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The effect of flue gas explosive decompression pretreatment on methane recovery from bioethanol production waste
Industrial Crops & Products 127 (2019) 66–72
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The effect of flue gas explosive decompression pretreatment on methane recovery from bioethanol production waste
T
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Lisandra Rocha-Menesesa, , Anastasia Ivanovab, Guilherme Atouguiac, Isaac Ávilac, Merlin Rauda, Kaja Orupõldd, Timo Kikasa a
Institute of Technology, Chair of Biosystems Engineering, Estonian University of Life Sciences, Kreutzwaldi 56, 51006, Tartu, Estonia School of Engineering, Tallinn University of Technology, Ehitajate tee 5, 19086, Tallinn, Estonia c Faculty of Agricultural and Environmental Sciences, University of the Azores, Rua Capitão João d’Ávila Pico da Urze, 9700-042, Angra do Heroísmo, Azores, Portugal d Institute of Agricultural and Environmental Sciences, Estonian University of Life Sciences, Kreutzwaldi 5, 51006, Tartu, Estonia b
A R T I C LE I N FO
A B S T R A C T
Keywords: Anaerobic digestion Bioethanol Biofuel Lignocellulose Production waste Zero-waste
Lignocellulosic biomass is an attractive feedstock for the production of liquid (eg. biofuel) or gaseous (eg. methane) fuels for the transportation sector. The bioethanol production process still produces a large quantity of production waste following the distillation process. Stillage consists mostly of lignin, hemicellulose, extractives, and yeast and therefore does not have any commercial value. The conversion of bioethanol production waste into gaseous biofuels like biogas or biomethane is a promising solution when it comes to transforming stillage into value-added products, enhancing the value of the biomass, and as a strategy for achieving zero-waste societies. This study aims to investigate the potential of bioethanol production waste for biomethane production. The results are compared with samples from different stages of the bioethanol production process. Milled barley straw (Hordeum vulgare) was used as a feedstock to produce energy in the form of methane, and the flue gas pretreatment method (with and without bubbling) was applied. The results show that the methane production yield of bioethanol production waste, which has been pretreated with flue gas without bubbling is 5% higher than that of untreated substrate, and can achieve 94% of the methane production of fermented samples. Bioethanol production waste from substrates, which have been pretreated with flue gas with bubbling have a methane production level that is 29% higher than that of untreated materials. The results suggest that methane yields are influenced by the bubbling process. It is reasonable to use bioethanol production waste for the production of energy in the form of methane and to increase the energy output from the biomass.
1. Introduction Economic development, along with population growth, has been contributing to an increase in the world’s energy production levels. Fossil fuels such as petroleum (32%), coal (28%), and natural gas (22%) are still the main non-renewable sources of energy being used in the world’s energy mix. These fossil fuels are essentially being consumed by two main sectors - transportation (29%) and industry (29%) (OECD/ IEA, 2017), and their utilisation brings with them environmental concerns and a depletion of those very fossils fuels. Due to these problems, a search is on for bio-based solutions that will decrease the share of fossil fuels being used in all of the energy-related segments (inclusive of the transportation sector), a decrease in the concentration of carbon dioxide in the atmosphere, diminishing the emission of greenhouse gases, and improving energy reliability (Mussatto and Loosdrecht,
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2016). A great deal of attention has been paid to biofuels being produced from lignocellulosic biomass due to its abundance, reduced costs, and renewability (Schmetz et al., 2016). Biofuels are a renewable energy source that can be used as a transport fuel. The conversion of lignocellulose into ethanol requires four sequential steps: pretreatment, hydrolysis, fermentation, and distillation. The pretreatment breaks down the plant cell walls and exposes cellulose for enzymatic hydrolysis. In the next step, the pretreated material is converted into fermentable sugars, using acids or commercial enzymes. In the fermentation process, yeasts or bacteria are used to ferment those sugars that are obtained from cellulose into ethanol. The material that is obtained from the fermentation process is a mixture of water and ethanol, and this must further be distilled in order to recover and purify the ethanol (Guragain et al., 2014; Rooni et al., 2017a). In all of the steps that are part of the bioethanol production process, great
Corresponding author. E-mail address: [email protected] (L. Rocha-Meneses).
https://doi.org/10.1016/j.indcrop.2018.10.057 Received 29 May 2018; Received in revised form 27 September 2018; Accepted 19 October 2018 0926-6690/ © 2018 Elsevier B.V. All rights reserved.
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attention has been paid to the pretreatment process. This increases the digestibility of the biomass and provides high sugar and ethanol yields. Pretreatment methods can be dived into physical, biological, chemical, and physio-chemical. Physical pretreatment includes chipping, milling and irradiation, and aims to reduce particle size and cellulose crystallinity, and to increase the surface area. Biological pretreatment uses micro-organisms, fungi, or enzymes to degrade lignin and hemicellulose. Chemical pretreatment consists of dilute acids, concentrated acids, alkali, ozone, ionic liquids, and Organosolv, and physio-chemical pretreatment includes uncatalysed steam, acid catalysed steam, liquid hot water/hydrothermal, ammonia fibre explosion, ammonia recycling percolation, soaking aqueous ammonia, wet oxidation, CO2 explosion, steam explosion, and nitrogen explosion (Rocha-Meneses et al., 2017; Rooni et al., 2017b). From the economic point of view, pretreatment methods represent 33% of the total costs of the process (Tomás-Pejó et al., 2008). The bioethanol production process generates a large quantity of production waste after the distillation process has been completed. This production waste is left unused. From the environmental perspective, it is a source of pollution due to the sheer quantity of leftover material that is produced and also due to its potential to cause environmental pollution. In addition, bioethanol production waste consists of a high level of latent energy, but its handling options may also have high energy-related requirements, resulting in lower net energy balances than expected. Therefore the anaerobic digestion of bioethanol production waste can be used as an effective handling option for these residues, as a way of reducing its environmental impact, thereby helping to offset the costs being incurred in bioethanol production, and as a solution to help improve the net energy balance of the production chain by means of the production of energy in the form of methane (Rocha-Meneses et al., 2017; Rabelo et al., 2011; Willington and Marten, 1982). Anaerobic digestion is a biological process that occurs in the absence of oxygen and which uses micro-organisms to convert organic waste into biogas and biofertiliser. Biogas is mainly composed of methane (50–70%) and carbon dioxide (30–50%). Minor compounds include vapour water, nitrogen, oxygen, hydrogen sulphide, ammonia, and silozanes (Angelidaki et al., 2018). Biogas production by anaerobic digestion involves four steps: hydrolysis, acidogenesis (fermentation), acetogenesis, and methanogenesis (Mehariya et al., 2018). In the first step, extracellular enzymes are used to convert complex compounds such as proteins, cellulose, polysaccharides, and lipids into more simple compounds such as amino acids, fatty acids, sugars, and alcohols. In the acidogenesis process, anaerobic bacteria is used to degrade the hydrolysed material into carbon dioxide, ammonia, hydrogen, and shortchain volatile fatty acids. In the acetogenesis phase, the products of the acidogenesis phase are further converted into acetate using acetogenic bacteria. In the last step (methanogenesis) microbes are used to metabolise the acetate and to produce methane (Rocha-Meneses et al., 2017). This article aims to estimate the potential of bioethanol production waste for methane recovery in order to use the results in the design for a pilot-size anaerobic digestion plant. The study uses flue gas explosive decompression (with and without bubbling) as a pretreatment method and barley straw as a biomass. The methane yields are evaluated using the biomethane potential assay (BMP). The biomethane potential of bioethanol production waste is compared to the biomethane potential of the raw materials and with samples from different stages of the bioethanol production process (pretreatment, hydrolysis, and fermentation).
Fig. 1. A schematic representation of a flue gas pretreatment system: 1 - flue gas tank, 2 - pressure control valve; 3 - manometer; 4 - modified pressure vessel cap; 5 - Parr instruments pressure vessel; 6 - ceramic contact heater; 7 - depressurisation valve; 8 - ventilation system; 9 - thermocouple; 10 - temperature controller unit (Raud et al., 2016a).
2. Material and methods 2.1. Bioethanol production 2.1.1. Biomass The barley straw (Hordeum vulgare) that has been used in the experimental procedure was collected in September 2013 from the plant field collection of the Estonian University of Life Sciences, near Tartu in Estonia. The straw was dried to a moisture content that was below 10%, and was further milled and sieved (to a particle size of 1–3 mm), with a Cutting Mill SM 100 comfort (from Retsch GmbH). The fibre composition of the barley straw (cellulose, hemicellulose, and lignin) was determined in an ANKOM 2000 analyser, using standardised procedures that are described by Van Soest, Robertson (Van Soest et al., 1991; AOAC, 1990).
2.1.2. Pretreatment The samples were pretreated using the explosive decompression pretreatment method with synthetic flue gas (composed of 20% CO2 and 80% N2) as a pressurising agent (Fig. 1). The gas was introduced into the reactor in two different ways, by bubbling it through the biomass-water mixture and by introducing it above the biomass-water mixture. In the bubbling process, the flue gas is equally distributed in the reaction medium, using a modified insertion tube with a circular shape at the bottom. The pretreatment without bubbling does not use an insertion tube, but introduces the gas into the reactor’s headspace. Thanks to this, the flue gas diffuses into the reaction mixture through the gas-liquid interface. In both pretreatment methods, 100 g of predried and milled barley straw is added into the pressure vessel and diluted in 800 g of distilled water. The samples were heated to a temperature of 150 ℃, and the flue gas was used to raise the pressure to 30 bar. After reaching the desired temperature, the vessel was cooled down to 80 ℃ and the pressure was released through the depressurisation valve. The pretreated biomass was cooled down to 50 °C prior to enzymatic hydrolysis. A CP-411 pH meter from Elmetron was used to measure the hydrogen-ion activity levels in the hydrolysate after flue gas pretreatment and after the enzyme addition.
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2.2.1. Calculations The molar concentration of initial methane gas in the test bottle [CH4 I ] is calculated by Eq. (1):
2.1.3. Hydrolysis The pretreated barley straw was introduced into a 1000 mL Erlenmeyer flask and 30 FPU g−1 cellulose of the enzyme complex Accellerase 1500 was added. The flask was filled with distilled water to a total volume of 1000 mL. The process of enzymatic hydrolysis was carried out in shaking incubator (Unimax 1010, Heidolph Instruments GmbH & Co KG) during a twenty-four hour period, at 50 rpm and a temperature of 50 °C. The effect of this pretreatment method on glucose yields has been described in a previous study (Raud et al., 2018).
