Fundamentals of Oxy-fuel Combustion

Chapter 2

Fundamentals of Oxy-fuel Combustion Sheng Chen Huazhong University of Science and Technology, Wuhan, China

2.1 INTRODUCTION In oxy-fuel combustion, chemical reactions take place in O2/CO2 atmosphere where O2 in the oxidant flow is diluted by CO2, rather than by N2 as in a conventional air-firing mode. Because the chemical and physical properties of CO2 are quite different from those of N2, the combustion characteristics and reaction pathways in an oxy-fuel combustion condition is altered more significantly than its air-firing counterpart [1]. These differences have a critical impact on the design, operation, and optimization of combustion systems; therefore it is crucial that relevant practitioners understand the chemical and physical effects of CO2 on combustion features, reaction kinetics, pollution generation, and so on. The purpose of this chapter is to present a brief, up-todate overview of the progress in these fundamental areas, using oxy-methane combustion as an example, first because methane is the main composition of many sustainable clean fuels (e.g., natural gas and landfill gas) [2] and second because it can help us understand the reaction mechanisms of volatiles released by solid fuels in O2/CO2 atmosphere [3].

2.2  EFFECTS OF CO2 ON COMBUSTION CHARACTERISTICS The effects of CO2 on combustion characteristics can be attributed to the following chemical and physical properties: (1) its low thermal diffusivity; (2) its high molar heat capacity; (3) its chemical path alteration, because it is not an inert gas in oxy-fuel combustion; and (4) its modified radiative heat transfer feature. Generally, the influences of CO2 on combustion characteristics fall into four categories as discussed next.

Oxy-fuel Combustion. http://dx.doi.org/10.1016/B978-0-12-812145-0.00002-5 Copyright © 2018 Chuguang Zheng and Zhaohui Liu. Published by Elsevier Ltd. All rights reserved.

13

14  Oxy-fuel Combustion

2.2.1  Burning Velocity in O2/CO2 Atmosphere Burning velocity, the rate of flame propagation relative to unburned gas ahead of the flame front, is a fundamental combustion property of all fuel/oxidant mixtures. A change in the burning velocity of a reactant mixture may cause poor combustion performance and a modified distribution of temperature and species in combustion chambers. Consequently, identifying the variation trend in the burning velocity of methane is very important for the oxy-methane combustion process. About three decades ago, it was found that substituting N2 with CO2 in the oxidizer led to a reduction of the flame speed [4]. Such reductions were observed over a wide range of experimental conditions, for example, with pressures varying from 0.25 atm to 2 atm (atmosphere) and with flame temperatures varying from 1550 to 2250 K [4]. Then Liu et al. gave an explanation for such phenomena from the viewpoint of the chemical properties of CO2; they believed that the decrease in burning velocity in O2/CO2 atmosphere could not be entirely described only by considering the physical properties of CO2 (the higher molar heat capacity of CO2 would cause a lower adiabatic flame temperature) [5]. With the aid of the software CHEMKIN, their analysis indicated that the most important reaction associated with the direct chemical participation of CO2 in these flames is CO + OH ↔ CO2 + H (2.1) which reduced the concentration of important radicals, for example, H, O, and OH, in the combustion chamber and thus reduced the burning velocity. In order to provide more detailed experimental support, Heil et  al. carried out an experimental study based on a lab-scale furnace at RWTH Aachen University [6]. Their well-designed experimental activities demonstrated that the chemical effects of CO2 had a significant impact on the production and consumption rates of carbon monoxide in oxy-fuel combustion. CO2 competed with O2 for atomic hydrogen and led to the formation of CO through the reverse reaction of reaction (2.1), which could decrease the burning rate of methane in O2/CO2 atmosphere. They also found that an increase of O2 concentration in oxy-fuel combustion might reduce this impact. Oh and Noh [7] also investigated the burning velocity of methane flame in O2/CO2 atmosphere, but with the aid of a lab-scale Bunsen burner. They conducted their experiments in atmospheric conditions (300 K and 1 atm), not only in fuel-lean but also in fuel-rich conditions. However, they observed that the laminar burning velocity of methane flame in O2/CO2 atmosphere is faster than its air-firing counterpart. In order to validate their experimental observations, the authors also conducted a numerical verification with the aid of the CHEMKIN software. They claimed that the laminar burning velocity of methane flame in O2/CO2 atmosphere depended on the temperature of the reactive mixture, the ambient pressure, the global equivalence ratio, and the mixture

