Life cycle emissions from biogas systems

show that, at a capacity of 1.69 million t/a (934 MWLHV), the production costs in the CO₂-based process (1.108 €/kgMeOH) are 3.3 times higher than in conventional production. H2 costs again dominate the costs of manufacturing. A study by PÉREZ-FORTES et al. (2016) investigated the methanol direct synthesis in a TEA which focussed primarily on the synthesis process as an option for carbon capture and utilisation (CCU), taking values for the upstream production of H2 and CO2

separation from the literature. The most expensive components of a plant, whose size is comparable to conventional methanol plant (440 ktMeOH/a), were the compressor system followed by the heat exchanger network. The results showed that the plant could be profitable if the methanol price increased by a factor two, the H2 costs decreased by a factor of 2.5 or the CO2 price decreased to 222 €/t (PÉREZ-FORTES et al., 2016). In fact, CO2 from flue gas can be obtained at costs for capture from the iron and steel industry between 50-70 €/tCO2 and in the cement industry between 70-150 €/tCO2 (BRYNOLF et al., 2018). VIEBAHN et al. (2018), e.g., give costs for direct air capture (DAC) of 480 €/tCO2. In other sources, however, both significantly higher and significantly lower costs can be found (SCHEMME, 2020).

3.4 Environmental assessments of biogas and Power-to-Fuel

important ones are CO2, CH4, nitrous oxide (N2O) and fluorinated compounds such as chlorofluorocarbons (CFCs). Most emissions stem from the provision of biomass, which can positively be influenced when residues and waste products are chosen. Agriculture in Germany is the source of numerous emissions caused by, e.g., the emissions from cultivation of land (CO2 and N2O) and the digestion of ruminants. However, plants also sequester CO2 during photosynthesis and contribute to CO2 emission reductions. Therefore, biogas and biomethane can present a valuable substitution option for fossil resources. Especially, the co-generation of heat and electricity offers a reduction in GHG emissions, as heat is used as a co-product. (FNR, 2012)

As an accounting example for agricultural emissions linked to BGPs, there is the German agricultural emission inventory by the KTBL¹ (WULF et al., 2019). The important emissions that are credited to BGPs are shown in Figure 3.4. CH4 can be emitted during the phase of AD from the fermenter either by unintended leakages in the cover or the controlled overpressure security valve and from the open storage of digestate. N2O and NH3 emissions from open storage and digestate application are also included in the inventory. CH4 and CO2 emissions by the CHP unit are usually assigned to the energy sector under the inventory. In addition, CO2

emissions from the conversion of plant biomass are usually not included, unless energy crops are part of the feedstock. However, these emissions cannot be specifically shown from the calculations, as the inventory does not distinguish between different utilisation paths of the harvested products. Emissions from crop cultivation are calculated primarily from the type of fertilisers used (mineral or organic) and their application technology.

The LCA literature proves to be inconsistent in their consideration of biogas system emissions, which is due to individual system boundaries based on different international and regional backgrounds and inventories as well as variations in the design of such systems. Therefore, it is difficult to compare the results of LCA studies with each other. System boundaries or functional units often vary as well as assumptions concerning feedstock and technical and agricultural aspects (BURATTI et al., 2013, HIJAZI et al., 2016). As stated by POESCHL et al.

(2012b), emissions of BGPs are connected to feedstock production, plant operation and construction, digestate handling and processing as well as application. The system boundaries around these emissions are set differently, meaning that restrictions to systems are common, cutting off certain production steps.

1 Kuratorium für Technik und Bauwesen in der Landwirtschaft

Furthermore, the environmental performance is based on many different choices such as the choice of the FU, various cultivation and feedstocks as well as location, the final utilisation of the generated energy and the treatment of digestate (PÉREZ-CAMACHO et al., 2019).

Figure 3.4: Emissions for biogas production.

Caption: WULF et al. (2019); CHP = Combined heat and power plant.

From the construction side, BGPs differ in their pre-storage of manure and storage of digestate, distinguishing between open and closed storage systems. This largely determines the emissions from the plant, as open storage has significantly higher emissions. There is unanimous agreement in LCAs of BGPs that open digestate storage negatively affects the GHG balance of a plant (ESTEVES et al., 2019, LANSCHE et al., 2012, PAOLINI et al., 2018). MEZZULLO et al. (2013) discovered that digestate storage tanks need to be covered in order to make the AD plant beneficial and avoid additional CH4, CO2 and NH3 emissions. HIJAZI et al.

(2016) also find that the GHG balance of a biogas plant can be improved by the collection of biogas during digestate storage or by using a gas flare during downtimes of the CHP. The German-wide survey for biogas plant operators in 2010 showed that two thirds of the German biogas plants have a gas-tight digestate storage. Besides, for newly-built plants it is mandatory to have such a storage (ZORN et al., 2014). As law requires a closed storage in recently-built BGPs, CH4

emissions from AD are close to zero (FNR, 2016).

