Research Article pubs.acs.org/journal/ascecg
Role of Steam Explosion on Enzymatic Digestibility, Xylan Extraction, and Lignin Release of Lignocellulosic Biomass Felicia Rodríguez,†,§ Arturo Sanchez,*,†,§ and Carolina Parra‡,§ †
Laboratorio de Futuros en Bioenergía, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, Unidad Guadalajara, Av. del Bosque 1145, Colonia El Bajío, 45019 Zapopan, Jalisco, México ‡ Laboratorio de Recursos Renovables, Centro de Biotecnología, Universidad de Concepción, Barrio universitario s/n, Concepción, Región del Bío Bío, Chile § Clúster de Bioalcoholes, Centro Mexicano de Innovación en Bioenergía, Guadalajara, Jalisco, México S Supporting Information *
ABSTRACT: This work studies the contribution of each step of the steam explosion (residence time and sudden decompression) of wheat straw. Raw wheat straw and wheat straw with different degrees of xylan extraction were subjected to a steam explosion without residence time at 160, 180, 200, or 220 °C, in order to identify the individual contribution of the sudden pressure change upon enzymatic digestibility, xylan extraction, and lignin release. In addition, the contribution of the residence time of the biomass in the reactor before the explosion was estimated using pretreated biomass prepared by autohydrolysis at 180−220 °C with a residence time of up to 56 min. Results show that steam explosion alone does not have significant effects on digestibility, xylan extraction, or lignin release of raw wheat straw. For autohydrolyzed biomass, steam explosion of 200 °C and above increases glucose release after 72 h of enzymatic hydrolysis at an enzymatic dose of 22 mg protein/g substrate. Digestibility at several enzymatic doses were also tested. No significant differences between autohydrolyzed biomass either before or after steam explosion were observed with an enzymatic dosage of 44 mg protein/g substrate. A simple mathematical model based on experimental results is proposed to describe the effect of steam explosion variables (residence time and temperature) on digestibility. KEYWORDS: Lignocellulosic biomass, Steam explosion, Pretreatment, Autohydrolysis, Enzymatic hydrolysis, Mathematical modeling
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INTRODUCTION Steam explosion without catalyst (SE) is considered to be one of the most sustainable biomass pretreatments of lignocellulosic biomass for bioethanol production, because of its impact on the reduction of greenhouse gases, and other environmental aspects.1 Also, SE has been extensively studied for the production of high-value-added products.2−4 One of the goals of pretreatments is to increase the enzymatic digestibility of cellulose with a minimal production of undesirable subproducts, thus reducing the total production cost. The success of this step is directly reflected in the decreased amount of enzyme required, which can contribute up to 16% of bioethanol’s total production cost.5 In addition, enzymatic hydrolysis time is also reduced, which, in turn, decreases the operational costs. The main SE operating variables are temperature and residence time (RT), ranging from 180 °C to 230 °C and from 3 min to 30 min,6 respectively. The SE effect reported in the literature is the increase of surface area due to breaking of the fibers and hemicellulose depolymerization.7 However, sugar degradation products and soluble phenolic compounds from biomass pretreatment by SE could © 2017 American Chemical Society
be produced at high temperatures or long RT. The production of these compounds reveals a decrease in hemicellulose recovery and the potential fermentation inhibitors.8 SE efficiency in batch reactors is influenced by the differential pressure, loading coefficient,9 number of cycles,10 and biomass treatments prior to the explosion (physical and thermochemical). Physical treatments include particle size reduction11 and soaking at different times and temperatures.12 Furthermore, thermochemical treatment (TCT) includes the chemical reactions that occur when the biomass is in the reactor before the sudden explosion (SE-RT). These reactions are promoted by temperature and steam (autohydrolysis). Common response variables used to evaluate SE are particle size and crystallization degree changes, wettability, hemicellulose and lignin depolymerization, formation of degradation products, enzymatic hydrolysis, and fermentation yields.13 Received: February 24, 2017 Revised: April 7, 2017 Published: April 11, 2017 5234
DOI: 10.1021/acssuschemeng.7b00580 ACS Sustainable Chem. Eng. 2017, 5, 5234−5240
Research Article
ACS Sustainable Chemistry & Engineering Table 1. AH and SE Conditions and Composition of Solid and Liquid Fraction of Pretreated Biomassa Experimental Conditionsb AH
a
Solid Fraction
SE
% Glucan
