Effects of Flue Gas Addition on the Premixed Oxy-methane Flames in Atmospheric Condition Academic research paper on "Chemical engineering"

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Energy Procedia 75 (2015) 3054 - 3059

The 7th International Conference on Applied Energy - ICAE2015

Effects of flue gas addition on the premixed oxy-methane flames in atmospheric condition

Yueh-Heng Lia'b*, Guan-Bang Chenb, Yei-Chin Chaoa

aDepartment of Aeronautics and Astronautics, National Cheng Kung University No.1, University Rd., Tainan 701, Taiwan. b Research Center for Energy Technology and Strategy, National Cheng Kung University No.1, University Rd., Tainan 701, Taiwan.

Abstract

This numerical study investigates the flame characteristics of premixed methane with various inert gas dilutions in order to simulate oxy-combustion of hydrocarbon fuels with flue gas recirculation system. In general, a flue gas consists of high concentration C02/H20 and high gas temperature. The diluent gas in oxy-combustion has been changed compared with air-combustion, so that the flame behavior and combustion characteristics of oxy-fuel must be influenced. By observing the resultant flame temperature and species concentration profiles can identify that the flame location shifts, and the concentration profile of major chemical reaction radicals varies, indicating the change of flame structure and flame chemical reaction paths. The dominant initial consumption reaction step of methane shifts from R53 (H+CH4=CH3+H2) to R98 (0H+CH4=CH3+H20) when nitrogen is replaced by the recirculated gases. It is because that the chemical effects of the recirculated gases changes the flame reaction pathway, and further affect reaction rate, species and radical concentrations.

Keywords: Hydrogen; porous medium; mild combustion; feedback combustion; high-temperature combustion

1. Introduction

Owing to the rising of the environmental protection awareness, oxy-combustion in the last decade has been widely discussed [1, 2]. The concept of combined post-combustion emission recycling with carbon dioxide capture has been the most attentive scheme for power generation system in present. Therefore, to retrofit the traditional coal-fire or nature gas-fire power generator to oxy-combustion has become an increasing subject. Oxy-combustion is to recuperate the flue gas as diluent gas to replace nitrogen in air, and to obtain high concentration of carbon dioxide and water steam in downstream flue gas [3, 4]. Nevertheless, carbon dioxide and water steam in hydrocarbon fuel as diluent gas would alter the flame

* Corresponding author. Tel.: +886-6-2757575-63632; fax: +886-6-2389940. E-mail address: yueheng@mail.ncku.edu.tw.

1876-6102 © 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of Applied Energy Innovation Institute

doi:10.1016/j.egypro.2015.07.623

behaviors via three effects, dilution effect, thermal effect and chemical effect. Principally, dilution effect is to reduce the concentration of oxygen in reaction zone and further influence the reaction rate, and thermal effect is caused by the difference in thermal capacity and thermal radiation, and the chemical effect is to increase the radical concentration and further shift the chemical pathway.

Kimura et al. [5] discovered that the ignition delay and flame instability would occur in oxy-combustion condition, and proposed that an increase of gas temperature and oxygen concentration in flue gas can mitigate the combustion instability. Besides, the increasing percentage of water steam in flue gas can improve the combustion efficiency of oxy-fuel flame. Payne et al. [6] use a pilot-scale furnace to compare the gas emission of oxy-combustion with dry and wet flue gas recirculation system. It is noted that the reduction of NOx emission in dry flue gas recirculation system approaches to 70% while the C02/02 mole fraction is 2.7. However, NOx emission in wet flue gas recirculation decreases 83% while the (C02+H20)/02 mole fraction is 3.2. In general, the combustion efficiency of wet flue gas recirculation is prior to that of dry flue gas recirculation. Compared to nitrogen, carbon dioxide and water steam would be pre-dissociated in a high temperature region of oxy-combustion, and lead to shift the chemical pathway. Haler et al. [7] experimentally and numerically investigated the flame behaviors of methane/air in C02-diluent, N2-diluent, and C02-N2-diluent (71.6%N2 + 28.4% C02). It found that C02-diluent has inherently high thermal capacity compared to N2-diluent, and induces pre-dissociation phenomenon.

Considered the effect of C02-diluent flames in flame reaction rate, Liu et al. [8] used fictitious inert gases (FC02), which only possess heat capacity and thermal transport, but do not participate in the chemical reaction, to compare with the real carbon dioxide. It appears that the flame burning velocity with fictitious inert gas is lower than that with real carbon dioxide. It points out that carbon dioxide participates in the chemical reaction by interfering with key chemical step R99, and leads to flame burning velocity reduction. Similarly, Mazas et al. [9] discussed the effect of water steam addition in premixed methane/air flame, and found the chemical effect in flame burning velocity, especially in fuel lean and near-stoichiometric condition.

