By minimizing, or even eliminating, the need for blowdowns in steam systems, precoat filter demineralizer systems can greatly contribute to water-conservation efforts
Steam is essential in any chemical process plant, primarily for generating heat and power. Power generation (the largest consumer of industrial water) and water-dependent thermal plants generate more than 81% of the world’s electricity using steam to rotate an electric generator. The steam can be generated in various ways, which include: by coal-fired boilers; waste heat from gas- or oil-fired turbine generators; and by direct or secondary generation in a nuclear reactor.
Because demand for electricity also drives demand for water, in a time when water as a resource is becoming less predictable and more stressed, power outages caused by water shortages are becoming a realistic threat. To help themselves out of this quandary, power plants, and other industrial plants that generate steam, stand to benefit from investment in technologies that conserve and make the greatest use of water.
Condensate contamination
FIGURE 1. Contaminated condensate can lead to several types of costly damage
The condensate water that results from steam-turbine operation is one area where water conservation can be realized. For both boilers and heat-recovery steam generators (HRSGs), condensed water collects in the system and can be reused. However, this fact is complicated by the fact that, over time, the condensate in the steam loop becomes increasingly contaminated, which will cause damage to plant components. This includes boilers, steam generators, reactors and turbines. Untreated condensate can damage the steam turbine by erosion through the impingement of abrasive particulate matter transported with the steam, or by deposition of soluble material, such as silica and copper, on the turbine blades. Other damage caused by contamination can include tube pitting, tube rupture and drum pitting (Figure 1). Contamination of boiler feedwater and condensate occur from four principal sources, as follows:
Makeup-water system leakage. Dissolved and colloidal solids that pass through the makeup water system can often cause contamination.
Condenser leakage. Mainly dissolved, including some suspended, solids can leak from the condenser. The amount of such solids is dependent on the condenser design, level of maintenance and the quality of cooling water present on the tubeside. In the case of seawater-cooled systems, the smallest leak can become a significant problem for high-pressure boiler systems.
Metallic corrosion. Metal-oxide-based suspended solids are created within the boiler steam cycle as byproducts of corrosion. These metal oxides are usually forms of iron oxide, but oxides of copper, chromium and nickel can also contribute.
Startup contaminants. Silica is a notorious contaminant due to its frequent presence during initial boiler startup, typically as residue from the system manufacture and assembly.
The high cost of blowdowns
The traditional method of removing contaminants is a blowdown, which involves essentially purging all the water from the condensate loop at once and replacing it with fresh water. For large plants, this can amount to tens of thousands of gallons of water per blowdown. This loss of water has numerous associated costs, not just due to the cost of fresh replacement water, which is rising steeply across all regions, but also the costly steps required to make it ready to convert into steam. These steps can include pre-treatments, such as clarifying, filtration, softening, dealkalyzing, reverse osmosis and deionization, and chemical treatment, such as neutralizing amines. In addition, the largest cost comes from the energy to heat the water. It takes 1 Btu of heat to raise 1 lb of water by 1ºF, and while the temperature of condensate typically hovers around 200ºF, makeup water usually enters the system at 55ºF. The cost of fuel required to raise the temperature of water by 145ºF is significant. Furthermore, there are significant wastewater-treatment costs associated with dumping the dirty condensate, as detailed in Table 1.
Recover condensate
One technology that offers a viable solution for minimizing blowdowns is a precoat filter demineralizer (Figure 2). This is a condensate polisher that removes contaminants from condensate while preserving the heat, allowing it to return to the process clean and warm.
FIGURE 2. A precoat demineralizer system can help to remove contaminants from condensate streams, making water reuse more economical
Precoat filter demineralizers combine two distinct technologies: ion-exchange and filtration in one vessel or application. They remove contaminants that are present in the steam condensate cycle by removing suspended solids using filtration and reducing ionized impurities to the parts-per-billion (ppb) range, and in some cases, to parts-per-trillion (ppt) levels. Precoat filter demineralizer systems are essentially vessels with similar filter septa to those used in pre-filters or condensate filters, but are specially designed to act as a support for a precoat material. Figure 3 illustrates the different parts of a precoat demineralizer vessel. Premixed resins available in various cation and anion ratios, with or without fiber material, are coated on the outside of the filter septa in a thin layer. These finely ground resins remove ionic contaminants while also effectively removing most suspended solids. Upon exhaustion (ion breakthrough) or high pressure drop (solids loading), the unit is backwashed, sending the exhausted precoat and contaminants out to drain. Subsequently, a new precoat is applied and the process is repeated.
FIGURE 3. A precoat demineralizer system combines filtration elements and ion-exchange resins within a specially designed process vessel
Precoat filter demineralizers remove iron, copper, silica, activated corrosion products and salts, whether soluble or suspended. Systems of this type have been in operation since the early 1960s, but have experienced advancements in precoating techniques, as well as improvements in a high-energy air-scour backwash for enhanced septa cleaning, reducing waste volumes by over 60% in certain applications. Figure 4 illustrates the progressive coating and cleaning process of the filter elements.
