An Engineer’s Perspective on Biogenic CO2

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By Anna Simet | November 01, 2013 The study that took the media by storm. BIOMASS WORSE THAN COAL BIOMASS DIRTIER THAN FOSSIL FUELS We all remember the ridiculous headlines surfacing during the month of June 2010. Even though the notorious Manomet Study’s release was over three years ago, it still gets plenty of attention, and it’s still intensely debated and referenced to. That’s expected, as the debate on carbon is seemingly more active than ever. It’s always interesting to hear new or different perspectives on Manomet and biogenic emissions, though the main or underlying points are usually the same. This week, I was contacted by Kirk Cobb, an engineer from Superior Process Technologies, and he wanted to share some of his thoughts. Here are some of the comments he made—some you may have heard, some perhaps not. A forest (or any biomass for that matter) is not a batch system, it is a continuous system. Maybe this is a uniquely chemical engineering perspective. When a person is trained in the art and science of chemical engineering, one of the fundamental approaches to understanding systems is the difference between batch systems and continuous systems. It does not matter whether a system is an industrial system, or a natural, biological system. In a batch system, we can observe how the chemical composition, or mass balance, of the system can change over time.  But in a continuous system, as older materials decay, new materials are added—or as mass enters the system, it also leaves the system at the same rate, and the system reaches a steady-state equilibrium balance.   This points to the fundamental error in thinking of the authors of the Manomet Study. They were looking at a small batch system of trees on a parcel of land, but they failed to realize that that small batch of trees was part of a much larger, continuous system. The study failed to properly evaluate the net carbon balance; they had mistakenly applied a batch carbon study, not realizing that the batch carbon data they presented in their study, was actually part of a much larger, continuous carbon system mass balance. This concept of a steady-state equilibrium mass balance applies to sustainable forestry, for example.  Whether the trees in a mature forest die naturally and decay, or are harvested and burned for fuel, the same equilibrium balance can be reached.  Assume that 4 percent of a commercial forest is cut down each year for fuel, on a 25 year cycle of harvesting and replanting.  During a typical year, that 4 percent of the forest is harvested, the other 96 percent of the forest continues to grow. If this sustainable forestry model is continuously observed for 25 years, after that time, the forest looks exactly the same as it did 25 years earlier. The total forest has the same sequestered biomass, the same biogenic carbon, that it had 25 years ago. In fact, during any given year during this cycle, the total forest has the same biogenic carbon at all times. The amount of carbon contained in the trees is the same at all times.  In a sustainable forest, the amount of trees, or the amount of carbon, being harvested on any given day is the same as the amount of carbon the rest of the forest has sucked out of the atmosphere on that particular day, or week, or month, or year, or any time unit that you wish to consider. Now, suppose this forest is 40 miles in radius, 80 miles in diameter, and a power plant is located in the center of this forest.  The trees are being harvested, chipped, stored, naturally dried, or even dried with the flue gas from the power plant, then used as fuel to generate electricity.  In place of the harvested trees, new seedlings are continuously replanted.  This forest will generate enough biomass from photosynthesis, from solar energy, to sustainably produce electricity at the power plant, forever.   As long as you don’t harvest the trees faster than they regrow, you are essentially producing electricity from solar energy. There is no net carbon emission from this model. But, assume 100 miles away, there is another power plant, burning coal to generate electricity.  Over the same unit of time, every ton of coal burned is taking sequestered carbon out of the ground and burning it, and generating CO2 for a one way trip to the atmosphere, to add to the GHG inventory. These two power plants could be identical in terms of their electrical power production. But the wood-based plant contributes no net CO2 to the atmosphere. The coal plant contributes all of its CO2 to the atmosphere. For comparison, consider a 1,000-MW power plant, using coal as its fuel.  A typical coal-fired power plant will consume 1 unit train of coal per day. That is 10,000 tons of coal; 100 coal cars with 100 tons of coal in each car.  About every 15 minutes, another 100 ton coal car is consumed by the plant.  When you burn coal, each pound of coal will generate 2.6 pounds of CO2. In one day, one unit train entering this plant will generate 2.6 “trains” of CO2, or 10,000 tons/day of coal will generate 26,000 tons of CO2.  (This is all assuming a typical bituminous coal, 12,000 BTU/pound; power generation at 10,000 BTU’s/kwhr of electricity generated, etc. Now, consider trying to sequester 26,000 tons/day of CO2 from this power plant. I defy the coal industry to prove this can be done on any thermodynamic basis that makes any sense. As a journalist, I often get lost in math, but those are the kinds of comparisons that I find really interesting. Because really, it is all about carbon math. Hopefully—but I won’t hold my breath—a new study on biogenic emissions will come out (hint: EPA?) in the near future, and it’ll get the same amount of attention that Manomet did and still does. That’s wishful thinking, though, because the headline won’t be nearly as sensational to the general media. Continue reading →

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