First, this study performed a technical assessment of preprocessing to investigate its impacts on the overall bioconversion yields using both feedstocks poplar whole-tree chips (WTC) and clean pulp chips (CPC). Total monomeric sugar yield (kilograms of monomeric sugars obtained per tonne of OD raw biomass) and recovery (percentage of monomeric sugars recovered from original sugars) after steam pretreatment and enzymatic hydrolysis were determined, and the ethanol fermentation yields of untreated and preprocessed samples were compared. It was also assessed if preprocessing could replace overliming detoxification in the ethanol bioconversion process. All percentage increase/decrease presented in this discussion were calculated based on the untreated biomass as the original value. Any data analysis mentioned as “significant” represents statistically significant (p <0.05). Second, an economic assessment was performed on three large-scale biorefinery scenarios: 1) CPC feedstock via pretreatment, enzymatic hydrolysis, overliming, and sugars to ethanol fermentation (base case scenario), 2) WTC feedstock via preprocessing, pretreatment, enzymatic hydrolysis, and sugars to ethanol fermentation, and 3) CPC feedstock via preprocessing, pretreatment, enzymatic hydrolysis, and sugars to ethanol fermentation.
TECHNICAL ASSESSMENT
Chemical composition of untreated and preprocessed biomass
Table 1 shows the chemical composition of untreated (original) and preprocessed poplar whole-tree chips (WTC) and clean pulp chips (CPC).
Table 1. Chemical composition of untreated and preprocessed poplar biomass (as a percentage of the OD weight).
|
|
Ash (%)
|
Extractives (%)
|
Glucan (%)
|
Xylan (%)
|
Total sugars* (%)
|
Total lignin (%)
|
Acetic acid (%)
|
Whole-tree chips (WTC)
|
Untreated
|
1.6 ± 0.1a
|
10.7 ± 0.1a
|
42.1 ± 0.9a
|
14.3 ± 0.3a
|
60.6 ± 1.2ab
|
28.6 ± 0.8a
|
4.8 ± 0.2a
|
Acidic
|
0.5 ± 0.1b
|
4.6 ± 0.3b
|
41.4 ± 0.9a
|
15.3 ± 0.2b
|
59.5 ± 1.2bc
|
28.1 ± 0.2ab
|
5.7 ± 0.7a
|
Alkaline
|
1.6 ± 0.1a
|
4.3 ± 0.4b
|
42.5 ± 0.5a
|
15.6 ± 0.1b
|
61.8 ± 0.7a
|
27.6 ± 0.2ab
|
2.7 ± 0.1b
|
Neutral
|
1.4 ± 0.1c
|
6.8 ± 0.2c
|
39.6 ± 0.3b
|
14.6 ± 0.1a
|
57.9 ± 0.5c
|
27.4 ± 0.2b
|
5.2 ± 0.1a
|
Clean pulp chips (CPC)
|
Untreated
|
0.6 ± 0.0a
|
3.5 ± 0.3a
|
47.9 ± 0.4a
|
14.4 ± 0.1a
|
64.7 ± 0.7a
|
26.1 ± 0.1a
|
5.0 ± 0.6a
|
Acidic
|
0.1 ± 0.1b
|
2.0 ± 0.1b
|
48.4 ± 1.3a
|
14.4 ± 0.4a
|
64.9 ± 1.7a
|
26.1 ± 0.2a
|
4.4 ± 0.2a
|
Alkaline
|
0.8 ± 0.1c
|
3.0 ± 0.0c
|
52.4 ± 0.9b
|
15.7 ± 0.3b
|
70.7 ± 1.3b
|
25.8 ± 0.1a
|
1.4 ± 0.1b
|
Neutral
|
0.4 ± 0.0d
|
2.6 ± 0.1d
|
49.4 ± 0.7a
|
14.9 ± 0.3a
|
66.8 ± 1.0a
|
26.3 ± 0.5a
|
4.8 ± 0.1a
|
Data represented as the mean values of triplicate analysis with standard deviation, extractives as duplicates.
Different superscript letters indicate statistically significant differences (p<0.05) within each column by Tukey’s test (WTC and CPC treatments were compared separately).
*Total sugars include glucan, xylan, arabinan, galactan, and mannan.
Non-structural components (ash and extractives)
Untreated WTC showed greater non-structural components (NSC) content (12% ash plus extractives) than CPC (4%) (Table 1) due to the chemical composition of different parts of the tree. The white wood fraction of untreated WTC contained approximately 7% NSC (including ash and extractives), while the bark fraction contained 35% (Supplementary Material, Table S.1). This finding agrees with earlier reports by Passialis et al., where they stated that bark of black locust has higher ash and extractives content than other wood components (20). Dou et al. (3) also compared the chemical characteristics of different fractions of 2-year-old poplar, and they reported that extractives content in bark was about two times higher than that in white wood, while ash content in bark was about five times higher than that in white wood. Furthermore, previous studies reported that juvenile wood has higher extractives content than mature wood (21). Since WTC poplar was harvested at a younger age than CPC, it can be inferred that WTC has a higher juvenile wood content than CPC (22). In good agreement with those studies, the white wood fraction of untreated WTC had 6.3% extractives (Supplementary Material, Table S.1), while the CPC had 3.5%.
