Overview of Biomass Preatreatment

8/12/2019 Overview of Biomass Preatreatment

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OVERVIEW OF BIOMASS

PRETREATMENT TECHNOLOGIES

YONGMING ZHUSCIENTIST

NovozymesTel. +4544460000Krogshjvej 36

2880 BagsvrdDenmark

[email protected]

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Wet oxidation

Wet oxidation was originally used as a means of wastewater treatment and soil

remediation, and later in the pretreatment of lignocellulosic biomass. Wet oxidation

involves water and air, oxygen, or hydrogen peroxide at elevated temperature and

pressure. Most of the hemicellulose is dissolved in the pretreatment, and it is possible

to achieve a moderate to high degree of delignification through oxidation (controlled

combustion). This process converts a large number of organic polymers (mainly

hemicellulose and lignin) to oxidized compounds, such as low-molecular-weight

carboxylic acids, alcohol, or even CO2and H2O. Wet oxidization is often carried out

with the addition of an alkali such as Na2CO3to reduce the reaction temperature and

the amount of hemicellulose being oxidized. The use of hydrogen peroxide hasattracted much interest in the past, but the high cost of this chemical could make this

technology economically unattractive. The introduction of pure oxygen to the reaction

has been challenged, since uncontrolled combustion can occur at the oxygen injection

points. In light of the facts mentioned above, it is unlikely that this pretreatment

method will find practical applications in biomass processing.

One advantage of the wet oxidation process is the lower production of furfural and 5-

hydroxymethylfurfural (HMF), which are strong inhibitors in the fermentation step, as

a result of oxygen degradation of these components. However, for the same reason,

the hemicellulose sugars are largely lost, and thus the overall process yield and

economy would be diminished. Another concern associated with the oxidation process

is its exothermal nature, which requires sophisticated control of the process

parameters.

Lime pretreatment

Lime is an inexpensive and safe chemical, and is recyclable using established

technology (Kiln process). The process of lime pretreatment involves slurrying the

lime (either quicklime (CaO) or slaked lime (Ca(OH)2) with water, spraying it onto the

biomass material, and storing or heating the material for a period of time. One

seemingly cost-effective method is to pile the biomass and recycle the water at

ambient temperature, but this requires a very long processing time, and the quality of

the pretreated substrate is strongly affected by environmental conditions.


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Alternatively, the pretreatment can be performed in a closed reactor with temperature

and flow controls.

The enhanced digestibility of lime-treated biomass results mainly from lignin removal.

However, because lime is a weak base and has low solubility, lime alone is not able to

remove enough lignin in some high-lignin biomass to achieve good digestibility. In this

case, an oxidant needs to be added to promote delignification, and the process

essentially turns into an alkaline wet oxidation process. Lime pretreatment retains the

hemicellulose in the biomass solids very well, but removal of acetyl and the various

uronic acid substitutions of hemicellulose is nearly complete. Due to the low severity

of the process, little sugar decomposition occurs. However, recycling of the lime (fromcalcium salt or dissolved Ca(OH)2) results in the loss of substantial heat from the

combustion of lignin, so the generation of electricity in the plant is not justified.

Dilute acid pretreatment

Pretreatment of lignocellulosic biomass with dilute sulfuric acid or SO2at low

concentration (both < 2%, w/w) and 160200 C can effectively remove

hemicellulose, producing C5 sugar monomers in the liquid. The porosity of the cell

walls increases, which in turn increases the enzymes access to the cellulose surface.

The digestibility of the remaining cellulose correlates well to the degree of

hemicellulose removal. The removal of lignin is insignificant, and most (> 70%) is

retained in solids after the pretreatment.

Because the acid pretreatment is carried out at high temperature, the corrosive action

of the acid on metal is a major consideration in the design of the pretreatment

reactor. A high-grade alloy resistant to hot acid corrosion must be used for the reactor

construction. The reactor cost is thus a significant element of the total capital cost.

Dilute acid pretreatment forms certain levels of furfural and lignin degradation

products that are inhibitory to fermentation organisms. A lime neutralization step is

usually used to both neutralize and detoxify the pretreatment stream. The generation

of calcium sulfate gypsum during neutralization creates a disposal problem. An

alternative detoxification option is to use ion exchange membrane, but this is very

expensive.


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Aqueous ammonia pretreatment

Aqueous ammonia pretreatment of lignocellulosic biomass can be performed either at

low temperature for a long time (SAA process; 25100 C, from 6 hr to 2 months) or

at high temperature for a short time (ARP process; 170190 C, 20 min 1 hr).

