Embed Size (px)
Citation preview
WASTEWATER MINIMIZATION, RECOVERY & RAINWATER
RECOVERY IN LAMMA POWER STATION Mr. Tsang Kwong Tim, Generation Chemist, The Hongkong Electric Co. Ltd. Tel. No.: 29826570, Email Address: [email protected] Postal Address: Lamma Power Station, GPO Box 915, Hong Kong. Mr. Terence Cheung Kwok Hung, Assistant Chemist, The Hongkong Electric Co. Ltd. Tel. No.: 29826593, Email Address: [email protected] Postal Address: Lamma Power Station, GPO Box 915, Hong Kong. Mr. Cheung Kam Hee, Assistant Chemist, The Hongkong Electric Co. Ltd. Tel. No.: 29826882, Email Address: [email protected] Postal Address: Lamma Power Station, GPO Box 915, Hong Kong. Abstract
Lamma Power Station is built on the Lamma Island of Hong Kong and supplies electricity to Hong Kong Island and Lamma Island. Lamma Power Station requires a lot of freshwater and at the same time produces considerable quantity of wastewater in the process of electricity generation. To reduce the consumption of freshwater will bring benefits both to the environment and the Station. With the installation of Submerged Scraper Conveyor (SSC) at the coal-fired units, the volume of effluent that has to be discharged is drastically reduced by more than 52,000 m3 per day. The conservation effort is further strengthened with a wastewater and rainwater recovery system that helps cut down the consumption of freshwater by more than 100,000 m3 every year. Key Words: Submerged Scraper Conveyor, Wastewater minimization, wastewater recovery,
rainwater collection.
WASTEWATER MINIMIZATION, RECOVERY & RAINWATER RECOVERY IN LAMMA POWER STATION
INTRODUCTION Lamma Power Station is built on the Lamma Island of Hong Kong (Figure 1) and supplies electricity to Hong Kong Island and Lamma Island. At the end 2006, Lamma Power Station has the following electricity generation units:
• 3 x 250 MW conventional coal-fired units (Units 1, 2 & 3); • 2 x 350 MW conventional coal-fired units (Units 4 & 5); • 3 x 350 MW coal-fired units with low nitrogen oxides burners and Flue Gas
Desulphurizaiton plant (Units 6, 7 & 8); • 1 x 55 MW aero-derivative gas turbine (GT 1); • 4 x 125 MW industrial gas turbines (GTs 2, 3, 4 & 6); • 1 x 365 MW combined - cycle unit converted from two industrial gas turbines (GTs 5 & 7); • 1 x 335 MW gas - fired combined-cycle unit (Unit 9).
Lamma Power Station uses a lot of water and also produces considerable quantity of wastewater in the process of electricity generation. To demonstrate commitment to environmental protection and conservation of resources, Lamma Power Station takes a proactive approach of continual improvement in minimizing wastewater discharge and use of fresh water, and maximizing the reuse and recovery of wastewater. The followings summarize the scheme adopted in these aspects.
Figure 1: Lamma Power Station
1
MINIMIZATION OF WASTEWATER DISCHARGE BY INSTALLATION OF SUBMERGED SCRAPER CONVEYORS (SSC) TO HANDLE FURNACE BOTTOM ASH (FBA) Coal-fired Units 1- 6 were originally installed with traditional once-through seawater Furnace Bottom Ash (FBA) handling system (Figure 22). The FBA from the furnace falls by gravity into a water impounded hopper directly underneath the furnace. The impounded hopper is filled with seawater all the time. FBA falling from the furnace is quenched by seawater continuously supplied by a cooling water pump. The latter also provides water to the furnace sealing trough for sealing off the furnace and also to the impounded water hopper for cooling purpose. Overflow from the impounded hopper is discharged to a Slurry Sump Pit where it would be pumped to an Ash Settlement Basin (ASB) regularly. FBA collected in the impounded hopper passes through an ash crusher and is then transported to the ASB by sluicing water provided by a High Pressure Ash Water Pumps. In the ASB, the sluicing water gradually clarifies as the FBA particles settle and the water over-flown from the ASB is discharged. In view of the huge volume of effluent generated by the impounded hopper system, alternative system using less water was considered for the two new coal-fired units (Units 7 & 8) at that time. Eventually, a much more environmental friendly FBA handling system employing Submerged Scraper Conveyor (SSC) was selected. Unlike the impounded hopper that works on a once-through seawater system, the SSC employs a closed-loop freshwater design. The SSC, consisting of a conveyor submerged in a trough of freshwater, is mounted underneath the furnace (Figure 2). Clinker fallen off the furnace drops through a transition chute into the trough and is immediately quenched by water to form FBA. The SSC, driven by moving chains, conveys the FBA to an ash crusher and then to storage bin. From the storage bin, the FBA is sluiced by a freshwater jet pump to the Filtering Basin where the ash is taken out by the filtering material (gravel and sand). The filtered water is drained into a filtered water basin and then recirculated back to the jet pump for another cycle of sluicing. A bleed off pumping system is provided to maintain the quality of the recirculating water. The bleed off water is recovered to the Reusable Effluent Storage Pit (RESP) for other uses including coal yard dust suppression water spraying, limestone slurry preparation and make-up for the Flue Gas Desulphurization (FGD) Plant (Figure 23).
