What does a coal-fired power plant produce? Well, electricity, of course, but it could also supply clean water. And so could many other types of factory. Technology developed by the EU-funded CapWa project extracts water from flue gas - more, in some cases, than was initially brought in. It also saves energy.
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Considering how many industrial processes use water that is eventually released as vapour, this advance could help whole sectors of the economy to avoid precious resources going up in smoke.
For the average coal-fired power plant, for example, the technology can extract more water than the plant can reuse, turning it from a consumer into a producer. This is due the fact that the plant ‘exhales’ a substantial amount of vapour.
“There is a lot of water in flue gas,” says project coordinator Ludwin Daal of DNV GL – Energy, formerly known as KEMA. “For a typical coal-fired power plant of 400 MW, there is about 150 cubic metres of water coming out per hour, and you only need about 20 % of that to make the plant self-sufficient.”
Recovering water from flue gas also helps to save energy, as it dispenses with the need to heat the gas to reduce corrosion in the stack. And in some cases, the de-watered warm air can actually be reinjected into the process.
These energy savings are, in fact, one of the main reasons why the technology quickly pays for itself. A paper production plant, says Daal, could expect return on investment within a year or two.
Waste not, want not
It’s an attractive prospect. If Europe’s power plants and paper factories combined were to adopt this technology, for example, they could provide water for two million households per year and save energy worth the annual consumption of three million homes, according to CapWa’s estimates.
Advances such as these do not materialise out of thin air. CapWa involved partners from Europe and beyond, bringing together a wide range of complementary skills towards a shared objective. Together, they upgraded and refined technology conceived several years earlier by lead partner DNV GL – Energy.
This teamwork, a cornerstone of EU-funded research, is truly unique, says Daal. Other parts of the world, he notes, “don’t have this approach of different ideas, different cultures all brought together and working as one”. Without it, he adds, these outcomes could not have been achieved.
Vapour abhors a vacuum
The resulting application is based on innovative hollow fibre membranes with a selective outer layer. Arrays of these tubes are placed into a humid gas stream. A vacuum maintained inside the tubes enables the arrays to suck water out the gas: the difference in pressure draws the molecules composing the air stream towards the vacuum, but the membranes’ selective coating will only let water molecules through.
The result is surprisingly clean water, extracted in a single step. This can be used for a variety of purposes, notably as high-purity boiler feed water.
The next stage in the deployment of this technology, says Daal, would be application in two or three large-scale demonstration sites, followed by commercialisation through a systems integrator. There is considerable interest from water-scarce areas around the world, notably in Africa, the Americas, Australia and China. But they are not the only ones who might benefit: given the potential of this elegant solution, clients in water-rich areas are also likely to take advantage of the technology.