|SPX Corp has recently completed several large dry cooling installations in China
Credit: SPX Corporation
Penny Hitchin examines the latest technology developments.
Despite political drives towards decarbonization of the power sector, coal continues to be a dominant force in the global energy market, second only to oil and accounting for one third of total energy consumption.
Using coal for power generation is water-intensive. Coal mines depend on water to extract, wash and process coal, while coal-burning power plants need water to create steam and for cooling.
Global demand for fresh water is rising at a time when global precipitation patterns are altering. The World Resources Institute (WRI) says that baseline water stress was more prevalent and more severe in every continent in 2010 than it was in 2000, notably in China, South and Central Asia, and the west coast of the US. It adds that currently, 1199 new coal-fired power plants with a total installed capacity of over 1400 GW have been proposed for construction in 59 countries – however, more than three quarters of this development is planned for China and India.
Global coal consumption grew by 3 per cent in 2013, the fastest-growing fossil fuel according to BP’s Statistical Review of World Energy 2014, and China, India, and the US remain the biggest coal producers and the biggest consumers.
Range of technologies
Coal is used to boil water to create steam, which then spins turbines to generate electricity. The power plant cannot operate without adequate cooling as steam from the turbine must be cooled to minimize back pressure on the turbine.
Effective cooling enables the plant to operate at an overall higher efficiency. Efficient plant uses less fuel and has fewer emissions per unit of electricity generated.
There are three main methods of cooling: once-through systems take water from nearby sources, circulate it through pipes to absorb heat from the steam in condensers, and then discharge heated water.
Wet re-circulating or closed-loop systems (wet cooling) incorporates a second cycle to reuse cooling water. Cooling towers expose the heated water to ambient air. Some of the water evaporates while the rest is returned to the power plant’s condenser. Wet re-circulating systems only withdraw water to replace any water that is lost through evaporation in the cooling tower.
Dry cooling technologies which use air to cool the steam from the turbine have been in use for several decades, reducing total power plant water consumption by more than 90 per cent. However, the water savings are offset by higher capital costs, lower thermal efficiency and higher operating costs per unit of generation.
Dry systems rely on air for cooling rather than on the evaporation of water. Because they need an air flow rate that is usually at least three times greater than in a wet cooling system, the physical footprint of the cooling technology is much bigger than the wet cooling equivalent. The footprint of a direct dry cooling system can be over twice the size and nearly twice the height of an equivalent wet cooling tower.
Dry cooling relies on the temperature of the ambient air, which is typically higher than the temperature to which water evaporates in wet cooling. This means that, depending on the local climate, the output of a plant with dry cooling will be slightly less than that of a similar plant with evaporative closed-loop cooling. The figure will depend on conditions: in hot and humid conditions, dry-cooled power-plant efficiency and plant output are likely to further decrease. Strength and direction of atmospheric wind can also affect the efficiency of air cooled systems.
|Simplifying cooling tower construction is an area for development
Credit: Mott MacDonald
There are a number of benefits of dry cooling. The cooling water intake flow of the power plant is reduced significantly compared to wet cooling systems. If water is scarce or expensive the savings can outweigh the potential increase in cost of the towers.
Discharging heated water impacts on the environment – in Southeast Asia dry cooling is used in order to protect fishing. The cost of water consumption and water discharge may make dry cooling more economic – developers in the western US use dry-cooled plants, because of high water costs. Installing a dry cooling system means that a plant does not have to be near a water source. This opens up the option to develop sites near transmission or rail lines, and may mean that land is more readily available for development. Air-cooled condensers do away with the visible water vapour plume – which can be important as in some countries a prerequisite for planning consent is that there should be no visible plume.
Ultimately, the choice of power plant cooling technology depends on a number of site-specific factors including climate, availability and cost of water, and environmental and other regulations. Dry and hybrid cooling systems have greater capital costs than wet cooling systems, but operational costs must be calculated for each individual location, taking into account the cost of water, the climate, the cost of emissions and the price of electricity as well as the capital cost of the equipment.
The main drawbacks of dry cooling systems are power capacity reductions and efficiency penalties during periods of hot weather, which leads to an increase in operating costs and emissions per MW of output.
