Wood gas condensate

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Wood gas condensate occurs during the production of wood gas in wood gasifiers . Wood gasification takes place at temperatures from around 400 ° C. In the absence of air ( pyrolysis ) or when the wood is burned in a lack of air, the desired volatile wood gas is produced and, in addition to water, a variety of organic substances, which depend in detail on the process, the type of wood and the process conditions (temperatures, lack of air, wood moisture, etc.).

Some data

The dry substance of wood (without bark) has the following mass components:

Carbon: 47%
Oxygen: 46%
Hydrogen: 6%
Ash: <1%

After gasification, in addition to the water vapor from the wood moisture (“air-dry” wood usually still has a wood moisture content of around 15–20%), the gas contains further water vapor, which is formed from the hydrogen and oxygen atoms in the wood.

When the gas is cooled, this water vapor and all other organic substances with a higher boiling point contained in the gas condense .

In the case of wood gas produced with pyrolysis , this is the wood tar .

In processes that work with a lack of air ( smoldering fire ), this condensate contains water in the countercurrent gasifier (the gas leaves the gasifier on the cold side and contains many substances that are also produced during pyrolysis), among other things:

Phenol , cresol , acetic acid and methanol with several grams per liter.

Values ​​that are important for wastewater technology were measured in a countercurrent gasifier ( Lit : P. Schulze Lammers, M. Leuchs):

Chemical oxygen demand ( COD ): 100 g / l
Biochemical oxygen demand in five days ( BOD 5 ): 50 g / l
Total carbon ( Total Organic Carbon - TOC): 50 g / l

Since even wood with 0% residual moisture produces around 0.25 l of water per kg in all processes, every gasification process produces around 0.5 l of waste water per kg of normally dried wood with 20 to 30% moisture content when the wood gas condenses. With the countercurrent gasifier, it is slightly acidic ( pH value approx. 4.0).

With cocurrent gasifiers (the gas leaves the gasifier in the hot, strongly reducing area), the waste water is slightly basic and contains, among other things, ammonium compounds. The wastewater pollution is not so high with this type of gasifier. In the 1930s, waste water from gasworks that resulted from coal gasification was partly used as fertilizer on fields because of its ammonium content. Because of the fungicidal ( fungicidal ) and bacterial ( bactericidal ) killing effects, this could only be done three weeks before the start of the growing season . The toxic effect on voles, white grubs, wireworms, slugs and larvae were present desired accompaniments ( Ref : F. Meink).

sewage

Every wood gasification plant that is to be used on a large scale for energy generation requires a suitable wastewater treatment plant , even if some plant manufacturers do not mention this point or even simply deny it.

The following are among others:

The effort for this reduces the overall efficiency of these systems.

literature

  • P. Schulze Lammers, M. Leuchs: Gasification of biomass and use of the gas to drive engines . Final report on the research project 03E-4469-A / B of the BMFT, 1984, research report BMFT-FB-T 85-066
  • F. Meinck, H. Stoof, H. Kohlschütter: Industrial wastewater . 4th edition. Gustav Fischer Verlag, Stuttgart 1968