Manure pit

Application CHP

unit

Digestate storage Fermenter

CH4

CH₄ CH4

CO2

NH₃

N₂O N₂O

NH₃

Environmental burdens for biogas plants with plant material are mainly due to the cultivation which can be omitted for manure. Emissions influencing eutrophication and acidification potential are lower for plants which are run exclusively with manure, as crops cause emissions during their production phase (LANSCHE et al., 2012). This is confirmed by an LCA from O'KEEFFE et al. (2019) who find that smaller scaled manure plants show GHG mitigation potential by negative kg CO2-eq. emissions as opposed to larger crop-based plants whose emissions are positive. The feedstock origin was also found to have an influence on the impact categories of eutrophication (freshwater and marine) and metal depletion by POESCHL et al. (2012a, p. 189). As long as straw and cattle manure are assumed to be waste, upstream processes such as cultivation processes and animal husbandry can be excluded from the analysis, as also done by POESCHL et al. (2012b). While some studies include e.g. animal husbandry, many omit this part, as well as the collection, transport and handling of the manure. It depends on the system boundaries again and can sometimes be due to lack of data or expected low impacts (ESTEVES et al., 2019). For instance, ZHANG et al. (2013), who looked at manure-treatment options in a cradle-to-gate LCA, assumed that the collection is already done by the livestock farm anyways. Therefore, they would not add this stage to the analysed system. A similar reasoning is followed by OEHMICHEN et al. (2017), who assume that manure emissions are ascribed to livestock production and hence inexistent until the BGP production stage, when the manure as feedstock is ready to be inserted into the fermenter. Emissions from pre-storage are thus non-existent. Therefore, CH4 and NH3 emissions occur only during the stages of AD and digestate storage (OEHMICHEN et al., 2017). ZHANG et al. (2013), on the other hand, included N and P emissions from pre-storage of manure, but did not specify the type of storage further. Direct emissions from raw manure storage were also considered by FUCHSZ et al. (2015), considering Western-European emission factors for numerous substances per animal and year. If storage or pre-treatment of manure together with other feedstock occurs at the BGP, it should be accounted for according to (LIEBETRAU et al., 2017). A review by PAOLINI et al. (2018) summarises the significant decrease in emissions when feedstock is stored in closed tanks.

Studies have also found that transport can contribute significantly to the climate change category in biogas systems (BURATTI et al., 2013, RAU, in press).

However, the inclusion of transport in studies varies as well, also depending on regional settings (ESTEVES et al., 2019). As this study neglects the application of digestate due to the system boundaries set, we do not go further into detail about them. While other studies have also decided to leave this step out (ESCOBAR et al., 2015, MEZZULLO et al., 2013), it is important to bear in mind that it causes

emissions dependent on the chosen technique (SOTERIADES et al., 2018).

However, its application is less polluting compared to raw manure application as digestate replaces mineral fertiliser and reduces NH3 and N emissions into the ground water to a minimum (SCHNEIDER-GÖTZ et al., 2007). The construction of the plant can also be included, but many sources found the relatively small contribution in the overall impact (ESTEVES et al., 2019, LANSCHE et al., 2012, MEZZULLO et al., 2013, RAU, in press). For the most part, emissions coming from plant manufacture are negligible for the whole life cycle impacts, as they only occur once in the lifetime of the plant whereas the emissions due to plant operation are reoccurring.

Credits and avoided emissions through manure credits

In general, the literature shows the importance of biogas systems in terms of credits which often make a difference in their environmental balance. As mentioned in section 3.2.1, environmental credits are linked to the avoided emissions from substituting the use of certain products for valuable co-products of an evaluated system (e.g., through a substitution or system expansion approach). Often considered are fertiliser credits from digestate as well as heat and electricity credits (ESTEVES et al., 2019). The replacement of mineral fertiliser by digestate as an organic product is found to decrease the overall environmental impacts of biogas systems (VAN STAPPEN et al., 2016). Moreover, digestate as opposed to raw manure has much lower odour emissions when applied in the fields (FACHVERBAND BIOGAS E.V., 2018). An LCA of manure-based biogas production by LANSCHE et al. (2012) found a relatively insignificant contribution by heat credits compared to electricity. However, the credit is dependent on the thermal efficiency of the CHP and also the chosen reference system, according to OEHMICHEN et al. (2017). They assumed to replace heat from the German heat mix which comprises a mix of NG and boilers run with fuel oil. Their calculated credits were also insignificant compared to the credits achieved by improved manure management which were almost six times as high.

A matter of debate are the avoided CH4 emissions from conventional manure storage that are often credited to BGPs. Also referred to as improved manure management or manure credit, the avoided emissions have been included in various LCA studies during the past years, some also carrying out system expansion over allocation (BÖRJESSON et al., 2010, EDWARDS et al., 2014, FNR, 2013, HIJAZI et al., 2016, LANSCHE et al., 2012, O'KEEFFE et al., 2019, OEHMICHEN et al.,

2017). In fact, it is possible to consider negative CH4 emissions of biogas systems for keeping the manure within a closed system and avoiding open manure storage.

These saved emissions are then considered within the biogas system as opposed to the conventional, open-air storage of raw manure that emits CH4 directly into the air (LIEBETRAU et al., 2017). However, environmental credits for using a more efficient process within a biogas system involves a double counting effect, as the substitution of renewable electricity already accounts for avoided emissions. This should be avoided by LCA standards (EC et al., 2010). Seemingly, the LCA literature does not concern itself too much with the double counting problem. Only few studies were found to include emissions from pre-storage of manure without considering the manure credit at all (FUCHSZ et al., 2015, ZHANG et al., 2013).

The eligibility of the method of counting avoided emissions could be confirmed after a phone call with an expert from DBFZ² (OEHMICHEN, 2020), such that a double purpose of a BGP is indeed existent. O'KEEFFE et al. (2019) even point out that there is more research required in replacing general statistics for manure credits with actual farm data, especially against the background of the RED II.