Sustainability of biodiesel production in Malaysia by production of bio-oil from crude glycerol using microwave pyrolysis: a review

Figures & data

Figure 1. Global biodiesel production and crude glycerol price from 2003 to 2020, source from ( 14).

Table 1. List of various applications of crude glycerol.

Products	Process type	End uses	References
Hydrogen	Steam reforming, partial oxidation, auto thermal reforming, aqueous-phase reforming and supercritical water, photo fermentation using a photosynthetic bacterium	New fuel and energy carrier that could be used in the transport sector, power generation, chemical industry, photovoltaic cells	( 28–35)
Fuel additive	Reactive of glycerol with acetic through acetylation or esterification process. Reaction of glycerol with ether substrate through the etherification process. Reaction of glycerol with acetone and acid anhydride through acetylation process Glycerol fermentation by Clostridium pasteurianum	These products may have suitable properties for use as solvents or additives in gasoline/petroleum engines without changes in design. Uses as brake fluids, as perfume-based, as paint thinner and hydraulic fluid	( 36–39)
Methanol	Via synthesis gas	An important chemical feedstock. Precursor in medical and industrial application	( 40–42)
Ethanol	Bioconversion of raw glycerol (glycerol fermentation by Escherichia coli)	Used as fuel in the space, industrial and transportation sector. Largely used in alcoholic beverages, medical applications and chemical feedstock	( 43,44)
Animal feed	_	Cow and other animal feed, pigs diet, poultry feed. Animal nutrition industry.	( 45–49)
Food Acrolein	On hot compressed water by micro- and mesoporous ZSM-5 Over-activated carbon-supported Over silica-supported heteropolyacid Reaction gas-phase glycerol/water mixture by zeolite Silicotungstic acids	Used as preservative, sweeteners and thickening agent. Applied in detergent, super absorber, acrylic acid, polymers	( 38,50,51)
Chemical industry products	Glycerol fermentation by Klepsiella pneumoniae. Glycerol selective dehydroxylation. Glycerol hydrogenolysis. Glycerol with CO2 (glycerol carbonate). Glycerol with heteropolyacid (DCP) Glycerol with hidrocloric acid catalyzed by acetic acid as acid catalyst	Polymer industry (use as monomer in the synthesis of several polyester and polymers, unsaturated polyester). Plastic industry (polyglycerol methacrylates). Textile industry (as a substitute for petroleum-based polypropelene, sizing and softening to yarn and fabric). Explosives industry (nitroglycerin), and antifreeze liquid. Additive for liquid detergent	( 38,52,53)
Pharmaceutical products	Glycerol oxidation for producing Dihydroxyacetone and crude glycerol with microalgal culture (DHA)	Used as a tanning agent in cosmetics industries and additive in drugs, love potion, health supplements, and nutrients	( 54,55)
Biogas	Co-digested in anaerobic digesters Syngas production	Fuel	( 56–59)

Table 2. Studies on the gasification of crude glycerol to produce syngas.

Method	Crude glycerol properties	Conditions	Production/yield	References
Air gasification with olive kernel	85.4% glycerol, 8.4% water, 6% other	Co-gasified glycerol with olive kernel in a fixed bed reactor at 750–850°C. 49% glycerol, 850°C, air ratio 0.4	Syngas production increased from 0.4 to 1.2 N m^3/kg, 19% to 33% (v/v) H2 increase and tar decrease from 19.5% to 2.4%	( 56)
Hydrothermal continuous gasification by supercritical water	68.53–71.18% glycerol, 2.62–4.19% methanol, 25.91–29.71% MONG, 1.49–2.51% ash, 0.01–0.04% water	650°C, 5% glycerol, continuous tubular reactor	Syngas 26.44–35.85 mmol/g produced. LHV: 8–10 MJ/m^3	( 63)
Gasification by supercritical water	_	500°C, 7% glycerol, 45 MPa and 2.39 mol/L KOH	H2 mole fraction yield 27.9 mol%, most significant	( 64)
Gasification with hardwood chips	_	Co-gasification of hardwood chips and crude glycerol	Up to 20% glycerol produce syngas within ICE standards, having higher heating values (HHV) of 18.71 MJ/Kg	( 65)
Steam gasification	60% glycerol, 31% methanol, 7.5% water, 1.05% KOH	Liquid hourly space velocity of 0.77/h, 800°C, Ni/Al2O3 catalyst, 1:3 steam to glycerol	69.4% yield of syngas	( 66)
Hydrothermal gasification in supercritical water	_	The impact of acid and base catalyst on the yield of syngas. Effectiveness of catalyst on gasification _	K3PO4 > K2HPO4 > H3PO4 > KH2PO4 for crude glycerol gasification. K2HPO4 and K3PO4 gave max H2 H3PO4 and KH2PO4 gave max CH4	( 67)
Air/O2 Gasification	60% glycerol, 20% MONG, 15% methanol, 5% ash;	Entrained flow gasifier, excess air ratio 0.35–0.4 Temperature of 950–1500°C	Syngas HVV of Syngas about 2500 kcal/N m^3 The fraction of syngas decreased as the oxygen- to-fuel ratio increased and atomization of glycerol using a nozzle improved the efficiency of the gasification process	( 68,69)

