Drying and combustion of an anaerobic digestate: Results and economical evaluation of a demonstrative-scale plant
Abstract
A demonstrative-scale drying-and-combustion plant, treating a third of the digestate produced in a large cattle slaughterhouse in Italy, was designed, built, operated, and monitored for two years in order to define its energetic, environmental, and economical performance. Raw and dried digestate, ash, as well as flue gases compositions, were monitored during the research period and mass balances were defined. Energy and chemicals consumption were accounted in order to obtain representative scenarios and to discuss the practicability of this in-situ approach. It resulted that the drying-combustion solution is interesting if: (a) the lower heating value (LHV) of the digestate is higher than 17,000-18,000 kJ per kg of dry matter, (b) the anaerobic plant is large, with a relevant production of digestate (more than 10 t d-1 ), (c) the cost of conventional disposal of the digestate is higher than 50 € t-1 . The chosen technologies appeared adequate to respect the regulatory limits regarding emissions
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Introduction
The continuously increasing organic solid waste generation worldwide calls for management strategies that integrate concerns for environmental sustainability [1]. Due to the growing demand of meat in the world, the contribution of organic solid waste from meat industries to the global waste production is increasing [2]. Approximately, a fraction between 20% and 50% of animals’ weight is not suitable for human consumption, and only a quota of this waste (depending on the applied technologies, the socio-economic context, etc.) can be used in other non-food applications. Considering that the bovine meat production reached 68 106 t (the overall meat production was estimated at 312·106 t) during the year 2014 [3], the amount of related organic waste is not negligible [4, 5].
It is well known that organic solid waste from meat producing industries is considered a suitable substrate for biogas production via anaerobic digestion, because of its high content of fat and protein [6, 7].
Nevertheless, anaerobic digestion process produces relevant amount of a solid residue (digestate) that contains all the recalcitrant substances that can’t be converted into biogas. Digestate gained a controversial reputation in the last years. On the on hand, it contains nutrients and stabilized organic matter, and can be used as organic fertilizer and/or soil conditioner [8].
On the other hand, an excessive or non-optimized use of digestate in agriculture can have a negative impact on the environment. For example, digestate can emit relevant amount of residual methane, ammonia, and nitrous oxide, contributing to climate change and local pollution [9]. Moreover, digestate can pollute surface water and groundwater through nitrate runoff and leaching [10] and can contain pathogens and antibiotic residues [11].
Up to now, the agricultural use of digestate derived from anaerobic digestion has not been specifically regulated by the European Community and only local guidelines have been proposed, with the exception of the application of Good Agricultural Practices [12], as well as the European “Nitrate Directive” (91/676/EEC) that strongly limits the amount of zootechnical nitrogen that can be applied per hectare of agricultural soil.
Conclusion
The present research shows that it is possible to minimize the production of solid waste from the anaerobic digestion process and, at the same time, to recover a fraction of nitrogen under a chemically stable form (ammonium sulfate). The process can be self-sufficient, even if it requires some particular attentions. It is in fact necessary to verify if the LHV of the dried digestate is high enough. From our results, it appears that the LHV should be higher than 17,000-18,000 kJ kg-1 in order to avoid the need of an additional fuel. Accordingly, an efficient thermal insulation is also required in order to minimize any heat loss. Nevertheless, the cost of the plant structure and of the reagents resulted high, and this solution appears feasible only for large anaerobic plants and/or particular conditions that imply an expensive disposal of the digestate (for example, when the digestate is considered a waste and cannot be used in agriculture). In the conditions of the present study, it was observed that the in-situ drying-combustion process is feasible for a digestate production higher than 10 t d-1 and for a conventional digestate disposal cost higher than 50 € t-1 .