Hybrid cooling refers to cooling systems with both dry and wet cooling elements, which are used individually or in tandem to achieve the best features of each: operating the wet cooling performance on the hottest days of the year and the water conservation capability of dry cooling during the remainder of the year.
Hybrid systems have the potential to halve water consumption compared to wet cooling towers. Hybrid cooling is becoming more popular because the tower sizes do not need to be as big as for dry cooling and performance is better than with air-cooling only.
China’s coal-water nexus
China is the world’s largest coal producer and consumer, accounting for more than half of the planet’s coal use. At the end of 2012 installed coal-fired power generation capacity of 758 GW made up two thirds of the nation’s total generation capacity. But more than half of China’s existing coal-fired power plants are located in areas of water stress.
China’s Ministry of Water Resources sets out water use efficiency and discharge requirements for existing coal bases. Proposed new power plants in major coal bases facing water scarcity must specify air cooling technology to dramatically reduce the overall water usage of new coal-fired power plant compared to old plant relying on water cooling.
China Electricity Coal Council figures for 2013 show a total of 150 GW of installed thermal power units with dry air cooling. Going forward, increased deployment of supercritical power plants, ultra-supercritical power plants, and various water-saving technologies will reduce the average water consumption.
|Eskom has implemented dry-cooling technology on power stations wherever feasible
Most dry cooling technology uses air-cooled condensers (ACC) to cool the turbine steam. Fans blow the ambient air on to the condensers which dissipate the heat in the atmosphere. Around 10 per cent of dry cooling uses a natural draft dry cooling tower. Condensers are installed around the periphery of a high (120-170 metre) concrete tower with natural draft replacing the fans. This technology is more expensive to install, with a slower return on capital.
SPX Corporation has recently completed a number of large dry cooling installations in China. These include the Zhenglan power plant ACC (6 x 600 MW) and natural draft dry cooling towers at Zuoquan (2 x 660 MW), Qinling (2 x 600 MW) and WuAn (2 x 300 MW).
South Africa pioneers dry cooling
South African utility Eskom is a large consumer of freshwater in South Africa. Eskom power stations supply almost all of South Africa’s electricity as well as more than half of the electricity used on the African continent.
Many of Eskom’s coal power stations are in areas of water stress and the utility sets each power station a water use target per unit of electricity generated for every year. Eskom has implemented dry-cooling technology on power stations wherever feasible, despite the higher costs. The company says this conservation effort results in an estimated combined water savings of over 200 Ml/day, over 70 million m3/year.
The 4.1 GW Kendal Power Station in Mpumalanga Province is the largest indirect dry-cooled power station in the world. SPX technology has been in use there for over 25 years. Eskom says that the station’s cooling towers are the largest structures of their kind in the world with a height and base diameter of 165 metres.
The 4 GW Matimba Power Station in the Limpopo Province is the largest direct-dry-cooled station in the world. Water consumption is in the order of 0.1 litres per kWh of electricity, compared with about 1.9 litres on average for the wet-cooled stations. The choice of dry-cooled technology for Matimba was largely influenced by the scarcity of water in the area.
SPX is currently installing a 4.8 GW air-cooled condenser at the new Kusile power plant. The physical size of the dry cooling system is around 700 metres long and 70 metres high.
The Electrical Power Research Institute (EPRI) is sponsoring research at the University of Stellenbosch in South Africa to develop a new design for an ACC dephlegmator which provides a secondary condenser to improve vapour flow through the primary condensers, flushing them of any non-condensable gases.
This project is developing a novel hybrid (dry/wet) dephlegmator that would replace the conventional all-dry dephlegmator unit in an ACC. The technology has the potential to increase power production on the hottest days and would use less makeup water than wet cooling tower systems and less water than is currently used by dry cooling with evaporative pre-cooling of the inlet air.
Water Research Centre
In the US the Water Research Centre (WRC) is conducting plant-based research into water use. The pioneering centre is based at at Georgia Power’s 3.5 GW coal-fired Bowen plant about 50 miles from Atlanta.
The WRC provides electric generating companies, research organizations and vendors access to a demonstration facility that has treatable water, monitoring and analysis facilities, and specialist staff.
