The large enterprises that mine coal and drill for oil and gas face a world
increasingly determined to use less of their products. The Climate Change
Convention, agreed at the Earth Summit last year, requires rich nations to
halt their rising emissions of carbon dioxide to end the threat of the
greenhouse effect. That means burning less fossil fuel, using energy more
efficiently and switching to electricity generation from ‘renewable’ sources
such as wind and solar power.
But energy conglomerates such as Exxon, Shell and British Coal think the
choice is not so stark. They believe that much of the 5 billion tonnes of
carbon dioxide gas emitted into the air every year as their products burn
could be captured, and then kept out of harm’s way – underground, on the
bottom of the ocean, in giant spheres of dry ice, or soaked up by living
organisms in the oceans or on land. Anything, in fact, but being left to
accumulate in the atmosphere.
A modestly-sized power station, generating around 500 megawatts, produces
about 500 tonnes of carbon, in the form of carbon dioxide, each hour, or 120
million tonnes in its 30-year lifetime. Technologies do exist that can
remove the gas from the flue emissions and store it under pressure or in
liquid form, though they consume about a fifth of the plant’s electricity. A
bigger problem is where to put the waste. Even when liquefied, it takes up
more space than the carbon fuel from which it comes – hence the industry’s
interest in solutions to bury it; dump it; freeze it; and encourage life
forms that will absorb it.
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Using these techniques, the oil and gas giants are trying to invent a whole
new waste-disposal industry that could eventually be as big as the business
of energy generation itself. They call it ‘carbon sequestration’.
The European Commission has already begun an international research project
to assess the storage capacity for waste carbon dioxide in holes in the
ground in member countries. ‘I’m pretty sure it’s feasible,’ says Sam
Holloway of the British Geological Survey. ‘We are looking for large natural
stores, including sealed traps such as natural gas fields.’
Beneath Britain, the prospects are not encouraging. The biggest single
onshore well, at Wytch Farm in Dorset, could take a mere seven years’ supply
from a 500-megawatt power station – too little, Holloway says, to make much
economic sense.
There is more space beneath the North Sea, where the oil and gas fields
could hold 4750 million tonnes of carbon, representing several decades’
storage of the entire national output of the gas. So might existing
production platforms one day pump the gas back into the depths? Some oil
companies are contemplating doing this anyway – to flush out residues of
oil. ‘This might offset some of the costs of the pipelines to deliver the
gas to the fields,’ says Holloway.
Another possible store around Britain’s shores is in porous water-bearing
underground rocks known as aquifers. Many, such as the chalk, limestone and
sandstone hills of lowland England, are unavailable because they are already
used to provide fresh drinking water. But offshore, where most aquifers are
unused and contain only saline water, Holloway says the potential is ‘truly
vast’ – perhaps 10 times that of the oil fields.
Under pressure
Other European countries have also investigated the prospects. Thomas Krom
of the environmental engineering division of the Danish company,
Elsamprojekt, says three saltwater aquifers beneath Jutland and the island
of Funen could store 30 years’ output of Danish carbon dioxide at a cost, he
estimates, of $14.5 per tonne. The Danes have thought more than
most about the possible hazards of storing so much carbon dioxide at
pressure underground. ‘We would need to monitor the facility for an
extremely long time. It would be like a nuclear waste facility,’ says Krom.
‘Compressed carbon dioxide is denser than air and any catastrophic leakage
contains a serious danger for the local inhabitants.’
The gas is not toxic, but if it emerged from below ground it would be denser
than air and could form a thick blanket of gas, suffocating every living
thing in its path. The effect, Krom suggests, could resemble the aftermath
of the spontaneous eruption of carbon dioxide trapped in the depths of the
volcanic Lake Nyos, in the Cameroon highlands in August 1986. There, the gas
rolled over the surrounding hills and suffocated 1750 people and an
uncounted number of animals.
An imaginative alternative to deep burial beneath the ground is to hold the
carbon dioxide on the surface, as giant insulated balls of dry ice – the
solid form of carbon dioxide that forms below -78.5 °C. This idea is
the brainchild of Walter Seifritz of the Institute of Energy Economics and
the Rational Use of Energy at the University of Stuttgart. He envisages
spheres of dry ice 400 metres across – higher than the Eiffel Tower –
dotted across the German landscape, each containing around 50 million tonnes
of carbon dioxide. ‘If 10 per cent of Germany’s carbon dioxide was disposed
of in this way,’ he says, ‘you would have to build two of these spheres
each year.’ Such outsize landmarks might be lined up in old opencast coal
mines, he suggests.
