The term “NOx” has appeared in several articles, but many individuals may not know what it is. Nitrogen Oxides is a harmful pollutant emitted during the combustion of fossil fuels such as natural gas and diesel. These emissions contribute to various environmental issues and have devastating effects on both the planet and living organisms. Nitrogen oxides is typically measured in terms of milligrams per kilowatt-hour (mg/kW.h) or parts per million (ppm). In an effort to mitigate these harmful emissions, scientists and engineers recommend techniques such as Flue Gas Recirculation (FGR), which helps reduce the concentration of Nitrogen Oxides released into the atmosphere. This article will provide a detailed explanation of NOx emissions, their impact, and the effectiveness of FGR and other innovative solutions.
Understanding Nitrogen Oxides and Its Impact on the Environment
NOx is a collective term that represents nitrogen oxides, including nitric oxide (NO) and nitrogen dioxide (NO2), produced during the combustion of fossil fuels. These pollutants are released into the environment through various industrial processes and power generation, entering the atmosphere primarily via boiler chimneys and exhaust stacks. The presence of Nitrogen Oxides in the atmosphere is particularly concerning due to its contribution to several environmental problems, including global warming, acid rain, and the depletion of the ozone layer. The effects of these pollutants are far-reaching; they can damage ecosystems, harm wildlife, and significantly impact human health.
The impact of NOx on public health is particularly alarming, as exposure to high levels of these pollutants can lead to serious respiratory issues, cardiovascular diseases, and premature deaths. According to studies conducted by the World Health Organization (WHO) in 2015, pollution-related diseases and deaths resulted in annual costs estimated at a staggering $1.6 trillion in the European Union alone. The role of NOx, along with other harmful gases like carbon monoxide, is clearly significant. This is especially critical considering that environmental regulations in European countries tend to be stricter than those in many developing nations.
In a landmark announcement on September 26, 2018, the European Union established new standards to limit nitrogen dioxide emissions from water heaters, mandating that they must not exceed 56 mg of NOx for every kilowatt-hour of fuel consumed. In response to this regulation, the Greater London Authority (GLA) implemented rules stipulating that the maximum allowable NOx emission limit for gas heating systems should be set at 40 mg/kWh. These regulations reflect a growing awareness of the necessity to control NOx emissions to protect public health and the environment.
The implementation of effective strategies to reduce this emission has gained momentum in recent years, particularly in countries like Germany and Denmark, which have successfully carried out projects aimed at lowering nitrogen oxide levels. As a result, the environmental requirements concerning NOx and carbon monoxide limits have been formalized as part of international agreements, including the Paris Agreement (or the 2020 Treaty). This treaty obliges participating countries, including Iran, to take measures to improve air quality and reduce pollution levels by the year 2020.
In alignment with global NOx reduction goals, companies such as Packman have taken proactive steps to address the challenges associated with nitrogen oxides. As a leading manufacturer of hot water and steam boilers in Iran, Packman is committed to developing technologies that not only comply with international standards but also contribute to a cleaner environment. Their initiatives aim to minimize the environmental impact of these emissions while enhancing the efficiency of their heating systems.
This article will further explore the mechanisms of Nitrogen Oxides formation, the effectiveness of Flue Gas Recirculation, and other technological advancements in reducing emissions. By understanding the complexities of Nitrogen Oxides and the solutions available, we can make informed decisions that promote a healthier planet for future generations.
NOx pollution reduction
NOx, or nitrogen oxides, is emitted by various industries, including the automobile sector, cooking appliances, heating systems, and power generation. Among these sources, heating systems that utilize gas fuel account for approximately 35% of total NOx emissions. This significant contribution underscores the need for targeted strategies to reduce this pollutant in the heating sector, as these emissions can have far-reaching consequences for both environmental and public health.
The Nitrogen Oxides generated during the combustion of natural gas primarily originates from three distinct sources: thermal NOx, fuel NOx, and prompt NOx. Each of these sources has different characteristics and formation mechanisms, which can inform effective strategies for emission reduction.
Thermal NOx:
This type of NOx is formed at high temperatures during the combustion process, primarily from the reaction between nitrogen and oxygen in the air. As combustion temperatures rise, the production of thermal NOx increases significantly. To mitigate thermal NOx emissions, technologies such as low-NOx burners, which operate at lower temperatures, can be employed. These burners modify the combustion process to reduce peak temperatures, thereby minimizing the formation of thermal NOx.
Fuel NOx:
Fuel NOx is generated from the nitrogen content present in the fuel itself. Natural gas, while typically having low nitrogen content compared to other fossil fuels, can still produce some NOx emissions through this mechanism. The level of fuel NOx can be reduced by using higher-quality fuels that contain minimal nitrogen or by employing combustion technologies that minimize nitrogen oxidation during the burning process.
Prompt NOx:
This type of NOx is formed through the rapid reaction of hydrocarbons with nitrogen in the combustion zone, particularly during the initial stages of combustion. Prompt NOx is generally less significant compared to thermal and fuel NOx, but its reduction is still crucial for comprehensive NOx control. Techniques to reduce prompt NOx include optimizing the air-fuel mixture and using staged combustion, where fuel and air are introduced in stages rather than all at once, allowing for more complete combustion and reduced formation of nitrogen oxides.
Efforts to reduce NOx emissions from heating systems are becoming increasingly important as governments and organizations recognize the environmental and health risks associated with air pollution. Regulatory measures, such as stricter emissions standards and incentives for cleaner technologies, are being implemented to encourage industries to adopt low-emission heating solutions.
