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Evaluation of electricity generation from lignin residue and biogas in cellulosic ethanol production
Accepted Manuscript Short Communication Evaluation of electricity generation from lignin residue and biogas in cellulosic ethanol production Gang Liu, Jie Bao PII: DOI: Reference:
S0960-8524(17)31108-2 http://dx.doi.org/10.1016/j.biortech.2017.07.022 BITE 18441
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
15 June 2017 29 June 2017 4 July 2017
Please cite this article as: Liu, G., Bao, J., Evaluation of electricity generation from lignin residue and biogas in cellulosic ethanol production, Bioresource Technology (2017), doi: http://dx.doi.org/10.1016/j.biortech. 2017.07.022
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Short Communication manuscript submitted to Bioresource Technology
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Evaluation of electricity generation from lignin residue and biogas in cellulosic ethanol
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production
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Gang Liu, Jie Bao*
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State Key Laboratory of Bioreactor Engineering, East China University of Science and
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Technology, 130 Meilong Road, Shanghai 200237, China
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* Corresponding author.
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Jie Bao, Tel/fax: +86 21 642151799, [email protected]
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Abstract
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This study takes the first insight on the rigorous evaluation of electricity generation
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based on the experimentally measured higher heating value (HHV) of lignin residue, as well
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as the chemical oxygen demand (COD) and biological oxygen demand (BOD5) of wastewater.
18
For producing one metric ton of ethanol fuel from five typical lignocellulose substrates,
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including corn stover, wheat straw, rice straw, sugarcane bagasse and poplar sawdust,
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1.26-1.85 tons of dry lignin residue is generated from biorefining process and 0.19-0.27 tons
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of biogas is generated from anaerobic digestion of wastewater, equivalent to 4,335-5,981 kWh
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and 1,946-2,795 kWh of electricity by combustion of the generated lignin residue and biogas,
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respectively. The electricity generation not only sufficiently meets the electricity needs of
24
process requirement, but also generates more than half of electricity surplus selling to the
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grid.
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Keywords: Electricity generation, Lignin residue, Biogas, Cellulosic ethanol, Lignocellulose
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1. Introduction Lignocellulose biomass is the most important feedstock option to compete with starch
5
and sugar feedstock for production of biofuels and bulk chemicals (Wyman and Dale, 2015).
6
Currently, the lignocellulose biorefining technology is in the developing stage. The energy
7
and water balance is obviously unfavorable to that of the mature corn ethanol production in
8
consuming more fresh water, steam, electricity (McAloon et al., 2000; Aden, 2007; Balan,
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2014), and generating more wastewater (Wingren et al., 2008; Sassner and Zacchi, 2008b;
10
Martin and Grossmann, 2011). Lignocellulose biorefining also lacks opportunity to recover
11
costs from high added value byproducts that may be available with other substrates such as
12
distillers dried grains with soluble (DDGS) from corn fermentation. On the other hand, lignin
13
residue generation adds a high credit of electricity to the cellulosic ethanol production by
14
combustion of lignin residue after cellulose and hemicellulose are converted into ethanol
15
(Humbird et al., 2011). Biogas (methane) from the anaerobic digestion of wastewater from
16
ethanol distillation provides an additional gas fuel for electricity generation (Humbird et al.,
17
2011). The electricity generation credit could balance the electricity consumption in cellulosic
18
ethanol production process and also sell the surplus electricity to grid.
19
In biorefining process, cellulose and hemicellulose in lignocellulose feedstock are
20
enzymatically hydrolyzed into fermentable sugars and then fermented into ethanol or
21
biochemicals, while the lignin portion is left behind as solid residue after ethanol is distillated
22
and water is filtrated from the residue. The higher heating value (HHV) of the pure lignin is
23
26.7 GJ per metric ton, which is competitive or even better than the raw coal (20.9 GJ/ton)
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(Archambault-leger et al., 2015). The practical lignin residue contains unconverted cellulose
25
and hemicellulose, fermenting microorganisms and inert solids, but its heating value is still
2
1
high enough to be used as boiler fuel. Wastewater is generated from the distillation of ethanol
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fermentation broth followed by a liquid-solid filtration. The wastewater is anaerobically and
3
aerobically digested to remove the chemicals indicated by chemical oxygen demand (COD)
4
value and generate biogas. Both the biogas and the sludge cake are capable of combustion use
5
for electricity generation in a combined heat and electricity facility.
6
Although the lignin residue and biogas demonstrated great potentials for electricity
7
generation, the previous studies did not deliver the rigorous evaluation based on the
8
experimentally measured data of practical lignin residues or biogas. For lignin residue, only
9
the HHV value of pure lignin component was applied for evaluation of electricity generation,
10
instead of the experimentally measured heat values using the practical lignin residues, the
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practical lignocellulose feedstocks, and the practical processing of cellulosic ethanol
12
production (Cardona and Sanchez, 2006; Archambault-leger et al., 2015). For biogas, the
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COD and BOD data were rarely experimentally measured and usually the rough estimation
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was made based on the limited COD data (Steinwinder et al., 2011; Humbird et al., 2011).
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In this study, the cellulosic ethanol production from five typical lignocellulose biomass
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including corn stover, wheat straw, rice straw, sugarcane bagasse and poplar sawdust was
17
performed by dry acid pretreatment and biodetoxification (DryPB), followed by a high solids
18
loading simultaneous saccharification and co-fermentation (SSCF) to achieve the high ethanol
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conversion efficiency (approximately 11% of ethanol titer by volumetric percentage in the
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fermentation broth). The mass balance of cellulosic ethanol production was calculated as the
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basis of electricity generation calculation. The higher heating values (HHV) of the lignin
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residues, the COD and BOD5 values of the wastewater from the distillation stillage from the
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five representative lignocellulose feedstocks were experimentally measured according to the
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standard methods. Based on these data, the electricity generation from lignin residue and
25
biogas were rigorously calculated and analyzed. This study took the first insight on the
3
1
rigorous evaluation of electricity generation in cellulosic ethanol production. The results
2
provided the important data for designing of commercial scale cellulosic ethanol plant and
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evaluating cellulosic ethanol economy.
