28 November 2011

Using Thermogravimetry for Carbon Accounting

Posted by John Freeland

Thermogravimetry-Derivative Thermogravimetry (TG/DTG) is a simple and inexpensive method to account for recent soil carbon sequestration. As such, it’s a good choice to be a standardized test in international carbon trading markets. More about TG/DTG shortly, but first, why is this important?

Here in the United States, clearly, any federal legislation that seeks to reduce carbon emissions, such as by taxation or carbon trading, has no chance of getting through congress. Fortunately, people think differently around the world and climate science can still find a “foot-hold.”

In an effort to reduce greenhouse gas (GHG) emissions, the Kyoto Protocols and United Nations Marrekesh Accords established “flexible mechanisms” to incentivise reductions, including market-based carbon trading. Here is a list of carbon trading systems passed by other governments:

California Global Warming Solutions Act of 2006 (AB 32) includes cap-and-trade.
Western Climate Initiative
EU Emissions Trading System (30 participating nations).
New Zealand Emissions Trading Scheme
Australia’s Carbon Reduction Laws

Critics of the power of emissions trading have a problem called history. Smithsonian has a good summary of sulfur dioxide cap-and-trade here. Sulfur-dioxide emissions trading was written into the Clean Air Act in 1990. Economist magazine called it “the greatest green success story of the past decade.”

Measuring Soil Carbon Pools: The TG/DTG Method

Soil carbon sequestration has been proposed as one option available to the coal industry to help offset its carbon dioxide emissions. Coal mine reclamation, including soil replacement and re-vegetation, offers a setting where the “new soil” can be evaluated for it’s carbon content, then monitored over time to record and track the accumulation of “new” carbon. Over time, and under other favorable environmental factors, the soil may accumulate enough organic matter to more closely resemble the soil that existed before the mine.

Sally Maharaj and Chris Barton, along with other researchers at the University of Kentucky,* wanted to find a way to distinguish “new” carbon accumulated in the soil as a result of recent biological activity, from “old” carbon stored in coal fragments and carbonate minerals present in the mine spoils used to make “soil” during mine reclamation. A short summary of their research project, including details on methods, is available here.

Basically, three carbon pools: “new” soil organic matter from recent biomass, “old” carbon in coal particles, and “old” carbon contained in carbonate minerals were heated to the point of pyrolysis, i.e., thermo-chemical decomposition in the absence of oxygen. The three groups reached pyrolysis at distinct termperature ranges:

“New” carbon from recent biomass: 270-395 degrees C;
“Old” carbon from coal: 415-520 degrees C;
“Old” inorganic carbon from carbonates: 700-785 degrees C.

With respect to soil carbon sequestration, TG/DTG answers the question: “how do we measure it?” The technique is simple, although I wonder if, in the interest of technology transfer, it might be simplified even further to a loss-on-ignition test requiring equipment less sophistiated than that used by Mahraj et al.

This carbon accounting method should work for other soils, not just mine reclamation sites. Soil carbon sequestration fits into the broader strategy of restoring and maintaining soil quality. Benefits of soil organic matter include higher plant nutrient availability, better soil structure, less soil erosion, higher water infiltration, less runoff, and greater water holding capacity.

Replacing soil carbon is a good idea. If the world gets serious about carbon trading, the term “land bank” may take on a whole new meaning.

Maharaj,S., C.D. Barton, A.D. Karathanasis, H.D. Rowe and S.M. Rimmer. (2007). Distinguishing “new” from “old” organic carbon in reclaimed coal mine sites using thermogravimetry: II. Field validation. Soil Science, 172 (4), 302-312

Related Paper Carbon Sequestration on Surface Mine Lands