The combustion temperature of coal is considered the main criterion that allows you to avoid mistakes when choosing fuel. The performance of the boiler and its quality work directly depend on this value.
Temperature detection option
In winter, the issue of heating residential premises is especially relevant. Due to the systematic increase in the cost of coolants, people have to look for alternative options thermal energy generation.
The best way to solve this problem would be to select solid fuel boilers that have optimal production characteristics, retain heat perfectly.
The specific heat of combustion of coal is a physical quantity that shows how much heat can be released during the complete combustion of a kilogram of fuel. In order for the boiler to operate for a long time, it is important to select the correct fuel for it. The specific heat of combustion of coal is high (22 MJ/kg), so this type of fuel is considered optimal for efficient work boiler
Characteristics and properties of wood
Currently, there is a tendency to switch from installations based on the gas combustion process to solid fuel heating household systems.
Not everyone knows that creating a comfortable microclimate in the house directly depends on the quality of the chosen fuel. As a traditional material used in such heating boilers, select the wood.
In harsh climatic conditions, characterized by long and cold winters, it is quite difficult to heat a home with wood for the entire heating season. When the air temperature drops sharply, the owner of the boiler is forced to use it to the brink of its maximum capabilities.
When choosing as solid fuel wood, serious problems and inconveniences arise. First of all, we note that the combustion temperature of coal is much higher than that of wood. Among the disadvantages and high speed combustion of wood, which creates serious difficulties in operating the heating boiler. Its owner is forced to constantly monitor the availability of firewood in the firebox; a fairly large amount of it will be required for the heating season.
Coal options
The combustion temperature is much higher, so this option fuel is an excellent alternative to conventional firewood. We also note the excellent heat transfer rate, the duration of the combustion process, and low fuel consumption. There are several types of coal, related to the specifics of mining, as well as the depth of occurrence in the bowels of the earth: hard, brown, anthracite.
Each of these options has its own distinctive qualities and characteristics that allow it to be used in solid fuel boilers. The combustion temperature of coal in a furnace will be minimal when using brown coal, since it contains a fairly large amount of various impurities. As for heat transfer indicators, their value is similar to wood. Chemical reaction combustion is exothermic, the calorific value of coal is high.
Coal has an ignition temperature of 400 degrees. Moreover, the calorific value of this type of coal is quite high, so this type of fuel is widely used for heating residential premises.
Anthracite has maximum efficiency. Among the disadvantages of such fuel, we highlight its high cost. The combustion temperature of this type of coal reaches 2250 degrees. No solid fuel extracted from the bowels of the earth has such an indicator.
Features of a coal-fired furnace
A similar device has design features, involves the pyrolysis reaction of coal. does not belong to minerals, it has become a product of human activity.
The combustion temperature of coal is 900 degrees, which is accompanied by the release of a sufficient amount of thermal energy. What is the technology to create such an amazing product? The essence lies in a certain processing of wood, due to which there is a significant change in its structure, separation from it excess moisture. A similar process is carried out in special ovens. The operating principle of such devices is based on the pyrolysis process. A charcoal furnace consists of four basic components:
- combustion chambers;
- reinforced foundation;
- chimney;
- recycling compartment.
Chemical process
After entering the chamber, gradual smoldering of the firewood occurs. This process occurs due to the presence in the firebox of a sufficient amount of oxygen gas that supports combustion. As the smoldering progresses, a sufficient amount of heat is released and excess liquid is converted into steam.
The smoke released during the reaction goes to the secondary processing compartment, where it burns completely and heat is released. performs several important functional tasks. With its help, charcoal is formed, and a comfortable temperature is maintained in the room.
But the process of obtaining such fuel is quite delicate, and with the slightest delay, complete combustion of the wood is possible. It is necessary to remove charred pieces from the oven at a certain time.
Application of charcoal
If the technological chain is followed, an excellent material is obtained, which can be used for full heating of residential premises during the winter. heating season. Of course, the combustion temperature of coal will be higher, but such fuel is not affordable in all regions.
Charcoal combustion begins at a temperature of 1250 degrees. For example, a smelting furnace runs on charcoal. The flame that is formed when air is supplied to the furnace easily melts the metal.