Delignification is effective, but hemicellulose solubilization is low or moderate. The

low-temperature treatment uses higher ammonia concentrations than the high-

temperature treatment (1530% vs. 510%, w/w). This pretreatment is an attractive

option because ammonia is cheap and safe, and is mostly recyclable. The

delignification in the low-temperature aqueous ammonia pretreatment (SAA process)

is highly selective, which means that the vast majority of carbohydrate remains in the

solids after the reaction. Water washing results in a clean and carbohydrate-rich

substrate with little need to worry about the sugar loss. This can simplify the

downstream bioprocess design but requires wastewater handling. The selectivity of

delignification at high temperature is not as good, that is, some hemicellulose sugars

are lost from the solids. A concern thus arises as to the recovery of the solubilized

hemicellulose sugars when using water washing.

Although most of the ammonia is recyclable after the pretreatment, some is not due

to its strong affiliation to water. In addition, some ammonia is consumed in the

pretreatment as a result of reaction with the acidic groups and lignin in biomass.

However, this portion of lost ammonia could be a nitrogen source for the fermentation

microorganisms. On-site storage of large quantities of liquid ammonia will be

associated with significant safety issues.

Ammonia fiber explosion (AFEX)

Instead of using aqueous ammonia, the ammonia fiber explosion (AFEX) process

treats lignocellulosic biomass (4050% moisture content) with pure liquid ammonia at

mild temperature (80100 C) and high pressure (4050 atm) followed by explosive

pressure release. Instantly releasing the pressure in the AFEX process disrupts the

fiber structure of biomass and increases the accessible surface area, which improves

the digestibility of the biomass. Since ammonia is easily evaporated from the biomass


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solids, the total solids content after this pretreatment can be very high (up to 60%,

w/w) compared to other thermochemical pretreatments.

Water washing does not remove much lignin from the treated substrate, and most of

the lignin and carbohydrates remains in the solids. The lignin goes through the

subsequent enzymatic hydrolysis together with the carbohydrates. Recycling ammonia

consumes a large amount of energy (mostly attributed to the evaporation of liquid

ammonia), and contributes significantly to the process cost. In addition, as in the

aqueous ammonia pretreatment, on-site storage of liquid ammonia in an AFEX-based

plant creates safety concerns.

Organosolv pretreatment

As the name indicates, organosolv pretreatment uses organic solvents (ethanol,

acetone, carboxylic acids, etc.) as the treatment agent. This type of pretreatment is

usually carried out at elevated temperature (e.g., 200 C) and high pressure. For the

ethanol-based organosolv pretreatment, the catalytic mechanism is identical to that of

autohydrolysis. The solvent (e.g., ethanol) plays a role only in improving the

solubilization of lignin and, probably, in protecting the cellulose from solubilization as

well. Lignin and hemicellulose can be completely solubilized from the solids in the

pretreatment process, and a significant percentage of the soluble carbohydrates is

further decomposed to by-products such as furfural or 5-hydroxymethylfurfural

(HMF). The dissolved lignin can be precipitated by evaporating the organic solvent or

diluting the stream with cold water. This lignin is of high quality because it contains

no sulfur and low ash, and may find utilization in specialty chemical production.

There is a significant concern regarding the use of flammable organic solvents in

biomass processing, because the high operating temperature and the mechanisms for

moving biomass increase the risk of combustion and explosion. Also, the recovery of

the organic solvents is highly energy intensive. Because of the high energy cost and

significant hemicellulose sugar decomposition, the organosolv process is more often

viewed as a fractionation method than a pretreatment technology.


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Mechanical pretreatment

The cellulose crystalline structure can be broken down by mechanical treatment such

as fine milling (nanomilling), sonication, and so forth. The size of the materials after

milling is usually 0.22 mm. Mechanical desizing of biomass feedstock will not

separate the different fractions (cellulose, hemicellulose, and lignin) to a significant

extent, since these are closely enmeshed in the structure of the biomass. However,

this type of treatment has been shown to improve the effect of downstream physical

and chemical pretreatments by increasing the contact surface area of the reactants.

The power required for mechanical pretreatment of lignocellulosic biomass depends on

the final particle size and the biomass characteristics, but is usually very high. This

largely prevents mechanical pretreatment from being considered an attractive optionin biomass processing.

Steam explosion/rapid expansion

In the steam explosion/rapid expansion operation, biomass treated with high-pressure

steam is rapidly discharged to a vessel operated at lower pressure (usually

atmospheric pressure). The sudden reduction in the pressure results in an explosion

effect on the biomass solids and physically destructs the fiber structure. The

crystallinity of the cellulose may decrease, and the surface of the substrate may

increase. The chemistry of steam processing involves the solubilization of

hemicellulose and lignin. These changes may help to improve cellulose digestibility.

