A biogas H₂S removal performance guarantee should state the maximum inlet concentration, the maximum gas flow and the outlet limit, and it should be proved by a continuous 24-hour test at maximum flow. In a proposal for a 600 to 800 Nm³/h bio-scrubber and activated carbon system, the outlet limit was below 8 ppm and the acceptance test was a 24-hour run measured with in-line cells or detector tubes.

The challenge: buying a result, not just equipment

When a plant owner buys a hydrogen sulphide removal system, the real requirement is a gas quality at the engine or boiler. Hydrogen sulphide corrodes engines, heat exchangers and exhaust systems, and many sites also have environmental limits. Equipment lists and vessel sizes are only the means.

The enquiry behind this example came from a client in South-East Asia. One version of the requirement listed an inlet of up to 12,000 ppm of H₂S, a flow in the region of 600 Nm³/h, and an outlet below 8 ppm. A second version of the same duty at a similar scale used a lower inlet of 2,000 ppm. Both ended at the same outlet figure, but the design burden of the first stage was very different. The point for any buyer is that the guarantee and the design must refer to the same numbers.

The approach: a two-stage design with a measurable guarantee

The design in brief

A biological scrubber is the first stage. It is a sealed GRP vessel with packing that supports sulphur-oxidising bacteria and takes the gas down to about 200 ppm. A granular activated carbon (GAC) system is the second stage. It is built as two vessels in lead-follow arrangement and brings the gas below 8 ppm. For a detailed look at sizing the carbon stage, see our post on activated carbon H₂S removal.

What the guarantee covers

The proposal’s performance guarantee reads in essence: based on the design specification, the supplier guarantees that the bio-scrubber and GAC system will reduce H₂S from a stated maximum inlet concentration to no more than 8 ppm by mass, provided that the maximum biogas flow rate is not exceeded. That wording does three useful things:

• It names the inlet limit, so a gas that is worse than the design basis is not covered.
• It names the flow limit, because contact time falls as flow rises.
• It sets the outlet limit as a hard number, so there is nothing to argue about at handover.

How performance is proved

During commissioning, the client accepts that the system has met its specified removal efficiency when it has done so for a continuous period of 24 hours at the specified maximum flow. The test is measured with proprietary H₂S monitoring equipment, such as an in-line chemical cell or a detector tube set. If the maximum flow is not available on the day of the test, the maximum available flow is used to show that the required reduction has been achieved.

That last point is practical. A new biogas plant may not yet be producing its design gas flow on the day of testing, and the guarantee needs a way to be tested without waiting.

The operating conditions that support the guarantee

A guarantee is only as good as the conditions it assumes. The proposal listed several:

• Temperature: the bacteria work best at about 35 to 40°C. In warm climates the heat needed is small, and in colder ones the vessel can be insulated, with heating coils fed by hot water at 70 to 90°C, for example from engine jacket cooling.
• Minimum flow: at least 20% of the design flow must pass through the bio-scrubber at all times to keep the bacteria active.
• Make-up water: the liquid should come from a source containing carbon, such as lagoon effluent, with low suspended solids and approved by the supplier.
• Position in the gas train: the bio-scrubber should sit before any gas drying equipment, because the gas leaving it is saturated.
• Servicing: the warranty normally depends on a service arrangement, either a service contract or operator training with full service records.

Why it works: instruments make the guarantee checkable

The standard scope includes the instruments that let the operator see whether the plant is meeting its duty:

• An oxygen cell and an H₂S cell on the gas leaving the system.
• A pressure switch on the biogas feed to detect a flow interruption.
• Pressure gauges that show the pressure drop across each stage.
• Level switches on the vessels, and a time-controlled auto-drain to remove excess bacteria.
• A weatherproof (IP56) PLC control panel, and an optional analyser panel with gas conditioning and 4-20 mA outputs for connection to a data logger.

The data logger itself, and any SCADA or telemetry, are normally excluded from the standard supply, so budget for them if you want a record of performance beyond the commissioning test. Because biogas and air can form an explosive mixture, electrical equipment in the gas line is protected to Zone 2 or intrinsically safe. The Health and Safety Executive’s EH40 workplace exposure limits list the limit for hydrogen sulphide that applies to anyone working near the equipment in the UK.

What to consider for your own project

1. Write down the design basis once. Flow, inlet H₂S, outlet limit, temperature and pressure should be identical in the enquiry, the design and the guarantee.
2. Specify the test. Duration, flow, instrument type and what happens if design flow is unavailable.
3. Check the assumptions. Ask what temperature, minimum flow and make-up water the guarantee relies on, and whether your site can provide them.
4. Decide on monitoring. Fit continuous analysers if you want to see performance after commissioning, and confirm who logs the data.
5. Plan the service arrangements. They are often a condition of the warranty.

FAQ

What should a biogas H₂S removal guarantee include?

A maximum inlet concentration, a maximum gas flow, a required outlet concentration and the method of proving it.

How long is a typical performance test?

In this proposal, 24 continuous hours at maximum flow, or the maximum available flow if the design flow is not yet produced.

How is H₂S measured during the test?

With in-line chemical cells or detector tube sets, both of which give a direct reading of H₂S in the gas.

Does the guarantee still apply if the gas is worse than the design?

No. A guarantee applies within the stated design conditions, so measure your gas properly before you specify.

Writing a gas cleaning specification?

Send us your gas data and we will help you set a design basis and a guarantee you can test.