Creating optimal conditions for combustion
Because of high temperature All internal elements The furnaces are made of special refractory bricks. Fireproof clay is used for their installation. If special conditions are created, it is quite possible to obtain a temperature in the furnace exceeding 2000 degrees. Each type of coal has its own flash point. After reaching this indicator, it is important to maintain the ignition temperature by continuously supplying excess oxygen to the firebox.
Among the disadvantages of this process, we highlight heat loss, because part of the released energy will escape through the pipe. This leads to a decrease in the temperature of the firebox. During experimental research Scientists were able to establish the optimal excess oxygen volume for various types of fuel. Thanks to the choice of excess air, you can count on complete combustion of the fuel. As a result, you can count on minimal losses of thermal energy.
Conclusion
The comparative value of fuel is assessed by its calorific value measured in calories. Considering the characteristics of its different types, we can conclude that hard coal is the optimal type of solid coal. Many owners of their own heating systems They try to use boilers that run on mixed fuels: solid, liquid, gaseous.
Different types of fuel (solid, liquid and gaseous) are characterized by general and specific properties. General properties of fuel include specific heat of combustion and humidity, specific properties include ash content, sulfur content (sulfur content), density, viscosity and other properties.
Specific heat of combustion of fuel is the amount of heat that is released during complete combustion of \(1\) kg of solid or liquid fuel or \(1\) m³ of gaseous fuel.
The energy value of a fuel is primarily determined by its specific heat of combustion.
The specific heat of combustion is denoted by the letter \(q\). The unit of specific heat of combustion is \(1\) J/kg for solid and liquid fuels and \(1\) J/m³ for gaseous fuels.
The specific heat of combustion is experimentally determined using rather complex methods.
Table 2. Specific heat of combustion of some types of fuel.
Solid fuel
Substance | Specific heat of combustion, |
| Brown coal | |
| Charcoal | |
| Dry firewood | |
| Wood chocks | |
Coal | |
Coal grade A-II | |
| Coke | |
| Powder | |
| Peat |
Liquid fuel
Gaseous fuel
(under normal conditions)
Substance | Specific heat of combustion, |
| Hydrogen | |
| Producer gas | |
| Coke gas | |
| Natural gas | |
| Gas |
From this table it is clear that the specific heat of combustion of hydrogen is the highest, it is equal to \(120\) MJ/m³. This means that with the complete combustion of hydrogen with a volume of \(1\) m³, \(120\) MJ \(=\)\(120\) ⋅ 10 6 J of energy is released.
Hydrogen is one of the high-energy fuels. In addition, the product of hydrogen combustion is ordinary water, unlike other types of fuel, where the combustion products are carbon dioxide and carbon monoxide, ash and furnace slag. This makes hydrogen the most environmentally friendly fuel.
However, hydrogen gas is explosive. In addition, it has the lowest density compared to other gases at the same temperature and pressure, which creates difficulties with the liquefaction of hydrogen and its transportation.
The total amount of heat \(Q\) released during complete combustion of \(m\) kg of solid or liquid fuel is calculated by the formula:
The total amount of heat \(Q\) released during complete combustion of \(V\) m³ of gaseous fuel is calculated by the formula:
Humidity (moisture content) of the fuel reduces its calorific value, as the heat consumption for evaporation of moisture increases and the volume of combustion products increases (due to the presence of water vapor).
Ash content is the amount of ash formed during the combustion of minerals contained in fuel. Mineral substances contained in fuel reduce its calorific value, since the content of combustible components decreases (the main reason) and the heat consumption for heating and melting the mineral mass increases.
Sulfur content (sulfur content) refers to negative factor fuel, since its combustion produces sulfur dioxide gases that pollute the atmosphere and destroy the metal. In addition, the sulfur contained in the fuel partially passes into the smelted metal and welded glass melt, reducing their quality. For example, for melting crystal, optical and other glasses, you cannot use fuel containing sulfur, since sulfur significantly reduces the optical properties and color of the glass.
Different types of fuel have different characteristics. This depends on the calorific value and the amount of heat released when the fuel is completely burned out. For example, the relative heat of combustion of hydrogen affects its consumption. Calorific value is determined using tables. They indicate comparative analyzes of the consumption of different energy resources.
There is a huge amount of combustibles. each of which has its own pros and cons
Comparison tables
With the help of comparison tables it is possible to explain why different energy resources have different calorific values. For example, such as:
- electricity;
- methane;
- butane;
- propane-butane;
- diesel fuel;
- firewood;
- peat;
- coal;
- mixtures of liquefied gases.
