Article pubs.acs.org/EF
Energy Recovery from Rice Straw through Hydrothermal Pretreatment and Subsequent Biomethane Production Leilei He,† He Huang,† Zhenya Zhang,*,† Zhongfang Lei,*,† and Bin-Le Lin‡ †
Graduate School of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8572, Japan Research Institute of Science for Safety and Sustainability (RISS), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8569, Japan
‡
S Supporting Information *
ABSTRACT: Rice straw is an abundant agricultural waste in Asia. Anaerobic digestion (AD) as an environmentally friendly process for bioenergy recovery is expected to solve the environmental issues brought about by open burning of rice straw. In order to test the feasibility and scalability of hydrothermal treatment (HTT) on rice straw for subsequent methane production from pretreated straw, this study attempted two peak HTT temperatures (150 and 210 °C, i.e. HTT150 and HTT210, respectively) for holding 0−30 min to pretreat rice straw which was then used for mesophilic methane fermentation. Thereafter energy balance and energy recovery were analyzed on HTT and subsequent AD of rice straw. Results show that HTT150 exhibited a positive effect on subsequent methane production, achieving the highest methane yield of 134 mL (STP) for per gram of added volatile solid (VSadded) of rice straw after being hydrothermally pretreated at 150 °C for 20 min. The maximum specific methane production rate (μ), around 17−40% higher than the control (without pretreatment), was achieved from HTT150 pretreated rice straw. HTT210 was found to have a negative effect on subsequent AD. Considering disposal of rice straw by HTT coupling with subsequent methane production, the highest net energy gain (ΔE = Eout − Ein), energy ratio (Eout/ Ein), and energy recovery (in comparison to direct combustion) were obtained at HTT150 for 20 min, about 2741 MJ/t, 2.7, and 30.7%, respectively. Results from this work imply that HTT temperature is critically important when subsequent AD for enhanced methane production and energy balance of the whole disposal system are targeted. enhanced methane production from five different types of organic wastes (cow manure, pig manure, municipal sewage sludge, fruit/vegetable waste, and food waste). According to their results, the methane yields of pretreated pig manure, fruit/ vegetable waste, and municipal sewage sludge were increased by 14.6, 16.1, and 65.8%, respectively, while those of pretreated cow manure and food waste were, respectively, decreased by 6.9% and 7.5%. Matsakas et al.7 also claimed that HTT is an efficient pretreatment for forest residues (such as spruce, pine, and birch) to achieve enhanced methane production. Results from these previous works indicate that the enhancement effect of HTT on subsequent methane production is closely relative to the characteristics of pretreated organic waste. In addition, as the optimum HTT temperature might be different for different agricultural wastes, more efforts should be made to investigate HTT conditions. For instance, 180 °C was reported to be the most suitable temperature to improve methane production from pretreated sunflower stalks,8 while as for sunflower oil cake, hydrothermal pretreatment at 100 °C was most appropriate for its subsequent AD.9 As seen, although there are many research works on hydrothermal pretreatment of biomass for enhanced AD process, up to the present, very limited information could be found on the effect of HTT operation conditions of rice straw for subsequent AD.10,11 Chandra et al.10 compared alkali
