Research Papers: Energy Systems Analysis

Effects of Preheating and CO2 Dilution on Oxy-MILD Combustion of Natural Gas

[+] Author and Article Information
Mohamad Hassan Moghadasi

Department of Aerospace Engineering,
Faculty of New Sciences and Technologies,
University of Tehran,
North Kargar Street,
Tehran 1439957131, Iran
e-mail: mh.moghadasi@ut.ac.ir

Rouzbeh Riazi

Department of Aerospace Engineering,
Faculty of New Sciences and Technologies,
University of Tehran,
North Kargar Street,
Tehran 1439957131, Iran
e-mail: ro_riazi@ut.ac.ir

Sadegh Tabejamaat

Department of Aerospace Engineering,
Amirkabir University of Technology,
Hafez Street,
Tehran 158754413, Iran
e-mail: sadegh@aut.ac.ir

Amir Mardani

Department of Aerospace Engineering,
Sharif University of Technology,
Azadi Street,
Tehran 1458889694, Iran
e-mail: amardani@sharif.edu

1Corresponding author.

Contributed by the Advanced Energy Systems Division of ASME for publication in the Journal of Energy Resources Technology. Manuscript received March 3, 2019; final manuscript received May 15, 2019; published online June 5, 2019. Assoc. Editor: Esmail M. A. Mokheimer.

J. Energy Resour. Technol 141(12), 122002 (Jun 05, 2019) (12 pages) Paper No: JERT-19-1114; doi: 10.1115/1.4043823 History: Received March 03, 2019; Accepted May 16, 2019

Oxy-moderate or intense low-oxygen dilution (MILD) combustion, which is a novel combination of oxy-fuel technology and MILD regime, is numerically studied in the present work. The effects of external preheating and CO2 dilution level on the combustion field, emission, and CO formation mechanisms are investigated in a recuperative laboratory-scale furnace with a recirculating cross-flow. Reynolds-averaged Navier–Stokes (RANS) equations with eddy dissipation concept (EDC) model are employed to perform a 3-D simulation of the combustion field and the turbulence–chemistry interactions. In addition, a well-stirred reactor (WSR) analysis is conducted to further examine the chemical kinetics of this combination when varying the target parameters. The simulations used the skeletal USC-Mech II, which has been shown to perform well in the oxy-fuel combustion modeling. Results show that with more preheating, the uniformity of temperature distribution is noticeably enhanced at the cost of higher CO emission. Also as inlet temperature increases, the concentration of minor species rises and CO formation through the main path (CH4→CH3→CH2O→HCO→CO→CO2) is strengthened, while heavier hydrocarbons path (C2H2→CO) is suppressed. Meanwhile, greater CO2 addition notably closes the gap between maximum and exhaust temperatures. In a highly CO2-diluted mixture, chain-branching reactions releasing CH2O are strengthened, while chain-terminating reactions are weakened. CH2O production through CH3O is accelerated compared with the straight conversion of methyl to formaldehyde. When diluting the oxidant, methylene CH2(s) plays a more influential role in CO formation than when pure oxygen is used, contributing to higher CO emission.

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Fig. 1

Schematic view of MCF (mm in unit)

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Fig. 2

Schematic view of (a) computational domain and the boundary conditions, (b) side view mesh, and (c) bottom view mesh

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Fig. 3

Axial velocity profiles (a) at z = 60.5 mm, X = 0, (b) at z = 176.5 mm, X = 0, and (c) at z = 176.5 mm, x = −10 mm for case 1 in Table 1 using three different grid resolutions

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Fig. 4

Temperature distribution for case 1, in Table 1 at (a) z = 42.5 mm and (b) z = 542.5 mm

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Fig. 5

CFD predictions and WSR results for (a) reference temperature and exhaust temperature and (b) variations of exhaust CO and CO2 emissions versus equivalence ratio. Experimental data are extracted from Figs. 4 and 5 of [22].

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Fig. 6

Contours of (a) axial velocity and flow streamlines, (b) temperature, and (c) oxygen mass fraction on yz plane (x = 0) for three inlet oxidizer temperatures

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Fig. 7

Variations of mean, exhaust, and WSR temperatures with To. Solid lines represent Φ = 1 and dashed lines represent Φ = 0.9 (cases 2–7).

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Fig. 8

Variations of mixture fraction profile with To along Y-axis at x = 0 and z = 342.5 mm (cases 2–4)

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Fig. 9

Variation of exhaust emissions of CO and CO2 with To for Φ = 1 and Φ = 0.9 (cases 2–7)

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Fig. 10

Major CO-containing reaction rates along the furnace height for different oxidizer inlet temperatures (cases 2–4)

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Fig. 11

(a) Mass fraction profiles of OH radical along y-axis at z = 342.5 and x = 0 mm (cases 3 and 4) and (b) mass fraction profiles of O and H radicals along y-axis at z = 342.5 and x = 0 mm (cases 2–4)

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Fig. 12

Methane oxidation pathways in a WSR analysis at Treactor = 1540 K, tR = 1 s, p = 1 atm for (a) To = 298 K and (b) To = 723 K

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Fig. 13

Temperature contours in different slices in xy plane at Φ = 1 and To = 298 K for different inlet mass fraction of CO2 in oxidizer mixture (cases 2, 8, 9, and 10 in Table 1)

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Fig. 14

Maximum, mean, and exhaust temperatures and exhaust emissions of CO and CO2 for different inlet oxygen volume fractions at Φ = 1, P = 13 kW (cases 2, 8, 9, and 10 in Table 1)

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Fig. 15

CO-containing reaction rates profile along the furnace for cases 2, 8, 9, and 10 in Table 1

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Fig. 16

Methane oxidation pathways in a WSR analysis at Treactor = 1540 K, tR = 1 s, and p = 1 atm for XCO2 = 50% (case 10)



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