Among the preprocessing conditions, acidic preprocessing was the most effective in removing ash from both WTC and CPC biomass (66% and 81% removal, respectively), followed by neutral preprocessing (11% and 32% removal, respectively) (Table 1). These findings are consistent with the literature, where Hӧrhammer et al. (17) reports 59% ash removal from poplar WTC using an acidic-neutral wash, and He et al. (6) reporter 20% ash removal after neutral washing of corn stover. Interestingly, alkaline preprocessed CPC biomass showed an approximate 40% increase in total ash content when compared to untreated CPC. This finding can be explained by an accumulation of sodium cations originated from the sodium hydroxide solution used during alkaline preprocessing (see results in Table 2). Although Kundu et al. (23) and Cho et al. (24) also studied the deacetylation of homogeneous yellow poplar biomass using dilute sodium hydroxide solution, they did not report its effects on the NSC content. Differently from CPC, the ash content of WTC did not significantly change (p<0.05) after alkaline preprocessing, showing only a 3% increase when compared to untreated.
Similarly, acidic preprocessing removed extractives to a greater extent, with 57% and 42% extractives removal from WTC and CPC biomass, respectively (Table 1). Neutral preprocessing removed 37% and 25% extractives from WTC and CPC, respectively. Similar results were reported by Hӧrhammer et al. (17), where acidic-neutral and neutral washes removed 43% and 51% extractives from poplar WTC, respectively.
Total sugars, lignin, and acetic acid
The total sugars of both types of biomass presented minor changes when comparing untreated and preprocessed samples (Table 1), with ranges of 57.9% - 61.8% total sugars for WTC and 64.8% - 70.7% for CPC. The fact that preprocessing did not compromise the sugar content to a big extent is favorable for its application in ethanol production. Similarly, the total lignin content also showed minimal changes after preprocessing, with numbers ranging from 27.4% - 28.6% total lignin content for WTC and 25.8% - 26.3% for CPC. Acetic acid, however, was extensively removed by alkaline preprocessing (48% and 73% from WTC and CPC, respectively). This finding is consistent with those from Chen et al. (19), where deacetylation removed 80% of acetyl groups from corn stover. No significant (p<0.05) removal of acetic acid was obtained with acidic and neutral conditions.
Elemental composition
Elemental analysis was performed to characterize the mineral composition of untreated and preprocessed biomass (Table 2). Calcium and potassium were predominant in both untreated WTC (3140 µg/g and 1893 µg/g, respectively) and CPC (831 µg/g and 793 µg/g, respectively), followed by magnesium, phosphorus, and sulfur. These findings are consistent with the literature, where the main inorganic components found in woody biomass are calcium, potassium, and magnesium (22). As expected, acidic preprocessing was more effective in removing minerals from both biomass: potassium and magnesium were completely removed, while calcium was partially removed (43% and 83% removal from WTC and CPC, respectively). Calcium is present in the biomass in different forms, such as acid-soluble salts, non-leachable salts, and organically bound metal ions which are very difficult to be removed (25). Alkaline preprocessing did not remove calcium but removed 89% and 100% of potassium from WTC and CPC, respectively. Not surprisingly, alkaline preprocessing added sodium to both WTC and CPC (692 µg/g and 1594 µg/g, respectively) due to sodium hydroxide diffusion into the wood during preprocessing, and the sodium cations bound to acid groups in the wood matrix.
Table 2. Elemental composition of untreated and preprocessed poplar biomass.
|
|
Ca (µg/g)
|
K (µg/g)
|
Mg (µg/g)
|
Na (µg/g)
|
P (µg/g)
|
S (µg/g)
|
Whole-tree chips (WTC)
|
Untreated
|
3140 ± 142
|
1893 ± 25
|
429 ± 10
|
0.0 ± 0.0
|
560 ± 35
|
228 ± 10
|
Acidic
|
1776 ± 56
|
0.0 ± 0.0
|
0.0 ± 0.0
|
0.0 ± 0.0
|
183 ± 8
|
152 ± 5
|
Alkaline
|
3248 ± 90
|
211 ± 19
|
358 ± 4
|
692 ± 0.0
|
250 ± 25
|
123 ± 7
|
Neutral
|
3256 ± 77
|
1015 ± 27
|
391 ± 23
|
0.0 ± 0.0
|
495 ± 11
|
196 ± 4
|
Clean pulp chips (CPC)
|
Untreated
|
831 ± 28
|
793 ± 12
|
237 ± 19
|
0.0 ± 0.0
|
170 ± 5
|
77 ± 2
|
Acidic
|
140 ± 6
|
0.0 ± 0.0
|
0.0 ± 0.0
|
0.0 ± 0.0
|
0.0 ± 0.0
|
66 ± 2
|
Alkaline
|
778 ± 52
|
0.0 ± 0.0
|
225 ± 20
|
1594 ± 24
|
57 ± 3
|
50 ± 6
|
Neutral
|
744 ± 20
|
0.0 ± 0.0
|
157 ±10
|
0.0 ± 0.0
|
108 ± 14
|
70 ± 4
|
Data represented as the mean values of duplicate analysis with standard deviation.
