Biochar

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Biochar (also biochar ) is produced by pyrolytic carbonization of plant raw materials. A traditionally very common form is charcoal .

use

In connection with other admixtures such as bones, fish bones, biomass waste, feces and ashes, it is, for example, part of the terra preta . In some countries (including Austria and Switzerland), biochar is approved in agriculture as a soil conditioner and carrier for fertilizers and as an additive for composting and fixing nutrients in liquid manure. Biochar is also used as a feed additive and dietary supplement . When used as a soil improver, it is attributed, among other things, great potential as a means of offsetting carbon dioxide emissions in view of global warming .

Biochar has numerous other uses, for example as an insulating material in building construction, in wastewater and drinking water treatment, as an exhaust gas filter and in the textile industry. Examples of current areas of application are: Ground biochar is used as a food coloring E  153 without a maximum quantity limit, e.g. B. as a coating for cheese . In medicine, it is used as medicinal charcoal for the treatment of diarrheal diseases . Similar to charcoal, biochar can also be used as activated charcoal .

In addition, biochar is used as an energy source by producing biochar from biogenic residues and later burning it in power plants, heating plants or industrial plants to generate thermal or electrical energy. It can also be burned directly, as a substitute for charcoal, only that it is considered too valuable for that. The carbon dioxide balance is different when burning biochar than when it is stored in the ground, since carbon dioxide is released during combustion.

Manufacturing

Biochar is produced in the absence of air at temperatures between 380 ° C and 1000 ° C (see  pyrolysis ). Under these process conditions, mainly water is split off, producing biochar, synthesis gas and heat. The minerals of the original biomass are bound in the pores and on the surface of the biochar.

Traditional production

Biochar has been produced in so-called coal piles since the beginning of the Iron Age . Wood was usually used as the starting material for this, so that the term charcoal came about. With this traditional process, the coal yield is relatively low and the pyrolysis gases escape unused into the atmosphere.

Technical pyrolysis

Biochar is usually made from the remains of plants growing on land. Other starting materials such as sewage sludge , microalgae or aquatic plants are also suitable.

Thanks to modern technical processes that have been developed since the 1990s, all vegetable raw materials with a moisture content of up to 50% can now be pyrolysed to biochar. The synthesis gases produced during pyrolysis can be burned with low emissions. Part of the resulting heat is used to heat the biomass that is then fed in . The much larger part of the waste heat can be used for heating purposes or partially converted into electricity by means of combined heat and power .

Pyrolysis is also used in wood gasification technology. The resulting gas is fed to an internal combustion engine . The efficiency of the system can be further improved by means of high and low pressure steam stages. The wood gasification technology is also used to generate electricity. A very fine-grained coal is also produced as a waste product . Two thirds of the energy accumulated through photosynthesis (mainly carbon formed through the reduction of carbon dioxide) is stored in the biochar produced.

Well-known manufacturers of pyrolysis systems are German companies such as Pyreg, Carbon Terra, BioMaCon, Regenis, Pyrotec Biomasseverwertung and the Australian companies Eprida, Pacific Pyrolysis (PacPyro). There are other industrial plant manufacturers in China and Japan . By the beginning of 2014, 10 industrial plants had been set up in composting plants, city garden centers, farms, communities, sewage treatment plants and waste disposal companies. In addition to the industrial systems mentioned above, numerous small and very small pyrolysis systems are currently being developed, which are to be used both in the home and in the garden and in developing countries.

Hydrothermal carbonization

Another method for producing coal from biomass is the so-called hydrothermal carbonization (HTC) with the addition of water under pressures of approx. 20 bar and temperatures of 180 ° C. The chemist Friedrich Bergius received the Nobel Prize in Chemistry in 1931 for this discovery . Compared to biochar, hydrocarbon is a related product, but chemically and physically different, which nevertheless has prospects for use in agriculture. In 2010 two industrial plants for the production of hydro coal ( HTC coal) were put into operation (Terra Nova Energy in Düsseldorf and AVA-CO 2 in Karlsruhe).

Pyrolysis at temperatures above 400 degrees results in very stable coals; on the other hand, HTC processes and torrefaction are carried out at a lower temperature and result in coals that are less stable. The longer the reaction time and the higher the temperature or pressure, the more stable the resulting carbon is against microbial degradation in the soil. The nomenclature is not uniform: Coals that are produced by HTC or torrefaction of organic material are sometimes also counted as biochars.

