Issue 41, 2014 - Research progress in coupling pretreatment of lignocellulose

Research progress of coupling method pretreatment of lignocellulose

□ Jilin Design Institute, Northeast Refining and Chemical Engineering Co.
, Ltd.
, CNPC Guan Liang

Since the 21st century, with the increase of oil prices, the use of lignin cellulose to ethanol has become a research hotspot
at home and abroad.
The main process routes of lignocellulosic ethanol production are: pretreatment of raw materials, cellulose hydrolysis, fermentation, purification and separation
.
Among them, pretreatment is very important
in the production of lignocellulosic ethanol.
Commonly used pretreatment methods include physical method, acid-base hydrolysis method, steam blasting method, biological method, etc.
, these treatment methods have certain defects and limit their industrial application
.
In recent years, the coupling pretreatment method is expected to overcome the shortcomings of the above methods, increase the yield of reducing sugars, reduce the formation of fermentation inhibitors, and shorten the process time
.
In this paper, the research progress of different coupling methods for pretreatment of lignocellulose is reviewed, and the advantages and disadvantages of various coupling methods are discussed, in order to provide some reference
for the development of cellulosic ethanol.

1.
The significance of lignocellulose pretreatment

The main components of straw are cellulose, hemicellulose and lignin
.
Cellulose is a polymer linear polymer formed by glucose through β-1,4-glycosidic bonding, which has high crystallinity and is not easy to be hydrolyzed; Hemicellulose has great changes in composition and structure, is a branched polymer, its monomers mainly include xylose, arabinose, glucose, galactose, mannose, etc.
, easy to be hydrolyzed; The basic unit of lignin is phenylpropane, a polymer compound linked by ether bonds and C-C bonds, which not only cannot be hydrolyzed by itself, but also forms a protective layer around cellulose and hemicellulose, affecting the hydrolysis
of cellulose and hemicellulose.
The raw material pretreatment technology can partially destroy the encapsulation of lignin and the crystalline structure of cellulose, partially hydrolyze cellulose and hemicellulose at the same time, increase porosity, increase specific surface area, and make hydrolase contact with cellulose surface more fully, thereby improving hydrolysis efficiency and subsequent ethanol yield
.
The intuitive pre-processing process is shown in
Figure 1.

2.
Coupling method

Acid-base coupling method The acid-base pretreatment method is one of the
earliest and most commonly used methods.
Acid hydrolysis converts some cellulose and hemicellulose into fermentable sugars, reducing the burden
of subsequent hydrolysis and fermentation processes.
It is that acid is not very destructive to lignin, simple acid treatment consumes a large amount of acid, environmental pollution is greater
.
Alkali pretreatment is the most effective chemical method
to hydrolyze the ester groups between lignin, cellulose and hemicellulose, thereby destroying the connection between lignin, cellulose and hemicellulose.
Therefore, pretreatment with lye before acid treatment can greatly reduce the amount of solvent and reduce the environmental load
.
Qi Wei et al.
used acid-base method to pretreat straw, and the lignin yield was about 80%, hemicellulose hydrolyzed to monosaccharides, and the cellulose retention rate was about
90%.
Lu et al.
pretreated sugarcane bagasse with dilute sodium hydroxide coupled with peracetic acid, and found that compared with the simple alkali pretreatment method or acid pretreatment, the coupling method had milder pretreatment conditions and higher
efficiency in removing lignin.

Dilute acid and gas burst coupling method The mechanism of the gas burst method is to use high-pressure steam to penetrate into the interior of lignocellulose, and then instantly reduce the pressure, so that the steam is released from the closed pores in the form of an air flow to achieve the destruction of
lignocellulose.
At the same time, the high temperature and high pressure operating conditions also destroy the hydrogen bonds inside some lignocellulose, free out new hydroxyl groups, make the structure loose, greatly increase the specific surface area and porosity of biomass, and greatly improve the enzymatic hydrolysis efficiency
of the subsequent process.
The coupling method of dilute acid and gas explosion pretreats biomass raw materials, which can increase the saccharification efficiency
.
The specific steps of this method are: first dilute acid hydrolysis to achieve the purpose of improving the conversion rate of xylose, followed by gas explosion to treat and destroy the structure of
lignocellulose.
Recently, Sun et al.
used dilute sulfuric acid coupled gas burst method to pretreat straw straw, and found that after coupling pretreatment, the yield of xylose was improved and the production
of inhibitors in subsequent processes was reduced.
Linde et al.
pre-treated wheat straw with 0.
2% H2SO4 before steam blast treatment, and treated it at 190 °C, 200 °C and 210 °C for 2min, 5 min and 10 min
, respectively.
The results showed that the yield of glucose and xylose was highest
when treated at 190 °C for 10min.

