Study on stretch extinction characteristics of methane/carbon dioxide versus oxygen/carbon dioxide counterflow non-premixed combustion under elevated pressures

https://doi.org/10.1016/j.jngse.2021.103994Get rights and content

Highlights

  • The extinction strain rate of oxy-enriched flame increases rapidly with pressure.

  • The extinction strain rate is proportional to OH formation rate at high pressures.

  • The OH formation rate of oxygen-enriched flame is higher than that of air flame.

  • The reasons for high extinction strain rate of oxygen-enriched flame were clarified.

Abstract

As a clean gaseous fuel, methane (the main constituent of natural gas) has been widely used in the lives of residents and industrial processes. The oxy-fuel combustion technology for methane/carbon dioxide is an effective way to reduce the pollutant emissions of NOx, which is also beneficial to the capture of carbon dioxide. Herein, the stretch extinction characteristics of non-premixed flames induced by methane/carbon dioxide mixtures versus oxygen/carbon dioxide mixtures counterflow combustion with various fuel concentrations were studied in high-pressure environments. The computed flame extinction strain rates of oxygen-enriched and air combustions in low fuel concentration range at pressures of 0.1, 0.5 and 0.7 MPa have a good consistency with the experiment results obtained from previous literature. The results show that the increase of flame extinction limit represented by the stretch rate for the oxygen-enriched combustion with increasing pressure is more significant than that for the air combustion. The analyses of reaction pathways, integral of reaction rates and sensitivity coefficients suggest that the OH formation rates of near extinction flames limits can be used to estimate the ratio of the flame extinction strain rate for oxygen-enriched combustion to that for air combustion under high pressures. The reasons for the enhancement of the OH formation rate aroused by increasing pressures were clarified.

Introduction

In recent years, the pollutant emissions due to the significant consumption of fossil fuels have driven the wide utilization of methane (CH4, the main constituent of natural gas) as a clean fuel (Li et al., 2020) and the rapid development of clean combustion technology in internal combustion engines (Al-Hamed and Dincer, 2020) and industrial production. Understanding and revealing the fundamentals knowledge of combustion characteristics plays a particularly important role in realizing the highly efficient utilization of fuel and the reduction of pollutant emission.

Currently, the concentration of carbon dioxide (CO2) in the atmosphere exceeds 410 ppm with an increase of 46.4% over the pre-industrial level (National Oceanic & Atmospheric Administration), which has become one of the most severe issues affecting global climate warming due to the greenhouse gas effect (Sharifi et al., 2015; Khallaghi et al., 2020). With the increasing requirement of the reduction of carbon dioxide emission from the human society, the combustion technology using O2/CO2 as oxidizer attracts more and more interests (Zhou et al., 2020). The oxy-fuel burning technology for solid fuels was proposed long time ago (Abraham et al., 1982; Nakayama et al., 1992; Payne et al., 1989; Wang et al., 1988), since solid fuel combustion contributes to the main part of the world energy consumption. The oxy-fuel combustion power plant has been realized and tested after comprehensive studies (Fujimori and Yamada, 2013). On the other hand, new burning technologies utilizing O2/CO2 as an oxidizing agent for gaseous fuels, which also play important role in the world energy-consuming, have also been proposed in recent years (Hargis and Petersen, 2015; Koroglu et al., 2016; Li et al., 2014, 2015; Mendiara and Glarborg, 2009; Park et al., 2004). New combustion technologies such as HiTOx (Research and development of high-temperature oxygen combustion technology, 2014) and MILD (Moderate or Intense Low-oxygen Dilution) oxy-combustion (Cheong et al., 2017; Li et al., 2013; Mardani and Ghomshi, 2016) for gas-fuel-fired furnaces have been developed. Some researchers also proposed the concept of oxy-fuel gas turbine cycles (Sundkvist et al., 2014). However, modifications of combustion facilities and operation parameters are required when it is switched to O2/CO2 combustion from conventional air combustion. This is because CO2 has different physical and chemical characteristics comparing with N2. Consequently, it is of great necessity to research the burning behaviors of gaseous fuels within the O2/CO2 environment and to compare with those of air flame.

