Influencing factors of liquefaction reaction
Effect of reaction time on liquefaction experiment
The liquid solid ratio was 5: 1, the catalyst mass fraction was 10%, the reaction temperature was 170 ℃. Fig. 1 shows, with the extension of the reaction time, the liquefaction rate of corn stalk increased gradually, it reached the maximum of 97.19% at 60 min. The main reason was that the corn stalk and the liquefaction agent were fully mixed in the reaction system with the increase of reaction time, so that the liquefaction rate was accelerated, which was conducive to the liquefaction of corn stalk. The liquefaction rate showed a slow downward trend when the reaction time lasted longer. It will produce a series of complex side effects as the reaction time was prolonged, such as polymerization, which resulted in the reduction of liquefaction rate. The reaction varied as time added up, there was a highest liquefaction rate at a certain time point, because side reactions took place when the degradation products were exposed too long to high temperatures [34].
Effect of reaction temperature on liquefaction experiment
The catalyst mass fraction was 10%, the liquid solid ratio was 5: 1, the reaction time was selected according to above optimum temperature of 60 min. As can be seen from Fig. 2, at 150℃ to 170℃, with the increase of temperature, the liquefaction rate increased significantly. This was mainly because the reaction temperature helped the acidic groups of concentrated phosphoric acid access to the fiber structure of the corn stalk and destroyed the crystal structure of cellulose, which made the corn stalk liquefaction rate speed up [35]. When the reaction temperature was more than 170 ℃, the liquefaction rate showed a downward trend. Until the temperature was further raised to 190℃, the liquefaction rate significantly reduced. Due to the high temperature resulting in condensation reaction between the liquefaction products, high temperature carbonization and partial product wall sticking phenomenon [36]. The reaction temperature plays a significant role in the control of the thermochemical conversion of biomass, and working at the appropriate temperature is a key factor in the improvement of the conversion and prevention of most side reactions, necessary to obtain high-purity and high liquefaction rate products [37].
Effect of catalyst content on liquefaction experiment
At the condition of the liquid to solid ratio was 5:1, the reaction time was 60 min, and the reaction temperature was 170 ℃. From Fig. 3, it could be seen that when the dosage of catalyst in the reaction system was relatively low, liquefaction reaction was relatively slow, and the liquefaction rate of corn stalk was relatively lower. With the increase of the amount of the catalyst, the liquefaction rate increased gradually. When the amount of the catalyst was 10%, the liquefaction rate of corn stalk was the highest, but the liquefaction rate decreased slightly when the amount of catalyst was over 10%.This was due to the degradation of lignin in acidic conditions into a free radical intermediate product, which can easily generate condensed residue, and the higher the concentration of acid, the more obvious of the condensation effect [38,39], in other words, lignin fragments are more apt to recondense in acid medium [40,41]. Therefore, the concentration of phosphoric acid did not only play a lytic role in the degradation reaction, but also played a catalytic role in the condensation reaction. To sum up, the liquefaction effect showed the optimum result when the catalyst dosage was 10%.
Effect of liquid solid ratio on liquefaction experiment
Liquid to solid ratio: it is the mass ratio of polyethylene glycol 400 and corn stalk powder in the liquefaction reaction. When the reaction temperature was 170 ℃, the reaction time was 60 min, and the catalyst dosage was 10%. As shown in Fig. 4, when the liquid to solid ratio was 1:1, the liquefaction rate was really low, when the liquid to solid ratio increased to 5:1, the liquefaction rate of corn stalk significantly increased and the liquefaction rate reached to the maximum. It can be seen that the liquid to solid mass ratio has a direct effect on the residue content and biofuel oil formation. The main reason was that, the liquid agent can fully make the raw material of corn stalk soaked with the increase of the ratio of liquid to solid, so that the raw material was in full contact with the agent, which was beneficial to the normal liquefaction reaction. Another reason is, with the increasing of liquid to solid mass ratio, degraded corn stalk components in the liquefied product’s fragments decreased, the recondensation of degradation products was impeded drastically. Hence the dosage of corn stalk should not be too high compared to liquid addition. As the liquid to solid ratio continued to increase on the basis of 5:1, the liquefaction rate didn’t raise obviously. Some side reactions might happen between lignin and liquefaction agent during the liquefaction process when the liquid to solid ratio is too high [42]. Considering the economic feasibility of liquefaction reaction, the liquid solid ratio 5:1 was chosen as the most suitable condition.
GC-MS analysis of liquefied biofuel oil
As shown in Fig. 5, in this study, GC-MS method was used to analyze the total ion flow diagram (TIC) of the biofuel oil prepared with polyethylene glycol 400 (PEG-400) as the agent. As shown in Table 1, the main components of biofuel oil and the relative peak area of biofuel oil were obtained by the spectral library retrieval and integration of the total ion flow chart.
