Where there is more gas than can be used in the energy recovery system (through unusually) high gas production rate or through breakdown/maintenance of the energy recovery system) then additional measures are necessary to eliminate the safety risks and protect the environment.

There are three options:

•  Storage
•  Additional energy recovery systems
•  flare

Storage of biogas is possible for short periods without compression, but for periods of more than a few hours is generally impractical due to the large volume. Compression and high pressure storage is performed but is always linked to biogas upgrading due the problems of corrosion and high cost. Additional energy recovery systems can be provided to give a level of redundancy to the boiler or gas engine. This may be a cost effective measure, but there have to be sufficient (and secure) outlet for the energy. Biogas flares are used to safely burn biogas that is surplus to the demand of energy recovery plant or where recovery plant fails. They may also provide the only means of safely disposing of biogas produced by anaerobic bioprocesses where the economics of energy recovery have not proved viable.

Types of Flare

Flare types can be divided into two main types, open flares and enclosed flares.

Open Flares

An open flare is essentially a burner with a small windshield to protect the flame. Gas control is rudimentary - a coarse manual valve in many cases and the rich gas mixture, lack of insulation and poor mixing lead to an incomplete burn and a luminous flame which is often seen abovethe windshield. Radiant heat loss is considerable and this leads to cool areas at the edge of theflame and quenching of combustion reactions to yield many undesirable reaction products. The heat loss is so severe that open flares are sited 5-6 metres above the ground to protect workers and supply pipework hence the term elevated flare is often applied. However the simplicity of open flares makes it impossible to engineer them to meet emission standards that are now being applied in most developed countries. The absence of an enclosure and intense heat associated with open flares also make it very difficult to monitor emissions. Historically, open flares have been popular because of their simplicity and low cost allied with the absence of emissions standards and controls. In the future the introduction of controls is likely to limit their use to mobile plant where simple skid mounted units can be deployed for safety and environmental control of biogas, particularly within landfill gas fields prior to the installation of permanent gas collection systems.

Enclosed Flares

Enclosed flares are usually ground based permanent plants which house a single burner or array of burners enclosed within a cylindrical enclosure lined with refractory material. Designed for purpose, the enclosure prevents quenching and as a result the burn is much more uniform and emissions are low. Monitoring of emissions is relatively straightforward and basic continuous monitoring of Temperature, hydrocarbons and carbon monoxide maybe incorporated as a means of process control. The increased engineering and process control provide greater flexibility in terms of turn down - the ratio of minimum biogas flow to maximum biogas flow under which satisfactory operating conditions are maintained. Manufacturers typically quote turndown of 4-5:1 for biogas quality of 20-60% methane (by volume). Higher turndown is achievable of up to 10:1 but at the expense of the quality of burn as the heat release does not enable adequate temperatures to be achieved.

A further sub-classification of enclosed flares is based on the method of introducing the air to the biogas.

1. Diffusion aeration (where the air and landfill gas is mixed at the burner – likened to the use of a Bunsen burner with the port closed – the flame is slow to propagate and this leads to high enclosures to achieve burn-out)
2. Pre-aerated (usually achieved through a venturi – likened to the use of a Bunsen burner with the port open - the aeration being proportional to the feedstock flow

http://www.iea-biogas.net/Dokumente/Flaring_4-4.PDF