Heat of combustion of diesel fuel. Calorific value of solid fuel for boilers. Where does fuel energy come from?
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.
The specific heat of combustion of a 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 a negative factor in 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.
5. THERMAL BALANCE OF COMBUSTION
Let's consider methods for calculating the heat balance of the combustion process of gaseous, liquid and solid fuels. The calculation comes down to solving the following problems.
· Determination of the heat of combustion (calorific value) of fuel.
· Determination of theoretical combustion temperature.
5.1. HEAT OF COMBUSTION
Chemical reactions are accompanied by the release or absorption of heat. When heat is released, the reaction is called exothermic, and when heat is absorbed, it is called endothermic. All combustion reactions are exothermic, and combustion products are exothermic compounds.
The heat released (or absorbed) during a chemical reaction is called the heat of reaction. In exothermic reactions it is positive, in endothermic reactions it is negative. The combustion reaction is always accompanied by the release of heat. Heat of combustion Q g(J/mol) is the amount of heat that is released during the complete combustion of one mole of a substance and the transformation of a combustible substance into products of complete combustion. The mole is the basic SI unit of quantity of a substance. One mole is the amount of substance that contains the same number of particles (atoms, molecules, etc.) as there are atoms in 12 g of the carbon-12 isotope. The mass of an amount of a substance equal to 1 mole (molecular or molar mass) numerically coincides with the relative molecular mass of this substance.
For example, the relative molecular weight of oxygen (O 2) is 32, carbon dioxide (CO 2) is 44, and the corresponding molecular weights will be M = 32 g/mol and M = 44 g/mol. Thus, one mole of oxygen contains 32 grams of this substance, and one mole of CO 2 contains 44 grams of carbon dioxide.
In technical calculations, it is not the heat of combustion that is most often used. Q g, and the calorific value of the fuel Q(J/kg or J/m 3). The calorific value of a substance is the amount of heat released during complete combustion of 1 kg or 1 m 3 of a substance. For liquid and solid substances, the calculation is carried out per 1 kg, and for gaseous substances - per 1 m 3.
Knowledge of the heat of combustion and calorific value of the fuel is necessary to calculate the combustion or explosion temperature, explosion pressure, flame propagation speed and other characteristics. The calorific value of the fuel is determined either experimentally or by calculation. When experimentally determining the calorific value, a given mass of solid or liquid fuel is burned in a calorimetric bomb, and in the case of gaseous fuel, in a gas calorimeter. These instruments measure the total heat Q 0 released during combustion of a sample of fuel weighing m. Calorific value Q g is found by the formula
The relationship between the heat of combustion and
calorific value of fuel
To establish a connection between the heat of combustion and the calorific value of a substance, it is necessary to write down the equation for the chemical reaction of combustion.
The product of complete combustion of carbon is carbon dioxide:
C+O2 →CO2.
The product of complete combustion of hydrogen is water:
2H 2 +O 2 →2H 2 O.
The product of complete combustion of sulfur is sulfur dioxide:
S +O 2 →SO 2.
In this case, nitrogen, halogens and other non-combustible elements are released in free form.
Combustible substance - gas
As an example, let us calculate the calorific value of methane CH 4, for which the heat of combustion is equal to Q g=882.6 .
· Let's determine the molecular weight of methane in accordance with its chemical formula (CH 4):
M=1∙12+4∙1=16 g/mol.
· Let's determine the calorific value of 1 kg of methane:
· Let's find the volume of 1 kg of methane, knowing its density ρ=0.717 kg/m3 under normal conditions:
.
· Let's determine the calorific value of 1 m 3 of methane:
The calorific value of any combustible gases is determined similarly. For many common substances, heat of combustion and calorific values have been measured with high accuracy and are given in the relevant reference literature. Here is a table of the calorific values of some gaseous substances (Table 5.1). Magnitude Q in this table is given in MJ/m 3 and in kcal/m 3, since 1 kcal = 4.1868 kJ is often used as a unit of heat.
Table 5.1
Calorific value of gaseous fuels
|
Substance |
Acetylene |
|||||
|
Q |
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Combustible substance – liquid or solid
As an example, let us calculate the calorific value of ethyl alcohol C 2 H 5 OH, for which the heat of combustion is Q g= 1373.3 kJ/mol.
· Let's determine the molecular weight of ethyl alcohol in accordance with its chemical formula (C 2 H 5 OH):
M = 2∙12 + 5∙1 + 1∙16 + 1∙1 = 46 g/mol.