1. INTRODUCTION Rice straw as one of the most abundant agricultural wastes in the world has an annual output of 973.9 million tons, of which 90.5% is from Asia, especially developing countries.1 Open burning is a common practice for disposal of rice straw in these countries, leading to serious air pollution.2 Even in developed countries like Japan, the most conventional methods for disposal of rice straw are composting and returning to fields. Most probably due to the recalcitrant nature of the lignin contained within, rice straw cannot be fully utilized via the above-mentioned methods. Thus, most countries are facing the challenge of rice straw disposal. On the other hand, being rich in cellulose and hemicellulose contents, rice straw is considered as a promising feedstock for bioenergy recovery, i.e., biogas production via anaerobic digestion (AD). The AD process is composed of four stages, i.e., hydrolysis, acidogenesis, acetogenesis, and methanogenesis, among which the first stage or hydrolysis is generally regarded as the rate-limiting step, especially for lignocellulosic feedstocks. Therefore, pretreatment is a prerequisite for high energy recovery from rice straw prior to energy production processes. Hydrothermal treatment (HTT) without chemical addition, an environmentally friendly and easy operation process, can be a prospective alternative for the pretreatment of rice straw to attain efficient hydrolysis of organic matters contained and thus enhanced biomethane production. As it is known, hydrothermal pretreatment has been trialed on various organic wastes like sewage sludge, microalgae, and municipal waste for subsequent AD.3−7 Qiao et al.6 applied HTT (170 °C for 1 h) and achieved © XXXX American Chemical Society
Received: May 13, 2017 Revised: August 30, 2017 Published: August 31, 2017 A
DOI: 10.1021/acs.energyfuels.7b01392 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels pretreatment and HTT on rice straw (only at 200 °C) and concluded that HTT was a feasible pretreatment method for biomethane production. In another research, Du et al.11 tried a mild HTT condition (only at 80 °C) for enhanced biomethane production from rice straw. In practice, more detailed HTT operation conditions are definitely necessary for the development and design of the HTT process for enhanced methane production from rice straw. Moreover, when considering the feasibility of applying HTT at a large scale, energy balance analysis is a prerequisite. Rather than rice straw, energy balance has been previously conducted on other biomasses like microalgae and corn stalks. For instance, Passos and Ferrer analyzed energy balance for AD of hydrothermally pretreated thickened microalgal biomass, and their results show that 0.01 GJ/t net energy could be achieved at 130 °C which is higher than that (−0.38 GJ/t) of the untreated one.4 Adl et al. conducted energy balance of AD of hydrothermally pretreated cotton stalks and pointed out that HTT followed by AD is feasible and viable only at feed streams with solid content >10% based on energy recovery from biogasification.12 Restated, HTT has been revealed as a promising pretreatment method for energy recovery from biomass by using subsequent AD. However, little information can be found on energy balance analysis of HTT and subsequent methane production from hydrothermally pretreated rice straw, especially under different HTT temperature and holding time conditions. In this work, the effect of different HTT conditions on subsequent biomethane production from pretreated rice straw was investigated. In order to utilize HTT in practice, hydrothermal pretreatment on rice straw in addition to subsequent methane production of pretreated straw was analyzed according to energy evaluation in this study.
and HTT210−30, respectively. The temperature variation in the HTT reactor under each operation condition was also described elsewhere.13 Batch biochemical methane potential (BMP) experiments were carried out in triplicate in 100 mL glass bottles. After HTT was performed under the above-mentioned conditions (different peak temperature and holding time), the resultant substrate, a mixture of pretreated rice straw and soluble substances (hydrolysate) produced during the HTT process, was mixed homogeneously and sampled and then inoculated with seed sludge at an inoculation ratio of around 30% (TSseed sludge/TSsubstrate, dry weight basis). Sodium hydroxide (2 M) or hydrochloric acid was used to adjust the initial pH of each bottle to be around 7.0. Their final volume was made up to 80 mL by using deionized water before fermentation. All the fermentation bottles (shown in Supporting Information, Table S1) with initial total solid (TS) of 4.1−6.3% (volatile solid (VS)/TS = 73.3−78.4%) were sealed with silica gel stoppers and placed in a thermostat controlled at (35 ± 1) °C after their headspace was flushed with nitrogen gas for 2 min. These bottles were labeled as R150-x (using HTT150 pretreated rice straw) and R210-x (using HTT210 pretreated rice straw), respectively, according to their feedstock used. x denotes the holding time for hydrothermal pretreatment, i.e., 0, 10, 20, and 30 min, respectively. 