Thursday, November 3, 2016

A reader responds to the wood vs. coal debate

In our October newsletter, we reprinted a column by Cory Hatch from the Jackson Hole News & Guide.  We found it to be a thoughtful piece by a person grappling with energy choices and trying to fairly weigh their costs and benefits.  We got a lot of positive feedback about the article but one reader, a former professor of thermodynamics thought it was important enough to clarify something the author said.

The column made many good points, and concluded:

"After you factor in the energy and associated emissions it takes to extract, refine and transport fossil fuels, wood starts to look pretty good again. If you harvest trees close to home, firewood is fairly efficient and renewable, even if storing that carbon again takes some time. Unfortunately, woodstoves have local impacts, too. Chimney smoke contains particulates, nitrogen oxides and other gasses that can degrade air quality and cause health problems, especially for people with respiratory or cardiovascular disease."

The author also noted: "according to the Sierra Club, burning coal for electricity is only about 35 percent efficient, whereas a modern wood stove is about 75 percent."

Professor Gael Ulrich took issue with that:

" It is true that wood is superior in that it is renewable and not a fossil fuel, and it does not contain sulfur or other elements that can be problems in coal stack emissions.  But his statement, attributed to the Sierra Club, implying that wood combustion is twice as efficient as coal combustion is incorrect.  The distinction is subtle and not appreciated by someone without a background in thermodynamics, but I will try to make the reason clear.  

Converting fuel energy to heat and then to electricity can never be done with 100% efficiency.  Even the most modern efficient power plants seldom succeed 40%.  Historically, that principle was elucidated by Carnot, and the theoretical maximum possible is known as the Carnot efficiency.  In short, electricity or "stored work” is a higher form of energy than heat.  Converting “heat" to “work” always represents an energy loss manifest as “waste heat.” 

Coal power plants probably don’t do much better that 35% as suggested, but wood-fired electricgenerators are even less efficient (for reasons that would require more explanation).  Converting the energy content of coal to heat can, on the other hand, be done with high efficiency, matching or exceeding that of biomass combustors.  

Thus, the statement as worded in Hatch’s column is misleading.  His arguments regarding global warming, etc. are ok.  

I thought you might be interested in the error in case someone else has not already called it to your attention."

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1 comment:

  1. I was concerned about about Professor Gael Ultrich not adding a rider to the concept of wood being a renewable fuel. theoretically it is but only if used on a limited scale. Even on the present scale the trees are going faster than they are growing and we will soon need to wait 50 years for them to re grow. It is good to note his comments about the pollution aspect of residential wood burning. With the new PurpleAir pocket particulate monitors I am receiving data indicating levels, as a result of having a wood burning neighbour, twenty times that at a central air quality monitoring unit. It is no surprise that the San Francisco Bay Area estimates the cost of wood burning to the health care system at one billion dollars annually.