What is fuel?
This is one component or a mixture of substances that are capable of chemical transformations associated with the release of heat. Different types fuels differ in the quantitative content of the oxidizing agent in them, which is used to release thermal energy.
AT broad sense fuel is an energy carrier, that is, a potential type of potential energy.
Classification
Currently, fuels are divided according to their state of aggregation into liquid, solid, gaseous.
To hard natural look include stone and firewood, anthracite. Briquettes, coke, thermoanthracite are varieties of artificial solid fuel.
Liquids are substances that contain substances organic origin. Their main components are: oxygen, carbon, nitrogen, hydrogen, sulfur. artificial liquid fuel there will be a variety of resins, fuel oil.
It is a mixture of various gases: ethylene, methane, propane, butane. In addition to them, gaseous fuels contain carbon dioxide and carbon monoxide s, hydrogen sulfide, nitrogen, water vapor, oxygen.
Fuel indicators
The main indicator of combustion. The formula for determining the calorific value is considered in thermochemistry. emit "reference fuel", which implies the calorific value of 1 kilogram of anthracite.
Domestic heating oil is intended for combustion in heating devices of low power, which are located in residential premises, heat generators used in agriculture for drying fodder, canning.
The specific heat of combustion of fuel is such a value that it demonstrates the amount of heat that is formed during the complete combustion of fuel with a volume of 1 m 3 or a mass of one kilogram.
To measure this value, J / kg, J / m 3, calorie / m 3 are used. To determine the heat of combustion, use the calorimetry method.
With an increase in the specific heat of combustion of fuel, the specific fuel consumption decreases, and the coefficient useful action remains the same value.
The heat of combustion of substances is the amount of energy released during the oxidation of a solid, liquid, gaseous substance.
It is determined by the chemical composition, as well as state of aggregation combustible substance.
Features of combustion products
The higher and lower calorific value is associated with the state of aggregation of water in the substances obtained after the combustion of fuel.
The gross calorific value is the amount of heat released during the complete combustion of a substance. This value includes the heat of condensation of water vapor.
The lower working calorific value is the value that corresponds to the release of heat during combustion without taking into account the heat of condensation of water vapor.
The latent heat of condensation is the value of the energy of condensation of water vapor.
Mathematical relationship
The higher and lower calorific value are related by the following relationship:
Q B = Q H + k(W + 9H)
where W is the amount by weight (in %) of water in the combustible substance;
H is the amount of hydrogen (% by mass) in the combustible substance;
k - coefficient of 6 kcal/kg
Calculation methods
The higher and lower calorific value is determined by two main methods: calculated and experimental.
Calorimeters are used for experimental calculations. First, a sample of fuel is burned in it. The heat that will be released in this case is completely absorbed by the water. Having an idea about the mass of water, it is possible to determine the value of its heat of combustion by changing its temperature.
This technique is considered simple and effective, it assumes only the knowledge of technical analysis data.
In the calculation method, the highest and lowest calorific value is calculated according to the Mendeleev formula.
Q p H \u003d 339C p + 1030H p -109 (O p -S p) - 25 W p (kJ / kg)
It takes into account the content of carbon, oxygen, hydrogen, water vapor, sulfur in the working composition (in percent). The amount of heat during combustion is determined taking into account the reference fuel.
The heat of combustion of gas allows you to carry out preliminary calculations, to identify the effectiveness of the application a certain kind fuel.
Features of origin
In order to understand how much heat is released during the combustion of a certain fuel, it is necessary to have an idea of its origin.
In nature there is different variants solid fuels, which differ in composition and properties.
Its formation is carried out through several stages. First, peat is formed, then brown and hard coal is obtained, then anthracite is formed. The main sources of solid fuel formation are leaves, wood, and needles. Dying, parts of plants, when exposed to air, are destroyed by fungi, forming peat. Its accumulation turns into a brown mass, then brown gas is obtained.
At high pressure and temperature, brown gas turns into coal, then the fuel accumulates in the form of anthracite.
Apart from organic matter, there is additional ballast in the fuel. An organic part is that part that was formed from organic matter: hydrogen, carbon, nitrogen, oxygen. In addition to these chemical elements, it contains ballast: moisture, ash.
Furnace technology involves the allocation of working, dry, as well as combustible mass of burned fuel. The working mass is called the fuel in its original form, supplied to the consumer. Dry weight is a composition in which there is no water.
Compound
The most valuable components are carbon and hydrogen.
