Oxy-Combustion of Hydrogen-Enriched Methane: Experimental

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Oxy-Combustion of Hydrogen-Enriched Methane: Experimental Measurements and Analysis Yinka S. Sanusi, Esmail M.A. Mokheimer, Mohammad Raghib Shakeel, Zubairu Abubakar, and Mohamed A. M. Habib Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b03118 • Publication Date (Web): 11 Jan 2017 Downloaded from http://pubs.acs.org on January 12, 2017

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Oxy-Combustion of Hydrogen-Enriched Methane: Experimental Measurements and Analysis

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Yinka S. Sanusi, Esmail M. A. Mokheimer1, Mohammad Raghib Shakeel, Zubairu Abubakar and

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Mohamed A. Habib

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Mechanical Engineering Department, College of Engineering, King Fahd University of Petroleum and Minerals,

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Dhahran 31261, Saudi Arabia

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1: Corresponding Author, e-mail: [email protected], Tel.: +966138602959, Fax: +9668602949

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Abstract

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Oxy-combustion characteristics of methane and hydrogen-enriched methane have been

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investigated experimentally in a non-premixed swirl stabilized combustor. Experiments were

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conducted at different firing rates ranging from 2.5 MW/m3-bar to 4.5 MW/m3-bar and 0-20%

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hydrogen content in methane/hydrogen fuel mixtures. When the combustor is operated under

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gas turbine conditions (≥3.5 MW/m3-bar), the flame transitions exhibit tri-modal regime (i.e.

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Attached flame → Lifted flame → No flame) below which the flame transitions exhibit bimodal

regime (i.e. Attached flame → No flame). Weak flames at the nozzle exit were generally

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observed to precede the Attached flame → Lifted flame transition. The weak flame is due to the

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entrainment of more CO2 containing oxidizer (O2/ CO2) to the fuel stream that reduces the flame

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burning rate. The Attached flame → Lifted flame transition occurs at a critical oxidizer velocity.

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The critical oxidizer velocity strongly depends on the fuel composition. Moreover, lifted flame

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oscillates about different points (stabilization points) within the combustor. These points can be

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interpreted as locations of lower scalar dissipation rate, where the leading edge flame speed

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matches the local flow velocity. Empirical equations presented in this study captured the trend 1 ACS Paragon Plus Environment

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of our experimentally normalized flame length. The predicted flame length based on the Near-

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field concept gave a good match with our experimentally observed flame length. Temperature

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data are also presented and can be used in the validation of numerical models to have further

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insight into the Oxy-combustion dynamics of methane and hydrogen-enriched methane in a cost

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effective way.

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Keywords: Oxy-combustion; hydrogen-enriched methane; flame transition; flame stability;

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flame length; Temperature coefficient

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1. Introduction

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There has been continuous increase in the global electricity demand. This is attributed to

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economic growth in developing countries and the electrification of heating and transport sectors

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in major economies. About 80% of the world electricity in 2011 was sourced from fossil fuels1,

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thus, contributing majorly to the global emission of greenhouse gases (carbon dioxide). The Paris

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Agreement of 20152 has renewed the Kyoto protocol with a pragmatic approach towards the

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reduction of greenhouse gases emission

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environmental regulations are expected in the nearest future. Strategies, such as the use of more

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efficient systems and fuel with low carbon content, deployment of renewable energies as well as

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the use of carbon capture and sequestration (CCS) techniques have been variously advocated to

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reduce the emission of carbon dioxide (CO2) to the atmosphere. Forecasts in the power sector

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have indicated that the world will still depend on fossil fuel sources for electricity production in

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the next three decades3. Therefore, CO2 capture and sequestration among other strategies should

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be vigorously pursued to militate against the effect of global warming. The approaches for

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possible carbon dioxide capture have been grouped into post-combustion, pre-combustion and

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oxy-combustion. The stake holders in the power industry see the Oxy combustion technique as

and carbon trading. Therefore, more stringent

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more promising not only in the recovery and capture of carbon dioxide from flue gases but also

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in the possible elimination of NOx emission. The technique involves the burning of fuel with

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pure oxygen and CO2 or recycled flue gas (RFG), such that CO2 can be recovered from the flue

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gases by condensing the water vapor. Most research on oxy-fuel combustion focuses on coal

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fired power plants and boilers4 with pilot scale plants developed and operated. The most recent

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pilot plants and demonstration projects for oxy-coal combustion are presented in previous study5.

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Data from international energy agency, however, revealed that the utilization of coal as an

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energy source has seen decreased from 22.6% in 1973 to 19.3% in 2014 while the use of natural

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gas increased from 18.9% to 25.6% over the same period6. This indicated that substantial CO2 is

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emitted from gas-fired power plants. Researchers and industrialist have started paying adequate

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attention to the implementation of oxy-fuel combustion in gas-fired power plants. In this regard,

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Siemens SGT 900 gas turbine was recently adapted to operate with a wide variety of

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hydrocarbon fuels and oxygen (O2) in the presence of recycled coolant (steam and CO2)7. This

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has opened new window in the retrofitting of existing gas turbine system for oxy-fuel

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application. Other researchers have investigated flame stability in separated-jets burner8 and

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swirl stabilized9 burners under lean conditions. They observed differences in the flame

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stabilization mechanism when compared to air combustion. These differences are attributed to

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the dissimilarity between N2 and CO2 properties such as specific heat capacity, viscosity, thermal

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diffusivity, radiative properties, and gas phase chemistry. Heil10, however, showed that the

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changes in the combustion dynamics is not only due the differences in the radiative and thermo-

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physical properties of CO2 but also due to the fact that CO2 participates directly in the chemical

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reaction. Studies have also shown that for oxy–fuel combustion to have a similar combustion

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dynamics with air combustion, the oxygen concentration in the oxidizer mixture should be at


