(Fig. 14.1 - Calorific value
fuel capacity)
Pay attention to the calorific value (specific heat of combustion) of different types of fuel, compare the indicators. The calorific value of the fuel characterizes the amount of heat released during the complete combustion of fuel with a mass of 1 kg or a volume of 1 m³ (1 l). The most common calorific value is measured in J/kg (J/m³; J/l). The higher the specific heat of combustion of fuel, the lower its consumption. Therefore, the calorific value is one of the most significant characteristics of the fuel.
The specific heat of combustion of each type of fuel depends on:
- From its combustible components (carbon, hydrogen, volatile combustible sulfur, etc.).
- From its moisture and ash content.
| Table 4 - Specific heat of combustion of various energy carriers, comparative analysis of costs. | |||||||||
| Type of energy carrier | Calorific value | Volumetric matter density (ρ=m/V) | Unit price reference fuel | Coeff. useful action (efficiency) systems heating, % | Price per 1 kWh | Implemented systems | |||
| MJ | kWh | ||||||||
| (1MJ=0.278kWh) | |||||||||
| Electricity | - | 1.0 kWh | - | 3.70 rub. per kWh | 98% | 3.78 rubles | Heating, hot water supply (DHW), air conditioning, cooking | ||
| Methane (CH4, temperature boiling point: -161.6 °C) | 39.8 MJ/m³ | 11.1 kWh/m³ | 0.72 kg/m³ | 5.20 rub. per m³ | 94% | 0.50 rub. | |||
| Propane (C3H8, temperature boiling point: -42.1 °C) | 46,34 MJ/kg | 23,63 MJ/l | 12,88 kWh/kg | 6,57 kWh/l | 0.51 kg/l | 18.00 rub. hall | 94% | 2.91 rub. | Heating, hot water supply (DHW), cooking, backup and permanent power supply, autonomous septic tank (sewerage), outdoor infrared heaters, outdoor barbecues, fireplaces, saunas, designer lighting |
| Butane C4H10, temperature boiling point: -0.5 °C) | 47,20 MJ/kg | 27,38 MJ/l | 13,12 kWh/kg | 7,61 kWh/l | 0.58 kg/l | 14.00 rub. hall | 94% | 1.96 rub. | Heating, hot water supply (DHW), cooking, backup and permanent power supply, autonomous septic tank (sewerage), outdoor infrared heaters, outdoor barbecues, fireplaces, saunas, designer lighting |
| propane butane (LPG - liquefied hydrocarbon gas) | 46,8 MJ/kg | 25,3 MJ/l | 13,0 kWh/kg | 7,0 kWh/l | 0.54 kg/l | 16.00 rub. hall | 94% | 2.42 rubles | Heating, hot water supply (DHW), cooking, backup and permanent power supply, autonomous septic tank (sewerage), outdoor infrared heaters, outdoor barbecues, fireplaces, saunas, designer lighting |
| Diesel fuel | 42,7 MJ/kg | 11,9 kWh/kg | 0.85 kg/l | 30.00 rub. per kg | 92% | 2.75 rub. | Heating (heating water and generating electricity are very costly) | ||
| Firewood (birch, humidity - 12%) | 15,0 MJ/kg | 4,2 kWh/kg | 0.47-0.72 kg/dm³ | 3.00 rub. per kg | 90% | 0.80 rub. | Heating (inconvenient to cook food, almost impossible to get hot water) | ||
| Coal | 22,0 MJ/kg | 6,1 kWh/kg | 1200-1500 kg/m³ | 7.70 rub. per kg | 90% | 1.40 rub. | Heating | ||
| MAPP gas (mixture of liquefied petroleum gas - 56% with methyl acetylene-propadiene - 44%) | 89,6 MJ/kg | 24,9 kWh/m³ | 0.1137 kg/dm³ | -R. per m³ | 0% | Heating, hot water supply (DHW), cooking, backup and permanent power supply, autonomous septic tank (sewerage), outdoor infrared heaters, outdoor barbecues, fireplaces, saunas, designer lighting | |||
(Fig. 14.2 - Specific heat of combustion)
According to the table "Specific calorific value of various energy carriers, comparative analysis of costs", propane-butane (liquefied hydrocarbon gas) is inferior in economic benefits and prospects of using only natural gas (methane). However, attention should be paid to the trend towards an inevitable increase in the cost of main gas, which today is significantly underestimated. Analysts predict an inevitable reorganization of the industry, which will lead to a significant rise in the price of natural gas, perhaps even exceed the cost of diesel fuel.