Accordingly, there are three parameters to influence the combustion efficiency of oxy-fuel, namely C02/02 mole fraction, temperature of flue gas and water steam content in flue gas. These parameters are crucial in design of oxy-fuel combustor. This study would numerically discuss the effect of flue gas additions in flame burning velocity of methane/oxygen flame. Various dilution concentrations of carbon dioxide, nitrogen, water steam, and flue gas (33%C02+67%H20) in oxy-combustion are considered. The shift of flame structure in various inert gas dilutions is also investigated.

2. Numerical method

The PREMIX code of Chemkin collection is used to calculate the adiabatic, unstrained, free propagation velocities of the laminar premixed CH4/air/H202 flames. It solves the equations governing steady, isobaric, quasi-one-dimensional flame propagation. An initial reactant mixture is specified and equilibrium of constant enthalpy and constant pressure is constrained. To obtain accurate adiabatic flame temperature, besides reactants and products, all radical species that might occur in the flame are also included.

The GRI-Mech 3.0 chemical kinetic mechanism composing of 53 chemical species and 325 reaction steps and detailed transport properties are used without any modifications. The reaction rate constant is represented by the modified Arrhenius expression.

At the cold boundary, the temperature of unburned reactants is 400K. The effects of different diluents (N2, C02, H20 or flue gas) on CH4 laminar burning velocity are investigated. The composition of flu gas is 33% C02 and 67% H20. The global reaction ofCH4/02/diluents is defined as: CH4 + 2(02+ y Di) ^ C02 + 2H20 + 2 y Di (1)

where 9 is the equivalence ratio and Di is the diluents. The dilution ratio y is defined as the mole ratio of diluents and oxidizer.

3. Results and discussion

3.1. Sensitivity analysis of the burning velocity

In order to further understand the effect of chemical reaction on the flame speed of methane flame with various inert gas dilutions, the first-order sensitivity analysis of laminar burning velocity is shown in Fig. 1. R38 is a dominant chain branching reaction step in sensitivity analysis, especially in C02-diluent, H20-diluent and flue gas-diluent. Besides, R52 is a second prominent reaction step in sensitivity analysis, and is an initiation reaction step of methane in low temperature surrounding. Regarding to R99, it is a key reaction of carbon monoxide oxidation, and it has high exothermicity. In nitrogen-diluent and H20-diluent conditions R99 is third prominent reaction step of methane combustion, but R119 would be a third prominent reaction step in a C02-diluent and flue gas-diluent

condition. In general, the present of R99 can improve the reaction, and enhance the flame speed. R119 is a key reaction step of CH3 oxidation close to ignition. It appears that high concentration of carbon oxide surrounding would modify the chemical reaction of methane flame.

3.2. Major species and intermediate radicals

Figure 2a displays the numerical results of carbon dioxide production rate along axial direction for various inert gas dilutions. Total rate of production (black line) is coincident with R99 (green line). It appears that R99 in N2-diluent is larger than others. Then, in C02-diluent and flue gas-diluent condition, R99 has apparently decreasing tendency. Especially, the amount of R99 in C02-diluent condition is much low than that in N2-diluent condition. When carbon dioxide is a diluent gas, it would promote the reverse reaction of R99, and decelerate the net reaction rate of R99. Nevertheless, R153 would be enforced as in C02-diluent condition. CO+OH = C02+H (R99)

CH2(S) + C02= CO + CH20 (R153)

Figure 2b displays the numerical results ofwater steam production rate along axial direction for various inert gas dilutions. It shows that total rate ofproduction in a N2-diluent condition is larger than others, and R84 (orange line) is dominant production rate step. In a H20-diluent condition R84 would apparently diminish, and R98 (purple line) would be prominent production rate step. However, in a C02-diluent condition R98 and R84 become equal. Reason is probably related to radical concentration of H and CH3.

«1 0 № o I 0J »4

>«n»lml «Mihih cwflkitihi>rlhiH>|Kfd

Figure 1. Sensitivity analysis of flame speed.

Otherwise, R86 (green line) is primary chain termination, and it would be enlarged in H20-diluent condition.

ОП + H2 = П + H20 (R84) 20H= О + H20 (R86)

OH + CH4 = CH3 + H20 (R98)

Figure 3a displays the numerical results of carbon monoxide production rate along axial direction for various inert gas dilutions. It shows that in a N2-diluent condition R284 and R144 are primary production rate step and secondary is R168 and R166. In a H20-diluent condition the total rate of production is decreasing, and R166 becomes a primary production rate step, and R284 and R144 are decreasing. In a C02-diluent condition the total production rate is sharply decreasing, but R166 is still a prominent production rate step. As to flue gas dilution, the total production rate is located between H20-diluent and C02-diluent condition, and R166 is also a dominant production rate step. Besides, due to reverse reaction of R99, CO concentration is increasing in a C02-diluent condition.

