While we wait to see if Copenhagen paves the way towards a long term fall in global carbon dioxide emissions, the short-term reality is that emissions of the pollutant will continue to rise.

So what do we do with such huge quantities of what is essentially a waste product?

The answer in most cases in recent history - think rubbish or nuclear - has been to bury it underground in landfill sites. The same theory is being suggested for carbon. 



With fossil fuel-burning power stations continuing to be built rather than closed down there has been focus on technologies that capture and store the carbon pollution produced. The resulting carbon is then compressed to a liquid so that it takes up less space, and then potentially buried deep underground.

As Robert Kunzig and Wallace Broecker point out in their book, Fixing Climate, Carbon Capture and Storage would mean landfill on a ‘stupifying’ scale:

‘If the twenty-nine gigatons produced by the world’s fossil-fuel burning in a single year were liquefied and spread over Manhatten, they would bury the island to about the eighty-fifth floor of the Empire State Building.’

Frank Zeman, from the Department of Earth and Environmental Engineering at Columbia University says we have not yet even started to address the huge issues involved with disposing of carbon.

‘We already have a huge waste problem: imagine what it is going to be like with CO2 as well. It’ll be nimbyism.’



Carbon resource

But some experts are now starting to ask: what if we used the carbon as a resource instead of disposing of it?

As far-fetched as it may sound there are already a number of experimental technologies that are making use of carbon either by absorbing it from the air or taking it in concentrated form from industry or power plants.


They may not all be carbon negative but they do at least make more use out of the carbon that we are producing through human activities.

Algae

Through the process of photosynthesis algae is able to use sunlight to absorb carbon dioxide and provide an end product – oil or feedstock.

Unlike agricultural crops grown for fuel, algae do not need good quality agricultural land: they can be grown anywhere and do not require as much water or even clean water.

In the US, the University of Minnesota has been demonstrating the technology using waste water from sewage works. The benefit is two-fold as municipal waste treatment plants often burn the waste sludge and left alone it will also emit carbon dioxide.

Although there is no reason why algae production could not be located next to any fossil-fuel burning power station, the best location may be in tropical countries.

‘[Algae's] water and land needs, combined with the fact that tropical climates bring the best yields, all point to developing nations,’ says Ben Graziano, research and development manager for the UK Carbon Trust’s Algae Biofuels Challenge.

The Carbon Trust’s project aims to develop low carbon microalgae biofuel technology to a commercial scale by 2020. There are also ongoing projects looking at using the oil produced by algae to make biodegradable plastics. 


‘In the short term the whole world sees carbon as a waste but we see that there are uses for carbon and opportunities to displace other fuels and reduce overall emissions,’ says Graziano.

‘Algae could absorb CO2 and displace fossil fuels – giving twice the energy for your emissions.’

Demonstration scale

Current yields for algae are already better than other biofuels: 10-15,000 litres of oil per hectare per year against 5-6,000 litres for palm oil plantations.

However, production is still only at demonstration scales of 10-20 hectares and needs to reach 1,000 hectares to become commercially viable. It is this scaling up that poses a challenge. Biological production is at risk from infection and maintaining the purity of the algae cultures is critical.

Despite the difficulties, Professor Roger Ruan, director of the Centre for Biorefining at the University of Minnesota, still expects commercial scale production within five years.

The Carbon Trust plans to have a pilot project up and running in a less-industrialised country by 2015.



The academic largely credited with designing the Kyoto protocol, Graciela Chichilnisky, believes carbon negative technology could be a financial coup for less-industrialised countries.

‘We need negative carbon technologies to bring most of the Clean Development Mechanism (CDM) investment to the regions that need it most: the low emitters in Africa, Latin America and the Small Island States that need energy to grow and to adapt and mitigate the catastrophic risks of climate change,’ says Chichilnisky.



Green cement

Cement production accounts for 5 per cent of greenhouse gas emissions and 50 per cent of the carbon emitted comes from converting limestone into cement.

So-called 'green cement' uses non-carbonate materials that cut out most of the CO2 emitted during production. It also absorbs CO2 during production to help make it carbon negative.

In the UK, researchers at Imperial College London have developed their own form of green cement and an independent company, Novacem Ltd, is now bringing it to commercial scale. They use a magnesium oxide composition in place of the limestone used in conventional cement. The final product is also recyclable, unlike conventional cement.

Novacem researchers estimate that for every tonne of ordinary Portland cement replaced by their green cement, around 0.75 tonnes of CO2 could be captured and stored indefinitely in construction products.

Although green cement plants could be located next to conventional cement plants and be potential absorbers of carbon dioxide, Novacem chairman Stuart Evans says the main benefit is likely to be in displacing carbon, rather than using it as a 'resource'.

Novacem expects to have an industrial scale plant up and running by 2011, and for the cement to be mainstream by 2014-15.

Hydrocarbons

Perhaps the most significant method for turning carbon into a resource is in the production of hydrocarbon fuels. You start with carbon dioxide, react it with hydrogen and make a fuel that can power existing vehicles.

Dr Frank Zeman, of Columbia University, co-authored a report on the potential for hydrocarbons made from captured and stored carbon dioxide.

‘We have shown that it is not technically difficult, the only challenge is the cost because at present it is cheaper to make cars more efficient than switching to hydrocarbons,’ says Zeman.

The big hope for a renewable hydrocarbons sector is the ability to use solar energy to split water (H2O) to make hydrogen.

‘The million-dollar question is solar-pv [photovoltaic] hydrogen. It’s too expensive at the moment but it is the panacea. This kind of closed loop system is the future if we are ever to reach zero carbon emissions,’ says Zeman.


Resource or waste?

With all carbon technologies there are questions about whether they are carbon negative (taking carbon out of the atmosphere) or simply carbon reducing, for example using pressurised CO2 to extract more oil which is then burned as fuel.

This doubt leaves some sceptical of whether carbon can ever become a true resource.

‘You can call it a resource, but it is one which we will have in abundance so I still see it as something we need to get rid of,’ says Professor John Shepherd, who chaired the Royal Society's recent study into the prospects for geoengineering the climate.

‘If we manage to capture as much as we could it’s hard to imagine us ever using up all the CO2. It is good to try and find a use for carbon. If you can use it to make a fuel through algae or hydrocarbons then great but equally I’d be happy to see it pumped into the ground,’ says Shepherd.

However, Dr Zeman, from Columbia University, says while fossil carbon will always be a pollutant, active carbon (already in the atmosphere) could be a resource if we can find uses for it through hydrocarbons, algae and the like.

‘We can’t permanently inject liquid carbon into the ground. Negative carbon technologies are a short-term solution that will help us get to a sustainable situation,’ says Zeman.

Useful links
Royal Society report on geoengineering
Carbon neutral hydrocarbons study
Carbon Trust's Algae Biofuels Challenge
The second green revolution? Plant-based biodegradable plastics
Novacem Ltd

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