Supercritical carbon dioxide enhanced pre-treatment of cotton stalks for methane production

Energy 194 (2020) 116903

Contents lists available at ScienceDirect

Energy journal homepage: www.elsevier.com/locate/energy

Supercritical carbon dioxide enhanced pre-treatment of cotton stalks for methane production Rafat Al Afif a, *, Martin Wendland a, Thomas Amon b, c, Christoph Pfeifer a a

Institute for Chemical and Energy Engineering, University of Natural Resources and Life Sciences, Vienna, Muthgasse 107, 1190, Vienna, Austria Department Engineering for Livestock Management, Leibniz-Institute for Agricultural Engineering, Max-Eyth-Allee 100, 14469, Potsdam-Bornim, Germany c €t Berlin, Robert-von-Ostertag-Str. 7-13, 14163, Institute of Animal Hygiene and Environmental Health, Department of Veterinary Medicine, Freie Universita Berlin, Germany b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 July 2019 Received in revised form 23 December 2019 Accepted 2 January 2020 Available online 3 January 2020

Cotton stalks (CSs) are an abundant, renewable lignocellulose residue, which is usually burnt in the field to prevent propagation of vegetal diseases, causing economic losses and environmental concerns. The production of biogas has been considered as an alternative. This work aimed to improve the biogas production from CS by steam or organosolv plus supercritical carbon dioxide (scCO2) pre-treatment. All samples were pre-treated in a 500 mL autoclave for 140 min at 180
C and fermented in 1 L eudiometer batch digesters for 42 days at 37.5
C. The biogas and methane yields achieved from the untreated CS were 250 and 137 norm litres per kg of volatile solid (LN kg1 VS), respectively. Pre-treatment of the CS samples with steam or the organosolv plus scCO2 process increased the methane yield by 20% compared to the untreated samples. The highest methane yield of 177 LN kg1 VS was achieved by organosolv plus scCO2 pre-treatment at 100 bar and 180
C for 140 min. Moreover, pre-treatment of the CS led to a significant reduction in the optimal digestion time from 30 days to 20 days for biogas production for the untreated CS. © 2020 Elsevier Ltd. All rights reserved.

Keywords: Methane Anaerobic digestion Cotton stalks Pre-treatment Organosolv plus supercritical carbon dioxide

1. Introduction Cotton is one of the most abundant crops in tropical and subtropical countries. Cotton influences the economies of nearly 90 countries. The major cotton-producing countries are India, China, the USA, Brazil, Uzbekistan, Turkey, Australia, Turkmenistan, Greece, Syria, and Egypt [1]. Syria has been chosen for this work as case country since the authors had comprehensive data on Syrian cotton production as well as samples from that region. Increases in cotton planting contributes to the economic growth of these countries, but increases other problems when, after harvesting, cotton stalks and leaves are left in the field and create a solid waste management problem and delay the subsequent plantation. Globally, about 80 Mt (dry mass) of cotton stalks (CS) are generated annually. CS as an agricultural residue is considered waste, although a part is used as fuel in the rural area. At present, CS is mulched with soil and removed from the cropland or burnt in the field to restrict future pest infestations, such as infestations from

* Corresponding author. E-mail address: rafat.alafi[email protected] (R. Al Afif). https://doi.org/10.1016/j.energy.2020.116903 0360-5442/© 2020 Elsevier Ltd. All rights reserved.

brown bollworm, which causes economic losses and environmental concerns [2]. Therefore, innovative technologies for CS treatment are urgently needed to avoid emissions from combustion in the field. The usage of cotton waste for energy production has become the subject of many studies in recent years [3e9]. However, the clean and energy efficient utilisation of CS in combustion plants is counter-indicated due to the high contents of elements such as Cl, K, and Na, which decrease the ash melting point and lead to fouling and corrosion [10]. On the other hand, due to the recalcitrant nature of the CS as lignocellulosic biomass, direct bioconversion to ethanol or biogas without pre-treatment always results in an extremely low yield [11e14]. Therefore, prior to bioconversion, lignocellulosic biomass needs to be pre-treated. In this regard, various attempts have been made by researchers around the world to develop a cheap, efficient, and environmentally friendly pre-treatment technique. The predominant methods for pre-treating lignocellulosic biomass include microwave, ultrasonic, chemo-mechanical methods [15], steam explosion [16e18], chemical [14,19,20], biological [21], hydrothermal [22,23] and thermo-chemical methods [24]. Organosolv is considered a thermo-chemical method for pretreatment of biomass for bio-refinery processes since organosolv

