3.1 Properties of tar
As shown in Table 3, the yield of tar cuts back with steam increasing. It is consistent with earlier reported results on biomass pyrolysis gasification with steam[10–12]. However, this trend is not maintained: the reduction rate reaches the maximum at S/F=0.8, declined by one third from 6.68% during pyrolysis, and then it is gradually slowing down. Similarly, the H/C atomic ratio of tar reduces after the addition of steam and reaches the minimum at S/F=1.2. And the oxygen element in tar reduces significantly with the growth of steam, which indicates better tar quality in terms of calorific value and stability[32, 33].
Table 3 Yield and elemental properties of tar.
|
S/F=0
|
S/F=0.8
|
S/F=1.2
|
S/F=1.6
|
Tar yield (g/g)
|
6.68±0.46
|
4.51±0.82
|
3.01±0.85
|
2.30±0.54
|
Ultimate analysis(wt.%)
|
C
|
62.72
|
69.31
|
76.37
|
74.53
|
H
|
5.34
|
5.75
|
5.29
|
5.62
|
N
|
0.36
|
0.33
|
0.37
|
0.35
|
Oa
|
31.58
|
24.61
|
17.97
|
19.5
|
H/Cb
|
1.022
|
0.996
|
0.831
|
0.905
|
a by difference; b atomic ratio.
3.2 Effect of steam on tar components
According to the measurement result of tar products (shown in the Appendix), the major compositions have been quantified and summarized in Table 4. Generally, O-heterocycles are highly corrosive to boilers, engines, and other equipment. Its yield is influenced significantly by temperature. Baldwin et al.[34] summarized the variation of biomass tar with temperature changing and thought there were more O-heterocycles in the tar at 500~600 °C, such as furan and pentose. They also reported the tar, like the O-heterocycles, cracked distinctly with the rise of temperature. However, the O-heterocycles yield is also affected by steam. After the addition of steam, the proportion of total O-heterocycles increases, and forming two new derivatives (Appendix, 24, 21/43/71). It can be attributed to the fact that steam can facilitate the dissociation of these O-heterocycles, which are formed by the polysaccharide structure of MCC after a series of reactions, such as decarboxylation, decarboxylation, and so on[35, 36]. In the reaction process, the compounds also crack into radical groups like the methyl radical to promote the substitution reaction and generate new derivatives.
Compared with complete pyrolysis, the steam stimulates the decline of open-chain compounds and increments alicyclic compounds in MCC pyrolysis tar significantly. However, the changing trend of alicyclic compounds is inversely proportional to the amount of steam. For example, with the continuous growth of steam, the cycloalkenes almost disappear after reaching the maximum value at S/F=0.8. Moreover, the rising rate of the cycloalkanes also slows down significantly. To be specific, the increase in steam has two effects: some of the olefin, alkyne produced from MCC pyrolysis are further decomposed into non-condensing gases (such as CO, CO2, C1-C4) or may react with aromatic compounds to form new compounds. On the other hand, O/H/OH active radicals generated by steam can adhere with tar fragment[12, 37], and produce many unsaturated open-chain compounds, which are likely to boost the forming of cyclohexene through the Diels-Alder reaction. At the same time, it is also accompanied by the hydrogenation reaction, which can further form cycloalkanes.
After the addition of steam, the proportion of aromatic compounds (the largest of the tar products) reduce slightly. For example, the yield of toluene is still over 10%, the same as during pyrolysis, while phosphonic acid, (p-hydroxyphenyl-) reduces and azobenzene disappears completely. In particular, the phosphonic acid, (p-hydroxyphenyl-) with multiple hydroxyls decreases by about 10% under the effect of steam; and this trend is more clearly with the increase of steam amount (S/F).
