Abstract

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.

摘要

欧盟 (EU) 以减少温室气体 (GHG) 排放为目标实施的严格法规要求沼气厂运营商正确管理沼气。沼液是沼气厂的副产品，被广泛用作有机肥料。消化物干燥是一种可商购的技术。根据 Fachverband Biogas（德国沼气协会）（2018 年）的数据，德国沼气行业使用了 500-700 台干燥机。热干燥既可以满足卫生要求，又可以生产可运输和可储存的肥料。此外，干消化物的温室气体排放量远低于湿消化物的排放量。除了干燥过程的高能量需求外，干燥过程中还会出现排放物，导致沼渣的肥料价值下降和对环境的一些负面影响。因此，需要对干燥动力学进行准确研究，以便在能耗、干燥时间和排放量方面优化过程。由于材料的特定特性以及环境问题，消化物的干燥面临许多困难。消化液往往会粘在高水分含量的干燥器上。尽管需要额外的能量，但通常使用返混或添加剂来解决这个问题。这些方法还对干燥速率产生积极影响，并显着缩短干燥时间。但是，应该对它们进行确切的调查，因为它们有时会在过度使用的情况下产生负面影响。干燥速率和氨排放也可以通过操作条件来控制。由于消化物种类繁多，通用可靠模型的可用性将是有益的，因为它允许工程师和设计师开发和优化干燥过程。但是，应将确定原始数据视为第一步。应该指出的是，即使在能源消耗和排放方面获得有效的干燥过程是不可能的，但可以实现相对承诺。由于计算资源的进步，如果可以获得可靠的输入数据，数值建模可以被认为是未来干燥过程设计和优化进展的关键工具。本文旨在回顾有关干燥技术、干燥动力学、