Figure 2: Submerged Scraper Conveyor System (SSC)
2
After successful implementation of SSCs in Units 7 & 8, the same system was retrofitted to Units 1 - 6 in the period 1999 - 2002. The benefits of SSC systems are: (i) The volume of effluent discharged to sea is drastically reduced by more than 52,000 m3 per
day; (ii) As SSC adopts freshwater for ash sluicing, the FBA produced can be utilized in the cement
industries.
WASTEWATER & RAINWATER RECOVERY SYSTEM In 2001, two water reservoirs, Effluent Separation Pit (ESP, 5,600 m3) and Reusable Effluent Separation Pit (RESP, 3,000 m3), were constructed at the east side of the Station. Since then, a systemic wastewater and rainwater recovery system was developed to recover and reuse wastewater while at the same time reduce the consumption of freshwater. These practices contribute to a higher efficiency in power generation and help preserve freshwater as a precious resource. In recent years, more plant modifications are continually carried out to expand the wastewater and rainwater recovery system to: (i) Increase the amount of recovered wastewater and rainwater. (ii) Efficiently utilize the recovered wastewater and rainwater.
Categorization & Characterization of Reused Water The wastewater produced from the Station are identified and segregated into 4 categories: (i) Demineralized (D/M) water grade with conductivity below 10 µS/cm; (ii) Raw water grade with conductivity below 2,000 µS/cm; (iii) Raw water grade with conductivity below 20,000 µS/cm; (iv) Seawater grade. Only the first three streams of wastewater as well as rainwater are considered reusable and therefore recovered. The fourth stream cannot be reused and is treated and discharged. The wastewater recovery system also implements a cascade-reuse concept, i.e. effluent from equipment that employs better grade wastewater is sent to equipment that can tolerate lower quality wastewater. Different sources of effluent and their quality are summarized in Table 1.
3
Sources of Effluent Conductivity(µS/cm)
Chloride (mg/L)
Total Suspended Solids (mg/L)
Remark
Sampling rack drainage water ~10 <1 <10 D/M water grade Filter plant regeneration water ~100 <30 <30 Rainwater ~100 <100 <10 Submerged Scraper Conveyor System bleed-off water
~1,000 <200 <30
Coal yard dust suppression water spray recirculation water
~1,000 <500 <500
Raw water grade with conductivity below 2,000 µS/cm
Demineralization plant rinsing water during regeneration
~3,000 <500 <30
Electrostatic Precipitator washing water
~5,000 <500 <20,000
Rotary Air Heater washing water
~5,000 <500 <20,000
Flue Gas Desulphurization Plant Gas-Gas Heater washing water
~5,000 <500 <20,000
Raw water grade with conductivity below 20,000 µS/cm
Table 1 - Sources and quality of the wastewater
Plant Equipment or Processes Using Wastewater & Rainwater Plant equipment or processes that can use recovered wastewater or rainwater are summarized in Table 2.
