On Sep 16, 2010, at 11:38 AM, David Keith wrote:
while cofiring biomass in coal-fired power plants without capture
does not rank on your list at all, because there is no capture, it
still avoids emissions of carbon. And because the capital cost of
retrofitting plants large coal plants for biomass co-feed is very
low and their combustion efficiency (for the biomass) is high this
can be a very cost-effective way to reduce carbon emissions with
biomass. Of course, it's not sexy and it's limited.
David this simply isn't right.
Firstly there IS carbon emission. Depending on how wet your feedstock,
is biomass can in some casess emit more co2 per kilowatt produced than
coal - thats real co2 aloft in the atmosphere for the next century.
Whether that Co2 can subsequently be quickly fixed by theoretical
replacement plants depends on what those plants are and how assiduosly
they are managed. At present most co-firing of coal for electricity is
using woodchips and other forest biomass. Releasing the carbon from a
mature tree takes seconds, refixing it in another mature tree takes
decades - maybe as long as seventy to a hundred years. All of that
time the initial Co2 released is still aloft and still impacting the
climate. That biomass can't even pretend to be carbon neutral for
several decades and as we all know timeframes are critical in
addressing climate change. Environmental journalist david baumann puts
it starkly: “it would take over 100,000 one-year old trees to equal
the weight of a 50 year old tree of similar species. Five year old
trees take around 30,000. So you see for every tree we cut down and
burn we’d have to plant 100,000 to resequester that much carbon in one
year, 30,000 in five years. We’d also have to find the space to plant
them.” .
The orthodoxy used to be that you can just plant fast growing
eucalypts to sequester the CO2 faster than the older trees but that
was based on a single study from the 1960’s only applicable to one
particular forest type and in fact its now understod that old growth
forests sequester much more carbon than young plantations and that for
the first 20 years or so a young plantation that replaces old growth
is stil giving more co2 into the atmosphere than its fixing.
Moreover the carbon footprint of bringing biomass to the power plant
is not insignificant as biomass is heavy stuff with low energy content
(heavier when wet but drying consumes space and energy too). Heres one
quick back of teh envelope calculation on that: a standard 40 ton
truck full of recently harvested woodchips will emit close to a
kilogram (0.91kg) of CO2 for every km that it drives when delivering
those woodchips for burning at a biomass electricity plant. Even a
smallish 50MW plant would require 12750 such truckloads per year. At
an average distance for sourcing woodchips of 68 km, that amounts to
almost 790 tons (788,970 kg) of extra CO 2 emissions per year just
for transport of wood chips alone. or put another way an additional 15
tonnes of Co2 per megawatt.
Lets say instead of wood you can use a biomass feedstock that
replenishes quickly such as annual crops, grasses or algae. (This
doesn't address the transport problem in fact it worsens it- you would
likely need more truckloads of crop residue than woodchips to get teh
same energy output). As Andrew correctly noted, production of
agricultural biomass feedstocks have significant greenhouse emissions
associated with them. The more material you take from the land for
burning, the less you have to replenish soil fertiity and hence the
more nitrogen fertiliser is required to maintain productivity.
Fertiliser production is responsible for approximately 1.2% of total
GHG emissions – equivalent to the full greenhouse gas emissions of
Indonesia or Brazil. In the US alone thirty percent of energy use in
agriculture is accounted by fertilizer use and production. Thats
before acount for nitrous oxide emission from fertiizer applications
and then again any methane or nitrous emissions form eutrophication
and runoff into dead zones such as the gulf of mexico. Algae is no
better - commercial freshwater algae systems require higher
concentrations of fertiliser than corn because they have no soil to
pull nutrients from and we know what large scale saltwater algal
production looks like - its called ocean fertilization and it raises
its own problems. You can harvest wild algae from eutrophic systems
and deadzones but thats building dependence on an ecoloically
unhealthy feedstock for your energy needs.
Even before you plant biomass feedstocks there will be significant
soil carbon release from land clearances and land use change
associated with turning so-called 'marginal'/unproductive lands over
to biomass cropping. The Stern report identified a full 18 percent of
climate gas emissions were the result of land use changes, second only
to emissions from the power sector. This is the reason why the folks
who initially argued for biomass to be counted as carbon neutral
within the IPCC last year admitted they had made a 'critical
accounting error' (searchinger et al)
and so on...
The claim that industrially burning biomass for power 'avoids
emissions of carbon' is at best unexamined wishful thinking, at worst
an outright lie.
Jim
Jim Thomas
ETC Group (Montreal)
[email protected]
+1 514 2739994
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