Cutting the cost of electricity

11 November 2014

Interview with

Mark Peplow, Science writer

Climate changeA large coal-fired power station can pump out up to one hundred thousand of tonnes of carbon dioxide per day, making it hard for countries like the UK to meet their obligations to cut greenhouse gas emissions.

There are ways to remove or scrub CO2 from what goes up the chimney, but they're very expensive to operate.

Now a research team in America have come up with a way to make this scrubbing process much more economical. They've found a bacterium that makes an enzyme, which is a kind of biological catalyst, that can turn carbon dioxide into a soluble chemical called bicarbonate.

By introducing random changes - or mutations - into the genetic instructions that tell the bacteria how to make the enzyme, they've been able to come up with a new super-powerful version of it, that might save us millions on our power bills.

Science writer Mark Peplow spoke with Naked Scientist Chris Smith about what this enzyme can do...

Mark -  It's called carbonic anhydrase.  This is an enzyme that's normally found in your body where it helps to transport carbon dioxide out of your tissue.  It does this by combining carbon dioxide with water to make soluble bicarbonates and a hydrogen ion.  Now enzymes are proteins and they'd normally be destroyed at the high temperatures and the harsh conditions that are normally used in carbon capture systems.  Just think about what happens when you put an egg in a frying pan.  The albumin in the egg white gets completely broken down and it goes solid and white and rubbery.

Chris -  So, when you would be applying this technology, you're saying that this enzyme would have to tolerate the flue gases coming out of a power station or something.  So, this can be really high temperatures.

Mark -  Well, that's right.  So, what happens normally when people are trying to set carbon capture systems on a fossil fuel power plant, it uses special solvents called amines.  They grab hold of carbon dioxide molecules before they fly up the chimney.  You then pipe the solvent away, carrying the carbon dioxide and you heat it up to release the carbon dioxide into a storage container so that it doesn't escape.  The solvent then gets cooled down and cycled around to do the same job again.  The trouble is that all this heating and cooling, and the equipment that you need to do it is really expensive and it potentially adds hundreds of millions of pounds to the cost of a powerplant which makes the electricity more expensive and it reduces the plant's power output as well.  That's been a huge barrier to widespread use of this system.  This is where the carbonic anhydrase comes in because it can help to capture the carbon dioxide and make this process much more efficient.  The trouble is that in the past, enzymes, because of their sensitivity, couldn't stand up to the hundred degree conditions and harsh alkaline conditions, of this heating and cooling of the amine solvent.

Chris -  So, where did these American researchers come by this super enzyme?  How did they make it?

Mark -  Well, they got have a bacterium and then they introduced random mutations into the enzyme to see if they could make it tougher in a process that's called directed evolution.  So, you get lots of the random mutations and...

Chris -  These are genetic changes aren't they?  You're adapting the DNA of the organism just by making mistakes if you like, spelling mistakes in the DNA.

Mark -  Yeah, you make deliberate spelling mistakes in the DNA and lots of different types of mistakes.  That gives you a whole pot of new mutated enzymes.  Most of them are rubbish but a few will be a bit tougher.  They did nine rounds of this directed evolution and each round, they picked the toughest enzyme that they got at each stage and then they made more mutations to that particular enzyme.  In all, it meant that they tested more than 27,000 different mutated enzymes.  At the end of that process, they end up with one which is the superman of all of these enzymes and it can withstand more than 100 degrees of temperature and a pH greater than 10, so that's almost as alkali as household bleach.

Chris -  That's almost the enzyme equivalent of the indestructible or invincible man, isn't it?  I mean, really, really tough.  So, have they actually got evidence that this can work?

Mark -  Well, this is the exciting thing about this paper which came out this week in the proceedings of the National Academy of Sciences.  They haven't just done this in the lab.  They tested this enzyme in a pilot carbon capture unit attached to a coal-fired plant.  They found that it improved the carbon dioxide absorption of the amine solvent system by 25 times.  It allowed it to remove about 60% to 70% of all the carbon dioxide from the plant's waste stream and they ran it for about 6 days with no loss in performance at all.  So clearly, the enzyme is working and it's really robust.  This faster absorption that it can allow you to do means that you could have much smaller processing units running at lower temperatures.  And that could potentially make carbon capture a lot cheaper.  They've done a cost estimate in fact and they think that if you scale this up, it could knock more than $100 million off the cost of a full scale unit.  That might sound like a huge figure and they're extrapolating quite a lot.  But it's important to say that the researchers that had been doing this, they're pretty big hitters.  They're involved big Industrial enzyme developers like Codexis and Novozymes, and they partnered with BP Biofuels on this.  They've been backed by research funding from the US Department of Energy.  All of that I think makes it quite promising that this is going to go forward for further development.

Science writer Mark Peplow.

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