The table shows the density of methane at different temperatures, including the density of this gas under normal conditions (at 0°C). Its thermophysical properties and characteristics of other methane gases are also given.
The following are presented thermophysical properties of methane gases: coefficient of thermal conductivity λ , η , Prandtl number Pr, kinematic viscosity ν , mass specific heat C p, heat capacity ratio (adiabatic exponent) k, thermal diffusivity coefficient a and density of methane gases ρ . The properties of gases are given at normal atmospheric pressure depending on temperature - in the range from 0 to 600°C.
Methane gases include hydrocarbons with the gross formula C n H 2n+2 such as: methane CH 4, ethane C 2 H 6, butane C 4 H 10, pentane C 5 H 12, hexane C 6 H 14, heptane C 7 H 16, octane C 8 H 18. They are also called the methane homologous series.
The density of methane gases decreases with increasing temperature due to thermal expansion gas This nature of the dependence of density on temperature is also typical. It should also be noted that the density of methane gases increases as the number of carbon and hydrogen atoms in the gas molecule increases (numbers n in the formula C n H 2n+2).
The lightest gas considered in the table is methane - Methane density under normal conditions is 0.7168 kg/m3. Methane expands when heated and becomes less dense. So, for example, at temperatures 0°C and 600°C, the density of methane differs by approximately 3 times.
The thermal conductivity of methane gases decreases with increasing number n in the formula C n H 2n+2. Under normal conditions, it varies in the range from 0.0098 to 0.0307 W/(m deg). According to the data in the table it follows that Gases such as methane have the highest thermal conductivity.— its thermal conductivity coefficient, for example at 0°C, is equal to 0.0307 W/(m deg).
The lowest thermal conductivity (0.0098 W/(m deg) at 0°C) is characteristic of octane gas. It should be noted that when methane gases are heated, their thermal conductivity increases.
The specific mass heat capacity of gases included in the homologous series of methane increases when heated. Their properties such as viscosity and thermal diffusivity also increase in value.
Basic Concepts
- Pressure is the force acting per unit area:
- P=F/S (Newton/m 2 = Kgm/sec 2 m 2 =kg/sec 2 m=Pa), where
- P - pressure (Pa - Pascal),
- F - force, F = ma (Kgm/sec 2, N - Newton),
- S - area (m2).
The technical atmosphere equal to pressure in I kgf/cm 2 is taken as the unit of pressure measurement. The technical atmosphere is measured in atm, or kgf/cm2.
A pressure of I at is capable of balancing a column of water 10 m high, i.e. 10,000 mm, or a column of mercury 735 mm high, since mercury is 13.6 times heavier than water.
I kgf/cm 2 = 10 m water column = 10000 mm water column = 735.6 mm Hg.
- Pressure unit ratio (SI):
- 1 kgf/cm 2 =9.8. 1O 4 Pa = 10 5 Pa = 0.1 mPa
- 1 mm water column = 9.8 Pa = 10 Pa
- 1 mm Hg = 133.3 Pa
- Multiples of units:
- Deca (YES) - 10
- Hecto (G) - 10 2
- Kilo (K) - 10 3
- Mega (M) - 10 6
- Giga (G) - 10 9
- Tera (T) - 10 12
- Submultiple units:
- Deci (D) - 10 -1
- Santi (C) - 10 -2
- Milli (M) - 10 -3
- Micro (MK) - 10 -6
- Nano (N) - 10 -9
- Pico (P) - 10 -12
Pressures can be excessive and absolute. If there is gas in the gas pipeline, then its pressure created inside the pipe will be absolute. From the outside, atmospheric air presses on the walls of the gas pipeline, so the gas pipeline is under the influence of excess pressure, i.e., the difference between internal and external pressures. The amount of excess pressure is measured by pressure gauges, and for absolute pressure it is necessary to excess pressure add atmosphere.
The temperature of gas transported through gas pipelines is measured by thermometers, the scale of which has two constant points, the melting point of ice (0°) and the boiling point of water (100°C). The distance on the scale between these points is divided into 100 equal parts with a division value of 1°C. Temperatures above 0°C are indicated by a “+” sign, and below by a “-” sign.
