Chemical Engineering May 2011 - 46

Feature Report
Burner Design for Fuel
Flexibility and Efficiency
Design of today's industrial combustion burners
must allow operation with multiple fuels
while keeping emissions low
David Littlejohn, Robert Cheng, Peter Therkelsen,
Kenneth Smith and Sy Ali
Lawrence Berkeley National Laboratory
C
ombustion provides heat for
many industrial processes and
accounts for over 80% of the
power used by turbines to generate
electricity. Over the past several
decades, regulatory action and concerns
over environmental air pollutants,
such as oxides of nitrogen (NOx),
have pushed industrial process designers
to develop burners that lower
pollutant emissions and prompted
manufacturers to switch to cleanerburning
fuels. Going forward, industrial
combustion - both for process
heat and power generation - will be
characterized by an increasingly diverse
fuel supply and a greater need
to reduce pollutants and carbon dioxide
emissions. As substitutes to natural
gas, coal and oil, alternative fuels
are being considered by the chemical
process industries (CPI) for power
generation and process heating. These
include low-heat-content fuels such as
landfill gas, biogas and synthesis gas
(syngas), as well as hydrogen.
To take full advantage of these alternative
fuels, CPI engineers are
exploring a number of approaches
aimed at reducing carbon emissions,
improving fuel flexibility and increasing
efficiency for combustion systems.
A key part of the effort is the development
of combustor designs that operate
effectively using a range of fuels
while maintaining low emissions and
stable performance.
In this article, we discuss the lowswirl
combustor design, its operation
and its performance with natural gas,
hydrogen and low-heat-content fuels.
We believe that the low-swirl combustor
design can make a substantial
contribution toward a reduced carbon
footprint and higher air quality.
New demands on burners
For many years, burners for industrial
process heating and power generation
were relatively simple devices. The
primary criteria for burners were good
heat distribution, good flame stability
and turndown capability. Until recent
decades, emissions were not a primary
concern in burner development.
Industrial burner manufacturers
have successfully responded to
the challenge of reducing emissions
from combustion systems. There are
a number of lean, premixed burner
designs for use with natural gas that
can achieve low (sub-15 ppm or even
single-digit) NOx emissions. However,
concerns about CO2 emissions and fossil
fuel availability, as well as discussions
over the creation of a carbon capand-trade
program or other carbon
tax, have created new challenges.
Burners that have been carefully optimized
to operate cleanly with natural
gas may not be capable of operating
with alternative fuels. The combustion
properties of these alternative fuels can
differ significantly from natural gas.
Combustors that have been optimized
for clean operation on natural gas may
not burn the alternative fuels cleanly,
or even worse, may become unstable
44 ChemiCal engineering www.Che.Com may 2011
Flame temperature
Figure 1. CO levels (red) rise as flame
temperature decreases. Increasing the
fuel-to-oxidizer ratio leads to higher flame
temperatures (blue) and higher NOx
and get damaged or destroyed.
The ideal burner design would be
able to operate cleanly on both natural
gas and alternative fuels, and be able
to switch between fuels with little or
no change in operating conditions.
Production of pollutants in flames
depends on several factors - fuel-air
mixing, fuel-air ratio, flame temperature
and fuel composition. Gaseous
fuels tend to burn more cleanly since
they can be mixed with air to produce
a uniform air-fuel mixture without
undergoing a phase change. It is more
difficult to produce uniform mixtures
from liquid fuels. Liquid fuels are
sprayed into the air for combustion,
and fuel droplets are surrounded by a
fuel-rich zone until the liquid fuel is
completely vaporized. The processes
involved with solid fuel combustion
are even more complicated. All of
these factors must be considered in
the development of cleaner-burning
combustion systems.
Stoichiometry and temperature
The temperature at which a flame
burns depends on the type of fuel used
and the flame stoichiometry. In combustion,
stoichiometry is a term that
is used to describe the ratio of fuel to
oxidizer consumed during the combustion
process. For example, in the combustion
of methane (CH4 + 2O2 ->
Pollutant concentration
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Chemical Engineering May 2011

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