Isra-Mart srl : Emission control: Turning carbon trash into treasure

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IT'S LIKE standing at the edge of a giant patchwork quilt. Stretching into the distance are broad bands of bright yellow alternated with patches of delicate white, all beneath a vast glass roof. This greenhouse full of flowers is just one of hundreds that dot the Dutch coast, where row after row of chrysanthemums, orchids and roses are fed carbon dioxide-enriched air, helping them to grow up to 30 per cent faster than normal.

While plenty of commercial greenhouses top up their air with extra CO2, what is unusual about this one is where its CO2 comes from. Until a few years ago, the greenhouse's operators used to burn natural gas for the sole purpose of generating CO2. Today it is piped from a nearby oil refinery. Each year, 400,000 tonnes of CO2 are captured and then piped to around 500 greenhouses between Rotterdam and The Hague, where it is absorbed by the growing plants before they are shipped for sale around the world (see "Cash for carbon").

As governments ramp up their efforts to cut carbon emissions, carbon capture is moving closer to the top of the agenda. The current plan to deal with all of our excess CO2 is to just pump the stuff underground - a kind of landfill for gases. Looking at this carpet of flowers, it is hard not to think that we are going about this in the wrong way. Shouldn't we look to pioneering schemes like the Dutch greenhouses to find ways to recycle the captured CO2 instead?

It turns out that a growing number of researchers, start-ups and even industry giants are also beginning to think like this. And not just for growing flowers; they believe whole cities could one day be built and powered with the help of exhaust fumes.

"It's time we stopped thinking of CO2 solely as a pollutant and viewed it as a valuable resource," says Gabriele Centi, a chemist at the University of Messina, Italy. "With carbon capture and sequestration, we'll essentially have a zero-cost feedstock."

There is certainly no shortage of CO2 to be captured and used - around 27 billion tonnes are released each year through human activity. For would-be recyclers, though, the sheer scale is part of the problem. To make any sort of dent in the amount of gas that would otherwise be sequestered underground, it would need to be recycled into something that we use a lot of. Luckily there is one area where CO2 may soon be used in huge quantities: cement manufacture.

Not much can match the output - or the carbon footprint - of the cement industry, which produces around 3 billion tonnes of Portland cement every year. To cook up a batch, manufacturers roast calcium carbonate (limestone) at around 1400 °C, which breaks down the carbonate to release CO2 (see diagram). What is left is calcium oxide, the main ingredient in cement. To reach these blistering temperatures, however, fossil fuels must be burned, releasing even more CO2. This all adds up: in total, cement production accounts for roughly 5 per cent of all human CO2 emissions - more than the airline industry.

But perhaps not for much longer. Several companies have proposed turning cement-making on its head, so that it captures more CO2 than it generates. A California-based firm called Calera says it has perfected a process to "mineralise" CO2 by bubbling gas from power plant chimneys through a calcium-rich mixture of sea water and fly ash, a by-product of coal-fired power plants. The end product is a mixture of calcium minerals that can be processed into cement. The idea should work, says Ken Caldeira at the Carnegie Institution for Science in Stanford, California, but whether it will work on an industrial scale remains to be seen.

A better approach might be to rip up the recipe book entirely and start again from scratch. One such idea is to replace the calcium oxide of traditional cement with magnesium oxide. The secret ingredient? A splash of fizzy water.
Break out the bubbly

The recipe starts with a lump of rock called serpentine, which contains magnesium silicate and, crucially, no carbon. Just like in traditional cement manufacture, this is roasted, only this time no CO2 is released from the rock. Instead, magnesium oxide is produced. That's not the only CO2 saving: temperatures of just 700 °C are needed, so less CO2 is released from burning fossil fuels.

The real trick comes in the last step, however. Here, the magnesium oxide is mixed with water to make cement. But by using water carbonated with captured CO2, the greenhouse gas can become trapped in the cement in the form of magnesium carbonate (see diagram).

"We're developing cement with the same properties and at the same cost as existing cement, but instead of emitting CO2 our cement actually absorbs it," says Nikolaos Vlasopoulos, chief scientist for Novacem, a London-based company developing magnesium oxide cement. Its cement can absorb as much as 50 kilograms of CO2 per tonne of cement, the company claims, compared with the 700 to 900 kilograms of CO2 released during normal cement production (see diagram).

The idea sounds viable, says Caldeira, although it might be a while before the construction industry dares to use the new cement to build a bridge or a skyscraper. A stepping stone towards such a use will soon been made, however. Novacem has joined up with the largest privately owned UK construction firm, Laing O'Rourke, to make masonry blocks, a building material that makes up 15 per cent of the UK's cement use but has relatively low performance requirements. If the new cement shows itself to be a strong and safe construction material, it could be adopted for a wider range of uses.

Still, even cement can only consume a small amount of the CO2 we release each year. There is only one way to reuse the majority of it, and that is to turn it back into fuel, says Michele Aresta, director of Italy's National Consortium on Catalysis. "If we could produce fuels at the same rate as we burn them, that would be the real solution."

That simple statement raises a big challenge, however. Carbon dioxide is carbon with all the energy wrung out; the stuff left over after burning a fossil fuel. Turning it back into something useful is an energy-intensive process, and if that energy comes from burning fossil fuels then the net reduction of CO2 is negligible, if any.

But what if everything needed to solve the problem is already in power plant flue gas? That is what Norwegian company RCO2, based in Langhus, believes. It is currently testing a process to pass the hot gas, which contains waste heat, water vapour and CO2, over a novel catalyst. This splits the water molecules to release hydrogen, which then combine with the CO2 to form methane - otherwise known as natural gas. "We don't need any other outside energy source, all we need is the heat that is left in the waste gas," says Erik Fareid, technical director at RCO2.

The firm won't say what the catalyst is but claims it can convert 20 per cent of a power plant's waste CO2 into methane. The company is now working on an improved version that would allow them to recycle between 50 and 55 per cent of a plant's CO2, Fareid says.

That still leaves a lot of spare CO2. To convert yet more into fuel would require extra energy, and in order to do that without burning fossil fuels, researchers are looking to nature for inspiration.

Plants are the obvious place to look, as they recycle CO2 into sugary fuel using just water and energy from the sun. Yet despite millions of years of evolution, they are still relatively inefficient at this process - less than 1 per cent of the sunlight that hits the typical agricultural plant over the course of a year is captured to make biomass (New Scientist, 11 September, p 40). Instead, researchers are concentrating their efforts on a more promising group of organisms: algae.