Effects of biomass densification on anaerobic digestion for biogas production

Abstract

In order to elucidate the possibility of applying biomass densification in anaerobic digestion, pelleting and briquetting were typically investigated for biogas production, and the anaerobic digestion using densified biomass at higher solid content was discussed as well. In addition, the logistic cost, including harvest, transportation, and storage, was also evaluated to check the economic feasibility once biomass densification was employed for biogas production. The results demonstrated that the main composition, including cellulose, hemicellulose and lignin, of the pelleted and briquetted corn stover exhibited some degradation compared with their corresponding undensified corn stover. Both pellets and briquettes had no adverse impacts on anaerobic digestion. The cumulative biogas production from pellets was 349.9 mL g⁻¹ VS (VS: volatile solid), which was 25.8% higher than unpelleted corn stover (278.1 mL g⁻¹ VS, respectively), moreover, the cumulative CH₄ production from pellets was 185.7 mL g⁻¹ VS compared with 135.1 mL g⁻¹ VS of unpelleted corn stover. Biogas production from briquettes was also slightly higher than that of the unbriquetted corn stover, however, improvement was not statistically significant. Increasing the organic loading, relatively higher biogas and CH₄ production can be achieved from briquettes compared with the unbriquetted corn stover. This was mainly attributed to more free water in the anaerobic digestion of briquettes. Especially, there was 11.7% improvement on biogas production as the organic loading was increased to 80 g TS per kg (TS: total solid). As the briquettes were employed for biogas production with the digestor scale of 3000 m³, the logistic cost could be curtailed by 36.1%. Evidently, the biomass densification can be employed in anaerobic digestion for biogas production to potentially solve the extra high cost of logistic issues.

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Introduction

Due to the rising energy demands, environmental and national security concerns, investigations on developing renewable energy resources have been raised in recent years. Lignocellulosic biomass is a promising energy source, because it is available in large quantities that will not conflict with food production and may contribute to environmental sustainability.¹ Typical lignocellulosic biomass includes agricultural residues, hardwood, and softwood, as well as dedicated energy crops.² Particularly, agricultural residues throughout the world is approximately 1.5 billion metric tonnes.³ These residues compose the most important biomass feedstocks in China owing to her vast agricultural scale. At present, agricultural residues have the potential to become a major source for bioenergy refinery, such as bioethanol, biogas and pyrolytic bio-oil, in which methane production is always a better option as most part of the biomass contents (carbohydrates, lignin, fats and proteins) in anaerobic digestion process can be converted into simple derivatives and finally into methane and carbon dioxide with the cooperation of a community of prokaryotic microorganisms.^4–7 Therefore, converting lignocellulosic biomass to biogas via anaerobic digestion is an important way to utilize agricultural residues.⁸ However, low bulk density of agricultural residues typically ranges from 40–80 kg m⁻³ which significantly augments the process of handling, transportation, and storage.^9–11 Moreover, it's a factor restriction use as a feedstock and directly impacts costs throughout the supply system.^12–14 One effective way to potentially overcome this limitation is to promote biomass density by densification, in which the unit density of feedstock can be increased as much as ten-fold and achieve consistent physical properties, such as size and shape, bulk and unit density, and durability.^12,15

Currently, the leading technologies for biomass densification mainly includes pellet mill, briquette press, screw extruder, tablet press, and agglomerator. Among these, briquette press and pellet mill are 2 dominating technologies that can increase biomass densification and help in solving logistic issues.^12,16–18 Pelleting is an agglomeration of ground particles into dense, free-flowing, durable pellets by means of mechanical or thermal processing.^9,19 Pelleting of biomass involves size reduction (using grinders, choppers, and hammer mills), conditioning of the ground biomass by applying heat and/or moisture, and extrusion of the ground biomass through a die.^20,21 The diameter and length of 6.3–6.4 mm and 13–19 mm pellets are the most common products with the unit density of 1125–1190 kg m⁻³ and cylindrical appearances.¹⁵ Briquetting is a mechanical process, in which biomass is first shredded with a low initial density and then submitted to high pressure, promoting its agglomeration and densification.¹⁷ Unlike the pelleting systems, briquetting can handle the biomass with larger-sized particles and wider moisture contents. The briquettes could be produced from various briquetting systems, such as piston press, tabletizer, cuber, roller press, and the produced briquettes have typical shapes of cylinder and cuboid. Generally, the cylindrical briquettes are 40–80 mm in diameter and 40–150 mm in length with unit density in the range of 800–1000 kg m⁻³.¹⁵ The cubic briquettes are from 12.7 × 12.7 mm to 38.1 × 38.1 mm in cross section, and from 25.4 to 101.6 mm in length with the unit density greater than 1000 kg m⁻³.^15,22