Table 3. The three different types of pyrolysis processes.

Type	Operating parameters	Yield
Slow pyrolysis	Temperature of 400°C. Residence time of more than 30 min and a heating rate of 0.1–1°C/s	35% bio-char (solid), 30% bio-oil (liquid) and 35% syngas (gas)
Fast pyrolysis	Temperature of 500°C. Residence time of 10–20 seconds and a heating rate of 1–200°C/s	20% bio-char (solid), 50% bio-oil (liquid) and 30% syngas (gas)
Flash pyrolysis	Temperature of 400°C. Residence time of about 1 second and a heating rate greater than 1000°C/s	2% bio-char (solid), 75% bio-oil (liquid) and 13% syngas (gas). Efficient decomposition of the feedstock components strongly influenced by the process parameters such as reaction temperature, pressure, residence time, reactor configuration, and catalyst

Table 4. Studies on the co-pyrolysis of crude glycerol with other biomass to produce bio-oil.

Method	Crude glycerol properties	Conditions	Char	Bio-oil	Gas	References
Pyrolysis of corn Straw with crude glycerol	56.2% glycerol, 7.3% methanol, 2.4–4.3% water, 3.5–7.2% ash	30°C/min to 550°C, use helium to remove air. 1:1 ratio of corn to glycerol	Low amount LHV: 24.84 MJ/kg	High yield LHV: 30.52 MJ/kg	High yield LHV: 28.65 MJ/kg	(74)
Pyrolysis of Greek Lignite with 20% crude glycerol	85.4% glycerol, 8.4% water, <6% methanol, <51.5% MONG, 0.2% FFA	850°C, 20% glycerol	_	_	71% conversion to volatiles. 65.44 v/v% hydrogen yield	( 75)
Pyrolysis of olive Kernel with crude glycerol	85.4% glycerol, 8.4% water, 6% other	720°C, 25% glycerol	_	_	Increase in H2 production by 11.6% (v/v)	( 76)
Pyrolysis of swine Manure with crude glycerol	55.09% glycerol/methanol/water 31.09% FFA, 13.81% salts	1:3 manure: glycerol, 7°C/min, 340°C, 0.65 MPa	_	68% yield. FFA was the key beneficial to oil yield. Yield enhanced 1.83 times. Lowers viscosity and density	_	( 77)
Pyrolysis of olive Kernel with crude glycerol	85.4% glycerol, 8.4% water, 6% other	Microwave pre-treatment of substrate (25% crude glycerol +75% olive kernel). followed by pyrolysis at 720°C	_	Bio-oil 59.53% v/v;	Enriched syngas (H2+CO) concentration (84.9% v/v)	( 78)

Table 5. Comparison between microwave heating and conventional heating ( 93).

Microwave heating	Conventional heating
Energetic coupling	Conduction/convection
Coupling at molecular level	Superficial/wall heating
Rapid	Slow
Volumetric and uniform heating at molecular level	Superficial heating
Selective	Non-selective
Dependent on material properties Higher electricity conversion efficiency Lower thermal inertia and faster response	Less dependent Lower electricity conversion efficiency Higher thermal inertia and slower response

Table 6. Product yields of lignocellulosic biomass using microwave pyrolysis.

