Experimental Study of the Laminar Flame Speeds of the CH4/H2/CO/CO2/N2 Mixture and Kinetic Simulation in Oxygen-Enriched Air Condition.

The experimental study on the laminar flame speeds of the CH4/H2/CO/CO2/N2 mixture was carried out in oxygen-enriched air condition. The laminar flame propagation velocities of the blended gas were measured in a range of equivalence ratios (from 0.6 to 1.4) and oxygen concentrations (from 21 to 33%) using a Bunsen flame. Comparisons between the experiments and calculations show that the GRI Mech 3.0 mechanism can well predict the laminar flame speed of the blended gas in oxygen-enriched conditions. The laminar flame propagation velocities were enhanced by the increasing oxygen concentration, while the reaction pathway of fuel changed little. The effects of each species of the CH4/H2/CO/CO2/N2 mixture on the laminar flame speeds were discussed. Results show that the laminar flame speed is promoted by the increase of H2 and CO, while the laminar flame speed is decreased by the increasing CH4, CO2, and N2 concentrations. The inhibition effect of CO2 on the laminar flame speed is bigger than that of N2, which is due to the difference in the properties of CO2 and N2.

Introduction

Oxygen-enriched combustion is a promising technology to reduce pollution. This technology uses oxygen-enriched air or pure oxygen instead of air,[1] which improves environmental protection significantly.[2] Because of the increase of oxygen content, the flue gas contains a lot of high concentration carbon dioxide, which is helpful for capturing carbon. The volume of the flue gas is reduced in the condition of high O2 concentration because of the decrease of nitrogen content, which reduced the heat loss of the flue gas. On the other hand, the proportions of CO2 and H2O in the combustion products increase, so the radiation capacity of the products is greatly improved compared with that of traditional combustion. Thus, combustion efficiency and radiative heat transfer are improved by increasing oxygen content. Some fuels with low calorific value can reach higher combustion temperatures.

The laminar flame speed is an important parameter for studying flame characteristics.[3,4] The enriched oxygen content in the oxidizer changes the combustion characteristics, including the laminar flame speed. There are a lot of research studies that focus on the combustion characteristics of fuels in oxygen-enriched air condition. Mazas et al.[5] measured the laminar flame speed of CH4/O2/N2/H2O. They found that the increasing O2 content increased the laminar flame speed, while the chemical effect of H2O is reduced. Similar conclusions were obtained by Chica Cano et al.[6] Takahashi et al.[7,8] studied the extinguishing features of the CH4 diffusion flame, which is changed by the O2 concentration. Zhou et al.[9] studied the effects of O2 and CO2 on the laminar burning velocity of the O2–N2–CO2–iso-octane mixture. Results show that the added CO2 and increasing O2 content changed the laminar flame speed significantly. Pio et al.[10] studied the laminar flame speeds of CH4/O2/air experimentally. They found that the increase of O2 content promoted the laminar propagation velocity and expanded the flammable range of CH4. Salzano et al.[11] and Xu et al.[12] studied the explosive characteristics of syngas and hydrous bioethanol in oxygen-enriched air conditions. Results show that the rising oxygen fraction promoted the deflagration index and explosive pressure. Boushaki et al. investigated the effect of the added O2 on the generation of CO, NOx, and CO2.[13] The results show that oxygen enrichment is favorable for the increase of CO2 and NO and the decrease of CO emissions. Alabaş B et al.[14] found that the oxygen enrichment of air reduces the CO and increases the NO generation for the H2/CO/CH4 mixture in a swirl burner.

Generally, the laminar burning velocity is affected by the compositions of fuels,[15,16] oxygen concentration,[17,18] dilution gas,[19−22] initial temperature,[23] and pressure.[24] For example, Nair et al.[25] studied the laminar flame propagating velocity of liquid petroleum gas in air condition. Weng et al.[26] measured the laminar flame speed of H2 in O2/N2 and O2/CO2 atmosphere using a Bunsen flame. Yelishala et al.[27] investigated the effects of the equivalence ratios, the initial pressure, and temperature on the laminar flame propagating velocities of propane/air/CO2 mixture. Wu et al.[28] measured the laminar flame propagation velocity of the CH4/air and the acetone/air mixture in a closed vessel. All of the above research shows that the laminar flame speeds of a gas mixture are affected significantly by the compositions of the gas mixture. The laminar flame speed of fuel in oxygen-enriched air condition is different from the air atmosphere because the high oxygen content of the mixture changes the combustion process.