Other elements were analyzed (including barium, iron, manganese, and silica), but they were either not detected or they were present at trace amounts (lower than 50 µg/g).
Buffering capacity
Buffering capacity of untreated and preprocessed biomass was measured to determine how the biomass pH changes with the addition of a dilute acid solution. The steam pretreatment is usually carried out under acidic conditions, and it has been suggested that ash can buffer the pH reduction during pretreatment, consequently decreasing the pretreatment efficacy (26). Figure 1 shows the titration curves for water extracts of untreated and preprocessed biomass, and DI water was used as a reference. Different preprocessing conditions had different initial pH for both WTC and CPC biomass due to the presence of residual chemicals from the preprocessing step. The pH of untreated WTC extract stayed quite stable with the continuous addition of a dilute-acid solution (pH dropped from 5.4 to 4.8), reflecting the high buffering capacity of the untreated biomass due to its higher ash content (Figure 1a). Acidic preprocessed WTC biomass displayed a similar behavior as the water blank, indicating a lower buffering capacity as a result of its low total ash content (Table 1). Alkaline preprocessed WTC started at pH 7.2 due to the presence of residual caustic from preprocessing, and the pH drop during the addition of the first 30 mL of acid solution illustrates the occurrence of neutralization reactions. Once the pH of all samples was stable, it was noted that all preprocessing conditions were able to reduce the buffering capacity of the biomass and achieve a lower pH than untreated biomass. Hörhammer et al. had similar results where acidic and neutral preprocessing decreased the buffering capacity of 2-year-old poplar whole-tree chips when compared to untreated biomass due to a lower ash content (17). The CPC samples presented similar trends as the WTC, as it is shown in Figure 1b. Alkaline preprocessed CPC started at a higher pH 9 as a result of greater residual caustic content in the biomass (1594 µg/g) when compared to the alkaline WTC sample (692 µg/g).
Figure 1. Titration curves of untreated and preprocessed poplar biomass of both (a) WTC samples and (b) CPC samples, along with DI water (blank) as a reference.
Liquid and solid fractions after steam pretreatment
Chemical composition of liquid fraction
Most sugars recovered in the liquid fraction after pretreatment were xylose, representing 50-70% of the total sugars (varying with the preprocessing condition), followed by glucose, which corresponded to 19-40%. Minor sugars, such as arabinose, galactose, and mannose were present at trace amounts. Table 3 shows the total sugar yield (kg/tonne of biomass) in the liquid fraction and the corresponding percentage of sugar present in monomeric form. For poplar WTC, acidic and neutral preprocessing had the highest total sugar yield, 258 and 257 kg/tonne respectively, thus obtaining approximately 8% more kilograms of sugars per tonne of biomass than untreated WTC. 91% of the total sugar released in the acidic preprocessed WTC liquid fraction was in monomeric form, approximately 12% greater than all other samples, including untreated biomass. Previous studies reported that the removal of NSC from biomass can enhance the hydrolysability of hemicellulose during the pretreatment step as a result of a lower buffering capacity effect (6,17). In good agreement with those studies, acidic preprocessing showed the highest removal of NSC (66% removal of ash) and lower buffering capacity, which improved the hemicellulose hydrolysis into monomers.
For CPC, acidic preprocessing had the highest total sugar yield of 213 kg/tonne of biomass, approximately 27% higher than untreated biomass (168 kg/tonne). Unlike WTC, all CPC samples had a monomeric sugar percentage above 93% as a result of the original low NSC of this biomass, with the highest percentage of ~100% for acidic preprocessing. Interestingly, CPC liquid fractions showed lower total sugar yield than WTC liquid fractions, which being possibly related to differences in anatomical properties between the WTC and CPC wood fibers. According to Bao et al., fibers in juvenile wood are about 24% shorter than those in mature wood, therefore being more susceptible to fractionation during steam pretreatment and resulting in better hemicellulose solubilization (21).