2

R. Al Afif et al. / Energy 194 (2020) 116903

pre-treatment is effective for cleaving alkali-labile ester bonds between monolignols and hemicelluloses [25]. As a material from a shrubby plant, CS contains a high amount of these ester linkages. Organosolv pre-treatment has been reported to be effective for delignification, leading to better enzymatic accessibility of the polysaccharide fraction. Supercritical carbon dioxide (scCO2), which is commonly used as an extraction solvent [26], has also been increasingly considered for other processes due to its many advantages. CO2, with its moderate critical temperature of 31.04
C and critical pressure of 7.36 MPa, can easily to be tuned in the near and supercritical state with a strong influence of pressure and temperature on its density and other thermophysical properties. This tunability can be exploited for the recovery of useful bioproducts in sustainable biorefinery processes [27]. ScCO2 is an inexpensive, clean, and environmentally benign solvent, and it is easy to recover after use. The unique properties of supercritical fluids (high diffusivity, low viscosity, high solving capacity, no liquidevapor meniscus, and, thus, no surface tension or capillary forces) are very useful for many processes. ScCO2 treatment has been shown to be an effective step in biorefinery applications for biomass pre-treatment, extractions, and chemical conversion [28,29]. The delignification effect produced by scCO2 pre-treatment in different types of lignocellulosic materials has been the subject of many studies and articles (e.g. Refs. [30e32]). These studies indicate that water in the biomass combined with scCO2 generate a carbonic acid mixture, which generates a kind of weak acidic environment, promoting hemicellulose hydrolysis and intensifying the mass transfer conditions [30]. In this context, it is worth noting that when water is mixed with another type of solvent, the delignification capability is enhanced; for instance, ethanol improves the solubility of lignin fragments [33]. Ethanol also drastically increases the very low solubility of water in scCO2. Other studies found that scCO2 explosion of pure cellulose or pre-treated cellulosic materials with the addition of aqueous buffer solutions improved the glucose yield from the enzymatic cellulose hydrolysis [30]. Srinivasana and Ju [34] reported that the most important advantage of the supercritical pre-treatment is that it is primarily a physical treatment, without appreciable loss/hydrolysis of cellulose and hemicelluloses. Krapf et al. [35] studied the effect of combining scCO2 with steam explosion or organosolv pre-treatment of rye straw on biogas production. They indicated that organosolv plus scCO2 pre-treatment [160 bar, 180
C for 140 min, 11% solid content, and 50% (v/v) ethanol content in the solvent] increased the methane yield by13% compared to the untreated samples. Presently, there are no systematic studies regarding organosolv and scCO2 pre-treatment of CS for biogas production. Therefore, studies for the optimisation of pre-treatment processes are required. This work seeks to improve biogas production from CS by organosolv plus scCO2 pre-treatment. 2. Materials and methods 2.1. Raw material CS (shredded and baled) was harvested in early October 2010 from the Research Station of Cotton Bureau in Syria. The moisture content of the CS was 4.8% [95.2% dry matter (DM)]. The samples for pre-treatment and anaerobic digestion experiments were chopped with a blender (Büchi B400) to a length of 1e3 mm and stored in airtight containers at room temperature until used for the experiments. 2.2. Inoculum Active sludge from a commercial biogas plant in Austria

(Table 1) was used as the inoculum. The substrates of the biogas plant were vegetables, maize silage, and sunflower silage. The inoculum was collected from the last section of the horizontal fermenter into a 50 L heatable container. 2.3. Analytical methods The nutritional composition of the CSs and inoculum were determined by analysing the following ingredients: DM, VSs, total nitrogen (TN), ammoniumenitrogen (NH4eN), raw protein (XP), raw fat (XL), crude fibre (XF), and nitrogen free extracts (XX). The DM content was analysed by drying the sample in a heating cabinet at 105
C until a constant weight was reached. The dried material was burned in a muffle furnace at 550
C for the determination of the raw ash content. The VS matter was calculated by subtracting the raw ash content from the DM [36]. The raw protein was analysed by digesting the sample with sulphuric acid in the presence of a catalyst. To make the acid solution alkaline, sodium hydroxide solution was added. The ammonia was distilled with a Büchi Distillation Unit B-324 and collected in a weighted amount of sulphuric acid. The excess was titrated with a standard solution of sodium hydroxide. The determined amount of nitrogen was multiplied by 6.25 [37,38]. The raw fat was analysed by extraction with diethyl ether [36]. The crude fibre was determined using standard procedures based on work by Naumann and Bassler [37] and Van Soest and Wine [39]. Elemental analyses for all samples were carried out with the PerkinElmer EA 1108 CHNSeO element analyser. All analyses were performed by the Microanalytical Laboratory at the University of Vienna [40]. The chemical composition and the elemental analyses of the CS are presented in Table 2. 2.4. Organosolv plus scCO2 pre-treatment of cotton stalks The effect of combining scCO2, steam, and organosolv pretreatment processes was studied along the experimental design in Table 3. Forty grams of milled, air-dry CS were mixed with a specific amount of water or watereethanol solution and placed in a 400 mL glass, bead-packed, high-pressure vessel. ScCO2 was delivered to the high-pressure vessel by a supercritical fluid control and delivery system (Separex SFP1). The high-pressure vessel was heated to the desired pre-treatment temperature with an electric heater. The pressure was kept at a constant level during the process. The pre-treatment scheme for the autoclavation of the CSs can be seen in Fig. 1. All samples were kept in a refrigerator until used for anaerobic digestion. The experimental design for the pre-treatment of CSs is shown in Table 3. Thus, untreated sample A was compared to 4 differently pretreated samples (B to E). Samples BeD were pre-treated with scCO2 plus steam, and sample E was pre-treated with scCO2 plus organosolv. In sample B, a pressure of 72 bar was subcritical for the mixture. For samples CeE, the pressure was supercritical, which allowed for the use of less water or organic solvent. One of the samples (D) was washed after pre-treatment to remove potential inhibitors, which might have been produced because of severe pre-treatment. After pre-treatment, this sample of the autoclaved CS was filtered with a fluted filter (Rotabilo; 150 mm; through-put time, 30 s) and washed with 1 L of a watereethanol solution (80:20, v/v). The pulp was oven-dried at 120
C for 24 h. The liquid phase containing the dissolved organic compounds, water, and ethanol was distilled to recover the ethanol [36]. After distillation, the water-insoluble lignin crystallised in the water phase and was separated by sedimentation. The recycled ethanol could then be reused for the washing process [26]. Organosolv lignin is often, because of its high quality, considered for