In fact, there are a large number of fuels with a low calorific value in the industrial field. A large number of gas fuels, such as the blast furnace gas, are produced in the process of iron-making in iron-steel plants. The compositions of the blended gas produced in steel and iron plants usually consist of CO, H2, and CH4, while the inert gas CO2 and N2 were mixed in the blended gas. If the oxygen-enriched combustion technology can be applied to the blended gas in iron-steel plants, it will reduce a large amount of energy consumption and CO2 emission. However, the studies on the laminar flame speeds of the blended gas in oxygen-enriched air are inadequate, especially for the complex blended gas. The laminar flame speeds of the blended gas are very useful for industrial applications, such as the blended gas burner design and safety prediction. In the present work, the laminar flame speeds of the blended gas were studied in oxygen-enriched air, which is helpful for the application of the blended fuel in the iron-steel field in oxygen-enriched air condition. The laminar flame speeds of the blended gas are new and the discussion about the factors on the laminar flame speed is useful for designing new burners or any other purpose.

The main components of the blended gas are CO, H2, CH4, CO2, and N2. The concentration of each species is shown in Table . Besides the species in Table , there is a very low proportion of O2 (less than 1.5%). Here, the residual O2 was considered in the oxidizer (O2/N2).

Table 1

Main Composition of the Blended Gas

species	CO	H2	CH4	CO2	N2
volume concentration (%)	17.34	24.55	9.30	10.87	36.56

Experimental Methods

Figure shows an experimental device, which consists of a gas supply system, a constant temperature water bath, a Bunsen burner, and a digital camera (Canon, 80D). The pure CO, H2, CH4, CO2, O2, and N2 came from the high-pressure gas cylinders. The purity of each species is above 99.99%. Six mass flow controllers (MFCs) were used to control the flow rate of each species. After MFC, six kinds of gases were mixed in a pipe sufficiently. The pipe is over 10 m long to mix the gas mixture thoroughly. Then, the blend gas was preheated in the water bath. The temperature of the water bath was controlled by the temperature controller. Finally, the premixed gas entered the burner, which was made of a straight tube. Three stainless steel tubes with 6, 8, and 12 mm inner diameter, respectively, were used to meet the requirement of different flow rates. The length of each stainless steel tube is about 1.0 m, which ensures that the laminar flow of the gas mixture is fully developed. A digital camera was used for recording the flame pictures. The flame contour was captured and the area of the flame surface (reaction zone) was calculated using these flame pictures. The experimental measurements were performed at 300 K, 1 bar.

Figure 1

Schematic Diagram of the Experimental Device.

Based on the mass balance, the formula for calculating the laminar flame speed SL of a one-dimensional laminar flame is as follows.[29,30]where Ab is the area of the reaction zone and Q̇ is the total volume flow rate of the premixed unburned gas. The value of Q̇ was the sum of the values of six MFCs, and Ab was obtained from the flame picture. The shape of the flame is a cone, and the area of the reaction zone is determined by the edge of the cone. The edge of the reaction zone is the position where the flame temperature is the highest. The brightest position in the vertical direction of the flame image corresponds to the hottest point of the flame. First, the brightest points in the flame image were captured using a piece of Matlab code, then the value of Ab was calculated based on the captured edge of the reaction zone.

The uncertainty of the measurement is mainly from two aspects. On the one hand, the uncertainty is from the experimental method, including the heat loss from the reaction zone to the unburned gas, the calculation of the flame area, and the estimation of SL using eq . The uncertainty in this aspect is about 3% concerning all of the influence factors. On the other hand, the uncertainty is from the error of the instruments, including the errors of the MFCs and the temperature fluctuation. The total error from the MFCs is about 3% and that from the temperature is about 1.5%. Thus, the total uncertainty of the measurement is expected to be 4.5% (eq ).

