All substances produced by combustion or pyrolysis are called combustion products.Usually refers to the gas, heat, visible smoke, etc. generated by combustion.Among them, the combustion products that can be seen in the air are calledsmokeIt is actually a mixture of suspended solids, liquid particles and gases produced by combustion.The particle size is generally between 0.01 and 10 microns.
Combustion products refer to all substances produced by combustion or pyrolysis.That is to say, when combustibles burn, the gas, solid, steam and other substances generated are combustion products.Combustion products can be divided into complete combustion products and incomplete combustion products according to the complete degree of combustion.
(1) If the products generated in the combustion process can no longer be burned, the combustion is called complete combustion, and the products are called complete combustion products.E.g. COtwo，SOtwo，HtwoO，PtwoOfiveAll are products of complete combustion.
Realistic significance: complete combustion products can dilute the oxygen content and inhibit combustion in the fire field.
(2) If the products generated during combustion can continue to burn, such combustion is called incomplete combustion, and the products are incomplete combustion products.For example, when carbon is burned under the condition of insufficient air, its combustion product iscarbon monoxide, because it can continue to burn with oxygencarbon dioxide, so it is the product of incomplete combustion.Incomplete combustion products are caused by low temperature or insufficient air.
Realistic significance: because the products of incomplete combustion can continue to burn, once mixed with air and encountering the fire source again, there is a possibility of explosion. Also, most of the products of incomplete combustion are toxic and easy to cause human poisoning.Therefore, it is necessary to find a way to change incomplete combustion products into complete combustion products.[1]
Combustion products of different substances
The quantity and composition of combustion products vary with the chemical composition of substances, temperature, air supply and other combustion conditions.
Combustion products of simple substances
Generally, a single substance burns completely in air, and its product is the oxide of the element.For example: carbon, hydrogen, phosphorus, sulfur, etccarbon dioxide、water、Phosphorus pentoxideandsulfur dioxideEtc.These products can no longer be burned and are called complete combustion products.
Combustion products of general compounds
Some compounds burn in the air to produce not only complete combustion products, but also incomplete combustion products, especially somePolymer compoundAfter being heated, it will produce thermal cracking, generating many different types of organic compounds, and the products can be further burned. The simplest incomplete combustion product is carbon monoxide, which can be further burned to generate carbon dioxide.
Wood combustion products
Wood is a kind of compound, which is the most widely used structural material. Wood combustion is often included in fire.Wood is mainly composed of carbon, hydrogen and oxygen, and is mainly composed of cellulose [(CsixHtenOfive）x]It also exists in the form of sugar, gum, ester, water and other molecules.
After heating, wood will produce pyrolysis reaction to produce small molecule products.At about 200 ℃, it mainly generatescarbon dioxide, steamformic acidAcetic acid and various flammable gases;Produce a small amount of water vapor at 200~280 ℃ andcarbon monoxide；At 280~500 ℃, combustible vapor and particles are generated;When the temperature is above 500 ℃, it is mainly carbon, and the free radicals produced have obvious acceleration effect on combustion.
The combustion of synthetic polymer materials is accompanied by thermal cracking. Some polymer materials also contain chlorine and nitrogen, so many toxic or irritating gases, such as hydrogen chloride (HCl), phosgene (COCltwo）、Hydrogen cyanide(HCN) andNitrogen oxide（NOX）Etc.[2]
Main combustion products
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(1) Carbon dioxide
carbon dioxideIt is the product of complete combustion. It is a colorless and incombustible gas, soluble in water, weakly acidic and suffocating.Several important data: 5% of the time, they are out of breath and cause poisoning within 30 minutes;At 6%,Shortness of breath, feel difficult;7-10%, lose consciousness within a few minutes, resulting in death.Carbon dioxide is liquid at normal temperature and 60 atm. When pressure is subtracted, this liquid carbon dioxide will vaporize quickly, absorb a lot of heat, and the temperature will quickly decrease to - 79 ℃ at most. Some of it will condense into snow like solids, so it is also called dry ice.
Realistic significance: carbon dioxide is often used as a fire extinguishing agent in fire safety, because carbon dioxide is a product of complete combustion, which can dilute oxygen content and absorb heat.But it should be noted that it can burn in carbon dioxide with potassium, sodium, calcium and other metal substances.Therefore, carbon dioxide can not be used to put out metal fire.
