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NOx removal in industrial flue gases

Improving our Environment

Air pollution remains a significant global challenge, impacting public health, and ecosystems, and contributing to climate change. Among the various air pollutants, nitrogen oxides (NOx) are particularly notorious, arising from industrial, waste incineration, and energy production processes.


This article delves into the innovative application of ozone for controlling NOx emissions, examining the technique's effectiveness and economic advantages for businesses, especially those directly involved in high-NOx emitting processes.


Understanding NOx and Its Environmental Impact

Nitrogen oxides, primarily nitrogen dioxide (NO2) and nitric oxide (NO), are byproducts of high-temperature combustion processes. These gases are mainly emitted from power plants, industrial furnaces, and waste incineration facilities, particularly those that burn fossil fuels or waste materials.


NOx pollutants are key contributors to the formation of ground-level ozone and particulate matter, both harmful to human health. Moreover, NOx emissions are involved in the development of acid rain, which adversely affects water bodies, forests, and agricultural lands while also corroding urban infrastructure.

Air pollution is a serious problem, and we should all be concerned. The World Health Organization (WHO) estimates 4.2 million deaths every year as a result of exposure to ambient (outdoor) air pollution, and 3.8 million deaths every year as a result of household exposure to smoke from dirty cookstoves and fuels. Worldwide, more deaths per year are linked to air pollution than to automobile accidents.

NO2 is both a primary and secondary pollutant.

Air pollution impacts everyone but some of us are more susceptible than others: children and seniors, people with existing lung and heart conditions, people working and exercising outdoors.

What are NOx emissions?

Emissions NOx refers to both nitric oxide (NO) and nitrogen dioxide (NO2). The environmental effects of releasing too much NOx into the atmosphere is listed below.

How can NOx emission be controlled?

NOx emissions can be controlled by limiting the amount of available O2, which can combine with nitrogen (N2) to form nitric oxide (NO). Because the mixing of air and fuel can never be perfect, some excess air is always required to ensure complete combustion. By limiting this excess amount of air, less NOx is likely to form.

How do you get rid of NOx emissions in flue gas?

There are several methods for controlling NOx emissions. Gas scrubbing is one of the most common forms of NOx treatment, with sodium hydroxide being the conventional scrubbing medium. However, the absorbed NOx is converted to nitrite and nitrate which may present wastewater disposal problems.

How do you remove NOx emission from the environment?

Processes for Nitrogen Oxide Removal. There are several methods for controlling NOx emissions. Gas scrubbing is one of the most common forms of NOx treatment, with sodium hydroxide being the conventional scrubbing medium. However, the absorbed NOx is converted to nitrite and nitrate which may present wastewater disposal problems.

The Impact of High NOx Emissions

Businesses that operate within sectors such as manufacturing, power generation, and waste incineration often face significant challenges due to high NOx emissions:


Strict limits on NOx emissions are enforced worldwide, with non-compliance resulting in heavy fines and


High NOx emissions often indicate inefficiencies in combustion processes, leading to increased fuel consumption and higher operational costs.

Public Image
and Marketability

In an era of heightened environmental awareness, companies with high emissions face consumer and shareholder scrutiny, which can affect market position and profitability.

Ozone-Based Control of NOx Emissions
Mechanisms and Technologies for Effective Implementation



The application of ozone in controlling NOx emissions is a cutting-edge approach, leveraging ozone’s potent oxidising properties to neutralize harmful nitrogen oxides. This process involves generating ozone and then facilitating a chemical reaction between the ozone and nitrogen oxides present in the emissions. Ozone (O3) is produced by splitting oxygen molecules, which then recombine to form ozone.


This ozone is introduced into the emission streams, where it reacts with nitrogen oxide (NO) to produce nitrogen dioxide (NO2) and oxygen (O2). The reaction continues as nitrogen dioxide further interacts with ozone, resulting in the formation of nitrogen (N2) and additional oxygen (O2), thereby effectively decreasing the overall concentration of NOx in the emissions. This sequential reaction not only reduces harmful pollutants but also converts them into harmless gases, enhancing air quality and contributing to environmental protection.