[CH4 I ] = MF
PI VHS R (273,15 + T )
(1) 3
where PI (Pa) is the initial partial pressure at the headspace, VHS (m ) is the volume of the headspace, MF is the methane fraction in the current time interval, R is the ideal gas constant (8314 Jmol−1 K−1), and T is the incubation temperature (ºC). The molar concentration of the final methane levels in the headspace of the bottle [CH4 F ] is defined by Eq. (2):
2.1.4. Fermentation To convert the glucose into ethanol, 2.5 g of dry yeast (Saccharomyces cerevisiae) was added to the hydrolysate. The flasks were sealed with a fermentation lock and the fermentation process was carried out under low oxygen conditions in 1000 mL flasks over a period of seven days at room temperature. The influence of this pretreatment on ethanol yields has been analysed in another publication (Raud et al., 2018).
[CH4 F ] = MF
PF VHS R (273,15 + T )
(2)
where PF (Pa) is the final partial pressure at the headspace, measured after the GC analysis. The cumulative methane produced in the current interval time [CH4 C ]t is given by Eq. (3): (3)
2.1.5. Separation by evaporation As the glucose and ethanol yields were studied previously (Raud et al., 2018), in this study the broth that was obtained from the fermentation process was separated by evaporation in order to mimic the stillage that would be left after the distillation step had been completed. Therefore the samples were placed into the oven for one hour at 95 °C, to be able to remove the ethanol content. The waste material from the distillation process (production waste), was further utilised in the biomethane potential analysis.
[CH4 C ]t = ([CH4 I ]t − [CH4 F ]t − 1) + [CH4 C ]t − 1
2.2. Biomethane potential
B = Bmax ( 1 – e−kt )
The biomethane yields of untreated, pretreated, hydrolysed, and fermented samples and bioethanol production waste were measured. The biomethane potential (BMP) was evaluated using a modified version of the procedures that were reported by Owen et al and Angelidaki et al (Owen et al., 1979; Angelidaki et al., 2009). The inoculum that was used in the experimental procedure came from the wastewater treatment plant in Tartu (Estonia). Before usage, the sludge was incubated for four days at 36 °C, for the purpose of degasification. The BMP assay was carried out in 575 mL plasma bottles for between 42–45 days, at mesophilic conditions (36 °C). The experiments were carried out in triplicate, and a blank test (using inoculum without substrate) was prepared for each batch. The bottles were filled up with substrate and inoculum to a total volume of 200 mL, and with a substrate to an inoculum ratio of 0.25. The required volume of inoculum and substrate was estimated from the analysis of the total solids (TS) and volatile solids (VS), which were determined according to standard method 1684 (the US Environmental Protection Agency – EPA). Subsequently, the bottles were flushed with N2 to guarantee the anaerobic conditions, and were sealed with rubber stoppers, mixed, and incubated. Over the first two weeks of the experiments, methane production was determined on a daily basis. In weeks three and four it was every other day, and in weeks five and six it was once a week. This process was done by measuring the pressure increase in the headspace, using a BMP-Testsystem WAL (WAL Mess- und Regelsysteme GmbH). The quantity of methane in the biogas was monitored chromatographically using a gas chromatograph (CP-4900 Micro-GC, Varian Inc). The final biogas and methane production rates for the substrates were calculated by subtracting the biogas and methane production levels respectively for the blank tests. The results were expressed in mol of CH4 per 100 g of raw biomass. The pH was measured at the end of the experiments using a SevenMulti™ S47 - dual pH/conductivity meter.
where B is the cumulative methane production at time (t), Bmax is the maximum methane yield, and k is the rate constant. The maximum methane yield (Bmax) was calculated from Eq. (4), and this corresponds to the maximum value of the fitted curve. The digestion time to achieve 85% and 95% of the methane potential was also calculated from the maximum methane yield. The lag time (λ) (the first stage of microbial growth, just before the exponential growth phase) was calculated by using the geometric method. In this technique, a line intersects the slope of the exponential curve, and the prolongation of this line to the initial methane concentration corresponds to the lag time (Prats et al., 2008).
where [CH4 I ]t is the initial concentration of methane in the headspace of the bottle in the current time interval, [CH4 F ]t − 1 is the concentration of final methane in the headspace of the bottle in the previous time interval, and [CH4 C ]t − 1 is the concentration of cumulative methane produced in the previous time interval. The methane results were modelled with a non-linear regression model and further fit into a first-order exponential association model (Eq. (4)) (Zeng et al., 2010; Wang et al., 2016), using the GraphPad Prism 5.0 software: (4)
2.3. Statistical analysis Shapiro-Wilk’s normality test was used to evaluate the normal distribution of the samples. The level of statistical significance was 95%, for a confidence interval of 0.05. The differences between both pretreatment methods (with and without bubbling) were analysed using the Krustal-Wallis test, and the post hoc test Dunn’s multiple comparison test to analyse the differences within the groups. The statistical analysis was carried out using the GraphPad Prism 5 software. 3. Results and discussion 3.1. Chemical composition The results of the fibre analysis of barley straw that was used in the experimental procedure are presented in Table 1. The percentage of cellulose is equivalent to the values that were reported in the available literature (being 1.6% higher) (Saini et al., 2015), while the relative proportion of lignin is particularly low (169% lower in comparison to the values described in the bibliography (Saini et al., 2015)). The composition in terms of hemicellulose and ash (3.8%) is within the range reported in the available literature (Saini et al., 2015). The 68
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biodegradability of the substrates and, therefore, improving biomethane production levels (Dai et al., 2017). Regarding the VS content, the lowest value was gained in that material that remained untreated (896 g/kgTS). For samples that were pretreated with flue gas without bubbling, it was found that the VS content varies between 990 g/kgTS (hydrolysed) and 996 g/kgTS (pretreated, fermented, and production waste). Statistically significant differences were found between the VS content of untreated versus pretreated (p ≤ 0.001), and untreated versus fermented/production waste (p ≤ 0.05). For samples that were pretreated with flue gas with bubbling, the VS content varies between 995g/kgTS (fermented, and production waste) and 996g/kgTS (pretreated and hydrolysed). Statistically significant differences were found between the VS content of untreated versus hydrolysed/pretreated (p ≤ 0.001). These results are particularly important since the production of biogas and its composition are mainly influenced by the quantity and quality of the substrates. Research has shown that the TS and VS contents affect the performance of anaerobic digestion. High reductions in the TS and VS content strongly affect specific biogas yields (Stan et al., 2018; AbbassiGuendouz et al., 2012; Haider et al., 2015). The pH values of the samples are reported in Table 2. For samples that were pretreated with flue gas without bubbling, the pH values for the hydrolysate after completion of the process were at 5.2 and after the enzyme addition they were at 4.7. For those samples that were pretreated without flue gas without bubbling, the pH value varied between 5.4 and 5.0. These values are within the optimal range for yeast growth (between 4 and 6) (Narendranath and Power, 2005).
Table 1 The content of cellulose, hemicellulose, and lignin in raw barley straw (n = 3, ± represents the standard deviation). Component
Content (%)
Hemicellulose Cellulose Lignin Ash
32.6 ± 0.5 45.7 ± 0.2 5.2 ± 0.0 3.8 ± 0.1
Table 2 The total solids (TS) and volatile solids (VS) content and pH levels of samples that were pretreated with flue gas, both with and without bubbling (n = 3). Pretreatment
Variable
TS g/kg
VS g/kgTS
– Flue gas without bubbling
Untreated Pretreated Hydrolysed Fermented Production waste Pretreated Hydrolysed Fermented Production waste
956 ± 2a,b,c,d,e 92 ± 5 87 ± 5b 85 ± 2c 85 ± 5a
896 996 990 996 996
± ± ± ± ±
12a,b,c,d,e 0.06a,d 1d 0.2b 0.03c
– 5.2 4.7 – –
93 93 79 80
996 996 995 995
± ± ± ±
0.08d 0.06e 0.08 41
5.4 5.0
Flue gas with bubbling
± ± ± ±
4 2 2e 8d
pH
Raised letters after some values indicate statistically significantly differences between the variables in the group, with a probability level of p ≤ 0.05.