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least 30%11. Researchers have also studied the chemical effects of CO2 concentration12 and

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radiation heat transfer13 in oxy-fuel combustor. The present work is motivated by the fact that in

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conventional gas turbines, combustion takes place under lean conditions to reduce NOx

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production. Most of the works on oxy-combustion in literature have similarly been conducted

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under lean conditions. These conditions present practical challenge in CCS due to the presence

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of oxygen in the exhaust that will make post-combustion carbon capture (O2/CO2 separation)

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necessary. Besides, lean oxy-combustion will lead to wastage of expensive oxygen produced via

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cryogenics or membrane technology. There is limitation on the purity of CO2 for transportation

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and storage aspects of CCS. For instance, the specification for CO2 transport in existing facilities

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by Sleipner, kinder Morgan and Weyburn are limited to more than 93, 95 and 96% CO2 by

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volume14, respectively. Therefore, concerted effort needs to be directed towards understanding

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the combustion dynamics of oxy-combustion for a global stoichiometric mixture. The combustor

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for such mixtures (global stoichiometric mixture) has to be carefully designed to ensure stability

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and efficient utilization of the oxygen. In this regard, we designed the combustor burner to

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discharge multiple high velocity fuel jets in circumferential manner into a swirling oxidizer

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(O2/CO2). This is to ensure stable flame at varying firing rate and fuel mixtures with oxygen

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concentration of even less than the minimum of 30% reported in literature11. Further, details of

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the burner and experimental set-up are given in section 2. There is also growing interest in

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operating power turbines using hydrogen rich fuels, e.g. syngas, biogas fuels, refinery waste gas

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etc. These fuels have varying hydrogen contents depending on their sources and processing

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techniques such as fuel reforming, IGCC, refinery waste, etc. The presence of hydrogen in the

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fuel mixtures has been previously reported to increase the flame stability15,

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flame extinction limit17 under air combustion. The extension of the flame stability is even more


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and extend the

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important under oxy-fuel combustion that contains CO2 diluents. This diluent (CO2) is

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characterized by high radiative properties which requires the increase of the oxygen

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concentration in the O2/CO2 mixture to about 30% to achieve similar combustion characteristics

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as compared to 21 % in air11. The effect of hydrogen contents in fuel mixture on the stability of

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O2/CO2 is rarely studied. In this study, we carried out experiments to examine the impact of CO2

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addition on the stability and thermal field of oxy-CH4 and oxy-CH4/H2 flames under non-

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premixed conditions. Experiments were carried out in a swirl stabilized model combustor at

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firing rates of 2.5-4.5 MW/m3-bar. Firing rate of 3.5-20 MW/m3-bar falls within operating

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condition of typical gas turbine combustor18. We also analyzed the effect of fuel (CH4/H2) and

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oxidizer (O2/CO2) compositions on the flame length scaling and proposed an empirical model

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constant for predicting the flame length.

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2. Experiments

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The schematic layout of the model of non-premixed swirl stabilized combustor used in the

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current study is shown in Fig. 1. The swirl combustor consists principally of the fuel nozzle,

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swirler, the air orifice and the combustion chamber as illustrated in Fig. 2. The fuel nozzle is

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installed on 6.35 mm diameter pipe and is made up of 16 fuel channels (0.13 mm × 0.45 mm)

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with a centered bluff body of 5 mm. The inserted bluff body was used to increase the fuel

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velocity and uniformity of the fuel injection. An eight-vane axial swirler with 45 degree vane

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angle was used to create a swirling ﬂow in the combustor corresponding to ~ 0.77 swirl number.

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The swirling flow enhanced fuel-oxidizer mixture and turbulence mixing in the combustor. The

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combustion chamber, where the ﬂame is anchored, is made of optically accessible quartz tube of

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300 mm. Fuel (methane and Hydrogen mixture) and the oxidizer (oxygen and carbon dioxide

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mixture) were delivered to the combustor via the fuel and oxidizer mixing chambers. The gases


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were regulated using mass flow controllers by Bronkhorst High-Tech with an uncertainty of ±

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0.5%. All gases used in the experiments were supplied from cylinders with purity of 99.99%.

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The fuel and oxidizer mixtures were allowed to flow through a separate vertical pipe of 2m

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height before being delivered to the combustor, thus, further enhancing mixing and homogeneity

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of the mixture. Oxy-combustion experiments of pure methane (0% H2) were initially carried out

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to study the flame stability and combustor thermal field under non-premixed conditions and at

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different CO2 concentrations in the oxidizer mixture. Experiments were, thereafter, carried out at

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combustor firing rates ranging from 2.5 - 4.5 MW/m3-bar and hydrogen contents of 10% H2 and

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20% H2 (on volume basis) in the fuel mixture. A sample of operating conditions at firing rate of

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4.5 MW/m3-bar is given in Table 1. Flame images were captured using high-speed digital

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camera. The flame length at different operating conditions was, thereafter, obtained using a

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MATLAB custom code. The radial and axial temperature profiles of the flame were measured at

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fixed location with reference to the combustor dump plane. A shielded type B thermocouple

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from Omega instruments with measuring range of up to 1993K and accuracy of ± 0.5% was used

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to measure the flame temperature. In order to obtain the flame transition points, CO2 sweep test

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was carried out. This was achieved by gradually increasing the CO2 concentration in the oxidizer

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mixture by 1 % (on volume basis) until flame transition occurs. The CO2 sweep test was repeated

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three times and the average transition point is reported. All experiments were carried out at

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ambient temperature and pressure.

