BioCycle April 2012, Vol. 53, No. 4, p. 60
Lines are being drawn. Battles are being fought. And all over the organic residuals that in many places, are still considered to be garbage. To understand the basis for these battles, it is really important to understand the role that fixed carbon plays in so much of the discussion about climate change, food security and the dwindling reserves of fossil fuels. And in order to understand that, you have to grasp some of the basics about carbon: about why this element is so special.
Carbon plagues us as CO2. Carbon feeds us as carbohydrates, sugars and proteins. Carbon fuels us as petroleum products. Carbon keeps our food fresh as plastic wrap. Carbon keeps our fingernails colorful as the phthalates in nail polish. And with age, rolls of carbon push us into those plus sizes. What is pretty amazing about carbon is the range of properties this one element can have. It is the most diverse element chemically and because of this, it is critical to everything we do.
Carbon is so special because its properties change so dramatically based on how many electrons it has, what it is bonded to and the nature of those bonds. When carbon is floating around in the atmosphere, it is normally attached to two oxygen atoms, aka carbon dioxide or C02. This carbon is the same carbon that forms the backbone of diamonds and oil.
As a gas floating around, this carbon is very light on electrons with a net charge of 4+. Being so light and free it takes an incredibly strong hand to bring this gas down to earth. Realize, and this is critical, that the only technology we know of that can bring the carbon down to earth, down to solid form, is photosynthesis. Plants do this using the energy from the sun. We don’t know how to do this. When the plants fix the carbon they turn it into leaves, roots and flowers. They use it to make themselves bigger. The only way that carbon comes from the sky as a gas to the wide range of forms and products listed in that second paragraph is via plants. And plants are the only things that understand how to turn the gas into leaves and fronds.
The carbon that is brought down from the sky via energy from the sun is referred to as fixed carbon. It has had electrons shoved onto its rings and has turned from a gas to a solid. Every growing season can be seen as the fixed carbon factory churning into operation. The process of turning the carbon from one state to another requires a lot of energy. Here it is the energy from the sun doing the work. While some of the energy required to do this is lost in the process, a good portion is stored in the newly fixed carbon. And this stored energy is why the fixed carbon is critical for two of the major problems we are currently faced with: food security and the diminishing supply of fossil fuels.
Food Security And Fossil Fuels
Let’s start with food security. We eat food to get both energy and mass. We eat plants or things that have grown by eating plants (think cows and chickens) and as our bodies digest the food, they break the bonds that have tied the carbon down as a solid and release the energy stored in those bonds. Those energy bars and energy drinks — all fixed carbon that gives us energy as the chemical bonds are broken. The calories in what we eat are the units used to express how much energy is stored in our foods. When we release the carbon, after we’re done burning those calories, much of it is exhaled as CO2. We don’t release all of it; that is true. Particularly for the young people who want to grow and us not so young people who grow in directions other than up, a portion of what we eat is turned into body mass. The vast majority of it however is used for energy. And while we may not precisely be what we eat, we are completely dependent on photosynthesis to meet our caloric requirements. With a population expected to hit 9 billion in the next few decades, growing sufficient plant material to meet the caloric requirements of all of us will be quite a challenge.
A version of those same energy bonds that humans break to get energy are broken by cars when they use gasoline for power or by power plants when they burn coal to make heat to produce electricity and by your wood stove that heats the house so nicely in the winter. Here, the heat energy stored in the bonds is quantified using units like BTUs (British Thermal Units). Even though crude oil looks and tastes nothing like ice cream, they both started out as fixed carbon in living plants. We have found some other ways besides fixed carbon to generate energy. Hydrogen fuel cells get energy by manipulating different sorts of chemical bonds. Both hydro and wind power transform kinetic energy to create electricity. Solar power, making leaps and strides of late, is able to take the energy from the sun and turn it into heat and electricity. Not stuff that we can eat, but at least a substitute for some of the energy we’ve traditionally gotten from fixed carbon.
A big concern with these alternative energy options is the quantity of energy that they can generate in comparison to current and expected demands. Another concern is that they can’t substitute for liquid fuels. A big emphasis in the search for fossil alternatives is to identify substitutes for liquid fuels. This is one of the reasons that ethanol (again from plants) has gotten so much attention. Now, it is quite possible that much of our transport industry can transition away from liquid fuel. I say this even though we didn’t win the Nissan Leaf in the school auction (not bitter at all). But certain types of equipment including planes and heavy trucks may not be able to make this transition so gracefully as they need more powerful (read higher energy intensity) fuels than our substitutes to date can provide.
Although the vast majority of our liquid fuels and even our electricity is currently derived from fossil fuels (things that all started out as plants millions an millions of years back), much of the new emphasis on ‘green’ energy is in fact based on figuring out how to get power from plants or plant material that recently was green and growing. This includes things like biodiesel, ethanol, biogas, pyrolysis and plain old combustion. In other words, our energy needs are competing with our food needs for access to plant biomass.
The other thing about carbon that makes it so special is its ability to change its properties so dramatically. The banana and the yellow diamond both started out as CO2 in the atmosphere. The banana is from the same family tree as the diamond. It is the plate and the platinum that are the alien species. The vast majority of all of the drugs we take, the plastics we use, the furniture we sit on is all derived from plant material and is all carbon based. So the third thing competing for the plant carbon is the stuff that you can make from fixed carbon. And while you can’t turn lead into gold, with carbon-based products, chemical engineers can turn crude oil into perfume, moisturizer and nondairy creamer, not to mention Glad wrap. Face it, no naturally occurring thing started out being anything like Glad wrap. By putting energy into a particular or group of particular carbon compounds, using that energy to change what the carbon is bonded to and the types of bonds that it has, it is possible to create Glad wrap. All you need to start is the energy and the fixed carbon.
So let’s bring this whole lecture back to organic residuals, all of which started as plants and all of which contain this valuable fixed carbon. I’m a soil scientist and a former chef, so I obviously think that a high priority for much of this stuff should be returning it to soil so that it can help soils grow more fixed carbon. Other people, say petroleum engineers, think that the best thing to do with it is make energy out of it. And there is yet another whole group who tend to be more like alchemists, who want to turn these residuals, often highly variable in types of bonded carbon, into gold. If not gold exactly, then a range of high value products. How this war for fixed carbon is won, or the outcome of the different battles within this war, can have an impact on that third issue mentioned early on: climate change. But you are best advised to go into this battle with a basic understanding of the element that is the key to the fight.
Sally Brown — Research Associate Professor at the University of Washington in Seattle — authors this regular column. Email Dr. Brown at firstname.lastname@example.org.