Water treatment systems for power stations

A review of the water purification needed for boiler feedwater has been published by Elga Process Water. The history of power generation in particular is inextricably linked with the development of water purification technology, and vice versa. However, the UK privatisation of power generation has meant a much greater awareness of economic factors, notably the need to maximise the utilisation of assets – so that currently the make-up water treatment plants are generally designed to just meet the normal operating demand of the station: they have become smaller and, instead of custom designed systems, most CCGT stations for example use standard, lower cost “packaged plant”. Elga Process Water has stepped in here to provide a further service, because many of these site installed units may not have adequate purification capacity during periods of high demand, such as commissioning and steam blowing. A temporary reverse osmosis or ion exchange plant in a mobile hired unit from Elga can provide that capacity and is a highly cost-effective option.

The review is as follows.

Back in the 1960s, the then nationalised power generation industry drove boiler pressures, in what were mostly huge, coal fired power stations, ever upwards in an attempt to improve thermodynamic efficiency.

Over the next fifty years on, privatisation and the advent of cheap N Sea gas changed the face of power generation, introducing smaller, high efficiency, combined cycle gas turbine stations.

The history of power generation is inextricably linked with the development of water purification technology, and the changes in the water treatment industry over that half century have been profound.

Thermodynamic efficiency depends on the temperature difference around the cycle, so a higher turbine inlet steam temperature will give better results.

A higher steam temperature means a higher steam pressure, hence the trend towards very high pressure and even supercritical boilers.

Unfortunately, as the boiler pressure increases so maintaining steam purity becomes a problem.

The density of water decreases with increasing temperature and the density of steam increases until, at the critical point (221bar pressure corresponding to a boiling point of 374C), they are the same.

Consequently, as the pressure and temperature increase, phase separation by gravity becomes more difficult and any impurities in the boiler water are likely to be carried over into the steam.

For this reason alone, power station chemists have always sought ultimate purity for their boiler feed water.

The challenge for water treatment engineers was to achieve this purity level, which they did by developing make-up water and condensate treatment plant capable of achieving conductivity less than 0.1uS/cm.

The process route that was developed was ion exchange demineralisation, using a two stage process comprising separate cation and anion exchange resin units, regenerated in reverse flow, followed by mixed bed polishing using cation and anion exchange resins mixed together in the same vessel.

This is still the most widely used scheme for producing power station boiler make-up water.

A bigger problem was that of silica.

Silica is anathema to power station chemists: at pressures over about 40bar it volatilises, passes over with the steam and sublimes from the vapour phase forming a solid deposit on any relatively cool surface.

If the cool surface happens to be the turbine blades the resulting solid deposit can have catastrophic effects on the turbine balance.

The solubility of silica in water increases with increasing temperature and pressure, so the higher the pressure the greater the risk of silica deposition.

Silica dissolves in water forming weakly ionised silicic acid which, in alkaline conditions, dissociates to form silicates: SiO2 + H2O gives H2SiO3, giving H+ + HSiO3- and 2H+ + SiO3–.

Because it is very weakly ionised, the silicate ion is difficult to remove from water and it was not until the development of high basicity Type 1 anion exchange resins that low silica residuals could be guaranteed.

The make-up water treatment plants were large, custom engineered systems generally designed with sufficient capacity to cover for loss of condensate.

This, of course, meant that they were under-utilised for most of the time.

Before the 1980s, power stations generally took their water from municipal supplies but increasing costs and the desire to have independent strategic supplies meant that, many stations adopted their own private water supplies.

This led to the installation of a number of schemes using reverse osmosis.

The process had been developed during the 1960s and 1970s for desalination that is producing drinking water from high TDS brackish and sea water.

Operating pressures were high and the membranes costly.

By the mid 1980s, membrane prices and operating pressures were both falling, and reverse osmosis had become competitive with ion exchange as a first stage demineralisation process at raw water TDS concentrations of about 400mg/l, with the advantage that the process also removed organic matter.

It also produced permeate that could be fed directly to a mixed bed unit, provided that the mixed bed was suitably designed which was not always the case.

Demineralised water from a two stage cation-anion exchange system typically has a pH of about 8.5 and contains about 0.1mg/l of sodium and 0.01mg/l of silica.

The anion load to the mixed bed polishing unit is, therefore, much lower than the cation load, which means that the pH at the exhaustion front of the resin bed is alkaline and, as was noted above, silicate ions remain in solution.