Thus, liquefied petroleum gas, the cost of which will remain virtually unchanged, remains extremely promising - the optimal solution for autonomous gasification systems.
thermal machines in thermodynamics, these are periodically operating heat engines and refrigerating machines (thermocompressors). A variety of refrigeration machines are heat pumps.
Devices that perform mechanical work due to the internal energy of the fuel are called heat engines (heat engines). The following components are necessary for the operation of a heat engine: 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, cooler and working medium .
As working body gas or steam is used, since they are highly compressible, 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 (A) is performed, then the working fluid gives off a certain amount of heat to the refrigerator (Q2) and cools down to the initial temperature. The described scheme represents the engine operation cycle and is general; in real engines, various devices can play the role of a heater and a refrigerator. 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 of the energy received by it from the heater goes to doing work. Respectively, efficiency engine (efficiency) is equal to the ratio of the work done (A) to the amount of heat received by it from the heater (Q1):
Internal combustion engine (ICE)
There are two types of internal combustion engines (ICE): carburettor and diesel. In a carburetor engine, the working mixture (a mixture of fuel with air) is prepared outside the engine in a 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
; the cylinder has two valves
, through one of which the combustible mixture is admitted into the cylinder, and through the other, the exhaust gases are released from the cylinder. Piston using crank mechanism
connects with crankshaft
, which comes into rotation during the translational movement of the piston. The cylinder is closed with a cap.
The cycle of operation of the internal combustion engine 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 inlet 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 the carburetor engine heats up to 300-350 °C, and the air in the 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 ignites spontaneously.
When the 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 its lowest point, the pressure in it will decrease. When the piston moves up, the valve opens and the exhaust gas is released. At the end of this cycle, the valve closes.
Steam turbine
Steam turbine represents the disk mounted on a shaft on which blades are fixed. Steam enters the blades. Steam heated to 600 °C is sent to the nozzle and expands in it. When the steam expands, its internal energy is converted into the kinetic energy of the directed motion of the steam jet. A jet of steam enters the turbine blades from the nozzle and transfers part of its kinetic energy to them, causing the turbine to rotate. Turbines usually have several discs, each of which receives a portion of the steam energy. The rotation of the disk is transmitted to the shaft, to which the 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 an energy-efficient fuel than firewood. This means that in order to obtain the same amount of heat, the mass of firewood to be burned must be significantly greater than the mass of natural gas. Consequently, various types of fuel from an energy point of view are characterized by a quantity called specific heat of combustion of fuel .
Specific heating value of fuel- a physical quantity showing how much heat is released during the complete combustion of fuel weighing 1 kg.
The specific heat of combustion is denoted by the letter q , its unit is 1 J/kg.
The value of specific heat is determined experimentally. The highest specific heat of combustion has hydrogen , the smallest - powder .
The specific heat of combustion of oil is 4.4 * 10 7 J / kg. This means that with the complete combustion of 1 kg of oil, the amount of heat 4.4 * 10 7 J is released. In the general case, if the mass of fuel is equal to m , then the amount of heat Q released during its complete combustion is equal to the product of the specific heat of combustion of the fuel q for its weight:
Q = qm.
Synopsis of a lesson in physics in grade 8 "Heat Machines. ICE. Specific calorific value”.
