Impact of Digestate formed during Biogas Generation Process on Hydrogen Production

Abstract

Hydrogen production from renewable sources has become a key objective in the energy transition to a more sustainable, and low-carbon economy. Among the many potential sources for hydrogen production, ammonia generated from anaerobic digestate presents a promising opportunity. Although digestate has its advantages, the presence of toxic ammonia poses environmental problems. Proper control of ammonia volatilization is essential to minimize risks. This study highlights the importance of side stream stripping as a viable method of recovering ammonia from digestate to support hydrogen production. In this study, the ammonium nitrogen (NH4-N) data of various substrates used for biogas production and its digestate was collected from a brochure on the results of biogas production measurements in Germany published by the Agency for Renewable Resources (FNR: Fachagentur Nachwachsende Rohstoffe). These data were used to estimate the amount of ammonia (NH3) contained in the digestate of various feedstocks for hydrogen production. Side-stream stripping has been shown to extract ammonia safely, facilitating handling and reducing gaseous and particulate emissions. In addition, the process reduces transport costs by removing excess water from the digestate, while the stripped digestate is returned to the anaerobic digester, ensuring a closed-loop system. The results of this study showed an increase in ammonia content as a function of the ammonium nitrogen content in the digestate. Digestate with a high ammonium nitrogen content produced more hydrogen respectively. The hydrogen potential produced depends on the amount of ammonia available in the digestate. This study explores the potential impact of anaerobic digestate on hydrogen production. Regarding the evaluation of catalysts used in ammonia decomposition. Ru-based catalysts are widely regarded as the best choice for highly efficient ammonia (NH3) decomposition. However, their considerable cost and limited availability are disadvantages for large-scale applications. In contrast, among non-noble metal catalysts, nickel-based catalysts show the highest activity, making nickel a promising alternative material for ammonia (NH3) decomposition due to its affordability. At present, challenges remain in improving the efficiency of Ru- and Ni-based catalysts.