Wood gasification boiler
The wood gasification boiler is a heating boiler and a variant of the log boiler .
In gasification boilers, the process finds wood gasification spatially separated from the wood gas combustion instead, allowing a separate and better control. Compared to other solid fuel boilers , very low pollutant emissions and a very high level of efficiency are achieved.
Coal gasification boilers and gasification boilers, which can use different fuels (for example logs , wood briquettes and lignite briquettes) , also work on the principle of the autothermal fixed-bed gasifier .
Wood gas produced in this way was also used (mainly because of a lack of fuel in the Second World War) to drive motor vehicles; the gasifier boilers were installed on cars and trucks for this purpose, see there .
As a special feature of the wood gasification boiler, the material burns downwards and the hot combustion gases collect in the lower area of the furnace. A furnace of this type is called a flue furnace. See also chimney stove # Lintel fire stoves .
technology
The wood is fed into the combustion chamber in batches and continuously gasified by the combustion heat generated during operation with a controlled supply of air. The resulting wood gas is burned immediately. The combustion gases heat the water pipes and thus the water that is pumped through radiators or consumed as hot water.
The wood gasification boiler is divided into an upper chamber and a lower chamber by a burner plate made of refractory ceramic. The burner plate has many small holes. In the upper chamber, the wood, charcoal and ashes lie on top of each other on the burner plate. Apart from the filling opening, which may only be opened briefly during operation, the upper chamber is tight at the top.
When the boiler is fired, all vaporised by the heat of the burning wood volatile wood constituents according to their evaporation points (e.g., terpenes ), which as wood gasification is referred to
The water contained in the wood (not chemically bound) also evaporates. The wood moisture is therefore withdrawn from the wood . Depending on how wood gas is defined, the water vapor and the inert atmospheric nitrogen are added to it or not.
The easily ignitable wood gases, which rise first due to the static buoyancy , burn in the upper chamber. Their combustion gases, together with wood gases that are more difficult to ignite, pass through the glowing charcoal and ash in the upper chamber and through the holes in the burner plate into the lower chamber. The draft through the combustion zone leads, on the one hand, to the heating of the glowing substances there due to residual oxygen and, on the other hand, to the turbulent mixing of the fuel gas sucked through at the same time as the ignition and combustion of all still combustible gas components.
This means that the wood gases that are more difficult to ignite and the carbon monoxide contained in the combustion gas are burned well at temperatures around 1100 ° C, gases that would otherwise have entered the chimney or the environment unburned and unused in the case of 'upper burn-up'. The exhaust gases flow from the lower chamber into the exhaust connection above.
This combustion technology significantly reduces the content of carbon monoxide and unburned substances in the exhaust gas. Otherwise, unburned substances would condense on entrained (or recombined ) ash dust and be adsorbed and increase the dust content in terms of mass.
Only with special natural draft wood gasifiers can the given draft of the chimney ( chimney effect ) be sufficient to convey the hot combustion gases against the natural buoyancy into the lower chamber (and finally further to the flue gas connection). As a rule, this requires a (mostly electrically driven) fan. Depending on the design, there is a heat-resistant induced draft fan in the exhaust gas flow or a pressure fan in the fresh air supply. Without a fan, the hot exhaust gases would accumulate in the upper combustion chamber and the stove would not be able to be lit. In operation, the chimney effect would be influenced by the amount and gas permeability of the combustion chamber filling, making it difficult to control the air throughput.
As with every wood-burning stove, the supply of combustion air is divided into primary and secondary air. The primary air is fed to the upper chamber, which controls the gasification and thus the boiler output. The secondary air is fed to the wood gas in the lower chamber for complete combustion. The primary and secondary air supply is regulated separately.
The air volumes are set manually or electronically depending on the manufacturer and type. In boilers with electronic control, either only the induced draft fan or the pressure fan is speed-controlled or the amount of secondary air is also controlled, whereby the residual oxygen content of the flue gases must be measured permanently with a lambda probe .
Like all wood boilers, a wood gasification boiler used to heat water also needs a return increase to avoid the formation of aggressive condensates and tar deposits at return temperatures below 55 ° C; But there are already more corrosion-resistant condensing boilers for logs on the market that can cool the exhaust gases more deeply. Like all solid fuel boilers, a wood gasification boiler must also be provided with a thermal discharge safety device .
Types:
- Hand-loaded solid fuel boilers
- Room air-dependent hand-fed boilers for solid fuels without motor drives
- Room air-dependent hand-fed boilers for solid fuels with motor drive
- Room air-dependent automatically charged boilers for solid fuels with motorized drives
Type test
Wood gasification boilers that are built from March 22nd, 2010 may only be operated if this can be proven by the manufacturer's type test . § 4 BImSchV . The type test (heating test) is carried out by an independent testing institute on behalf of the manufacturer. Test item : DIN 303-5. Compliance with the standard is publicly confirmed by the manufacturer's declaration of conformity in accordance with ISO / IEC 17050-1.
