Substrate (biogas plant)

from Wikipedia, the free encyclopedia
Maize silage, the most common biogas substrate in Germany, in the feed device to the biogas fermenter

As a substrate or biogas substrate is - in accordance with the micro- and cell biology  in a designated raw material, - the biogas plant for the generation of biogas is used. Occasionally the term fermentation substrate , which is not exactly defined, is used.

properties

Biomass with a high water content , which cannot be directly thermally utilized , is particularly suitable as a substrate. Biomass, which is rich in cellulose (e.g. straw) and lignocellulose (wood), is difficult to access for microbial degradation and is therefore not suitable as a substrate for biogas production without prior digestion of the cellulose by bioextrusion .

use

Anaerobic microbial degradation ( fermentation ) of the substrate takes place in biogas plants . The substrate serves as a nutrient and energy source for the microorganisms. The gaseous methane separates from the liquid to solid substrate and can, for example, be used in a combined heat and power plant to generate electricity and heat. The main components of the biogas, methane (CH 4 ) and carbon dioxide (CO 2 ), are on the one hand metabolic waste of the microorganisms, on the other hand the high-energy methane is the main product of a biogas plant. The liquid to solid portion remaining after fermentation is referred to as digestate and can be used as nutrient-rich organic fertilizer, provided there is no increased exposure to harmful substances or germs.

In addition to a main substrate, other raw materials are often used, which are cofermented as cosubstrate . Especially in biogas plants that are mainly operated with liquid manure , cosubstrates such as fat separator residues can provide a large proportion of the methane yield.

Substrates for biogas production

Comparison of biogas raw materials
material Biogas yield
in m 3 per ton of
fresh mass 		Methane
content
Corn silage 202 52%
Grass silage 172 54%
Rye GPS 163 52%
Pressed sugar beet
pulp
ensiled		 125 52%
Fodder beet 111 51%
Biowaste 100 61%
Chicken litter 80 60%
Pig manure 60 60%
Cattle manure 45 60%
Grain Meat 40 61%
Pig manure 28 65%
Cattle manure 25th 60%
In order to correctly carry out the billing for biogas raw materials between the cultivating farmer and the consuming biogas plant, the dry matter is determined for the raw materials (here: maize chaff)

In biogas plants, renewable raw materials (Nawaro) as well as residues from animal husbandry and biogenic waste are used as substrates.

In principle, almost all substances of organic origin are suitable for fermentation in biogas plants. The less water and inorganic substance and the more easily degradable substances such as fats, proteins and carbohydrates there are in the substrate, the more methane can potentially result from it. Fibrous substrates with poorly degradable carbon compounds such as lignocelluloses (wood) are unsuitable as they are almost exclusively and very slowly degraded by aerobic fungi. In order to estimate the gas formation potential, the methane yield is related to the organic portion of the dried input mass.

Renewable raw materials

In the case of biogas plants that ferment renewable raw materials, the Renewable Energy Sources Act provides not only a fixed feed-in tariff (EEG tariff) but also a renewable energy bonus for the electricity fed in. This makes the cultivation of energy crops attractive for use as a substrate. Maize, whole plant silage, sugar beet and grass silage are among the most commonly used substrates. Some energy crops (e.g. sunflower ) are used individually or regionally. The suitability of numerous other energy crops as a substrate is currently being tested.

Corn

Corn is the most important renewable raw material in biogas production. The reasons are the high yields per hectare, which should be improved even further by growing so-called energy maize , the efficient harvest with forage harvesters and the good storability as maize silage . In addition, it can be easily introduced into the biogas plant, easily degradable due to its high starch content and does not contain any long fibers that could interfere with the plant technology. It is used all year round as maize silage.

Whole plant silage

Whole-crop silage (GPS), mainly made from rye and triticale, is a substrate of increasing importance . The dry matter yields per hectare are around 20% lower than with maize. By growing a subsequent crop, such as millet , the yields per hectare and year can be increased significantly. With early harvest of winter rye as so-called green rye , a subsequent cultivation of maize is also possible.

sugar beet

High yields per hectare can be achieved with sugar beets . In addition, the degradability in biogas plants is good. Soil buildup on the beet, which can disrupt the operation of the biogas plant, is problematic. Storage of the sugar beet is only possible until spring, as preservation by ensiling is not feasible.

