Firewood

With wood and firewood wood is called, which is used for heating or cooking.

Dry wood is used for combustion in a friendly fire. It is the oldest fuel of mankind and has been used for about 400,000 years. While it came off in the 20th century in industrialized countries by the use of firewood in favor of internal funds with higher energy density and lower prices, is since the beginning of the 21st century increasingly gained heat with firewood. In developing countries, for cooking - to prevent smoke from open fires - used instead of green fresh wood often produced by charcoal burners charcoal.

Firewood, or firewood are broad colloquial terms for fuel wood, which is the generic term for the various forms of trade, such as Oven ready piece of wood, firewood, logs and burning brushwood. Wood pellets and wood briquettes are made from wood, but do not count colloquially for firewood. In this case, the resultant structure is decisive.

In addition, it may be:

• Untreated wood ( lumps or chips)
• Residual wood (from building sites, production waste from wood processing industries)
• Waste wood ( building demolition, furniture, wooden packaging)
• Problematic waste wood ( intensively treated with wood preservatives, halogen- organic coated)

Central to the value of firewood is the calorific value. Other relevant properties can be the focal length, and the look and the smell when burned.

• 3.1 Commercial forms
• 3.2 Brennholzmaß
• 3.3 Old Brennholzmaß
• 3.4 Other
• 3.5 workup 3.5.1 Occupational Safety

Properties

Calorific value

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

In the heating value per unit mass (kWh / kg or MWh / t ) plays the different density of the wood species not matter. It is important however, the water content, it is specified as water content w %

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

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

From the example calculation shows that the decrease of the mass-based heating value mainly due to increasing level of water on the reduction of the dry mass fraction, and only secondary to the increasing heat of vaporization of water (which in burning reduces the energy yield).

Fuel oil equivalent, and energy density

As fuel oil equivalent is defined as the quantity of fuel oil, which has the same calorific value as the predetermined amount of fuel. Since the firewood heating value depends on the water content, it must be used with every heating value can be specified. Absolutely dry wood (absolutely dry) with 0 % water content can not be reached by natural drying, but only by kiln drying. The end point of the natural drying is " air-dry = lutro " the state with about 15 % water content. The fuel oil equivalent may be used if one wants to compare the purchase of wood with the cost of the equivalent quantity of fuel oil. It should be noted, however, that the calorific value per cubic meter (Rm ) of a type of wood has a strong fluctuation resulting from the variation of wood density and the variation of the conversion factor cubic meters (Fm, m3 ) results by cubic meter. The table below contains the average of the heating value per Rm one type of wood.

A cubic meter of dry hardwood replaced about 200 liters of heating oil or 200 m³ of natural gas. Contrast, softwoods have a slightly higher calorific value per unit weight, but take due to their lower mass density more space and burn faster.

Inflammation

Before lighting is stacked in the kiln to firing first highly flammable dry wood rich newspaper and it loosely layered remains of corrugated board or equal fine dry wood chip or wood chips, as well as an ignition aid equivalent wax-impregnated wood wool. Through the use of highly flammable paper, the flame of a match to light is sufficient. Due to the poor thermal conductivity of wood connected to the thus rapidly reach the flash point of the timber or outgassing wood components ( terpenes in coniferous wood, Essential Oils in birch or beech wood, wax vapor at ignition aids ) the tension wood ignite quickly and put in the following coarser log timber or Brown coal briquettes on fire.

If the log of wood stacked on the ignition aid above, there is a top burnout, in which unburned leave from the logs outgassing wood components prior to ignition of flue gases up the chimney, the fire is lit on the thick log of wood, then the outgassing volatile compounds are represented by the directed fire zone, a lower burnup equivalent. In this case, the fuel is used more efficiently. The burning of wet wood these volatiles condense together with the outgassing of water vapor or fog and are perceived together with carbon black particles as smoke.

Combustion

Wood combustion is a two stage process with gasification of wood as the first and the oxidation of gases and charcoal as the second part of the process.