Figure 3b displays the numerical results of О production rate along axial direction for various inert gas dilutions. In a H20-diluent condition R86 becomes a consumption reaction, and yields a lot of OH radicals. It leads to enhance the reverse reaction of R38, and further reduces the concentration of OH radical. In a C02-diluent condition the reverse reaction of R99 would be increased and competes with R38 for radical П. It affects the concentration of О and OH radicals, and limits to offer О radicals to RIO and R3. In a flue gas-diluent condition R86 is also a primary consumption reaction step and apparently increased due to water addition. 0 + H2 = П + OH (R3)

0 + CH3 = H + CH20 (RIO)

CO + OH = TO2 + H (R38)

20П = 0 + H20 (R86)

Figure 2. Effect of inert gas dilution on different production rate, (a) CO2 production rate; (b) H20 production rate.

Figure 4a displays the numerical results of H production rate along axial direction for various inert gas dilutions. In a H20-diluent condition the total production rate becomes weak, and R166 becomes a primary production reaction, and R84 becomes a secondary production reaction. It appears that adding water in inert gas results in increasing reverse reaction of R84 and both reactions of R166. In a C02-diluent condition the total production rate is extensively reducing, and R86 is still a primary production rate step. However, R38 is decreasing due to decreasing OH and H radicals. It causes to reduce the production of H radicals providing from R84. In a flue gas-diluent condition the total production rate is located between H20-diluent and C02-diluent condition, and R166 becomes a secondary production rate step.

CO + OH = œ2 + H (R38)

OH + H2=H + H20 (R84)

HC0+H20=H+C0+H20 (R166)

Figure 4b displays the numerical results of OH production rate along axial direction for various inert gas dilutions. In a H20-diluent condition the total production rate becomes weak, and R38 becomes a primary production reaction, and R86 replaces R3 to become a secondary production reaction step due to water addition. R98 becomes the primary consumption reaction and R84 becomes a secondary one. In a C02-diluent condition the total production rate is extensively reducing, and R38 is still a primary production rate step. However, R98 and R84 are fairly equal due to OH radical competition. In a flue gas-diluent condition the total production rate is located between H20-diluent and C02-diluent condition, and R38 and R86 are top two production reaction step, while R98 and R84 are top two consumption reaction step.

0 + H2= H + OH (R3)

C0 + 0H = C02 + H (R38)

20H = 0 + H20 (R86)

OH + CH4 = CH3 + H20 (R98)

Axial distance(cm) Axia l distance (cm)

Figure 3. Effect ofinert gas dilution on different production rate, (a) CO production rate; (b) O production rate.

004 0.03 ТоШ ra» of product

0 .02 - «6*-^ i"* ^RIM 1

001 J 0 ■0 01 : RIOJ* 'У - i *

-0 02 Л'л i l" (/

-0,03 - \J

■0 04 - \ --— N2

-O OS - l \ t - H20 - C02 ---Fiu*a»s 1

■0.06 - \ t R38 1

■0.07 -

Axial distance(cm)

4. Effect ofinert gas dilution on different production rate, (a) H production rate; (b) OH production rate.

Figure 4. Conclusions

In this work, we discuss the various dilutions in methane oxy-combustion, C02-diluent, H20-diluent, N2-diluent and flue gas-diluent case, respectively. According to sensitivity analysis, R38 is a dominant chain branching reaction step, while R52 is a prominent reaction step to restrain flame speed for all dilution cases. R99 is a third reaction step and provides principal exothermicity for N2-diluent and H20-

diluent cases, but in the cases of C02-dilution and flue gas -dilution the third reaction step is switched to R119. Besides, production rates of major species and intermediate radicals are further discussed and compared with various gas dilutions. Results show the primary production and consumption reaction steps for different inert gas dilution are shifted.

Acknowledgements

This research was partially supported by the National Science Council ofRepublic ofChina under Grant numbers NSC 99-2221-E-006 -051-MY3. Computer time and numerical packages provided by the National Center for High-Performance Computing, Taiwan (NCHC Taiwan), are gratefully acknowledged.

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Biography

Yueh-Heng Li is currently Assistant Professor of the Department of Aeronautics and Astronautics at National Cheng Kung University (NCKU). He received Ph.D. degree in Mechanical Engineering from National Cheng Kung University, Taiwan. From 2013 to 2014, he was an assistant researcher at Research Center for Energy Technology and Strategy, NCKU. He participates in researches related to oxy-coal combustion and coal co-firing with biomass granted by National Energy Program (Taiwan).