3.2. The effect of bubbling in methane recovery chemical composition of the barley straw predicts that it is a suitable material for biofuel production, since bioethanol and biomethane yields rise with an increase of cellulose content and with the reduction of lignin proportions (Herrmann et al., 2016; Raud et al., 2016b). In the analysis of the content of total solids (TS) and volatile solids (VS) in the raw material, samples which were pretreated with flue gas without bubbling and samples which were pretreated with flue gas bubbling are presented in Table 2. Untreated barley straw had the highest TS content (956 g/kg) when compared to samples that were gathered after both pretreatment, due to the dilution of the samples in distilled water. In samples that were pretreated with flue gas with bubbling and without bubbling, the TS content was higher in the former for material that was simply pretreated (93 g/kg as opposed to 92 g/kg) and material that was further hydrolysed (93 g/kg as opposed to 87 g/ kg), and lower in the case of the material after fermentation (79 g/kg as opposed to 85 g/kg), and production waste (80 g/kg as opposed to 85 g/ kg). Taking into account the fact that TS content in raw material is 10 times higher than in each step of bioethanol production, these results indicate that the flue gas pretreatment method is effective in the reduction of the TS content. Consequently, this serves to enhance the accessibility of anaerobic micro-organisms to cellulose, improving the
Fig. 2 shows the experimental results for BMP tests with untreated, pretreated, hydrolysed, fermented, and bioethanol production wastes, and the respective fitting curves. Methane yields are presented in mols of methane per 100 g of substrate in order to enable a comparison with the initial raw biomass. As can be seen in Fig. 2 and Table 3, untreated barley straw has the lowest methane yield (0.93 mol CH4/100 g), when compared with samples that were pretreated with flue gas with and without bubbling. For samples that were pretreated with flue gas without bubbling, the pretreatment step had the lowest methane yield (0.97 mol CH4/100 g), while fermented barley straw had the highest methane values (1.1 mol CH4/100 g). For samples that were pretreated with flue gas with bubbling, the methane yields varied between 0.97 mol CH4/100 g (pretreated) and 1.3 mol CH4/100 g (fermented). The maximum methane yield of bioethanol production waste was 20.99% higher than that of samples that were pretreated alone. Hydrolysed material had a maximum methane yield of just 3.4% higher than that of production waste. Statistically significant differences were found in production waste versus untreated/pretreated/fermented (p ≤ 0.0001), and production waste versus hydrolysed (p ≤ 0.05), untreated versus pretreated
Fig. 2. Experimental results for BMP tests involving untreated, pretreated, hydrolysed, fermented material, and bioethanol production waste, and respective fitting curves. A - Flue gas without bubbling; B - Flue gas with bubbling. 69
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production from fermentation residues. The methane yield of the steam-exploded samples was at 120 mL/gVS, and at 69 mL/gVS for organosolv pretreated material (three and 5.2 times lower that the results that were obtained in our study). Safari et al (Safari et al., 2016) also analysed integrated ethanol and biogas production, using softwood pine as a feedstock and diluted sulfuric acid as a pretreatment method. The results showed that biogas production from the pretreatment hydrolysates is an efficient method for converting these compounds into biofuel. Similar conclusions were found by Liu et al (Liu et al., 2015). The authors obtained approximately 307 mL/gVS of methane by means of the biogas digestion of sugarcane bagasse (with results that were similar to those of our study). Berlowska et al (Berlowska et al., 2017) concluded that stillage from sugar beet pulp waste contains unfermented sugars that can be used as a substrate for methane production, achieving a reading of 444 mL/gVS. The results also showed that the system’s energy balance was improved with the integrated ethanol fermentation and anaerobic digestion. The results show that the flue gas explosive decompression pretreatment method (with or without bubbling) works better for cellulases than it does for anaerobic bacteria. The differences between pretreated and untreated samples is higher in the bioethanol process (Raud et al., 2018) than in the anaerobic digestion. The pretreatment opens the structure enough for the enzymes, but not enough for the bacteria to efficiently reach the cellulose. Therefore the pretreatment method is more effective after the hydrolysis step has been taken, facilitating bioethanol production rather than methane production. This may also be due to the presence of inhibitory compounds (such as furan derivatives and phenolic acids) which are released after the pretreatment process, and which have more of an effect on methane producing bacteria than they do on the enzymes and yeasts being used in the bioethanol production process. Furthermore, samples that were pretreated with flue gas with bubbling provided higher maximum methane yields (18.8%) than samples that were pretreated without bubbling. Statistically significant differences were found between hydrolysed/ fermented/production waste (pretreated without bubbling) versus hydrolysed/fermented/production waste (pretreated with bubbling) (p ≤ 0.0001). These differences between the two pretreatment methods may be due to the dissolution of the carbon dioxide within the reaction medium. When bubbling of the flue gas is not applied, the dissolution of carbon dioxide molecules in water is hindered as the contact surface is much smaller than in the case of bubbling. However, bubbling flue gas through the reaction medium improved CO2 dissolution because in this case the contact area for carbon dioxide molecules with water is much greater, thereby ensuring a uniform distribution in the reaction mixture. Furthermore, dissolved carbon dioxide possibly served to influenced the pH value of the solution (involving the acidification of the biomass), which affects the production of inhibitory compounds in the fermentation process, along with bacterial growth and, consequently, the biomethane yields (Espinoza-Escalante et al., 2009; Palmqvist and Lidén, 2014). In Table 3 the best fit-values and the respective standard deviations (for maximum methane yield, the rate constant, and the correlation coefficient) of the fitting curves can be seen for those samples which have been produced during the various steps of the bioethanol production process, whether pretreated with flue gas with or without bubbling. The results show that the equation that was used to generate the theoretical curves (the one-phase exponential association equation) successfully expresses the variation in the data. For samples that were pretreated without bubbling, the results vary between 0.96 (hydrolysed) and 0.99 (pretreated). For substrates that were pretreated with bubbling, the correlation coefficient is 0.99.
Table 3 Maximum methane yield (Bmax), rate constant (k), and correlation coefficient (R2) for untreated, pretreated, hydrolysed, fermented barley straw and bioethanol production waste ( ± represents the standard deviation). R2
Pretreatment
Variable
Bmax mol CH4/100 g
k
– Flue gas without bubbling
Untreated Pretreated Hydrolysed Fermented Production waste Pretreated Hydrolysed Fermented Production waste
0.93 ± 0.021a 0.97 ± 0.013b 0.99 ± 0.022c 1.1 ± 0.013d 0.98 ± 0.015e
0.18 0.17 0.25 0.25 0.25
± ± ± ± ±
0.019 0.015 0.037 0.018 0.021
0.98 0.99 0.96 0.98 0.98
0.97 ± 0.013 1.2 ± 0.011 1.3 ± 0.013 1.2 ± 0.010f
0.19 0.21 0.22 0.23
± ± ± ±
0.011 0.0097 0.011 0.0097
0.99 0.99 0.99 0.99
Flue gas with bubbling
The superscripts in the values indicate statistically significantly differences between the variables (explained below), with a probability level of p≤0.05. a Untreated sample versus pretreated/hydrolysed/fermented (flue gas without bubbling), and untreated sample versus hydrolysed/fermented/production waste (flue gas with bubbling). b Pretreated sample versus hydrolysed/fermented (flue gas without bubbling), and pretreated sample versus untreated/hydrolysed/fermented. c Hydrolysed sample versus untreated/pretreated/hydrolysed/fermented (flue gas with bubbling). d Fermented sample versus untreated/pretreated/hydrolysed/fermented (flue gas with bubbling). e Production waste versus untreated/fermented (flue gas without bubbling) and production waste versus hydrolysed/fermented (flue gas with bubbling). f Production waste versus untreated/pretreated/ fermented (flue gas with bubbling).
(p ≤ 0.05), and untreated versus hydrolysed/fermented (p ≤ 0.0001). Statistically significant differences between methane yields from untreated and pretreated biomass were also reported in the available literature. Menardo et al(Menardo et al., 2012) studied the influence of the pretreatment method on methane yields in four agricultural byproducts (barley, wheat, rice straw, and maize stalks). When compared with untreated samples, mechanical pretreatment significantly increased methane yields of barley and wheat straw by 54.2% and 83.5% respectively. When treated with the thermal pretreatment method, the methane yields of these biomasses increase by 41.7% and 64.3% respectively. Similarly, Bauer et al (Bauer et al., 2009) investigated methane yields in wheat straw that was pretreated using steam explosion. The results show a significant increase (by up to 20%) when compared to those for untreated wheat straw. In addition, fermented samples presented higher methane yields when compared to hydrolysed material, probably due to the production of lactic acid in the fermentation step. Lactic acid can inhibit or stimulate the reaction. In this case, it probably interfered with the process of yeast metabolism and stimulated the growth of the anaerobic bacteria. Another hypothesis is the limited hydrolysis of the lignocellulose. As can be seen in Table 3, in the flue gas with bubbling the hydrolysis step has a lower kinetic rate when compared to fermented samples. Despite this fact, this pretreatment method has been reported as being suitable for biomass destruction (with an hydrolysis efficiency rate of 46.7%), this rate is lower than in other pretreatment methods, such as the N2 explosive decompression method (74.1%) (Raud et al., 2016a, 2018). Some authors consider that enzyme hydrolysis is still a ratelimiting step in the conversion of lignocelluloses into biogas (Čater et al., 2014). Regarding biomethane production from bioethanol production waste, similar studies on integrated ethanol and biogas production have been reported in the available literature. Katsimpouras et al (Katsimpouras et al., 2017) used corn stover which was pretreated with steam explosion and organosolv to investigate ethanol and biomethane
3.3. Digestion time The rate constant (k) (Table 3) of those samples that were pretreated with bubbling varies between 0.17 (pretreated material) and 70
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(CH4:Biogas = 0.76), hydrolysed (CH4:Biogas = 0.70), pretreated (CH4:Biogas = 0.54) and untreated material (CH4:Biogas = 0.61, at t = 5 days). At the end of the anaerobic digestion process, fermented samples and production waste have the highest methane ratio (0.68 and 0.60 respectively). Statistically significant differences were found between the CH4:Biogas of untreated/pretreated versus fermented (p ≤ 0.0001), hydrolysed versus fermented (p ≤ 0.01), pretreated versus production waste (p ≤ 0.0001), and untreated versus production waste (p ≤ 0.01). For samples that were pretreated with bubbling, the ratio of methane varies between 0.60 (pretreated, t = seven days; and production waste, t = four days) and 0.61 (untreated, t = five days; hydrolysed, t = seven days; and fermented, t = seven days). There are statistically significant differences between pretreated waste and production waste (p ≤ 0.05), and pretreated versus hydrolysed (p ≤ 0.01). Also, statistically significant differences were found between fermented samples of both pretreatment methods (p ≤ 0.001). These differences in the methane/biogas ratios may be due to the CO2 fixation process. Under anaerobic conditions, the CO2 could have been reduced into methane by the methanogenic bacteria, leading to an increase in daily methane production (Al-mashhadani et al., 2016). These results are particularly important in the biorefinery concept. Despite all of the efforts that have been made to increase the cellulose conversion rate and to make bioethanol production economically competitive, its production is still expensive. Combining lignocellulosic bioethanol production with anaerobic digestion of its production waste can be used to increase the energy output from the biomass, improve the efficiency of the process, and can also provide a solution when it comes to fully valorising the side-streams from biorefineries, a point that is fundamental to zero-waste societies (Rocha-Meneses et al., 2017). The explosive decompression method that has been applied in this study is an economically and environmentally attractive one since, instead of using chemicals, it uses pressure and water to break down the plant cell wall (Raud et al., 2018). The characterisation of the biomethane potential of production waste from this study will be further utilised to design a pilot-scale biogas plant. Holliger et al (Holliger et al., 2017b) concluded that a biomethane potential test can be used with confidence to estimate methane production for the design and operation of anaerobic digestion plants. For this purpose the substrates should be well characterised, and the operating parameters should be properly adapted to the substrates and the anaerobic process.