Table 3. Chemical composition of liquid and solid fractions after steam pretreatment, and enzymatic hydrolysis conversion.
|
|
Liquid fraction
|
|
Solid fraction
|
|
|
|
|
|
|
Chemical composition
|
|
|
Elemental
composition 3
|
Enzymatic hydrolysis
|
|
|
pH
|
Total sugars 1
|
|
Glucan
|
Xylan
|
Lignin
|
Ash
|
Ca
|
S
|
Glucose conversion 4
|
|
|
|
kg/tonne
|
mono% 2
|
|
%
|
%
|
%
|
%
|
µg/g
|
µg/g
|
%
|
Whole-tree chips (WTC)
|
Untreated
|
1.8
|
239 ± 7
|
79.0%
|
|
60.7 ± 0.5
|
2.4 ± 0.0
|
36.6 ± 0.3
|
1.0 ± 0.3
|
3000 ± 171
|
1062 ± 13
|
67.7 ± 1.4
|
Acidic
|
1.4
|
258 ± 2
|
91.1%
|
|
62.7 ± 0.1
|
1.3 ± 0.0
|
38.4 ± 0.9
|
0.2 ± 0.0
|
1349 ± 55
|
811 ± 22
|
75.7 ± 1.2
|
Alkaline
|
1.6
|
245 ± 5
|
78.7%
|
|
63.1 ± 0.7
|
2.6 ± 0.0
|
35.2 ± 0.1
|
0.5 ± 0.2
|
2315 ± 10
|
1084 ± 16
|
72.7 ± 1.2
|
Neutral
|
1.6
|
257 ± 9
|
78.9%
|
|
61.9 ± 0.5
|
1.6 ± 0.1
|
37.6 ± 0.3
|
1.0 ± 0.1
|
2766 ± 81
|
1144 ± 19
|
72.8 ± 0.6
|
Clean pulp chips (CPC)
|
Untreated
|
1.4
|
168 ± 5
|
96.2%
|
|
68.0 ± 1.1
|
1.2 ± 0.1
|
34.4 ± 0.4
|
<0.1%
|
126 ± 16
|
486 ± 20
|
78.2 ± 2.3
|
Acidic
|
1.4
|
213 ± 4
|
101.2%
|
|
66.8 ± 1.1
|
0.0 ± 0.0
|
33.6 ± 0.2
|
<0.1%
|
97 ± 19
|
408 ± 12
|
76.7 ± 1.2
|
Alkaline
|
1.6
|
176 ± 1
|
93.4%
|
|
72.9 ± 0.6
|
2.4 ± 0.0
|
28.3 ± 0.2
|
<0.1%
|
99 ± 10
|
562 ± 21
|
69.8 ± 1.4
|
Neutral
|
1.5
|
201 ± 5
|
93.5%
|
|
70.6 ± 0.3
|
1.4 ± 0.1
|
31.5 ± 0.5
|
<0.1%
|
57 ± 6
|
431 ± 11
|
73.2 ± 0.1
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Data represented as the mean values of triplicate analysis (except elemental composition that was done in duplicate), which in turn comes from two steam pretreatments replicates.
1 Total sugars (kg/tonne of original biomass) include glucan, xylan, arabinan, galactan, and mannan
2 Monomeric sugar percentage of the total sugars
3 Magnesium, potassium, phosphorus, and sodium were completely removed (0 µg/g) during steam pretreatment from both WTC and CPC. Other elements were analyzed (including barium, iron, manganese, and silica), but they were either not detected or they were present at trace amounts (lower than 50 µg/g).
4 Maximum glucose conversion: WTC samples after 96h of hydrolysis, and CPC samples after 48h.
Chemical composition of solid fraction and enzymatic hydrolysis conversion
Table 3 also shows the chemical composition of the solid fractions and maximum cellulose to glucose conversion after enzymatic hydrolysis (EH) of WTC (after 96h of reaction) and CPC solids (after 48h of reaction). For WTC, all preprocessed samples had higher EH conversions than untreated. Untreated WTC had the lowest conversion of 68% as a result of the combination of higher xylan and ash contents (2.4% and 1.0%, respectively) when compared to the preprocessed samples. Previous studies (27–29) have reported that xylan has a negative effect on cellulose digestion because it behaves like a physical barrier blocking the access of enzymes to the cellulose fibers. Furthermore, He et al. (6) and Bin (30) reported that certain cations, including calcium, can negatively affect the hydrolysis by inhibiting the activity of endoglucanases and exoglucanases. Accordingly, acidic preprocessed WTC resulted in the highest conversion of 76% (8% improvement when compared to untreated WTC) due to its lower xylan and ash contents (1.3% and 0.2%, respectively). The lower xylan content in acidic preprocessed solids is associated with greater solubilization of the hemicellulose during pretreatment (31).
