As we decarbonise, does NOx become the next focus for hydrogen combustion research?

How do we decide whether electrification or hydrogen is the more suitable option for reducing emissions in various industrial applications and market sectors? NOx could hold the answer.

SHARE:

With the move away from fossil fuels towards a low carbon energy future, we primarily consider either electrification or hydrogen. In many instances, electrification can be adopted with no emissions at the point of use, although there may be some from the electricity generation process. For hydrogen there may be emissions associated with the production process and then also with the end use. So, it’s possible for emissions to switch from end use to production, although, overall the aim is to reduce them. So how do we decide which are suitable options?  For a number of industrial applications and in some market sectors, electrification may not be feasible and the development of different hydrogen technology options is important to maintain choice, viability and suitability.

We are not starting afresh here, hydrogen has been a viable fuel for many industries for a number of years and the absence of carbon dioxide emissions at the point of use is particularly beneficial. However, it is recognised that hydrogen combustion does raise potential challenges with increased nitrogen oxides (NOx)[1] emissions compared to natural gas. Although NO is the major component in exhausts, it readily converts to NO₂ in the atmosphere. The UK government has long recognised the impact of NOx[2]  as a pollutant and has set specific emission limits. Legislation and emission limits from the Industrial Emissions Directive (IED) and Large Combustion Plant Directive (LCPD) are also in place and there are also ones for some domestic appliances to ensure they comply with certification criteria[3]. NOx emissions have an impact on indoor air quality, local outdoor (and regional) air quality and wider environmental atmospheric changes. NO₂ can affect respiratory systems and be especially of concern for people with lung conditions. NOx can also form low level ozone which can adversely impact overall air quality.

These known health impacts, and the fact that in many cases hydrogen combustion increases NOx emissions, could be considered a significant challenge to the wider development of hydrogen use. However, combustion scientists and engineers are aware of these and are working on low-NOx developments that could include both burner and combustion system modifications or exhaust gas clean-up. Research is well underway and early indications are that new systems will be able to achieve low emissions, meeting existing NOx limits for natural gas and going further to lower NOx emission overall.

Professor Lewis from the University of York has found[4] that “Minimising NOx as a by-product from hydrogen boilers and engines is possible through control of combustion conditions, but this can lead to reduced power output and performance. After-treatment and removal of NOx is possible, but this increases cost and complexity in appliances. Combustion applications therefore require optimisation and potentially lower hydrogen-specific emissions standards if the greatest air quality benefits are to derive from a growth in hydrogen use.”

So the combustion community is working on technology solutions to lower emissions and that NOx from hydrogen combustion needs to be treated on a ’level playing field’ with regard to emissions measurements. The Electric Power Research Institute (EPRI)[5] has highlighted that NOx measurements on a volumetric basis, measured “dry” and at a specified oxygen reference content can result in “high” NOx emission values compared to natural gas whereas it might be more appropriate to report NOx measurements on a mass basis. So rather than ppm(v) we could be best served by using g/MWh. This will mean that we are comparing ’like-for-like‘ and it ensures that hydrogen is not overlooked as a potential future energy option.

There’s lots to highlight here (lean burn systems, flue gas recirculation, steam injection, new burner designs) and the developments are leading to better, lower emission systems but the overall message is clear, NOx emissions are not a new phenomenon, and we’ve got a track record for development of systems to limit NOx formation or remove it from flue gases or exhaust systems.

We know that there are technology challenges to overcome but to come right back to question posed in the title – “Yes” NOx emissions could change if we move to hydrogen as a low-carbon fuel option, but scientists and engineers are developing better systems that will enable it to be used in a wide range of end-use applications. The clear steps for the energy community are to accelerate research in NOx emission reduction, focusing on what we can change or control, including combustion conditions, or using current (or new) technologies to encourage post combustion removal in-flue. The challenge is clear and, whatever the approach, rest-assured, we can and will control NOx emissions in future. 

Authored/Written by Martin Brown, Principal Specialist  

2/4/2025 4:59:00 PM