Propane is one of the popular types of fuel
Tables can demonstrate not only, for example, the specific heat of combustion of diesel fuel. Other indicators are also included in the comparative analysis reports: calorific value, volumetric densities of substances, price for one part of conditional power supply, efficiency of heating systems, cost of one kilowatt per hour.
In this video you will learn about how fuel works:
Fuel prices
Thanks to the reports comparative analysis determine the prospects for using methane or diesel fuel. Gas price in a centralized gas pipeline tends to increase. It may be higher even than diesel fuel. That is why the cost of liquefied petroleum gas will hardly change, and its use will remain the only solution when installing an independent gasification system.
There are several types of names for fuels and lubricants (fuels and lubricants): solid, liquid, gaseous and some other flammable materials, in which, during the heat-generating reaction of oxidation of fuels and lubricants, its chemical heat energy is converted into temperature radiation.
The heat energy released is called the calorific value of various types of fuel during complete combustion of any flammable substance. Its dependence on chemical composition and humidity is the main indicator of nutrition.
Thermal susceptibility
Determination of the OTC of fuel is carried out experimentally or using analytical calculations. The experimental determination of thermal susceptibility is carried out experimentally by establishing the volume of heat released during fuel combustion in a heat store with a thermostat and a combustion bomb.
If necessary, determine the specific heat of combustion of fuel from the table First, calculations are made according to Mendeleev's formulas. There are higher and lower grades of OTC fuel. At the highest relative heat, a large amount of heat is released when any fuel burns out. This takes into account the heat spent on evaporating the water in the fuel.
At the lowest degree of burnout, the TTC is less than at the highest degree, since in this case less evaporation is released. Evaporation occurs from water and hydrogen when fuel burns. To determine the properties of a fuel, engineering calculations take into account the lowest relative calorific value, which is important parameter fuel.
The following components are included in the tables of the specific heat of combustion of solid fuels: coal, firewood, peat, coke. They include the values of the GTC of solid flammable material. The names of fuels are entered in the tables alphabetically. Of all solid forms of fuel and lubricants, coking, stone, brown and lubricants have the greatest heat transfer capacity. charcoal, as well as anthracite. Low productivity fuels include:
- wood;
- firewood;
- powder;
- peat;
- combustible shale.
Indicators of alcohol, gasoline, kerosene, and oil are entered in the list of liquid fuels and lubricants. The specific heat of combustion of hydrogen, as well as different forms of fuel is released when one kilogram, one cubic meter or one liter is completely burned out. Most often these physical properties measured in units of work, energy and the amount of heat released.
Depending on the degree to which the OTC of fuel and lubricants is high, this will be its consumption. This competence is the most significant parameter of the fuel, and this must be taken into account when designing boiler installations using different types of fuel. Calorific value depends on humidity and ash content, as well as from flammable ingredients such as carbon, hydrogen, volatile combustible sulfur.
SG (specific heat) of burnout of alcohol and acetone is much lower than classic motor fuel and lubricants and it is equal to 31.4 MJ/kg; for fuel oil this figure ranges from 39-41.7 MJ/kg. The indicator of combustion efficiency of natural gas is 41-49 MJ/kg. One kcal (kilocalorie) is equal to 0.0041868 MJ. The caloric content of different types of fuel differs from each other in terms of burnout. How more heat gives off any substance, the greater its heat transfer. This process is also called heat transfer. Liquids, gases and hard particles take part in heat transfer.
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 their quantitative content of oxidizer, which is used to release thermal energy.
In a broad sense, fuel is an energy carrier, that is, a potential type of potential energy.
Classification
Currently, fuel types are divided according to their state of aggregation into liquid, solid, and gaseous.
Natural hard materials include stone, firewood and anthracite. Briquettes, coke, thermoanthracite are types of artificial solid fuel.
Liquids include substances containing substances of organic origin. Their main components are: oxygen, carbon, nitrogen, hydrogen, sulfur. Artificial liquid fuel will be a variety of resins and fuel oil.
It is a mixture of various gases: ethylene, methane, propane, butane. In addition to them, gaseous fuel contains carbon dioxide and carbon monoxide, hydrogen sulfide, nitrogen, water vapor, and oxygen.
