3.4. Consideration towards the cost of pre-treatment and the potential
for its improvement to electricity production
When making a evaluation of treatment costs and the resulting
biomethane potential yields, the use of Ca(OH)2 pre-treatment to
3 mm milled wheat straw is recommended. As shown in Fig. 2, this
treatment combination performed well, in terms of potential gas
yields, at every time point examined. Moreover, when using
Ca(OH)2 pre-treatment no potential yield benefits from more
extensively milling the straw to 3 mm or below were apparent.
Although the addition of enzyme was also able to improve biomethane
potential yields, this would be heavily counteracted by
the cost of the enzyme dosage which is a much more expensive
ingredient than the amount of Ca(OH)2 used for pre-treatment.
However, it should be acknowledged that as enzyme production
technology is constantly being developed and driven by increased
global demand the price of cellulase should decline (Sarrouh et al.,
2012).
At the time of writing Ca(OH)2 can be bought for $140 US per
tonne, which equates to $10.36 US for the amount required per
tonne of wheat straw VS for the lime treatments described here.
According to the equation calculated by Mani et al. (2004), the
electricity demand of milling wheat straw (containing 87.9% TS)
to 3 mm can be calculated to be 27.76 kW h t1. Menardo et al.
(2012) calculated that 4 kWh of electricity can be recovered from
1 Nm3 of methane.
Based on biomethane production over 30 days in this study, the
combination of alkaline pre-treatment and milling to 3 mm would
improve the electrical energy potential yield by 284 kW h t-VS1
wheat straw, when compared to the 936 kW h t-VS1 potential
electrical energy produced from the 10 mm milled wheat straw
control. When considering the use of a shorter 15-day batch
anaerobic digestion period, the same pre-treatment would increase
the average electrical energy generated by 74% (480 kW h t-VS1
wheat straw), on top of the 648 kW h t-VS1 obtained from the
10 mm wheat straw control.
The potential improvement in electricity production from
wheat straw calculated from batch anaerobic digestion after
milling and Ca(OH)2 pre-treatment in this study is promising.
However, further work which involves the continuous feeding of
Ca(OH)2 pre-treated wheat straw to steady-state anaerobic digestion
and a subsequent full economic input–output life-cycle-analysis
is now required. This is essential to establish if the results are
transferable to continuous operation and if the pre-treatment is
commercially cost effective.