However, it has long been disputed whether or not explosion is necessary in biomass

pretreatment. Despite this, some studies indicate that the main benefit of steam

explosion is to reduce the energy input for pumping and mixing of the substrates after

pretreatment. Steam explosion is often used as a technique to supplement acid or

base pretreatment at high temperatures.

Less developed pretreatment methods

A number of less developed pretreatment methods have also been pursued by

research institutes, academia, and industry; however, these technologies are currently

at an early stage compared to the pretreatment processes reviewed above. Because

of the limited information available, most of these technologies have not been


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reviewed in detail in the present paper. Below are a few comments on two emerging

technologies: ionic pretreatment and microwave pretreatment.

I o n i c l i q u id p r e t r e a t m e n t

Ionic liquids (ILs) are considered efficient and green cellulose solvents. They can

dissolve large amounts of cellulose in extremely mild conditions, with the possibility of

recovering nearly 100% of the ILs used at their initial degree of purity. This

technology has been used in the modern fiber-making industry to directly dissolve

cellulose. The dissolution mechanism involves the opening of the hydrogen bonds

between molecular chains of the cellulose using ILs. The solubilized cellulose was

found to have the same DP and polydispersity as the initial cellulose, but significantlydifferent macro- and microstructure, particularly the decreased degree of crystallinity.

As a result, the enzyme can easily access the dissolved cellulose, and hydrolysis is

extremely fast.

Application of ionic liquids has opened up a new method of biomass pretreatment and

fractionation. However, there are many challenges (including the high cost of ILs and

the methods for regeneration of the expensive solvent)to be addressed before this

technology can be put into practical use.

B io l o g ic a l p r e t r e a t m e n t

Biological pretreatment employs wood-degrading microorganisms, including white,

brown, and soft rot fungi, and bacteria to modify the chemical composition and/or

structure of the lignocellulosic biomass so that the modified biomass is more

amenable to enzyme digestion. Most biological pretreatment so far has focused on the

degradation of lignin in lignocellulosic biomass. However, degradation of lignin usually

accompanies the loss of cellulose and hemicellulose. In order to reduce and eliminate

the sugar loss during biological pretreatment, the microbial strains should have low

cellulase activity. White rot fungi are the most widely studied for biological

pretreatment since they can degrade lignin more effectively and more specifically.

Biological pretreatment appears to be a promising technique and has very clear

advantages, including no chemical requirement, low energy input, mild environmental


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conditions, and an environmentally friendly working manner. However, biological

pretreatment is very slow (taking from weeks to a year) and requires careful control

of growth conditions and a large amount of space to carry out. In addition, most

lignolytic microorganisms solubilize/consume not only lignin but also hemicellulose

and cellulose. Biological pretreatment therefore faces substantial techno-economic

challenges.

CONCLUSION

All of the pretreatment methods described above can lead to a high yield of glucosefrom cellulose as long as suitable feedstock and sufficient enzyme activities are used

in hydrolysis. In other words, it is not the enzymatic accessibility that matters in the

overall cost of biomass processing. However, the other factors such as enzyme

dosing, equipment cost, total recovery of sugars (especially hemicellulose sugars),

energy cost, and so forth, can vary dramatically among the various types of

pretreatment technologies and will result in different overall process economics. Also,

it is evident that the solid substrates obtained from different pretreatment methods

vary greatly in composition and properties, which implies that the optimal enzyme

recipes could be very different for each of the substrates. An in-depth understanding

of the substrates and how they affect the enzyme functions is important.

RECOMMENDED READING

Mosier N, Wyman C, Dale B, et al (2005), Features of promising technologiesfor pretreatment of lignocellulosic biomass. Bioresource Technology96 (6):

673686

Sun Y, Cheng JY (2002), Hydrolysis of lignocellulosic materials for ethanolproduction: a review. Bioresource Technology83 (1): 111

Eggeman T, Elander RT (2005), Process and economic analysis of pretreatmenttechnologies. Bioresource Technology 96 (18): 20192025


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Wyman CE, Dale BE, Elander RT, et al (2005), Coordinated development ofleading biomass pretreatment technologies. Bioresource Technology96 (18):

19591966

Yang B and Wyman CE (2007), Pretreatment: the key to unlocking low-costcellulosic ethanol. Biofuels,Bioproducts and Biorefining 2 (1): 2640

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Overview of Biomass Preatreatment