration and different H_{2}O concentration conditions, and explores the influence characteristics of H_{2}O content on combustion temperature, CO volume fraction, CO_{2} volume fraction and pollutant (NOx, carbon black) concentration. By analyzing the differences of combustion temperature, CO volume fraction, CO_{2} volume fraction and pollutant (NOx, carbon black) generation under different conditions of O_{2}/H_{2}O atmosphere and O_{2}/FH_{2}O atmosphere with the same concentration proportion, the influence of chemical action of H_{2}O additive on oxygen enriched diffusion combustion was studied. Furthermore, the optimal concentration range of O_{2} and H_{2}O in oxidant was determined by overall comparative analysis.
2.1 Mathematical model
In the process of numerical simulation, the standard model and the general finite rate model are selected as the turbulent flow model and the combustion model respectively; each radiation heat transfer model needs to be comprehensively considered in the applicable occasions, calculation amount, calculation accuracy, and hardware CPU performance requirements. Thus, the p1 model is defined as the radiation heat transfer model; the probability density function (pdf) model is used as a mathematical model of the nitrogen oxide pollutant reaction mechanism and information transmission process, and the NO concentration transport equation provided by Fluent is used to predict the emission of NOx and pollutants.
$\frac{\partial }{\partial t}\left( \rho {{Y}_{NO}} \right)+\nabla \cdot \left( \rho \bar{v}{{Y}_{NO}} \right)=\nabla \cdot \left( \rho D\nabla {{Y}_{NO}} \right)+{{S}_{NO}}{{Y}_{NO}}$ (1)
In the above formula, r is the density, Y_{NO} is the volume fraction of NO, D is the diffusion coefficient, S_{NO} is the generation rate of NO, t is the time, $\bar{v}$ is the average rate;
The singlestep Khan and Greeves model is selected for the carbon black generation model. In terms of the gas phase chemical reaction mechanism, it adopts the same fourstep simplified reaction mechanism as the combustion form of CH_{4}.Separation solver, implicit format, finite volume method, and SIMPLE algorithm are used to solve the calculation.
2.2 Geometric model
2.2.1 Model simplification
In this paper, the cylindrical burner is used in the simulation, and the geometric model is established by using fluent preprocessing software gambit. The simplified geometric shape and size are shown in Figure 1. It can be clearly seen from the figure that the whole length of the burner is 3 m, the radius is 0.3m, and there is a nozzle with a radius of 0.006m in the middle. The fuel methane enters the burner along the nozzle, and the oxidant enters the annular area of the combustion chamber inlet (excluding the nozzle), mixes with methane and burns, diffuses into the combustion chamber at the same time, and finally produces turbulent diffusion flame. In order to obtain a clearer diffusion flame, an outer wall is installed to separate the flammable methane and oxidizer, and the length of the outer wall is specified as 0.05m.
Figure 1. Schematic diagram of burner structure
2.2.2 Grid division
After the model is built, perform numerical calculation and mesh division. The detailed steps are to divide the spatially continuous solution area into multiple small subareas, and confirm each node separately. There are many factors that need to be considered in the division of the grid, so the workload will be relatively large. The grid format (such as structured and unstructured grid) of the division can directly affect the accuracy and complexity of the calculation results. Generally, the accuracy of the calculation result can be judged by the number of grids: the more grids, the higher the accuracy of calculation, but the calculation efficiency is low; the number of grids is small, the calculation accuracy is low and even the convergence cannot be achieved. Thus, the correct and reasonable grid division is the prerequisite for ensuring highquality grids.
Gambit software is used to divide the overall grid of the solution area, and the grid division diagram is shown in Figure 2. Considering that solving the regional data structure is relatively simple, a quadrilateral structured grid is adopted to ensure the formation of highquality grids in a short time. The overall presentation is graded grid, that is, along the axial gas flow, the grid is increasingly sparse, while considering a number of important areas have great impacts on the calculation results, such as gas inlet, exterior in the vicinity and inside the burner, the divided grids need to be encrypted. A total of 20300 nodes and 20075 grid cells are divided.
Figure 2. Grid division diagram
2.3 The mechanism of gas phase chemical reaction and solving method
(1) To simplify the calculation complexity and improve the efficiency, the fourstep simplified chemical reaction mechanism of methane combustion is adopted:
$\begin{align} & C{{H}_{4}}+2H+{{H}_{2}}O=CO+4{{H}_{2}} \\ & CO+{{H}_{2}}O=C{{O}_{2}}+{{H}_{2}} \\ & H+H+M={{H}_{2}}+M \\ & {{O}_{2}}+3{{H}_{2}}=2H+2{{H}_{2}}O \\ \end{align}$ (2)
