‘Reject gas’ causes plant operators problems – reasons for a triple membrane gas store
Biomethane industry expert Mr Owen Yeatman of Farmergy Ltd explains why the Triple Membrane Roof Gas Store is an innovation and benefit to the biogas and biomethane industries.
“As a consultant who has worked on several biogas to biomethane projects in recent years, from the second gas to grid plant commissioned in the UK, to projects currently underway, I have gained much experience over the issues around reject gas.
Reject gas occurs when the grid entry unit, which monitors the gas quality before allowing its entry into the grid operators network, rejects upgrade gas for a range of reasons. It rejects gas until the upgraded gas (along with added propane) finally meets the grid standards, when the valve then opens, and the upgrade gas is allowed into the network.
Gas can be rejected for many reasons, but the most frequent reason for rejection is related to energy content and the “Wobbe index.” Put simply, the gas has to meet both minimum, and maximum energy content standards.
In the event of gas falls outside of these standards, the grid valve shuts down, and the gas is then sent to reject. Reject, can mean either it is sent directly to the flare, where it is burned off, or as the gas has cost money to acquire (through feedstock and processing costs) it can be recirculated back into the biogas store, where it is then recirculated back into the upgrade plant. It is this last operation where problems start to occur.
The upgraded and propanated gas is then mixer with the unprocessed biogas, meaning the upgraded gas which has a CH4 content of over 95% and the Biogas having a CH4 content of between 50%-60% are mixed.
The issue here is that the gasses don’t mix evenly, and so the CH4 content of the gas that then feeds into the upgrade unit, is no longer stable, and therefore the system has to cope with extreme variations in energy content from minute to minute. The sampling and propane adding systems cannot react quickly enough to counter these fluctuations, and so the reject valve operate again, and the cycle is repeated.
Inevitably this results in the rejected gas being sent directly to flare, where it is then wasted, producing CO2 emissions for no economic benefit.
By adding a separate store for this upgraded gas, the site operator can choose to utilise this gas, by slowly feeding it back into the biogas supply line to the upgrade plant, in a controlled and measured way, such that the analysing system can then manage the variations in energy content, as they are more predictable and managed.
Or the gas can be blended into the biogas that supplies the on-site CHP unit, again this can be done in a controlled manner, and fully utilises the gas that would otherwise have been flared. By utilising this gas, a bio-methane plant operator can improve his efficiency, eliminate waste, reduce unnecessary CO2 emissions, and improve the sustainability of the plant.
The triple membrane roof enables plants with existing double membrane roofs to be able to benefit, without taking up further on-site space, or having to reengage with the planning authorities for planning permission, as the triple membrane roof is visually identical to the standard double membrane type roof.” Owen Yeatman
Farmergy are the UK sales and distribution agents for Wiefferink.