R. Al Afif et al. / Energy 194 (2020) 116903

3

Table 1 Parameters of the biogas plant from which the inoculum was taken. Parameter

Biogas plant

Digester type Digester

Horizontal shot plug flow digester 1 Mixing tank (193 m3) 4 Horizontal digesters (160 m3) 1 Vertical second stage digester (1885 m3) 1 Storage tank (opened) (4825 m3) Energy crops, vegetable matter 37
C z70 days 330 kW 2475 MWh

Used substrate Temperature in digester Average hydraulic retention time Electrical output of gas engine Electrical energy production per year

Before sampling, the transport container was filled with argon to ensure anaerobic conditions. The specific methane potential of the inoculum was measured as well. The inoculum showed a low specific methane potential of only 15 LN kg1 volatile solid (VS).

Table 2 Nutrient composition and the elemental analyses of the cotton stalk (CS) and inoculum. Substrate DM % FM VS % DM TN % FM NH4eN % FM XP % DM XL % DM XF % DM XX % DM Sta. DM Sug.% DM C % VS H % VS N % VS S % VS O % VS C/N CS In

95.2 2.2

91.6 52.8

0.88 e

0.18 e

6 14.5

0.7 0.8

45.7 10

43.9 27.5

1.25 e

4.3 e

55.8 27.7

6.4 e

0.95 6.3

0.06 e

36.8 e

58.7 4.4

DM¼dry matter; VS¼volatile solid; TN¼ total nitrogen; NHN¼ammoniumenitrogen; XP¼raw protein; XL¼crude fat; XF¼raw fibre; XX¼nitrogen free extracts; Sta. ¼Starch; Sug. ¼Sugar; C¼carbon; H¼hydrogen; N¼nitrogen; 4 S¼sulphur; O¼oxygen; C/N¼C/N ratio; CS¼cotton stalk; In¼inoculum.

Table 3 Experimental design of the pre-treatment process. Variants of the CS samples

CS [g]

H2O [g]

C2H5OH [g]

scCO2 Treatment p [bar]

T [
C]

Time [min]

Washing with H2O þ C2H5OH (20:80, v/v)

A B C D E

40 40 40 40 40

0 90 40 40 20

0 0 0 0 16

Without 72 100 100 100

_ 180 180 180 180

_ 140 140 140 140

Without Without Without With Without

CS¼cotton stalk; scCO

supercritical carbon dioxide; CHOH¼ethanol; HO¼water. 2¼ 25 2

37.5
C. The samples and inoculums were weighed out in a ratio of 1:3 (based on the DM). The produced amount of biogas was monitored on a daily basis. The biogas and methane production is given in norm litres (273 K and 1013 mbar) per kg of volatile solid (LN kg1 VS). The biogas composition (CH4 and CO2) was determined with a GC-SRI instruments Multiple Gas Analyser #2 (separating column FS-FFAP-CB, d. f. ¼ 0.5 mm). The gas analyser could also be equipped with a helium ionisation detector (HID) in addition to a thermal conductivity detector (TCD). The results of the elemental analyses can be used for calculating the theoretical biogas and methane yield as well as the concentration of the trace gases ammonia and hydrogen sulphide. The theoretical methane yield was calculated with the Buswell equation, as modified by Boyle [43]: Fig. 1. Pre-treatment scheme for the autoclavation of cotton stalks.

further commercial use. 2.5. Anaerobic digestion experiments: determination of the biochemical methane potential Anaerobic digestion experiments to measure the biochemical methane potential (BMP) were conducted by the anaerobic digestion laboratory of the University of Natural Resources and Life Sciences, Vienna. The experiments were carried out in accordance with VDI [41] and DIN standards [42]. In detail, eudiometer batch digesters of 1 L capacity were used. The temperature was set to

    b c d e a b c d e ca Hb Oc Nd Se þ a   þ 3 þ H2 O/ þ  3  C H4 4 2 4 2 2 8 4 8 4   a b c d e  þ þ 3 þ C O2 þ d N H3 þ e H2 S þ 2 8 4 8 4 This equation is used for balancing the carbon converted into methane during anaerobic fermentation. Therefore, the methane yield measured in batch fermenter experiments was compared to the theoretical methane yield.