(2) The solver used in the simulation is a separate pressurebased separation solver, which solves the control equations in sequence. In the process of discretizing nonlinear partial differential equations, to directly transform the nonlinear partial differential equations into linear algebraic equations on a grid element, the finite volume method can be considered. We can choose the standard format as the pressure interpolation format. The momentum equation, the turbulence kinetic energy equation, the turbulence dissipation rate equation, the component mass conservation equation and the energy equation can all be selected as the firstorder upwind scheme, because it has the remarkable characteristics and advantages of the secondorder upwind scheme such as high stability and fast calculation speed. SIMPLE algorithm is widely used in the actual research and calculation of fluid mechanics and computational heat transfer, and its subrelaxation factor is a numerical calculation method that can be widely used to calculate the flow of objects at any velocity, and the range of subrelaxation factor can be expanded in an appropriate amount. Accordingly, this paper will use the SIMPLE algorithm to solve the pressurevelocity coupling scheme. In the setting of the residual value, the residual value of the energy equation and the P1 radiation heat transfer equation is set to 10^{6}, and the other governing equations are all set to 10^{4}. In setting the sub relaxation factor, the subrelaxation factors of pressure, momentum, $k$ and $\varepsilon $ are set to 0.3, 0.7, 0.8 and 0.8 respectively.
2.4 Governing equation
Establishing the governing equation is the first step of numerical simulation. A general form of the governing equation is established to facilitate the analysis of each governing equation [1216]:
$\begin{align} & \frac{\partial }{\partial x}\left( \rho u\phi \right)+\frac{\partial }{r\partial r}\left( r\rho v\phi \right)+\frac{\partial }{r\partial \theta }\left( \rho w\theta \right)= \\ & \frac{\partial }{\partial x}\left {{\Gamma }_{\phi }}\frac{\partial \phi }{\partial x} \right+\frac{\partial }{r\partial r}\left r{{\Gamma }_{\phi }}\frac{\partial \phi }{\partial r} \right+\frac{\partial }{{{r}^{2}}\partial \theta }\left {{\Gamma }_{\phi }}\frac{\partial \phi }{\partial \theta } \right+{{s}_{\phi }} \\ \end{align}$ (3)
In the above equation, j is the universal dependent variable, G_{j }is the transport coefficient, S_{j }is the source term, r is the density, x, r, q respectively represent the axial coordinates, radial coordinates, circumferential coordinates, u, v, w respectively represent the component of the velocity vector in the x, r, q direction. (1) When j=1, G_{j}=0, it represents the continuity equation; (2) When j=u, v, w, it represents the momentum equation corresponding to the direction; (3) When j=k, it represents the turbulent energy equation; (4) When j=e, it represents the turbulent energy dissipation rate equation; (5) When j=f, it represents the equation of conservation of component mass; (6) When j=h, it represents the energy equation. Where k is the turbulent kinetic energy, eis the turbulent energy dissipation rate, f is the mass fraction, and h is the enthalpy.
3.1 Boundary conditions
The fuel methane nozzle was set as the velocity inlet boundary, and the velocity was 70m/s. In this case, the turbulence intensity and hydraulic diameter were set at 10% and 0.006m respectively, and the temperature was set at 300K. The velocity inlet boundary was set at the oxidant inlet. When H_{2}O was added to the oxidant, it was necessary to ensure that the content of O_{2} added each time remained unchanged, while the content of the additive H_{2}O gradually increased (or decreased). Taking the working condition CH_{4}/21%O_{2}/79%H_{2}O and the flow rate of the oxidant 0.6m/s as the benchmark. The flow rates of the oxidant in the set conditions were successively calculated as 0.42m/s, 0.36m/s, 0.315m/s, 0.28m/s and 0.252m/s. The turbulence intensity and hydraulic diameter were set at 10% and 0.294m, respectively, and the temperature was set at 300K.The outlet of the burner was set as the pressure outlet boundary, the gauge pressure was set as 0, and the turbulence intensity and hydraulic diameter were set as 10% and 0.3m respectively. At the same time, the backflow at the pressure outlet should also be considered. The other boundaries were set as wall boundaries, and since burner walls usually have cooling devices, the wall temperature was specified as 300K.
3.2 Calculation conditions
In view of the particularity of the chemically active substance H_{2}O, it is necessary to consider its chemical action as the key point. Thus, a fictional substance FH_{2}O is set up, which has the same thermal properties, transport properties and radiation properties as H_{2}O, but does not participate in chemical reactions.
Six working conditions (21%O_{2}, 30%O_{2}, 35%O_{2}, 40%O_{2}, 45%O_{2}, 50%O_{2}) were selected for the numerical simulation of natural gas oxygenenriched diffusion combustion in the O_{2}/H_{2}O atmosphere. FH_{2}O was added to the oxidant instead of H_{2}O, and the same boundary conditions and working conditions were set. The specific calculation working conditions are shown in Table 1.
Table 1. Calculation conditions
Calculation conditions 
Gas species 
Velocity, m/s 