137 138 139 140


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Table 1. Operating conditions at 0% H2 and 20% H2: firing rate of 4.5 MW/m3-bar

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0% H2 % CO2 in oxidizer mixture

0 10 20 30 40 50 691 842

20% H2

Volume of CH4 (lit/min)

Volume of O2 (lit/min)

Volume of CO2 (lit/min)

% CO2 in oxidizer mixture

Volume of CH4 (lit/min)

Volume of H2 (lit/min)

Volume of O2 (lit/min)

Volume of CO2 (lit/min)

9.54 9.54 9.54 9.54 9.54 9.54 9.54 9.54

19.08 19.08 19.08 19.08 19.08 19.08 19.08 19.08

2.12 4.77 8.18 12.72 19.08 42.47 100.16

0 10 20 30 40 50 79.31 89.52

8.87 8.87 8.87 8.87 8.87 8.87 8.87 8.87

2.22 2.22 2.22 2.22 2.22 2.22 2.22 2.22

18.85 18.85 18.85 18.85 18.85 18.85 18.85 18.85

2.09 4.71 8.08 12.56 18.85 72.2 160.4

146 147

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Attached flame → Lifted flame transition point, 2Lifted flame → No flame transition point.


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Fig. 1. Experimental set-up of oxy-combustion of hydrogen-enriched methane


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Fig. 2. Combustor layout

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3. Results and discussion

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3.1

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In Fig. 3, we present the visible flame images for oxy-combustion of methane and hydrogen

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enriched methane of a swirl stabilized jet flame. The flame generally transits through the

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following flame regimes:

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turbine operating conditions.

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flame, while flame stabilized at a distance away from the nozzle is referred to as lifted flame.

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The regime when no visible flame is observed in the combustor is the No flame regime. For the

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base case of pure methane (0% H2), a long stem flame with a reddish plume in the far burner

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region (i.e. towards the combustor exit) was observed. The reddish plume suggests the presence

Flame structure

Attached flame → Lifted flame → No flame, especially under gas

Flame anchored to the fuel nozzle is referred to as the attached


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of unburned hydrocarbon and formation of pollutant emission (such as soot) in the flame

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implying an incomplete combustion.

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increasing CO2 in the oxidizer mixture and a more compact flame was, thereby, observed. The

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observed flame length at different CO2 is discussed in section 3.4.

The luminosity of the reddish glow decreases with

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Fig. 3. Flame images for different oxidizer (O2/CO2) mixtures and fuel compositions at a firing rate of 4 MW/m3-bar. 10 ACS Paragon Plus Environment

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The addition of CO2 increases the oxidizer mass flow rate (i.e. velocity) and, consequently, the

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Reynolds number. This enhances turbulent mixing in the combustor that results in a more

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efficient chemical reaction. A further increase in the CO2 in the oxidizer leads to transition of the

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flame from the attached flame condition to lifted flame state, forming a shrinked-balloon-type

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flame. The lifted flame was observed to move to and from within the combustor. More details on

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the stability of the lifted flames are discussed in section 3.3. CO2 also serves as diluents in the

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combustion process leading to a decrease in the flame temperature and flame burning velocity19,

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thus reducing the resistance of the flame to strain. The strain rate used in this article is the

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hydrodynamic gradient in the flame. The flame resistance to large hydrodynamic gradient is used

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to explain the flame transition and stability. The expression for the global strain rate for diffusion

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flame is given in20. At a critical CO2 contents in oxidizer mixture the flame cannot withstand the

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strain imposed by the flow at the flame root. The flame is thereby lifted (stabilized) to a distance

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away from the fuel nozzle. Further increase in the CO2 composition in the oxidizer leads to the

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extinction of the flame. More insight into the dynamics of the flame transition is given in section

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3.2. The addition of hydrogen to the methane has earlier been reported to extend the flame

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stability range15. We similarly observed that the Attached flame → Lifted flame and Lifted

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flame → No flame transition regimes occur at higher CO2 composition in the oxidizer mixture.

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The extension of the transition by hydrogen addition can be attributed to hydrogen higher

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resistance to strain compared to methane fuel21, thus, enabling the flame to be sustained at higher

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CO2 content in the oxidizer. A lifted flame was observed when the CO2 composition in the

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oxidizer is 69%, 74% and 76% for 0% H2, 10% H2 and 20% H2, respectively. It is worth

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mentioning here that in the neighborhood of the flame transition regime, there is abrupt change


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in the acoustic level of the combustor. The acoustic response of the combustion was, however,

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not quantitatively measured and not reported in this paper.

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3.2

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In this section, we examine the transition regimes of oxy-combustion of methane and hydrogen

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enriched methane flames under varying operation conditions. Figure 4 shows the stability map of

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the combustor for oxy-combustion of the base case (0% H2) at different firing rates. The figure

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illustrates the flame transition points observed in the combustor. The observed transition points

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were obtained by gradually increasing the percentage of CO2 in O2/CO2 mixture until transition

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occurs. As mentioned earlier, three flame regimes were generally observed: the attached flame

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regime, the lifted flame regime and No flame regime. At firing rate of 2.5 MW/m3-bar and 3

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MW/m3-bar, the flame transitions exhibit bimodal regime (i.e. attached flame and No flame

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regime). Thus, the flame transits directly from attached flame to No flame regime. At firing rates

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of ≥3.5 MW/m3-bar, the flame transitions exhibit tri modal regime (i.e. attached flame, Lifted

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Flame Transition regimes

flame and No flame regime) such that, the flame transits from Attached flame → Lifted flame → No flame regime. These firing rates (≥3.5 MW/m3-bar) fall within the range of industrial gas

turbine combustor of 3.5 - 20 MW/m3 -bar 18. For industrial gas turbine firing rate (≥3.5 MW/m3-

bar), we observed the Attached flame → Lifted flame transition point decreases with increasing

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firing rate, while the Lifted flame →No flame transition point increases with increasing firing

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rate. This increased the lifted flame window.