It is also well known that the formation of solid bridges is the main binding mechanism during densification. These solid bridges are developed by chemical reactions and sintering solidification, hardening of the melted substances, or crystallization of the dissolved materials.^16,23 During densification, the moisture in the biomass forms steam under high pressure and temperature, which may hydrolyze the hemicellulose and lignin into lower molecular carbohydrates, lignin products, sugar polymers, and other derivatives.²⁴ In contrast to the undensified biomass, the densified biomass will result in different behaviors as they are employed for subsequent conversion for bio-methane by anaerobic digestion.^17,25

However, as 2 leading densification technologies, the effect of briquetting and pelleting of agricultural residues on anaerobic digestion have been scarcely evaluated. In order to clarify the responses of pelleting and briquetting to the anaerobic digestion, pellets and briquettes made from corn stover were employed, and their corresponding undensified biomass were also anaerobically digested for comparison. Organic loading of 30, 50, and 80 g TS per kg were investigated for biogas production as well to elucidate the possibility of applying densified biomass for the digestion at higher solid content. Besides, the logistic cost, containing the streams of harvest, transportation, and storage, was evaluated to clarify the economic feasibility of applying biomass densification for biogas production.

Material and methods

Feedstocks and inoculum

The employed briquettes and pellets, and their corresponding raw materials in this work were collected from Heilongjiang Province in China. The raw materials for briquettes mainly consisted of corn leaves and corn stalks. The raw materials were pressed with a ring die briquette machine (cuber) (9SYH-1200, WanGuo Bioenergy Technology Co., Ltd., Xuzhou, Jiangsu Province, China) with the working pressure of approximately 2600 kg cm⁻² and capability of approximately 1000–1600 kg h⁻¹. The diameter of die ring was 960 mm with size of 30 × 30 mm in cross section. The raw materials moisture and size should be controlled in the range of 15–30% (wet basis) and less than 50 mm, respectively, before they were briquetted. The produced briquettes presented as cubic shape of approximately 30 × 30 mm in cross section, and 50–150 mm in length with the unit density of 1484 kg m⁻³.

The corn stalks were main raw materials for the pellets, which were produced by a ring die pelletizer with double rollers (FTHBCX350, Futen New-energy Technology Co., Ltd, Sanmenxia, Henan Province, China) with the capability of approximately 700–1200 kg h⁻¹. The diameter of the die ring was 350 mm with die size of 10.0 mm. The raw materials should be grounded with size of 10–30 mm and the moisture was required in the range of 15–30% for pelleting. The produced pellets were cylindrical in shape with 10.0 mm in diameter and 15–20 mm in length, and the unit density was determined as 1176 kg m⁻³.

The determined moisture content of the briquettes and pellets and their corresponding raw materials were in the range of 8.4–10.27% (dry basis). The characteristics of the densified and undensified corn stover were presented in Table 1.

Table 1 The main composition of the pellets, briquettes, and their corresponding undensified biomass

Composition (%)	Unpelleted corn stover	Pellets	Unbriquetted corn stover	Briquettes
a The values in the table were listed in the form of “Mean ± Standard deviation”.
Total solid	89.7 ± 0.3^a	91.5 ± 0.0	90.97 ± 0.1	90.2 ± 0.0
Volatile solid	96.4 ± 0.1	94.8 ± 0.0	94.9 ± 0.3	88.0 ± 1.1
Ash	3.6 ± 0.1	5.2 ± 0.0	5.1 ± 0.3	12.0 ± 1.1
Cellulose	35.2 ± 0.7	33.9 ± 1.5	34.5 ± 0.1	37.2 ± 1.1
Hemicellulose	17.8 ± 0.3	15.6 ± 0.8	21.4 ± 1.9	19.1 ± 1.8
Acid-insoluble lignin	17.1 ± 0.6	15.0 ± 2.0	17.7 ± 0.1	13.2 ± 0.1
Acid-soluble lignin	4.1 ± 0.8	6.6 ± 0.4	5.4 ± 0.0	6.9 ± 0.1
Ethanol extractives	6.5 ± 0.4	1.7 ± 0.8	2.2 ± 0.4	1.8 ± 0.7

