Flexible package designed to cope with rising gas volumes

Summerleaze RE-Generation Ltd is in the process of installing a gas control system including a 2,000 m3/hr high-temperature flare at Onyx Total Waste Management Ltd's landfill site at Candles, Telford. The operational requirements of this site have required an original design which may have wider applications.

Candles is a modern, engineered landfill site in the early days of operation. It has already taken a significant volume of waste, but not sufficient to allow the first cells to be capped. Nevertheless, landfill gas has already started to be produced, and needs to be controlled. There is therefore a requirement for a gas collection and flaring system for the uncapped cells to control odour and emissions. Only small volumes of gas can be collected, as applying significant suction pressure to the uncapped cells would draw oxygen into the waste, with the consequent risk of creating tip fires. However, once the first cells are completed and capped, the amount of available gas will increase dramatically. The expectation is that gas volumes at the site may rise from 200 m3/hr at present to 2,000 m3/hr at peak gas production.

This is a common scenario. Historically, this would have been dealt with, either by allowing the cells to vent passively until capped, or by use of a temporary low-capacity exposed flare, to be replaced by a high-temperature flare when the cells were capped. However, the operators are now under pressure to provide high-temperature combustion of their landfill gas from the start. It is clearly uneconomic to install a small high-temperature flare only to replace it or augment it within a short period with a larger high-temperature flare. But it is by no means simple to design a single system that can cope with significant variations in gas quantity and quality, and meet emissions requirements at all levels.

This was the problem that Summerleaze RE-Generation Ltd put to Hofstetter Umwelttechnik AG. Hofstetter was able to provide a simple solution based on its standard models, with only minor modifications.

The skid-mounted gas delivery unit is based on Hofstetter's flexible LCM model, with a top-of-the-range LFM4c flarestack. The LFM4c provides combustion at 1,200°C with a retention time of greater than 0.3s. This specification exceeds all current or proposed emission standards. The LFM4c comes as standard with a 5:1 turn-down ratio, but with the optional 10:1 turn-down enhancement, the flare would be able to cope with volumes of gas ranging from 2,000m3/hr (its design capacity) to as little as 200m3/hr. The use of the 1,200°C flare, rather than a standard 1,000°C unit, combined with minor modifications to the burners, compensates for the less complete combustion that would otherwise occur when small volumes of gas are burnt in a chamber designed for larger volumes.

The gas delivery unit needed to match this flexibility. A typical package would include a single pump sized to the maximum volume of the flare. However, a 2,000 m3/hr pump could only reduce to a minimum flow of 400 m3/hr, still too much for this application. Hofstetter therefore adapted its LCM delivery unit to provide an extra set of flanges to add a second pump at a later stage. The original pump was sized to 1,000 m3/hr which could operate at the initial expected flow rate of 200 m3/hr. When gas volumes reach 1,000 m3/hr, a second identical pump can be added to double the capacity. This not only provides flexibility but also the security to know that gas extraction can continue should one pump fail.

Flexibility
The flexibility of this package is not only an advantage in the early stages of gas production at the site. Candles has a NFFO contract for electricity generation. Once generation is operational, gas volumes through the flare may vary from nothing, when all the gas is being used for generation, to small quantities if conditions produce a temporary surge in gas output, to larger volumes when an engine is down for maintenance. The system will be able to cope with all these scenarios, and with the gradual reduction in gas volumes that will occur after the site has finished filling. This flexibility, combined with the reputation of Hofstetter equipment, means that a single system should be sufficient to cater for the gas control requirements through the whole of the site's gas-producing lifespan.



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