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Fig. 4. Flame stability map at different firing rate; 0 % H2 .Dashed line: Lifted flame →No flame transition boarder

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Similar trend was observed when the hydrogen content in the fuel mixture was increased to 20%

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H2 (see Fig. 5).The addition of hydrogen, however, shifts the stability map of the combustor to

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region of higher CO2 concentration in the oxidizer mixture. Hydrogen has been previously

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reported to have higher laminar flame speed, lower ignition energy, wide flammability limit as

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well as higher resistance to strain compared to methane21-23. Thus, the addition of hydrogen to

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methane increased both the Attached flame → Lifted flame and lifted flame→No flame transition

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regimes. This implies that the addition of hydrogen enables the flame to withstand higher CO2

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concentration in the oxidizer.


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Fig. 5. Flame stability map at different firing rate; 0 %H2: 20 %H2. Dashed line: Lifted flame →No flame transition boarder Increasing the combustor firing rate is expected to increase the local flame temperature and,

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thereby, extends the transition regime (Attached flame → Lifted flame) to region of higher CO2

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concentration in the oxidizer. Remarkably, we observed a decrease in flame resistance to CO2 at

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higher firing rate. It is important to state that by increasing the firing rate, the amount of fuel

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supplied is increased, thus, increasing the turbulence at the fuel nozzle. The increased turbulence

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at the nozzle enhances the entrainment of the oxidizer (containing CO2) into the fuel stream. CO2

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has been reported to decrease flame reactivity24. Thus, the entrainment of more CO2 containing

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oxidizer at higher firing rate (due to enhance turbulence) reduces the flame burning rate (weak

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flame) at the flame root (see Fig. 8). We generally observed that weak flames with low strain

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resistance at the nozzle preceded the Attached flame → Lifted flame transition. Flame

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characterized with weak flame root under high flow strain rate (high fuel velocity) transits from

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Attached flame → Lifted flame at lower CO2 composition in the oxidizer. Interestingly, we observed a nearly constant oxidizer velocity (Uox) at the Attached flame → Lifted flame 14

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transition regime suggesting that there is a critical velocity for this particular transition (see Fig.

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6). At this critical velocity, the rate of flow strain imposed on the flame in the vicinity of the

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nozzle is more than the strain the flame can resist (extinction strain rate), this results in the local

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flame extinction at the fuel nozzle leading to the stabilization of the flame at a distance from the

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nozzle. Um et.al.25 have earlier reported that the lifted flame occurs when the local strain rates

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are too high to match the local burning velocity. In another work26, the imbalance in the flow

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time scale and chemical time scale have been identified as the main cause of the flame

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transition. Results presented in this paper suggest that the Attached flame → Lifted flame

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transition strongly depends on the flame strain resistance such that fuel composition that

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produces flames with higher strain resistance will similarly have higher critical Uox. This implies

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that the critical oxidizer velocity at which the flame transition occurs will increase with

253

increasing hydrogen contents in the fuel mixture. The critical velocity for 0% H2 and 20% H2 is

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20.43 m/s ± 5% and 25.58 m/s ± 4%, respectively.

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Fig. 6. Velocities of the fuel and oxidizer at the transition points; 0 %H2: line: Lifted flame →No flame transition boarder 15 ACS Paragon Plus Environment

20 %H2 .Dashed

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Increasing the combustion firing rate increases the local temperature in the combustor. The

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increased flame temperature increases the flame burning velocity and consequently the flame

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resistance to extinction. This implies that flame at higher firing rate will be able to accommodate

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more CO2 concentration in the oxidizer before extinction (see Fig. 5). Thus lifted flame→No

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flame transition regimes extend to region of higher CO2 in the O2/CO2 mixture. The addition of

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20 % H2 further increases the flame burning speed and resistance to extinction strain compared to

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the case of 0 % H221-23. The flame extinction regime similarly extends to region of higher CO2

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in the O2/CO2 mixture. The addition of hydrogen is observed to have stronger effect in delaying

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of the Lifted flame → No flame transition points when compared to the combustor firing rate.

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For instance, increasing the firing rate from 3.5 MW/m3-bar to 4.5 MW/m3-bar (about 29%

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increase) increases the Uox by about 7% while increasing the hydrogen content in the fuel

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mixture by 20% increases the Uox by about 24%. We, thereafter, isolated the effect of combustor

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firing rate to study the effect of hydrogen enrichment on the flame stability (see Fig. 7). Addition

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of 20% H2 extends the lifted and No flame regime by 7% and 4 %, respectively, when compared

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to the case of 0% H2. Over this range of hydrogen addition, the fuel flow velocity also increased

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by 16% due to lower density of hydrogen. This implies that the increased flow strain imposed on

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flame by the increase in fuel flow velocity could be responsible for marginal extension limit

275

observed. Further studies will be required to isolate the effect of Reynolds number of the flow to

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effectively quantify the contribution of H2 on the flame stability in oxy-combustion application.


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277 278 279 280 281 282 283

Energy & Fuels

Fig. 7. Flame stability map at different hydrogen contents in the fuel mixture; firing rate is 4 MW/m3-bar. 3.3 Stability of lifted non premixed flame Figure 8 shows sequential images of the flame near the lift off point. As stated earlier, weaker

284

flames at the base (Flame. II - V) were observed to precede the flame lift off (Flame VI). The

285

weak flame at the base indicates reduction in the chemical activity leading to local flame

286

extinction at the flame base (Flame V) and eventual flame lift off (Flame. VI).

287 288

289 290 291 292

Fig. 8. Sequential images of the flame base close to lift of point for 0 % H2 and 64-69% CO2 in oxidizer (O2/CO2) mixtures.


