How to calculate your NOx emissions for your CHP project planning

Discover the relevance of your NOx emissions for your business and its environmental responsibility.

Cogeneration will play a key role in your energy strategy. It will help you to improve energy efficiency, lower costs and reduce emissions. CHP emissions are most frequently addressed from the perspective of managing emissions of carbon dioxide. Emissions of other gases associated with energy use must also be considered – especially as they are increasingly subject to regulation. Emissions of nitrous oxide (NOx) is one such example.

Measuring and regulating your NOx emissions

It is widely accepted that reduced NOx emissions are one of the many benefits available from cogeneration, however it can be challenging to work out how to quantify this element of the benefits, using a recognised formula can help.

The basic formula for calculating NOx savings is:

NOx from mains electricity displaced + NOx from boiler heat displaced – CHP unit NOx output = Net NOx saving

If you have the correct data to hand, this calculation can be straightforward. The Building Research Establishment Environmental Assessment Method (BREEAM) provides a clear definition of the first part of the equation: NOx from mains electricity displaced + NOx from boiler heat displaced.

The difficulty in calculating NOx levels can arise because suppliers use different formats for presenting CHP units. According to BREEAM, NOx units must be measured in mg/kWh (heat); CHP manufacturers typically publish NOx data as exhaust gas emissions (mg/Nm³). You will need to liaise with your CHP manufacturer for help in calculating BREEAM-approved NOx emissions data. 

How does planning affect NOx emissions and measurement?

BREEAM is the principle NOx emission guideline for planning permission applications. The new design construction supplementary planning guidance (SPG) has prioritised the importance of reducing NOx emissions – particularly in Greater London. Developers and planners are now required to meet a much lower emission rating – potentially as low as 95 mg/Nm³. Note that the SPG uses different NOx measurement units (mg/Nm³) to those used in BREEAM approval.

Many manufacturers are now reassessing their product lines in order to assist with reducing NOx emissions. As the new planning guidelines are implemented, expect to see manufacturers employing more NOx abatement technologies to assist. To keep up to date with future changes, we strongly suggest building close relationships with your CHP manufacturers.

NOx emissions are just one part of planning an efficient CHP project. Find out more by reading about Creating buildings fit for the future