CHP NOx emissions: A guide to understanding NOx calculations and the cost/value of NOx reduction

Posted by Ian Hopkins on 23-Sep-2014 10:00:00

Discover the relevance of NOx emissions in an organisation's prospective CHP project, as well as issues surrounding environmental responsibilities.

CHP NOx emissions   A guide to understanding NOx calculations and the cost value of NOx reduction   article image

Cogeneration can have a key role to play in an energy strategy that focuses on the increasingly important areas of improved energy efficiency, lower cost and reduced emissions. The CHP emissions angle is most frequently addressed from the perspective of managing emissions of carbon dioxide, but emissions of other unwelcome gases associated with energy use are also on the CHP radar. They are also increasingly subject to regulation. Emissions of nitrous oxide (NOx) fall into this category.


How are an organisation's emissions of NOx measured and regulated?

Whilst it's clear and widely accepted that reduced NOx emissions are one of the range of benefits available from implementation of a cogeneration project, it can be challenging to work out how to quantify this element of the benefits.

How to calculate NOx savings:

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

This appears (and is) a relatively straightforward calculation. The 'NOx from mains electricity displaced + NOx from boiler heat displaced” are clearly defined within BREEAM. However, the NOx emissions level of CHP units may be presented in different formats by different CHP suppliers. The NOx units required for the BREEAM calculation are mg/kWh (Heat), yet CHP manufacturers have conventionally presented CHP NOx output in mg/Nm3 of exhaust gas and often without a common oxygen reference. Manufacturers should be able to provide you with support in calculating NOx emissions in the format required by BREEAM. Further guidance on the calculation of CHP NOx emissions can be found here.


How does planning affect NOx emissions and measurement?

Currently as part of planning permission BREEAM is considered to be the principal NOx emission guideline. However, with the new design construction supplementary planning guidance (SPG) the importance of achieving low NOx has become a heightened priority, particularly in the Greater London area. The new guidelines mean that developers/planners are now required to meet a much lower emission rating, which can be as low as 95 mg/Nm3. Here we see again a new piece of planning guidance which defines NOx measurement in units that are different to BREEAM’s requirement for mg/kWh (Heat)!

In response to the requirement for lower NOx emission levels, many manufacturers are now analysing their product offering and looking at various NOx abatement technologies that can be implemented to assist with achieving these ratings. As the new planning guidelines are implemented there are likely to be further updates and changes, so a close relationship with manufacturers is recommended to keep up to date with further information and guidance on how this can impact upon CHP as it becomes available.


NOx emissions are just one part of planning an efficient CHP project. Find out more in your free eGuide: CHP project planning: Assessing planning and regulatory issues


Topics: Energy Management, CHP / Cogeneration, Consultants, Guides

Ian Hopkins

Ian Hopkins is a technical sales professional and business leader with more than 15 years’ experience in delivering energy efficiency projects and strategy in Europe and the United States. Ian currently heads up the Sales and Marketing function as one of the board directors at ENER-G Combined Power Ltd.