The Hongkong Electric Co., Ltd. (HK Electric) is one of the two power utilities in Hong Kong and has been generating and delivering electricity to Hong Kong Island and Lamma Island of the Hong Kong Special Administrative Region for over 100 years. Number of employees in HK Electric as of end 2006 is around 1,900.
Being an environmentally responsible corporate citizen, HK Electric has long been committed to protecting and preserving Hong Kong’s environment and in support of sustainable development, while providing the city, which is an international financial and commercial centre, with a reliable world-class supply of electricity.
Case Background/Context
At HK Electric, electricity is generated at its Lamma Power Station which has an installed capacity of 3,756 MW as of end 2006. The Station has 8 coal-fired units, 5 oil-fired open-cycle gas turbines, one oil-fired combined-cycle-gas-turbine block, one gas-fired combined-cycle gas turbine and one wind turbine. Three coal-fired units have been installed with flue gas desulphurisation (FGD) plant which employs limestone slurry to scrub the flue gas and remove more than 90% of its sulphur dioxide (SO2) content. The first FGD plant began its operation in 1993 and at that moment was the first unit of its kind in this region.
The three FGD plants remove SO2 at the expense of consuming a huge volume of freshwater. It would be beneficial to both the community and HK Electric if there are means of reducing the FGD’s demand for freshwater. A smart idea later evolved when employees witnessed the collection of thousands of cubic meters of rainwater on occasions of heavy rainfall which was then drained to the sea. This inspired us tao collect rainwater for use by the FGD plants. On the other hand, various parts of the power station generate wastewater of different grades and some of them can become feed water for the FGD plants before the effluent that subsequently produced is treated and discharged. These ideas were eventually integrated into a Rainwater & Wastewater Collection & Reuse Project.
The Main Objective of the Case
- To control the volume of freshwater consumed by Lamma Power Station by re-using rainwater and wastewater as far as is practicable. A target of collecting 100,000 m3 rainwater and wastewater per year was set up later on.
- To minimise the discharge of effluent, after treatment, to the environment.
Problems and Difficulties
- Extensive Infrastructure
The collection and transfer of a significant volume of rainwater would require an extensive network of infrastructure. Thanks to the storm-drain system that had been built into the buildings and tank farms of the power station and with a small diversion of the storm drain pipe to the wastewater collection system, the rainwater collected from the roof of the buildings and tank farms could be easily recovered for use. A few sump pits used for other purposes previously were converted to form part of this collection system and this also helped cut down additional infrastructure required.
- Segregation of Different Grades of Wastewater
Not all streams of wastewater can be reused. Those that contain salt content approaching the level of seawater virtually have no reuse value whereas those of freshwater base can be reused. In some cases, the wastewater could be reused more than once by stages before it is treated and discharged.
- Cascade Mode of Reuse
Different equipment within the power station can use water of different quality. Hence, effluent from equipment that produces better grade of wastewater is sent to equipment that can tolerate lower quality feed-water. With careful analysis, planning and modification, a cascade mode of wastewater reuse was implemented. It helped reduce the volume of final effluent and treatment chemicals as well.
- Drastic Cut in Effluent Discharge
This was a big challenge to us as the coal-fired units were originally designed to employ an once-through sea water sluicing system to handle the furnace bottom ash (FBA) generated from coal firing. It was an inefficient process as a huge volume of seawater had to be used to sluice the FBA to an Ash settlement Basin (ASB) where the solid particles were allowed to settle and the supernatant liquid discharged. It constituted a very large portion of the effluent produced by the power station.
With an innovative design, beginning a decade ago, this seawater sluicing system was converted into a closed-loop recirculation system that employs freshwater as the sluicing medium. The new system allowed the slurry to be filtered and the filtered water could then be re-circulated back to the boiler for sluicing purpose. Working on this principle, the system produces a very small quantity of effluent.
Outcomes
- More than 100,000 m3 of rainwater and wastewater is collected and reused every year. The same volume of freshwater is thus saved.
- The volume of effluent that has to be discharged is drastically reduced by more than 52,000 m3 per day with the conversion of the once-through FBA sluicing system into the closed-looped FBA sluicing system.
Lessons Learned
We learnt from this Rainwater & Wastewater Collection & Reuse Project that, with some modifications, new application could be introduced to facilities not originally designed for such purpose. The storm-drain system thus became an important component of the rainwater collection facility. This rainwater collection feature is now a standard design of new buildings in our Station.
Documentation
A paper on the above scheme was granted The Hong Kong Institution of Engineers Environmental Paper Award 2007. The full paper is attached for reference.