In general, the EH of all CPC samples was faster than that of WTC, which may be associated with the overall lower ash and lignin content of CPC solids. Surprisingly, alkaline preprocessed CPC solids had the lowest EH conversion (70% after 48 h of hydrolysis) among all CPC samples (Table 3). The yields (kg/tonne) of sugars, lignin, and ash of the solid fractions are shown in Supplementary Material, Table S.3.
The elemental composition of the solid fraction is also presented in Table 3. Potassium, magnesium, sodium, and phosphorus were utterly removed in all WTC and CPC samples during steam pretreatment. Calcium, conversely, was removed to a lesser extent due to its lower solubility (25). Overall, WTC solids showed lower calcium removal during pretreatment than CPC solids. Compared to the original calcium content of each sample before pretreatment (Table 2), alkaline preprocessed WTC had 29% removal, followed by acidic with 24% removal, neutral with 15%, and finally untreated WTC with a minor removal of 4%. In contrast, untreated CPC solids showed a calcium removal of 85%, which is about 20 times greater than the removal in untreated WTC solids. Neutral preprocessing had the highest calcium removal of 92%. The difference in calcium removal between WTC and CPC biomass can be attributed to the presence of bark in WTC, which is where the majority of calcium is encountered (Supplementary Material, Table S.2). Finally, an increase in sulfur content when compared to samples before pretreatment (Table 2) was observed in the order of 4 to 9-fold for WTC and 6 to 11-fold for CPC, which was originated from the SO2 used during biomass impregnation.
Monomeric sugar recovery and yield after preprocessing, steam pretreatment, and enzymatic hydrolysis
Figure 2 illustrates the total monomeric sugar recovery (percentage of monomeric sugars recovered from original sugars) and yield (kilograms of monomeric sugars obtained per tonne of OD raw biomass) after steam explosion (SE) and enzymatic hydrolysis (EH) for both WTC (Figure 2a) and CPC (Figure 2b). Error bars indicate standard deviation from duplicate measurements. The complete data, including statistical analysis, can be found in the Supplementary Material, Table S.3.
The monomeric sugar recovery of WTC significantly (p<0.05) improved when preprocessing was done (Figure 2a). Acidic preprocessing achieved remarkable 87.5% monomeric sugar recovery while untreated WTC had a recovery of 73%. Neutral and alkaline preprocessed WTC had monomeric sugar recovery of 82% and 81%, respectively. CPC samples presented only minor differences in monomeric sugar recovery (Figure 2b), with the highest being achieved by acidic preprocessing (80%), only 5% higher than untreated CPC. Overall, WTC samples recovered more monomeric sugars after SE and EH than CPC samples, which could be related to the different morphology of the biomass and its effects on hemicellulose solubilization during pretreatment, as discussed previously.
In like manner, Figure 2a shows that untreated WTC had the lowest monomeric sugar yield of 493 kg/tonne, while WTC preprocessed under acidic condition resulted in a significantly (p<0.05) higher yield of 578 kg/tonne, followed by alkaline (553 kg/tonne) and neutral (529 kg/tonne). Hӧrhammer et al. (17) reported similar results, where the acidic wash of poplar WTC resulted in a 90 kg increase in monomeric sugar yield when compared to untreated. For the CPC samples (Figure 2b), acidic (577 kg/tonne), alkaline (573 kg/tonne), and neutral (580 kg/tonne) preprocessing demonstrated significant (p<0.05) improvement in monomeric sugar yield when compared to untreated (539 kg/tonne). There was no statistical difference (p<0.05) in monomeric sugar yield between the three CPC preprocessing conditions.
Figure 2. Total monomeric sugar recovery and yield of (a) WTC samples and (b) CPC samples.
Fermentation and detoxification
Ethanol fermentation was performed separately on the liquid fraction after pretreatment and the liquids resulted from EH of solids. The initial concentrations of sugars (glucose and xylose) and maximum concentration of ethanol obtained in the fermentation experiments can be found in the Supplementary Material, Table S.5. An additional detoxification step was performed on the CPC liquid fractions after pretreatment prior to fermentation to simulate the base case scenario of an ethanol biorefinery. Ammonia conditioning, as described in the NREL 2011 report (15), was first tested as the detoxification step. However, this method was not effective in poplar CPC liquid fractions, resulting in an unfermentable liquid (ethanol fermentation yield of only 1% – data not shown). It appears that ammonia conditioning might not be able to remove certain types of fermentation inhibitors present in poplar liquid fraction after pretreatment. This inference goes along with a research study by Persson et al. (32), which reported that conditioning spruce liquid fraction after pretreatment to pH 10 was far more effective in removing inhibitors (mostly furfural, HMF, and phenols) than only neutralizing to pH 5.5 using four different bases (NaOH, KOH, Ca(OH)2, and NH3). For this reason, the present study chose the well-established overliming as the one-step detoxification method used for the CPC liquid fractions after pretreatment, based on the NREL 2002 report practices (10).