fuel 
CH_{4} 
70 

oxidizer 
O_{2}/H_{2}O atmosphere 
case1 
21%O_{2}/79%H_{2}O 
0.6 
case2 
30%O_{2}/70%H_{2}O 
0.42 

case3 
35%O_{2}/65%H_{2}O 
0.36 

case4 
40%O_{2}/60%H_{2}O 
0.315 

case5 
45%O_{2}/55%H_{2}O 
0.28 

case6 
50%O_{2}/50%H_{2}O 
0.252 

O_{2}/FH_{2 }Oatmosphere 
case7 
21%O_{2}/79%FH_{2}O 
0.6 

case8 
30%O_{2}/70%FH_{2}O 
0.42 

case9 
35%O_{2}/65%FH_{2}O 
0.36 

case10 
40%O_{2}/60%FH_{2}O 
0.315 

case11 
45%O_{2}/55%FH_{2}O 
0.28 

case12 
50%O_{2}/50%FH_{2}O 
0.252 
4.1 Effect of additive H_{2}O content on oxyfuel diffusion combustion
Figure 3 shows the distribution curve of combustion temperature on the central axis under 6 working conditions.
As shown in the figure, the combustion temperature gradually decreases with the increase of the content of additive H_{2}O.In the axial direction, the effect of H_{2}O addition on combustion temperature increases first and then decreases. According to calculation, the maximum combustion temperature under six working conditions is 1517.4K, 1838.7K, 2261.8K, 2409.3K, 2622.8K and 2752.9K respectively. As shown in Figure 4, when the proportion of additive H_{2}O is in the range of 65%70%, the decrease rate of the maximum combustion temperature is the fastest, which indicates that the addition of H_{2}O in this concentration range has the greatest impact on the combustion temperature in the whole burner.
Figure 3. Distribution of combustion temperature on the central axis
Figure 4. Maximum combustion temperature in six operating conditions
As products in the combustion process, the production of CO and CO_{2} will inevitably be affected by the concentration of reactants H_{2}O. Figure 5 shows the distribution curves of CO volume fraction and CO_{2} volume fraction on the central axis under 6 working conditions. As shown in Figure 5(a), within the zones Z=00.25m and Z=2.53.0m, CO production remained almost unchanged with the increase of additive H_{2}O content; within the zones Z=0.251.5m, CO production decreased with the increase of additive H_{2}O content; within the zones Z=1.52.5m, with the increase of the content of additive H_{2}O, the amount of CO production also increases. As shown in Figure 5(b), CO_{2} production decreases with the increase of the content of additive H_{2}O, that is, the addition of H_{2}O inhibits CO_{2} generation. In the axial direction, the influence degree of H_{2}O addition on CO_{2} production shows a trend of first increasing, then decreasing and then increasing.
Figure 6 shows the distribution curve of NOx volume fraction on the central axis under 6 working conditions. As shown in the figure, the volume fraction of NOx in the condition of 21%O_{2}/79%H_{2}O spikes in the axial direction, which may be due to the combustion temperature below 1600K in the condition. The factor affecting the generation of NOx is the length of time that the reactants stay in the high temperature zone, while the high concentration of H_{2}O in the oxidizer does not participate in the reaction at the beginning. As a result, the production of NOx will increase rapidly with the extension of retention time before the combustion reaction reaches chemical equilibrium [1720]. The overall analysis of the other 5 working conditions shows that the NOx production decreases with the increase of the content of H_{2}O additive. And when the proportion of additive H_{2}O is in the range of 55%60%, the addition of H_{2}O has the greatest influence on NOx generation. In the axial direction, the effect of H_{2}O addition on NOx generation increases at first and then decreases [21].
Figure 7 shows the distribution curve of carbon black volume fraction on the central axis under 6 working conditions. According to the figure, within the zone Z=00.25m, the amount of carbon black production almost remained unchanged with the increase of the additive H_{2}O content; within the zone Z=0.251.5m, the amount of carbon black production decreased with the increase of the additive H_{2}O content; within the zone Z=1.53.0m, the amount of carbon black production decreased with the increase of the additive H_{2}O content. In the axial direction, the effect of the addition of H_{2}O on the carbon black production increased at first and then decreased until it was almost unaffected. As shown in Figure 8, the change curve of the maximum volume fraction of pollutant NOx and carbon black with the increase of additive H_{2}O content shows that, when the proportion of additive H_{2}O is within the range of 55%60%, the addition of H_{2}O inhibits the generation of pollutants in the whole burner, and the inhibition effect is the largest.
(a) COVolume fraction
