What is the calorific value of fuel? How to calculate this value, where can it be used? Together we will seek answers to these important and relevant questions for humanity.
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 of fuel differ in their quantitative content of oxidizing agent, which is used for the release of thermal energy.
In a broad sense, fuel is an energy carrier, that is, a potential type of potential energy.
Classification
Currently, fuels are divided according to the state of aggregation into liquid, solid, gaseous.
Coal and brown coal, firewood, anthracite are ranked as a solid natural species. Briquettes, coke, charcoal, thermoanthracite are varieties of artificial solid fuel.
To liquids are substances containing organic substances in their composition. Their main components are: oxygen, carbon, nitrogen, hydrogen, sulfur. A variety of resins and fuel oil will be artificial liquid fuel.
Gaseous fuel is a mixture of various gases: ethylene, methane, propane, butane. In addition to them, gaseous fuels contain carbon dioxide and carbon monoxide, hydrogen sulfide, nitrogen, water vapor, and oxygen.
Fuel performance
The main indicator is the calorific value. The formula for determining the calorific value is considered in thermochemistry. emit "standard fuel", which implies a calorific value of 1 kilogram of anthracite.
Domestic heating oil is intended for incineration in heating devices of insignificant power, which are located in residential premises, heat generators used in agriculture for drying feed, and canning.
The specific heat of combustion of the fuel is such a value that it demonstrates the amount of heat that is generated during the complete combustion of fuel with a volume of 1 m 3 or one kilogram mass.
To measure this value use J / kg, J / m 3 , calorie / m 3 . To determine the calorific value, use the method of calorimetry.
With an increase in the specific heat of combustion of the fuel, the specific fuel consumption decreases, and the efficiency remains unchanged.
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 the state of aggregation of the combustible substance.
Features of combustion products
Higher and lower heat of combustion is associated with the state of aggregation of water in substances obtained after fuel combustion.
Higher calorific value is the amount of heat released during the complete combustion of a substance. This value also includes the heat of condensation of water vapor.
The lower working heat of combustion is the value that corresponds to the heat generated during combustion without taking into account the heat of condensation of water vapor.
The latent heat of condensation is considered the value of the energy of condensation of water vapor.
Mathematical relationship
Higher and lower calorific value are related by the following ratio:
Q B = Q H + k (W + 9H)
where W is the amount by weight (in%) of water in a combustible substance;
H is the amount of hydrogen (% by weight) in the combustible substance;
k - coefficient of 6 kcal / kg
Calculation Methods
Higher and lower heat of combustion is determined by two main methods: calculated and experimental.
For experimental calculations, calorimeters are used. First, a portion of fuel is burned in it. The heat that will be released in this case is completely absorbed by water. Having an idea of the mass of water, you can determine by the change in its temperature, the value of its calorific value.
This technique is considered simple and effective, it involves only ownership of information about technical analysis data.
In the calculation methodology, the higher and lower heat of combustion is calculated according to 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 conditional fuel.
The calorific value of gas allows preliminary calculations, to identify the effectiveness of a particular type of 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 are different types of solid fuels that differ in composition and properties.
His education is carried out through several stages. First peat is formed, then brown and black coal is obtained, then anthracite is formed. The main sources of solid fuel formation are leaves, wood, needles. When they die off, parts of plants under the influence of air are destroyed by fungi and form peat. Its accumulation turns into a brown mass, then brown gas is obtained.
At high pressure and temperature, brown gas passes into coal, then the fuel accumulates in the form of anthracite.
In addition to organic matter, there is additional ballast in the fuel. The part considered to be organic is that formed from organic substances: 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 combusted fuel. The working mass is called the fuel in its original form, supplied to the consumer. Dry mass is a composition in which there is no water.
Composition
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. Contacting with oxygen, it forms moisture, which significantly reduces the thermal value of any fuel.
Its percentage varies from 3.8 in oil shale to 11 in fuel oil. The ballast is oxygen, which is part of the fuel.
It is not a heat-generating chemical element; therefore, it negatively affects the value of its heat of combustion. The combustion of nitrogen, contained in free or bound form in the products of combustion, is considered harmful impurities, therefore, its amount is clearly limited.
Sulfur is part of the fuel in the form of sulfates, sulfides, as well as sulfur dioxide. During hydration, sulfur oxides form sulfuric acid, which destroys boiler equipment, negatively affects vegetation and living organisms.
That is why sulfur is the chemical element whose presence in fossil fuels is highly undesirable. If it gets inside the working room, sulfur compounds cause significant poisoning to the operating staff.
Three types of ash are distinguished depending on its origin:
- primary;
- secondary;
- tertiary.
The primary species is formed from minerals that are contained in plants. Secondary ash is formed as a result of the ingress of sand and soil during plant formation.
Tertiary ash is included in the fuel during production, storage, as well as 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. A huge amount of ash negatively affects the operation of the boiler.
Finally
Volatile substances have a significant effect 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, light up easily, the process is accompanied by slight heat loss. Coke, which 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 of the formation of solid fuel: peat, brown, coal.
Natural wood is used in small boiler plants. They mainly use wood chips, sawdust, slabs, bark, and firewood themselves are 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: fast flammability, minimal ash content, and no trace of sulfur.
Reliable information about the composition of natural or synthetic fuels, their calorific value, is an excellent way to conduct thermochemical calculations.
Currently, there is a real opportunity to identify those basic options for solid, gaseous, liquid fuels that will become the most effective and inexpensive to use in a particular situation.