firewood

from Wikipedia, the free encyclopedia

With firewood or firewood is wood that designates, for heating is used or cooking.

Logs as firewood before removal

Dry wood is used for burning in a utility fire . It is the oldest fuel known to man and has been used for around 400,000 years. While the industrialized countries abandoned the use of firewood in the 20th century in favor of fuels with higher energy density and lower prices, since the beginning of the 21st century, more and more heat has been obtained with firewood. In developing countries is for cooking - to prevent smoke from open fires - instead of green fresh wood often by charcoal burners produced charcoal used.

Firewood or firewood are broad colloquial terms for " energy wood ", which is the generic term for the various forms of trade, such as "kiln-ready logs", " logs " and " firewood ". Wood pellets and wood briquettes are also made of wood, but colloquially they do not count as firewood. The structure obtained is decisive here.

The calorific value is of central importance for the value of firewood . Other relevant properties can be the burning time as well as the look and smell when burning.

Pile of firewood
Unsplit firewood in an open fire

properties

calorific value

Since wood is a natural product, its structure and composition are subject to fluctuations. This can also affect the calorific value per mass (for example in kWh / kg ) or per volume (for example in kWh / cubic decimeter ).

When it comes to the calorific value per unit of mass (kWh / kg or MWh / t) , the different densities of the wood species are irrelevant. However, the water content is important; it is given as the water content  w% .

The calorific value of moist wood results from the calorific value of the dry matter contained in it , from which the energy that is needed to evaporate the water content must be subtracted. This is 0.63 kilowatt hours per kg of water.

Absolutely dry hardwood has a calorific value of around 5 kWh / kg. The calorific value of coniferous wood is slightly higher at 5.2 kWh / kg due to the different chemical composition (higher resin content) of the wood.

Calorific value per unit of weight (softwood / hardwood) depending on the water content
Example calculation for the calorific value
of 1 kilogram of firewood with 20% water content:
80% * 5.2
kWh
   -    20% * 0.63
kWh
   =    4.03
kWh

Calorific value of the
absolute
dry matter
minus Evaporation
heat of the
water part
equal Calorific value
normal
Calorific value of 1 kg of firewood ( dry matter ): 5.2 kWh.
Energy to evaporate 1 kg of water: 0.63 kWh

The example calculation shows that the decrease in the mass-related calorific value with increasing water content is mainly due to the decrease in the dry matter content and only secondarily to the increasing heat of evaporation of the water (which reduces the energy yield during combustion).

Heating oil equivalent and energy density

The heating oil equivalent is the amount of heating oil that has the same calorific value as the specified amount of fuel. Since the calorific value of the firewood depends on the water content, this must be included with every calorific value. "Absolutely dry" wood (= atro) with 0 percent water content cannot be achieved through natural drying, but only through technical drying. The end point of natural drying is the state "air dry = lutro" with around 15 percent water content. The heating oil equivalent can be used if you want to compare the wood purchase with the cost of the equivalent amount of heating oil. It should be noted, however, that the calorific value per cubic meter (Rm) of a type of wood has a strong fluctuation range, which results from the fluctuation range of the wood density and the fluctuation range of the conversion factor solid cubic meters (Fm, m³) to cubic meters. The table below contains the mean value of the calorific value per cubic meter of a type of wood.

Type of wood air dry Calorific value
(in kWh / kg)
Calorific value
(in MJ / kg)
Calorific value
(in MWh / Rm)
Gross density
(in kg / dm³)
Retail density
(in kg / Rm)
Beech, ash 4.2 15th 2.0 0.74 480
Oak 4.2 15th 2.0 0.69 470
birch 4.2 15th 1.9 0.68 450
larch 4.3 15.5 1.8 0.58 420
jaw 4.3 15.5 1.6 0.51 360
Spruce 4.3 15.5 1.4 0.44 330
Heating oil 12 43 10 0.84 840
Lignite briquettes 5.3 19th 2.2 0.60