Another scale is also used - the Kelvin scale. On this scale, point “0” corresponds to absolute zero, i.e., the degree of cooling of the body (body temperature) at which all movement of the molecules of any substance stops. Absolute zero, used as the reference point for temperatures in the SI system, in technical system equal to 273.1b°C (the temperature measured from - 273.16° is called absolute and is designated by the letter T and °K)
T = t 0 C + 273.2 = 100° + 273.2° = 373.2°K at t = 100°C
Measurement of quantity, heat, measured (Cal)
A calorie is the amount of heat that must be imparted to I g. clean water to increase its temperature by 1°, or Kcal is the amount of heat that must be supplied to 1 kg of distilled water to increase its temperature by 1°.
Calorific value gas fuel is the amount of heat that is released during complete combustion of I m of gas. The heat of combustion of gaseous fuel is measured in Kcal per I m 3. For ease of comparison various types fuel, the concept of standard fuel was introduced, the calorific value of which is assumed to be 7000 Kcal.
The value showing how many times the calorific value of a given fuel is greater than the calorific value of the standard fuel is called thermal equivalent. For methane the thermal equivalent will be equal to:
E = 8558/7000 = 1.22 kg, i.e. 1 m3 of methane is equivalent to 1.22 kg of standard fuel.
Specific gravity of combustible gases
The specific gravity of flammable gases is usually called the weight of one cubic meter of gas in kilograms, taken at a temperature of 0° and a pressure of 760 mm Hg. (nm 3 / kg).
Different gaseous fuels have different weights. So, for example, I nm 3 of coke oven gas weighs 0.5 kg, and I nm 3 of generator steam-air gas weighs 1.2 kg. This is explained not only by the fact that various gaseous fuels differ from each other in their composition, but also by the different weights of their constituent gases. Hydrogen is the lightest gas, nitrogen is 7 times heavier, oxygen and methane are 8 times heavier, carbon monoxide is 14 times heavier, carbon dioxide 22 times, some heavy hydrocarbons 29 times. Almost all gaseous fuels are lighter than air, 1 nm 3 of which weighs 1.29 kg. It follows that in the room into which he entered flammable gas, it will tend upward, because the density will be less than the density of air.
The specific gravity of the gas indicated above is called the absolute specific gravity, in contrast to the relative specific gravity of the gas, which expresses the weight of 1 nm of gas compared to the weight of 1 nm of air. To determine the relative specific gravity of a gas, its absolute specific gravity must be divided by the specific gravity of air. For example, the relative share of Stavropol natural gas will be equal to: 0.8/1.29 = 0.62.
In order to promptly detect a gas leak, it is subjected to odorization, i.e., it is given a sharp, specific odor. Ethyl mercaptan is used as an odorant; the odor should be felt when the gas content in the air is no more than 1/5 of the lower flammability limit. In practice, natural gas, which has a lower explosive limit of 5%, should be felt in indoor air at a concentration of 1%.
Unfortunately, if there is a gas leak from underground gas pipeline, odorized gas is filtered when passing through the ground, i.e., it loses its odorant and its smell may not be noticeable in a gas-filled room. Therefore, gas leaks from an underground gas pipeline are very dangerous and require service personnel increased attention.
Combustible gas composition
The composition of any gaseous fuel includes combustible and non-combustible parts. The larger the combustible part, the higher the calorific value of the fuel.
Combustible components include:
Carbon monoxide (CO). Colorless gas, odorless and tasteless; mass 1 Nm 3 is 1.25 kg; calorific value Q = = 2413 kcal/kg.
Stay in a room whose air contains 0.5% CO for 5 minutes. life threatening. The maximum permissible concentration (MPC) when using gas in everyday life is 2 mg/m3.
Hydrogen (H2) is a colorless, non-toxic gas. The mass of 1 Nm 3 is equal to 0.09 kg, it is 14.5 times lighter than air. Calorific value Q = 33860 kcal/kg. It is highly reactive, has wide flammability limits, and is highly explosive.