However, as shown in Fig 2, the trend of monocyclic aromatics compounds is enhancing with the amount of steam growing, and the yield of monocyclic aromatics compounds changes from 28.32% (S/F = 0) to 45.73% (S/F = 1.6). The amount of unsaturated monocyclic aromatics, which are the critical products in the pyrolysis-gasification process, is also proportional to steam. For example, alkenyl benzene increases significantly and reaches the maximum increments of 18.88% at S/F=1.6. However, it is challenging to generate polycyclic aromatic hydrocarbons (PAH) by the Diels-Alder reaction because the styrene is not a dienophile[38]. And the styrene is more likely to react with hydrogen radicals to form alkylbenzenes rather than cyclization and aromatization. It also can be proved by the increment of xylene and other alkylbenzenes in Fig 2. Significantly, the other alkylbenzenes increase considerably, which comes from 0.95% at S/F=0 to a maximum of 6.40% at S/F=1.2. Besides, monocyclic aromatics compounds, such as phenol, increase with the addition of steam and then slightly decrease at S/F=1.6, but all of these have a little fluctuation. It indicates that phenols and its derivatives are stable, and only a part participates in the reaction[39, 40]. Therefore, there are two possible pathways to reduce the yield of phenols: firstly, the phenols could be a precursor to form PAHs[41, 42], as shown in Fig 3; secondly, the reaction of substitution or polycondensation occurs.
Except for the toluene and phenol, naphthalene is also the most representative aromatic compound in biomass tar, whose molecular amount is the smallest among PAHs[43]. In combination with the previous analysis, naphthalene may be formed through a series of polymerization by the phenol precursor. Naphthalene, unlike benzene, is more likely to participate in electrophilic substitution as its poor stability. Therefore, the naphthalene is inclined to react with the H radical to form naphthalene,1,2-dihydro-. Furthermore, steam can facilitate the formation of compounds with caged scaffolds (Appendix, 37/63, 38/90), but their yields are inversely proportional to steam. Furthermore, the anthracene derivatives (Appendix, 68/93) also increases with the amount of steam growing after the first appearance at S/F=1.2.
Based on the above analysis, the transformation path of compounds can be summarized, as shown in Fig 4. In short, steam has two functions: it can promote compounds cracking, especially the compounds with double or triple bonds; on the other hand, it stimulates polymerization, which means the increment of steam leads to the increase of PAHs and facilitates the formation of the caged scaffold.
Table 4 The main compounds distributed at different S/F ( the first stage was 600℃, and the second stage was 800 ℃)
|
S/F=0
|
S/F=0.8
|
S/F=1.2
|
S/F=1.6
|
Main types of compounds(%)
|
Open chain compounds
|
5.39
|
-
|
1.16
|
-
|
Alicyclic compounds
|
3.74
|
10.33
|
8
|
7.36
|
O-heterocycles a
|
4.95
|
9.01
|
6.14
|
8.03
|
Aromatic compounds
|
85.92
|
80.66
|
84.7
|
84.61
|
Open chain compounds(%)
|
n-Hexane
|
-
|
-
|
1.16
|
-
|
1,5-Hexadiyne
|
5.39
|
-
|
-
|
-
|
Alicyclic compounds(%)
|
Cyclaneb
|
1.71
|
3.24
|
5.6
|
5.81
|
Cycloalkenec
|
2.03
|
3.61
|
-
|
-
|
Other alicyclic compoundsd
|
-
|
3.48
|
2.4
|
1.55
|
Aromatic compounds(%)
|
Benzene
|
3.47
|
8.46
|
5.41
|
6.53
|
Toluene
|
12.88
|
15.49
|
10.59
|
12.3
|
Benzene, (2-nitroethyl)-
|
-
|
5.76
|
