Organic fuel: types, composition and classification

In traditional energy, 2 types of fuel are used - organic and nuclear. Despite the fact that since the second half of the XX century. Nuclear energy is developing very actively, the share of organic fuel in the overall structure prevails. Currently, it is the main source for the production of thermal and electric energy. In total, a person uses about two hundred of its types, each of which has its own characteristics and indicators.

Kinds

There are several classifications of fossil fuels:

• By origin: natural (natural); artificial (obtained by processing natural).
• By area of ​​use: energy (for generating electricity and heat); technological (for the production of various industrial products).
• According to the physical state of the substance (the most common are indicated in brackets): liquid (fuel oil); solid (fossil coals); gaseous (natural gas).
• By “life span”: renewable (wood, plants); conditionally renewable, in which the accumulation period in the earth's crust is several thousand years (peat); non-renewable (coal, shale, oil, gas).

For non-renewable sources of fuel, the accumulation period is many times greater than the expected consumption period.

Natural fuel

Natural types of fossil fuels are divided into the following groups:

• Fossils (extracted from the bowels of the earth): coal and lignite; natural gas; peat; anthracite; oil; oil shale and others.
• Artificial: gasoline; kerosene; shale oil; fuel briquettes; charcoal; hydrolysis lignin; waste from the food, agricultural and pulp and paper industries; fuel oil; gas fuel, obtained as a by-product in the processing of oil shale, smelting cast iron, pyrolysis and other technological processes; waste from wood processing industries (dry sawdust, shavings, lumpy waste).

Organic fuel from agricultural waste

Of agricultural waste, the following are most often used:

• husk of sunflower seeds;
• buckwheat husk;
• rice husk;
• straw.

Since the volume of these sources is small, they are most often used as fuel for local boiler houses.

Origin

According to scientific ideas, all types of fossil fuels were formed from plant debris and microorganisms that existed from 500 thousand to 500 million years ago. Their accumulation occurred in those parts of the earth's crust that were protected from active oxidation (shallow coastal zones of water bodies, swamps, the bottom of the seas). The chemical composition of these residues includes 4 basic elements:

• carbohydrates;
• lignin (intercellular substance of higher plants);
• fat-like substances (resins, waxes, glycerol esters);
• squirrels.

The remains of higher plants and mosses that accumulated in the swampy areas of land became the basis for the formation of humolites (fossil coals), and microalgae and bacteria at the bottom of water bodies - sapropelites. Under the influence of high pressure and temperature, organic matter was converted (coalification).

Humolites with a low degree of coalification are called brown coals. At higher temperatures, humic acids turned into neutral humins. In coal, there is a complete absence of humic acids.

In mild conditions, under mild conditions, the polymerization of unsaturated hydrocarbons predominantly occurred with the formation of oil shales, which, when distilled, produce a large amount of resin similar in composition to oil. Metamorphoses of sapropel at high temperatures and the catalytic participation of rocks led to the formation of a mixture of hydrocarbons in a liquid and gaseous state (oil, natural gas, associated gas).

Solid fuel

Solid fossil fuels are capillary-porous heterogeneous materials. Their structure contains a large number of pores and cracks. Before burning at thermal power plants, the raw materials are crushed to size 15-25 mm (layered burning in boilers) or to a pulverized state to reduce losses from underburning.

Liquid and solid fossil fuels are based on 5 combustible chemical elements: C, H ₂ , O ₂ , S. External (ash residue after burning, moisture) and internal (nitrogen and oxygen) ballast degrade the quality of the fuel.

Solid fuel characteristics

The main types of combustible organic fuels, their brands and a brief description are presented in the table below.

Moisture effect

The high moisture content makes it difficult to ignite combustible materials, reduces the temperature in the furnace, and increases heat loss. Fuels, which are characterized by a large geological age, have little water in their composition (brown coal, peat).

There are several types of moisture:

• sorption, accumulating at the boundary of the solid and gaseous phases;
• capillary (pore);
• superficial (found on the outer surface of the pieces);
• hydrated (part of crystalline hydrates).

The first 3 types of moisture can be removed from solid fossil fuels by drying at a temperature of 105 ° C, the latter - only through chemical reactions when heated to 700-800 ° C. During transportation and storage in the open air, the water content may increase significantly, impairing the quality of the fuel.

Mineral impurities

All types of solid fuel have mineral impurities, mainly consisting of the following compounds:

• silicates;
• sulfides;
• carbonic salts of Ca, Mg and Fe;
• phosphates;
• chlorides;
• sulfates of calcium and iron.

In the process of burning fossil fuels, they undergo a high-temperature transformation, as a result of which solid non-combustible ash remains. Its composition is very different from the starting materials due to the following reactions:

• the conversion of iron oxide salts to oxides;
• dehydration of silicate compounds;
• decomposition of carbonates, evolution of CO ₂ , formation of oxides;
• oxidation of sulfur compounds, evolution of sulfur dioxide;
• evaporation of alkali metal salts.