The combustion of wood following sub-processes run partly simultaneously and partly consecutively from:

• Heating of the fuel by re-radiation of heat from flame, ember bed and firebox walls and due to flow of hot exhaust
• Evaporation of volatile wood constituents ( terpenes, etc. )
• Drying by evaporation and removal of the water ( from 100 ° C)
• Decomposition of the wood by the effect of temperature ( 250 ° C)
• Gasification of the wood with the primary air to gas, and solid carbon ( 250 ° C)
• Gasification of carbon ( from 500 ° C)
• Oxidation of the combustible gases to carbon oxides (carbon monoxide and carbon dioxide) and water at temperatures from 700 ° C to 1500 ° C (maximum around 2000 ° C)
• Heat release of the flame to the surrounding walls and the newly supplied fuel
• All drying and evaporating processes lead to the reduction of the flame temperature and the exhaust gas that is the calorific value of the fuel reduction

In a wood burning the release of these materials by gasification of wood takes place (for lack of air, ie combustion air ratio lambda < 1) in the combustion zone. For this purpose, " primary air " supplied. Upon heating 80 to 90 percent by weight of the dry mass of wood to be released as gases. Primarily carbon monoxide (CO ), hydrogen ( H2) and hydrocarbons ( CmHn ).

Subsequently, the gases are mixed with combustion air and burned in the combustion chamber in a long flame. For the combustion of the gases is " secondary air " supplied in the rule. As the gases burn out in a long flame, wood is called langflammiger fuel. The charcoal burns in the combustion zone, however, slow and low flame formation from (with formation of more carbon monoxide in the exhaust ).

Ovens " with upper combustion " can cool escaping gases and rather incomplete combustion in furnaces " lower combustion " means the gases are passed through the fire bed, thereby heating intense and oxidized completely.

Emissions

During combustion are released as the main components of carbon dioxide (CO2 ) and water vapor (H2O). Wood contains small amounts of nitrogen (~ 900 mg / kg). This is - as well as the nitrogen contained in the combustion air - converted during combustion to form nitrogen oxides which react further with water ( steam ) to acids and pollute the environment. The moreover, contained in the wood sulfur ( ~ 120 mg / kg) is predominantly bound in the ash, leaving only little sulfur dioxide is emitted.

The greater the moisture content is, the more heat is required for the evaporation of this water, characterized - but also in excess of air ( from the installation suctioned secondary air at an " fireplace " ) - cool the flames and there is " incomplete burn " this refers on the one hand an incomplete oxidation as well as the reduction of organic compounds or carbon dioxide to carbon black or tar. Also lack of air ( through poor chimney draft or shut off the air supply ) or poor combustion control ( too little turbulence in the combustion chamber ) may lead to incomplete combustion. Here, in varying degrees, new compounds are formed, for example:

• All the above -mentioned compounds, which do not burn up, but blows up as unused volatile fuel through the chimney into the environment
• Carbon monoxide ( CO)
• Shining soot (C )
• Hydrocarbons ( C x H y )
• Hydrogen ( small amounts, from the reduction to carbon black )
• Ash particulate matter
• Mineral products

Emitted.

Condensables can condense at cold spots (heat exchanger for boilers, long stovepipes, in the fireplace ) and deposit. The deposits are ( also because of the condensed water ) sticky, it shall remain the hanging dust, in turn, attract other dusts by agglomeration and entanglement.

Recent studies show that the total load of fine dust that is produced when burning wood, exceeds the sum of particulate emissions allowed in Germany motor vehicles. However, the emissions from wood heaters can be influenced by the selection of appropriate boiler.

Firewood is a renewable resource compared to fossil fuels (eg oil, coal, natural gas ) has the advantage that it is CO2 neutral because the amount equivalent of carbon dioxide released to the amount that has withdrawn during the growth phase of the air of the tree.

Untreated wood has low heavy metal and chlorine contents, during the combustion of contaminated waste wood can be quite heavy metals (arsenic, lead, cadmium, chromium, copper, nickel, mercury, zinc, and others ) and dioxin via flue gas and ash are emitted.

Types of wood

For heating purposes will find different types of wood use. A distinction is mainly on the heating value, burning time and ease of use ( flames, smell).

Per volume ( cubic meter ) deciduous hardwoods and have a much higher calorific value than deciduous softwoods or conifers. Per weight but is the calorific value of softwood something about the hardwood. Softwood burns faster and development of higher temperatures than hardwood. This is due mainly in the higher resin content.

For heating purposes usually a continuous heat is desired. In particular, the combustion technique decides which types of wood are suitable in each case better. In modern wood gasification boilers for pure heat recovery all types of firewood can be optimally used without restriction by the hochtemperaturige combustion.

For fireplaces or stoves are all deciduous hardwoods are very well suited as an energy source. It burns slower and more lasting than softwood, but is a little more ash (maintenance). In larger systems therefore cheaper softwood is the preferable method.

