Getting the best water for Indian Queens1 August 2007
Power stations without consent to discharge can exploit the possibilities of rental de-ionisation sets.
International Power plc is a still growing, independent power generation company with 18 GW net (30 GW gross) ownership of capacity worldwide. In 2006, it acquired the 140 MW Indian Queens oil fired open cycle gas turbine station in St Austell, Cornwall, UK, from AES. The power plant (Figure 1) was constructed by John Brown Engineering in 1996 and, under the terms of its integrated pollution control application, it operates as a peak lopping station, which means that it runs only infrequently, and predominantly in the winter months when demand is high.
In order to suppress NOx emissions, water is sprayed directly into the burner to reduce the flame temperature. As this water evaporates it leaves behind its dissolved salts which deposit in the burner and need to be removed regularly. In order to avoid this maintenance headache and any problems it might bring, Indian Queens uses de-ionised water, characterised by 0.1µS/cm conductivity and a silica level less than 20ppb, which is very low in dissolved salts. During the summer, the station uses about 40 m3 per day of de-ionised water, but in winter the demand rises to about 170 m3/day. To produce this high quality water, an ion exchange plant with its associated regenerant chemicals and effluent discharge is required. Back in 1996, Indian Queens did not have a Consent to Discharge and, since the station would operate with a very low utilisation, its then owners, Indian Queens Power, opted to use a mobile rental de-ionisation system with off-site regeneration and zero on-site effluent discharge.
The ten year rental contract allowed Indian Queens Power to obtain de-ionised water from any source available during the summer months. Water costs were reviewed every year and, when the economics moved in favour of on-site regeneration, the company purchased a 9 m3/h Rapide de-ioniser made by ELGA Process Water. The Rapide uses state-of-the-art ion exchange technology, with a highly efficient regeneration process that produces such a small quantity of effluent that it can be economically tankered off site to a merchant waste disposal facility. During the winter months, with the higher demand for water, this becomes uneconomical and the station reverts to the mobile rental solution.
The Elga plant had performed well, and its after sales service had also been successful, ensuring that the de-ionisation process was problem free, so when the rental contract was due for renewal, Elga was given a chance to bid. The company operates a fleet of AQUAMOVE mobile trailer mounted treatment plants that includes MODI de-ionisation units. These provide two streams of cation, anion and mixed bed ion exchange vessels housed in custom built, 45 ft trailers together with automated controls, remote telemetry and all the necessary safety equipment. Each MODI trailer is capable of producing in excess of 150 m3/h of high purity water – more than enough for Indian Queens’ needs. Once exhausted the complete trailer is returned to Elga’s Stoke-on-Trent facility for regeneration, which means zero discharge on site and no problems of handling or disposal of regenerant chemicals. But the MODI trailers are intended mainly for emergencies or planned maintenance shutdowns and are an expensive option for long term hire.
Elga’s Aquamove team carried out a detailed review of the Indian Queens site and concluded that, rather than a trailer-mounted mobile unit, a lower cost option would be to install a custom designed 30 m3/h containerised, de-ionisation plant. They proposed two 20 ft ISO containers each fitted out with a cation exchanger, two anion exchangers and a mixed bed polisher. One container is also equipped with the controls, quality monitors and remote monitoring equipment needed for both streams (Figures 2, 3). Rather than shipping the complete unit back to Elga for regeneration when the resins are exhausted, the resin vessels are simply exchanged for regenerated ones by the Aquamove service team. Each containerised unit will treat about 1200 m3 of mains water between regenerations; enough for a fortnight’s usage at average station generating capacity.
This proposal was considered to offer a worthwhile saving compared to the mobile trailers case and International Power signed up to a 5-year contract which comes into effect in 2008. In the meantime (because the order came too late for Elga to build, deliver and commission the new containerised plant on a schedule that would dovetail with the end of the existing contract) Elga is to provide a MODI trailer at the same contract price, ensuring that the station’s water supply is secured without affecting its economic forecasts, and at a familiar level of service and maintenance.