Pyrolysis in domestic solid fuel boilers – myths and reality

Pyrolysis is a term that homeowners who use solid fuel boilers to heat their homes frequently come across. However, there is a lot of misinformation and confusion surrounding this term. By debunking myths and providing clarification on the implications for your heating system, our goal in writing this piece is to bring pyrolysis in residential solid fuel boilers down to earth.

The process of pyrolysis is what happens when solid fuels like coal, wood, or biomass are burned. It’s an important part of the burning process where the fuel decomposes due to high temperatures and a lack of oxygen. Volatile gases are released during this breakdown and can be burned to generate heat. Anyone who depends on solid fuel boilers for their home heating needs needs to understand this process.

Despite its significance, pyrolysis is frequently misinterpreted, which results in false beliefs about its effectiveness and effects on the environment. We’ll dispel some common misconceptions about pyrolysis in residential solid fuel boilers in this post and examine the relevant evidence. You’ll have a better knowledge of pyrolysis’s operation and function in your home’s heating system by the end.

What is pyrolysis – a description of the process

It is theoretically possible to burn any material that contains carbon-hydrogen compounds, including:

• coal;
• natural gas (methane, propane and so on);
• biomass – fresh, dry;
• wood products, cellulose, ordinary firewood;
• Different types of plastics;
• natural or artificial rubber;
• oil, its derivatives;
• other carbon-containing wastes.

Depending on the mass to be burned’s initial moisture content, a specific amount of heat energy will be produced. We’ll utilize the following chemical formula to explain the procedures:

The process of combustion is quick oxidation. In a perfect world, every carbon atom interacts with two oxygen particles and every hydrogen atom with one oxygen particle. Innocent substances like water and carbon dioxide (CO2) are created as a result. When heated, the latter vaporizes, removing some of the heat released.

Important point. Due to the deficiency of oxygen molecules, not every hydrogen and carbon atom in the real world finds a partner. As a result, some dangerous combustible substances, such as carbon monoxide (CO), free hydrogen (H2), and carbon in the form of soot, are included in the combustion products.

Pyrolysis is a chemical reaction that occurs when a material is heated and isn’t exposed to free oxygen. In installations involving gas generators, the following idea is applied:

1. Fuel (in particular, wood) is placed inside a closed metal vessel – a reactor.
2. The tank is heated from outside up to 500…900 degrees Celsius, a dosed amount of air and water is supplied through special holes – tuyeres.
3. Under the influence of high temperature the substance decomposes into 3 main components – carbon monoxide (CO), hydrogen (H₂) and solid or liquid carbon residue. A small amount of carbon dioxide and water vapor is generated in parallel.
4. Volatile products make up pyrolysis gas – a combustible mixture of hydrogen and carbon monoxide, leaving the vessel through a separate pipeline. The released gaseous fuel is purified, cooled and then pumped into the tank.

Synopsis: The same gas generator tank receives the synthesis gas for heating under production conditions.

Pyrolysis and combustion are two distinct processes that can happen at the same time. As an illustration, during the intense burning of wood in the boiler furnace, a small amount of CO2—harmless CO2—is produced. On the other hand, when firewood is smoldering, it releases a lot of hydrogen and carbon monoxide, some of which has time to oxidize and become CO2. In other words, everything is dependent upon how much oxygen is used in the process.

Effect of high moisture content

High levels of moisture in the source material are bad for pyrolysis and combustion processes in equal measure. Let’s examine the procedures using burning wood as an example:

1. During combustion, the energy released is used to vaporize the water contained in the wood. The amount of heat output is significantly reduced, the fuel is wasted.
2. Moisture strongly slows down the thermal decomposition of the substance. Part of the heat consumed for heating is taken away by evaporating water, the required temperature (at least 500 °C) is not reached. Pyrolysis of wood containing more than 50% moisture is practically impossible.

