Drying of biogas digestate: A review with a focus on available drying techniques, drying kinetics, and gaseous emission behavior

Strict regulations imposed by the European Union (EU) with the target of reducing greenhouse gas (GHG) emissions oblige the biogas plant operators to manage digestate properly. Digestate is a by-product of biogas plants and is used extensively as an organic fertilizer. Digestate drying is a commercially available technology. According to the Fachverband Biogas (German Biogas Assiciation) (2018), 500–700 dryers are in use in Germany's biogas industry. Thermal drying can meet the requirements for hygienization and produce a transportable and storable fertilizer as well. Moreover, the GHG emissions from dried digestate are considerably lower than emissions from moist digestate. In addition to the high energy demand of the drying process, emissions occur during drying, resulting in both a decrease in the digestate's fertilizing value and some negative effects on the environment. Consequently, accurate investigation of the drying kinetics is needed in order to optimize the process with regard to energy consumption, drying time, and the amount of emissions. Drying of digestate faces many difficulties because of material specific properties as well as environmental issues. Digestate tends to stick to the dryer in high moisture content. In spite of additional energy requirements, back-mixing or additives is usually utilized to cope with this problem. These methods also have a positive effect on drying rate and considerably reduce drying time. However, they should be investigated exactly because they can sometimes have negative effect in case of excessive usage. Drying rate and also ammonia emission can be controlled by operational conditions, as well. On account of wide variety of digestate, availability of a generalized reliable model will be of benefit as it allows the engineers and designers to develop and optimize the drying processes. However, determination of primary data should be regarded as the first step. It should be noted that even though obtaining an efficient drying process with regards to both energy consumption and emission is not possible, a relative commitment can be achieved. Thanks to the advances of computational resources, numerical modeling could be considered a key instrument for future progress in designing and optimization of drying process provided that reliable input data are obtainable. This paper aims to review the literature on drying techniques, drying kinetics, and emission behavior of digestate.