4.74
|
2.85
|
Phosphonic acid,(p-hydroxyphenyl-)
|
15.21
|
5.94
|
4.35
|
-
|
Xylenese
|
6.56
|
8.61
|
8.86
|
9.59
|
Other alkylbenzenesf
|
0.95
|
1.13
|
6.41
|
4.97
|
Alkenyl benzeneg
|
7.93
|
15.07
|
7.86
|
18.88
|
3-Methylphenylacetylene
|
-
|
-
|
1
|
-
|
Benzene-alcoholh
|
1.04
|
0.74
|
-
|
-
|
Indenesi
|
3.12
|
-
|
6.39
|
3.83
|
Phenols j
|
10.93
|
12.98
|
12.41
|
9.38
|
Naphthalene and its derivativesk
|
4.1
|
3.92
|
11.18
|
9.6
|
Azobenzene
|
18.8
|
-
|
-
|
-
|
Other aromatic compoundsl
|
0.93
|
2.56
|
5.5
|
6.68
|
a O-heterocycles (Furfural,2-Furancarboxaldehyde,5-methyl-,Furan,2,5-dimethyl-,)
b Cyclane (cyclopentane,methyl-;cyclohexane)
c Cycloalkene (1,3-cyclopentadiene,5-methyl-,1,3,5,7-Cyclooctatetraene)
d Other alicyclic compounds (Tetracyclo[5,3,0,0,<2,6>,0<3,10>]deca-4,8-diene; 10-Hydroxytricyclo[4,2,1,1,(2,5)]dec-3-en-9-one)
e Xylenes (p-Xylene;Benzen,1,3-dimthyl)
f Other alkylbenzenes (Benzene,1-ethyl-3-methyl-;Benzene,(1-methylethyl)-;Benzene,1,2,3-trimethyl-)
g alkenyl benzene (Styrene; Benzene,2-propenyl-; Benzene,1-propenyl-; Benzene,2-propenyl-)
h Benzene-alcohol (Benzyl alcohol;2,5-dimethylphenyl methyl carbinol)
i Indenes (Indene; 1H-Inden,3-methyl-)
j Phenols (phenol,2-methyl-;phenol,3-methyl-;Phenol,4-(3-hydroxy-1-propenyl-);phenol,2,5-dimethyl-,2-Propenal,3-pheny;-;phenol,2,4-dimethyl-;1-Phthalanol,1,3,3-trimethyl-)
k Naphthalene and its derivatives (Naphthalene;Naphthalene,1,2-dihydro-;Naphthalene,1-methyl-;1-Naphthalene-1-yl-ethylideneamine)
l Other aromatic compounds (Benzofuran,2-methyl-; Benzocycloheptatriene; 1,4-Dihydro-1,4-ethanoanthracene)
3.3 The reaction mechanism of tar
Given the experimental results in 3.2, the Gibbs free energy of main tar components (yield > 4%) under pyrolysis conditions is summarized as listed in Table 5. The results show that the phosphonic acid, (p-hydroxyphenyl-), yield =15.21%, has the lowest Gibbs free energy (- 875.45883 Hartree), which means it has strong reactivity. And as shown in Table 4, phosphonic acid, (p-hydroxyphenyl-), is likely to react with steam, cracking and completely disappeared.
The CDD is widely used to predict the reaction sites[44], and the electrophilic reaction is more likely to happen when the value of CDD is smaller. As shown in Table 6, the O7 connected with a benzene ring is most likely to be attacked. The other atoms( C1, O11, C4, O10, H16, P8, and O9) are also possibly attacked, but the activity decreases in turn, which is consistent with the experimental results. Besides, there is a direct correlation between LBO and bond dissociation energy, which is also instrumental in predicting reaction sites. The LBOs between atoms are listed in Fig 5. Small LBOs of C4-C7, P8-O9, and P8-O10 indicate that the hydroxyl groups connected with C4 or P8 are probably to break. The following analysis will focus on the removal mechanism of the phosphonic acid group.
Table 5 The Gibbs free energy of main tar components (yield > 4%) formed from MCC pyrolysis at 800 ℃
Components
|
1,5-hexadiyne
|
Toluene
|
Styrene
|
P-Xylene
|
1,3-cyclopentadiene,5-methyl-
|
Free energy(Hartree)
|
-232.00
|
-271.66823
|
-309.77283
|
-310.98244
|
-233.51467
|
Components
|
Furfural
|
Phenol,3-methyl-
|
Naphthalene
|
Azobenzene
|
Phosphonic acid,
(p-hydroxyphenyl-)
|
Free energy(Hartree)
|
-343.52177
|
-346.92599
|
-386.02201
|
-572.95485
|
-875.45883
|
Table 6 The electron density of phosphonic acid, (p-hydroxyphenyl-) by CDD
NO.
|
|
NO.
|
|
NO.
