Flowmeter on Bio Gas Measurement

Thermal mass type (hot wire anenometer). Endress & Hauser supply them, among many others.

Has the advantage of reading mass (or normal) flow directly, and does not need correction for actual pressure and temperature. Gives good turndown.

You don't mean specific gravity, this is a ratio, for gases taken as air = 1. Density is in kg/Nm3, but your 1.261 looks high, biogas would be ~ 55% CH4, 45% CO2 to give that density. More typiclal biogas is 70% CH4, 30% CO2, density ~ 1.1 kg/Nm3, though it does vary.

Density = mass/volume. I may be dense, but in kg/Nm3 what is the N doing?

Nm3 is a normal cubic meter. Like a standard cubic foot, but more... metric.

Thermal mass types will work if the gas is dry, however moisture can be a problem for that principal. I would suggest you look at Ultrasonic type, see http://krohne.com/en/products/flow-measurement/ultrasonic-flowmeters/optisonic-7300-biogas/

Yes you are absolutely correct, the end user is already using E&H thermal mass flowmeter but due to moisture & dust, the end user is asking for alternative solution.

I seem to recall that we used a Rebel Pont wet gas meter, but that was only in lab testing when we were doing anaerobic digestion tests to determine flow rates/total volumes of digesting hog manure. Not sure if they have large enough models for your flow rate. And the lab units are really not designed for anything else as they are not very robust.