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Methane enhancement through sequential thermochemical and sonication pretreatment for corn stover with anaerobic sludge Muhammad Hassan, Muhammad Umar, Tursun Mamat, Furqan Muhayodin, Zahir Talha, Esmaeil Mehryar, Fiaz Ahmad, Weimin Ding, and Chao Zhao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00478 • Publication Date (Web): 19 May 2017 Downloaded from http://pubs.acs.org on June 5, 2017
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Methane enhancement through sequential thermochemical and
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sonication pretreatment for corn stover with anaerobic sludge
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Muhammad Hassan a, Muhammad Umar b, Tursun Mamat c, Furqan Muhayodin d,e,
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Zahir Talha a, Esmaeil Mehryar a,g, Fiaz Ahmad f, Weimin Ding a, * , Chao Zhao h, *
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a
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210031, China
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b
Department of Food Engineering, University of Agriculture, Faisalabad 38000, Pakistan
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c
School of Mechanical and Traffic Engineering, Xinjiang Agricultural University, Urumqi
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830052, China
College of Engineering, Nanjing Agricultural University, Nanjing, Jiangsu province
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d
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Pakistan
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e
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Germany
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f
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Pakistan
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g
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Technology Isfahan 84156, Iran
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h
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Engineering, Zhejiang A&F University, Linan, Zhejiang 311300, China
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*
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(Weimin Ding), Tel +86- 571-63746877,
[email protected] (Chao Zhao)
Department of Farm Machinery and Power, University of Agriculture, Faisalabad 3800,
Chair of Thermal Process Engineering, Otto von Guericke University, Magdeburg 39106,
Department of Agricultural Engineering, Bahauddin Zakariya University, Multan 60800,
Department of Biosystem Engineering, College of Agriculture, Isfahan University of
National Engineering Research Center for Wood-based Resource Utilization, School of
Corresponding author: Tel +86-25 5860 6502, E-mail address:
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Graphical abstract
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Abstract
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The present research emphasized the utilization of a novel sequential thermochemical and
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sonication pretreatment technology to enhance methane production from corn stover. The
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corn stover was thermo-chemically pretreated with sodium hydroxide to enhance its
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lignocellulosic digestibility. Due to thermo-chemical pretreatment, 65.45 % lignin removal
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and 36.33 % hemi-cellulose solubilization was observed and further five sonication levels
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were employed (25min, 45min, 60min, 90min and 120min). All pretreatments were found
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significant (P< 0.05) to enhance methane production from 14.78 % to 73.72 % while
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thermo-NaOH pretreatment with 90 minutes sonication time was proved as the optimum
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pretreatment with specific methane production of 320 mL/g.VS. Anaerobic digestion
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process stability was deeply monitored at each three days interval like TVFAs, alcohol
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production, pH, CODs and VS removal.
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words:
Sonication
pretreatment,
Methane
enhancement,
Corn
stover,
1
Key
2
Thermochemical pretreatment.
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1. Introduction
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The sustainable energy resource is the main fields of socio-economic interest due to
5
limited supply and resources of fossil fuels worldwide. The biogas production from
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agricultural waste is considered as green energy resource and has greater importance in
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contrast to other renewable resource like solar and wind power 1. The limited supply of
8
fossils and arising global warming situation has strongly diverted the researchers in
9
alternative renewable energy resources
2, 3
. The condition becomes very challenging to
10
cope with the rising energy demand in the rural sector of the developing countries; hence
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lignocellulosic biomasses had emerged as an environmental friendly option with onsite
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solution of green energy production during last decades. Lignocellulosic biomasses are an
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abundant agricultural resource found worldwide with an annual growth of 170-200 billion
14
tons
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million metric tons of corn stover 5.
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Anaerobic digestion (AD) is widely adopted waste treatment technology on commercial
17
scale that finally produces biogas, consisting 50 % to 70 % methane with large reduction in
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sludge volume
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energy production but it also proved helpful to reduce the risk of environmental pollution
20
and greenhouse gas emissions
21
acetogenesis and methanogenesis
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and China as being the leader of agricultural production, has annual yield of 216
2, 6
. Anaerobic digestion is not only a useful method for the renewable
7-9
. It consisted of four steps; hydrolysis, acidogenesis, 10-13
and in case of lignocellulosic fed material to
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anaerobic digestion, hydrolysis is considered as the rate limiting stage during the anaerobic
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digestion process
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macromolecular structure of plant tissues could take place to easily digestible products like
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peptides, amino acids, alcohol, proteins and fatty acids. However, there are some
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drawbacks of using corn stover as direct AD feed material on commercial scale, because
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they resulted in poor anaerobic digestion efficiencies with less volatile solids reduction and
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methane production
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contains high contents of lignin and complex hemi-cellulosic texture that retarded the
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hydrolysis stage of the AD process 5, 16, 20.
