NO_x/Particulate Removal from Exhaust with a Plasma

With the recent changes to the Clean Air Act in the United States and progressive tightening of the emission standards in Europe, Japan, and throughout the rest of the world, there are more demands placed on reducing the exhaust emissions from all engine types. The diesel engine has been targeted because of its tendency to produce smokey and odorous exhaust as well as the contribution to smog and acid rain. Emission reductions have been achieved by modifying the engine, utilizing exhaust aftertreatment devices, and reformulating diesel fuel. Diesel engine manufacturers have made great strides in achieving lower particulate and oxide of nitrogen (NO_x) emissions in current technology engines by carefully controlling the combustion in the cylinder and the oil consumption. Fuel manufacturers have agreed to remove most of the sulfur from fuel for further particulate emission reduction. Despite reductions in these areas, additional emission reductions are needed to meet future emission standards. To meet these demands, exhaust aftertreatment devices will be required. However to date, no singular aftertreatment system has proved superior to all others.

Toyota Pick-Up Truck on the Chassis Dynamometer

The U. S. Environmental Protection Agency (EPA) established 1991 particulate emission standards of 0.25 g/bhp-hr for heavy-duty trucks and 0.10 g/bhp-hr for urban buses. These standards were changed by the Clean Air Act Amendments of 1990. In 1994, heavy-duty trucks are required to meet a 0.10 g/bhp-hr standard, and urban buses are required to achieve an additional 50 percent reduction. In addition, heavy-duty diesel trucks will be required to meet a 4.0 g/bhp-hr NO_x standard for the 1998 model year. The significance of a lower NO_x standard will include more difficulty in achieving the particulate standard because of the NOx/particulate trade-off which occurs when the combustion is further modified to reduce either of these emissions. To meet these demands, heavy-duty diesel engine manufacturers have taken numerous steps in achieving required levels through improvements in engine design and electronic controls. Exhaust aftertreatment technologies such as diesel particulate traps and diesel tlowthrough catalysts have also been considered to meet these future demands. Even the most advanced engine designs may require some method of exhaust aftertreatment to meet the 1998 standards.

Plasma Technology for Exhaust Aftertreatment

No existing exhaust aftertreatment technique for diesel exhaust has the combined effect of reducing the particulate emissions while also affecting the gaseous emissions, specifically NO_x. However, a plasma reaction bed is a possible alternative which has the potential of reducing both NO_x and particulate. A plasma is a gas which has been a1 least partially ionized, and is comprised of an essentially charge-neutral mixture of atoms. molecules, free radicals, ions. and electrons in various states of excitation. Plasmas are reactive because of the free radicals and electrons that are present, and plasmas can be produced chemically through chemi-ionization in a flame front and more commonly by the application of an electric field. With an electric field, the process of initiating the plasma is called "striking" or "striking an arc." The electric field accelerates free electrons, which cause ionization and dissociation upon collision with any neutral compounds in the gas stream.

Two forms of plasma occur when generated by an electric field. One is a "thermal" plasma: and the other is a "non-thermar, plasma. With a "thermal" plasma, a thermal equilibrium exists between all constituents in the plasma. This situation tends to be the natural state of a plasma at atmospheric pressures due to:

• High electric field required to break down gases at this pressure
• High power input required to sustain the plasma
• Rapid collisional redistribution of energy between electrons and neutral gas atoms.

A good example of a "thermal" plasma is a direct current arc used for welding, curing, or melting metals. Very high temperatures are produced with very high input powers, and these plasmas essentially convert electrical energy into heat.

At reduced pressures (510 tort), "non-thermal" plasmas are easily produced. Under these conditions:

• Lower electric fields are required to break down the gas • Lower power tends to be required to sustain the plasma
• Collisional redistribution of energy from the electrons is less effective due to the reduced collision frequency
• Increased mean free path of the electrons results in the electrons gaining large amounts of energy between collisions as they accelerate in the field; therefore, the electrons have an excess temperature relative to the other constituents of the plasma.

A good example of a "non-thermal" plasma is a household fluorescent lamp. For a 40-watt mercury/argon plasma in a lamp, the background gas temperature may reach about 40°C, while the electron temperature in the system is close to 11,000°C. If "non-thermal" plasmas could be produced economically and conveniently at atmospheric pressure, then these plasmas would represent a new technology for the removal of many pollutants, through reaction, from a gas stream: and these plasmas may be useful when applied to diesel exhaust aftertreatment.

Result from Evaluation of Plasma Aftreatment System

A plasma reaction bed provided by AEA Technology was evaluated for its potential in removal of particulate and NO_x in the exhaust of two different light-duty diesel vehicles: an older technology indirect injection Toyota truck and a newer technology direct injection Dodge truck. Flow rates from 1 to 7 Umin. were examined with dilule exhaust to determine the effect of space velocity on the operation of the plasma aftertreatment system. With successful results obtained using dilute exhaust, experiments were continued with raw exhaust using steady state conditions. Particulate removal efficiencies and NO_x and CO conversion efficiencies were determined at space velocities up to about 20,000/hr. Particulate removal efficiencies were above 70 percent for most conditions. but decreased with increasing space velocities and concentrations of exhaust constituents. Carbon monoxide and NO_x conversion efficiencies were also dependent on the space velocity through the plasma afterteatment system. Oxides of nitrogen conversion efficiencies were greater than 60 percent between space velocities of 4000 and 7000/hr. For CO, the concentration after the plasma aftertreatmem system was higher than entering the reaction bed, which indicated that CO was produced by reactions in the plasma; but the increase in CO concentration was less with higher space velocities. Since concentrations of CO in diesel exhaust are typically low, this result may have little consequence. Several other experiments included evaluating the effectiveness of the plasma on exhaust while operating the test vehicles over a light-duty transient cycle, degradation of the plasma reaction bed over time, and the effect of the plasma on gasoline exhaust.

Several other observations related to the use of the plasma aftertreatment system which were not directly related to the test results were;

• This system could operate as a passive system similar to a gasoline catalyst.
• An electrical heater could prevent water condensation if that became a problem.
• The system could operate independent of the exhaust concentrations, flow rates, and engine operating conditions.
• This system avoids many of the problems with exhaust restriction which occur with current diesel particulate trap systems.
• Traps require periodic regeneration the bum off of particulate from the trap, which requires elaborate sensors and control systems to initiate and control the regeneration.
• This system is projected to be relatively inexpensive when mass produced.

In general, the results from the plasma aftertreatment system showed that a plasma was capable of reducing both particulates and NO_x simultaneously, which is not currently possible with a single technology.

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