Biomass conversion

Burning wood - an everyday form of biomass conversion

Biomass conversion or biomass conversion refers to the processing and refinement of biomass into energetically and / or materially usable products such as fuels or chemicals .

Depending on the type of conversion, a distinction is made between thermal-chemical conversion, biochemical conversion and physical-chemical conversion.

General

Conversion types to produce
some products
from biomass
product	Thermo- chemical	bio- chemical	Physico- chemical
Fuel additives	J	J	J
Charcoal	J	-	-
Diesel-like fuels	-	-	J
BtL fuel	J	-	-
Heavy fuel oil	J	-	-
petrol	J	-	-
hydrogen	J	J	-
methane	J	J	-
acetone	-	J	-
Methanol	J	-	-
Ethanol	J	J
Butanol	-	J	-
Activated carbon	J	-	-
fertilizer	J	-	J
Fine chemicals	J	J	J

In the course of moving away from fossil fuels, renewable raw materials (biomass) play an important role. These can be used both materially and energetically, however, the underlying biomass for energetic use often first has to be processed into more practical secondary energy sources or for material use via various platform chemicals into end products. There are numerous technical possibilities for these conversions, which can be roughly classified into three main categories, in thermo-chemical, biochemical and physical-chemical conversions. It should be noted that the same products cannot be obtained through all types of conversion.

Depending on the type of conversion, biomass is converted into various intermediate products (selection shown in green), which can be further processed in several steps to form different end products (selection shown in yellow)

The term is often in the context of the concept of the biorefinery , which in the future will be seen as a place for the various conversions.

Thermo-chemical conversion methods

The thermo-chemical conversion methods include all processes that mainly use heat for biomass conversion. This includes, for example, all types of pyrolysis , biomass gasification , torrefaction, but also the direct combustion of biomass. Depending on the process, the goal can be the production of platform chemicals for material use or the production of solid, liquid or gaseous fuels for energetic use. Compared to biochemical methods, these conversion methods are fast and produce rather complex product mixtures.

In the complete thermochemical conversion, different phases can be distinguished in which different processes take place. These are run through one after the other depending on the temperature and the amount of oxygen present ( combustion air ratio λ), but they cannot be completely separated from one another, instead there are flowing transitions. During combustion, all phases are run through in parallel in the same place in the presence of oxygen. By setting the appropriate conditions, however, a specific phase can also be aimed for. Since this is also done technically, the names of the phases partially overlap with the names of technical processes.

Schematic representation of the phases of thermo-chemical transformations. The transitions between the phases are fluid, so the limits shown are not exact. The exothermic phase is shown darker than the endothermic phases.

Heating up

During the heating process, the mass is heated and thereby dried, as stored water evaporates. This phase is usually endothermic and the fuel itself only heats up slowly, as the evaporating water draws energy from it. No oxidation processes take place in this phase. Heating is usually used at temperatures between 100 and 200 ° C, but the transition to the following phase is fluid, since the first decomposition processes can already take place here.

Charcoal - a solid product of pyrolytic decomposition

Pyrolytic decomposition

This phase is reached between 150 and 700 ° C and without external oxygen. The energy supplied decomposes the dried material, as bonds are broken at the particle level. Numerous different products are created in all aggregate states. The proportions of solid (pyrolysis coke ), liquid ( pyrolysis ) and gaseous products ( fuel gas ) can vary depending on the precise conditions. Even though there is no external oxygen present, oxygen-induced reactions can still take place, since oxygen atoms are present in the biomass. Since the resulting compounds are often reactive, secondary reactions can occur, especially with longer periods and higher temperatures. Like heating, pyrolytic decomposition is an endothermic process.

Pyrolysis processes (short: pyrolysis) are processes that are stopped at this stage.

gasification

During gasification, the products of the pyrolytic decomposition are heated up even more (approx. 700 to 1000 ° C) and the solid pyrolysis coke reacts with an external gasifying agent that contains oxygen or oxygen atoms (e.g. air, oxygen or water) to form flammable gases like carbon monoxide and hydrogen. The degree of oxidation increases further. In this phase, too, numerous different reactions take place depending on the gasification medium and temperature. Like the previous steps, the gasification is endothermic.

