Chemistry

How Urea Reduces Diesel Emissions



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For many years, diesel has been overshadowed by petroleum as the dominant vehicle fuel. Despite having a higher energy density than gasoline (engines that run on diesel are nearly 30% more efficient than gasoline based engines), the pollutants released by diesel, which include several known carcinogens, have turned public interests away from utilizing diesel as the main form of fuel. However, this is beginning to change. Many car companies are now utilizing one of the most unlikely compounds, urea (one of the main components of organic waste), to reduce the emission of nitric oxides from diesel vehicles.

Nitric oxides are one of the most deadly pollutants produced by diesel vehicles. Inhaling nitric oxides in high concentrations is known to cause respiratory disorders, including asthma attacks, and nitrate particles have also been linked to heart attacks. Humans are far from the only ones to be affected by nitric oxides; nitrate particles can react with water in the atmosphere to produce nitric acid, resulting in acid rain which destroys wildlife and pollutes drinking water. Because of its wide ranging environmental and health effects, the U.S. Environmental Protection Agency (EPA) has imposed strict regulations on the acceptable output of nitric oxides.

Urea reduces the output of nitric oxides from the exhaust by selective catalytic reduction (SCR), a process which breaks nitric oxides down into harmless nitrogen and water through a redox reaction with ammonia. The exhaust system of diesel vehicles is injected with a mixture of urea and de-ionized water, known as diesel urea. The aqueous urea evaporates, forming ammonia.  In the presence of a catalyst, ammonia donates hydrogen atoms to the nitric oxides, acting as a reductant in the reaction which results in the production of nitrogen gas and water. The SCR process is believed to remove anywhere from 70% to 95% of nitric oxides from exhaust.

Although there are several other ways to reduce nitric outputs of diesel vehicles, the SCR method using urea has proven to be the most efficient. Other popular methods of reducing nitric oxide emission have been found to be far less effective and more costly than SCR. For example, nitric oxide absorbers utilize precious metals such as platinum, and are thus impractical because of the cost. On the other hand, urea is significantly cheaper, with costs at about $4 per gallon. While this may still be quiet pricey for many users of diesel vehicles, the added efficiency to vehicles may help make up for the extra costs, and urea enabled SCR is currently one of the few viable ways for diesel vehicles to comply with the stringent limits on nitric oxides emission imposed by the EPA.

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