Thermochemical processes, such as pyrolysis, have the potential to convert anaerobic digestion effluent into higher value products, including biochar and biooil. Biochar is a solid product with high content of stable carbon that is used as soil amendment. Biochar benefits to the soil are numerous and include increase in nutrient availability, improvement of physical and microbiological properties, soil remediation, and providing a long-term carbon sink. Biochar from anaerobic digestate has enhanced properties over the dried digestate, including higher surface area, improved concentration of nutrients, and higher adsorption capacity. However, digestate drying is required for the pyrolysis process. Hydrothermal carbonization (HTC) is performed in the presence of subcritical liquid water at high pressure, in the absence of oxygen. Thus, it can process the digestate directly without dewatering. HTC produces a ‘hydrochar’ and a liquid stream. The hydrochar is highly hydrophobic, which facilities its separation from the liquid stream. Hydrochar properties and potential uses are very similar to those of biochar.  When land applied, biochar and hydrochar, have a potential to improve greenhouse gases (GHG) emission and reduce nutrients runoff. However, the properties of these two products differ significantly. Hydrochar appears to be less stable than biochar, and to significantly increase the microbial activity, which can increase the GHG emissions. In addition, the energy required for drying the digestate and for the thermochemical conversion can have a detrimental effect on the environmental sustainability of these processes. Thus, in this work, we perform a life cycle assessment (LCA) of the production and land application of biochar via slow pyrolysis and hydrochar via HTC from anaerobic digestate. We compared these two processes with the direct land application of the digestate, with and without dewatering, and with direct combustion of the digestate. LCA is performed according to the standard ISO 14040: 2006 using OpenLCA software. The system boundary include from the collection of anaerobic digestate to the land application of the final product. Data from literature and experiments, along with the Ecoinvent database are used for the inventory analysis, and the impact assessment is performed using TRACI 2.1. With this work we will determine the environmental impacts of these thermochemical processes for anaerobic digestion management.

Environmental assessment of pyrolysis and hydrothermal carbonization of anaerobic digestion effluent

2019 Fall National Meeting

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