CO is the product of incomplete combustion.It is a colorless, tasteless, highly toxic flammable object, and is difficult to dissolve in water.As carbon monoxide is toxic, it can replace oxygen from the oxygen heme in the blood and combine with heme to form carbon monoxide heme, which makes people feel seriously hypoxic.
Sulfur dioxide is the product of combustion of combustible sulfur.It is a colorless gas with irritating odor.The special smell of sulfur combustion is the smell of sulfur dioxide.It is easily soluble in water. In some aspects, 40 volumes of sulfur dioxide can be dissolved in 1 volume of water at 20 ℃.Sulfur dioxide is toxic, which is one of the most harmful gases in air pollution. It seriously damages plants, stimulates people's respiratory tract, and corrodes metals.
Phosphorus pentoxide is the combustion product of combustible phosphorus.It is a white solid powder under normal temperature and pressure, which can be soluble in water and generate metaphosphoric acid (HPOthree）Or orthophosphoric acid (HthreePOfour）Phosphorus pentoxide is toxic and can stimulate respiratory organs, causing cough and vomiting.
(5) Hydrogen chloride
Hydrogen chloride is the combustion product of chlorine containing combustibles.It is an irritant gas. It absorbs water in the air and becomes an acid mist. It has strong corrosivity. At higher concentrations, it will strongly stimulate people's eyes, causing respiratory tract inflammation and pulmonary edema
(6) Oxides of nitrogen
Nitrogen oxides in combustion products are mainlynitric oxideandNitrogen dioxide。Decomposition of nitric acid and nitrate, containing nitrate andnitriteIn the explosion process of explosives, nitric acid fibers and other nitrogen containing organic compounds will produce NO and NO during combustiontwo。NO is a colorless gas;NOtwoIt is brownish red gas.They all have an unpleasant smell and are poisonous.[3]
Main characteristics
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Combustion products are closely related to fire fighting.It has both advantages and disadvantages for fire fighting.
Favorable aspects
(1) Flame retardant under certain conditions.The combustion products of complete combustion are incombustibleinert gas, e.g. COtwo, steam, etc.In case of an indoor fire, with the increase of these inert substances, the oxygen concentration in the air will be relatively reduced, and the combustion speed will also be slowed down. If the ventilation doors, windows and holes can be closed, the combustion speed will be slowed down until the combustion stops.
(2) Provide basis for fire reconnaissance.The smell, color, concentration and flow direction of smoke are different for different substances, different combustion temperatures and different wind directions.Through these characteristics of smoke, firefighters can roughly judge the type of combustion materials, fire development stage, fire spread direction, etc.Due to the combustion of different substances, the smoke has different colors and smells, so the smoke produced by Huamiaobi at the initial stage of a fire can provide people with fire alarm. People can roughly determine the fire location and the type of combustor according to the smoke location, scale, color and smell, so as to implement correct fire fighting methods.
(3) The color and smell of smoke can be used to determine what substances are burning.Smoke is a solid and liquid particle produced by combustion or pyrolysis and suspended in the atmosphere. It is actually a small particle group suspended in the air, with a particle size of 0.01~10 µ m.Large diameter particles are easy to fall down from the smoke and become smoke or carbon black.The composition of substances is different, and the color and smell of smoke with different components are also different.According to this feature, in the process of fire fighting, we can judge what substance is burning according to the color and smell of smoke.For example, the combustion of white phosphorus generates thick white smoke and garlic flavorPhosphorus trioxide。[1]
Adverse aspects
Harmfulness of combustion products: the experiment shows that the most direct combustion product on the fire site is smoke.General fires are always accompanied by thick smoke, which produces a large number of toxic and harmful smoke to human body.It can be said that smoke is the greatest threat to people in a fire.Therefore, it is very important to understand the dangerous characteristics of combustion products.
(1) Cause poisoning and suffocation.Many of the combustion products are toxic gases, such as CO, HCl, HCN and NOX。It has the function of anaesthesia, suffocation and stimulation to human body: these gases hinder people's normal breathing and escape, and also bring difficulties to firefighters' fire fighting work.
(2) People may be injured.The smoke of combustion products contains a lot of heat.People are easily scalded in this hot and humid environment.
(3) Affects vision.The burning produces a large amount of smoke, which affects people's sight.The visibility is greatly reduced, and people often can't distinguish the direction in the thick smoke, which brings difficulties to fire fighting and personnel evacuation.
(4) It has become a factor of fire development and spread.The combustion products have high heat energy, which is very easy to cause flashover or new fire point due to convection or thermal radiation.[1]