These elements are found in any type of fuel. In peat and wood, the percentage of carbon reaches 58 percent, in black and brown coal - 80%, and in anthracite it reaches 95 percent by weight. Depending on this indicator, the amount of heat released during the combustion of fuel changes. Hydrogen is the second most important element of any fuel. Contacting with oxygen, it forms moisture, which significantly reduces the thermal value of any fuel.
Its percentage ranges from 3.8 in oil shale to 11 in fuel oil. Oxygen, which is part of the fuel, acts as ballast.
It is not a heat-generating chemical element, therefore it negatively affects the value of its combustion heat. Combustion of nitrogen contained in free or bound form in combustion products, is considered harmful impurities, so its quantity is strictly limited.
Sulfur is included in the composition of the fuel in the form of sulfates, sulfides, and also as sulfur dioxide gases. When hydrated, sulfur oxides form sulfuric acid, which destroys boiler equipment, negatively affects vegetation and living organisms.
That is why sulfur is the chemical element, the presence of which in natural fuel is highly undesirable. When getting inside the working room, sulfur compounds cause significant poisoning of the operating personnel.
There are three types of ash depending on its origin:
- primary;
- secondary;
- tertiary.
The primary view is formed from minerals that are found in plants. Secondary ash is formed as a result of ingestion of plant residues by sand and earth during formation formation.
Tertiary ash turns out to be part of the fuel in the process of extraction, storage, and also its transportation. With a significant deposition of ash, there is a decrease in heat transfer on the heating surface of the boiler unit, reduces the amount of heat transfer to water from gases. Great amount ash negatively affects the operation of the boiler.
Finally
Volatile substances have a significant impact on the combustion process of any type of fuel. The larger their output, the larger the volume of the flame front will be. For example, coal, peat, easily catch fire, the process is accompanied by insignificant heat losses. The coke that remains after the removal of volatile impurities contains only mineral and carbon compounds. Depending on the characteristics of the fuel, the amount of heat varies significantly.
Depending on the chemical composition There are three stages in the formation of solid fuels: peat, lignite, coal.
Natural wood is used in small boiler plants. Mostly wood chips, sawdust, slabs, bark are used, firewood itself is used in small quantities. Depending on the type of wood, the amount of heat released varies significantly.
As the calorific value decreases, firewood acquires certain advantages: rapid flammability, minimal ash content, and the absence of traces of sulfur.
Reliable information about the composition of natural or synthetic fuel, its calorific value, is great way carrying out thermochemical calculations.
At present, there is a real opportunity to identify those main options for solid, gaseous, liquid fuels that will be the most efficient and inexpensive to use in a particular situation.
Gas fuel is divided into natural and artificial and is a mixture of combustible and non-combustible gases containing a certain amount of water vapor, and sometimes dust and tar. Quantity gas fuel expressed in cubic meters normal conditions(760 mm Hg and 0 ° C), and the composition is in percent by volume. Under the composition of the fuel understand the composition of its dry gaseous part.
natural gas fuel
The most common gas fuel is natural gas, which has a high calorific value. The basis of natural gas is methane, the content of which is 76.7-98%. Other gaseous hydrocarbon compounds are part of natural gas from 0.1 to 4.5%.
Liquefied gas is a product of oil refining - it consists mainly of a mixture of propane and butane.
Natural gas (CNG, NG): methane CH4 more than 90%, ethane C2 H5 less than 4%, propane C3 H8 less than 1%
Liquefied gas (LPG): propane C3 H8 more than 65%, butane C4 H10 less than 35%
Combustible gases include: hydrogen H 2, methane CH 4, Other hydrocarbon compounds C m H n, hydrogen sulfide H 2 S and non-combustible gases, carbon dioxide CO2, oxygen O 2, nitrogen N 2 and a small amount of water vapor H 2 O. Indices m and P at C and H characterize compounds of various hydrocarbons, for example, for methane CH 4 t = 1 and n= 4, for ethane С 2 Н b t = 2 and n= b etc.
Composition of dry gaseous fuel (in percent by volume):
CO + H 2 + 2 C m H n + H 2 S + CO 2 + O 2 + N 2 = 100%.
The non-combustible part of dry gaseous fuel - ballast - is nitrogen N and carbon dioxide CO 2 .
The composition of the wet gaseous fuel is expressed as follows:
CO + H 2 + Σ C m H n + H 2 S + CO 2 + O 2 + N 2 + H 2 O \u003d 100%.