FIGURE 4. The filter elements are coated with resins to remove ionic contaminants and suspended solids, including iron, copper and silica. Once the resin is exhausted, the elements can be cleaned and re-coated
Water conservation
Using precoat filter demineralization provides a string of benefits, starting with water conservation. By avoiding blowdowns, plants can save millions of gallons of water per year, as well as the costs associated with treating both blowdown water and makeup water, including the related labor costs. There are also growing regulatory compliance implications, especially in water-stressed areas. Some plants are already facing increased pressure to reduce water waste, and every other plant should be preparing now for this eventuality.
But even in the absence of regulation, there are stout economic advantages to installing this kind of condensate polisher. A large cost benefit is that precoat filter demineralization helps protect assets. Less contaminated water will result in less damage to system components, extending the life and performance of those components and reducing maintenance requirements. Plants that do not install a condensate polisher may save money in the short-term, but pay more in the future for equipment damage, boiler cleaning and replacement fees. Though many natural-gas combined-cycle (NGCC) plants are still relatively new, corrosion has already become nearly ubiquitous in steam boilers and heat-recovery boilers in those plants without high-quality condensate-polisher systems. Furthermore, the air-cooled condenser (ACC) design paradigm contributes substantial corrosion-product contamination to cycle water. Data from the Electric Power Research Institute (EPRI; Palo Alto, Calif.; www.epri.com) indicate that four of the top five causes of HRSG tube failure have been tied to steam-cycle chemistry related to flow-accelerated corrosion, corrosion fatigue, under-deposit corrosion and pitting.
Returning more high-quality condensate back to the boiler system can improve feedwater quality, increasing the number of boiler cycles of concentration, faster startup times and extending the time between blowdowns.
For some plants, when it is time to replace their 40- to 50-year-old boilers, they are finding that manufacturers have more stringent condensate-quality requirements in order to protect the equipment from contamination damage. In these scenarios, plant engineers are turning to condensate polishers to help them maintain the water quality specified by the modern equipment.
Enhanced startup and efficiency
For various maintenance and repair reasons, a turbine occasionally has to be taken offline. When this happens, the condensate in the system can become contaminated very quickly due to the stagnation of the water inside metal pipes, combined with the introduction of air into the system.
When the condensate is highly contaminated, a blowdown is required before the system can be restarted, significantly delaying the achievement of full generating capacity. During this startup delay, it can cost the operator between $25 and $35 per MWh — a substantial outlay each time the plant cycles. Further, chemistry fluctuations make it impossible to determine how long it will take for the plant to reach full power each time it starts, complicating budgeting for auxiliary power purchase. Systems that use a precoat filter demineralizer can avoid this blowdown, resulting in a process that can return the turbine to service much more quickly and less expensively. Additionally, the elimination and control of harmful impurities, both dissolved and insoluble, can improve thermal efficiency (by up to 1%) and can increase overall plant efficiency (by approximately 1–3%).
Water scarcity will continue
While the cost of fresh water is among the smaller pieces of the total cost of makeup water, this is not likely to remain so. The average price of water increased by 60% in the 30 largest U.S. cities between 2010 and 2019, according to data compiled by Barclays, while California water futures have regularly jumped as much as 300% in recent years. The growing water crisis is requiring all industries to rethink their water use, especially those in essential service sectors that depend so heavily on water.
Real-world examples
Small power plant
A small, regional NGCC power plant (air-cooled condenser configuration; 180-MW capacity) in Alaska needed to remove suspended and dissolved iron and hardness from condensate to prevent contamination. The plant was seeking a condensate polisher that was compact, simple to operate and with a low capital cost. There was also interest in a system that enabled a quick startup and improved uptime. The plant’s engineers chose a precoat filter demineralizer system consisting of two 30-in. diameter vessels, each designed to treat 100% of the condensate flow of 474 gal/min (108 m3/h). The condensate to the system inlet header had a maximum pressure of 260 psig (17.9 bars) and a maximum temperature of 140ºF (60ºC). The system was assembled complete on one structural steel skid for ease of installation.
Large power plant
A major NGCC power plant (air-cooled condenser configuration; 1,704-MW capacity) in Texas was seeking a condensate polisher system that would remove suspended and dissolved iron and hardness from condensate, as well as reduce condensate-loop maintenance and decrease downtime. The plant chose a precoat filter demineralization system consisting of two 84-in.-dia. modular assemblies (each designed to handle 100% of the effluent flow), each with a hold pump, valves, piping and instrumentation; two effluent-resin basket strainers (one for each polisher vessel); one precoat skid with a precoat injection pump, a precoat recycle pump, a precoat tank, a precoat tank mixer, an auxiliary tank, valves, piping and instrumentation; an air surge tank, a spent-resin backwash tank; and a control panel. ❑
Edited by Mary Page Bailey
Author
John Yen is the director of strategic marketing and innovation at Marmon Industrial Water LLC (30 Technology Drive Suite 2F, Warren, NJ 07059; Phone: 908-516-1400; Email: info.graver@marmonwater.com). With over 30 years of experience in the water industry, Yen drives innovation and revenue growth for the industry through capital equipment, aftermarket parts and service solutions. He previously held roles at BASF and Siemens Water (now Evoqua). He has a B.S.Ch.E. from Rutgers University and an M.S. degree in management science from the University of Massachusetts, Lowell.