Biomass	Conditions used for the experiment	Yield (wt %)			References
		Gas	Liquid	Solid
Larch	Sample: 191 g; microwave power: 1.5 kW; reaction time: 18 min	60.8	18.1	21.1	( 101)
Pine wood sawdust	Sample: 15 g; microwave power: 0.3–1 kW; reaction temperature: 470°C; reaction time: 12 min	60.0	22.7	17.3	( 102)
Peanut shell	Sample feeding rate: 2 kg/h; reaction temperature: 500°C; particle size: 0.5–1.0 mm	17.1	52.4	30.5	( 103)
Rice straw	Sample: 3–5 g; microwave power: 300 W; reaction temperature: 400°C; reaction time: 30 min; particle size: <0.425 mm	49.4	22.6	28.1	( 104)
Corn stover	Sample: 100 g; microwave power: 875 W; reaction temperature: 450–550°C; reaction time: 20 min; particle size: 6.2 mm in diameter and 10–20 mm in length	34.4	37.0	28.6	( 96)
Wheat straw	Sample :100–150 g; microwave power: 1.2 kW (max); reaction temperature: 180°C (max); reaction time: 10 min	14	57	29	( 105)
Oil palm shell	Microwave power: 450 W; reaction time: 25 min; particle size: 1–100 mm	19	13	68	( 106)
Oil palm fiber	Microwave power: 450 W; reaction time: 25 min; particle size: 0.3–0.6 mm	12	8	80	( 106)

Table 7. Biomass feedstock properties and microwave-assisted pyrolysis ( 100).

Biomass feedstock property	Remarks	References
Particle size	Smaller feedstock particle size – generally improves the microwave-assisted pyrolysis process (higher heating rates). Very small particles lead to very high external surface area and give complete pyrolysis, whereas too large particles may result in incomplete pyrolysis. Each of these would result in a low bio-oil yield for the microwave-assisted pyrolysis of biomass. The biomass particle sizes of 2–4 mm are better for microwave-assisted pyrolysis. However, the optimal particles sizes for the conventional heating pyrolysis are between 0.85 and 2 mm.	( 107) (108,109) ( 107,110)
Moisture content	Water has a very good microwave absorbability (because of its high tangent lost – tan δ) and renders the feedstock a high temperature during the process. With increasing temperature, the moisture evaporates, the biomass becomes less microwave absorptive, and the temperature rising rate slows down. Reaction temperatures of the process also effect the tangent loss value of a biomass feedstock. When tangent loss value decreases, the microwave absorbability of the biomass becomes lower and lower. This makes the achievable temperature of the pyrolysis process low (e.g. less than 200°C) even if the microwave power is increased. Microwave absorbers which can absorb microwave energy (those with high tangent lost) are used to enhance heating and make the process more efficiently.	( 111) (112,113) ( 114,115)
Inorganics	Ash is another main inorganic matter in a biomass. The ash components in biomass feedstock vary significantly, e.g. rice straw and wood sawdust have high contents of SiO2 (52.66–69.52%) and K2O (10.30–40.13%) while sewage sludge have low contents of SiO2 (1.62–26.40%) and K2O (0.05–1.62%). Generally, some ashes are good microwave absorbers and some are not. For example, Fe2O3 and TiO2 have higher tan δ values (0.001–0.05) whereas MgO and SiO2 have lower tan δ values (0.0002–0.0005). The ash components themselves do not convert to bio-oil. The high ash content would therefore reduce the bio-oil yields. Studies on microwave-assisted heating have shown that the high ash content (60.5–70.9%) will give low bio-oil yields (0–0.24 wt%).	( 105,116–118) ( 117) (119)
Organics	Lignocellulosic biomass is mainly composed of cellulose (23–60%), hemicellulose (25–44%), and lignin (12–49%). Generally, cellulose and hemicellulose result in more bio-oil than lignin. With conventional heating, the decomposition temperature ranges are cellulose (315–390°C), hemicellulose (250–350°C), and lignin (200–550°C). However, with microwave-assisted pyrolysis, the lignocellulose components decomposed at lower temperature (around 100–150°C lower). Generally, the lignocellulosic biomass components under microwave-assisted pyrolysis would produce more bio-oil than conventional heating pyrolysis. Algae biomass is rich in protein (9–30%), lipids (30–62%), and some carbohydrates (2–18%). Algae biomass is usually not a good microwave absorber, even if it has high lipid content (among carbohydrate, protein, and lipid, the good microwave absorber is lipid). Microwave absorbers are generally required to obtain high temperatures for microwave-assisted pyrolysis of an algal biomass. The bio-oil yield from the microwave-assisted pyrolysis algal is lower. However, algal biomass produces bio-oil that has much better quality by having higher H/C and H/O ratios and thereby showing higher HHVs. Plastics and rubbers (high hydrocarbon) are also widely used as absorbers in microwave pyrolysis for bio-oil production. Compared with the plastic, rubber is a better microwave absorber. Rubber can achieve a high temperature of 500°C and give high bio-oil yields during microwave pyrolysis. Due to the low microwave absorbability of the plastic components (e.g. polypropylene, polystyrene, polyethylene), the plastic cannot achieve a high temperature (lower than 180°C) if no microwave absorber is added but can increase significantly when microwave absorbers are added. Due to the high hydrocarbon content of plastics, the microwave-assisted pyrolysis of plastics would result in high bio-oil yields.	( 120) (111,121) (113,122) (121,123) (124,125) (126–131)

