Scholarly article on topic 'Demand-driven biogas production by flexible feeding in full-scale – Process stability and flexibility potentials'

Demand-driven biogas production by flexible feeding in full-scale – Process stability and flexibility potentials Academic research paper on "Chemical engineering"

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{"Demand-oriented feeding" / Monitoring / "Gas storage" / "Sugar beet silage" / Bioenergy / "Balancing power"}

Abstract of research paper on Chemical engineering, author of scientific article — E. Mauky, S. Weinrich, H.F. Jacobi, H.J. Nägele, J. Liebetrau, et al.

Abstract For future energy supply systems with high proportions from renewable energy sources, biogas plants are a promising option to supply demand-driven electricity to compensate the divergence between energy demand and energy supply by uncontrolled sources like wind and solar. Apart expanding gas storage capacity a demand-oriented feeding with the aim of flexible gas production can be an effective alternative. The presented study demonstrated a high degree of intraday flexibility (up to 50% compared to the average) and a potential for an electricity shutdown of up to 3 days (decreasing gas production by more than 60%) by flexible feeding in full-scale. Furthermore, the long-term process stability was not affected negatively due to the flexible feeding. The flexible feeding resulted in a variable rate of gas production and a dynamic progression of individual acids and the respective pH-value. In consequence, a demand-driven biogas production may enable significant savings in terms of the required gas storage volume (up to 65%) and permit far greater plant flexibility compared to constant gas production.

Academic research paper on topic "Demand-driven biogas production by flexible feeding in full-scale – Process stability and flexibility potentials"

Accepted Manuscript

Demand-driven biogas production by flexible feeding in full-scale - Process stability and flexibility potentials

E. Mauky, S. Weinrich, H.F. Jacobi, H.J. Nägele, J. Liebetrau, M. Nelles

PII: S1075-9964(17)30053-7

DOI: 10.1016/j.anaerobe.2017.03.010

Reference: YANAE 1706

To appear in: Anaerobe

Received Date: 15 November 2016 Revised Date: 24 February 2017 Accepted Date: 7 March 2017

Please cite this article as: Mauky E, Weinrich S, Jacobi HF, Nägele HJ, Liebetrau J, Nelles M, Demand-driven biogas production by flexible feeding in full-scale - Process stability and flexibility potentials, Anaerobe (2017), doi: 10.1016/j.anaerobe.2017.03.010.

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Demand-driven biogas production by flexible feeding in full-scale - Process stability and flexibility potentials

Mauky E. ab*, Weinrich S. ab, Jacobi H.F. c, Nägele H.J. d, Liebetrau J. a, Nelles M. ab

a Department of Biochemical Conversion, DBFZ - Deutsches Biomasseforschungszentrum, Torgauer Straße 116, D-04347 Leipzig, Germany (E-mail: eric.mauky@dbfz.de)

b Faculty of Agricultural and Environmental Sciences, Chair of Waste Management, University of Rostock, Justus-von-Liebig-Weg 6, 18059 Rostock, Germany

c Hessian State Laboratory, Fachgebiet IV.5.3 renewable energies, Schubertstraße 60, 35392 Gießen

d State Institute of Agricultural Engineering and Bioenergy, University of Hohenheim, Garbenstraße 9, Stuttgart 70599, Germany

* Corresponding author

Abstract For future energy supply systems with high proportions from renewable energy sources, biogas plants are a promising option to supply demand-driven electricity to compensate the divergence between energy demand and energy supply by uncontrolled sources like wind and solar. Apart expanding gas storage capacity a demand-oriented feeding with the aim of flexible gas production can be an effective alternative. The presented study demonstrated a high degree of intraday flexibility (up to 50 % compared to the average) and a potential for an electricity shutdown of up to 3 days (decreasing gas production by more than 60 %) by flexible feeding in full-scale. Furthermore, the long-term process stability was not affected negatively due to the flexible feeding. The flexible feeding resulted in a variable rate of gas production and a dynamic progression of individual acids and the respective pH-value. In consequence, a demand-driven biogas production may enable significant savings in terms of the required gas storage volume (up to 65 %) and permit far greater plant flexibility compared to constant gas production.

Keywords: demand-oriented feeding; monitoring; gas storage; sugar beet silage; bioenergy; balancing power

1 Introduction

The share of energy - especial electricity - produced from renewable energy is constantly increasing [1]. The major challenge for the next decades is to further expand the amount of renewable energies in the grid and an efficient integration into energy system. Due to the fluctuation within production of solar and wind energy it is essential to evolve also the potential of capacity utilization management, powerful transmission networks, as well as new storage technologies [2]. Bioenergy and especial the biogas technology can play a significant role in such smart energy grids [3-5]. The strong political support for biogas production by the German Renewable Energy Act (REA, German "EEG") over the past decade has greatly affected agricultural sectors in Germany, which led to a boost in biogas production [6]. Particularly between 2006 and 2011, the total number of plants doubled and the total capacity increased by more than 150 %. In 2015, more than 8800 biogas plants with a total capacity of 4000 MW produced renewable energy in Germany [7]. This immense and decentral potential could be used to stabilize the grid.

Agricultural biogas plants in Germany are mainly based on continuous stirred tank reactor (CSTR) systems and have originally been designed for a constant energy output (base load energy). Up until today the substrate is fed semi-continuously in intervals between 15 minutes and three hours usually. However, the requirements for bioenergy currently change and the cost pressure increases (i.e. decreasing remuneration and increasing substrate prices). A promising economically feasible and sustainable perspective for biogas is a flexible and demand-oriented energy production [8,9].

