Table 8 Summary of some benefits and limitations of various lignocellulosic biomass pretreatment methods

1

Mechanical milling

(i) It reduces the particle size and increases the available surface area

(ii) Cellulose crystallinity of the lignocellulose is reduced

(iii) The final particle size of the feedstock can be controlled

(iv) It makes materials handling easy

(i) The energy required for this method is high and most times energy consumption is more than the energy released by the lignocellulose feedstock

2

Extrusion

(i) The fiber is shortened and defibrillated

(ii) It releases low inhibitory products

(iii) The method operates at higher solid loadings

(iv) The treatment time required is short

(i) The method required high energy

(ii) The impact of the method is limited without chemical addition

(iii) It is mostly efficient on herbaceous lignocellulose materials

3

Pulsed electrical field

(i) It is operated at ambient conditions

(ii) Lignocellulose cells are interrupted

(iii) The equipment for the process is simple

(i) There is need for more research in this area because there is little information about it

4

Pyrolysis

(i) It releases both liquid and gaseous products

(i) It requires very high temperature

(ii) The system produce ash

(iii) Formation of inhibitory compound is possible

5

Acid hydrolysis

(i) Hemicelluloses are hydrolyzed to xylose and other sugars

(ii) It modifies the lignin arrangement

(iii) The acid itself is not needed always needed for enzymatic hydrolysis

(iv) It hydrolyzes lignocellulose materials into fermentable sugars

(i) The cost of setting up the process is high

(ii) Equipment with high resistance to corrosion is needed because of the corrosive properties of the acid

(iii) The system produces toxic substances

(iv) The process release inhibitory compounds

6

Alkaline hydrolysis

(i) It dislodges lignin and hemicellulose

(ii) It improves the available surface area of the feedstock

(iii) The sugar degeneration is lesser when compared with acid pretreatment

(i) The residence time needed is long

(ii) The alkaline used is not recoverable

(iii) The salts liberated during the process are consolidated into the feedstock

(iv) The process liberated some inhibitors

7

Ozonolysis

(i) It reduces lignin component drastically

(ii) The process does not generate toxic substances

(i) The quantity of ozone needed is high

(ii) The method is expensive

8

CO₂ explosion

It improves the available surface area of the feedstock

It is economical

It does not produce inhibitory substances

(i) It has no effect on lignin and hemicelluloses

9

AFEX

(i) It improves the available surface area

(ii) Certain percentage of lignin and hemicellulose are eliminated

(iii) The process does not produce inhibitory compounds that can affect the downstream process

(iv) It is very effective and selective for reaction with lignin

(i) It is not effective for lignocellulose with higher percentage of lignin

(ii) The effectiveness on softwood is poor

(iii) High cost of ammonia and the environment is a major challenge

10

Biological

(i) It degrades both hemicellulose and lignin

(ii) It requires low energy

(i) Hydrolysis rate is very low

11

Steam explosion

(i) It degrades hemicellulose and softens the lignin

(ii) It is economical

(iii) It required low initial capital investment

(iv) It requires low energy

(v) It has little or no effects on the environment

(i) The process destructs certain percentage of xylan

(ii) The debasement of lignin-carbohydrate matrix is not complete

(iii) Certain compounds that hinder the microorganisms are released

(iv) It is not effective for softwood treatment

12

Deep eutectic solvents

(i) It removes lignin and some portions of hemicellulose

(ii) It is a green solvent that is biodegradable and compatible

(i) The process is not effective when the pretreatment temperature is high

13

Supercritical fluids

(i) It removes the lignin and reduces the cellulose content

(ii) The process does not degrade sugars since the solvent used is green

(iii) This technique is appropriate for mobile biomass pretreatment processors

(i) The total costs of setting up the process are too high

14

Microbes

(i) It degrades lignin and hemicellulose selectively

(ii) It is eco-friendly

(i) Due to slow yield, the process can take a longer period (several weeks)

15

Organosolv

(i) It dislodges lignin and hemicellulose disintegration

(ii) It generate feedstock with low lignin residue that lower unneeded adsorption of enzymes

(iii) The chemical can be retrieved and reuse

(i) The capital investment for the method is very high

(ii) Difficulties in the handling of harsh organic solvents

(iii) Inhibitory compounds are generated from this method

16

Oxidation

(i) It dislodges lignin and hemicelluloses

(ii) It degrades cellulose partially

(iii) By-products produced are usually low

(i) The cost of setting up the process is high

17

Ionic liquid

(i) It reduces the crystallinity and slightly removes hemicellulose and lignin

(ii) The process requires low pressure solvent equipment

(iii) The process takes place under modest reaction conditions

(i) High cost of the process

(ii) Synthesis and purification of the liquid is complex

(iii) The process is toxic

(iv) Biodegradability is poor

(v) An inhibitory compound that hinders enzymatic hydrolysis is released

18

Liquid hot water

(i) It eliminates soluble lignin and hemicelluloses

(i) The leftover lignin hinders the subsequent hydrolysis by the enzymes

(ii) The quantity of water is needed

(ii) It is energy intensive

19

SPORL

(i) It eliminates lignin

(ii) It is efficient for softwood and hardwood

(iii) It requires low energy

(i) It required size reduction prior to pretreatment and this size reduction required high energy

20

Hydrothermal

(i) It does not require the use of chemical

(ii) It hydrate cellulose

(iii) It removes hemicellulose part

Certain percentage of lignin is removed

(i) There is formation of inhibition in the process

(ii) High temperature is required

21

Combined process

(i) They are more effective than a single process

(i) They are often complex

(ii) They are not always economical

22

Microaerobic

(i) It is considerably faster

(ii) There is no process challenges associated with fibers and big chunks in the biogas digester

(i) Some of the organic matters that are supposed to be digested to methane are digested to carbon dioxide if the pretreatment time is too long

(ii) The leach bed reactor needs to be emptied and the solid fractions removed of

23

Enzymes

(i) Higher substrate solubilization

(ii) It is sustainable and ecological

(i) Relatively high price of enzymes for a limited improvement in biogas yield

(ii) The process is slow

24

Ultrasound and irradiations

(i) It increases the pore size and surface area

(ii) It disrupts feedstock crystallinity and reduces polymerization

(i) It consumes high electricity

(ii) High cost of equipment

(iii) Maintenance of the equipment is expensive

25

Nanoparticles

(i) It has high surface area to volume ratio

(ii) It has high selectivity, specificity and potential catalytic activity

(iii) It is an eco-friendly method

(iv) It does not produce inhibitory compound

(i) Some nano-particles inhibit biogas producing microorganisms

(ii) Operation cost is high

(iii) Some nanoparticles are toxic

(iv) They have low stability and reusability

26

Wet oxidation

(i) It is very effective in pretreatment of feedstock with high lignin content

(i) Cellulose is less affected

(ii) Possibility of producing inhibitory products at high temperature is high

(iii) It is not advisable at the industrial scale because of the combustible nature of oxygen and high cost of hydrogen peroxide used in the process

27

High hydrostatic pressure

(i) The pressure is distributed proportionally in all parts of the feedstock irrespective of its shape and size

(ii) Pressure favors all structural reaction and changes that involve a decrease in volume

(i) Exorbitant cost of the equipment

28

High-pressure homogenizer

(i) It has high disintegration potential

(ii) It requires minimal operation cost

(iii) It is easy to operate and handle

(iv) It does not require chemical addition

(i) The initial cost is high

(ii) The method depends on the shear stress as a result of pressure gradient