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2. Materials and methods
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2.1. Raw materials
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Five lignocellulose feedstocks were collected from their dominant growing regions in
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China. Corn stover was harvested from Bayan Nur, Inner Mongolia, China in fall 2015.
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Wheat straw was harvested from Dan Cheng, Henan, China in fall 2013. Rice straw was
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harvested from Chang Zhou, Jiangsu, China in fall 2014. Sugarcane bagasse was obtained
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from the sugar plant of Bei Hai, Guangxi, China in fall 2014. Italian poplar sawdust was
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obtained from the wood factory in Yan Cheng, Jiangsu, China in fall 2015. The field dirt,
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sands, metal pieces and other impurities were carefully avoided during the collection and then
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screened during the prehandling. The collected corn stover, wheat straw, rice straw and
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sugarcane bagasse were ground coarsely using a beater pulverizer and screened through a
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mesh with the circle diameter of 10 mm. Poplar sawdust was used directly without grinding.
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The composition of the lignocellulose feedstocks was measured by the two-step acid
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hydrolysis method according to National Renewable Energy Laboratory (NREL) protocols
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(Sluiter et al., 2008; 2012). On the dry weight base (w/w), corn stover contained 35.4% of
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cellulose, 24.6% of hemicellulose, 16.1% of lignin, and 3.5% of ash; wheat straw contained
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38.7% of cellulose, 25.9% of hemicellulose, 14.9% of lignin, and 5.2% of ash; rice straw
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contained 35.3% of cellulose, 18.4% of hemicellulose, 22.5% of lignin, and 9.3% of ash;
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sugarcane bagasse contained 38.8% of cellulose, 23.9% of hemicellulose, 26.4% of lignin,
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and 1.3% of ash; poplar sawdust contained 39.7% of cellulose, 16.6% of hemicellulose, 29.4%
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of lignin, and 3.2% of ash.
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2.2. Dry acid pretreatment and biodetoxification (DryPB) biorefining process
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1
Ground lignocellulosic biomass is pretreated using the dry acid pretreatment method at
2
175 oC for 5 min with 2.0 g sulfuric acid per 100 g dry biomass in the 20 L pretreatment
3
reactor (Zhang et al., 2011; He et al., 2014a; 2014b). All the dilute acid solution and the
4
condensed water were completely adsorbed into the solids to form approximately 50% (w/w)
5
of the dry pretreated feedstock solids with the pH around 2.0, and no free wastewater stream
6
was generated. The sulfuric acid in the pretreated biomass solids was neutralized to 5.5 by the
7
addition of 20% (w/w) Ca(OH)2 suspension slurry. The pretreated biomass solids were briefly
8
milled by a disk milling machine (PSB-80JX, Fleck Co., Nantong, Jiangsu, China) to remove
9
the extra-long fibers to avoid the blockage of pipelines and valves in the downstream flow of
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the hydrolysate slurry and broth. Then the pretreated solids were aerobically biodetoxified using Amorphotheca resinae
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ZN1 in a 15 L bioreactor to remove the inhibitors generated during the dry acid pretreatment
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operation at 28 oC the water saturated aeration of 0.8 vvm for 36-48 h (Zhang et al., 2010a; He
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et al., 2016). The major inhibitors were completely assimilated and degraded. Xylose and
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glucose released during the pretreatment were preserved without observable loss. And no
16
cellulose degradation was observed during the biodetoxification period.
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The pretreated and biodetoxified lignocellulose feedstock solids were enzymatically
18
hydrolyzed into liquid hydrolysate slurry containing both mono and oligomer sugars in the
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specially designed 5 L bioreactors equipped with helical ribbon impeller at 50 oC, pH 4.8 and
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150 rpm for 12 hours (Zhang et al., 2010b). Then glucose and xylose were co-fermented into
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ethanol simultaneously with the hydrolysis of cellulose and oligomer sugars at high solids
22
loading (30%, w/w) and low cellulase dosage (10 mg protein/g cellulose) at 30 oC for 96
23
hours by inoculating the shortly adapted yeast seed cells S. cerevisiae XH7 into the
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hydrolysate at 10% (v/v). pH was maintained at 5.5 by adding 5 M NaOH solution.
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Fermentation broth from corn stover, wheat straw, rice straw, sugarcane bagasse, or
5
1
poplar sawdust was directly distillated to recover ethanol to about 35% (w/w) distillate in a
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glass distillation equipment with the inner column diameter of 40 mm filling with stainless
3
theta ring packings (3 mm in diameter). The stillage from the distillation column was filtrated
4
by a press filter machine through a fabric cloth to yield the wastewater stream and the lignin
5
residue cake.
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2.3. Determination of higher heating value, COD and BOD5
7
Lignin residue cake with the thickness of 4 mm contained approximately 65% (w/w) of
8
dry solids. It was dried at 105 oC until constant weight and sent for determination of higher
9
heating value on a calorimeter (MMC 274 multi-module, Netzsch Co., Fresitaat Bayerm,
10
Germany) according to the China National Standard Method GB/T213-2003 (Determination
11
of Calorific Value of Coal).
12
The wastewater stream was sent for determination of chemical oxygen demand (COD)
13
and biochemical oxygen demand after 5 days (BOD5). The COD of the wastewater was
14
determined according to the China National Environmental Protection Standard Method
15
HJ/T399-2007 (Water Quality Series, Determination of Chemical Oxygen Demand, COD).
