Table 2: Different pretreatment technologies used in biorefinery and their advantages and disadvantages.

PretreatmentChemicals usedCatalyst recoveredSolidLiquidProcess conditionsWater usedAdvantagesDisadvantagesReference

(I) Physical
(1) Disk millingNANAWhole biomassNAMilling (10–30 mm) and grinding, particle size (0.2–2 mm)No(i) No chemical used,
(ii) Scalable 
(i) Poor sugar conversion,
(ii) Highly energy intensive process
[87]
(2) ExtrusionNANAWhole biomassNAScrew speed, 350 rpm, barrel temperature, 80°C, 40% moistureMinimal(i) Low temperature pretreatment and no degradation products formation,
(ii) No washing and conditioning,
(iii) No effluent and
(iv) Can be made continuous
(i) High energy cost,
(ii) More aberration of metal surface need frequent replacement 


[88]
(3) MicrowaveNANAEnriched cellulose and hemicelluloseGlucose, XyloseMicrowave 680 W, irradiation time 24 min and substrate concentration 75 g/LHigh(i) Short processing time,
(ii) High uniformity and selectivity,
(iii) Less energy input than the conventional heating
(i) Cost of reactor system will be high,
(ii) Additional safety requirements needed,
(iii) Low sugar conversion and low substrate concentration.
[117]
(4) Acidic
 (i) Dilute sulfuric acidDilute sulfuric acidNoEnriched cellulose + some hemicelluloseSoluble xylose 140–190°C, 0.4–2% sulfuric acid, 1–40 min. resident time High(i) Can be used for wide range of materials,
(ii) Produce hydrolyzed xylose during pretreatment





(i) Need to use expensive hastealloy reactor,
(ii) Difficult to controlling reaction condition,
(iii) Toxic degradation products,
(iv) Expensive to remove salt while recycling water
[99, 179]
 (ii) Organic acidAcetic acid or Fumaric acid or maleic acid, and so forth,NoEnriched cellulose and some hemicelluloseSoluble lignin and hemicellulose130–190°C, 50–90 mM of organic acidHigh(i) Fractionation of biomass in to soluble lignin rich hemicellulose stream,
(ii) Low pressure reactions
(i) Recovering acid is an expensive process,
(ii) High water usage to clean substrate after pretreatment
[100, 101]
 (iii) Concentrated acidSulphurous sulfuric, HF, HCl, phosphoric acid, nitric and formic acidYesCondensed ligninSoluble glucose and xylose or Soluble cellulose which is precipitatedShorter residence time(i) Few cases no need enymes to depolymerize cellulose,
(ii) When phosphoric acid is used cellulose I is converted to highly reactive amorphous cellulose,
(iii) Effective on soft wood
(i) Corrosion,
(ii) Energy intensive acid recovery step








[102]
 (iv) Acidic organosolvMethanol, ethanol, acetone, ethylene glycol and tetrahydrofurfuryl alcohol, water mixture, organic or inorganic acidSolvent recovered, No catalyst recoveryEnriched cellulose and most of the hemicelluloseLignin and some soluble hemicelluloseAcetone-water pretreatment (acetone : water molar ratio of 1 : 1) at 195°C, pH 2.0, 5 minutes residence timeMedium(i) Can separate pure lignin stream,
(ii) Lignin removal leads to increase cellulose digestibility
(i) High risk of high-pressure operation,
(ii) Flammability,
(iii) Volatility of solvents
[103]
 (v) SPORLdilute sulfuric acid, NaHSO3 and disc millingYesEnriched cellulose and some hemicelluloseLignin and hemicellulose180°C, 25 min. liquor/wood = 3 : 1 v/wHigh(i) High sugar yields,
(ii) Effective lignin removal, and
(iii) Recovery of biomass components in less chemically transformed forms.


(i) Sugar degradation at severe conditions,
(ii) Large volumes of process water used in postpretreatment washing,
(iii) High costs of recovering pretreatment chemicals.
[104]
(5) Neutral
 (i) Ionic liquid1-allyl-3-methylimidazolium-chloride ([AMIM]Cl), 1-ethyl-3-methylimidazolium-acetate ([EMIM]Ac).99%Enriched cellulose and hemicelluloseLignin and some hemicellulose100–150°C, few min. to hrs.High(i) Carbohydrate losses are generally low and
(ii) Degradation products are significant only at severe conditions.
(i) High solvent cost,
(ii) High solvent loading,
(iii) Cost of solvent regeneration,

[97]
 (ii) Liquid hot water WaterNAEnriched celluloseSolubilized hemicellulose160–220°C, 15 min. residence time High(i) No external chemical added,
(ii) Simple reactor system

