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Read a quick summary of how diesel exhaust fluid works and the chemistry behind it.
The U.S. Environmental Protection Agency (EPA) sets standards for the emissions of nitrogen oxides (NOx) for Medium and Heavy-Duty vehicles. The EU and Canada also have their own limits. For the regulations, reference EUR-Lex and the Canadian Justice Laws website. Manufacturers for these types of engines examined a number of technologies to meet these requirements. Selective Catalytic Reduction (SCR) was seen as a means of meeting the continually improving environmental standards.
SCR converts nitrogen oxide compounds (generically referred to as NOx) with the use of a catalyst into nitrogen (N2); which is rather harmless as it composes about 78 percent of the atmosphere.
Diesel Exhaust Fluid (DEF) which is composed of 32.5 percent urea and 67.5 percent deionized water. For those who like trivia, urea is considered to be the first organic compound to be synthesized from inorganic chemicals.
Unlike the safe and fairly inert N2, NOx causes a whole host of issues such as health problems when inhaled, it can turn into nitric acid, both create and destroy ozone and cause a number of other issues. So, less NOx in the atmosphere, particularly around the areas where we live and work, is a very good thing. DEF is used with diesel engines to achieve this effect.
Petrol/gasoline engines run cooler, so they tend not to produce as much NOx as their diesel counterparts. The result of the lower operating temperature for petrol/gasoline engines is less pollution from NOx compounds and therefore they do not need to use SCR to reduce the NOx in the exhaust.
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Let’s start at the beginning of the process and discuss how the NOx compounds are created in the first place. Diesel engines run hotter than their petrol/gasoline based counterparts. At temperatures above 1600℃, which diesel engines can exceed during combustion, Nitrogen and Oxygen react. The reactions follow the Zel’dovich mechanism:
N2 + O → NO + N
N + O2 → NO + O
N + OH → NO + H
(N / N2 = Nitrogen O / O2 = Oxygen H = Hydrogen C = Carbon)
Which can further react to create:
2 NO + O2 → 2NO2
Now that we have created NOx compounds, they need to be dealt with. This is where the DEF and catalyst come in. The DEF is added to the exhaust gas and then passed through the catalytic chamber. The catalyst itself can be composed of a variety of materials such a metals, metal oxides, perovskites and zeolites.
The urea (CO(NH2)2) from the fluid reacts with the NO by:
4NO + 2CO(NH2)2 + O2 → 4N2 + 4H2O + 2CO2
The ammonia (NH3) can then react with the NOx compounds by:
4NO + 4NH3 + O2 → 4N2 + 6H2O
6NO2 + 8NH3 → 7N2 + 12H2O
As you can see there are a lot of reactions going on to break down the NOx compounds into N2 (nitrogen), CO2 (carbon dioxide) and H2O (water). So, with a little catalytic chemistry the bulk of the NOx pollution can be reduced in diesel emissions.
If you were paying attention, you would have noticed that the ammonia can react with both NO and NO2 and does not create any CO2. However, urea releases about 0.75 grams of CO2 per gram of urea used.
So why not use ammonia in place of urea when urea that much CO2?
Well, ammonia has a few problems that make it difficult to use in vehicles. Pure ammonia boils at -33℃, so to store it at room temperature as a liquid you would need to keep it at high pressure. This type of storage has the potential to leak, particularly in the event of a road accident. It does not take a significant quantity of ammonia fumes to create an experience that is unpleasant to downright dangerous, and in liquid form it is highly corrosive.
Contrast this this a urea solution, which can be stored at room temperature and pressure, has low reactivity and it does not evaporate quickly. However, it will corrode some metals, which is why it is often stored in plastic. It is also nontoxic, with urea being used in other applications such as fertilizer, animal feed and even some cosmetic products.
With these advantages over ammonia it is understandable why it is the material of choice. As always, there is room for improvement, and we can look forward to seeing this replaced by even better technologies.
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