Nitric oxide NO and nitrogen dioxide NO2 are the two gases that make up nitrogen oxides, as defined by emission rules and regulatory measurement techniques. In the table on the right, the physical properties of both gases are listed. There are various more nitrogen oxides from a chemical standpoint. Nitrous oxide (N2O) is one of them, and it is described further down.
NOx is one of the most dangerous pollutants found in the exhaust of all internal combustion engines. Nitrogen oxides are highly active ozone precursors that play a key role in the chemistry of smog. In the atmosphere, they can also generate secondary nitrate particles.
NOx concentrations in untreated diesel exhaust typically range from 50 to 1000 parts per million. NOx is commonly stated as NO2 equivalent when concentrations are given in mass units.
The gas nitric oxide (NO) is odorless and colorless. It can be made in the lab directly from nitrogen and oxygen at high temperatures and pressures:
How do you reduce NOx in a diesel engine?
The most common method of removing NOx from diesel engine exhaust emissions is selective catalytic reduction (SCR). With the help of a reducing agent, usually ammonia, and a catalyst, SCR converts NOx to nitrogen and water. For stationary diesel engines and industrial NOx emissions, ammonia-SCR is commonly employed.
How do diesel engines produce nitrogen oxides?
Prompt NOx is created when molecular nitrogen in the air reacts with gasoline in fuel-rich circumstances, which occur in all combustion to some extent. During combustion, this nitrogen oxidizes with the fuel and produces NOx, much as fuel NOx.
How much NOx does a car produce?
The real-world nitrogen oxide (NOx) emissions behavior of heavy-duty diesel vehicles in the United States, as measured by portable emissions measuring equipment, is examined in this research (PEMS). This conclusion is based on 160 PEMS tests from engines that have been certified to emit 0.2 grams of NOx per brake horsepower-hour (g/bhp-hr). Data from eight manufacturers and 26 distinct engine families certified between 2010 and 2016 is included in the testing. The impact of vehicle speed, vehicle type, and manufacturer on real-world NOx emissions was assessed using second-by-second data.
- To determine compliance for heavy-duty in-use NOx emissions, the Not-to-Exceed (NTE) methodology evaluates less than 10% of total emissions data. The average emission value of 0.18 g/bhp-hr obtained from the NTE analysis is much lower than the 0.42 g/bhp-hr figure obtained from a total route analysis.
- The low-speed operation characteristic of urban driving emits a disproportionate quantity of NOx emissions from heavy-duty vehicles. For the average heavy-duty vehicle in the study, operating at speeds less than 25 mph resulted in NOx emissions that are more than five times the certification limit.
- Average NOx emissions from heavy-duty vehicles (HDVs) are 2.7 times the certification limit at mid-speed driving conditions, between 25 and 50 mph, which is typical of suburban travel. HDVs only emit average NOx emissions at the certification limit and below the in-use NTE emissions limit of 0.3 g/bhp-hr at highway speeds above 50 mph.
- At speeds below motorway speeds, line-haul trucks have the highest average NOx emissions. In urban driving, their average NOx emissions of 1.41 g/bhp-hr are more than 7 times the engine certification limit, while in suburban driving, they are more than 3 times the limit. Line-haul trucks only generate NOx at engine certification limit levels during high-speed operation.
- In urban travel, a single line-haul truck emits the NOx equivalent of 100 cars each mile driven. Line-haul trucks release an average of 7.0 g/mi of NOx in urban driving conditions, compared to less than 0.07 g/mi for a gasoline automobile. According to PEMS data, these vehicles, which are designed for highway travel, spend 43 percent of their time in urban-like operations, including low-speed driving and idling, and generate 40 percent of the total mass of NOx.
The present NTE in-use testing technique is insufficient to assess HDV performance in the United States, particularly at low speeds. The possibility of future NOx requirements based on low-load cycle and idle tests, in addition to the typical federal test technique, necessitates the development of a new instrument for determining correct in-use compliance. That tool should make sure that not only highway data is used to assess in-use compliance, but also low-speed, low-load, and idle data. This would ensure that laboratory-based engine dynamometer emission results translate to real-world benefits.