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Inoculum for the batch anaerobic digestion was obtained from Biogas Institute of Ministry of Agriculture, Chengdu, China. The inoculum was pre-incubated for 15 days at mesophilic temperature in order to deplete the residual biodegradable organic materials (degasification). The basic characteristics of inoculum were total solids (TS) of 3.7%, volatile solids (VS) of 52.2% (dry basis), ash content of 47.8% (dry basis), cellulose of 9.3% (dry basis), hemicellulose of 4.2% (dry basis), lignin of 19.4% (dry basis), C/N of 6.6 and pH of 7.42.

Anaerobic digestion in batch

1.0 L glass bottles were used as reactor for anaerobic digestion and the total working weight was 800 g for the batch digestion. When the responses of pelleting and briquetting to the anaerobic digestion were investigated, the organic loading of pellets and briquettes were controlled as 30 g TS per kg. Afterwards, the organic loading was increased to 50 and 80 g TS per kg to investigate the digestion performances using densified biomass at higher solid content. The seeding sludge for inoculation in all digestion was 15 g TS per kg, and all runs were investigated in triplicate. In addition, 3 blanks were also taken with only inoculum. After the substrates and inoculum were loaded, each bottle was sealed with a rubber stopper, and the headspace was flushed with pure N₂ for 2.0 min. The bottles were then incubated at a mesophilic temperature (35.0 ± 1.0 °C) in the thermostat water bath, and the bottles were shaken with frequency of 24 times per day lasting for 5 min for each shaking. The whole duration of anaerobic digestion was arranged for 46 days. The biogas production was daily determined via water displacement, and 1.0 mL biogas was sampled using a syringe for the composition determination.

Analysis

The total solid (TS) and volatile solid (VS) were analyzed according to the standard methods in reference.²⁶ The Klason insoluble lignin and carbohydrates content in the undensified corn stover, densified corn stover and the residual solids after digestion were analyzed according to the method in the Tappi-T-22 om-88 as described in reference.²⁷ The hydrolysate from the Klason analysis was retained and analyzed for acid-soluble lignin by an UV spectrophotometer at 205 nm. Glucose and xylose in the hydrolysate from the Klason analysis were measured by a high-performance liquid chromatograph (HPLC) (Flexar, PerkinElmer, Inc., Waltham, MA, USA). 100 μL sample was injected into the HPLC with lactose (∼0.5 g L⁻¹, Sigma, Sigma-Aldrich Co., Ltd., USA) as an internal standard. The sugars were separated by a sugar column (SH1011, Shodex, Showa Denko America, Inc., New York, USA) at 60 °C using 0.05 mol L⁻¹ H₂SO₄ as mobile phase (flow rate of 0.8 mL min⁻¹), and the separated sugars were detected at 50 °C using a refractive index detector. The obtained glucose and xylose contents were used for calculating glucan and xylan contents, which represented the contents of cellulose and hemicellulose, respectively. The lignin and carbohydrates content in the biomass and the residual solids were determined with 3 repetitions.

The degradation percentage of VS, cellulose, hemicellulose, and lignin after the anaerobic digestion were calculated according to the following equations.

where, W_s is the weight of loaded substrate in the reactor (g, dry basis); W_d is the weight of effusive digestates (g, dry basis); D_VS is the VS content of the effusive digestates (%); S_VS is the VS content of the loaded substrate (%).where, D_Cellulose is the cellulose content in the effusive digestates (%, dry basis); S_Cellulose is the cellulose content in the loaded substrate (%).where, D_{Hemicellulose} is the hemicellulose content in the effusive digestates (%, dry basis); S_{Hemicellulose} is the hemicellulose content in the loaded substrate (%).where, D_Lignin is the lignin content in the effusive digestates (%, dry basis); S_Lignin is the lignin content in the loaded substrate (%).

The composition of the daily sampled biogas, including CH₄, CO₂, N₂ and H₂, were measured using a gas chromatograph (SP-2100A, Beifenruili Analytical Instrument Co., Ltd, Beijing, China) equipped with a molecular sieve packed stainless-steel column with the length and diameter of 2.0 m × 3.0 mm (TDX-01) and a thermal conductivity detector (TCD). The temperatures of the detector, injector, and oven were set at 100 °C, 50 °C, and 50 °C, respectively. The injection volume of sample to the column was 1.0 mL. A standard gas consisting of 52.81% (v/v) CH₄, 32.27% (v/v) CO₂, 4.93% (v/v) N₂ and 9.99% (v/v) H₂ was used for calibrating reads from the gas chromatograph.