Vapothermal carbonization

A further development of hydrothermal carbonization is vapothermal carbonization (VTC), in which biochar is produced in a steam atmosphere . As a result, the reaction conditions can be better controlled and the process can be carried out faster and more energy-efficiently and thus more cost-effectively. Vapothermal carbonization is an exothermic process that takes place at temperatures between 180 and 250 ° C and pressures of 16 to 42  bar . Vapothermal carbonization is suitable for recycling biological waste products with a moisture content of over 50%.

Charging and activation

Biochar is not a fertilizer, but primarily a carrier for nutrients and a habitat for microorganisms. If left untreated in the soil, it would absorb and fix nutrients and water from the soil; it could inhibit plant growth for months. In order to bring its soil-improving properties to effect, the biochar must first be physically charged with nutrients and / or biologically activated.

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Biochar is porous and has a high specific surface area of sometimes over 300 m² per gram. Due to its high porosity, biochar can absorb up to five times its own weight in water and the nutrients it contains. This property is called the adsorption capacity (AK) of the biochar for hydrophobic substances, which depends on the one hand on the pyrolyzed biomass and on the other hand on the pyrolysis conditions. In the range from 450 ° C to 700 ° C, biochar is produced with the highest adsorption capacity.

Another important property to explain the special nutrient dynamics of biochar is the high cation exchange capacity (KAK). The KAK depends on the surface of the biochar, but is a chemical quantity that increases through oxygen and contact with the soil and only reaches its maximum value after a while. A high KAK prevents mineral and organic nutrients from being washed out and ensures that nutrients are more readily available . A high KAK also favors the binding of heavy metal ions, which protects the soil flora and fauna .

The high adsorption and cation exchange capacities of biochar mean that biochar is suitable as a nutrient carrier. The nutrients absorbed by the biochar mean that microorganisms find habitats in and around the biochar. This leads to microbial revitalization of the soil, which can benefit symbioses of microorganisms and plant roots.

"Charging" can also be done on a small scale yourself by adding charcoal to the raw compost. Here a ripe compost with visible pieces of coal (arrows)

properties

The properties of biochar vary greatly depending on the starting material and the conditions of the pyrolysis.

  1. C content > 50% The carbon content of pyrocarbons varies between 25 and 95% depending on the biomass used and the process temperature. (e.g .: chicken manure: 26%, beech wood: 86%). In the case of very mineral-rich biomasses such as cattle manure, the pyrolysis product predominates in the ash content, and accordingly these products fall under the category of ashes with a more or less high proportion of biochar. Such mineral-rich biomasses should be composted or fermented rather than pyrolysed in order to achieve the most efficient material flows possible, so that the nutrients are made available to the plants again as quickly as possible.
  2. Molar H / C ratio between 0.1 and 0.6. The degree of carbonization and thus also the stability of the biochar can be derived from the molar H / C ratio. The ratio is one of the most important properties of biochar. The values ​​vary depending on the biomass and the chosen process. Values ​​outside this range indicate inferior coals and inadequate pyrolysis processes.
  3. Nutrient content The fluctuations in the nutrient content of different biochars are very high (between 170 g / kg and 905 g / kg). According to the Federal Soil Protection Act (BBodSchG), the nutrient content must be determined. This results in the maximum permissible quantities that can be worked into the soil. However, it is not the absolute nutrient content that is decisive, but the respective nutrient availability, which is difficult to determine (e.g. the nutrient availability of phosphorus is around 15%, that of nitrogen is sometimes less than 1%). According to the Federal Soil Protection Act, however, only the total content of nutrients is taken into account.
  4. Heavy metal content in mg / kg: Cadmium (Cd) 0.8 / chromium Cr 50 / copper Cu 50 / mercury Hg 0.5 / nickel Ni 20 / lead Pb 67 / zinc Zn 200 / arsenic As 10. As in the case of composting Almost the entire amount of heavy metals in the biomass originally used is retained in the final substrate even with pyrolysis. However, the heavy metals are fixed very efficiently and in the long term by the biochar. How permanent this fixation is has not yet been clarified. Since biochar, unlike compost, is only introduced into the soil once (or several times up to a maximum final concentration), an accumulation with heavy metals is unlikely.
  5. PAH contents (sum of the 16 lead compounds of the EPA) <12 mg / kg DM / PCB content <0.2 mg / kg DM. Biochar fixes PAHs very efficiently. The effects of a potential PAH exposure are therefore relatively minor. It should be noted that due to the high adsorption power of biochar, most of the standard methods for analyzing PAHs are not suitable for biochar and only give values ​​in the range of less than 10% of the real value. Pyrocarbons are therefore to be analyzed according to the DIN ISO 13887: B method (Soxhlet extraction with toluene).
  6. Furans <20 ng / kg (I-TEQ OMS);
  7. pH value - the pH values ​​fluctuate between 6 and 10 and do not constitute an exclusion criterion for certification. They must, however, be specified, as a shift in the soil pH value has a major influence on soil culture
  8. Specific surface - its value depends on both the pyrolysed biomass and the pyrolysis process used (above all maximum temperature, residence time, particle size). Typical values ​​for biochar vary between 100 and 300 m² / g.

