Recycling of Treated Sewage water for Ash Handling A Case study

Transcription

1 Recycling of Treated Sewage water for Ash Handling A Case study in 2 x 800 MW Krishnapatnam, Nellore Thermal Power station Venkatesh.M Deputy General Manager Power Generation Tata Projects Limited, Hyderabad 1

2 APPDCL Power Plant Introduction First 800 MW commissioned in Govt. sector Imported coal as primary fuel. Sea water intake near plant is primary source of water. Designed with multiple redundancy concepts for very high level of operation. Power Plant 2*800 MW Won the coveted Dun and Bradstreet award for: Infrastructure for start upon Brackish / sewage mix water as feed and recycle of total Desailnated water generated in the plant. Won many safety awards, best project awards Central Control Room 2

3 Power Plant Technical highlights Use of Sea water for most of the purposes. For critical cooling purpose DM water is used. Service water, Cooling water, Drinking water, DM water generated from a 6 MLD SWRO plant (with highest level of automation in the country). Challenge: in designing, commissioning and operations the plant with varying salinity from 9000 ppm till 45000ppm (plant was taken into operation with inputs upto 50000ppm under emergency for short time) Largest Electro-chlorination plant with highest automation. Biggest ventilation system in the country. Generating transformers catering to One phase. More than 8500 piles used for foundations with depths upto 50 meters. Coal by a cross country pipe conveyor from Krishnapatnam port. Biggest cooling towers. Chimney construction was a challenge due to poor soil conditions. The power plant has a total water recycle concept built into the designs so that no treated water is ever wasted. The water from ETP and STP is totally recycled to ash handling plant for reuse. 3

4 Power Plant Key photographs 4

5 Basic philosophy of Recycle Contract required Tata Projects to build a STP to cater to the power station and the township. For 2500 people, working out to 15 cum/hr. The treated water is sent to the plant guard pond where it is mixed with treated water from plant effluent treatment plant (ETP) and sent to the Ash handling plant (AHP). The vacuum pumps of the dry ash system designed to use this recycled processed mixture of ETP and sewage water for the seal water and was a continuous requirement. There are 12 vacuum pumps 8W + 4 Standby in the AHP, and each pump requiring an average of 5 cum/hr working out to 40 cum/hr and the treated effluent and sewage put together could cater to this requirement. The water after performing the critical purpose of ash vacuum seal was either recycled back to the ETP or recycled back to the ash pond. The details are presented else where on the control philosophy. 5

6 The selection of technology & design basis for STP The following technologies were considered for the STP, ASP (conventional), SBR, MBBR. The plant was designed for 15 cum/hr. Plant operates at cum/hr. With maximum design BOD 275 cum/hr but operating from 80 to maximum 180 and was never above 200. Considering the above scenario, we decided to go for a conventional ASP with 10 hours retention time, aeration upto 20 hours, primary clarifier, Secondary clarifier, Chlorination and then to a storage tank followed by the tertiary process. Based on preliminary trails considering abnormal BOD despite chlorination and proper aeration it was decided that secondary biocide was a must. Based on the pathological analysis, we decided to add a secondary biocide dosing ( could be triclosan or Glutaraldehyde ) test done at PHD. We achieved less than 10 ppm which was acceptable for human contact even. We had achieved as low as 2.5 ppm BOD and an average of 4.5 ppm BOD in operation. The treated sewage was led to CMB of ETP and blended with the treated effluent before ph and Biocide dosing. 6

7 The STP Process 7

8 STP recycle the concept Treated STP Water Treated ETP water CMB BOD Re- Monitoring & Enhanced Biocides ph Correction Ash Handling pump vacuum Water seal ( Dry ash system ) Wet ash slurry system Ash dyke (Sea water ) 8

9 The Key plant photos of the paper STP AHP Vac PH Electrochlorination plant 9 Ash dyke

10 Challenges in operations with recycled water There was a problem in the mind set of recycling the water, with all, including the customer There was a phase where it was observed that the seal water ph fell as low as 3 and the blame was on recycled water. A detailed study was done and the actual root cause analysis revealed that it was a power plant operational problem as the plant as operated below Sulfur dew point and the acidic fumes were sucked into the vacuum pump causing acidity. When the plant was operated beyond the sulfur dew point the problem persisted but was reduced. But we proved that this problem was there even with the service water. The advanced biocides used had no problem in the plant operation. 10

11 Conclusion Industries today are not normally allocated fresh water or power for operations, so the promoter or owner needs to take care of all his needs. Nowadays power plants are built near sea as this is perineal source of water. This plant was built with this concept. So,when the promoters spend lot of money on de salination plants it becomes imperative that we look into the option of recycling the water to the extent possible. In the project we have put in all our efforts and achieved 100% de salinated water recycle. for the which we were awarded the D & B Award for water recycle in the infrastructure industry. Do note that we did not have an ph, BOD, or LSI related issues in this recycle attempt, though were faced in the process water / service water / drinking water --- which was the initial make up for ash water vacuum loop. As designers / engineers we need to go of the way, think unconventionally,convince customers, consultants, contractors and even own management to achieve economic solutions to achieve total water recycle. 11

12 It is there but isn t It isn t there but is ----Robert Ludlum Thank You Thank You Best Project Award By D&B 12

13 Thank You 13