Fuel indicators
The main indicator of combustion. The formula for determining the calorific value is considered in thermochemistry. emit “standard fuel”, which implies the calorific value of 1 kilogram of anthracite.
Household 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 feed, canning.
The specific heat of combustion of a fuel is a value that demonstrates the amount of heat that is generated 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/m3, calorie/m3 are used. To determine the heat of combustion, the calorimetry method is used.
With an increase in the specific heat of combustion of fuel, it decreases specific consumption fuel, and the efficiency remains unchanged.
The heat of combustion of substances is the amount of energy released during the oxidation of a solid, liquid, or 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 values are related to the state of aggregation of water in the substances obtained after combustion of fuel.
The higher calorific value is the amount of heat released during complete combustion of a substance. This value also includes the heat of condensation of water vapor.
The lowest working heat of combustion 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 amount of energy of condensation of water vapor.
Mathematical relationship
The higher and lower calorific values are related by the following relationship:
QB = QH + k(W + 9H)
where W is the amount by weight (in %) of water in a flammable substance;
H is the amount of hydrogen (% by mass) in the combustible substance;
k - coefficient equal to 6 kcal/kg
Methods for performing calculations
The higher and lower calorific values are determined by two main methods: calculation and experimental.
Calorimeters are used to carry out experimental calculations. First, a sample of fuel is burned in it. The heat that will be released is completely absorbed by the water. Having an idea of the mass of water, you can determine by the change in its temperature the value of its heat of combustion.
This technique is considered simple and effective; it only requires knowledge of technical analysis data.
In the calculation method, the higher and lower calorific values are calculated using the Mendeleev formula.
Q p H = 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 equivalent fuel.
The heat of combustion of the gas allows preliminary calculations, identify the effectiveness of using a certain type of fuel.
Features of origin
In order to understand how much heat is released when a certain fuel is burned, 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 occurs through several stages. First, peat is formed, then brown and hard coal are obtained, then anthracite is formed. The main sources of solid fuel formation are leaves, wood, and pine needles. When parts of plants die and are exposed to air, they are destroyed by fungi and form peat. Its accumulation turns into a brown mass, then brown gas is obtained.
At high blood pressure and temperature, brown gas turns into coal, then the fuel accumulates in the form of anthracite.
In addition to organic matter, the fuel contains additional ballast. Organic is considered to be that part that is formed from organic substances: hydrogen, carbon, nitrogen, oxygen. In addition to these chemical elements, it contains ballast: moisture, ash.
Combustion technology involves the separation of the working, dry, and combustible mass of burned fuel. The working mass is the fuel in its original form supplied to the consumer. Dry mass is a composition in which there is no water.
Compound
The most valuable components are carbon and hydrogen.
These elements are contained in any type of fuel. In peat and wood, the percentage of carbon reaches 58 percent, in hard and brown coal - 80%, and in anthracite it reaches 95 percent by weight. Depending on this indicator, the amount of heat released during fuel combustion changes. Hydrogen is the second most important element of any fuel. When it binds 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. The oxygen contained in the fuel acts as ballast.
It is not heat generating chemical element, therefore negatively affects the value of its heat of combustion. The combustion of nitrogen, contained in free or bound form in combustion products, is considered harmful impurities, therefore its quantity is strictly limited.
Sulfur is included in 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 a chemical element whose presence in natural fuel is extremely undesirable. If sulfur compounds get inside the work area, they cause significant poisoning of operating personnel.
There are three types of ash depending on its origin:
- primary;
- secondary;
- tertiary
The primary species is formed from minerals found in plants. Secondary ash is formed as a result of plant residues entering sand and soil during formation.
Tertiary ash appears in the composition of fuel during extraction, storage, and transportation. With significant ash deposition, a decrease in heat transfer on the heating surface of the boiler unit occurs, reducing the amount of heat transfer to water from gases. A huge amount of ash negatively affects the operation of the boiler.
Finally
Volatile substances have a significant influence on the combustion process of any type of fuel. The greater their output, the larger the volume of the flame front will be. For example, coal and peat ignite easily, the process is accompanied by minor heat losses. The coke that remains after removing volatile impurities contains only mineral and carbon compounds. Depending on the characteristics of the fuel, the amount of heat changes significantly.
Depending on the chemical composition There are three stages of solid fuel formation: peat, brown coal, and coal.
