The pyrolysis phenomenon always accompanies the burning of solid fuel in heating furnaces and boilers. The scale of the process depends on two factors - the combustion mode and the design of the home heat power plant. We offer a detailed look at the pyrolysis of wood or coal, options for its use in industrial and domestic conditions. The goal is to dispel the myths invented by sellers and artisan manufacturers of expensive "pyrolysis" equipment designed to heat private homes.
What is pyrolysis - process description
Theoretically, any substance including carbon compounds with hydrogen can be burned, for example:
- coal;
- natural gas (methane, propane and so on);
- biomass - fresh, dry;
- wood products, cellulose, ordinary wood;
- various types of plastics;
- rubber from natural or artificial rubber;
- oil, its derivatives;
- other carbon-containing waste.
At the exit, get a certain amount of thermal energy, depending on the initial moisture content of the burned mass. To describe the processes, we use the chemical formula:
Combustion is a quick oxidation reaction. Under ideal conditions, each carbon atom combines with two oxygen particles, and 2 hydrogen atoms interact with 1 oxygen particle. As a result, harmless compounds are formed - carbon dioxide CO2 and water. The latter evaporates when heated, taking away part of the released heat.
An important point. In real conditions, not all hydrogen and carbon atoms find a mate due to a lack of oxygen molecules. Therefore, the composition of the combustion products includes a small fraction of harmful combustible compounds - carbon monoxide (CO), free hydrogen (N2) and carbon in the form of soot.
Pyrolysis is a decomposition reaction of a substance that occurs when heated and lack of free oxygen. The indicated principle is used in gas generating units:
- Fuel (in particular, wood) is placed inside a closed metal vessel - reactor.
- The capacity is heated externally to 500 ... 900 degrees, through special openings - tuyeres a metered amount of air is supplied.
- Under the influence of high temperature, the substance decomposes into 3 main components - carbon monoxide (CO), hydrogen (N2) and solid or liquid carbon residue. A small amount of carbon dioxide and water vapor are formed in parallel.
- Volatile products make up pyrolysis gas - a combustible mixture of hydrogen and carbon monoxide, leaving the tank through a separate pipeline. The released gaseous fuel is cleaned, cooled, and then pumped into the tank.
Reference. Under production conditions, the resulting synthesis gas is sent to heat the same capacity of the gas generator.
Combustion and pyrolysis are 2 different processes that can occur simultaneously. Example: during intense burning of firewood, a small amount of carbon monoxide, a harmless CO, is formed in the boiler furnace2 much bigger. And vice versa, in the smoldering regime of firewood a lot of hydrogen and fumes are released, part of which manages to turn into CO2 - oxidize. That is, it all depends on the amount of oxygen involved in the reaction.
The effect of high humidity
The high moisture content in the starting material equally affects the combustion and pyrolysis reactions. Consider the processes of wood burning as an example:
- When burning, the energy released is spent on evaporating the water contained in the wood. The amount of heat at the outlet is significantly reduced, fuel is wasted.
- Moisture greatly slows down the thermal decomposition of a substance.Evaporated water takes part of the heat spent on heating; the required temperature (minimum 500 ° C) is not achieved. Pyrolysis of wood containing more than 50% moisture is almost impossible.
The best indicator of humidity for fruitful burning or decomposition of wood in a gas generator is 8 ... 15%. At home, it is unrealistic to achieve such indicators, prolonged drying of firewood under a canopy allows you to achieve 20-25% moisture content.
Reference. In the manufacture of fuel pellets and briquettes at the plant, sawdust is dried to an indicator of 8-10%. The maximum humidity of the finished granules is 15%.
Why use thermal decomposition
The scope of pyrolytic processes is quite wide:
- Production of propylene and ethylene for the chemical industry by processing liquid hydrocarbon feedstocks (oil).
- Obtaining charcoal by the method of oxygen-free decomposition of wood processing waste.
- The same technological process, but with a limited air supply allows the generation of combustible synthesis gas from a tree - a mixture of methane, hydrogen, carbon monoxide and neutral nitrogen.
- Pyrolysis of coal - brown and stone - a whole area of processing. The resulting compounds are synthetic gasoline, coke, ammonia, coal tar. Toluene, benzene, naphthalene and various phenols used in the chemical industry are mined from the latter.
- New developments - commercial utilization of municipal solid waste, car tires, plastics, organics.
Note. The most famous methods for using pyrolytic reactions are listed here. In reality, there are many more use cases. Wikipedia claims that pyrolysis processes are not fully understood, many projects are under development.
For thermal decomposition, pyrolysis furnaces and various reactors are used in industry. The diagram above shows a gas generator that processes wood waste and sawdust into gaseous fuel. The main role here is played by the direct dry distillation reactor, where the prepared raw materials are processed into synthesis gas by slow combustion.