Free water holdings of densified and undensified corn stover were estimated according to the unabsorbed water in a set duration. 12.0, 20.0 and 32.0 g densified and undensified corn stover were added in 400.0 g distilled water to simulate the organic loading of 30, 50 and 80 g TS per kg in the batch digestion. The water-absorbed biomass was filtered with a Buchner funnel for 2.0 h so that the unabsorbed water can be separated completely for collection. Afterwards, the unabsorbed water in each group was weighted to calculate the free water holding of densified and undensified corn stover. In this part, 3 repetitions were performed on each group.

Results and discussion

Main composition of the densified and undensified corn stover

Table 1 presents the main composition of the densified and undensified corn stover for anaerobic digestion. The total solids of the densified and undensified corn stover did not display significant differences due to their close moisture content in same storage condition. However, the volatile solids displayed decreases as the corn stover were densified, especially after the corn stover was briquetted. This was mainly attributed to part of organic fractions degradation during the process of briquetting and pelleting by the high temperature and pressure.²⁸ Of course, the increased ash content after densification also can be explained by this reason. In addition to the degradation of organic fractions, some inorganic matters, such as bentonite, is generally supplemented as a binder (approximately 2.5–6.0%) during the biomass briquetting according to the communication with the supplier of briquettes.²⁹

As the richest fraction in lignocellulosic biomass, cellulose in the pellets was a little lower than the unpelleted corn stover. Unlike pellets, its content in briquettes presented to be a little higher comparing with the unbriquetted corn stover. The decrease of hemicellulose content could be observed after pelleting and briquetting. Similar results have been reported in reference, in which the mannan (mainly representing hemicellulose of softwood) in pellets of Doulas-fir was decreased obviously in contrast to the unpelleted chips.³⁰ The hemicellulose was much more sensitive to thermo-shock than the fraction of cellulose and lignin and the degradation of hemicellulose will occur easily at high temperature and pressure during the process of briquetting and pelleting.¹² Consequently, it was believed that degradation of hemicellulose was beneficial to natural bonding for densification due to the formation of adhesive products.¹⁶ Moreover, the relatively increased ash content after biomass densification also partially related to hemicellulose degradation. The decrease of acid-insoluble lignin content could be observed after the raw feedstocks were densified into briquettes and pellets. On the contrary, the acid-soluble lignin content was increased in some degree. The lignin content (the sum of acid-insoluble lignin and acid-soluble lignin) was decreased from 23.1% to 20.1% after briquetting, however, the changes of lignin before/after pelleting was not significant. This could be easily understood that densification intensity of briquetting was typical higher than that of pelleting resulting in higher pressure and temperature during briquetting process, and the lignin degradation may be intensified.¹² Thereby, it can be deduced that the subsequent biological conversion in anaerobic digestion may be potentially affected by the changes of main composition of biomass after densification, which deserves a detailed investigation.