The heat of combustion, kJ / m (kcal / m 3), 1 m 3 of pure dry gas under normal conditions is determined as follows:
Q n s \u003d 0.01,
where Qco, Q n 2 , Q with m n n Q n 2 s. - heat of combustion of individual gases that make up the mixture, kJ / m 3 (kcal / m 3); CO, H 2, Cm H n , H 2 S - components that make up gas mixture, % by volume.
The heat of combustion of 1 m3 of dry natural gas under normal conditions for most domestic fields is 33.29 - 35.87 MJ / m3 (7946 - 8560 kcal / m3). Characteristics of gaseous fuel is given in table 1.
Example. Determine the net calorific value of natural gas (under normal conditions) of the following composition:
H 2 S = 1%; CH 4 = 76.7%; C 2 H 6 = 4.5%; C 3 H 8 = 1.7%; C 4 H 10 = 0.8%; C 5 H 12 = 0.6%.
Substituting into formula (26) the characteristics of gases from Table 1, we obtain:
Q ns \u003d 0.01 \u003d 33981 kJ / m 3 or
Q ns \u003d 0.01 (5585.1 + 8555 76.7 + 15 226 4.5 + 21 795 1.7 + 28 338 0.8 + 34 890 0.6) \u003d 8109 kcal / m 3.
Table 1. Characteristics of gaseous fuel
|
Gas |
Designation |
Heat of combustion Q n s |
|
|
KJ/m3 |
kcal/m3 |
||
| Hydrogen | H, | 10820 | 2579 |
| carbon monoxide | SO | 12640 | 3018 |
| hydrogen sulfide | H 2 S | 23450 | 5585 |
| Methane | CH 4 | 35850 | 8555 |
| Ethane | C 2 H 6 | 63 850 | 15226 |
| Propane | C 3 H 8 | 91300 | 21795 |
| Butane | C 4 H 10 | 118700 | 22338 |
| Pentane | C 5 H 12 | 146200 | 34890 |
| Ethylene | C 2 H 4 | 59200 | 14107 |
| Propylene | C 3 H 6 | 85980 | 20541 |
| Butylene | C 4 H 8 | 113 400 | 27111 |
| Benzene | C 6 H 6 | 140400 | 33528 |
Boilers of the DE type consume from 71 to 75 m3 of natural gas to produce one ton of steam. The cost of gas in Russia in September 2008 is 2.44 rubles per cubic meter. Consequently, a ton of steam will cost 71 × 2.44 = 173 rubles 24 kopecks. The real cost of a ton of steam at factories is for DE boilers at least 189 rubles per ton of steam.
Boilers of the DKVR type consume from 103 to 118 m3 of natural gas to produce one ton of steam. The minimum estimated cost of a ton of steam for these boilers is 103 × 2.44 = 251 rubles 32 kopecks. The real cost of steam for plants is at least 290 rubles per ton.
How to calculate the maximum consumption of natural gas for a steam boiler DE-25? it technical specifications boiler. 1840 cubes per hour. But you can also calculate. 25 tons (25 thousand kg) must be multiplied by the difference between the enthalpies of steam and water (666.9-105) and all this divided by the boiler efficiency of 92.8% and the heat of combustion of gas. 8300. and all
Artificial gas fuel
Artificial combustible gases are fuel local importance because they have a much lower calorific value. Their main combustible elements are carbon monoxide CO and hydrogen H2. These gases are used within the limits of the production where they are obtained as fuel for technological and power plants.
All natural and artificial combustible gases are explosive, capable of igniting on an open flame or spark. There are lower and upper explosive limits of gas, i.e. the highest and lowest percentage concentrations in the air. Lower explosive limit natural gases ranges from 3% to 6%, and the top - from 12% to 16%. All combustible gases can cause poisoning of the human body. The main toxic substances of combustible gases are: carbon monoxide CO, hydrogen sulfide H2S, ammonia NH3.
Both natural combustible gases and artificial ones are colorless (invisible), odorless, which makes them dangerous when they penetrate into interior boiler room through leaks in gas pipeline fittings. To avoid poisoning, combustible gases should be treated with an odorant - a substance with an unpleasant odor.
Obtaining carbon monoxide CO in industry by gasification of solid fuel
For industrial purposes, carbon monoxide is obtained by gasification of solid fuel, i.e., its transformation into gaseous fuel. So you can get carbon monoxide from any solid fuel - fossil coal, peat, firewood, etc.
