Can We Reduce Carbon Emissions by Burning Dead Trees?

A recent study finds that specially treated wood from beetle-killed trees could replace some of the coal in existing power plants, reducing emissions of fossil carbon.

According to the Intergovernmental Panel on Climate Change (IPCC), the carbon dioxide emitted from burning wood (biomass) doesn't count. That's because it is not fossil carbon but is biogenic "fast" carbon. The best explanation comes from the International Energy Agency:

"Burning fossil fuels releases carbon that has been locked up in the ground for millions of years, while burning biomass emits carbon that is part of the biogenic carbon cycle. In other words, fossil fuel use increases the total amount of carbon in the biosphere-atmosphere system while bioenergy systems operates within this system; biomass combustion simply returns to the atmosphere the carbon that was absorbed as the plants grew."

The IPCC differentiates between "the slow domain" where carbon turnover takes more than 10,000 years, and the "fast domain" of a hundred years or less with vegetation. So it's apparently OK to burn lots of wood in power plants and not have it count against your carbon budget. That's what they do at the Drax power station in the United Kingdom and call it green

I will admit that I have been reading about this for years, and have never understood it. CO2 is CO2 and we have a hard ceiling of how much we can add to the atmosphere to keep under 2.7 degrees Fahrenheit (1.5 Degrees Celsius)—and the atmosphere doesn't differentiate between biogenic and fossil carbon. The Carbon Cycle assumes that the tree will eventually fall, rot, and release its carbon back into the atmosphere, but that could take another 50 years. That may not be slow carbon, but it is certainly meandering. Burning it now, before 2030, now that's really fast carbon.

That's why this new research out of Brigham Young University and the University of Utah is so interesting. The team went after dead trees killed by bark beetles. According to Brigham Young University chemical engineering professor Andrew Fry, there are some forests where over 70% of the trees are dead and standing. These trees are a source of fuel for wildfires, and soon will be falling over and rotting, releasing their carbon into the atmosphere over the next few years. So you might as well throw them into the power plant and get something out of them; the carbon is just a bit faster. From the press release:

“This project is really useful from two perspectives,” Fry said. “If we can reduce the wildland fire potential and offset some carbon emissions, it has more advantages.”

The problem is that you can't just throw it into the furnace and mix it with coal; it has different properties. Katie Child of Brigham Young explains:

"These trees are biomass, renewable organic material that comes from plants, but can cause problems for traditional power stations not equipped to deal with it. Biomass fuel cause jams and blockages in the power plants, and minerals released from burning the wood can also coat mechanisms with ash, which can be dangerous."

“Ash is a bad thing in a combustor because it coats surfaces of heat transfer and places where heat needs to escape,” Fry said. “It reduces the efficiency of the whole power plant and can eventually shut you down if you don’t handle it right.”

Fry tried two different techniques, including torrefaction, which is often used to treat lumber for decking. You cook the wood at 400 degrees Fahrenheit in a low-oxygen atmosphere. It reduces the strength a bit but breaks down the lignin structure of the wood and makes it unappetizing to fungus and insects. Fry found that torrefied wood burned nicely with no ash.

"In a demonstration at the Hunter Power Plant in Emery County, Utah, Fry and his team proved it possible. There they burned 900 tons of biomass mixed with coal for 24 hours with great success. This demonstration was a major milestone, largely because it revealed that there were no significant changes in ash deposition from coal vs. co-fired coal and biomass material and showed that biomass fuel can be safely and effectively used in power plants."

According to the abstract of the paywalled paper, the experiment replaced 15% of pulverized coal with the woody biomass feedstock and the furnace didn't notice. "Results show no significant changes in ash transformation when switching from coal combustion to co-firing with torrefied wood."

Carbon is still carbon, and we would obviously prefer that they covered Utah in solar panels; but then again, this is Utah. If 15% less coal is burned, and the dead beetle-killed wood now has value and is removed before it burns or rots, that's a win.