Ultra-Clean Low Swirl Combustion
Combustion provides 83% of the energy consumed in the U.S. For the past three decades the reduction of harmful pollutants from combustion systems has been the major driver of combustion technology development. In 1991, Robert Cheng, a Berkeley Lab scientist, conceived a combustion method that emits a very low level of pollutants. His low swirl combustion method operates on a new basic principle that can be broadly applied to heat and electric power generating systems of all sizes. It is a simple, robust, and very cost-effective technology that has been commercialized in burners for industrial heating and drying. For electric power generation, the technology is being adapted to micro-turbines (< 20 kW) and megawatt size gas turbines (< 10 MW) that operate on natural gas. Current research is focused on advancing the technology for the utility size gas turbine (> 200 MW) being developed for Department of Energy's FutureGen zero-emissions coal power plants that operate on gaseous fuels with high-hydrogen contents produced by gasification of coal.
In 1991, Robert Cheng discovered the low-swirl flame stabilization method while conducting experimental research for the Department of Energy (DOE), Office of Basic Energy Sciences to study the dynamic and chaotic interactions between turbulent fluid motions and premixed combustion. His method utilizes an aerodynamic turbulent flow mechanism called divergence to produce a "floating" flame that enables sophisticated laser diagnostics to study velocity temperature, gas density, and concentrations of chemical species within the very hot turbulent flame.
Despite a "floating" flame appearance that is normally indicative of unstable behavior, flame generated by low-swirl is robust. This method permits the study of turbulent flame behavior over a wide range of experimental conditions and has since become a benchmark experimental configuration for basic studies. Cheng soon realized that the capability of low-swirl combustion to burn robust and stable ultra-lean flames that emit very low concentrations of pollutants such as oxides of nitrogen (NOx) can be exploited for clean combustion systems.
The operating principle of low-swirl combustion is counterintuitive to the conventional high-swirl combustion method practiced for nearly a century. Understandably, most researchers and engineers were initially skeptical, treating it as an unconventional and unproven experimental method because the concept was demonstrated in laboratory flames whose outputs were orders of magnitudes smaller than the levels required for most industrial applications. Questions also arose on the flames' capability to withstand the complex flow environment inside a combustion chamber, and the effects of burner size and flow throughputs on the divergent flow mechanism.
In 1994, with the support of DOE, Office of Science, Laboratory Technology Research, Cheng began to adapt the low-swirl combustion concept to small domestic water heaters of 50,000 Btu/hr (14.6 kW). Through analysis of experimental measurements, Cheng and his team gained the scientific underpinnings for this method and used the understanding to design a simple swirler that has since become the key component for adaptation of the low-swirl combustion method to heating and power generation systems. With additional support of the California Institute of Energy Efficiency (CIEE) and DOE Energy Efficiency and Renewable Energy (EERE), Industrial Technology Program, the swirler design was scaled and tested successfully at the level of a typical small industrial systems of 2 MMBtu/hr (586 kW). Using basic combustion principles, a set of engineering guidelines was also developed for scaling the swirler to even larger capacities. Berkeley Lab received the U.S. patent for the low-swirl flame stabilization method in 1998 and for the design of the swirler in 1999.
When a small low-swirl burner with the patented swirler was featured on Discovery Channel's "Your New House" TV program in 2000, an engineer at Maxon Corp. was intrigued and impressed by the technology. Maxon soon became the low-swirl burner's first partner in development and commercialization.
Berkeley Lab signed a licensing agreement with Maxon in 2001 and Maxon commercialized the low-swirl burner as its ultra-low emissions M-PAKT burner with output capacities ranging from 0.4 to 5.8 MMBtu/hr. In late 2005, Maxon introduced a second line of larger burners of up to 45 MMBtu/hr called OPTIMA-SLS. Jeffery Rafter, the Director of Marketing at Maxon writes, "The burner design scaled governing equations that is a radical departure from the experimentation approach...The market response to this product has been good and several hundred M-PAKT burners installed to date. The M-PAKT and OPTIMA-SLS burner products...produce 'industry best' emissions without sacrificing cost or performance." Even in states without stringent air-quality rules, customers purchase M-PAKT burners because the low levels of pollutants improve their product quality by virtually eliminating the discoloring of the dried spray painted parts and the baked food products.