By comparison, reverse osmosis permeate typically contains sodium and chloride ions plus silica and dissolved carbon dioxide.

The anion load to the polishing mixed bed is higher than the cation load and, unless a significant excess of anion exchange resin is provided in the mix, the pH at the exhaustion front is acidic resulting in the potential for precipitation of insoluble, colloidal silica.

More recently the move to combined cycle gas turbine power stations has led to a reduction in boiler pressures to, typically, around 60 – 80bar, and the steam rates are considerably lower than those of traditional steam turbine stations.

Consequently the water treatment plants have become smaller and, instead of custom designed systems, most CCGT stations use “packaged plant”.

These are standard products and, although cheaper in capital cost than bespoke units, may not always have the design features required to meet power station standards.

Many standard two-bed deionisation plants, for example, have similar volumes of cation and anion exchange resin.

Depending on the water analysis this may mean that the anion resin bed becomes exhausted before the cation, and this can result in increased silica leakage at the end of the run, placing a high load onto the polishing mixed bed and putting steam purity at risk.

Privatisation of power generation has meant a much greater awareness of economic factors, notably the need to maximise utilisation of assets.

One result of this is that make-up water treatment plants are generally designed to just meet the normal operating demand of the station.

Consequently they may not be adequate during periods of high demand such as commissioning and steam blowing.

But this is not a criticism.

In fact it has been instrumental in one of the biggest changes in the approach to water treatment over the last twenty years: mobile plant.

The periods of high demand are normally planned, predictable and of short duration.

Hiring in a temporary, trailer-mounted, mobile demineralisation plant is a very cost effective solution for these occasions.

When Siemens Power Generation was commissioning the new E.ON 44MW biomass fired power station in Lockerbie the installed make-up water treatment plant did not have sufficient capacity for steam blowing the SST-800 steam turbine so, in August Elga Process Water supplied Aquamove MODI trailers to provide the additional make-up water needed.

Pre-commission flushing of the Air Cooled Condensers (ACC) was also supported by a novel Elga Process Water MOFI mobile filtration and polishing ion exchange plant.

Ordinarily, ACCs are “flushed to drain” for up to 24 hours, generating huge effluent volumes and wasting vast amounts of irrecoverable energy, before operating in a recycle mode.

The novel Elga approach using a closed-loop filtration and ion exchange polishing system at full flow (140 m3/h at 85C), provided very substantial energy and effluent disposal cost savings in addition to overcoming other logistical issues.

Temporary treatment plant can be rented to cover planned maintenance of make-up water treatment systems, for example resin or membrane changes, and can also be used to handle short term variations in raw water quality.

Such changes are becoming more frequent in mains water supplies as water suppliers switch between sources in order to meet their own distribution needs.

Typically this might mean a change from a surface water source with low TDS to a hard and alkaline groundwater source.

This can, obviously, have major repercussions on the capacity of an ion exchange plant.

A temporary reverse osmosis or ion exchange plant can restore that capacity and, once the raw water quality is back to normal, the mobile unit is returned to the supplier, making it a highly cost-effective option.

Elga Process Water has a range of Aquamove MORO mobile reverse osmosis units and MODI ion exchange systems.

These mobile plants are self contained units installed in standard 40′ insulated trailers with heating, lighting and all necessary safety equipment.

All that is required on site is a connection to the water supply.

The ion exchange units are not regenerated on site: once they are exhausted, the complete trailer is returned to the central regeneration facility, which means zero discharge on site and no problems of handling or disposal of regenerant chemicals.

Pre-treatment, if required, is usually provided in a separate MOFI pressure vessels that can be can be charged with a wide variety of filtration, ion exchange, adsorption or conditioning media.

The simplicity of the concept makes it very versatile, with process options to suit almost any application.

The water purification industry has responded to the changing demands of power station steam raising plant over the last fifty years by developing new technologies and new ways of applying existing technologies.

Capital cost is, currently, one of the main factors affecting the choice of equipment, but cost effective solutions depend on engineering expertise and an in-depth understanding of both the water quality requirements and the performance of water purification technologies under all conditions if generation efficiency is not to be compromised.

Elga Process Water brings together the expertise of three companies – Wm Boby, Permutit and Dewplan – which have served the modern power generation industry for over half a century and which, between them, were responsible for many of the advances in ion exchange technology including air hold down and split flow regeneration, HiPol and SCION short cycle.

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