Effects of pelleting and briquetting on anaerobic digestion

The anaerobic digestion of densified and undensified corn stover was performed at (35.0 ± 1.0 °C) in 1.0 L reactors for 46 days, and the daily biogas production were plotted in Fig. 1a. Overall, the daily biogas production displayed 3 peaks during the whole anaerobic process, which was similar with that of anaerobic digestion of most lignocellulosic biomass.³¹ The first peak of biogas production can be observed during 1–3 days after the biomass was inoculated, and approximately 13.9, 19.2, 20.7, and 22.9 mL g⁻¹ VS biogas can be collected from the pellets, briquettes and their corresponding undensified corn stover. The second peak of biogas production appeared at 4–7 days in the groups of the unpelleted, briquetted and unbriquetted corn stover, and the corresponding biogas production was 25.2, 24.4, and 26.0 mL g⁻¹ VS. However, the obvious valley of biogas production was observed on the pellets, and the second peak was postponed to 5–12 days. Subsequently, the third peak of biogas production from the unpelleted, briquetted and unbriquetted corn stover were 14.3, 17.2, 15.1 mL g⁻¹ VS, which all appeared at the 9^th day. However, the third peak of biogas production from the pelleted corn stover was 24.1 mL g⁻¹ VS, which was postponed by 8 days consequently. Based on these results, it was obvious that biomass briquetting did not affect the anaerobic process, however, the pelleting retarded the digestion process in some degree. Furthermore, when the cumulative biogas production was evaluated (see Fig. 1b), it could be found that biogas production from pellets was 349.9 mL g⁻¹ VS, which was significantly higher than that of the corresponding unpelleted cover stover (278.1 mL g⁻¹ VS) (p < 0.001). Moreover, the briquettes also exhibited a slightly higher cumulative biogas production comparing with the unbriquetted corn stover. This result implied that biomass densification could not negatively affect the biogas production although a little delay appeared in the digestion process of pellets. Instead, approximately 25.8% and 0.6% improvement on the cumulative biogas production can be achieved. As stated above, the thermal degradation always involves in biomass densification, which may facilitate the digestion by anaerobic microorganisms and improve enzyme accessibility.^12,17,32 As the VS degradation was calculated, it was 61.3% and 57.5% for the pellets and briquettes comparing with the unpelleted and unbriquetted corn stover (57.5% and 55.8%, respectively), which again proved the anaerobic digestion performance of cover stover can be improved by biomass densification.

Fig. 1 Effects of densification on biogas production during the anaerobic digestion. (a) Daily biogas production; (b) cumulative biogas production; (c) cumulative methane production; in (a) and (c), □ unpelleted corn stover; ○ pellets; △ unbriquetted corn stover; ▽ briquettes; in (b), *** refers to p < 0.001, and ns refers to p > 0.05.

Furthermore, the cumulative methane yield of undensified and densified corn stover with a total solid loading of 30 g TS per kg was plotted in Fig. 1c. It could be found that the pellets yielded 185.7 mL g⁻¹ VS over 46 days HRT (hydraulic retention time) comparing with the unpelleted corn stover of 135.1 mL g⁻¹ VS, in which the difference was significant (p < 0.001). However, an obvious retard of special methane yield during the first 15 days can be observed on the pellets, which can be supported by the daily biogas production of pellets. As for the briquettes, methane yielded 131.7 mL g⁻¹ VS, which was a slight higher than that of unbriquetted corn stover (129.3 mL g⁻¹ VS), and the difference was not significant (p > 0.05). According to the above results, it was suggested that the biomass densification was feasible to be applied in anaerobic digestion as no negative effects on CH₄ production.

Anaerobic digestion of briquettes with increasing organic loading

Increasing the organic loading rate for anaerobic digestion has been claimed to be advantageous over low-solid anaerobic digestion for several reasons, such as reducing reactor volume, lowering energy requirements for heating, improving substrate handling efficiency, and so on.³³ However, it is hard to enhance to organic loading rate greatly for agricultural residues due to the very lower bulk density and the nature of suspension during digestion.^8,34 It is well known that bulk density of densified biomass will be greatly promoted, and the suspension during digestion thereby can be definitely avoided, potentially resulting in the improvement on biogas production.

As far as the technologies for biomass densification were concerned, the biomass need to be cut into smaller sizes for pellets production. Unlike pellet mills, briquetting machines can handle larger-sized particles, which meant more energy will be consumed for grinding biomass prior to densification for pellets.^2,9,35 Moreover, the briquettes have been proved to be a promising low-cost, low-energy, high-capacity approach for densifying corn stover for renewable energy applications in contrast to the pellets.²⁰ Thus, the briquetted corn stover was employed to investigate the digestion performances at the increasing organic loadings of 30, 50, and 80 g TS per kg. The daily biogas production, cumulative biogas production, and cumulative CH₄ production were displayed in Fig. 2.

Fig. 2 Effects of organic loadings on biogas production during the anaerobic digestion using briquettes. (a) Daily biogas production; (b) cumulative biogas production; (c) cumulative methane production; in (a) and (c), □ unbriquetted corn stover (30 g TS per kg); ○ briquettes (30 g TS per kg); △ unbriquetted corn stover (50 g TS per kg); ▽ briquettes (50 g TS per kg); ◇ unbriquetted corn stover (80 g TS per kg); ◁ briquettes (80 g TS per kg); in (b), ns refers to p > 0.05; * refers to p < 0.05; ** refers to p < 0.01.

