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Solutions to increase the methane yield from biomass: preliminary results for the application of enzymatic pre-treatment in a full scale biogas plant


As part of an experimental program conducted by the ENEA Research Centre in Bologna (UT VALAMB-IDR) and Biological Care S.r.l, the efficacy of applying an enzyme mixture as pretreatment in a full scale biogas plant constructed on the premises of Società Agricola Salera Michela e Anna Lisa & C.S.S. (CR) was evaluated. The plant was operating under a system involving co-digestion of livestock effluent and dedicated crops.

By starting enzyme mixture dosing (DSM MethaPlus® L 100), it was possible to reduce the daily quantities of maize silage introduced into the primary digester, reducing the quantity from an average of 25.8 t/day to 23.6 t/day. With the aim of maintaining the same hydraulic retention times (HRT) in the transition between the stages, as the flow of maize silage was reduced, the relative quantity of livestock manures was increased.

Laboratory batch tests carried out with the aim of determining the residual biomethanation potential are showing for the biogas reactors with an application of enzymes a 21% (first reactor) and 36% (2nd reactor) lower specific residual methane production potential.

Economy and operability of enzymatic pre-treatment
With the aim of evaluating the economic benefits that can be achieved by applying enzymatic pretreatment, an economic-financial analysis was drawn up with reference to a 15 year period, taking into account all the management costs incurred and avoided following implementation of pretreatment, based on the experimental results described previously. This analysis led to the calculation of a net annual value (NAV), in other words a sum of net cash flows arising out of the application of enzymatic pretreatment updated to the present, which was found to be equal to €280,149. The positive value of the NAV, combined with the other management advantages offered by enzymatic pretreatment (simplicity of application, lack of onerous and invasive adjustments to the plant line, absence of additional management costs) confirms the worth and economy of implementing this type of pretreatment.

Although these preliminary results only relate to a research period of limited duration, activities carried out on a full scale biogas plant allowed an initial quantification of the effective increase in terms of specific methane yield that can be obtained from the mix of substrates fed to the plant (maize silage and cow manure). For the same assumed energy production, enzymatic pretreatment allowed a reduction in the energy crops fed into the plant (maize silage) and a concomitant significant reduction in residual SGP values measured inside the primary and secondary digesters.

The above effects indicate that enzymatic pretreatment (DSM MethaPlus® L 100) can represent a beneficial solution from both an environmental and a financial viewpoint. It not only offers greater environmental sustainability of the biogas supply chain but can also offer financial advantages in terms of operational management of company plants by reducing matrix supply charges.

Source: Technica e Tecnologia

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