One cubic meter of dry hardwood replaces around 200 liters of heating oil or 200 m³ of natural gas . Conifers , on the other hand, have a slightly higher calorific value per unit of weight, but due to their lower mass density they take up more space and burn off faster.

inflammation

Kindling wood

Before lighting is layered in the firing furnace to first highly flammable dry wood rich newsprint and easy to layered remains of corrugated cardboard or equal fine dry chipboard or wood chips, as equal starting aid is also wax -impregnated wood shavings used. By using the easily combustible paper, the flame of a match is enough to light it. Due to the poor thermal conductivity of the wood, combined with the rapid reaching of the flash point of the wood or outgassing wood components ( terpenes for coniferous wood, essential oils for birch or beech wood, wax vapor for ignition aids), the chipwood ignites quickly and consequently sets in coarser logs Fire.

When it comes to central heating boilers that do not yet function as so-called wood gasification boilers (these are, however, models that are being phased out, as they are excluded from public subsidies, for example KfW and Bafa , and are increasingly being used in existing buildings by the regulation on small and medium-sized combustion systems (1st BImSchV) problems), the following heating-up variant would be more correct instead of layering a stove with wood and then plugging it in from below. However, the dilemma remains that after heating up, the subsequent filling is “forced” to be added on top and then the problem arises again. So it's better to operate a wood gasifier stove / boiler:

If the firewoods are stacked on top of the ignition aid, there is an upper burn , in which wood components degassed from the firewood leave the chimney unburned before these flue gases ignite ; If the fire is kindled on the thick logs, their outgassing volatile compounds are passed through the fire zone, which is equivalent to a lower burn . The fuel is used more efficiently. When moist wood is burned, these volatile substances condense together with the outgassing water vapor or water mist and are perceived as smoke together with soot particles .

combustion

Wood combustion is a two-step process with gasification of the wood as the first and oxidation of gases and charcoal as the second sub-process.

When wood is burned, the following sub-processes run partly simultaneously and partly one after the other:

  • Heating of the fuel through reflection of heat from the flame, embers and furnace walls as well as through the flow of hot exhaust gas
  • Evaporation of volatile wood components ( terpenes and so on)
  • Drying through evaporation and removal of the water (from 100 ° C)
  • Decomposition of the wood due to the effect of temperature (from 250 ° C)
  • Gasification of the wood with primary air to form gases and solid carbon (from 250 ° C)
  • Gasification of the carbon (from 500 ° C)
  • Oxidation of the flammable gases to carbon oxides (carbon monoxide and carbon dioxide) and water at temperatures from 700 ° C to around 1500 ° C (maximum around 2000 ° C)
  • Heat transfer from the flame to the surrounding walls and the newly supplied fuel

All drying and vaporizing processes lead to a reduction in the temperature of the flame or the exhaust gas, i.e. a reduction in the calorific value of the fuel.

In a wood furnace, these substances are released through gasification of the wood (if there is a lack of air, i.e. combustion air ratio lambda <1) in the ember bed. "Primary air" is supplied for this purpose. When heated, 80 to 90 percent by weight of the dry wood mass is released as gases. These are primarily carbon monoxide (CO), hydrogen (H 2 ) and hydrocarbons (C m H n ). The remaining solid fraction remains as ash, settles as soot or is released into the environment in the form of particles.

Then the gases are mixed with combustion air and burned in the combustion chamber in a long flame. "Secondary air" is generally supplied to burn out the gases. Since the gases burn out in a long flame, wood is called a long-flame fuel. The charcoal in the bed of embers, on the other hand, burns slowly and with little flame formation (with the formation of more carbon monoxide in the exhaust gas).

Ovens "with upper burnout" can cool down escaping gases and burn them incompletely, with ovens "with lower burnout" the gases are led through the ember bed, thus heated more intensively and oxidized more completely.

