Biogas is produced as a byproduct of wastewater treatment plants. The amount and quality of the gas depend on the quantity and composition of the effluent being treated by the plant. The gas can be utilised to produce electricity, using positive displacement engines (same type as a car engine). Burning the gas in an engine also produces heat which can be utilised elsewhere in the plant – for example to heat up the digester to optimum temperature. Where biogas is not sufficient natural gas is used to augment biogas as fuel for the engines. Where excess biogas is produced it is flared. Due to the complexity of the process and the disturbances caused by variable biogas production the controls for the engines can become unstable, causing unnecessary engine trips and biogas flaring with a simultaneous high natural gas usage.
The total process consists of:
> Seven digesters producing biogas. Each digester has a biogas flowmeter.
> Gas is fed into a low pressure gas holder, intended to be a buffer in the system
> Four flares that will flare of gas once the low pressure gas holder reaches its upper limit
> Four gas blowers to raise the pressure of the biogas from 2.5kPa to 45kPa required for optimum engine performance
> Pipe lines transferring the gas across the plant to where it is being consumed by the engines
> Two engines that can blend natural gas and biogas – these are primary engines for controls
> Two engines which can either run on 100% natural gas or 100% biogas
> All engines are equipped with a flowmeter for biogas and natural gas consumption flow measurement.