Methane (CH 4) is a colorless, non-toxic gas, odorless and tasteless. The composition includes 75% carbon and 25% hydrogen. 1 Nm 3 weighs 0.717 kg. Calorific value Q = 13200 kcal/kg. Explosive, explosive limits 5–15.
Nitrogen (N 2) is the non-combustible part of gaseous fuel, colorless, odorless and tasteless, does not react with oxygen, it is considered an inert gas.
Carbon dioxide (C0 2) is colorless, heavy, low-reactive, has a slightly sour smell and taste, the mass of 1 Nm 3 is 1.98 kg. At concentrations up to 10% in the air it causes severe poisoning.
Oxygen (0 2) - odorless, color and taste, mass of 1 Nm 3 is 1.43 kg. The oxygen content in the gas reduces its calorific value and makes the gas explosive; according to GOST, it should not exceed no more than 1% by volume in the gas.
Hydrogen sulfide (H 2 S) is a heavy gas with strong unpleasant smell, 1 Nm 3 is 1.54 kg, strongly corrodes gas pipelines, when burned it forms sulfur dioxide (SO 2) harmful to health, the hydrogen sulfide content should not exceed 2 g per 100 m 3 of gas; Harmful impurities include hydrocyanic acid NS, the content of which should not exceed 5 g per 100 m 3 of gas.
Gas humidity - according to the current GOST, the moisture saturation of gas when entering city gas pipelines d.6. no more than maximum gas saturation at a temperature of 20°C in winter and 35°C in summer (the higher the gas temperature, the more moisture contained in a unit volume of gas).
Composition and calorie content of real network gas in Moscow
Table No. 1
|
Sampling address from gas station. |
Carbon dioxide (C0 2) |
Oxygen (0 2) |
Methane (CH 4) |
Ethane (C 2 H 6) |
Propane (C 3 H 8) |
calorie content |
|
|
Karacharovskaya |
|||||||
|
Ochakovskaya |
|||||||
|
Golovinskaya |
|||||||
Features of the physico-chemical properties of liquid (liquefied) gas
It is known that all substances (bodies) consist of individual particles (molecules) placed in a certain order. The closer these molecules are located to each other and the greater their interaction with each other, the closer the body is in its state to a solid. Therefore, a state of matter is called solid when the distances between its molecules are negligible and the interaction forces are enormous. Characteristic feature solids is that they have their own shape and volume. Solid fuels found in nature are, for example: wood, coal, shale. The liquid state of a substance is characterized by the fact that the distance between the molecules in it is relatively small and the forces of their interaction are small. A peculiarity of liquid bodies is their lack of their own volume and shape. All liquids take on the shape of the container in which they are placed. Liquid fuels are gasoline, kerosene, liquid (liquefied) gas, etc.
Gaseous (vapor) is a state of matter when the distances between the molecules in it are enormous and the forces of their interaction are negligible. Gases, like liquids, do not have their own volume and shape. Among a wide variety of solid, liquid and gaseous fuels special place occupies liquid gas.
Liquid is a gas that, at normal temperature (+20°C) and atmospheric pressure (760 mmHg), is in a gaseous state, having the ability to turn into liquid with a slight increase in pressure and, conversely, to quickly evaporate when the pressure decreases. Liquid gases used in everyday life should be understood as a mixture of propane and butane with a small content of ethane, pentane, butylene and some other gases.
The main raw materials for producing liquid gas are oil, natural gases and coal.
When using liquid gas in everyday life, you have to deal with its liquid and gaseous phases. The specific gravity of the liquid phase is determined in relation to specific gravity water, equal to one, and varies depending on the composition of the gas from 0.495 to 0.570 kg/l. The specific gravity of the gaseous (vapor) phase is taken in relation to the specific gravity of air taken equal to one, and depending on the composition of the gas, ranges from 1.9 to 2.6 kg/m 3, i.e., liquid gas vapor used in household gas appliances is approximately twice as heavy as air.