16
Briefly, the oxidation was carried out at 165 oC for 15 min in a digester (Hach DRB200, Hach
17
Inc, NY, USA) and the absorbance was measured at 600 nm on a spectrophotometer (Hach
18
DR5000, Hach Inc, NY, USA). Potassium dichromate was used as oxidizing agent and
19
potassium biphthalate was the control. The BOD5 of the wastewater from the same source was
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determined according to the China National Environmental Protection Standard Method
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HJ505-2009 (Water Quality Series, Determination of Biochemical Oxygen Demand after 5
22
Days, BOD5). Briefly, the wastewater was cultured at 20 oC for 5 days in glass bottles in an
23
anaerobic flask inoculated with the standard microbe seed and then the dissolved oxygen
24
consumption was determined.
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3. Results and discussion
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1 2
3.1. Mass balance in cellulosic ethanol production Corn stover, wheat straw, rice straw, sugarcane bagasse, and poplar sawdust were dry
3
acid pretreated and biodetoxified to yield the inhibitor free and high xylose containing
4
feedstock for simultaneous saccharification and co-fermentation (SSCF). SSCF from the five
5
feedstocks achieved high ethanol titer of 85.1, 87.0, 71.9, 78.3, and 79.1g/L (9.1%-11.0% in
6
volumetric percentage), respectively (Liu et al., 2017). Ethanol in the fermentation broth was
7
distillated. The mixed lignin residue and wastewater slurry was recovered as the bottom
8
stream of the distillation column. The slurry was filtrated to get the lignin residue cake and
9
wastewater liquid. The mass balance of biomass, water, ethanol and lignin residue in the
10
biorefining process was calculated based on the experiment results (Fig. 1). Processing one
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metric ton lignocellulosic feedstocks, ethanol yield is 0.215-0.260 ton, in which the highest
12
one is 0.260 ton ethanol per ton of wheat straw and the lowest is 0.215 ton of ethanol per ton
13
of rice straw, mainly due to the different cellulose and xylan content among the lignocellulose
14
feedstocks. Meanwhile, 0.546-0.658 ton of lignin residue (with the water content of 35%) and
15
2.204-2.250 tons of wastewater are generated from one metric ton of feedstocks. On the basis
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of ethanol fuel product, production of one metric ton of ethanol approximately generates
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1.26-1.85 tons of dry lignin residue and 8.66-10.27 tons of wastewater.
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3.2. Electricity generation form lignin residue
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The higher heating values (HHV) of lignin residues were experimentally determined
20
according to the standard methods and the equivalent electricity generation was calculated
21
(Table 1). The HHV values are in the range of 17.12-18.88 GJ per dry ton of lignin residues
22
from corn stover, wheat straw, rice straw, sugarcane bagasse, or poplar sawdust. The HHV
23
values are approximately 70% of the pure lignin (26.7 GJ/ton), and 85% of the raw coal (20.9
24
GJ/ton), due to the existence of the impurities such as the unreacted cellulose, hemicellulose,
25
residual sugars, inert soil solids and minor amount of fermenting yeast cells and cellulase
7
1 2
enzyme proteins. Production of one metric ton of fuel ethanol generates 1.26-1.85 tons of lignin residues
3
(dry base) according to the mass balance (Fig. 1). Combustion of 1.26-1.85 tons of lignin
4
residue generates 22.95-31.66 GJ of higher heating values. 1 GJ of heating value is equivalent
5
to 277.8 kWh of electricity based on the theoretical energy equivalence. The boiler efficiency
6
(80%) and the generator efficiency (85%) of the turbine shaft conversion to electricity using
7
the lignin residue fuels are cited from Humbird et al (2011). Therefore, practically 1 GJ of the
8
higher heating value generates 277.8 0.85 0.80 = 188.9 kWh of electricity, and
9
22.95-31.66 GJ of higher heating value generates 4,335-5,981 kWh of electricity. In summary,
10
for producing one metric ton of ethanol fuel, 4,335-5,981 kWh of electricity is generated by
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combustion of the correspondingly generated lignin residue.
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3.3. Electricity generation form biogas which produced after wastewater treatment
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The wastewater was collected after the filtration of the stillage liquid and the chemical
14
oxygen demand (COD) and the biological oxygen demand after 5 days (BOD5) were
15
experimentally determined (Table 2). The COD values of the wastewater are in the range of
16
80-137 g/L for the five lignocellulose feedstocks. The BOD5 values are in the range of 19-44
17
g/L for the feedstocks. These values are in agreement of the NREL data (87 g/L of COD)
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(Humbird et al., 2011) and in the general range of fermentation wastewater (50-120 g/L of
19
COD and 20-60 g/L of BOD5) (Jin and Zhao, 1987). The ratio of COD/BOD5 is in the range
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of 0.22-0.39 and fit the general ratio for anaerobic digestion (> 0.3) (Steinwinder et al., 2011).
21
Approximately 0.35 standard cubic meter of biogas or 0.27 kg of methane is obtained
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from 1 kg of COD removed in the anaerobic digestion of the stillage wastewater (Wingren et
23
al., 2008). Production of one metric ton of ethanol generates 0.19-0.27 ton of biogas
24
according to the mass balance (Fig. 1) resulted from the 91% reduction of the COD in
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anaerobic digestion (Humbird et al., 2011). Combustion of 0.19-0.27 tons of biogas generates
8
1
10.30-14.80 GJ of higher heating values based on the HHV of methane of 55.5 GJ/ton.
2
Similar to lignin residue, 1 GJ of heating value generates 188.9 kWh of electricity, and
3
10.30-14.80 GJ of higher heating value generates 1,946-2,795 kWh of electricity. In summary,
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for producing one metric ton of ethanol fuel, 1,946-2,795 kWh of electricity is generated by
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combustion of the correspondingly generated biogas form wastewater.
