Cooling Tower Blowdown Treatment and Reuse in a Coal Fired Power Plant in India Reducing the water footprint in coal fired power plants

Transcription

1 Cooling Tower Blowdown Treatment and Reuse in a Coal Fired Power Plant in India Reducing the water footprint in coal fired power plants Author name Deepak Kachru Company Aquatech Systems (Asia) Pvt. Ltd. Country Pune, India

2 Contents Executive summary & Introduction Typical Power plant WBD (Water Balance Diagram) Current water conservation practices Cost Benefit around current practice Integrated ETP WTP concept for a power plant Constraints The Scheme Approaching zero Liquid Discharge Cost Benefit around integrated scheme (without zero liquid discharge) Conclusion Page 2

3 Executive Summary Several studies have been made in the past and continue to be made on water usages and water losses within a power plant. Most of the studies have detailed on the complete water balance within various configurations of power plants identifying losses, internal generation, discharges, etc to give a complete water usage picture for these plants. There have also been studies focusing on optimizing water usage within a given configuration by internal recycling and reuse of effluent within plant areas. Most of the subcritical and supercritical power plants are utilizing wastes for scrubbing and dust suppression applications, thereby significantly reducing their dependence on fresh water. This practice alone has nearly halved the water consumption in such plants from a substantial 5 liters/unit of power produced to now almost liters/ unit of power produced. It is the intent of the paper here to (1) highlight the possibilities of using the power plant effluent for more critical applications namely, boiler feed water, (2) evaluate the benefits and value proposition of such an approach and (3) understand the pros and cons The present paper is also based on actual under construction power plants and their finalized water usage models or Water balance diagrams. Introduction Water is a key component in the smooth functioning of any power plant. A power plant requires a majority of its water for various cooling purposes besides the all important boiler grade water. A coal fired power plant of MW capacity would normally require about MLD of water considering a COC of 5 6 in the cooling tower. Of this requirement cooling tower losses account for close to 80 90% of the water requirement, DM water requirement is about 5%, ash handling, coal dust suppression, service water requirements consume the balance 10 15% of the water collectively. Most of the coal fired power plants located inland have developed a model WBD to optimize intake of fresh water by maximizing reuse of the wastewater by utilizing the same in various auxiliary applications within the plant, namely coal handling, ash handling, etc. Despite these best practices there is always some amount of water that will be required to be disposed off with or without treatment, typically this quantity can be pegged at about 120 liters/mwh. Most of the power plants today proactively envisage an ETP with membrane based recycle of the cooling tower blowdown and other streams. This water so produced can be reused for CT make up requirements. Page 3

4 Typical Power Plant WBD Fig 1.0 (All figures are in m3/hr) Evap Losses 100 Raw water from river or Raw water canal 3333 reservoir 30 Clarifier Clarified water tank days storage Waste sludge CT Make up 218 WTP (150) Cooling/ventilation and Service water usage (68) C.O.C = 5 Recirculation rate = Condenser cooling Drift 150 Cooling Towers Evap losses Auxiliary cooling CT Blowdown 450 Recycled effluent 100 RO Reject 25 Ash handling system 400 Coal handling system Boiler blowdown , clarified water 10 to ETP 60 to ETP DM Plant based WTP 135 DM Tank Power Cycle 15 to ETP 125 RO Based ETP 25 to Ash handling system CPU Regeneration ETP treated water for reuse as CT make up 100 5, Regen waste to ETP 10, Hydrogen Generation plant 10 Make up & chemical feed, etc. Page 4

5 Current water conservation practice As indicated in Figure 1.0 above a lot of water reuse maximization is possible and is currently being practiced in some coal fired thermal power plants. The key areas for water reuse maximization are, 1. Ash handling 2. Coal handling 3. Effluent treatment plant In the above case, CTBD (cooling tower blowdown) of about 450 m3/hr combined with boiler blowdown of 50 m3/hr can be completely reused with ash handling consuming close to 400 m3/hr and coal handling consuming about 30 m3/hr, i.e. almost 95% of the blowdown water can be reused within the plant. This saves an equivalent amount on the intake of fresh water for the power plant. Most of the power plants will have a final effluent discharge to ETP, which will be mostly the residual CTBD and boiler blowdown as continuous streams. The other intermittent discharges to ETP would also include regeneration wastes, transformer yard discharges, floor washings, service water, etc. It has become imperative for many in land power plants to maximize reuse of wastewaters within plant, in order to be eligible for fresh water permits for expansions. In light of this and also due to increased awareness of power developers to the reality of depleting fresh water resources, there has been a renewed focus on achieving near zero liquid discharge. This development has led to an increased focus on adopting membrane based recycle systems into the ETP, with the treated water being recycled to augment CT make up. The combined efforts at maximizing reuse of water will save close to 15% in terms of lesser requirement of fresh water. The below table indicates the average specific water consumption in different configurations of power plants. Table 1.0: Specific water consumption in power plants Power plant type Range M 3 /MWh Gas based power plants Total dry ash handling power plants MW coal based thermal power plants with once trough system 200 MW coal based thermal power plants Page 5