Table 4. Ethanol fermentation yields of solid fraction after EH and liquid fraction after pretreatment (with and without overliming), and total sugar loss after with overliming.
|
|
Ethanol fermentation yield 1 (Y%T)
|
|
|
|
Solid fraction after EH
|
Liquid fraction
|
Overlimed liquid fraction
|
Total sugar loss after overliming 3 (%)
|
|
Control 2
|
84.6 ± 3.8
|
79.1 ± 1.6
|
77.7 ± 2.1
|
NA
|
Whole-tree chips (WTC)
|
Untreated
|
82.2 ± 2.7
|
1.9 ± 0.2
|
NA
|
NA
|
Acidic
|
82.8 ± 0.6
|
53.2 ± 1.3
|
NA
|
NA
|
Alkaline
|
82.3 ± 0.9
|
55.0 ± 0.2
|
NA
|
NA
|
Neutral
|
83.4 ± 0.6
|
49.0 ± 0.4
|
NA
|
NA
|
Clean pulp chips (CPC)
|
Untreated
|
83.7 ± 0.8
|
1.1 ± 0.0
|
45.3 ± 0.5
|
29.5
|
Acidic
|
82.5 ± 1.4
|
52.0 ± 3.2
|
53.3 ± 1.0
|
19.4
|
Alkaline
|
81.8 ± 1.7
|
55.1 ± 1.5
|
64.6 ± 0.6
|
20.4
|
Neutral
|
85.5 ± 2.4
|
55.6 ± 1.9
|
54.6 ± 1.3
|
19.2
|
Data represented as the mean values of duplicate analysis with standard deviation
NA = Not Applicable
1 Ethanol fermentation yield is expressed as a percent of theoretical yield (Y%T), according to Equation 1
2 The fermentation control contained reagent-grade sugars at similar concentrations to those in experimental samples
3 Accounting for glucose and xylose only (main sugars present in the liquid fraction after steam pretreatment)
Fermentation of solids after enzymatic hydrolysis
Since the liquid obtained after EH of the solid fraction contained only monomeric sugars and no inhibitors, all WTC and CPC samples reached a maximum ethanol conversion after 8h of reaction with very similar yields ranging from 82 to 85% (Table 4).
Fermentation of liquid fractions
General trends were observed in fermentation yields of both WTC and CPC liquid fractions after pretreatment without detoxification (Table 4). First, untreated samples resulted in negligible ethanol yields (1.9% and 1.1% of theoretical ethanol yield for untreated WTC and CPC, respectively). In contrast, liquid fractions from preprocessed WTC and CPC presented a significant improvement in ethanol conversion, with yields ranging from 49% to 56%. This finding could be associated with the removal of specific types of extractives with antimicrobial characteristics during preprocessing (Table 1), which were inhibiting the fermentation of untreated samples (9,33). Similar trends were reported by Hӧrhammer et al. (17), who observed a 50% increase in ethanol yield when acidic preprocessing was performed with 2-year-old whole-tree chips poplar biomass.
Fermentation of overlimed CPC liquid fractions
Since CPC was chosen as the base case feedstock due to its superior quality, overliming was performed in these liquid fraction samples to replicate existing ethanol biorefinery models. Not surprisingly, overliming increased the ethanol fermentation yield of untreated CPC from 1.1% to 45% (Table 4). The fermentation yields of acidic and neutral preprocessed CPC samples with overliming did not demonstrate improvements compared to the samples without overliming (53% and 55% ethanol yield, respectively). In contrast, alkaline preprocessed CPC sample had a 10% improvement on fermentation yield when overliming was done. It should be highlighted that, when comparing the increase in CPC fermentation yield exclusively via overliming (from 1.1% to 45%) with that via preprocessing only (from 1.1% to 52-55%), similar improvements were observed by both methods. Most importantly, overliming resulted in a sugar loss of approximately 30% in untreated CPC and 19-20% in all preprocessed CPC samples when compared to the original sugar content in the liquid fractions (Table 4). Hence, even though the fermentation yield increased with overliming (in the case of untreated and alkaline preprocessed CPC), the associated sugar loss results in a reduction of the final ethanol production per tonne of biomass. Consequently, preprocessing alone seems to be a better option than overliming because it improves the ethanol fermentation yield to similar extents without compromising the initial concentration of sugars.
Finally, it should be noted that the present study used non-genetically engineered microorganisms for the fermentation experiments, thus the fermentation yields shown in Table 4 were not optimized. NREL, for example, commonly uses recombinant co-fermenting bacteria to maximize the yield (15). Henceforth, in an actual large scale biorefinery where the fermentation is optimized, the concentration of monomeric sugars available for fermentation is the determining factor for the final ethanol production yield. With this in mind, this study demonstrated that acidic preprocessing could substantially improve the ethanol production of a biorefinery because of its higher monomeric sugar yield using both types of poplar biomass.