2.3. Analytical Methods. Total solid (TS) and volatile solid (VS) were determined in accordance with the standard methods.14 Total organic carbon (TOC), total organic hydrogen (TOH), and total organic nitrogen (TON) of solid samples were measured by means of PerkinElmer 2400 CHN Elemental Analyzer (PerkinElmer, Japan). Soluble TOC (STOC) analysis was conducted with a TOC-V analyzer (Shimadzu, Japan). Volatile fatty acid (VFA) determination has been described elsewhere.13 Before and after fermentation, pH values of the substrate were measured using an FE20-Kit FiveEasy pH meter (Mettler Toledo, USA). During fermentation, daily biogas production was directly read from the scale on the syringe connected to the fermentation bottle after being shaken manually for 2 min. In order to make the results comparable with previous studies, all the results of biogas production (expressed as mL/g-VSadded) from the glass bottles were converted to standard temperature and pressure conditions (STP, 273.15 K and 101.325 kPa) and used for energy calculation. Biogas composition was analyzed by using a gas chromatograph (GC8A, Shimadzu, Japan) equipped with a thermal conductivity detector (80 °C) and a Porapak Q column (60 °C). Data were expressed as mean ± standard deviation (SD) for all the results from the above tests. In this work, total biogas or methane yield refers to the yield with the biogas or methane production from seed sludge being included, while actual biogas or methane yield from rice straw is the yield with the biogas or methane production from seed sludge being subtracted. As for the BMP results and the following energy recovery and balance analysis, the latter or the actual biogas or methane yields from rice straw or hydrothermally pretreated rice straw were used. 2.4. Calculation and Modeling. 2.4.1. Methane Production Performance. In this study, the effective biogasification period (τe, days) and averagely effective biogas production rate (re, mL/g-VS/d) as defined by Huang et al.15 were used to indicate the performance of each fermentation bottle, which can reflect the process duration and effectiveness for achieving 80% of total biogas production. In addition, the first-order kinetic model (eq 1) and Gompertz model (eq 2) were applied for modeling the methane production process from rice straw or hydrothermally pretreated rice straw, respectively.
2. MATERIALS AND METHODS 2.1. Rice Straw and Seed Sludge. Rice straw used in this study was collected from a farm field in Tsukuba (Ibaraki, Japan) and then air-dried and milled according to He et al.13 The seed sludge for AD was obtained from a semicontinuously operated mesophilic anaerobic digester in the lab (with a working volume of 2.5 L and retention time of 60 days) by using the raw straw. This reactor has been operated for 1 year before this test. The physical and chemical characteristics of raw rice straw and seeded anaerobic sludge are shown in Table 1. 2.2. HTT Procedure and Biochemical Methane Potential (BMP) Test. All the HTT experiments were conducted in triplicate as described by He et al.13 Two different peak temperatures (150 and 210 °C) were applied in HTT experiments at different holding time, i.e., 0, 10, 20, and 30 min, which were labeled as HTT150−0, HTT150−10, HTT150−20, HTT150−30, HTT210−0, HTT210−10, HTT210−20,
Table 1. Main Physicochemical Characteristics of Rice Straw and Anaerobic Digested Sludge (Seed Sludge) Used in This Work items
unit
rice straw
pH total solid (TS) volatile solid (VS, of TS) total organic carbon (TOC) soluble total organic carbon (STOC) total organic nitrogen (TON) total organic hydrogen (TOH)
% % % TS mg/L
n.d.a 90.11 ± 0.32 79.37 ± 1.07 35.18 ± 0.14 n.d.a
