Exploring the four biggest NOx emission hot spots

Air pollution claims the lives of millions worldwide each year, with the World Health Organisation (WHO) estimating this number at seven million annually. Among the key contributors to this crisis are nitrogen oxides (NOX) emissions, which not only exacerbate respiratory diseases but also play a central role in the formation of harmful pollutants like ozone and particulate matter. Here, Dr. Alexander Krajete, an expert in emissions treatment at Krajete, explains what NOX emissions actually are and why they matter.

NOX is a group of highly reactive gases primarily composed of nitric oxide (NO) and nitrogen dioxide (NO₂). NO is initially emitted from combustion processes and subsequently oxidized in the atmosphere to form NO₂, a key contributor to air pollution.

Although NOX emissions originate from both natural and anthropogenic sources, human activity has significantly disrupted this balance.

The transportation sector remains the dominant emitter, with substantial contributions from aviation, cars and maritime transport. Additionally, stationary combustion sources, such as offshore oil refineries, play a significant role in NOx emissions.

Road transport NOX emissions

In 2022, roadside NO₂ levels in the UK exceeded legal limits in multiple urban areas, with 65 per cent of NOX pollution linked to road transport. Diesel and petrol vehicles remain major contributors due to high combustion temperatures and inefficient catalytic conversion.

To mitigate these emissions, Euro 6 regulations enforce NOX caps of 80 mg/km for diesel and 60 mg/km for petrol vehicles. Real Driving Emissions (RDE) testing ensures compliance by measuring NOX output under actual driving conditions.

Aviation high and low NOX emissions

Aviation contributes significantly to NOX emissions, affecting both the upper and lower atmosphere. The aviation sector’s impact was first recognised in an early investigation in 1997, when the AERONOX project examined NOX emissions from aircraft engines.

Approximately 90 per cent of aircraft NOX emissions occur above 3,000 feet, where they enhance ozone (O₃) formation in the upper troposphere and lower stratosphere, amplifying climate and air quality impacts.

At lower altitudes, NOX emissions from landing and take-off (LTO) cycles contribute to ground-level air pollution. Airports are hotspots for NO2, O3 and PM2.5, exposing local populations to health risks. Regulatory bodies have responded by enforcing stringent NOX standards, prompting advancements in low-emission combustor technology.

Maritime shipping: coastal NOX hotspots

Shipping accounts for around 15 per cent of global NOX emissions. About 70 per cent of maritime NOX emissions occur within 400 km of coastlines. Satellite data shows port cities have particularly high concentrations due to docking and loading.

Because of their transboundary nature, maritime NOX emissions contribute to inland air pollution hundreds of kilometres from coastal areas.

Regulatory frameworks such as the International Maritime Organization (IMO) Tier III standards mandate an 80 per cent reduction in NOX emissions by 2050 relative to 2019 levels.

One industry response is the adoption of selective catalytic reduction (SCR) technology. Companies like Damen have developed modular NOX reduction systems, utilising urea injection to neutralise NOX in exhaust streams, achieving reductions of up to 80 per cent.

Offshore oil refineries: stationary NOX hotspots

Offshore oil and gas operations generate NOX through gas flaring, diesel-powered drilling rigs and turbine emissions. High-temperature combustion in these processes produces significant NOX, worsening regional air quality.

To curb emissions, regulatory bodies are mandating best available control technologies, including low-NOX burners, dry low-emission gas turbines and selective catalysis reduction systems. Electrification of offshore platforms and carbon capture integration are emerging strategies for long-term NOX reduction.

While existing regulations aim to reduce NOX emissions, innovative treatment technologies can further drive reductions. For instance, Damen’s SCR technology significantly cuts maritime emissions, while Krajete employs zeolite-based NOX treatment.

Zeolites are natural materials with a unique porous structure that are effective at trapping gases. Krajete’s emissions technology captures NOX molecules through physisorption, removing 90 per cent of NOX emissions and converting them into valuable byproducts like nitric acid. These can be sold to a third party or fed back into industrial processes.

By combining regulatory frameworks with advanced NOX treatment technologies, these industries can significantly lower emissions, mitigating air pollution and improving environmental and public health outcomes.

Discover how Krajete’s advanced zeolite-based technology can help your industry reduce emissions by up to 90 per cent.