The process of gasification of solid fuel is shown in a laboratory experiment (Fig. 1). Having filled the refractory tube with pieces of charcoal, we heat it up strongly and let oxygen pass through the gasometer. Let the gases coming out of the tube pass through a lime water washer and then set it on fire. Lime water becomes cloudy, the gas burns with a bluish flame. This indicates the presence of CO2 dioxide and carbon monoxide CO in the reaction products.
The formation of these substances can be explained by the fact that when oxygen comes into contact with hot coal, the latter is first oxidized into carbon dioxide: C + O 2 \u003d CO 2
Then, passing through hot coal, carbon dioxide is partially reduced by it to carbon monoxide: CO 2 + C \u003d 2CO
Rice. 1. Obtaining carbon monoxide (laboratory experience).
Under industrial conditions, gasification of solid fuels is carried out in furnaces called gas generators.
The resulting mixture of gases is called producer gas.
The gas generator device is shown in the figure. It is a steel cylinder with a height of about 5 m and a diameter of approximately 3.5 m, lined inside with refractory bricks. From above, the gas generator is loaded with fuel; From below, air or water vapor is supplied by a fan through the grate.
Oxygen in the air reacts with the carbon of the fuel, forming carbon dioxide, which, rising up through a layer of hot fuel, is reduced by carbon to carbon monoxide.
If only air is blown into the generator, then a gas is obtained, which in its composition contains carbon monoxide and nitrogen of the air (as well as a certain amount of CO 2 and other impurities). This generator gas is called air gas.
If, however, water vapor is blown into the generator with hot coal, then carbon monoxide and hydrogen are formed as a result of the reaction: C + H 2 O \u003d CO + H 2
This mixture of gases is called water gas. Water gas has a higher calorific value than air gas, since, along with carbon monoxide, it also contains a second combustible gas - hydrogen. Water gas (synthesis gas), one of the products of gasification of fuels. Water gas consists mainly of CO (40%) and H2 (50%). Water gas is a fuel (calorific value 10,500 kJ/m3, or 2730 kcal/mg) and at the same time raw material for the synthesis of methanol. Water gas, however, cannot be obtained long time, since the reaction of its formation is endothermic (with the absorption of heat), and therefore the fuel in the generator cools down. In order to keep the coal in a hot state, the injection of water vapor into the generator is alternated with the injection of air, the oxygen of which, as is known, reacts with the fuel with the release of heat.
AT recent times steam-oxygen blast began to be widely used for fuel gasification. Simultaneous blowing of water vapor and oxygen through the fuel layer makes it possible to carry out the process continuously, significantly increase the generator productivity and obtain gas with a high content of hydrogen and carbon monoxide.
Modern gas generators are powerful devices of continuous action.
So that when fuel is supplied to the gas generator, combustible and toxic gases do not penetrate into the atmosphere, the loading drum is made double. While fuel enters one compartment of the drum, fuel is poured out of the other compartment into the generator; when the drum rotates, these processes are repeated, while the generator remains isolated from the atmosphere all the time. Uniform distribution fuel in the generator is carried out using a cone, which can be installed at different heights. When it is lowered, the coal lies closer to the center of the generator; when the cone is raised, the coal is thrown closer to the walls of the generator.
Removal of ash from the gas generator is mechanized. The cone-shaped grate is slowly rotated by an electric motor. In this case, the ash is displaced to the walls of the generator and is thrown into the ash box with special devices, from where it is periodically removed.
The first gas lamps were lit in St. Petersburg on Aptekarsky Island in 1819. The gas that was used was obtained by gasification hard coal. It was called light gas.
The great Russian scientist D. I. Mendeleev (1834-1907) was the first to express the idea that the gasification of coal can be carried out directly underground, without lifting it out. The tsarist government did not appreciate Mendeleev's proposal.
The idea of underground gasification was warmly supported by V. I. Lenin. He called it "one of the great triumphs of technology." Underground gasification was carried out for the first time Soviet state. Already before the Great Patriotic War, underground generators were operating in the Donetsk and Moscow region coal basins in the Soviet Union.
Figure 3 gives an idea of one of the methods of underground gasification. Two wells are laid in the coal seam, which are connected at the bottom by a channel. Coal is set on fire in such a channel near one of the wells and blast is supplied there. Combustion products, moving along the channel, interact with hot coal, resulting in the formation of combustible gas, as in a conventional generator. The gas comes to the surface through the second well.