Fundamentals of Oxy-fuel Combustion  Chapter | 2  15

fraction of the diluent gases. The maximum burning velocity of methane flame in O2/CO2 atmosphere appeared in a fuel-rich condition, in the vicinity of a stoichiometric reaction. It is very interesting that Oh and Noh’s major observations are quite opposite those shown in previous literature [4,5]. The discrepancy may result from the differences between their research prototypes. In Refs. [4,5] the adiabatic flame temperature in O2/CO2 atmosphere was lower than its air-firing counterpart. On the contrary, in Ref. [7], it was reported that, for their investigated configuration, the adiabatic flame temperature in O2/CO2 atmosphere was higher than its air-firing counterpart. The effects of water vapor on the burning velocity of methane flame in O2/ CO2 atmosphere was reported in Ref. [8]. It is well known that in the wet cycling of power plants, the concentration of water vapor is so high that its influence cannot be ignored. The authors observed that the laminar burning velocity of methane flame in O2/CO2 atmosphere would experience a quasi-linear decrease when the steam molar fraction was increased. Furthermore, it was found that, compared with its air-firing counterpart, such a decrease would be more pronounced in oxy-fuel combustion. Mazas et al. presented an explanation of this phenomena by analyzing the physical and chemical effects of adding steam [9]. They claimed that the effects were mainly attributed to steams high chaperon efficiency in the recombination reaction H + O2 + M ↔ HO2 + M (2.2) which competes with the chain-branching reaction H + O2 ↔ OH + O (2.3) They underlined that methane oxidation in the reaction CH 4 + O ↔ CH 3 + OH (2.4) would be suppressed because steam also favors O radical consumption and OH production through the reaction H 2 O + O ↔ OH + OH (2.5) More recently, Xie et al. [10] investigated, experimentally and numerically, the burning velocity of oxy-methane flame under various pressures. Their results were consistent with those in Refs. [4,5] in that the laminar burning of methane flame in O2/CO2 atmosphere decreased with an increase in the CO2 fraction. Furthermore, the authors found that although the calculated data agreed well with the experimental measures under the atmospheric pressure, the discrepancy became greater with an elevated pressure. They also observed that the discrepancy became greater with a decrease in the CO2 fraction. The authors explained that the discrepancy was partially caused by radiation reabsorption of the flames, which had a positive effect on the laminar burning velocity when the mixtures CO2 fraction was high. They emphasized that the GRI-3.0 mechanism, which was used regularly for methane combustion simulation, needed to be

16  Oxy-fuel Combustion

revised and validated for the calculation of oxy-methane flames, since it might account for the main part of the discrepancy.

2.2.2  Adiabatic/Maximum Flame Temperature in O2/CO2 Atmosphere Adiabatic flame temperature is a fundamental parameter for theoretical combustion. Maximum flame temperature is another important quality in combustion science and engineering and is more useful for realistic combustion systems. In fact, adiabatic flame temperature is just the maximum temperature of reactants in an adiabatic condition. Generally, the adiabatic flame temperature decreases with the addition of CO2 in the mixture for all equivalence ratios. It was reported that the adiabatic flame temperature did not decrease linearly with the increase of the CO2 fraction because its thermal effects became stronger with a higher concentration [10]. To reveal which effects are the dominant ones for determining the maximum flame temperature of oxy-methane combustion, a detailed comparison analyzing the difference between the physical and chemical effects of CO2 was presented in Ref. [1]. In their study, the analysis was based on the software CHEMKIN. The research prototype was counterflow diffusion combustion, a benchmark in combustion science. The effects of added steam on the maximum flame temperature was considered; as previously mentioned, the effects of steam on oxyfuel combustion cannot be omitted in wet cycling of a power plant where the mole fraction of H2O in the oxidant stream may be as high as 30 vol%. To aid the analysis, artificial species X, X′, Y, and Y′ were introduced. In their research, X and Y had the same thermal and transport properties of CO2 and H2O, respectively, but they were not allowed to be a part of a chemical reaction. Therefore X and Y were regarded strictly as chemically inert species. Moreover, artificial species X′ and Y′ had the same transport properties as those of CO2 and H2O, respectively, while their thermal properties remained the same as N2, and they were not part of a chemical reaction. Therefore X′ and Y′ were regarded strictly as thermal and chemical inert species. A total of 11 cases were designed to conduct a comprehensive comparison analysis, as shown in Table 2.1. Fig. 2.1 illustrates the variation of the maximum flame temperature at various mole fractions of O2 in the oxidizer stream, with CO2, X, and X′ as additives. The differences in the predictions for the maximum flame temperature between X′ and X additive cases are due to the thermal effect. Thus the difference between the additive cases of CO2 and the artificial species X is due to the chemical effect. The results show that both the thermal and chemical effects of CO2 decrease the flame temperature. It is clear that the result of the thermal effect is due mainly to the higher heat capacity of CO2. The main factor for the chemical effect is the thermal dissociation of CO2, which can be explained by two characteristics: (1) the thermal dissociation of the CO2 process is endothermic, which is quite noticeable at high mole fractions of CO2 under oxy-fuel conditions; and

