A good tender specification for biogas desulphurisation states the flow, composition, inlet and outlet H₂S, dew point and site conditions, and asks the supplier to fill in the rest. This post uses a requisition for a 900 Nm³/h biological scrubber and dehumidifier package at a palm oil mill biogas plant as a checklist.
The design basis
- Flow 900 Nm³/h, running 24 hours a day.
- Methane 50 to 60%, CO₂ 40 to 50%, nitrogen up to 10%.
- Inlet H₂S 2,500 to 3,000 ppm, outlet below 200 ppm.
- Inlet gas below 50°C, outlet dew point below 15°C.
- Ambient 20 to 40°C, wind load 125 km/h, seismic zone 3 (0.3 g).
Desulphurisation package
A single 100% biological scrubber in a vertical tank. A key requirement is online cleaning, so the package need not stop while the packing media is cleaned. The vendor must state media, dimensions, materials, pressure drop (clean and dirty), effluent pH, nutrient supply (nitrogen, phosphate, potassium) and power.
Included: oxygen analyser, nutrient and circulation pumps, make-up water tank, gas intercooler, moisture separator, pressure/vacuum relief valve (with the flame arrester built in), control panel and differential pressure connections. Excluded: a separate flame arrester and a gas flow meter.
Dehumidifier package
- A shell and tube biogas cooler, with chilled water on the shell side (7°C in, 10°C out) and biogas on the tube side leaving at 15°C.
- A chilled water generation system: compressor, condenser, evaporator, circulation pump and cold water tank.
Why these details matter
- Online cleaning avoids engine downtime.
- The dew point sets the engine’s humidity margin.
- Site loads (wind, seismic) set the structural design.
Checklist
- Flow range, not only maximum.
- Composition range.
- Inlet and outlet H₂S.
- Dew point and temperature.
- What is included and excluded.
- Utilities: power, water, nutrients.
See a worked conditioning train.
FAQ
Why a 15°C dew point?
It is the engine requirement in this specification.
What nutrients are needed?
Nitrogen, phosphate and potassium for the bacteria.
Why a relief valve?
To protect the tank from over- and under-pressure.
Is a standby unit required?
The specification is for one 100% package, with online cleaning instead.