Current Challenges in Commercially Producing Biofuels from Lignocellulosic Biomass

Table 2

Different pretreatment technologies used in biorefinery and their advantages and disadvantages.

Pretreatment	Chemicals used	Catalyst recovered	Solid	Liquid	Process conditions	Water used	Advantages	Disadvantages	Reference
(I) Physical
(1) Disk milling	NA	NA	Whole biomass	NA	Milling (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) Extrusion	NA	NA	Whole biomass	NA	Screw speed, 350 rpm, barrel temperature, 80°C, 40% moisture	Minimal	(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) Microwave	NA	NA	Enriched cellulose and hemicellulose	Glucose, Xylose	Microwave 680 W, irradiation time 24 min and substrate concentration 75 g/L	High	(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 acid	Dilute sulfuric acid	No	Enriched cellulose + some hemicellulose	Soluble 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 acid	Acetic acid or Fumaric acid or maleic acid, and so forth,	No	Enriched cellulose and some hemicellulose	Soluble lignin and hemicellulose	130–190°C, 50–90 mM of organic acid	High	(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 acid	Sulphurous sulfuric, HF, HCl, phosphoric acid, nitric and formic acid	Yes	Condensed lignin	Soluble glucose and xylose or Soluble cellulose which is precipitated	Shorter 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 organosolv	Methanol, ethanol, acetone, ethylene glycol and tetrahydrofurfuryl alcohol, water mixture, organic or inorganic acid	Solvent recovered, No catalyst recovery	Enriched cellulose and most of the hemicellulose	Lignin and some soluble hemicellulose	Acetone-water pretreatment (acetone : water molar ratio of 1 : 1) at 195°C, pH 2.0, 5 minutes residence time	Medium	(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) SPORL	dilute sulfuric acid, NaHSO3 and disc milling	Yes	Enriched cellulose and some hemicellulose	Lignin and hemicellulose	180°C, 25 min. liquor/wood = 3 : 1 v/w	High	(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 liquid	1-allyl-3-methylimidazolium-chloride ([AMIM]Cl), 1-ethyl-3-methylimidazolium-acetate ([EMIM]Ac).	99%	Enriched cellulose and hemicellulose	Lignin and some hemicellulose	100–150°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	Water	NA	Enriched cellulose	Solubilized hemicellulose	160–220°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) Ozonolysis	Ozone	No	Enriched cellulose and hemicellulose	Soluble lignin degraded products and some hemicellulose	Room temperature, Ozone sparging	High	(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) AFEX	Liquid or gaseous anhydrous ammonia	Up to 97%	Whole biomass	NA	100–140°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) ARP	Ammonium hydroxide	No	Enriched cellulose and some hemicellulose	Soluble lignin and hemicellulose	160–180°C, 10–30 min. residence time, 0.5 g ammonium hydroxide per g of biomass	High	(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) SAA	15% ammonia solution	No	Enriched cellulose and some hemicellulose	Soluble lignin and hemicellulose	Solid 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) NaOH	NaOH	NaOH recovered	Cellulose II formation and some hemicellulose	Soluble lignin and hemicellulose		High	(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 H2O2	NaOH, H2O2	NaOH recovered	Enriched Cellulose and some hemicellulose	Soluble degraded lignin and hemicellulose	0.5–2% 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) Lime	CaO with and without oxygen	No	Whole Biomass	NA	25–160°C, 120 min. to weeks, 0.07–0.2 g CaO/g biomass	High	Pretreatment 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  oxidation	Oxygen or air	No	Whole Biomass	NA	>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 SO2	No	Enriched cellulose	Soluble hemicellulose	180–210°C, 1–120 min. residence time	High	Works well both for hardwood and herbaceous biomass	Expensive reactor system requirement due to high pressure operation	[105]
(2) Supercritical CO2	CO2 (21.37 Mpa) + Water = Carbonic acid	NA	Whole biomass	No	112–165°C, 0–73% moisture, 10–60 min. residence time	Medium	(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 bacteria	No	Whole biomass (with reduce cellulose and hemicellulose content)	NA	25–30°C, solid state fermentation, 80–120% moisture, 10–15 days residence time	High	(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.