In a gas generator, 8–15% moisture content is ideal for the successful combustion or decomposition of wood. Achieving such indicators in domestic settings is unfeasible; however, firewood that has been dried for a long time under a shed can attain a moisture content of 20–25%.

Reference: Sawdust is dried to 8–10% in the factory before being used to make fuel pellets and briquettes. The final pellets have a maximum moisture content of 15%.

What thermal decomposition is used for

The range of uses for pyrolytic processes is extensive:

1. Production of propylene and ethylene for the chemical industry by processing liquid hydrocarbon raw materials (oil).
2. Production of charcoal by oxygen-free decomposition of woodworking waste.
3. The same technological process, but with a limited air supply, allows to produce combustible synthesis gas from wood – a mixture of methane, hydrogen, carbon monoxide and neutral nitrogen.
4. Pyrolysis of coal – lignite and hard coal – is a whole area of processing. The compounds produced are synthetic gasoline, coke, ammonia, coal tar. From the latter, toluene, benzene, naphthalene and various phenols used in the chemical industry are extracted.
5. New developments – commercial recycling of municipal solid waste, car tires, plastics, organics.

Note: This is a list of the most well-known uses for pyrolytic reactions. There are actually a ton more uses for this. According to Wikipedia, numerous projects are being developed, and the pyrolysis processes are not fully understood.

Industry uses a range of reactors and pyrolysis furnaces for thermal degradation. A gas generator plant that produces gaseous fuel from sawdust and wood waste is depicted in the diagram above. The direct process dry distillation reactor, which slowly burns the prepared raw material to produce synthesis gas, is the key player in this process.

Important nuance. Before being put into the gas generator or pyrolysis furnace, the wood is always chopped and dried until the moisture content is 10% or less.

Fast pyrolysis technology, in which the reactor is quickly heated to a temperature of 700–900 °C, is also used in industrial chemistry. The goal is to expedite processing and boost equipment productivity.

Domestic use

Pyrolysis is useful in the home to address the following issues:

• cleaning the oven or roaster from sticky fatty deposits that cannot be removed mechanically;
• obtaining charcoal;
• heating of a private house with a pyrolysis solid fuel boiler.

Putting the pan in the oven, adjusting the temperature to 200–250 °C, and leaving it there for 30 minutes is the most effective way to clean it. The deposits will be destroyed and reduced to ash in the absence of oxygen, and the kitchen hood will remove the pyrolysis gases.

Overview. Certain oven models come equipped with an integrated pyrolytic cleaning feature. All that’s left to do after the "frying" is through is wipe the inside surfaces and discard the ashes that have formed.

In addition to being used for blacksmithing and grilling, charcoal can also be refueled for automobile gas generators (the mechanics of which are covered in a different section). The process, known as slow pyrolysis, involves burning wood waste in a closed container.

The practicality of purchasing and using pyrolysis boilers is a contentious topic. What’s concerning is that not even the salespeople showcasing gas-generator heating apparatus at the renowned "Aquatherm" exhibition knew what pyrolysis was. If you don’t believe me, view the video:

We provide an in-depth analysis of the issues related to pyrolytic wood heat generators.

Myths about pyrolysis TT-boilers

A gas-generator heater and a conventional direct combustion boiler differ primarily in that the former has two chambers while the latter only has one. Between each of the two fireboxes is a ceramic nozzle through which air is driven by a fan. Refractory brick lining protects the metal walls of the pyrolysis unit. How it functions

1. Wood or coal is put into the upper (primary) chamber and ignited.
2. Automation starts the blower fan.
3. When the temperature in the fuel box rises to 500 degrees Celsius, pyrolysis gases start to release.
4. Attracted by the general flow of combustion products, these volatile compounds fall into the lower secondary chamber, where they are (allegedly) afterburning in the presence of oxygen.

In fact, because the fan provides excess air, the produced synthesis gas begins to burn even in the primary furnace. There is only one thing pointed toward the second chamber: the flame torch. Additional combustion byproducts travel through the heat exchanger’s flame pipes, heat the coolant, and then exit through the chimney.