For the faster cooking stoves burning softwood is desirable because it provides fast heat ( " heat-up " a cold furnace, more direct control of the hot plate temperature). However, it is langflammiger and therefore needs more flash space and higher oxygen supply. Therefore, kitchen stoves are usually entirely designed differently stoves.

The various types of wood have when used as firewood pros and cons:

• Spruce is a relatively fast and combusting wood and therefore is well suited for burning. Frequently, it is also used in masonry heaters / boilers carburetor. In Europe, spruce forest is widespread and to acquire the wood favorably. For the fireplace it is not suitable, since aufplatzende resin bubbles lead to " splash " of embers.
• Pine burns as fast as spruce, but is caused by the reduced expression of resin bubbles significantly less sparks. Fir is conventional firewood in the Alpine region for open hearth fire, but hardly to get sorted.
• Pine and larch are - with a similar burning behavior - of much better quality, but only play a role regionally as the heating medium.
• Birch is often used for open fires. Even though often beech or ash will be called in the first place, so ( quite bright, bluish) but is birch wood ' the ' classic firewood, since it does not form sparks -causing resin blisters and next to his beautiful flame pattern mainly because of (instead of resinous products ) contained essential oils and smells very pleasant. Although birch wood burns slightly faster than beech or ash, but much slower than softwoods.
• Beech is considered to be a very suitable firewood, because it has a beautiful flame pattern and good Glutentwicklung. At the same time it shows only very slight Sparks ( spatter ) and has a very high calorific value. The condensing / heating value of beech wood is often used as a reference value when compared to other woods. Due to the estimated smell and taste mostly beech wood is used for smoking foods. Buchholz is highly sought after and therefore is in the upper price range.
• White beech or hornbeam is often also called beech, but is its own type of wood. Hornbeam is dried extremely difficult and therefore, based on the volume ( such as oak) a very high calorific value. Hornbeam has a beautiful flame pattern, little funk splashes and burns very long. It's especially hard to saw and split.
• Oak is used in all ovens ( stove, fireplace, workshop stove) that actually serve the heat. For open fires it is not preferred because, although it developed good glow, but not as nice flame pattern. The calorific value is slightly higher than that of beech, and the focal length is very long. Oak wood contains a relatively large amount of tannic acid, which could result from improper combustion (insufficient air supply ) exhaust pipes attacks ( sooting ). It is therefore well suited for furnaces, but not for open fireplaces. The tannin content can be reduced if the (already split ) wood is first stored outdoors without cover; rain, much of the tannin is washed out.
• Ash has a similar calorific value such as beech and birch developed alongside the most beautiful flame pattern. It is equally well suited for open fires, as it is also hardly injected spark. Ash wood is hard and tough ( easy to saw but difficult to split ) and thus similar to high priced such as beech.

The hardwoods poplar or willow are firing characteristics similar to softwoods, because they have a similar low energy density and burn relatively quickly. In the energy sector, however, the poplar is by her enormously rapid growth of wood in a very economical hybrid varieties. It is preferably used as wood chips in large combustion plants with controlled fuel supply.

Trade, processing and storage

Commercial forms

Wood can be purchased fresh or impact but dry as fresh wood shortly after the cut principle. Fresh wood overlaid by at least one, better two winters. The higher the water content of the wood, the longer it must be stored to burn without smoke and with as little soot formation.

Commercial forms are for example:

• Roundwood sawlog ( Austrian ): elongated but not cleaved
• Chopped firewood, yard stake: roughly split, elongated approximately einmetrig Logs, fuel logs: about drittelmetrig lengthened
• Piece of wood: oven ready, halbmetrig (50 cm), drittelmetrig (33 cm) and viertelmetrig (25 cm) cut to length; it is also called just " firewood ", but also includes wood for charcoal production

Brennholzmaß

Traditionally, firewood is traded and settled in the area or volume measure. The calorific value per volume of firewood is influenced a lot of different minor moisture values ​​than in measure by weight. In addition, the end user can determine the volume better than the weight. Common dimensions are:

Old Brennholzmaß

Various measures of firewood were used: The cords of firewood was expected wide with 5 feet high and 5 feet. As a billet length it should have 3 feet. It was called the Nuremberg work size. Taking into account the drying of the wood a log was defined as excess. The fathoms without excess Nuremberg had 75 cubic feet, which were 2.1066 Steren. In Würzburg Government Gazette on November 6, 1811, the Grand Duke laid down this measurement.

