Biofuels in Perspective

Biomass Digestion to Methane in Agriculture 193
should have the following characteristics: it should mainly produce rapidly convertible
carbohydrates (sugars, starch, cellulose) and contain a minimum of lignin, hemi-cellulose,
proteins, minerals, pigments. Equally important is that the mass of produced organic matter
per unit are is high, which depends basically on three factors: (1) the climate (2) the land
and agricultural techniques applied and (3) the nature of the plant and quality of the seed.
In other words, it must maximize the conversion of sunlight to digestible carbohydrate.
At present, of the 15,000 kW power of sunlight which one ha of corn receives, hardly
5 is trapped as potential bioethanol power, and a mere 1.5 kW (1 out of 10 000) is
recovered as effective bioethanol fuel. To the best of our knowledge, no conventional
breeding programme to generate an optimal energy/biogas producing crop has ever been
set up. New methods of crop ‘design’ implementing various genetic potentials in a specific
plant used for biogas production, should be able to considerably increase the harvesting
of the sun energy. At present, considerable effort is underway to design new types of food
crops; it stems to reason to expect rapid advances in the design of new biogas-dedicated
crops.
A second line of progress will be the development of new technologies to produce
biogas. Certainly, the current techniques to harvest and preserve the biomass before it is
submitted to biogas digestion need to be tuned to the subsequent and overall rate limiting
methane production process. With respect to anaerobic digestion itself, two lines of reactor
configuration are of value A first line relates to intensive digestion in highly technical
hardware, as described above. Plenty of novel concepts will certainly arise and allow to
enhance the overall energy recovery. Of special interest is for instance the coupling of
anaerobic digestion with subsequent further removal of the residual organics in microbial
fuel cells (Rabaey and Verstraete, 2005) Yet, this approach will demand major investments.
Although on average a digester constitutes only a relative simple amount of infrastructure
representing on average an all-in capex of 1000 Euro per m 3 , its payback at the current
energy price of methane (0.2 Euro per m 3 ) and reactor rate of methane production (10
volumes of methane per volume of reactor per day) is still of the order of several years.
Hence, it stems to reason to also consider the possibility to harvesting biomass, bringing it
together in large scale landfill type reactor system and then utilize this biomass, stored in
a less expensive way, over a longer time scale (Verstraete et al., 2005).
A third line of improvement must be sought in the better positioning of the biogas
produced. At present, the biogas is generally combusted to render electricity. Technology
to upgrade it to vehicle fuel is already long time available (Henrich, 1981). Yet, provided it
is produced at sufficiently large scale, a thermal conversion to syngas (H2 and CO) is quite
possible (Effendi et al., 2005; Wang and Chang, 2005). The advantage of this route is that
the conventional petrochemistry can directly tap on this resource and produce from it all
possible commodities. Once biogas digestion is linked to conventional petrochemistry, it
will have a major inroad to the chemical industry and hence the overall industrial society.
10.6 Conclusions
Biogas from agriculture has several strong points. It can deal with a variety of materials and
it thus fits perfectly in the various routes of downstream processing of the agro-business.
Secondly, it allows recovering energy with a maximum of efficiency because its end product,
methane, distils by itself from the mixture which is used to produce the fuel. Third, it can