本文作者：小红猪小分队

Source：http://www.newscientist.com/article/mg21628941.600-biofuel-thats-better-than-carbon-neutral.html?full=true

The race is on to create a biofuel that sucks carbon out of the sky and locks it away where it can't warm the planet

THE green sludge burbles away quietly in its tangle of tubes in the Spanish desert. Soaking up sunshine and carbon dioxide from a nearby factory, it grows quickly. Every day, workers skim off some sludge and take it away to be transformed into oil. People do in a single day what it took geology 400 million years to accomplish.

Indeed, this is no ordinary oil. It belongs to a magical class of "carbon negative" fuels, ones that take carbon out of the atmosphere and lock it away for good. The basic idea is fairly simple. You grow plants, in this case algae, which naturally draw CO₂ from the atmosphere. After you extract the oil, you're left with a residue that holds a substantial portion of the carbon. This residue is the key to carbon negativity. If you can store the carbon where it won't decompose and return to the air, more CO₂ is taken out of the atmosphere than the fuel emits.

Such carbon negative fuels are no accounting sleight of hand - they could be the most realistic short-term solution we have to curb climate change. And although it is still early days, companies like General Electric, BP and Google are putting their money behind the idea.

Every time you drive your car or hop on a plane to somewhere sunny you're adding a little more carbon to the atmosphere and bringing a global warming crisis just a little bit closer. Biofuels are one way of reducing the problem, as plants draw CO₂ from the atmosphere as they grow, thereby not adding to the carbon footprint. Today, the most popular biofuel is ethanol made from corn.

In theory, such a fuel should be carbon neutral: that's to say, for every 100 carbon atoms it draws from the atmosphere, it returns exactly 100 when burned. Unfortunately, however, it's not that simple. By the time farmers have tilled the soil, poured on fertiliser and harvested the crop - not to mention the natural gas and coal burned to run the ethanol plant itself - they've used an awful lot of fossil fuel, leaving them well short of carbon neutral.

You might think the problem could be simply solved by capturing the carbon emitted during the biofuels production process. The fermentation process used to produce ethanol, for example, generates an almost pure stream of CO₂ as a by-product. So, earlier this year, agricultural giant Archer Daniels Midland (ADM) started building the US's first large-scale carbon capture and storage project in Decatur, Illinois. It will siphon CO₂ from the company's ethanol plant, compress it and store it underground nearby. It plans to store over 1 million tonnes of CO₂ annually (see diagram).

However, ADM's ethanol still isn't carbon neutral: instead, thanks to all the energy costs of making the ethanol, it's likely to reduce emissions by only about 20 or 30 per cent compared with fossil fuel.

You might be able to solve the problem if you replaced all the fossil fuels used to run the ethanol plant with renewable energy. But that doesn't solve the other major issue for crop-based biofuels: they compete with food crops for land. In 2010, corn-based ethanol accounted for 8 per cent of US transport fuel, but consumed almost 40 per cent of the country's corn. If ethanol replaced all fossil fuels, it would either push food prices into the stratosphere or force farmers onto new land - most likely both. To make a dent in the amount of greenhouse gas in the atmosphere, we need to find ways around this. "The question is how many of these situations we can find without infringing on other services that the biomass or the land is supplying," says Johannes Lehmann, a soil scientist at Cornell University in Ithaca, New York.

This is exactly why algae is so promising, notably the single-celled, blue-green variant now referred to as cyanobacteria. They grow much faster than terrestrial crops, potentially yielding 20 times more biomass per day than soybeans; their oil production is easy to ramp up through genetic engineering; best of all, they can grow in seawater or brackish groundwater on non-arable land, so they don't take land away from food production or forest (Science, vol 314, p 1598).

These qualities were especially appealing to Bio Fuel Systems, (BFS) a small company in Alicante, Spain, that uses cyanobacteria to make its "Blue Petroleum". The company's prototype plant, in the Spanish coastal desert, is piggybacked on a cement factory, which emits the CO₂ the algae need to grow.