Location to use
Source of wastewater
Submerged Scraper Conveyor System make-up
Sampling Rack drainage water
Irrigation Rainwater Floor cleaning Rainwater Barge ash conditioning Submerged Scraper Conveyor System
bleed-off water, filter plant regeneration water, rainwater
Coal yard dust suppression water spray Submerged Scraper Conveyor System bleed-off water, filter plant regeneration water, rainwater, coal yard dust suppression water spray recirculation water
Limestone slurry preparation & make-up for Flue Gas Desulphurization Plant
Rainwater, filter plant regeneration water, Demineralzation plant regeneration rinsing water, Electrostatic Precipitator washing water, Rotary Air Heater washing water, Flue Gas Desulphurization Plant Gas-Gas Heater washing water
Table 2 – Plant equipment or process using wastewater and rainwater
4
Process of Using Wastewater & Rainwater (W&R) The process of W&R recovery and reuse is depicted in Figure 24. To facilitate the using of wastewater, the following 3 water reservoirs are used to receive different kinds of wastewater: (i) Reusable Effluent Separation Pit with capacity of 3,000 m3 (Figure 3) is used to receive
wastewater of raw water grade, with conductivity below 2,000 µS/cm, mainly for coal yard dust suppression water spray (Figure 4), floor cleaning (Figure 5) and barge ash conditioning (Figure 6).
(ii) Effluent Separation Pit with capacity of 5,600 m3 (Figure 3) is used to receive wastewater of raw water grade, with conductivity below 20,000 µS/cm, mainly for limestone slurry preparation & make up for Flue Gas Desulphurization Plant (Figure 7) which adopts a design of single-looped, co-current, wet limestone-gypsum process with forced in-situ air oxidation.
(iii) Common Equipment Sump Pit with capacity of 900 m3 and equipped with agitators (Figure 8) is used to receive wastewater of raw water grade, with conductivity below 20,000 µS/cm and with total suspended solids below 20,000 mg/L, mainly for limestone slurry preparation for Flue Gas Desulphurization Plant.
Effluent Separation Pit
Reusable Effluent Separation Pit
Figure 3: Reusable Effluent Separation Pit and Effluent Separation Pit
5
Figure 4: Coal yard dust suppression water spraying
Figure 5: Bowser for floor cleaning
Figure 6: Loading port for ash barge
Figure 7: Flue Gas Desulphurization Plant
Figure 8: Common Equipment Sump Pit with 8 agitators
6
The Effluent Treatment Plant (Figure 9) has 4 neutralization basins with a total capacity of 3,600 m3 and equipped with agitation air, sulphuric acid and sodium hydroxide solution dosing equipment to neutralize the wastewater before use.
Figure 9:
Effluent Treatment Plant The drainage of the sampling rack (Figure 10) for boiler water and steam are collected and directed to the Submerged Scraper Conveyor System as make-up. The bleed-off water of the Submerged Scraper Conveyor System is directed to the Reusable Effluent Separation Pit.
Figure 10: Sampling Rack
The Electrostatic Precipitators, EP (Figure 11), of the coal-fired units remove pulverized fuel ash particulates in the flue gas before the flue gas is discharged. The EPs are washed when they are fouled or when inspection and maintenance on the EP have to be carried out. In this case, the washing wastewater is directed to the Common Equipment Sump Pit after neutralization at the Effluent Treatment Plant.
Figure 11: Electrostatic Precipitator
7
The Rotary Air Heater, RAH (Figure 12), of the coal-fired units uses the residual heat of flue gas to preheat the air for coal combustion. Washing the RAH is required when it is fouled or when inspection and maintenance on the RAH have to be carried out. The washing wastewater is directed to the Common Equipment Sump Pit after neutralization at the Effluent Treatment Plant.
Figure 12: Rotary Air Heater
The Gas-Gas Heater, GGH (Figure 13), of the Flue Gas Desulphurization (FGD) Plant uses the flue gas leaving the FGD Plant to cool the flue gas entering into the absorber of the FGD Plant while heating the flue gas to avoid condensation inside the chimney. Washing the GGH is required when the GGH is fouled or when inspection and maintenance on the GGH have to be carried out. The washing wastewater is directed to the Common Equipment Sump Pit.
Figure 13: Flue Gas Desulphurization Plant Gas-Gas Heater
The Water Treatment Plant consists of two sections, viz. Filter Plant and Demineralization (D/M) Plant. The Filter Plant (Figure 14) has bed of sand filter and anthracite filter to filter the town water before it enters into the D/M Plant. Regeneration of the filter plant by backwashing is regularly required when the Filter Plant is fouled and the regeneration effluent is directed to the Effluent Separation Pit.
Figure 14 Water Treatment Plant – Filter Plant
8
The D/M Plant (Figure 15) operates on the ion-exchange principle and consists of cation resin bed, anion resin bed, degassed unit and mixed resin bed to purify town water into D/M water for use in the boilers of the electricity generation units. Regeneration of the resin beds is regularly required by dosing sulphuric acid and sodium hydroxide solution when the resin is exhausted. The regeneration rinsing wastewater is directed to the Effluent Separation Pit.
