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Tag Archives: biosequestration

Bioenergy: new blooms, but also prunings

By Dave Elliott

In an opinion survey by YouGov for the Energy Technologies Institute, 80% of respondents supported an increase in bioenergy use in the UK. Around 74% supported producing bioenergy from biomass and 81% backed producing biomass from waste, comparable to levels of support seen for other renewables. However, some environmental groups strongly oppose some types of biomass use and there is a sometimes rather heated debate underway, as this extended post explores.


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The biochar debate

There is one way we
could save ourselves, and that is through the massive burial of charcoal.

James Lovelock

Converting biomass into
charcoal type char which can be used to improve soil fertility, while also
trapping carbon dioxide, certainly has major attractions. But a key issue is whether, in net
climate terms, the loss of (some)
biomass for direct conversion to energy is balanced by the gain from CO2
entrapment and extra CO2 absorption by more fertile soils – especially if the
combustion route also used geo-sequestration i.e. CCS?

parametric study of bio-sequestration by Malcolm Fowles at the Open University,
suggested that from a global warming perspective we should displace coal with
biomass if the latter’s conversion efficiency is much over 30%. Otherwise we
should sequester carbon from biomass rather than generate energy.

this was only a preliminary study and he felt that a more comprehensive
analysis might shift the balance more towards bio-sequestration. He did not
include carbon savings from hydrogen and other pyrolysis products, or crucially
from reduced soil emissions- that’s hard to assess after all. And costs were not included in his
model, although qualitatively and intuitively he felt bio-sequestration should
be cheaper than geo-sequestration by CO2 capture and storage. (Fowles, M. (2007), “Black carbon sequestration as an
alternative to bio-energy”, Biomass and Bioenergy 31: 426–432, doi:10.1016/j.biombioe.2007.01.012).

Clearly though there are lot
of unknowns – for example as to the permanence of bio-sequestration – how long
will the carbon stay trapped in the soil? Some say thousand of years, based on historical examples of charcoal
use. But then that was in traditional “no til” agricultural contexts: farming methods would now have to change
if we wanted to avoid releasing the stored carbon.

are also strong views about the likely impact if biochar production was adopted
on a wide scale. While some see it
as a major way to deal with climate problems, the fear of vast agri-business
plantations worries some people, Guardian correspondent George Monbiot
especially, although even he accepts that there could be niche uses.

can be produced by pyrolysis at around 500 degrees C, either slowly (over days,
the traditional approach e.g. in kilns), which results in about equal amounts
of biochar (about 35% of the original biomass), liquid and gaseous fuels; or
rapidly (e.g. flash pyrolysis, in
seconds), which gives less biochar (about 15% converted) less gaseous products, but more liquid “bio-oil”
products (about 75%). In addition there is high temperature (800 °C)
gasification, which typically, over hours, yields a low proportion of solids
(only about 10% biochar), but a high proportion of gaseous products (about

with fast pyrolysis or gassifiation the processing throughputs can be larger,
but slow pyrolysis gives you more biochar in the mix. For example, BEST Energy
in Australia, have developed a slow pyrolysis approach called Argichar, in
which between
25 and 70% by weight of the dry feed material is converted to a high-carbon
char material, while also generating
syngas: see


How much carbon
sequestration might be achieved? Globally, according to Professor Tim
Lenton, from UEA: “Biochar has the potential to sequester
almost 400 billion tonnes of carbon by 2100 and to lower atmospheric carbon
dioxide concentrations by 37 parts per million.” How does that
compare to other approaches, like Carbon Capture and Storage? Biochar production removes CO2 from the
air, while CCS aims to remove it from the exhaust gases of power plants – in
large quantities. According to Bruce Tofield, from the Low Carbon Innovation
Centre, UEA: “In the UK biochar
might yield a few million tonnes CO2 saving with current biomass
sources – CCS needs to aim for over 100 m tonnes.”

However, that doesn’t mean
turning biomass into biochar is a bad idea, and some environmenalists are quite
enthusiastic. In The Renewable World, a new book from the World Future
Council, Herbie Girardet and Miguel Mendonca (Green Books) are very keen on
techniques for improving soil fertility and biological carbon dioxide absorption,
and talk of “carbon farming”. They
note that “by pyrolysing one tonne of organic material which contains about
half a tonne of carbon, about half a tonne of CO2 can be removed from the
atmosphere and stored in the soil, while the other half can be used as carbon
neutral fuel”. However they add that “a major question that needs an urgent answer is how enough organic matter
can be made available to produce significant amounts of biochar. Opponents
argue that farming communities in developing countries may be forced to produce
fast-growing tree monocultures on precious agricultural land to produce biochar
to counter climate change for which they are not even responsible”. But
they point to sewage as an example of a less contentious feedstock.

are no doubt many other niche sources of biomass like this, as well as novel
sources like algae, although there may also be competing uses (e.g. sewage gas
is one of the cheapest renewable energy sources for electricity generation). But then we are back with the question
of which is most effective at reducing carbon dioxide?

Royal Society’s recent review of Geoengineering commented: “It remains questionable
whether pyrolysing the biomass and burying the char has a greater impact on
atmospheric greenhouse gas levels than simply burning the biomass in a power
plant and displacing carbon-intensive
coal plants.” It concludes: “Biomass
for sequestration could be a significant small-scale contributor to a
geoengineering approach to enhancing the global terrestrial carbon sink, and it
could, under the right circumstances,
also be a benign agricultural practice. However, unless the sustainable
sequestration rate exceeds around 1 GtC/yr, it is unlikely that it could make a
large contribution. As is the case with biofuels, there is also the significant
risk that inappropriately applied incentives to encourage biochar might
increase the cost and reduce the availability of food crops, if growing biomass
feedstocks becomes more profitable than growing food.”

That is a point picked up by
James Bruges in the new Schumacher society report The Biochar Debate (Green Books). He argues for a global
Carbon Maintenance Fund, rather than just awarding carbon credits. But that is rather going ahead of
ourselves. First we have to see if the biochar option makes sense. The Royal
Society pointed out that so far there was not enough research on the topic. Defra has commissioned the UK
Biochar Research Centre (UKBRC) to review the impacts of biochar. Hopefully
that will provide some answers.

More at

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