Emissions

The main components of the combustion are carbon dioxide (CO 2 ) and water vapor (H 2 O). Wood contains a small amount of nitrogen (≈900 mg / kg). This - like the nitrogen contained in the combustion air - is partially converted into nitrogen oxides during combustion , which react with water (vapor) to form acids and pollute the environment. The sulfur that is also present in the wood (≈120 mg / kg) is mainly bound in the ash, so that only little sulfur dioxide is emitted.

The greater the moisture content of the wood , the more heat is required for the evaporation of this water, as a result - but also in the case of excess air (secondary air extracted from the installation room in the case of an "open fireplace" ) - the flames cool down and "incomplete combustion" occurs, This means, on the one hand, incomplete oxidation and also the reduction of organic compounds or of carbon dioxide to soot or wood tar . A lack of air (due to poor chimney draft or blocking the air supply) or poor combustion management (too little turbulence in the furnace) can lead to incomplete combustion. In the process, new connections are formed and emitted to varying degrees, for example:

  • all of the above-mentioned compounds that do not burn, but instead are released into the environment as unused volatile fuel through the chimney
  • Carbon monoxide (CO)
  • Black carbon (C)
  • Hydrocarbons (C x H y )
  • Hydrogen (small amounts, from reduction to soot)
  • Ash dust
  • mineral substances

Condensable substances can condense in cold places (heat exchangers in boilers, long stove pipes, in the chimney ) and deposit. The deposits are sticky (also because of the condensation), dust remains on them, which in turn attract other dusts by agglomeration and entanglement.

More recent studies show that the total amount of particulate matter caused by burning wood exceeds the total amount of particulate matter emissions from vehicles registered in Germany. However, the emissions from wood heating systems can be influenced by the choice of suitable boiler .

As a renewable raw material, firewood has the advantage that the carbon converted into CO 2 was absorbed much more quickly while the tree was growing than with fossil fuels ( e.g. crude oil , coal , natural gas ).

Natural wood has a low heavy metal and chlorine content; When contaminated waste wood is burned, heavy metals ( arsenic , lead , cadmium , chromium , copper , nickel , mercury , zinc and others) and dioxins can be emitted via exhaust gas and ash. The same applies to wood-based materials such as chipboard or plywood , where the adhesives, coatings or varnishes used can release toxins.

Types of wood

Beech wood is a popular firewood

Various types of wood are used for heating purposes . A distinction is to be made mainly according to calorific value, burning time and comfort of use (flame image, smell).

Deciduous and hardwoods have a significantly higher calorific value per volume (cubic meter) than hardwoods or softwoods. However, per weight, the calorific value of softwood is slightly higher than that of hardwood . Softwood burns faster and develops higher temperatures than hardwood. This is mainly due to the higher resin content .

For heating purposes, continuous heat generation is usually desired. The combustion technology in particular decides which types of wood are more suitable. In modern wood gasification boilers for pure heat recovery, all types of firewood can be optimally used without restriction due to the high-temperature combustion.

All hardwoods are very suitable as energy sources for open chimneys or stoves . They burn more slowly and persistently than softwood, but form a little more ash ( maintenance ). In larger plants, therefore, cheaper coniferous wood is preferred.

Coniferous wood, which burns faster, is desirable for kitchen stoves, as it provides heat quickly (“heating up” a cold oven, more direct control of the hotplate temperature). But it is langflammiger and therefore needs more combustion zone and higher oxygen supply. Therefore, kitchen stoves are usually designed completely differently than heating stoves.