Therefore, bioenergy have to supply demand-driven electricity to compensate the divergence between energy demand and energy supply by uncontrolled sources like wind and solar power, because the energy conversion from biomass is - in opposite to wind and solar power - weather independent.

In general there are different options for increasing flexibility along the biogas production chain, e.g., substrate management, storage of intermediates (i.e. acids), heat and/or gas use within a further expansion of gas storage and CHP capacities and upgrading the generated biogas to bio-methane and its subsequent feed into the gas grid [10]. However, the possible extent of flexibilisation depends on the design and the dimensions of plants, for example in additional CHP capacity and available gas storage. One example in [11] shows an integrated double membrane gas storage on top of the fermenter which costs about 51,000 Euro for a capacity of 3000 m3.

Besides the extension of gas storage capacity, a promising part of the solution to improve the flexibility of biogas plants is the direct regulation of the biological process, e.g. by individual feeding management. Barchmann et al. [11] and Grim et al. [12] showed that flexible biogas production also benefit economically from a reduced gas storage demand by a demand-oriented feed management. Different laboratory-scale studies by Lv et al., Mauky et al. and Mulat et al. [13-15] demonstrated that flexible feeding resulted in high dynamic biogas production and does not have a negative impact on the stability of the process. Although these promising results are achieved under controlled lab scale conditions the question remains, if these results can be repeated and transferred at full-scale. Only a few studies [8,16,17] developed methods to operate biogas plants demand-driven by flexible feeding in full-scale. According to Gaida et al. [18], the main challenges for upscaling advanced control procedures into practice are a lack of robust and reliable process monitoring tools as well as the

difficulty of convincing a conservative industry of the benefits of monitoring and feed control. Reservation of plant operators in flexible biogas production is largely due to fear of process disturbances caused by changing feeding regimes. And obviously, a lot of questions are still open about possibilities and limitations of plant flexibility by demand-oriented feeding.

The scope of this work is to investigate the flexibility of a full-scale biogas production and to compare the results with laboratory-scale studies. The focus is on the achieved dynamics of gas production rates intraday and for longer periods up to 3 days. The effect of flexible feeding on daily and longtime process stability will be assessed. Furthermore, the saving effect on gas storage demand based on the achieved flexible biogas production versus a constant production will be discussed and compared with literature values.

2 Materials and Methods

2.1 Experimental Setup and Substrates 2.1.1 Research Biogas Plant A

The two main CSTR-digesters at research biogas plant A have each an active volume of

165 m3 (208 m3 total). The digesters were equipped with central agitators (Centromix;

7.5 kW, Karl Buschmann Maschinenbau GmbH, Germany). To ensure a constant gas

space volume for accurate gas quality measurement, they had a fixed flat ceiling. For

piping and instrumentation diagrams and further setup information, see Mauky et al.

[17]. The digester temperatures were maintained at mesophilic conditions (38 ± 1 °C).

The feeding substrates were cattle slurry and maize silage at the primary digester and

sugar beet silage at the secondary digester (results of Weender analysis given in Table

1). The digestate from the primary digester to the secondary digester were pumped

103 once a day prior the first feeding of the primary digester. Before feeding the secondary

104 digester the digestate from the secondary digester was also transferred to the digestate

105 storage tank. Table 2 shows the different experimental phases with the respective

106 operation time, used substrates and organic loading rates (OLR).

107 2.1.2 Research Biogas Plant B

108 The CSTR-digester at research biogas plant B had a volume of 923 m3 with an active

109 volume of 800 m3. The digester was equipped with a submersible motor mixer (4670,

110 13 KW, ITT Flygt AB, Sweden) and a propeller incline shaft agitator (Biogator HPR 1,

111 15 kW, Ellzee-Hausen, Germany). A fixed flat ceiling ensured a constant gas space

112 volume for accurate gas quality and volume measurement. For additional information

113 for piping and instrumentation, see Mauky et al. [17]. The digester temperatures were

114 maintained at mesophilic conditions at 40.5 ± 1 °C. At full-scale plant B maize silage,

115 grass silage and ground wheat grain were used as feedstock (results of Weender

116 analysis given in Table 1). Table 2 summarizes the experimental phases with the

117 respective operation time, used substrates and OLR. The total flexible operation time

118 was 10 months for experiments A, respective 7 months for experiments B. The courses

119 of the gas production rate of both full-scale plants is already published partly in [17].

121 2.1.3 Referential Laboratory Experiments

122 To evaluate the full-scale experiments, additional data of already published laboratory-

123 scale experiments were used. Table 3 gives an overview of information characterizing

124 these experiments. The total flexible operation time was 8 and 5 months of experiment

125 A and B. Further information on the experiments can be found in Mauky et al. [14].

126 Table 1 shows additional the results of the Weender analysis for the used feedstock in

127 the laboratory experiments.