Fundamentals of Oxy-fuel Combustion  Chapter | 2  17

TABLE 2.1  Cases Designed for Comparison Analysis Between Physical and Chemical Effects of CO2 and H2O Case

O2

CO2

H2O

X

X′

Y

Y′

Sim1

20%

–

–

–

80%

–

–

Sim2

25%

–

–

–

70%

–

5%

Sim3

25%

–

–

70%

–

–

5%

Sim4

25%

70%

–

–

–

–

5%

Sim5

25%

70%

–

–

–

5%

–

Sim6

25%

70%

5%

–

–

–

–

Sim7

30%

–

–

–

50%

–

20%

Sim8

30%

–

–

50%

–

–

20%

Sim9

30%

50%

–

–

–

–

20%

Sim10

30%

50%

–

–

–

20%

–

Sim11

30%

50%

20%

–

–

–

–

3000 a a

Maximum temperature (K)

a a

2500

a a a a a

2000

1500 0.0

0.2

0.4 0.6 aCO2' aX ' aX '

O2 O2 O2 O2 O2 O2 O2 O2 O2

0.8

= 0.2 CO2 = 0.2 X = 0.2 X ' = 0.3 CO2 = 0.3 X = 0.3 X ' = 0.4 CO2 = 0.4 X = 0.4 X '

1.0

FIG. 2.1  Variation of the maximum flame temperature with the mole fraction of CO2, X, and X′ for various mole fractions of O2 in the oxidizer stream. (Reprinted with permission from Wang L, Liu Z, Chen S, Zheng C, Li J. Physical and chemical effects of CO2 and H2O additives on counterflow diffusion flame burning methane. Energy Fuels 2013;27:7602–11. Copyright (2013) American Chemical Society.)

18  Oxy-fuel Combustion

1800 AIR OCDR OCWR 1600

Base point

Chemical effect of H2O

Thermal effect of H2O

Chemical effect of CO2

Chemical effect of H2O

2000

Thermal effect of H2O

Temperature (K)

2200

Chemical effect of CO2

Thermal effect of CO2

2400

Thermal effect of CO2

(2) the thermal dissociation of CO2 breaks the chemical equilibrium of some reactions related to the production of CO. These factors increase the time for the production and oxidation of CO, thus slowing the release of heat. In Ref. [1], in addition to the thermal effect of CO2 on the maximum flame temperature, the influence of the mass diffusion of CO2 was also covered. In most available literature discussing the physical effects of CO2 on oxy-fuel combustion, the influence of the latter usually was ignored. In Ref. [1], a type of oxy-methane counterflow combustion was adopted, where 21% O2 was diluted by 79% CO2 or CO2∗. CO2∗ is an artificial species whose diffusion property is the same as N2. The results indicated that although the lower diffusion coefficient of CO2 would not change the maximum flame temperature obviously, it would alter the temperature profile significantly. In addition, it was observed that the thermal effects of H2O decreased the maximum flame temperature because of its higher heat capacity. The difference between the additive cases of H2O and the artificial species was due to chemical effects. The results indicated that the chemical effects of H2O slightly increased the flame temperature. The overall physical and chemical effects of CO2 and H2O on the maximum flame temperature of oxy-methane combustion is summarized in Fig. 2.2, where AIR, OCDR, and OCWR denote conventional air-firing, oxy-fuel combustion with dry cycling, and oxy-fuel combustion with wet cycling, respectively. For either OCDR or OCWR, both the thermal and chemical effects of CO2 induce a decrease in the flame temperature, and the thermal effect shows the greatest importance. However, for OCWR, the proportion of the chemical effect of CO2

Base point

Sim1 Sim2 Sim3 Sim4 Sim5 Sim6 Sim7 Sim8 Sim9 Sim10 Sim11

FIG. 2.2  Overall effect for both CO2 and H2O on maximum flame temperature of oxy-methane counterflow combustion. (Reprinted with permission from Wang L, Liu Z, Chen S, Zheng C, Li J. Physical and chemical effects of CO2 and H2O additives on counterflow diffusion flame burning methane. Energy Fuels 2013;27:7602–11. Copyright (2013) American Chemical Society.)

