3.1. Semi-Continuous Flexible Biogas Production
Three different feeding strategies (feeding 1x, 3x and 9x per day) were applied to assess their effects on the process performance of a semi-continuous biogas system. Additionally, two different feedstocks were used at an OLR of 1.4 kg vs. m
−3 d
−1 for feedstock 1 and 2.15 kg vs. m
−3 d
−1 for feedstock 2. The performance of biogas and methane production was analyzed hourly and daily.
Figure 3 (a. to c.) illustrates the average hourly biogas production (l
N m
−3 h
−1) of the three feeding strategies over a period of 24 h.
For the 1x feeding strategy (
Figure 3a), biogas production from both feedstocks was immediately increased within the first hour after the feeding event, which took place 25 minutes before hour 1. The maximum was achieved at hour 2 with a biogas production of 86 and 82 l
N m
−3 h
−1 and a methane production of 61 and 63 l
N m
−3 h
−1 for feedstocks 1 and 2, respectively. These values make up 128% of the average daily biogas production for both feedstocks. After this peak, the production rate decreased until the next feeding event. The decline was initially more rapid compared to the decline during the last two-thirds of the 24-h period. However, a high amount of biogas and methane production was recorded during the first eight hours. At the end of the 24-h period, the biogas production was at 66% for feedstock 1 and at 68% for feedstock 2 compared to their highest levels at hour 2.
Figure 3b presents the results from the 3x feeding strategy, where fermenters were fed 25 minutes before hour 1, hour 5 and hour 9. Both feedstocks showed a similar pattern of biogas production. Compared to the 1x feeding strategy, the fast increase in biogas and methane production did not occur after the first feeding event. The highest level of biogas production was achieved at about hour 3 after the first feeding event, whereas the highest level was already reached within one hour after the second and third feeding event. However, a similar deterioration rate of biogas production was measured after each feeding event. Both feedstocks had their overall peaks of biogas production at hour 10, where the data show a biogas and methane production of 81 and 57 l
N m
−3 h
−1 for feedstock 1 and 82 and 51 l
N m
−3 h
−1 for feedstock 2. These values constitute 122% and 129% of their average daily biogas production. From hour 11, biogas production decreased continuously until the next feeding event. At the end of the 24-h period, the biogas production capacity was at 73% for feedstock 1 and at 64% for feedstock 2 compared to their highest capacities at hour 10.
The biogas production for the 9x feeding strategy is shown in
Figure 3c, suggesting a continuous increase in biogas production up to hour 10. The first feeding event started 30 min before hour 1, proceeded at hourly feeding intervals and included a feeding break at hour 5. The biogas and methane production patterns from both feedstocks were similar. Biogas and methane production were gradually accumulated until hour 4, which was followed by a slight decrease at hour 5, due to the feeding break. Thereafter, the biogas and methane production further increased until hour 10, where the biogas production peaks for both feedstocks were registered (115% for feedstock 1 and 121% for feedstock 2, compared to the average daily biogas production rate). Feedstock 1 achieved at its maximum point a biogas and methane production of 74 and 52 l
N m
−3 h
−1, whereas the maximum point was 77 and 48 l
N m
−3 h
−1 for feedstock 2. At the end of the 24-h period, the biogas production capacity was 78% for feedstock 1 and 71% for feedstock 2 compared to their highest production rates at hour 10. The slight increases in biogas and methane production describe the different behavior of a more frequent feeding strategy compared to the more rapid increases in biogas production that occurred in the 1x and 3x feeding strategies.
Heterogeneity of biogas production after the feeding events was observed between the three feeding strategies, which could result from a different response of microorganisms to the applied feeding quantity. However, regarding the accumulated biogas production, all feeding strategies showed roughly the same amounts of biogas at the end of the day. This may imply that under the investigated conditions, the feeding events could be shifted to another time (earlier or later) without negatively affecting the overall productivity. The 9x feeding strategy, which represents a conventional biogas performance, showed a more continuous, but slow increase of biogas production than the 1x or 3x feeding strategies. Conversely, the latter two strategies recorded a larger amount of biogas production after the feeding event(s) and a smaller amount during the non-feeding period. These findings might indicate possible suitability for flexible biogas production in practice.
Similar findings regarding more/less frequently-fed strategies were achieved by Mauky et al. and Mulat et al. [
7,
8]. They reported on differences in biogas production after the feeding events based on the feeding strategy, but the accumulated biogas production remained approximately equal, which is comparable to the present study. Additionally, a sharp increase in biogas production after the feeding events for the less frequently-fed strategies was also observed in these studies, although the kinetic characteristics, as well as the number of feedstocks fed into fermenters, were the determining factors [
8]. Eltrop et al. [
41] examined the consequences of an intermittent feeding in full scale. The results show peaks of 140–150% of the average daily biogas production, which was achieved by feeding with maize silage three times per day. Peaks of 130–140% of the average daily biogas production resulted from two feeding times (per day) of grain. Similar peaks (140–150%) were achieved by Mauky et al. [
8]. These peaks from the similar findings are considerably higher than those from our experiments (1x feeding: 128%, 3x feeding: 122% and 129%, und 9x feeding: 115% and 121% for feedstock 1 and 2, respectively). One explanation could be the slower degradability of feedstocks with a high raw fat content, which was used in this study. The high raw fat content may be very effective (high amounts of biogas and methane production), but the degradability of feedstocks with an easily degradable composition (e.g., high content of sugar, starch, pectin, proteins) is much faster.
