04. Biogas Processing

• Raw biogas from digester is 100% saturated with steam and has high content of hydrogen sulphide.

• Cooling below dew point

• Water vapour condenses and fall out from the gas

• Usually sufficient (economical) for engine use

Cooling by..

• Underground (or outdoor) pipes

• Chiller unit

  
  

—> Biological desulphurisation in the digester by adding air

• Products: Elemental sulphur and water

• For engines usually sufficient (desulphurisation up to 95 %)

Advantages

+ Simple construction

+ No chemicals

+ Additional fertilizer

Disadvantages

- Natural gas quality not achievable

- Difficult regulation

• Sulfid precipitation in the digester —> Internal chemical desulphurisation

  H₂S is precipitated as insoluble metal sulfides, which can then be removed from the system.

• Adsorption on activated carbon

—> Done in a bioscrubber, also known as bio filter

• Better adjustment of the air supply

• Biogas and air are fed in the container

• Product: Sulphur oxidizing bacteria

—> Adding iron compounds (iron chloride, ferrouss sulfate, iron hydroxide)

• Formation of hardly soluble ferrous sulfide compounds discharge with digestate

• Often used additionally if air desulphurisation is not sufficient

• Attention to heavy metal contamination by iron compounds

Advantages

+ Simple —> no technical effort

+ High deposition rates

+ No digester disturbance by oxygen

Disadvantages

- Use of chemicals

- Current costs

- Dosing difficult —> mostly overdosing

—> Steam and oxygen are needed

• Pre-cleaning for lower costs for activated carbon

Advantages

+ Very high deposition rate

+ Moderate investment costs

+ No digester disturbance by oxygen

Disadvantages

- High running costs (regeneration/disposal)

- Often necessary for the use of catalytic converters in engines

Pressure swing adsorption (PSA)

• Uses different bond strengths of CO2 (+) and CH4 (-) to adsorbers

• Adsorbents: activated carbon, zeolites

• Previous drying necessary

• Methane slip approx. 3% requires post-combustion of the exhaust gas

• Good for small plants

• At the same time drying

3 steps

1. adsorption at higher pressure (6 – 9 bar)

2. desorption by pressure reduction and purging

3. pressure build-up

Pressurized water scrubbing

• Higher pressure —> more can be dissolved (in water)

• Trickle-bed reactor

• Regeneration by pressure reduction

• Methane slip ≈ 2 % —> requires post-combustion of exhaust gas

• Operating pressure 6-9 bar

  
  

Membrane technology

• E.g. polymer membrane / hollow fiber membrane

• Driving force: partial pressure gradient top / bottom side

• Combustion engine

• Microgas turbine

• Feed into the natural gas grid

• Very well suited for combustion in CHP

• Has mainly 2 concepts - gas engine and dual fuel engine

• Natural gas grid is a huge energy store

• Electricity and especially heat can be produced where it is needed

• Economically attractive

Digestate is the byproduct of the anaerobic digestion process.

Digestate can be used as fertilizer like slurry plant nutrients stay retained.

Advantages of fermentation

• Reduction of bad smell

• Improvement of the flow properties

• Nutrients better soluble/available in plants

—> But heavy metals stay retained

• Electrolysis —> hydrogen is produced with excess electricity

• Methanation —> hydrogen is converted with CO2 into CH4 —> biological or chemical

• Storage in the natural gas grid (conversion into electricity, fuels, etc.)

Storage

• Catching the digestate methane with gas-tight cover of the digestate tank

• GHG balance greatly improved

• possible economic advantage

In comparison to slurry, digestate has reduced ammonia and reduced methane.

• Spreading of digestate during storage is desired —> circulation

• Digestate usually fertilizer

• digestate has some nutrient and pollutant limits

• Blocking periods of spreading is the time when it is not good to spread the digestate(Jan-Nov)

—> Storage necessary