From biogas to biomethane.

Overview

As fossil fuel demand and prices climb, upgrading biogas to fuel quality, biomethane, turns a waste stream into one of the cleanest fuels in existence. In South Africa it is being used as vehicle fuel, injected into the natural gas grid, burned for heat and steam and used as a chemical feedstock.

Membrane separation

The membrane consists of a cylinder packed with hollow porous fibres. Under pressure, carbon dioxide permeates the fibre walls faster than methane, so the two gases leave in separate streams. Multi-stage arrangements recover more than 99% of the methane.

Water absorption

Water scrubbing upgrades biogas by washing out H₂S and CO₂ in a circulating water column. Methane, which is poorly soluble, passes through while the hydrogen sulphide and carbon dioxide dissolve and are stripped from the water in a second column so it can be reused.

The product

Upgraded biogas is cleaned to a level of about 97% methane. Compressed, it stores and transports like natural gas, and one cubic metre of biomethane holds roughly the same energy as one litre of diesel.