How nitrogen oxides are formed and what are the main causes of nitrogen oxides in the air

Nitrogen oxides. It is a product of complete combustion of fuel, and a large amount of nitrogen oxides will be produced when the fuel is burned at high temperatures.

Smoke from smoking also contains nitrogen oxides, which enter the room from outside.

Of the nitrogen oxides, NO, which accounts for more than 90%, is nitrogen dioxide.

Accounting for 5%-10%, the generation mechanism is generally divided into the following three types:

1. Thermal type.

When burned, nitrogen in the air is oxidized at high temperatures, and the generation process is an unbranched chain reaction. The mechanism of its generation can be expressed by the Zeldovich reaction.

As the reaction temperature t increases, its reaction rate.

Increases exponentially. When t < 1500, the amount of NO is small, while when t > 1500, the reaction rate increases by 6-7 times for every 100 increase in t.

Mechanism of thermodynamic NOx formation: O2 + N ->2O + N

o + n2 ->no + n

n + o2 ->no + o

Total formation at high temperature: N2 + O2 ->2NO

2no + o2 ->no2

2. Instantaneous reaction type (rapid type).

Type NOX was discovered experimentally by Fenimore in 1971. in hydrocarbons.

When the fuel is burned, NOx is rapidly generated near the reaction zone when the fuel is too thick.

CH radicals due to the pyrolysis of hydrocarbons in fuel volatiles.

It can react with nitrogen in the air to form HCN and N, and then further react with oxygen to generate at a very fast rate, its formation time is only 60ms, and the generated is proportional to the power of the furnace pressure, and has little relationship with temperature.

Neither of the above NOx makes up the majority of NOx and is not the main one**.

Branching chain reactions. The mechanism of its formation can be expressed by the Zeldovich reaction, with a 6-7-fold increase in rate.

3. Fuel-type NOx

It is formed by the oxidation of nitrogen compounds in the fuel during combustion. Since the thermal decomposition temperature of nitrogen in the fuel is lower than the combustion temperature of pulverized coal, the fuel type will be generated at 600 800, which accounts for 60 80 of the NOx product of pulverized coal combustion.

In the generation of fuel-based NOx, intermediate product groups such as N, CN, HCN, and other intermediate product groups are first produced by thermal cracking of nitrogen-containing organic compounds, and then oxidized to NOX. Since the combustion process of coal is composed of two stages: volatile combustion and coke combustion, the formation of the fuel type is also composed of the oxidation of gas-phase nitrogen (volatile) and the oxidation of the remaining nitrogen in coke (coke) When combustion, especially when the combustion temperature is higher than 800, a large amount of nitrogen oxides (NOx) will be generated. And the combustion temperature of the boiler of the power plant is very high, so the nitrogen oxides are produced.

Nitrogen oxides are produced in three ways.

Nature's thunder and lightning produce small amounts in the air.

Artificial fertilizer plants, etc., also produce nitrogen oxides.

And the largest amount is biological nitrogen fixation.

Rhizobia of the legume family.

Downstairs said car exhaust.

In the final analysis, it is still the category of biological nitrogen fixation.

Biological nitrogen fixation forms plants (trees, etc.) over billions of years to form petroleum, coal, etc.

Therefore, it should also be the above three points.

The nitrogen oxides NOx produced in the coal combustion process mainly include nitric oxide (NO2), nitrogen dioxide (NO2), and a small amount of N20, etc. At present, NOx generated by coal-fired power plants according to the conventional combustion mode, NO90, NO2 accounts for 5-L0, and N20 only accounts for about 1 Therefore, the generation and emission of NOx mainly depend on NO According to the NOx generation mechanism, the amount of nitrogen oxides produced in the coal combustion process and the coal combustion mode, combustion temperature, excess air Factors such as gas coefficient and flue gas residence time in the furnace are closely related

Recommended NOx formation: Naturally emitted NOx, mainly from the decomposition of organic matter in the soil and ocean, is part of the natural nitrogen cycle. Most of the NOs emitted from anthropogenic activities come from the combustion process of fossil fuels, such as the combustion process of automobiles, aircraft, internal combustion engines and industrial kilns; It also comes from the production and use of nitric acid, such as nitrogen fertilizer plants, organic intermediate plants, non-ferrous and ferrous metal smelters, etc.