Table 1, Table 2 and 3

2.2 Analytical Methods

At the research biogas plant A the gas production was measured by a dynamic pressure probe sensor (S.K.I. Schlegel & Kremer GmbH, Germany). The biogas composition (CH4, CO2, H2S, H2, O2) at research biogas plant A was measured with an AWIFLEX gas analyzer (Awite Bioenergy GmbH, Germany). At the research biogas plant B the biogas production was recorded continuously with a gas flowmeter (GD 300, Esters Electronik GmbH, Rodgau, Germany). The biogas quality (CH4, CO2, H2S, O2) was measured by a multisensor analyzing system (INCA 4000, Union ^Instruments GmbH, Karlsruhe, Germany). The gas yields were corrected to standard conditions (0 °C, 1013 hPa). The preparation and measurement of the sum parameter of volatile organic acids (VOA) by means of titration (Kapp method, Mettler Toledo Typ Rondo 60/T90), ammonia nitrogen (photometric by Nessler method with Hach DR3900), and pH (WTW Typ pH 3310 SenTix 41) are described in [19]. At research biogas plant A the redox potential was measured by an online sensor (Hach Lange, type DRD2P5.99). At both plants the samples of input substrates and digestate were taken twice a week and analyzed both for dry matter (DM) and organic dry matter (oDM). The composition of the individual feedstocks was analyzed by Weender method [19]. For evaluation of the process stability the relationship of VOA and the reactor buffer capacity relative to calcium carbonate (FOS/TAC according to [19]) was used. In general, reactor samples were taken and prepared for further measurement prior to the first feeding. Additionally, at particular days the course of the acids was monitored by frequent sampling throughout the day. The samples at Research Biogas Plant B were immediately frozen for transportation

and further analyzation at DBFZ. The concentration of individual volatile fatty acids (VFA) was determined by using an Agilent 7980A gas chromatograph (Agilent, USA) equipped with a Turbo Matrix 110 automatic headspace sampler (Perkin Elmer, USA) and an Agilent HP-FFAP column (30 m x 0.32 mm x 0.25 mm) for chromatographic separation. Sample preparation for VFA analysis (1 ml H3PO4 and 1 ml internal standard) was performed according to [19].

2.3 Data Processing

For illustrating the intraday distribution of the gas production (chapter 3.2), the experimental data was split into two periods every day. Figure 1 shows the subdivision of an exemplary daily gas production course. In a practical context the first half (0 -12 h) can be understood as a utilization phase in which biogas is consumed and electrical current is produced. The second half (12 - 24 h) represents the storage phase when no biogas is utilized. The real daily time sequence of the experiments can be slightly different. However, the starting point for calculation (0 h) is set by the first feeding event of a day.

Figure 1

To assess the temporal degradation characteristics and the dynamics of biogas production in different experimental scales, a model-based evaluation was performed. The utilized model is based on the stoichiometric structure of the Anaerobic Digestion Model No.1 (ADM1, [20]). Furthermore, the complex model structure of the ADM1 is simplified to simulate the complete anaerobic digestion of particulate carbohydrates, proteins and lipids to biogas by the superposition of three brutto reactions. Detailed information on the model derivation procedure or the simplified model structure, as well

as the implemented kinetic and physico-chemical model parameters can be obtained in Weinrich et al. 2015 [21] and Mauky et al. 2016 [17].

For each experimental setup the model parameters were manually adjusted to simulate the measured biogas production rate in a semi-stationary process state, see Figure 2. Then the modelled feed was suspended and the simulated gas production rate subsides. The total amount of biogas produced by the last feed is determined by subtracting the current from the previous degradation curves (dashed area in Figure 2). On this basis, the time periods were calculated when 15, 25 and 50 % of the total amount of biogas is produced by the last feed. The calculated area can be assumed to be the biogas potential of each substrate at infinite retention time. Compared to the practical semi-continuous process, there is a small difference because of the effect of the hydraulic retention time (partial substrate washout at each feeding event in the CSTR).

Figure 2

In order to evaluate the theoretical effect of flexible compared to constant feeding on the necessary gas storage demand (chapter 3.3), a third method was introduced. The comparison was done by four gas utilization scenarios. Scenarios A, B and C assumes gas utilization phases of 8, 12 and 16 h a day. The storage phase thus stretched over 16, 12 and 8 h per day, respectively. Finally, a scenario with a feeding break of 72 h was assumed to simulate a low demand at the weekend or high electricity production by i.e. wind power [22]. The flexible gas production for calculating the utilization scenarios was based on the measured gas production dynamics of the laboratory-scale experiment B and full-scale experiment B. The comparison of the scenarios was extrapolated up to gas production rates of 1400 m3 h-1. The necessary gas storage demand was calculated

200 for the gas production rate of the respective experiment and was furthermore

201 extrapolated to gas production rates of up to 1400 m3 h-1. In order to broaden the

202 spectrum of experiments and reactor technologies, literature results for an 8 h and a

203 72 h scenario implemented with alternative hydrolysis/fixed bed configuration (called

204 ReBi) [22] were integrated in the comparison. Results by Lemmer and Krümpel [23]

205 based on flexible feeding of comparable anaerobic filter concepts affirm such high

206 gradients in the gas production. Both approaches realizes a highly dynamic gas

207 production rate (up to factor 8 between minimal and maximal rate) by addition of

208 energy rich hydrolysate into a methanization reactor.

209 Based on regulations of the German Federal Pollutant Control Act (BImSchV) an upper

210 limit for on-farm biogas storage capacity was specified for evaluation. This act defines

211 limits for on-farm biogas storage capacity as a threshold for the type of permission of

212 operation. With more than 50 tons on-farm biogas storage capacity the plant needs to

213 deal with stronger safety regulations and expensive permit procedures with high

214 administrative effort. With a density of raw biogas of approximately 1.3 kg m-3 the limit

215 of 50 tons corresponds to a volume of 38,000 m3. In consequence, plant operators try

216 to operate below this biogas storage capacity limit.