The chemical reaction kinetic effects of CO2 on the combustion velocities of natural gas and hydrogen premixed combustion processes at normal pressure were investigated numerically by Liu et al. (2003). Prathap et al. (2012) measured the flame speeds of syngas/air flame at different dilutions in the laminar flame region. Hu et al. (2014) compared the flame speeds of CH4/O2/CO2 combustion with that of CH4/O2/N2 combustion by employing a Bunsen burner. It was found that CO2 could accelerate reaction rates of CO2-related reactions, which decreases the mole concentration of active radicals. The previous studies on premixed flames with CO2 dilutions reveal that the flame speed at normal pressure can be decreased by the kinetic effect of carbon dioxide.

As one of the significant combustion characteristics, flame extinction has an impact on the combustion efficiency and safety. For non-premixed coaxial swirl combustion, Jerzak (2018) effectively evaluated the extinction limits of fuel-rich and fuel-lean flames of methane and two mixtures of CH4/CxHy/N2 by analyzing the limit ratios of equivalence. Li et al. (2014) investigated the stretch extinction limits of CH4/CO2 versus O2/CO2 counterflow flames within the oxidant temperatures ranging from 300 K to 1000 K. It was found that the increasing of the CO2 concentration and the elevating of the oxidizer temperature have significant effects on decreasing effect on the stretch extinction limit of the oxy-fuel flame. The research work by Song et al. (2015) shows that the temperature of oxy-fuel counterflow flame is lower compared with that of the air flame when the oxygen mole concentrations are the same. This was caused by the radiation and chemical kinetic reaction effects of carbon dioxide. Both of the investigations carried out by Kim et al. (2016) and Park and Fisher (2016) revealed that the third-body effects and the radiation effects of CO2 are insignificant when the flow residence time is short.

The influences of carbon dioxide on the combustion behavior of jet fire in the O2/CO2 atmosphere were also discussed extensively. Through numerical research, Xu et al. (2017) analyzed the influences of carbon dioxide on flame height and temperature by suspending chemical and physical effects of carbon dioxide, respectively. And, the results presented that the psychical thermal effect of carbon dioxide on the combustion process is remarkable (Xu et al., 2017). The effects of CO2 dilutions in co-flow air on the lifting behaviors were studied experimentally by Min et al. (2011, 2010) and Min and Baillot (2012). These studies showed that highly CO2 dilution leads to a shorter flame length and a higher lift-off height as compared to the N2 and Ar dilutions. Research work by Nada et al. (2014) demonstrated that the conventional premixed flame correlation is incapable of predicting the lift-off height behaviors of flame formed in the O2/CO2 co-flow combustion system, and CO2 is the primary cause of this problem.

Generally, the majority of previous researches concerning the burning behaviors of gaseous fuels in O2/CO2 environments were concentrated on the flame at normal pressure. This knowledge is useful for the design and operation of industrial furnaces by applying O2/CO2 combustion technologies. Also, it is meaningful for the development of combustion technology using O2/CO2 as an oxidizer at high pressures, i.e., O2/CO2-flame-fired gas turbine (Krieger et al., 2015; Liu et al., 2012; Saanum and Ditaranto, 2017). Nevertheless, the research on the flame in O2/CO2 atmosphere under elevated pressures is quite insufficient. Due to the effect of CO2 diluted and high pressure, the flame extinction and flashback are likely to occur and it is of difficulty to form stabilized flame in the combustion chamber, which induces that the performance of experiments under high pressures is challenging.

Xie et al. (2013) studied the speeds of laminar flame for CH4/O2/CO2 combustion in a constant volume chamber at the pressure of 0.3 MPa. It could be found that the laminar flame speed for CH4/O2/CO2 decreases with the increasing pressure. Additionally, research performed by Kobayashi et al. (2013) explored the flame characteristics for the turbulent combustion of CO/H2/O2 mixtures diluted by CO2 at 1.0 MPa pressure. They indicated from OH–PLIF images that in the turbulent flame region, the deep flame wrinkles could be observed at 0.5 MPa while the depth of the large-scale flame wrinkles decreases at 1.0 MPa. These researches indicated that the combustion characteristics of the flame with CO2 dilution at high pressures show significant differences.

