Combined Heat and Power Installations – Talk by R.G. Bailey, member, at the Durrington Community Centre on 5th October, 1994.
CHP is about fuel efficiency and the environment. For the purposes of this talk combined heat and power is the output of a gas turbine.
Before talking about gas turbines the differences compared to steam turbines should be considered. Both steam and gas turbines convert heat energy to mechanical [hence electrical] energy and at similar cycle efficiency [broadly 30% to 40%]. However the modern condensing steam turbine then rejects about 65% of its heat to the environment by way of warmed cooling water [a few degrees]. The gas turbine rejects the 65% of its heat in the form of hot gas to the environment at a temperature of 500oC. This heat can be captured in a waste heat recovery boiler to produce steam which can then supply a condensing steam turbine to produce more power and thus give an overall thermal efficiency of 55%. This is termed Combined Cycle Power or CCP and is what the present “dash for gas” is all about in the UK today.
When some or all of the steam from a CCP steam turbine is taken further to supply heat at a lower temperature for industrial use or district heating then we have what is termed a Combined Heat and Power system or CHP which can give thermal efficiency in the 80-90% region.
The story starts with the discovery of the first oil in the Middle East in Bahrain in 1932. There never was a lot of oil but a huge field of natural gas was discovered. This gas supplied all the country’s fuel for power but this was only scratching the surface of its energy potential.
In the late 1960’s someone had the idea of using this gas in a large gas turbine power station to supply electric power to an aluminium production plant. Aluminium is produced by passing a huge direct current through the molten aluminium ore. A typical “pot line” has 250 cells, or pots, connected in series with a current of 100,000 amps flowing and a volt drop on each cell of 4.00 volts. Hence one potline consumes 100 MW of power. The power needs to be maintained continuously since if it is off for more than say one hour the cells start to solidify, an irreversible condition which entails removal and reconstruction and can cost £30,000.00 per cell.
It was vital for the power station to be 100% reliable and in 1968/9 World Market the gas turbine with the longest accumulated operational hours was the G USA Frame 5 machine [rated at 15-18 MW].
19 of these turbines were ordered from John Brown Engineering on Clydebank who were an Associated Manufacturer for G USA. Two potlines were built and the plant started up in 1970. There were tremendous problems in keeping the whole plant operation going and in the power station the punishing duty on the turbines revealed numerous shortcomings. These were dealt with as they arose and information fed back to the designers and builders resulted in a continuous improvement of the machine which laid the foundation for the range of bigger and more reliable turbines which developed during the 1970’s.
The ALBA project was a successful struggle and today it has quadrupled in size and boasts the largest potline in the world. It now has CCP with air cooled condensers but no CHP.
The DUBAL project was designed as a CHP operation and followed on the concept from ALBA but with a more complex sequence of fuel and heat utilisation.
The story again starts with the discovery of oil. Dubai’s oil was found offshore in the lower Arabian Gulf in the 1970’s. It was excellent quality light crude but had a lot of associated gas which had to be flared to waste.
To use this gas it was piped ashore to a gas treatment plant which condensed out the LPG and then passed the remaining natural gas, methane, to the DUBAL gas turbines to produce electrical power for aluminium production.
By this time G USA had developed its range of gas turbines from the Frame 5 up to the Frame 9 which was a 3000 rpm direct drive 85 MW machine. The DUBAL power station had 8 Frame 5 turbines and 5 Frame 9 turbines, again manufactured by John Brown on Clydebank. There were 3 potlines with an electrical load of 120 MW each.
In Dubai the need for water was paramount to all planning and so each gas turbine had a waste heat recovery boiler which supplied dry saturated steam to a large multi-stage flash sea water desalination plant. This is the only Combined Heat and Power installation of this type in the world.
The project was successfully completed by 1982 but with many problems left to overcome. The gas turbines proved quite reliable but the boilers suffered disastrous failures due to the severe cyclical stresses placed on them by the operational requirements of the aluminium process. The Sub-Contractor Foster-Wheeler UK had to carry out a rework programme on these boilers which imposed severe constraints on the plants overall operation for some years.
It had been apparent from the initial tests on the boilers that they were in fact able to deliver quite a lot more steam than the rated value. Since the steam was delivered to the desalination plant at 20 bar and was then dropped down to 2 bar before entering the brine heaters the idea of installing back pressure turbines to gain more power and utilise the extra steam seemed quite feasible. This project went ahead in 1987 with the insertion of two 35 MW steam turbines and the total capital costs had been recovered in fuel saved in less than 2 years.
The DUBAL plant continues to operate very successfully and has been expanded to four potlines. The metal produced is of the highest quality and sells at top premiums on the world market.
This is a unique example of a Combined Heat and Power system with an overall thermal efficiency around 80%. However this is not the end of the story. The final heat rejected from the desalination plant takes the form of warmed concentrated sea water and plans are in hand to pipe this liquid to solar evaporation ponds where it gives a substantial increment to the input of feedstock for a chemical industry based on the minerals available in the sea.