Biomethane is becoming an established part of the UK’s bus scene, but how is it produced and how is it distributed into the national gas grid? Our industry energy expert considers how it all works out
In the previous column, the arrival of biomethane-fuelled bus fleets in the UK was discussed.
Gas bus technology is well-established, but the availability of renewable biomethane from the national gas grid is a game-changer for low-emission bus fleets. In this column, its manufacture and distribution is considered.
Biomethane is manufactured from organic waste. Food waste, agricultural waste, the residue from brewing and distilling: Almost any organic material can be used.
The waste material is mixed into a slurry with water, and processed in an anaerobic digester (AD). The AD is an air-tight tank not unlike an old-fashioned gasometer, and it operates without air in contact with the slurry.
The slurry is heated to 38oc to start the digestion process. Once commenced, the process generates enough hear to self-sustain. The products of the digestion process are methane and carbon dioxide at a ratio of approximately 60:40.
Some of this raw gas is used to run a combined heat and power unit that provides all of the heat and power for operating the AD gas production and purification processes. The carbon dioxide is removed by chemical absorption and is available as another product stream and the pure biomethane is introduced into the national gas grid for distribution to customers.
Biomethane is a completely renewable fuel with a minimal carbon footprint. Natural gas is a fossil fuel and is on average 90% methane in the UK. Gas used by bus operators will mostly be natural gas, but the equivalent amount of biomethane is injected into the grid on a daily basis, and biomethane suppliers provide green certification to their customers.
Biomethane-fuelled buses reduce their carbon footprint to almost zero, and they have significantly lower emission of NOx, particulates and hydrocarbons when compared even to Euro 6 diesel engines.
Exhaust treatment for biomethane buses is a simple three-way oxycat, with no DOC, no DFP, no SCR converter, no catalytic convertor prone to blocking and thus no Ad-Blue or the associated control equipment. Gas bus operators report 20-30% fuel savings.
The next column will look at biomethane filling stations, their design, operation, reliability and safety.
Our Industry Expert speaks with authority and can back up all of his facts, but what do you think? Email editorial@divcom.co.uk if you agree or disagree with him.