% TS % TS
0.66 ± 0.04 5.01 ± 0.18
a
seed sludge 7.52 1.16 69.02 33.07 190
± ± ± ± ±
0.11 0.02 0.53 0.12 27
G = GT[1 − exp(− kt )]
(1)
⎛ ⎛ μ(λ − t )e ⎞⎞ G = GT exp⎜⎜ − exp⎜ + 1⎟⎟⎟ ⎝ GT ⎠⎠ ⎝
(2)
where G (mL/g-VSadded) is the cumulative methane production; GT (mL/g-VSadded) is the total methane production; and t (days) is the operation duration. k (d−1) is the specific growth rate constant under designed methane fermentation condition calculated from the firstorder model; e is Euler’s number; and μ (mLCH4/g-VSadded/d) and λ
5.53 ± 0.06 5.25 ± 0.10
n.d., no determination. B
DOI: 10.1021/acs.energyfuels.7b01392 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels ⎛ 4c − h − 2o + 3n ⎞ ⎜ ⎟H O ⎝ ⎠ 2 4 ⎛ 4c + h − 2o − 3n ⎞ ⎛ 4c + h − 2o − 3n ⎞ ⎟CH + ⎜ ⎟ →⎜ 4 ⎝ ⎠ ⎝ ⎠ 8 8
(days) are the maximum methane production rate and lag phase time from the Gompertz model, respectively.16 2.4.2. Energy Balance. In order to evaluate the process scalability, energy balance of AD of rice straw with and without hydrothermal pretreatment was also analyzed. The parameters used for full-scale reactors were estimated from the above experimental data using the method modified from Passos and Ferrer.4 The daily disposal capacity of rice straw was assumed to be 1 ton in this work. The energy assessment included two aspects: (1) energy input mainly involving heat consumption by hydrothermal pretreatment and temperature maintaining for AD and (2) the energy output estimated from the methane production from the BMP tests. All the parameters used for energy balance analysis are summarized in the Supporting Information (Table S2). The energy consumption and production were estimated according to the following equations
CC HhOoNn +
CO2 + n NH3
TMP = 22.4 ×
((4c + h − 2o − 3n)/8) (1/g − Cc HhOoNn) 12c + h + 16o + 14n (8)
In order to estimate energy recovery, the heating value of rice straw (Q) was calculated according to the empirical heating value equation proposed by Dmitri Mendeleev20 as follows
Q = 4.187 × [81C + 300H − 26 × (O−S)] E in,HTT = [m w γw(Tp − Ta) + mr γr(Tp − Ta)] − [m w γw(Tp − Td)ϕ + mr γr(Tp − Td)ϕ]
energy recovery (%) =
(3)
(4)
where Ein, AD (MJ) is the heat consumption for AD, m (t) the digestate weight, γ (kJ/kg °C) the specific heat capacity of digestate and estimated to be the same as water (4.18 kJ/kg °C) in this study, Td (°C) the AD temperature (35 °C), and Ta (°C) the ambient temperature (25 °C in this study). Besides, 5% of the heat consumption by AD was estimated as heat loss.18 The energy output was calculated according to the methane yield from the BMP test according to eq 5. Eout = ηmPCH4ξCH4
Eout /E in =
Eout E in,HTT + E in,AD
× 100
(10)
Q = 4.187 × [81C + 300H − 26 × (O + N − S)]
(11)
3. RESULTS AND DISCUSSION 3.1. HTT Pretreatment for Enhanced Hydrolysis of Rice Straw. As reported elsewhere,13 the rice straw used contains 28.2% of cellulose, 17.5% of hemicellulose, and 14.6% of lignin. After HTT pretreatment, TS and VS contents of rice straw decreased under all tested conditions. The hydrothermal temperature has a significant effect on both TS and VS reduction of rice straw. 10−21% reductions of TS and 3−7% reductions of VS were achieved at HTT210, respectively. At HTT150, only 4−5% and 1−2% reduction in TS and VS were achieved. In addition, the main VFAs produced from hydrothermal hydrolysis of rice straw were acetate and propionate. In the hydrolysates from HTT150, VFAs were lower than 10 mg COD/g-VSadded, while HTT210 achieved VFAs about 40−82 mg COD/g-VSadded. When comparing HTT150 with HTT210, more acetate (45−68% of VFAs) was produced at HTT150, while more propionate (47−64% of VFAs) was detected in HTT210 hydrolysates. Meanwhile, it was found that HTT indeed opened the surface of rice straw with porous structure formed in the pretreated straw.13 3.2. Biochemical Methane Potential (BMP) Test. Figure 1 shows the cumulative methane production by using the resultant substrates from HTT pretreated rice straw during the 45 days’ BMP tests, and Table 2 summarizes the average performance of biogas and methane production from the pretreated rice straw in the subsequent AD after different HTT conditions.
