Perspectives on Bioenergy and Biofuels - Sustainable Biotechnology: Sources of Renewable Energy - page 186

2 Technical Details and Status of Technological Implementation

Consider a biomass type (typical of a lignocellulosic material) with a composition
of ca. 55% carbohydrates, ca. 15% lignin, ca. 10% protein and ca. 20% other organic
components with a calorific value of 15-18 GJ of energy per tonne (dry weight) [23].

2.1 Q: Burn or Bioethanol?

• Assuming use for incineration for heat and electricity, comparisons with coal

should be made. The biomass could be expected to generate 15-18 GJ of energy
which could replace ca. 0.6 tonnes of coal. 1 Assuming coal has an average carbon
content of 80%, then combustion of one tonne biomass could also save ca. 1.8
tonnes of CO 2 emissions [24] 2 . Due to the lower cost of coal compared to other
fossil materials, C3 per GJ 3 [25], the biomass has its lowest financial value. In
this example it would have a potential value (based on coal) of ca. C50 per tonne.

•

Should the same biomass be used for the production of ethanol then, assuming
full conversion of all carbohydrates, 275 kg of ethanol could be produced (with
450 kg of unused organic material). The use of the ethanol 4 as a fuel could gen-
erate ca. 8.3 GJ of energy and combustion of the unused organic material could
generate up to 8.1 GJ of heat energy. In terms of energy (providing no losses due
to process) then this is comparable with the example above. However when the
costs per GJ for the applications are considered this changes. Fuel sources such
as petroleum have a value of ca. C8 per GJ. Thus the biomass could have a total
value of ca. C90 per tonne for example (ca. C66 for fuel application and ca. C24
for heat application).

Thus one may conclude that the use of lignocellulosic material could generate
more added value if used to produce fuel and more effectively if all components are
used for an application providing the costs for isolation and/or transformation are
favourable.
Is it possible to develop processes for the conversion of lignocellulose to
bioethanol without (or reduce) the use of large quantities of corrosive sulphuric acid
(for pretreatment and hydrolysis) circumventing the need for recycling of mineral
acids and use of other chemicals such as lime?
One possible approach is a short, (low concentration organic) acid pretreatment
followed by hydrolysis with an aqueous solution of cellulase enzymes. In such an
approach there are several issues that need to be explored.

1 Calorific value of ca. 24 GJ per tonne
2 600 kg

1,776 kg CO 2
3 usually expressed as currency per British thermal unit (Btu), 2007 figures
4 Calorific value of ca. 30 GJ per tonne

× 0.8 = 480 kg C, combustion of C results in 480 kg × 44/12 =