Activated carbon H₂S removal is the polishing stage that takes biogas from about 200 ppm hydrogen sulphide down to below 8 ppm. In a proposal for a 730 Nm³/h biogas plant, a biological scrubber removed most of the 2,000 ppm in the raw gas, and a pair of granular activated carbon (GAC) vessels, arranged lead and follow, did the rest, with the carbon changed about once a year.
The challenge: a very low outlet limit
Hydrogen sulphide forms when proteins and sulphates break down during anaerobic digestion. Raw biogas can carry anything from a few ppm up to about 4% by volume, and some wastes produce more. It has to be removed to meet environmental standards and to prevent corrosion in gas engines and boilers.
Here the client specified an outlet below 8 ppm, from an inlet of up to 2,000 ppm at a maximum flow of 730 Nm³/h. A biological scrubber is a low-cost way to remove most of the sulphide, and the proposal used it for that job. For the final step to single-figure ppm it added a second, carbon-based stage.
The approach: bio-scrubber first, carbon second
Stage 1: removing the bulk
The bio-scrubber is a sealed GRP vessel (about 3.5 m in diameter and 7 m tall in this design) filled with packing media. Bacteria grow on the packing and consume hydrogen sulphide, using air supplied by a blower. It needs no regular chemical dosing, which keeps running costs low. It takes the gas from about 2,000 ppm down to about 200 ppm.
Two operating notes from the proposal matter for the carbon stage:
- The bio-scrubber must always have at least 20% of its design flow to keep the bacteria active.
- The gas leaving it is saturated with moisture, so the scrubber should sit before any gas drying equipment.
Stage 2: adsorbing the remainder
Activated carbon has a very large surface area. Hydrogen sulphide molecules stick to it and are held there. The proposal used a working figure that carbon can adsorb about 25% of its own mass in H₂S. As an example, a yearly loading of 2,000 kg of H₂S would need about 8,000 kg of carbon.
That rule of thumb is why the first stage matters. If the bio-scrubber removed less, the carbon would be loaded faster and need changing more often. Taking 90% of the sulphide out biologically leaves the carbon with a manageable job.
Two vessels in lead-follow
The carbon was split between two vertical GRP vessels, each about 3.5 m high and 1.6 m in diameter. The gas passes through the lead vessel first and then the follow vessel. Each vessel holds about six months of carbon, so each is changed once a year, but the changes are staggered:
- The lead vessel becomes saturated first. Its outlet H₂S begins to rise.
- The follow vessel catches what the lead vessel lets through, so the gas leaving the system stays below the limit.
- The lead vessel is refilled with new carbon and then becomes the follow vessel, while the former follow vessel becomes the lead.
The two vessels are there, as the proposal puts it, to stagger the replacement of carbon, so the plant is never relying on a single bed that is close to exhaustion.
Gas velocity and internal layout
Gas velocity through the bed should stay between 0.25 and 0.45 m/s. Too fast and gas channels through the carbon without contact, and pressure drop increases. Too slow and the vessel is unnecessarily large. To keep velocity in range without making a tall, thin vessel, each vessel contains two circular rings that divide it into three flow sections. The carbon is split over several layers with flanged access so it can be removed and refilled easily.
Safety and instruments
Biogas and air can form an explosive mixture, so electrical equipment in the gas and air lines is protected to Zone 2 or powered by an intrinsically safe supply, and over-pressure and under-pressure relief valves protect the GRP vessels. In the UK, equipment in explosive atmospheres is governed by the Dangerous Substances and Explosive Atmospheres Regulations (DSEAR), and the hazard zone is set by a risk assessment. The system carries an oxygen cell and an H₂S cell on the outlet and pressure gauges to monitor pressure drop across each stage. The H₂S reading is the early warning that the lead vessel is near saturation.
Why it works
| Bio-scrubber (stage 1) | Activated carbon (stage 2) | |
|---|---|---|
| Job | Remove the bulk of the H₂S | Polish to below 8 ppm |
| Typical duty | About 2,000 ppm to 200 ppm | About 200 ppm to below 8 ppm |
| Consumable | Make-up liquid and nutrient | Carbon, changed about annually |
| Chemical dosing | None regular | None |
Each unit does the job it is cheapest at. Biology handles the high concentration at low cost, and carbon handles the last couple of hundred ppm down to the specified limit. Because all components are GRP and fit within standard ISO containers, shipping is straightforward, which helps on remote sites.
What to consider for your own project
- Ask what limit you really need. Engines usually have a manufacturer limit for H₂S, and a permit may set another. A tighter limit adds a polishing stage.
- Estimate the annual H₂S load. Multiply flow, concentration and hours, then apply the carbon loading rule to estimate the carbon mass.
- Size for a change interval you can manage. Annual changes were chosen here, but a smaller bed means more frequent visits.
- Plan the carbon logistics. Loading, removal and disposal or regeneration of spent carbon are normally outside the equipment supply.
- Keep the bio-scrubber running. It needs minimum flow to stay biologically active, so check low-load operation.
FAQ
How does activated carbon remove hydrogen sulphide?
The carbon’s large internal surface adsorbs H₂S from the gas. The carbon eventually fills up and must be replaced or regenerated.
Why use two carbon vessels?
In a lead-follow arrangement, the follow vessel protects the outlet while the lead vessel is replaced, and the changes are staggered.
How often does the carbon need replacing?
This design was sized for about one change a year per vessel. Your interval depends on the H₂S load.
Can activated carbon be used on its own?
For low sulphide loads, yes. At 2,000 ppm the carbon would need changing very often, so a first stage is more economical.
Need biogas below 8 ppm H₂S?
Send us your gas flow and H₂S range and we will outline a bio-scrubber and carbon design.