2.6. Statistical data analysis The data given in tables and figures is of the means and standard

4

R. Al Afif et al. / Energy 194 (2020) 116903

hydrolysis by acting as a shield, preventing hydrolysis of the digestible parts of the substrate [49]. The decrease in methane yield after washing was due to the fact that soluble carbohydrates are also removed together with the lignin. The same observation was made by Krapf et al. [36], who stated that the separation process after pre-treatment of the rye straw, which included washing, distillation, lignin precipitation, and drying steps, negatively affected the microbial fermentation process. The highest specific methane yield of 177.4 LN kg1 VS (Tukey’s test, p < 0.05) was found for CSs pre-treated with organosolv plus scCO2 (variant E) at 100 bar and 180
C for 140 min. Ethanol is often used as a co-solvent for the hydrophobic scCO2 to improve the solubility of the polar components in the supercritical gas phase. Organosolv pre-treatment itself has advantages in terms of steam explosion because lignin is not solvable in pure water. The ethanol content prevents re-precipitation of lignin to cellulose fibres due to the pressure and temperature decrease after pre-treatment. The mixture is also much less aggressive than pure water, which likely ~e s et al. [50] yields to the production of less inhibitors. Montan showed that the solubility of tagatose, galactose, lactulose, and lactose in scCO2 with ethanolewater mixtures as a co-solvent was in the range of 0.02e1.09 mg g1. Water itself has very low solubility in scCO2, which increases substantially with the addition of a polar co-solvent [51]. Thus, water and ethanol together can be transported into the fibres by the supercritical phase much better than just pure water alone. This allows for lower temperatures, lower amounts of water, and lower pre-treatment times, as already shown by Krapf et al. [36] for rye straw. In comparison to the other studies, the digestion CS pre-treated with organosolv plus scCO2 in the present study resulted in a higher methane yield of 177.4 LN kg1 VS. This value was higher than the methane yields (122.7, 123, and 144.4 LN kg1 VS) obtained using alkaline treatment þ co-digestion with cow dung, hot water pretreatment, and dilute ammonia pre-treatment, respectively [52,53]. However, studies regarding the utilisation of CS for methane production are still rare. As seen from Table 2, the C/N ratio of the CS of 58.7 was much higher than the optimal C/N ratio of about 40 [13]. Therefore, in addition to improving the pretreatment processes, studies of the enhancement in anaerobic digestion through co-digestion of CS with nutrient-rich feedstock are desirable for future work. In this work, the increase in methane yield due to pre-treatment shows that, with further development, biogas production from pretreated CS could be a valuable alternative in cotton-producing countries. The possible impact is shown in Table 5. CS in Syria, with an annual generation of 2.6  106 tonnes [54], has the potential to generate about 313.5  106 CH4 per year (without pre-treatment) and up to the range from 296  106 to 404  106 m3 CH4 per year (with different pre-treatment conditions), if anaerobic digestion of CSs is applied. This could substitute 328.3  106 to 448.6  106 L of diesel per year. Or, if used in standard technology as engine-based CHP with z40% el. efficiency,

deviations of the performed experiments. Statistical analysis was carried out using SPSS Version 15. The data were analysed by oneway ANOVA, followed by Tukey’s test for post hoc comparison. The level of significance was set at p < 0.05. 3. Results and discussion 3.1. Biogas and methane production from pre-treated cotton stalks To determine the effect of pre-treatment on methane and biogas production, the CS samples were pre-treated under different conditions (Table 3) and then anaerobically digested. Table 4 shows the accumulated biogas and methane yield in norm litres per kg of volatile solids, obtained during anaerobic digestion of untreated and pre-treated CSs, which ranged from 221 to 295 to 129.8e177.4 LN kg1 VS, respectively. It is worth noting that the methane yield for the untreated CS reached 137.5 LN kg1 VS, whereas Krapf et al. [36] reported a substantially higher methane yield from untreated rye straw of 262 LN kg1 VS. The difference in yield can be attributed to the difference in biochemical compositions. In particular, the high lignin concentration (21 wt% dry) in CS compared to its concentration in rye straw (17 wt% dry) [44] contributes to the poor € degradation of CS during anaerobic digestion. Ohgren et al. [45] showed that the presence of lignin is considered a major hindrance to enzymatic hydrolysis. Due to the existence of ligninecarbohydrate complexes, the polysaccharides are chemically linked to lignin. The cellulose fibrils are physically encased by the surrounding lignin, which constitutes a barrier against enzymatic degradation of the polysaccharides in the cell walls [46]. This shielding effect reduces the rate and extent of lignocellulose hydrolysis and, therefore, limits the transformation of polysaccharides into biogas [47]. The biogas and methane yields for the three cases (A, B, and C) were 137.5, 147.1, and 163.0 LN kg1 VS, respectively. The changes in the process conditions were the following: sample A, untreated; sample B, pre-treated under slightly subcritical; or sample C, under supercritical conditions. The improvement was small under subcritical conditions with only 7%. Under supercritical conditions an improvement of 20.5% was reached, even with the addition of much less water. This clearly shows the benefit of scCO2, which, thanks to its high diffusivity and low viscosity, is able to penetrate and transport water into the biomass much better than a liquid solvent. It was reported that water and scCO2 can form weak carbonic acid at high pressure [30,33]. Also, Gao et al. [48] concluded for rice straw that scCO2 pre-treatment rendered fibres relatively fluffy and soft, which enhanced cellulose enzymatic hydrolysis. The effect of washing pre-treated CS with a watereethanol mixture (20:80, v/v) on the biogas and methane yield was shown with sample D. Washing resulted in a decrease in the methane yield of 26% compared to sample C, which did not involve washing. Nevertheless, removal of lignin from the CS should increase hydrolysis since lignin limits the rate and extension of (enzymatic)

Table 4 Specific biogas and methane yield of untreated and pre-treated CSs, as well as the methane concentration in biogas. Variants of the CS Samples

n

Biogas yield [LN kg1 VS] Av.