What is the biggest source of NOx?
The majority of NOx in the air comes from human-caused combustion-related emissions, such as fossil fuel burning in electric utilities, high-temperature operations in various industrial sources, and motor vehicle operation.
Do diesel engines produce more NOx?
NOx emissions and particulates are the two most major (anthropogenic) pollutants created by humans.
Nitrogen oxides are what it’s all about. Purists will argue that it solely refers to nitric oxide (NO) and nitrogen dioxide (NO2), but most people will include nitrous oxide (N2O) as well. Other versions exist, but their atmospheric concentrations are insufficient.
- High-temperature combustion of fuels occurs when the temperature rises above 1300°C/2370°F, allowing some nitrogen in the air to be oxidized to NOx gases. Because hydrogen burns at such a high temperature, it is included. Below are some comments about diesel engines.
- Because all plants contain nitrogen, burning plant material releases nitrogen oxides.
- NOx gases are produced by chemical and industrial operations that involve nitric acid, nitrates, or nitrites.
Q. What is the difference between a diesel engine and a petrol/gasoline engine in terms of combustion?
A mixture of fuel and air is injected into the combustion chamber of a petrol/gasoline engine. This is compressed, and then a spark plug ignites it.
In a diesel engine, air is pumped into the cylinder and compressed twice as much as in a gasoline or petrol engine. Because of the heat generated by compression, diesel fuel burns spontaneously when injected.
A. Diesel engines are hotter and have a higher pressure than gasoline ones. NOx gas generation is favored under these conditions. The volume and duration of the hottest section of the flame determine the quantity.
A. Diesel fuel produces more energy per unit volume (diesel has a lower calorific value than petrol/gasoline but a higher density). A diesel engine is also more efficient due to the increased combustion temperature. Heat engines that run at greater temperatures can produce more productive work.
A. By reducing the combustion temperature, which is usually accomplished through Exhaust Gas Recirculation (EGR). A portion of the exhaust gas is cooled and reintroduced into the combustion chamber. Because some of the oxygen in the exhaust gas has been consumed by prior combustion, there is less to feed the flame. The heat capacity of exhaust gas is more than that of air, hence it takes longer to heat up.
Yes, there is a disadvantage. The power and fuel economy both decrease when the combustion temperature drops.
A. Depending on the application, different strategies are used, albeit a lot of effort is put into designing burners that limit NOx emissions in the first place.
- The most frequent approach in diesel car exhausts is selective catalytic reduction (SCR), although it is expensive and hence not used in tiny, inexpensive automobiles. The exhaust flow can be injected with a variety of unique ammonia and urea mixtures. Over a catalyst, they react with NOx gases, converting them to harmless nitrogen and water.
- SNCR (Selective Non-Catalytic Reduction) occurs in ducting at a temperature of around 1000°C (1800°F). The NOx emissions are converted to nitrogen without the use of a catalyst when urea or ammonia is introduced.
- On a large scale, chemicals like sodium hydroxide, hydrogen peroxide, or a combination of hydrogen peroxide and nitric acid can be used to scrub exhaust emissions. The NOx gases are removed by these chemicals as they react with them.
- Because of its radiative effect and the time it takes to break it down, it is classified as a significant greenhouse gas that is 298 times as hazardous as CO2.
- It is used as an anesthetic and is generally non-toxic. It reacts with vitamin B12, which can be problematic for persons who are weak in the vitamin.
- It decomposes in the stratosphere and catalyzes the decomposition of ozone. Ozone is necessary for absorbing UV radiation in the high atmosphere, but it is damaging at the earth’s surface.
- It is non-toxic in modest amounts and plays an important role as a regulator in the human body.
- A important pollutant and smog component. Its dark emissions may remind you of chemistry experiments in school.
