Before a palm oil mill biogas plant is built, a front-end feasibility study (FEFS) and front-end engineering design (FEED) package confirms the real effluent data, the ground conditions and the design that will be priced. A proposal for a mill processing about 60 tonnes of fruit an hour shows what goes into each, and what the supplier and owner each provide.
The challenge: a design that rests on assumptions
A biogas plant is designed from effluent flow, strength and temperature, and built on ground whose bearing capacity must be known. At quotation stage most of these are estimates. The FEFS and FEED turn them into confirmed figures before the full capital cost is fixed.
Stage one: the feasibility study
The FEFS covered:
- A ten-day sampling campaign. Effluent flow, COD, total solids, temperature and pH are recorded. Ten days captures variation across production cycles.
- Biogas production rate. Confirmed from the measured effluent and the reactor design.
- A topographical survey of the reactor location.
- Soil tests at the reactor location.
- The electrical single line diagram for the plant’s power supply.
- Confirmation of design configuration, scope, capital cost and delivery time.
The output is a report. The price was for the scoped items only, with extras added by agreement.
Stage two: front-end engineering design
The FEED package produces the documents that fix the design:
- Heat, energy and mass balance.
- Process flow diagram.
- Process and instrument diagram.
- General arrangement drawing.
- Data sheets for each major component.
What the plant itself looks like
The design was a covered lagoon reactor with a floating gas collection cover, geomembrane liner and baffles. Upstream, hot effluent passed through a cooling tower with a bar screen, then a mix tank with two mixers and feed pumps. A sludge sump handled sludge removal. Downstream were two centrifugal biogas blowers sized for a peak flow of about 1,100 m³/h, an enclosed flare with flame detection and a safety shut-off valve, designed for over 95% methane destruction, and a full PLC and motor control package. Engines were excluded.
The spare-parts and options list
Options included standby pumps and blower, telemetry alarms, a data logger for gas flow, temperature, pressure and composition, and infrared and chemical-cell gas analysers.
Acceptance: proving trials
After installation and commissioning, the plant had to meet its process guarantees over five consecutive days, with the client providing operators and the supplier supervising. If any guarantee was missed, adjustments were made and the five-day trial restarted.
Two ways of buying the same plant
The mill owner issued a request for quotation for a design-and-build contract, with civil and structural works (piling, buildings, drainage), mechanical and electrical works, and a commissioning period. The supplier’s offer was narrower, covering the proprietary biogas equipment, with exclusions such as engines, electrical supply connection, site investigation, buildings and import duties. The gap between the two documents is typical: owners should line up scope boundaries before comparing offers.
What to consider for your own project
- Do the sampling campaign. Ten days of data are worth more than any rule of thumb.
- Get the ground tested. Piling can change civil costs sharply.
- Define scope boundaries. Who builds the powerhouse, grid link and pond?
- Specify the acceptance test. Five days of guaranteed performance is a clear target.
- Add instrumentation. Gas analysis and logging make future troubleshooting possible.
For how the digester itself is sized, see sizing a covered lagoon for palm oil mill effluent.
FAQ
What is the difference between FEFS and FEED?
FEFS confirms the data and viability. FEED produces the design documents.
How long is the sampling campaign?
Ten days in this proposal.
What does a proving trial involve?
Five consecutive days meeting the process guarantees.
Starting a biogas project at your mill?
Tell us about your mill and what data you have and we will outline the study.