Wood Smoke Worse than Originally Thought

A high-efficiency woodstove my actually create more greenhouse gasses than a traditional fire

At the World Conference on Solar Cooking in 1992, Dr. Daniel M. Kammen, of the Department of Physics at Harvard University, gave a presentation on the emissions from traditional cooking fires and the effects they may have on the atmosphere and global warming. We spoke with Dr. Kammen at the conference.

 SBJ: Dr. Kammen, what is it that you have discovered about the smoke from traditional cooking fires?

 Daniel Kammen: While everyone knows that carbon dioxide produced from burning wood contributes to the greenhouse effect. What we discovered was that other gasses--carbon monoxide, methane, sulfur dioxide, nitrogen dioxide, etc.--are produced in much greater quantities than previously thought.

 SBJ: What does this mean?

 DK: When you calculate the contribution that these gasses make to global warming, they rival or exceed the greenhouse effect produced by carbon dioxide alone.

 The typical kilogram of wood is roughly 50% carbon. When it is burned under ideal conditions, the smoke consists mostly of water and carbon dioxide. But in traditional cooking practices, where there is not enough oxygen, those 500 grams of carbon produce 50 to 60 grams of carbon monoxide, 20 to 30 grams of methane, and 30 to 40 grams of other nasty stuff. Each of these non-CO2 products of combustion have a larger greenhouse effect, molecule per molecule, than carbon dioxide. Carbon monoxide's effect is five times greater, methane is 23 times greater, and nitrogen dioxide is 280 times more serious as a greenhouse gas.

 When you multiply the amount of emissions by the global warming potential for each gas, and add it up, you find that the non-C02 gasses produce a C02 equivalent of 1687 grams. The amount of C02 directly released by burning one kilogram of wood is about 1613 grams, since oxygen from the air is added. (Coment: the original article reported weights of carbon required to produce the CO2 - eg, with completely efficient combustion, 440 g of carbon produces 1613 g of CO2. The change was made after consulting with Prof Kammen, to make the text easier to understand.)

 SBJ: So in essence we have twice the greenhouse effect?

 DK: That's right. Now the key item is, how long do these gasses stay in the atmosphere? The half-life of each gas is not known precisely, but half the C02 released today will still be in the atmosphere 150 to 200 years from now. In another 150 to 200 years after that, only a quarter of it will still be up there. Methane has a half life of about 7 years.

 SBJ: It seems fortunate that the more powerful greenhouse gasses have shorter half lives.

 DK: Yes, but remember, these gasses don't suddenly vanish. When a molecule of methane stops being methane, it becomes C02. It is still a bad thing. Now, if you multiply how much of each gas is in the atmosphere by how reactive it is, then subtract what is decaying due to its half life, you find that over the first 20 years the non-C02 gasses have had the biggest effect.

 SBJ: And this is not generally known?

 DK: The atmospheric chemistry has been known for quite a while. The big surprise is that small-scale burning produces more non-C02 gasses than originally thought. Scientists just had not looked that closely at inefficient, smoldering cookfires.

 SBJ: And you have actually measured the production of these gasses?

 DK: That's right. And something I hope to look at more closely at this summer, while in Kenya, is the effect oxygen availability has on non-CO2 gas production. This could be significant. The high efficiency ceramic stoves currently being disseminated, reduce wood consumption by reducing oxygen supply. We might be producing just as much or more of greenhouse gasses with less wood. We will also be looking at baking bricks in a large solar box, eliminating another smoldering burn that produces lots on non-CO2 gasses.

 SBJ: So, could you sum up the ramifications of all this for solar cooking?

 DK: The contribution cooking fires make to the global "greenhouse budget" has been underestimated. The potential contribution solar cooking can make is bigger than we'd thought. The scary thing is this: even if by the year 2000 we wise up and do all the right things, we still have a long time to live with what we have done already. That is why it is so important that we get moving right away!

Prof. Kammen's home page:-