Mankind, in the course of its evolution, has learned to obtain thermal energy by burning different types of fuel. The simplest example is a fire made of wood, which was kindled by primitive people, and since then peat, coal, gasoline, oil, natural gas are all types of fuel, by burning which a person receives thermal energy. So what is the specific heat of combustion?
Where does heat come from during combustion?
The process of fuel combustion itself is a chemical, oxidative reaction. Most fuels contain large amounts of carbon C, hydrogen H, sulfur S and other substances. During combustion, C, H, and S atoms combine with O 2 oxygen atoms, resulting in CO, CO 2, H 2 O, SO 2 molecules. In this case, a large amount of thermal energy is released, which people have learned to use for their own purposes.
Rice. 1. Types of fuel: coal, peat, oil, gas.
The main contribution to the heat release is made by carbon C. The second largest contribution is made by hydrogen H.
Rice. 2. Carbon atoms react with oxygen atoms.
What is the specific heat of combustion?
The specific heat of combustion q is a physical quantity equal to the amount of heat released during complete combustion of 1 kg of fuel.
The formula for specific heat of combustion looks like this:
$$q=(Q \over m)$$
Q is the amount of heat released during the combustion of the fuel, J;
m is the mass of fuel, kg.
The unit of q in the international system of units SI is J/kg.
$$[q]=(J \over kg)$$
Large quantities of q are often denoted by off-system units of energy: kilojoules (kJ), megajoules (MJ), and gigajoules (GJ).
The q values for different substances are determined experimentally.
Knowing q, we can calculate the amount of heat Q, which will result from the combustion of fuel of mass m:
How is the specific heat of combustion measured?
To measure q, devices called calorimeters are used (calor - heat, metreo - measure).
A container with a portion of fuel is burned inside the device. The container is placed in water with a known mass. As a result of combustion, the released heat heats the water. The value of the mass of water and the change in its temperature allow us to calculate the heat of combustion. Next, q is determined by the above formula.
Rice. 3. Measurement of specific heat of combustion.
Where to find q values
Information on the values of specific heat of combustion for specific types of fuel can be found in technical reference books or in their electronic versions on Internet resources. They are usually presented in the form of a table like this:
Specific heat of combustion, q
Resources of explored, modern types of fuel are limited. Therefore, in the future they will be replaced by other energy sources:
- atomic, using the energy of nuclear reactions;
- solar, converting the energy of sunlight into heat and electricity;
- wind;
- geothermal, using the heat of natural hot springs.
What have we learned?
So, we have learned why a lot of heat is released during the combustion of fuel. To calculate the amount of heat released during the combustion of a certain mass m of fuel, it is necessary to know the value q - the specific heat of combustion of this fuel. The q values were determined experimentally by calorimetry methods and are given in reference books.
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In this lesson, we will learn how to calculate the amount of heat that fuel releases during combustion. In addition, consider the characteristics of the fuel - the specific heat of combustion.
Since our whole life is based on movement, and movement is mostly based on the combustion of fuel, the study of this topic is very important for understanding the topic "Thermal phenomena".
After studying the issues related to the amount of heat and specific heat capacity, we turn to the consideration the amount of heat released during the combustion of fuel.
Definition
Fuel- a substance that in some processes (combustion, nuclear reactions) releases heat. Is a source of energy.
Fuel happens solid, liquid and gaseous(Fig. 1).
Rice. 1. Types of fuel
- Solid fuels are coal and peat.
- Liquid fuels are oil, gasoline and other petroleum products.
- Gaseous fuels include natural gas.
- Separately, one can single out a very common lately nuclear fuel.
Fuel combustion is a chemical process that is oxidative. During combustion, carbon atoms combine with oxygen atoms to form molecules. As a result, energy is released, which a person uses for his own purposes (Fig. 2).
Rice. 2. Formation of carbon dioxide
To characterize the fuel, such a characteristic is used as calorific value. Calorific value shows how much heat is released during the combustion of fuel (Fig. 3). In calorific physics, the concept corresponds specific heat of combustion of a substance.
