The specific heat of combustion of firewood is the amount of heat that is released during the combustion of one kilogram (unit volume) of gaseous, liquid, solid fuel. As an international unit of measure for a given quantity, j / kg or j / m3 is used.
What is fuel
This substance (mixture), which is capable of chemical reactions accompanied by heat.
If the fuel does not have an oxidizing agent, it is called combustible. In a general sense, it includes not only different types of wood, but also different fuel mixtures.
Classification
Depending on the state of aggregation, gaseous, liquid, solid fuel is emitted.
The hard look includes stone and brown coals, anthracite, peat, firewood. Charcoal, thermoanthracite, coal and lignite dust, and coke are considered artificial types of such fuel.
Liquid species are organic substances that contain oxygen, hydrogen, carbon, sulfur, nitrogen. Artificial liquid fuel is fuel oil and various resins.
Gaseous species are mixtures of ethylene, methane, and other hydrocarbons. They may include carbon oxides and dioxides, hydrogen sulfide, water vapor. Artificial gaseous fuels are gases that are obtained in coke ovens and gas generators.
Fuel performance
The specific heat of combustion of firewood is an important quantity that characterizes the quality of fuel.
In order to make comparisons between different species, the concept of fuel equivalent was introduced. 29.3 mJ (7000 cal) corresponds to the lowest calorific value of anthracite, this indicator is the calorific value of 1 kg of “standard fuel”.
Domestic heating oil is used for burning in heating devices of insignificant power, which are located in residential premises. In addition, it is in demand in heat generators used in agriculture for drying fruits, grain, and feed.
The specific heat of combustion of firewood depends on the type of wood. The larger this value, the less fuel will be required for an equal value of the efficiency.
Separation of the calorific value
This value is determined by the chemical composition of the combustible substance. Allocate lower and higher heat of combustion.
The highest value is the amount of heat that is released during the combustion of a substance, including the heat of condensation of water vapor during the cooling of combustion products.
The lowest indicator of this physical quantity corresponds to the energy that is released during complete combustion, not taking into account the heat of condensation of water vapor.
Correlation of quantities
There is a mathematical relationship between the higher and lower heat of combustion:
Q B = Q H + k (W + 9H), where k is 6 kcal / kg (coefficient); W-% amount by weight of water in the combustible compound; H-% hydrogen content .
The specific heat of combustion of firewood is calculated by calculation or experimental methods.
Fuel burning
In the case of adiabatic combustion of a combustible mixture, the heat released, as well as the composition of the products, can be determined. With the known composition of the fuel components, with a constant volume and pressure, the thermal effect of the chemical reaction can be calculated . Thanks to the thermodynamic calculation, only some information about the process can be obtained: product temperature, equilibrium composition.
For a complete description of combustion, including the speed of the process, critical conditions, it is necessary to consider the relationship between the transfer of matter and energy.
With preliminary mixing of the oxidizer and fuel, the process is carried out throughout the space, it is called volumetric combustion. In unmixed systems, diffusion combustion is observed when the fuel is separated from the oxidizing agent.
Chemical age of solid fuels
There are three stages of its formation: brown, peat, coal.
Wood is mainly used in small boiler plants. Since the specific heat of combustion of firewood in joules is of high importance, waste from woodworking production (sawdust, shavings, wood chips, slabs, bark) are used in modern boiler plants.
The specific heat of combustion of dry pine firewood significantly exceeds the value characteristic of a wet tree.
Firewood that is harvested in the spring is considered semi-dry. Dry are those that lay in the forest after logging, their humidity does not exceed 30%.
The specific heat of combustion of dry firewood depends on the breed. Note that this indicator is the same for all wood waste: sawdust, wood chips.
Firewood with a reduced rate has certain advantages: low ash content, low flammability.
That is why they can be burned in simple furnace devices that function effectively.
With proper drying, the specific heat of combustion of firewood is 20,000 kJ / kg, which corresponds to 5.5 kW * hours / kg.
Types of firewood
Pine firewood has a higher specific heat of combustion than birch wood. But it has a higher density, so when you burn one meter cubic, it will generate more heat than pine.
The specific heat of combustion of birch firewood is similar in value to alder, aspen, and oak. The group of hardwoods includes elm, maple, ash, linden, apple, cherry, walnut, acacia. They are characterized by high density. That is why the specific heat of combustion of dry birch firewood allows them to be used in the heating of baths.
Of the conifers, the most common are pine and spruce. Despite the fact that juniper is also a conifer, they are practically not used as fuel.
The high specific heat of combustion of pine firewood is explained by their resinousness. This quality allows the use of conifers as an excellent kindling material. Resins emit a significant amount of soot during combustion, as a result of which a chimney is quickly clogged. When selecting a specific furnace design, the output of a rational combustion mode, excessive soot can be avoided.
The high specific heat of combustion of dry firewood of pine and spruce for many centuries has been used by residents of the taiga zone to heat homes.
There is a division of rocks by density into three groups:
- soft are spruce, pine, aspen, cedar, linden, fir;
- consider plum, juniper, birch, elm as average in hardness;
- solid rocks recognize apple, ash, hornbeam, maple, oak.
For practical use, it is important to have knowledge of several parameters. The calorific value of firewood of different species per unit of weight is almost the same, but heavier and denser wood emits more heat during combustion. For example, birch firewood can provide 20-30 percent more heat than its pine counterparts.
In addition, this value is associated with wood moisture. The drier the wood, the more heat can be obtained from it. Professionals consider humidity to be the optimal value in the range of 20-25 percent.
Why is the specific calorific value of raw firewood less? They contain about 50 percent moisture, so they give off three times less heat than dry ones.
For many owners of private houses, the cost of firewood is not the last indicator. Pine and spruce are more affordable, therefore it is precisely such firewood that a large part of the population acquires.
Alternative fuel
In addition to firewood, peat has excellent calorific values. Depending on the method of production, it is divided into three types: baggage (machine-forming), milling, hydraulic.
The first option is characterized by the extraction of peat mass from the quarry with the help of excavators, feeding it to a special press, where a tape shape is formed. It is cut into small bricks, which, after drying, are stacked in special stacks.
Hydraulic production is based on the erosion of the peat massif by a powerful water stream, which flows under high pressure. The resulting slurry is passed through special rubbers, pumped to the site with pumps, and then dried.
The milling version is associated with the consistent development of peat bogs with special machines, drying the resulting mass, and stacking it in stacks.
Conclusion
All types of solid fuels existing on our planet owe their appearance to chlorophyll and solar energy. In the course of complex organic interactions, the green parts of the plants turned into brown coal, anthracite, and peat.
Despite this diversity, firewood remains one of the most common types of fuel these days.
In combustion technology, combustible, dry, and also working masses of minerals are isolated.
The amount of heat released after complete combustion depends on these quantities. The specific heat of combustion is that physical quantity by which it is possible to determine the rationality of the choice of wood for heating boilers. To obtain maximum thermal energy, it is necessary to take into account not only the type of wood, but also its moisture and density.
Due to the serious deterioration of the environmental situation on our planet, a decrease in the number of minerals, there has recently been a tendency to increase the share of alternative energy sources. Traditional firewood, hard coal and lignite, is gradually supplanted by the energy of the wind, tides.