Hanovia UV Ensures Low TOC Levels In Boiler Feedwater At Kaeng Khoi 2 Power Plant In Thailand

The Kaeng Khoi 2 power plant in Saraburi, Thailand, is using medium pressure UV technology from UK company Hanovia to reduce total organic carbon (TOC) in boiler feedwater. Situated in Saraburi province north of Bangkok, Kaeng Khoi 2 is a gas-fired thermal power plant with a generating capacity of 1468 MW, 7.5% of Thailand’s total power consumption.

Kaeng Khoi UV systems

This gas fired, combined cycle type plant uses 2 x 734 MW trains of combined cycle gas turbines and two heat recovery and steam generation systems, feeding one common steam turbine. The facility was designed to meet Thai and World Bank environmental regulations and the electricity generated is being sold to the Electricity Generating Authority of Thailand (EGAT) under a 25-year power purchase agreement.

The success of the Kaeng Khoi 2 project, which had to overcome a number of obstacles, including changes in the project’s fuel type from coal to natural gas and relocation from Prachuab Khiri Khan Province to Saraburi Province, and is widely seen by industry observers as strong sign of confidence in Thailand’s long-term strategy for expanding electricity generation. The power plant will play a vital role in meeting Thailand’s future electricity demand, which is expected to reach approximately 41,000 MW in 2015, based on forecast annual growth of 6 to 7 percent.

Kaeng Khoi panels

On a lower level but equally important to the long term success of the project is the elimination of TOC from boiler feedwater. This is essential for a high pressure boiler’s long-term operation and performance, especially modern boilers like the one at Kaeng Khoi 2, which have a high evaporative rate. TOC can causes corrosion in boiler systems as the break down of  organic compounds in heated water forms carbonic acid in the steam and condensate.

Corrosion is a serious problem which can lead to failure of critical parts of the system and overall efficiency loss. Most plants set maximum limits for TOC in boiler feedwater, the level depending on the pressure of the boiler – the higher the pressure, the higher the purity requirements.
The level and type of organic material in feedwater depends on the source and can vary during the year as the seasons change. To make things more complicated, TOC is an aggregate measure of organic carbon which does not differentiate between simple low molecular weight molecules and more complex high molecular weight compounds.

The water treatment designer has a number of tools in his armoury to remove TOC, including flocculation, filtration, ultrafiltration, reverse osmosis, ion exchange and UV, each of which can remove some of the TOC, but seldom all. The designer is therefore often faced with a poorly characterised feed water quality which will vary over time, with the requirement to deliver and warranty a low TOC level from a system in which no single process will guarantee the result.

The process design strategy  therefore has to accommodate this uncertainty and very often relies on a combination of processes to ensure the final result. UV is one of the tools employed, offering a unique ability to break down trace organics which cannot be removed by  other processes.

UV technology

A typical UV system consists of a UV lamp housed in a protective quartz sleeve which is mounted within a cylindrical stainless steel chamber. The water to be treated enters at one end and passes along the entire length of the chamber before exiting at the other end.

There are two main types of UV technology, based on the type of UV lamps used: low pressure and medium pressure. Low pressure lamps have a monochromatic UV output (limited to a single wavelength at 254nm), whereas medium pressure lamps have a polychromatic UV output (with an output between 185-400nm). Medium pressure UV systems also allow the higher energy input, often required for more difficult organic compounds, in a compact and cost effective package.

How UV works on TOCs

Research by Hanovia has shown that medium pressure UV works in two ways:

• The first action of UV on TOCs is an oxidation process caused by UV breaking down water molecules at low wavelengths. This creates hydroxyl radicals (free OH- radicals), strong oxidising agents which readily combine with other molecules, such as the hydrocarbon molecules that make up TOCs. When hydroxyls combine with TOCs they form H2O and CO2 molecules – the TOCs are destroyed and oxidation is complete.

• The second type of reaction occurs when UV is absorbed directly by the organic molecules, breaking down their chemical bonds. This occurs across a wider range of wavelengths depending on the chemical bonds being broken.

This ‘two pronged’ attack is very effective and can result in dramatic reductions in TOC concentrations. It is particularly useful in addressing seasonally varying feed water quality, as the two mechanisms are complimentary and may address different TOC molecules. Only medium pressure UV provides this dual mechanism.