Figure 15: Water Treatment Plant – Demineralization Plant
Rainwater is collected at the following areas : (i) Fuel Oil Tank Farm (Figure 16) and transferred to the Reusable Effluent Separation Pit. (ii) Coal jetty drainage system (Figure 17) and transferred to the Reusable Effluent Separation Pit. (iii) Buildings roof (Figure 18) and transferred to the Reusable Effluent Separation Pit. (iv) Coal yard and transferred to the Coal Yard Spray Water Pump Pit (Figure 19). (v) Lamma Extension Drain Pit (Figure 20) for irrigation (Figure 21).
Figure 16: Fuel Oil Tank Farm
9
Figure 17: Coal Jetty Drainage System
Figure 18: Boiler House Roof Top
Figure 19: Coal Yard Spray Water Pump Pit
10
Figure 20: Lamma Extension Drain Pit
Figure 21: Irrigation System at Lamma Extension
The water for suppressing fugitive coal dust is stored in the Coal Yard Spray Water Pump Pit (Figure 19) and transferred to the water sprayers by the Coal Yard Spray Water Pumps for water spray (Figure 4). The spray water returns to the Coal Yard Spray Water Pump Pit from the coal yard drainages for recirculation. Results of Wastewater & Rainwater Recovered With continual plant modification on the wastewater and rainwater collection and transfer system, the volume of wastewater and rainwater recovered increases continually in recent years and amounts to more than 100,000 m3 every year as tabulated below:
Year Wastewater and Rainwater Reused (m3)
2002 ~102,700
2003 ~121,000
2004 ~131,600
2005 ~135,500
2006 ~137,300
11
CONCLUSION Power station is a big consumer of freshwater. It also generates a considerable volume of wastewater. The two processes are inter-related and cutting down the discharge of wastewater can reduce the consumption of freshwater. Therefore, implementing these conservation practices will bring double benefits to the power station. In Lamma Power Station, the conversion of the once-through ash sluicing system employing seawater to a closed-loop ash sluicing system employing freshwater not only drastically reduces the volume of effluent discharged by the Station but also permits a more effective cascade mode of effluent recycling and reusing. Together with increasingly more collection equipment added to the system, the volume of wastewater and rainwater recovered increases continually in recent years. An equivalent quantity of freshwater is thus saved at the same time. ACKNOWLEDGEMENT The authors would like to thank the management of The Hongkong Electric Co., Ltd. for their support and approval on publishing this paper.
12
Hi
Ash Cooling WatePump
Sea Water
Sea Water
F
Furnace Hopper
Ash Crusher
Clinker Jet
Slurry Pump Pit
Ash Settlement Basin
Pump Pit
Effluent Pumpgh Pressure Ash Pump
r
Sea via Cooling Water Culvert
igure 22: Once-through seawater Furnace Bottom Ash (FBA) handling system
13
Submerged Scraper
Conveyor
JetPum
Scree
TransfeBin
Transfer Pump
OverflowTransfer
Tank
Transition
Chute
Boiler
Make-up
Figure 23: Submerged Scraper Conveyor Sys
14
Bleed off to Reusable Effluent
Separation Pit
High PressureWater Pump
p
Cooling Water Pump
Ash Conveyor
To Barge
Ash Gantry Crane
Filtered Water Basin
Filtering Basin
n
Crush
r
tem
15
Sampling Rack
Drainage
Submerged Scraper
Conveyor
ES
(5
CommonEquipment Sump Pit(900 m3)
Rainwater
Collected at coal yard
Collected at Fuel Oil Tank Farm
Collected at coal jetty drainage
Collected at buildings
roof
Collected at Lamma Extension drain pit
Irrigation
Effluent Treatment
Plant (3,600 m3)
Electrostatic Precipitator washing
Rotary Air Heater washing
Filter plant regeneration
Demineralization Plant regeneration
M
Flue Gas Desulphurization Plant Gas-Gas Heater washing
Reusable Effluent Separation Pit (3,000 m3)
ffluent eparation
Pit ,600 m3)
Coal Yard Spray
Water Pump Pit
Coal yard dust suppression
water spray
Floor cleaning
Barge ash conditioning
Limestone slurry Preparation & make up for
Flue Gas Desulphurization
Plant
ake-up
Bleed-off
Figure 24: Wastewater & Rainwater Recovery System