Physicochemical characteristics basic: liquid and hydrocarbon gases
Table No. 2
|
The name of indicators |
Propylene |
|||||
|
Chemical formula |
||||||
|
Specific gravity of gas_at 760 mm Hg. and 0°C, kg/m 3 |
||||||
|
Specific volume of gas at 760 mm Hg. and 0°C, M 3 /KG |
||||||
|
Ratio of gas volume to liquid volume |
||||||
|
Calorific value kcal; lowest/highest |
22359 |
29510 32010 |
I 5370 |
14320 15290 |
21070 22540 |
10831 |
|
Explosion limits of a mixture of gas vapors and air % lower/upper |
Note:
Knowing the ratio of the volume of gas to the volume of liquid (Table 2, item 4), you can determine the volume of evaporated gas (m 3) of a container filled with liquid gas.
Pressure and vapor pressure of liquid gas
It is known that there is always water vapor above the surface of various bodies of water (rivers, lakes, seas, etc.). The higher the air temperature surrounding water bodies, the more vapors there are above their surface. The same phenomenon is observed if kerosene, gasoline or liquid gas is placed in any vessel - liquid vapors will always be above its surface, and the higher the temperature, the greater the number of them.
and the larger the surface (mirror) of liquid evaporation. Naturally, if you place liquid gas in a vessel and close it, the vapors of this gas will begin to exert a certain pressure on the walls of the vessel.
The excess pressure that can create liquid gas vapor in a closed vessel is called the vapor pressure of this gas.
Approximate values of the vapor pressure of some hydrocarbon gases in absolute atmospheres, depending on temperature.
Table No. 3
|
Temperature, °C |
Propylene |
|||||
From Table 3 it can be seen that the main gases that make up liquid gas used in everyday life - propane and butane - have a sharp excellent elasticity vapors even at the same temperature. Therefore, in the cold season (winter), the gas with the highest vapor pressure is used, namely gas containing 70–85% propane. The use of gas with low vapor pressure at this time of year, i.e. with a high butane content, can cause interruption in operation gas appliances, due to its poor volatility.
- Note:
- The presence of ethane and ethylene in liquid gases is undesirable, since they have high vapor elasticity and lead to excessive pressure in cylinders and other containers.
- Liquid gas has a high coefficient of volumetric expansion. This means that with increasing temperature, its volume in the vessel increases, and therefore the containers for transport and storage are filled no more than 84–90%, otherwise, when the temperature rises, rupture of these vessels may occur.
- (When overfilled cylinders were stored, there were cases of their rupture, which caused major accidents with human casualties).
- Liquid gas vapor mixed with air in the zone between the upper and lower explosive limits forms explosive explosive mixtures (Table 2).
Gas combustion and gas burners
The occurrence of combustion and its progression are possible only under certain conditions. Supplying flammable gas to the combustion site, thoroughly mixing it with required quantity air, as well as achieving a certain temperature level. For normal combustion you need 1 part gas to 10 parts air. As a result of the combustion of 1 m 3 of methane, 1 m 3 of carbon dioxide, 2 m 3 of water vapor and 7.52 m 3 of nitrogen are obtained. The more C0 o in the combustion products, the less carbon monoxide CO they contain, i.e., the more complete the combustion and the less unburned hydrogen (Hg). (CO + H^. - the most favorable combustion. When the needle is at zero. Gas combustion is accompanied by a flame, i.e., the zone in which combustion reactions occur. There are two types of flame propagation: slow and detonation. Slow is called normal - the normal speed of flame propagation The magnitude of the flame propagation speed has a very important For proper organization gas combustion process.
If the speed of flame propagation of the gas-air mixture leaving the burner is less than the speed of movement of this mixture, then flame separation will occur.
Flame breakthrough occurs if the speed of flame propagation is greater than the speed of movement of the gas-air mixture. A breakthrough may be accompanied by combustion of gas inside the burner itself.
Detonation (explosion) is a type of flame propagation in which the propagation speed is the highest - several thousand meters per second. During detonation, the highest explosive pressures arise (20 atm and above), leading to severe destruction.
Gas combustion methods
Gas can be burned with luminous and non-luminous flames, as well as flameless combustion. The methods of gas combustion depend on the method of mixing gas with air due to the ability of gas and air particles to penetrate each other. This phenomenon is called diffusion, and burners working on this principle are called diffusion - luminous flame.
Diffusion-kinetic combustion - non-luminous flame - injection with primary and secondary air intake from the environment.