Table 4 The lag-phase and the digestion time for untreated, pretreated, hydrolysed, fermented substrates and bioethanol production waste (= lag-phase; NE = non-existent). Pretreatment
– Flue gas without bubbling
Flue gas with bubbling
Variable
Untreated Pretreated Hydrolysed Fermented Production waste Pretreated Hydrolysed Fermented Production waste
85% Bmax
λ
95% Bmax
mol CH4/ 100 g
Days
mol CH4/ 100 g
Days
mol CH4/ 100 g
Days
0.030 NE NE NE NE
1
0.79 0.83 0.84 0.89 0.83
11 11 7 8 8
0.88 0.92 0.94 0.99 0.93
17 18 12 12 12
0.79 1.0 1.1 1.0
11 9 9 8
0.89 1.1 1.2 1.1
17 14 14 13
NE NE NE NE
0.25 (other steps). It achieves its maximum value for production waste. The results show that pretreatment method successfully breaks down the plant cell walls and makes the substrates more accessible to degradation. Similar results regarding the kinetics of the anaerobic process have been reported in the available literature. Borja et al discovered that the kinetic constant increases with a reduction of phenolic compound contents (Holliger et al., 2017a; Borja et al., 1995). Table 4 summarises those parameters that are important to biogas production - the lag phase ( λ ) and the digestion time to reach 85% Bmax and 95% Bmax of the biomaterial. Only untreated barley straw has a lagphase on day one (0.030 mol CH4/100 g). For those samples that were pretreated without bubbling, production waste achieves 95% of the maximum methane yield around six days before the pretreated samples, while for those samples that were pretreated with bubbling it takes just three days less to reach 95% of the maximum methane yield. Therefore, despite pretreatment with bubbling improving the overall methane yield, the time taken for the anaerobic digestion process is also increased, resulting in a slower rate of production for higher volumes of methane. These results may be due to the effect of the pretreatment method upon the biomass. Pretreatment with bubbling is more effective than pretreatment without bubbling in degrading lignin and hemicellulose, meaning that there is access to a higher proportion of the substrate for anaerobic digestion. More time will be needed to degrade and convert the substrate’s volume into methane.
4. Conclusion The effect was analysed of the flue gas explosive decompression pretreatment method (with and without bubbling) on methane yields of barley straw samples from different steps in the bioethanol production process (untreated, pretreated, hydrolysed, fermented, and production waste). A biomethane potential test was used to estimate methane production from bioethanol waste in order to utilise these results in the design of a pilot-scale anaerobic digestion plant. Methane yields were
3.4. Relationship between methane and biogas production The ratio of methane in the produced biogas volume is presented in Fig. 3. For flue gas pretreatment without bubbling, the highest production ratio is achieved on day one in fermented samples (CH4:Biogas = 0.79), followed by production waste
Fig. 3. Ratio of methane volume in the overall biogas volume. A - flue gas without bubbling; B - flue gas with bubbling. 71
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influenced by the bubbling process. Samples that were pretreated with bubbling provided higher methane yields than samples that were pretreated without bubbling. Statistically significant differences were found between the methane production of hydrolysed, fermented barley straw and bioethanol production waste of both pretreatment methods (p ≤ 0.0001). The results show that bioethanol production waste has a high energetic value and can contribute to an increase in the overall energy that is output from the biomass. Its methane production is 94% of the methane production of fermented samples. The flue gas explosive decompression method is an effective one in the pretreatment of biomass. It is more advantageous for bioethanol yields (cellulases) than it is for the anaerobic digestion process (anaerobic bacteria). In order to make this pretreatment method competitive (meaning that it has to be cost-effective), it requires further research. The digestion time for the samples should be reduced and the methane yields should be improved.
and substrate to inoculum ratio on biogas production. Bioresour. Technol. 190, 451–457. Herrmann, C., Idler, C., Heiermann, M., 2016. Biogas crops grown in energy crop rotations: linking chemical composition and methane production characteristics. Bioresour. Technol. 206, 23–35. Holliger, C., Fruteau de Laclos, H., Hack, G., 2017a. Methane Production of Full-Scale Anaerobic Digestion Plants Calculated from Substrate's Biomethane Potentials Compares Well with the One Measured On-Site. Front. Energy Res. https://doi.org/ 10.3389/fenrg.2017.00012. Holliger, C., Fruteau de Laclos, H., Hack, G., 2017b. Methane production of full-scale anaerobic digestion plants calculated from substrate’s biomethane potentials compares well with the one measured on-site. Front. Energy Res. 5 (12). Katsimpouras, C., et al., 2017. Sequential high gravity ethanol fermentation and anaerobic digestion of steam explosion and organosolv pretreated corn stover. Bioresour. Technol. 244, 1129–1136. Liu, Y., et al., 2015. Sequential bioethanol and biogas production from sugarcane bagasse based on high solids fed-batch SSF. Energy 90, 1199–1205. Mehariya, S., et al., 2018. Co-digestion of food waste and sewage sludge for methane production: current status and perspective. Bioresour. Technol. 265, 519–531. Menardo, S., Airoldi, G., Balsari, P., 2012. The effect of particle size and thermal pretreatment on the methane yield of four agricultural by-products. Bioresour. Technol. 104, 708–714. Mussatto, S.I., Loosdrecht, M., 2016. Cellulose: a key polymer for a greener, healthier, and bio-based future. Biofuel Res. J. 3 (4) 482-482. Narendranath, N.V., Power, R., 2005. Relationship between pH and medium dissolved solids in terms of growth and metabolism of lactobacilli and Saccharomyces cerevisiae during ethanol production. Appl. Environ. Microbiol. 71 (5), 2239–2243. OECD/IEA, 2017. Key World Energy Statistics. pp. 1–97. Owen, W.F., et al., 1979. Bioassay for monitoring biochemical methane potential and anaerobic toxicity. Water Res. 13 (6), 485–492. Palmqvist, B., Lidén, G., 2014. Combining the effects of process design and pH for improved xylose conversion in high solid ethanol production from Arundo donax. AMB Exp. 4 (1), 1–9. Prats, C., et al., 2008. Analysis and IbM simulation of the stages in bacterial lag phase: Basis for an updated definition. J. Theor. Biol. 252 (1), 56–68. Rabelo, S.C., et al., 2011. Production of bioethanol, methane and heat from sugarcane bagasse in a biorefinery concept. Bioresour. Technol. 102 (17), 7887–7895. Raud, M., Olt, J., Kikas, T., 2016a. N2 explosive decompression pretreatment of biomass for lignocellulosic ethanol production. Biomass Bioenergy 90 (Supplement C), 1–6. Raud, M., et al., 2016b. Dependence of the hydrolysis efficiency on the lignin content in lignocellulosic material. Int. J. Hydrogen Energy 41 (37), 16338–16343. Raud, M., Rooni, V., Kikas, T., 2018. The efficiency of nitrogen and flue gas as operating gases in explosive decompression pretreatment. Energies 11 (8), 2074. Rocha-Meneses, L., et al., 2017. Second-generation bioethanol production: A review of strategies for waste valorisation. Agronomy Res. 15 (3), 830–847. Rooni, V., Raud, M., Kikas, T., 2017a. The freezing pre-treatment of lignocellulosic material: a cheap alternative for Nordic countries. Energy 139, 1–7. Rooni, V., Raud, M., Kikas, T., 2017b. Technical solutions used in different pretreatments of lignocellulosic biomass: a review. Agronomy Res. 15 (3), 848–858. Safari, A., Karimi, K., Shafiei, M., 2016. Integrated ethanol and biogas production from pinewood. RSC Adv. 6 (43), 36441–36449. Saini, J.K., Saini, R., Tewari, L., 2015. Lignocellulosic agriculture wastes as biomass feedstocks for second-generation bioethanol production: concepts and recent developments. 3 Biotech 5 (4), 337–353. Schmetz, Q., et al., 2016. Comprehension of an organosolv process for lignin extraction on Festuca arundinacea and monitoring of the cellulose degradation. Ind. Crops Prod. 94, 308–317. Stan, C., et al., 2018. Pilot-scale anaerobic Co-digestion of the OFMSW: improving biogas production and startup. Sustainability 10 (6), 1939. Tomás-Pejó, E., Oliva, J.M., Ballesteros, M., 2008. Realistic approach for full-scale bioethanol production from lignocellulose: a review. J. Sci. Ind. Res. (India) 67 (11), 874–884. Van Soest, P.J., Robertson, J.B., Lewis, B.A., 1991. Methods for Dietary Fiber, Neutral Detergent Fiber, and Nonstarch Polysaccharides in Relation to Animal Nutrition. J. Dairy Sci. 74 (10), 3583–3597. Wang, Q., et al., 2016. Polyhydroxyalkanoates in waste activated sludge enhances anaerobic methane production through improving biochemical methane potential instead of hydrolysis rate. Nature 6, 19713. Willington, I.P., Marten, G.G., 1982. Options for handling stillage waste from sugar-based fuel ethanol production. Resour. Conserv. 8 (2), 111–129. Zeng, S., et al., 2010. Effect of inoculum/substrate ratio on methane yield and orthophosphate release from anaerobic digestion of Microcystis spp. J. Hazard. Mater. 178 (1), 89–93.
Acknowledgements The authors gratefully acknowledge the financial support of the European Regional Development Fund via the Mobilitas Pluss (project MOBERA1) of the Estonian Research Council and the base financed project of EULS P170025 TIPT.