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Combining lignin residue and biogas, the electricity generation from the five typical
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lignocellulose biomass is in the range of 7,121-8,180 kWh for producing one ton of ethanol in
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the present dry acid pretreatment and biodetoxification (DryPB) process. The general energy
9
requirement is 12 GJ of steam (Hinman et al., 1992; Wingren et al., 2008; Piccolo et al., 2009)
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and 1,306 kWh of electricity (Humbird et al., 2011) for producing one metric ton ethanol.
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Translating the steam heating into electricity, the total electricity consumption is around 3,500
12
kWh for producing one ton ethanol. Therefore, the electricity generation not only sufficiently
13
meets the electricity need of process requirement, but also generates more than half of the
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electricity surplus for selling to the grid.
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4. Conclusion
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Production of one ton of cellulosic ethanol generates 1.26-1.85 tons of dry lignin residue
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and 0.19-0.27 tons of methane after ethanol recovery from the five typical lignocellulose
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biomass. The electricity generation from combustion of lignin residue and biogas is in the
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range of 7,121-8,180 kWh per ton of ethanol produced in the present DryPB process. The
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electricity generation sufficiently meets the cellulosic ethanol production needs and generates
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surplus to the grid.
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Acknowledgements
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This research was partially supported by the National Basic Research Program of China
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(2011CB707406).
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Table 1 Electricity generation from lignin residue combustion Lignin residue generation
Higher heating value of lignin residue
Electricity equivalent a
(ton/ton dry solids)
(ton/ton ethanol)
(GJ/ton dry lignin residue)
(kWh/ton ethanol)
Corn stover
0.384
1.51
18.878
5,385
Wheat straw
0.355
1.26
18.215
4,335
Rice straw
0.428
1.85
17.116
5,981
Sugarcane bagasse
0.395
1.57
17.979
5,332
Poplar sawdust
0.388
1.52
17.992
5,166
Feedstock
a
Boiler efficiency (80%) for lignin residue or methane combustion and the generator efficiency of the turbine shaft conversion to electricity
(85%) were cited from Humbird et al (2011). 1 GJ of heating value is equivalent to 277.8 kWh of electricity.
13
Table 2 Electricity generation from biogas combustion after wastewater anaerobic digestion Feedstock
Wastewater generation
Chemical oxygen
Biological oxygen Methane
Electricity
demand (COD)
demand (BOD5)
generation
equivalent a
(g/L)
(kg/ton ethanol)
(kWh/ton ethanol)
(ton/ton dry solids)
(ton/ton ethanol)
Corn stover
2.227
8.768
124
44
267
2,795
Wheat straw
2.250
8.664
137
30
266
2,786
Rice straw
2.204
10.271
80
29
186
1,946
Sugarcane bagasse
2.216
9.481
113
19
242
2,538
Poplar sawdust
2.227
9.433
97
38
207
2,166
a
(g/L)
The boiler efficiency of the lignin residue or methane feedstocks (80%) and the generator efficiency of the turbine shaft conversion to electricity
(85%) are cited from Humbird et al (2011). 1 GJ of heating value is equivalent to 277.8 kWh of electricity.
14
Corn Stover: 0.391 t Wheat Straw: 0.382 t Rice Straw: 0.459 t Bagasse: 0.426 t Poplar Sawdust: 0.422 t
Corn Stover: 1.673 t Wheat Straw: 1.673 t Rice Straw: 1.673 t Bagasse: 1.673 t Poplar Sawdust: 1.673 t
Water and Steam: 0.382-0.459 t
a
Dry Acid Pretreatment
Feedstocks : 1.000 t H2O: 0.250 t Corn Stover: 1.000 t Wheat Straw: 1.000 t Rice Straw: 1.000 t Bagasse: 1.000 t Poplar Sawdust: 1.000 t
Water: 1.673 t
Biodetoxification
SSCF
CO2: 0.243-0.265 t Corn Stover: 0.243 t Wheat Straw: 0.248 t Rice Straw: 0.254 t Bagasse: 0.260 t Poplar Sawdust: 0.265 t
Fig. 1 a
Corn Stover: 0.433 t Wheat Straw: 0.442 t Rice Straw: 0.365 t Bagasse: 0.398 t Poplar Sawdust: 0.402 t Distilled Water (Recycled): 0.365-0.442 t
Recovery
Wastewater: 2.204-2.250 t Corn Stover: 2.227 t Wheat Straw: 2.250 t Rice Straw: 2.204 t Bagasse: 2.216 t Poplar Sawdust: 2.227 t
Ethanol Product: 0.215-0.260 t Corn Stover: 0.254 t Wheat Straw: 0.260 t Rice Straw: 0.215 t Bagasse: 0.0.234 t Poplar Sawdust: 0.236 t Lignin Residue: 0.546-0.658 t Corn Stover: 0.591 t Wheat Straw: 0.546 t Rice Straw: 0.658 t Bagasse: 0.608 t Poplar Sawdust: 0.597 t
Mass balance of ethanol production from five lignocellulose feedstocks
One ton of dry lignocellulose feedstock is companied with 0.250 ton of water (20% of water content)
15
Corn Stover Wheat Straw Rice Straw Bagasse Poplar Sawdust
Dry Acid Pretreatment
Biodetoxification
SSCF
Recovery
16
Ethanol Lignin residue Biogas
1.00 ton of ethanol
0.67 ton of gasoline equivalent
1.26-1.85 tons of lignin residue
4,335-5,981 kWh of electricity
0.19-0.27 ton of biogas
1,946-2,795 kWh of electricity
Highlights
Mass balance of cellulosic ethanol production from five feedstocks was established.
HHV of lignin residue, COD and BOD5 of wastewater were measured.
Electricity generation from lignin residue and biogas was calculated and analyzed.