Fundamentals of Oxy-fuel Combustion  Chapter | 2  19

on the flame temperature increases. Moreover, the thermal and chemical effects of H2O on the flame temperature almost cancel each other out; therefore the existence of H2O hardly affects the flame temperature. It is well known that pressure has a significant influence on flame temperature. The effects of various pressures on the adiabatic flame temperature of methane in O2/CO2 atmosphere were numerically investigated by Seepana and Jayanti [11]. They found that the adiabatic flame temperature increased monotonically with pressure, ranging from 1 to 30 bar. The authors also made a comparison with its air-firing counterpart. Their results showed that under the same pressure, the adiabatic flame temperature of methane in O2/CO2 atmosphere is much lower than its air-firing counterpart because of the higher specific heat of CO2.

2.2.3  Ignition, Extinction, Flammability, and Flame Instability in O2/CO2 Atmosphere Knowledge about ignition, extinction, and flame stability is crucial for any combustion technology because of performance and safety considerations. Unfortunately, so far the relevant literature on oxy-methane combustion is quite sparse. A detailed experimental study on the ignition times of oxy-methane combustion [12] was published last year. According to the best knowledge of the present author, before Ref. [12] there was no publication on the influence of high concentration CO2 on ignition delay times for methane, although there are several studies on the ignition of propane in O2/CO2 atmosphere [13]. The experiments were carried out in a shock tube facility, with temperatures ranging from 1577 to 2144 K and pressures varying from 1 to 4 atm. The experimental data in Ref. [13] showed that the ignition of methane was postponed with an increasing CO2 concentration, although the delay was not significant. An empirical correlation for the ignition delay time was proposed based on the experimental data, which read as d τ = Ae E / RT P bϕ C X CO (2.6) 2

where τ, A, E, R, T, P, φ, XCO2, b, c, and d are the ignition delay time, preexponential factor, activation energy, universal gas constant, pressure, equivalent ratio, methane mole fraction, and fitting parameters, respectively. The authors identified the most dominant reaction in their case to be: H + O2 ↔ O + OH (2.7) As in oxy-fuel combustion, CO2 competed for the H radicals through the reaction listed in Eq. (2.1), whereas the reaction presented by Eq. (2.7) was inhibited. The authors claimed that the ignition delay of methane in high concentration CO2 atmosphere was attributed to the reaction pathway Eq. (2.7). Besides the chemical effects, the authors also thought that the physical effects of CO2, namely its lower diffusion coefficient and higher heat capacity, c­ ontributed to ignition delay. Because there were some uncertainties in the measurements of the ignition delay time, further studies are necessary.

20  Oxy-fuel Combustion

Comparing Refs. [12,13], it seems that the ignition delay times of fuels in O2/CO2 atmosphere are extremely sensitive to fuel types. However, as the available open data on this topic are too limited, extensive future investigation on the dependence of ignition delay times on fuel types. The effects of CO2 addition on nonpremixed flame extinction was investigated numerically by Seepana and Jayanti [14]. They indicated that the peak flame temperature and the mole fractions of the intermediate radicals H, O, OH, and CH3 all decreased with the addition of CO2. They also conducted a comparison between oxy-methane flame extinction and its air-firing counterpart. They found that for their investigated cases, the extinction features of oxy-methane flames were nearly the same with the N2-diluted air-methane flames. The effects of radiative heat loss and stretch rate on oxy-methane counterflow flame extinction were reported by Ref. [15]. Their research prototype was a lab-scale counterflow burner. They claimed that the effect of radiative heat loss on stretch extinction limits was not significant in the high oxidizer temperature. Their results also showed that the extinction stretch rate was mainly determined by the total fuel enthalpy flux and specific chemical kinetics. Moreover, the contributions of physical and chemical properties of dilution gases to flame extinction were analyzed numerically. Through a comparison analysis, they claimed that the extinction curves of air-methane flames were close to those of oxy-methane counterflow flames, which was consistent with the conclusion in Ref. [14]. Kim et  al. [16] also investigated the effects of chemical and radiation on oxy-methane counterflow flame extinction by numerical simulation. They found that the effect of radiative heat loss on the critical dilutent mole factor for flame extinction was not significant, which agreed with the statement in Ref. [15]. In particular, they studied the flame extinction limit under a high strain rate. Through a scaling analysis, they proposed a correlation to quantify the relationship between the extinction stretch rate, the oxidizer Lewis number, and a specific chemical kinetic term. They suggested that the intermediate radical, OH, was a good indicator for flames, not only in oxy-fuel combustion but also in air-firing scenes. The extinction characteristics of oxy-methane counterflow flames under elevated pressures were reported by Ref. [17]. Their study was carried out with the aid of a lab-scale experimental apparatus. Numerical simulation was also conducted to check and explain their experimental data. The authors observed that the conventional C-shape extinction curves caused by the upstream conduction loss at a lower stretch rate could also be observed under low pressures (0.1–0.2 MPa for their research schematic configuration) in oxy-­ methane counterflow combustion. However, under high pressures (0.5–0.7 MPa), the extinction characteristics of oxy-methane counterflow flames were different from their air-firing counterpart, and their extinction curves illustrated a monotonously broadening trend. The authors believed the difference could be

