Low Swirl Combustion

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3.2.1.4.2
Low Swirl Combustion
Robert K. Cheng
Lawrence Berkeley National
Laboratory
MS70-108B, 1 Cyclorton Road
Berkeley, CA 94720
phone: (510) 486-5438
email: rkcheng@lbl.gov
241 241
3.2.1.4.2-1 Introduction
Lean-premixed (LP) combustion technologies have been adopted by virtually
every industrial gas turbine manufacturer as a Dry Low NOx (DLN) method to meet
emissions regulations which are being implemented in the US and in many regions
worldwide. But to meet more stringent ultra-low emissions standards being proposed,
the DLN combustors have to operate at conditions near the lean limit of their stability
envelopes where noise, instability, fl ame blowoff, and fl ashback can seriously affect
engine performance. To mitigate these potential problems much effort has been devoted
to explore passive control, e.g., fuel and/or air staging, and active control, e.g., feed-
back loop, strategies. Other alternatives invoke more costly exhaust gas clean up or
catalytically assisted combustion. Undoubtedly, utilization of these new schemes would
lead to more complex combustion devices consisting of tightly controlled sensors and
actuators as well as many auxiliary components. For coal-based syngas engines the
instability problems are further exacerbated due to the variability of the fuel contents.
Therefore the injectors as well as the combustors have to be optimized or re-engineered
to accommodate the changes in the combustion properties. Because most turbine
combustors are designed for natural gas, they may not be readily adaptable or scalable
to burn IGCC fuels.
One promising solution to resolve fuel fl exibility issues of IGCC turbines
is a novel premixed combustion technology that operates on a unique low-swirl
combustion (LSC) concept. Originally developed at the Lawrence Berkeley National
Laboratory as a small laboratory research burner (about 15 kW) for fundamental
studies, a good understanding of its operating principle has been obtained1. This
patented combustion concept is based on exploiting the aerodynamic properties of the
propagating premixed fl ames2. It is a simple, robust, and readily adaptable technology
for industrial process burners and gas turbine combustors to meet stringent emissions
targets without signifi cantly altering their system confi gurations, effi ciencies, turndown,
and costs. LSC has been commercialized for industrial process heaters as low-swirl
burners (LSB). Products of 150 kW to 7.5 MW (0.5 to 25 MMBtu/hr) with ultra-low
emissions of 4 – 7 ppm NOx and CO (both @3% O2) have been available since late
2003. Central to the commercialization pathway was the scientifi c knowledge obtained
from laboratory studies that has provided critical information for scaling as well as
resolving system integration issues. LSC is also being adapted for natural gas turbines.
Rig tests of prototype low-swirl injectors (LSI) for 10 MW size engines show it to be a
very promising and cost-effective solution as “plug-in” injector replacements to enable
current DLN turbines to meet the emission targets of < 5 ppm (@ 15% O2) for both NOx
and CO.
The LSC concept is readily adaptable for burning other hydrocarbons and
hydrogen enriched fuels. Its operating principle is based on matching the fl owfi eld
to the turbulent premixed fl ame speeds of ultra-lean premixed fl ames. Laboratory
measurements of fl ame speeds and fl ame temperatures for the alternate fuels will be
necessary to obtain basic information for optimizing the LSC design. This strategy has
already been applied to develop fuel-fl exible industrial LSBs. Prototypes have been
tested with propane, ethylene, natural gas diluted with fl ue gases (up to 40%), and refi nery
gases with large hydrogen constituencies (up to 50% H2). The main technical issue for
adapting LSI to IGCC syngas turbines concerns the signifi cantly different combustion
properties of the two principal types of gasifi ed coal fuels. Typical compositions of
the syngas from oxygen blown coal gasifi cation are 25% H2, 40% CO, 20% H2O,
with a lower heating value of 200 BTU/ft3. With the addition of CO2 separation and
sequestration, the syngas composition shifts to mostly hydrogen at 65-85% H2, and
15-35% H2O. These syngases have diverse combustion properties and the LSI needs
to be optimized for the slower and faster burning fl ames (compared to natural gas) at
operating conditions where the fl ame temperatures are suffi ciently low to prevent NOx
formation. Other concerns stem from the high H2 diffusivity and short auto-ignition
delay time. Therefore, issues on system integration will need to address the impact
on fl ashback, blow-off, light-off, shut-down, off-load, and load following. Currently,
the research activities have been limited to proof-of-concept laboratory experiments
using hydrogen and hydrogen/hydrocarbon blended fuels. More extensive laboratory
studies will be necessary to develop basic LSI designs optimized for syngases and the
accompanying scaling rules and engineering guidelines.