Table 8. Properties of bio-oils produced by microwave and conventional pyrolysis ( 142).

Property	Microwave pyrolysis bio-oil					Conventional pyrolysis bio-oil
	Pine wood sawdust	Wheat straw	Waste office paper	Corn stover	Coffee hulls
Carbon (wt%)	48.8	58.9	49.9	60.7	74.8	54–58
Hydrogen (wt%)	6.8	6.9	5.8	7.7	8.3	5.5–7.0
Oxygen (wt%)	43.5	33.2	44.2	–	8.5	35–40
Nitrogen (wt%)	0.9	1.2	ND^a	2.0	7.8	0–0.2
Sulfur (wt%)	0	0.02	0.04	0.15	0.6	–
Ash (wt%)	–	–	1.2	0.04	–	0–0.2
Water (wt%)	26.2	<1	2.6	15.2	–	15–30
pH	2.5	1.4	–	2.9	–	2.5
Density (g/ml)	1.15	1.2	–	1.25	–	1.2
Flash point (°C)	63.5	–	64	–	79	72
High heating value (MJ/kg)	15.0	16–22	21.8	17.5	34.4	16–19

^aND: Not detected.

Table 9. Standard test for pyrolysis oil for boiler fuel application by ASTM D 7544 ( 156).

Property	Test method	Grade G	Grade D
Gross heat of combustion, MJ/kg, min	D240	15	15
Water content, wt%	E203	30	30
Pyrolysis solids content, wt%	D7579	2.5	0.25
Kinematic viscosity at 40°C, mm^²/s	D445	125	125
Density at 20°C, kg/dm^³	D4052	1.1–1.3	1.1–1.3
Sulfur content, wt%	D4294	0.05	0.05
Ash content, wt%	D482	0.25	0.15
pH	E70	Report	Report
Flash point, °C	D93	45	45
Pour point, °C	D97	−9	−9

Table 10. Standard enthalpies of formation for major liquid components’ presence in the bio-oil during the pyrolysis of glycerol.

Reactant/Product name	Formula	Enthalpies of Formation Η° at 25°C (kJ/mol)	Energy required to raise the product temperature from 25°C to 400°C [Q = CpΔT] (kJ/mol)
Glycerol liquid	C3H8O3	−669.6	84.71
Acetaldehyde	CH3CHO	−191.70	33.40
Acetone	CH3COCH3	−247.48	47.06
Methanol	CH3OH	−238.42	29.81
Ethanol	C2H5OH	−277.05	41.80
Water	H2O	−241.8	28.48
Acetic acid	CH3COOH	−484.93	46.31
Acrolein	CH2CHCHO	−120.0	26.73

Table 11. The details of calculation for enthalpy of formation for the reactants and products.

