Methane combustion in MILD oxyfuel regime: Influences of dilution atmosphere in co-flow configuration

doi:10.1016/j.energy.2017.01.011

Energy

Volume 121, 15 February 2017, Pages 159-175

https://doi.org/10.1016/j.energy.2017.01.011 Get rights and content

Highlights

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A comprehensive comparison on methane MILD oxyfuel combustion.
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Combustion characteristics in various dilution atmosphere are analyzed.
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Some conflicted conclusions in previous publications are analyzed.
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New questions are raised, which may inspire future research activities.

Abstract

MILD (moderate or intense low oxygen dilution) oxyfuel combustion is a recently proposed clean combustion mode which can remedy the shortcomings of the standard oxyfuel combustion technology. Nowadays most available studies on MILD oxyfuel combustion focus on how to realize this new combustion regime in O₂/CO₂ atmosphere. The open research on methane MILD oxyfuel combustion in O₂/H₂O atmosphere is quite sparse. In the present work, we carry out a comprehensive comparison study on methane MILD oxyfuel combustion in different dilution atmosphere for the first time. The JHC (jet in hot co-flow) burner is adopted as a research prototype. The investigation is based on numerical simulation, so firstly the adopted numerical approach is validated by some experimental data in open literature. The numerical comparison is conducted by varying the mass fraction of oxygen in the co-flow and the temperature of the hot co-flow, two key parameters affecting fine reaction structures in JHC. Through the present investigation, a number of findings are reported for the first time and some conclusions presented in previous publications are checked with analyses, especially on some conflicted claims between the previous publications. In addition, several new questions are raised, which may inspire further research activities in future.

Introduction

MILD oxyfuel combustion [1], [2] is a recently emerging term which can be regarded as an organic combination of two promising clean combustion technologies, MILD (moderate or intense low oxygen dilution) combustion and oxyfuel combustion. Originally, some of the present authors proposed this new idea in order to utilize biogas with a higher efficiency [3]. Soon after, it was extended to various fuels [2], [4], [5], [6], [7], [8]. Through these preliminary studies, it was found that the MILD combustion regime could be established more easily in oxyfuel condition [1], [3], [7] and meanwhile a number of shortcomings of the standard oxyfuel combustion technology could be remedied straightforwardly by the introduction of MILD combustion regime [4]. Especially, the experimental efforts [4], [7] further demonstrated there was no obvious technical difficulty to establish and to sustain MILD oxyfuel combustion in industrial furnaces. Consequently, MILD oxyfuel combustion may become one of the next generation clean combustion technologies for carbon capture which is crucial to the sustainable development of human society [9]. For this purpose, consecutive research on MILD oxyfuel combustion is essential as our knowledge, as well as available open literature, on it is quite limited [1], [2].

Originally, the research on MILD oxyfuel combustion focused on how to realize this new combustion regime in O₂/CO₂ atmosphere, namely oxygen in oxidant flow being diluted by carbon dioxide rather than nitrogen in conventional air-firing mode [2], [4], [5], [6], [7], [8]. Recently, the present authors discussed the possibility to establish and to sustain MILD oxyfuel combustion in O₂/H₂O atmosphere where oxygen in oxidant flow is diluted by steam rather than carbon dioxide [1]. As shown in Ref. [1], compared with its O₂/CO₂ counterpart, there are at least three advantages to realize MILD oxyfuel combustion in O₂/H₂O atmosphere, such as simpler plant configuration, lower operation cost and high power-generation efficiency. In the oxyfuel combustion research community, the approach to realize oxyfuel combustion in O₂/H₂O atmosphere is named as steam-moderated oxyfuel combustion or oxy-steam combustion [1], [10]. As the chemical and physical properties of steam are quite different from those of CO₂, inevitably, compared with its O₂/CO₂ counterpart, combustion behavior may be significantly altered in the steam-moderated oxyfuel scenario. Consequently, comprehensive comparison of combustion characteristics between in O₂/CO₂ and in O₂/H₂O atmosphere is necessary, as it has done between in O₂/CO₂ (standard oxyfuel combustion) and in O₂/N₂ (air-firing mode) condition [11]. Unfortunately, nowadays the essential studies on this critical topic are extremely sparse. Some of the present authors compared the effects of CO₂- and H₂O-dilution on combustion temperature and reaction kinetics of methane [12]. It was observed that the chemical and thermal effects of CO₂ and of H₂O on combustion behavior of methane are quite different and consequently they will alter combustion temperature and reaction paths of methane in the oxyfuel combustion regime by different ways. Zou et al. investigated steam's effect on temperature distribution in methane oxy-steam combustion [13]. With the aid of numerical simulation, they found out the key elementary reaction step which determined the combustion temperature. In Refs. [14], [15], [16], wet recycle of oxy-coal combustion was investigated, not only by numerical simulation but also by experimental approaches. As steam is rich in wet recycle of oxyfuel combustion, it was observed that high concentration H₂O in recycled flue gas could influence combustion characteristics of pulverized coal significantly [14], [15], [16]. However, these studies [12], [13], [14], [15], [16] all are limited in the so-called “feed-back” combustion regime rather than MILD combustion regime [1], so whether the conclusions made in these studies are tenable in the MILD oxyfuel combustion regime is still an open question. To the best knowledge of the present authors, on comparison study between CO₂ and H₂O on establishing and sustaining MILD oxyfuel regime, until now perhaps there are only three open publications [1], [16], [17]. In Ref. [1], the present authors compared the effects of CO₂ and of H₂O on establishing biogas MILD oxyfuel combustion with the aid of a counter-flow configuration. It was found that biogas MILD oxy-fuel combustion would be established more easily in O₂/H₂O atmosphere but meanwhile the reaction zone would become more complicated. Sabia et al. discussed propane auto-ignition delay time in MILD combustion regime, where reactants were diluted by CO₂ and H₂O, respectively [16]. In Ref. [16], a cross-flow configuration was adopted. The authors claimed that in the O₂/H₂O option the auto-ignition delay time would be a little shorter than its O₂/CO₂ counterpart. Recently, some of the present authors conducted a numerical investigation about the influence of H₂O addition on MILD oxy-coal combustion [17]. The concentration of H₂O in oxidant flow varied from 0% (standard O₂/CO₂ condition) to 70% (oxy-steam atmosphere). It was observed that NO emission could be suppressed and heat transfer would be enhanced in O₂/H₂O atmosphere. As the IFRF (International Flame Research Foundation) semi-industrial scale co-flow furnace adopted in Ref. [17] is not an ideal MILD oxyfuel combustion research prototype and the extreme complication of coal combustion, Ref. [17] failed to reveal the influence of different types of dilution gases (H₂O or CO₂) on fine reaction structures. In our latest work [1], it has been underlined that further research on this topic is necessary as co-flow is more popularly found in practical combustion systems. Especially, through our recent research [12], [19], it was observed that the effect of dilution gas on combustion performance in a co-flow configuration may differ from its counter-flow counterpart because flow-reaction interaction, which is excluded in a one-dimension model (e.g. a counter-flow configuration), will play an important role in a co-flow configuration. Consequently, in order to deepen our knowledge in this emerging area so to advance its application in energy industry, a systematic comparison between the performance of co-flow MILD oxyfuel combustion in O₂/H₂O condition and that in O₂/CO₂ atmosphere, is essential.