Recirculating Combustion Products
In this method that is the main topic of this article, by adding neutral gases such as carbon dioxide, nitrogen, steam or … to the combustion site, the average flame temperature and consequently the thermal NOx will be decreased. One of the perfect sources of neutral molecules is the combustion products passing through the boiler chimney which has lower temperature compared to the combustion chamber. By adding or recirculating 10% of the combustion products, it is possible to reduce the flame temperature by 7%.
The recirculation system is shown in Figure 4 schematically. At the beginning, the combustion products recirculation is not active. The equation of methane combustion at 20% excess air is indicated in Equation 1. As it can be seen, the total number of moles resulting from combustion which absorb the combustion energy, equal 12/424 mole for burning one mole of methane. This causes a temperature in the range of 1300 degrees Celsius. This temperature is the main cause of forming thermal NOx.
Equation 1
If recirculation is 10%, in fact 10% of the combustion products, which mainly include neutral moles, will be transferred to the burner inlet. This transfer is done by pipeline connecting from the boiler chimney to the FGR inlet on the burner. The transmission path needs to be completely insulated thermally so that condensation not occur along it. The burner inlet also needs a discharge port for the possible condensation of water vapor combustion. The transmission is done by the burner’s fan in small capacities. Separated fans which are function of central control system, are used in large capacities to push the combustion products toward the burner. The combustion products contain some oxygen, too. Therefore, by transferring a percentage of combustion products, the inlet air to the burner is reduced so that the combustion operation is fully observed at 20% excess air. In this case, the reaction equation will be changed and the moles resulting from combustion equal to 13/47 mole for burning one mole of methane. This mole increase means the particles in the combustion chamber of the boiler are increased. The energy in the combustion chamber has not changed since the mole of Methane is constant. This means that the previous cycle heat (without FGR system) is distributed 13/47 instead of 12/424 which means a 7-8 % decrease in flame temperature. This can be repeated in the following cycles in the same way and the system reaches balance after passing some cycles in short time. Reducing the flame temperature strongly reduces the thermal NOx. The combustion equation in the initial cycle of FGR running is indicated in Equation 2 and the process is shown schematically in Figure 5.
Equation 2
The applicable percent for recirculation depends on burner type and its head mixing power.
The diagram in Figure 6 shows the NOx reduction in terms of recirculation percentage. As it can be observed, this method can reduce NOx by 50%. Generally, it is recommended to do the experiment to 20% for the burner manufacturers.
If the recirculation with the aim of NOx reduction exceeds this value, the burner flame becomes very voluminous and causes negative effects on heat transfer levels in the boiler combustion chamber. In this case, the user has to reduce the capacity.
Conditions for FGR installation
If the boiler has two or more burners, each burner will have separate FGR port. The management system adjusts the recirculation amount of each separately with the aim of reducing the flame’s general NOx. Recirculation has insignificant negative effects on efficiency since the flame temperature is lower and this will decrease thermal capacity a little. The general solution to compensate this reduction is to increase the mixing in the heat transfer paths. Another negative effect is the actual price of the burner. Setting up the FGR system will cost about 20% of total actual price of the burner. This cost includes piping, sensor, modular motor connected to the burner port and etc.
This cost will be more for one per kw fuel consumption in low capacity. According to the report by the US Environmental Protection Agency, among all NOx reduction methods, FGR is the most economical method for burners with capacity of over 500 kW.
All the operations described in this report, which are in sync with the researchers in the field of facilities in American and European Unions, are set up in Packman and are practically applicable for the burners with capacity of over 1700kw. Figure 7 shows an example of FGR application on two burners with capacities of 10Mw and 5Mw.
It is hoped that a strong and effective step can be taken to preserve the environmental health and protect it for the future generation by reducing NOx from heating and steam systems by 50% which include 35% of the country’s pollution.
In addition to technological innovations, public awareness and education play vital roles in NOx reduction efforts. Encouraging consumers to choose energy-efficient appliances and promote regular maintenance of heating systems can further decrease emissions. For instance, well-maintained gas furnaces and boilers not only operate more efficiently but also produce lower levels of NOx.
Furthermore, transitioning to alternative energy sources, such as electricity from renewable resources or hydrogen fuel, can significantly reduce reliance on gas-fired heating systems, thereby cutting down on NOx emissions. Governments are increasingly investing in infrastructure for cleaner energy sources and incentivizing the adoption of electric or hybrid heating systems that contribute to lower overall emissions.
As we strive to combat air pollution and its associated health risks, a comprehensive approach to NOx reduction is essential. By combining advanced technologies, regulatory frameworks, consumer education, and a shift toward cleaner energy sources, we can make significant steps in minimizing NOx emissions from heating systems and other sources, ultimately leading to a healthier environment and improved public health outcomes.
Authors: The Research Department of Burner Factory, Packman Company, Managed by Vahid Azizi
Reference:
- Economic cost of the health impact of air pollution in Europe – accessed 9 November 2016.
- Nitric oxide: Toxicity – accessed 9 November 2016.
- Commission Regulation (EU) No 814/2013 of 2 August 2013 implementing Directive 2009/125/EC on ecodesign requirements for water heaters and hot water storage.
- Sustainable design and construction, Greater London Authority, 2014.
- Implementation of Clean Air Zones in England – accessed 9 November 2016.
- City of London Air Quality Strategy 2015 – 2020, City of London Corporation.
- Hazelhurst J, Tolley’s Industrial and Commercial Gas Installation Practice – Gas Service Technology Volume 3, Taylor & Francis, 2012.