6 500 MW coal based super thermal power plants MW coal based power plants with ash water recycling 500 MW coal based super thermal power plants with ash water recycling This paper is an attempt to highlight the limited value proposition of recycling the effluent for CT make up and will try to highlight the clear value proposition in recycling the recovered water for boiler feed instead. Cost benefit around current practice The total CT make up requirement in the cooling tower in Figure 1 stands at 3100 m3/hr, with a blowdown of about 450 m3/hr. The total wastewater being sent to ETP is 125 m3/hr, which after passing through a membrane system get converted to good quality CT grade water. This water is then recycled back to augment CT make 100 m3/hr. The below Table 2.0 illustrates the value proposition in terms of a cost Vs benefit analysis for this approach. Table 2.0: Cost Benefit analysis of current practice Source Quantity, m3/day Cost/m3, INR Cost per day, INR Fresh water ,000 Recycled effluent ,200 Net Savings in water costs, INR/ day Net savings in water costs, INR/ year Impact of Improvement in COC in cooling tower 4,800 17,52,000 NIL 4 Note 1. Fresh water off take will be reduced to the extent of wastewater being recycled for CT make up. 2. This per m3 rate is considered based on applicability of a cess for withdrawal of water from natural sources including pumping and treatment cost upto CT make up. Page 6

7 3. This per m3 treatment rate is considered based on a membrane based ETP incorporating UF RO treatment and operating at a maximum efficiency of 75 80%. 4. The permeate quantity from RO is only 100 m3/hr, which is only 3.2% of the total make up water requirement thereby making a negligible impact on COC in cooling tower. Page 7

8 Integrated ETP WTP concept for a power plant An integrated scheme across the ETP WTP tries to maximize the value proposition of the ETP by not only maximizing throughput but also utilizing it for a boiler feed application instead of CT make up. Essentially the wastewater from the power plant, to the tune of 125 m3/hr (as shown in figure 1.0) is pushed through a recycle system, to condition the water making it suitable for feeding to a polishing MB and then for boiler feed. This in other words means that adopting an integrated scheme with maximum recovery will eliminate the need for a separate WTP dedicated to produce BFW (Boiler feed water) To better understand the challenges and constraints of utilizing wastewater for critical boiler feed applications requires an overview of the CTBD analysis the major component of wastewater from a power plant. Constraints A typical CTBD analysis is indicated in Table 3.0 below. Table 3.0: Typical CTBD analysis Constituents Units Value Flow m3/hr 125 ph 8.0 Turbidity NTU 1 TSS ppm 104 Ammonia as NH4 ppm 0 Potassium as K ppm 14 Na as Na ppm 502 Mg as Mg ppm 47 Ca as Ca ppm 126 Sr as Sr ppm 0 Barium as Ba ppm 0 Total cations ppm 688 Carbonate as CO3 ppm 3 M Alkalinity as HCO3 ppm 617 Nitrate as NO3 ppm 8 Chloride as Cl ppm 485 Fluoride as F ppm 1.37 Sulphate as SO4 ppm 96 Page 8

9 Barium as B ppm 0 Total Anions ppm 1210 Calculated TDS ppm 1900 Constituents Units Value Silica as SiO2 ppm Carbon dioxide as CO2 ppm 3 BOD ppm 19 COD ppm 132 The above table indicates the following parameters as critical to the design of an integrated ETP WTP operating at high efficiency, a. Total hardness b. Silica c. Alkalinity d. Trace organics And in some cases even oil & grease. From the above list it is imperative to understand that the above contaminants can be addressed in a low efficiency conventional membrane recycle process done by physical removal of these contaminants. This would however involve use of higher chemical dosages of lime, dolomite, and proprietary antiscalants. Having done that will yet offer a recovery across the system of not more than 80% in the best case. Our objective here is to maximize recovery (Value proposition no. 1) and then utilize the water so recovered for boiler feed, thereby eliminating the need for a separate WTP (Value proposition no.2). A high efficiency process pushes the recovery to over 90% across the membrane based recycle system, which is achieved by operating the system at a higher ph and ensuring that most of the contaminants are addressed without extensive chemical and or precipitation requirements. The Integrated Scheme A typical WBD for the scheme is indicated in Figure 2.0 below, highlighting the stages of treatment in the scheme and the overall recovery across the system. Page 9