Blue petroleum

The numbers given to New Scientist by BFS president Bernard Stroiazzo illustrate the fraction of carbon that can be trapped by the process. To make a single barrel of oil, the algae suck a little over 2 tonnes of CO₂ from the smokestack of the cement works. Not all of that stays out of the atmosphere, though. The algal cultures need regular mixing, which takes energy, as does supplying fertiliser and creating the oil through a patented process involving high heat and pressure. All the fossil fuels needed for these processes release about 700 kilogrammes of CO₂. Burning the oil itself - in car engines, say - emits another 450 kg. The rest of the carbon - the equivalent of about 900 kg of CO₂ - stays in the leftovers, an inorganic carbonate sludge that can be buried or mixed into concrete. "That will never go back in the atmosphere," says Stroiazzo.

BFS's pilot plant produces about 2.5 barrels of crude oil per hectare of algae each day. At that rate, Stroiazzo says, a system like BFS's could replace the world's entire crude oil consumption, using an area just a quarter the size of the Libyan desert. Thirty-five million hectares is a lot of land, to be sure, but not overwhelming if it replaces the 90 million barrels of oil we use each day. It is also about 1 per cent of the world's pasture area; spread over many plants worldwide it quickly becomes feasible.

But there are a few more factors to consider. Though they are not selling the oil yet, cost will likely be an issue: BFS's equipment is by no means cheap. The polycarbonate tubes that house the cultures cost upwards of $1 million per hectare, and stirring the algae requires large amounts of electricity. This is likely to push the cost of algal biofuel to at least $5 per litre, according to a2010 International Energy Agency report.

To stay solvent, BFS sells its high-value algal by-products as nutritional supplements, such as omega-3 fatty acids. While this may work in a nascent biofuels industry, demand for nutritional supplements will falter when the products flood the market, and anyway it doesn't get to the heart of the problem.

Other companies are trying to do that, though. Algae Systems, near San Francisco, suggests cutting costs by culturing its algae in the ocean, in 25-metre plastic bags floating near the shore. The bags keep the algae at the surface, where the light is most intense, and natural wave action does the mixing. The firm plans to pipe in nitrogen-rich wastewater to fertilise the algal growth.

Algae Systems is now constructing a pilot plant covering several hectares in Mobile Bay, off the coast of Alabama, which should be operational early next year. If all the component processes work as well as they have in the research lab, the result should be carbon-negative fuels, says company president Matthew Atwood. This fuel should be able to undercut fossil petroleum prices within three or four years, he adds.

However, they will need to solve another problem for algal biofuels: fertiliser. Algae are gorge on expensive nutrients like nitrogen and phosphorus. At relatively small scales, wastewater from cities and croplands can easily supply these, as in Algae Systems's design. But scale up and there simply isn't enough wastewater to go around. "Human nutrient loading is simply not sufficient," says Stefan Unnasch, an energy analyst and engineer at California consultancy Life Cycle Associates. "You put more in your car every day than into your toilet." Indeed, producing even a tenth of the US's liquid fuel from algae would consume more than the entire US supply of both nitrogen and phosphorus, according to calculations by Ronald Pate, an algal biofuels specialist at Sandia National Laboratory in New Mexico (Applied Energy, vol 88, p 3377).

Researchers may some day find a way to solve the nutrient problem by extracting and reusing nitrogen and phosphorus from the algal residue, but the biggest difficulty to scaling up is more intractable: how do you get your hands on all that CO₂? Even if algae-growers could tap every last smokestack in the US, that would only be enough to produce about 75 billion litres of algal biofuel per year, according to Pate's calculations. That's less than 10 per cent of the world's current transport fuel needs. Moreover, tying biofuel production to fossil-fuel-burning industrial smokestacks merely wrings a second round of energy out of CO₂. "This just postpones emissions," says Jonas Helseth, director of Bellona Europa, an environmental foundation based in Brussels, Belgium.