Grass silage

A small amount of grass silage is often used in biogas plants . The long fibers contained require a suitable stirring technique . In addition to grass from agricultural areas, green waste from landscape management is also available. Especially for viehlos managed farms of organic farming , the use of can clover grass silage to be attractive since it supports green manure is used directly redeeming bringing and is obtained simultaneously with the fermentation residue, a flexible operating time fertilizer.

Residues from animal husbandry

In the early agricultural biogas plants, manure was used as the main substrate. It occurs in livestock farms and is usually available free of charge. Difficult to digest parts of the cattle feed give the manure a certain gas potential. Further arguments in favor of the fermentation of liquid manure are the reduction of odor emissions from liquid manure through fermentation. It is also an important cosubstrate in plant fermentation plants, as it stabilizes the fermentation processes through its high buffer capacity and supplies nitrogen compounds (ammonium: NH 4 + ) and trace elements that optimize the activity and growth of microorganisms. It should be noted that contamination of the manure with antibiotics and disinfectants from livestock farming can inhibit the microbiological activity in the fermenter . For manure the same applies as for manure. The firmer consistency requires a different stirring technique in the fermenter; on the other hand, fermentation using dry fermentation is also possible. The straw it contains does not make a significant contribution to the gas yield, as it mainly consists of cellulose, which is hardly broken down in conventional biogas plants.

Proper fermentation of manure and liquid manure can reduce emissions of the highly climate-damaging methane from animal husbandry, and unlike input materials from renewable raw materials, the substrate does not affect the competition for land in arable farming. Therefore, in the version of the Renewable Energy Sources Act (EEG) that has been in effect since 2009, a bonus payment has been specified (“slurry bonus”). This is paid for electricity from smaller biogas plants that are operated with liquid manure as a substrate. In addition, liquid manure is counted among the renewable raw materials in the sense of the EEG, so that the “ Nawaro bonus ” is also paid for electricity from manure-powered biogas plants . In 2008, the Biogas Association expected an expansion of such plants due to the bonus regulation .

The gas yield per ton of liquid manure is much lower than the yield of most other substrates, partly because of its high water content. For a given system performance, more fermentation space must be used than z. B. be provided as a substrate for maize, and manure as a fermentation substrate is hardly worth transporting . In the case of untreated manure, transport as a substrate is usually only economical over a few kilometers. The first projects on manure separation, in which only the portion of the manure relevant for fermentation is used as substrate, are carried out.

Organic residues

Agricultural industry residues

When processing agricultural products, residues arise that are suitable for the production of biogas. These are for example

The gas yields per ton of fresh matter are very different for the various residues, but correlate strongly with the dry matter content . The residues are not considered renewable raw materials in the sense of the EEG. However, they can be used in biogas plants that ferment renewable raw materials to produce electricity without endangering the renewable energy bonus for the entire plant.

Waste

Different types of biogenic waste such as

and others can be used in biogas plants. They often occur irregularly and are therefore used as a cosubstrate. Certain hygienization steps are required by law. Biowaste is mostly recycled in dry fermentation plants because of its high dry matter content. The fermentation properties of biowaste often fluctuate strongly, so that the process control is relatively demanding. No waste may be used in plants that receive the Nawaro bonus. As a rule, the plant operator receives remuneration from the substrate supplier for recycling the waste.

Other substrates

Many other substrates are currently being investigated for their suitability for biogas production, for example to avoid monocultures or to achieve higher yields through certain crop rotations . Residues from the industrial processing of biomass, for example from the paper industry, are also examined for their suitability as a substrate. The technology and legal classification of the biogas plant can be limiting. Since the microbiology of a biogas plant takes a long time to adapt to a substrate, the short-term and short-term use of a new substrate can be problematic.

literature

Individual evidence

  1. ↑ Fachagentur Nachwachsende Rohstoffe e. V. (FNR): Biogas basic data Germany. (PDF; as of October 2008). Source for all information except for pressed pulp.
  2. ↑ Biogas yields of different substrates, division potatoes / beets lfl.bayern.de, see pressed pulp ensiled .
  3. ↑ Company advisory board of the Fachverband Biogas e. V. expects a new upswing in the biogas industry ( Memento of December 13, 2010 in the Internet Archive ). Fachverband Biogas, press release 08/2008, July 14, 2008; Retrieved April 15, 2010.
  4. ↑ Information on the project development and comparison of optimized cultivation systems for the agricultural production of energy crops under the different site conditions in Germany (EVA) ( Memento from August 31, 2012 in the Internet Archive ) of the Fachagentur Nachwachsende Rohstoffe e. V. (FNR).