As long as the power industry has been putting steam to work, ultrapure boiler feedwater has been a critical part of the process. Water purity translates directly to boiler efficiency, reliability and service life. It correlates to the same characteristics in turbines and other downstream components as well.
Producing this vital ingredient has traditionally been a complex, multi-stage process that involves filtration, water conditioning, bulk reduction of suspended solids and organics, degasification, and final polishing to remove ionized contaminants.
The Final Step is the Hardest
While each of these steps has its challenges, the final step, polishing, can be the greatest source of both cost and opportunity. Boiler and turbine water quality requirements are stringent, and it’s critical to achieve low conductivity, silica, sodium and total organic carbon (TOC) in the final treated water.
Purification at the ion level is what puts the “ultra” in ultrapure water. Traditional methods of achieving this state have involved chemically intense processes that have high financial, environmental and safety costs. The good news is that purification technologies have evolved in recent years to provide solutions that yield better water quality at lower operating cost.
Currently Available Technologies
Ion Exchange & Its Costs
For the last 70 years, power plant operations have relied on a chemically regenerated ion exchange process to produce ultrapure boiler feedwater. The process, which uses ion-attracting resins to remove contaminants, is still in use in many operations today. Its costs and safety challenges stem from the regeneration process, which must take place periodically as the resins become loaded with contaminants and lose their effectiveness. Regenerating these resins consumes both caustic and acid—hazardous substances that require special storage and handling. Managing the waste stream from this process poses additional costs.
Operators can address these challenges by using a service deionization (SDI) contractor for off-site regeneration, but the tradeoff is adding an extra link to the supply chain and an element of risk that may not be tolerable.
RO Pretreatment: A Step Up
In recent decades, the labor and chemical intensity of this process has been partially mitigated by the introduction of reverse osmosis (RO) as a pretreatment to ion exchange. As a response to tighter purity specifications in general, and to higher demand for removing organics and inorganics in particular, RO pretreatment provides greater overall purity while reducing the need for regeneration chemicals and their associated costs—a major change that made has RO pretreatment + ion exchange a dominant technology in the power industry.
However, with this alternative, some costs and operating challenges persist. While RO pretreatment removes many contaminants prior to ion exchange, it doesn’t get them all, and the need for ion exchange and its chemical costs remains. Furthermore, it’s still a batch process in which residual contaminants gradually rise to a breakthrough point where they approach specification limits, triggering a regeneration cycle. The inconsistent nature of this process requires an operator to monitor water quality, constantly watching for breakthrough. In addition, water quality isn’t consistent.
CEDI: The Emerging Leader
In recent years a new technology, continuous electrodeonization (CEDI) has offered an alternative that takes ultrapure boiler feedwater production to a new level of quality and cost effectiveness. First commercialized by the former Ionpure Technologies Corp in 1987, now Evoqua’s Ionpure®, CEDI has proven itself as an effective way to eliminate chemicals and reduce costs in a variety of industries, including power generation.
A Chemical-Free Process
CEDI replaces chemical-based ion exchange with an electrochemical process that uses no hazardous substances. Instead, it uses self-regenerating ion exchange resins, held within membranes in an electrically charged chamber, to purify water. In the process, direct current applied to the feedwater stream drives positively and negatively charged ions through negative and positive ion exchange membranes, where they are captured and directed to the waste stream. What’s left behind is product water than can reach 18 megohm-cm resistance in purity - well within today’s tolerances for ultrapure boiler feedwater.
This process eliminates the need for traditional ion exchange resin beds and chemical regeneration. Its waste stream, instead of being transported offsite for processing, can be managed internally and, in some cases, treated for reuse.
Continuous, Consistent Purity
In addition to being chemical free, CEDI is a continuous process in which the ion exchange resins continuously regenerate themselves under the control of sophisticated power management technology. Purity remains at a consistent, high level, and there’s no need to monitor the process for breakthrough.
As the originator of CEDI for the commercial market, Evoqua’s Ionpure product has designed CEDI technology into a wide range of compact modules for applications in research, electronics, industrial, pharmaceutical and power generation. All of these systems deliver higher throughput within a smaller footprint compared to traditional ion exchange, making it relatively simple to upgrade to this advanced technology. And the technology is still breaking new ground. Current designs offer 20 times the flow rate of previous-generation CEDI in just three times the footprint.
For the power industry, Evoqua’s VNX Series CEDI modules provide high flow rates from 15 up to 100 gpm and are available in nine models. In addition, Evoqua provides compatible ancillary systems for power management and pre-programmed touchscreen control. These systems are proven in the field, with thousands of installations worldwide.
As the energy market transitions to a new era of regulations and requirements for efficiency, the pressure to improve boiler performance can only grow. At the same time, it’s a safe bet that requirements for environmental compliance and workplace safety will inevitably increase. In this environment, CEDI offers new-generation technology that delivers the best of all worlds: safety, compliance, and efficiency - all while dramatically reducing OPEX.