Advanced Vortex Combustion
Conventional Combustion Approaches
Since the mid 1970’s the emissions control from combustion processes has been a area of great public concern due to its impact on health and the environment. As a result the combustion industry has experienced rapid changes in both the regulations for controlling emissions and in the technologies used to meet them. The emissions of principal concern include carbon monoxide (CO), unburned hydrocarbons (UHC), particulate matter, oxides of sulfur, and NOx (nitrogen monoxide and nitrogen dioxide). Carbon dioxide and water vapor are not yet regarded as pollutants; however, they both contribute to global warming which is now receiving tremendous international attention.
The reduction of NOx emissions has received particular attention due to its toxicity, itself a precursor to chemical smog and acid rain, and its direct impact on the depletion of ozone in the stratosphere. Many scientists and consultants believe that NOx emissions pose more a threat to public health and the environment than the other combustion emissions, although actual consequences to be more indirect and long-term.
Since the mid 1970’s, NOx emissions from “conventional” combustion systems have been decreasing from levels between 90 to 250 ppmv, parts per million by dry volume at 15 % oxygen, to less than 50 ppmv, using various water and steam injection techniques.
The dry low emissions (DLE) approach to control NOx emissions was introduced
in the late 1980’s, in an attempt to eliminate the costly onsite
requirement for water or steam injection. This introduction and its continued
development has resulted in further reductions of industrial gas turbine
NOx emissions to levels between 10 and 25 ppmv.
The majority of industrial gas turbines are unable to achieve NOx emissions below 10 ppmv and require the addition of expensive exhaust gas after-treatment, such as selective catalytic reduction (SCR). The SCR technique requires the injection of highly toxic ammonia into the exhaust stream to neutralize those remaining emissions. At a minimum, this is a very costly, but it can also create environmental hazards that must also be managed, and does require the storage and handling of ammonia in densely populated regions.
NOx emissions form in the primary zone of the combustion chamber. The highest gas temperature and subsequent NOx emissions occur at the “stoichiometric” condition, which is the single point between too much fuel, or “rich” conditions, and not enough fuel, or “lean” conditions. A decrease in temperature reduces the NOx emissions, but increases the CO emission as the combustor approaches “lean extinction.”
In order to meet <10 ppmv ultra low NOx emissions requirements without a SCR, the combustor must be operated near the lean extinction limit. The lean extinction limit is that area where the combustor temperature is low and the flame is extremely weak, which then becomes very susceptible to sudden “flameouts.” These instabilities occur when fluctuations in the heat release rate couple with the acoustics of the combustion system to produce pressure oscillations.
Industry Norm Swirl Stabilized Combustion
The industrial gas turbine community has battled with balancing extremely low emissions, with engine flameouts and the unstable pressure fluctuations that can cause great damage to the engine since the early 1990’s.
In contrast to conventional combustion systems, which rely on swirl stabilization, the AVC employs cavities to stabilize the flame. Much of the literature on cavity flows examines the flow field dynamics established by the cavities, as demonstrated in aircraft wheel wells, bomb bay doors and other external cavity structures. Cavities have also been studied for a wide range of applications that include such diverse uses as a means of cooling and reducing drag on projectiles and for scramjets, to waste incineration.
The actual stabilization mechanism facilitated by the AVC is relatively simple. A conventional bluff or fore body is located upstream of a smaller bluff body - commonly referred to as an aft body. The flow issuing from around the first bluff body separates as normal, but instead of developing shear layer instabilities, which in most circumstances is the prime mechanism for initiating blowout, the alternating array of vortices are conveniently trapped or locked between the two bodies.
In an AVC concept, the re-circulation of hot products into the main fuel-air mixture is accomplished by incorporating two critical features.
First, a stable recirculation zone must be generated adjacent to the
main fuel-air flow. If the vortex region, or cavity region, is designed
properly, the vortex will be stable and no vortex shedding will occur.
This stable vortex is generally used as a source of heat, or hot products
The very stable, yet more energetic, primary/core flame zone is now very resistant to external flow field perturbations, extending the lean and rich blowout limits relative to its simple bluff body counterpart. Early research has demonstrated that the AVC configuration can withstand through-put velocities near Mach 1.
This system configuration also has greater flame holding surface area and hence will facilitate the more compact primary/core flame zone essential to promoting high combustion efficiency and reduced CO emissions.
Ramgen Power Systems has developed an advanced, high-velocity combustor technology that can have a significant impact on conventional gas turbine and plant design as well the design and operation of advanced IGCC Clean Coal power plants and hydrogen based fuel cells and fuel cell systems.
High hydrogen content fuels present a particular problem in that the flame speed of the hydrogen is approximately 6x that of natural gas. To prevent flashback, the through flow velocity needs to be greater than the flame speed, but this creates problems in establishing and maintaining the swirl-stabilized effect.
This problem is compounded in lean pre-mix designs since flashback of the flame into the fuel injector (shown above) will cause severe damage to the hardware and potential engine failure. As a result of these factors there are no high-hydrogen syngas, lean pre-mix gas turbines in operation anywhere in the industry.