Selective catalytic reduction (SCR) is the name given to the post-combustion technology treatment introduced for use within Tier 4 final engines to ensure the reduction of harmful emissions, such as nitrogen oxide (NOx).

This technology requires the use of an additive called diesel exhaust fluid (DEF), a product known commonly as AdBlue. Almost all of the main engine manufacturers use this technology to meet the requirements of the legislation to reduce the levels of NOx emissions.

Components

The main components of an SCR system are the:

  • SCR catalyst.
  • DEF fluid injection unit.
  • DEF tank.
  • DEF injection control unit.

    • Where did SCR come from?

    Up until Stage IV, otherwise known in the US as Tier 4 final, engines had to meet stage 3B or Tier 4 interim standards, which did not have the same degree of regulation as that of stage 4. Stage 3B could be made compliant by means of exhaust gas recirculation (EGR) and the use of what is called a diesel particle filter (DPF).

    EGR takes a portion of the engine’s exhaust gas and re-circulates it with fresh air intake. The exhaust air reduces the level of oxygen in the combustion chamber. As result, when the oxygen-reduced air ignites on the compression stroke, the exhaust contains less NOx. This lowers the combustion temperature and reduces the nitrous oxide formation. This system works in combination with a DPF to clean up the particulate matter.

    DPF works by use of a mechanical filter to trap soot particles after they have been partially oxidised by a catalyst. At certain intervals during operation, the trapped particles are incinerated during a process called “regeneration”, whereby the exhaust system is heated to a significantly higher temperature, burning off built-up soot. This process happens automatically while the tractor is working and you will notice it as the engine seems to rev itself automatically.

    The only issue with DPF systems is that they create a back pressure on the engine, making them work harder and burning more diesel as result.

    This was all well and good until 2014 when legislation was changed and engines then had to be Stage IV-compliant. This saw a further drop in NOx limits, from 3.30g/kWh to 0.40g/kWh within engines ranging from 75hp up to 175hp.

    While particle matter (PM) remained the same for both stages at 0.025g/kWh, PM can be best described as the “black smoke” which is not seen on the newer tractors of today due to the compliance of emission regulations.

    What is AdBlue?

    AdBlue, a name commonly given to the colourless liquid that is diesel exhaust fluid (DEF) and that name has managed to stick over time. It is a non-hazardous, high-purity, odourless solution containing 32.5% urea, with deionised water accounting for the remaining 67.5%.

    Urea is made of synthetic ammonia and carbon dioxide. It is used as an artificial fertiliser and it is also found in cosmetics, among many other products.

    AdBlue is stored in a separate tank on the tractor, usually located beside the diesel tank.

    Typically, AdBlue is refilled once every three diesel fills on average for most tractors, but this is dependent on machine application and workload.

    SCR systems with which DEF is used are very sensitive to potential chemical impurities that can contaminate the urea solution. So it is important to purchase only a product that bears the name AdBlue and is manufactured according to the ISO 22241- 1 to 4 standard. Even small amounts of impurities can dramatically damage your SCR catalyst performance. Using a quality product will avoid the high costs of having to replace a damaged catalyst. DEF has a shelf life of between six and 12 months.

    The engine process

    The process is simple – DEF is used in Stage IV diesel engines with selective catalytic reduction (SCR) systems. Since SCR is an after-treatment technology, DEF is sprayed into the hot exhaust gases. This mixture passes through a catalytic converter. When the hot exhaust gases combine with the DEF within the catalytic converter, the exhaust gases are broken down into harmless nitrogen and water vapour – two natural components of the air we breathe.

    With SCR, the NOx reduction process takes place within the exhaust line so as to not diminish engine performance.

    Have all these regulations made a difference?

    Over the years, the changes in regulation and the advancement in engine technology have made a huge difference in terms of efficiency and emission levels. According to the European Agricultural Machinery Manufacturers Association (CEMA), “a modern agricultural tractor emits about 95% less NOx and particulate matter (PM) than a comparable machine did 20 years ago”. In other words, taken together, 25 newly built tractors in 2016 emit less NOx and PM than one single tractor built in 1997. The goal for the future is to have air of breathable quality from the exhaust pipe.

    The graphs illustrate a significant reduction in engine emission limits for tractors in the 100hp to 175hp bracket from the introduction of Stage I, up to the recently announced Stage V.

    The emission levels have reduced drastically since the introduction of Stage I regulations in 1999.

    Stage V emission standards

    The next stage of EU emission standards comes into force in 2019 and 2020 for non-road diesel engines across the power spectrum. Machines for construction and agriculture are two sectors to be included in these standards.

    Stage V calls for a further reduction in particulate matter and NOx emissions. The main difference with Stage V is that for the first time there will be a limit on the number of particulates.

    Particulate matter levels have reduced from 0.025g/kWh seen in Stage IV engines to 0.015g/kWh.

    Stage V diesel engine technology provides the performance and machine capability required for near-zero emissions.

    There continues to be a very strong case for the use of diesel to meet the varying power demands of construction, as well as agriculture. European emissions legislation for off-highway applications has driven a 96% NOx and 97% PM reduction since it was first introduced in 1999.

    The latest phase, Stage V, warrants for the first time, ultra-clean diesel technology and introduces a particulate matter number count which is only achievable by the use of a DPF.

    Tips for storing AdBlue

  • As DEF is a water-based solution, it will freeze. Take precautions when using DEF in extreme low temperature conditions. The process of freezing DEF has no effect on its quality or performance.
  • DEF is mildly corrosive to copper and many other materials, so storage tanks should be made from a suitable material. Materials such as plastic or a suitable grade of stainless steel is advised.
  • Leave room for expansion in the DEF tank. It is reported that DEF will expand upon freezing by at least 7%. When storing equipment overnight or longer in temperatures that could result in DEF freezing, it is important to make sure that the DEF tank on the equipment is not completely full to allow for this expansion. This will also help to prevent cracking of the storage container. In winter, once the tractor is started, DEF will thaw quickly and return to its usable state.
  • The injectors that meter DEF into the exhaust stream have strict tolerances, just like tractor engine fuel injectors. Many tractor SCR systems have a cartridge filter inside the pumping unit to protect these injectors and they have a recommended 1,200-hour service interval. Change the filter on the pumping unit at the time recommended.
  • The storage life of DEF will vary. As it is made from urea, it will degrade in sunlight and higher temperatures. In good storage conditions, where there is a proper container, away from direct sunlight, in an even temperature, DEF can maintain its qualities for about 12 months.
  • You need to handle DEF with the same care as diesel. Tractors use DEF sensors to continually analyse the quality of the DEF being consumed. These sensors will trigger a fault code whenever an imbalance, such as dirt or water, is detected. So only transfer DEF with dedicated equipment, keeping it free of any contaminants and moisture and store it in a proper container kept in a cool, shaded place.

    • AdBlue prices

    Unlike diesel, AdBlue is sold in a range of container sizes from small five-litre containers to 1,000-litre tanks as well as bulk loads. It is sold like this due to the fact that such small quantities are used compared with diesel (roughly, one fill of AdBlue to three fills of diesel).

    Tanks on machines hold only a small number of litres which also play a part in the way that it is sold. Most reasonable-sized tractors hold on average 15 to 50 litres, depending on brand, etc. The larger the quantity purchased, the cheaper it will be.

    At current prices, it costs €468 including VAT to buy 1,000 litres.