Boiler feed water is a power-plant efficiency problem

Ask a plant team what limits thermal efficiency and the answers will usually be about combustion, turbine condition or condenser vacuum. Water quality appears further down the list, if at all — which is odd, because it degrades all three.

The mechanism is not complicated. Raw water contains hardness, silica and dissolved ions. In a boiler these deposit on the tubes. Deposits reduce heat transfer. Reduced heat transfer means more fuel is burned for the same steam output, equipment corrodes, and maintenance cost rises. Every one of those consequences is permanent until the water is fixed.

Where high-quality water is actually needed

A thermal plant has several distinct water requirements, and conflating them leads to either over-treating or under-protecting:

• Boiler feed water — the steam generation system: boiler, steam drum, economiser and feedwater pump. Quality here is critical for stable operation.
• Demineralised water — the steam cycle needs water with very low electrical conductivity; dissolved ions damage steam equipment.
• Condensate — returning condensate may carry metallic impurities, corrosion products or cycle contamination that reduce steam quality and equipment life.
• Cooling water — poor quality causes scaling, corrosion and fouling in heat exchangers and condensers; a degraded condenser directly reduces plant efficiency.

Large plants relying on the Rankine cycle need high-quality condensate specifically to maintain cycle efficiency, which is why condensate polishing is not an optional refinement on a plant of any size.

The treatment train

Producing demineralised water is one of the main processes in a steam plant, and it typically combines reverse osmosis, ion exchange units and a mixed bed polisher with storage. The question that follows is how the final polishing stage should be done.

EDI or mixed bed: a real comparison

Both reach high purity. They differ in how they get there, and the differences compound over the life of the plant.

The capital comparison favours mixed bed and the lifetime comparison favours EDI, which is the usual shape of this kind of decision. What often settles it is neither number: EDI produces ultrapure water through a continuous, chemical-free process, so there is no hazardous regeneration waste to handle and no acid and alkali storage to site, permit and maintain.

On a plant where chemical handling is already a compliance burden, removing a regeneration step is worth more than the operating-cost differential suggests.

The two effluents nobody plans for

Two streams in a thermal plant regularly arrive as surprises because conventional treatment does not handle them.

FGD wastewater. Flue gas desulfurization removes SOx from the exhaust, and produces a complex effluent carrying sulfate, chloride, heavy metals and high TDS. Coal-fired and some industrial plants face this stream, and ordinary treatment is not sufficient for it.

Ash-handling water. Wet ash handling and slurry transport generate streams with high suspended solids and inorganic contaminants that need managing both to reduce water consumption and to prevent environmental contamination.

Both are candidates for advanced treatment, and where discharge is restricted or the site is water-stressed, both are candidates for ZLD.

The recovery opportunity

Cooling towers are among the largest water consumers in a thermal plant. To prevent TDS building up, part of the circulating water is discharged as blowdown — a stream with high dissolved solids that represents a substantial volume of lost water.

Recovering blowdown reduces both fresh water intake and effluent disposal cost, and it is frequently the most straightforward water-saving project available on an operating plant. MLD, ZLD and membrane treatment all have a role depending on the recovery target.

Why this is an energy conversation

The reason to treat water quality as an efficiency topic rather than a utilities topic is that its effects are cumulative and invisible. Scale does not announce itself; it degrades heat transfer gradually, and the additional fuel is burned quietly every hour. By the time it is diagnosed as a water problem, it has usually been paid for several times over.