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LONDON: As well as being extremely costly proposed carbon capture technology could more than double water consumption by conventional coal-fired power plants, adding to reasons for an urgent demonstration program.
Carbon capture and storage (CCS) technology is central to cutting carbon emissions from fossil fuel power plants as the only known way to slash these from gas and coal combustion.
But no commercial-scale demonstration plant exists in the power generation sector, and just one is under construction at Boundary Dam Power Station in Saskatchewan, Canada.
A major problem is the extra capital cost of trapping greenhouse gas emissions, normally vented into the atmosphere, and piping it underground, estimated at about $ 1.5 billion for a medium-sized coal power plant.
There are also public acceptance concerns regarding the possibility that the stored CO2 may leak and suffocate people above ground, a risk often discounted by experts. Another potential deal-breaker, and far less discussed, is a so-called water penalty, which is particularly relevant in water-stressed India and China where most new coal plants will be built in the coming decades.
Concerns about water availability are growing worldwide in response to rising populations, more frequent heatwaves and ground-water depletion.
Ways to mitigate the problem include a parallel support for wind and solar power, which have negligible water consumption.
In thermal power generation, a fuel source such as gas, coal or a nuclear reactor is used to boil steam and drive a turbine-generator.
Typically, steam exhaust from the turbine is condensed and recycled back to the boiler, repeating the process.
That condensation requires significant cooling water, given that more efficient power generation depends on a cooler condensate.
A 500 megawatt coal-fired power plant uses more than 12 million gallons of water per hour, according to U.S. Department of Energy data.
There is an important distinction between water withdrawal and consumption: withdrawal refers to how much water is diverted for example from a river or the sea, while consumption refers to whether that is then made permanently unavailable for example through evaporation, or else returned to its original source.
The two main types of cooling systems are once-through and recirculating.
Once-through cooling pulls in water, to cool the steam exhaust, and then returns it to the river or sea.
Instead of releasing the water back into the environment, wet recirculating systems pump it to a cooling tower or pond where some evaporates and the rest is condensed and recycled.
They have higher consumption and lower withdrawal because of this evaporation and recycling.
At present US power generation is split equally between the two but recirculating systems will increasingly dominate because of the 1972 Clean Water Act, with the aim of protecting aquatic creatures sucked into power plants during water withdrawal, a process which does them no good at all.
CCS adds to water consumption from coal combustion, according to the technology of the original power plant.
In the case of conventional pulverized coal power plants, the CCS process (called post-combustion) uses solvents to absorb the CO2 from the power plant exhaust gases and significantly raises water consumption in two ways.
First, the process requires additional water to cool the power plant exhaust gases to below 40 degrees Celsius, as well as to cool the solvent, and then the concentrated CO2 prior to compression and dehydration.
"The CDR (carbon dioxide recovery) facility requires a significant amount of cooling water for flue gas cooling, water wash cooling, absorber intercooling, reflux condenser duty, reclaimer cooling, the lean solvent cooler, and CO2 compression interstage cooling," reported a US National Energy Technology Laboratory (NETL) study, published in 2010, "Cost and Performance Baseline for Fossil Energy Plants."
Second, the CCS process itself is energy hungry, sapping around a quarter of the power plant output, called "parasitic load" because it cuts net electricity available to the grid.
That raises water consumption per unit of exported power.
Energy is consumed, for example, to strip the CO2 from the solvent by applying heat, to compress the CO2, and then to pump it underground.
In the case of advanced, integrated gasification combined cycle (IGCC) plants, the impact of adding CCS on water consumption is far less.
Higher water demand comes from its reaction with carbon monoxide to produce a hydrogen-rich syngas plus CO2, in a so-called water gas shift reactor, and secondly to cool the power plant gases prior to trapping the CO2.
A NETL study in 2008 estimated that CCS more than doubled the water consumption and withdrawal of conventional coal power plants, but had a far smaller impact on IGCC.
The trouble is that IGCC is still only an emerging technology which is far more expensive than conventional coal.
There are several ways to mitigate the water penalty.
First, carbon injection has long been used to prolong the production of oilfields, by displacing hydrocarbons.
But they also produce water, given an average water-to-oil ratio of 9.5 in 2002 in such operations, according to NETL data.
Such CO2 has historically been derived from purification of natural gas or coal gasification, but could equally come from CCS with power generation.
Water production from such displacement could even exceed consumption by the coal power plant, but there are question marks over its usability and in particular salt water from saline formations.
Another approach is to increase the efficiency of CCS, reducing the sacrificed power generation, focusing on CO2 compression designs and improved solvents.
A third option is to use dry cooling systems instead of wet recirculation, where the exhaust steam is cooled by air rather than water, but such systems are three times more costly.
A fourth is to replace the sacrificed power generation with wind or solar power, which has negligible water consumption.
CCS is in the unusual position of being both critical and undesirable, given its role in avoiding serious climate change, barring miracle geoengineering cures, and cost.
Like its high costs, its water penalty demand an urgent global demonstration program which is still absent.
— The author is a Reuters market analyst. The views expressed are his own.