Kinetic combustion (almost no flame) - preliminary 100% mixing of gas with air, combustion surrounded by hot refractories and is called flameless combustion of gas.
Natural gas has no color, smell or taste.
Main indicators of combustible gases used in boiler houses: composition, calorific value, specific gravity, combustion and ignition temperatures, explosive limits and flame propagation speed.
Natural gases from purely gas fields consist mainly of methane (82-98%) and other hydrocarbons.
The composition of any gaseous fuel includes flammable and non-flammable substances. Combustibles include: hydrogen (H2), hydrocarbons (CnHm), hydrogen sulfide (H2S), carbon monoxide (CO); non-flammable - carbon dioxide (CO2), oxygen (02), nitrogen (N2) and water vapor (H20). Natural and fuel gases have different hydrocarbon compositions.
Heat of combustion- this is the amount of heat that is released during the complete combustion of 1 m3 of gas. Measured in kcal/m3, kJ/m3 of gas. In practice, gases with different calorific values are used. Fuel gas has a higher calorific value than natural gas.
Specific gravity gaseous substance - this is a quantity that is determined by the ratio of the mass of a substance to the volume occupied by it. The basic unit of measurement for specific gravity is kg/m3. The ratio of the specific gravity of a gaseous substance to the specific gravity of air under the same conditions (pressure and temperature) is called relative density. Natural gas is lighter than air, while fuel gas is heavier. The density of natural gas (methane) under normal conditions is 0.73 kg/m3, and the density of air is 1.293 kg/m3.
Combustion temperature is the maximum temperature that can be achieved by complete combustion of a gas if the amount of air required for combustion is exactly the same chemical formulas combustion, and the initial temperature of gas and air is 0. The combustion temperature of individual gases is 2000 - 2100°C. The actual combustion temperature in boiler furnaces is lower than the heat productivity (1100-1400°C) and depends on the combustion conditions.
Flash point is the minimum initial temperature at which combustion begins. For natural gas it is 645°C.
Explosive limits.
Gas-air mixture in which the gas is:
up to 5% - off;
From 5 to 15% - explodes;
More than 15% - burns when air is supplied.
Flame propagation speed for natural gas - 0.67 m/sec (methane CH4).
Combustible gases are odorless. To timely determine their presence in the air and quickly and accurately detect leaks, the gas is odorized (it gives off an odor). Ethyl mercaptan is used for odorization. The odorization rate is 16g per 1000 m3 of gas. Odorization is carried out at gas distribution stations (GDS). If there is 1% natural gas in the air, you should smell it.
The use of natural gas has a number of advantages compared to solid and liquid fuel:
No ash or release of solid particles into the atmosphere;
High calorific value;
Ease of transportation and combustion;
The work of service personnel is made easier;
Sanitary and hygienic conditions in the boiler room and in the surrounding areas are improved;
Various possibilities for automating work processes are emerging.
However, the use of natural gas requires special measures caution, because it may leak through leaks at the junction of the gas pipeline and equipment with fittings.
The presence of more than 20% of the gas in a room causes suffocation; its accumulation in a closed volume of 5 to 15% can lead to an explosion of the gas-air mixture; if combustion is incomplete, it is released carbon monoxide CO, which even at low concentrations (0.15%) is poisonous.
Gas burning
Combustion is a reaction in which the chemical energy of a fuel is converted into heat. Combustion can be complete or incomplete. Complete combustion occurs when there is sufficient oxygen. Its lack causes incomplete combustion, which releases less heat than complete combustion and carbon monoxide (CO),
It is necessary to ensure that the excess air ratio is not less than 1, as this leads to incomplete combustion of the gas. An increase in the excess air ratio reduces the efficiency of the boiler unit. The completeness of fuel combustion can be determined using a gas analyzer and visually - by the color and nature of the flame.
The combustion process of gaseous fuels can be divided into four main stages:
1) gas flowing out of the burner nozzle into the burner device under pressure at an increased speed (compared to the speed in the gas pipeline);
2) formation of a mixture of gas and air;
3) ignition of the formed combustible mixture;
4) combustion of a flammable mixture.
