Technology and operationAnalysis
Chemical desulphurisation and activated carbon: iron beds, scrubbers and filters
When biological desulphurisation is not enough, iron oxide beds, alkaline scrubbers and activated carbon are used. A comparison of methods, an example calculation of bed life and the rules for safe replacement.
Other language versions: polski.
When biological desulphurisation does not deliver the required H₂S level, chemical and sorption methods are used: iron oxide or hydroxide beds, caustic scrubbers and activated carbon filters. The most common arrangement at agricultural biogas plants is biological desulphurisation as the first stage and an activated carbon filter as the last, so that the sorbent is consumed more slowly and the engine receives gas with a low H₂S content. Below is a comparison of the methods and an example calculation of bed life.
Methods and their characteristics
| Method | Mechanism | Advantages | Disadvantages |
|---|---|---|---|
| Iron oxide/hydroxide bed | H₂S reacts with iron oxides or hydroxides, forming iron sulphides | effective at higher concentrations, cheap sorbent | requires moisture, spent sorbent can self-heat on contact with air |
| Activated carbon (usually impregnated) | adsorption and catalytic oxidation of H₂S, often to sulphur | low outlet level, compact design | cost of sorbent, need for oxygen and humidity depending on the type, not for high concentrations |
| NaOH scrubber (with an oxidant) | absorption in an alkaline solution | high effectiveness at large loads | effluent, consumption of chemicals, handling of chemicals |
| Dosing of iron salts into the digester | precipitation of FeS | simple, reduces the load | cost, sludge, effect on the process |
| Ion-exchange packings / other sorbents | specialised | specific applications | cost |
Activated carbon — how it works and what it requires
Impregnated carbon (e.g. with potassium iodide or potassium hydroxide) acts catalytically: H₂S is oxidised to elemental sulphur on the surface, which requires the presence of trace oxygen. "Plain" carbon adsorbs more slowly. Efficiency depends on temperature, relative humidity (gas that is too wet floods the pores; too dry limits the reaction), flow velocity and contact time. For this reason the gas is dried and heated before the filter to a relative humidity below about 60–80% (the sorbent manufacturer's recommendations take precedence).
Example: carbon life
Assumptions (illustration): gas flow 500 m³/h, H₂S concentration at the filter inlet 100 ppm, sorption capacity of the carbon 0.15 kg S per kg of sorbent (the typical value depends on the product). H₂S stream: 500 × 0.0001 = 0.05 m³/h; at a density of about 1.54 kg/m³ this is 0.077 kg H₂S/h, or about 0.072 kg S/h. Carbon consumption: 0.072/0.15 ≈ 0.48 kg/h ≈ 11.5 kg/d ≈ 4.2 t/year. If the inlet concentration rises to 500 ppm, consumption rises fivefold – this shows the importance of the first stage. It is therefore worth comparing the cost of carbon (price per kg) with the cost of maintaining biological desulphurisation.
Iron beds
Iron oxide beds take larger loads than carbon, but their capacity is limited and the spent material requires replacement or regeneration. Regeneration with air is exothermic and can lead to self-heating; spent beds can ignite after removal. Replacement procedures should include wetting, safe storage and management as waste.
Caustic scrubbers
Effective at large loads, but they require handling of chemicals, pH control and management of effluent; they are encountered less often at agricultural biogas plants.
Safe sorbent replacement
- Isolate and degas the filter in accordance with the procedure; take an atmosphere measurement.
- Use spark-proof tools and personal protective equipment (dust, sulphides).
- Treat the spent sorbent as waste (the code and method of management are determined under the waste regulations), and avoid contact with damp air near ignition sources.
- Store the stock of fresh sorbent in a dry, ventilated place.
- Check tightness after replacement.
Choosing a method — criteria
| Question | Conclusion |
|---|---|
| What is the H₂S load after the first stage? | the higher it is, the greater the need for a cheaper sorbent (iron) or a scrubber |
| What H₂S level does the engine require? | the manufacturer's requirements (often several dozen to several hundred ppm) |
| What is the annual cost of the sorbent? | compare with the cost of iron in the digester |
| What operating risks are there? | self-heating, chemicals, waste |
Monitoring
Measure H₂S before and after the filter; a fall in effectiveness (breakthrough) shows that the sorbent is close to exhaustion. Good practice is to have two filters in the system, in series or in parallel, which allows the sorbent to be replaced without stopping the CHP. Record the results so as to predict replacement dates and negotiate prices with the supplier.
Common mistakes in filter selection
The most common mistake is excessive trust in activated carbon as the only desulphurisation method: when H₂S at the inlet is thousands of ppm, the sorbent is exhausted within weeks and the operating cost becomes dominant. The second mistake is ignoring the humidity of the gas, which shortens the life of the sorbent. The third is the lack of a spare filter, so that replacement means engine downtime. The fourth is the lack of measurement downstream of the filter, so that the moment of breakthrough is visible only after a jump in H₂S in the engine oil. It is also worth agreeing the limit concentration and the method of documentation with the engine manufacturer beforehand, so as not to lose the warranty.