(b) CO_{2}Volume fraction
Figure 5. Volume fraction distribution on the central axis
Figure 6. Distribution of NO_{x} volume fraction on the central axis
Figure 7. Distribution of soot volume fraction on the central axis
Figure 8. Volume fraction of maximum pollutants (NOx, soot) in six operating conditions
4.2 Effect of chemical action of additive H_{2}O on oxyfuel diffusion combustion
In order to distinguish the influence of chemical action on oxygenrich diffusion combustion, the simulation results of six working conditions in O_{2}/H_{2}O atmosphere were compared with those in O_{2}/FH_{2}O atmosphere, and the difference between the two was caused by chemical action. Figures 9 and 10 are the comparison of the maximum combustion temperature under two atmospheres and the fitting curve of temperature difference respectively. The temperature differences under conditions of 50%H_{2}O, 55%H_{2}O, 60%H_{2}O, 65%H_{2}O, 70%H_{2}O and 79%H_{2}O were calculated as 193.4K, 259.6K, 176.7K, 121.5K, 61.3K and 110.8K, indicating that when high concentration of H_{2}O is added to the oxidizer, in the process of CH_{4} oxyfuel diffusion combustion, there will be chemical action, which has a negative impact on combustion temperature. With the increase of additive H_{2}O content, the temperature difference decreases first, then gradually increases, and then decreases. The chemical action has the greatest impact on combustion temperature when the proportion of additive H_{2}O is in the range of 65%79%.
Figure 9. Comparison of the highest combustion temperatures
Figure 10. Fitting the temperature difference
Figure 11 shows the contrast curves of the maximum CO volume fraction and the maximum CO_{2} volume fraction under the O_{2}/H_{2}O atmosphere and the O_{2}/FH_{2}O atmosphere respectively. The comparison shows that the chemical effect of H_{2}O has a great influence on the generation of CO, and can promote the generation of CO to a large extent in the reaction process. On the whole, the chemical effect of H_{2}O has little effect on the generation of CO_{2}, and it can be considered that it is completely affected by physical effects, and the chemical effect can be ignored. More specifically, it has a slight inhibition effect on CO_{2} generation when the additive H_{2}O proportion is in the range of 50%65%, and can slightly promote CO_{2} generation when the additive H_{2}O ratio is in the range of 65%79%.
Figure 12 shows the contrast curves of the maximum NOx volume fraction and the maximum carbon black volume fraction at the outlet under six working conditions under two atmospheres. As shown in Figure 12(a), the chemical effect of H_{2}O promotes the generation of NOx when the content of H_{2}O is in the range of 50%55% and 70%79%; the chemical effect of H_{2}O has an inhibitory effect on the generation of NOx, and the inhibitory effect is the most obvious when the content of H_{2}O is in the range of 55%70%. As shown in Figure 12(b), the chemical action of H_{2}O also inhibits the generation of carbon black, and the influence is greatest when the content of H_{2}O is in the range of 65%70%. In general, the addition of H_{2}O can inhibit the generation of pollutants (NOx, carbon black) in the combustion process when the content of H_{2}O is in the range of 55%70%, and the inhibition effect on pollutants is the best when the content of H_{2}O is in the range of 65%70%.
(a) CO volume fraction
(b) CO_{2} volume fraction
Figure 11. Highest volume fraction comparison
In order to more intuitively observe the effect of chemical action on methane oxyfuel diffusion combustion degree, and compared with the physical action, determine its primary and secondary position in the reaction process. Table 2 shows the proportion of chemical inhibition on combustion temperature and pollutants (NOx, carbon black) when the proportion of additive H_{2}O is 65% and 70%. In the table,j_{T}, j_{NOx} andj_{Soot} represent the corresponding proportions respectively. As can be seen from the table, in the inhibition effect of H_{2}O on combustion temperature, physical effect plays a dominant role (accounting for more than 85%), while chemical effect has a small effect (accounting for less than 10%). In the inhibition effect of H_{2}O on the production of pollutants (NOx, carbon black), chemical action and physical action play an equal role (both are about 50%). In the process of oxyfuel diffusion combustion, the generation of pollutants is inhibited at the same time.
(a) Maximum NOx volume fraction
(b) Maximum soot volume fraction
Figure 12. Comparison of maximum volume fraction at the outlet
Table 2. Proportion of chemical inhibition

