麻豆传媒

Carbon dioxide may be bad for the climate, but it鈥檚 good for the roses. Perhaps it鈥檚 time we rehabilitated this gaseous villain

IT鈥橲 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鈥檚 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 鈥淐ash for carbon鈥�).

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 鈥� 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鈥檛 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.

鈥淚t鈥檚 time we stopped thinking of CO₂ solely as a pollutant and viewed it as a valuable resource,鈥� says Gabriele Centi, a chemist at the University of Messina, Italy. 鈥淲ith carbon capture and sequestration, we鈥檒l essentially have a zero-cost feedstock.鈥�

鈥淚t鈥檚 time we stopped thinking of CO₂ solely as a pollutant and viewed it as a valuable resource鈥�

There is certainly no shortage of CO₂ to be captured and used 鈥� 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 鈥� or the carbon footprint 鈥� 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 掳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 鈥� 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 鈥渕ineralise鈥� 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鈥檚 not the only CO₂ saving: temperatures of just 700 掳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).

鈥淲e鈥檙e developing cement with the same properties and at the same cost as existing cement, but instead of emitting CO₂ our cement actually absorbs it,鈥� 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鈥橰ourke, to make masonry blocks, a building material that makes up 15 per cent of the UK鈥檚 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鈥檚 National Consortium on Catalysis. 鈥淚f we could produce fuels at the same rate as we burn them, that would be the real solution.鈥�

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 鈥� otherwise known as natural gas. 鈥淲e don鈥檛 need any other outside energy source, all we need is the heat that is left in the waste gas,鈥� says Erik Fareid, technical director at RCO₂.

The firm won鈥檛 say what the catalyst is but claims it can convert 20 per cent of a power plant鈥檚 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鈥檚 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 鈥� 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 (麻豆传媒, 11 September, p 40). Instead, researchers are concentrating their efforts on a more promising group of organisms: algae.

Farming fuels

Photosynthesising microalgae can harness as much as 8 per cent of sunlight to make biomass, turning CO₂ into oils that can be processed into biodiesel. They have already caught the eye of oil companies. In 2009, ExxonMobil announced a $300 million deal with Synthetic Genomics of La Jolla, California, a biotech company led by geneticist Craig Venter, to genetically engineer microalgae to boost their oil output.

Other companies are taking a different approach. Instead of engineering common microalgae, they are hunting for any highly productive strains that are already out there. An example is Shell, which in 2007 formed a joint venture called Cellana with biofuel company HR BioPetroleum to search for naturally productive strains of marine microalgae. According to Mark Huntley, HR BioPetroleum鈥檚 chief scientific officer, only about 3000 of the 100,000 to 1 million algae species thought to exist in nature have ever been studied. 鈥淭here鈥檚 an awful lot of natural variability out there that no one has even looked at yet,鈥� he says.

Once a promising species of microalgae has been found, the next step would be to farm them on an industrial scale. However, efforts to farm algae have been plagued by either high costs or low yields. The cheapest method is to farm them outdoors in open ponds. Unfortunately, the ponds are quickly contaminated by unproductive wild algae strains, and infiltrated by predatory microorganisms hungry for an oily meal. Locking up algae inside plastic 鈥渂ioreactors鈥� prevents this contamination, but makes large-scale production prohibitively expensive.

Huntley says the solution is to develop a hybrid system. In Cellana鈥檚 system, algae are initially grown in high concentrations in closed bioreactors, where they divide rapidly but take on little mass. The organisms are then moved into open ponds where individual algae grow quickly, doubling in mass several times each day. After two or three days the algae are harvested for their oil, before contaminants have time to infiltrate the pond.

Production costs are still high, but the technique is beginning to gain a foothold in areas with high electricity costs. For example, Hawaii depends on imported diesel to run power plants, so its electricity rates are more than double the US national average. That is why HR BioPetroleum has chosen Hawaii as the location of its first commercial operation. The company will work with the Maalaea power plant, a 215-megawatt facility that currently burns diesel and provides 85 per cent of electricity to the Hawaiian island of Maui.

The plan is to siphon CO₂ from the existing plant鈥檚 smokestack and pump it into bioreactors and ponds where it will be absorbed by the algae. Cellana hopes to absorb 60 per cent of the power plant鈥檚 CO₂ emissions this way. Oil produced by the algae and processed into biodiesel will be fed straight back into the plant, where it will be burned in place of the imported diesel.

Even this won鈥檛 let us recycle all the CO₂ we produce, though. In order to do that, it seems we will have to do better than nature. That might not be as difficult as it sounds. After all, says Aresta, 鈥渨e can fly much better than birds, so why not try to make a synthetic process that turns carbon dioxide and sunlight into energy better than a leaf?鈥�

Heinz Frei, deputy director of the Helios Solar Energy Research Center in Berkeley, California, agrees. He is developing light-activated catalysts that take in sunlight, CO₂ and water, and spit out fuel.

The field scored its first major breakthrough in 1998 when researchers at the National Renewable Energy Laboratory in Golden, Colorado, developed a cell that captured 12 per cent of the sunlight that hit it, using the energy to make hydrogen. 鈥淚t was an important proof of concept, but it was hopeless in terms of large-scale manufacturing and durability,鈥� Frei says. The device began to corrode after just 20 hours of use.

Frei is now working on a next-generation cell that produces liquid fuel. In one prototype, each cell will consist of a pair of cadmium-based nanorods which absorb light and emit electrons, separated by a membrane. On one side of the cell, a catalyst captures the electrons emitted by the nanorod and uses them to split water and make hydrogen. The hydrogen passes through the membrane to the other side of the cell, where another catalyst surrounding the other rod converts CO₂ into carbon monoxide. The hydrogen and carbon monoxide combine to form methanol, a liquid fuel that could be used to fuel cars. Frei鈥檚 team has already tested several components of the system, including the water-splitting nanorods (), and is working towards a fully functioning device.

The goal, Frei says, is to develop a system that is at least 5 to 6 per cent efficient at capturing sunlight to turn CO₂ and water into liquid fuel, and cheap enough to be deployed over millions of acres of non-arable land. If this or any other team succeeds, the results would be vital to our fight against climate change. 鈥淚f we鈥檙e creative enough and invest enough in new technology we can find a solution that will replace fossil fuels completely,鈥� Frei says.

鈥淚f we鈥檙e creative enough, and invest enough in new technology, we can replace fossil fuels completely鈥�

In the meantime, like those who own the Dutch greenhouses, we have just got to wait for the technology to bloom.

When this article was first posted, we attributed the wrong company to the Maalaea power plant project in the paragraph beginning 鈥淧roduction costs are still high鈥︹��