Pyrolysis is thermal decomposition of organic compounds without access for air. Pyrolysis can also be called a dry run-down, and raw materials can consist of various gaseous hydrocarbons (ethane, propane), crude oil, the compounds containing organic components such as wood, coal, turf etc.

Pyrolysis of wood is one of the first chemical processes taken up by people. Since the 12th century, it is widely used in Russia to produce pine resin (used to pitch wooden ships and impregnate ropes), this process is called resinous wood distillation. Sweden is considered to have been the first to use the pyrolysis technology on an industrial scale since XVI century. Swedes exploited technology of ship wood impregnating by tanning pine resins resulted from simple pyrolysis. Resinous wood was placed in a copper vessel to be sealed air-proof to achieve no air access. . The vessel was heated up to 400-500 °C in the open fire;this would cause oily resins to accumulate in the bottom and merge through a hole. As metallurgy developed, it gave start to another industry based on wood pyrolysis technology –i.e. charring. Burning of wood without access of air creates charcoal.

Russian chemists Yulia Vsevolodovna Lermontova and Alexander Alexandrovich Letniy were the first in the history of chemistry to note that black coal yields coal gas (a mixture of hydrogen, methane, carbon monoxide and other combustible gases produced during pyrolysis of coal or oil), worse than gas of oil origin. And in 1877, hydrocarbon pyrolysis effect was patented by Alexander Alexandrovich Letniy.

Back in 1875, he found that heavy oil residues are partially decomposed into lighter products (gasoline, kerosene, gas) at temperatures above 300 °C. In 1877, aromatic hydrocarbons (benzene, toluene, xylene, anthracene, etc.) were first extracted from petroleum and he established important regularities in oil pyrolysis. Almost at the same time, the first factories were built in Kiev and Kazan. Kerosene was pyrolyzed to obtain gas for lighting. World War I has contributed much to develop  industrial applications of pyrolysis technology, when there was a great need in toluene which is raw material for production of TNT. In XX century, a significant contribution to the development of new areas of wood pyrolysis was introduced by Professor Tishchenko.

Pyrolysis of the solid fuel

Pyrolysis of solid fuels has been a very important part of Russian energy sector for a long time (until use of natural gas). Many enterprises acquired powerful gas generator equipment, and vehicles were equipped with gas-based engines using pyrolysis technlogy.

In the 30-ies of XX century USSR has witnessed a very rapid industrial development with many vehicles to require an enormous amount of hydrocarbon fuels - gasoline and diesel fuel. Lack of oil was permanent.

Mobile gas generators invented during World War I have been widely used to solve this problem. Design of a gas generator unit is simple. Fuel loaded into a gas generator unit is ignited through an air vent using a torch. Air required for gasification is sucked into the chamber through the tuyere openings due to dilution created by the suction effect from the engine. Moreover, the amount should be insufficient for complete combustion. Carbon combines with oxygen to fuel the air to form carbon dioxide (CO2) and carbon monoxide (CO). Then they fall in the reduction zone to pass through live coal lying on the grate . As a result, non-flammable CO2 is converted into combustible CO. Hydrogen being part of fuel partially combines with oxygen to form water which is added to the fuel moisture and the remainder is released in a pure form. Under the influence of high temperatures in the gasification chamber part of the moisture combines with carbon to form carbon monoxide and hydrogen. Carbon monoxide, together with a previously formed and recovered moisture passes into the gas generator . Water-decomposition hydrogen is added to the free hydrogen; part of which goes into the gas generator , and the other part enters a chemical reaction with carbon in the fuel to form methane. Theoretically, all of the oxygen in the air must expend itself during gasification, but in fact, some of it is stored and transferred in the gas generator. Water, which failed to decompose by gasification, goes into generator gas as steam. Fuel pyrolysis process (dry run-down) starts in the fuel layer situated directly above the combustion zone, i.e. heating without access of air . Dry   run-down comprises wood charcoal or coke, and volatiles, resins and moisture being an output as gas and steam. All dry run-down products described in the described type of generator entirely pass through the combustion and recovery zone where they are exposed to gasification process a bit more complicated than this, but giving the same basic products. Drying zone is located over the run-down zone, where fuel dries. Gas leaving the generator is highly heated and polluted by ash and coal particles. It cannot be used in an engine in this form and it shall be cleaned and cooled before it enters the cylinder.

Pyrolysis gas comes into use worldwide as an alternative source of energy, especially  thermo-based energy. In many European countries  pyrolysis gas has long been common fuel, which generated power, steam and hot water for decades. Presently thermal pyrolysis of hydrocarbon raw materials is the main method for producing of ethylene and propylene.

Power station on plant’s pyrolysis gas in Aceralia, Spain

Thermal power station on pyrolysis gas in Fondototse, Italy

Pyrolysis of Wood is air-tight decomposition of wood at 450 ° C to form gaseous and liquid products and a solid residue. Hardwood (seldom - softwood) is now commonly used for pyrolysis process.

Pyrolysis of solid fuel has the same essence as a fluid-like, as Letniy wrote. When splitting molecules of a solid fuel, there is a carbon-rich solid phase (coke, coal) and a gaseous phase containing hydrocarbon vapors (pyrolysis gas). Pyrolysis gas is obtained during thermal chemical transformations of solid fuel air-tight and heated to 500-1000 °C with a calorific value of 3000-4000 kkal/nm3, and in the combustion process with a deficiency of air  as in C + O2 = CO2 + Q, then CO2 + C = 2CO -Q, C + H2O = CO + H2 -Q with calorific value of 900-1600 kcal/nm3.

Pyrolysis boilers

Pyrolysis (gas generator ) boiler is just based on the principle of pyrolysis combustion where at high temperature and no air access  wood divides into a volatile part – (pyrolysis gas) and a solid residue (ash) . After combustion in the boiler furnace solid (smoldering)  pyrolysis gas enters the post-combustion chamber, after which it burns in a very clean flame  The energy of pyrolysis gas combustion goes from the post-combustion chamber through the heat exchanger and is used to heat  the heating carrier that participates in the combustion process . Thus, solid fuel pyrolysis boilers have a higher efficiency and allow you to adjust the power.

As power carriers like gas and liquid fuel go up, solid fuel gasification become more relevant, especially wood and coal. Russia is one of the most forest-rich countries. We have world best scientists related to pyrolysis, including pyrolysis of wood. Pyrolysis boilers areprofitable, efficient and very economical thermal equipment for any room!