TucuruÃ, Brazil's second largest dam has many times the GHG emissions of a natural gas plant of the same capacity -- though there is fierce argument over whether that output substantially exceeds what a natural watercourse would produce. (The emissions are due to methane from trapped organic matter in the dam.)
There is now a proposal to tap that methane to run gas turbines and produce electricity, reducing the emissions many times, since CO2 from burning the methane has a much lower impact than the methane itself. It would also close to double the electrical output from the dam. This seems very close to an acknowledgment that critics of methane from dams are correct. Outside of estuaries, I don't know many natural water courses that might be tapped in such a way. I have to admit that it is an ingenious solution to the problems of dams as methane sources.
How complete a solution is another question: I'd be curious to know what percent of methane production they are proposing to capture. They don't have to capture all of it to reduce greenhouse impact to zero -- some of the methane would produced anyway, though how much is under dispute. At any rate, I'd be curious see estimates of net methane production minus the dam, compared to net methane production with the dam plus methane capture and burning.
Methane capture won't undo any of other damage dams often do, of course, but it may be a solution to the problem of dams as net greenhouse emitters.
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paddymccully Posted 10:47 am
08 Jun 2007
There's no serious debate that the emissions from Tucuruí are much higher than the "natural watercourse". The hydro industry-linked scientists in Brazil say that the net emissions may be much lower than natural gas alternatives, but even they would admit that the emissions are much higher than pre-dam ones. (And they can only claim that the emissions are lower than natural gas by ignoring the main source of emissions, which is the degassed methane at the turbine outlets, below the spillway and immediately downstream). There is a lot of debate over what proportion of CO2 emitted from reservoirs would have been released from the river w/out the dam (or represent carbon which was in any case sequestered from the atmosphere by biological productivity in the reservoir) - but informed agreement that most methane emissions are produced only because of the reservoir.
The scientists working on reservoir methane capture from the Brazilian Inst. for Space Research (INPE) believe that they could capture around 60% of methane in the reservoir deep water. Some of this methane would not have been released to the atmosphere - a lot of the methane is oxidized to CO2 as it rises to the reservoir surface. This is why the sudden release of methane when deep water is released through turbines/spillways is so significant - ie there is no chance for it to be oxidized before release.
Patrick
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