Wood chip heating
A heating system that is operated with wood chips as a biogenic solid fuel is called a wood chip heating system . Larger heating systems for wood chips that supply heating networks or large customers are called wood chip heating plants .
Procedure
Similar to pellet heating systems , the fuel is delivered periodically and automatically fed into the combustion chamber as required by means of feed devices. The fixed-bed firings common in wood heating systems are used for the combustion . The heat generated is used to heat water from the heating circuit in the boiler of the wood chip heating system. As with other central heating systems, the heat is distributed through the heated water. In contrast to oil or gas heating systems, a hot water storage tank must be integrated into the heating system with wood chip heating systems, which stores the heat generated in the combustion process with little loss until the heating system calls for it.
Fuel storage and supply
Depending on the fuel quality requirements of the heating system, the wood chips are delivered fresh to moist or pre-dried in a wood chip drying system and are usually stored under a roof, but sometimes also under fleece or foil, or in the open air. The storage space required for wood chips is relatively large. In comparison to oil, wood chips require about ten times the storage volume, but with less expenditure for the room, since wood chips are not water-polluting substances. Compared to pellets , about three times the storage space must be taken into account with wood chips, as these have a lower bulk density.
The removal of the fuel from the wood chip store is facilitated by various technical systems such as screws, agitators with screw conveyors or sliding floors ; the supply to the combustion chamber is usually via screw conveyors . Hydraulic feed systems with delivery pistons, which can cut through protruding material and are therefore less prone to failure, are also used for uneven qualities. The heating system regulates the amount of fuel supplied depending on the heat demand of the heating system.
Firing
The furnace, designed as a fixed bed reactor , is automatically supplied with material to be fired. The fuel is fed into the combustion chamber either from the side (cross feed firing) or from below (under feed firing). In the case of very uniform wood chip qualities (sieved), drop firing is also possible, in which the wood chips fall into the combustion chamber from above.
The control and regulation technology of the plant gradually supplies the fuel in the amount that corresponds to the required heat output. Depending on the design, the wood chips fed in are either automatically ignited with hot air blowers or a bed of embers is kept permanently in the combustion chamber.
The ashes fall into an ash box. Especially when using ash-rich fuels such as wood chips with a high bark content, ash removal systems are also used, in which the combustion residues are transported to collecting containers by means of screw conveyors.
Heat transfer and storage
Just as with the use of other fuels, the combustion of the energy carrier in the boiler heats the water, which serves as a heat exchanger for a heating and / or hot water system and transports the heat energy to the place of consumption via pumps and pipes. Since a largely complete combustion of the wood chips is only possible in regular operation and greater losses and higher emissions occur during the warm-up and burn-out phases in boilers with steel combustion chambers, the heated water is usually first fed into a heat store, as is the case with log heating systems, from where it is accessed by consumers as needed. In the case of boilers with fully firebricked combustion chambers and lambda probe control, a heat storage device is not absolutely necessary, as this can also be operated in long-term partial load operation with low emissions at the same time.
Measurement, control and regulation
The measuring, control and regulation technology of the wood chip heating is usually more complex than the comparable heating systems with fossil fuels. On the one hand, the integration of one or more heat accumulators requires regulation of hot water storage, delivery and replenishment; on the other hand, the regulation of fuel supply, combustion air supply and firing is more complex, among other things because the fuel is less uniform than crude oil or natural gas.
Safety devices
Because of the special features of the fuel, wood chip heating systems have different safety devices than oil or gas burners. The fuel feed is equipped with a burn-back feed. Extinguishing water systems or systems for separating the flow of material such as gate valves or rotary valves prevent fire from spreading from the combustion chamber via the feed line to the fuel store. Vacuum controls in the furnace prevent toxic or flammable gases from escaping into the boiler room. An overheat protection is provided by Security heat exchanger ensures the cold automatically when overheated water passes through a heat exchanger.
Degree of automation, support and maintenance
Modern wood chip systems work largely fully automatically. The regular work on the heater is limited to filling the store, removing the ashes and, in the case of simpler models, cleaning the flues. Due to the low uniformity of the fuel, however, the susceptibility to failure is higher than with comparable pellet firing systems, which is why regular control of the system operation is advantageous. Like other systems, wood chip heating systems are serviced annually.
Power range and efficiency
Wood chip heating systems are available in all power ranges from approx. 10 kW. Most of the systems available today have a capacity control for the fuel and combustion air supply, so that they can be operated at full load as well as at part load. Wood chip boilers currently achieve a combustion efficiency of around 80–93% when operating at full load (nominal heat output), which is slightly less than the values for pellet boilers (85–96%).
fuel
Wood chip heating systems are designed to use wood chips as fuel. The fuel quality requirements vary depending on the system technology. In some cases, other biogenic fuels can also be used.
Wood chip qualities
Smaller plants in particular need dry, sieved goods with defined sizes of the individual cossettes for trouble-free operation.Fuels with a particle length of the main fraction of 3.15 to 30 mm and a residual moisture of less than 30% (class G30 S30 according to EN) are common 14961-2 ). Larger systems can often also use coarser goods with larger deviations in edge lengths. The systems are usually designed for specific fuel moisture ranges. So should z. B. Systems for firing fresh wood chips with higher moisture contents cannot be operated with dry material at full load, since damage can occur at the higher combustion chamber temperatures that then prevail.
An important quality feature for incineration is the ash content of the wood chips, which is significantly influenced by the bark content. With high ash contents, such as B. with material from short rotation plantations or from landscape maintenance (hedge and shrub cut), the higher ash accumulation requires frequent emptying of the ash box or automatic ash removal. With a higher ash content, there are also higher demands on the design of the combustion chamber in order to avoid slagging and corrosion of the materials. In addition, emissions of pollutants, especially fine dust, are increasing. The limit values for small-scale combustion systems that will apply from 2015 can now be met by conventional wood chip heating systems if high-quality, low-ash fuel is used. Otherwise flue gas cleaning is required.
