The environmental strong points of biomethane are well-known, but what’s involved in getting it to where it needs to be: In buses’ fuel tanks? Our Industry Expert discloses the details of this process
Here we will examine the delivery of biomethane to customers and the filling stations that supply the biogas to buses’ fuel tanks.
Biomethane is delivered directly to bus depots via the national gas grid. It arrives at a low pressure (typically 350millibar – about 4psi) from the pipe in the street. It has to be delivered to bus fuel tanks at 200bar (about 3,000psi) and the filling station compresses it to the required pressure.
The main components of a gas bus filling station are the compressor(s); high pressure gas storage bottles; and the fuel dispensers.
The compressor(s) raise the gas pressure to typically 250bar (3,600psi) and fill the gas storage bottles. When 250bar is reached, the compressors shut down. Fuel is delivered into the bus tanks via a gas dispenser that is identical in looks and operation to a traditional liquid fuel pump, apart from its high pressure connector.
Fuel delivery is metered in kg to a maximum pressure of 200bar, and it is monitored by a standard fuel management system.
When a gas bus is connected to a fuel dispenser and the compressor(s) are not running, gas is immediately available from the storage bottles and filling begins at once. The pressure in the storage bottles will then fall, triggering the compressor(s) to start to maintain the transfer of fuel.
A safety interlock system prevents filling with the engine running and the same system prevents drive-off with the hose still connected. An urban bus typically uses 100-130kg of gas per day, and it takes less than five minutes to fill.
The filling operation is leak- and spill-free via easy-to-use connectors. After connection, the filling operation is entirely automatic.
In the unlikely event of a leak, methane is lighter than air and so will disperse quickly. It has a low flammability range, 5-15% concentration in air and the highest self-ignition temperature of 580oc. That compares with diesel at 336oc and petrol at 400oc.
The fuel stations are designed and constructed to well-established international rules and safety standards, and they have reliability and availability comparable with diesel dispensers. Inbuilt telemetry allows remote monitoring of performance, and it will automatically call out a fast-response maintenance team if required.
Our Industry Expert speak 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.