The final composition of the ash residue depends on the conditions of combustion of fossil fuels. At high temperatures, it can melt and go into a liquid state (slag). Part of the ash is removed from the furnaces along with volatile combustion products, which leads to pollution, slagging and corrosion of the furnace equipment.

Thermal decomposition

Solid fossil fuel during combustion passes through several stages of decomposition:

• bertinization (temperature up to 300 ° , carbon dioxide and carbon monoxide, hydrogen and hydrocarbons, pyrogenetic water are released);
• semi-coking (400-450 ° , the main volume of combustible gas is released);
• coking (700-1100 ° C, completion of the process for the release of volatile compounds).

Combustion products of fossil fuels are respectively called bertinates, semi-coke, coke.

The lowest calorific value for high-ash shales, wet peat and brown coal, and the highest for anthracite. The net calorific value, at which water vapor enters the atmosphere rather than condenses, is 4.6-26 MJ / kg for solid fuels.

Liquid fuel

Liquid organic fuels for the energy industry are obtained from oil using its thermochemical decomposition. Fuel oil is used at large facilities (thermal power plants, boiler houses), and for domestic purposes, distillate fractions of oil products (gasoline, kerosene, diesel fuel, diesel fuel) are used.

Fuel oil, like oil, is a complex colloidal compound. Its chemical composition varies in the following ranges (in percent):

• carbon 86-89;
• hydrogen 9.6-12;
• sulfur - 0.3-3.5;
• oxygen and nitrogen - 0.5-1.7.

Types of Fuel Oil

The classification of fuel oil is as follows:

• By sulfur content: low sulfur (<0.5%); low sulfur (0.5-1%); sulphurous (1-2%); high sulfur (2-3.5%).
• By viscosity (brands are indicated in brackets): light or naval (5, 12); medium (40V, 40); heavy (100, 200 and 100V); coal and shale (formed during the processing of oil shale and coal).

The calorific value of liquid fuel varies between 39-41 MJ / kg.

Gaseous fuel

The composition of gaseous fuels includes the following substances:

• combustible (saturated hydrocarbons, H ₂ , CO, H ₂ S) and non-combustible (carbon and sulfur dioxide, nitrogen, oxygen, atmospheric air) gases;
• water vapor;
• resins;
• dust.

The following fossil fuels are most widely used:

• Natural gas. The main component is methane. Before consumption, the gas is drained, dedusted, and harmful impurities of hydrogen sulfide are removed.
• Associated gas generated during oil production. Hydrocarbons are separated from the liquid phase in separators. The volume fraction of methane is less than in natural gas, and that of heavy hydrocarbons is higher. In this regard, when burning fossil fuels, more heat is released.
• Liquefied gas. The main components are propane and butane, as well as impurities of heavy hydrocarbons. At a temperature of 20 ° C and atmospheric pressure, it takes on a gaseous state. With increasing pressure or decreasing temperature, the gas passes into the liquid phase, which is used for its transportation. The raw material for this type of fuel is associated gas and gas obtained during oil refining.
• Coke oven gas. It is a by-product of coal coking. The initial product is purified from harmful impurities, ammonia, aromatic hydrocarbons. The output is up to 3000 cubic meters of 1 ton of coal.
• Blast furnace gas. It is formed as a result of the interaction of coke and iron ores when they are purged in blast furnaces. The output is 2200-3200 m ³ per 1 ton of smelted cast iron.

The calorific value of gas fuel depends on its chemical composition and is in the range of 4-47 MJ / m ³ . Almost all types of flammable gases are lighter than air and accumulate under leaks during leakage. The lowest concentration in the mixture with air required for ignition is in pentane (1.4% by volume).

The given characteristics

For a comparative analysis of the properties of various types of fuel, the above characteristics are used, defined as the ratio of the quality index of the working fuel to its specific lower heat of combustion.

The main calculated indicators are:

• humidity;
• ash content;
• sulfur and nitrogen content.

In the fuel and energy industry, the concept of conventional fuel is also used to compare the efficiency of the fuel used. It is a fuel in which the lowest specific heat of combustion in working condition is 7000 kcal / kg. For each type of fuel, it is possible to calculate the dimensionless heat coefficient in the form of the ratio of the specific heat of combustion to this value for conventional fuel.

With the complete combustion of fossil fuels, triatomic gases (carbon dioxide and sulfur dioxide) and water are formed. The consumption of substances (for 1 mole of fuel) involved in the combustion is calculated according to the formulas based on the condition that all oxygen supplied with air will react. Such equations are called the material balance of combustion.

Under real conditions, the calculated values ​​are corrected with the help of coefficients, since full combustion always requires more air. To determine the temperature of the combustion products, the heat balance of the oxidation reaction is calculated (per 1 kg of liquid or solid organic fuel or 1 m ³ for gaseous). From the point of view of physics, the heat balance equation is nothing more than a form of writing the law of conservation of energy.