Generator gas is widely used for heating industrial furnaces - metallurgical, coke and as a fuel in cars (Fig. 4).
Rice. 3. Scheme of underground gasification of coal.
A number of organic products, such as liquid fuels, are synthesized from hydrogen and carbon monoxide of water gas. Synthetic liquid fuel - fuel (mainly gasoline) obtained by synthesis from carbon monoxide and hydrogen at 150-170 degrees Celsius and a pressure of 0.7 - 20 MN / m2 (200 kgf / cm2), in the presence of a catalyst (nickel, iron, cobalt ). The first production of synthetic liquid fuels was organized in Germany during the 2nd World War due to the shortage of oil. Synthetic liquid fuels have not received wide distribution due to their high cost. Water gas is used to produce hydrogen. To do this, water gas in a mixture with water vapor is heated in the presence of a catalyst and as a result, hydrogen is obtained in addition to that already present in water gas: CO + H 2 O \u003d CO 2 + H 2
The heat of combustion is determined by the chemical composition of the combustible substance. The chemical elements contained in the combustible substance are designated by the accepted symbols FROM , H , O , N , S, and ash and water are symbols BUT and W respectively.
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The heat of combustion can be related to the working mass of the combustible Q P (\displaystyle Q^(P)), that is, to a combustible substance in the form in which it enters the consumer; to dry matter Q C (\displaystyle Q^(C)); to the combustible mass of matter Q Γ (\displaystyle Q^(\Gamma )), that is, to a combustible substance that does not contain moisture and ash.
Distinguish higher ( Q B (\displaystyle Q_(B))) and lower ( Q H (\displaystyle Q_(H))) heat of combustion.
Under higher calorific value understand the amount of heat that is released during the complete combustion of a substance, including the heat of condensation of water vapor during cooling of the combustion products.
Net calorific value corresponds to the amount of heat that is released during complete combustion, without taking into account the heat of condensation of water vapor. The heat of condensation of water vapor is also called latent heat of vaporization (condensation).
The lower and higher calorific value are related by the ratio: Q B = Q H + k (W + 9 H) (\displaystyle Q_(B)=Q_(H)+k(W+9H)),
where k is a coefficient equal to 25 kJ/kg (6 kcal/kg); W - the amount of water in the combustible substance,% (by weight); H is the amount of hydrogen in the combustible substance, % (by mass).
Calculation of heat of combustion
Thus, the higher calorific value is the amount of heat released during the complete combustion of a unit mass or volume (for gas) of a combustible substance and cooling the combustion products to the dew point temperature. In heat engineering calculations, the gross calorific value is taken as 100%. Latent heat gas combustion is the heat that is released during the condensation of water vapor contained in the combustion products. Theoretically, it can reach 11%.
In practice, it is not possible to cool the combustion products to complete condensation, and therefore the concept of net calorific value (QHp) is introduced, which is obtained by subtracting from the higher calorific value the heat of vaporization of water vapor both contained in the substance and formed during its combustion. 2514 kJ/kg (600 kcal/kg) is spent on vaporization of 1 kg of water vapor. The net calorific value is determined by the formulas (kJ / kg or kcal / kg):
Q H P = Q B P − 2514 ⋅ ((9 H P + W P) / 100) (\displaystyle Q_(H)^(P)=Q_(B)^(P)-2514\cdot ((9H^(P)+W^ (P))/100))(for solid)
Q H P = Q B P − 600 ⋅ ((9 H P + W P) / 100) (\displaystyle Q_(H)^(P)=Q_(B)^(P)-600\cdot ((9H^(P)+W^ (P))/100))(for liquid substance), where:
2514 - heat of vaporization at 0 °C and atmospheric pressure, kJ/kg;
H P (\displaystyle H^(P)) and W P (\displaystyle W^(P))- the content of hydrogen and water vapor in the working fuel,%;
9 is a coefficient showing that when 1 kg of hydrogen is burned in combination with oxygen, 9 kg of water is formed.
The heat of combustion is the most important characteristic fuel, since it determines the amount of heat obtained by burning 1 kg of solid or liquid fuel or 1 m³ of gaseous fuel in kJ / kg (kcal / kg). 1 kcal = 4.1868 or 4.19 kJ.