- It produces nitric acid when it combines with water, which is why it irritates the eyes and respiratory tract so much.
A. Any sulphur in gasoline is transformed to sulphur dioxide (SO2) gas when it is burned in an engine. This easily dissolves in water to form an acid, which is why inhaling it causes irritation to your respiratory tract. It also has an impact on the environment. Large amounts of sulphur can be found in crude oil and gas, which must be eliminated in the refinery. Sulphur content in fuel is regulated laxly in some nations, resulting in excessive emission levels.
- Smog caused by photochemical reactions. In the presence of sunlight, nitrogen dioxide and another pollutant, volatile organic compounds (VOCs), mix to form ozone and a range of other chemicals. These are quite harmful to the respiratory system.
Do petrol cars emit NOx?
According to the testing, the cleanest 10% of new diesel cars generate an average of 70mg of NOx per kilometre, while the dirtiest 10% of new petrol models produce 129mg/km, in contrast to the official tests, which are conducted under laboratory settings.
Several municipalities have increased parking fees for diesel vehicles, regardless of emissions. From January, the London borough of Islington will levy a £2 per hour penalty on diesel vehicles of any age that park in short-term parking lots.
From April 2019, Sadiq Khan, the mayor of London, intends to expand the toxicity charge, which was implemented last month, to include all pre-2016 diesels and raise the daily rate from £10 to £12.50.
Many modern diesel models, according to Nick Molden, founder of Emissions Analytics, are still highly polluting, but the results demonstrate that this is incorrect “Demonize” all diesel-powered vehicles.
“These real-world findings call into question whether blanket diesel taxes are the optimal policy,” he remarked. “It makes no sense to penalise or tax diesel as a technology in and of itself, only unclean cars of any kind.”
He said that by installing more effective pollution controls on the cleanest diesels, vehicle manufactures have cut NOx emissions. These are usually selective catalytic reduction systems that neutralize NOx by injecting urea solution into the exhaust gases. AdBlue, as the commercial name for the solution is known, requires a 10-20 litre tank.
Many vehicle companies had previously chosen less effective methods, such as lean NOx traps, partially to save money but also because they believed drivers would object to having a urea tank that needs to be refilled on a regular basis and takes up room in the boot, according to Mr Molden.
According to Edmund King, head of the Automobile Association, there is now a danger of moving too far in the opposite direction, 17 years after the Labour administration neglected air pollution issues by supporting diesel. “He warned, “We could be shooting ourselves in the foot again.”
“For the next ten years, we’ll need a variety of fuels, including diesel, because it’s still king and there’s nothing better for large trucks and pulling vehicles.
“If you don’t live in a city and drive a lot of highway miles, a clean modern diesel may be the greatest option for the environment, helping to balance climate change and air pollution.
He claimed that extra sanctions for diesel owners were unnecessary. According to data from What Car?, sales of new diesels plunged 30% last month compared to October last year, with discounts 25.6 percent higher on average than for diesels’ petrol equivalents.
Is N2O included in NOx?
The word “nitrogen oxides” (NOx) refers to two gases: nitric oxide (NO), a colorless, odorless gas, and nitrogen dioxide (NO2), a reddish-brown gas with a distinct odor. NO3 (nitrogen trioxide), N2O (nitrous oxide), N2O4, and N2O5 are some of the other nitrogen oxides.
How do you calculate NOx emissions?
The amount of NOx emissions is noted on the vehicle’s Certificate of Conformity for new cars (CoC). This paperwork should be obtained from the seller or the manufacturer of the car. Prior to a vehicle’s registration inspection, a CoC must be uploaded.
How do I get rid of NOx?
Because of its proven role in the creation of ground-level ozone, reducing NOx emissions is a significant priority of the Clean Air Act Amendments. The United States Environmental Protection Agency (EPA) believes that establishments can minimize NOx emissions from their most prevalent source, combustion equipment, by employing reasonable methods.
