AskPablo: Katrina’s Trees

katrina%20trees.jpgWhen hurricane Katrina hit the Gulf Coast it not only uprooted and killed citizens, but it also killed millions of trees. In fact, Tulane University researchers estimated the number at 320 million. Not only does this loss decrease the amount of CO2 sequestered from the atmosphere, but the trees will actually contribute greenhouse gases to the atmosphere as they decay. I will examine just how big this problem is and explore some possible solutions.

It is estimated that the 320 million trees represent 105 million metric tons (MMT) of carbon. As these trees decompose, the carbon, now stored in cellulose, will become CO2. Every 12 grams of carbon will be joined by 32 grams of oxygen to make 44 grams of CO2. This means that the trees will eventually turn into almost 385 MMTCO2 (105 x 44/12). US emissions from petroleum, coal, and natural gas are almost 6 billion metric tons per year. The decomposition of these trees is around 6.5% of that.
So what can be done to decrease this further threat to our climate? The Arbor Day Foundation has a campaign called Trees for Katrina and they will plant one tree for each $1 donation received. This would make a great holiday gift for the climate conscious people in your life, including me! Planting trees will help restore the sequestering capacity of the affected area but it won’t prevent the decay of the dead trees and the release of the greenhouse gases.
What if the dead trees are harvested, shredded, and used as land-fill in the low-lying areas of New Orleans? The problem I see with this idea is that landfilling results in anaerobic conditions, in which the cellulose will decay into methane rather than carbon dioxide. With methane every 12 grams of carbon will join with 4 grams of hydrogen to make 16 grams of CH4. Anaerobic decomposition would then result in 140 million metric tons of methane (105 x 16/12). Well, 140 is lower than 385 so it’s a better option, right? Unfortunately not. Methane has a much higher global warming potential (GWP) than carbon dioxide. In fact, it is 23 time as bad for the climate as CO2. The 140 MMT of CH4 would equal 3.22 billion metric tons of CO2-equivalent (CO2e), or almost 10x as bad.
Combustion of the biomass in a biomass electricity plant is another potential solution but with only 252 MW of installed capacity the 9 biomass plants in the US could probably not handle this additional feedstock. The wood could be used for heating homes, which would still release CO2 but at least it would serve a beneficial use and possibly reduce the combustion of fossil fuels. But those homes that currently use wood for heating are finite and it would represent a significant air-quality issue as well due to the particulate matter.
One innovative idea that I have heard is to transport the biomass to a very dry region of the country and lock it away in an old pit mine. Let’s just arbitrarily pick the Bagdad pit mine in Bagdad, Arizona (I know, it must be a charming town…). Bagdad is 2,575 km away from New Orleans (1,600 miles). The trees, which contain 105 MMT carbon are mostly made out of cellulose (C6-H10-O5). This means that 72 grams of carbon, 10 grams of hydrogen, and 80 grams of oxygen combine to make each 162 grams of tree. The 105 MMT of carbon actually represent 236 million metric tons of trees (105 x 162/72), at 740 kg per tree (236/320).
Trucking emissions are based on weight and distance. In this case, we are sending 236 MMT over 2,575 km, which equals 607,700,000,000 ton*km. According to one source the greenhouse gas emissions from 1 ton*km is 100g so the total emissions from transporting the biomass to Arizona would be 60.77 MMTCO2 (0.0001 x 607.7b). I am totally surprised that this is actually less than the natural decay of the trees, making it strangely feasible. In fact it is only 16% (61/385) of the emissions from decay. Any residual moisture in the wood would cause some anaerobic decay in the pit mine but this could be easily captured and cleanly burned (releasing only CO2 and H2O) in a co-generation plant to offset the need for fossil fuels. This added benefit would only sweeten the deal for the entrepreneur that takes on this massive project, because he/she stands to make at least $810 million from selling the credits from the 324 MMT emissions reduction.
Pablo Päster
Sustainability Engineer