Computing Methods

In the present work, the PREMIX code[31] in Chemkin package was employed to calculate the laminar flame speed. A freely propagating flame was chosen in the kinetic simulation. In the calculation, the maximum grid number was set to be 400. Both the GRAD and CURV values were 0.01. The length of the reaction zone was 10 cm to ensure that the chemical reactions have been completed in the reaction zone. For comparison, the GRI Mech 3.0[32] and USC Mech II mechanism[33] were used in the calculation. GRI Mech 3.0 was usually used for describing methane oxidation. This mechanism contains 53 components and 325 elementary reactions. The USC II mechanism was proposed for describing the combustion of CO/H2/hydrocarbon fuels (C1–C4). This mechanism contains 111 components and 784 elementary reactions.

Results and Discussions

Effect of the Equivalence Ratios

Figure shows the measurements and calculations of SL of the blended gas versus the equivalence ratios ϕ at different oxygen concentrations (21, 25, 27, 29, 33%). Here, the oxygen concentration is the mole fraction of O2 in the oxidizer (O2/N2). As seen in Figure , both the experimental and calculated SL increased with the increase in the equivalence ratio at the lean-fuel side and decreased at the rich-fuel side. The peak value of SL appears at the equivalence ratio from 1.0 to 1.2. The peaks of the experimental laminar flame speeds are 43.6, 62.1, 77.3, 87.2, and 102.7 cm/s at 21, 25, 29, 33, and 37% oxygen concentrations, respectively. Figure shows the measured laminar flame speeds of pure fuels and syngas in air condition. Through comparison, the peak position of the blended gas is close to that of pure CH4,[5] C3H8,[25] and H2[19] and different from that of syngas.[26] The position of the maximum SL of H2/CO mixture appears at around ϕ = 2.4 because the content of CO (75%) in the H2/CO mixture is much higher than the H2 content (25%). These experiments indicate that the position of the peak value is closely related to the composition of fuels. The fractions of H2 and CH4 are higher than that of CO in the blended gas, thus the change trend and the position of the peak value of the blend gas are mainly determined by the content of H2 and CH4.

Figure 2

Measurements and calculations of SL of the blended gas versus the equivalence ratios ϕ.

Figure 3

Experimental SL of H2, CH4, C3H8, and syngas versus the equivalence ratios ϕ.

The effect of the equivalence ratio on the SL is mainly due to the change in the flame temperature. As the equivalence ratio increased from the lean-fuel to the rich-fuel sides, the flame temperature increased first and then decreased. Near the stoichiometric point (ϕ = 1.0), the fuel and oxidizer are completely consumed, and the flame temperature reaches the maximum value. Thus, the peak point of the laminar flame speed appears near the stoichiometric point.

It is found that SL has a nearly quadratic function relationship with the equivalence ratio. Through comparison, the measured laminar flame speeds are found to be close to the calculated results using GRI 3.0. The average difference between the measurements and the calculation with GRI 3.0 is about 3%, while the average difference is about 7% for USC II. Thus, the discussions were performed with the calculated results using GRI 3.0 in the following sections.

Effect of Oxygen Concentration

Figure shows SL versus oxygen concentrations in some cases. The results show that under a certain equivalence ratio, SL changes approximately linearly with increasing oxygen concentration, which is different from the change trend of some fuels. For example, SL of methane has a “u” (approximately quadratic function) relationship with oxygen concentrations.

Figure 4

SL versus oxygen concentrations at different equivalence ratios.

The effects of the oxygen concentration on SL are from two sources. On one side, the increasing oxygen concentration reduces the N2 content, which decreases the amount of the exhaust and reduces the heat loss significantly. Thus, the adiabatic flame temperature is promoted by the increasing oxygen concentration. On the other side, the O2 molecule enhances the free radical content in the reaction zone, such as O radical, which increases the collision probability of free radicals and promotes the branch reactions.

