Carbon reuse

There are only two ways to offset the effect of carbon emitted by burning fossil fuels. Either the carbon must be locked away so it can't enter the biosphere, ideally for as long as the coal, oil and gas were kept separate since the Carboniferous Era: this is likely to be expensive in both energy and money and offers no obvious way to recoup these costs. The alternative is to find some use for the combustion products that avoids the need to extract more fossil fuels and, ideally, covers the cost of capturing and processing the carbon.

A major requirement of carbon reuse is that it must recycle very large amounts of carbon, 27 billion tonnes of carbon dioxide are emitted annually (2010 figures) and combined recycling should process at least this amount if it is to be worthwhile.

These uses as well as other ways of reducing carbon emissions are starting to appear.

New industrial processes

Cement manufacture

Traditional cement manufacture consists of roasting limestone at 1400°C, which drives off carbon dioxide to leave calcium oxide which, when mixed with water and sand or crushed rock, forms concrete This process emits 800kg of carbon dioxide for every 1000kg of cement produced.

Instead, 'green' serpentine cement is made by heating magnesium silicate to 700°C to produce magnesium oxide. This is used to make concrete in the same way that traditional cement is used, except that the carbonated water is used and the resulting concrete is bound with a magnesium carbonate matrix rather than the traditional calcium hydroxide binder. Overall, producing serpentine cement and using it to make concrete absorbs 50kg of carbon dioxide for every 1000kg of cement produced.

Halving the temperature needed to make the cement represents a huge energy saving while the change of chemistry avoids the emission of 850kg of carbon dioxide per tonne of cement.

Fuel use reduction

Carbon dioxide enrichment to boost plant growth

Dutch greenhouse operators have discovered that enriching the greenhouse air with carbon dioxide can make plants grow up to 30% faster. Formerly this was generated by burning natural gas to generate carbon dioxide, but now, in the Rotterdam area, it is extracted from the flue gasses of a local oil refinery and piped to the greenhouses, replacing 400,000 tonnes of carbon dioxide from burning natural gas. Everybody wins: the refinery makes a profit from this carbon capture while still managing to sell the gas to the greenhouse operators for less than they formerly paid for natural gas.

Other research projects are looking at using the carbon dioxide from flue gasses to increase the growth rate in algal biodiesel production. Cellana has a pilot plant at Kona on Hawaii's Big Island. The intention is to use 60% of the carbon dioxide in the flue gas from the 215MW Maalaea diesel generator to boost biodiesel production. The biodiesel will be used to replace imported diesel oil burnt in the plant.

Carbon recycling

Methane production

RCO2 is developing Catalytic CO2 Recycle (CCR) technology. This passes hot power plant flue gas, which contains water vapour and carbon dioxide over a catalyst. This converts 20% of the carbon dioxide into methane, using the waste heat in the flue gas to drive the reaction. They hope that an improved catalyst will allow 50-55% of the carbon dioxide to be recycled.