Occupational safety and health protection
Chemical process combustion. Factors ensuring the combustion process. For the combustion process to occur, three factors are required: flammable substance oxidizer and ignition source. When there is an excess of oxygen, combustion products are not capable of further oxidation.
74. Chemical combustion process. Factors ensuring the combustion process. Basic principles of fire extinguishing.
Combustion- This is a complex, rapidly occurring physical and chemical transformation of substances, accompanied by the release of heat and light. For the combustion process to occur, the presence of three factors is required: a combustible substance, an oxidizer and an ignition source.
Oxidant - air oxygen or some other substances: chlorine, fluorine, bromine, nitric oxide.
Ignition source- random sparks of various origins (electrical, static, etc.)
A distinction is made between complete and incomplete combustion. Complete with an excess of oxygen, combustion products are not capable of further oxidation. Incomplete occurs when there is a lack of oxygen and toxic and flammable products are formed.
Based on the speed of flame propagation, they are distinguished: deflagration combustion propagation speed of tens of m/s; explosive hundreds of meters per second; detonation (thousands of meters per second)
Depending on combustible mixture combustion occurs: homogeneous (one state of aggregation for the oxidizer); heterogeneous.
Combustion processes:
Flash- rapid combustion of the combustible mixture, not accompanied by the formation of compressed gases.
Fire the occurrence of combustion under the influence of an ignition source.
Ignition fire accompanied by the appearance of a flame.
Spontaneous combustionthe phenomenon of a sharp increase in the rate of exothermic reactions, leading to combustion of a substance in the absence of an ignition source.
- spontaneous combustionspontaneous combustion accompanied by the appearance of a flame.
One of effective means Fire extinguishers are used to extinguish fires. Currently, the manual fire extinguisher OHP-10, air-foam fire extinguisher OVP-10 (Figure 10), carbon dioxide fire extinguishers OU-2, OU-5, OU-8, mobile carbon dioxide fire extinguisher UP-2M and powder fire extinguishers-OP-1, OPS-6, OPS-10 (Figure 11).
The chemical foam manual fire extinguisher OHP-10 is designed to extinguish fires in the initial stages of their occurrence.
To activate the fire extinguisher, it is necessary to grasp it by the side and lower handles, turn the fire extinguisher over with the lid down and the handlerotate 180°. In this case, the valve of the acid glass opens, the acidic part of the charge flows out of the glass and mixes with the alkaline part. Foam forms and pressure increases in the fire extinguisher body. Under pressure foam through spraysis thrown out. The duration of the fire extinguisher is about 1 minute, the jet length is 6 x 8 m, the productivity is 90 liters of foam.
Air-foam fire extinguishers are used to extinguish fires of various substances and materials, except for alkali metals, electrical installations under voltage, and substances that burn without air access.
To activate the fire extinguisher, press the trigger lever. At the same time, carbon dioxide compressed in the cylinder through the socketdiscards the foaming agent solution. The fire extinguisher operates for 20 s, the jet length is 4.5 m.
Except foam fire extinguishers apply carbon dioxide fire extinguishers OU-2, OU-5 and OU-8
Various types of fires are used to extinguish fires.fire extinguishing agents. The most common is water. In addition to it, sand and other types of soil, various foams and powders are used.
Water should not be used to extinguish oil products, fires in live electrical equipment, sodium, calcium and potassium carbides. Petroleum products and other substances whose density less water, float above it and spread over a large area, which can cause the fire to intensify. Water is a conductor of electric current, so do not direct a stream of water at electrical equipment, as damage may occur. electric shock. Water reacts with alkali metal carbides to form flammable and explosive substances.
Sand and all other types of soil universal remedy extinguishing small fires. It is thrown onto the fire with shovels, scoops or buckets so as to first localize the fire and then cover it up.
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LECTURE 1
SECTION 1. Basic concepts of combustion
The phenomena observed when a candle burns are such that there is not a single law of nature that would not be affected in one way or another.
