Natural Gas
Gas is also considered to be the ‘cleanest’ fuel because its exhaust contains lower levels of potentially harmful gases.
Natural gas is extracted from underground sources and distributed by pipework throughout most parts of the UK. It consists of a high proportion of hydrocarbon gases – mainly methane – with small quantities of non-fuel gases such as nitrogen and carbon dioxide. Natural gas contains virtually no sulphur or contaminants. Although distribution pipework is required on-site, together with a housing for metering and pressure regulating equipment, there are no handling or storage costs.
Natural gas is widely available in most parts of the UK. It is purchased from one of a number of suppliers and is transported through the national gas distribution system. Since packaged CHP units do not have the capability to operate on other fuels, purchasers/installers must make an economic choice between an interruptible or a firm gas supply. If the packaged CHP purchaser/installer opts for interruptible supplies, a cheaper tariff is granted in exchange for the right of the gas shipper or National Grid plc to interrupt supplies on a certain number of days in each year. During each interruption, all electricity requirements must be imported, and heat must be available from other sources such as oil-fired boilers or gas boilers on a firm supply. It is possible to have both firm and interruptible supplies to a site.
If a firm supply is adopted, then CHP availability will not be compromised but gas costs will be higher. The cost of the gas is determined on an individual contract basis and will reflect factors such as location and the required quantity and capacity of supply. In all cases, the actual prices charged are a result of contract negotiation and can be fixed for periods of up to three years or more, thereby providing a relatively stable fuel supply cost.
The quality and consistency of natural gas are high, and the fuel can be used directly in a CHP package without any treatment or filtration. The composition of the natural gas supply at any location will vary slightly from time to time, according to a number of supply and operational factors, but the variations are not significant and do not usually affect the operation of a gas-fired CHP package.
For the larger packaged CHP units, a compressor may be necessary to raise the incoming gas supply to a suitable pressure for injection into the engine.
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