Many NOx-control systems have been applied to stationary combustion sources with great effectiveness. The type of facility (for example, an industrial boiler, a gas turbine, or a municipal-waste combustor), site-specific conditions, and regulatory and economic considerations will all influence the technique chosen.
Thermal NOx and fuel-bound NOx are the two types of NOx produced by two basic mechanisms. A third mechanism, known as “prompt NOx,” is responsible for a small portion of NOx generation.
When dissociated nitrogen from combustion air interacts with oxygen atoms to make nitrogen oxides such as NO and NO2, thermal NOx generation happens only at high flame temperatures. The formation of thermal NOx is proportional to the square root of the amount of oxygen in the combustion zone and grows exponentially with combustion temperature.
The generation of fuel-bound NOx is not confined to high temperatures, although it is influenced by the nitrogen content of the fuel.
Reduced flame temperature, reduced surplus oxygen, and/or the use of low nitrogen-containing fuels are the best ways to limit NOx generation.
The following are NOx-reduction tactics and technologies for combustion sources that have been demonstrated and are currently accessible.
Changing the fuel source. Switching to a cleaner fuel is the easiest and most cost-effective strategy to minimize NOx emissions. Switching to a lower-nitrogen fuel is the most effective way to reduce fuel-bound NOx generation. To reduce NOx emissions, No. 6 fuel oil or another residual fuel with a high nitrogen content can be substituted with No. 2 fuel oil, another distillate oil, or natural gas (which is essentially nitrogen-free).
Recirculation of flue gases (FGR). Flue gas recirculation is the process of removing some flue gas from the stack and recirculating it with the combustion air supplied to the burners. By diluting the combustion air with flue gas, the procedure lowers both the oxygen concentration and the temperature at the burners. There have been reductions in NOx emissions of 30 to 60%.
Burners with low NOx emissions. NOx emissions can be reduced by up to 50% by installing burners specifically designed to limit NOx generation. Combining a low-NOx burner with FGR can result in higher reduction efficiencies, albeit the reduction efficiencies are not cumulative. By generating recirculation zones, staging combustion zones, and lowering local oxygen concentrations, low-NOx burners are aimed to reduce peak flame temperature.
Derating. Some industrial boilers can be derated, resulting in less steam or hot water being produced. Derating reduces the temperature of the flame within the unit, which reduces the generation of thermal NOx. Derating can be performed by lowering the firing rate or inserting a permanent restriction in the fuel line, such as an orifice plate.
Injection of steam or water.
By injecting a little amount of water or steam into the flame’s near proximity, the flame temperature will be lowered and the local oxygen concentration will be reduced. As a result, the generation of thermal and fuel-bound NOx is reduced. Please be aware that this method reduces the combustion efficiency of the unit by 1 to 2%.
Combustion in stages. A fuel-rich zone followed by an air-rich zone, or an air-rich zone followed by a fuel-rich zone, can be created using either air or fuel injection.
Installing a low-NOx staged combustion burner or retrofitting the furnace for staged combustion are two options for achieving staged combustion. With staged combustion, NOx reductions of more than 40% have been demonstrated.
Fuel is re-burned. The method of fuel reburning can be used to generate staged combustion by generating a gas-reburning zone above the primary combustion zone. Additional natural gas is delivered into the gas-reburning zone, generating a fuel-rich region where hydrocarbon radicals react with NOx to generate molecular nitrogen. Natural gas reburning (NGR) has resulted in NOx reductions of 40 to 75 percent in field tests on many full-scale utility boilers.
Oxygen content is low. Excess air is removed from the combustion zone, which reduces the amount of oxygen available and lengthens the flame, resulting in a lower heat-release rate per unit flame volume.
NOx emissions decrease in a nearly linear fashion when surplus air is reduced. However, as surplus air falls below a certain threshold, imperfect mixing reduces combustion efficiency, and CO emissions rise.