Table 2 indicates the average daily biogas and methane production from the semi-continuous test of feedstock 1 and 2 at the three feeding strategies (feeding 1x, 3x and 9x per day). The daily biogas production from the semi-continuous experiment ranged from 1524-1,623 l
N m
−3d
−1, with a methane production of 947-1,138 l
N m
−3 d
−1, considering both feedstocks and strategies. Similar results were obtained from the Bruck an der Leitha biogas plant. However, a very high standard deviation in the full-scale performance was measured. This arose from the daily variation in the feedstock composition, which is followed by a diverging OLR. Thus, comparing the volume-specific biogas production rate between full scale and laboratory results seems to be difficult.
The results from the 1x feeding strategy demonstrate the highest average daily biogas and methane production, followed by the results from the 3x feeding strategy. This applies for feedstocks 1 and 2, respectively. The 9x feeding strategy shows the lowest gas production. However, the gas production rates for all feeding strategies were very close to each other. In other words, the average daily gas production indicates roughly the same amounts, which were slightly higher for feedstock 1. The mean values may suggest that the fewer fermenters are fed, the higher the biogas and methane production is. Nevertheless, the results from the t-test for the mean biogas and methane production per day show no significant differences between the three feeding strategies for both feedstocks (p < 0.05). Conversely, the t-test (p < 0.05) for the peak values per hour (biogas and methane) demonstrate a high significance between the feeding strategies, which applies to both feedstocks. This implies that production peaks in biogas and methane can be shifted to another time via changing feeding intervals.
The results of our experiment confirm the assumptions of Mauky et al. [
7,
8] that a flexible feeding regime causes no significant reduction in biogas production. This contradicts the previous assumptions that only continuous feeding can ensure stability [
42,
43]. Mulat et al. [
7] describe two potential factors that may explain a stable biogas and methane production under a flexible feeding regime. First, they note the ability of microorganisms to adapt to changing conditions; second, an extended retention time enables the organic substances to degrade more efficiently. Both factors could also be valid for the semi-continuous experiments in this work.
According to the gas composition, the methane content was around 70 vol.-% for feedstock 1 and 64 vol.-% for feedstock 2 (
Table 3).
Feedstock 1 obtained a 7% higher methane concentration than feedstock 2. This can be explained by the high amount of raw fat contained in feedstock 1 (
Table 1) [
44,
45]. The methane content level within the three feeding strategies from both feedstocks was approximately the same with no significant difference. The H
2 concentrations show a large variance between the different feeding strategies; this applies to both feedstocks, whereas the highest concentrations were observed with the 1x feeding strategy. According to Speece [
46], an H
2 concentration greater than 100 ppm can influence the biogas process negatively, whereas Drosg [
42] defines a risk at concentrations of more than 500 ppm. However, given the high methane production rate, no inhibition of the methanogenesis over the acetoclastic path was observed. The H
2S concentration of both feedstocks was lower than the average H
2S concentration of the biogas plant Bruck an der Leitha in the year 2015 (139 ppm), which applies to all feeding strategies.
3.3. Effects on Biogas Storage Capacities
The obtained biogas and methane production curves for the two investigated feedstocks and three flexible feeding strategies were implemented in the process simulation model of the biogas plant. The three power generation scenarios described in
Section 2.7 were simulated, and the reduction potential of the needed biogas storage capacity compared to continuous feeding was determined.
Figure 7 and
Figure 8 show the biogas storage levels during the simulated power generation scenarios for feedstock 1 (OLR 1.4 kg vs. m
−3 d
−1) and feedstock 2 (OLR 2.15 kg vs. m
−3 d
−1), respectively.
Continuous feeding was compared to the three investigated feeding strategies (1x, 3x and 9x per day) for two feedstock mixtures with different OLRs (1.4 and 2.15 kg vs. m−3 d−1). The results for scenarios SCE and SCE-12H show that the needed biogas storage capacity could be reduced by demand-oriented feeding for all of the three investigated feeding strategies in comparison to continuous feeding.
The results from scenario 12H and continuous feeding reveal for both OLRs that the installed biogas storage capacity at the biogas plant was not sufficient to provide enough biogas for power generation in peak times while continuously producing biomethane (the minimum biogas storage level of 5% was reached, which led to negative and unreasonable mass flow results). However, the biogas storage capacity proved sufficient for all of the investigated feeding regimes at an OLR of 1.4 kg vs. m
−3 d
−1 and when feedstock was fed once per day with an OLR of 2.15 kg vs. m
−3 d
−1. These results indicate that certain power generation strategies that cannot be executed when the feedstock is fed continuously, due to limited biogas storage capacity may be executable when the feeding regime is changed.
Table 6 summarizes the reduction potential of the needed biogas storage capacity for the simulated scenarios.
Biogas storage investment costs significantly contribute to the biogas plant’s economic viability. A biogas storage volume reduction of several hundred cubic meters leads to savings of tens of thousands of Euros if compared to common cost functions in literature, e.g., in Lauer et al. [
59].
In this study, the highest biogas storage reduction potential with 73.7% was observed for scenario SCE, when feedstock was fed 3x per day with an OLR of 2.15 kg vs. m−3 d−1. This difference in biogas storage level between continuous and flexible feeding occurred during the end of the simulated period, when control energy reserves were activated exceptionally frequently (more than fifty times in the simulated last week). There was hardly any difference between continuous and flexible feeding strategies during the rest of the simulated period, except for small fluctuations in the biogas storage level that occurred during flexible feeding strategies.
For scenario SCE-12H biogas storage savings range between 25.2–38.9%. Similar values were reported in other studies that assessed demand-oriented feeding strategies. Mauky et al. [
5] demonstrated that the needed biogas storage capacity could be reduced by up to 45% within the flexible power generation context. Barchmann et al. [
17] reported biogas storage saving potentials of up to 65.1% for a weekly optimized power generation schedule and results from Grim et al. [
13] showed that biogas storage investment costs could be reduced by 24–53% by flexible feeding strategies.