Selective Catalytic Reduction to Reduce NOx from Power Generation

Selective catalytic reduction, often abbreviated to SCR, is a technology used to reduce NOx emissions from a variety of sources, including thermal power plants, internal combustion engines, and gas turbines. It is used where the flue gas contains excess oxygen.

Nitrogen oxides are formed at high temperatures during combustion processes. Unlike other pollutants, they are not derived from the fuel being burnt, but from reactions between components of the combustion air. They have a negative effect on human health, primarily affecting the lungs and breathing passageways, but can also cause irritation to the eyes. There are a variety of nitrogen oxides, including NO, NO₂, and N₂O, but NO₂ (nitrogen dioxide) is one of the most damaging because it is highly reactive and therefore oxidises human tissues.

The aim is to break down nitrogen oxides created during combustion processes back into their constituent atoms. A chemical reaction in which oxygen atoms are stripped from another element is known as ‘reduction’. Reducing nitrogen oxides using a catalyst is a relatively simple matter, but the problem is that if there is excess oxygen in the flue gas, as is the case with lean-burn engines, the nitrogen and oxygen atoms rapidly recombine.

An important feature of selective catalytic reduction is the introduction of hydrogen into the exhaust gas. This then reacts with the oxygen atoms liberated from the nitrogen oxides to produce water, preventing them from forming again. The reaction therefore converts nitrogen oxides plus hydrogen into water plus nitrogen. The word ‘selective’ in selective catalytic reduction means that the hydrogen atoms ‘select’ the surplus oxygen atoms before the nitrogen atoms get a chance to react with them.

SCR and Selective Non-Catalytic Reduction

The reduction of nitrogen oxides can occur without a catalyst if the flue gas temperature is high enough. In this case, the process is known as ‘selective non-catalytic reduction’ or ‘SNCR’. However, the temperature may need to reach 800 °C or higher, which is not attainable in many applications, so a catalyst is used to lower the temperature at which the reaction occurs. Selective catalytic reduction involves mixing hydrogen with flue gas and then passing the mixture through a catalytic converter, where the nitrogen oxides are broken down.

Choosing a Hydrogen Source for Selective Catalytic Reduction

In most situations, hydrogen is either not available or is too dangerous to store, so other chemicals containing hydrogen may be used instead. One option is hydrocarbon fuel, but this is not very economical, except perhaps in the oil and gas industry, and it also results in additional carbon emissions.

The most popular option is ammonia (NH₃), but this also poses problems with safe handling and storage, so a compound containing ammonia is normally used instead. The two most suitable compounds are ammonium carbamate and urea, both of which can be dissolved in water. Ammonium carbamate solution is relatively easy to store and handle, although it does smell of ammonia, so spills need to be avoided, especially in confined spaces. Urea solution is the safest of all and is therefore most suitable for road vehicles. Urea solution is usually referred to as ‘AdBlue’ in Europe or Diesel Exhaust Fluid in the USA.

SCR Reductants: AdBlue, Ammonium Carbamate, and Hydrogen

Using AdBlue as the source of the required hydrogen presents several challenges. A number of reactions need to take place:

• Vapourising the AdBlue
  
• Removing the water from the AdBlue to leave urea (known as hydrolysis)
  
• Converting the urea into ammonia (known as thermolysis)
  
• Breaking down the ammonia into hydrogen and nitrogen

Each of these reactions requires a certain amount of time. Although this is a fraction of a second, there needs to be enough space in the exhaust system for the exhaust gas to remain there long enough for the reactions to occur. Each reaction also requires a specific temperature range.

catalytic converter price

Another problem with using AdBlue is that urea can convert into solids at temperatures over 400 °C, and these solids can block injector nozzles and catalytic converters.

Replacing AdBlue with ammonium carbamate solution reduces the space required in the exhaust system, increases the temperature range of operation, and also avoids the problem of blockage caused by the production of solids. It may also be less expensive for large-volume users; however, it is only appropriate where procedures are in place to prevent spillage.

Hydrogen, particularly green hydrogen produced using renewable electricity to electrolyse water, is another alternative to AdBlue. This solution can reduce NOx emissions from a modern power plant while enabling the use of a very compact SCR system that can operate over a wide temperature range and with very high reliability. The oxygen emitted from the electrolyser can even be fed into the engine to increase combustion efficiency.