M. Faraday
TOPIC 1. BASICS OF COMBUSTION PROCESSES
Questions:
1. Definition of the combustion process, necessary and sufficient conditions for combustion. Types of combustion.
2. Basic characteristics of the flame. Flame temperature.
3. Classification of flammable substances, oxidizers and ignition sources. Chemical reactions, occurring during combustion.
In the middle of the 18th century, M.V. Lomonosov first suggested that the combustion process is the process of interaction of a combustible substance with oxygen in the air, i.e. oxidation. The French scientist A. Lavoisier experimentally confirmed this in 1772-76. In 1883, French chemists Malyar and Le Chatelier measured the normal speed of flame propagation. Representatives of the Russian and Soviet schools made an outstanding contribution to the creation and development of combustion theory. Our compatriot, physicist and meteorologist V.A. In the early 1900s, Michelson established the dependence of the speed of propagation of the flame front on the composition of the combustible mixture, laid the foundations of the thermal theory of explosive combustion, and developed the theory of gas combustion in a Bunsen burner.
Founder of the Soviet school of combustion, laureate Nobel Prize, academician N.N. Semenov developed the theory of branched chain reactions and thermal self-ignition (explosion). Academician Ya.B. Zeldovich and Professor D.A. Frank-Kamenetsky created a theory of flame propagation. Our scientists' seminal research has received worldwide recognition.
Combustion is fast (seconds or fractions of seconds), redox, exothermic,
a self-sustaining process, often accompanied by glow and flame formation.
The absence of any of these signs will indicate that the process in question does not relate to combustion, for example, corrosion of metals, glow of a light bulb, phosphorescence, etc.
The concept of combustion does not include slow reactions (low-temperature oxidation, biochemical oxidation) and very fast ones (explosive transformations). Combustion occurs not only due to the formation of oxides, but also due to the formation of fluorides, chlorides and nitrides. It has been established that oxygen-containing anhydrides, salts and acids of elements of variable valency (sulfur, nitrogen, chromium, manganese, chlorine, etc.) can act as oxidizing agents in combustion reactions.
Oxidation reactions are exothermic; therefore, combustion releases a large amount of heat. This determines the high temperature of combustion processes, for example, wood - 700-800°C, oil products - 1300-1500°C. According to Van't Hoff's rule, with every 10°C increase in temperature, the reaction rate increases by 2-4 times, that is, the oxidation reaction rate must be high. It follows that combustion processes are based on high-speed and high-temperature oxidation reactions. During combustion, heated to high temperature volatile products: C0 2, H 2 0, CO, etc. The density of hot combustion products is 3-5 times less than the density of the surrounding air. Therefore, they are forced upward by fresh air, i.e. above the combustion site there is a continuously rising convective flow of hot temperature determined by T c. Starting from certain limiting values, mixtures, both lean and rich, are unable to ignite. This is confirmed experimentally. For example, the dependence curve T c = f(C) for the oxide
on the composition of the mixture
4. The rate of combustion reaction depends on pressure and catalysts, so the auto-ignition temperature also depends on these factors (Table 1). Table 1 Change in auto-ignition temperature depending on pressure
Catalysts, as is known, are divided into positive (accelerating) and negative (slowing down the reaction). Positive catalysts reduce the auto-ignition temperature, while negative catalysts increase it.
The walls of the vessel containing the combustible mixture may have catalytic properties. As the catalytic activity of the vessel wall material increases, T c decreases.
The self-ignition temperature of a mixture of flammable substances usually does not obey the additivity rule. For example, the autoignition temperature of a mixture of methanol and diethyl ether different composition always below the additivity calculated by the rule.