The net calorific value is determined experimentally for each substance and is a reference value. It can also be determined for solid and liquid materials, with a known elemental composition, by calculation in accordance with the formula of D. I. Mendeleev, kJ / kg or kcal / kg:
Q H P = 339 ⋅ C P + 1256 ⋅ H P − 109 ⋅ (O P − S L P) − 25.14 ⋅ (9 ⋅ H P + W P) (\displaystyle Q_(H)^(P)=339\cdot C^(P)+1256\ cdot H^(P)-109\cdot (O^(P)-S_(L)^(P))-25.14\cdot (9\cdot H^(P)+W^(P)))
Q H P = 81 ⋅ C P + 246 ⋅ H P − 26 ⋅ (O P + S L P) − 6 ⋅ W P (\displaystyle Q_(H)^(P)=81\cdot C^(P)+246\cdot H^(P) -26\cdot (O^(P)+S_(L)^(P))-6\cdot W^(P)), where:
C P (\displaystyle C_(P)), H P (\displaystyle H_(P)), O P (\displaystyle O_(P)), S L P (\displaystyle S_(L)^(P)), W P (\displaystyle W_(P))- the content of carbon, hydrogen, oxygen, volatile sulfur and moisture in the working mass of fuel in% (by mass).
For comparative calculations, the so-called Conventional Fuel is used, which has a specific heat of combustion equal to 29308 kJ/kg (7000 kcal/kg).
In Russia thermal calculations(for example, calculating the heat load to determine the category of a room for explosion and fire hazard) is usually carried out according to the lowest calorific value, in the USA, Great Britain, France - according to the highest. In the UK and USA before the introduction of the metric system of measures specific heat combustion was measured in British thermal units (BTU) per pound (lb) (1Btu/lb = 2.326 kJ/kg).
Substances and materials Net calorific value Q H P (\displaystyle Q_(H)^(P)), MJ/kg Petrol 41,87 Kerosene 43,54 Paper: books, magazines 13,4 Wood (bars W = 14%) 13,8 Natural rubber 44,73 Polyvinyl chloride linoleum 14,31 Rubber 33,52 Staple fiber 13,8 Polyethylene 47,14 Styrofoam 41,6 Cotton loosened 15,7 Plastic 41,87 Substances of organic origin include fuel, which, when burned, releases a certain amount of thermal energy. Heat generation should be characterized high efficiency and the absence of side effects, in particular, substances harmful to human health and the environment.
For ease of loading into the furnace, wood material is cut into individual elements up to 30 cm long. To increase the efficiency of their use, firewood should be as dry as possible, and the combustion process should be relatively slow. In many respects, firewood from such hardwoods as oak and birch, hazel and ash, hawthorn is suitable for space heating. because of high content resin, increased speed combustion and low calorific value coniferous trees are significantly inferior in this respect.
It should be understood that the density of wood affects the value of the calorific value.
it natural material plant origin mined from sedimentary rock.
This type of solid fuel contains carbon and other chemical elements. There is a division of material into types depending on its age. The youngest is considered brown coal, followed by stone, and older than all other types - anthracite. The age of the combustible substance is also determined by its moisture content, which in more present in young material.
In the process of burning coal, the environment is polluted, and slag is formed on the grate of the boiler, which, to a certain extent, creates an obstacle to normal combustion. The presence of sulfur in the material is also an unfavorable factor for the atmosphere, since in airspace this element is converted to sulfuric acid.
However, consumers should not be afraid for their health. Manufacturers of this material, taking care of private customers, seek to reduce the sulfur content in it. The calorific value of coal can differ even within the same type. The difference depends on the characteristics of the subspecies and the content of minerals in it, as well as the geography of production. As a solid fuel, not only pure coal is found, but also low-enriched coal slag pressed into briquettes.
Pellets (fuel pellets) is a solid fuel created industrially from wood and plant waste: shavings, bark, cardboard, straw.
The raw material crushed to the state of dust is dried and poured into the granulator, from where it already comes out in the form of granules certain form. To add viscosity to the mass, a vegetable polymer, lignin, is used. Complexity production process and high demand form the cost of pellets. The material is used in specially equipped boilers.
The types of fuel are determined depending on what material they are processed from:
- round timber of trees of any species;
- straw;
- peat;
- sunflower husk.
Among the advantages that fuel pellets have, it is worth noting the following qualities:
- environmental friendliness;
- inability to deform and resistance to fungus;
- ease of storage even outdoors;
- uniformity and duration of burning;
- relatively low cost;
- the possibility of using for various heating devices;
- suitable pellet size for automatic loading into a specially equipped boiler.
