Lean Pre-Mixed Combustion

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3.2.1.2
Lean Pre-Mixed Combustion 3.2.1.2-1 Introduction
Gas turbine designers are continually challenged to improve cycle effi ciency
while maintaining or reducing emissions. This challenge is made more diffi cult by
the fact that these are often confl icting goals. The path to improved effi ciency is
higher working fl uid temperatures, but higher temperatures promote NOx
formation
and at 2,800 F the threshold for thermal NOx formation is reached. Furthermore,
reducing available oxygen to reduce NOx can result in higher carbon monoxide (CO)
and unburned hydrocarbon emissions due to incomplete combustion. Moreover,
increasing fi ring temperatures above 2,350 F represents a signifi cant materials science
challenge.1
To achieve lower pollutant emission rates, a variety of pre-formation and post-
formation control technologies have been utilized either individually or in combination,
including:
• Wet controls (water or steam injection)
• Dry combustion controls (lean combustion, reduced residence time, lean
premixed combustion, and two-stage rich/lean combustion)
• Selective catalytic reduction
• SCONOX catalytic absorption
• Catalytic combustion (e.g. Xonontm )
• Rich Quench Lean Combustors
• CO oxidation catalysts
This section of the Handbook focuses on Lean Premixed (LPM) combustion,
a pre-formation control strategy that has become the standard technique employed
by gas turbine original equipment manufacturers (OEM), particularly for natural gas
applications.
OEMs have developed processes that use air as a diluent to reduce combustion
fl ame temperatures and reduce NOx by premixing fuel and air before they enter the
combustor. This lean premixed combustion process is referred to by a variety of trade
names including General Electric’s and Siemens-Westinghouse’s Dry Low NOx (DLN)
processes, Rolls-Royce’s Dry Low Emissions (DLE) process and Solar Turbines’ SoLo
NO
x process. When fi ring natural gas, most of the commercially available systems are
guaranteed to reduce NOx emissions within the 15 to 25 parts per million by volume,
dry (ppmvd) range, depending on the OEM, turbine model and application. A few
OEM’s have guaranteed single digit NOx
emissions.
3.2.1.2-2 Emissions Overview
The primary pollutants emitted by gas turbine engines are NOx, CO and to a
lesser extent, unburned hydrocarbons (UHC). Sulfur dioxide, particulate matter (PM)
and trace amounts of hazardous air pollutants may also be present when liquid fuels are
fi red.
Both CO and UHC are the products of incomplete combustion. Given
suffi cient time and at high enough temperatures, these two pollutants will be further
oxidized to carbon dioxide and water. In the proposed standards of performance for new
stationary combustion turbines (40 CFR 60, subpart KKKK, dated February 18, 2005),
EPA states, “Turbine manufacturers have signifi cantly reduced CO emissions from
combustion turbines by developing lean premix technology. Lean premix combustion
design not only produces lower NOx than diffusion fl ame technology, but also lowers
CO and volatile organic compounds (VOC), due to increased combustion effi ciency.”2
The proposed rulemaking concludes that “Stationary combustion turbines do not
contribute signifi cantly to ambient CO levels.”3 Accordingly, the primary pollutant of
concern from gas turbines continues to be NOx
.
There are two sources of NO
x emissions in the exhaust of a gas turbine. Most
of the NO
x is generated by the fi xation of atmospheric nitrogen in the fl ame, which is
called thermal NO
. Thermal NO
x
x production rates fall sharply as either the combustion
temperature decreases, or as the fuel to air ratio decreases. Nitrogen oxides are also
generated by the conversion of a fraction of any nitrogen chemically bound in the fuel.
Emissions of NO
x from fuel bound nitrogen are insignifi cant when fi ring natural gas,
but must be considered when fi ring lower quality distillates and syngas.4
William R. Bender
Technology & Management
Services, Inc.
Gaithersburg, MD 20879
phone: (301) 670-6390 x144
email: wbender@tms-hq.com