The implementation of ozone-based NOx control technologies involves the use of industrial ozone generators and direct injection systems, complemented by advanced control and monitoring systems. Industrial ozone generators are designed to produce large volumes of ozone, making them essential for treating emissions from extensive industrial processes. The ozone is then introduced directly into the exhaust or emission streams through direct injection systems, ensuring thorough mixing and effective reaction with NOx.


This process is typically automated, allowing for seamless integration into existing emission control frameworks. Additionally, automated control systems play a crucial role by monitoring NOx levels in real-time and adjusting the ozone injection rate to maintain optimal reduction efficiency. These systems are supported by advanced sensors and feedback loops, which provide continuous data on NOx and ozone concentrations, ensuring that the treatment remains balanced and effective throughout the operation.

Typical point of Ozone injection


Theory NOx removal using Ozone by Oxidation

Ozone technology is an end of pipe system that removes NOx by adding ozone to oxidise insoluble NO, NO2 to N2O which is a highly soluble species of NOx.

The NOx removal theory, unlike most techniques for NOx removal by ozone dose not occur by reduction but by oxidising the NOx compounds. The NO and NO2 are oxidised by O3 to N2O5 in the following subsequent steps:

NO + O3 --------- NO2 + O2

NO2 (Nitrogen Dioxide) + O3 (Ozone)------- NO3 (Nitric Acid) + O2 (Oxygen)

NO2 + NO3 (Nitrogen Dioxide + Nitric Acid) ----- N2O5 (Nitrogen Pentoxide)

The oxidisation from NO2 to NO3 is the slowest reaction in the chain, N2O5 which is the anhydride of nitric acid, can easily be removed from the flue gas by contact with water in a scrubber unit according to the following reaction to form Nitic acid HNO3

B2O5(g) + H2O ------ 2HNO3(ag)

oxidisation process.png
  • NOx emissions can be reduced by more than 95% through the addition of ozone to the flue gas.

  • The reduction of NOx emission is more effective at lower temperatures(110oC) than higher temperatures (160oC)

  • The technique is applicable on flue gases from biomass combustion despite the presence of compounds such as SO2 and HCI

  • Reduction in NOx emissions requires approximately half as much O3 when it is applied to the natural gas fired engines compared to straw fired boilers and fossil fuel boilers

  • The higher the O3 consumption is due to both the higher flue gas temperatures and the larger NOx fluctuations.

  • Ozone is highly selective for NOx relative to other combustion products.

  • The resulting N2O5 is readily adsorbed by aqueous scrubbing solutions or absorbed by dry/semi dry scrubber adsorbents.

Typical Plant Layout


Economic Benefits of Ozone-Based NOx Control for Businesses

  • Implementing ozone technology not only assists in achieving regulatory compliance but also delivers several economic benefits:

  • Reduction in Regulatory Fines: Adhering to emission standards helps avoid costly penalties.

  • Operational Efficiency: Enhanced combustion efficiency reduces fuel costs.

  • Enhanced Corporate Image: Embracing green technology improves public perception and investor confidence.

  • Market Opportunities: Sustainability leaders often gain access to new markets and customer segments.

  • Long-term Cost Savings: The initial costs are generally offset by mid-term and long-term savings from operational efficiencies and fewer penalties.

Overall Benefits of ozone Applications

  • Lower achievable level of NOx emissions

  • Easily adjusted for variable loads

  • Low temperature operation

  • Fully automated system

  • No secondary gaseous pollutants generated.

  • Consistent reliable performance regardless of fuel type

  • No effect of particle matter on NOx removal

  • Simultaneous oxidation of Hg and other contaminants

  • Easily integrated with existing wet scrubbers

Piloting 1,5...2 – Optimum O3 / NO ratio

Skyline panorama

What’s the Problem with NOx?

Nitrogen dioxide is an irritant gas, causing respiratory conditions and allergies from inflammation of the airways and lungs when found in high concentrations. In heavily populated urban areas with extreme emission concentrations, NOx poses a significant health concern.