As presented in Fig. 2a, the biogas overall could be produced as the biomass was inoculated, and the biogas releasing exhibited a retard as the organic loading was increased, especially at 80 g TS per kg. The anaerobic digestion did not appear apparent difference between briquettes and the unbriquetted corn stover when the organic loading of 30 and 50 g TS per kg were employed. However, as the organic loading was enhanced to 80 g TS per kg, approximately a delay of 6 days can be observed (see the second peak and the third peak) as unbriquetted corn stover was employed. As the cumulative biogas production based on VS was evaluated (see Fig. 2b), it was no surprising that the biogas production decreased with the increase of organic loading. Moreover, the biogas production from briquettes with organic loading of 30 g TS per kg was improved by 0.6% comparing with the unbriquetted corn stover (p > 0.05). Furthermore, the improvement was intensified as the organic loading was increased. Especially, there was 11.7% improvement on biogas production from briquettes in contrast to the unbriquetted coren stover (p < 0.01) as 80 g TS per kg substrates were loaded. When the cumulative CH₄ production based on VS was investigated (see Fig. 2c), the lower organic loading exhibited the higher CH₄ production. However, the briquettes released more CH₄ comparing with the unbriquetted corn stover. Moreover, the difference on CH₄ production between briquettes and unbriquetted corn stover appeared to be more significant with the higher organic loading. According to these results, it was proved that the densification can promote the anaerobic digestion, although the biogas production generally reduced by increasing organic loading.

Apparently, the improvement on biogas releasing may relate to the higher bulk density of briquettes, in which water in anaerobic digestion could not be adsorbed too much, and more free water may potentially retain in the digestion system comparing the unbriquetted corn stover. Thus, it could be speculated that mass transfer and the accessibility of microorganisms and enzymes to substrates facilitating could be improved as briquettes were employed. In order to prove this speculation, the free water in the anaerobic digestion with different organic loadings was determined as displayed in Fig. 3. The retained free water in the anaerobic digestion was reduced with the increasing loading, which well responded to the decreased biogas production at higher loading. Moreover, the free water in the briquettes was also more than that of unbriquetted corn stover. When the weight differences of free water between briquettes and unbriquetted corn stover were correlated with the differences of biogas (CH₄) production between briquettes and unbriquetted corn stover, the positive correlation could be observed with correlation coefficient (R²) of 0.77 and 0.86 for biogas and CH₄ respectively, indicating the biogas production from anaerobic digestion may partially relate to the free water. Thus, it could be substantially proved the improvement on biogas production from briquettes was mainly due to the retained free water in the digestion slurry.

Fig. 3 Free water in anaerobic digestion using briquettes and unbriquetted corn stover at organic loading of 30, 50, and 80 g TS per kg. *** refers to p < 0.001; ** refers to p < 0.01.

According to Table 2, the VS degradation after anaerobic digestion of the briquettes at organic loading of 30, 50, and 80 g TS per kg appeared to be higher than that of the unbriquetted corn stover. Moreover, increasing organic loading caused the decrease of VS degradation. This result could be a supplementary evidence for the intensified mass transfer and accessibility of microorganisms and enzymes to substrates in the digestion of briquettes. As the degradation of lignocellulosic fraction was calculated, the hemicellulose and cellulose were 2 dominant fractions for biogas conversion, which was consistent with the reported work.³⁶ Moreover, the degradation of lignocellulosic fractions, including hemicellulose, cellulose and lignin, all decreased as the organic loading was promoted. Their degradation of briquettes after digestion at these 3 loadings were also higher than that of unbriquetted corn stover. These results also greatly responded to the results of retained free water in Fig. 3. Thus, the degradation of lignocellulosic fractions could be another supplementary proof that the anaerobic digestion was intensified by the more retrained free water during the anaerobic digestion. In addition to the free water, the thermal degradation during briquetting was attributed to the analysis of lignocellulosic fractions (see Table 1), which can be another important reason for improving the anaerobic digestion and biogas production.^17,34

Table 2 The degradation of VS and lignocellulose fractions after anaerobic digestion