17
S0960-8524(17)31108-2 http://dx.doi.org/10.1016/j.biortech.2017.07.022 BITE 18441
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
15 June 2017 29 June 2017 4 July 2017
Please cite this article as: Liu, G., Bao, J., Evaluation of electricity generation from lignin residue and biogas in cellulosic ethanol production, Bioresource Technology (2017), doi: http://dx.doi.org/10.1016/j.biortech. 2017.07.022
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1
Short Communication manuscript submitted to Bioresource Technology
2 3
Evaluation of electricity generation from lignin residue and biogas in cellulosic ethanol
4
production
5 6
Gang Liu, Jie Bao*
7 8
State Key Laboratory of Bioreactor Engineering, East China University of Science and
9
Technology, 130 Meilong Road, Shanghai 200237, China
10 11
* Corresponding author.
12
Jie Bao, Tel/fax: +86 21 642151799, [email protected]
13 14
Abstract
15
This study takes the first insight on the rigorous evaluation of electricity generation
16
based on the experimentally measured higher heating value (HHV) of lignin residue, as well
17
as the chemical oxygen demand (COD) and biological oxygen demand (BOD5) of wastewater.
18
For producing one metric ton of ethanol fuel from five typical lignocellulose substrates,
19
including corn stover, wheat straw, rice straw, sugarcane bagasse and poplar sawdust,
20
1.26-1.85 tons of dry lignin residue is generated from biorefining process and 0.19-0.27 tons
21
of biogas is generated from anaerobic digestion of wastewater, equivalent to 4,335-5,981 kWh
22
and 1,946-2,795 kWh of electricity by combustion of the generated lignin residue and biogas,
23
respectively. The electricity generation not only sufficiently meets the electricity needs of
24
process requirement, but also generates more than half of electricity surplus selling to the
25
grid.
1
1
Keywords: Electricity generation, Lignin residue, Biogas, Cellulosic ethanol, Lignocellulose
2 3 4
1. Introduction Lignocellulose biomass is the most important feedstock option to compete with starch
5
and sugar feedstock for production of biofuels and bulk chemicals (Wyman and Dale, 2015).
6
Currently, the lignocellulose biorefining technology is in the developing stage. The energy
7
and water balance is obviously unfavorable to that of the mature corn ethanol production in
8
consuming more fresh water, steam, electricity (McAloon et al., 2000; Aden, 2007; Balan,
9
2014), and generating more wastewater (Wingren et al., 2008; Sassner and Zacchi, 2008b;
10
Martin and Grossmann, 2011). Lignocellulose biorefining also lacks opportunity to recover
11
costs from high added value byproducts that may be available with other substrates such as
12
distillers dried grains with soluble (DDGS) from corn fermentation. On the other hand, lignin
13
residue generation adds a high credit of electricity to the cellulosic ethanol production by
14
combustion of lignin residue after cellulose and hemicellulose are converted into ethanol
15
(Humbird et al., 2011). Biogas (methane) from the anaerobic digestion of wastewater from
16
ethanol distillation provides an additional gas fuel for electricity generation (Humbird et al.,
17
2011). The electricity generation credit could balance the electricity consumption in cellulosic
18
ethanol production process and also sell the surplus electricity to grid.
19
In biorefining process, cellulose and hemicellulose in lignocellulose feedstock are
20
enzymatically hydrolyzed into fermentable sugars and then fermented into ethanol or
21
biochemicals, while the lignin portion is left behind as solid residue after ethanol is distillated
22
and water is filtrated from the residue. The higher heating value (HHV) of the pure lignin is
23
26.7 GJ per metric ton, which is competitive or even better than the raw coal (20.9 GJ/ton)
24
(Archambault-leger et al., 2015). The practical lignin residue contains unconverted cellulose
25
and hemicellulose, fermenting microorganisms and inert solids, but its heating value is still
2
1
high enough to be used as boiler fuel. Wastewater is generated from the distillation of ethanol
2
fermentation broth followed by a liquid-solid filtration. The wastewater is anaerobically and
3
aerobically digested to remove the chemicals indicated by chemical oxygen demand (COD)
4
value and generate biogas. Both the biogas and the sludge cake are capable of combustion use
5
for electricity generation in a combined heat and electricity facility.
6
Although the lignin residue and biogas demonstrated great potentials for electricity
7
generation, the previous studies did not deliver the rigorous evaluation based on the
8
experimentally measured data of practical lignin residues or biogas. For lignin residue, only
9
the HHV value of pure lignin component was applied for evaluation of electricity generation,
10
instead of the experimentally measured heat values using the practical lignin residues, the
11
practical lignocellulose feedstocks, and the practical processing of cellulosic ethanol
12
production (Cardona and Sanchez, 2006; Archambault-leger et al., 2015). For biogas, the
13
COD and BOD data were rarely experimentally measured and usually the rough estimation
14
was made based on the limited COD data (Steinwinder et al., 2011; Humbird et al., 2011).
15
In this study, the cellulosic ethanol production from five typical lignocellulose biomass
16
including corn stover, wheat straw, rice straw, sugarcane bagasse and poplar sawdust was
17
performed by dry acid pretreatment and biodetoxification (DryPB), followed by a high solids
18
loading simultaneous saccharification and co-fermentation (SSCF) to achieve the high ethanol
19
conversion efficiency (approximately 11% of ethanol titer by volumetric percentage in the
20
fermentation broth). The mass balance of cellulosic ethanol production was calculated as the
21
basis of electricity generation calculation. The higher heating values (HHV) of the lignin
22
residues, the COD and BOD5 values of the wastewater from the distillation stillage from the
23
five representative lignocellulose feedstocks were experimentally measured according to the
24
standard methods. Based on these data, the electricity generation from lignin residue and
25
biogas were rigorously calculated and analyzed. This study took the first insight on the
3
1
rigorous evaluation of electricity generation in cellulosic ethanol production. The results
2
provided the important data for designing of commercial scale cellulosic ethanol plant and
3
evaluating cellulosic ethanol economy.
