Parameters for sustainable and demand-oriented biogas production

Abstract

Biogas production offers a possibility to generate electricity and heat by renewable sources. It has a great potential in terms of availability. Unlike solar radiation and wind energy, biogas production does not underlie natural fluctuations. Biogas substrates can be stored and are therefore available at all times. In an ideal scenario, biogas production should meet certain criteria. It should be (1) sustainable, (2) provide the highest possible system effectiveness and (3) must be available in times of high electricity demand. This work concentrates on aspects of all three criteria. Biogas production in a highly sustainable way can be found on organic farms (Manuscript 1). Therefore, we analysed 13 biogas plants on organic farms and looked for improvement potentials. The survey concentrated on biogas plant specifications, fermentation characteristics and feedstock compositions. The biogas plants were tested repeatedly to investigate the impact of fermentation characteristics on volatile solids (VS) degradation, indicating the effectiveness of the anaerobic digestion process. The total feedstock composed mainly of livestock residues (61 %) and grass silage (14 %). Such substrates are considered ecologically beneficial (Danner & Kilian 2012; Insam et al. 2015), when compared to possible negative effects of maize (Svoboda et al. 2015). By comparing the feedstock composition to the biogas plant specifications we found significant correlations between the amount of livestock residues and the dimension of the combined heat and power (CHP) unit. With an increasing share of livestock residues in the feedstock the installed electrical capacity of these biogas plants became smaller. Within the fermentation characteristics, the process temperatures ranged between 17 49 °C. Low temperatures were found to be the driving factor for low VS degradation (range 11.67 - 87.70 %) in the residue storage. Significantly better VS degradations were found in biogas plants with two reactor stages compared to one reactor stage. Effluents with high VS concentrations can cause harmful greenhouse gas emissions (Ruile et al. 2015). Backfitting a cover on the residue storage is highly recommended, as most biogas plant residue storages were found to be installed without (> 84 %). To maintain a highly sustainable biogas production on organic farms, the maximisation of VS degradation is required. Therefore, the biogas plants’ VS degradation could be raised by increasing the process temperatures and/or the hydraulic retention time. Most biogas plants in Germany are, however, installed on conventional farms, accounting for > 99 % of the total installed electrical capacity. Here, maize silage is the most important substrate with the highest share on the total feedstock (ca. 40 % fresh weight) (FNR 2014). During the anaerobic digestion in biogas plants, the maximisation of carbon transformation from substrate to biogas is a desired goal of every biogas plant operator. It increases the system effectiveness of the biogas plant and therefore its economical sustainability. The carbon degradation can be quantified by common carbon degradation determination methods (C degmix) (VDLUFA 1997). With this method, it is not possible to quantify substrate-bound carbon degradation independently of the inoculum. Therefore, we tested an isotope-based method in reactor mixtures of maize silage and inoculum from agricultural biogas plants (Manuscript 2) to identify solely substrate-bound carbon degradation (C deg ƒMS). As stable isotopes underlie natural variability, the method was tested on six different maize silages of different farming systems (organic or conventional farming) and maize silages of high (MS high) and low (MS low) qualities. A total of 19 lab-scale batch reactors were analysed for digestion parameters, specific biogas (sby) and methane yields (smy), stable isotopes and carbon degradations. Reactors with MS high showed up to 23.8 % higher sby and smy. The carbon degradations in reactors with MS low were significantly lower compared to reactors containing MS high. The C deg ƒMS values were, however, significantly higher compared to C degmix, as it excludes the masking effect of the inoculum. At the start of the experiments, the stable carbon isotope values (δ13C) in the reactors were highly variable. Nonetheless, C deg ƒMS showed reliable values independently of the maize silage qualities. The effect of the farming system of the maize silages was found negligible in both carbon degradation methods. We found that the isotope-based carbon degradation determination method offers a possibility to assess the substrate-bound carbon degradation in more detail and contributes to our understanding of the total system effectiveness. In a future scenario of 100 % renewable energy in Europe, a large contribution will come from wind and photovoltaic energy generation (Steinke et al. 2013). Such resources underlie, however, strong temporal fluctuations. This creates balancing challenges in the energy systems (Hahn et al. 2014). In times of sun and wind deficits energy shortage could partly be filled in by biogas plants (SRU 2011), having relatively short start-up phases. The biogas production is, however, dependent on the microbial transformation of the substrate. Therefore, a quickly adaptable microbial community is necessary to cope with flexible substrate inputs. In our survey (Manuscript 3) we focused on structural changes of microbial communities in four lab-scale reactors. Every reactor was continuously fed with different mixtures of maize silage (MS) and sugar beet silage (SBS). The mixing ratio was 1:0 (VS ratios of MS and SBS) in continuous fermenter one (CF1), 6:1 (CF2), 3:1 (CF3) and 1:3 (CF4), respectively. The organic loading rates were equal in all reactors with 1.25 kg VS m 3 d 1. Bacterial and archaeal communities’ compositions were analysed with 454 amplicon sequencing technique on the basis of 16S rRNA genes. With increasing amounts of SBS, the bacterial and archaeal communities’ compositions shifted. Despite the shifts, biogas production rates and methane concentrations were similar in all reactors. This showed that the communities adapted to different environmental conditions induced by different substrate mixtures and ensured biogas production efficiency. In conclusion, my work contributes to parameters for optimized biogas production scenarios. For sustainable biogas production on organic farms deficits were detected, which could be reduced by increasing the VS degradations during the fermentation process. High VS degradation is a crucial factor for biogas system effectivity. In conventional biogas plants high VS degradation of maize silage is also a desired goal, which is resembled by carbon degradation. The application of a stable isotope-based method for carbon degradation determination of maize silage showed reliable and more detailed results. This increases our understanding of anaerobic degradation effectiveness in biogas reactors. For demand-oriented biogas production, quickly adaptable microbial communities are crucial prerequisites. Different substrate feeding regimes in biogas reactors resulted in compositional changes of microbial communities by simultaneously ensuring biogas production efficiency. This implicates the functional redundancy of microbial communities within biogas plant reactors. The outcome of this thesis contributes to future-oriented biogas production. Optimised biogas production can improve environmental sustainability and national energy sufficiency. This supports the creditability of biogas production and emphasises its importance among all renewable energies.