Liquid Fraction % Xylan
% Xylan Extraction
% Lignin
pH
ID
T
t
T
AH
AH + SE
AH
AH + SE
AH
AH
AH + SE
AH
AH + SE
1-SE 2-SE 3-SE 4-SE 5-SE 6-SE 7-SE 8-SE 9-SE 10-SE 11-SE 12-SE 13-SE 14-SE
180 180 180 180 200 180 200 200 200 220 180 180 200 200
12 22 35 46 35 55 11 25 56 25 14 21 16.5 44
160 160 180 180 180 190 200 200 200 200 220 220 220 220
45.59 47.20 56.21 56.90 61.8 52.45 54.60 46.73 58.07 56.43 50.26 46.00 50.76 60.90
53.50 51.00 51.09 61.28 62.64 48.42 53.73 64.15 59.54 63.65 58.07 56.09 62.65 61.68
19.26 15.95 11.89 10.40 3.63 9.46 13.32 6.17 1.82 1.12 19.64 15.35 9.37 2.39
18.85 14.65 9.64 8.87 3.13 7.57 9.89 5.08 2.30 1.31 14.30 10.85 6.94 2.20
40.80 53.44 63.12 71.56 90.39 73.86 70.00 84.06 95.32 97.69 38.56 54.50 74.61 93.83
5.98 5.49 5.55 4.10 5.70 2.87 5.83 5.87 5.00 5.12 5.28 5.21 6.16 6.22
5.96 6.10 5.42 7.48 8.30 5.35 6.10 8.71 5.97 8.00 12.47 11.11 9.06 6.55
4.16 3.91 3.72 3.77 3.42 3.68 3.87 3.39 3.19 3.33 4.12 3.90 3.51 3.26
4.17 4.01 3.75 3.82 3.39 3.75 3.84 3.54 3.34 3.36 4.14 4.03 3.73 3.31
AH = autohydrolysis, SE = steam explosion, and AH + SE = autohydrolysis + steam explosion. bT = Temperature (°C) t = time (min).
(SER) without RT. The impact of sudden decompression was calculated as the difference between the results from autohydrolyzed samples before and after SE. Furthermore, the effect of SE on nonautohydrolyzed biomass (raw wheat straw) and the enzymatic dosage effect on random samples are also discussed. An semiempirical mathematical model with SE variables for predicting enzymatic digestibility of exploded biomass was developed from the experimental results.
In order to model the effects of SE, biomass characteristics such as particle radius, particle size, moisture content, porosity, and heat capacity have been considered. 14,15 Physical phenomena that occur during the explosion, including steam diffusion, heat transference, and SE reactor design (discharge port area) and operation (explosion duration ratio16) are also taken into account. Although such mathematical models consider important SE variables and related phenomena, the measurement of some variables is difficult and the response variables (such as theoretical explosion power density) do not provide direct information regarding structural changes in biomass. SE effects on xylan extraction, enzymatic hydrolysis, fermentation yields,17 and cellulose stability18 have been modeled using the severity factor, where only autohydrolysis (AH) variables were considered (temperature and RT). In addition, strategies based on statistical analysis19 and computational tools inspired by biological systems as artificial neural networks20 have also been applied to modeling and optimizing the SE effects on enzymatic hydrolysis and fermentation yields. However, these approaches are not capable of predicting the effects of degradation of sugars, because of long reaction times or high temperatures nor variations in the reactor heating rate and temperature. Also, it is not possible to easily identify the individual contribution of AH and the decompression phenomenon. Although AH has been studied extensively (including kinetics and mechanisms of formation of degradation products21,22), the individual effect of quick depressurization is scarcely reported. Therefore, there is not enough information to design optimization and control strategies to increase lignin release, hemicellulose depolymerization, or enzymatic digestibility, nor to decrease energy costs in SE batch reactors or continuous pretreatment reactors, where SE is one of the main process stages.23,24 This paper evaluates the individual contribution of SE-RT and the quick decompression on hemicellulose depolymerization, lignin release, and enzymatic hydrolysis yield of wheat straw. First, the SE-RT effects were analyzed by subjecting the raw wheat straw to an AH reaction, where temperature and reaction time were the operational variables. Then, the autohydrolyzed biomasses were exploded in a SE reactor
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MATERIALS AND METHODS
Lignocellulosic Biomass. The raw wheat straw used for this work was harvested in the summer of 2015 in the Los Á ngeles province of VIII Region, Chile (37°43′04.4″ and 72°14′14.6″). The straw was milled with a hammer mill (Maestranza Proinco S.A.) and classified using a set of ASTM sieves (10, 12, 14, 16, 18, 20, and 40 mesh). The fractions collected in the 14, 16, and 18 mesh sieves (1.4 mm to 1.0 mm) were used for the study. The composition was glucan 38.5%, xylan 23.7%, and lignin 23.8% on dry weight basis (dwb). Autohydrolysis. The AH reactions were performed in a 4581 series 1-gal Parr batch reactor equipped with a stainless-steel anchor stirrer, a temperature controller, an electric furnace chamber and an internal cooling