Thus, the data presented show that the temperature
spontaneous combustion is really not a constant, but depends on
many factors. Its true value at point C in Fig. 2 can be determined experimentally only by direct temperature measurement. However modern means measurements do not yet allow this to be done with a sufficient degree of accuracy, since it is unknown at what point in the volume of the combustible mixture the initial source of combustion occurs. The thermal theory of self-ignition suggests a way out of this situation. At the point of contact C, on the one hand, there is equality of heat release and heat removal. On the other hand, at point C each function is tangent to the other, i.e. temperature derivatives of q+ and q_ must also be equal to each other B mathematical form it will look like this:
Qrop - V-k 0 -C r0 p-C 0 K-exp(-E/RT c) = a (T-To)-S (27)
and for derivatives:
Q r0p -V-k o -C r0p -C 0K -exp(-E/RTc)-E/RT c 2 = a-S (28)
Dividing (27) by (28), we get:
RT c 2 /E = T c - T 0. (29)
Using simple mathematical transformations from this quadratic equation, you can find an expression for Tc, which will look like: T c = To + RT c 2 / E. (30)
From Fig. 2 it is clear that during self-ignition the mixture in the vessel is heated from temperature T 0 to T c. Calculations show that the difference between them is small. For example, for hydrocarbons it is only 30 °C.
This circumstance is used in practice: the auto-ignition temperature is taken to be the lowest temperature of the vessel wall at which auto-ignition occurs.
Since the self-ignition temperature depends on the conditions of its determination (on the material of the vessel, its shape, size, etc.), in order to eliminate this point, in our country and abroad, testing conditions that are identical for all laboratories are established by law, recorded in GOST 12.1.044. It should be noted that this technique is universal and is used to determine the self-ignition temperature of gases, liquids and solid combustible substances. Currently, the auto-ignition temperature has been determined for many substances, and it can be found in reference literature. For alkanes, aromatic hydrocarbons and aliphatic alcohols, it can be approximately calculated from the conditional average length of the compound molecule.
Ø presence of flammable substance,
Ø presence of oxidizing agent
Ø presence of an ignition source.
The combustible substance and oxidizer must be heated to a certain temperature by the ignition source. In a steady combustion process, the constant source of ignition is the combustion zone, i.e. the area where the reaction occurs produces heat and light.
Ignition sources:
Ø open fire,
Ø heat heating elements and devices,
Ø electrical energy,
Ø energy of mechanical sparks,
Ø static electricity and lightning discharges,
Ø energy of processes of self-heating of substances and materials (spontaneous combustion), etc.
Combustion of substances can be complete or incomplete. With complete combustion, products are formed that are not capable of further combustion (CO 2, H 2 O, HCl); when incomplete, the resulting products are capable of further combustion (C, CO, CH, H 2 S, HCN, NH 3); as a rule, the products of incomplete combustion are toxic. A sign of incomplete combustion is the presence of smoke containing unburned carbon particles (soot). Combustion products are gaseous, liquid and solids, formed as a result of the combination of a combustible substance with oxygen during the combustion process. Their composition depends on the composition of the burning substance and its combustion conditions. In fire conditions, organic substances (wood, fabrics, gasoline, plastic, rubber, etc.) most often burn, which consist mainly of carbon, hydrogen, oxygen and nitrogen. Less commonly, inorganic substances such as sulfur, phosphorus, sodium, potassium, aluminum, titanium, magnesium, etc. burn during a fire.
When the concentration of oxygen in the air changes, the intensity of combustion also changes. The combustion of most substances stops when the oxygen content in the air is less than 16%.
When heated, all liquid flammable substances and most solid substances, evaporating or decomposing, turn into gaseous substances, which form flammable mixtures with oxygen or other oxidizing agent. In order for combustion of the gas-air mixture to begin, the presence of an external ignition source is not necessary; an increase in temperature to a certain limit is sufficient.
A fire, in addition to combustion, includes mass and heat transfer phenomena that develop in time and space. These phenomena are interrelated and are characterized by fire parameters: burnout rate, temperature, etc. and are determined by a number of conditions, many of which are random.
The phenomena of mass and heat transfer are called general phenomena , i.e. characteristic of any fire, regardless of its size and location. Only eliminating the fire can lead to their cessation. During a fire, the combustion process is not controlled by humans for a sufficiently long period of time. The consequence of this process is large material losses.
