Residues from agriculture include a variety of products, such as straw from cereal and oilseed cultivation, tops from potato and sugar beet cultivation, and manure from animal husbandry. All of these can be used for biofuel production in different ways, as shown in the flow chart below. Using residues is a way of increasing energy production from agriculture without competing with food production.
Straw
Straw is the stalks from cereal or oil plants. Straw can be harvested or left in the field. The reason for leaving it in the field can be to maintain soil quality, that there is no demand for it, or that the weather or time does not allow for collection. The straw harvested in Sweden is largely used as bedding material, but the excess could be used for energy purposes. Fuel quality for different types of straw vary and wheat straw is commonly considered as suitable due to e.g. high yields and low content of ash. The content of alkali metals and chlorine can be reduced if the straw is left in the field and exposed to rain before harvesting.
Harvesting of cereal is done with a harvesting combine that cuts the plants and feed them into a thresher where grains are separated from the straw. The straw is then placed in rows on the field for collection. The straw yield varies with respect to species and stubble height but is generally in the range of 1-5 tonnes dm/hectare.
There are many systems to collect and store straw. Capacity is always important when collecting straw, due to economic and time constraints. The straw can be pressed into square or round bales, with square bales usually having a higher density and being easier to transport and store because of their shape. Another method for collection is to load chopped straw directly onto a collecting trailer. The expenses for baling can then be avoided, but chopped straw has low density, making transportation and storage expensive. When straw that is not dry enough is stored, it can lead to molding and spores can spread that could cause health problems such as lung disease. It also leads to dry matter losses, heat development and risk of self-ignition. The straw can hold sufficiently low moisture content at the point of collection, but the moisture content is very weather dependent.
Potato and sugar beet tops
Tops that remain from cultivation is a non-utilized feedstock with potential for harvesting as substrate for anaerobic digestion. Potato and sugar beet tops represent the main quantities. In potato cultivation the tops are terminated about three weeks prior to harvest, to prevent mold contaminated green tops to cause damage to the potatoes, and to make the harvesting easier. In conventional farming this is done by spraying the tops with herbicides, and in organic farming it is done mechanically by breaking or burning of the tops. In most cases, the tops are left on the field, but technology for collection exists (Energikontor Syd 2024). Harvest is estimated to vary between 1-4 tonnes dm/hectare. In sugar beet cultivation the tops are generally left on the field but can be collected during the harvest. Many beet harvesting machines can separate the tops from the beet, and by using an elevator they can be collected in a trailer. The harvest of beet tops varies between 3-8 tonnes dm/hectare. However, removing tops from the field also removes nutrients. This could be compensated for by returning sludge from the anaerobic digestion to the field.
Manure
Manure is the feces and urine from livestock. There is a distinction between liquid manure with a low content of dry matter, and solid manure with a higher content of dry matter that is mixed with bedding materials like straw and feeding residues. In practice all manure in Sweden is used as a fertilizer on farmland, with or without previous anaerobic digestion. Storing of manure leads to emissions of methane as the organic matter decomposes. Therefore, manure is a good substrate for anaerobic digestion as this method avoids emissions of methane to the atmosphere, and at the same time allows for energy production and provides a digestate that is used as fertilizer. However, manure often needs to be co-digested with other substrates as its methane yield is rather low.
Exemption from taxes
Since 2003, Sweden has granted exemptions from energy and CO2 taxes for sustainable, non-food-based biogas and bio-propane used as motor fuel if they meet sustainability and GHG saving criteria as set out in the Swedish Act on sustainability criteria for biofuels, bioliquids and biomass fuels. In 2007, the exemption was extended to heat generation. The exemptions, that apply to both domestically produced and imported biogas and bio-propane, have been continuously extended over the years and accepted by the European Commission. In 2020, the European Commission approved an extension of the tax exemption schemes until 2030. However, in 2022, the Court of Justice of the European Union annulled the Commission’s 2020 decisions on procedural grounds. After a formal investigation, the Commission concluded in October 2024 that the tax exemption for non-food-based biogas and bio-propane used for heating or as motor fuel are in line with EU State aid rules and prolonged the exemption until 31 December 2030 (European Commission 2024, Greppa Näringen 2024).
Current production and potential
As mentioned in the introduction, using residues such as straw, tops and manure is a way of increasing energy production from agriculture without competing with food production. However, not all straw available on Swedish fields can be used for energy production. Due to weather conditions and time constraints during harvest season it is not possible to harvest all of it. Furthermore, straw left in the field or returned to the field as biochar after pyrolysis contributes to carbon sequestration and soil quality. Straw is also frequently used as bedding material for animal husbandry. This considered, the potential for use of straw for energy purposes in Sweden is estimated to around 3.2 TWh per year (calculated based on Lindfors & Feiz 2023). However, a large share of straw harvested as bedding material for animal husbandry will end up as manure that can also be used for energy purposes.
The annual potential for energy production from potato tops and sugar beet tops is estimated to about 0.3 TWh per year, and the biogas potential from collected manure amounts to 4.4 TWh per year (calculated based on Lindfors & Feiz 2023). About 13% of the total production of biogas in Sweden 2023 (2,3 TWh) was produced from manure in farm-based digesters. Upgrading of the biogas takes place in very few (7) of the 74 farm-based biodigesters, where the biogas is mainly used for heat and power production. 95 biodigesters use manure as substrate (Energigas Sverige 2024).
Properties
Depending on the crop, straw can originate from different cereals and oil crops and have different properties. The lower heating value, ash content and lignin content have a mean value and range (in parentheses) of 17 (14-19) MJ/kg dm*, 6.4 (1.3-14)% and 18 (8-30)% respectively (source: Phyllis database**). For tops there is no reliable data, and for manure the data varies depending on animal type and management system. To illustrate the varying properties of different agricultural residues, their potential methane yield from anaerobic digestion is compiled in the table below.
* dm stands for dry matter, i.e. represents the weight percentage of dry material (including ash)
** European Commission 2013 https://knowledge4policy.ec.europa.eu/dataset/beo-ecnphyllis_en