References Abbassi-Guendouz, A., et al., 2012. Total solids content drives high solid anaerobic digestion via mass transfer limitation. Bioresour. Technol. 111, 55–61. Al-mashhadani, M.K.H., Wilkinson, S.J., Zimmerman, W.B., 2016. Carbon dioxide rich microbubble acceleration of biogas production in anaerobic digestion. Chem. Eng. Sci. 156, 24–35. Angelidaki, I., et al., 2009. Defining the biomethane potential (BMP) of solid organic wastes and energy crops: a proposed protocol for batch assays. Water Sci. Technol. 59 (5), 927–934. Angelidaki, I., et al., 2018. Biogas upgrading and utilization: current status and perspectives. Biotechnol. Adv. 36 (2), 452–466. AOAC, 1990. Official Methods of Analysis of the Association of Official Analytical Chemists, vol. 1. Bauer, A., et al., 2009. Analysis of methane potentials of steam-exploded wheat straw and estimation of energy yields of combined ethanol and methane production. J. Biotechnol. 142 (1), 50–55. Berlowska, J., et al., 2017. Integrated bioethanol Fermentation/Anaerobic digestion for valorization of sugar beet pulp. Energies 10 (9), 1255. Borja, R., et al., 1995. Influence of different aerobic pretreatments on the kinetics of anaerobic digestion of olive mill wastewater. Water Res. 29 (2), 489–495. Čater, M., Zorec, M., Marinšek Logar, R., 2014. Methods for improving anaerobic lignocellulosic substrates degradation for enhanced biogas production. Springer Sci. Rev. 2 (1), 51–61. Dai, B.-l., et al., 2017. Comparison of different pretreatments of rice straw substrate to improve biogas production. Waste Biomass Valorization. Espinoza-Escalante, F.M., et al., 2009. Anaerobic digestion of the vinasses from the fermentation of Agave tequilana Weber to tequila: The effect of pH, temperature and hydraulic retention time on the production of hydrogen and methane. Biomass Bioenergy 33 (1), 14–20. Guragain, Y.N., et al., 2014. Low-lignin mutant biomass resources: effect of compositional changes on ethanol yield. Ind. Crops Prod. 61, 1–8. Haider, M.R., et al., 2015. Effect of mixing ratio of food waste and rice husk co-digestion
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Industrial Crops & Products journal homepage: www.elsevier.com/locate/indcrop
The effect of flue gas explosive decompression pretreatment on methane recovery from bioethanol production waste
T
⁎
Lisandra Rocha-Menesesa, , Anastasia Ivanovab, Guilherme Atouguiac, Isaac Ávilac, Merlin Rauda, Kaja Orupõldd, Timo Kikasa a
Institute of Technology, Chair of Biosystems Engineering, Estonian University of Life Sciences, Kreutzwaldi 56, 51006, Tartu, Estonia School of Engineering, Tallinn University of Technology, Ehitajate tee 5, 19086, Tallinn, Estonia c Faculty of Agricultural and Environmental Sciences, University of the Azores, Rua Capitão João d’Ávila Pico da Urze, 9700-042, Angra do Heroísmo, Azores, Portugal d Institute of Agricultural and Environmental Sciences, Estonian University of Life Sciences, Kreutzwaldi 5, 51006, Tartu, Estonia b
A R T I C LE I N FO
A B S T R A C T
Keywords: Anaerobic digestion Bioethanol Biofuel Lignocellulose Production waste Zero-waste
Lignocellulosic biomass is an attractive feedstock for the production of liquid (eg. biofuel) or gaseous (eg. methane) fuels for the transportation sector. The bioethanol production process still produces a large quantity of production waste following the distillation process. Stillage consists mostly of lignin, hemicellulose, extractives, and yeast and therefore does not have any commercial value. The conversion of bioethanol production waste into gaseous biofuels like biogas or biomethane is a promising solution when it comes to transforming stillage into value-added products, enhancing the value of the biomass, and as a strategy for achieving zero-waste societies. This study aims to investigate the potential of bioethanol production waste for biomethane production. The results are compared with samples from different stages of the bioethanol production process. Milled barley straw (Hordeum vulgare) was used as a feedstock to produce energy in the form of methane, and the flue gas pretreatment method (with and without bubbling) was applied. The results show that the methane production yield of bioethanol production waste, which has been pretreated with flue gas without bubbling is 5% higher than that of untreated substrate, and can achieve 94% of the methane production of fermented samples. Bioethanol production waste from substrates, which have been pretreated with flue gas with bubbling have a methane production level that is 29% higher than that of untreated materials. The results suggest that methane yields are influenced by the bubbling process. It is reasonable to use bioethanol production waste for the production of energy in the form of methane and to increase the energy output from the biomass.
1. Introduction Economic development, along with population growth, has been contributing to an increase in the world’s energy production levels. Fossil fuels such as petroleum (32%), coal (28%), and natural gas (22%) are still the main non-renewable sources of energy being used in the world’s energy mix. These fossil fuels are essentially being consumed by two main sectors - transportation (29%) and industry (29%) (OECD/ IEA, 2017), and their utilisation brings with them environmental concerns and a depletion of those very fossils fuels. Due to these problems, a search is on for bio-based solutions that will decrease the share of fossil fuels being used in all of the energy-related segments (inclusive of the transportation sector), a decrease in the concentration of carbon dioxide in the atmosphere, diminishing the emission of greenhouse gases, and improving energy reliability (Mussatto and Loosdrecht,
⁎
2016). A great deal of attention has been paid to biofuels being produced from lignocellulosic biomass due to its abundance, reduced costs, and renewability (Schmetz et al., 2016). Biofuels are a renewable energy source that can be used as a transport fuel. The conversion of lignocellulose into ethanol requires four sequential steps: pretreatment, hydrolysis, fermentation, and distillation. The pretreatment breaks down the plant cell walls and exposes cellulose for enzymatic hydrolysis. In the next step, the pretreated material is converted into fermentable sugars, using acids or commercial enzymes. In the fermentation process, yeasts or bacteria are used to ferment those sugars that are obtained from cellulose into ethanol. The material that is obtained from the fermentation process is a mixture of water and ethanol, and this must further be distilled in order to recover and purify the ethanol (Guragain et al., 2014; Rooni et al., 2017a). In all of the steps that are part of the bioethanol production process, great
Corresponding author. E-mail address: [email protected] (L. Rocha-Meneses).
https://doi.org/10.1016/j.indcrop.2018.10.057 Received 29 May 2018; Received in revised form 27 September 2018; Accepted 19 October 2018 0926-6690/ © 2018 Elsevier B.V. All rights reserved.
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attention has been paid to the pretreatment process. This increases the digestibility of the biomass and provides high sugar and ethanol yields. Pretreatment methods can be dived into physical, biological, chemical, and physio-chemical. Physical pretreatment includes chipping, milling and irradiation, and aims to reduce particle size and cellulose crystallinity, and to increase the surface area. Biological pretreatment uses micro-organisms, fungi, or enzymes to degrade lignin and hemicellulose. Chemical pretreatment consists of dilute acids, concentrated acids, alkali, ozone, ionic liquids, and Organosolv, and physio-chemical pretreatment includes uncatalysed steam, acid catalysed steam, liquid hot water/hydrothermal, ammonia fibre explosion, ammonia recycling percolation, soaking aqueous ammonia, wet oxidation, CO2 explosion, steam explosion, and nitrogen explosion (Rocha-Meneses et al., 2017; Rooni et al., 2017b). From the economic point of view, pretreatment methods represent 33% of the total costs of the process (Tomás-Pejó et al., 2008). The bioethanol production process generates a large quantity of production waste after the distillation process has been completed. This production waste is left unused. From the environmental perspective, it is a source of pollution due to the sheer quantity of leftover material that is produced and also due to its potential to cause environmental pollution. In addition, bioethanol production waste consists of a high level of latent energy, but its handling options may also have high energy-related requirements, resulting in lower net energy balances than expected. Therefore the anaerobic digestion of bioethanol production waste can be used as an effective handling option for these residues, as a way of reducing its environmental impact, thereby helping to offset the costs being incurred in bioethanol production, and as a solution to help improve the net energy balance of the production chain by means of the production of energy in the form of methane (Rocha-Meneses et al., 2017; Rabelo et al., 2011; Willington and Marten, 1982). Anaerobic digestion is a biological process that occurs in the absence of oxygen and which uses micro-organisms to convert organic waste into biogas and biofertiliser. Biogas is mainly composed of methane (50–70%) and carbon dioxide (30–50%). Minor compounds include vapour water, nitrogen, oxygen, hydrogen sulphide, ammonia, and silozanes (Angelidaki et al., 2018). Biogas production by anaerobic digestion involves four steps: hydrolysis, acidogenesis (fermentation), acetogenesis, and methanogenesis (Mehariya et al., 2018). In the first step, extracellular enzymes are used to convert complex compounds such as proteins, cellulose, polysaccharides, and lipids into more simple compounds such as amino acids, fatty acids, sugars, and alcohols. In the acidogenesis process, anaerobic bacteria is used to degrade the hydrolysed material into carbon dioxide, ammonia, hydrogen, and shortchain volatile fatty acids. In the acetogenesis phase, the products of the acidogenesis phase are further converted into acetate using acetogenic bacteria. In the last step (methanogenesis) microbes are used to metabolise the acetate and to produce methane (Rocha-Meneses et al., 2017). This article aims to estimate the potential of bioethanol production waste for methane recovery in order to use the results in the design for a pilot-size anaerobic digestion plant. The study uses flue gas explosive decompression (with and without bubbling) as a pretreatment method and barley straw as a biomass. The methane yields are evaluated using the biomethane potential assay (BMP). The biomethane potential of bioethanol production waste is compared to the biomethane potential of the raw materials and with samples from different stages of the bioethanol production process (pretreatment, hydrolysis, and fermentation).
Fig. 1. A schematic representation of a flue gas pretreatment system: 1 - flue gas tank, 2 - pressure control valve; 3 - manometer; 4 - modified pressure vessel cap; 5 - Parr instruments pressure vessel; 6 - ceramic contact heater; 7 - depressurisation valve; 8 - ventilation system; 9 - thermocouple; 10 - temperature controller unit (Raud et al., 2016a).
2. Material and methods 2.1. Bioethanol production 2.1.1. Biomass The barley straw (Hordeum vulgare) that has been used in the experimental procedure was collected in September 2013 from the plant field collection of the Estonian University of Life Sciences, near Tartu in Estonia. The straw was dried to a moisture content that was below 10%, and was further milled and sieved (to a particle size of 1–3 mm), with a Cutting Mill SM 100 comfort (from Retsch GmbH). The fibre composition of the barley straw (cellulose, hemicellulose, and lignin) was determined in an ANKOM 2000 analyser, using standardised procedures that are described by Van Soest, Robertson (Van Soest et al., 1991; AOAC, 1990).
2.1.2. Pretreatment The samples were pretreated using the explosive decompression pretreatment method with synthetic flue gas (composed of 20% CO2 and 80% N2) as a pressurising agent (Fig. 1). The gas was introduced into the reactor in two different ways, by bubbling it through the biomass-water mixture and by introducing it above the biomass-water mixture. In the bubbling process, the flue gas is equally distributed in the reaction medium, using a modified insertion tube with a circular shape at the bottom. The pretreatment without bubbling does not use an insertion tube, but introduces the gas into the reactor’s headspace. Thanks to this, the flue gas diffuses into the reaction mixture through the gas-liquid interface. In both pretreatment methods, 100 g of predried and milled barley straw is added into the pressure vessel and diluted in 800 g of distilled water. The samples were heated to a temperature of 150 ℃, and the flue gas was used to raise the pressure to 30 bar. After reaching the desired temperature, the vessel was cooled down to 80 ℃ and the pressure was released through the depressurisation valve. The pretreated biomass was cooled down to 50 °C prior to enzymatic hydrolysis. A CP-411 pH meter from Elmetron was used to measure the hydrogen-ion activity levels in the hydrolysate after flue gas pretreatment and after the enzyme addition.
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2.2.1. Calculations The molar concentration of initial methane gas in the test bottle [CH4 I ] is calculated by Eq. (1):
2.1.3. Hydrolysis The pretreated barley straw was introduced into a 1000 mL Erlenmeyer flask and 30 FPU g−1 cellulose of the enzyme complex Accellerase 1500 was added. The flask was filled with distilled water to a total volume of 1000 mL. The process of enzymatic hydrolysis was carried out in shaking incubator (Unimax 1010, Heidolph Instruments GmbH & Co KG) during a twenty-four hour period, at 50 rpm and a temperature of 50 °C. The effect of this pretreatment method on glucose yields has been described in a previous study (Raud et al., 2018).