65%H_{2}O 
70%H_{2}O 
j_{T} 
6.76% 
11.34% 
j_{NOx} 
49.97% 
49.73% 
j_{Soot} 
43.37% 
48.24% 
In this paper, based on the principle of constant O_{2} concentration and gradient change of H_{2}O concentration, the boundary conditions and calculation conditions required in the simulation were determined. The combustion process under six different conditions of O_{2}/H_{2}O atmosphere and O_{2}/FH_{2}O atmosphere were numerically simulated. From the characteristics of combustion temperature, CO volume fraction, CO_{2} volume fraction and pollutant (NOx, carbon black) production, the horizontal analysis was carried out, and the influence rules of additive H_{2}O content and chemical action on the above characteristics were deeply explored. The conclusions were as follows:
(1) The chemical effect of H_{2}O has a negative influence on the combustion temperature, and the physical action plays a dominant role. The chemical action has the greatest influence on the combustion temperature when the proportion of H_{2}O is in the range of 6579%.
(2) The chemical effect of H_{2}O has a great influence on the generation of CO, and has a significant promotion effect on the production of CO; the chemical effect of H_{2}O has little effect on the generation of CO_{2}, which is almost negligible, and it is completely controlled by the physical effect.
(3) The chemical effect of H_{2}O has a significant inhibitory effect on the formation of NOx when the proportion of H_{2}O is in the range of 55%70%; the chemical effect of H_{2}O inhibits the formation of carbon black; the chemical effect has the greatest influence on the formation of dyes (NOx, carbon black) when the proportion of H_{2}O is in the range of 6570%.
Based on the above research and analysis, the optimal concentration range of H_{2}O can be determined to be 6579% based on the influence of H_{2}O content and chemical effect on combustion temperature and pollutant generation.
The authors gratefully expressed their thanks for the financial support for this research from the National Natural Science Foundation of China (Grant No.: 51974033), from the Yangtze Youth Talents Fund (Grant No.: 2015cqt01), from College Students' Innovation and Entrepreneurship Project (Grant No.: 2019314, Yz2020272).
Y 
volume fraction 
D 
diffusion coefficient 
S t 
generation rate time, s 
$\overline{v}$ 
average rate, m. s^{}^{1} 
Greek symbols 

r 
density, kg. m^{3} 
j 
universal dependent variable 
G_{j} 
transport coefficient 
x, r, q 
axial coordinates, radial coordinates, circumferential coordinates 
u, v, w 
the component of the velocity vector in the x, r, qdirection, m. s^{}^{1} 
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