|A sketch of an embodiment of the aftertreatment system.|
A recent US patent award to Honda provides some insight into the approaches the automaker is taking to be able to meet both California LEV II LEV and EPA Tier 2 Bin 5 diesel emissions requirements in the US—thereby giving it a “50-state diesel.” The patent describes a diesel emissions aftertreatment system that combines a small plasma reactor with catalytic units to reduce NOx emissions.
NOx reduction to the level required by the regulations is the US is one of the thornier issues automakers must solve. DaimlerChrysler became the first to announce a 50-state solution when it announced its E320 BLUETEC and VISION GL320 BLUETEC earlier this year. (Earlier post.)
DaimlerChrysler is using two different technologies to bring NOx down to compliance levels. For the E320—due to be introduced later this year—the company is using a newly-developed NOx adsorber, a catalytic device that converts NOx to nitrogen.
For the larger GL320, DaimlerChrysler plans to use a urea-based injection system (using an aqueous urea solution called AdBlue, the genesis of the BLUETEC name). Both are combined with Selective Catalytic Reduction systems, which, while in principle are the same, differ in application design based on vehicle parameters and emissions targets. (DaimlerChrysler also needs to have the EPA buy into the notion of using the urea-based injection system.)
BLUETEC is a good example, however, of the current technology applied to NOx reduction.
In its patent filing, Honda acknowledges both approaches. The company also notes problems with each: that the adsorber can impose a fuel penalty due to the regeneration strategy, and the urea-injection approach requires the development of an infrastructure for another fluid.
In the proposed Honda system, the electrically-powered plasma reactor first converts oxides of nitrogen other than NO2 to NO2. In addition, and in conjunction with a reducing agent injected upstream of the reactor, it can also oxidize PM. Multiple reactors could be placed in series or in parallel, if needed.
The NO2 exhaust stream then flows to the catalyst units where it is adsorbed or reduced by alkali metals and silver.
Other companies and laboratories are exploring the use of plasma-catalyst combinations for NOx reduction.
Research funded by the DOE and later by ArvinMeritor led to the deployment of a plasma reformer (Plasmatron) for use with heavy-duty diesel engines. In development since the 1990s, the system reduced NOx emissions by up to 90% when used with an adsorber catalyst. It operated effectively at lower temperatures than other NOx removal systems, and it reduced the amount of fuel required for adsorber regeneration in half.
GM researchers have developed a plasma-assisted catalyst system (PAC) capable of reducing NOx under highly lean conditions using E-diesel or ethanol as the reductant. The system consists of a compact, energy-efficient hyperplasma reactor followed by a dual-bed catalytic reactor. They also demonstrated good NOx conversion (above 90% on average) over a wide temperature range of 200-400° C under steady-state optimum operating conditions.
Pacific Northwest National Laboratory also developed a two-phase approach to a plasma catalysis system, also producing reduction of NOx emissions by as much as 90%.
Caterpillar has looked at reformer-assisted lean NOx catalysis as well as plasma-facilitated catalysis.
Researchers from Ford, GM, DaimlerChrysler and PNNL collaborated on a three-phase plasma-catalyst system. (They found that with hexene as a reductant, the system reduced NOx by more than 90%; with diesel or Fischer-Tropsch reductant, however, the catalyst efficiency rapidly dropped off.)
The key to successful commercialization will be developing the right catalytic units, solving the problem of generating and maintaining the electricity required for the plasma, packaging it such as way that it works within vehicle form factors, manufacturing it cost-effectively and delivering the required low emissions.
Honda has said that it will introduce a 4-cylinder clean-diesel engine into the US market within the next three years. (Earlier post.)
US Patent # 7,043,902: Exhaust Gas Purification System
GM: Knocking out NOx