Ecological potential

The ecological and economic balance of biochar depends on the type of biomass used and on its use, as well as on economic policy framework conditions. Since the production of renewable raw materials can be expensive, mainly residues are used which have a very low value or whose disposal would otherwise cause difficulties or costs.

Biochar as a soil conditioner

Biochar has been contributing to soil improvement in numerous regions of the world for over 2500 years. The biochar was usually introduced into the soil in combination with other organic residues such as cattle manure, compost or bokashi , which are commercial mixtures of different, universally occurring aerobic and anaerobic microorganisms from the food industry. The biochar served primarily as a carrier for nutrients and as a microhabitat for soil microorganisms such as bacteria and fungi. The best-known example of the use of biochar for the sustainable improvement of weathered soils is terra preta .

The introduction of activated biochar into agriculturally used soils can have an impact on soil activity, soil health and yield capacity. In scientific studies, the following advantages for soil cultures have been demonstrated:

  • Improvement of the water storage capacity of the soil
  • Increase in soil bacteria, which find a protected habitat in the niches of the highly porous coal, which promotes the conversion of nutrients for the plants.
  • Increase in mycorrhizae , which ensures improved water and mineral absorption and effective protection against plant pests.
  • Adsorption of toxic soil substances such as organic pollutants and heavy metals, which improves food quality and groundwater protection.
  • Higher soil aeration and better activity of N-bacteria and thus a significant reduction in climate-damaging methane and nitrous oxide emissions.
  • More efficient nutrient dynamics, which ensures both increased plant growth and reduced nutrient leaching
  • Improving plant health through induced resistance

The German Federal Environment Agency (UBA) and the Federal Institute for Geosciences and Raw Materials (BGR) warn of potential risks with regard to the formation of organic pollutants during biochar production and the effects on soils and crops in view of the large number of starting materials, manufacturing processes and areas of application. In 2016, the German UBA recommended further systematic investigations and the establishment of a certification system.

Carbon sink

Biochar mainly consists of pure carbon, which microorganisms can only break down very slowly. If this biochar is incorporated into agricultural soils, a proportion of over 80% of the carbon remains stable for more than 1000 years and thus represents a way of removing the CO 2 originally assimilated by plants from the atmosphere in the long term and thereby slowing down climate change.

In the context of pyrogenic CO 2 capture and storage , corresponding processes could be used in the fight against global warming .

Biological residues such as green cuttings, pomace or manure are currently either sent to composting, fermentation or rotting. During composting and rotting, around 60% of the carbon contained in the biomass escapes as CO 2 and methane. In decentralized pyrolysis, approx. 30% biochar is produced from the original biomass. Since the energy of the synthesis gas can also be used to generate electricity and thus replace fossil fuels , the carbon footprint of the pyrolysis of biological residues is climate- positive compared to their mere rotting. Pyrolysis can also be used in the recycling of residues. Residues from biogas plants, press residues from sunflower, rapeseed or olive oil production and fermentation residues from bioethanol production can be used.

Using a pyreg pyrolysis system, for example, around one ton of CO 2 can be extracted from the atmosphere over the long term from every two tons of green waste . All energy expenditure such as for the transport of the green material, its crushing, the operation of the plant and the introduction of the biochar into the soil are already taken into account. The pyrolysis system used is energy self-sufficient and is operated in a continuous process. The energy required to heat the biomass to over 400 degrees Celsius comes from the biomass itself and is generated by burning the gas produced during pyrolysis. Some systems use the waste heat from other systems to carbonize the biomass. Such systems are e.g. B. biogas plants. The hot exhaust gases from the combustion engines are used here to carbonize the biomass. All of the gas produced by the pyrolysis is fed to the combustion engines for climate-positive electricity generation, as it is no longer needed to carbonize the biomass. The pyrolysis system can be operated both continuously and discontinuously, since the system is always kept at operating temperature through the use of waste heat, thus eliminating the need for heating phases.

Biochar introduced into the ground can survive there for thousands of years.