Rice. 3. Specific heat of combustion
Definition
Specific heat of combustion- the physical quantity characterizing the fuel is numerically equal to the amount of heat that is released during the complete combustion of the fuel.
The specific heat of combustion is usually denoted by the letter . Units:
In units of measurement, there is no , since the combustion of fuel occurs at an almost constant temperature.
The specific heat of combustion is determined empirically using sophisticated instruments. However, there are special tables for solving problems. Below we give the values of the specific heat of combustion for some types of fuel.
|
Substance |
|
Table 4. Specific heat of combustion of some substances
From the given values it can be seen that during combustion a huge amount of heat is released, therefore the units of measurement (megajoules) and (gigajoules) are used.
To calculate the amount of heat that is released during the combustion of fuel, the following formula is used:
Here: - mass of fuel (kg), - specific heat of combustion of fuel ().
In conclusion, we note that most of the fuel that is used by mankind is stored with the help of solar energy. Coal, oil, gas - all this was formed on Earth due to the influence of the Sun (Fig. 4).
Rice. 4. Formation of fuel
In the next lesson, we will talk about the law of conservation and transformation of energy in mechanical and thermal processes.
Listliterature
- Gendenstein L.E., Kaidalov A.B., Kozhevnikov V.B. / Ed. Orlova V.A., Roizena I.I. Physics 8. - M.: Mnemosyne.
- Peryshkin A.V. Physics 8. - M.: Bustard, 2010.
- Fadeeva A.A., Zasov A.V., Kiselev D.F. Physics 8. - M.: Enlightenment.
- Internet portal "festival.1september.ru" ()
- Internet portal "school.xvatit.com" ()
- Internet portal "stringer46.narod.ru" ()
Homework
When a certain amount of fuel is burned, a measurable amount of heat is released. According to the International System of Units, the value is expressed in Joules per kg or m 3. But the parameters can also be calculated in kcal or kW. If the value is related to the unit of measure for the fuel, it is called specific.
What is the calorific value of different fuels? What is the value of the indicator for liquid, solid and gaseous substances? The answers to these questions are detailed in the article. In addition, we have prepared a table showing the specific heat of combustion of materials - this information will be useful when choosing a high-energy type of fuel.
The release of energy during combustion should be characterized by two parameters: high efficiency and the absence of the production of harmful substances.
Artificial fuel is obtained in the process of processing natural -. Regardless of the state of aggregation, substances in their chemical composition have a combustible and non-combustible part. The first is carbon and hydrogen. The second consists of water, mineral salts, nitrogen, oxygen, metals.
According to the state of aggregation, fuel is divided into liquid, solid and gas. Each group further branches into a natural and artificial subgroup (+)
When burning 1 kg of such a "mixture", a different amount of energy is released. How much of this energy will be released depends on the proportions of these elements - the combustible part, humidity, ash content and other components.
The heat of combustion of fuel (HCT) is formed from two levels - higher and lower. The first indicator is obtained due to water condensation, in the second this factor is not taken into account.
The lowest TCT is needed to calculate the need for fuel and its cost, with the help of such indicators, heat balances are compiled and the efficiency of fuel-operated installations is determined.
TST can be calculated analytically or experimentally. If the chemical composition of the fuel is known, the Mendeleev formula is applied. Experimental procedures are based on the actual measurement of heat during fuel combustion.
In these cases, a special combustion bomb is used - a calorimetric bomb together with a calorimeter and a thermostat.
Features of calculations are individual for each type of fuel. Example: TCT in internal combustion engines is calculated from the lowest value, because liquid does not condense in the cylinders.
Parameters of liquid substances
Liquid materials, like solid ones, are decomposed into the following components: carbon, hydrogen, sulfur, oxygen, nitrogen. The percentage is expressed by weight.
