Environmental Engineering Reference
In-Depth Information
2.2 Pretreatment
•
Can acid pretreatments only be carried out with sulphuric acid or is it also pos-
sible with other acids for example with (low levels) of less corrosive organic
acids?
•
What does the effect of the potential pretreatment conditions have on carbohy-
drate decomposition?
•
What are the relative costs of traditional pretreatments (including capital costs)
compared to potential alternatives?
•
Are there possibilities to reduce costs?
2.3 Pretreatment Experiments
From preliminary studies it was found that pretreatment of various lignocellulose
materials that a variety of acids, acid concentrations, temperatures and times could
lead to interesting levels of glucose (after enzymatic hydrolysis) [26]. Based on this
a more detailed study was performed where wheat straw was pre-treated with water
(reference solution), sulphuric acid, or various organic acids (maleic and fumaric
acid), at acid concentrations of 50 mM, over a temperature range of 130-170
◦
Cfor
a 30 min period. The effect of such conditions on furfural and HMF formation and
fermentable sugar yields (after enzymatic hydrolysis) were studied [27].
After pretreatment only a small amount of glucose was formed, with most being
formed, together with low levels of HMF, under higher temperature conditions using
sulphuric acid. The production of xylose from hemicellulose (and degradation to
furfural), was more pronounced under increasingly acidic conditions. It was found
that the use of acid was in all cases superior to that of only water in increasing
the susceptibility of lignocellulose for enzymatic hydrolysis to glucose. In general
increasing temperature and use of acids, with increasing acid strength (Pk
a
), led to
higher levels of glucose being produced. Encouragingly the use of both sulphuric
acid and maleic acid yielded near stochiometric amounts and the weakest (organic)
acid used, fumaric acid, also resulted in very high levels (ca. 85%).
2.4 Pretreatment Costs and Acid Recovery
To determine if potential process costs could be reduced by decreasing the heat-
ing requirements by the use of smaller reactor volumes in the pretreatment stage,
higher concentration of straw were pretreated, keeping the weight ratio of straw:acid
constant. The results on glucose yield (after enzymatic hydrolysis) mirrored those
previously observed using lower concentrations of straw, so such an approach could
be a positive method of reducing costs.
However, compared to the reported traditional routes using sulphuric acid, the
costs of organic acid per tonne ethanol still remain high as a method of pretreatment.
It is estimated that the costs of the organic acid are ca. C300 per tonne ethanol
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