Chemistry: electrical and wave energy – Processes and products – Electrostatic field or electrical discharge
Reexamination Certificate
1997-07-18
2002-11-05
Mayekar, Kishor (Department: 1741)
Chemistry: electrical and wave energy
Processes and products
Electrostatic field or electrical discharge
C204S177000, C588S249000
Reexamination Certificate
active
06475350
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the removal of pollutants from gases, and more particularly to the removal of nitrogen oxides, such as NO and NO
2
, and other pollutants including particulates from exhaust gases or other industrial gases such as produced by internal combustion engines using a plasma-assisted catalytic surface, and to industrial processes generating such gases.
BACKGROUND OF THE INVENTION
Carbonaceous fuels are burned in internal combustion engines and other equipment, including boilers, furnaces, heaters, incinerators, and the like (i.e., in a wide variety of industrial processes). Excess air frequently is used to complete the oxidation of combustion byproducts such as carbon monoxide (CO), hydrocarbons, and soot. High temperature combustion using excess air, however, tends to generate nitrogen oxides (often referred to as NOx). In addition, a number of fossil fuel combustion sources result in polluted exhaust streams. These sources include internal combustion engines such as diesel, natural gas, and lean burn gasoline as well as external combustion sources such as boilers, incinerators, and other NOx, particulate and hydrocarbon containing streams. The polluted exhaust streams from such sources also may contain high O
2
(0-18%) levels. Reducing NOx can be particularly difficult for such gases containing high O
2
levels.
Emissions of NOx include nitric oxide (NO) and nitrogen dioxide (NO
2
). During combustion, it is believed that free radicals of nitrogen (N
2
) and oxygen (O
2
) combine chemically primarily to form NO at high temperatures. Mobile and stationary combustion equipment are concentrated sources of NOx emissions. If discharged to the environment, NO emissions oxidize to form NO
2
, which tends to accumulate excessively in many urban areas. In sunlight, the NO
2
reacts with volatile organic compounds to form ground level ozone, eye irritants, and photochemical smog. These adverse effects have prompted extensive efforts for controlling NOx emissions. Despite advancements in fuel and combustion technology, ground level ozone concentrations still exceed federal guidelines in many urban areas. Under the Clean Air Act and its amendments, these ozone nonattainment areas must implement strategies for low NOx, which can only be attained by exhaust aftertreatment.
Exhaust aftertreatment techniques tend to remove NOx using various chemical or catalytic methods. Such methods are known in the art and typically involve either reduction to N
2
or oxidation to NO
2
and subsequently to HNO
3
. The former reduction processes generally involve either nonselective catalytic reduction (NSCR), selective catalytic reduction (SCR) or selective noncatalytic reduction (SNCR). Alternatively, NO may be oxidized to NO
2
for removal by wet scrubbers. Such aftertreatment methods typically require some type of additional reactant to remove the NOx emissions. The use of these reactants often results in safety problems in addition to the added cost of the reactant. It would be more desirable to utilize reduction as opposed to oxidation because reduction of NO results in benign N
2
, while oxidation or NO results in NO
2
. Furthermore, it would be desirable to achieve reduction of NO to N
2
without the use of additional reactants or additives.
Although a number of different catalytic and non-catalytic postcombustion technologies have been used for NO removal, none have been able to convert NO to N
2
to an acceptable degree in the presence of large amounts of O
2
and/or H
2
O. Additives such as nitrogen based chemicals (NH
3
) and hydrocarbons also have been used to yield NOx reduction to N
2
, but such techniques tend to result in higher cost and are undesirable as they tend to present storage, safety, and by-product slippage problems.
Conventional catalytic technologies for the selective removal of NOx tend to operate at temperatures between 600-1000° F. and require the use of additives such as NH
3
(toxic) or hydrocarbons, often with undesirable by-products and safety concerns. Non-catalytic technologies tend to require much higher temperatures (above 1300° F.), requiring accessory equipment to increase its temperature and needing toxic additives such as NH
3
.