Degradation (%)	30 g TS per kg		50 g TS per kg		80 g TS per kg
	Unbriquetted corn stover	Briquettes	Unbriquetted corn stover	Briquettes	Unbriquetted corn stover	Briquettes
a The values in the table were listed in the form of “Mean ± Standard deviation”.
Volatile solid	55.8 ± 0.3^a	57.5 ± 0.2	41.9 ± 0.1	43.0 ± 1.0	32.5 ± 0.1	34.8 ± 0.1
Cellulose	57.8 ± 0.1	87.3 ± 0.1	39.2 ± 0.0	51.3 ± 0.2	33.4 ± 0.2	50.5 ± 0.5
Hemicellulose	63.3 ± 1.9	89.5 ± 0.0	49.1 ± 0.1	53.6 ± 0.9	43.6 ± 0.1	48.8 ± 0.2
Lignin	51.6 ± 2.1	61.4 ± 0.4	30.2 ± 0.8	46.8 ± 2.1	25.4 ± 1.0	35.67 ± 0.3

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Estimating logistic cost applying briquettes for biogas production

As stated above, it was technically feasible to apply biomass densification for biogas production. In order to clarify the economic feasibility, a preliminary estimation on the logistic cost with briquettes was investigated from the aspects of harvest, transportation, and storage based on a 3000 m³ anaerobic digestor for biogas production. According to our experiences, the suitable radius for transporting biomass of such scale plant was 5.0 km in China, and it thereby was defined for the estimation. In addition, the consumed corn stover was estimated as approximately 3900 tonnes per year. Based on these informations, the logistic cost was evaluated, and results was presented in Table 3.

Table 3 Logistic cost estimation for briquetted and unbriquetted corn stover^a

Items in logistic cost	Unbriquetted corn stover	Briquettes	Comparison
a The detailed estimation for these items were listed in ESI.
1. Harvest cost (¥ Yuan per ton)	137.5	156.3	↑ 13.7%
1.1 Collection from field	80.0	80.0	→ 0.0%
1.2 Size-reduction and densification	7.5	26.3	↑ 250.7%
1.3 Labour cost	50.0	50.0	→ 0.0%
2. Transportation cost (¥ Yuan per ton)	17.8	7.5	↓ 57.9%
3. Storage cost (¥ Yuan per ton)	126.43	16.14	↓ 87.2%
3.1 Construction for storage	80.0	10.0	↓ 87.5%
3.2 Fire-fighting equipment	38.5	5.13	↓ 86.7%
3.3 Land rent for storage	2.0	0.25	↓ 87.5%
3.4 Labour and operation cost	5.93	0.76	↓ 87.2%
Total cost (¥ Yuan per ton)	281.73	179.94	↓ 36.1%

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As indicated in Table 3, it could be easily found that the harvest cost will be definitely increased by 13.7% due to 3.5-fold increase on size-reduction and densification as briquettes was employed. However, the transporting cost could be obviously reduced by 57.9% as the biomass volume can be densified to 1/8 comparing with the undensified corn stover. Even more importantly, the reduction of biomass volume also greatly curtailed the cost on construction and fire-fighting equipment in the stream of storage by 87.5% and 86.7%, respectively. Correspondingly, the land rent, labor and operation cost were also reduced by 87.5% and 87.2%, respectively. Overall the total logistic cost of briquettes for a 3000 m³ anaerobic digestor could be reduced to 179.94 ¥ Yuan per ton, which was decreased by 36.1% comparing with the unbriquetted corn stover. As a result, it was a feasible way to apply biomass densification in anaerobic digestion potentially to solve extra high cost of logistic issues.

Conclusions

Biomass degradation, especially hemicellulose degradation, can be achieved by pelleting and briquetting. Moreover, the pellets and briquettes did not exhibit negative effects on biogas production. Instead, the biogas production could be improved by densification, especially, there were approximately 25.8% improvement on the cumulative biogas production from pellets, although a time-delay can be observed. As increasing organic loading for anaerobic digestion was investigated using briquetted corn stover, more free water can be retained in anaerobic digestion using briquettes in contrast to the unbriquetted corn stover, by which biogas production can be improved, especially at higher organic loadings. Moreover, approximately 36.1% cost reduction on logistics can be achieved as briquettes were employed for biogas production. Thus, biomass densification can be potentially employed for anaerobic digestion of lignocellulosic biomass for solving logistic issues to some extent.

Acknowledgements

The Department of Science and Technology of Sichuan Province is appreciated for funding support (No. 2014JQ0037, 2015NZ0100). This work was also supported by the National Natural Science Foundation of China (21306120) and the “Program for Changjiang Scholars and Innovative Research Team in University” (IRT13083) from the Ministry and Education of China. We also appreciated Dr Tao Luo, in Biogas Institute of Ministry of Agriculture of China, for the logistic cost estimation.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra17298a

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