Biogas processing
Raw biogas from digester is 100% saturated with steam and has high content of hydrogen sulphide.
Dewatering by condensation drying
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
Desulphurisation by biological processes
—> 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
Desulphirisation by chemical-physical processes
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
Desulphurisation by biological process in the external digester
—> 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
Chemical-physical processes - Internal chemical desulphurisation
—> 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
+ Simple —> no technical effort
+ High deposition rates
+ No digester disturbance by oxygen
- Use of chemicals
- Current costs
- Dosing difficult —> mostly overdosing
Chemical-physical processes - Adsorption on activated carbon
—> Steam and oxygen are needed
Pre-cleaning for lower costs for activated carbon
+ Very high deposition rate
+ Moderate investment costs
- High running costs (regeneration/disposal)
- Often necessary for the use of catalytic converters in engines
CO2 separation by Pressure swing adsorption
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
Biogas utilization
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 utilization
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
Power to gas
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.)
Properties to be considered in digestate utilization Storage
Storage
Catching the digestate methane with gas-tight cover of the digestate tank
GHG balance greatly improved
possible economic advantage
Bearing
In comparison to slurry, digestate has reduced ammonia and reduced methane.
Spreading
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
Last changed3 months ago