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Research Papers: Gas Turbines: Combustion, Fuels, and Emissions

# A Novel Low $NOx$ Lean, Premixed, and Prevaporized Combustion System for Liquid Fuels

[+] Author and Article Information
P. Gokulakrishnan1

Combustion Science and Engineering, Inc., 8940 Old Annapolis Road, Suite L, Columbia, MD 21045gokul@csefire.com

M. J. Ramotowski, G. Gaines, C. Fuller, R. Joklik, L. D. Eskin, M. S. Klassen, R. J. Roby

Combustion Science and Engineering, Inc., 8940 Old Annapolis Road, Suite L, Columbia, MD 21045

1

Corresponding author.

J. Eng. Gas Turbines Power 130(5), 051501 (May 30, 2008) (7 pages) doi:10.1115/1.2904889 History: Received September 24, 2007; Revised October 05, 2007; Published May 30, 2008

## Abstract

Dry low emission (DLE) systems employing lean, premixed combustion have been successfully used with natural gas in combustion turbines to meet stringent emission standards. However, the burning of liquid fuels in DLE systems is still a challenging task due to the complexities of fuel vaporization and air premixing. Lean, premixed, and prevaporized (LPP) combustion has always provided the promise of obtaining low pollutant emissions while burning liquid fuels, such as kerosene and fuel oil. Because of the short ignition delay times of these fuels at elevated temperatures, the autoignition of vaporized higher hydrocarbons typical of most practical liquid fuels has been proven difficult to overcome when burning in a lean, premixed mode. To avoid this autoignition problem, developers of LPP combustion systems have focused mainly on designing premixers and combustors that permit rapid mixing and combustion of fuels before spontaneous ignition of the fuel can occur. However, none of the reported works in the literature has looked at altering fuel combustion characteristics in order to delay the onset of ignition in lean, premixed combustion systems. The work presented in this paper describes the development of a patented low $NOx$ LPP system for combustion of liquid fuels, which modifies the fuel rather than the combustion hardware in order to achieve LPP combustion. In the initial phase of the development, laboratory-scale experiments were performed to study the combustion characteristics, such as ignition delay time and $NOx$ formation, of the liquid fuels that were vaporized into gaseous form in the presence of nitrogen diluent. In the second phase, a LPP combustion system was commissioned to perform pilot-scale tests on commercial turbine combustor hardware. These pilot-scale tests were conducted at typical compressor discharge temperatures and at both atmospheric and high pressures. In this study, vaporization of the liquid fuel in an inert environment has been shown to be a viable method for delaying autoignition and for generating a gaseous fuel stream with characteristics similar to natural gas. Tests conducted in both atmospheric and high pressure combustor rigs utilizing swirl-stabilized burners designed for natural gas demonstrated an operation similar to that obtained when burning natural gas. Emission levels were similar for both the LPP fuels (fuel oils 1 and 2) and natural gas, with any differences ascribed to the fuel-bound nitrogen present in the liquid fuels. An extended lean operation was observed for the liquid fuels as a result of the wider lean flammability range for these fuels compared to natural gas.

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## Figures

Figure 1

Schematic of the premixing section of the flow reactor used for ignition delay time measurements

Figure 2

Atmospheric pressure ignition delay time measurements of stoichiometric n‐heptane∕O2∕N2, mixture as a function of the inlet O2 composition. Key: symbols, experimental data; lines, ignition delay time model predictions using the detailed kinetic model of Curran (18).

Figure 3

Comparison of ignition delay time measurements of n-heptane, fuel oil 1, and fuel oil 2 as a function of the inlet O2 composition at 900K inlet temperature, 1atm pressure, and 1.0 equivalence ratio

Figure 4

Cutaway view of the bench-scale, high pressure swirl burner used to investigate NOx emissions from the LPP combustion

Figure 5

The effect of N2 to fuel dilution ratio on NOx and CO for fuel oil 2 at 5atm and 0.6 equivalence ratio

Figure 6

Comparison of NOx measurements for methane and fuel oils 1 and 2 as a function of equivalence ratio at varying pressures. Key: methane at 2atm (*) and 5atm (●) pressures; fuel oil 1 at 3atm (◇), 4atm (◻), and 5atm (○) pressures; fuel oil 2 at 5atm (△), 8atm (▲), and 10atm (+) pressures. The lines indicate the trends. The fuel to N2 molar dilution ratio was 1–5.

Figure 7

Comparison of NOx measurements for methane and fuel oil 2 as a function of pressure. The fuel to N2 molar dilution ratio was 1–5.

Figure 8

NOx data obtained in the atmospheric pressure test rig for methane, fuel oil 1, and fuel oil 2. The data are compared to the methane data of Leonard and Stegmaier (19) as well as to the NOx data obtained in the high pressure swirl burner described in Fig. 6 at 2atm and 5atm.

Figure 9

Comparison of natural gas and fuel oil 1 flames at atmospheric pressure for Centaur 50 fuel nozzle at full load conditions

Figure 10

Comparison of NOx emission measurements for fuel oil 2, fuel oil 1, and natural gas as a function of measured exhaust gas temperature for a single fuel nozzle at Centaur 50 full load conditions (100%). The combustion air temperature was 613K, the combustor pressure was 1atm, and the fuel dilution was 6:1 (molar basis).

Figure 11

Comparison of CO emission measurements for fuel oil 2, fuel oil 1, and natural gas as a function of measured exhaust gas temperature for a single fuel nozzle at Centaur 50 full load conditions (100%). The combustion air temperature was 613K, the combustor pressure was 1atm, and the fuel dilution was 6:1 (molar basis).

Figure 12

Comparison of NOx and CO emission measurements for fuel oil 2 and natural gas as a function of measured exhaust gas temperature for a single fuel nozzle at Taurus 60 full load conditions (100%). The combustion air temperature was 648K, the combustor pressure was 12.6atm, and the fuel dilution was 5:1 (molar basis).

Figure 13

Comparison of NOx and CO emission measurements for fuel oil 2 as a function of measured exhaust gas temperature for a single fuel nozzle at Taurus 60 (T60) and Taurus 70 (T70) full load conditions (100%). The combustion air temperatures were 648K (T60) and 706K (T70), the combustor pressures were 12.6atm (T60) and 16.2atm (T70), and the fuel dilution was 5:1 (molar basis).

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