ECONOMIC ASSESSMENT
Large-scale biorefinery ethanol production
Among all scenarios investigated so far, three of them were chosen to be further assessed regarding ethanol production in a large-scale biorefinery. The chosen scenarios were: 1) CPC feedstock via pretreatment, enzymatic hydrolysis, overliming, and ethanol fermentation (as a base case scenario); 2) WTC feedstock via acidic preprocessing, pretreatment, enzymatic hydrolysis, and ethanol fermentation (to assess the effects of preprocessing using biomass with high NSC content); and 3) CPC feedstock via acidic preprocessing, pretreatment, enzymatic hydrolysis, and ethanol fermentation (to assess the effects of preprocessing using biomass with low NSC content). This way, it is possible to determine the effects of preprocessing using both types of biomass and compare it to the base case using overliming detoxification.
Figure 3 shows the ethanol yield (liters per tonne of biomass) of each scenario calculated based on the total monomeric sugar yield obtained in the experimental part of this work (Figure 2) from both liquid and solid fractions after SE and EH. For this assessment, the fermentation was assumed to be performed using a recombinant co-fermenting bacteria with 95% glucose conversion and 85% xylose conversion to ethanol (15). Next, the large-scale biorefinery ethanol production (million liters per year) was calculated by combining the ethanol yield and a feedstock usage of 250,000 dry tonnes/year (Figure 3).
Figure 3. Annual large-scale ethanol production of the three scenarios assessed.
It can be seen in Figure 3 that the base case scenario 1 using poplar CPC as feedstock had the lowest ethanol production (74.3 MM L/year) mostly due to the 30% sugar loss in the liquid fraction after pretreatment associated with overliming. Scenario 2, on the other hand, used poplar WTC as feedstock with acidic preprocessing and no overliming, resulting in ethanol production of 84.8 MM L/year. This increase of 10.5 MM L/year compared to scenario 1 was due to both a higher amount of monomeric sugars available for fermentation and no sugar loss resulting from overliming. Similarly, scenario 3 used CPC via acidic preprocessing and resulted in the highest ethanol production of 86.6 MM L/year as a result of using low-NSC biomass with higher initial sugar content. It is clear that acidic preprocessing has the potential to increase the ethanol production of a large scale biorefinery using both types of poplar feedstocks. To have a more complete picture of the economics of each scenario, this study compared the costs of the unit process involved (either preprocessing or overliming) and the different feedstocks used.
Cost assessment of preprocessing and overliming
First, the capital costs were assessed: the overliming unit operation was assumed to be the same as the one in the 2002 NREL report (10), while the preprocessing unit was assumed to have a similar configuration as the deacetylation process from the 2015 NREL report (34). Chemical Engineering Plant Cost Index (CEPCI) was used to properly adjust the price of the equipment to the year 2018, and a scaling exponent of 0.6 was used to adjust the capital cost of each equipment to our biorefinery size. The biorefinery size ratio was calculated based on the feedstock consumption rate (dry tonne/day). Second, the direct operating costs of both unit processes were calculated: the direct operating cost of overliming included chemicals (calcium hydroxide and sulfuric acid) and gypsum disposal to a landfill; while the direct operating cost of acidic preprocessing included water and sulfuric acid for a final liquid-to-biomass ratio of 4:1. All operating costs were calculated based on the latest pricing quotes and properly scaled to the biorefinery flow rates. The complete individual equipment and chemical prices used can be found in the Supplementary Material, Tables S.6 and S.7.
The calculated capital and direct operating costs of both processes are presented in Table 5. It can be seen that the capital cost of preprocessing was about twice higher than that for overliming, while the direct operating cost of preprocessing using fresh water was approximately 4 times higher than that of overliming due to the high cost of fresh water. Chen et al. (19) performed a techno-economic analysis of deacetylation of corn stover using a liquid-to-biomass ratio of 3:1, and for every gallon of ethanol produced their process required 3.5-4.5 gallon of fresh water, thus increasing the total costs. For this reason, the present study also considered the use of process water recycled from the system as a more cost-effective alternative for preprocessing, which lowered its direct operating cost by 45-fold (Table 5). The source of process water will be further discussed in the section “Proposed process model”.