Basic Concepts: .

Nitrogen oxides in the air are mainly caused by the following three types:

1. When lightning occurs: the temperature of the air rises sharply, and the nitrogen and oxygen in the atmosphere are intensified to an active state, and the reaction becomes NO.

2. Automobile exhaust: Although the car contains catalysts that make nitrogen oxides and CO react with each other to become CO2 and N2, the reaction efficiency is not 100%, so some nitrogen oxides are directly emitted into the atmosphere without reaction.

3. Nitric acid production plant: Nitric acid production plant mainly produces concentrated nitric acid, and concentrated nitric acid is not stable, and it is easy to disintegrate and form NO and NO2 by itself, so it is considered to be a major pollution source.

Nitrogen oxides refer to compounds composed of only two elements, nitrogen and oxygen, including a variety of compounds, such as nitrous oxide (N2O), nitric oxide (NO), nitrogen dioxide (NO2), nitrogen trioxide (N2O3), nitrogen tetroxide (N2O4) and nitrogen pentoxide (N2O5). Except for nitrous oxide and nitrogen dioxide, other nitrogen oxides are unstable and become nitrogen dioxide and nitric oxide when exposed to light, humidity or heat, and nitric oxide becomes nitrogen dioxide. As a result, occupational environments are exposed to a mixture of several gases, often referred to as gunpowder fumes (gases), mainly nitric oxide and nitrogen dioxide, with nitrogen dioxide as the main body.

Nitrogen oxides all have varying degrees of toxicity.

Nitrogen oxides are nitric oxide, nitrogen dioxide, nitrous oxide, nitrous pentoxide, nitrous trioxide.

The valency of nitrogen can be +1, +2, +3, +4, +5.

The compounds composed of nitrogen and oxygen are as follows:

Nitrous oxide (N2O), nitric oxide (NO), nitrogen dioxide (NO2), nitrogen trioxide (N2O3), nitrogen tetroxide (N2O4) and nitrogen pentoxide (N2O5), etc.

Nitrogen is a chemical element with its chemical symbol n and its atomic number is 7. Nitrogen is the most abundant element in the air, which is widely found in nature and has a great effect in living organisms, and is one of the basic elements that make up amino acids.

Nitrogen and its compounds are widely used in production and life.

A brief history of the discovery: It was discovered in 1772 by the Swedish pharmacists Scherer and Rutherford, respectively, and later determined by the French scientist Lavoisier that it was an element.

Proposed in 1787 by Lavoisier and other French scientists, the English name for nitrogen, is"saltpeter composers". Xu Shou, a chemist and enlightener in the late Qing Dynasty, first translated nitrogen into Chinese as "light qi," meaning that it "diluted" oxygen in the air.

The name of the element is in Greek, which originally meant "saltpeter".

Content distribution: The content of nitrogen in the earth's crust is very small, and most of the nitrogen in nature is in the form of elemental molecular nitrogen in the atmosphere, and nitrogen accounts for 78 of the volume of air. The most important mineral of nitrogen is the nitrate of chingham.

The total weight percentage of nitrogen leakage in the earth's crust is about 4 1 012 tonnes. Proteins in both animal and plant bodies contain nitrogen. There are nitrates in the soil, such as KNO3.

In Chile, South America, there is saltpeter ore (nano3), which is the only deposit of its kind in the world and is a rare nitrogen-containing deposit. Nitrogen-containing molecules have been found in the universe, such as NH3 and HCN.

At present, there are three main types of nitrogen oxides in the atmosphere**.

When lightning occurs, the temperature of the air rises sharply, and the nitrogen and oxygen in the atmosphere are intensified to an active state, and the reaction becomes NO.

Automobile exhaust gas, although the car contains a catalyst that causes nitrogen oxides and CO to react with each other to become CO2 and N2, but the reaction efficiency is not 100%, and some nitrogen oxides are directly emitted into the atmosphere without reaction.

Nitrogen oxides refer to compounds that are made up of only two elements, nitrogen and oxygen.

Nitrous pentoxide is a solid, and the rest is a gas. Among them, nitrogen tetroxide is a dimer of nitrogen dioxide, which is often mixed with nitrogen dioxide to form an equilibrium mixture.

A mixture of nitric oxide and nitrogen dioxide, also known as nitrate gas (gunsmoke).