A landfill gas flare in a seismically active region has to be designed to the local earthquake code, not just to wind loads, and it has to take only the gas that the site’s engines cannot use. For a 3,750 Nm³/h enclosed flare at a landfill in New Zealand, that meant a seismic design review against NZS 1170, a back-pressure regulator that spills only surplus gas to the flare, and a series of detailed choices on instruments and controls that the operator raised during the project.
The challenge: a tall flare, a regulated site and a gas engine plant
The site generated landfill gas at up to 3,750 Nm³/h, with a 10:1 thermal turndown wanted at the low end for later years. Gas went first to engines. The flare had to deal with whatever the engines could not take, and had to do so within the site’s licence conditions.
The gas design basis was typical landfill gas: methane 30 to 70% (design 50%), carbon dioxide 25 to 40%, oxygen up to 2%, saturated at about 35°C, with a minimum inlet pressure of 50 mbar g.
The project had two phases. The original flare and blower skid were supplied and commissioned in the early 2010s. Roughly a decade later, a further flare of the same rating was quoted, built to the same seismic basis. It reused the existing condensate knockout pot, and a blower was offered as an option to fit the existing blower skid.
The approach: what the design had to do
Burn at 850°C or above with 0.5 seconds of residence
The enclosed ground flare is a 304 stainless steel shroud lined with 100 mm of ceramic block. Automatic temperature control uses a PID controller and combustion air louvres to hold a combustion chamber temperature of at least 850°C when methane is above 25%. Below that, the flare runs cooler, down to 20% methane. Minimum residence time is 0.5 seconds. Ignition is automatic, using an igniter, a UV flame sensor and a secondary air damper whose position is set at commissioning and then handed over to PID control.
For the UK expectations on enclosed flares, see the Environment Agency’s monitoring guidance (LFTGN05). Our worked example of ground and elevated flares compares the two temperature classes.
Design for the local earthquake code
The flare was designed to the earthquake actions standard for the region, NZS 1170 Parts 2 and 5. The original design review statement was kept as a reference, and a new review was planned for the new flare to ensure it met the current standard. In the original enquiry, an allowance was made for a local structural engineer to carry out the seismic evaluation, and foundation and holding-down bolt design was left to a local chartered engineer using the site’s geotechnical bearing pressures.
The design consequences are mostly structural: a tall, slender shroud on a skid, anchored to a concrete base that has to be designed for the local ground. The supplier provided footprint and load data after order, and the client wanted to cast the fixing bolts into the slab, at least at each corner, so the footprint drawing mattered.
A back-pressure regulator so the flare only takes surplus
A back-pressure regulator assembly (a butterfly valve with an actuator and positioner, a PID controller and a pressure transducer) sits at the flare inlet. It controls the gas pressure on the line to the client’s engines at a set point. Only gas above what the engines need spills to the flare. This protects engine output, because the flare does not compete with the engines for gas.
Metering, data and remote operation
During the enquiry the operator raised a series of questions:
- Flow meters. The first proposal used an orifice plate meter. The operator reported that these had clogged in service with deposits on the plate and asked about a thermal mass flow meter. A thermal mass meter was proposed instead, with one in the feed-gas train and a second on the gas inlet.
- Display and logging. The operator wanted temperature, pressure and flow visible in one place. A multi-channel data logger with a screen was proposed, with 4-20 mA signals from the transmitters.
- Remote start and stop. This was confirmed as possible but needing clear safety procedures, with a siren and a flashing beacon recommended. Auto-dial alarms were offered through the data logger, with a landline or GSM connection.
- Carbon monoxide monitoring. Monitoring was required under the operator’s licence. The supplier explained that a basic CO monitor has a limited life on landfill gas, so a cheaper unit would be a consumable rather than a lasting component.
Why it works, and the trade-offs
Shroud height and emissions
The client considered a shorter shroud, about 11 m. They noted that at another landfill, a flare of around 11 m had failed its emissions test, and an extra section had been added to reach about 12.8 m before it passed. The client’s position was that a reduced height would need evidence it still met emissions requirements. The lesson is that shroud height is part of the residence time and mixing that make the emissions pass, not just a visual dimension.
Blower and knockout pot
The blower skid was sized for a flow of 3,750 Nm³/h at 300 mbar lift with a variable speed drive, and the knockout pot was sized for a maximum of 8,000 Nm³/h. Pipework between flare and blower skid, and the pressure of the inlet suction, were items the operator wanted confirmed. A high suction pressure shut-down was handled as a formal variation.
Documents the operator will ask for
Before handover, the operator asked for a destruction efficiency certificate, a commissioning method statement, and the documents needed to compile a hazardous area dossier (electrical wiring diagrams, a zoning diagram showing the hazardous area around pipes, and electrical certificates of compliance). Providing these early avoids delays at commissioning. In the UK, the equivalent framework is the HSE’s DSEAR guidance.
What to consider for your own project
- Specify the seismic code up front and budget for a local engineer to review the design and the foundation.
- Decide how the flare shares gas with engines. A back-pressure regulator is the usual way to make the flare take only surplus.
- Choose a flow meter for wet, dirty gas. Ask what happens to an orifice plate in your gas.
- Check CO monitoring against your permit and treat the sensor as a consumable if it has a short life.
- Ask for the shroud height basis. A lower flare may need emissions test evidence.
- Agree who does the bolting-down and when the footprint drawing is due.
FAQ
Do landfill gas flares need seismic design?
In earthquake zones, yes. The flare, skid and anchor bolts are designed to the local seismic standard, and the foundation is designed by a local engineer for the site’s ground.
Why use a back-pressure regulator on a flare?
It holds the pressure on the engine line at a set point, so only gas surplus to engine demand reaches the flare.
Are orifice plate meters suitable for landfill gas?
They can be, but the operator in this project reported clogging from deposits and asked for a thermal mass flow meter.
What is a turndown ratio?
It is the ratio between the highest and lowest flows a flare can burn stably. A 10:1 turndown means it can run at one tenth of its maximum.
Specifying a flare for a regulated site?
Tell us your gas flow, engine arrangement and local code and we will advise on the design.