The different types of wood have advantages and disadvantages when used as firewood:

  • Spruce is a wood that burns on and off relatively quickly and is therefore very suitable for burning. It is also often used in basic furnaces / gasification boilers. In Europe, spruce forest widespread and purchase the wood favorably. It is less suitable for the open fireplace, as bursting resin bubbles can cause embers to splash.
  • Fir burns just as quickly as spruce, but causes significantly fewer flying sparks due to the lower level of resin bubbles. Fir is the classic firewood used in the Alpine region for open hearth fires, but it is hardly possible to obtain a single variety.
  • Pine and larch are - with similar burning behavior - of much better quality, but only play a regional role as heating means.
  • Birch is often used for open fireplaces . Even if beech or ash are often mentioned first, birch wood is 'the' classic firewood, as it does not form resin bubbles that could cause flying sparks and, in addition to its beautiful flame image (quite light, bluish), mainly because of the (instead of resinous substances) contained essential oils also smells very pleasant. Birch wood burns a little faster than beech or ash, but much more slowly than conifers.
  • Beech is considered to be a well-suited firewood because it has a beautiful flame pattern and good embers. At the same time, it shows only very few sparks (splashes) and has a very high calorific value. The calorific value / calorific value of beech wood is often used as a reference value compared to other types of wood. Due to its valued smell and taste, beech wood is mostly used for smoking food. Beech wood is very popular and is therefore in the upper price range. However, it is often difficult to get good beech wood; healthy logs are mostly used for furniture or veneers. Often only crown wood (with relatively more bark, i.e. less calorific value and more ash) or sticky trunks (with poor calorific value) are available as firewood.
  • White beech or hornbeam is often also called beech, but has nothing to do with beech (Fagaceae), but belongs to the birch family (Betulaceae). White beech is extremely heavy even when dried and therefore has a particularly high calorific value in terms of volume (just like oak). Hornbeam has a nice flame pattern, few sparks and burns for a long time. It is especially difficult to saw and split.
  • Oak can be used in all stoves (tiled stoves, chimney stoves, workshop stoves ) that actually serve to generate heat. It is not preferred for open fireplaces as it emits good embers but does not produce such a beautiful flame. The calorific value is a little higher than that of beech and the burning time is very long. Oak wood contains a relatively large amount of tannic acid , which if burned improperly (insufficient air supply) attacks exhaust pipes ( soot ). It is therefore well suited for stoves, but not for open fireplaces. The tannin content can be reduced if the (already split) wood is first stored outdoors without a cover; A large part of the tannins is washed out by rain.
  • Red oak is a species of tree that originated in America and was only introduced to Europe about 250 years ago. Red oak can hardly be compared with oak. As firewood, it is comparable to beech. Red oak is difficult to saw and is very heavy. It can be split easily (with straight trunks) and should be dried for at least two years.
  • Ash has a calorific value similar to beech and develops the most beautiful flame picture next to the birch. It is equally well suited for open fireplaces, as it hardly sparks either. Ash wood is hard and tough (easy to split but difficult to saw) and therefore just as expensive as beech.
  • Maple , robinia and elm are ideal as firewood, but are also suitable for all types of stoves. At 4.1 kWh / kg, the calorific value is slightly below that of beech or oak.
  • Linde has a low calorific value per unit volume, but a high calorific value per kg.

The hardwoods poplar or willow are similar to conifers in terms of burning behavior (actually even worse), as they have a similarly low energy density and burn down relatively quickly. In the energy industry, however, the poplar is a very economical wood species in hybrid varieties due to its extremely rapid growth. It is preferably used as wood chips in large-scale fire systems with a controlled fuel supply, but only in summer, because poplar and willow cannot achieve this when there is a high heat demand.

Trading, processing and storage

Forms of trade

In principle, wood can be bought as fresh wood shortly after cutting, fresh or dry . Fresh wood is stored for at least one, better two winters. The higher the water content of the wood, the longer it has to be stored in order to burn without smoke and with as little soot formation as possible.

Burning sticks
Waves : bundles of burning sticks

Commercial forms are for example:

  • Round wood , log wood ( Austrian ): elongated but not split
  • Split wood, meter log : roughly split , about one meter long
    • Firewood , burning logs: about elongated drittelmetrig
    • Logs : ready to use, half-meter (50 cm), third-meter (33 cm) and quarter-meter (25 cm) cut to length; it is also just called “firewood”, but also includes wood for making charcoal
  • Burning brushwood is wood, which is not solid wood strength reached of 7 cm diameter ( twigs and branches )
    • Waves are bundles of wood, which consist of a mixture of brushwood and trunk wood and are tied together in a bundle, the wave.