5, 14-17
. During the hydrolysis stage, the transformation of complex
18, 19
. The phenomena could be well emphasized that the corn stover
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To overcome this problem, different physical, chemical, thermo-chemical and biological
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pretreatment for corn stover were reported in the literature 5, 11. The major objective of all
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these pretreatments was to reduce the lignin barrier, crystallinity of cellulose and ease
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hydrolysis during the AD process 5, 21. The main factors that can influence the selection of
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suitable pretreatment method are the effectiveness, feasibility and cost. In utilizing the
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ultrasonic pretreatment, the initial cost is significantly higher than the other pretreatment
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methods, but it was found more advantageous over other pretreatments, because of zero
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chemical accumulation in the downstream of the AD process. As conclusion, a significant
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potential of the ultrasonic pretreatment was found in the case of corn stover and other
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lignocellulosic biomasses to enhance their cellulosic accessibility 22.
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Ultrasounds frequencies of higher than 20 kHz produces cavitation’s phenomena in
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cellulosic texture, furthermore, these forces disrupt the cell wall and binding tissues in the
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liquid state that leads to enhanced hydrolysis rate of the biomass 23. The sonication energy
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consisted of cyclic and pressurized sound that produces cavities and raptures in the cell
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walls, as a result of combination of shearing forces, chemical radical generation and
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transformations, loosening of the fibrous network took place. During the sonication
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process, micro bubbles are produced that results in violent collapses and intense specific
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energy transfer to the interior texture of the corn stover. The sonication activities results in
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production of free radicals like hydroxyl (OH·), hydroperoxy (HOO·) that helps further to
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release more volatile and accessible products from the substrate 24. Ultrasound emerged as
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a leading pretreatment technology because of its improved substrate bio-degradability,
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increased methane contents in biogas, environmental friendly/no chemical usage and 25, 26
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improved digestion period
. In the literature, sonication was mostly referred for
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micro/macro algae, waste activated sludge, waste water, oil mil effluents and landfill
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leachate and significant research gap was found to study the sonication future for
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lignocellulosic biomasses prior to anaerobic digestion process.
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However, sonication pretreatment was found quit fit and reported for the liquid based
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waste products like waste water and industrial effluents but for the lignocellulosic
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biomasses only sonication pretreatment proved least effective. Another well-known
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pretreatment called thermo-chemical pretreatment, have its own importance in lignin
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removal, acetyl-aryl linkage swellings, reduced cellulosic crystallinity and hemicellulosic
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solubilization from the corn stover 5. If thermo-chemical and sonication pretreatments were
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combined together, the higher methane production could be achieved. In the literature, a
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few studied were reported, like sonication was combined with the thermal pretreatment for
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waste activated sludge and cumulative methane production was found 13.6 % enhanced
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than the control
. Another study reported more than 100 % methane enhancement from
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wheat straw when hydrodynamic cavitation and KOH pretreatment was combined together
3
28
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pretreatment was carried out for the dairy waste activated sludge 29.
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Therefore, in the present study, thermo-NaOH followed by sonication pretreatment was
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carried out to enhance lignocellulosic digestibility and methane production capability of
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the corn stover. The main objective of the present study was to enhance the methane
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production from corn stover. The process biochemistry and stability like total volatile fatty
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acids production, alcohol production, soluble chemical oxygen demand and pH values
. Rani et al., reported methane enhancement of up to 80 %, when sono-alkalization
10
were deeply monitored during the anaerobic digestion period.
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2. Material and methods
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2.1. Substrate collection and sludge activation
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Corn stover was collected from Pukou agricultural research station, Nanjing, China. It was
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dried in open sunshine for fortnight after that it was oven dried at 105 °C for 24 hours. The
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dried corn stover was chopped into small pieces and further grinded by desk type
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continuous miller (LH-08B, China), further wards, it was sieved through 1mm sieve to
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achieve the homogeneous size of corn stover. This mechanical pretreatment supported the
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thermochemical pretreatment to achieve the maximum available surface area of the corn
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stover to achieve maximum homogeneity. The chemical composition of the corn stover
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was determined and results were presented in Table 1.
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Table 1: Chemical composition of corn stover (CS) and seed sludge Parameters
Units
CS untreated
TS VS TN TP TK TOC OM C/N Cellulose Hemi cellulose Lignin pH CODs TVFAs
% % % % % % % % % % mg/L mg/L
99.17±0.04 97.26 ±0.01 0.72±0.01 0.30±0.12 0.16± 0.01 42.31±2.20 72.95±0.04 59.10±0.35 39.63±0.96 27.13±0.02 9.53±0.17 -
CS Pre-treated with Thermo-NaOH 98.62±0.48 98.03 ±0.01 0.80±0.03 31.82±0.86 52.90±0.08 40.03±0.69 45.12±1.18 19.90±0.38 5.76±0.29 -
seed sludge 2.87±0.18 37.28±0.11 3.38±0.13 1.78±0.24 2.13±0.23 34.28±0.43 59.11±0.13 10.14±0.45 7.67±0.02 4021.33±112 344.56±10.23
2
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The sludge was collected from a working hog manure feed biogas plant located in the
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vicinity of Pukou, Nanjing, China. The waste sludge was collected and stored in 20 liter
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vessel for activation process. It was fed with daily glucose dose of 2g/L per day for about
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fortnight 5. This process was adopted to make sure the maximum methanogenic population
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growth. After that the sludge was passed through a polyester screen of 1mm to remove all
8
the grits and foreign substances. Chemical characteristics of the waste activated sludge
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were measured and presented in Table 1.