The Chemical Composition of CO2 and NOx

As fleet managers and heavy equipment operators, it’s important to understand the difference between the most common emissions and why they are so heavily regulated. If you are a business that operates heavy machinery or a fleet of trucks, you are most likely familiar with CO2 and NOx emissions.


In the modern world where it seems as though everything is increasingly regulated, CO2 and NOx are perhaps the most highly regulated emissions of all due to their association with greenhouse gases, global warming, and climate change.


In general, emissions such as CO2 and NOx are described as gases and particulate matter that are emitted into the air and atmosphere by a wide spectrum of sources, such as vehicles, trucks, refineries, and heavy equipment.

Carbon Dioxide – CO2

The Earth’s atmosphere is made up of a diverse mix of gases including 78% nitrogen (N), 21% oxygen (O) and roughly 1% additional gases that include about 0.04% carbon dioxide (CO2). Carbon dioxide is a highly common, naturally occurring gas that is colourless, odourless and consists of one carbon atom bonded to two oxygen atoms. CO2 plays an important role in the regulation of the Earth’s surface temperature through what is known as the Greenhouse Effect – the process whereby radiation from our planet’s atmosphere warms the surface of the planet to a temperature that exceeds what it would be without the presence of its atmosphere.


Carbon dioxide is also a key player in the carbon cycle, a biochemical cycle where the element of carbon is exchanged between biological systems – from ocean to land, land to sky and atmosphere, and then sky and atmosphere back to the ocean. We exhale CO2, contributing to its presence in our atmosphere when we breathe, and plants absorb this exhaled CO2 through the process of photosynthesis to grow. When there is too much carbon dioxide in the atmosphere as a greenhouse gas it can pose problems for the liveable temperature of the planet by trapping the energy of the sun and causing global warming.

Oil Refinery

What’s the Problem with CO2?

The release of extra CO2 into the atmosphere through the burning of fossil fuels, for example, traps heat within the thin protective atmospheric film blanketing the earth, which causes a global warming phenomenon linked to climatic instability and major shifts in weather and natural conditions and formations. 

Nitrogen oxide, or oxides of nitrogen are atmospheric pollutants formed when fuel is burned at extremely high temperatures, and it represents a highly reactive and poisonous family of gases. NOx is a pollutant and is emitted into the atmosphere through the exhaust of vehicles, trucks, heavy equipment, watercraft and industrial equipment and locations such as refineries and power plants.


Brownish in colour, NOx is considered an oxidizing agent that reacts with other atmospheric gases and particles as well as the volatile organic compounds that produce smog, ground level ozone, haze, acid rain and fine particulates associated with a spectrum of health concerns.

What’s the difference between NOX, NO and NO2?

  • Nitrogen oxides (NOx) is a collective term used to refer to nitrogen monoxide (nitric oxide or NO) and nitrogen dioxide (NO2)

  • Nitrogen monoxide (NO) is a colourless gas and one of the principal oxides of nitrogen.

  • Nitrogen dioxide (NO2) is a reddish-brown gas with a pungent, acrid odour and one of the several oxides of nitrogen.

Sources of NOX – NO and NO2


Now that you’ve been formally introduced to NOx, NO and NO2 – how are nitrogen oxides produced and where do they come from?

Nitrogen oxides are produced from the reaction of nitrogen and oxygen gases in the air during combustion, especially at high temperatures. At normal temperatures, oxygen and nitrogen gases do not react together.

Nitrogen oxides are produced naturally from a lightning strike. A bolt of lightning can reach temperatures of roughly 30,000 kelvins (53,540 degrees Fahrenheit). How many times does lighting strike the earth you might ask? About 100 lightning bolts strike the earth’s surface every second. There are 86,400 seconds in a day and 365 days in a year – you do the math…let’s just say it’s a lot.