Briquettes
Briquettes are called solid fuel, in many respects similar to pellets. For their manufacture, identical materials are used: wood chips, shavings, peat, husks and straw. During the production process, the raw material is crushed and formed into briquettes by compression. This material also belongs to environmentally friendly fuel. It is convenient to store even outdoors. Smooth, uniform and slow burning of this fuel can be observed both in fireplaces and stoves, and in heating boilers.
The varieties of environmentally friendly solid fuels discussed above are a good alternative to generating heat. Compared with fossil sources of thermal energy, which adversely affect the combustion of environment and being, in addition, non-renewable, alternative fuels have clear advantages and relatively low cost, which is important for certain categories of consumers.
At the same time, the fire hazard of such fuels is much higher. Therefore, some precautions must be taken regarding their storage and the use of fire-resistant wall materials.
Liquid and gaseous fuels
As for liquid and gaseous combustible substances, the situation is as follows.
PHYSICAL AND CHEMICAL PROPERTIES OF NATURAL GASES
Natural gases have no color, smell or taste.
The main indicators of natural gases include: composition, heat of combustion, density, combustion and ignition temperature, explosive limits and explosion pressure.
Natural gases from pure gas fields mainly consist of methane (82-98%) and other hydrocarbons.
Combustible gas contains combustible and non-combustible substances. Combustible gases include: hydrocarbons, hydrogen, hydrogen sulfide. Non-flammables include: carbon dioxide, oxygen, nitrogen and water vapor. Their composition is low and amounts to 0.1-0.3% CO 2 and 1-14% N 2 . After extraction, toxic hydrogen sulfide gas is extracted from the gas, the content of which should not exceed 0.02 g/m3.
The calorific value is the amount of heat released during the complete combustion of 1 m3 of gas. The heat of combustion is measured in kcal/m3, kJ/m3 of gas. The calorific value of dry natural gas is 8000-8500 kcal/m 3 .
The value calculated by the ratio of the mass of a substance to its volume is called the density of the substance. Density is measured in kg/m3. The density of natural gas depends entirely on its composition and is within c = 0.73-0.85 kg/m3.
The most important feature of any combustible gas is the heat output, i.e. the maximum temperature reached with complete combustion of the gas, if required amount combustion air exactly matches the chemical formulas of combustion, and the initial temperature of the gas and air is zero.
The heat capacity of natural gases is about 2000 -2100 °C, methane - 2043 °C. The actual combustion temperature in furnaces is much lower than the heat output and depends on the combustion conditions.
The ignition temperature is the temperature of the air-fuel mixture at which the mixture ignites without an ignition source. For natural gas, it is in the range of 645-700 °C.
All combustible gases are explosive, capable of igniting with an open flame or spark. Distinguish lower and upper concentration limit of flame propagation , i.e. the lower and upper concentrations at which an explosion of the mixture is possible. The lower explosive limit of gases is 3÷6%, the upper limit is 12÷16%.
Explosive limits.
Gas-air mixture containing the amount of gas:
up to 5% - does not burn;
from 5 to 15% - explodes;
more than 15% - burns when air is supplied.
The pressure during the explosion of natural gas is 0.8-1.0 MPa.
All combustible gases can cause poisoning of the human body. The main toxic substances are: carbon monoxide (CO), hydrogen sulfide (H 2 S), ammonia (NH 3).
Natural gas has no smell. In order to determine the leak, the gas is odorized (i.e., they give it a specific smell). Carrying out odorization is carried out by using ethyl mercaptan. Carry out odorization at gas distribution stations (GDS). When 1% of natural gas enters the air, its smell begins to be felt. Practice shows that average rate ethyl mercaptan for the odorization of natural gas that enters the city networks should be 16 g per 1,000 m3 of gas.
Compared to solid and liquid fuels, natural gas wins in many ways:
Relative cheapness, which is explained by an easier way of extraction and transport;
No ash and carryover particulate matter in atmosphere;
High heat of combustion;
No preparation of fuel for combustion is required;
The work of service workers is facilitated and the sanitary and hygienic conditions of their work are improved;
Facilitate the automation of work processes.
Due to possible leaks through leaks in gas pipeline connections and fittings, the use of natural gas requires special care and caution. The penetration of more than 20% of the gas into the room can lead to suffocation, and if it is present in a closed volume from 5 to 15%, it can cause an explosion of the gas-air mixture. Incomplete combustion produces toxic carbon monoxide CO, which even at low concentrations leads to poisoning of the operating personnel.