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2.2. Thermo-chemical and sonication pretreatment details
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To increase the anaerobic performance and methane production from the corn stover, two
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stage pretreatment was carried out. Thermo-NaOH pretreatment details and selected
13
concentration level of the NaOH were based on the previous research works 5, 30. The corn
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stover was homogenized with deionized water in the ratio of 1:6 on weight basis in an
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Erlenmeyer flask and heated at 80 °C for 15 minutes. Afterwards, 7.5 % NaOH
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added on weight basis into the flask as pretreating agent and it was autoclaved at 100 °C
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for 20 minutes. The thermochemical pretreatment was carried out according to the
5
previous work 5. Pretreatment after effects of the corn stover were analyzed and the results
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were presented in Table 1 with details. The pretreated corn stover was cooled down and
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used further for sonication. The sonication pretreatment was carried out by a bath type
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sonicator (Model-KQ5200DE, China) with an ultrasonic power of 200W and sonication
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frequency of 40 kHz. The sonication was carried out at five different time levels, (25, 45, 31
5
was
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60, 90 and 120 min) on the basis of a previous study
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presented in Table 2. The only sonication and thermo-NaOH pretreatment of the corn
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stover was also prepared in order to compare the other treatments while the untreated corn
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stover was termed as control. Specific input energy applied (Ei) during pretreatment was
14
determined by equation 132 while specific output energy (Eo) was calculated by using
15
equation 2 33.
16
E (/ ) =
()× () ( !)
and sonication details were
/10$ ………………1
17
%& (/ ) = ∆()*+ (,-/ . ) ∗ € (1/,$ )/102 ……………2
18
Specific input energy (Ei) applied during pretreatment was the summation of energy
19
utilized during mechanical, thermal and ultrasonic pretreatments, while ∆PCH4 was the
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extra methane production in ml/gVS due to pretreatment as compared with control and €
21
was the lower heating value of methane (35,800 kJ/m3CH4).
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2.3. Batch AD experiments
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One liter laboratory fabricated batch anaerobic digesters were used to evaluate the
3
significance of all the pretreatments in triplicates. The pretreated corn stover was co-
4
digested with anaerobic sludge. The complete mechanism of the anaerobic digesters was
5
shown in Figure 1. The activated sludge was also digested separately for measuring the
6
exact methane production from all the treatments. Each digester contained 700 mL of
7
working volume and 5 % TS level was maintained into each digester. Mesophilic (37±1)
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°C anaerobic digestion was carried out for 40 days. Brine saturated water displacement
9
principle was used to measure the biogas production. Stratification was avoided by manual
10
agitation which was adopted on the daily basis. Biogas production and composition was
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measured on daily basis while the liquid digestate was withdrawn from each digester to
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measure the process chemistry parameters like TVFAs (total volatile fatty acids), alcohol,
13
CODs (chemical oxygen demand) and pH during the whole anaerobic digestion cycle.
14 15
Figure 1: Batch anaerobic digester mechanism
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2.4. Sampling and analytical techniques
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The all parameters like TS, VS, TN, TP, TK, CODs, TOC and OM were measured
3
according to the standard methods prescribed by APHA 2006
4
determined by GC (Renhua, GC 9890A, China) equipped with thermal conductivity
5
detector (TCD), (Ф4mm×1 m, Shimadzu, Japan), while hydrogen was the carrier gas
6
during the biogas analysis. The oven, injector temperature was kept 150, 100, 120 °C
7
respectively and 0.5 mL of the biogas sample was injected. Total volatile fatty acids and
8
alcohol concentration was determined by the GC-2014, Shimadzu, Japan equipped with
9
TCD and column specification of (DA- Stabilwax, 30 m×0.53 mm×1µm). The injector and
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detector temperature of the GC was kept at 150 °C and 240 °C respectively. The pH was
11
measured by the digital pH meter Mettler-Toledo (FE20K, Mettler-Toledo, Switzerland).
12
Lignocellulosic composition of the corn stover was determined by the standard protocol
13
described by Van Soest
14
Scientifica Company, Italy). The CODs removal was determined by the equation proposed
15
by 5 and the volatile solids removal was calculated by the equation 3 36.
16
35
34
. Methane contents were
and raw fiber extractor was used for this purpose (VELP
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