Supercritical CO2 and gas explosion coupling method The advantage of this method is that the addition of CO2 to form carbonic acid during treatment can significantly improve the hemicellulose hydrolysis efficiency
.
Pretreatment with CO2 has the advantages of acid catalysis, without the disadvantages of acid catalysis, does not produce compounds that inhibit subsequent hydrolysis and fermentation, while the viscosity and diffusion coefficient of supercritical CO2 are close to gas, while the density and solvation ability are close to liquid, and have good solubility
for hemicellulose in lignocellulose.
Alinia et al.
pretreated wheat straw by coupling supercritical CO2 and gas explosion method, and compared
it with the traditional supercritical CO2 method.
The results show that the coupling method is divided into two steps, namely steam injection, the operating conditions are: (200 °C, the operation time is 15 minutes), and the supercritical CO2 gas burst, the operating conditions are: (190 °C, 12 MPa, the operation time is 60 minutes).

Compared with the traditional supercritical CO2 method (190°C, 12 MPa, operation time 30 minutes), the coupling method has higher
pretreatment efficiency.

Dilute acid coupled biological pretreatment method The principle of biological pretreatment method is to use microorganisms that can decompose lignin and remove lignin to destroy its encapsulation of
cellulose.
In biological pretreatment, white rot, brown rot and soft rot and other microorganisms are often used to degrade lignin to improve the enzymatic hydrolysis saccharification rate of cellulose and hemicellulose, biological treatment method has the advantages of mild action conditions, low energy consumption, strong specificity, no environmental pollution, low treatment cost, etc.
, but there are few types of lignin degradation microorganisms, low activity of decomposition enzymes, long action cycle and other problems
.
The acid pretreatment method can dissolve hemicellulose well, but it has the disadvantages
of large environmental pollution and high energy consumption.
Therefore, some scholars combine biological method with acid treatment method, in order to prevent acid from destroying microorganisms, the use of weak acids with relatively mild acidity, the results of the study found that this method can greatly improve the saccharification efficiency and ethanol yield
.

Biological method and gas explosion method coupling method Since white rot bacteria can only destroy lignin, the degradation efficiency of hemicellulose and cellulose is low, and the cycle of biological treatment of lignocellulose is too long, and some cellulose and hemicellulose will be consumed by bacteria during treatment, reducing the yield of monosaccharides, which will affect the yield of ethanol
.
Therefore, some scholars have proposed that the combination of biological method and gas explosion method shows that compared with the traditional biological method, the coupling method reduces the pretreatment time, reduces the loss of cellulose in the process, and improves the yield
of monosaccharides.

Microwave and acid/base treatment coupling method Microwave is a new energy-saving, temperature-free heating technology
.
Microwave by changing the structure of lignocellulose, lignocellulose decrystallization, can degrade part of lignin and hemicellulose, increase the specific surface area, improve the efficiency of enzymatic hydrolysis (saccharification) of plant cellulose, microwave pretreatment has a short time, simple operation, saccharification effect is obvious, but the cost is high, difficult to achieve industrial application
。 Yue Jianzhi et al.
compared the reducing enamel content of sorghum straw enzyme hydrolysis in microwave radiation pretreatment and microwave radiation combined with acid and alkali pretreatment, and the results showed that microwave radiation pretreatment alone had little effect on the enzymatic hydrolysis of sorghum straw, and microwave combined alkali treatment had a greater
effect on the hydrolysis of sorghum straw enzyme than microwave combined acid pretreatment.
Deng Hui et al.
used alkali method and microwave/alkali coupling method to pretreat cotton straw, and found that compared with alkali method, microwave/alkali coupling method greatly shortened the pretreatment time
.

3.
Conclusion

As a key step in converting lignocellulosic biomass into fuel ethanol, pretreatment technology currently has problems such as high economic cost and serious pollution, and the technology that can be applied to industrial production is not mature
.
Therefore, combined with the structural characteristics and component properties of raw materials, exploiting strengths and avoiding weaknesses, complementing each other's advantages, and adopting multi-method coupling pretreatment technology with low cost, good treatment effect, less environmental pollution, and non-toxic to subsequent processes as much as possible is the development trend
of wood fiber raw material pretreatment in the future.
In this paper, the coupling pretreatment method is reviewed and analyzed, which provides a basis for selecting reasonable treatment methods, and then lays a solid foundation
for the industrialization of ethanol.