Fundamentals of Oxy-fuel Combustion  Chapter | 2  21

attributed to radiation reabsorption, which would play a significant role at high-pressure conditions. Hu et al. [18] investigated the flammability limits of oxy-methane flames experimentally, as well as numerically. Their experimental measurements were based on a cylinder reactor. Their results showed that CO2 had a dramatic decrease effect on the upper flammability limits but had only a slight influence on the lower flammability limits. The flammability limit of oxy-methane jet flames was studied in Ref. [19]. Their experimental research was carried out with the aid of a nonpremixed oxymethane jet in a lab-scale furnace. For a jet flame, the flammability can be identified by the flame stabilization map, which consists of three parts: the attached flame region, the lifted flame region, and the blow-off region. The authors drew the flame stabilization map according to their experimental data. They observed that the addition of CO2 had a strong effect on the flammability of oxy-methane jet flames, especially when the concentration of CO2 in the reactants was less than 20%. The influences of the addition of steam on the flammability limits of oxymethane flames were reported by Ref. [20]. According to their numerical simulation of oxy-methane counterflow combustion, the flammability limits broadened with the addition of steam because the molecular specific heat capacity of steam is lower than that of CO2. It is well known that the flame instability is determined by three mechanisms: hydrodynamic instability, diffusive-thermal instability, and buoyancydriven instability [21]. To date only a few publications are available about the effects of the first two mechanisms on oxy-methane flames. Ditaranto and Hals [22] investigated experimentally the oxy-methane flame instability in a sudden expansion combustor system. The flame instability was attributed to the hydrodynamic instability. The authors observed that for the oxy-methane flames, a higher oxygen concentration was required to avoid the instability. Furthermore, they claimed that there were several different instability modes for their research prototype, depending on the ratio of flame speed to inlet flow velocity. The instability of a cellular oxy-methane flame was investigated in Ref. [23]. It was observed that the hydrodynamic instability was responsible for the onset of flame instability, while the diffusive-thermal instability affected the flame instability at large standoff distances. In the lean oxy-methane flames, the diffusive-thermal effect tended to enhance the flame instability, whereas in the stoichiometry and rich oxy-fuel flames, it suppressed the flame instability. Xie et al. [10] investigated experimentally the flame instability of stretched spherical oxy-methane flame. They observed that the addition of CO2 suppressed the hydrodynamic instability, while enhancing the diffusive-thermal instability. The overall effect was that the addition of CO2 could suppress the flame instability.

22  Oxy-fuel Combustion

2.2.4  Flame Radiation in O2/CO2 Atmosphere Flame radiation characteristics are also critically important for the design of a furnace, besides combustion featuress chamber. As previously mentioned, most fundamental parameters of combustion, such as the burning rate and maximum flame temperature, depend closely on flame radiation characteristics. Tan et al. [24] measured the temperature profiles in a pilot-scale furnace, with different dilution gases. Their experimental results indicated that the temperature distributions were quite different between oxy-fuel combustion and air-firing. Xie et  al. [10] compared the methane flame radiative heat flux in different dilution atmosphere. They found in general that the flame radiative heat flux of oxy-methane flames was much greater than its air-firing counterpart. Ditaranto and Oppelt [25] investigated the radiative heat flux characteristics of oxy-­methane flames. Their experimental observations underlined that the radiative heat flux was closely dependent on the oxygen concentration and inlet Reynolds number. Because measuring the flame radiative heat flux in large-scale furnaces is very challenging, numerical simulation is indispensable for providing the related necessary information. Since the radiative characteristics of CO2, a kind of triatomic gas molecule, are quite different from those of N2, the popularly used gas radiation models for air-firing should be carefully checked and revised. Consequently, the efforts on gas radiation models in O2/CO2 atmosphere simulation have become a “hotspot” in the oxy-fuel combustion community. Rajhi et al. [26] made a comprehensive comparison of the commonly used gas radiation models. Their research covered the simple gray gas model, the exponential wide band model, the Leckner and the Perry models, and the weighted-sumof-gray-gases (WSGG) model. Through the comparison between numerical predictions and experimental measurements of a full-scale furnace fueled by methane diluted by CO2, they concluded that the Leckner model and the Perry model overpredicted the temperature compared to the exponential wide band model. The WSGG model could predict accurate results compared to the benchmark model; however, its parameters should be revised according to different ratios of H2O and CO2 in the furnace. One of the limitations of the WSGG and Perry models is that they were not applicable for pressures other than one atmosphere. The exponential wide band model and the Leckner model both were valid for use for various pressures and ratios of H2O and CO2. Now a consensus has been reached that the WSGG model is a good option for oxy-fuel combustion simulation because of the balance between numerical accuracy and computational cost. Abdul-Sater and Krishnamoorthy [27] conducted an assessment of gray and nongray formulations of a WSGG model for oxy-methane combustion simulation. They focused on the effects of the thermal boundary conditions, soot, and turbulence radiation interactions. They found that the nongray model's prediction provided a lower radiant fraction than the gray model calculations. Recently, Guo et al. proposed a full spectrum k-distribution-based