In order to bridge the aforementioned gap, in this work we numerically investigate methane combustion in MILD oxyfuel regime, diluted by carbon dioxide and steam, respectively. The JHC (jet in hot co-flow) burner developed in Ref. [20] is adopted in the present study as the research prototype. Besides the JHC burner proposed by Dally's group [20], there is another popularly used JHC burner developed by the researchers in Delft [21], [22]. Within a JHC burner the influence of surrounding atmosphere on fine reaction structures can be prevented, so it is an ideal benchmark for a comparison study on MILD oxyfuel combustion in various dilution gases. The investigation is based on numerical simulation, so firstly the adopted numerical approach is validated by the experimental data [20]. In the present work, besides the influences of various dilution atmospheres, the effects of temperature of co-flow on MILD oxyfuel combustion are also investigated as until now no open effort reported on this important issue. Through the present study, a number of findings are reported for the first time and some conclusions presented in previous publications are checked with analyses on the differences, especially on some conflicted claims. In addition, several new questions are raised, which may inspire future research activities.

Section snippets

Configuration of the JHC burner and numerical conditions

The configuration of the JHC burner is illustrated by Fig. 1 and the detailed description on it please refer to Ref. [20]. As the JHC burner is axisymmetric, in order to reduce numerical simulation cost, the investigated domain can be simplified as a two-dimensional case, as shown by Fig. 2. In the JHC burner, fuel is injected through the central jet pipe whose inner diameter reads 4.25 mm. The fuel jet pipe is surrounded by an annulus oxidant co-flow pipe with an inner diameter 77.75 mm. The

Results and discussion

As shown in previous research [2], [6], [19], for JHC combustion, the temperature of the co-flow ( $T_{c o f}$ ) and the oxygen mass fraction in the co-flow ( $f_{o 2}$ ) are the key parameters that affect fine reaction structures. Therefore, in the present work we compare the MILD oxyfuel combustion characteristics in different dilution atmosphere by adjusting these two parameters, respectively. Firstly we try to reveal the MILD oxyfuel combustion characteristics in different dilution atmosphere with a

Conclusion

In order to deepen our insight into MILD oxyfuel combustion, a recently emerging idea for next generation clean combustion technology, in the present work we carry out a comprehensive comparison study on methane MILD oxyfuel combustion in different dilution atmosphere (O₂/H₂O and O₂/CO₂). The JHC burner is adopted as a research prototype. The comparison is conducted by varying the mass fraction of oxygen in the co-flow ( $f_{o 2}$ ) and the temperature of the hot co-flow ( $T_{c o f}$ ), two key parameters

Acknowledgments

This work has received funding from the Universidad Carlos III de Madrid, the European Unions Seventh Framework Programme for research, technological development and demonstration under grant agreement No. 600371, el Ministerio de Economa y Competitividad (COFUND2014-51509), el Ministerio de Educacin, cultura y Deporte (CEI-15-17) and Banco Santander. We also acknowledge the support from the British Newton Alumni Fellowship Scheme, the National Natural Science Foundation of China (Grant No.

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Methane combustion in MILD oxyfuel regime: Influences of dilution atmosphere in co-flow configuration

Highlights

Abstract

Introduction

Section snippets

Configuration of the JHC burner and numerical conditions

Results and discussion

Conclusion

Acknowledgments

Energy Convers Manag

Energy

Int J Hydrogen Energy

Combust Flame

Fuel Process Technol

Int J Hydrogen Energy

Energy Convers Manag

Prog Energy Combust Sci

Int J Heat Mass Transf

Energy Procedia

Appl Energy

Appl Energy

Combust Flame

Proc Combust Inst