(5)
where Eout (MJ) and PCH4 (m3CH4/t-VSadded) are output energy and methane yield from the AD system, respectively. ξCH4 (MJ/m3 CH4), m (t), and η (%) are lower heating value of methane, digestate weight, and energy conversion efficiency. The lower heating value (ξCH4) of methane was assumed to be 35.8 MJ/m3 CH419 with 90% of energy conversion efficiency being considered. Finally, results were expressed as energy balance or net energy gain (ΔE) and energy ratio (Eout/Ein) for both the control and HTT pretreated rice straw reactors. The energy balance was calculated as the difference between the energy output and energy input (heat and electricity, eq 6), while the energy ratio was calculated from the energy output over the energy input (heat and electricity, eq 7). ΔE = Eout − (E in,HTT + E in,AD)
Q
where Q (kJ/kg) is the heating value of rice straw through direct combustion, and QBMP (kJ/kg) is the heating amount of rice straw calculated from the above experimental BMP (through methane combustion). C, H, O, and S are the mass fractions of carbon, hydrogen, oxygen, and sulfur in the dry matter of rice straw, respectively. According to the results from elemental analysis, the mass fractions of organic C, H, N, and O (dry mass basis) were determined as 35.18%, 5.01%, 0.66%, and 38.55%, respectively, in the dry rice straw used in this study (Table 1). Due to the fact that S mass fraction is usually very low in rice straw (≤0.1%),21,22 which cannot be quantified by the elemental analyzer used in this work, S mass fraction was assumed to be 0.1% in all the related calculations. Besides, in order to well compare the estimation from eq 8 including the contributions from C, H, and N, eq 9 was slightly modified by introducing N fraction into O fraction as eq 11.
where Ein,HTT (MJ) is the heat consumption for HTT, mr (t) the disposal capacity of rice straw, mw (t) the water usage, γw (kJ/kg °C) the specific heat capacity of water (4.18 kJ/kg °C), γr (kJ/kg °C) the specific heat capacity of rice straw (1.67 kJ/kg °C),17 Tp (°C) the hydrothermal pretreatment temperature (150 or 210 °C), Ta the ambient temperature (25 °C in this study), Td (°C) the temperature for AD (35 °C in this study), and ϕ (%) the heat recovery efficiency (assumed at 85%). Because a short holding time (0−30 min) was applied13 and the out wall of the HTT reactor would be equipped with thermal insulation material when being utilized in practice, heat loss through the reactor wall during the HTT process was not accounted for in this work. E in,AD = mγ(Td − Ta) + 5%mγ(Td − Ta)
Q BMP
(9)
(6)
(7)
2.4.3. Theoretical Methane Production and Energy Recovery from AD of Rice Straw. According to carbon, hydrogen, oxygen, and nitrogen contents in rice straw, theoretical methane production (TMP) can be calculated according to the following chemical equation C
DOI: 10.1021/acs.energyfuels.7b01392 Energy Fuels XXXX, XXX, XXX−XXX
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Restated, R150s exhibited not only higher methane production potential but also much faster effective biogas production rate and shorter biogasification period. HTT210 for holding 0 min has been determined as the optimum condition for subsequent biohydrogen production, while a longer holding time (10−30 min) at HTT210 seemed to have negative effect on biohydrogen production. 