Steam and electricity generation

Steam and electricity are generated together. This is energy efficient because it makes use of the hot by-products put out by the generating process. The electricity is sold to the National Grid. However, the steam is all used on site.

Photo of SP4
The SP4 steam and electricity generator.

Fawley oil refinery uses steam for a number of processes:

  • in reactions, such as steam cracking and stripping
  • for cleaning vessels
  • for heating chemicals
  • for lowering the viscosity of products in pipes using steam tracers.

The refinery can use over a thousand tonnes of steam per hour. This steam is produced in 3 large steam generators, including the SP4 and the COGEN.

The steam is generated at 900 kPa (9 times atmospheric pressure) and 2000 kPa (20 times atmospheric pressure). At these pressures the steam can be heated to 180°C and 270°C respectively.

The steam is piped into a steam main that runs around the site. This is rather like the electricity main that runs round your house. But, instead of plugs, the steam main has connectors and valves. These allow engineers to tap into the steam supply wherever they need it.

Both the SP4 and the COGEN are types of Combined Heat and Power (CHP) plants. They generate electricity and steam from the same fuel. Any power station generating electricity will also heat up something in its surroundings. In a CHP plant, the hot by-product is put to good use rather than wasting it.

Let's see why we cannot avoid making something hot.

The picture represents a generalised power plant:

  • a hot gas (often steam) is used to turn a turbine (a bit like a propellor in reverse);
  • the turbine drives a generator;
  • the hot gas is cooled in a heat exchanger once it has passed through the turbine;
  • the heat exchanger uses water from a river (or the air) to cool the hot gas;
  • having cooled the hot gas, this water is slightly hotter than it was before.

We have to cool the hot gas to reduce its pressure. If we didn't, then the gaswould stop flowing. It flows from the hot side to the cold side because of the pressure difference.

Therefore, we cannot avoid heating something up on the colder side.

Graphic of power station
A generalised power station. The hot gas has to be cooled to produce a pressure difference across the turbine and to keep the gas flowing.

In a conventional power station, the slightly hot water is usually wasted. However, in some CHP plants, it used to heat local factories or houses. In the Fawley generators (the SP4 and the COGEN), it is, in effect used to produce steam. Though they each do this in a different way.

The generators are connected directly to the National Grid. At the same time, the site is supplied with electricity from the National Grid. This arrangement means that the generators can work at a constant rate and don't need to match their output to the demands of the site. If the requirements increase, then the site simply takes more current from the National Grid. Over time, the average output and input are pretty evenly matched.