Energy & Fuels

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293

The numerical results24 have similarly shown that the addition of diluent reduces the chemical

294

activity due to reduced temperature at the base of the flame. They concluded that the reduced

295

temperature at the flame base is due to their observed H-atom destruction rate (i.e. reaction rate)

296

that leads to local flame extinction and flame lift off. Different theories have been advanced to

297

explain the stability of a lifted non-premixed flame and summarized in previous work27:

298

Premixed Flame Theory27, The Edge-Flame Concept28, and The Critical Scalar Dissipation

299

Concept29. In The Premixed Flame Theory, the base of a non-premixed flame is premixed27 and

300

burns locally at a burning velocity that stabilizes the flame. The Edge-Flame Concept28,

301

however, claims that the leading edge of the flame is partially premixed and propagates upstream

302

to counter the effect of the flow field. The Critical Scalar Dissipation Concept29 suggests that

303

lifted flames are stabilized at a location where Scalar Dissipation rate is lower than a critical

304

value. The Edge-Flame Concept and Scalar Dissipation Concept were collapsed in Takahashi

305

and Katta30 theory that suggests that lifted flame stabilization is due to a dynamic balance in the

306

scalar dissipation rate and characteristic reaction rate. This theory was reported to be consistent

307

with their numerical observations. In our study, we observed that the leading edge of the lifted

308

flame oscillates to and fro at different points within the combustor indicating that the

309

stabilization points are the points where the edge flame speed balances the local flow velocity.

310

This is in agreement with the Takahashi and Katta30 theory since the edge flame speed is a

311

function of the scalar dissipation rate. The unsteady nature of the flow-flame interaction will

312

change the distribution of the scalar dissipation rate within the combustor and the lifted flame

313

stabilization points. From our study, we can conclude that the flame stabilization location is not

314

fixed in a turbulent non-premixed flame as the flame leading edge will continue to identify

315

locations with lowest scalar dissipation to stabilize.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

316

Energy & Fuels

3.4 Flame length

317

We have earlier observed that the addition of CO2 results in a more compact flame. In this

318

section, we discuss the variation of the flame length with changes in the oxidizer and fuel

319

compositions. For each operating condition, about 10 flame images were averaged based on

320

which the flame length was obtained using custom MATLAB code. The averaged flame images

321

were converted to a binary form based on the flame intensity. The binary image was, thereafter,

322

used to determine the flame edges and consequently the flame length. The flame length was

323

normalized with the effective fuel nozzle diameter and presented in Fig. 9.

324 325 326

Fig. 9. Normalized flame length for different oxidizer (O2/CO2) mixtures and fuel compositions at a firing rate of 4 MW/m3-bar.

327

Previous studies have shown that the rate of flame propagation and density change in the flame

328

surface are factors that affect the flame surface area31. It can be seen that increasing the CO2

329

composition in the oxidizer alters the flame propagation rate and leads to a decreased normalized

330

flame length. Reasons for the observed decrease in the flame length were discussed earlier and

331

can be summarized as: (1) Increasing the CO2 content increases the oxidizer Reynolds number 19 ACS Paragon Plus Environment

Energy & Fuels

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332

and, thereby, increases the turbulence level in the combustor. This improves the oxidizer-fuel

333

mixing and increases the burning intensity of the flame, thus, making the flame to become

334

compact as observed in Fig.3. (2) The addition of CO2 causes convection and radiation of heat

335

away from the reaction zone. High heat transfer rates from the reaction zone could reduce the

336

flame propagation rate and further contribute to decreased flame length. (3) The addition of CO2

337

lowers the molecular diffusivity of O2 to the fuel stream, thus, affecting the rate of flame

338

propagation. (4) The chain branching reaction is suppressed by the addition of CO2 leading to

339

flame extinction even at higher flame temperature12, 32. Also from Fig.9, the flame length for

340

pure methane was observed to be marginally longer than those of hydrogen enriched flame.

341

Similar observations were reported33 for increasing concentration of H2 in H2 –Natural gas fuel

342

mixture. They suggest that fuel mixture containing hydrogen burns faster resulting in lower

343

residence and convective time scale. This implies that fuel-containing hydrogen will burn (faster)

344

over a shorter distance, thereby, shortening the flame length. The addition of hydrogen was also

345

reported to increase the radical pool of OH and H that increases the rate of combustion and

346

reduces the flame length26. The flame length was observed to decrease by about 4% with

347

addition of 20% H2. This is comparable with the 12% flame length reduction when the hydrogen

348

percentage in natural gas - air flame is increased from 0 % to 50%26.

349

3.5

350

In the results presented in Fig. 9, the flame length was normalized with the nominal diameter of

351

the fuel jet. The procedures for computing the effective diameter using the Near-field and Far-

352

field concept can be found in34. The normalized flame length obtained using this approach is the

353

characteristics length of the flame that will conceptually allow the same mass and momentum

354

flux as the actual34.

Empirical model of the flame length


Page 21 of 33

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355

Energy & Fuels

In the Near-field concept, the effective mixture density

≈ is assumed to be

356

approximately the fuel density, the effective nozzle diameter for the Near-field is given as34:

357

=

358

While in the Far-field concept, the effective mixture density

≈ is assumed to be that

359

(1)

of the ambient air and the effective nozzle density for Far-field concept is calculated as 34:

360

=

361

The effective density is computed as given in Eq. (3):

362

=

363

The calculated effective nozzle diameter using the Near-field and Far-field concept is then used

364

to normalize the experimental flame length as:

365

"#,% =

366

(4)

367

Where L is the measured flame length, "#,% is the normalized measured flame length.

368

These are compared to those obtained using nominal diameter of the nozzle as presented in Fig.

369

10 and as explained hereunder.

(2)

. . ! . . . ! .

(3)

&

'


Energy & Fuels

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370

In modeling the flame length, Chen et al.,35 proposed an empirical model for predicting the flame

371

length. Analysis by Dahm et al.,36 results in a similar model as given in Eq. (5). The model is

372

developed to predict the flame length based on nominal nozzle diameter. The empirical

373

normalized flame length ("#, ( ) is computed as:

374

"#, ( =

.!