Figures –7 show the reaction pathway of the blended gas in different oxygen concentrations (21, 25, and 29%). In these figures, the arrow indicates the oxidation process of the major intermediates. The reaction numbers were marked along the arrows. The numbers in the brackets are the contribution rates of the related reactions. CO is an important intermediate in the oxidation process of CH4 because the oxidation process of CO is involved in that of CH4. H2 is an important combustible species in the blended gas. Thus, the reaction pathway of the blended gas consists of the oxidation process of CH4 and H2. It is noted that several “CO” and “H2” are used in these figures to express the pathway more clearly. Here, these “CO” or “H2” are the same substance.

Figure 5

Reaction pathway of the blended gas at 21% oxygen concentration.

Figure 7

Reaction pathway of the blended gas at 29% oxygen concentration.

Reaction pathway of the blended gas at 25% oxygen concentration.

The major pathway of the oxidation of CH4 is as follows: CH4 → CH3 → CH2O → HCO → CO → CO2, which is consistent with our previous results.[34] This major oxidation pathway is not affected by the increasing oxygen concentration, but some contribution rates of element reactions are changed by elevated oxygen concentration. There are three major element reactions in the first step of dehydrogenation of CH4: O + CH4 ↔ OH + CH3(R11), H+CH4 ↔ CH3 + H2(R53), and OH + CH4 ↔ CH3 + H2O(R98). Most of CH4 is oxidized into CH3 through the three reactions. The sum of the contribution rates of these elementary reactions is 98–99%, which is not changed by the oxygen concentration. H+CH3(+M) ↔ CH4(R52) is the most important reaction for CH4 generation. This reaction is also an important elemental reaction that decreases SL because of the prevention of CH4 dehydrogenation.

The largest H2 consumption is from OH + H2 ↔ H + H2O(R84). This reaction is the major pathway to generate H radical and H2O from H2 oxidation. Because of the generation of H, this reaction plays an important role in promoting SL. However, the contribution rate of this reaction is slightly reduced by the rise of O2 concentration, which is due to the competition of H2 with other radicals such as O. The second important reaction for H2 oxidation is O + H2 ↔ H + OH(R3). This reaction increases H and OH, which increases SL. The contribution rate of this reaction was enhanced with the increased oxygen concentration. Besides the addition from the blended gas, H2 is generated from the CH4 dehydrogenation through the reaction H + CH4 ↔ CH3 + H2(R53), H + CH4 ↔ CH3 + H2(R45), H + CH2O ↔ HCO + H2(R58), and O + CH3 → H + H2 + CO(R284). The contribution rates of these reactions are also changed by the oxygen concentration, but both the generation and consumption pathways of H2 are changed little by the oxygen concentration.

Almost all of the CO was oxidized into CO2 through the reaction OH + CO ↔ H + CO2(R99), which has a more than 97% contribution rate. However, the effect of R99 was decreased by the increase in oxygen concentration. Because the contribution rate of the reaction O + CO(+M) ↔ CO2(+M)(R12) was enhanced by the increasing O, which was closely related to O2 concentration.

Influence of the Composition of the Blended Gas

The blended gas composed of combustible components (H2, CO, CH4) and incombustible components (N2 and CO2). The effects of each species on SL are discussed in this section. Here, the concentration of each species is the volume fraction of the species in the blended gas. If one component concentration increases, the concentrations of other components are reduced in the initial proportions.

Effect of H2, CO, and CH4

Figure shows SL of the blended gas versus the H2 concentration at the stoichiometric ratio (ϕ = 1.0). The results show that SL was enhanced by the increase in H2 concentration from 0 to 60%. The rate of increase in SL increased with an increase in H2 content. The increase in H2 concentration promotes the calorific value of the blended gas. Figure also displays variation in the adiabatic flame temperature versus H2 concentration. The equilibrium gas model coupled with the detailed thermal data of GRI 3.0 is used to estimate the adiabatic flame temperature. The result shows that the increasing H2 concentration increased the adiabatic flame temperature because the combustion heat release was increased by H2. The increasing H2 increased the H and OH radicals through reactions such as R3, which accelerates the combustion rate.

Figure 8

SL versus H2 concentrations at different oxygen concentrations.

