At its flagship Medupi power plant, South African utility Eskom is keenly aware of the need to reuse as much water as possible, writes Drew Robb
Medupi is one of Eskom’s flagship power plants
Drought currently plagues much of South Africa. It is one of a couple of dozen countries in the world facing extreme water stress.
That is one of the reasons why the country’s state-owned electricity provider Eskom elected not to build a water-cooled coal plant to solve its ongoing electricity shortage. Instead, it is well on the way to building the world’s largest air-cooled coal plant.
Known as the Medupi Power Plant, it still has to use river or reservoir water to produce steam for its six massive Alstom turbines. But actual water usage is minimized by reusing as much of the water as possible. The lack of abundant water means Eskom must carefully utilize every drop of water it draws from local supplies – any waste must be minimized.
To ensure it achieves that goal, Eskom has installed an Ovivo Condensate Polishing Plant (CPP) at all six units. This technology eliminates the requirement for boiler blowdown by removing contaminants from condensate water so that clean water can be safely fed back into the condensate circuit and to the boiler for the production of steam. Thus steam is maintained at the highest quality which, in turn, maximizes power output and longevity of the complete steam-condensate circuit including turbine and condenser.
The CPP is designed to produce treated condensate with cation conductivity less than 0.1 àŽ¼S/cm, chloride less than 1 àŽ¼g/l (less than 1 ppb) and silica less than 5 àŽ¼g/l (less than 5 ppb).
The Medupi plant is located at Lephalale in Limpopo Province, in the northern part of South Africa. Once complete, this baseload facility is expected to provide about 12 per cent of the nation’s power generation capacity.
The 4790 MW plant will be the biggest dry-cooled power station in the world. It is composed of six Alstom STF100 steam turbines, each capable of producing 794 MW. Alstom – which is now part of GE – is also supplying its ALSPA Series 6 control system, while GEA – recently renamed Kelvion – is providing the air cooled condensers (ACCs), and Hitachi is suppling its supercritical boilers.
The plant will have six separate power islands, known as Units 1 through 6, with Unit 6 recently beginning commercial operation. Each unit consists of a boiler, turbine, generator and balance of plant equipment. The steam is superheated to 560à‚°C at a pressure of 241 bar. Reheat steam conditions are 50.5 bar and 570à‚°C. Steam flow at nominal load is around 617 kg per second. The condenser back pressure is 141 bar at a temperature of 23.7à‚°C.
The Hitachi supercritical boilers are once-through Benson type and at Medupi they burn local bituminous coal. Their capacity for steam is 2288 t/h per boiler. A series of low-NOx burners with staged combustion are used to burn the coal.
After using all the useful energy from the steam, it is sent horizontally to the air cooled condensers. A massive steel and concrete structure supports the ACC modules which use GEA’s A-tube arrangement in order to maximize the volume of galvanized tubing subjected to cooling.
Enormous fans continually blow air across these tubes to cool the steam down to the desired temperature to bring about condensation. The condensate feed extraction pump has both a fixed and a variable speed drive.
Eskom selected Ovivo after having successfully used its technology at other sites. That gave the utility the confidence that its condensate would be treated effectively to achieve the feedwater quality required in the water-steam cycle.
The CPP treats 100 per cent of the condensate flow under normal operating conditions. This removes iron and copper corrosion products, as well as dissolved cationic and anionic impurities, while allowing ammonia ions to pass through, when operated in the ammonia cycle, maintaining a high pH to protect the condenser.
A closed-loop system exists where recycled condensate is sent to the coal-fired boilers which produce steam. This steam is fed through the turbines to generate electricity. The ACCs cool the exhaust steam, causing it to condense. That condensate, however, contains contaminated water which must be cleaned before the loop is continued. The first stage of condensate polishing utilizes cartridge filters to remove most of the particulates (see Figure 1).
Service vessels containing ion exchange resins are used to remove impurities before the water is fed back into the boiler. Eskom chose a system with separate vessels for cation and anion resins to give maximum flexibility of operation.
The resins remove contamination in the condensate by exchanging dissolved impurities for hydrogen and hydroxide ions, as the water passes through the service vessels. The resin also acts as a filter to remove any remaining particules.
By utilizing separate vessels for resin, the anion vessel can be bypassed at those times when condensate temperatures rise higher than those recommended by the anion resin manufacturer. Failure to place the anion vessel offline during short periods when temperatures spike can shorten the longevity of the resin.
As the resin becomes exhausted, it slowly loses its capacity to exchange ions which reduces its ability to exchange contaminants. Eventually, it reaches the point when the quality of the water deteriorates. Technicians can detect such an occurrence by measuring water quality parameters such as cation conductivity.