Fundamentals of Oxy-fuel Combustion  Chapter | 2  23

WSGG model for oxy-fuel combustion simulation [28]. In their strategy, which was different from previous efforts where new parameters for modified WSGG models came from data fitting, the modified parameters were obtained by a full spectrum k-distribution model. The latest efforts on modeling oxy-methane flame radiation was summarized by Abdul-Sater et al. [29].

2.3  EFFECTS OF CO2 ON REACTION KINETICS The purpose for revealing the effects of CO2 on the reaction kinetics of oxy-­ methane combustion are threefold: (1) to construct reduced reaction mechanisms, as currently it is still impossible to adopt the detailed reaction mechanism of methane to full-scale furnace simulation; (2) to understand pollution-­generating mechanisms in O2/CO2 atmosphere; and (3) to explain the chemical effects of CO2 on oxy-fuel combustion characteristics, such as burning rate and ignition delay. Because we previously discussed the third topic, this section focuses on the latest progress of the first two. Andersen et al. [30] proposed a global reaction mechanism for oxy-methane combustion simulation. They compared the hydrocarbon oxidation mechanisms by Westbrook and Dryer (WD) and by Jones and Lindstedt (JL). Andersen et  al. found that the CO concentration could not be predicted accurately via these mechanisms, after which they proposed an improved version to solve the problem. With the aid of the software CHEMKIN, the present authors compared the influences of various dilution atmospheres on the oxidized pathways of methane. The research prototype was a counterflow burner, and the strain rate was fixed at 100 s−1 [31]. As shown in Fig.  2.3A, for air-firing scenes, the major reaction pathway consisted of two C1-branch paths and one C2-branch path: (1) CH3 → CH2OH → CH2O → HCO → CO → CO2, (2) CH3 → CH2(s) → CH2  → CH → CH2O → HCO → CO → CO2, and (3) CH3 → C2H6 → C2H5 → C2H4 →  C2H3 → C2H2 → HCCO → CO → CO2. C1-branch paths represent the low temperature oxidization processes, while the C2-branch path indicates the hightemperature oxidation processes. The ratio between C1- and C2-branch paths was 1.23, which implied that in air-firing conditions, the C1-branch path was the dominant one. For OCDR cases, illustrated in Fig. 2.3B, the ratio between C1- and C2-branch paths increased to 1.88 as the contributions of CH3 → CH2 OH,CH3 → CH2O,CH3 → CH2(s) increased. The C1-branch path played a more important role. Another difference, compared with its AC counterpart, was that in the OCDR condition, the contribution of CH2(s) → CH2OH increased from 15% to 61%, which indicated that the chain from CH4 to CO was shortened. In the OCWR condition, shown in Fig. 2.3C, the ratio between C1- and C2-branch paths was the highest. The role of the intermediate radical H was suppressed, while that of OH was enhanced. Table 2.2 shows the detailed information. Based on the preceding analyses, Wang et al. [31] and Chen et al. [2] proposed a global reaction mechanism for oxy-methane combustion simulation,

24  Oxy-fuel Combustion

respectively. Their results showed that their reduced reaction mechanism could reproduce sufficiently accurate numerical predictions in many scenarios. Most of the available research on the effect of CO2 addition on the generation of pollution focused on NOx, as air leakage is inevitable for any realistic combustion system. Park et al. [32] numerically investigated NO emission behavior in oxy-methane counterflow combustion. They claimed that the Fenimore mechanism mainly contributed to NO destruction and that the dominant source of NO formation in oxy-fuel combustion was

%

00%

H 65% OH 24%

H 3%

(M) 95%

C2H2

H 84%

OH 14%

OH 4% OH 50% O2 34%

CH 38% CH2(S) 55%

H 85% O 5%

CH 38%

CH2 7%

CO2

OH 6%

C

OH 14%

HCCO

OH 4% H 14% O2 4%

HCO

CH3 15%

CH

C2H3

O 51%

CH3 4%

HCCOH

CH2(s)