217 3 Results and Discussion

218 3.1 Process Dynamics and Stability

219 3.1.1 Full-scale Biogas Plant A

220 Figure 3a shows the gas production rate and gas quality achieved with flexible feeding

221 at full-scale plant A (full-scale Experiment A) between day 9 and 36. The results show

222 that by flexible substrate feeding, the daily gas production rate can be modulated up to

223 ± 50 % of the daily average gas production rate (e.g. day 33 with min/max values of

14 m3 h-1 and 38 m3 h-1). By targeted reduction of the feeding quantity at the weekend, the gas production could be reduced even below 12 m3 h-1. The results show the effects of flexible substrate feed on the gas production rate and thus the flexibility of the process itself. The process is reacting to the feeding event within several minutes with a significant jump in the gas production rate. Thereby, presumably the volatile components in the substrate are initially degraded; CO2 is released due to the prevalence of hydrolytic activity and a slight pH drop within the digestate is measured. A considerable increase in the gas production rate can be observed within an hour. After a feeding event the CH4 concentration drops below 50 % and the CO2 share rises up. But in the further progress, the methane content increases again, exceeding 55 % and returns to the initial percentage in the course of the day. Figure 3c shows the biogas and the methane production rate normalized to the respective average gas production rate and the deviation to each other. Thus, the effect of CO2 can be separated and the metabolic activity can be better described. It can be seen that the methane potential slightly shifts into the second half of the day. The behavior of gas concentrations can be explained by the immediate onset of hydrolysis, where organic acids and CO2 are produced. Once these intermediates are available, the downstream processes and finally methanogenesis follow, leading all measured values back to the initial levels. It is assumed that processes fed in a flexible manner as presented alternate between phases with on the one hand increased hydrolysis / acido- / acetogenesis and on the other hand methanogenesis processes throughout the course of roughly 24 h after a feeding event. The anaerobic system reacts, buffers the disturbance and gains stability again. Phases with higher gas production after feeding events yielded poorer gas qualities regarding the methane content. At this full-scale experiment a fixed roof with a relatively small gas volume, similar to laboratory-scaled digesters, was used. However, in conventional full-scale plants the gas is collected in larger gas storages holding

capacity for several hours. Thus, different qualities are mixed, leading to lower variations in the gas quality. Furthermore, modern combined heat and power (CHP) units with combustion control can process gas of varying quality within the observed margin of 45 % methane. However, the engine needs more gas at low CH4 contents to generate the wanted output, may start worse and the exhaust gas values qualities might also deteriorate (NOx can increases).

Figure 3

A reduction of gas production rate over several days was tested three times (day 16, 23, 30) in this experimental period (day 9 to 36) to simulate a period of lower demand. The ability to reduce the gas production rate over several days is considered equally important in the context of the requirements from the energy system as the short term "on/off" flexibility of intraday gas production [11]. Findings indicate that the biogas production can be highly varied and ramp up and down within a few hours.

With a view to process stability, also the courses of acetic and propionic acid concentrations are presented in Figure 3a. The samples for analyses were usually taken prior to the first daily feeding event and the results show no significant long term accumulation of VFAs. In laboratory experiments [14], a highly dynamic behavior and stability at the same time was detected in a CSTR-system. In the full-scale experiments on days 15 and 21 the course of the acids was monitored by frequent sampling throughout the day. Figure 3b gives a detailed segment from day 20 to 28 with gas production rate, gas quality (CH4 and CO2) and the course of the feeding rate. Additionally, in Figure 3d the courses of the acetic and propionic acid concentration and the pH-value are indicated. A parallel rise in acid concentrations and gas production rate can be seen. Any increased acid concentration should lead to an increased gas

production as long as the risen concentration does not lead to an inhibition. During the day (see day 21), the acetic acid concentration reaches a maximum of 800 mg l-1 and then decreases until below 200 mg l-1. In the course of the day, the propionic acid does not rise above 50 mg l-1. The fluctuations of the acid concentration in the indicated ranges are not critical for the process [24]. The FOS/TAC ratio varies between uncritical values of 0.17 and 0.29 (not shown). This also corresponds to the observations made in laboratory-scale [14]. Feeding pauses (see day 23 - 25) lead to a substantial reduction of the acids. The subsequent accumulation of acids which is often feared could not be observed. Other analyzed acids, (butyric-, valeric- and hexanoic acid) also remained below a level of 50 mg l-1 (not shown). The pH-value corresponds inversely proportional to the acid concentrations. Hence, the pH can be suitable for an easy-to-measure indicator, as already recommended by [25].

The experiments show that the stop of substrate addition for 2 days does not harm the general responsiveness of biogas production. However, after longer periods (from 2 days) without feeding a delay in before the rise of the gas production rate could be observed (e.g. 8h delay after feeding on day 25 in Figure 3b and day 32 in Figure 3a). In consequence, only 50 % of daily gas production can be found in the first half of the day. In comparison, on day 26 the proportion of daily gas production within the first 12 h is 65 %. The CO2-fraction on day 25 (see Figure 3b) rises in a typical way during the first feeding events. But after the fourth feeding the CO2-fraction begins to decrease, followed by a second peak, which is untypical.

On one hand, the delayed CO2 formation could be due to the degradation process itself (i.e. hydrolysis gas from a fraction with a delayed degradation). Furthermore, only small changes in the hydrogen measurement (gas phase between 100 and 150 ppm) and a constant low redox potential (< -300 mV) were observed. These are not typical signs of

299 inhibition [26]. In the considered time period, no online-measured values were available

300 for pH-value and acids which could confirm this hypothesis. Nevertheless, a slight

301 inhibition or metabolic state change by the high feed after this long hunger phase is

302 possible. The feeds were calculated by a model predictive control (already published

303 description in [17]) in order to fulfill a gas demand timetable. In doing so, the model

304 possibly overestimated the responsiveness of the process directly after a longer pause,

305 which leads to a daily overfeed. However, on the next day (day 26), the analysis of the

306 acids, pH value and the course of the gas production and quality showed no long-term

307 inhibition.