The stretch extinction limits of flame at high pressures are important parameters to determine operation parameters of pressurized O2/CO2 combustion technologies (Kapadia et al., 2011). Whereas, literatures focused on this topic are few. Maruta et al. (2007) measured the extinction limits of flames produced by CH4/N2 versus O2/N2 and CH4/CO2 versus O2/CO2 counterflows combustion under normal and elevated pressures by experimental study. However, the maximum pressure in the experiments was 0.7 MPa (Maruta et al., 2007), which is much less than the working pressure in the gas turbine. Actually, the operating pressures in gas turbine combustors of civilian and military aircrafts could reach 2.0 MPa and 4.0 MPa respectively (Karataş and Gülder, 2012). Moreover, the variations of the flame extinction limit of CH4/CO2 versus O2/CO2 counterflow combustion with increasing pressures were not clarified till now. Namely, the knowledge about the stretch extinction characteristic of flame in CH4/CO2 versus O2/CO2 counterflow combustion under the elevated pressures larger than 0.7 MPa is still limited. On the other hand, the research conducted by Maruta et al. (2007) lacks the detailed analysis on the chemical reaction kinetic mechanism for flame stretch extinction limit under elevated pressures. Accordingly, it is meaningful and necessary to further study the stretch extinction characteristic for the CH4/CO2 versus O2/CO2 counterflow flames at elevated pressures larger than 0.7 MPa.

Therefore, the current work would like to reveal the evolution of stretch extinction limit of non-premixed flame for CH4/CO2 versus O2/CO2 counterflow combustion (oxy-combustion flame) in a wide fuel concentration range under high pressures whose maximum value is 4.0 MPa through numerical computation and compare to that for CH4/N2 versus air counterflow non-premixed combustion (air flame). The influence mechanism of pressure on the stretch extinction limit (represented by extinction strain rate) of non-premixed flames in CH4/CO2 versus O2/CO2 counterflow combustion was clarified by analyzing the reaction pathways, contributions of important radicals and reactions. This work aims at providing new knowledge involving CH4/CO2 versus O2/CO2 counterflow non-premixed combustion at elevated pressures, which could be the guidance for the development of the CH4 combustion technology.

Section snippets

Numerical computations

In the present study, numerical computation utilizing the OPPDIF module of the CHEMKIN-PRO (CHEMKIN-PRO 15131) was chosen to study the counterflow non-premixed combustion process. Fig. 1 depicts the physical model of the counterflow non-premixed flame in this study. The spacing of the fuel and oxidizer outlets (L) was set as 10 mm. The previous studies of CH4 flames with different CO2 dilutions (Hu et al., 2014; Li et al., 2014, 2015; Song et al., 2015) suggest that the GRI-Mech 3.0 (Smith et

Influence of radiation heat loss on the computed extinction strain rate

As is well known, adding carbon dioxide to the flame can induce the increasing of radiation heat loss because the gaseous carbon dioxide has a relatively high Planck mean absorption coefficient (Ju et al., 1997; Tien, 1969). For clarifying the influence of radiation heat loss on the computed values of flame extinction strain rates, OTM and ADI models separately were employed to perform the computations for the oxy2 and air combustions at the pressures of 0.7 and 4.0 MPa. Fig. 2 presents the

Extinction characteristics at elevated pressures

The flame extinction strain rates for the combustion processes occurring in the oxygen-enriched and air atmosphere with a wide fuel concentration range under 0.1, 0.7, 2.0 and 4.0 MPa pressures were obtained by numerical computation with ADI model, as shown in Fig. 4. For the scenarios of normal pressure (Fig. 4a), the extinction strain rate of the oxy2 flames is higher than those of the oxy1 and air flames with the same fuel mole fraction. Furthermore, the extinction strain rate of the oxy1

Conclusions and remarks

Under normal and elevated pressures, the stretch extinction limits of the non-premixed flames in CH4/CO2 versus O2/CO2 counterflow combustion with the oxygen mole fractions of 0.35 and 0.40 for different fuel concentrations were studied by numerical computation. Major findings include:

  • (1)

    The extinction strain rates of the non-premixed flames for CH4/CO2 versus O2/CO2 and CH4/N2 versus air counterflow combustion processes increase with the increasing of pressure. With the same mass fraction of

Credit author statement

Xiaochun Zhang: Methodology, Investigation, Writing- Original draft, Writing- Reviewing and Editing. Zijian Zhang: Investigation, Writing- Original draft, Writing- Reviewing and Editing. Formal analysis. Xing Li: Conceptualization, Data curation Writing- Reviewing and Editing, Funding acquisition. Yufang Chen: Software, Validation, Writing- Original draft. Xiaohan Wang: Supervision, Formal analysis.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by National Natural Science Foundation of China under No. 51506204.

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