Fuel alternatives and supplements
Wood pellets can also be used in wood chip systems as a fuel alternative that is simpler in terms of conveying and combustion technology, but significantly more expensive, but vice versa, wood chips cannot generally be used for pellet heating. Plants that, subsidy and emission of technically advanced solid biofuels such as IC engine to use grain combustion or Halmgütern such as straw, hay or Miscanthus are designed to be generally operated with wood chips.
The combination of wood chip heating with domestic water heating and heating support through solar collectors ( solar thermal ) is advantageous for two reasons: On the one hand, the systems, which often work less efficiently during partial load operation and short operating times, can be taken out of service for months when there is little heat demand. On the other hand, with the buffer storage that is sometimes required for wood chip heating (some manufacturers do not necessarily need a buffer storage), a significant proportion of the systems required for heat supply via solar thermal systems are already available.
Diffusion and development
Mainly because of the high storage space requirements for wood chips, but also because of the low transportability by land compared to other fuels, wood chip heating systems are particularly widespread in rural regions. These heating systems are particularly common in regions with a high proportion of forest. In Bavaria, wood chip heating systems with an output of 23 kW are installed per thousand inhabitants, while in Schleswig-Holstein, the least forested area, only 2 kW per 1000 inhabitants were installed (as of 2009).
In recent years, the proportion of wood chip heating has increased. However, the market development remains well behind the strong growth in pellet heating. The largest proportion of the wood chips produced for energy recovery is currently consumed by biomass heating plants and biomass heating power plants .
economics
In comparison to oil or gas heating systems, wood chip heating systems are currently economically viable from a power requirement of 20 kW and are therefore particularly suitable for larger old buildings, apartment buildings or building complexes. The acquisition costs are usually significantly higher than with conventional oil, pellet or gas heating systems. The fuel costs, on the other hand, are much lower than the latter. The period after which the investment in a wood chip heating system pays off financially is therefore heavily dependent on the relation of the raw material market prices.
Compared to oil or gas heating systems, more electrical energy is required for operation, e.g. B. for the drive of the agitator, the screw conveyor and the automatic ash discharge.
advancement
The investment in automatically fed wood chip heating is significantly subsidized by the federal government. The Federal Office of Economics and Export Control (BAFA) grants investment grants of € 1,400 per system between 5 and 100 kW output. Larger heating systems are subsidized by the Kreditanstalt für Wiederaufbau (KfW) with investment grants. The programs grant additional funding if the systems are installed in energy-efficient buildings or if a solar system for hot water generation is installed at the same time .
sustainability
In comparison to heating systems for fossil fuels, wood chip heating systems are considered environmentally friendly. The amount of CO 2 that is released when wood chips are burned corresponds exactly to the amount of CO 2 that was incorporated into the wood as it grew. Even if the CO 2 release through forest management, transport, processing, electricity consumption of the heating system, etc. ("gray energy ") is factored in, wood chips produce significantly fewer climate-relevant emissions than fossil fuels in sustainable forest management. The CO 2 emissions from wood chips are around 35 g / kWh, with heating oil the value is 303 g / kWh.
The use of wood chips as a biogenic energy source reduces the dependency on fossil energy sources. In addition, in contrast to fossil fuels, when using wood chips, regionally produced fuels can be used. The expenditures for the fuel then remain largely in the own region and increase the added value there.
Emissions
The emissions of carbon monoxide, volatile organic hydrocarbons, nitrogen oxides and dust in the flue gases are higher than those from heating oil or gas boilers. B. far below those of old design stoves. The pollutant emissions of modern wood chip heating systems are far below the current limit values, even after the reduction in exhaust gas values in 2015 in accordance with the 4th BImschV, the systems can comply with the limit values. With poor fuel quality or inadequate maintenance, however, exhaust emissions can rise sharply.
Systems for flue gas cleaning can further reduce emissions. Filter systems for fine dust and nitrogen oxides are used in larger heating systems for wood chips that are subject to the stricter exhaust gas regulations of the TA Luft. The first systems for exhaust gas purification for fine dust are on the market for wood chip heating systems below 100 kW.
Electrostatic filter to reduce fine dust
A biomass heating system with an integrated fine dust filter is now also available on the market. The fine dust filter is designed as an electrostatic dust filter. It is located in the last flue gas passages of the wood chip heating. The electrostatic fine dust filter is controlled by the existing regulation of the biomass heating. The advantage of an integrated fine dust filter is its compact design, since neither additional space for the electrostatic fine dust filter nor an external control is required.
The biomass heating with electrostatic filter to reduce fine dust was also tested by TÜV Süd according to EN 303-5: 2012. The electrostatic fine dust filter integrated in the biocompact “ECO 80 E” wood chip heating system achieved a dust value of 0.9 mg / MJ at full load and even a dust value of less than 0.1 mg / MJ at partial load.
literature
- Information brochure of the Swiss Federal Office of Energy ( PDF )
- Fachagentur Nachwachsende Rohstoffe eV (FNR), 4th, updated edition 2012: Market overview of wood chip heating systems. PDF
- Fachagentur Nachwachsende Rohstoffe eV (FNR), 2007: Handbook of Small Bioenergy Systems. ISBN 3-00-011041-0 ( pdf )
Web links
swell
- ↑ BAFA: Funding for Biomass Plants , accessed on August 26, 2012.
- ↑ IER Stuttgart, A. Voß: The Energy Challenge (PDF file; 597 kB), June 10, 2009.
- ↑ www.biokompakt.com ( Memento of the original dated November 10, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
