Analyzing key constraints to biogas production from crop residues and manure in the EU—A spatially explicit model

This paper presents a spatially explicit method for making regional estimates of the potential for biogas production from crop residues and manure, accounting for key technical, biochemical, environmental and economic constraints. Methods for making such estimates are important as biofuels from agricultural residues are receiving increasing policy support from the EU and major biogas producers, such as Germany and Italy, in response to concerns over unintended negative environmental and social impacts of conventional biofuels. This analysis comprises a spatially explicit estimate of crop residue and manure production for the EU at 250 m resolution, and a biogas production model accounting for local constraints such as the sustainable removal of residues, transportation of substrates, and the substrates’ biochemical suitability for anaerobic digestion. In our base scenario, the EU biogas production potential from crop residues and manure is about 0.7 EJ/year, nearly double the current EU production of biogas from agricultural substrates, most of which does not come from residues or manure. An extensive sensitivity analysis of the model shows that the potential could easily be 50% higher or lower, depending on the stringency of economic, technical and biochemical constraints. We find that the potential is particularly sensitive to constraints on the substrate mixtures’ carbon-to-nitrogen ratio and dry matter concentration. Hence, the potential to produce biogas from crop residues and manure in the EU depends to large extent on the possibility to overcome the challenges associated with these substrates, either by complementing them with suitable co-substrates (e.g. household waste and energy crops), or through further development of biogas technology (e.g. pretreatment of substrates and recirculation of effluent).


Source code and interactive visualizations
Source code and interactive visualizations of some results from the paper can be found online at: https://rasmuse.github.io/biogas-residues-manure 2 Crop residues Data on harvests were collected from three different tables in Eurostat: • agr_r_crops, listing harvested amounts of some of the investigated crops (wheat, barley, sugar beet, rapeseed and turnip rape, sunflower seed) for most NUTS2 regions (NUTS1 for Germany and the UK) in each year; • ef_oluaareg, listing planted areas of all the investigated crops on NUTS2/NUTS1 level, as measured in the triannual farm structure survey (years 2007, 2010, 2013, etc); • apro_cpp_crop, listing harvested amounts of all the investigated crops on NUTS0 (national) level in each year.
Where available, we used data from agr_r_crops, averaged over the years 2009-2011.
When not available, we estimated the subnational harvest data by assuming a constant area yield in each country, i.e., distributing the total NUTS0 harvest (apro_cpp_crop data) proportional to the NUTS2/NUTS1 planted areas (ef_oluaareg data) for the year 2010.
The nomenclature in these table is not completely identical across tables. We connect crops to statistical codes as shown in Table 1. 3 Manure

Animal populations
Animal populations were taken from the triannual farm structure survey, Eurostat table ef_olsaareg. We used data from 2010.
The classification of animals used for manure management systems in the National Inventory Reports (NIRs) is more aggregated than the Eurostat population data, so we aggregated the animal populatios from ef_olsaareg into the NIR manure management classes as follows. We used the average manure management reported in the NIRs for years 2009-2011 (in the 2014 submissions). The two different options for cattle classification were both included because manure management systems are reported in the NIRs according to either one of them (chosen by the reporting country).
The population data was multiplied by the excretion factors (see Table 2) to obtain total excreted amounts per animal category and manure management class. Before downscaling the manure amounts spatially, we aggregated them to the GLW2 classes (cattle, pigs, chickens).

Manure management systems
In the NIRs, seven manure management systems are included, which we aggregated into three classes (liquid, solid, unavailable) according to Table 3.
Countries did not seem to use the National Inventory classifications completely in the same way. For example, some countries (e.g. Sweden and Denmark) seemed to use the category "Other" to indicate deep bedding (a.k.a. bedded pack) systems, while others (e.g. UK, Germany and France) appear to include such systems in the "Solid storage" category. In face of these ambiguities, we chose to only include one type of solid manure system. We also noted that Spain reports 100% of its manure management in the "Other" category. Instead of interpreting this as 100% solid manure, we assigned Portugal's distribution of manure management systems also to Spain.
The description of solid manure systems is especially problematic since there seems to be a very wide variation within this category. The word "solid manure" may be used to describe everything from relatively wet (around 15% DM) mixtures of manure and straw, removed daily, to deep bedding (roughly 25-30% DM) which is stored and partially composted for several months before removal. We have only encountered anecdotal evidence on the amounts and types of bedding material used in "solid" systems. Loosely based on such evidence we chose to assume that in solid manure systems for cattle and pigs, bedding straw was added to excretions in the ratio 1:1, counted in volatile solids basis. For chickens in solid manure systems, we assumed the bedding material was wood chips and that the wood chips did not add any volatile solids.