Table 5. Capital and operating costs of overliming and preprocessing units.
|
Capital cost (MM $)
|
Direct operating cost with fresh water (MM $/year)
|
Direct operating cost with process water (MM $/year)
|
Overliming
|
$ 1.38
|
$ 0.96
|
$ 0.96
|
Preprocessing
|
$ 2.68
|
$ 4.18
|
$ 0.16
|
Overall cost assessment of scenarios
It is important to realize that Table 5 provides an incomplete picture of the overall process economics of the scenarios since they use different feedstocks and have different ethanol production yields. With this in mind, first, the annual cost of each feedstock was calculated and added to the operating cost of each scenario (Table 6). Based on the feedstock usage of 250,000 dry tonnes/year, the annual feedstock cost of poplar CPC was calculated as $30.2 MM, while the cost of poplar WTC was $20.1 MM. Second, to define the annual revenue of each scenario, their final ethanol production presents in Figure 3 (liters per year) was multiplied by the cellulosic ethanol selling price of $0.94 per liter. This price included the selling price of the fuel plus the cellulosic waiver credit (CWC) and D5 RIN, as regulated by the Renewable Fuel Standard program (35), and the CWC and D5 RIN prices were determined using the latest U.S. EPA 2019 guidelines (36). A summary of the capital and total operating costs (including the feedstock cost and using process water for preprocessing), as well as the annual revenue of the three scenarios, are presented in Table 6. It can be noted that the total operating cost of scenario 2, using WTC feedstock via acidic preprocessing, was $11 million cheaper than the base case biorefinery scenario 1 because of the cheaper feedstock. Besides, preprocessing considerably increased the final annual revenue in scenarios 2 and 3 when compared to the base case scenario 1 by $9.8 and $11.5 million, respectively.
Table 6. Capital cost, total operating cost, and revenue of the three scenarios proposed.
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Description
|
Capital cost (MM $)
|
Total operating cost*
(MM $/year)
|
Annual revenue (MM $/year)
|
Scenario 1
|
CPC feedstock, with overliming
|
$ 1.38
|
$ 31.20
|
$ 70.02
|
Scenario 2
|
WTC feedstock, with acidic preprocessing using process water
|
$ 2.68
|
$ 20.24
|
$ 79.85
|
Scenario 3
|
CPC feedstock, with acidic preprocessing using process water
|
$ 2.68
|
$ 30.41
|
$81.53
|
* Total operating cost including the feedstock cost.
Finally, to assess the economic benefits of switching from the base case scenario that uses CPC via overliming to the new proposed processes, the incremental return on investment (ROI) associated with making these process changes was calculated pairwise between scenarios 1 and 2, and between scenarios 1 and 3 (Equation 2). The ROI between scenarios 1 and 2 was determined to be 1600%, meaning that there is an enormous return on using WTC feedstock via acidic preprocessing using process water instead of the base case process. This astounding ROI is due to four main reasons: a) WTC feedstock is substantially cheaper than CPC, and the feedstock was the biggest contributor to the operating cost; b) using process water in the preprocessing step significantly decreased the operating cost of preprocessing; c) acidic preprocessing resulted in higher ethanol production and consequently higher revenue; d) the absence of overliming prevented the sugar loss and its resulting lower ethanol yields. In like manner, the ROI between scenarios 1 and 3 was 948%, demonstrating that by keeping the same feedstock and just switching the process from overliming to preprocessing still results in much higher revenue.
Proposed process model
As has been noted, preprocessing is a superior approach to conditioning the liquid fraction after pretreatment for subsequent fermentation compared to overliming because it eliminates the large sugar loss, results in higher ethanol production, and enables the use of low quality, but much cheaper, biomass feedstock. It also should be noted that biomass preprocessing will frequently be necessary for a biorefinery to remove the dirt and grift from the feedstock that would erode the downstream process equipment. In the present work, we propose that the preprocessing be engineered such that it cleans the feedstock and eliminates the need for the overliming process.
A new process design was proposed by the authors based on the NREL 2002 design (10) with some key modifications (Figure 4): a preprocessing unit was included prior to pretreatment, the overliming unit was removed, and process water was recycled to feed the preprocessing unit. Because of the simplicity of this process and the absence of ammonia conditioning, the evaporator design from the NREL 2002 report was maintained. In the proposed model, the evaporator had two main outlet streams: the stillage containing the organic and inorganic compounds in syrup form (which was directed to the combustor to generate electricity for the whole system), and the vapor condensate. The evaporator condensate stream was then directed to the preprocessing unit and contained enough water to reach the required liquid-to-biomass ratio of 4:1, while the excess of water was sent to the WWT plant. Because the evaporator was kept and the loading sent to WWT was minimized, the WWT plant in our process was assumed to be the same as the NREL 2002 design. According to NREL, this WWT configuration had an installed equipment cost of only $3.3 million (10). Therefore, our research group believes that by substituting overliming by preprocessing not only solves the problem with sugar loss and gypsum formation but also enables the adoption of a much simpler and cheaper WWT design.
Figure 4. Simplified process flow of proposed biorefinery using poplar feedstock via acidic preprocessing using process water.