Investigation of nitrogen loss during laboratory scale fixed-bed drying of digestate

Abstract

In addition to the drying kinetics of digestate, a comprehensive knowledge is necessary on nitrogen loss and ammonia emissions in order to appropriately develop and design a dryer. Otherwise, these phenomena would result in environmental concerns and reduce the nutrient value of digestate. Expectedly, drying parameters considerably affect drying time. In this study, drying experiments were conducted at a laboratory scale fixed-bed dryer. The drying experiments were performed on dewatered digestate that mainly consisted of grass and maize silage (initial moisture content 76.4% w.b., 0.026 kg N kg^-1 DM). The drying air temperatures were adjusted to 60, 70 and 80 °C, the airflow rate varied between 150 and 250 m³ h^-1, respectively. As the results have shown, the total nitrogen was reduced by 29 to 42% during drying. Almost all ammonium nitrogen was lost (92%). No relationship between nitrogen loss and drying air temperature or airflow rate could be proved. These results will be used as a basis for further investigation of continuous digestate drying at a semi-technical scale.

Keywords: Ammonia emission; Digestate; Laboratory fixed-bed dryer; Nitrogen loss.