Towards a circular bio-economy: Evaluating ideals versus constraints for biogas feedstock availability in the Netherlands

Biogas production can play an important role in the circular economy due to its ability to consume diverse inputs and produce multifunctional outputs. Of the resulting products, methane is both an energy carrier and a potential chemical feedstock and the bio-slurry can be used as a source for biofertilizers. Many governments consider biogas as an important measure to reach the climate targets. In the Netherlands, the target of biogas is almost equal to the estimated maximum production potential from biomass flows in the country. However, we question whether this target would be feasible as the estimation does not include characteristics beyond biogas yield and the physical availability of the potential feedstock.
In this research, we applied the concept of circular economy through four parameters to re-evaluate this estimation. We first considered (1) the competing uses of the various biomass sources by other industries. If these alternative applications are higher on the biomass value pyramid than energy, they may decrease the practical chances of obtaining the feedstocks for making biogas. Next, we considered two factors limiting the practical applicability: (2) the contamination level which affects the desirability of the cascading use of bio-slurry and (3) the nitrogen content of the feedstock which in excess leads to critical environmental impacts. In the Netherlands, strict regulations are in place for these two parameters. Lastly, we looked at (4) greenhouse gas emissions to compare the climate change benefit of different biogas feedstocks.
Insights in competing uses of biogas feedstock is built by using co-occurrence analysis. This machine learning technique has been used in the field of industrial symbiosis to calculate the frequency that a biomass source and an application are mentioned together in a large collection of scientific articles. The analysis of contamination level and nitrogen content is derived from technical literature. For calculating greenhouse gas emissions, we performed a simplified carbon footprint inventory of the generation, processing and transportation of the biomass sources.
The result reveals that around one-fifth of the Dutch feedstock types for biogas have high competing uses, especially with the bio-ethanol and chemical industries. Also, a few feedstocks are the by-product of further processing other potential feedstocks. Additionally, certain feedstocks may have decent biogas yields, but their high contamination levels and nitrogen content make them less likely to be used by biogas practitioners. These analyses together with the differences in greenhouse gas emissions suggest the amount of feedstock available for Dutch biogas production will be far lower than estimated in policy documents. From a broader perspective, our study shows that circular economy involving biomass should take into account other aspects and uses when estimating the potential productions.