New valorization outputs for solid anaerobic digestate: biofuels and pyrolysis process
Résumé
In many European countries, biogas production through anaerobic digestion (AD) is a very fast
growing market in the agricultural sector. AD converts biologically organic matrix into biogas and
digestate, the latter corresponding to the anaerobically not degraded fraction. Until yet, digestate is
mostly used as soil improver or fertilizer at farm scale, but its increasing amount causes problems
relative to its transport cost, greenhouse gases emissions during its storage and its used is regulate by
the European Nitrate Directive.
In this study, we investigated two energetically valorization routes (i.e. biofuels, pyrolysis) to
maximize the energy recovery from the initial feedstock and reduce the digestate amount.
In the first experiment, digestate that contains still a high amount of sugars especially cellulose was
investigated for soluble sugars production for further biofuels used (i.e. bioethanol, biohydrogen…).
For this purpose, digestate was mechanically treated (i.e. centrifugal milling, 0.12 mm) and further
submitted to enzymatic hydrolysis. Energy requirement of mechanical pretreatment and enzymatic
hydrolysis yields were compared between the anaerobic digestate and the original feedstock. First,
energy requirement for mechanical pretreatment was less important for digestate (1.3 kWh.kg-1) than
original feedstocks (3.3 kWh.kg-1). AD has a positive effect on the physicochemical structures of the
original feedstocks and consequently leads to a reduction of the energy requirement. Then, the
enzymatic hydrolysis yield of digestate (63%) was increased compared to raw feedstock (54%). These
results suggests that digestate from AD process could represent a suitable residues for sugar release in
the optic of biofuels production.
In the second experiment, a new concept of coupling AD with digestate post-pyrolysis was
investigated. Pyrolysis of digestate at 500°C results to 8.8 wt. %, 58.4 wt. % and 32.8 wt. % of syngas,
oil and pyrochar, respectively. Like methane produced from AD process, syngas and bio-oil can be
further converted into heat and electricity through a combined Heat and Power system. By coupling
anaerobic digestion with pyrolysis process the electricity producted raised from 9896 KWhel day-1 to
14066 KWhel day-1 corresponding to an increase of 42% compared to AD process alone. On the other
hand, pyrochar can be used as soil amendment in order to preserve the fragile soil quality, a practice
which is in line with the principles of ecology and sustainable agriculture.