A Compact Brayton-Cycle Engine and Biomass Process for Mixed-Alcohol Fuels
28 April 2006
|The StarRotor compact Brayton cycle engine.|
A Texas A&M chemical engineering professor has developed a process to convert biomass to a mixed alcohol fuel that contains more energy than fuel ethanol. He has also developed a compact Brayton-cycle engine (the same thermodynamic cycle employed by jet engines) capable of being powered by any type of fuel—including his MixAlco mixed alcohol fuel.
Prof. Mark Holtzapple projects that his StarRotor engine, which is being developed by a company of the same name, could deliver efficiencies of 49–55% applied in a passenger car—about 2.5 to 3 times more efficient than a conventioanl gasoline engine.
The StarRotor engine. In the classic Brayton-cycle engine, ambient air is pressurized in a compressor, passed to a mixing chamber where fuel is added, and then ignited in an expansion chamber. It then expands through a piston/cylinder.
|The Brayton cycle.|
As applied to gas turbines, the Brayton engine has a compressor, a burner and an expansion turbine. Ambient air is compressed and passed through a heat exchanger for pre-heating. The pre-heated charge goes to a combustor where fuel is ignited, and the hot compressed air then flows to an expander where the thermal energy is converted to shaft work. The hot exhaust gases from the expander are sent to the heat exchanger where they are cooled and then discharged.
Brayton cycle engines have a high power density (hence their use in jet aircraft), compared to the lower power density of Otto (spark ignition) and Diesel engines.
The major challenge in implementing Brayton cycle engines, according to an analysis done for the Defense Advanced Research Projects Agency by Holtzapple, is to find a means to process large volumes of air to achieve a desired power output.
Traditionally, this is accomplished using dynamic (i.e., axial or centrifugal) compressors and expanders. The devices, however, require very high speeds—e.g., 100,000 rpm for a 30kW unit—to develop the desired pressure and flow. They also operate efficiently at only one speed, and are affected by changes in air density.
The patented StarRotor Brayton cycle engine uses gerotors for both the compressor and expander. (A gerotor is a positive displacement pump mechanism that delivers a known, predetermined quantity of fluid in proportion to speed.)
The StarRotor compressor has an inner gerotor with n teeth and an outer gerotor with n + 1 teeth. As the gerotors rotate, the void that opens draws air in through the inlet port. As the rotation continues, the void closes and compresses the air. When the air is compressed enough, the compressed air exhausts through the outlet port.
Because the void opens n + 1 times per revolution of the outer gerotor, the gerotor compressor is able to process enormous volumes of gas in a very compact size. The expander operates similarly to the compressor, except in reverse.
|The StarRotor applied in a vehicle could yield efficiencies of 49 to 55% and fuel economy of 75 to 100 mpg, according to Holtzapple.|
The gerotor teeth must be dry—lubricants are not compatible with the high temperatures. To prevent wear and friction, there must be no physical contact between the teeth of the inner and outer gerotors. StarRotor employs an inexpensive surface treatment to minimize gas leakage through the small gap, and an external synchronization mechanism ensures proper motion of the inner and outer gerotors.
The StarRotor, according to Holtzapple, can offer power ranges from 50W to 50,000kW. Lower-power versions employ a single stage that compresses air from 1 to 6 atm. The medium-power engines employ a second stage that compresses air from 6 to 36 atm. The high-power engines employ a third stage that compresses air from 36 to 216 atm. The power density is improved by using small-diameter rotors that rotate rapidly.
|Energy Content of Fuels|
|MixAlco Blend 1||29.0||104,000|
|MixAlco Blend 2||26.5||95,000|
MixAlco. The MixAlco process converts biomass into organic chemicals and alcohols with a multi-stage process that includes lime pretreatment, non-sterile acidogenic digestion, product concentration, thermal conversion to ketones and their subsequent hydrogenation to create mixed alcohol end products.
We can use anything that biodegrades. If you put it outside and it rots, we can use it. So we can use trees, grass, manure, sewage sludge or garbage.—Mark Holtzapple (The Eagle)
The MixAlco process consumes about 90% of the raw material substrate, and the process recycles all of its water and primary reagents. It can be tuned to produce the chemicals most in demand at a given time.
|MixAlco Chemical flowchart. Click to enlarge.|
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