When site data is limited, size a landfill gas flare from a modelled gas production curve, then add a margin for uncertainty. In a project for a landfill in Portugal, the modelling used UK default values for waste biodegradability and a 75% gas collection efficiency, and the engineers advised adding 30% to the maximum predicted flow. The flare quoted was a 250 m³/h ground flare designed to hold 1,000°C.

The challenge: no local data

An agent for a landfill operator in Portugal asked for a price on a flare designed for a 1,000°C combustion temperature. To size it, the supplier needed a forecast of how much gas the site would produce.

There was a problem. The supplier had a breakdown of the waste types but no Portuguese data on how quickly that waste would degrade. Many landfill projects face this problem.

The approach: model, state assumptions, add a margin

The gas production curve

The supplier produced a gas production curve using its own modelling programme. The model uses biodegradability half-lives for each waste type, so default values had to be chosen. The engineers used UK defaults and said so plainly.

They also set out the limits of the result:

• The figures probably represent a lowest case of production, so they advised adding 30% to the maximum figure to allow for uncertainty.
• A 75% gas collection efficiency was assumed. This depends on how the site and its extraction system are designed.
• With a breakdown of waste types available, the approach was judged likely to be “reasonably accurate”.

Stating assumptions makes the number defensible. The client can see which input to challenge if the real gas does not match.

The flare that followed

An SC Range ground flare was quoted at 250 m³/h:

• Temperature: at least 1,000°C when methane is above 25%, falling to 20% methane.
• Residence time: at least 0.3 seconds. The Environment Agency’s LFTGN05 uses 0.3 seconds at 1,000°C as the indicative standard for enclosed flares.
• Turndown: 5:1, so the minimum flow is about 50 m³/h.
• Height: 6 m, with an adjustable muffle to help retain heat.
• Booster: a 5.5 kW direct-drive explosion-protected motor with a stainless steel impeller, delivering 250 m³/h at about 150 mbar.

Why it works: the margin and the turndown work together

Adding 30% to the peak means the flare may be larger than the early gas supports. A 5:1 turndown covers that. The unit stays within its design range at 50 m³/h, so it can handle the early years when gas production is still building, and the later years when it falls.

The sizing question is not just “what is the peak?” It is “what range will the flare see over its life, and can it operate across all of it?”

What to consider for your own project

1. Ask for the model’s assumptions in writing. Waste composition, half-lives and collection efficiency each move the result.
2. Treat the first curve as a lower case. If the model says so, believe it and add a margin.
3. Check collection efficiency against your design. A well-engineered site may collect more than 75%. A poorly capped one may collect less.
4. Size across the whole life. Confirm the flare can operate at both the peak and the tail.
5. Use measured data when you have it. Pumping trials give real flows that can replace modelled ones.
6. Allow for delivery and installation. These were priced separately from the equipment, with commissioning expected to take about a week.

FAQ

How do you estimate landfill gas production?

With a model that uses waste type, quantity, age and biodegradability. Where local data is missing, defaults from another country can be used, with the assumption stated.

What is gas collection efficiency?

The share of generated gas that the extraction system actually captures. It depends on the site design and the gas collection system. A value of 75% was assumed here.

Why add a margin to a modelled peak?

Because model inputs are uncertain. The engineers on this project advised adding 30% to the maximum figure.

What turndown should a flare have?

Enough to cover the range of gas flows you expect. A 5:1 turndown, as here, allows a flare to operate down to a fifth of its rated flow.

Need a gas forecast to size your flare?

Contact our engineers with your waste data and we can produce a gas production curve and recommend a flare size.