[CH4 I ] = MF
PI VHS R (273,15 + T )
(1) 3
where PI (Pa) is the initial partial pressure at the headspace, VHS (m ) is the volume of the headspace, MF is the methane fraction in the current time interval, R is the ideal gas constant (8314 Jmol−1 K−1), and T is the incubation temperature (ºC). The molar concentration of the final methane levels in the headspace of the bottle [CH4 F ] is defined by Eq. (2):
2.1.4. Fermentation To convert the glucose into ethanol, 2.5 g of dry yeast (Saccharomyces cerevisiae) was added to the hydrolysate. The flasks were sealed with a fermentation lock and the fermentation process was carried out under low oxygen conditions in 1000 mL flasks over a period of seven days at room temperature. The influence of this pretreatment on ethanol yields has been analysed in another publication (Raud et al., 2018).
[CH4 F ] = MF
PF VHS R (273,15 + T )
(2)
where PF (Pa) is the final partial pressure at the headspace, measured after the GC analysis. The cumulative methane produced in the current interval time [CH4 C ]t is given by Eq. (3): (3)
2.1.5. Separation by evaporation As the glucose and ethanol yields were studied previously (Raud et al., 2018), in this study the broth that was obtained from the fermentation process was separated by evaporation in order to mimic the stillage that would be left after the distillation step had been completed. Therefore the samples were placed into the oven for one hour at 95 °C, to be able to remove the ethanol content. The waste material from the distillation process (production waste), was further utilised in the biomethane potential analysis.
[CH4 C ]t = ([CH4 I ]t − [CH4 F ]t − 1) + [CH4 C ]t − 1
2.2. Biomethane potential
B = Bmax ( 1 – e−kt )
The biomethane yields of untreated, pretreated, hydrolysed, and fermented samples and bioethanol production waste were measured. The biomethane potential (BMP) was evaluated using a modified version of the procedures that were reported by Owen et al and Angelidaki et al (Owen et al., 1979; Angelidaki et al., 2009). The inoculum that was used in the experimental procedure came from the wastewater treatment plant in Tartu (Estonia). Before usage, the sludge was incubated for four days at 36 °C, for the purpose of degasification. The BMP assay was carried out in 575 mL plasma bottles for between 42–45 days, at mesophilic conditions (36 °C). The experiments were carried out in triplicate, and a blank test (using inoculum without substrate) was prepared for each batch. The bottles were filled up with substrate and inoculum to a total volume of 200 mL, and with a substrate to an inoculum ratio of 0.25. The required volume of inoculum and substrate was estimated from the analysis of the total solids (TS) and volatile solids (VS), which were determined according to standard method 1684 (the US Environmental Protection Agency – EPA). Subsequently, the bottles were flushed with N2 to guarantee the anaerobic conditions, and were sealed with rubber stoppers, mixed, and incubated. Over the first two weeks of the experiments, methane production was determined on a daily basis. In weeks three and four it was every other day, and in weeks five and six it was once a week. This process was done by measuring the pressure increase in the headspace, using a BMP-Testsystem WAL (WAL Mess- und Regelsysteme GmbH). The quantity of methane in the biogas was monitored chromatographically using a gas chromatograph (CP-4900 Micro-GC, Varian Inc). The final biogas and methane production rates for the substrates were calculated by subtracting the biogas and methane production levels respectively for the blank tests. The results were expressed in mol of CH4 per 100 g of raw biomass. The pH was measured at the end of the experiments using a SevenMulti™ S47 - dual pH/conductivity meter.
where B is the cumulative methane production at time (t), Bmax is the maximum methane yield, and k is the rate constant. The maximum methane yield (Bmax) was calculated from Eq. (4), and this corresponds to the maximum value of the fitted curve. The digestion time to achieve 85% and 95% of the methane potential was also calculated from the maximum methane yield. The lag time (λ) (the first stage of microbial growth, just before the exponential growth phase) was calculated by using the geometric method. In this technique, a line intersects the slope of the exponential curve, and the prolongation of this line to the initial methane concentration corresponds to the lag time (Prats et al., 2008).
where [CH4 I ]t is the initial concentration of methane in the headspace of the bottle in the current time interval, [CH4 F ]t − 1 is the concentration of final methane in the headspace of the bottle in the previous time interval, and [CH4 C ]t − 1 is the concentration of cumulative methane produced in the previous time interval. The methane results were modelled with a non-linear regression model and further fit into a first-order exponential association model (Eq. (4)) (Zeng et al., 2010; Wang et al., 2016), using the GraphPad Prism 5.0 software: (4)
2.3. Statistical analysis Shapiro-Wilk’s normality test was used to evaluate the normal distribution of the samples. The level of statistical significance was 95%, for a confidence interval of 0.05. The differences between both pretreatment methods (with and without bubbling) were analysed using the Krustal-Wallis test, and the post hoc test Dunn’s multiple comparison test to analyse the differences within the groups. The statistical analysis was carried out using the GraphPad Prism 5 software. 3. Results and discussion 3.1. Chemical composition The results of the fibre analysis of barley straw that was used in the experimental procedure are presented in Table 1. The percentage of cellulose is equivalent to the values that were reported in the available literature (being 1.6% higher) (Saini et al., 2015), while the relative proportion of lignin is particularly low (169% lower in comparison to the values described in the bibliography (Saini et al., 2015)). The composition in terms of hemicellulose and ash (3.8%) is within the range reported in the available literature (Saini et al., 2015). The 68
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biodegradability of the substrates and, therefore, improving biomethane production levels (Dai et al., 2017). Regarding the VS content, the lowest value was gained in that material that remained untreated (896 g/kgTS). For samples that were pretreated with flue gas without bubbling, it was found that the VS content varies between 990 g/kgTS (hydrolysed) and 996 g/kgTS (pretreated, fermented, and production waste). Statistically significant differences were found between the VS content of untreated versus pretreated (p ≤ 0.001), and untreated versus fermented/production waste (p ≤ 0.05). For samples that were pretreated with flue gas with bubbling, the VS content varies between 995g/kgTS (fermented, and production waste) and 996g/kgTS (pretreated and hydrolysed). Statistically significant differences were found between the VS content of untreated versus hydrolysed/pretreated (p ≤ 0.001). These results are particularly important since the production of biogas and its composition are mainly influenced by the quantity and quality of the substrates. Research has shown that the TS and VS contents affect the performance of anaerobic digestion. High reductions in the TS and VS content strongly affect specific biogas yields (Stan et al., 2018; AbbassiGuendouz et al., 2012; Haider et al., 2015). The pH values of the samples are reported in Table 2. For samples that were pretreated with flue gas without bubbling, the pH values for the hydrolysate after completion of the process were at 5.2 and after the enzyme addition they were at 4.7. For those samples that were pretreated without flue gas without bubbling, the pH value varied between 5.4 and 5.0. These values are within the optimal range for yeast growth (between 4 and 6) (Narendranath and Power, 2005).
Table 1 The content of cellulose, hemicellulose, and lignin in raw barley straw (n = 3, ± represents the standard deviation). Component
Content (%)
Hemicellulose Cellulose Lignin Ash
32.6 ± 0.5 45.7 ± 0.2 5.2 ± 0.0 3.8 ± 0.1
Table 2 The total solids (TS) and volatile solids (VS) content and pH levels of samples that were pretreated with flue gas, both with and without bubbling (n = 3). Pretreatment
Variable
TS g/kg
VS g/kgTS
– Flue gas without bubbling
Untreated Pretreated Hydrolysed Fermented Production waste Pretreated Hydrolysed Fermented Production waste
956 ± 2a,b,c,d,e 92 ± 5 87 ± 5b 85 ± 2c 85 ± 5a
896 996 990 996 996
± ± ± ± ±
12a,b,c,d,e 0.06a,d 1d 0.2b 0.03c
– 5.2 4.7 – –
93 93 79 80
996 996 995 995
± ± ± ±
0.08d 0.06e 0.08 41
5.4 5.0
Flue gas with bubbling
± ± ± ±
4 2 2e 8d
pH
Raised letters after some values indicate statistically significantly differences between the variables in the group, with a probability level of p ≤ 0.05.