The WRC is funded by a consortium of the Electric Power Research Institute (EPRI), Georgia Power and 14 other companies. The two areas of development are water optimisation in advanced cooling and waste water treatment. The collaboration aims to demonstrate power plant cooling innovations. Potential areas of research include novel air-cooled condenser advancements, high-efficiency adsorption chiller technology and advanced indirect dry cooling tower design.
|The 582 MW coal gasification plant at Kemper County will use municipal wastewater for cooling
Credit: Mississippi Power
Jeff Wilson, manager of the internal environmental R&D group at WRC explains: “Traditionally, water has been considered abundant, but over the last five to ten years we have started to see that in certain locations, at certain times of year, water can become a bit stressed. We talked to companies around the world to come up with the areas we wanted to focus on in this facility.”
Since 2011 WRC has installed infrastructure which re-routes some hot water away from the traditional cooling towers at Plant Bowen to two test areas where advanced or hybrid cooling systems can be deployed.
Wilson explains: “We have put in the infrastructure needed to accelerate technology evaluation and improvements. We haven’t changed plant operations, but we pull off slipstreams to test the technology.”
The new centre is currently gathering performance and operation data under varying loads and weather conditions for two new hybrid technologies. Johnson Controls’ Thermosiphon Cooler technology has been installed in parallel with an existing cooling tower. It uses a dry-heat-rejection technology originally developed for space conditioning in buildings. An air-cooled refrigerant in a closed loop is used to cool water from the steam condenser prior to the wet cooling tower. Atmospheric conditions dictate which system is used; on a hot or muggy, humid day the cooling tower is used while in cooler dry conditions the Thermosiphon kicks in.
Richard Breckenridge, programme manager at EPRI Generation, says: “The overall concept is to look at the amount of water that can be saved without any operational impact on the plant. The concern with dry cooling is the efficiency loss that can occur with the systems.
“Depending on the conditions, sometimes there is an energy penalty of 10 to 15 per cent. The premise behind the hybrid cooling is that by marrying the two systems we will minimise the energy penalty. We want to minimise the cost of dry cooling, so that we can maybe get the same type of performance at a minimal economic penalty.”
Evapco’s Eco-WD Cooler (wet-dry cooling tower) employs an innovative wet-dry cooling tower technology. The technology works in wet-dry mode during the hot summer months and in dry mode the remainder of the year. In wet-dry mode, hot water is initially cooled through air-cooled heat exchangers and further cooled through heat exchanger bundles sprayed with treated water. In dry mode, the spray system is turned off, and the system uses no water for evaporative cooling.
Jeff Wilson says: “We have looked at these technologies and found that costs for retrofitting would be prohibitive. However, for new plants, these technologies could make a lot of sense. We hope that this type of work will accelerate the advancement of technologies, drive costs lower. We believe that going forward water is going to become more and more precious and more and more scrutinised.”
Other WRC projects are studying advanced water treatment technologies to enable the use of degraded water sources for cooling systems a area that is of increasing interest worldwide. The new state-of-the-art 582MW coal gasification plant at Kemper County in Mississippi will use municipal wastewater for cooling and look to minimise water withdrawal and maximise water use.
Water availability is set to become an increasing concern in the decades ahead. The power industry has traditionally required huge quantities of water, but the development of advanced cooling technologies can reduce this. Environmental policy and regulatory requirements are dictating many changes in coal-power generation.
In Europe, compliance with the Industrial Emissions Directive means that coal power plants must be fitted with selective catalytic reduction to reduce nitrogen emissions; flue gas desulphurisation to remove sulphur dioxide emissions; and electrostatic precipitators to mitigate particulate matter emissions. In an era of water stress, advanced cooling technology could be another essential addition.
As EPRI’s Richard Breckenridge says:”Water availability is something that will become increasingly important in the future. Decreased availability and competing demands for freshwater mean we need to develop advanced cooling technology.”
The cooling technology for coal power plant is specified on an individual basis. Going forward the development of cost-effective systems for reducing water use is a priority. Simplifying cooling tower construction, reducing evaporative loss in cooling towers and advancing hybrid wet-dry cooling systems are areas for development.
The focus of advances in dry cooling technology is likely to be reduced steam condensation temperature without significantly increasing fan power consumption or the size of air-cooled condensers, and development of alternative dry cooling technologies.
Penny Hitchin is a freelance journalist specializing in the energy sector.
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