coil. The reactor loading was 100 gr (dwb) of raw wheat straw and 1000 mL of water. Reactions were carried out with a stirring speed of 10 rpm at 180, 200, and 220 °C. The reaction durations were from 12 min to 56 min. The initial reaction time was fixed when the reactor temperature reached 170 °C. The reactor heating rate was 4.5 °C/min, reaching 170 °C at 34 min. Representative heating profiles are shown in Figure S1 in the Supporting Information (SI). Upon reaction completion, the reactor was cooled to room temperature at a cooling rate of 12.5 °C/min. A control sample (ACS) was analyzed to evaluate the effect of the reactor preheating time at 170 °C. The resulting slurry was centrifuged and stored at 4 °C for further analysis. Steam Explosion. SE was carried out in a 5 L steam explosion reactor (SER) equipped with a 121 L expansion chamber. Raw wheat straw and autohydrolyzed biomass were soaked, for 2 h, in water and autohydrolysis liquor, respectively. After loading the SER with a sample equivalent to 60 g (dwb), it was fed with saturated steam reaching 160, 180, 200, or 220 °C (equivalent of 89, 146, 230, or 348 psig, respectively) and then immediately depressurized to atmospheric pressure. The biomass was no longer exposed to steam after the heating time (SE without RT). The reactor’s heating time from room temperature to explosion temperature was 40−60 s. The slurry was centrifuged and stored at 4 °C for further analysis.
5235
DOI: 10.1021/acssuschemeng.7b00580 ACS Sustainable Chem. Eng. 2017, 5, 5234−5240
Research Article
ACS Sustainable Chemistry & Engineering Enzymatic Hydrolysis. Enzymatic hydrolysis was carried out in 250 mL Erlenmeyer flasks at 50 °C in a shaking incubator (Labtech LSI-4018A) at 150 rpm for 72 h. All experiments were run in duplicate with a total volume of 100 mL, composed of 2% solids (w/v), an enzymatic load of 22 mg protein/g substrate and 0.05 M sodium citrate pH 4.8 buffer. The enzyme used was Cellic Ctec3 (Novozymes, Denmark). Raw wheat straw was used as a blank sample (BS). The reactions were sampled at multiple time points. Samples were centrifuged, filtered (0.22 μm), and frozen for further analysis. Enzymatic digestibility was measured using the following enzymatic hydrolysis yield:
EH yield (%) =
Gs × 100 Gi
Glucan was not detected by HPLC in the liquid fraction after AH, probably because of the reaction conditions not promoting cellulose depolymerization. The glucan content in the solid fraction is shown in Table 1. The glucan recovery in the solid fraction after AH was 90% ± 7%, with the exception of sample 10-SE, which, in particular, showed a darker color, suggesting that, under these operating conditions and xylan extraction (>95%), part of the solid fraction was carbonized, thus decreasing recovery. Enzymatic Digestibility of Autohydrolyzed Biomass. ACS, BS, and wheat straw pretreated by AH was enzymatically hydrolyzed according to the procedure described in the earlier section entitled “Enzymatic Hydrolysis”. The kinetic profile of enzymatic hydrolysis of BS was very similar to that of ACS. The difference between the digestibility values of BS and ACS was 0.12% after 72 h of enzymatic hydrolysis. ACS digestibility value, coupled with the liquid fraction analysis, suggest that no significant changes to the biomass composition and structure occur during the reactor preheating time from room temperature to 170 °C. The enzymatic digestibility value after 72 h of enzymatic hydrolysis of autohydrolyzed samples (Ga(t) exp) is shown in the left-hand side of Table 2. The maximum digestibility at 180,
(1)
where Gs is the amount of glucose released (grams) and Gi is the initial amount of cellulose expressed as grams of monomer. The percentage of contribution of each pretreatment to digestibility was calculated by eq 2:
% contribution =
d pt dt
× 100
(2)
where dpt is the digestibility value achieved due to each treatment (AH or SE) and dt is the digestibility value achieved after SE. Analytical Techniques. The glucan, xylan, and lignin contents in the solid and liquid fractions were measured according to NREL laboratory analytical procedures.25,26 Cellobiose, glucose, and xylose were determined by HPLC (Merck Hitachi) equipped with a refractive index detector and an Aminex HPX-87H (Bio-Rad, USA) column. Furfural and hydroxymethylfurfural were analyzed by HPLC equipped with an UV-vis detector and an Aminex HPX-87H column. The percentage of reaction products (glucan, xylan, and lignin) on solid and liquid fractions were calculated based on the theoretical initial amount of each component loaded into the Parr reactor (w/w dry basis). The moisture content of the solid fraction was determined using a moisture analyzer (Sartorius MA35). pH was measured using a pH meter (Orion 5-Star Plus).