According to their origin, natural gases are divided into two groups: dry and fatty.
Dry gases are gases of mineral origin and are found in areas associated with present or past volcanic activity. Dry gases consist almost exclusively of methane alone with negligible amounts of ballast constituents (nitrogen, carbon dioxide) and have a calorific value Qн=7000÷9000 kcal/nm3.
fatty gases accompany oil fields and usually accumulate in the upper layers. By their origin, fatty gases are close to oil and contain many easily condensable hydrocarbons. Calorific value liquid gases Qн=8000-15000 kcal/nm3
The advantages of gaseous fuels include the ease of transportation and combustion, the absence of moisture ash, and the significant simplicity of boiler equipment.
Along with natural gases artificial combustible gases are also used, obtained during the processing of solid fuels, or as a result of the operation of industrial plants as waste gases. Artificial gases consist of combustible gases of incomplete combustion of fuel, ballast gases and water vapor and are divided into rich and poor, having an average calorific value of 4500 kcal / m3 and 1300 kkam3, respectively. Composition of gases: hydrogen, methane, other hydrocarbon compounds CmHn, hydrogen sulfide H 2 S, non-combustible gases, carbon dioxide, oxygen, nitrogen and a small amount of water vapor. Ballast - nitrogen and carbon dioxide.
Thus, the composition of dry gaseous fuel can be represented as the following mixture of elements:
CO + H 2 + ∑CmHn + H 2 S + CO 2 + O 2 + N 2 \u003d 100%.
The composition of the wet gaseous fuel is expressed as follows:
CO + H 2 + ∑CmHn + H 2 S + CO 2 + O 2 + N 2 + H 2 O \u003d 100%.
Heat of combustion dry gaseous fuel kJ / m3 (kcal / m3) per 1 m3 of gas under normal conditions is determined as follows:
Qn \u003d 0.01,
Where Qi is the calorific value of the corresponding gas.
The heat of combustion of gaseous fuel is given in table 3.
Blast furnace gas formed during iron smelting in blast furnaces. Its yield and chemical composition depend on the properties of the charge and fuel, the operating mode of the furnace, methods of intensifying the process, and other factors. The gas output ranges from 1500-2500 m 3 per ton of pig iron. The proportion of non-combustible components (N 2 and CO 2) in blast-furnace gas is about 70%, which causes its low thermal performance (the lowest calorific value of gas is 3-5 MJ/m 3).
When burning blast-furnace gas, the maximum temperature of the combustion products (excluding heat losses and heat consumption for the dissociation of CO 2 and H 2 O) is 400-1500 0 C. If the gas and air are heated before combustion, the temperature of the combustion products can be significantly increased.
ferroalloy gas formed during the smelting of ferroalloys in ore reduction furnaces. The exhaust gas from closed furnaces can be used as fuel SER (secondary energetic resources). In open ovens due to free access air gas burns on the top. The yield and composition of ferroalloy gas depends on the grade of the smelted
alloy, charge composition, furnace operation mode, its power, etc. Gas composition: 50-90% CO, 2-8% H 2 , 0.3-1% CH 4 , O 2<1%, 2-5% CO 2 , остальное N 2 . Максимальная температура продуктов сгорания равна 2080 ^0 C. Запылённость газа составляет 30-40 г/м^3 .
converter gas formed during steel smelting in oxygen converters. The gas consists mainly of carbon monoxide, its yield and composition during melting change significantly. After purification, the composition of the gas is approximately as follows: 70-80% CO; 15-20% CO 2 ; 0.5-0.8% O 2 ; 3-12% N 2. The heat of combustion of the gas is 8.4-9.2 MJ/m 3 . The maximum combustion temperature reaches 2000 0 С.
coke oven gas formed during the coking of coal charge. In ferrous metallurgy, it is used after the extraction of chemical products. The composition of coke oven gas depends on the properties of the coal charge and coking conditions. Volume fractions of components in the gas are within the following limits, %: 52-62H 2 ; 0.3-0.6 O 2 ; 23.5-26.5 CH 4 ; 5.5-7.7 CO; 1.8-2.6 CO 2 . The heat of combustion is 17-17.6 MJ / m ^ 3, the maximum temperature of the combustion products is 2070 0 С.