SD

P

A B C D E

3 3 3 3 3

251.0 264.8 286.0 221.0 295.0*

8.7 13,7 20.7 6.6 21.2

_ 0.18 0.998 0.308 0.037

n ¼ number of replications; Av ¼ Average; SD ¼ standard deviation; P ¼ P evalue significance. * Significant difference (p<0.05) between untreated and pre-treated CS within one column (Tukey’s test for post hoc comparison).

n

Methane yield [LN kg1 VS] Av.

SD

P

%

3 3 3 3 3

137.5 147.1 163.0 129.8 177.4*

4.8 9.0 13.0 3.9 12.8

_ 0.28 0.074 0.877 0.007

54.8 55.5 57.0 58.7 60.0

R. Al Afif et al. / Energy 194 (2020) 116903

5

Table 5 Estimated quantity of cotton stalks in Syria and the production of methane, total energy and electrical energy expected to be produced from anaerobic digestion of untreated and pre-treated cotton stalks. Variants of the CS samples

Approx. quant. VS Spec. methane yield kt/year m3NkgVS1

a Methane expected 106 m3/year

b

Total energy expected GWh/year

c

A B C D E

2279.6 2279.6 2279.6 2279.6 2279.6

313.44 335.33 371.57 295.89 404.40

3135.5 3340.0 3716.1 2957.2 4041.6

1097.4 1169.0 1300.7 1035.0 1414.6

a b c

0.138 0.147 0.163 0.130 0.177

Electrical energy expected Equivalent of diesel GWh/year 106 il.L/year 348.04 370.75 412.50 328.25 448.62

1 m3 methane ¼ 10 kWh. 1 m 3 methane ¼ 1.11 L diesel. 1 m 3 methane ¼ 3.5 kWh electricity; VS ¼ ¼ volatile solid.

could produce 1034 to 1414.5 GWh of electricity. Furthermore, if pre-treatment by organosolv plus scCO2 (variant E) is applied, an additional 91  106 m3 of methane could be obtained (Table 5). In addition to generating energy, the quantity of fertiliser gained from anaerobic digestion of CSs could reach up to 2.35  106 tonnes per year. The present results with pre-treatment by organosolv plus scCO2 show the potential for more efficient conversion of CSs to energy or fertiliser. However, such a course of action must be evaluated by a thorough energy balance, which requires additional further research. The energy quantities determined in these studies represent a theoretical potential. The share of this potential that can actually be implemented in practice depends in particular on current or expected price-cost relations on the market. Furthermore, for investigating the feasibility of carrying out such a technical solution on a real scale, in-depth studies of the techno-economic and environmental challenges of the proposed pre-treatment technology on a pilot/demonstration scale are highly recommended. 3.2. Quality of biogas The quality of the biogas produced from the different pretreated samples is illustrated in Table 4. The methane concentration (by volume) in the biogas was between 54.8 and 60.1%; the rest of the gas mostly consisted of CO2. The highest methane concentration was obtained from CS pre-treated with organosolv plus scCO2 (variant E). These results show that the quality of biogas produced by digestion of pre-treated and untreated CSs was good, and it increased with better pre-treatment methods. Nevertheless, further process optimisation is needed. 3.3. Energy turnover Specific methane yields from untreated CS and pre-treated samples measured in the eudiometer batch digesters were compared with the values estimated with the Buswell equation [40]. As shown in Table 6, the estimated values differed considerably from the measurements.

The coefficients of anaerobic energy turnover were between 23.1 and 31.5%. These results show the potential for obtaining more methane yield from CS by pre-treatment. Additional experiments are necessary to further improve the methane production potential. Future investigations need to focus on the following: optimisation of physicalethermal or biochemical pre-treatment, which break up the long chains of the molecules and the solid components of the CSs, and optimisation of the enzyme mixture, co-fermentation, and the organic loading rates, which would enhance methane production.