308 On the other hand, the second peak in the CO2-content could have led to a dissociation

309 shift in the direction to CO2. In consequence more CO2 is set free in the liquid phase

310 which leads to an increased contribution to the gas phase. By the presumed acid

311 formation the pH value might be reduced which leads to the dissociation shift and CO2

312 is ejected from the liquid phase. In this case, the second peak can be explained with the

313 delayed degradation of the acids. Further reasons for these behaviors may be found

314 also in rheological-induced local disturbances which have still to be investigated.

315 The described behavior was not observed in previous laboratory experiments (10 and 35

316 liter volume) [14]. Therefore, it should be investigated whether similar situations can be

317 reproduced in a laboratory fermenter, if possible in larger scale.

318 3.1.2 Full-scale Biogas Plant B

319 Figure 4a shows the course of gas production rate and gas quality (CH4 and CO2) at full-

320 scale experiment B during a period of 11 weeks. The results evince that by flexible

321 substrate feeding, the intraday gas production rate can be modulated between

322 min/max values of 75 m3 h-1 and 140 m3 h-1 (e.g. day 53). This daily spread of 65 m3 h-1

(±30 % variation of gas production rate based on the daily average) is lower than the intraday dynamic observed in full-scale experiment A (full-scale Exp. A gives a variation of ±50 %). Causes of this observation may be found in the different substrates, geometrical, rheological and procedural conditions. In general, the gas production rate rises within 2 hours after feeding by a rate of change of 10 m3 h-2 to a significantly higher level. Shea et al. [27] suggests a comparable 2-hour lead-time for feeding of grass silage in advanced of required electricity production.

Figure 4

Figure 4b and c show a single week in detail. The gas production rate as well as the CH4 and CO2 percentage show a highly dynamic behavior as already observed in full-scale experiment A and lab-scale [14]. Between day 79 and 85 as well as between 100 and 107 samples were taken several times a day in order to analyze the dynamics of intermediate formation (Figure 4a and c shows the course of the acetic and propionic acid concentrations). During this period, again a parallel progression of the individual acid concentrations and the gas production rate can be observed. The basic level in the acid concentrations at full-scale experiment B is higher than in full-scale experiment A. Acetic acid concentration peaks of over 1500 mg l-1 was measured. During the feeding pauses (i.e. day 85 and 105), the acid concentration decreased significantly. The FOS/TAC ratio varies between 0.18 and 0.30 throughout the experiment. However, the values are below a potentially process jeopardizing limit of 2500 mg l-1 for acetic acid and FOS/TAC ratio of < 0.4 [24]. Therefore a stable anaerobic digestion process can be stated with no long-term inhibitions.

Furthermore, pH and acids concentration show the same inversely proportional behavior as in full-scale experiment A and laboratory. However, the pH-value should nevertheless be considered critically, since the accuracy related to the measured alterations is low. Furthermore, the pH-value depends on the plant-specific buffer capacity affected e.g. from substrates and procedural conditions. Therefore it should be used only in combination with other parameters (i.e. according to Bensmann et al. [25] with gas composition measurement) for monitoring short term changes of process stability at biogas plants.

During feeding pauses of 2 days (day 63 - 65; 70 - 72; 77 - 80) it was possible to reduce gas production rate by more than 70 % compared to the average over a time of up to 72 h based on the used substrates and their proportional variation. In comparison to the full-scale experiment A, a similar delayed gas production (maxima 5 to 7 h later than expected) is observed on days 101, 102 and 103. The same causes can be stated as already discussed in the previous section (3.1.1). Thus, a minor inhibition or acidification could cause the delay. Furthermore, rheologically induced effects and general activity changes in the microbial community are possible. This delay effect should be reproduced and investigated more in detail.

It is thus also apparent in full-scale that the anaerobic process can be operated stable even in the case of flexible feeding including high short-term organic loads. The pH and acids concentrations show an inverse behavior, as already seen in laboratory-scale in Mauky et al.[14]. However, different levels of acid concentrations (i.e. average acetic acid concentration of around 200 mg l-1 in full-scale experiment A and 1000 mg l-1 in full-scale experiment B) could be observed. In comparable laboratory experiments only an average of 55 mg l-1 acetic acid was measured. However, during the entire period no

process conditions were observed which endangered the long-term stability of the biogas process.

3.2 Effect of Demand-Oriented Feeding at Different Scales

Figure 5 illustrates the intraday flexibility potential compared for different scales and substrates. According to the methodology in chapter 2.3, the percentage of the cumulated daily gas production (0-24 h) is shown, which is produced in the first half (0 -12 h after first feed).

Figure 5a compares the gas production rate from experiments at different scales where maize silage was the main substrate. The impact on process dynamics of the co-substrate cattle slurry can be neglected, since it contributes to the OLR with less than 5 %. Grass silage (GS) shows a nearly similar behavior as maize silage regarding kinetic behavior [28]. In laboratory-scale experiment A in the first daily half a spread in the flexibility of biogas production between 58 and 66 % was observed. The median is at 62 % gas production in the first half of the day in percentage of the cumulated daily gas production. In both full-scale experiments (A and B) nearly the same level (median at 56 and 55 % of cumulated percentage in the first daily half) can be observed. The comparison shows a reduction of the intraday flexibility potential from laboratory to full-scale. Explanations for that different behavior could be found in the rheology and fermenter dimensions as well as in the general process conditions in practice against laboratory conditions. Thereby, in full-scale lower mixing times and outgassing is reached compared to laboratory-scale. The full-scale system responds more slowly to changes in process conditions and thus a reaction to feedings will take longer.