However, even with a 2-metre-thick coat of glass wool insulation, the dry
ice would gradually evaporate, releasing the carbon dioxide into the
atmosphere. Half would have evaporated in 800 years; the rest after 4000
years. But the delay could give humanity several centuries’ breathing space
in which to convert to power sources that did not produce carbon dioxide.
The disadvantage, Seifritz admits, is that ‘you’d consume roughly half the
energy produced in a power station in order to solidify the carbon
dioxide’. So Germany would need twice as many power stations, and one of
every two spheres would be a by-product of making the other. And, though the
emissions of carbon dioxide would be delayed and occur over a longer period,
ultimately the system would release twice as much gas into the air.
In these terms, it sounds crazy. But if, in the future, national governments
or energy companies were given limits to how much carbon dioxide they could
emit, or faced punitive carbon taxes for exceeding them, then accountants
could argue that postponing and spreading emissions might be worthwhile.
What the sequestration gurus want most is a cheap, efficient way of dumping
carbon dioxide in the planet’s biggest dustbin – the bottom of the ocean.
This offers almost unlimited space – provided you can be sure the gas will
never return to the surface. The oceans are already the greatest natural
sink for carbon dioxide: at their surface they absorb between 25 and 40 per
cent of all the carbon dioxide we put into the atmosphere. In all, they
contain some 40 trillion tonnes of carbon (not including sediments), a
figure that chiefly reflects the overwhelmingly presence of carbon in the
form of dissolved carbon dioxide though it does also include the
contribution of life forms. So, the argument goes, why not encourage them
to take a little more?
Journey under the sea
Iain Summerfield, of the Coal Research Establishment run by British Coal in
Gloucestershire, has drafted a map showing how he might dispose of the
carbon dioxide from a standard 500-megawatt power plant in the East
Midlands. A pipeline half a metre in diameter would take the compressed gas
300 kilo-metres across country to a terminal on the South Wales coast,
somewhere near Milford Haven, before extending out along the sea bed into
the North Atlantic. Here, it could discharge its daily load of 10 000 tonnes
of carbon dioxide into ocean more than 2000 metres deep, not far west of
Ireland. Or the pipe could extend further, until it could discharge into an
enclosed basin about 4000 metres deep near the Mid-Atlantic Ridge – a total
ocean journey of some 1100 kilometres.
‘Nobody has ever needed to lay pipelines at this depth,’ admits
Summerfield. ‘But I see no reason why it can’t be done.’ Nonetheless, at
more than £500 000 per kilometre of pipeline, it would be a
major capital investment to cope with the output of one modestly sized power
plant. Summerfield puts the total bill for separation and disposal at
between £20 and £100 for each tonne of
carbon. Costs might be kept nearer the low estimate if a pipeline network
gathered the gaseous effluent from several power plants as it crossed
Britain. Even so, the figure is at least double the Danish estimate for
disposing of the waste in aquifers.
Ever more ambitious pipeline networks are sprouting on the backs of
envelopes, and on computers in laboratories across Europe and the US. Many
are mirror images of the continent-wide pipe systems that currently
transport oil, gas and some chemical feedstocks. To fuel companies, this
makes them reassuringly familiar.
Otto Skovholt, of the research and development centre for the Norwegian
state enterprise Statoil, says; ‘If one could imagine a large-scale carbon
dioxide transportation system serving a large area – such as one country or
several countries – the costs might be brought down considerably.’ The aim,
he says, would be to make the cost of the pipeline lower than the likely
carbon emissions tax that any company releasing the gas into the atmosphere
would have to pay.
A map produced by Skovholt shows pipes meeting in southwest France, near
Bordeaux, where a tongue of the deep Atlantic penetrates the Bay of Biscay:
within 300 kilometres of the French coast the ocean is 4000 metres deep.
