Combined Heat & Power (CHP) is a favourite with design engineers when it comes to specifying a low carbon energy source on a new build project to help comply with building regulations. However, with ever increasing legislation and environmental demands, the requirement for CHP engines with low NOx emissions has never been greater. Therefore, gaining an understanding of what makes a low NOx emission engine is important.

NOx is the chemical abbreviation of nitric oxide and nitrogen dioxide which are poisonous gasses produced by internal combustion engines burning a fossil fuel such as natural gas.

The only way to filter out NOx from exhaust gasses is by means of a catalytic converter, or a catalyst as they’re more commonly known.

micro CHP

In order for a catalyst to work effectively, the exhaust gasses passing through it from the engine must be as a result of perfect (or near perfect) combustion. Perfect combustion is called stoichiometric combustion and this is when the exact required amount of air is mixed with the fuel during the combustion process. Often a combination of a stoichiometric engine and catalyst can result in NOx emissions as low as 20mg/Nm3

Not all engines are able to be configured with stoichiometric combustion and the reason for this is that they burn very hot and therefore can’t be placed under too heavy a load, otherwise they will overheat and cause damage to the engine components. In terms of CHP, this means we can only use an engine with stoichiometric combustion on the smaller range of sizes.

For larger CHP systems we need to use an engine with a lean burn combustion process, as this runs a lot cooler and can therefore, can be put under much heavier loads.

The challenge with lean burn combustion is that the air-to-fuel ratio is much higher, resulting in non-stoichiometric combustion. This means a conventional catalyst, as described earlier, will not be effective in removing the poisonous NOx gasses. Instead we use a process called selective catalytic reduction or SCR.

SCR uses urea as a reductant and it is injected into the exhaust pipeline where the aqueous urea vaporises to form ammonia. Within the SCR catalyst the NOx gasses are catalytically reduced by the ammonia into water and nitrogen both of which are harmless. Put simply the injection of urea treats the exhaust gasses to make them effective when passed through a catalyst.

For advice on the best low NOx CHP system for your project, call us on 0844 888 444 5 and ask to speak to one of our CHP technical advisers who will be glad to help.