(i) Large water use,
(ii) Some hemicellulose lost in water stream,
(iii) Low solids loading
[97]
 (iii) OzonolysisOzoneNoEnriched cellulose and hemicelluloseSoluble lignin degraded products and some hemicelluloseRoom temperature, Ozone spargingHigh(i) Effective removal of lignin and
(ii) Very low production of inhibitory products,
(iii) Reactions performed at atmospheric conditions
(i) Expensive due to large requirement of ozone
(ii) Some portion of lignin is lost due to cleavage during ozone pretreatment process
[98]
(6) Alkaline
 (i) AFEXLiquid or gaseous anhydrous ammoniaUp to 97%Whole biomassNA100–140°C, 1 : 1–2 : 1 ammonia to biomass loading, 30–60 min. residence time, 60–100% moisture.Medium(i) Volatile ammonia can be recovered and reused,
(ii) Lesser degradation product formation,
(iii) Dry to dry process,
(iv) Lignin relocation to surface help to densify the biomass
(i) Safety precautions for handling ammonia,
(ii) Ammonia recovery step is added cost,
(iii) Not efficient for hardwood biomass.



[89, 126, 132]
 (ii) ARPAmmonium hydroxideNoEnriched cellulose and some hemicelluloseSoluble lignin and hemicellulose160–180°C, 10–30 min. residence time, 0.5 g ammonium hydroxide per g of biomassHigh(i) Recalcitrant lignin can be removed,
(ii) Works very well for grasses

(i) High amount of water used in the process,
(ii) Energy intensive process,
(iii) Not efficient for hardwood biomass.
[90]
 (iii) SAA15% ammonia solutionNoEnriched cellulose and some hemicelluloseSoluble lignin and hemicelluloseSolid to liquid ratio 1 : 11, 60°C, 8–24 h, High(i) Lower reaction temperature


(i) Longer Residence time,
(ii) Large water usage,
(iii) Scale-up issues.
[91]
 (iv) NaOHNaOH NaOH recoveredCellulose II formation and some hemicelluloseSoluble lignin and hemicelluloseHigh(i) Conversion of highly reactive cellulose II
(ii) Solubilization of lignin

(i) Longer residence time,
(ii) Large water usage,
(iii) Scale-up issues,
(iv) Expensive catalyst recovery
[92]
 (v) Alkaline H2O2NaOH, H2O2NaOH recoveredEnriched Cellulose and some hemicelluloseSoluble degraded lignin and hemicellulose0.5–2% sodium hydroxide, 0.125 g H2O2/g biomass, 22°C, and atmospheric pressure for 48 h.High(i) Milder pretreatment condition.
(ii) Commercially used in paper industry and scalable,
(i) Large water use,
(ii) Catalyst recovery is expensive,
(iii) Energy content of lignin is lost due to oxidation.
[93]
 (vi) LimeCaO with and without oxygenNoWhole BiomassNA25–160°C, 120 min. to weeks, 0.07–0.2 g CaO/g biomassHighPretreatment can be done using inexpensive pretreatment reactor system(i) Large water use,
(ii) Catalyst recovery is expensive,
(iii) Residence time is longer
[94]
 (vii) Alkaline wet  oxidationOxygen or airNoWhole BiomassNA 120°C, 0.5–2 Mpa, 30 min. residence time.Less(i) Dry to dry process
(ii) Lesser degradation products formation



(i) Require high pressure equipment,
(ii) High cost of oxygen that is used as a catalyst,
(iii) Oxidation of lignin makes is lesser dense in energy
[95]
(II) Physiochemical
(1) Steam explosion (catalyzed using SO2)Steam and SO2NoEnriched celluloseSoluble hemicellulose180–210°C, 1–120 min. residence timeHighWorks well both for hardwood and herbaceous biomass
Expensive reactor system requirement due to high pressure operation[105]
(2) Supercritical CO2CO2 (21.37 Mpa) + Water = Carbonic acidNAWhole biomassNo112–165°C, 0–73% moisture, 10–60 min. residence timeMedium(i) Less corrosiveness,
(ii) Nontoxic chemical,
(iii) Non-flammability,
(iv) No waste stream
(i) High pressure reactions,
(ii) Need expensive reactor system.


[106, 107]
(III) Biological Microbes like fungus or bacteriaNoWhole biomass (with reduce cellulose and hemicellulose content)NA25–30°C, solid state fermentation, 80–120% moisture, 10–15 days residence timeHigh(i) Mild pretreatment condition,
(ii) Low energy consumption,
(iii) No chemicals needed


(i) Slow process and slow throughput,
(ii) Sugar conversion after pretreatment is not high,
(iii) Larger space requirement and
(iv) Need continuous monitoring
[108, 109]

Abbreviations: rpm: revolutions per minute; W: watt; NA: not applicable; L: liter; g: gram; C: centigrade; mm: millimeter; min: minutes; mM: millimolar; h/hrs: hour/hours; V/W: volume/weight; Mpa: mega Pascal, ha: hectare.