The Kaeng Khoi 2 water treatment system

The original source of the boiler feedwater is water from a local river, which is stored in a raw water pond before being pumped into the water treatment plant. Following softening and clarifying pre-treatment, the water passes through fine sand filters and then undergoes reverse osmosis to remove TDS (total dissolved solids) before passing through two medium pressure UV chambers. Positioned in series, each chamber contains six medium pressure UV lamps. Following UV treatment the water passes through mixed bed deionisers prior to use.

The two Hanovia UV chambers treat a combined water flow of up to 400m3 per hour. Actual TOC input to the UV chambers is 183ppb and output is 58ppb – a 68% reduction and well below the required minimum required concentration.

The contractors who supplied the demineralisation plant decided to use Hanovia UV technology because of the company’s international reputation for reliability and customer service. Other important factors were the low pressure drop through the medium pressure UV chambers and the system’s overall ease of use compared with competitors’ equipment.

UV system design requirements

The optimum location for the UV systems depends on the overall deionisation process selected, but generally it is deployed after the primary deionisation but before the polishing deionisation, as the polishing stage will be required to remove the break-down products from the UV.

The UV dose received by a TOC molecule is dependent on the energy output of the UV lamp, the flow rate of the water, the ability of the water to transmit UV (its transmittance value) and also the geometry of the treatment chamber. Proper design of the treatment chamber must take all of these factors into account. Transmittance is especially important and is a measure of the amount of UV light absorbed or scattered by both suspended and dissolved material in the water. This can vary considerably depending on the source of the water and its level of purity, so a transmittance test should always be carried out to determine correct UV system design.

As flow rates increase, chamber size and lamp power output can be increased as required. For larger flows, multiple chambers are used, in series or in parallel, until the required degree of TOC reduction is reached.

Reliable TOC reduction requires that a minimum UV dose is applied to the water.  Hanovia medium pressure UV lamps are powered by constant wattage transformers, which supply constant power to the UV lamp(s), despite fluctuations in power supply. This is especially important in regions where power supply may be intermittent or unreliable. Power switching options are available, adjusting the lamp power on-line as the water flow or the quality of the water changes. The power switching option maintains a constant, pre-determined UV dose level whilst ensuring maximum energy efficiency.

In most UV systems an instantaneous means of monitoring UV intensity is desirable. If a minimum dose of UV energy, calculated from the maximum flow rate and taking into account the transmittance of the water, can be shown to have reached the outer surface of the treatment chamber (where the UV monitor is situated) then the necessary level of TOC reduction has taken place. The UV monitor can detect variations in the transmittance value of the water and helps to adjust the UV output accordingly, ensuring consistent UV levels at all times.

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The Use Of UV For Dechlorination

For many years chemical disinfection techniques have been used to provide microbiologically pure water for industrial and domestic use. Free chlorine, typically introduced by municipal water treatment plants in gaseous form, has been employed for many decades as a primary oxidizing agent for the control of microbiological growth. Free chlorine can also be introduced through the injection of sodium hypochlorite, chlorine dioxide and other chlorine compounds.

Hanovia UV chamber

When chlorine is injected into waters with naturally occurring humic acids, fulvic acids or other naturally occurring material, trihalomethane (THM) compounds are formed. Approximately 90% of the total THMs formed are chloroform, with the remaining 10% consisting of bromodichloromethane (CHCl2Br), dibromochloromethane (CHBr2Cl) and bromoform (CHBr3). Since THMs have been shown to be cancer-causing to laboratory animals in relatively low concentrations, there is concern about limiting their prevalence. The United States Environmental Protection Agency  (USEPA), for example, has set the maximum contaminant level in primary drinking water to be 100 parts per billion (ppb).

Although chlorine is widely used in industry, many processes cannot tolerate it because of contamination and unwanted chemical reactions. It can accelerate corrosion of process vessels and piping and also causes damage to delicate process equipment such as reverse osmosis (RO) membranes and deionization (DI) resin units. It can also affect the taste, flavour and smell of drinks and liquids. It therefore must be removed once it has performed its disinfection function.

To date, the two most commonly used methods of chlorine removal have been granular activated carbon (GAC) filters or the addition of neutralizing chemicals such as sodium bisulphate. Both of these methods have their advantages, but they also have a number of significant drawbacks.