4
2. Materials and methods
5
2.1. Raw materials
6
Five lignocellulose feedstocks were collected from their dominant growing regions in
7
China. Corn stover was harvested from Bayan Nur, Inner Mongolia, China in fall 2015.
8
Wheat straw was harvested from Dan Cheng, Henan, China in fall 2013. Rice straw was
9
harvested from Chang Zhou, Jiangsu, China in fall 2014. Sugarcane bagasse was obtained
10
from the sugar plant of Bei Hai, Guangxi, China in fall 2014. Italian poplar sawdust was
11
obtained from the wood factory in Yan Cheng, Jiangsu, China in fall 2015. The field dirt,
12
sands, metal pieces and other impurities were carefully avoided during the collection and then
13
screened during the prehandling. The collected corn stover, wheat straw, rice straw and
14
sugarcane bagasse were ground coarsely using a beater pulverizer and screened through a
15
mesh with the circle diameter of 10 mm. Poplar sawdust was used directly without grinding.
16
The composition of the lignocellulose feedstocks was measured by the two-step acid
17
hydrolysis method according to National Renewable Energy Laboratory (NREL) protocols
18
(Sluiter et al., 2008; 2012). On the dry weight base (w/w), corn stover contained 35.4% of
19
cellulose, 24.6% of hemicellulose, 16.1% of lignin, and 3.5% of ash; wheat straw contained
20
38.7% of cellulose, 25.9% of hemicellulose, 14.9% of lignin, and 5.2% of ash; rice straw
21
contained 35.3% of cellulose, 18.4% of hemicellulose, 22.5% of lignin, and 9.3% of ash;
22
sugarcane bagasse contained 38.8% of cellulose, 23.9% of hemicellulose, 26.4% of lignin,
23
and 1.3% of ash; poplar sawdust contained 39.7% of cellulose, 16.6% of hemicellulose, 29.4%
24
of lignin, and 3.2% of ash.
25
2.2. Dry acid pretreatment and biodetoxification (DryPB) biorefining process
4
1
Ground lignocellulosic biomass is pretreated using the dry acid pretreatment method at
2
175 oC for 5 min with 2.0 g sulfuric acid per 100 g dry biomass in the 20 L pretreatment
3
reactor (Zhang et al., 2011; He et al., 2014a; 2014b). All the dilute acid solution and the
4
condensed water were completely adsorbed into the solids to form approximately 50% (w/w)
5
of the dry pretreated feedstock solids with the pH around 2.0, and no free wastewater stream
6
was generated. The sulfuric acid in the pretreated biomass solids was neutralized to 5.5 by the
7
addition of 20% (w/w) Ca(OH)2 suspension slurry. The pretreated biomass solids were briefly
8
milled by a disk milling machine (PSB-80JX, Fleck Co., Nantong, Jiangsu, China) to remove
9
the extra-long fibers to avoid the blockage of pipelines and valves in the downstream flow of
10 11
the hydrolysate slurry and broth. Then the pretreated solids were aerobically biodetoxified using Amorphotheca resinae
12
ZN1 in a 15 L bioreactor to remove the inhibitors generated during the dry acid pretreatment
13
operation at 28 oC the water saturated aeration of 0.8 vvm for 36-48 h (Zhang et al., 2010a; He
14
et al., 2016). The major inhibitors were completely assimilated and degraded. Xylose and
15
glucose released during the pretreatment were preserved without observable loss. And no
16
cellulose degradation was observed during the biodetoxification period.
17
The pretreated and biodetoxified lignocellulose feedstock solids were enzymatically
18
hydrolyzed into liquid hydrolysate slurry containing both mono and oligomer sugars in the
19
specially designed 5 L bioreactors equipped with helical ribbon impeller at 50 oC, pH 4.8 and
20
150 rpm for 12 hours (Zhang et al., 2010b). Then glucose and xylose were co-fermented into
21
ethanol simultaneously with the hydrolysis of cellulose and oligomer sugars at high solids
22
loading (30%, w/w) and low cellulase dosage (10 mg protein/g cellulose) at 30 oC for 96
23
hours by inoculating the shortly adapted yeast seed cells S. cerevisiae XH7 into the
24
hydrolysate at 10% (v/v). pH was maintained at 5.5 by adding 5 M NaOH solution.
25
Fermentation broth from corn stover, wheat straw, rice straw, sugarcane bagasse, or
5
1
poplar sawdust was directly distillated to recover ethanol to about 35% (w/w) distillate in a
2
glass distillation equipment with the inner column diameter of 40 mm filling with stainless
3
theta ring packings (3 mm in diameter). The stillage from the distillation column was filtrated
4
by a press filter machine through a fabric cloth to yield the wastewater stream and the lignin
5
residue cake.
6
2.3. Determination of higher heating value, COD and BOD5
7
Lignin residue cake with the thickness of 4 mm contained approximately 65% (w/w) of
8
dry solids. It was dried at 105 oC until constant weight and sent for determination of higher
9
heating value on a calorimeter (MMC 274 multi-module, Netzsch Co., Fresitaat Bayerm,
10
Germany) according to the China National Standard Method GB/T213-2003 (Determination
11
of Calorific Value of Coal).
12
The wastewater stream was sent for determination of chemical oxygen demand (COD)
13
and biochemical oxygen demand after 5 days (BOD5). The COD of the wastewater was
14
determined according to the China National Environmental Protection Standard Method
15
HJ/T399-2007 (Water Quality Series, Determination of Chemical Oxygen Demand, COD).
