Guidelines to Reduce NOx Emissions

Nitrogen oxides (NO_x) are formed when nitrogen (N₂) and oxygen (O₂) are combined at high temperatures and pressure during the combustion of fuel. All fuels, such as gasoline, diesel, biodiesel, propane, coal, and ethanol, emit NO_x when burned. The EPA estimates that 49% of NO_x emissions come from on-road and off-road vehicles, 27% from power generation (electric utilities) and the remaining 24% from industrial, commercial and residential sources. Due to the many compounds that are a part of NO_x (predominantly nitrogen dioxide and nitric oxide), the pollutant contributes to a wide variety of health and environmental problems. NO_x is also a main component of ground-level ozone and contributes to global warming. Since the passage of the Clean Air Act in 1970, all primary air pollutants have decreased - except NO_x, which has increased by 10%. Due to its serious health and environmental impact, the reduction of NO_x in our atmosphere has now become a major focus in the fight against air pollution.

Exposure to diesel PM may result in both cancer and non-cancer health effects. Non-cancer health effects from one or more of these compounds may include irritation to the eyes and lungs, allergic reactions in the lungs, asthma exacerbation, blood toxicity, immune system dysfunction, and developmental disorders.

In 2004 the EPA introduced stringent air emission standards for on-road vehicles. Any pre-existing vehicle is not required to comply with these newer standards. Diesel vehicles from older model years will have higher non-methane hydrocarbon and particulate matter emissions.

Typically, diesel retrofit involves the addition of an emission control device to remove emissions from the engine exhaust. Retrofits can be very effective at reducing emissions, eliminating up to 90 percent of pollutants in some cases. Some examples of emission control devices used for diesel retrofit include diesel oxidation catalysts, diesel particulate filters, NO_x catalysts, selective catalytic reduction, and exhaust gas recirculation. Devices to control crankcase emissions also exist.

Significant improvement in diesel emission levels, in both light- and heavy-duty engines, was achieved in the 1970 - 2000 period. PM, NO_x, and HC emissions were cut by one order of magnitude. Most of that progress was achieved by emission-conscious engine design, such as through changes in the combustion chamber design, improved fuel systems, implementation of low temperature charge air cooling, and special attention to lube oil consumption.

However, more progress was still required, as the NO_x and PM emissions from diesels remained higher than those from Spark Ignited (SI) engines. A new series of diesel emission regulations was developed with implementation dates around 2005-2010, which require the introduction of exhaust gas aftertreatment technologies in diesel engines, as well as fuel quality changes and additional engine improvements.

Available Diesel Retrofit Technologies

The array of emission control methods provides the designer with building blocks which need to be chosen and combined into the emission control system, which in turn is integrated with the engine to achieve a given emission target. A system approach is necessary to develop the clean emission diesel engine. There is no miraculous “plug-in” device available which could be installed on a particular engine and effectively clean emissions. An effective emission control strategy has to combine elements of engine design with the use of appropriate fuels and exhaust aftertreatment methods.

Selective catalytic reduction (SCR) of NO_x by nitrogen compounds, such as ammonia or urea—commonly referred to as simply “SCR”—has been developed for and well proven in large-scale industrial stationary applications. The SCR technology was first applied in thermal power plants in Japan in the late 1970s, followed by widespread application in Europe since the mid-1980s. In the USA, SCR systems were introduced for gas turbines in the 1990s, with increasing potential for NO_x control from coal-fired powerplants. In addition to coal-fired cogeneration plants and gas turbines, SCR applications also include plant and refinery heaters and boilers in the chemical processing industry, furnaces, coke ovens, as well as municipal waste plants and incinerators. The list of fuels used in these applications includes industrial gases, natural gas, crude oil, light or heavy oil, and pulverized coal.[1]

SCR is the only proven catalyst technology capable of reducing diesel NO_x emissions to levels required by a number of future emission standards. Urea-SCR has been selected by a number of manufacturers as the technology of choice for meeting the Euro V (2008) and the JP 2005 NO_x limits—both equal to 2 g/kWh—for heavy-duty truck and bus engines. First commercial diesel truck applications were launched in 2004 by Nissan Diesel in Japan and by DaimlerChrysler in Europe.

SCR systems are also being developed in the USA in the context of the 2010 NO_x limit of 0.2 g/bhp-hr for heavy-duty engines, as well as the Tier 2 NO_x standards for light-duty vehicles.

The technologies and strategies being developed for the 2007/2010 heavy-duty highway diesel engine and Tier 4 nonroad diesel engine standards may be applicable stationary diesel engines provided adequate lead-time is given. The issue is to match the right technologies to the right applications. Reduction of emissions is influenced by the duty cycle of the engine.