In large cities, nitrogen oxides are produced from fuel combustion in mobile and stationary sources. The combustion of gasoline in automobiles emit nitrogen oxides into the atmosphere (mobile source). Stationary emissions come from coal fired power plants, electric power plant boilers.

Meet the nitrogen oxide family: NOX, NO and NO2


Meet nitrogen, commonly referred to simply with the chemical symbol N. When two nitrogen atoms bond together, they form nitrogen gas (N2). Nitrogen gas is odourless, colourless, and tasteless. It is non-flammable and will not support combustion.

Meet nitrogen’s friend oxygen (O), a highly reactive non-metal, oxidizing agent that readily forms oxides with most elements and other compounds.

At normal pressure and temperature, molecules of oxygen bind to form dioxygen (O2) which is present in the atmosphere.

Oxygen is the third most abundant element in the universe, and of course is crucial to our continued existence. Interesting fact: the most common auroral colour (pale yellowish green) in the northern lights is produced by oxygen molecules located about 60 miles above the earth. How cool is that!


Fun fact


Nitrogen makes up 78% of the Earth’s atmosphere. You probably remember learning about the nitrogen cycle in school and how it is crucial to all life on earth.

Natural sources: Mainly thunderstorms due to the extreme heat of lightning. Forest fire is another natural source.

Biogenic: Agricultural fertilization and the use of nitrogen fixing plants (through nitrogen fixation by microorganisms).

Industrial sources: Thermal NOx, fuel NOx and prompt NOx.

  • Thermal NOx – most relevant source of natural gas combustion

  • Fuel NOx – transportation fuel is estimated to be 54% of NOx caused by humans.

  • Prompt NOx – occurs in the early stage of combustion, forming fixed species of nitrogen (like NH…etc) which can oxidize to NO

Although naturally produced nitrogen oxides outweigh man-made emissions, NOx from natural sources is typically found at altitudes higher than 5km. Man-made emissions, due to fossil fuel combustion either from stationary sources (24%) and mobile sources (49%) are typically found near the sources at near surface elevation where it can cause significant health issues


Do NOx emissions impact the environment?

High levels of NOx can have a negative effect on vegetation by making it more susceptible to disease and frost damage. When NOx reacts with other pollutants in the presence of sunlight, it forms ozone. Ozone at high concentrations also damage vegetation. 

Nitrogen Oxides (NOx) react to form smog and acid rain. NOx reacts with ammonia, moisture, and other compounds to form nitric acid vapour and related particles. The impacts of NOx on human health include damage to the lung tissue, breathing and respiratory problems.

Nitric Oxide (NO) is not considered to be hazardous to health at typical ambient conditions. However, excess nitric oxide and its products may cause respiratory ailments, hematologic side effects, metabolic disorders, low blood pressure, nausea, vomiting and diarrhoea.

Nitrogen Dioxide (NO2) at high concentrations causes inflammation of the airways. Breathing in high levels of NO2 can increase the likelihood of respiratory problems: wheezing, coughing, colds, flu, and bronchitis. People with asthma are prone to have more intense attacks. Prolonged exposure to high levels of NO2 can cause irreversible damages to the respiratory system.

While ozone-based NOx control offers significant benefits, it's essential to consider certain factors. Initial setup costs for ozone generation and injection systems may require careful financial planning, although these investments can yield long-term savings and regulatory compliance. Additionally, integrating new ozone technology with existing emission control setups may involve some adjustments, but with the right expertise, this process can be streamlined. It's worth noting that while ozone effectively neutralizes NOx emissions, occasional secondary pollutants like nitrogen dioxide or ozone itself may arise, prompting the need for additional treatment measures to ensure minimal environmental impact.

Utilising ozone for NOx control presents a viable strategy for businesses to address stringent environmental regulations and reduce their ecological footprint. Despite the challenges, the long-term economic and environmental benefits of adopting ozone technology make it an attractive solution for industries committed to sustainable practices.


By proactively managing NOx emissions, businesses not only comply with global standards but also enhance their operational efficiency and corporate image, aligning themselves with modern environmental expectations.

Brave thinking.

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