Supercritical fuel

Dr. Neil Canter, Contributing Editor | TLT Tech Beat February 2010

A new process transforms diesel fuel in a way that reduces emissions and increases brake thermal efficiency.

KEY CONCEPTS
• A fourth state known as supercritical is reached when a substance exhibits temperature and pressure values above the critical point.
• Deficiencies in the diesel engine can be overcome by moving the fuel into a supercritical state.
• CFD modeling shows that a supercritical fluid based on a mixture of diesel fuel and an EGR stream has the potential to lead to reduced emissions and increased brake thermal efficiency in a diesel engine.

A wide range of approaches have been undertaken to determine how to reduce emissions from automotive engines. In a previous TLT article, the development of the homogeneous charge compression ignition (HCCI) engine was discussed (1).

When used in place of a conventional gasoline engine, an HCCI engine can produce a 15% to 20% improvement in fuel economy and a significant reduction in No_x emissions. The HCCI process relies on spontaneous ignition of the gasoline, air mixture similar to the auto-ignition process in a diesel engine.

As we all know, three specific states of matter (solid, liquid and gas) exist, and most substances can be found in one of these states at any given combination of temperature and pressure. When the temperature and pressure is increased above a value known as the critical point, the substance reaches a fourth state known as supercritical.

A supercritical fluid exhibits properties somewhere between a liquid and a gas. It is a homogeneous fluid that is maintained in a single phase. Dr. George Anitescu, research scientist in the department of biomedical and chemical engineering at Syracuse University, in Syracuse, N.Y., says, “A supercritical fluid cannot be turned into a liquid solely by compression/expansion (e.g., air in the engine cylinders).”

Two supercritical fluids also mix easily with one another. They exhibit molecular diffusivities and no surface tension, which facilitates the mixing process. Supercritical fluids are used in applications such as decaffeination and power generation.

Anitescu says, “The diesel engine displays greater efficiency than the gasoline engine. The air-fuel mixture in the latter, although homogeneous, needs a spark to ignite due to low reactivity. In contrast, the mixing of air and fuel in a diesel engine is not uniform. Diesel fuel, which is composed of heavier hydrocarbons, is more viscous and less volatile than gasoline. The resulting incomplete combustion usually leads to the generation of undesirable levels of particulate matter, while high-temperatures at the flame front generate No_x.”

These deficiencies in the diesel engine can be overcome if the diesel fuel is moved into a supercritical state. Anitescu says, “Enabling diesel fuel to become supercritical will allow for mixing with air in a homogeneous phase upon injection, in a similar fashion to a gasoline engine.”

An approach has now been devised to use diesel fuel in a supercritical state.

FUEL–EGR FUEL MIXTURES 
Anitescu has developed a process for making diesel fuel supercritical. He says, “We need to mix diesel fuel with a diluent to prevent coking, which is the formation of carbon deposits. Coking of pure diesel fuel can occur at temperatures above 200 C. The logical diluents to use are water and carbon dioxide. Both are readily available, relatively inert at the temperatures where fuel combustion occurs and environmentally acceptable.”

A mixture of diesel fuel and carbon dioxide is shown at various points on the way to a supercritical state in Figure 1. The first image shows a blend of the two at a temperature well below critical. In the second image, the blend is seen just below the critical temperature.


Figure 1. Image 1 shows a blend of diesel fuel and carbon dioxide well below the critical temperature. The mixture in Image 2 is just below the critical temperature. Image 3 shows the two phases as they transition to a supercritical state. The uniform mixture of two components in a supercritical state is seen in Image 4. (Courtesy of Syracuse University)

The two phases, as they transition to a supercritical state, are shown in the third image. Finally, the fourth image shows a uniform mixture in a supercritical state. Note there is no sign of a meniscus between the two liquids.

Anitescu determined that the logical source of the water and carbon dioxide is the exhaust gas recycled (EGR) stream that is already generated by a diesel engine. He conducted a series of experiments in mixing diesel fuel No. 2 or hexadecane (cetane) with a blend of nitrogen, carbon dioxide and water meant to be comparable to EGR. The composition of the blend is 74% nitrogen, 13.5% carbon dioxide and 12.5% water.

Tests to evaluate the feasibility of preparing a supercritical fluid mixture were conducted using a benchtop laboratory apparatus. The mole fraction of the fuel was varied between 0.100 and 0.786. Experimental conditions to produce diesel fuel-EGR mixtures ranged up to a pressure of 600 bar and a temperature of 450 C. Anitescu found that a mixture of either diesel fuel No. 2 or hexadecane and the synthetic EGR can be heated up to a supercritical state without coking.

Promising results have been found in initial CFD modeling for both emissions reduction and brake thermal efficiency. Anitescu says, “An independent assessment showed that an 80% reduction in both No_x and particulate matter can be achieved by the combustion of supercritical fuel-air mixtures. In fact, in laboratory combustion experiments almost no particulate matter and No_x were observed, as the emission stream captured in hexane was as clean as the pure solvent.” The reason for the lower No_x result is that combustion was achieved at a lower temperature than a conventional diesel engine.

Brake thermal efficiency represents the percentage of fuel converted into useful work. Anitescu found that this figure could be increased for the diesel engine from approximately 40% to 50%.

Anitescu believes that the EGR stream should be mixed with diesel fuel at a temperature between 100 C and 200 C. But the EGR stream leaves the engine manifold at a temperature of approximately 500 C.

Anitescu envisions that a portion of the EGR stream should initially be sent through an existing heat exchanger to be cooled down prior to mixing with the diesel fuel. From there, the mixture is heated up to approximately 400 C by the hot EGR to a supercritical state prior to injection into the combustion chamber.

One of the biggest problems facing Anitescu is the need for development of a suitable injector that can handle the fuel-EGR mixture. This remains an objective as the process is optimized for use in a diesel engine. The Achilles’ heel here is the low lubricity of supercritical fuel-diluent mixtures. One idea is to keep the mixture in a liquid form below the critical point since liquids can be handled better by current injectors. The fuel will then rapidly transition to a supercritical state upon injection.

Anitescu says, “We now know that a supercritical fluid blend can be prepared, but we need to find out experimentally how it performs in the combustion chamber. The spraying of the supercritical fuel mixture into a view cell is currently being studied. Finally, we need to find out what are the performance differences between conventional fuel and supercritical fuel in the diesel engine.”

Additional information can be found in a recent paper (2) and patent (3) or by contacting Anitescu at ganitesc@syr.edu.

REFERENCES
1. Canter, N. (2008), “HCCI Gas Engine,” TLT, 64 (1), pp. 16–17.
2. Anitescu, G., Tavlarides, L. and Geana, D. (2009), “Phase Transitions and Thermal Behavior of Fuel-Diluent Mixtures,” Energy and Fuels, 23, pp. 3068–3077.
3. Anitescu, G. and Tavlarides, L., (2009), “Supercritical Diesel Fuel Composition, Combustion Process and Fuel System,” U. S. Patent 7,488,357 B2.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.