Abstract
Anaerobic digestion is an established technological option for the treatment of agricultural residues and livestock wastes beneficially producing renewable energy and digestate as biofertilizer. This technology also has significant potential for becoming an essential component of biorefineries for valorizing lignocellulosic biomass due to its great versatility in assimilating a wide spectrum of carbonaceous materials. The integration of anaerobic digestion and pyrolysis of its digestates for enhanced waste treatment was studied. A theoretical analysis was performed for three scenarios based on the thermal needs of the process: The treatment of swine manure (scenario 1), co-digestion with crop wastes (scenario 2), and addition of residual glycerine (scenario 3). The selected plant design basis was to produce biochar and electricity via combined heat and power units. For electricity production, the best performing scenario was scenario 3 (producing three times more electricity than scenario 1), with scenario 2 resulting in the highest production of biochar (double the biochar production and 1.7 times more electricity than scenario 1), but being highly penalized by the great thermal demand associated with digestate dewatering. Sensitivity analysis was performed using a central composite design, predominantly to evaluate the bio-oil yield and its high heating value, as well as digestate dewatering. Results demonstrated the effect of these parameters on electricity production and on the global thermal demand of the plant. The main significant factor was the solid content attained in the dewatering process, which excessively penalized the global process for values lower than 25% TS.
Highlights
The production of biogas by anaerobic digestion and its subsequent valorization either into electricity or up-grading, via gas clean-up technologies followed by injection into the natural gas grid, makes this process a key biological conversion technology
One of the main disadvantages of valorizing swine manure by anaerobic technology is the low productivity of the reactor, which has a close relationship with the organic loading rate
An average farm of large dimensions was assessed to reflect the technical feasibility of a centralized waste treatment installation
Summary
The production of biogas by anaerobic digestion and its subsequent valorization either into electricity or up-grading, via gas clean-up technologies followed by injection into the natural gas grid, makes this process a key biological conversion technology. The presence of lignocellulosic material in agricultural wastes poses challenges to this process due to the recalcitrant nature of lignin, which can result in lower biogas yields than expected when evaluated in terms of the organic loading rate of the feed, and the need of operating at a higher solid content. Recent interest has been focused on the effect of carbon-conductive materials on digestion performance, where it has been proven to increase biogas yields and enhance the activity of methanogenic microbial communities [41,42,43] In this manuscript, the performance of anaerobic digestion followed by pyrolysis of digestates was evaluated for three different scenarios conceived to produce char and biofuels for electricity production once the on-site thermal demand is fully met. The effect on the global process of bio-oil yields and its energy content was estimated considering the valorization of digestion gas and pyrolysis fuel products by means of a combined heat and power (CHP) unit
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