Excess air values must be calculated experimentally and are dependent on the fuel and combustion system design. A feedback control system can be added to monitor the levels of oxygen or combustibles in the flue gas and change the combustion-air flow rate until the desired target is met. A solution like this can cut NOx emissions by up to 50%.
Catalytic reduction with selectivity (SCR). SCR (selective catalytic reduction) is a NOx-control method that uses a catalyst to facilitate a chemical reaction between NOx and ammonia, resulting in nitrogen and water.
The exhaust gas is injected with an ammonia/air or ammonia/steam mixture, which subsequently passes over the catalyst, reducing NOx. The temperature of the exhaust gas as it travels through the catalyst bed must be within a specified range to optimize the reaction.
Regardless of the combustion procedure or fuel type employed, removal efficiencies of more than 80% can usually be reached. SCR has a number of drawbacks, including the need for additional space for the catalyst and reactor vessel, as well as a system for storing, distributing, and injecting ammonia. Ammonia storage may also necessitate the preparation and submission of a Risk Management Plan (RMP) in accordance with Federal Accidental Release Prevention guidelines.
Ammonia injection must be precisely controlled. Excess ammonia can cause ammonia “slip,” or the release of unwanted ammonia into the atmosphere, whereas insufficient ammonia can cause unacceptable high NOx emission rates.
Non-catalytic reduction with selectivity (SNCR). A reducing agentammonia or ureais injected into the flue gas for selective non-catalytic NOx reduction. When employing ammonia, the ideal injection temperature is 1850oF, at which 60 percent NOx removal can be achieved. When employing urea, the ideal temperature range is expanded.
Ammonia forms below the ideal temperature range, and NOx emissions rise above it. The capacity of the agent to mix sufficiently with flue gas, as well as the injection temperature, determines the efficacy of NOx removal.
As a result of legislation limiting emissions of precursors of ground-level ozone, many facilities are compelled to minimize NOx emissions. For NOx reduction and control, there are a variety of approaches with varying success rates. As a result, the technical decision-making process requires considerable consideration.
Reduced flame temperature, reduced surplus air, and/or the use of low-nitrogen-containing fuels can all help to reduce NOx generation. Technology for post-formation control is also available.
The various choices must be thoroughly examined in terms of the required amount of NOx reduction, the individual combustion source, the potential rise in other pollutant emissions, site-specific limits, and economic viability.
The authors are: Malcolm Pirnie, Inc. employs Marc Karell and Amit Chattopadhyay. The former is based in the company’s White Plains, NY, office and has more than 15 years of experience in the field of air quality engineering; the latter is based in the Mahwah, NJ, office and has more than 25 years of experience in the field of combustion engineering. They’re both licensed professional engineers.
Which is worse CO2 or NOx?
According to a new study published in the journal Nature, the amount of nitrous oxide in the atmosphere is gradually increasing. Nitrous oxide is 300 times more detrimental to the environment than carbon dioxide.
While nitrous oxide is created in a variety of ways, the study discovered that agriculture is the major contributor, as it is produced as a by-product of nitrogen, which is mostly utilized as a fertilizer in agriculture.
According to the study, nitrous oxide levels in the atmosphere were 270 parts per billion in 1750 and had climbed to 331 parts per billion in 2018. In the previous five decades, the rate of increase has been the quickest.
According to the multinational team of writers, if current trends continue, more nitrous oxide could boost global temperatures up to 3 degrees Celsius over pre-industrial levels by 2100, well beyond the 1.5 or 2 degrees Celsius limit set by the Intergovernmental Panel on Climate Change (IPCC).
“Currently, emissions are on track to cause a global temperature increase of more than three degrees by the end of this century,” said Hanqin Tian, co-lead author of the study and director of Auburn University’s School of Forestry and Wildlife Sciences in Alabama’s International Center for Climate and Global Change Research.
“It emphasizes the urgency… and it’s necessary to think about this,” says the author.
Three main greenhouse gases are of special importance when it comes to climate change: carbon dioxide (CO2), methane, and nitrous oxide (N2O).