3.4. Specific methane yield per hour The effect of the pre-treatment of CS under different conditions on the digestion time (DT) was investigated. As seen in Fig. 2, during the first two days of digestion, the specific methane yield per hour of the pre-treated samples increased to the range between 1.9 and 2.8 LN kge1 VS h1, while the methane yield per hour for untreated sample A increased to only 0.6 LN kge1 VS h1 in the first five days. After this observed increase, the methane production per hour from all samples decreased exponentially, and at the end of the 20th day, the digestion reached the stationary phase. The same observations have also been made by Chen et al. [8] and Isci et al. [11]. This observation may be described by rapid bioconversion of organic acids and sugars in the feed during the first five days, whereas the biodegradation rate of hemicellulose is slower than for the extractives but faster than for the cellulose [55]. Pre-treatment decreased the digestion period to 27 days (down from 34 days), when the minimum methane yield was approximately 0.03 LN kg1 VS h1. The amount of methane produced from the pre-treated samples (C, D, E) during the first 20 days equaled 89% of the total methane yield (42 days). In comparison, 90% of the total methane yield from the untreated samples was achieved during the first 30 days. The cost of the reactor is the major investment cost in anaerobic digestion [56]. Therefore, because of the significantly shorter DT for pre-treated CS, reduction of the reactor volume could be useful for obtaining an economically competitive digestion process. Hence,

Table 6 Comparison between the measured and estimated methane yields. Variants of The CS samples Specific methane yield Measured

A B C D E

Specific methane yield Difference between measured and estimated Coefficient of anaerobic energy turnover

h

Estimated

[LN kg1 VS] [LN kg1 VS] [LN kg1 VS]

%

137.5 147.1 163.0 129.8 177.4

24.4 26.1 29.0 23.1 31.5

562.9 562.9 562.9 562.9 562.9

425.4 415.8 399.9 433.1 385.5

6

R. Al Afif et al. / Energy 194 (2020) 116903

Fig. 2. Specific methane yield of the digested samples per hour. All treated samples (BeE) were pre-treated at 180
C for 140 min.

the necessary DT to achieve a high rate of biodegradation is about 20 days for CS pre-treated with organosolv plus scCO2 and about 30 days for the untreated CS. Based on these findings, if this technology is scaled up, the pre-treatment of CS by oganosolv plus scCO2 could result in faster methane production rates, corresponding to a shorter DT. This could lead to a significant economic benefit through the increase in methane production efficiency or through the treatment capacity of the digester, which uses a shortened DT. The necessary process costs of pre-treatment must be included in the economic consideration. A profitability of the pre-treatment variants examined here is given, if the additional energy yield and the savings of the fermenter costs are higher than the necessary costs for the pre-treatment. 4. Conclusions The aim of this study was to investigate the feasibility of organosolv plus scCO2 based pre-treatment processes to increase the methane production from CS. The experimental results are already promising since our study has shown that pre-treatment of CS using scCO2 plus water increased the biogas and methane yield by 20 and 29%, respectively, compared to the untreated samples. On the other hand, washing pre-treated CS samples led to a decrease in methane yield of 6%. It is worth noting that the quality of biogas was good, and it increased with pre-treatment from 50 to 60% CH4. Regarding methane production, 2.6  106 tonnes of CSs are generated annually in Syria, with the potential to yield 313.5  106 m3 methane or, in the case of organosolv plus scCO2 pre-treatment, even another 90.5  106 m3. Which part of this theoretical energy potential can be implemented in practice depends largely on the price-cost structure on the market. Nevertheless, pre-treatment conditions have to be optimised in order to maximise methane production from CS since the coefficients of anaerobic energy turnover are not high (ranging between 23.1 and 31.5%). Furthermore, in-depth studies of the techno-economic and environmental challenges of the proposed pre-treatment technology on a pilot/demonstration scale are highly recommended.

Acknowledgements The research was supported by the University of Natural Resources and Life Sciences, Vienna, Austria. We thank Prof. Dr. Tobias €ll for his comments, which greatly improved the manuscript, Pro and special thanks is due to Dr. Alexander Bauer for his continues help. References [1] Shaikh AJ, Gurjar RM, Patil PG, Paralikar KM, Varadarajan PV, Balasubramanya RH. Particle boards from cotton stalk. accessed 25.07.2011, http://www.icac.org/tis/regional_networks/asian_network/meeting_5/ documents/papers/PapShaikhA.pdf; 2011. [2] Kaur U, Oberoi HS, Bhargav VK, Sharma-Shivappa RR, Dhaliwal SS. Ethanol production from alkali- and ozone-treated cotton stalks using thermotolerant Pichia kudriavzevii HOP-1. Ind Crops Prod 2012;37:219e26. [3] Al Afif R, Anayah S, Pfeifer P. Batch pyrolysis of cotton stalks for evaluation of biochar energy potential. Renew Energy 2020;147:2250e8. €ll T. Bioenergy recovery from cotton stalk. Intech 2019. [4] Al Afif R, Pfeifer C, Pro https://doi.org/10.5772/intechopen.88005. [5] Algin HM, Turgot P. Cotton and limestone powder wastes as brick material. Constr Build Mater 2008;22:1074e80. [6] Zheng J, Yi W, Wang N. Bio-oil production from cotton stalk. Energy Convers Manag 2008;49:1724e30. € ll T, Al Afif R, Schaffer S, Pfeifer C. Reduced local emissions and long-term [7] Pro carbon storage through pyrolysis of agricultural waste and application of pyrolysis char for soil improvement. Energy Procedia 2017;114:6057e66. [8] Chen X, Chen Y, Yang H, Chen W, Wang X, Chen H. Fast pyrolysis of cotton stalk biomass using calcium oxide. Bioresour Technol 2017;233:15e20. [9] Christopher M, Mathew AK, Kumar MK, Pandey A, Sukumaran RK. A biorefinery-based approach for the production of ethanol from enzymatically hydrolysed cotton stalks. Bioresour Technol 2017;242:178e83. € ll T, Al Afif R, Pfeifer C. A mass- and energy balance-based [10] Schaffer S, Pro process modelling study for the pyrolysis of cotton stalks with char utilization for sustainable soil enhancement and carbon storage. Biomass Bioenergy 2019;120:281e90. [11] Isci A, Demirer GN. Biogas production potential from cotton wastes. Renew Energy 2007;32:750e7. [12] Al Afif R, Linke B. Biogas production from three-phase olive mill solid waste in lab-scale continuously stirred tank reactor. Energy 2019;171:1046e52. [13] Al Afif R, Amon T. Mesophilic anaerobic co-digestion of cow manure with three-phase olive mill solid waste. Energy Sources, Part A Recovery, Util Environ Eff 2018. https://doi.org/10.1080/15567036.2018.1549157. [14] Silverstein RA, Chen Y, Sharma-Shivappa RR, Boyetteand MD, Osborne J. Improvement of dry simultaneous saccharification and fermentation of rice straw to high concentration ethanol by sodium carbonate pretreatment.

