The ABC of SCRs

But in tractors we're already seeing New Zealand distributors landing many high powered machines that use the simple chemistry of SCR (selective catalytic reduction) to meet emissions regulations.

While NZ has no specific legislation on emission levels, farm tractors made in Europe, the US and Japan all must conform to these countries' stricter emission levels; Tier 5 regulations are due to cut in as early as 2020, affecting all off-road vehicles with power greater than 15kW.

In diesel engines there are generally two problems in respect of emissions. Problem number one is minute soot and carbon particles caused by incomplete combustion, in big urban areas causing smog and breathing related illnesses. Problem number two is nitrogen oxides (NOX) -- 'greenhouse gases' degrading the ozone layer and putting humans at risk from higher concentrations of UV light.

Tier/Stage 1 emission rules kicked in in 1993, and now we have reached Tier/Stage 4 for off-road vehicles, which have seen particulates drop by around 95% and NOX by 75%; so we have got to know acronyms starting with D -- DOC (diesel oxidisation catalyst), DPF (diesel particulate filter) and DEF (diesel exhaust fluid).

Ever more stringent emission rules require engines that more precisely meet fuel injection parameters via improved combustion -- notably higher combustion temperatures at which particulate matter gets burned off – and lower fuel consumption. Problem number one is sorted.

But there is a flip side -- problem number two: higher combustion temperatures bring a marked increase in NOX levels – the unhelpful greenhouse gas. This is where DEF (diesel exhaust fluid) plays its part in sorting out this second emissions problem. DEF is a colourless liquid made up of 32.5% urea (the same stuff as in fertiliser) and 67.5% demineralised water; it is the catalyst for SCR.

SCR takes place downstream of the combustion process, so it has no effect on the performance of the engine, unlike the former emission control process EGR (exhaust gas recirculation), which restricted gas flow and resulted in 'dirty' engines. Because SCR equipped engines are set up more precisely and breathe more freely, manufacturers and users are getting fuel consumption down by as much as 15%.

In operation -- after combustion and as the exhaust gases enter the exhaust stack -- a stream of DEF is injected into the gas stream in minute quantities appropriate to the levels of NOX (determined by sensors). High temperatures cause the DEF to react and release ammonia molecules into the gas stream. These combine with the nitrogen oxides to cause a chain reaction that produces harmless nitrogen and water in the form of steam. Problem number two is sorted.

The likelihood at Tier 5 is that all manufacturers will need to use some form of SCR technology to meet the emission requirements – in greater or lesser amounts. So if you are likely to buy a new tractor in the next few years it would be wise to factor SCR into the purchase decision.

The simple chemistry of the process can give rise to urban myths that need to be dispelled.

The classic, "I've got urea on the farm so I'll buy some mineral water from New World and make my own blend", is not a good idea. The demineralised water used is 100 times purer than the bottled stuff – meaning all trace elements and minerals that might react with the DOC filter are removed using gel filter technology.

The other favourite, "it's mainly water so I'll just fill the tank with tap water", is also likely to result in an engine shutting down as the injection of water results in no reaction and unacceptable levels of NOX detected by the system.

Of course, there's the bloke in the white van who says he can turn the system off. He probably can, but would you let such a person loose on your $200,000 pride and joy and risk losing the manufacturer's warranty?