In Figure 5b, a comparison of intraday flexibility potential based on the sugar beet

experiments is given. In laboratory-scale experiment A II a high percentage of 69 %

395 biogas with a spread between 62 and 77 % could be produced in the first half of the

396 day. In laboratory-scale experiment B with an exclusive digestion of sugar beet silage

397 the highest flexibility potential with a median of 72 % and a spread between 66 % and

398 77 % was reached. In full-scale only a flexibility of 59 % (median) was measured, but the

399 spread was the smallest of all experiments. The results confirm the expectations that

400 with sugar beet silage a higher flexibility is possible as this substrate has a high content

401 of quickly degradable components and thus faster kinetics. In all experiments, the

402 percentages for intraday methane production for this consideration are lowered the

403 values by 2.5 % compared to biogas.

404 Figure 5

405 In Table 4 the simulated degradation time, which is needed to produce 15, 25 and 50 %

406 of the expected biogas yield of each substrate is shown. For each scale differences in

407 the dynamics of biogas production can be observed. Within the first few hours after the

408 substrate addition, large proportions of the substrates are already degraded (e.g. 15 %

409 after 2 h and 25 % after 4 h using sugar beet silage in laboratory scale experiment B). In

410 full-scale experiment it takes more than twice as long for a comparable degradation of

411 sugar beet silage (4.5 and 8 h for 15 %, respective 25 %). However, due to the different

412 substrate qualities and process conditions, the results can only be compared with each

413 other to a limited extent and should only give an orientation. The shorter this time span,

414 the larger is the possibility to substantially shift the daily gas production into the periods

415 of demand. A substrate degradation of 50 % within one day is reached (besides the

416 general results in laboratory scale) only with the fast degradable sugar beet silage

417 (20.5 h) and ground wheat grain (20 h). In the first 30 minutes of all experiments a peak

418 in the measured biogas production rate can be observed. This can probably be explained

by the proportion of volatile acids in the respective ensiled substrates and the content of fast degradable components. This behavior can already comprehend within the model.

Table 4

In general, the full-scale experiments could confirm the practical possibilities of flexible biogas production, albeit at a lower level. Therefore, in particular the transferability to even larger plants, the degradation process itself and higher OLRs should be further investigated.

3.3 Theoretical Saving of Gas storage Capacity Based on Flexible Feeding

Figure 6 compares the theoretical savings of gas storage capacity caused by flexible versus continuous biogas production based on laboratory and full scale experiment B. Scenario A, B, C (Figure 6 a, b, c) assumed a gas utilization phases of 8, 12 and 16 h a day, respectively a storage phases of 16, 12 and 8 h (see methodology in chapter 2.3). The thin gray line describes the respective basis of a continuous biogas production. The dashed line and bolt line give the demands of gas storage capacity based on the laboratory and full-scale flexible potentials at CSTR experiments. The resulting savings in storage capacity were described by the differences to the gray line (continuous biogas production).

In scenario A (8 h utilization, 16 h storage) a high reduction of the necessary storage

demand was possible by 35 % in laboratory-scale and 11 % in full-scale was possible

with flexible feeding. However, with the high dynamics of a fixed-bed reactor [22]

(dotted line) a further decrease of more than 35 % could be achieved. In scenario B

(12 h utilization, 12 h storage) the necessary storage demand could be reduced by 45 %

and 15 % at laboratory respective full-scale (Figure 6b). The necessary storage demand, assuming the scenario C (16 h utilization, 8 h storage) could be reduced by 15 % (full-scale) and 17 % (laboratory-scale) with a flexible feeding of CSTR-systems. For the scenarios B and C (12 h and 16 h utilization), no additional data for hydrolysis/fixed-bed configuration could be found in the literature.

Figure 6

At last - in scenario D, in Figure 6d - a period without biogas consumption during 72 hours (i.e. weekend from Friday noon to Monday noon) was assumed. Biogas plants can have an economic benefit in times of low demand, if they can reduce the feed-in energy into the grid for longer periods. Furthermore, the limiting factor of 50 tons on-site biogas storing capacity (see methodology in chapter 2.3) is pictured as bolt dashed/dotted line. With the ReBi-configuration a high decrease of the necessary gas storage demand until below the 50-ton-limit is possible for much larger biogas plants, i.e. higher average biogas production rates than with constant biogas production (see Figure 6d). The ReBi concept generally shows a lower storage demand and additionally benefits at higher gas production rates over 1400 m3 h-1 in these scenarios and undercuts the on-farm biogas storage requirement. However, the ReBi-results need to be validated at larger scale, taking into account that these results are based on laboratory-scale experiments. With a full-scale CSTR-system also a substantial reduction of the necessary gas storage demand by over 65 % can be reached in scenario D. Thereby, the 50-tons-limit can be undercut up to an average biogas production rate of 1400 m3 h-1. This allows a considerably expanded range of flexibility options to be offered without the need for an additional high investment for complex structural changes. The calculation is based on the dynamic seen in the full-scale experiment B (experimental day 60 to 80 in Figure 4a).

