Is coal or wood worse for the environment? Which fuel produces greater air pollution? Should wood, or any bio-mass, be considered an acceptable fuel? These questions are matters of hot debate in the Twin Cities today, and I wish I could say we have tracked down the answer. I can't say that, but I can describe the process of turning what looked like an easy answer into a series of unanswered questions.
Dan Gordon's District Energy: An empire built on wood is well-researched and sheds light on important energy and environmental issues. While digging for answers, Dan came up with interesting statistics from MPCA, which looked like proof for one side in the wood-burning vs. non-woodburning debate. The MPCA statistics pointed toward a conclusion that wood generates less of some greenhouse gases, but more particulate emissions (PM-10) than coal.
The MPCA statistics on District Energy showed that the total amount of particulate matter the plant’s smokestacks have released has more than doubled since 1990. The amount of ultra-fine particulates under 10 micrometers, which the EPA labels “inhalable coarse particles”, has also increased. In 1990, when District Energy was still burning coal, it produced about 14 tons of these particles. In 2005, the most recent year in which data for the plant is available, it released just over 176 tons.
I have a nasty, suspicious mind, which is a great asset in this job. When I read this paragraph in the original article, I immediately wondered whether the increase in PM-10 emissions actually was attributable to the switch from coal to wood, or whether other factors were in play.
Dan had already spent far more hours on the article than justified by the amount we pay free-lance writers. Moreover, the statistic affected just one paragraph in a lengthy article. I asked him if it would be okay if I tracked this statistic the rest of the way, and he agreed.
I checked the PCA website and verified the numbers, but that raised more questions. The PCA figures showed 176 tons of PM-10 emissions – ultra-fine particulates under 10 micrometers. But it showed TOTAL particulate emissions as 120 tons. How can the total be less than one of the components?
Since the PCA website raised more questions than it answered, I called District Energy. Four or five phone calls and e-mails focused on both the general question—Why are particulate emissions so much higher in 2005 than in 1990?—and detailed, specific questions about the numbers.
More information generated more questions, and fewer answers.
1) Back in 1990 (and probably continuing through 2001), regulators used a set of guidelines called AP 42, which list different kinds of combustion technologies and what industry-standard emissions rates were for each kind of technology. In other words, the amounts in the PCA tables for early years were arrived at by multiplication rather than measurement. Today's numbers are based on sampling (how often? what kind?) and on modeling from that sampling (how accurate?)
2) District Energy today produces MUCH more heat and energy than it did in 1990. How much more? Is the increase in emissions directly proportional to the increase in heat and energy?
3) Even though District Energy today burns wood to generate heat and electricity, it also burns coal. DE personnel say that most of the PM-10 emissions today are atrributable to the coal-burning, rather than the wood-burning operation. (Non-District Energy sources confirm this.) Skip the next quote if you want to avoid head-spinning numbers:
District Energy says:
For example in 2006, particulate emissions (PM-10) resulting from burning wood at the wood-fired combined heat and power facility in St. Paul comprised only 25 percent of the total PM-10 emitted at the facility (29 tons of total of 117 tons in 2006) and yet wood was 63 percent of the total fuel consumed at the facility (on a heat input basis). Coal is utilized to a limited extent (only 13% of the total fuel consumed at the facility in 2006 on a heat input basis) in separate boilers designed for coal and which supplement the primary fuel source, wood. The use of coal makes up the majority of the remaining PM-10.
Complicated? Oh, yes! And that's only the beginning. District Energy and independent sources agree in saying that the real determinant for PM-10 emissions is not the type of fuel used, but rather the type of emissions control system in place. Better emissions control = less PM-10 emissions. What emissions controls are out there? What is the state of the art?
Stay tuned – this is not the final word on the issue.
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