( - ).*! + , -

- ./ 0 1 0 23 - ./ .0

(

(5)

4

375

Kim et al.,37 also reported a theoretical model that predicts the flame length based on the Near-

376

field The normalized flame length based on the Near-field concept ("#, )is:

0 23 / ! . / 0

377

"#, = 5. *1 + 89 , :

378

The normalized flame length based on the Far-field concept ("#, < ) is:

0 23 / ! . . - / 0

;

0 23 / ! . . - / 0

(6)

379

"#, < = 5. *1 + 89 ,

380

One of the drawbacks in the above flame length models is the arbitrary constant C given in the

381

models and which varies in all the models and changes with changes in fuel composition. In the

382

present study, we suggest that that the constant C can be calculated as:

383

5=

=

#=

'>?

:

0 23 / ! . / 0

;

(7)

(8) 22 ACS Paragon Plus Environment

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Energy & Fuels

384

By incorporating the obtained C in Eq. 5, 6 and 7 the models' equations can be re-written as:

385

For Chen et. al.35 :

386

387

"#, ( =

=

#=

'>? .

( - *! + , -

- ./ 0 1 0 23

.!

- ./ .0

(

(9)

4

For Near-field concept37 :

=

388

"#, =

389

For Far-field concept37:

#=

'>?

. *1 + 89 , :

0 23 / ! . / 0

0 23 / !- ./ . 0

;

0 23 / ! . . - / 0

(10)

390

"#, < = *1 + 89 ,

391

These equations (Eqs. (9-11)) are then used to predict the flame lengths. The predicted flame

392

lengths are compared to the normalized experimentally measured flame lengths as shown in Fig.

393

10. For both cases of 0% H2 and 20 % H2, all the models captured the trend of the normalized

394

flame length. The Far-field, however, over-predicts the flame length while the model by Chen35

395

under-predicts the flame length. The predicted flame length based on the Near-field concept gave

396

good fit of the experimentally measured flame length. This is in agreement with Kim et al.

397

who have also reported that the Near-field model fitted their experimental measured data.

.

=

#=

'>?

.:

0 23 / !/ . 0


;

(11)

37

Energy & Fuels

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398 399

a) Case of 0 %H2

400 401 402 403 404 405

b) Case of 20 %H2 Fig. 10. Experimental and modeled normalized flame length for different oxidizer (O2/CO2) mixtures and fuel compositions at a firing rate of 4 MW/m3-bar; Near-field concept, Farfield concept, Chen model : experiment-symbol, empirical model- line. 24 ACS Paragon Plus Environment

Page 24 of 33

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406

Energy & Fuels

3.6 Combustor thermal field

407

The axial temperature along the combustor axis for different operating conditions is given in Fig.

408

11. We isolated the effect of the firing rate (i.e. keeping it constant at 4 MW/m3-bar) to study the

409

impact of CO2 addition and/or hydrogen addition on the combustor temperature distribution. We

410

generally observed that the temperature decreases with increasing distance from the combustion

411

burner. This is expected due to the convection and radiation heat loss to the ambient. The

412

addition of CO2 to the oxidizer mixture monotonically decreases the temperature distribution in

413

the combustor for 0% H2 and 20% H2 operating conditions. CO2 acts as a diluent that lowers the

414

flame temperature. The fact that CO2 has a high molar heat capacity and radiative properties

415

facilitates the convection and radiation of heat away from the flame zone. Moreover, the addition

416

of CO2 increases the oxidizer mass and volume flow rates (and consequently, the velocity and

417

the Reynolds number) of the oxidizer mixture, leading to a better mixing with fuel which results

418

in a more compact flame (see Fig. 3). Such that, at the mid plane of the combustor, flame

419

containing 0 % CO2 have higher temperature than those of 50 % CO2 by more than 100 K.

420 421

a) 0% H2 in the fuel mixture 25 ACS Paragon Plus Environment

Energy & Fuels

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422 423 424 425 426

Page 26 of 33

b) 20% H2 in the fuel mixture Fig. 11. Axial temperature along the combustor axis for different CO2 composition in the oxidizer mixture and firing rate – 4 MW/m3-bar

427 428

In Fig. 12, we showed that the addition of H2 increases the flame temperature at all axial

429

locations. This could be attributed to: (1) H2 higher adiabatic flame temperature compared to

430

methane, (2) increased fuel jet velocity with addition of hydrogen. Cozzi38 has earlier reported

431

that the addition of hydrogen shifts the location of maximum temperature downstream of the

432

nozzle in a non-premixed Hydrogen /Natural gas-air flame.


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433 434 435 436

Energy & Fuels

Fig. 12. Effect of hydrogen content on the combustor axis temperature at 0% CO2: 4 MW/m3-bar We normalized the combustor temperature distribution with the adiabatic temperature by

437

computing the temperature coefficient (β) as given in Eq. (11).

438

@=

439

Similar temperature coefficient for 0% H2 and 20% H2 was observed at the same O2/ CO2 ratio.

440

This implies that by knowing the adiabatic flame temperature of the fuel mixture, the local

441

temperature distribution in the combustor can be determined for other H2-CH4 fuel compositions.

442

It can also be inferred from Fig. 13 that at higher CO2 content in the oxidizer, the flame burns

443

closer to the corresponding adiabatic flame temperature of the fuel-oxidizer mixture. This is due

444

to the fact that increasing the CO2 composition enhances turbulence mixing in the combustor.