467 Besides the intraday flexibilisation, the possibility to reduce gas production within CSTR-

468 systems over several days shows a large potential to substitute necessary gas storage

469 volume. The investigation demonstrates a significant contribution of flexible feeding to a

470 demand-driven energy production at full-scale biogas plants.

471 4 Conclusion

472 This study showed that by dynamic substrate feeding flexible biogas production can be

473 achieved in full-scale biogas plants. Flexible feeding in anaerobic digestion results in an

474 accordingly variable rate of gas production (up to 50 % of daily average). At the same

475 time alterations of methane, carbon dioxide and acid concentrations occur within

476 acceptable ranges. Also the pH- value corresponds to the flexible feeding. However the

477 long-term process stability was not negatively affected by flexible feeding. In result, a

478 flexible biogas production can enable significant savings regarding the required gas

479 storage volume (in intraday context up to 45 %) and thus allows a far greater flexibility

480 in power production than at a constant gas production rate. Particularly the decrease of

481 gas production during periods without any feeding of up to 3 days, the investigated

482 processes showed great savings potentials (of up to 65 %) regarding necessary gas

483 storage volume.

484 5 Acknowledgements

485 Parts of the work were funded by European Regional Development Fund (ERDF) through

486 the Sächsische Aufbaubank - Förderbank with grant 100143221.

487 6 References

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Tables:

Table 1 Composition of feedstocks (Maize silage, Grass silage, Cattle Slurry, Sugar beet silage and Ground wheat grain) based on Weender analysis; Acronyms: fresh matter (FM)

Table 2 Overview of the experiments performed during the study (acronyms: MS-maize silage; SBS-sugar beet silage; CS-cattle slurry; DP-digestate primary; GS-grass silage; GWG-ground wheat grist; VR_liq: active liquid volume; VS: volatile solids)

Table 3 Overview of already published laboratory experiments for comparison based on [14] (Acronyms: MS-maize silage; SBS-sugar beet silage; CS-cattle slurry; DP-digestate primary)

597 Table 4 Comparison of the duration from feeding event to the point when 15 %, 25 %

598 and 50 % of biogas were produced from a substrate portion

600 Figures:

601 Figure 1 schematic illustration of determining the flexibility potential by comparing the

602 gas production generated within the first half of the day (0-12 h) related to the total

603 daily biogas volume (0-24 h); In this example 70 % of the daily total biogas production

604 were produced in the first half of the day. In result the cumulative intraday flexibility

605 potential is 70 %. In the second half of the day only 30 % of the total daily sum of biogas

606 were produced; every day's calculation is beginning with the first feeding event

607 Figure 2 Illustration of determining the duration from feeding event to the point where

608 15, 25 and 50 % of cumulated total biogas is produced; The total amount of biogas

609 produced by the last feeding is calculated by subtracting the current (blue line) from the

610 previous degradation curves (gray overlaid lines)

611 Figure 3 Experimental results at full-scale plant A - period I: a) (experimental day 9 to

612 36) biogas production, methane and carbon dioxide concentration and acid

613 concentration b) biogas production, methane, carbon dioxide concentration and feeding

614 events from experimental day 20 to 28 c) normalized biogas and methane rate and

615 deviation of normalized methane rate from normalized biogas rate d) acid concentration

616 and pH- value from experimental day 20 to 28

617 Figure 4 Experimental results at full-scale plant B: a) (experimental day 45 - 122) biogas

618 production rate, average biogas production rate, methane and carbon dioxide

619 concentration, acid concentration, b) biogas production rate, average biogas production

620 rate methane, carbon dioxide concentration and feeding events at experimental day

621 100 - 109 c) acid concentration and pH- value at experimental day 100 - 109

622 Figure 5 Flexibility potential as percentage of daily gas production (0-24 h) cumulated in

623 the first half of the day after first feeding (0-12 h) at different scales and substrates; a)

624 Comparison of experiments based mainly on maize silage; b) Comparison of

625 experiments based mainly on sugar beet silage; Abbreviations: MS = maize silage; CS =

626 cattle slurry; GS = grass silage; SBS = sugar beet silage; DG = digestate from a primary

627 digester; The box's give the upper and lower quantile (25 % and 75 % range), the

628 median line, the whiskers give the 99 % and 1 % quantile; the white square stands for

629 the arithmetic average

630 Figure 6 Comparison of necessary gas storage volume based on continuous and flexible

631 biogas production by CSTR system and hydrolysis/fixed bed configuration; a) Scenario A

632 with 8 h utilization /16 h storing profile; b) Scenario B with 12 h utilization /12 h storing

633 profile; c) Scenario C with 16 h utilization /8 h storing profile; d) Scenario D with a 72 h

634 storing phase; (ReBi: alternative hydrolysis/fixed bed configuration. Description and

635 values of ReBi-configuration adapted from [22])

Table 1 Composition of feedstocks (Maize silage, Grass silage, Cattle Slurry, Sugar beet silage and Ground wheat grain) based on Weender analysis; Acronyms: fresh matter (FM)

Component

Maize silage

MS I MS II MS

Grass silage

GS CS I

Cattle slurry CS II

Sugar beet silage

Ground wheat grain

SBS I SBS II SBS III GWG

Dry mass (DM)

[% FM]

28.4 33.3 39.5 44.9 5.9 6.9 19.4 20.3 14.1 88.6

Organic dry mass (VS)

[% DM]

96.4 96.3 98.6 96.0 77.9 72.1 82.2 86.2 71.4 97.5

Nitrogen free Extracts (NFE) [g kg -1 DM] 595.3 621.0 869.0 770.0 343.8 324 707.2 753.7 617.0 810.0