16
Briefly, the oxidation was carried out at 165 oC for 15 min in a digester (Hach DRB200, Hach
17
Inc, NY, USA) and the absorbance was measured at 600 nm on a spectrophotometer (Hach
18
DR5000, Hach Inc, NY, USA). Potassium dichromate was used as oxidizing agent and
19
potassium biphthalate was the control. The BOD5 of the wastewater from the same source was
20
determined according to the China National Environmental Protection Standard Method
21
HJ505-2009 (Water Quality Series, Determination of Biochemical Oxygen Demand after 5
22
Days, BOD5). Briefly, the wastewater was cultured at 20 oC for 5 days in glass bottles in an
23
anaerobic flask inoculated with the standard microbe seed and then the dissolved oxygen
24
consumption was determined.
25
3. Results and discussion
6
1 2
3.1. Mass balance in cellulosic ethanol production Corn stover, wheat straw, rice straw, sugarcane bagasse, and poplar sawdust were dry
3
acid pretreated and biodetoxified to yield the inhibitor free and high xylose containing
4
feedstock for simultaneous saccharification and co-fermentation (SSCF). SSCF from the five
5
feedstocks achieved high ethanol titer of 85.1, 87.0, 71.9, 78.3, and 79.1g/L (9.1%-11.0% in
6
volumetric percentage), respectively (Liu et al., 2017). Ethanol in the fermentation broth was
7
distillated. The mixed lignin residue and wastewater slurry was recovered as the bottom
8
stream of the distillation column. The slurry was filtrated to get the lignin residue cake and
9
wastewater liquid. The mass balance of biomass, water, ethanol and lignin residue in the
10
biorefining process was calculated based on the experiment results (Fig. 1). Processing one
11
metric ton lignocellulosic feedstocks, ethanol yield is 0.215-0.260 ton, in which the highest
12
one is 0.260 ton ethanol per ton of wheat straw and the lowest is 0.215 ton of ethanol per ton
13
of rice straw, mainly due to the different cellulose and xylan content among the lignocellulose
14
feedstocks. Meanwhile, 0.546-0.658 ton of lignin residue (with the water content of 35%) and
15
2.204-2.250 tons of wastewater are generated from one metric ton of feedstocks. On the basis
16
of ethanol fuel product, production of one metric ton of ethanol approximately generates
17
1.26-1.85 tons of dry lignin residue and 8.66-10.27 tons of wastewater.
18
3.2. Electricity generation form lignin residue
19
The higher heating values (HHV) of lignin residues were experimentally determined
20
according to the standard methods and the equivalent electricity generation was calculated
21
(Table 1). The HHV values are in the range of 17.12-18.88 GJ per dry ton of lignin residues
22
from corn stover, wheat straw, rice straw, sugarcane bagasse, or poplar sawdust. The HHV
23
values are approximately 70% of the pure lignin (26.7 GJ/ton), and 85% of the raw coal (20.9
24
GJ/ton), due to the existence of the impurities such as the unreacted cellulose, hemicellulose,
25
residual sugars, inert soil solids and minor amount of fermenting yeast cells and cellulase
7
1 2
enzyme proteins. Production of one metric ton of fuel ethanol generates 1.26-1.85 tons of lignin residues
3
(dry base) according to the mass balance (Fig. 1). Combustion of 1.26-1.85 tons of lignin
4
residue generates 22.95-31.66 GJ of higher heating values. 1 GJ of heating value is equivalent
5
to 277.8 kWh of electricity based on the theoretical energy equivalence. The boiler efficiency
6
(80%) and the generator efficiency (85%) of the turbine shaft conversion to electricity using
7
the lignin residue fuels are cited from Humbird et al (2011). Therefore, practically 1 GJ of the
8
higher heating value generates 277.8 0.85 0.80 = 188.9 kWh of electricity, and
9
22.95-31.66 GJ of higher heating value generates 4,335-5,981 kWh of electricity. In summary,
10
for producing one metric ton of ethanol fuel, 4,335-5,981 kWh of electricity is generated by
11
combustion of the correspondingly generated lignin residue.
12
3.3. Electricity generation form biogas which produced after wastewater treatment
13
The wastewater was collected after the filtration of the stillage liquid and the chemical
14
oxygen demand (COD) and the biological oxygen demand after 5 days (BOD5) were
15
experimentally determined (Table 2). The COD values of the wastewater are in the range of
16
80-137 g/L for the five lignocellulose feedstocks. The BOD5 values are in the range of 19-44
17
g/L for the feedstocks. These values are in agreement of the NREL data (87 g/L of COD)
18
(Humbird et al., 2011) and in the general range of fermentation wastewater (50-120 g/L of
19
COD and 20-60 g/L of BOD5) (Jin and Zhao, 1987). The ratio of COD/BOD5 is in the range
20
of 0.22-0.39 and fit the general ratio for anaerobic digestion (> 0.3) (Steinwinder et al., 2011).
21
Approximately 0.35 standard cubic meter of biogas or 0.27 kg of methane is obtained
22
from 1 kg of COD removed in the anaerobic digestion of the stillage wastewater (Wingren et
23
al., 2008). Production of one metric ton of ethanol generates 0.19-0.27 ton of biogas
24
according to the mass balance (Fig. 1) resulted from the 91% reduction of the COD in
25
anaerobic digestion (Humbird et al., 2011). Combustion of 0.19-0.27 tons of biogas generates
8
1
10.30-14.80 GJ of higher heating values based on the HHV of methane of 55.5 GJ/ton.
2
Similar to lignin residue, 1 GJ of heating value generates 188.9 kWh of electricity, and
3
10.30-14.80 GJ of higher heating value generates 1,946-2,795 kWh of electricity. In summary,
4
for producing one metric ton of ethanol fuel, 1,946-2,795 kWh of electricity is generated by
5
combustion of the correspondingly generated biogas form wastewater.