445

This increases the burning intensity as earlier observed in Fig.3. A similar temperature

446

coefficient was also observed for different H2 contents as shown in Fig.14, further confirming

447

our earlier observation. The temperature coefficient at higher CO2 is also closer to the flame

448

adiabatic temperature compared to these at 0% CO2. The temperature data presented in this work

ABACD

AE0 BACD

(11)


Energy & Fuels

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Page 28 of 33

449

can be used in the validation of numerical models. The validated numerical models can be used

450

to gain further insight in the dynamics of the oxy combustion of methane and hydrogen- enriched

451

methane in a more cost efficient way. Finally, we carried out uncertainty analysis for the

452

measured and observed values using the method presented by Holman39. The uncertainty in the

453

observed phenomenon and measured data are due to uncertainty in the mass flow controller, data

454

acquisition system and thermocouple. The uncertainty in the observed phenomenon (such as

455

transition point etc.) is primarily due to the uncertainty in the mass flow controller, which is ±

456

0.5 % (manufacturer value), while, the uncertainty in the flame length is computed as an average

457

of ± 3.2 % . The uncertainty in the measured temperature is ± 0.6 % of the full-scale reading.

458 459 460 461

Fig. 13. Axial distribution of the Combustor temperature coefficient at firing rate of 4 MW/m3bar: -0% CO2, -30% CO2, -50% CO2: Solid lines-0% H2, Dashed lines-20% H2


Page 29 of 33

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Energy & Fuels

462 463 464 465 466 467

4. Conclusions

468 469 470

The oxy-combustion characteristics of methane and hydrogen enriched methane have been experimentally studied under non-premixed conditions. The following conclusions are drawn from this study:

471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491

Fig. 14. Radial distribution of the Combustor temperature coefficient at firing rate of 4 MW/m3bar: LC/DC=3.57.

1. The flame exhibits tri modal regime (i.e. Attached flame → Lifted flame → No flame), when the combustor is operated under gas turbine conditions (≥3.5 MW/m3-bar), below which the flame exhibit bimodal regime (i.e. Attached flame → No flame). 2. Weak flames at the nozzle exit were generally observed to precede the Attached flame → Lifted flame transition. The weak flame is due to the entrainment of more CO2 containing oxidizer (O2/ CO2) to the fuel stream that reduces the flame burning rate. 3. There exists a critical oxidizer velocity (Uox) at which the Attached flame → Lifted flame transition regime occurs. The transition regime strongly depends on the fuel composition such that fuel with higher strain resistance (such as H2 containing fuel) similarly has higher critical Uox. 4. The leading edge of the lifted flame was observed to oscillate about different points (stabilization points) within the combustor. These are points of lowest scalar dissipation rate where the edge flame speed balances the local flow velocity. 5. We also presented an empirical equation to compute the arbitrary constant given in the models of Chen 35 and Kim 37. The predicted flame length based on the Near-field concept (Eq. (8)) gave a good fit of our experimentally observed flame length. 6. Results presented in this study have shown that addition of hydrogen extends the previously reported stability limit (CO2 concentration) of oxy-methane flame under gas turbine conditions. The results can further be used to validate numerical models to have further insight into the Oxy-combustion dynamics of methane and hydrogen-enriched methane. 29 ACS Paragon Plus Environment

Energy & Fuels

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492 493 494 495

Nomenclatures

496

F

497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519

F

Area of the oxidizer nozzle

5

Area of the fuel nozzle

Diameter of the fuel nozzle

GH

Arbitrary constant

IJ

Diameter of the oxidizer orifice

89 "

Stoichiometric oxidizer to fuel mass ratio Flame length

"#,%

Experimental normalized measured flame length

LM

NO'

Number of mole of oxygen per unit mole of the fuel mixture

NP# N

Combustor temperature inlet

Q

Temperature

R

Oxidizer velocity

@

Thermal diffusivity of the fuel mixture

"#, ,"#, , "#, K "H

R

RIJ

Combustor diameter

Effective diameter of the fuel nozzle

Empirical normalized flame length

Length of the combustor

Adiabatic flame temperature

Thickness of the fuel nozzle

Fuel velocity

S

Oxidizer velocity

IJP

Density of the fuel

Temperature coefficient

Density of the oxidizer 30 ACS Paragon Plus Environment

Page 30 of 33

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520 521 522 523

Energy & Fuels

Density of oxidizer

Effective mixture density

M

Density of oxygen

524

Acknowledgment

525

This project was funded by the National Plan for Science, Technology and Innovation

526

(MAARIFAH)–King Abdulaziz City for Science and Technology, through the Science &

527

Technology Unit at King Fahd University of Petroleum & Minerals (KFUPM)–the Kingdom of

528

Saudi Arabia, award number 14-ENE67-04.

529 530

References:

531 532 533 534 535 536 537 538 539 540 541 542 543

1. Saygin, D.; Kempener, R.; Wagner, N.; Ayuso, M.; Gielen, D., The Implications for Renewable Energy Innovation of Doubling the Share of Renewables in the Global Energy Mix between 2010 and 2030. Energies 2015, 8, (6), 5828-5865. 2. Deprez, A.; Colombier, M.; Spencer, T., Transparency and the Paris Agreement: driving ambitious action in the new climate regime. 2015. 3. Walker, M. E.; Abbasian, J.; Chmielewski, D. J.; Castaldi, M. J., Dry gasification oxy-combustion power cycle. Energy & Fuels 2011, 25, (5), 2258-2266. 4. Wall, T.; Liu, Y.; Spero, C.; Elliott, L.; Khare, S.; Rathnam, R.; Zeenathal, F.; Moghtaderi, B.; Buhre, B.; Sheng, C., An overview on oxyfuel coal combustion—State of the art research and technology development. Chemical Engineering Research and Design 2009, 87, (8), 1003-1016. 5. Wall, T.; Yu, J., Coal-fired oxyfuel technology status and progress to deployment| NOVA. The University of Newcastle's Digital Repository. 2009. 6. Key world energy statistics 2015. International Energy Agency.

