NOx emissions – formation, reduction and abatement
The two most significant pollutants produced by humans (anthropogenic) are NOx emissions and particulates.
Q. What does the term NOx mean?
A. It refers to nitrogen oxides. The purists would say that it refers to nitric oxide (NO) and nitrogen dioxide (NO2) only, but most also include nitrous oxide (N2O) in this description. There are some other variants, but their concentrations in the atmosphere are too low.
Q. Why are NOx gases produced?
A. There are three main causes of NOx emissions:-
- High temperature combustion of fuels where the temperature is hot enough (above about 1300°C/ 2370°F) to oxidise some of the nitrogen in air to NOx gases. This includes burning hydrogen, as it burns at a very high temperature. Comments on diesel engines are shown below.
- Burning plant material releases nitrogen oxides, as all plants contain nitrogen.
- Chemical and industrial processes which use nitric acid, nitrates or nitrites will release NOx gases.
Q. What is the difference in combustion between a diesel engine and a petrol/gasoline engine?
A.
In a petrol/gasoline engine, a mix of fuel and air is injected into the chamber. This is compressed and then ignited by a spark plug.
In a diesel engine air is injected into the cylinder, and is compressed by around twice as much as in a petrol/gasoline engine. This compression generates heat, so that diesel fuel burns spontaneously when it is injected.
Q. Why do diesel engines produce more NOx than petrol engines?
A. Diesel engines operate at a higher temperature and pressure than petrol engines. These conditions favour the production of NOx gases. The quantity depends on the volume and duration of the hottest part of the flame.
Q. Why are diesel cars more fuel efficient than petrol/gasoline cars?
A. Diesel fuel produces more energy for a given volume (diesel has a lower calorific value, but a higher density than petrol/gasoline). Also the higher combustion temperature in a diesel engine makes it more efficient. Heat engines can generate more useful work if they operate at higher temperatures.
Q. How do you reduce NOx emissions from diesel engines?
A. By lowering the combustion temperature, typically by Exhaust Gas Recirculating (EGR). Some exhaust gas is cooled and injected back into the combustion chamber. There is less oxygen in the exhaust gas because some has been consumed by previous combustion, so there is not as much to feed the flame. The exhaust gas also has a higher heat capacity than air, so it takes longer to heat up.
Q. Are there any other consequences of using EGR?
A. Yes, there is a downside. As the combustion temperature drops, so does the power, and the fuel economy.
Q. How can you remove NOx from exhaust gases?
A. There are various techniques, depending on the applications, although a lot of effort goes into designing burners which reduce NOx emissions in the first place.
- Selective Catalytic Reduction (SCR) is the most common method in diesel vehicle exhausts, but it is expensive so isn’t use in small cheap vehicles. There are various proprietary blends of ammonia and urea which can be injected into the exhaust flow. These react with NOx gases over a catalyst, which turns them into harmless nitrogen and water.
- Selective Non-Catalytic Reduction (SNCR) – takes place in ducting where the temperature is about 1000°C (1800°F). Urea or ammonia is injected, and the NOx gases are reduced to nitrogen without the need for a catalyst.
- On an industrial scale, exhaust gases can be scrubbed with chemicals such as sodium hydroxide, hydrogen peroxide, or a mixture of hydrogen peroxide and nitric acid. These chemicals react with the NOx gases and removes them.
Q. Why are NOx gases harmful?
A. Internal combustion engines can produce all three nitrogen oxides.
Nitrous oxide (N2O), also known as 'laughing gas'.
- It is a serious greenhouse gas, and is defined as being 298 times as bad as CO2 because of its radiative effect, and the time taken to break it down.
- Used as an anaesthetic and generally considered to be non-toxic. It does react with vitamin B12, which may be a problem for those who are deficient.
- It is broken down in the stratosphere, and catalyses the breakdown of ozone. Ozone in the upper atmosphere is vital for absorbing UV rays; at the earth’s surface, it is harmful.
Nitric oxide (NO).
- Readily oxidised in the atmosphere to nitrogen dioxide.
- Non-toxic in small quantities, infact it serves a vital role as a regulator within the human body.
Nitrogen dioxide (NO2).
- A major pollutant and component of smog. Its brown fumes may be familiar from school chemistry experiments.
- It reacts with water to produce nitric acid, which is why it is so irritating to the eyes and respiratory tract.
Q. What are SOx emissions?
A. When fuel is burnt in an engine, any sulphur will be converted into sulphur dioxide (SO2) gas. This readily dissolves in water to produce an acid, which accounts for the irritation to your respiratory tract if you inhale it. It also affects the ecology. Oil and gas in the ground can contain large quantities of sulphur, which have to be removed in the refinery. Some countries have lax regulations on sulphur content in fuel, with resulting high pollution levels.
Q. What is smog?
A. Fog or haze intensified by smoke or other pollutants.
- Traditional smog was caused by burning coal, particularly high-sulphur coal.
- Photochemical smog. Nitrogen dioxide, and a fellow pollutant volatile organic compounds (VOCs), combine in the presence of sunlight to produce ozone and a variety of other compounds. These have a nasty affect on the respiratory system.
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