Firewood measure

Traditionally, firewood is traded and charged in terms of room or volume. The calorific value per volume of firewood is much less influenced by different moisture values ​​than by measure by weight. In addition, the end user can determine the volume better than the weight. Common dimensions are:

  • 1 solid cubic meter = 1 m³ wood mass without spaces, is calculated from the thickness and length of the logs before splitting.
  • 1 cubic meter or sterile = 1 m³ of layered logs 1 m long with spaces in between and corresponds to about 0.7 solid cubic meters.
  • 1 cubic meter (SRM) = 1 m³ of poured, unstacked logs, corresponds to about 0.71 to 0.82 cubic meters of re-installed, oven-ready wood or about 0.4 solid meters.

Old firewood measure

Different measures for firewood were common: The fathom of firewood was counted as 5 feet high and 5 feet wide. The length of the log should be 3 feet. It was called the Nuremberg factory measure. Taking into account the drying of the wood, one log was defined as oversize. The fathom without excess was 75 Nuremberg cubic feet , that is 2.1066 steren. In the Würzburg government gazette of November 6, 1811, the Grand Duke stipulated this measure.

A cart of firewood in Würzburg was set 4½ feet wide and 5½ feet high. Later the cart of firewood was 4 feet 19 inches wide and high by the old Nuremberg measure. The log length was then 3 feet. The cart now had 1.9685 stereos. From 1822 the firewood was sold according to the Bavarian ½ fathom. One measuring frame contained 18 Bavarian square feet. In the Kingdom of Bavaria itself, the fathom was defined as 6 × 6 × 3½ feet, that is 3.1325 steren or 126 cubic feet. Firewood was also measured by thread and Reep . After the thread was measured with 6 × 6 × 2 feet in the clear frame. The result was 72 cubic feet, or 1.7442 French stere (stert). The Reep was reserved for larger quantities of wood. The length was 2½ feet, or 2.45 stars. Grindelein was also a Bavarian measure of firewood. In the firewood trade, the fathom or mass was divided into quarters, eighths and corner (1/16). The Isenburg firewood measure was assigned 6 × 6 shoes and 3½ feet in length. Many firewood dimensions were subject to regional characteristics.

Work-up

Log splitter for mechanically chopping firewood

Firewood can best be processed into logs in the form of meter logs and is also offered in this form by forestry . If the meter log is too big for the end user, it is cut to the required length with a saw (mainly a tilting saw ).

To split firewood by hand , tree slices of around 30 cm are first cut off with a chainsaw , for example , and split when they are damp (freshly felled). If the wood is dried first, which takes considerably longer because of the larger pieces, it is much more difficult to split for most species. When splitting, it is advantageous to split the wood from top to bottom (crown → root), because this requires less force. A rough motto is: "The wood tears like a bird."

A motorized log splitter or a splitting hammer can be used for splitting .

Occupational safety

When processing firewood is for reasons occupational safety to the personal protective equipment ( PPE-forestry to pay). This includes, for example, work gloves , safety shoes , hearing protection and protective goggles . When using chainsaws, cut protection trousers of the appropriate protection category must also be worn. There is an increased risk of accidents, especially when working with a circular saw , but also with a wood splitter or splitting hammer / splitting ax . Hardwood dust (beech, oak) generated during processing can have a carcinogenic effect.