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http://www.iea.org/publications/freepublications/publication/KeyWorld_Statistics_2015.pdf. 7. Krieger, G.; Campos, A.; Takehara, M.; da Cunha, F. A.; Veras, C. G., Numerical simulation of oxyfuel combustion for gas turbine applications. Applied Thermal Engineering 2015, 78, 471-481. 8. Boushaki, T.; Sautet, J.; Salentey, L.; Labegorre, B., The behaviour of lifted oxy-fuel flames in burners with separated jets. International Communications in Heat and Mass Transfer 2007, 34, (1), 818. 9. Kutne, P.; Kapadia, B. K.; Meier, W.; Aigner, M., Experimental analysis of the combustion behaviour of oxyfuel flames in a gas turbine model combustor. Proceedings of the Combustion Institute 2011, 33, (2), 3383-3390. 10. Heil, P.; Toporov, D.; Förster, M.; Kneer, R., Experimental investigation on the effect of O 2 and CO 2 on burning rates during oxyfuel combustion of methane. Proceedings of the Combustion Institute 2011, 33, (2), 3407-3413. 11. Ditaranto, M.; Hals, J., Combustion instabilities in sudden expansion oxy–fuel flames. Combustion and Flame 2006, 146, (3), 493-512.


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12. Glarborg, P.; Bentzen, L. L., Chemical effects of a high CO2 concentration in oxy-fuel combustion of methane. Energy & Fuels 2007, 22, (1), 291-296. 13. Yin, C.; Rosendahl, L. A.; Kær, S. K., Chemistry and radiation in oxy-fuel combustion: a computational fluid dynamics modeling study. Fuel 2011, 90, (7), 2519-2529. 14. De Visser, E.; Hendriks, C.; Barrio, M.; Mølnvik, M. J.; de Koeijer, G.; Liljemark, S.; Le Gallo, Y., Dynamis CO 2 quality recommendations. International Journal of Greenhouse Gas Control 2008, 2, (4), 478-484. 15. Sanusi, Y. S.; Habib, M. A.; Mokheimer, E. M., Experimental study on the effect of hydrogen enrichment of methane on the stability and emission of nonpremixed swirl stabilized combustor. Journal of Energy Resources Technology 2015, 137, (3), 032203. 16. Di Sarli, V., Stability and emissions of a lean pre-mixed combustor with rich catalytic/lean-burn pilot. International Journal of Chemical Reactor Engineering 2014, 12, (1), 77-89. 17. Shanbhogue, S.; Sanusi, Y.; Taamallah, S.; Habib, M.; Mokheimer, E.; Ghoniem, A., Flame macrostructures, combustion instability and extinction strain scaling in swirl-stabilized premixed CH 4/H 2 combustion. Combustion and Flame 2016, 163, 494-507. 18. Nemitallah, M. A.; Habib, M. A., Experimental and numerical investigations of an atmospheric diffusion oxy-combustion flame in a gas turbine model combustor. Applied Energy 2013, 111, 401-415. 19. Halter, F.; Foucher, F.; Landry, L.; Mounaïm-Rousselle, C., Effect of dilution by nitrogen and/or carbon dioxide on methane and iso-octane air flames. Combustion Science and Technology 2009, 181, (6), 813-827. 20. Williams, B.; Fleming, J., DETERMINATION OF THE STRAIN IN COUNTERFLOW DIFFUSION FLAMES FROM FLOW CONDITIONS. 21. Hawkes, E. R.; Chen, J. H., Direct numerical simulation of hydrogen-enriched lean premixed methane–air flames. Combustion and Flame 2004, 138, (3), 242-258. 22. Di Sarli, V.; Di Benedetto, A.; Long, E. J.; Hargrave, G. K., Time-Resolved Particle Image Velocimetry of dynamic interactions between hydrogen-enriched methane/air premixed flames and toroidal vortex structures. International journal of hydrogen energy 2012, 37, (21), 16201-16213. 23. Di Sarli, V.; Di Benedetto, A., Effects of non-equidiffusion on unsteady propagation of hydrogenenriched methane/air premixed flames. International journal of hydrogen energy 2013, 38, (18), 75107518. 24. Briones, A. M.; Aggarwal, S. K.; Katta, V. R., A numerical investigation of flame liftoff, stabilization, and blowout. Physics of Fluids (1994-present) 2006, 18, (4), 043603. 25. Um, D.; Joo, J.; Lee, S.; Kwon, O., Combustion stability limits and NO x emissions of nonpremixed ammonia-substituted hydrogen–air flames. International Journal of Hydrogen Energy 2013, 38, (34), 14854-14865. 26. El-Ghafour, S.; El-Dein, A.; Aref, A., Combustion characteristics of natural gas–hydrogen hybrid fuel turbulent diffusion flame. International Journal of Hydrogen Energy 2010, 35, (6), 2556-2565. 27. Lyons, K. M., Toward an understanding of the stabilization mechanisms of lifted turbulent jet flames: experiments. Progress in Energy and Combustion Science 2007, 33, (2), 211-231. 28. Buckmaster, J., Edge-flames. Progress in Energy and Combustion Science 2002, 28, (5), 435-475. 29. Peters, N.; Williams, F. A., Liftoff characteristics of turbulent jet diffusion flames. AIAA journal 1983, 21, (3), 423-429. 30. Takahashi, F.; Katta, V. R., A reaction kernel hypothesis for the stability limit of methane jet diffusion flames. Proceedings of the Combustion Institute 2000, 28, (2), 2071-2078. 31. Filatyev, S. A.; Driscoll, J. F.; Carter, C. D.; Donbar, J. M., Measured properties of turbulent premixed flames for model assessment, including burning velocities, stretch rates, and surface densities. Combustion and Flame 2005, 141, (1), 1-21.


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