The report on the type test (heating test) must contain the following, among other things:
- the fuel analysis
- the total fuel supplied over the test period in kilograms
- the supplied fuel in kilograms per hour
- the test duration
- the water cycle
- the water temperature when entering the boiler
- the water temperature at the boiler outlet
- the resulting temperature difference
- the measured carbon monoxide emissions with the corresponding oxygen content
- Boiler efficiency
- the measured dust emissions with the corresponding oxygen content [mg / m³]
- the fuel heat output [kW]
- the heat output made available by the boiler
- with autom. sent systems a detailed system description
- In the case of manually loaded systems, a detailed system description (combustion air fan, combustion control, lambda probe or temperature sensor behind the combustion chamber)
The design requirements must be listed in the system description of the test report.
In the case of a manually loaded system (wood gasification boiler), an unequivocal, concrete statement from the testing institute must be provided, which shows how the system tested is also suitable for burning coal, coke, etc. or only logs can be burned.
On the basis of this test report, the specifications of the applicable directives with regard to carbon monoxide emissions, dust emissions, boiler efficiency and design requirements must be verified.
Buffer storage
In Germany, wood boilers with an output of more than 4 kW require a buffer according to the Federal Immission Control Ordinance (BImSchV) . This reduces emissions, as the intermediate storage of the generated heat allows the boiler to always be operated at nominal load, i.e. in the optimal operating state. Comfort is also increased, as the boiler does not have to be constantly lit, but is only put into operation when required to heat the buffer tank. The buffer size according to BImSchV and the BAFA funding guidelines must be at least 55 l per kW boiler output and 12 liters per liter of the filling shaft volume. It should preferably be above 75 l per kW boiler output and 17-20 liters of buffer per liter of filling shaft volume. The boiler output should also not be selected too low in order to achieve reasonable buffer heating times and to generate sufficient heating output in high winter while buffer is being heated at the same time. As a guideline, 50% should be added to the output of an oil / gas boiler according to the conventional calculation method. This guide value only applies to hand-loaded wood gasification boilers.
Grants
Germany: The Federal Office of Economics and Export Control (BAFA) gives grants for the installation of certain log gasification boilers and other biomass burners. In addition, low-interest loans are available from the Kreditanstalt für Wiederaufbau (KfW) with corresponding conditions. The federal states and municipalities also occasionally offer relevant funding.
criticism
According to a study by the Austrian Society for Environment and Technology , which compares the capital costs and running costs of heating systems under different heat consumption and energy price scenarios , “log heating (including buffer storage etc.) 'proves to be the most cost-effective heating system in eight of the nine scenarios examined. ". Only with a low consumption of 20,000 kW / h and a consistently low energy price level of 70 $ / bbl is it cheaper to continue operating an old oil boiler over a period of 20 years.
The European Environment Agency warns that increased combustion of biomass in private heating systems could worsen air quality, as wood smoke contains fine dust and soot and can contain toxic substances such as dioxins . From around 2000 to 2005, reductions in fine dust by means of lower-emission forms of wood combustion were nullified by an increase in wood combustion systems. According to an investigation by the Federal Environment Agency, the fine dust emissions from wood combustion systems exceeded the emissions from road traffic (incineration only) by 22,700 tons.
See also
Web links
- BAFA: subsidies for renewable energies
- List of wood gasification boilers funded by BAFA (PDF; 257 kB)
- 1. BImSchV: Ordinance on small and medium-sized combustion systems
https://www.beuth.de/de/norm-entwurf/din-en-303-1/249287799
Individual evidence
- ↑ http://atmos-zentrallager.de/produkte/atmos-kc-kohlevergaser
- ↑ bafa.de
- ↑ Michael Cerveny, Thomas Sturm: Full cost comparison of heating systems for single-family houses - comparison of the life cycle costs of heating oil, natural gas, pellet and log heating for old single-family houses in nine scenarios. Austrian Society for Environment and Technology, Vienna 2011, p. 9. Retrieved on August 25, 2020 (PDF).
- ↑ Air quality in Europe - 2017 report apren.pt (PDF); European Environment Society; EEA Report No 13/2017, ISSN 1977-8449 .
- ↑ Timothy Spence: Doubts cast on biofuels' air quality claims ; at euractiv.com
- ↑ The side effects of comfort: fine dust from the fireplace and wood stove. Background paper of the Federal Environment Agency, March 2006.