Furthermore. There is another type of heater where the secondary chamber is at the top and there is no fan. Pyrolysis-wise, the idea is worthless; the device works like a traditional wood-fired hot-water boiler, but it costs twice as much as its traditional counterparts.

Those who support pyrolysis heat generators (manufacturers, retailers, and do-it-yourselfers) credit their TT-boilers with the following benefits:

• the fuel is burned completely, the residue in the ashtray is practically zero;
• Duration of combustion – 10 hours or more;
• low emissions into the atmosphere;
• high economic efficiency due to the efficiency of 86…90% (manufacturers" indicators) compared to traditional boilers efficiency of 75%.

Let’s attempt to ascertain whether the aforementioned claims are accurate. First, there will be fine ashes left behind after combustion if the fuel box is filled with dry firewood, as specified in the heater’s operation manual. The light residue will simply be blown into the chimney by the air flow that is produced by the fan and intensified in the nozzle.

As a result, the ash pan appears nearly empty and full combustion is created. A similar residue, with a small amount of ash at the bottom, will result from adding dry wood to a traditional turbocharged TT-boiler. In other words, fuel quality determines how fully a fuel burns, not the heat generator’s construction.

Note: Any boiler will experience adverse effects if raw wood with a moisture content greater than 50% is laid. It makes no sense to think about these options.

Let us now quickly address the remaining statements:

1. Burning duration 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 and more), which fits a lot of firewood. Pyrolysis has absolutely nothing to do with it.
2. Assurances about the environmental friendliness of the boiler are true. The fan blows air in excess, toxic gases are formed very little. In standby mode, no oxygen enters the furnace, the wood slowly smolders and the amount of harmful emissions increases.
3. Boiler efficiency 90% – fairy tales. In the active combustion mode, the boiler operation principle is similar to the turbo versions of traditional units, whose efficiency does not exceed 75%. When the fan is turned off, the flame dies out, the smoldering embers give off little heat.

In conclusion. Purchasing a solid fuel boiler’s gas generator model is a very questionable decision. The lining makes the unit twice as heavy and three times more expensive than standard versions. Usually more affordable and dependable than factory-made heat generators, but too big. Their efficiency and other features are not superior to the traditional TT-boilers that have a chain draught regulator or a turbine.

In his video, a renowned expert practitioner will validate our opinion:

The use of pyrolysis in residential solid fuel boilers has drawn interest recently due to its potential to lower emissions and increase efficiency. But pyrolysis is surrounded by a lot of false information, so it can be challenging for homeowners to decide if it’s the best choice for their heating requirements.

The idea that pyrolysis boilers are hard to maintain and operate is one that is frequently spread. Although it’s true that these boilers need to be cleaned and maintained on a regular basis, contemporary models are made to be effective and user-friendly. Pyrolysis boilers can supply dependable and reasonably priced home heating with the right maintenance.

Another myth is that traditional solid fuel boilers emit fewer emissions than pyrolysis boilers. As pyrolysis technology burns wood at a higher temperature and captures more of the released gases, it can actually significantly reduce emissions. As a result, heating your home with pyrolysis boilers is more ecologically friendly and cleaner.

All things considered, pyrolysis technology has the power to completely transform the way we heat our houses, providing increased convenience, reduced emissions, and increased efficiency for homeowners. You can decide whether or not this technology is right for you by busting myths and learning the truth about pyrolysis boilers.

Pyrolysis in residential solid fuel boilers has drawn notice recently as a cost-effective and green heating solution. Myths, however, abound regarding its efficacy and safety. We’ll examine pyrolysis in solid fuel boilers in detail in this article, sorting fact from fiction. In order to empower homeowners to make knowledgeable decisions regarding insulation and heating for their properties, we’ll go over the advantages of pyrolysis as well as how it operates.

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