The internal organic fuel ballast is formed from oxygen and nitrogen; these components do not burn and are included in the composition conditionally. The outer ballast is formed from moisture and ash.
High specific heat of combustion is observed in gasoline. Depending on the brand, it is 43-44 MJ.
Similar indicators of the specific heat of combustion are also determined for aviation kerosene - 42.9 MJ. Diesel fuel also falls into the category of leaders in terms of calorific value - 43.4-43.6 MJ.
Relatively low TST values are characteristic of liquid rocket fuel, ethylene glycol. Alcohol and acetone differ in the minimum specific heat of combustion. Their performance is significantly lower than that of traditional motor fuel.
Properties of gaseous fuel
Gaseous fuel consists of carbon monoxide, hydrogen, methane, ethane, propane, butane, ethylene, benzene, hydrogen sulfide and other components. These figures are expressed as a percentage by volume.
Hydrogen has the highest heat of combustion. When burning, a kilogram of a substance releases 119.83 MJ of heat. But it has a high degree of explosiveness.
High calorific values are also observed in natural gas.
They are equal to 41-49 MJ per kg. But, for example, pure methane has a higher heat of combustion - 50 MJ per kg.
Comparative table of indicators
The table shows the values of the mass specific heat of combustion of liquid, solid, gaseous fuels.
| Type of fuel | Unit rev. | Specific heat of combustion | ||
| MJ | kW | kcal | ||
| Firewood: oak, birch, ash, beech, hornbeam | kg | 15 | 4,2 | 2500 |
| Firewood: larch, pine, spruce | kg | 15,5 | 4,3 | 2500 |
| Brown coal | kg | 12,98 | 3,6 | 3100 |
| Coal | kg | 27,00 | 7,5 | 6450 |
| Charcoal | kg | 27,26 | 7,5 | 6510 |
| Anthracite | kg | 28,05 | 7,8 | 6700 |
| wood pellet | kg | 17,17 | 4,7 | 4110 |
| Straw pellet | kg | 14,51 | 4,0 | 3465 |
| sunflower pellet | kg | 18,09 | 5,0 | 4320 |
| Sawdust | kg | 8,37 | 2,3 | 2000 |
| Paper | kg | 16,62 | 4,6 | 3970 |
| Vine | kg | 14,00 | 3,9 | 3345 |
| Natural gas | m 3 | 33,5 | 9,3 | 8000 |
| Liquefied gas | kg | 45,20 | 12,5 | 10800 |
| Petrol | kg | 44,00 | 12,2 | 10500 |
| Diz. fuel | kg | 43,12 | 11,9 | 10300 |
| Methane | m 3 | 50,03 | 13,8 | 11950 |
| Hydrogen | m 3 | 120 | 33,2 | 28700 |
| Kerosene | kg | 43.50 | 12 | 10400 |
| fuel oil | kg | 40,61 | 11,2 | 9700 |
| Oil | kg | 44,00 | 12,2 | 10500 |
| Propane | m 3 | 45,57 | 12,6 | 10885 |
| Ethylene | m 3 | 48,02 | 13,3 | 11470 |
The table shows that hydrogen has the highest TST of all substances, and not only of gaseous ones. It belongs to high-energy fuels.
The combustion product of hydrogen is ordinary water. The process does not emit furnace slag, ash, carbon monoxide and carbon dioxide, which makes the substance an environmentally friendly fuel. But it is explosive and has a low density, so such fuel is difficult to liquefy and transport.
Conclusions and useful video on the topic
On the calorific value of different types of wood. Comparison of indicators per m 3 and kg.
TST is the most important thermal and operational characteristic of fuel. This indicator is used in various areas of human activity: heat engines, power plants, industry, home heating and cooking.
Calorific values help to compare different types of fuel in terms of the degree of energy released, calculate the required mass of fuel, and save on costs.
Do you have something to add, or do you have questions about the calorific value of different types of fuel? You can leave comments on the publication and participate in discussions - the contact form is located in the lower block.