3.2. The effect of bubbling in methane recovery chemical composition of the barley straw predicts that it is a suitable material for biofuel production, since bioethanol and biomethane yields rise with an increase of cellulose content and with the reduction of lignin proportions (Herrmann et al., 2016; Raud et al., 2016b). In the analysis of the content of total solids (TS) and volatile solids (VS) in the raw material, samples which were pretreated with flue gas without bubbling and samples which were pretreated with flue gas bubbling are presented in Table 2. Untreated barley straw had the highest TS content (956 g/kg) when compared to samples that were gathered after both pretreatment, due to the dilution of the samples in distilled water. In samples that were pretreated with flue gas with bubbling and without bubbling, the TS content was higher in the former for material that was simply pretreated (93 g/kg as opposed to 92 g/kg) and material that was further hydrolysed (93 g/kg as opposed to 87 g/ kg), and lower in the case of the material after fermentation (79 g/kg as opposed to 85 g/kg), and production waste (80 g/kg as opposed to 85 g/ kg). Taking into account the fact that TS content in raw material is 10 times higher than in each step of bioethanol production, these results indicate that the flue gas pretreatment method is effective in the reduction of the TS content. Consequently, this serves to enhance the accessibility of anaerobic micro-organisms to cellulose, improving the
Fig. 2 shows the experimental results for BMP tests with untreated, pretreated, hydrolysed, fermented, and bioethanol production wastes, and the respective fitting curves. Methane yields are presented in mols of methane per 100 g of substrate in order to enable a comparison with the initial raw biomass. As can be seen in Fig. 2 and Table 3, untreated barley straw has the lowest methane yield (0.93 mol CH4/100 g), when compared with samples that were pretreated with flue gas with and without bubbling. For samples that were pretreated with flue gas without bubbling, the pretreatment step had the lowest methane yield (0.97 mol CH4/100 g), while fermented barley straw had the highest methane values (1.1 mol CH4/100 g). For samples that were pretreated with flue gas with bubbling, the methane yields varied between 0.97 mol CH4/100 g (pretreated) and 1.3 mol CH4/100 g (fermented). The maximum methane yield of bioethanol production waste was 20.99% higher than that of samples that were pretreated alone. Hydrolysed material had a maximum methane yield of just 3.4% higher than that of production waste. Statistically significant differences were found in production waste versus untreated/pretreated/fermented (p ≤ 0.0001), and production waste versus hydrolysed (p ≤ 0.05), untreated versus pretreated
Fig. 2. Experimental results for BMP tests involving untreated, pretreated, hydrolysed, fermented material, and bioethanol production waste, and respective fitting curves. A - Flue gas without bubbling; B - Flue gas with bubbling. 69
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production from fermentation residues. The methane yield of the steam-exploded samples was at 120 mL/gVS, and at 69 mL/gVS for organosolv pretreated material (three and 5.2 times lower that the results that were obtained in our study). Safari et al (Safari et al., 2016) also analysed integrated ethanol and biogas production, using softwood pine as a feedstock and diluted sulfuric acid as a pretreatment method. The results showed that biogas production from the pretreatment hydrolysates is an efficient method for converting these compounds into biofuel. Similar conclusions were found by Liu et al (Liu et al., 2015). The authors obtained approximately 307 mL/gVS of methane by means of the biogas digestion of sugarcane bagasse (with results that were similar to those of our study). Berlowska et al (Berlowska et al., 2017) concluded that stillage from sugar beet pulp waste contains unfermented sugars that can be used as a substrate for methane production, achieving a reading of 444 mL/gVS. The results also showed that the system’s energy balance was improved with the integrated ethanol fermentation and anaerobic digestion. The results show that the flue gas explosive decompression pretreatment method (with or without bubbling) works better for cellulases than it does for anaerobic bacteria. The differences between pretreated and untreated samples is higher in the bioethanol process (Raud et al., 2018) than in the anaerobic digestion. The pretreatment opens the structure enough for the enzymes, but not enough for the bacteria to efficiently reach the cellulose. Therefore the pretreatment method is more effective after the hydrolysis step has been taken, facilitating bioethanol production rather than methane production. This may also be due to the presence of inhibitory compounds (such as furan derivatives and phenolic acids) which are released after the pretreatment process, and which have more of an effect on methane producing bacteria than they do on the enzymes and yeasts being used in the bioethanol production process. Furthermore, samples that were pretreated with flue gas with bubbling provided higher maximum methane yields (18.8%) than samples that were pretreated without bubbling. Statistically significant differences were found between hydrolysed/ fermented/production waste (pretreated without bubbling) versus hydrolysed/fermented/production waste (pretreated with bubbling) (p ≤ 0.0001). These differences between the two pretreatment methods may be due to the dissolution of the carbon dioxide within the reaction medium. When bubbling of the flue gas is not applied, the dissolution of carbon dioxide molecules in water is hindered as the contact surface is much smaller than in the case of bubbling. However, bubbling flue gas through the reaction medium improved CO2 dissolution because in this case the contact area for carbon dioxide molecules with water is much greater, thereby ensuring a uniform distribution in the reaction mixture. Furthermore, dissolved carbon dioxide possibly served to influenced the pH value of the solution (involving the acidification of the biomass), which affects the production of inhibitory compounds in the fermentation process, along with bacterial growth and, consequently, the biomethane yields (Espinoza-Escalante et al., 2009; Palmqvist and Lidén, 2014). In Table 3 the best fit-values and the respective standard deviations (for maximum methane yield, the rate constant, and the correlation coefficient) of the fitting curves can be seen for those samples which have been produced during the various steps of the bioethanol production process, whether pretreated with flue gas with or without bubbling. The results show that the equation that was used to generate the theoretical curves (the one-phase exponential association equation) successfully expresses the variation in the data. For samples that were pretreated without bubbling, the results vary between 0.96 (hydrolysed) and 0.99 (pretreated). For substrates that were pretreated with bubbling, the correlation coefficient is 0.99.
Table 3 Maximum methane yield (Bmax), rate constant (k), and correlation coefficient (R2) for untreated, pretreated, hydrolysed, fermented barley straw and bioethanol production waste ( ± represents the standard deviation). R2
Pretreatment
Variable
Bmax mol CH4/100 g
k
– Flue gas without bubbling
Untreated Pretreated Hydrolysed Fermented Production waste Pretreated Hydrolysed Fermented Production waste
0.93 ± 0.021a 0.97 ± 0.013b 0.99 ± 0.022c 1.1 ± 0.013d 0.98 ± 0.015e
0.18 0.17 0.25 0.25 0.25
± ± ± ± ±
0.019 0.015 0.037 0.018 0.021
0.98 0.99 0.96 0.98 0.98
0.97 ± 0.013 1.2 ± 0.011 1.3 ± 0.013 1.2 ± 0.010f
0.19 0.21 0.22 0.23
± ± ± ±
0.011 0.0097 0.011 0.0097
0.99 0.99 0.99 0.99
Flue gas with bubbling
The superscripts in the values indicate statistically significantly differences between the variables (explained below), with a probability level of p≤0.05. a Untreated sample versus pretreated/hydrolysed/fermented (flue gas without bubbling), and untreated sample versus hydrolysed/fermented/production waste (flue gas with bubbling). b Pretreated sample versus hydrolysed/fermented (flue gas without bubbling), and pretreated sample versus untreated/hydrolysed/fermented. c Hydrolysed sample versus untreated/pretreated/hydrolysed/fermented (flue gas with bubbling). d Fermented sample versus untreated/pretreated/hydrolysed/fermented (flue gas with bubbling). e Production waste versus untreated/fermented (flue gas without bubbling) and production waste versus hydrolysed/fermented (flue gas with bubbling). f Production waste versus untreated/pretreated/ fermented (flue gas with bubbling).
(p ≤ 0.05), and untreated versus hydrolysed/fermented (p ≤ 0.0001). Statistically significant differences between methane yields from untreated and pretreated biomass were also reported in the available literature. Menardo et al(Menardo et al., 2012) studied the influence of the pretreatment method on methane yields in four agricultural byproducts (barley, wheat, rice straw, and maize stalks). When compared with untreated samples, mechanical pretreatment significantly increased methane yields of barley and wheat straw by 54.2% and 83.5% respectively. When treated with the thermal pretreatment method, the methane yields of these biomasses increase by 41.7% and 64.3% respectively. Similarly, Bauer et al (Bauer et al., 2009) investigated methane yields in wheat straw that was pretreated using steam explosion. The results show a significant increase (by up to 20%) when compared to those for untreated wheat straw. In addition, fermented samples presented higher methane yields when compared to hydrolysed material, probably due to the production of lactic acid in the fermentation step. Lactic acid can inhibit or stimulate the reaction. In this case, it probably interfered with the process of yeast metabolism and stimulated the growth of the anaerobic bacteria. Another hypothesis is the limited hydrolysis of the lignocellulose. As can be seen in Table 3, in the flue gas with bubbling the hydrolysis step has a lower kinetic rate when compared to fermented samples. Despite this fact, this pretreatment method has been reported as being suitable for biomass destruction (with an hydrolysis efficiency rate of 46.7%), this rate is lower than in other pretreatment methods, such as the N2 explosive decompression method (74.1%) (Raud et al., 2016a, 2018). Some authors consider that enzyme hydrolysis is still a ratelimiting step in the conversion of lignocelluloses into biogas (Čater et al., 2014). Regarding biomethane production from bioethanol production waste, similar studies on integrated ethanol and biogas production have been reported in the available literature. Katsimpouras et al (Katsimpouras et al., 2017) used corn stover which was pretreated with steam explosion and organosolv to investigate ethanol and biomethane
3.3. Digestion time The rate constant (k) (Table 3) of those samples that were pretreated with bubbling varies between 0.17 (pretreated material) and 70
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(CH4:Biogas = 0.76), hydrolysed (CH4:Biogas = 0.70), pretreated (CH4:Biogas = 0.54) and untreated material (CH4:Biogas = 0.61, at t = 5 days). At the end of the anaerobic digestion process, fermented samples and production waste have the highest methane ratio (0.68 and 0.60 respectively). Statistically significant differences were found between the CH4:Biogas of untreated/pretreated versus fermented (p ≤ 0.0001), hydrolysed versus fermented (p ≤ 0.01), pretreated versus production waste (p ≤ 0.0001), and untreated versus production waste (p ≤ 0.01). For samples that were pretreated with bubbling, the ratio of methane varies between 0.60 (pretreated, t = seven days; and production waste, t = four days) and 0.61 (untreated, t = five days; hydrolysed, t = seven days; and fermented, t = seven days). There are statistically significant differences between pretreated waste and production waste (p ≤ 0.05), and pretreated versus hydrolysed (p ≤ 0.01). Also, statistically significant differences were found between fermented samples of both pretreatment methods (p ≤ 0.001). These differences in the methane/biogas ratios may be due to the CO2 fixation process. Under anaerobic conditions, the CO2 could have been reduced into methane by the methanogenic bacteria, leading to an increase in daily methane production (Al-mashhadani et al., 2016). These results are particularly important in the biorefinery concept. Despite all of the efforts that have been made to increase the cellulose conversion rate and to make bioethanol production economically competitive, its production is still expensive. Combining lignocellulosic bioethanol production with anaerobic digestion of its production waste can be used to increase the energy output from the biomass, improve the efficiency of the process, and can also provide a solution when it comes to fully valorising the side-streams from biorefineries, a point that is fundamental to zero-waste societies (Rocha-Meneses et al., 2017). The explosive decompression method that has been applied in this study is an economically and environmentally attractive one since, instead of using chemicals, it uses pressure and water to break down the plant cell wall (Raud et al., 2018). The characterisation of the biomethane potential of production waste from this study will be further utilised to design a pilot-scale biogas plant. Holliger et al (Holliger et al., 2017b) concluded that a biomethane potential test can be used with confidence to estimate methane production for the design and operation of anaerobic digestion plants. For this purpose the substrates should be well characterised, and the operating parameters should be properly adapted to the substrates and the anaerobic process.
Table 4 The lag-phase and the digestion time for untreated, pretreated, hydrolysed, fermented substrates and bioethanol production waste (= lag-phase; NE = non-existent). Pretreatment
– Flue gas without bubbling
Flue gas with bubbling
Variable
Untreated Pretreated Hydrolysed Fermented Production waste Pretreated Hydrolysed Fermented Production waste
85% Bmax
λ
95% Bmax
mol CH4/ 100 g
Days
mol CH4/ 100 g
Days
mol CH4/ 100 g
Days
0.030 NE NE NE NE
1
0.79 0.83 0.84 0.89 0.83
11 11 7 8 8
0.88 0.92 0.94 0.99 0.93
17 18 12 12 12
0.79 1.0 1.1 1.0
11 9 9 8
0.89 1.1 1.2 1.1
17 14 14 13
NE NE NE NE
0.25 (other steps). It achieves its maximum value for production waste. The results show that pretreatment method successfully breaks down the plant cell walls and makes the substrates more accessible to degradation. Similar results regarding the kinetics of the anaerobic process have been reported in the available literature. Borja et al discovered that the kinetic constant increases with a reduction of phenolic compound contents (Holliger et al., 2017a; Borja et al., 1995). Table 4 summarises those parameters that are important to biogas production - the lag phase ( λ ) and the digestion time to reach 85% Bmax and 95% Bmax of the biomaterial. Only untreated barley straw has a lagphase on day one (0.030 mol CH4/100 g). For those samples that were pretreated without bubbling, production waste achieves 95% of the maximum methane yield around six days before the pretreated samples, while for those samples that were pretreated with bubbling it takes just three days less to reach 95% of the maximum methane yield. Therefore, despite pretreatment with bubbling improving the overall methane yield, the time taken for the anaerobic digestion process is also increased, resulting in a slower rate of production for higher volumes of methane. These results may be due to the effect of the pretreatment method upon the biomass. Pretreatment with bubbling is more effective than pretreatment without bubbling in degrading lignin and hemicellulose, meaning that there is access to a higher proportion of the substrate for anaerobic digestion. More time will be needed to degrade and convert the substrate’s volume into methane.