Table 2. Experimental and Simulated Digestibility after 72 h of Enzymatic Hydrolysis of Autohydrolyzed and Exploded Biomass Experimental
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RESULTS AND DISCUSSION Carbohydrates and Lignin Extraction of Autohydrolyzed Biomass. The results of the analysis of liquid fraction of ACS showed that only 0.8% of xylan was released during reactor preheating time. A total of 14 samples were autohydrolyzed with the reaction conditions described in Table 1. The effect of temperature and reaction time on xylan extraction is shown in Table 1. The xylan extraction at 180 °C and 55 min was 73.86%, which a value that is similar to that registered at 200 °C and 16.5 min. 95.32% of the xylan was extracted at 200 °C and 56 min, while 97.69% was registered at 220 °C and 25 min. The observed reaction rates at all temperatures were slower than those previously reported by Sidiras.22 This may be due to differences in solids loading, stirring speed, and particle size, which could affect the masstransfer rate. The pH values of the liquid fraction of each experiment are shown in the right-hand side of Table 1. The pH decrease is a result of the acetic acid generation by the hydrolysis of acetyl groups during the hemicellulose depolymerization.27 The lowest pH value was reached at 200 °C and 56 min. Another reported effect of AH is lignin depolymerization.28 The percentage of lignin released in the liquid fraction was 5.7%, on average. The maximum lignin released in the liquid fraction was observed in sample 14-SE (6.22%). However, no significant correlation between lignin content in liquid fraction and temperature or time reaction was found.
Simulation
ID
Ga(t) exp
Ga(SE) exp
Ga(t) sim
Gc(t) sim
Ga(SE) sim
% error
1-SE 2-SE 3-SE 4-SE 5-SE 6-SE 7-SE 8-SE 9-SE 10-SE 11-SE 12-SE 13-SE 14-SE
46 65 75 69 73 70 63 71 66 50 43 57 77 71
49 69 90 81 86 87 95 90 80 73 100 101 99 88
35 65 76 77 73 77 60 77 64 50 42 63 79 70
65 34 24 23 20 23 40 20 20 17 58 37 20 20
49 72 86 87 84 88 93 94 81 71 97 98 105 96
0.69 6.14 4.65 7.19 2.82 1.32 1.35 3.93 1.22 3.04 2.50 2.90 6.60 8.80
200, and 220 °C was observed after 35, 16.5, and 25 min of AH time, respectively. The digestibility values then decreased gradually with the increase of AH time. The sample autohydrolyzed at 220 °C showed a lower value compared with samples autohydrolyzed at 180 and 200 °C. This behavior was consistent with that reported by Ertas.29 The no-available glucans due to high reaction temperatures and long reaction time of AH were encompassed in the term “glucan loss”. Glucan loss could be a result of the cellulose degradation, bioproducts formation, or enzyme access restriction due to depolymerization or repolymerization of lignin. Steam Explosion of Nonautohydrolyzed Biomass. SE without RT was performed at 160, 180, 200, and 220 °C using raw wheat straw. Changes in particle size distribution were observed in samples exploded at 200 and 220 °C. The initial particle size of wheat straw was in the range of 14−18 mesh (1.40−1.0 mm), but after the explosion, 7% of the sample treated at 200 °C and 60% of the sample treated at 220 °C 5236
DOI: 10.1021/acssuschemeng.7b00580 ACS Sustainable Chem. Eng. 2017, 5, 5234−5240
Research Article
ACS Sustainable Chemistry & Engineering
depolymerization. Significant lignin release was recorded at 200 and 220 °C with a maximum of 12.47% in sample 11-SE. The difference between xylan content in solid fraction before and after SE was