The use of non-thermal plasmas for NOx and particulate removal at low temperatures is described in the literature. Without being bound by theory, a non-thermal plasma consists of high energy electrons that are highly reactive, but thermally cool (hence “non-thermal”). It is believed that these reactive electrons collide with the primary components of the polluted gas stream to form the active species in-situ, which in turn may remove NOx and particulate emissions.
Attempts to remove NOx from exhaust gases using various types of plasma reactors has been explored. A variety of reactors, which differ primarily in the mode of generating electrons through an electrical discharge, have been used for NOx removal. These include the following: (1) corona (DC or pulsed); (2) dielectric barrier discharge; and (3) dielectric packed bed reactor. In general, the polluted gas stream is passed through each of the reactors in which a non-thermal plasma is generated, leading to the in-situ formation of the desired active species. In the presence of O
2
(as in typical diesel exhaust), studies conducted to date using these discharge reactors for NOx removal have reported the oxidation of NO to NO
2
with very poor selectivity to the desired species, N
2
.
Mathur et al. (U.S. Pat. Nos. 5,240,575 and 5,147,516) and Breault et al. (U.S. Pat. No. 5,458,748) have discussed using a corona as well as a “catalyzed” corona reactor to treat simulated exhaust. The general thrust of such disclosures is that NO is primarily removed by oxidation to NO
2
in the presence of O
2
, with subsequent absorption as HNO
3
. A number of prior art studies referenced in Mathur and Breault also describe the removal of NO by oxidation to NO
2
. Other studies, such as Penetrante et al. (
NOx Reduction by Compact Electron Beam Processing
, Proceedings of the 1995 Diesel Engine Emissions Reduction Workshop, University of California, San Diego, Jul. 24-27, 1995, p. IV75-85), Wallman et al. (
Nonthermal Aftertreatment of Diesel Engine Exhaust
, Proceedings of the 1995 Diesel Engine Emissions Reduction Workshop, University of California, San Diego, Jul. 24-27, 1995, p. V33-39), Civitano et al. (
Flue Gas Simultaneous DeNOx/DeSOx by Impulse Corona Energization,
3rd International Conference on Electrical Processing, 1987), Mizuno et al. (
Application of Corona Technology in the Reduction of Greenhouse Gases and Other Gaseous Pollutants.
, Non-Thermal Plasma Techniques for Pollution Control-Part B: Electron Beam and Electrical Discharge Processing, (Edited by B. M. Penetrante and S. E. Schultheis), Springer-Verlag, Heidelberg, 1993), and Fujii et al. (
Simultaneous Removal of NOx, COx, SOx and Soot in Diesel Engine Exhaust.
, Non-Thermal Plasma Techniques for Pollution Control-Part B: Electron Beam and Electrical Discharge, (Edited by B. M. Penetrante and S. E. Schultheis), Springer-Verlag, Heidelberg, 1993, 257-279), which used a diesel film present the shift in the NO removal to NO
2
instead of the desired product N
2
, with the introduction of less than 2% O
2
in the feed gas.
Similarly, Gentile et al. (
Microstreamer Initiated Advection in Dielectric Barrier Discharges for Plasma Remediation of NxOy: Single and Multiple Streamers
, Proceedings of the 1995 Diesel Engine Emissions Reduction Workshop, University of California, San Diego, Jul. 24-27, 1995, p. V45-56, and
Microstreamer Dynamics During Plasma Remediation of NO using Atmospheric Pressure Dielectric Barrier Discharges: Single and Multiple Streamers
, Proceedings of the Eight ONR Propulsion Meeting, San Diego, Calif., 1995, p. 64-69) used a dielectric barrier discharge, resulting in NO removal by oxidation to NO
2
.
The average kinetic energy of the electrons in a conventional gas phase plasma discharge (such as described in the above studies) is less than 10 eV. Under such co
Palekar Vishwesh
Slone Ralph J.
Loudermilk & Associates
Mayekar Kishor
Noxtech Inc
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