A rise in cation conductivity indicates that more contaminants are present in the treated condensate and the resins require regeneration. When cation conductivity or other process parameters, such as silica or sodium, indicates quality of treated condensate is deteriorating, the cation or anion resin is automatically transferred from the service vessel to one of the external regeneration stations and a spare charge of regenerated resin is sent back to the service vessel. Each resin charge can last approximately three months between regenerations.
Medupi water treatment plant
There are four Ovivo external regeneration stations, two for cation resin and two for anion resin. Each regeneration station has a regeneration vessel and a hold vessel. The regeneration vessels feature an optimized dished nozzle-plate design on which resin is supported.
This arrangement retains the resin while allowing free flow of air or liquid. The open nature of the dished nozzle-plate also allows removal of fine particulate matter using a combined air scour/rinse down technique. An acid and caustic regeneration chemical recovery system cuts down on the use of virgin resin regeneration chemicals.
The regeneration system maximizes recovery of resin transfer, resin cleaning and resin rinse water for reuse during the next regeneration in order to minimize the generation of waste water and the need for fresh demineralized water replenishment. Towards the end of regeneration, an optional step enables circulation of dilute ammonia solution through the resin to improve treated condensate quality.
Eskom places a heavy emphasis on obtaining the best quality demineralized water for boiler make-up for supercritical boilers. Accordingly, the full spectrum of water treatment methods is utilized. Pre-treatment processes include clarification and ultra-filtration, while the demineralization process includes gas transfer membrane (GTM) technology followed by continuous electro de-ionization (CEDI).
|Figure 1: Condensate polishing stages|
Initially conceptualized as a Zero Liquid Effluent Discharge (ZLED) plant, all liquid effluents from the water treatment process at Medupi were going to be treated in an evaporator/crystallizer system, aiming to recover as much water to the demineralized water circuit as possible and co-disposing the final crystalline solution with fly ash tailings.
But after considering the capital and operating costs associated with these thermal processes, the decision was taken to sink all water treatment plant effluent into the station’s wet ashing system. This is Eskom’s standard approach for the disposal of saline liquid effluent at their existing coal-fired power stations.
Currently, all effluent from the water treatment plant is collected in the neutralization sump. This includes spent backwash and chemically enhanced backwash effluent from the ultra-filters, brine from reverse osmosis (RO) and concentrate from the CEDI modules.
Sections for clarifier/thickener and dissolved organics scavenging anionic ion exchange address suspended solids and organic carbon concentrations. The underflow from the clarifier and spent regenerant from the ion exchange units are discharged into the fly ash tailings and neutralization sump respectively.
The neutralization sump comprises two equi-sized cells, each with a wet volume of approximately 750 m3. Basic neutralization using caustic lye or sulphuric acid is employed as the only treatment effected in the effluent system. Once the batch pH reaches a terminal value inside the target range (6 < pH < 8), the effluent is discharged to Eskom for use in its boiler ashing system. A significant volume – up to 60 per cent – of this effluent is chemically bound on the tailings dam in reaction with course and fly ash coming from the boilers.
Unit 6 commissioning
Despite some delays that have impacted the plans for Medupi to come online, Unit 6 is now in commercial operation by Eskom. The results of the condensate polishing process are the removal of 99.99 per cent of iron and copper corrosion products, as well as cationic and anionic impurities.
As most condensate contaminants enter the system via the air-cooled condensers, the CPP is located immediately downstream of the ACC tank. This takes care of contaminants before they can impact materials and equipment in the rest of the circuit, while minimizing water wastage.
As much water as possible is reused at Medupi power plant
Further benefits include a reduction in water side corrosion, turbine deposits, turbine corrosion and deposition of solids in the reheater. In addition, startup times are improved as the plant manager is able to verify water quality almost immediately upon startup. This contributes to a lowering of fuel consumption as less time is spent idling before moving the plant up to full load.
Another gain from the Ovivo system is that it has allowed Eskom to harness alkaline oxygenated treatment (AOT) cycle chemistry. This allows the plant operator to maintain the condensate at the higher pH levels recommended for air cooled condensers. In addition, this oxidizing environment provides a protective oxide fill on metal surfaces in contact with corrosive pre-polished condensate.
Eskom’s preference for separate resin vessels means no resin separation is required so regeneration time is minimized. The use of an external regeneration system eliminated any risk of chemicals being inadvertently introduced into the steam-water cycle. By employing auxiliary cation units in the anion regeneration stations, anion rinse water can be recycled.
Now that Unit 6 of Medupi is running, Eskom will gradually commission and bring the other units onto the grid. When completed, almost 4800 MW of power will be available. This provides a much needed boost to South Africa in the face of occasional load shedding and a dwindling reserve margin.
Drew Rob is a freelance journalist specializing in the energy sector.