MIT RESEARCH DEMONSTRATES HYDROGEN MANUFACTURE WITH NON-THERMAL PLASMA

A paper by L. Bromberg et al., presented at the American Chemical Society National Meeting in San Diego, California, in March, describes research carried out in the Plasma Science and Fusion Center of the Massachusetts Institute of Technology (MIT). They have found that hydrogen-rich gas can be efficiently produced in compact plasma boosted reformers by conversion of a variety of hydrocarbon fuels including natural gas and diesel. A novel type of plasma reformer, using a low-current, high voltage non-thermal plasmatron has been investigated. Use of a low-current non-thermal plasmatron greatly reduces the specific electrical energy consumption and the electrode wear relative to thermal arc plasma reformers.

According to the authors, plasma technology has potential advantages over conventional means of manufacturing hydrogen. The shortcomings of conventional reformers include the need for large-scale plants, cost and deterioration of catalysts; size and weight requirements; limitations on rapid response; and limitations on reformation of heavy hydrocarbons.

The main disadvantage of plasma reforming is the dependence on electrical energy. The new low-current plasmatron developed at MIT makes it possible to overcome this problem and drastically decrease energy consumption.

The combination of new low-current plasma boosted reformers with rapid start-up metallic catalyst and a simple counterflow heat exchanger make it possible to develop a conceptual design for a compact highly efficient multifuel plasma processor for fuel-cell power generation and other applications.

Plasma boosted reforming may allow economically attractive operation at small hydrogen production levels. The plasmatron fuel converter technology is being developed for hydrogen manufacturing for a variety of stationary applications including distributed, low pollution electricity generation from fuel cells; hydrogen-refueling gas stations for fuel cell-powered cars; and decentralized hydrogen generation for industrial processes. It is also being developed for potential use with internal combustion engine vehicles.

Plasmatron Fuel Converter

Plasmatrons are electrical devices that take advantage of the finite conductivity of gases at elevated temperatures. At these temperatures, the gas is partially ionized and electrically conducting. Plasmatron fuel converters provide electrical discharges in flowing gases of hydrocarbon fuels and air (and/or other oxidants). The resulting generation of reactive species in the flowing gases along with increased mixing accelerates reformation of hydrocarbon fuels into hydrogen-rich gas.

Plasmatron fuel converters may also be utilized for increasing the gas stream enthalpy, further accelerating the reaction rates. The high temperatures can be used for reforming a wide range of hydrocarbon fuels into hydrogen-rich gas without the use of a catalyst. With the use of a catalyst, the plasma is an excellent fuel preprocessor, because it can vaporize even heavy hydrocarbons and thus could provide a source of hydrogen that can be used with many fuels.

The plasmatron would be used to boost the temperature and kinetic reactions in a reformer, resulting in hydrogen-rich gas production throughout a wide range of operation, from partial oxidation to steam reforming. The boosting of the conversion process would occur as a result of the creation of a small region with high electron temperatures (3,000 to 10,000 K) where radicals are produced and as a result of increasing the average temperature in an extended region.

The additional heating provided by the plasmatron would serve to ensure a sufficiently high number of chemically reactive species, ionization states, and temperatures for the partial oxidation or other reforming reaction to occur with negligible soot production and a high conversion of hydrocarbon fuel into hydrogen-rich gas.

The boost provided by the plasma can facilitate partial oxidation reactions with negligible soot production and efficient conversion of hydrocarbon fuel into hydrogen-rich gas. Plasmatron fuel converters can alleviate problems associated with catalytic reformation, such as response time limitations, sensitivity to fuel composition, poisoning, soot formation and a narrow operational temperature range.

The plasmatron employs a discharge mode with non-equilibrium features allowing operation at much reduced plasma current relative to compact arc plasmatron fuel reformers previously developed at MIT. The discharge is a non-thermal plasma, with electrons at much higher temperature than the ions and neutrals, which are at near room temperature. This plasma generates relatively low levels of plasma heating. Air and fuel are continuously injected in a plasma region provided by a discharge established across an electrode gap. The device operates at atmospheric pressure, with air as the plasma forming gas. When operating DC, the cathode can be a heavy-duty sparkplug. The ground electrode of the sparkplug would have been removed. The anode can be a steel or copper cylinder. Neither electrode is water cooled. A variety of electrode and injection geometries are possible.

The electrical power loss results in a drain of 3 to 5 percent of the chemical power of the fuel in order to produce the electricity. Most of the heating is provided by the exothermicity of the partial oxidation reaction. The partial oxidation heating can be increased by operating with increased oxygen-to-carbon ratio. However, this additional heating decreases the hydrogen yield.

The plasma source is followed by a reaction extension cylinder. For tests with the addition of water, a simple heat exchanger was added downstream from the reaction extension cylinder, both to cool and reformate and to produce steam.

Test results indicate a better than 70 percent conversion efficiency, with an electrical energy input of less than 3 percent of the heating value of the fuel. They also demonstrate high efficiency water shifting in a compact unit.

Hydrogen yields higher than 100 percent (H2 in product gas divided by hydrogen in fuel) can be achieved with the use of a catalyst and water shifting. Power conversion efficiencies of close to 90 percent have also been obtained under certain conditions. For onboard applications, it is difficult to provide the required water onboard vehicles, and this issue could be a major deterrent to the use of the water-shift reaction for onboard applications for internal combustion engines.

Typical second-generation low-current plasmatron fuel converter parameters were power levels of 300 watts to 600 watts, oxygen/carbon ratios of 1.2 to 1.5, and fuel rates of 0.3 to 0.5 grams per second (corresponding to about 10 to 20 kilowatts of fuel power).

The energy conversion efficiency, defined as the ratio between the heating value of the reformate and the heating value of the initial fuel, is on the order of 80 percent. This occurs without the use of heat exchangers and improved process parameters. Further improvements in the case of methane are expected in the near future. It is estimated that with improved process parameters, the energy efficiency in the case of natural gas could be as high as 90 to 95 percent.

Hydrogen Generation From Diesel

Preliminary reforming experiments with the plasmatron fuel converter have also been carried out with diesel. In this case, although the diesel conversion is high, the efficiency of the water shifting section of the reactor is not as efficient as in the case with methane. The CO concentration at the output of the reformer is on the order of 6 percent, due in part to the high gas temperature (less low temperature water shift). In the near future, experiments with diesel will be carried out with better temperature and space velocities in the water shifting reactor.

In summary, experimental studies of low-current non-thermal plasmatron fuel converter operation indicate that this approach may provide a more compact, robust, responsive and lower cost means of converting a variety of hydrocarbon fuels into syngas or hydrogen.


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