Natural wood is used in small boiler installations. They mainly use wood chips, sawdust, slabs, bark, and the firewood itself is used in small quantities. Depending on the type of wood, the amount of heat generated varies significantly.
As the heat of combustion 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 in a great way carrying out thermochemical calculations.
Currently, there is a real opportunity to identify those main options for solid, gaseous, liquid fuels that will be the most effective and inexpensive to use in a certain situation.
Thermal machines in thermodynamics, these are periodically operating heat engines and refrigeration machines (thermocompressors). Variety refrigeration machines are heat pumps.
Devices that perform mechanical work due to internal energy fuels are called heat engines (heat engines). For the operation of a heat engine, the following components are required: 1) a heat source with a higher temperature level t1, 2) a heat source with a lower temperature level t2, 3) a working fluid. In other words: any heat engines (heat engines) consist of heater, refrigerator and working fluid .
As working fluid gas or steam are used, since they are well compressed, and depending on the type of engine, there may be fuel (gasoline, kerosene), water vapor, etc. The heater transfers a certain amount of heat (Q1) to the working fluid, and its internal energy increases due to this internal energy, mechanical work is performed (A), then the working fluid gives off a certain amount of heat to the refrigerator (Q2) and is cooled to the initial temperature. The described diagram represents the engine operating cycle and is general; in real engines, the role of a heater and a refrigerator can be played by various devices. The environment can serve as a refrigerator.
Since in the engine part of the energy of the working fluid is transferred to the refrigerator, it is clear that not all the energy it receives from the heater is used to perform work. Respectively, efficiency engine (efficiency) is equal to the ratio of the work done (A) to the amount of heat it receives from the heater (Q1):
Internal combustion engine (ICE)
There are two types of engines internal combustion(ICE): carburetor And diesel. IN carburetor engine the working mixture (a mixture of fuel and air) is prepared outside the engine in special device and from it enters the engine. In a diesel engine, the fuel mixture is prepared in the engine itself.
ICE consists of cylinder
, in which it moves piston
; there are in the cylinder two valves
, through one of which the combustible mixture is admitted into the cylinder, and through the other, exhaust gases are discharged from the cylinder. Piston using crank mechanism
connects with crankshaft
, which begins to rotate with the translational movement of the piston. The cylinder is closed with a lid.
The internal combustion engine operating cycle includes four bars: intake, compression, stroke, exhaust. During intake, the piston moves down, the pressure in the cylinder decreases, and a combustible mixture (in a carburetor engine) or air (in a diesel engine) enters it through the valve. The valve is closed at this time. At the end of the intake of the combustible mixture, the valve closes.
During the second stroke, the piston moves up, the valves are closed, and the working mixture or air is compressed. At the same time, the gas temperature rises: the combustible mixture in a carburetor engine heats up to 300-350 °C, and the air in a diesel engine - up to 500-600 °C. At the end of the compression stroke, a spark jumps in the carburetor engine and the combustible mixture ignites. In a diesel engine, fuel is injected into the cylinder and the resulting mixture spontaneously ignites.
When a combustible mixture is burned, the gas expands and pushes the piston and the crankshaft connected to it, performing mechanical work. This causes the gas to cool.
When the piston reaches the lowest point, the pressure in it will decrease. When the piston moves upward, the valve opens and exhaust gas is released. At the end of this stroke the valve closes.
Steam turbine
Steam turbine It is a disk mounted on a shaft on which the blades are mounted. Steam enters the blades. Steam heated to 600 °C is directed into the nozzle and expands in it. When steam expands, its internal energy is converted into kinetic energy of the directed movement of the steam jet. A jet of steam comes from the nozzle onto the turbine blades and transfers part of its kinetic energy to them, causing the turbine to rotate. Typically, turbines have several disks, each of which transfers part of the steam energy. The rotation of the disk is transmitted to a shaft to which an electric current generator is connected.
When different fuels of the same mass are burned, different amounts of heat are released. For example, it is well known that natural gas is energetically more profitable fuel than firewood. This means that to obtain the same amount of heat, the mass of wood that needs to be burned must be significantly greater than the mass of natural gas. Hence, different kinds fuels from an energy point of view are characterized by a quantity called specific heat of combustion of fuel .
Specific heat of combustion of fuel - physical quantity, showing how much heat is released during complete combustion of fuel weighing 1 kg.