13 In addition, compared to HTT210, HTT150 did not show an advantageous effect on biohydrogen fermentation. In the present study, all the HTT150 pretreatments rather than HTT210 have shown favorable subsequent biomethane production. Similarly, a longer holding time (10−30 min) at higher HTT temperature (HTT210) exerted negative effect on biomethane production. Generally, higher HTT temperature above 200 °C is considered as the threshold for subsequent methane production from the resultant hydrolysate.23−25 This phenomenon is probably attributed to the formation of phenolic compounds and furan derivatives under high HTT temperatures, which may inhibit the activity of methanogens.26,27 As reported by Budde et al.,28 ∼160 °C is a more appropriate HTT temperature than other tested conditions (200−220 °C) for biomethane production from cattle waste due to the abundance of inhibitors and other nondigestible substances produced under HTT at higher temperature, leading to lower methane yields. After checking the AD of pretreated sugar beet pulp at 120−200 °C, HTT at 160 °C achieved the highest methane yield, while HTT at 200 °C negatively affected methane production, most probably attributable to high concentration of p-hydroxybenzoic acid and other phenolic compounds which may inhibit methanogenesis.29 A similar phenomenon was also obversed by Hesami et al.8 who tested the effect of hydrothermal pretreatment on sunflower stalks for biogasification, indicating that HTT at temperature of 140−180 °C had a positive effect on methane yield which decreased when HTT temperature was elevated to 200 °C. According to eq 8, the theoretical methane production (TMP) of rice straw used in this study was calculated as 177 mL/g-VSadded. The experimental methane yields from HTT150 pretreated rice straws were about 45−60% of TMP (Table 2). Nevertheless, most of the experimental methane yields from HTT210 pretreated rice straws were less than 23% of TMP which were also much lower than that of the Control (46% TMP). This result was in accordance with effective biogas production rate and biogasification period, meaning that HTT210 to some extent had a negative effect on AD of rice straw for methane production.
Figure 1. Cumulative methane production from anaerobic digestion of the resultant substrate from hydrothermally pretreated rice straw. Data are expressed as the average values of triplicate tests.
As it can be seen, HTT150 pretreated rice straws exhibited much higher biogas and methane production potentials than HTT210 pretreated ones. The rice straw after HTT150 for 20 min (R150−20) attained the highest biogas yield of 289 mL/gVSadded with the maximum methane yield of 134 mL/g-VSadded. In comparison to the Control (rice straw without pretreatment), this hydrothermal pretreatment condition (HTT150− 20) achieved the highest increase in methane yield by 23% and biogas yield by 12%. It was found that 80% of total biogas production was completed within 27 days in all R150s. In contrast, only a very small amount of methane was detected from R210s during the first 20 days (Figure 1), and their correspondingly final methane yields varied from 28 to 68 mL/ g-VSadded (Table 2). Compared to the Control, R210s exhibited decreased biogas yield by 40−66% and decreased methane yield by 37−74%, respectively. The effective biogasification periods (τe) of R150s and R210s varied from 26−27 days to 27−38 days (Table 2), indicating that HTT210 pretreated rice straw needs longer biogasification period which might result from the byproducts produced during the HTT process. Besides, the effective biogas production rates (re) of rice straw after HTT150 and HTT210 were in the range of 9.2−10.7 mL/ g-VSadded/d or 3.2−4.7 mL/g-VSadded/d (Table 2), respectively.
Table 2. Average Performance for Methane Production from Hydrothermally Pretreated Rice Straw reactors control R150−0 R150−10 R150−20 R150−30 R210−0 R210−10 R210−20 R210−30
total biogas yield (mL/g-VSadded) 257 270 239 289 287 122 156 86 146
± ± ± ± ± ± ± ± ±
40 23 44 40 54 42 13 20 51
total methane yield (mL/g-VSadded)
τe (days)
re (mL/g-VSadded/d)
BMP of rice strawa (mL/g-VSadded)
BMPrice straw/ TMPrice strawb (%)
± ± ± ± ± ± ± ± ±
28 27 26 27 27 35 38 27 31
9.2 10.0 9.2 10.7 10.6 3.5 4.1 3.2 4.7
81 93 79 106 95 9 40 0 27
45.8 52.5 44.6 59.9 53.7 5.1 22.6 0.0 15.3
109 120 108 134 124 37 68 28 55
16 12 22 24 30 9 5 8 20
