Most straw is incorporated into the soil of agricultural land as a source of humus and nutrients , either immediately after harvest or - mixed with animal excrement as manure - after it has been used as litter . Some types of straw also serve as feed that is rich in crude fiber but low in nutrients . Straw is also used energetically as fuel and as a raw material for synthetic biofuels . Moreover straw is a building material ( straw bale ), as raw material for ( art ) handicraft work (z. B. straw weaving ), and earlier as a material for the covering of roofs used.
Straw can be used both materially and energetically. The use of straw can be traced back to the prehistoric phases of human history with the beginnings of the agricultural use of cereals. The straw of various types of grain, mainly wheat , rye , barley and triticale , is a by- product when the grain is used for nutrition and starch production. In Germany, around 23 million t of wheat are produced on 3.2 million ha of land each year, plus 11.5 million t of barley on 2 million ha, 2.8 million t of rye on 550,000 ha, 2.6 Million t of triticale on 380,000 ha and 1.2 million t of oats on 210,000 ha. t rye, 1.3 million t oat and 1.2 million t triticale straw (calculated on the basis of the grain-to-straw ratio). Of this, around 20–30% can be taken from the material cycle for energetic or material use without harming the soil fertility, otherwise a supply of comparable organic matter to the cultivated areas is necessary to ensure a balanced humus balance .
Bedding and horticultural use
Most of the straw that is brought in serves as litter for large animal husbandry (cattle, horses and pigs) and is returned to the nutrient cycle after use. In the private sector, straw is used to keep rabbits and other small animals: it can be used here as an alternative to sawdust or shives . Barley straw has a significantly lighter color than straw made from other types of grain and does not absorb moisture. For this reason, it is not very suitable for bedding. It is the same with oat straw, which is also very soft and is therefore mainly used as feed straw. Rye and wheat straw, on the other hand, are suitable for both feeding and bedding. Smaller amounts of straw are used in mushroom cultivation as a growing substrate or in fruit growing , especially with strawberries, for underlaying the fruits so that the fruits do not lie on the ground and get dirty. It also serves as protection against the cold.
Construction and insulation material
|raw materials||Bales of straw|
|Thermal conductivity λ||fixed: 0.038 W / (m K )|
|Specific heat capacity c||2100 J / (kg K)|
|Bulk density ρ||90-120 kg / m³|
|Vapor diffusion resistance μ||2.0|
|Building material class||B2|
|Areas of application||Insulation of walls, ceilings, roofs|
|material costs||3–5 € / m² at 0.1 W / (m² · K)|
A material use of wheat straw in particular is its use as a building and insulation material. In house construction, straw bale construction is established, which has been practiced in the United States since around 1890 and has experienced a renaissance since the 1980s. Today, straw bales are used as building material in North America, Europe and Asia. With the load-bearing straw bale construction, the walls consist entirely of straw bales and the roof load is carried by the straw bales. In the non-supporting construction forms a timber framework , the support structure and the spaces ( compartments ) to be filled with straw. This type of construction largely corresponds to the timber frame construction or the classic half-timbered house and is usually preferred in Germany, since straw bales are not yet approved as load-bearing building materials here by the building authorities. Another possibility of insulating with straw is blowing straw, which can be used like cellulose. Furthermore, straw in the form of straw building panels is used for the purpose of dry construction.
Straw is a good insulation material that has a WLG value of 046 in bale form , 043 as blow-in insulation and has general building authority approval, more details in straw bale construction . Properly installed bales of straw are mold-resistant, are categorized as normally flammable building material (B2) and, with settlement values of 2.3% in the component, are on a par with other insulating materials. The bulk density of the bales is between 90 and 150 kg / m³.
There are also numerous mixed forms. Traditionally , straw was often mixed with clay in earth building , it increases strength and improves thermal insulation. In addition, thatched roofs used to be made in many regions .
Sack of straw and other material uses
Historically in particular, the use of straw for a number of products has been of much greater importance, but has been replaced by other materials. Woven straw was used to make shoes, so-called straw shoes . Another use was in beehives made of woven straw, which are still occasionally made today.
The use of drinking straws is also outdated , although plastic drinking straws are still often referred to as straws . In the past, straws were used as packaging material to dampen vibrations. Today, this function is mainly fulfilled by polystyrene and other plastics, although there are also new products based on straw as packaging filler material. Sometimes straw is also used to make paper . Also targets for archery are made of straw.
In Japan, the living rooms and bedrooms are covered with mats made of rice straw, so-called tatamis . In Europe tatamis are used, among other things, as a base for the practice of judo and other Japanese martial arts .
Around 1800, straw was used as a weft material for making marling fabrics .
For centuries, straw sacks were used instead of mattresses . Straw or hay was exchanged several times a year and the straw sack was washed. As the long stalks broke quickly, the camp became hard and uncomfortable. Today, the natural product in straw core mattresses is experiencing a small renaissance: cleaned straw is pressed, surrounded with a fabric, quilted with yarn and often combined with layers of other natural materials such as natural rubber . The straw swab was used as a field sign.
In addition to wood , especially fast-growing wood from short rotation plantations , grain straw is a potential supplier of lignocellulose as a raw material for use in biorefineries for the production of various platform chemicals. Corresponding projects are, however, still being planned.
In Germany, straw is approved as a standard fuel for heating systems between 15 kW and 100 kW in accordance with the Small Firing Systems Ordinance, so that it can be burned in systems for domestic use without a special permit. For systems of z. B. 15 to 50 kW result from this regulation exhaust gas limit values (per m³ exhaust gas) of 0.15 g / m³ fine dust and 4 g / m³ carbon monoxide (CO), but the use in heating for individual households is not very widespread because the system technology in Compared to other fuels (pellets, wood) is more expensive. In Denmark, the burning of straw for decentralized heat and power generation is already relatively widespread. The first three straw heating power plants in Germany are planned in Northern Germany, each with a thermal output of 49.8 MW. It has to be differentiated from the grain burning , in which the grain is used for fueling. The production of energy pellets from straw for combustion is practiced occasionally, but most pellet heating systems are only suitable for wood pellets .
The combustion properties of straw differ significantly from those of wood as a fuel. Among other things, the ash content is significantly higher, while at the same time the ash melting point is reduced and the airborne dust emissions are higher. In addition, the concentrations of nitrogen, sulfur and chlorine are less favorable for combustion. Compliance with applicable emission limit values is associated with considerable additional expenses, especially in small systems. It has a calorific value of 18.3 to 18.5 MJ / kg.
After entire rows of houses burned down in many villages, strict regulations were issued in the 18th century under Count Palatine Karl IV to prevent a fire, which also regulated the proper handling of straw and hay.
As a raw material rich in cellulose, straw is also suitable for the production of biofuels that use cellulose as a starting product (for example BTL fuel and other synthetic biofuels, cellulose ethanol ). These fuels are still largely in the development phase.
In 2012, the first plant went into operation in Zörbig ( Saxony-Anhalt ), which can produce 20,000 tons of straw annually from biomethane . The German Biomass Research Center in Leipzig estimates the potential in Germany at 8-13 million tonnes, in Eastern Europe at 240 million tonnes, with 8 million tonnes corresponding to 2.5 gigawatts or clearly the energy requirements of 4 million natural gas vehicles. In contrast to the cultivation of energy crops , the energetic use of straw does not entail any competition for land for food production. Since the fermentation residues can be spread on the fields, there is also only a limited loss of nutrients.
Etymologically the term depends straw with the old- and Middle High German expression STRO (related to ströuwen : scatter ') together, so actually means Starting interspersed or Spreading , (stall) scattering .
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