According to model calculations, with sustainable biochar production it is theoretically possible to compensate for CO 2 , methane (CH 4 ) and nitrous oxide (N 2 O) emissions of up to 6.6 Pg of CO2 equivalent (CO 2 e), which corresponds to 12 % of annual anthropogenic greenhouse emissions. Over the course of a century, a quantity of biochar could be produced that corresponds to total emissions of 480 Pg CO 2 e without endangering food security , biodiversity and the stability of ecosystems . Only part of this potential biochar production is economically feasible. Estimates for Germany showed that - if the emission of one tonne of CO 2 costs around 75 euros in 2050 - around a third of the potential available in Germany could be produced economically.

With regard to the question of whether soils emit a larger or smaller amount of the greenhouse gases carbon dioxide, methane and nitrous oxide after the introduction of biochar than before, the results of studies show a mixed picture.

Possible use in carbon fuel cells

In coal-fired power plants , carbon (previously from fossil coal) is burned, and electrical energy can be obtained from the heat with a heat engine . But it is also possible to convert the chemical energy of carbon in a fuel cell , in this case a carbon fuel cell , directly into electrical energy , which theoretically results in a higher efficiency. The use of biochar as a regenerative energy source for this conceivable application is being intensively researched, as a review from 2018 shows.

Other uses

Due to its adsorption capacity, biochar is suitable for use in water treatment , especially for removing heavy metals.

In California, wood is processed into biochar by means of thermochemical gasification, which is used for soil improvement or as filter material. Electrical energy is generated, but the maximum possible energy yield of the fuel is dispensed with in order to obtain the biochar. The raw material used is wood, which is felled for firebreaks . In California, such wood would have been burned on the spot earlier.

Regulations on biochar

Guidelines and Certification

The International Biochar Initiative has been developing its IBI Guidelines for Biochar since 2009 , and independently of this, the Ithaka Institute has been developing the European Certificate for Biochar (EBC) since 2010 . Both were first published in March 2012.

National regulations and European approaches

In Japan, biochar was approved as a soil improver in 1984. In Switzerland , on April 23, 2013, the Federal Office for Agriculture issued a permit for the use of certified biochar in agriculture.

In Germany, the Fertilizer Ordinance (DüMV) does not yet list biochar; it only permits lignite and charcoal as a starting material for growing media and as a carrier substance in connection with the addition of nutrients via approved fertilizers. Biochar and HTC charcoal are (as of 2018) not permitted as a component of fertilizers, as soil additive or as a growing medium.

In the current regulation (EC) No. 2003/2003 on fertilizers, biochar is not yet provided for (status: 2019). The REFERTIL project was set up to prepare for a revision, with a duration of 4 years from October 2011. This project dealt in particular with compost and the use of biochar as organic fertilizer ("ABC biochar" from animal bones) or as a soil additive (" PBC biochar "from plants). In the summer of 2018, a working group presented its preliminary final report with a proposal for a revised regulation. a. the EBC certificate included.

The EU feed regulation permits the use of biochar as feed; However, in addition to the EBC certificate, further conditions according to Directive 2002/32 / EC and Regulation (EC) No. 396/2005 must be met. When used as animal feed, biochar can then (indirectly) be composted as manure and applied to the fields.

In the USA, funds intended to improve the soil normally do not require a permit, even if they are intended for large-scale use.

Possible contribution to the climate crisis

If biochar is introduced into the ground, it is stored there in a stable manner for several millennia, similar to petroleum or lignite. That part of the carbon of the plants that was bound in biochar is thus withdrawn from the carbon cycle, since it is not converted into CO 2 or methane either through combustion or rotting . The soil input of biochar can turn agricultural soils into carbon sinks .

Even from the point of view that around half of the carbon stored in the starting material escapes during the production of biochar, the sequestration of CO 2 in the form of biochar is viewed as positive in the medium and long term compared to other, non-pyrolyzed forms of biomass.

In the special report 1.5 ° C global warming published in October 2018, biochar was first mentioned by the IPCC as a promising negative emission technology (NET). Studies on the climate impact of the production and use of biochar are, however, in the background compared to other NETs. At the last World Climate Conference in Katowice, December 2018, there was no decision to include such sequestrations in a global carbon trade .

In view of the scarcity of the biomass that can be sensibly used for charring, there is a risk that valuable wood stocks or even contaminated chargeable waste will be used if the pyrolysis of biochar is widely used - and possibly promoted.

Web links

Individual evidence

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  63. ↑ BUND: Terra Preta / Pyrolysekohle: BUND assessment of their environmental relevance. Retrieved February 19, 2020 .