Other researchers have proposed European networks that meet near Gibraltar,
where every second the strong ocean current carries around 2 million tonnes
of relatively saline water out of the shallow Mediterranean Sea and down
into the Atlantic depths, below 1500 metres. In 1977 Cesare Marchetti of the
International Institute for Applied Systems Analysis in Austria first
proposed that if a stream of carbon dioxide could hitch a ride with this
current, the expensive construction of a pipeline to the ocean floor could
be avoided. Other possible ‘sinking regions’ identified by Marchetti
included the Norwegian and Red Seas. In the US, pipeline systems stretching
half across the North American continent could collect carbon dioxide at a
few points along the eastern seaboard before piping it perhaps 300
kilometres out to sea and pouring it over the edge of the continental shelf.
Such proposals usually rely on the fact that under growing pressure, carbon
dioxide becomes increasingly more dense than water. Below around 3000
metres, liquid carbon dioxide dumped into the ocean would probably form a
dense stream and sink to the ocean floor. Some think that as the liquid left
the pipe or diffuser it would form solid particles, known as hydrates, that
would be heavier than sea water and would sink like a marine snow to the
ocean bed. Hydrates might form and descend in this way at depths of as
little as 700 metres and so greatly reduce the cost of ocean disposal, says
Masao Morishita of the Tokyo Electric Power Company.
But according to Guttorm Alendal of the Nansen Environmental and Remote
Sensing Center in Norway, who has examined the likely properties of a
carbon dioxide ‘plume’ in the deep ocean, ‘there is no doubt that, around
the plume, fish would be wiped out.’ And, says Dan Golomb of the University
of Massachusetts, the water around an outlet would be acidic, with a pH of
around 3. It would also lack oxygen and be very turbulent. Golomb warns that
‘sinking hydrate particles may bury benthic (bottom-dwelling) creatures, and
may be mistaken for food by swimming predators, thus filling their guts and
causing starvation’.
But before then, the protagonists of deep-ocean disposal must answer the
most basic question: will the carbon dioxide stay down? And if so, for how
long?
Brian Flannery of the Exxon Research and Engineering Company warns that
what goes down will eventually come up. Carbon dioxide buried in the ocean
at depths greater than 1000 metres or so could stay down for hundreds, even
thousands, of years, he says, but not forever. And, as with the dry ice
option, the penalty of this system is that you typically create about 20 per
cent more carbon dioxide than you would have done if you had simply released
the gas into the sky. All this, he says, ultimately returns to the
atmosphere.
What goes down . . .
But how quickly? Where the gas is dumped into the sea at depths of less than
500 metres, it could be less than 100 years before there is more carbon
dioxide in the atmosphere than there would have been if you had simply
released the gas in the first place, says Flannery. Ultimately, even carbon
dioxide buried in the deepest ocean will surface with the water it mixes
with, bubbling up in the slow, thousand-year mixing of the ocean. ‘In nearly
all cases,’ he says, ‘after sufficient time, atmospheric emissions actually
exceed those in the base case. It is critical to decide just how much time
is enough.’
A third route to carbon dioxide disposal is to encourage the planet’s
natural sinks for carbon dioxide: its living matter, or biomass. Since
living organisms are composed of carbon compounds, as they grow they extract
more carbon either from the atmosphere or the oceans.
Unfortunately, biomass also gives up its carbon when it burns or rots. So,
in order to sequester carbon from the air in plant biomass, planners must
either harvest and store the plant material (keeping it in dry air to
minimise the risk of it rotting), or increase the total growing stock of
plants. Alternatively, they could harvest and use biomass as a substitute
for fossil fuels.
In recent years some power generators have promised to plant forests
calculated to soak up the amount of carbon equivalent to that emitted from
their power plants. But there is growing support for the idea of power
stations having their own ‘in-house’ biomass sink. One suggestion is to
grow enormous amounts of microalgae in large shallow ponds attached to power
plants, and bubble the flue gases through the ponds. The basic requirements
for growing lots of microalgae, says Lewis Brown of the National Renewable
Energy Laboratory in Colorado, are sunlight, large amounts of flat land,
water – fresh or salty – and a carbon dioxide feedstock. Deserts – either on
the coast or with saline groundwaters underneath – fit that bill, and pilot
projects have been carried out in recent years in Israel’s Negev Desert and
in the US desert states of New Mexico and Arizona.
According to Brown, an 800-megawatt power plant could recycle its carbon
dioxide through a pool of microalgae covering about 130 square kilometres.