Crude protein

[g kg -1 DM] 77.4 67.0 35.0 49.0 190.4 147 46.1 42.3 63.9 118.0

Crude lipids

Crude fiber Ash

[g kg -1 DM] 11.5 8.0 15.0 14.0 5.3

[g kg -1 DM] 280.1 267.0 67.0 [g kg -1 DM] 35.7 37.0 14.0

4,5 2.9 4.5 19.0

127.0 239.3 247 64.6 63.3 136.6 28.0 40.0 221.2 279 177.6 137.8 178.0 25.0

Table 2 Overview of the experiments performed during the study (acronyms: MS-maize silage; SBS-sugar beet silage; CS-cattle slurry; DP-digestate primary; GS-grass silage; GWG-ground wheat grist; VR_iiq : active liquid volume; VS: volatile solids)

Experiment Period Days Digester VR_liq OLR Substrates 1

[d] [m3] [kgvS m-3 d-1]

Full-scale experiment A i 70 Primary 165 4 MS II + CS II

II 70 Secondary 165 2 DP + SBSIII

Full-scale experiment B 190 Primary 800 2.8 - 3.5 GS + MS III + GWG

1 maximal feeding velocity at both plants is 60 kg min-1 fresh matter

Table 3 Overview of already published laboratory experiments for comparison based on Mauky et al. [14] (Acronyms: MS-maize silage; SBS-sugar beet silage; CS-cattle slurry; DP-digestate primary)

Experiment

Period Days

Digester VR_iiq OLR

Substrates

[m3] [kgvs m-3 d-1]

Laboratory-scale experiment A I

Laboratory-scale experiment B

107 109

Primary

Secondar y

Primary

3.0 - 4.0 3.0 - 4.0

0.035 2.0 - 5.0

MS I + CS I DP + SBS II

Table 4 Comparison of the duration from feeding event to the point when 15 %, 25 % and 50 % of biogas were produced from a substrate portion

Full-scale A

Lab-scale exp. A Lab-scale exp. B

Full-scale B

Duration from feeding event to:

Mix of Maize

Maize silage Sugar beet silage Maize silage Sugar beet silage (MS III) Ground wheat (MS I) (SBS I) (MS II) silage (SBS III) and Grass silage grain (GWG)

15 % of cumulated total biogas production

from a substrate portion (t15%) [h] 2.5

25 % of cumulated total biogas production

from a substrate portion (t25%) [h] 5

50 % of cumulated total biogas production

from a substrate portion (t50%) [h] 15

First half (0-12h)

Second half (12-24h)

\ \ 1 \

\ \ \ 1

Intraday flexibility potential

■-Biogas production rate — — Daily average gas production

Sum of biogas at the first / / /and second \ \ daily half ^ Feeding events

-Resulting biogas production curve by

overlaying past — degradation curves

Current degradation curve //// Total amount of biogas by the last feed

I 111 H

Substrate feeds

24 25 26

Time [d]

Biogas production rate ----Average biogas rate .........Methane fraction

Acetic acid ♦ Propionic acid Maize silage-Normalized biogas rate

Deviation of nomalized methane rate from normalized biogas rate * pH-value

Carbon dioxide fraction Normalized methane rate

~l 1I 1r 75 81 87 Time [d]

i 1 i 1 i 1 r 99 105 111 117

i-1-1-1-1-1-1-1-1-1-r

100 101 102 103 104 105 106

Time [d]

-Biogas production rate----Average biogas production rate..........Methane

♦ Propionic acid

I Ground wheat arain

Carbon dioxide • Acetic acid Mix of 40 % Maize and 60 % Grass silaae & oH-value

Lab-scale exp.A I

(0.01 m3, MS+CS)

Full-scale exp.A I

(165 m3, MS+CS)

Full-scale exp.B

(800 m3, MS+GS)

•1 k..

Lab-scale exp.A II

(0.01 m3, SBS+DG)

Lab-scale exp.B

(0.035 m3, SBS)

Full-scale exp.A II

(165 m3, SBS+DG)

a) Scenario A (8h/16h profile)

:5000-r—r-

Continuous biogas production Flexible CSTR lab-scale

---Flexible CSTR full-scale

..........Flexible ReBi lab-scale

co" 25000 £

| 20000

15000-

w 10000-

200 400 600 800 1000 1200 1400

Average biogas production rate [m3 h"1] Scenario C (16h/8h profile)

-Continuous biogas production

Flexible CSTR lab-scale ---Flexible CSTR full-scale

cv 100000

80000-

60000-

2 40000-

20000-

200 400 600 800 1000 1200 1400 Average biogas production rate [m3 h"1]

Scenario B (12h/12h profile)

Continuous biogas production Flexible CSTR lab-scale ---Flexible CSTR full-scale

200 400 600 800 1000 1200 1400 Average biogas production rate [m3 h"1]

Scenario D (72h storage demand)

-Continuous biogas production

-----Safety limit on-site biogas

---Flexible CSTR full-scale

.........Flexible ReBi lab-scale

200 400 600 800 1000 1200 1400 Average biogas production rate [m3 h"1]

Highlights

• Flexible biogas production demonstrated in full-scale by demand-oriented feeding

• Savings in necessary gas storage volume of up to 65% by flexible feeding

• Intraday dynamic in biogas production rate of up to ±50 % based on average

• Intraday alteration of methane, carbon dioxide and acid concentrations, as well as the pH- value corresponding to the flexible feeding

• Comparison with laboratory-scale results based on CSTR and hydrolysis/fixed bed configuration