Let's determine the calorific value of 1 kg of ethyl alcohol:
The calorific value of any liquid and solid combustibles is determined similarly. In table 5.2 and 5.3 show the calorific values Q(MJ/kg and kcal/kg) for some liquids and solids.
Table 5.2
Calorific value of liquid fuels
|
Substance |
Methyl alcohol |
Ethanol |
Fuel oil, oil |
||||
|
Q |
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Table 5.3
Calorific value of solid fuels
|
Substance |
The tree is fresh |
Dry wood |
Brown coal |
Dry peat |
Anthracite, coke |
||
|
Q |
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Mendeleev's formula
If the calorific value of the fuel is unknown, then it can be calculated using the empirical formula proposed by D.I. Mendeleev. To do this, you need to know the elemental composition of the fuel (equivalent fuel formula), that is, the percentage content of the following elements in it:
Oxygen (O);
Hydrogen (H);
Carbon (C);
Sulfur (S);
Ashes (A);
Water (W).
The products of fuel combustion always contain water vapor, which is formed both due to the presence of moisture in the fuel and during the combustion of hydrogen. Waste combustion products leave an industrial plant at a temperature above the dew point. Therefore, the heat that is released during the condensation of water vapor cannot be usefully used and should not be taken into account in thermal calculations.
The net calorific value is usually used for calculation Q n fuel, which takes into account heat losses with water vapor. For solid and liquid fuels the value Q n(MJ/kg) is approximately determined by the Mendeleev formula:
Q n=0.339+1.025+0.1085 – 0.1085 – 0.025, (5.1)
where the percentage (wt.%) content of the corresponding elements in the fuel composition is indicated in parentheses.
This formula takes into account the heat of exothermic combustion reactions of carbon, hydrogen and sulfur (with a plus sign). Oxygen included in the fuel partially replaces oxygen in the air, so the corresponding term in formula (5.1) is taken with a minus sign. When moisture evaporates, heat is consumed, so the corresponding term containing W is also taken with a minus sign.
A comparison of calculated and experimental data on the calorific value of different fuels (wood, peat, coal, oil) showed that calculation using the Mendeleev formula (5.1) gives an error not exceeding 10%.
Net calorific value Q n(MJ/m3) of dry combustible gases can be calculated with sufficient accuracy as the sum of the products of the calorific value of individual components and their percentage content in 1 m3 of gaseous fuel.
Q n= 0.108[Н 2 ] + 0.126[СО] + 0.358[СН 4 ] + 0.5[С 2 Н 2 ] + 0.234[Н 2 S ]…, (5.2)
where the percentage (volume %) content of the corresponding gases in the mixture is indicated in parentheses.
On average, the calorific value of natural gas is approximately 53.6 MJ/m 3 . In artificially produced combustible gases, the content of methane CH4 is insignificant. The main flammable components are hydrogen H2 and carbon monoxide CO. In coke oven gas, for example, the H2 content reaches (55 ÷ 60)%, and the lower calorific value of such gas reaches 17.6 MJ/m3. The generator gas contains CO ~ 30% and H 2 ~ 15%, while the lower calorific value of the generator gas is Q n= (5.2÷6.5) MJ/m3. The content of CO and H 2 in blast furnace gas is lower; magnitude Q n= (4.0÷4.2) MJ/m 3.
Let's look at examples of calculating the calorific value of substances using the Mendeleev formula.
Let us determine the calorific value of coal, the elemental composition of which is given in table. 5.4.
Table 5.4
Elemental composition of coal
· Let's substitute those given in the table. 5.4 data in the Mendeleev formula (5.1) (nitrogen N and ash A are not included in this formula, since they are inert substances and do not participate in the combustion reaction):
Q n=0.339∙37.2+1.025∙2.6+0.1085∙0.6–0.1085∙12–0.025∙40=13.04 MJ/kg.