4. Conclusion The effect was analysed of the flue gas explosive decompression pretreatment method (with and without bubbling) on methane yields of barley straw samples from different steps in the bioethanol production process (untreated, pretreated, hydrolysed, fermented, and production waste). A biomethane potential test was used to estimate methane production from bioethanol waste in order to utilise these results in the design of a pilot-scale anaerobic digestion plant. Methane yields were
3.4. Relationship between methane and biogas production The ratio of methane in the produced biogas volume is presented in Fig. 3. For flue gas pretreatment without bubbling, the highest production ratio is achieved on day one in fermented samples (CH4:Biogas = 0.79), followed by production waste
Fig. 3. Ratio of methane volume in the overall biogas volume. A - flue gas without bubbling; B - flue gas with bubbling. 71
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influenced by the bubbling process. Samples that were pretreated with bubbling provided higher methane yields than samples that were pretreated without bubbling. Statistically significant differences were found between the methane production of hydrolysed, fermented barley straw and bioethanol production waste of both pretreatment methods (p ≤ 0.0001). The results show that bioethanol production waste has a high energetic value and can contribute to an increase in the overall energy that is output from the biomass. Its methane production is 94% of the methane production of fermented samples. The flue gas explosive decompression method is an effective one in the pretreatment of biomass. It is more advantageous for bioethanol yields (cellulases) than it is for the anaerobic digestion process (anaerobic bacteria). In order to make this pretreatment method competitive (meaning that it has to be cost-effective), it requires further research. The digestion time for the samples should be reduced and the methane yields should be improved.
and substrate to inoculum ratio on biogas production. Bioresour. Technol. 190, 451–457. Herrmann, C., Idler, C., Heiermann, M., 2016. Biogas crops grown in energy crop rotations: linking chemical composition and methane production characteristics. Bioresour. Technol. 206, 23–35. Holliger, C., Fruteau de Laclos, H., Hack, G., 2017a. Methane Production of Full-Scale Anaerobic Digestion Plants Calculated from Substrate's Biomethane Potentials Compares Well with the One Measured On-Site. Front. Energy Res. https://doi.org/ 10.3389/fenrg.2017.00012. Holliger, C., Fruteau de Laclos, H., Hack, G., 2017b. Methane production of full-scale anaerobic digestion plants calculated from substrate’s biomethane potentials compares well with the one measured on-site. Front. Energy Res. 5 (12). Katsimpouras, C., et al., 2017. Sequential high gravity ethanol fermentation and anaerobic digestion of steam explosion and organosolv pretreated corn stover. Bioresour. Technol. 244, 1129–1136. Liu, Y., et al., 2015. Sequential bioethanol and biogas production from sugarcane bagasse based on high solids fed-batch SSF. Energy 90, 1199–1205. Mehariya, S., et al., 2018. Co-digestion of food waste and sewage sludge for methane production: current status and perspective. Bioresour. Technol. 265, 519–531. Menardo, S., Airoldi, G., Balsari, P., 2012. The effect of particle size and thermal pretreatment on the methane yield of four agricultural by-products. Bioresour. Technol. 104, 708–714. Mussatto, S.I., Loosdrecht, M., 2016. Cellulose: a key polymer for a greener, healthier, and bio-based future. Biofuel Res. J. 3 (4) 482-482. Narendranath, N.V., Power, R., 2005. Relationship between pH and medium dissolved solids in terms of growth and metabolism of lactobacilli and Saccharomyces cerevisiae during ethanol production. Appl. Environ. Microbiol. 71 (5), 2239–2243. OECD/IEA, 2017. Key World Energy Statistics. pp. 1–97. Owen, W.F., et al., 1979. Bioassay for monitoring biochemical methane potential and anaerobic toxicity. Water Res. 13 (6), 485–492. Palmqvist, B., Lidén, G., 2014. Combining the effects of process design and pH for improved xylose conversion in high solid ethanol production from Arundo donax. AMB Exp. 4 (1), 1–9. Prats, C., et al., 2008. Analysis and IbM simulation of the stages in bacterial lag phase: Basis for an updated definition. J. Theor. Biol. 252 (1), 56–68. Rabelo, S.C., et al., 2011. Production of bioethanol, methane and heat from sugarcane bagasse in a biorefinery concept. Bioresour. Technol. 102 (17), 7887–7895. Raud, M., Olt, J., Kikas, T., 2016a. N2 explosive decompression pretreatment of biomass for lignocellulosic ethanol production. Biomass Bioenergy 90 (Supplement C), 1–6. Raud, M., et al., 2016b. Dependence of the hydrolysis efficiency on the lignin content in lignocellulosic material. Int. J. Hydrogen Energy 41 (37), 16338–16343. Raud, M., Rooni, V., Kikas, T., 2018. The efficiency of nitrogen and flue gas as operating gases in explosive decompression pretreatment. Energies 11 (8), 2074. Rocha-Meneses, L., et al., 2017. Second-generation bioethanol production: A review of strategies for waste valorisation. Agronomy Res. 15 (3), 830–847. Rooni, V., Raud, M., Kikas, T., 2017a. The freezing pre-treatment of lignocellulosic material: a cheap alternative for Nordic countries. Energy 139, 1–7. Rooni, V., Raud, M., Kikas, T., 2017b. Technical solutions used in different pretreatments of lignocellulosic biomass: a review. Agronomy Res. 15 (3), 848–858. Safari, A., Karimi, K., Shafiei, M., 2016. Integrated ethanol and biogas production from pinewood. RSC Adv. 6 (43), 36441–36449. Saini, J.K., Saini, R., Tewari, L., 2015. Lignocellulosic agriculture wastes as biomass feedstocks for second-generation bioethanol production: concepts and recent developments. 3 Biotech 5 (4), 337–353. Schmetz, Q., et al., 2016. Comprehension of an organosolv process for lignin extraction on Festuca arundinacea and monitoring of the cellulose degradation. Ind. Crops Prod. 94, 308–317. Stan, C., et al., 2018. Pilot-scale anaerobic Co-digestion of the OFMSW: improving biogas production and startup. Sustainability 10 (6), 1939. Tomás-Pejó, E., Oliva, J.M., Ballesteros, M., 2008. Realistic approach for full-scale bioethanol production from lignocellulose: a review. J. Sci. Ind. Res. (India) 67 (11), 874–884. Van Soest, P.J., Robertson, J.B., Lewis, B.A., 1991. Methods for Dietary Fiber, Neutral Detergent Fiber, and Nonstarch Polysaccharides in Relation to Animal Nutrition. J. Dairy Sci. 74 (10), 3583–3597. Wang, Q., et al., 2016. Polyhydroxyalkanoates in waste activated sludge enhances anaerobic methane production through improving biochemical methane potential instead of hydrolysis rate. Nature 6, 19713. Willington, I.P., Marten, G.G., 1982. Options for handling stillage waste from sugar-based fuel ethanol production. Resour. Conserv. 8 (2), 111–129. Zeng, S., et al., 2010. Effect of inoculum/substrate ratio on methane yield and orthophosphate release from anaerobic digestion of Microcystis spp. J. Hazard. Mater. 178 (1), 89–93.
Acknowledgements The authors gratefully acknowledge the financial support of the European Regional Development Fund via the Mobilitas Pluss (project MOBERA1) of the Estonian Research Council and the base financed project of EULS P170025 TIPT.
References Abbassi-Guendouz, A., et al., 2012. Total solids content drives high solid anaerobic digestion via mass transfer limitation. Bioresour. Technol. 111, 55–61. Al-mashhadani, M.K.H., Wilkinson, S.J., Zimmerman, W.B., 2016. Carbon dioxide rich microbubble acceleration of biogas production in anaerobic digestion. Chem. Eng. Sci. 156, 24–35. Angelidaki, I., et al., 2009. Defining the biomethane potential (BMP) of solid organic wastes and energy crops: a proposed protocol for batch assays. Water Sci. Technol. 59 (5), 927–934. Angelidaki, I., et al., 2018. Biogas upgrading and utilization: current status and perspectives. Biotechnol. Adv. 36 (2), 452–466. AOAC, 1990. Official Methods of Analysis of the Association of Official Analytical Chemists, vol. 1. Bauer, A., et al., 2009. Analysis of methane potentials of steam-exploded wheat straw and estimation of energy yields of combined ethanol and methane production. J. Biotechnol. 142 (1), 50–55. Berlowska, J., et al., 2017. Integrated bioethanol Fermentation/Anaerobic digestion for valorization of sugar beet pulp. Energies 10 (9), 1255. Borja, R., et al., 1995. Influence of different aerobic pretreatments on the kinetics of anaerobic digestion of olive mill wastewater. Water Res. 29 (2), 489–495. Čater, M., Zorec, M., Marinšek Logar, R., 2014. Methods for improving anaerobic lignocellulosic substrates degradation for enhanced biogas production. Springer Sci. Rev. 2 (1), 51–61. Dai, B.-l., et al., 2017. Comparison of different pretreatments of rice straw substrate to improve biogas production. Waste Biomass Valorization. Espinoza-Escalante, F.M., et al., 2009. Anaerobic digestion of the vinasses from the fermentation of Agave tequilana Weber to tequila: The effect of pH, temperature and hydraulic retention time on the production of hydrogen and methane. Biomass Bioenergy 33 (1), 14–20. Guragain, Y.N., et al., 2014. Low-lignin mutant biomass resources: effect of compositional changes on ethanol yield. Ind. Crops Prod. 61, 1–8. Haider, M.R., et al., 2015. Effect of mixing ratio of food waste and rice husk co-digestion
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