A landfill gas fuel skid takes raw gas from the collection system and delivers it to an engine at a steady pressure, cooled, dried and filtered. The design here, rated at 500 Nm³/h for a landfill in East Asia, refined an existing skid by adding a buffer tank, a speed-controlled blower and a finer filter. Each change tackles a specific cause of engine trips.

The challenge: unstable gas, sensitive engines

Landfill gas engines expect a stable supply pressure. Landfill gas is rarely stable: methane content varies, wells are adjusted, and the supply can dip briefly. If the pressure to the engine falls or surges, the engine may trip. The operator had an existing skid and wanted a new one with the same basic design, improved in five ways.

Gas conditions

• Methane 30 to 50%, with the balance carbon dioxide, air and inert gas.
• Lower heating value 3 to 5 kWh/m³.
• Gas saturated with moisture at 25 to 35°C before the blower and 45 to 60°C after it.
• Maximum ambient 40°C, with the cooled gas leaving within 5°C of ambient.

The approach: five changes to the standard skid

1. A gas buffer tank

The new skid has a 3 m³ outlet vessel. It holds enough gas to run a 1 MW engine for about three seconds at full load, while keeping the delivery pressure above 200 mbar. That is enough to ride through a short dip in supply, such as a valve adjustment or a blower restart.

2. A mechanical pressure regulator

A regulator downstream of the tank can be set between 100 and 210 mbar g and was set at about 200 mbar g. It gives the engine a stable supply even when the tank pressure moves.

3. A variable speed drive on the blower

A pressure transmitter on the buffer tank tells the drive how fast to run the blower to hold a fixed tank pressure. The drive also provides an automatic reset and a ‘flying start’ after a power loss, which smooths the change-over after a power failure.

4. A 3-micron particulate filter

The existing oil filter was replaced. Oil did not appear to be a problem in this gas, so a disposable rigid filter was fitted instead: EN 779 class F9, with an initial resistance below 175 Pa and average efficiency above 95%. Two spares are supplied, and a bypass valve allows filter changes without stopping the engine.

5. An optional air blast cooler bypass

In cold weather an air blast cooler may chill the gas below its dew point and cause excess condensation. A valved bypass lets the operator avoid this. The site is hot, so it was offered as an option only.

What else is on the skid

• A 304 stainless steel inlet knockout pot with a polypropylene demister.
• A twin-stage centrifugal blower with a 304 stainless steel casing and impeller, rated for 500 Nm³/h and a lift of about 300 mbar, driven by a 22 kW explosion-protected motor.
• A stainless steel air blast cooler with aluminium fins.
• An orifice plate flowmeter, stainless steel pressure and temperature gauges at each stage, and a high-temperature switch on the cooler outlet.
• An IP56 control panel with a PLC and HMI for control and alarms.
• A hot-dip galvanised skid, with gas-contact pipework in 304 stainless steel flanged to DIN standards.

What to consider for your own project

1. Ask what the engine needs. Get the supplier’s gas pressure and cleanliness requirements first.
2. Size the buffer by time. The question is how many seconds of supply the engine needs, not just how many cubic metres.
3. Match the filter to the gas. Where oil is not an issue, a rigid particulate filter is simpler.
4. Keep the engine running during maintenance. A filter bypass saves a shutdown.
5. Think about climate. Cooling and condensation behave differently in hot and cold sites.

Gas utilisation equipment on UK sites sits within the permit regime. See the Environment Agency’s guidance on LFTGN03 for the framework.

FAQ

Why cool landfill gas before an engine?

Cooling condenses moisture, which protects the engine and gas train.

What does a buffer tank do?

It holds a short reserve of gas at pressure so brief supply dips do not trip the engine.

Why use a variable speed drive on the blower?

It holds the buffer tank at a fixed pressure by changing the blower speed, rather than throttling.

What was left out of the scope?

Installation, foundations, drainage, engine and field pipework, gas analysis and data recording were excluded.

Upgrading a fuel skid?

Tell us about your engine and gas and we will propose a skid.