Powering CHP by gas expansion18 May 2001
The first Atlas Copco gas turboexpander in Belgium’s public distribution network has been commissioned by Electrabel, Belgium’s main electricity generator and the largest distributor of gas and electricity in the country; and it will use the energy of expansion to generate electricity.
In combination with two gas engines, which preheat the incoming gas, it forms a 5.3 MW combined heat and power station based on a single stage turboexpander connected to a 2.6 MWe generator via a fixed speed gearbox. The two Cummins V16 gas engines are each connected to 1.36 MW alternators.
Gas arrives at the plant at a pressure of 9 bar/8°C and the first stage of the process involves heating it to 90°C. Expanding the gas without preheating would result in an outlet temperature too low for the turbine, in this case –61°C, which would embrittle the turbine blades and incidentally create free ice in the form of “snow”. It would also be too low for the gas distribution network; gas as supplied is not dry, and if the expansion process reduces its temperature below dew point, hydrates will be formed that are difficult to remove.
The gas then passes through the expander, exiting at a pressure of 1.7 bar at approximately 7°C. Final pressure reduction to 24 mbar is done remotely at local sites. Electrical production efficiency is around 70 per cent, and total efficiency around 80 per cent, this figure being inflated slightly by the decompression energy gained from the gas itself.
The turboexpander runs continuously at its nominal load during the winter months, from November to February, at a nominal flow rate of 75 000 m3/h. It has been sized on the basis of the maximum flow that the network can take, and the gas engines sized accordingly.
Working continuously with a nominal flow makes the system simpler and more reliable and less costly to install. But the economics of the situation, which Electrabel first studied in 1996, don’t allow summer running. This unit is most suitable for peak shaving, and summer time prices are too low to allow economic running and do not attract the peak shaving fee. In fact the economics of the installation are quite critical. It is necessary that the installation be relatively low cost, and attract low maintenance costs, so a simple installation in which the engines do not have to be modulated is an important factor. Similarly it proved more economic to use the cogenerated heat to preheat the gas rather than use it to provide hot water.
The two gas engines produce 3.7 MW of heat, 3 MW of which is required by the turbine. The surplus heat provides base load heating for Electrabel’s offices, supplemented by three conventional boilers each of 1.5 MW which are connected into the heating circuit in such a way that they can also provide heat for the turbine should the gas engines fail. Additional security of supply is provided by a roof mounted cooling tower with the capacity to dump excess heat should the turbine fail.
Local legislation sets a low maximum noise level, with the result that this plant is well insulated and very quiet – less than 45 dBA at a 1m distance from the plant room outside wall.
Electrabel has financed the plant in a joint venture with Sibelgas,the local distributor. When they first started looking at the possibility of using an expander in this way they chose this site because it is at the centre of the network, has large volumes of available gas at high pressure and benefits a large number of customers. Now Electrabel plans to build the same kind of plant in Antwerp.