Table 1 Analysis of main components of liquefied biofuel oil
Serial number
|
molecular
|
compound name
|
relative peak area%
|
1
|
C2H6O
|
ethanol
|
1.027
|
2
|
C2H6O2
|
Glycol
|
28.879
|
3
|
C3H6O3
|
lactic acid
|
0.473
|
4
|
C5H8O3
|
Acetyl propionate
|
0.429
|
5
|
C4H10O3
|
Diethylene glycol
|
26.572
|
6
|
C12H22O8
|
Polyethylene glycol succinate
|
0.310
|
7
|
C3H8O3
|
Glycerol
|
1.52
|
8
|
C6H8O4
|
2-methyl-5-oxo-tetrahydrofuran-2-carboxylic acid
|
3.984
|
9
|
C6H14O4
|
Triethylene glycol
|
12.0135
|
10
|
C4H6O2
|
3-hydroxyl-3-butene-2-ketone
|
1.091
|
11
|
C8H18O5
|
Tetraethylene glycol
|
6.206
|
12
|
C16H32O2
|
Palmitic acid
|
2.552
|
13
|
C18H36O2
|
stearic acid
|
1.202
|
14
|
C12H26O7
|
hexaethylene glycol
|
0.459
|
15
|
C22H46O12
|
Eleven glycol
|
4.579
|
16
|
C18H38O10
|
Nine glycol
|
1.818
|
17
|
C29H50O
|
beta- sitosterol
|
0.452
|
18
|
C24H50O13
|
Twelve glycol
|
4.030
|
Note: Table 1 only shows the peak area of the compound group, which is more than 0.3%
The biofuel oil obtained in a dark brown organic liquid form, similar morphology to many researches’ [43-45], results from GC-MS analysis showed that the composition of corn stalk liquefaction was quite complex. Prepared with polyethylene glycol as liquid agent of biofuel oil were detected, there were 31 kinds of organic compounds. The main ingredients included alcohols, organic acids, esters, ketones and sugars compounds. The biofuel oil components were close to those from deoxy-liquefaction and fast pyrolysis, which contain organic compounds that belong to alkanes, aromatic hydrocarbons, phenol derivatives and little amounts of ketones, esters, ethers, sugars, amines and alcohols [46, 47].
Table 1 lists the 18 major compounds (peak area of compound group was greater than 0.3% of total peak area). As can be seen from Table 1, a large number of oxygens containing functional groups existed in biofuel oil, they made the biofuel oil with a high oxygen content. This did not only reduce the calorific value of biofuel oil, but also affect the stability of biofuel oil [48, 49]. Also, can be seen from Table 1, the biofuel oil alcohols accounted for a large proportion, consisted of 11 species, of which the most abundant were ethylene glycol and diethylene glycol, 28.879% and 26.572%, respectively. The biofuel oil contained a large number of hydroxyls, which could be used for the preparation of polyurethane foam and polyurethane adhesive materials etc. [50]. There were 5 kinds of organic acids, the 2-methyl-5-oxo-tetrahydrofuran-2-carboxylic acid was the topest, 3.984%. In addition, the biofuel oil also contained a small number of esters and ketones, respectively, polyethylene glycol single amber esters and 3-hydroxyl-3-butene-2-ketone.
Infrared spectrum analysis of liquefaction residue and biofuel oil
By comparing the infrared spectra of the raw materials and the liquid products, the changes of the composition and functional groups of the corn stalk were investigated before and after the experiment. The composition of corn stalk mainly consisted of 3 main components, namely, cellulose, hemicellulose and lignin. The structure and properties of corn stalk in the process of liquefaction have been changed. liquefaction have been changed.
As shown in Fig. 6, raw material (curve B): 3398cm-1 absorption peak was O-H stretching vibration; 2918cm-1 absorption peak was C-H stretching vibration; 1731 cm-1 absorption peak was C=O stretching vibration, which was the characteristic absorption peak of hemicellulose. 1601 cm-1 and 1512 cm-1 absorption peak was lignin benzene skeleton stretching vibration; C-O stretching vibration (methoxy benzene) was 1251cm-1 absorption peaks; beta glycosidic bond stretching vibration was 894 cm-1 absorption peaks, which was the characteristic absorption peak of cellulose. Liquefaction residue (curve A): 3398 cm-1, 2918 cm-1, 894cm-1 absorption peak was narrow and the intensity was weakened, which was caused by the liquefaction of cellulose in the raw material [51,52]. The disappearance of 1731 cm-1 absorption peak showed that the raw material in the hemicellulose has been basically liquefied. The liquefaction residue had absorption peak at 1601cm-1, which proved the presence of lignin, and there was no absorption peak at 1251 cm-1 and 1512 cm-1, which proved that the C-O of aromatic cyclic lignin was degraded [53]. Liquefied biofuel oil (curve C): there was a strong absorption peak at 3398cm-1 and 2918 cm-1, which showed the reaction generated a large number of hydroxyl groups, that was, biofuel oil contained more alcohol. These signs were consistent with the results of GC-MS analysis. The absorption peak of 1635 cm-1 was the stretching vibration of C = O bond, which indicated that the reaction produced ester compounds.
SEM analysis of raw material and liquefaction residue
The main components of corn stalk powder were cellulose, hemicellulose and lignin. As can be seen from the Fig. 7 (a), the structure of the internal structure of stalk was layered structure, it was arranged in order, and the gap between two layers was large, and the combination was not tight [54]. Seen from Fig. 7 (b), fiber structure could not be observed from the liquefaction residue, and the residue was basically loose bulk and granular, which indicated that the corn stalk fiber was almost totally liquefied and had been destroyed in the process of liquefaction. It has reached a consensus for researchers that the key challenges in the material and biofuel utilisation of lignocellulose include: its resistance to breaking down into its components cellulose, hemicellulose and lignin [55]. Here, the atmospheric catalytic liquefaction showed great breakdown ability is of considerable economic importance for biofuel oil production.