To co-fire biogas in a palm oil mill boiler you need duty and standby blowers, a flare for gas the boiler cannot take, and a burner package with its own gas and air trains for each boiler. In a design for a mill producing about 1,250 Nm³/h of biogas at 55% methane, the blowers and flare were rated at 1,375 Nm³/h, and two burners of 1,250 Nm³/h each were specified.
The challenge: a continuous gas supply for a boiler that is not always hungry
A covered lagoon produces gas around the clock, but the mill boilers burn it only when they are running and when their demand allows. The gas system must therefore keep the lagoon at a safe pressure, deliver gas to the boilers at the right pressure and send the excess to a flare, without any manual intervention.
The approach: the gas path from lagoon to burner
Sizing the gas equipment
The mill operated 22 hours a day, giving about 790 m³ of effluent a day and about 47 tonnes of COD removed daily. That produced 30,000 Nm³ of biogas a day, or 1,250 Nm³/h at 55% methane, which is about 6.8 MW of fuel energy. The installed blower and flare rating was increased 10% to 1,375 Nm³/h.
Equipment in the gas path
- Two multistage centrifugal blowers, one duty and one standby, each rated 1,375 Nm³/h at about 300 mbar total head and about 40 kW, with a turndown to roughly 275 Nm³/h.
- Flame arresters on the blower train and on each burner line.
- Demisting and condensate collection, plus an air compressor and dryer for instrument air. A chiller was not included.
- An automatic open flare rated at the installed 1,375 Nm³/h, with automatic ignition, flame detection, a fail-safe quick-closing valve, manual emergency capability and a protected access ladder.
- Two burner packages, each rated 1,250 Nm³/h, with a gas train, an air train, control panel, valves, instruments, UV sensor, ignition pilot and air blower.
- Metering: three ultrasonic flowmeters on the biogas branches and two magnetic flowmeters on the effluent.
Safe disposal route
The flare is the safety route. If a boiler trips or demand falls, the blowers send the gas to the flare, so the lagoon cover does not over-pressurise.
Where the supply stops
The burner supply ends at the boiler. Changes to the boiler itself, such as pressure-part modifications and the final connection, remain with the mill and a boiler specialist unless added to the order. This interface is the most commonly missed item in co-firing projects.
Why it works
Duty and standby blowers keep the gas moving when one fails. A flare rated for the full gas flow means gas can always be disposed of safely. Burners with their own air and gas trains and flame supervision are designed to start and stop on their own. A 5-year warranty was offered on the burner packages.
In the UK, the Health and Safety Executive’s guidance on DSEAR covers explosive atmospheres around biogas equipment.
What to consider for your own project
- Rate the flare for the full gas flow, not for the surplus only.
- Specify duty and standby blowers. A single blower is a single point of failure.
- Define the boiler interface in writing before you order.
- Check boiler turndown. The boiler must be able to burn the gas it is offered.
- Plan the gas drying. Condensate in the pipe disturbs burners.
For how the gas flow is calculated, see our post on covered lagoon sizing for palm oil mill effluent.
FAQ
What is biogas co-firing?
Burning biogas in a boiler alongside the mill’s usual fuel to reduce fuel use.
Why is a flare needed if the gas is used?
The boilers may not accept all the gas at all times. The flare disposes of the rest safely.
Why 10% above the calculated flow?
It gives a margin for variation in the effluent.
Do the burners need modification to the boiler?
The boiler needs an opening for each burner. The mill or a boiler specialist is responsible for the boiler itself.
Planning to burn biogas?
Tell us your boiler duty and gas flow and we will advise on the gas train.