storage

Storage of firewood in a timber rental
Beech firewood processed in the forest

Freshly felled softwood has a wood moisture content of around 55 to 70 percent (water content 35 to 41 percent), with hardwood the value is between 70 and 100 percent (water content 41 to 50 percent). Therefore, the wood moisture should be reduced through storage or technical drying to the residual value of less than 20 percent (water content <16 percent) that is usual for wood combustion. The initial moisture content of the wood is decisive for the duration of the dry storage. This can vary depending on the weather and tree species and possible preliminary storage (trunk storage in the forest or on log yards). Usually, however, a period of at least one year to two years is set for storage drying. The type of storage - for example stacked, heaped or in a silo - depends on how the firewood is processed. With optimal conditions for firewood (finely split and not too long logs in covered, open to the wind lattice boxes or firewood containers outdoors) sometimes even seven months are enough. Firewood can also be stored outdoors in a timber pile or under a roof with good ventilation at the same time. The lack of wind flow in cellars and garages, for example, is a crucial requirement for drying, which is why the wooden sheds that were used in the past often have walls made of laths with a certain distance to allow ventilation. On an (ideally southern) house wall under a canopy, you should therefore keep a distance of at least 5 to 10 cm from the house wall. Technical drying makes it possible to dispense with longer storage, but it has the disadvantage that the burning properties deteriorate compared to the slowly dried wood. Chamber or drum drying systems can bring the wood to the ideal humidity in about a week, depending on the initial humidity. In order to ensure the energy efficiency and economy of the systems, waste heat from other facilities is often used.

Others

Timber collectors in Mozambique
Wood market in Africa

The Bavarian State Institute for Forests and Forestry has published a leaflet on the subject of firewood, which includes a conversion table for the various units of measurement.

The unit kilogram is becoming increasingly important in the modern energy industry, home delivery on pallets and in the use of dried pellets ( wood pellets or wood briquettes ). In terms of weight, the water contained in the wood (residual moisture, water content) plays a significantly greater role than in terms of room dimensions. A purchase by weight should only be considered if the options for weighing and for laboratory testing of quality (composition, residual moisture, water content) are given.

The basis for calculating the price is delivered from the forest, from the forest road / forest road or from the warehouse (pick-up, depending on accessibility) or free domicile, but increasingly also in retail, for example in hardware stores.

The fall of a pile of firewood declared a building by the Potsdam city administration became known nationwide.

See also

literature

Web links

Commons : Firewood  - collection of images, videos and audio files
Wiktionary: Firewood  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. Anja Behnke: The side effects of comfort: fine dust from the fireplace and wood stove. Federal Environment Agency, December 2007 (PDF; 77 kB).
  2. Emissions and material flows from (residual) wood-burning systems, measurement methods, evaluation and results , Eidgenössische Materialprüfungs- und Forschungsanstalt EMPA, Report No. 880'002 / 1.
  3. ^ Karl Wilhelm Ludwig Heyse , Johann Christian August Heyse : Concise Dictionary of the German Language, Volume 2, Part 2 . Wilhelm Heinrichshofen, Magdeburg 1849 ( full text in the Google book search).
  4. Jutta Schütz: "Making a wave, what is it actually?" In: Badische Zeitung. February 4, 2014, online at Badische-Zeitung.de, accessed on January 10, 2017.
  5. Markus Kreusch: solid meters, cubic meters, bulk meters & Co. April 25, 2016, online at Wald-Prinz.de, accessed on January 10, 2017.
  6. ^ Georg K. Chelius , Johann F. Hauschild, Heinrich Christian Schumacher: Measure and Weight Book. Jäger, Frankfurt am Main 1830.
  7. G. Buchner: The most worth knowing from the mass, weight u. Coin history in tabular form with special consideration of the Bavarian. Measurement and weight system. J. Paul'sche Buchdruckerei, Günzburg 1853, p. 4.
  8. LWF Leaflet 20: "Firewood - Production, Storage, Key Figures". (PDF; 897 kB) Bavarian State Institute for Forests and Forestry, July 2014, accessed on January 10, 2017 .
  9. Dispute over building permit for wood piles. In: welt.de, September 17, 2016, accessed on May 18, 2017.