6
Combining lignin residue and biogas, the electricity generation from the five typical
7
lignocellulose biomass is in the range of 7,121-8,180 kWh for producing one ton of ethanol in
8
the present dry acid pretreatment and biodetoxification (DryPB) process. The general energy
9
requirement is 12 GJ of steam (Hinman et al., 1992; Wingren et al., 2008; Piccolo et al., 2009)
10
and 1,306 kWh of electricity (Humbird et al., 2011) for producing one metric ton ethanol.
11
Translating the steam heating into electricity, the total electricity consumption is around 3,500
12
kWh for producing one ton ethanol. Therefore, the electricity generation not only sufficiently
13
meets the electricity need of process requirement, but also generates more than half of the
14
electricity surplus for selling to the grid.
15
4. Conclusion
16
Production of one ton of cellulosic ethanol generates 1.26-1.85 tons of dry lignin residue
17
and 0.19-0.27 tons of methane after ethanol recovery from the five typical lignocellulose
18
biomass. The electricity generation from combustion of lignin residue and biogas is in the
19
range of 7,121-8,180 kWh per ton of ethanol produced in the present DryPB process. The
20
electricity generation sufficiently meets the cellulosic ethanol production needs and generates
21
surplus to the grid.
22
Acknowledgements
23
This research was partially supported by the National Basic Research Program of China
24
(2011CB707406).
25
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Table 1 Electricity generation from lignin residue combustion Lignin residue generation
Higher heating value of lignin residue
Electricity equivalent a
(ton/ton dry solids)
(ton/ton ethanol)
(GJ/ton dry lignin residue)
(kWh/ton ethanol)
Corn stover
0.384
1.51
18.878
5,385
Wheat straw
0.355
1.26
18.215
4,335
Rice straw
0.428
1.85
17.116
5,981
Sugarcane bagasse
0.395
1.57
17.979
5,332
Poplar sawdust
0.388
1.52
17.992
5,166
Feedstock
a
Boiler efficiency (80%) for lignin residue or methane combustion and the generator efficiency of the turbine shaft conversion to electricity
(85%) were cited from Humbird et al (2011). 1 GJ of heating value is equivalent to 277.8 kWh of electricity.
13
Table 2 Electricity generation from biogas combustion after wastewater anaerobic digestion Feedstock
Wastewater generation
Chemical oxygen
Biological oxygen Methane
Electricity
demand (COD)
demand (BOD5)
generation
equivalent a
(g/L)
(kg/ton ethanol)
(kWh/ton ethanol)
(ton/ton dry solids)
(ton/ton ethanol)
Corn stover
2.227
8.768
124
44
267
2,795
Wheat straw
2.250
8.664
137
30
266
2,786
Rice straw
2.204
10.271
80
29
186
1,946
Sugarcane bagasse
2.216
9.481
113
19
242
2,538
Poplar sawdust
2.227
9.433
97
38
207
2,166
a
(g/L)
The boiler efficiency of the lignin residue or methane feedstocks (80%) and the generator efficiency of the turbine shaft conversion to electricity
(85%) are cited from Humbird et al (2011). 1 GJ of heating value is equivalent to 277.8 kWh of electricity.
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Corn Stover: 0.391 t Wheat Straw: 0.382 t Rice Straw: 0.459 t Bagasse: 0.426 t Poplar Sawdust: 0.422 t
Corn Stover: 1.673 t Wheat Straw: 1.673 t Rice Straw: 1.673 t Bagasse: 1.673 t Poplar Sawdust: 1.673 t
Water and Steam: 0.382-0.459 t
a
Dry Acid Pretreatment
Feedstocks : 1.000 t H2O: 0.250 t Corn Stover: 1.000 t Wheat Straw: 1.000 t Rice Straw: 1.000 t Bagasse: 1.000 t Poplar Sawdust: 1.000 t
Water: 1.673 t
Biodetoxification
SSCF
CO2: 0.243-0.265 t Corn Stover: 0.243 t Wheat Straw: 0.248 t Rice Straw: 0.254 t Bagasse: 0.260 t Poplar Sawdust: 0.265 t
Fig. 1 a
Corn Stover: 0.433 t Wheat Straw: 0.442 t Rice Straw: 0.365 t Bagasse: 0.398 t Poplar Sawdust: 0.402 t Distilled Water (Recycled): 0.365-0.442 t
Recovery
Wastewater: 2.204-2.250 t Corn Stover: 2.227 t Wheat Straw: 2.250 t Rice Straw: 2.204 t Bagasse: 2.216 t Poplar Sawdust: 2.227 t
Ethanol Product: 0.215-0.260 t Corn Stover: 0.254 t Wheat Straw: 0.260 t Rice Straw: 0.215 t Bagasse: 0.0.234 t Poplar Sawdust: 0.236 t Lignin Residue: 0.546-0.658 t Corn Stover: 0.591 t Wheat Straw: 0.546 t Rice Straw: 0.658 t Bagasse: 0.608 t Poplar Sawdust: 0.597 t
Mass balance of ethanol production from five lignocellulose feedstocks
One ton of dry lignocellulose feedstock is companied with 0.250 ton of water (20% of water content)
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Corn Stover Wheat Straw Rice Straw Bagasse Poplar Sawdust
Dry Acid Pretreatment
Biodetoxification
SSCF
Recovery
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Ethanol Lignin residue Biogas
1.00 ton of ethanol
0.67 ton of gasoline equivalent
1.26-1.85 tons of lignin residue
4,335-5,981 kWh of electricity
0.19-0.27 ton of biogas
1,946-2,795 kWh of electricity
Highlights
Mass balance of cellulosic ethanol production from five feedstocks was established.
HHV of lignin residue, COD and BOD5 of wastewater were measured.
Electricity generation from lignin residue and biogas was calculated and analyzed.
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