10 Figure 2.0: Water balance diagram for an integrated ETP WTP scheme incorporating high efficiency membrane recycle process Approaching zero liquid discharge It is evident that maximizing recoveries across membrane recycle systems augurs well for any downstream evaporation process whether based on natural or thermal evaporation. In other words enhancing the recovery across the recycle system from 80% to 90% halves the requirement of water to be evaporated; in a natural or solar evaporation pond, this translates to a reduction in area requirement by 50% and in case of a thermal evaporation system, this would essentially reduce the sizing requirement by half. This translates to value proposition no. 3 on adoption of an integrated ETP WTP scheme. Cost benefit around the integrated scheme The below Table 4.0 highlights the tremendous value in altering the reuse potential of the recovered wastewater and maximizing the efficiency of the recycle system. Page 10

11 Table 4.0: Cost Benefit analysis of integrated scheme Source Quantity, m3/day Cost/m3, INR Cost per day, INR Fresh water ,000 Impact of reduction in DM plant opex due to elimination of WTP ,200 Recycled effluent ,280 Net Savings in water costs, INR/ day Net savings in water costs, INR/ year Impact of Improvement in reduction in size of evaporation pond /system Reduction in overall water requirement m3/ year 13,920 51,00,000 Intangible benefit 31,53,600 Note 1. DM plant feed requirement of 150 m3/hr for an output of 135 m3/hr of DM water. 2. DM plant output of 135 m3/hr. 3. This per m3 treatment rate is considered based on a membrane based WTP incorporating UF RO treatment due to presence of colloidal silica and TOC in fresh water, and operating at a maximum efficiency of 75 80%. 4. This per m3 treatment rate is considered based on membrane based ETP with high efficiency of atleast 90% across RO, followed by a 2 nd Pass RO system and a Mixed bed polisher. Conclusion It is evident from the above discussion that an integrated scheme combining ETP & WTP into a single unit offers the following advantages. Page 11

12 1. Lesser fresh water off take to the tune of almost 4 Lakh cubic metre per year a significant saving when compared to current recycle practice. Thereby reducing the specific power consumption of the facility. 2. Three times more savings in the operating cost of the water system as compared to the practice of recycle to CT make up. 3. Significant advantage in case of zero liquid discharge requirements from the facility, by reducing the net inflow to the evaporation system being envisaged. In other words with increasing water costs and scarcity, it is important to innovate and adopt schemes that are capable of further reducing the water footprint of the power plant. By using less water and doing it the right way will only mean more savings and provide a significant value proposition to the end user in adopting recycling and zero discharge concepts into the water balance of the power plant. Page 12

13 References 1. Mukhopadhyay; Debasish, US patent , July 20, Mukhopadhyay; D.,, Whipple; S. S., RO Systems that Reduces Membrane Scaling and Fouling Tendencies" Ultrapure Water, October Allen Boyce, Michael Ferringo, Devesh Sharma, Zero Discharge Strategy Boiler Makeup from Cooling Tower Blowdown, 60th IWC, October BetzDearbon Handbook of Industrial Water Conditioning, Ninth edition, Copyright 1991, Betz Laboratories, Inc. 5. Reverse Osmosis A Practical Guide for Industrial Users, Byrne W., Tall Oak Publishing, Inc., Boyce; A., Ferrigno; M., Zero Discharge Strategy Boiler Makeup from Cooling Tower Blowdown, The 60th IWC, Pittsburgh October 19, Bradley, R., Pilot Testing High Efficiency Reverse Osmosis Das Well Produced Water, 61 st IWC, October Demineralization by Ion Exchange, Applebaum; S. B., Academic Press, Inc., 9. Dennis Mcbride, Deb Mukhopadhyay, 450 ppm Silica sustained in Innovative Reverse Osmosis Technology, International Water Conference (IWC), Fritz; C. H., Ranade; B., HERO Process Volume Reduction of Cooling Tower Blowdown as Preconcentrator for ZLD Application, The 62nd International water Conference, Pittsburgh, October Bulletin of energy efficiency, Volume 7, Issue 3, December Page 13