R. Al Afif et al. / Energy 194 (2020) 116903 Energy 2019;167:654e60. [15] Li Y, Chen MQ, Li QH, Huang YW. Effect of microwave pretreatment on the combustion behavior of lignite/solid waste briquettes. Energy 2018;149: 730e40. €ger A, Kikas T. Nitrogen explosive decompression [16] Raud M, Krennhuber K, Ja pre-treatment: an alternative to steam explosion. Energy 2019;114:175e82. [17] Khoshnevisan B, Shafiei M, Rajaeifar MA, Tabatabaei M. Biogas and bioethanol production from pinewood pre-treated with steam explosion and N-methylmorpholine-N-oxide (NMMO): a comparative life cycle assessment approach. Energy 2016;114:935e50. € sch P, Friedl A, Amon T. Analysis of methane potentials of steam[18] Bauer A, Bo exploded wheat straw and estimation of energy yields of combined ethanol and methane production. J Biotechnol 2009;142:50e5. [19] Yao R, Hu H, Deng SH, Wang H, Zhu H. Structure and saccharification of rice straw pretreated with sulfur trioxide micro-thermal explosion collaborative dilutes alkali. Bioresour Technol 2011;102:6340e3. [20] Zhang Q, Tang L, Zhang J, Mao Z, Jiang L. Optimization of thermal-dilute sulfuric acid pretreatment for enhancement of methane production from cassava residues. Bioresour Technol 2011;102:3958e65. [21] Zhang J, Li W, Lee J, Loh K-C, Dai Y, Tong YW. Enhancement of biogas production in anaerobic co-digestion of food waste and waste activated sludge by biological co-pretreatment. Energy 2017;137:479e86. [22] Hashemi SS, Karimi K, Mirmohamadsadeghi S. Hydrothermal pretreatment of safflower straw to enhance biogas production. Energy 2019;172:545e54. [23] Tayeh HA, Levy-Shalev O, Azaizeh H, Dosoretz CG. Subcritical hydrothermal pretreatment of olive mill solid waste for biofuel production. Bioresour Technol 2016;199:164e72. [24] Jiang D, Ge X, Zhang Q, Li Y. Comparison of liquid hot water and alkaline pretreatments of giant reed for improved enzymatic digestibility and biogas energy production. Bioresour Technol 2016;216:60e8. [25] Sun RC, Sun XF, Wang SQ, Zhu W, Wang XY. Ester and ether linkages between hydroxycinnamic acids and lignins from wheat, rice, rye, and barley straws, maize stems, and fast-growing poplar wood. Ind Crops Prod 2002;15:179e88. [26] Melo MMR, Silvestre AJD, Silva CM. Supercritical fluid extraction of vegetable matrices: applications, trends and future perspectives of a convincing green technology. J Supercrit Fluids 2015;92:115e76. [27] Chew KW, Yap JY, Show PL, Suan NH, Juan JC, Ling TC, Lee DJ, Chang JS. Microalgae biorefinery: high value products perspectives. Bioresour Technol 2017;229:53e62. [28] Morais AR, da Costa Lopes AM, Bogel-Lukasik R. Carbon dioxide in biomass processing: contributions to the green biorefinery concept. Chem Rev 2015;115:3e27. [29] Yildiz-Ozturk E, Ilhan-Ayisigi E, Togtema A, Gouveia J, Yesil-Celiktas O. Effects of hydrostatic pressure and supercritical carbon dioxide on the viability of Botryococcus braunii algae cells. Bioresour Technol 2018;256:328e32. [30] Zhao M-J, Xu Q-Q, Li GM, Zhang Q-Z, Zhan HS. Pretreatment of agricultural residues by supercritical CO2 at 50e80
C to enhance enzymatic hydrolysis. J Energy Chem 2019;31:39e45. [31] Narayanaswamy N, Fail A, Goetz DJ, Cu T. Supercritical carbon dioxide pretreatment of corn stover and switchgrass for lignocellulosic ethanol production. Bioresour Technol 2011;102:6995e7000. [32] Gao M, Xu F, Li S, Ji X. Effect of SC-CO2 pretreatment in increasing rice straw biomass conversion. Biosyst Eng 2010;106:470e5. [33] Lü H, Ren M, Zhang M, Chen Y. Pretreatment of corn stover using supercritical CO2 with watereethanol as co-solvent. Chin J Chem Eng 2013;21:551e7. [34] Srinivasana N, Ju L. Pretreatment of guayule biomass using supercritical carbon dioxide-based method. Bioresour Technol 2010;101:9785e91. [35] Krapf C, Bauer A, Amon T, Haimer E, Wendland M. Hochdruckverfahren zum Aufschluss von Lignocellulose aus Stroh. Chem Ing Tech 2009;81:1213e4. Krapf C. Pre-treatment of rye straw for biogas fermentation - Usage of different mixing ratio of water, ethanol and supercritical CO2. Master Thesis