An important nuance. Before loading into a pyrolysis furnace or gas generator, the wood is always crushed and dried to a moisture content of 10% or less.
In industrial chemistry, rapid pyrolysis technology is also used, when the reactor is heated to a temperature of 700 ... 900 ° C for a short period of time. The goal is to increase equipment productivity and speed up processing.
Domestic use
At the household level, pyrolysis helps to solve the following problems:
- cleaning the oven or fryer from sticky fat deposits that cannot be removed mechanically;
- charcoal production;
- heating a private house with a pyrolysis solid fuel boiler.
The best method to clean the pan is to place it in the oven, set the temperature to 200 ... 250 ° C and let stand for half an hour. Without oxygen, sediment destruction will occur, only ash will remain, and the cooker hood will take away the pyrolysis gases.
Reference. There are models of ovens with a built-in pyrolytic cleaning function. At the end of the "roasting", it remains only to wipe the internal surfaces and discard the resulting ash.
Charcoal is used for frying barbecue, blacksmithing and more exotic purposes - refueling an automobile gas generator (how it works, read in a separate material). The method of obtaining - burning wood waste inside a closed container, that is, slow pyrolysis.
We propose to analyze in detail the problems associated with pyrolysis wood heat generators.
Myths about pyrolysis TT boilers
The main structural difference between a gas generator heater and a traditional direct combustion boiler is 2 chambers instead of one.A ceramic nozzle is arranged between both fire chambers; air is forced by a fan. The metal walls of the pyrolysis unit are protected by a refractory brick lining. How does he work:
- Firewood or coal is laid in the upper (primary) chamber and set on fire.
- Automation starts the boost fan.
- When the temperature in the firebox rises to 500 degrees, the release of pyrolysis gases begins.
- Carried away by the general flow of combustion products, these volatile compounds enter the lower secondary chamber, where they are burned in the presence of oxygen (supposedly).
In fact, the resulting synthesis gas begins to burn even in the primary furnace, since the fan supplies excess air. Only a torch of flame is directed into the second chamber ... and that’s it. Further, the combustion products move through the heat pipes of the heat exchanger, heat the coolant and fly into the chimney.
Addition. There is another design of heaters - without a fan, the secondary chamber is located at the top. From the point of view of pyrolysis, the concept is inoperative, the unit functions like a regular wood-fired hot water boiler, although it costs twice as much as classic counterparts.
Supporters of pyrolysis heat generators (such as the manufacturers of this equipment, sellers and home craftsmen) attribute the following advantages to their TT boilers:
- the fuel is burned completely, the remainder in the ash pan is practically zero;
- burning time - 10 hours or more;
- low amount of harmful emissions into the atmosphere;
- high efficiency due to the efficiency of 86 ... 90% (manufacturers' indicators) compared to traditional boilers with an efficiency of 75%.
Let's try to figure out the veracity of these statements. The first moment: if the firebox is loaded with dry wood (such is required according to the instructions for use of the heater), then fine ash will remain after burning. The air flow created by the fan and accelerating in the nozzle simply blows a light residue into the chimney.
The result is an almost empty ash pan, an illusion of complete combustion. If you lay dry wood in a classic turbocharged TT boiler, you get a similar residue - a little ash at the bottom. That is, the completeness of combustion depends on the quality of the fuel, and not on the design of the heat generator.
Comment. The laying of raw firewood with a humidity of over 50% will give a negative result in any boiler. Considering such options is pointless.
Briefly give answers to the remaining statements:
- The burning time of 10-12 hours corresponds to reality. Another thing is that the indicator is achieved due to the size of the fuel chamber (100 liters or more), which holds a lot of firewood. Pyrolysis has absolutely nothing to do with it.
- The environmental assurance of the boiler is true. The fan pumps excess air; very few toxic gases are formed. In standby mode, oxygen does not enter the furnace, firewood slowly smolders and the amount of harmful emissions increases.
- The boiler efficiency of 90% is a fairy tale. In active combustion mode, the principle of operation of the boiler is similar to the turbocharged versions of traditional units, whose efficiency does not exceed 75%. When the fan is turned off, the flame dies, the embers emit little heat.
Conclusion. The acquisition of a gas-generating model of a solid fuel boiler is a very dubious undertaking. The unit is three times more expensive than regular versions and twice as heavy due to the lining. Homemade heat generators, as a rule, are more reliable and cheaper than factory ones, but are too bulky. In terms of efficiency and other characteristics, they do not outperform classic TT boilers with a turbine or chain traction control.
A well-known expert practitioner will confirm our opinion in his video:
Conclusion
In general, pyrolysis is a rather useful phenomenon that is widely used in industrial chemistry.At the household level, pyrolytic processes are used infrequently, although the generation of combustible gases occurs in any wood-burning stove or boiler. So it’s pointless to buy expensive pyrolysis models.