Subtracting the methane production from seed sludge, on average 28 mL CH4/g-VS from triplicate tests. The average final biogas production from seed sludge was about 70 mL/g-VS. bTMP, theoretical methane production. τe and re are effective biogasification period and effective biogas production rate, i.e., the duration and average biogas production rate up to 80% of total biogas production, respectively. a
D
DOI: 10.1021/acs.energyfuels.7b01392 Energy Fuels XXXX, XXX, XXX−XXX
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Table 3. Maximum Biogas Yields and Methane Yields from Crop Residues through Batch Anaerobic Digestion Tests after Hydrothermal Pretreatmenta methane fermentation conditions crop residues
inoculum
methane yield (mL/g-VSadded)
biogas yield (mL/g-VSadded)
pretreatment method
temperature (°C)
HRT (d)
initial pH
reference
35
45
7.0
187b
HTT at 150 °C for holding 20 min no HTT pretreatment
35
45
7.0
133c ---d
316c 286c
200 °C for 10 min 80 °C for 24 h
37 37
40 20
---d ---d
this work this work 10 11
296c
---d
35.1
45
---d
30
digested manure
396c
---d
55
60
---d
31
AD sludge
---d
144c
200 °C for 5 min + steam explosion 80 °C for 6 min + 180 °C for 15 min + 190 °C for 3 min 134 °C for 20 min
37
30
7.5
32
digestate of agricultural waste and sewage sludge
251c
---d
190 °C for 30 min
38
34
---d
33
rice straw
AD sludge
106b
219b
rice straw
AD sludge
81b
rice straw rice straw wheat straw
AD sludge digestate of swine manure AD sludge
wheat straw olive husks (mixed with olive mill wastewater and dairy wastewater) barley straw a
AD, anaerobic digested; HTT, hydrothermal treatment; HRT, hydraulic retention time which is fermentation duration in this table. bSubtracting the biogas or methane production from seed sludge. cNo indication about whether the yield was subtracted from seed sludge or not. dNo data in the literature.
Figure 2. Initial and final VFAs and STOC concentrations in the fermentation reactors using hydrothermally pretreated rice straw for methane production. VFAs, volatile fatty acids; HAc, acetic acid; HPr, propionic acid; HBu, butyric acid; STOC, soluble total organic carbon.
HTT has also been regarded as a promising pretreatment method for enhanced biogasification of other crop residues.30−32 Table 3 lists the maximum methane yields from crop residues with and without hydrothermal pretreatment. Compared with other HTT conditions (mainly relating to temperature and reaction time), the hydrothermal pretreatment at 150 °C in this study possesses the advantages including relatively low temperature, shorter duration, simple and easy operation, and lower energy input. Results from this study also indicate that moderate hydrothermal temperature rather than higher temperature above 200 °C is more appropriate for biomethane production from rice straw. Figure 2 illustrates the changes of initial and final VFAs and STOC concentration in the fermentation bottles before and
after methane production from HTT pretreated rice straw. The major initial VFA was found to be acetic acid after HTT150 pretreatment (before AD). Correspondingly, after high methane production from AD of the HTT150 pretreated rice straw, the concentrations of VFAs in all the R150s dramatically decreased from 486−662 mg COD/l to 69−148 mg COD/l. On the contrary, a large amount of VFAs were still detected in the R210s after the same fermentation duration (from 836−2594 mg COD/l to 1081−5153 mg COD/l), yielding much lower methane production. This phenomenon is most probably attributable to the inhibition effect of produced inhibitory substances at HTT210 on methanogens that cannot effectively consume VFAs generated from the previous AD stages with less methane production. E
DOI: 10.1021/acs.energyfuels.7b01392 Energy Fuels XXXX, XXX, XXX−XXX
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Table 4. Parameter Estimation from the First-Order and Gompertz Models for Methane Production of Rice Straw and Hydrothermally Pretreated Rice Straw reactors model
parameters
control
R150−0