Let us determine the amount of firewood required to heat 50 liters of water from 10° C to 100° C, if 5% of the heat released during combustion is consumed for heating, and the heat capacity of water With=1 kcal/(kg∙deg) or 4.1868 kJ/(kg∙deg). The elemental composition of firewood is given in table. 5.5:
Table 5.5
Elemental composition of firewood
|
· Let's find the calorific value of firewood using the Mendeleev formula (5.1): Q n=0.339∙43+1.025∙7–0.1085∙41–0.025∙7= 17.12 MJ/kg. · Let's determine the amount of heat spent on heating water when burning 1 kg of firewood (taking into account the fact that 5% of the heat (a = 0.05) released during combustion is spent on heating it): Q 2 =a Q n=0.05·17.12=0.86 MJ/kg. · Let's determine the amount of firewood required to heat 50 liters of water from 10° C to 100° C:
Thus, about 22 kg of firewood is required to heat water. |
kg.
Recently, due to the regular increase in the price of natural gas, the issue of both installation and conversion/modernization of heating systems to alternative (renewable) energy sources, such as coal, firewood, pellets, solar and wind energy, has become relevant.
In this section we will focus on solid fuel boilers.
Depending on the type of fuel, they can be divided into solid fuel boilers (fuel - coal, wood) and pellet boilers (fuel - pellets). In turn, solid fuel boilers are made of cast iron and steel. Each of them is designed to burn a specific type of fuel.
In cast iron boilers, the main type of fuel is coal. Therefore, the rated power of such boilers according to the passport, as a rule, is indicated based on the combustion of coal in cast iron boilers. But, in addition to coal, cast iron boilers can operate on wood and briquettes. But in this case, you need to understand that the rated power of the bute boiler is somewhat less than that stated in the manufacturer’s passport.
Steel boilers are designed for burning brown coal and wood. As a rule, the rated power of such boilers is indicated based on the use of brown coal as fuel. When using wood, depending on its calorific value, the rated power of a steel boiler may differ slightly. Brown coal as a fuel is, as a rule, widespread in Europe (Germany, Poland, etc.) due to its fairly large deposits in this area. In view of the fact that brown coal is not relevant for Ukraine, wood should be taken as a basis.
Since we are talking about the calorific value of solid fuel, I propose to consider this concept and compare different types of fuel according to their calorific value.
Specific calorific value of fuel is a physical quantity that shows how much heat is released during the complete combustion of fuel weighing 1 kg or volume 1 m3. The specific heat of combustion is measured in J/kg (J/m3) or calorie/kg (calorie/m3). To experimentally measure this quantity, calorimetric methods are used.
The higher the specific heat of combustion of the fuel, the lower the specific fuel consumption for the same value of the coefficient of performance (efficiency) of the boiler.
The table below shows the main types of boiler fuel used in everyday life, common in Ukraine.
| Type of energy carrier | Specific calorific value | Implemented systems | ||
| MJ |
Kcalories | kWh | ||
| (1MJ=0.239006 calories) | (1MJ=0.278 kWh) | |||
| Brown coal, briquette | 21 | 5019 | 5,84 |
Heating, hot water supply (DHW) |
| Unprocessed brown coal | 14,7 | 3513 | 4,09 | |
| Charcoal | 31 | 7409 | 8,62 | |
| Oak | 13 | 3108 | 3,61 | |
| Birch | 11,7 | 2804 | 3,25 | |
| Pine | 8,90 | 2127 | 2,47 | |
| Alder | 8,77 | 2097 | 2,43 | |
| Spruce | 7,72 | 1846 | 2,15 | |
| Aspen | 7,40 | 1768 | 2,06 | |
| Coal | 29,3 | 7003 | 8,14 | |
| Coke | 29 | 6931 | 8,06 | |
| Dry peat | 15 | 3585 | 4,17 | Heating |
This table gives a distinctive idea of the maximum possible level of that energy, which is often called the specific heat of combustion for dry (when it makes sense to talk about it) fuels.
Also, from the values presented in the table, you can determine how much the rated power of the boiler will change depending on the type of fuel used. So, for example, if the rated power of a boiler using unprocessed brown coal is 20 kW, then if oak is used as fuel, the rated power of the same boiler will decrease to 17.7 kW.
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 comparative analysis reports, the prospects for using methane or diesel fuel are determined. 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 the fuel, engineering calculations take into account the lower relative calorific value, which is an important parameter of the 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 fuels and lubricants, coking, hard coal, brown and charcoal, as well as anthracite, have the greatest heat transfer capacity. 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 various forms of fuel, is released with the unconditional combustion of one kilogram, one cubic meter or one liter. Most often, such physical properties are 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. The more heat any substance gives off, the greater its heat transfer. This process is also called heat transfer. Liquids, gases and hard particles take part in heat transfer.
