Pyrolysis boiler

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In this article: how generator gas is formed in pyrolysis boilers; how the pyrolysis boiler works and works; its positive and negative characteristics; manufacturers, prices and selection criteria; installation and operation of pyrolysis boilers

Pyrolysis boiler

An indicator of the complete combustion of fuel in the furnace and, accordingly, its complete transformation into thermal energy for heating living quarters is … the smoke coming from the chimney. If this smoke is visible, as they say, with the naked eye, furnace gases escape through the pipe, which could be converted into energy. Today there is only one type of boilers capable of completely burning the solid fuel contained in them, including generating and burning its gas phase, and these are pyrolysis boilers..

Where does the gas phase come from when burning solid fuel?

To see the gas phase with your own eyes, light a regular match and extinguish it after the flame has ignited. Did you notice the white smoke coming from the match as soon as it is extinguished? This is the very gas phase that occurs when the wood is strongly heated. Pyrolysis (generator) gas consists of dioxide, carbon monoxide, methane and other hydrocarbons. By the way, it is on the pyrolysis of wood that the work of smokers and smoking itself is based.

How do ordinary household stoves, which are traditionally equipped in private houses in rural areas, work? The first fuel filling is a wood-burning one, it heats the stove to the required temperature, then coal is laid. When the first bookmark is ignited, smoke comes out of the chimney in clubs, because the furnace temperature is too low for high-quality combustion of fuel, then the intensity of the smoke output decreases (coal is fired), but complete combustion of the gas phase of the coal does not occur and these gases leave the furnace into the chimney, then into the atmosphere.

To obtain and ignite the gas phase, a temperature of 450 to 1100 ° C is required; as a result, additional heat energy is generated, which makes it possible to increase the efficiency of the heating boiler. But only a high temperature will not be enough, a forced supply of carefully calculated portions of air will be required (a small amount of it will not allow the fuel to burn completely, but if there is a lot of air, it will simply heat up and evaporate into the pipe along with the heat) and a gas combustion chamber that has no length less than half a meter. Adjustable air supply in manual mode is not possible, here you need an automation that can monitor the temperature inside the boiler and, according to these data, reduce or increase the air supply. In the absence of a combustion chamber, pyrolysis gases will cool down, having passed only 400 mm along the cold walls of the boiler, their combustion will stop and they will still evaporate into the pipe.

Pyrolysis boiler – device and principle of operation

Its design contains two chambers – the upper one is designed for stowing and burning solid fuel, the lower one is used for burning generator gases. The wood fuel placed in the upper chamber is additionally dried in the process of combustion and the formation of the gas phase (the process of wood pyrolysis is exothermic), in the same chamber the air forcibly forced by the fan is heated. Between the upper and lower chambers of the pyrolysis boiler, there is a ceramic grate nozzle capable of withstanding high temperatures – through the holes in it, the gases formed during the pyrolysis of wood in conditions of a lack of oxygen penetrate into the lower chamber. Ashes remaining after complete combustion of the fuel also accumulate in the lower chamber of the boiler.

Pyrolysis boiler
1 – fuel, 2 – primary air, 3 – secondary air, 4 – exhaust gases, 5, 6 – water inlet and outlet
A – gasification chamber, B – grate, C – afterburner, D – flue gas duct, E – smoke exhauster

Forced air supply with regulation of its volume is produced by a fan supplying air through the air duct located in the upper chamber of the pyrolysis boiler, or with the help of a smoke exhaust fan installed inside the exhaust pipe.

The pyrolysis boiler works like this: fuel (wood) is put in the upper chamber, ignited, the chamber door closes and the boiler fan turns on. As soon as the temperature inside the upper chamber rises above 250 ° C, the wood begins to char, releasing generator gas that flows through the nozzle into the lower chamber. Here, secondary air is added to the gases obtained as a result of pyrolysis by pressurization and they burn, returning some of the heat to the lower level of the fuel fill. Secondary air – air flow that bends around the fuel in the upper chamber on the sides or enters through special slots in the door of the lower chamber of the pyrolysis boiler.

The intake of heat from the pyrolysis boiler and its transfer to the coolant (water or air) is carried out through the contact of the pipeline with the coolant inside with the bottom of the lower chamber of the boiler.

With one full load of the fuel (upper) chamber, the pyrolysis boiler generates heat from 6 to 12 hours, depending on the fuel quality and heat needs.

If we divide the processes taking place in the pyrolysis boiler in stages, then the following happens:

  1. Temperature 450 ° С – wood fuel is dried and degassed;
  2. Temperature 560 ° С – generator gas connected to secondary air is incinerated;
  3. Temperature 1100 ° С – the remains of pyrolysis gas are burned out, heat is returned to the lower layer of fuel in the upper chamber;
  4. Temperature 160 ° С – exhaust of afterburning products into the chimney.

