[Digestion] Bio methane economy?

[Björn Dahlroth]

Comment to Cezar and others
 
There is no doubt that increased production of ethanol from grain has
importance for the price increase of food but there are also other reasons
like increased demand for more advanced food from rising economies like
China and others. Most of the grain from corn and beans from soy is used for
animal feed because we prefer more meat and more dairy products when we
become a little richer rather than eating much of the corn and the soybeans
ourselves(which of course is quite possible to do).

But the question was whether biogas crops are more food crop friendly and
some time ago there was some question about if LCA-analysis could not
clarify what is best.  It is difficult to give precise figures but there are
indications from different sources - many available on the web if you surf
around - that biogas in theory could be better than making ethanol. But one
can not make a general statement as it all depends on different
circumstances. But first one important statement about LCA-analysis.

LCA-analysis is very valuable tool to make you understand some of the
consequences if you do one thing or another, but it doesn't consider market
conditions nor agricultural politics, but market forces and politics are
important realities that we have to live with. Another shortcoming in
LCA-analysis is that it is practically necessary to limit the system and to
make assumptions about the outside world. A very common assumption is that
electricity is produced in some special way, base power, mixed system power,
marginal short term power or marginal long term power - of different kinds.
What you assume there can change the result completely. It is the same with
fuel and the fuel market. Replace one fuel and try to understand how the
market reacts - on the margin. The effect of ethanol production on food
prices and agricultural production in other parts of the world has probably
not been part of any LCA-analysis and has not been pointed out until
recently. The fact that the European decision to mix 5% bio-oil in the
diesel oil would lead to more land clearing, burning of trees and brush, and
more oil palm planting in tropical countries was probably also not
considered. It is the same for bio-oil that much of it is not used for food.


One problem with making a good LCA-analysis on digestion is that it seems
difficult to get reasonably good and representative information on the
electricity demand and heating demand of bigger digestion plants. There are
indications though that some plants use a lot of electricity and a quite a
bit of the produced gas for pasteurisation and keeping the temperature level
in cool climates. For the cases where the substrate is food waste from
industry or from source separated household waste it seems difficult to find
good information on the energy content of the raw material that is used.
(The reason is probably that LCA-specialists erroneously do not consider
food waste to have any heat value.) Another problem with the efficiency of a
digestion plant that for instance is using food waste from households is
that the business economy depends on the fee that they charge the waste
supplier and the income they have from the sales of gas. If they let the
substrate be more completely digested it will take a longer time, the
capacity will go down and the economy will suffer. So often the residue
still has biodegradable matter left which is treated in a composting
process. This is another loss of efficiency and can also lead to release of
some methane if the process is not monitored and managed well. In the
upgrading process there is also some consumption of electricity and risk of
losing some methane. 

To cultivate a crop and digest it would normally yield more biogas-energy
per hectare than what you can get as ethanol-energy if you ferment the same
crop. Digestion can use cellulose, starch and sugar whereas you can not
ferment cellulose unless it is treated in a special way. Fermentation
requires a lot of energy for distillation. Digestion also requires energy
for electricity and heating but probably less than a fermentation plant with
the same energy otput. The best crop for digestion might not be the best for
fermentation so comparison is not that easy. In Sweden we have a plant
producing ethanol from wheat for mixing with gasoline. We have also a plant
for producing biogas from pasture crops. So how can these be
compared.(Neither plant would have been built without subsidies of different
kinds.)

An alternative that will always give more vehicle fuel per hectare is to
grow the kind of crop which locally - depending on soil and climate - will
yield the maximum quantity of dry matter biomass per hectare and then use
that in a big thermal gasification plant and produce methane, methanol, DME
or ethanol according to market demand. However this is still only theory.
There are practical problems to be solved. With thermal gasification you can
also used wood and forestry residues. (You can also make ethanol from
cellulose but it seems some more research remains to be done. Very important
is the market for the by-products just as it is if you produce ethanol from
grain.)

If the main reason for using biomass for energy is to decrease the release
of CO2 from the use of fossil fuels, then the future most efficient way is
to burn the biomass in a combined heat and power plant and replace
electricity that on the margin is produced from coal condensing plants. The
heat from the CHP-plant will also replace other fuels for heating and those
fuels can be used somewhere else to decrease the release of fossil CO2. If
you already have biogas it may actually be better to use the gas in a
stationary engine and produce electricity and heat instead of using it for
cars and buses but the tricky thing is that vehicle owners might be willing
to pay a much higher price then the value of electricity and heat and this
depends very much on taxation. However - CHP is senseless if you don't have
any heat demand.

What we must remember is that it is agricultural land and forest land that
are the ultimate limited resource. There is always competition about land,
and that will limit the available quantity of biomass even more - be it
wood, leguminous plants, sugar cane,grain, animal manure, food waste etc. As
biomass is likely to become a limited resource - it might start within just
a couple of decades - we will have to use whatever biomass that will be
reasonably available and that will depend on local conditions. What will be
locally available? What must be imported? What are the local and the global
markets going to be like?  

So in the future we will see many kinds of bio-fuels for vehicles.
Even if ethanol from fermentation or biogas from digestion are not the
ultimate big solutions for fuelling vehicles they will be around in the
future and have some share of the market. For this reason one must continue
striving to achieve lower cost and higher energy efficiency. One must also
not forget that we will have agricultural politics in the future. 

General answers are not possible. It all depends. 

This, shall I call it, discussion club for biogas is quite interesting to
follow. I have picked up some good grains from it and there will probably be
more coming.

Regards to all
Bjorn Dahlroth

-----Ursprungligt meddelande-----
Från: digestion-bounces at listserv.repp.org
[mailto:digestion-bounces at listserv.repp.org] För Cezar T
Skickat: den 27 februari 2008 22:52
Till: digestion at listserv.repp.org
Ämne: [Digestion] Bio methane economy?

Hi everyone,
I agree "hydrogen economy" is a losing proposal, but
how much better is biogas? How does its economy and
energy balances comparing with other biofuels, or
other renewable energy resources?

There is a recent trend to blame food increase prices
or  rainforest cuts and even increased greenhouse
gases upon the increased demand of "evil" biofuels -
like ethanol or biodiesel. Can biogas be considered
more food-crop friendly?

thanks,
cezar


 
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