Your Frequently Asked Methane Stripping Plant Questions Answered
What is the Quantity of Methane Gas Emitted from a typical Landfill Methane Stripping Plant?
At standard temperature and pressure, about 25mg/l of methane gas can dissolve in water. In a landfill environment, where the atmosphere may typically comprise 60% methane and 40% carbon dioxide by volume, then this figure would reduce to a maximum dissolved methane concentration of about 15mg/l.
In practice and in our experience, concentrations of methane dissolved in leachate very rarely exceed this value in actual samples, although they can frequently approach it.
If more than 99% of this gas is stripped from the leachate during passage through the plant, then this represents up to about 15 grammes of methane per cubic metre of leachate treated, or up to 18kg of methane gas per day.
Since the gramme molecular weight of methane is 16, then 16 grammes of methane will occupy 22.4 litres at STP. So, the 18kg of methane emitted each day will occupy about 25 cubic metres. In annual terms, if operating continuously at maximum rate, with leachate containing maximum concentrations of dissolved methane (obviously an absolute worst case), then the plant will emit about 6.5tonnes, or 9000m3 of methane gas.
To put this into context, a landfill might emit about 10m3 of methane gas from every tonne of landfilled waste it contains, in a year. So, 9000m3 of methane represents the gas from 900 tonnes of landfilled waste - a very small proportion of the gas being generated by a typical modern landfill in every year.
Whether this is nevertheless considered a significant emission of methane in a year in terms of carbon emissions from the stripped landfill methane gas, must be determined by the landfill site owner/operator in a discussion with the local regulatory authority.
What are the Concentrations of Methane in the Offgas from a Typical Methane Stripping Plant and will they be explosive?
On average, the concentration of methane in off-gas in our plants would be 25m3 in 12,000m3 of air, or about 2,080 ppm methane by volume. The lower explosive limit (LEL) for methane in air is about 5 percent by volume, or 50,000 ppm, so the concentration of methane in off-gas is about 4 % of the LEL.
This means that there is not enough methane in the offgas from our plants to pose a significant risk of explosion while in normal operation.
You Have Just Described the Average Concentrations in the Mixed Off-gas. But Won’t the first tank be a Worst Case, and Won’t the Off-gas from this Tank Pose an Explosion Risk?
No. (But don’t assume this will always be the the case for Methane Stripping Plants by different designers.)
Two things can affect the above calculation. First, in the initial stripping reactor that receives incoming leachate directly, it is likely that between 60 and 70 percent of all methane will be removed, because of the "half-life" nature of the stripping process.
Therefore, in off-gas from this tank, in the worst case calculation being carried out above, up to 15m3 of methane gas will be diluted into the flow of air from the air delivery system, each day. This generally represents a concentration of methane gas in air of about 5,000 ppm by volume, which is still a factor of ten below the LEL.
A safety factor of ten times is widely used for gas safety in the mining industry, and indeed is exactly the reason that a discharge limit of 0.14 mg/l of dissolved methane in water is almost universally adopted for discharge of leachates and other liquids into the public sewer (since 1.4mg/l of dissolved methane is the lowest value at which an explosive headspace atmosphere is likely to be possible).
What are the Effects of Unusual Operational Conditions on the Plant. Is it Possible for Explosive Methane Concentrations to Ever Occur?
Yes, it is. Do always be diligent in the presence of landfill gas/methane, and obtain advice from experts.
One possibility, in terms of adverse effects on emissions from the plant, is the situation where the plant has been allowed to be filled entirely with untreated leachate before being turned on.
In such a situation, the initial flush of gas from each reactor, as the first air passes through the leachate, will undoubtedly contain potentially methane concentrations exceeding the LEL if unventilated. As this mixes with air explosive concentrations of methane gas (between the lower and upper explosive limits) will then be present. This is in exactly the same manner as any pumping chamber, or confined leachate storage tank etc, will also be liable to contain an explosive and hazardous atmosphere.
Indeed, each methane stripping reactor will contain such a a potentially explosive atmosphere within it, before the air supply is even turned on.
This potential situation must obviously be borne in mind when operating any methane stripping plant, and will need to be covered in the plant operations manual.
This Makes a Methane Stripping Plant Appear to be Quite Dangerous then?
We apologise if we have given that impression. We would like to emphasise that Methane Stripping Plants are not dangerous when the legal requirements which apply throughout the European Union which require all owners/employers to comply with the ATEX Directives, are met.
Application of the ATEX Directives will ensure that procedures are in place which identify the risks, ensure that the risks are marked, and ensure that all personnel are adequately trained in the risks. (The choice of the equipment chosen is also covered by ATEX to ensure that the equipment in contact with the methane gas will not cause an explosion or fire.)
After all is considered, a methane stripping plant installation which is properly designed, operated and maintained contains a lower explosion risk material than any petrol station. Petrol stations are safe to use because we all know and appreciate the risks, and we certainly don’t smoke while filling our tanks!
How Do I Comply with the ATEX Directive?
If you have a site which is prone to an explosion risk there should have been explosion risk assessment carried out, any “ATEX” explosion zones identified, and the zones delineated etc. Contact Us via the email address on the left panel for expert advice through this web site, or visit our ATEX and DSEAR compliance web site here for further information on our service.