R150−10
R150−20
R150−30
R210−0
R210−10
R210−20
R210−30
first-order
k (d−1) R2 μ (mL CH4/g-VSadded/d) λ (d) R2
0.080 0.864 4.14 2.44 0.931
0.092 0.887 5.20 3.79 0.950
0.097 0.905 4.86 3.66 0.962
0.090 0.902 5.75 3.95 0.967
0.090 0.907 5.32 3.95 0.967
0.050 0.579 1.39 17.24 0.786
0.046 0.547 2.53 17.70 0.823
0.087 0.795 1.43 12.06 0.924
0.084 0.716 2.82 13.76 0.885
Gompertz
Table 5. Energy Balance and Energy Recovery from Methane Production of Rice Straw with and without Hydrothermal Pretreatmenta reactors parameters
control
R150−0
R150−10
R150−20
R150−30
R210−0
R210−10
R210−20
R210−30
Ein, HTT (MJ) Ein, AD (MJ) Eout (MJ) ΔE (MJ) Eout/Ein energy from methane production (QBMP, MJ/t) energy recovery (%)b,c
0 1075 3512 2437 3.3 3287 23.5
501 1075 3866 2290 2.5 3733 26.7
501 1075 3480 1904 2.2 3246 23.2
501 1075 4317 2741 2.7 4301 30.7
501 1075 3995 2419 2.5 3896 27.8
667 1075 1192 −550 0.7 365 2.6
667 1075 2191 449 1.3 1623 11.6
667 1075 902 −840 0.5 0 0.0
667 1075 1772 30 1.0 1096 7.8
a
Heating value of rice straw (Q) was calculated as 14 000 MJ/t or 14 MJ/kg according to element contents of dried rice straw. QBMP (MJ/t) is the heating amount calculated from experimental BMP. bEnergy recovery (%) = 100 × QBMP/Q. cAll the energy calculations were conducted according to the results of BMP tests after being converted to standard temperature and pressure conditions (STP, 273.15 K and 101.325 kPa).
at HTT150. On the other hand, according to the results from the Gompertz model, the λ (lag-phase time) for R150s varied from 3.66 to 3.95 days, much shorter than those for R210s (12.06−17.70 days). This observation implies that HTT210 pretreated rice straw needs longer start-up time for methane fermentation. Moreover, the maximum specific methane production rate (μ) was 1.39−2.82 mL CH4/g-VSadded/d for HTT210 pretreated rice straw, much lower than those of Control (4.14 mL CH4/g-VSadded/d) and HTT150 pretreated rice straw (4.86−5.75 mL CH4/g-VSadded/d). These results indicate that HTT150 pretreatment could improve methane production from rice straw, while HTT210 may exert some negative effect on this process leading to the prolonged lagphase time and decreased methane production rate. Although HTT at higher temperature (e.g., HTT210 in this study) could enhance the solubility of rice straw, undesirable substances might be generated resulting in the decreased growth rate of methanogens and productivity of cells.26,27 Results from kinetic modeling further indicate that an appropriate HTT temperature is critically important for the enhanced methane production from rice straw and its AD efficiency. 3.4. Energy Balance and Energy Recovery. Whether HTT can be really applied for the full-scale AD of rice straw or not, energy evaluation is necessary. Table 5 summarizes the results of energy calculation according to the assumptions in sections 2.4.2 and 2.4.3. As can be seen, 1 ton of rice straw without HTT achieved 2824 MJ net energy gain (ΔE) (Ein,AD = 1075 MJ for maintaining AD, and Eout = 3512 MJ from methane production). After pretreatment at HTT150, the net energy gain (ΔE) was generally increased, achieving the highest ΔE (2741 MJ) at HTT150−20. Compared to the input energy (Ein = Ein,HTT + Ein,AD, 1576 MJ) for R150−20, the output energy (Eout = 4317 MJ) was 2.7 times the input energy (Ein). On the contrary, compared to the ΔE of Control (2437 MJ), the ΔE of HTT210 declined at least by 82%. Due to its increased energy input (667 MJ) for pretreatment and much
As for the variation of STOC, the initial STOC concentrations in R210s were much higher than those in R150s, and higher concentrations of STOC (2500−3900 mg C/l) remained after the 45 days’ methane production, too (Figure 2). This observation is possibly due to an incomplete methane fermentation (with longer lag time as discussed in 3.3) or inhibited methane fermentation process. In contrast, stable and low STOC concentrations (