As yet, this problem has no robust solution. A few companies are developing technologies to extract and concentrate CO₂ from the air. Global Thermostat, based in New York, has patented a process that uses chemicals and low-temperature waste heat - about 90 °C - to capture CO₂ from a stream of air. Its pilot plant has been operating near San Francisco for more than a year, and a second is on the way, says co-founder Graciela Chichilnisky. The company has already signed an agreement to supply its technology to Algae Systems and is in talks with several other algal biofuel companies, she says.

Biofuels franchise

Solve these problems, and algae may yet be vindicated as the most promising path to carbon negative biofuels. But until then, a less glamorous method is poised to take off.

The cheapest, most low-maintenance feedstock for biofuels is waste biomass, such as the cobs and straw left over after corn harvest, perennial grasses such as giant miscanthus, or dead trees. This raw material has been used to make ethanol, but its efficiency has been stymied by the difficulty of breaking down the materials. Cool Planet Energy Systems, based just north of Los Angeles in Camarillo, California, has found a better way to process it. It has developed a variant of a process called pyrolysis, in which heat, pressure and catalysts convert the biomass directly into the hydrocarbons found in gasoline, diesel oil and jet fuel. This means the company's fuel can be mixed into regular gasoline to reduce the overall amount of fossil fuel, or in other words, it lowers the carbon intensity of the gasoline.

Earlier this year, researchers at Google - one of the company's investors - road-tested a blend of 5 per cent Cool Planet fuel and 95 per cent gasoline in its GRide cars at its headquarters in Mountain View, California. The mix reduced the carbon intensity of gasoline by 10 per cent, says vice-president Mike Rocke, meeting California's 2020 Low Carbon Fuel Standard eight years early.

Better yet, carbon gets sequestered. Along with fuel, Cool Planet's pyrolysis process yields large amounts of biochar, a carbon-rich compound that resembles charcoal. Instead of burying this residue deep underground like ADM or mixing it into cement, however, Cool Planet returns the biochar to the soil.

This has several advantages. It does not depend on the presence of suitable geological formations, and it is easier to transport. Best of all, the biochar enriches the soil and enhances crop yields because its high surface area helps hold water and nutrients. "It's like a molecular sponge," says Rocke. Lehmann, a biochar expert, says the stuff can persist in the soil for centuries, which qualifies as carbon sequestration as set by the Intergovernmental Panel on Climate Change.

That's not the only trick that makes the biofuel carbon negative. Instead of wasting fossil fuel on transporting the biomass to a centralised factory to be made into fuel, Cool Planet will build 400 modular units, each capable of producing between 40 and 200 million litres of gasoline per year. These will use whatever biomass is available within about a 50-kilometre radius. "Wherever the biomass is, we're going to roll out these plants," says Rocke. "They're like a Starbucks."

Cool Planet's process only returns half the carbon to the atmosphere and stores the other half as biochar, making the fuel what Rocke terms "100 per cent carbon-negative". To break into the market, however, the company plans to make a version that is 60 per cent carbon-negative, storing only about a third of the carbon in the plant matter. At this sweet spot, Rocke reckons the company should be able to sell its fuel for about 40 cents a litre.

To date, the research facility has produced only a few thousand litres of fuel. However, a pilot plant - bankrolled by investors including Google, BP and GE - will start operation near Los Angeles this month, producing nearly a million litres per year. And within 20 years, they intend to build 2000 of their modules, enough to supply about 10 per cent of the world's current liquid fuel needs.

Cool Planet's results are encouraging. In 2007, the IPCC reported that for the world to escape catastrophic climate change, carbon emissions would have to begin declining by 2015, with an 85 per cent reduction by 2050. We haven't even started.

Since we can't seem to keep the CO₂ from entering the atmosphere, we're left with only two ways to avoid trouble. We could embark on grand geoengineering schemes to cool the planet, all of which bring huge risks of unintended consequences (New Scientist, 22 September, p 30). Or we could try to pull some of the CO₂ back out of the atmosphere, one car trip at a time. "Even if carbon-negative biofuels turns out to be just a bit player, they will have done at least a little to reduce carbon emissions," says Lehmann. "It's a no-regret strategy."

Bob Holmes is a consultant for New Scientist