H 7% OH 5% OH 100%

C2H5

)1

(M

H 85%

CO2 15% (M) 89%

23%

C2H4

CH2 5%

CO2 18%

C2O CH3CO

(M) 100%

(A)

OH 23% H 19% CH3 54%

CH2O 3%

H 80% OH 18%

CH3 29% H2O 28% N2 44%

(M) 10%

CO

C2H6 CH 3

H 42%

CH2

CH3 OH 2% H 2% H 4% OH 5%

OH 2%

O2 6% OH 22%

H2O 52%

%

15

2

H 50% OH 41% CH3 7%

O 5%

CO

CH2(S) 55%

%

27

CH2O

CH3 54%

CH4 OH

(M) 91%

CH2OH

H+(M) 8% CH 3+ (M )1 7

O 22% OH 4%

(2.8) H + NO + M ↔ HNO + M

H 48%

CH2CO

OH 14%

H 45% (M) 7%

5e-8-1e-7 1e-7-5e-7 5e-7-1e-6 1e-6-5e-6 5e-6-1e-5 1e-5< 5e-5

FIG. 2.3  Oxidized pathways of methane in different dilution atmosphere: (A) AC, (B) OCDR, and (C) OCWR. (Reprinted with permission from Wang L, Liu Z, Chen S, Zheng C, Li J. Physical and chemical effects of CO2 and H2O additives on counterflow diffusion flame burning methane. Energy Fuels 2013;27:7602–11. Copyright (2013) American Chemical Society.)

Fundamentals of Oxy-fuel Combustion  Chapter | 2  25

(M) 98% O 53%

OH 27%

OH 6%

CH 20% CH2(s) 75%

H 61% O 8%

CH 20%

CH2 11%

CO2

C2O CH3CO

(M) 98% 1e-7-5e-7

H 73%

5e-7-1e-6

OH 12%

C

C2H2

OH 3%

OH 23% O2 76%

5e-8-1e-7

C2H3

HCCO

CH

H 3%

HCO

H 5% OH 8% OH 94% O 6%

H 45% OH 50%

HCCOH

CH2(s)

O2 2%

(M) 85%

00%

)1

(M

H 61%

CO2 61%

(B)

C2H5

CH3 5%

H 33%

(M) 21%

9%

C2H4

CH2 4%

CO2 63%

CO

OH 33% H 17% CH3 39%

CH2O 3%

OH 2% H 2%

CH3

CH 3

CH3 25% H2O 19%

CH2

C2H6

% H+(M) 5% CH 3+ (M )2 0

H 52%

OH 42%

OH 2%

O2 26%

H2O 32% CO2 14% OH 28%

%

61

2

H 31% OH 59% CH3 7% O 4%

O 9%

CO

CH2(S) 70%

%

31

CH2O

CH3 39%

CH4 OH

(M) 72%

CH2OH

OH 11%

O 22% OH 3%

Mendiara and Glarborg [33] experimentally studied the NO reduction mechanism in O2/CO2 atmosphere. They observed that in stoichiometric and fuellean conditions, the NO reduction in oxy-fuel combustion was higher than its air-firing counterpart. The deduction was attributed to the suppression of the O/H radical pool in O2/CO2 atmosphere. The authors noted that the recirculation of flue gas had a significant influence on NO formation and destruction. The effect of elevated pressures on NO generation in oxy-methane combustion was reported by Seepana and Jayanti [11]. Their numerical results showed that NO concentration increased with a high pressure. The increase was attributed to the fact that the flame temperature increased with pressure, which enhanced the thermal and prompt route of NO formation.

1e-6-5e-6

H 59%

CH2CO

OH 10%

H 37% OH 5% 5e-6-1e-5

1e-5< 5e-5

5e-5<

FIG. 2.3, CONT’D (Continued)

26  Oxy-fuel Combustion

In year 2015, a study on SOx formation in oxy-methane combustion was published [34]. The aim of Ref. [34] was to explore the possibility of utilizing “sour” natural gas resources, which consist of highly concentrated H2S. Their numerical predictions indicated that the addition of CO2 increased SOx generation. But the authors emphasized that further experimental data are needed to validate their predictions.