- Maxon Corporate web site
- Low-Swirl Combustion Clears the Air
- Available for licensing: Ultraclean Low Swirl Combustion
Progress in the development of low-swirl combustion for industrial heating systems also generated interest from gas turbine equipment manufacturers. Since 1996, with the support of the DOE Office of Electricity, Distributed Energy Resources Program (DER), Berkeley Lab collaborated with engineers at Solar Turbines, Inc. in San Diego to adapt the low-swirl combustion principle for the high pressure and temperature operating conditions of a gas turbine.
In a gas turbine, the injector serves a critical role of delivering a stable and clean flame into the combustion chamber and handling the swings in conditions at different load points and during load transitions. The Berkeley Lab/Solar team designed a low-swirl injector (LSI) that is capable of meeting these performance metrics. It is configured from an existing component of Solar's SoLoNOx injector but changing its fundamental operating principle from high-swirl to low-swirl. The LSI is a "drop-in" retrofit for a seven-megawatt gas-turbine engine and is simpler and less costly to manufacture than current SoLoNOx injectors. Recent tests in an engine demonstrate its potential to lower the emissions of oxides of nitrogen from current levels of 15 to 25 parts per million to below 5 parts per million (corrected to 15 percent O2). The joint testing and development of this technology will help gas turbine equipment operators to meet stringent air quality standards being implemented in many urban areas in the US.
Berkeley Lab and Solar Turbine are continuing their collaboration on the development of "fuel-flexible" gas turbines with LSI. The technology allows gas turbine operators to choose among such fuels as natural gas, propane, waste gases, biogases, and petroleum refinery gases—a significant competitive advantage as more environmentally sustainable fuels grow in availability. Use of gaseous hydrocarbon fuels generated in carbon-neutral processes helps reduce net greenhouse gas emissions to the atmosphere, and therefore, reduces global climate change potential. LSI gas turbines can also be adjusted to operate on pure hydrogen.
With the support of the California Energy Commission (CEC), Berkeley Lab is also working with Elliot Energy Systems to develop microturbines (100 kilowatts) with the low-swirl injector technology. Recent testing in an engine showed that the LSI can lower the NOx emissions of theses small engines to below 4 ppm (corrected to 15% O2).
For much larger utility-size gas turbines approaching 250 MW, the Department of Energy is supporting research to evaluate the low-swirl combustion technology as a candidate for the hydrogen turbines in DOE Fossil Energy's FutureGEN program. FutureGen is a DOE initiative to build the world's first zero-emissions fossil fuel power plant, using the Integrated Gasification Combined Cycle (IGCC) approach to produce hydrogen, which is separated from a concentrated CO2 stream. The CO2 is then sequestered in the earth preventing emissions to the atmosphere that contribute to climate change.
Burning of hydrogen in a gas turbine presents many technical and engineering challenges because of the high flame speed and the propensity of the H2/air mixture to auto-ignite. Cheng approaches these problems from a scientific perspective and is developing a first-order analytical model to show the coupling of the LSI flowfield with the hydrogen and natural flames. The model then provides a guide to engineer the LSI design to accommodate the changes. Preliminary tests results are encouraging and show that the LSI engineered for hydrogen can accept fuels with over 90% H2 at simulated gas turbine conditions.
Cheng is optimistic that the low-swirl combustion method can overcome the challenges associated with the very energetic hydrogen flames. His effort is one of many among universities, national laboratories and equipment manufacturers to address the challenges of IGCC. The impact of IGCC will be significant. He calculates that it would reduce greenhouse gas emissions by an average of 1.8 million metric tons of CO2, and by 4,000 metric tons of NOx per 250 MW power plant per year.