Pros and cons of pyrolysis boilers

First of all, only pyrolysis boilers are capable of burning generator gas, i.e. similar capabilities to afterburning pyrolysis gases advertised by the manufacturers of Buleryan type convection boilers do not correspond to reality.

Pros and cons of pyrolysis boilers

Positive characteristics:

  • high efficiency, achieved by two stages of fuel combustion, high efficiency – up to 90%;
  • heat generation from one fuel tab lasts about 12 hours (a conventional wood-burning stove works on one tab for no more than 4 hours);
  • low waste due to the almost complete combustion of wood fuel, cleaning of the ash pan in the lower chamber and chimney channels is rarely required;
  • the combustion process allows for automation, is easily regulated and controlled;
  • integrates into almost any heating system without requiring any major changes;
  • not chopped logs are allowed;
  • environmental friendliness achieved by burning more harmful substances in the lower chamber, due to which they are not emitted into the atmosphere.

Negative characteristics:

  • the need for electricity necessary for the operation of the fan (smoke exhauster);
  • high cost, in comparison with conventional heating boilers – about 2 times;
  • the need for low moisture content of the fuel, i.e. firewood must be dry, have a moisture content of no higher than 20%;
  • requires a high load in operation, when it drops below 50%, combustion stability is disturbed, tar accumulates in the smoke channel.

Price and manufacturers, criteria for choosing a pyrolysis boiler

Pyrolysis boilers of the Czech companies “Dakon”, “OPOP” and “Atmos”, Slovenian “Attack”, German “Viessmann”, Austrian “Wirbel” are presented on the Russian market. The cost of a pyrolysis boiler with a capacity of 20 kW will average 45,000 rubles.

Pros and cons of pyrolysis boilers

When choosing a boiler of this type, it is better to stop at European manufacturers, because domestic pyrolysis boilers often have low quality characteristics. Take a close look at the boiler, in particular the lower chamber casing and the nozzle – the boiler will last longer if the mentioned elements are made of ceramics that can withstand high temperatures. Make sure that there are two combustion chambers in the boiler, that there is an adjustable air supply to both chambers. The channel in which the coolant circulates should be in contact only with the lower chamber of the pyrolysis boiler and the chimney channel, otherwise the high temperatures required for pyrolysis and afterburning of gases in the furnace chambers will not be formed – the coolant will actively take away heat.

The manufacturer’s warranty for the operation of the pyrolysis boiler must be more than one year, for possible burnout of the chamber walls – at least 10 years.

Pay attention to the guarantees for the boiler automation and the nozzle – their cost in case of repair can be up to 50% of the cost of the boiler itself.

When selecting the power of the boiler, one should proceed from the area to be heated. The choice of a low-power or excessively powerful pyrolysis boiler will lead to its premature failure: in the first case, due to extreme loads; in the second – due to the accumulation of condensate in the chimney channel.

Meticulously evaluate the appearance of the boiler, inspect the welds, find out the thickness and grade of metal used in its construction. The inner walls of the boiler must be made of high-alloy boiler steel with a thickness of 4 mm or more – if the seller called the steel from which the chambers of the pyrolysis boiler are formed “structural”, then refuse to buy and leave this point of sale (structural steel will not withstand high temperatures in furnaces boiler and burns out in two to three years).

The design of the pyrolysis boiler described in this article has been tested by European and domestic users and is the most effective – improvised models of some domestic and European manufacturers, whose design differs from the “classical” one, most often do not have the ability to either generate or burn pyrolysis gases.

How to install and operate a pyrolysis boiler

These boilers must be placed outside residential areas. According to fire safety standards, the base for the pyrolysis boiler must be made of brick or stone, the floor area in front of the furnace chambers must be covered with a sheet of metal 1.5-2 mm thick. The minimum distance between walls or objects in the boiler room and the boiler drum is 200 mm. Manufacturers of pyrolysis boilers recommend insulating the chimney using foil mineral wool – cooling the flue gases will cause the accumulation of condensate and tar.

How to install and operate a pyrolysis boiler

Pyrolysis boilers, however, like any other heating boilers operating on combustible fuel, need a constant supply of fresh air – a ventilation hole into the boiler room with a minimum area of ​​100 cm2 absolutely necessary.

Keep an eye on the moisture content of wood logs placed in the upper furnace of the boiler – in the operation of pyrolysis boilers this is an important factor in their successful and long service.

Important: manufacturers categorically prohibit burning any synthetic materials in pyrolysis boilers. the resulting combustion products will actively corrode the internal boiler body, drastically reducing its service life.

During operation, it is required to remove ash from the bottom of the furnace chambers (about once a week, the frequency depends on the intensity of work and the power of this boiler), make sure that the holes in the nozzle located between the upper and lower chambers are not clogged with ash. Once a month it is required to completely clean the furnace chambers, once a week – the doors of the furnaces using a scraper.

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