Anaerobic digestion in sustainable biomass chains

This thesis evaluates the potential contribution of anaerobic digestion (AD) to the sustainability of biomass chains. Results provide insights in the technological potential to recover energy and valuable by-products from energy crops and residues, and evaluate biomass cascades involving AD technology for their feasibility and desirability. Embedding AD in biomass chains addresses current constraints towards increased use of biomass for energy production considering land competition and environmental pollution. In this context, so far the major advantages of AD to improve energy efficiency and closing material cycles have received, thus far, limited attention. As part of the experimental research an Oxitop® protocol was refined for screening plant material suitable for anaerobic digestion based on their energy content. Environmental factors influencing the test outcome are the use of NaOH pellets for CO2 scavenging, substrate pretreatment, microbial culture, and type of buffer. The use of NaOH pellets and substrate pretreatments were most influential on the results. By means of the developed Oxitop® protocol the relationship between plant ligno-cellulosic composition and the Biochemical Methane Potential (BMP) and first-order hydrolysis constant (kh) was researched. The Acid Detergent Fibre (ADF) and the Neutral Detergent Fibre (NDF) as analyzed by the van Soest method were proposed as suitable plant characterization techniques for predicting BMP and kh, respectively. The model proposed was further used to predict the biodegradability of 114 European plant samples identifying interesting crops and crop residues suitable for anaerobic digestion. Batch experiments on digestate quality during codigestion of maize silage and manure showed an increase of 20-26% and 0-36% in solublised NH4+ and PO43-, respectively, after 2 months of digestion. The largest fraction of the inorganic nutrients was found in the liquid fraction of the digestate, i.e. 80-92% NH4+ and 65-74% PO43-. Increase in manure content in the mixture showed a positive effect in the methane production rate. Digestion time and increased proportion of maize silage in the mixture positively influenced the availability of PO43-. The added value of AD within different biomass cascades was evaluated by means of a sustainability framework developed for the purpose. The sensitivity analysis of the energy balance of an AD facility showed that the most important energy loss when a high value substrate such as energy maize is employed are heat losses induced by restricted reuse possibilities within the cascade. In contrast, when low energy substrates such as manure are used, indirect energy inputs embedded in infrastructure become significant. The developed sustainability framework was applied for the Colombian case. Results show that production of bio-ethanol from cassava is only sustainable from an energy and greenhouse gas (GHG) perspective when energy recovery from the process residues, using AD, is part of the process. The exact outcome of the evaluation largely depends on variables like substrate drying, type of fuel used, reuse possibilities for the digestate and type of applied AD system. During the study of other Colombian biofuel cascades the contribution of by-products was shown to be crucial, constituting 41-68% of the sum of all energy flows. For oil palm, sugarcane, panelacane and cassava, the estimated energy contribution of the by-products to the different biofuel systems fluctuate between 51-158, 122-290, 71-170, and 36-71 GJ.ha-1yr-1, respectively. AD had also a positive impact on nutrient recovery and water savings in the studied chains. The energy, nutrient and water benefits were set in perspective by giving an indication on the economic benefits and land savings potentially attainable under Colombian conditions.