Gasification processes such as biomass gasification are processes that are stopped at this stage.

oxidation

During the oxidation, all of the oxidizable products from the previous phases are completely oxidized by oxygen, with water and carbon dioxide being formed. There must be enough oxygen for this. In order for the oxidation to proceed properly, it is important that the temperature is high enough that the combustible gases and oxygen are constantly mixed and that there is enough time. This phase is strongly exothermic and can thus provide energy for the endothermic phases in an ongoing combustion process.

Image of a biogas plant - a biochemical biomass conversion takes place here

Biochemical conversion methods

The biochemical conversion methods include all types of fermentations and fermentations , for example the production of biogas , the production of alcohol through alcoholic fermentation, but also composting . These methods are more tedious compared to thermochemical methods, but more targeted products are obtained.

Physico-chemical conversion methods

The physico-chemical conversion methods include all processes that emanate from oil plants . Vegetable oils can be obtained from these by pressing and / or extraction , whereby the remaining mass (press cake or extraction meal) can be used as material or as animal feed. The vegetable oils can be used energetically as fuels in internal combustion engines, however, depending on the engine, further processing (e.g. transesterification ) is necessary.

Individual evidence

↑ Biomass conversion. Institute for Energy and Environmental Research Heidelberg, accessed on October 30, 2019 .
^ ^A ^b ^c Tony Bridgwater: Review Biomass for energy . In: Journal of the Science of Food and Agriculture . tape 86 , 2006, pp. 1755-1768 , doi : 10.1002 / jsfa.2605 .
↑ ^a ^b ^c ^d Martin Kaltschmitt: Biomass as a renewable energy source . In: Martin Kaltschmitt, Hans Hartmann & Hermann Hofbauer (eds.): Energy from biomass - fundamentals, techniques and processes . 3rd updated and expanded edition. Springer Vieweg, Berlin 2016, ISBN 978-3-662-47437-2 , 1.1, p. 1-8 .
↑ ^a ^b ^c ^d ^e ^f Veronika Wilk, Hermann Hofbauer & Martin Kaltschmitt: Thermo-chemical conversion processes . In: Martin Kaltschmitt, Hans Hartmann & Hermann Hofbauer (eds.): Energy from biomass - fundamentals, techniques and processes . 3rd updated and expanded edition. Springer Vieweg, Berlin 2016, ISBN 978-3-662-47437-2 , 11.2, pp. 646-683 .

[ifeu-1] Biomass conversion. Institute for Energy and Environmental Research Heidelberg, accessed on October 30, 2019 .

[Bridgwater-2] A ^b ^c Tony Bridgwater: Review Biomass for energy . In: Journal of the Science of Food and Agriculture . tape 86 , 2006, pp. 1755-1768 , doi : 10.1002 / jsfa.2605 .

[Kaltschmitt-3] Martin Kaltschmitt: Biomass as a renewable energy source . In: Martin Kaltschmitt, Hans Hartmann & Hermann Hofbauer (eds.): Energy from biomass - fundamentals, techniques and processes . 3rd updated and expanded edition. Springer Vieweg, Berlin 2016, ISBN 978-3-662-47437-2 , 1.1, p. 1-8 .

[Wilk-4] ↑ ^a ^b ^c ^d ^e ^f Veronika Wilk, Hermann Hofbauer & Martin Kaltschmitt: Thermo-chemical conversion processes . In: Martin Kaltschmitt, Hans Hartmann & Hermann Hofbauer (eds.): Energy from biomass - fundamentals, techniques and processes . 3rd updated and expanded edition. Springer Vieweg, Berlin 2016, ISBN 978-3-662-47437-2 , 11.2, pp. 646-683 .