2.4  FACTORS AFFECTING OXY-FUEL COMBUSTION BEHAVIOR

(M) 98% O 53%

CH 17% CH2(s) 74%

H 80% O 7%

CO2

C 2O CH3CO

(M) 98% 5e-7-1e-6

OH 8%

OH 27%

OH 6%

CH2 1%

FIG. 2.3, CONT’D

H 73%

HCCO

H 3%

C

C2H2

OH 9%

OH 53% O2 46%

1e-7-5e-7

C2H3

CH3 5%

CH 17%

OH 94% O 6%

H 53% OH 43%

CH

O2 2%

HCO

H 5% OH 6%

5e-8-1e-7

00%

)1

(M

HCCOH

CH2(s)

CO2 55%

(C)

C2H5

H 80%

CO2 47%

(M) 89%

16%

CH2 4% C2H4

H 34%

(M) 16%

CO

OH 31% H 17% CH3 48%

CH2O 4%

OH 2% H 1%

CH3

CH 3

CH3 27% H2O 28%

CH2

C2H6

% H+(M) 4% CH 3+ (M )1 5

H 64%

OH 34%

OH 2%

O2 7%

H2O 47% CO2 8% OH 30%

%

55

2

H 36% OH 55% CH3 7% O 2%

O 2% CO

CH2(S) 70%

%

41

CH2O

CH3 48%

CH4 OH

(M) 91%

CH2OH

OH 7%

O 20% OH 6%

Realistic oxy-fuel combustion systems, their performance and behavior may be closely dependent on two factors, namely composition of reactants and hydrodynamics of combustible mixture.

H 50%

CH2CO

OH 19%

H 44% OH 6%

1e-6-5e-6

5e-6-1e-5

1e-5< 5e-5

5e-5<

Fundamentals of Oxy-fuel Combustion  Chapter | 2  27

TABLE 2.2  Contribution Weight of Intermediate Radical Species in Different Dilution Atmosphere Radical Species

AC

OCDR

OCWR

H

80%

52%

64%

OH

18%

42%

34%

CH3 → CH2OH

OH

27%

31%

41%

CH3 → CH2O

O

5%

9%

2%

CH3 → CH2(s)

H

4%

−

−

OH

5%

11%

7%

M

91%

72%

91%

O2

6%

26%

7%

H

50%

31%

36%

OH

41%

59%

55%

O

−

7%

7%

CH3

7%

4%

2%

M

89%

85%

89%

H

7%

5%

5%

OH

5%

8%

6%

CH2(s) → CO

CO2

15%

61%

55%

CH → CO

CO2

18%

63%

47%

CH3 → C2H6

CH3 + M

17%

20%

15%

CH3 → C2H5

CH3

23%

9%

16%

CH3 → C2H4

CH2

5%

4%

4%

HCCO → CO

H

85%

61%

80%

C2H6 → C2H5

OH

23%

33%

31%

H

19%

17%

17%

CH3

54%

39%

48%

OH

24%

45%

53%

H

65%

50%

43%

CH3

4%

5%

4%

OH

100%

94%

94%

O

−

6%

6%

CH4 → CH3

CH2OH → CH2O

CH2O → HCO

HCO → CO

C2H4 → C2H3

CO → CO2

28  Oxy-fuel Combustion

For oxy-methane combustion, the composition of reactants mainly depends on the injection rate of oxygen and the recirculation rate/mode of flue gases. Many fundamental combustion characteristics, such as pollution generation and flame radiation, are influenced significantly by the reactants composition. These issues were previously covered, so they are not repeated here. The key to organizing good hydrodynamics is in the design of the burner and furnace chamber. Unfortunately, to the knowledge of this books authors, the open studies on this topic are very sparse, partially because of commercial competition. For oxy-methane combustion, Tan et  al. [24] investigated experimentally the influence of a swirl burner on flame behavior in a pilotscale furnace “CANMET.” The observed NO generation was sensitive to the swirl number. Hwang and Gore studied the effects of the inlet Reynolds number on oxy-­methane flames [35]. Through the change of the inlet Reynolds number and of the oxidant inject configuration, four different types of flames were formed with the burner that they used. The radiation intensities of various flames were distinctly different. With the aid of numerical simulation, Habib et  al. [36] investigated a typical natural gas-fired package boiler operated in O2/CO2 atmosphere. Their numerical results indicated that the mass and heat transfer characteristics in oxy-fuel combustion were completely different from its air-firing counterpart. Unfortunately, guidelines on how to organize better hydrodynamics through burner and chamber optimization are still absent.

2.5 SUMMARY In this chapter, using methane as an example, some fundamentals of oxy-fuel combustion were briefly discussed. Because the characteristics of oxy-fuel combustion are definitely different from its air-firing counterpart, to make this new clean combustion strategy strongly competitive with other clean technologies, combustion experts and practitioners need to dig deeper into their understanding of it. Although the results of numerous efforts related to oxy-fuel combustion have been published, our knowledge of many important aspects of oxy-fuel combustion is still deficient, such as burner and furnace chamber design. In addition, more experimental data are needed in order to check and improve certain theoretical achievements. The deployment of oxy-fuel combustion technology will be boosted only when its potential advantages are completely harnessed. That goal can be achieved only by thoroughly understanding the fundamentals of oxy-fuel combustion technology.

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