|
|
C1
|
-0.0669
|
O7
|
-0.0875
|
H13
|
0.0179
|
C2
|
0.0611
|
P8
|
-0.0122
|
H14
|
0.0157
|
C3
|
0.0458
|
O9
|
-0.0088
|
H15
|
0.0211
|
C4
|
-0.0398
|
O10
|
-0.0177
|
H16
|
-0.0163
|
C5
|
0.041
|
O11
|
-0.0426
|
H17
|
0.0058
|
C6
|
0.068
|
H12
|
0.0184
|
H18
|
-0.003
|
Combined with the analysis of active sites, the possible reaction pathways are shown in Fig 6. R-1 and phosphonic acid may have a substitution reaction to form P-1 and H2O during cellulose pyrolysis (path 1). Moreover, R-2 is produced from cellulose pyrolysis, which can further react with OH radicals to form P-1 and H radical (path 2). But when the steam is added, the R-2 also probably reacts with H2O to form P-2 and phosphonic acid (path 3). Thus, there is the competition of OH radical and H2O to react with R-2. The Gibbs free energy of each possible reaction path is listed in eq. ( 1 )-( 3 ), all of these values are negative. It indicates that the reactions of different ways can be spontaneous. In particular, the Gibbs free energy of path 3 is -61.568kj/mol, which means that H2O is most likely to set a dominant position in competing with OH radicals for reacting with R-2. It also verifies the experimental result of cellulose pyrolysis tar that phosphonic acid, (p-hydroxyphenyl-) decreases sharply with the addition of steam.
The energy barriers of the three paths are listed in Fig 7, where also shows molecular structures (including reactants, transition states, products) involved in the reaction process. To further analyze the removal mechanism of the phosphonic acid group, this part will focus on the competition between H2O and OH radical for R-2 (path 2 and 3).
Path 2 is the reaction path of hydroxyl (O9-H17) attacking the P8-H19 bond of R-2 and then substituting the H19 atom. The bonds of O9-H17 and C1-P8 are slightly extended due to the mutual attraction between O9 and P8 atoms during the substitution of OH radical for H19 atom. At the same time, the H19 atom gradually separates from R-2 due to the offensive OH radical. When R-2 transforms to the transition state(TS5), the bond length of O9-H17 extends marginally from 0.96 Å to 0.981 Å, and the bond distance of the C1-P8 bond is stretched 0.05 Å to 1.85 Å, while the bond length of P8-H19 is lengthened from 1.410 Å to 1.488 Å. Besides, the angle between O9-P8 and P8-H19 is 58.93°, and the angle between P8-H19 and benzene ring plane is also 89.41°. All the changes of bond distance and angle promote the breaking of the P8-H19 bond and substitution of OH radical. Then P8 atom and O9 atom gradually form bond 7, the bond length shrinks to 1.61 Å (during the transition state that is 1.97 Å), and P8-H19 bond breaks, H19 gradually deviates from the benzene ring plane, C1-P8 bond also shrinks to 1.79 Å, finally forming a stable product system P-1.
Path 3 is the reaction process of H2O attacking the C1-P8 bond of R-2 and then substituting phosphonic acid. In the process of attacking the bond orbital, the H2O molecule gradually approaches the C1-P8 bond, and the bond length of H19-O18 stretches with the molecular angle expands. When the R-2 converters to transition state (TS3), the bond distance of H19-O18 extends to 1.44 Å, and the molecular angle extends from 109.50° to 113.69°. What is more, the bond length of C1-P8 is stretched from 1.80 Å to 2.08 Å, while the phosphonic acid group deviates from the benzene ring plane, and the C1-P8 bond forms an angle of 40.84 °with the benzene ring plane. All of these make for the substitution of the H atom. After that, the H19-O18 bond breaks to produce the H19 atom and OH radical (O18-H20). The H19 atom further attacks C1, and the P8 is also attacked by OH radical. In this case, the atom distance between C1 and H19, and between O18 and P8 is 1.22 Å and2.03 Å, respectively. And then, the C1-P8 bond breaks to forms C1-H19 and P8-O18 bond, which bond length shrinks to 1.09 Å and 1.62 Å, respectively. Finally, the compound forms a stable structure (P-2).