Granular Activated Carbon (GAC)
Activated carbon is frequently used in industrial applications such as beverage and pharmaceutical manufacturing and in point-of-use units for residential and commercial applications. However, GAC filters, which are usually located upstream of the RO membranes, also can serve as an incubator of bacteria because of their porous structure and nutrient-rich environment. Additional problems encountered with the use of GAC filters are:

• Increased head loss
• Regeneration costs
• Unpredictable chlorine breakthrough

Sodium Metabisulphite or Sodium Bisulphate
This is either purchased in solution or bought as a dry powder and then mixed on site. It is commonly injected in front of RO membranes used in the pharmaceutical and semiconductor industries. One common problem with this approach is that the solution itself becomes an incubator of bacteria, causing biofouling of the membranes. It is also another chemical that has to be documented in use, handling and storage for regulators such as environmental protection or health and safety agencies. Additional problems encountered with the use of sodium metabisulphite are:

• Maintenance of dosing equipment
• Hazardous material to handle
• Scaling of RO membranes
• Sodium sulphate can be formed, acting as a stimulant to sulphate reducing bacteria
• Odour and taste implications also arise

The UV Alternative
An increasingly popular dechlorination technology, with none of the above drawbacks, is ultraviolet (UV) treatment. High intensity, broad-spectrum UV systems (also known as medium pressure UV) reduce both free chlorine and combined chlorine compounds (chloramines) into easily removed by-products.

Between the wavelengths 180 nm to 400 nm UV light produces photochemical reactions which dissociate free chlorine to form hydrochloric acid. The peak wavelengths for dissociation of free chlorine range from 180 nm to 200 nm, while the peak wavelengths for dissociation of chloramines (mono-, di-, and tri-chloramine) range from 245 nm to 365 nm. Figure 1 shows the UV output of a high intensity Hanovia medium pressure UV lamp. Up to 5ppm of chloramines can be successfully destroyed in a single pass through a UV reactor and up to 15ppm of free chlorine can be removed.

Many water treatment systems include RO units, which commonly use thin-film composite membranes because of their greater efficiency. However, these membranes cannot tolerate much chlorine, so locating the UV unit upstream of the RO can effectively dechlorinate the water, eliminating or greatly reducing the need for neutralizing chemicals or GAC filters.

The UV dosage required for dechlorination depends on total chlorine level, ratio of free vs. combined chlorine, background level of organics and target reduction concentrations. The usual dose for removal of free chlorine is 15 to 30 times higher than the normal disinfection dose. Membranes therefore stay cleaner much longer because the dose for dechlorination is so much higher than the normal dose used if dechlorination was not the goal. Additional important benefits of using UV dechlorination are:

• High levels of UV disinfection
• TOC destruction
• Eliminate safety hazard associated with mixing bisulphate
• Eliminate risk of introducing micro-organisms into RO (via GAC or injection of neutralizing chemicals)
• Overall improved water quality at point-of-use

As with other dechlorination technologies, the UV dosage required at a given flow rate is dependent on several process parameters, including:

• Process water transmittance level
• Background organics level
• Influent chlorine level and target effluent chlorine concentration level

UV Applications
Successful UV dechlorination applications range from pharmaceutical, food and beverage processing to semiconductor fabrication and power generation. In all these industries, dissatisfaction with conventional dechlorination methods has encouraged alternative methods to be found. The following are examples of some applications in which high-intensity, broad-spectrum output (medium-pressure) UV has been successfully used for dechlorination:

Pharmaceutical industry
A Hanovia UV dechlorination unit was installed at a Procter & Gamble manufacturing plant in the Georgia. The unit was installed before two banks of RO membranes; prior to this dechlorination was achieved using sodium bisulphate. Trials run soon after the UV system’s installation showed a dramatic reduction in the RO membrane wash frequency – down from an average of eight cleanings per month to only two per month – amounting to annual savings of $70,000. The number of shutdowns for RO membrane maintenance has also been significantly reduced.

Brewing industry

Many breweries, soft drinks manufactures and other processors use UV for general disinfection of product make-up and process water. UV kills all known spoilage microorganisms, including bacteria, viruses, yeasts and moulds (and their spores) and has many advantages over alternative methods. Unlike chemical biocides, UV does not introduce toxins or residue into process water and does not alter the chemical composition, taste, odour or pH of the fluid being disinfected.

One example is Shepherd Neame brewery in the United Kingdom, one of the oldest in the country. It uses a UV system to treat water drawn from a private well and used for deoxygenated beer cutting. The water passes through the UV treatment chamber before entering a storage tank, and from here it passes through a series of sterile filters before use. In addition to treating cutting water, the UV system also disinfects water used for bottle rinsing.

As can be seen from the above examples, the potential applications for high-intensity, medium-pressure UV for dechlorination and disinfection, and the benefits it brings, cover a wide variety of industries and processes. UV dechlorination offers real opportunities for those willing to invest in this innovative technology.

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