Thermochemical conversion and utilization of digestates from anaerobic digestion of lignocellulosic biomass

Abstract: Anaerobic digestion can be widely used to convert the lignocellulosic biomass into biogas, particularly with the by-product digestates. A large amount of digestate discharge has been one of the most limiting factors for the promotion and application of anaerobic digestion, with the development of large-scale biogas engineering in recent years. It is highly urgent to rapidly and effectively treat the digestate. Alternatively, the thermochemical conversion can be selected to realize the harmless treatment and resource utilization of lignocellulosic digestates. The digestates still retain most of the carbon elements and energy in the original material before digestion. The content of lignocellulosic is also very considerable for a large potential of thermochemical conversion. Therefore, this review aims to focus on the digestates forming fuel, pyrolysis, and hydrothermal carbonization. The forming performance of digestates was better than that before digestion, but the NOx emission and slagging phenomenon during the combustion were outstanding to be monitored and controlled. In pyrolysis, the lignocellulosic digestates presented the application potential in the preparation of liquid fuels, phenolic compounds, carbon-based adsorbents, soil amendments, and syngas synthesis. Specifically, the digestate derived bio-oil behaved a much higher calorific value while a lower acid content, where the relative content of 4-vinylphenol reached 60.9%. The pore structure of the biochar was also developed to contain more nutrients, such as P and K. The gaseous product presented a more suitable H2 to CO molar ratio with less tar. However, the conversion application in the combustion and pyrolysis was confined to the drying energy consumption caused by the high water content of digestates. In comparison, the drying energy consumption was ignored before hydrothermal carbonization. At the same time, hydrothermal carbonization was used to improve the quality of digestates fuel, including the removal of alkali and alkaline earth metals, the higher calorific value, as well as the improved hydrophobicity, grindability, and fluidizability. Another potential was to prepare the functionalized carbon materials for soil improvement. But, there were still some challenges to the disposal of water phase products after hydrothermal carbonization. In addition, the potential ecological hazards of biochar and hydrochar derived from digestates for farmland application also needed to be paid enough attention, including heavy metals, and polycyclic aromatic hydrocarbons. Overall, the anaerobic digestion and thermochemical conversion presented complementary advantages in energy utilization efficiency, large-scale treatment, and the removal of biotoxicity. Additionally, the thermochemical conversion products of digestates also showed great potential for recycling in anaerobic digestion processes. For instance, the high-temperature flue gas produced by combustion can be recycled to the insulation of the biogas engineering, while the ash residue and biochar can be used to purify the biogas slurry, and the water-phase by-products also have a certain methane production potential. Consequently, a combination of lignocellulosic biomass anaerobic digestion and digestates thermochemical conversion can greatly contribute to the promotion and application of large-scale biogas engineering.