If 30 per cent of Arizona, or around 100 000 square kilometres of land, were
given over to ponds of microalgae, it could absorb all the US’s current
carbon dioxide emissions from power stations. The large amounts of
microalgae harvested from such a system could also fuel power stations,
completing a benign carbon circle.
Less work has gone into identifying which microalgae to use, although
Japanese scientists have come up with the idea of growing chlorella, a
carbon-guzzling alga first isolated from rice paddies, and now widely grown
in Japan as a health food.
A related idea, proposed last year by Edward Glenn of the Environmental
Research Laboratory in Tucson, Arizona, involves growing large acreages of
salt-tolerant plants, or halophytes, on coastal deserts and salt flats
irrigated with sea water or salty underground water. Glenn estimates that
more than 750 million hectares of arid land – in Saudi Arabia, Lake Eyre in
Australia, the Aral Sea basin in the former Soviet Union, Pakistan’s Indus
Valley and the American desert regions – could be suitable. Suitable crops,
such as the oilseed halophyte, Salicornia bigelovii, could absorb up to 6
tonnes of carbon per hectare each year. If all their straw were then used to
replace fossil fuels in generating electricity, the system could soak up
perhaps 10 per cent of humanity’s current carbon dioxide releases.
The idea of stimulating the oceans to absorb more carbon by ‘fertilising’
marine organisms is attractive, but hard to translate into reality. The most
obvious organisms to encourage would be plankton, the ‘grass’ of the oceans
and base of the marine food chain. But nobody knows quite what limits the
growth of algae, and so how to stimulate growth.
However, the most direct and obvious route to increasing nature’s ability
to absorb carbon dioxide from the atmosphere is to plant more trees, and
conserve those we have. Forests are by far the largest land-based sinks for
carbon.
David Hall, professor of biosphere science at Kings College London, says the
top priority is to stop deforestation, which releases up to 2 billion tonnes
of carbon into the atmosphere each year. ‘After that, we need to substitute
coal in our power stations with biomass.’ Every tonne of coal burnt in power
plants adds to the planet’s load of atmospheric carbon. But every tonne of
wood burnt, provided it is replaced by new growing trees, is neutral in
greenhouse terms. The new trees, like all growing plants, absorb carbon
dioxide during the process of photosynthesis, in which they use the energy
of sunlight to manufacture nutrients.
‘If you want a cheap means of removing carbon from the atmosphere, there
is absolutely no doubt you should take the biomass route,’ says Hall.
Estimates of the cost of sequestering carbon in this way begin at Dollars
5 per tonne, considerably less than piping it to the ocean bed or down oil
wells.
One constraint, as with other biomass alternatives, could be a shortage of
land. It takes one hectare of forest land to absorb five tonnes of carbon a
year. Densely populated countries, such as Germany and the Netherlands,
could not grow enough trees to absorb all their present carbon dioxide
emissions, even if every existing hectare was set aside for the purpose.
Rooted in trees
Yet the countries with the worst records of carbon dioxide pollution, such
as the US, Canada and parts of Eastern Europe, do have the land. The US,
despite being responsible for more than one-fifth of the world’s carbon
dioxide emissions, could absorb every last tonne if one-third of its land
were forested for carbon sequestration.
Donald Rosenthal of the US Department of Energy says tree planting could be
the cornerstone of a future US strategy to stabilise the nation’s emissions
of carbon dioxide at 1990 levels to at least the year 2030 at a cost to the
economy of only one-fifth as much as other measures. Tree planting within
US borders could offset up to a quarter of US carbon emissions in coming
decades, says Kenneth Richards of the US Department of Agriculture. Trees,
says Rosenthal, ‘allow the US energy policy to go on much as before, with
business as usual out to 2015′.
Few scientists involved in the greenhouse debate believe that long-term
stabilisation of emissions is enough. Certainly for the US, the world’s
largest per-capita source of carbon dioxide, a ‘business-as-usual’ approach
to energy hardly seems adequate. A 60 per cent cut in global emissions is
necessary to stabilise the amount of greenhouse gases in the atmosphere at
current levels. But Rosenthal’s calculations do show the potential
importance of trees to any strategy. Tree planting, more than anything else,
is likely to bring about a meeting of minds between the ‘mega-tech’
engineers and environmentalists.