Figure shows the change in SL versus CO concentration. The laminar flame speed was increased with the rise in the combustion release from increasing CO. However, the increased rate was reduced with the increasing CO until it became negative in the condition of high CO concentration. On the one hand, the calorific value of CO is lower than that of H2 and CH4. The concentrations of H2 and CH4 decreased with the increasing CO, so the increased rate of the total heat release of the blended gas was reduced. On the other hand, the amount of some free radicals such as H decreased with the decrease of H2 and CH4 content, because the free radicals containing H were generated from the dehydrogenation. The decrease in free radicals slowed down the laminar flame speed.

Figure 9

SL versus CO concentrations at different oxygen concentrations.

Figure shows that SL of the blended gas decreased with the increase in the CH4 concentration from 0 to 60%. The decrease in SL under the influence of CH4 was because of the increase in the N2 concentration in the oxidizer (O2/N2). For example, under the condition of 30% oxygen concentration, the chemical reaction equations of CH4, CO, and H2 were as follows.According to the above equations, about 4.66 mol N2 is added in the premixed mixture from the oxidizer for 1 mol CH4, while 1.17 mol N2 is added for 1 mol H2 or CO. It is inferred that there is more N2 in the combustion process of CH4 than that of H2 or CO. N2 is an inert gas that reduces the laminar flame speed through the thermal and dilution effects.[35] The adiabatic flame temperature of the blended gas is promoted by the CH4 content, which indicates that the thermal property of the premixed gas is not the major reason for the decrease of SL. Thus, the dilution effect of N2 is the main factor in the reduction of SL. Two N atoms connected with each other by bonds in the N2 molecule. The N2 molecule acts as a barrier to isolate the free radicals, the fuel, and O2. Thus, the reaction rates and the laminar flame speeds are decreased by the elevated CH4 concentration.

Figure 10

SL versus CH4 concentrations at different oxygen concentrations.

Effect of N2 and CO2

Figures and 12 show the effects of N2 and CO2 concentrations on SL, respectively. The increase in N2 and CO2 concentrations decreases the laminar flame speed. This is mainly because both N2 and CO2 are incombustible species. First, the increase in N2 or CO2 content reduces the amount of H2, CO, and CH4, which decreases the total heat release of the blended gas. Second, the dilution effect of N2 or CO2 reduces the collision possibility of free radicals. Third, the added N2 or CO2 decreased the adiabatic flame temperature through the thermal property. Through comparison, the inhibition effect of CO2 on SL is bigger than that of N2. On the one hand, the specific heat capacity of CO2 is higher, thus the flame temperature of the gas mixture diluted by CO2 is lower. On the other hand, the CO2 molecule has a larger diameter and a larger surface area. CO2 reduces more possibility of collision between active molecules. In addition, CO2 is not an inert gas but participates in chemical reactions, such as R99, which reduces the H concentration and combustion reaction rate.[36] For the above reasons, the inhibition effect of CO2 on SL of the blended gas is bigger than N2.

Figure 11

SL versus N2 concentrations at different oxygen concentrations.

Figure 12

SL versus CO2 concentrations at different oxygen concentrations.

Conclusions

In this paper, SL of the CH4/H2/CO/CO2/N2 mixture has been investigated using a Bunsen flame and the kinetic simulation. These results are very useful for the oxygen-enriched combustion technology of the flexible blended gas with low calorific value in the industrial field. The following conclusions are formulated from the experiments and calculations. First, the peaks of the experimental laminar flame speed of the CH4/H2/CO/CO2/N2 mixture are 43.6, 62.1, 77.3, 87.2, and 102.7 cm/s at 21, 25, 29, 33, and 37% oxygen concentrations, respectively, which usually appeared at the equivalence ratio from 1.0 to 1.2. Second, the elevated oxygen concentration promoted the laminar flame speed. Third, the reaction rates and the contribution rates of the reactions were changed by the increased oxygen concentration of an oxidizer, but the reaction pathway changed little. Next, the laminar flame speed increased with the increase in H2 and CO concentrations but decreased with the increase in CH4, CO2, and N2 contents. Lastly, the inhibition effect of CO2 on SL is bigger than that of N2, which is due to the higher specific heat and chemical effect of CO2.