7

2008. University of Natural Resources and Life Sciences, Vienna. [36] Naumann C, Bassler R. Die chemische untersuchung von Futtermittel. third ed. Darmstadt: VDLUFA-Verlag; 1993. [37] Commission of the European Communities. Commission directive 93/28/EEC of 4 June 1993 amending Annex I to the third Directive 72/199/EEC establishing community methods of analysis for the official control of feeding stuffs. Official Journal of the European Communities 1993;L375. [38] Conklin-Brittain NL, Dierenfeld ES, Wrangham RW, Norconk M, Silver SC. Chemical protein analysis: acomparison of Kjeldahl crude protein and total ninhydrin protein from wild, tropical vegetation. J Chem Ecol 1999;25: 2601e22. [39] Van Soest PJ, Wine RH. Use of detergents in the analysis of fibrous feeds. IV. Determination of plant cell-wall constituents. J Assoc Off Anal Chem 1967;50: 50e5. [40] Theiner J. Elemental C/H/N/S analysis. Vienna, Microanalytical Laboratory, University of Vienna, [Austria]. [41] Verein Deutscher Ingenieure. Vdi 4630 - fermentation of organic materials. In: Characterisation of substrate, sampling, collection of material data, Fermentation Tests [1872]. VDI Gesellschaft Energietechnik. [42] DIN standard. German standard methods for the examination of water, waste water and sludge; sludge and sediments (group S). Determination of the Amenability to Anaerobic Digestion (S 8). Berlin: Deutsches Institut für Normung e.V.; 1985. 38 414e8. [43] Boyle WC. Energy recovery from sanitary landfills - a review. In: Schlegel HG, Barnea J, editors. Microbial energy conversion. Oxford: Pergamon Press; 1976. p. 119e38. [44] Phyllis2 - database for biomass and waste e ECN. https://www.ecn.nl/ phyllis2. € [45] Ohgren K, Bura R, Saddler J, Zacchini G. Effect of hemicellulose and lignin removal on enzymatic hydrolysis of steam pretreated corn stover. Bioresour Technol 2007;98:2503e10. [46] Yang B, Wyman CE. Pretreatment: the key to unlocking low-cost cellulosoic ethanol. Biofuels, Bioprod Bioref 2008;2:26e40. [47] Grabber JH, Hatfield RD, Ralph J. Ferulate cross-links limited the enzymatic degradation of synthetically lignified primary walls of maize. J Agric Food Chem 1998;46:2609e14. [48] Gao M, Xu F, Li S, Ji X, Chen S, Zhang D. Effect of SC-CO2 pretreatment in increasing rice straw biomass conversion. Biosyst Eng 2010;106:470e5. [49] Chang VS, Holtzapple MT. Fundamental factors affecting enzymatic reactivity. Appl Biochem Biotechnol 2000;84:5e37. ~e s F, Fornari T, Stateva R, Olano A, Ib ~ ez E. Solubility of carbohydrates [50] Montan an in supercritical carbon dioxide with (ethanol þ water) cosolvent. J Supercrit Fluids 2009;49:16e22. [51] Adrian T, Wendland M, Hasse H, Maurer G. High-pressure multiphase behaviour of ternary systems carbon dioxide-water-polar solvent: review and modeling with the Peng-Robinson equation of state. J Supercrit Fluids 1998;12:185e221. [52] Adl M, Sheng K, Gharibi A. Technical assessment of bioenergy recovery from cotton stalks through anaerobic digestion process and the effects of inexpensive pre-treatments. Appl Energy 2012;93:251e60. [53] Radwan AM, Sebak HA, Mitry NR, El-Zanati EA, Hamad MA. Dry anaerobic fermentation of agricultural residues. Biomass Bioenergy 1993;5:495e9. [54] National cotton council of americ rankings. http://www.cotton.org/econ/ cropinfo/cropdata/rankings.cfm; 2010. re H. Lignocellulosic [55] Monlau F, Barakat A, Trably E, Dumas C, Steyer JP, Carre materials into biohydrogen and biomethane: impact of structural features and pretreatment. Crit Rev Environ Sci Technol 2013;43:260e322. [56] Hilkiah IA, Ayotamuno MJ, Eze CL, Ogaji SOT, Probert SD. Designs of anaerobic digesters for producing biogas from municipal solid-waste. Appl Energy 2008;85:430e8.