A cart of firewood in Würzburg was 4 ½ feet wide and 5 ½ feet high fixed. Later the foot of the carts firewood 4 was 19 inches after the old Nuremberg measure wide and high. The log length was then 3 feet. The cart had now 1.9685 Stere. From 1822 the firewood was sold to the Bavarian half fathoms. A measuring frame contained 18 Bavarian square feet. In the Kingdom of Bavaria itself was the fathoms with 6x6x3 / 2 feet, which were 3.1325 Steren or 126 cubic feet of firewood also was measured after thread and Reep. After the thread has been measured in the clear frame with 6x6x2 foot. Result was 72 cubic feet or 1.7442 French Stere ( Stert ), The Reep was larger quantities of wood reserved. The length was 2 ½ feet, so 2.45 Stert. Also Grindelein was a Bavarian Brennholzmaß. In Brennholzhandel the fathoms or measurement in quarters, eighths and Ecklein ( 1/16) was divided. The Isenburg Brennholzmaß were assigned length 6x6 shoe and 3 ½ feet. Many firewood dimensions subject to regional characteristics.

Others

The Bavarian State Institute of Forestry has a fact sheet on the topic of firewood brought out, in which, among other things, a conversion table for the various units is included.

The unit kilogram gets in the framework of modern power industry, home delivery on a pallet and in the use of dried pellets ( pellets or briquettes ) are becoming increasingly important. When weight of the water contained in the wood (residual moisture content, water content ) plays a much greater role than in cubic measure. A purchase by weight should be considered only when the possibilities of weighing and for laboratory verification of the quality (composition, residual moisture content, water content ) are given.

Calculated on the basis of the award is from the forest, from the forest road / forest road or from stock ( Customer pickup, depending on accessibility) delivered or free home, but increasingly also in the retail business, including home improvement stores.

Oven wood is a project of the Forest Association of Chamber of Agriculture Austria to offer a quality seal reliable and standardized quality, in addition to the minimum criteria to dryness and format well -defined types of wood (oak, beech / hornbeam, ash hardwood / maple and other softwood, birch) and the criteria a includes delivery service, and is a supplement to the quality standards to pellets and briquettes.

Work-up

Firewood can be best worked as a meter wood firewood and is offered in this form of forestry. Is the meter of wood for the consumer too large, it is with a saw brought (mainly a circular saw ) to the desired length.

Initially for the splitting firewood by hand, for example, with a chainsaw, cut tree slices of about 30 cm and in the wet ( freshly cut ) state split. If the product is first dried, which takes considerably longer because of the larger pieces, it's much harder to be cleaved in most species. The columns, it is advantageous to divide the timber from top to bottom (crown → root ) as less force is required. A coarse Wish sentence reads: " The wood breaks like the bird craps. "

For splitting you can use a motor-driven log splitter or a splitting maul.

Occupational safety

In the workup of firewood to look for reasons of occupational safety on the Personal Protective Equipment (PPE - forestry ). This includes, for example, work gloves, safety shoes, ear protection and safety glasses. When using power saws also a safety trousers appropriate protection category is to be created. Especially when working with a circular saw, but also the wood splitter or Maul / Log Splitter is an increased risk for accidents. When processing incurred hardwood dust (beech, oak) may have a carcinogenic effect.

Storage

Freshly cut softwood has a moisture content of about 55 to 70% (water content 35-41 %), with hardwoods, the value is between 70 and 100% ( water content 41-50 %). Therefore, the moisture content should be reduced by storage or kiln drying on the usual for the combustion of wood residual value of less than 20 % (water content <16 %). Crucial for the duration of the drying storage is initially the initial moisture content of the wood. This can vary ( in the woods or on round wood yards root storage) depending on weather conditions and tree species and any preliminary storage. Usually during storage, however, a drying period of at least one year is set up to two years. The type of storage - stacked, for example, or poured in the silo - depends on the processing form of firewood. Under optimum conditions for burning wood ( finely divided and not too long logs in covered, the wind open mesh boxes or firewood containers outdoors ) rich sometimes even seven months. Very good firewood stored outdoors in a wooden rent or under a roof with good ventilation. The air flow with wind is a crucial requirement for drying. At one (ideally southern ) house wall under a canopy you should also keep at least 5-10 cm from the wall. The kiln drying eliminates the need for longer storage, but it has the disadvantage that worsen the burning characteristics compared to the slowly dried wood. Chamber or drum drying systems can in about 1 week the wood, depending on the initial moisture, bring to the ideal humidity. To ensure energy efficiency and sound systems, waste heat from other institutions is often used.