General phenomena can lead to private phenomena , i.e. those that may or may not occur in fires. These include: explosions, deformation and collapse of technological devices and installations, building structures, boiling or release of petroleum products from tanks and other phenomena. The emergence and progression of particular phenomena is possible only when certain conditions favorable for this are created during fires.
A fire is also accompanied by social phenomena that cause not only material damage to society. Death of people, thermal injuries and poisoning by toxic combustion products, panic at sites with large numbers of people, etc. – also phenomena that occur during fires. And they are also private, since they are secondary to the general phenomena accompanying the fire. This is a special group of phenomena that causes significant psychological overload and even stressful conditions in people.
Conditions for combustion:
Ø presence of flammable substance,
Ø presence of oxidizing agent
Ø presence of an ignition source.
The combustible substance and oxidizer must be heated to a certain temperature by the ignition source. In a steady combustion process, the constant source of ignition is the combustion zone, ᴛ.ᴇ. the area where the reaction occurs produces heat and light.
Ignition sources:
Ø open fire,
Ø heat of heating elements and devices,
Ø electrical energy,
Ø energy of mechanical sparks,
Ø static electricity and lightning discharges,
Ø energy of processes of self-heating of substances and materials (spontaneous combustion), etc.
Combustion of substances must be complete and incomplete. With complete combustion, products are formed that are not capable of further combustion (CO 2, H 2 O, HCl); when incomplete, the resulting products are capable of further combustion (C, CO, CH, H 2 S, HCN, NH 3); as a rule, the products of incomplete combustion are toxic. A sign of incomplete combustion is the presence of smoke containing unburned carbon particles (soot). Combustion products are gaseous, liquid and solid substances formed as a result of the combination of a combustible substance with oxygen during the combustion process. Their composition depends on the composition of the burning substance and its combustion conditions. In fire conditions, organic substances (wood, fabrics, gasoline, plastic, rubber, etc.) most often burn, which consist mainly of carbon, hydrogen, oxygen and nitrogen. Less commonly, inorganic substances such as sulfur, phosphorus, sodium, potassium, aluminum, titanium, magnesium, etc. burn during a fire.
When the concentration of oxygen in the air changes, the intensity of combustion also changes. The combustion of most substances stops when the oxygen content in the air is less than 16%.
When heated, all liquid flammable substances and most solid substances, evaporating or decomposing, turn into gaseous substances, which form flammable mixtures with oxygen or other oxidizing agent. In order for combustion of the gas-air mixture to begin, the presence of an external ignition source is not necessary; an increase in temperature to a certain limit is sufficient.
A fire, in addition to combustion, includes mass and heat transfer phenomena that develop in time and space. These phenomena are interrelated and are characterized by fire parameters: burnout rate, temperature, etc. and are determined by a number of conditions, many of which are random.
The phenomena of mass and heat transfer are called general phenomena , ᴛ.ᴇ. characteristic of any fire, regardless of its size and location. Only eliminating the fire can lead to their cessation. During a fire, the combustion process is not controlled by humans for a sufficiently long period of time. The consequence of this process is large material losses.
General phenomena can lead to private phenomena , ᴛ.ᴇ. those that may or may not occur in fires. These include: explosions, deformation and collapse of technological devices and installations, building structures, boiling or release of petroleum products from tanks and other phenomena. The emergence and progression of particular phenomena is possible only when certain conditions favorable for this are created during fires.
A fire is also accompanied by social phenomena that cause not only material damage to society. Death of people, thermal injuries and poisoning by toxic combustion products, panic at sites with large numbers of people, etc. – also phenomena that occur during fires. And they are also private, since they are secondary to the general phenomena accompanying the fire.
Posted on ref.rf
This is a special group of phenomena that causes significant psychological overload and even stressful conditions in people.
Conditions for the occurrence of combustion: - concept and types. Classification and features of the category “Conditions for the occurrence of combustion:” 2014, 2015.