	Calculation	Total (kJ)
∑nrΔHḟ (reactants)	6*–669.6	−4017.6
∑npΔHḟ (products) at 25°C	1* − 191.70 + 1* − 247.48 + 1* − 238.42 + 2* − 277.05 + 7* − 241.8 + 1* − 484.93 + 1* − 120 + 1* − 669.6	−4198.83
∑npΔHḟ (products), Q4 from 25°C to 400°C	1*33.40 + 1*47.06 + 1*29.81 + 2*41.80 + 7*28.48 + 1*46.31 + 1*26.73 + 1*84.71	550.98

Table 12. Comparison of propylene-based and bio-based acrolein production (10,000 ton/year) regarding feedstock and energy consumption.

Raw material	Price	Propylene process (/year)	Bio-based process (/year)
			From pure glycerol	From crude glycerol
Propylene	$1770.9/ton^a	9375 ton^b	–	–
Pure glycerol	$ 550–580/ton^c	–	20,536 ton^d	–
Crude glycerol	$275–300/ton^c	–	–	25,669 ton^e
Steam	$7.7/ton^f	20,089 ton^g	82,144 ton^h	82,144 ton^h
Energy	$0.07/kWh^i	0	2.67 × 10^d kWh^j	2.67 × 10^d kWh^j
Total cost (Million)		$16.76^k	$12.11–12.73^l	$7.88–8.52^m

^aThe price reported in January 2014 by ICIS ( 164).

^bCalculated based on the 10,000-ton annual acrolein production and 80 mol% acrolein yield: .

^cThe price of pure glycerol and crude glycerol reported in June 2017 by Oleo line ( 165).

^dCalculated based on the 10,000-ton annual acrolein production, 80 mol% acrolein yield, and the stoichiometry of glycerol dehydration to acrolein: /.

^eCalculated based on the same assumptions as 6 and the assumption that the crude glycerol contains 80% glycerol: /.

^fThe cost of the steam per ton is reported for 2003 by U.S. Department of Energy ( 166).

^gCalculated based on the feed molar ratio of propylene-air-steam as 1:10:5 and the calculated propylene requirement: .

^hCalculated based on the calculated requirement for glycerol and the assumption that the feed has a concentration of 25 wt% glycerol: .

ⁱThe price of energy reported in 2017 by EIA ( 167).

^jCalculated based on the heat requirement for the endothermic dehydration reaction of glycerol to acrolein, from [168] and the annual working hours of : .

^kThe total cost of acrolein production from Propylene-based process: .

^lThe total cost of acrolein production from pure glycerol-based process at low glycerol price value: The total cost of acrolein production from pure glycerol based process at high glycerol price value: .

^mThe total cost of acrolein production from crude glycerol-based process at low glycerol price value: kWh) = $7.88 Million. The total cost of acrolein production from crude glycerol-based process at high glycerol price value: kWh) = $8.52 Million.

Table 13. Advantages, disadvantages, and bio-oil yield from different pyrolysis reactors ( 100).

Reactor type	Advantages	Disadvantages	Bio-oil yield (%)
Fixed bed	Simple design Reliable Biomass size independent	High carbon conservation Long solid residence time Low ash carry over Difficult to remove char	35–50%
Bubbling fluidized bed	Simple design Easy operation Good temperature control Suitable for large scale	Small particle sizes are needed	70–75%
Circulating fluidized bed	Well-understood technology Good thermal control Large particle sizes can be used	Unlikely to be suitable for large scale Complex hydrodynamics Char is finer	70–75%
Rotating cone	Centrifugal force moves heated sand and biomass No carrier gas required Less wear	Complex process Small particle sizes needed Not proven yet for large scale	65%
Vacuum	Produces clean oil Can process larger particles of 3–5 cm No carrier gas required Lower temperature required Easier liquid product condensation	Slow process Solid residence time is too high Require large-scale equipment Poor heat and mass transfer rate Generates more water	35–50%
Ablative	Inert gas is not required Large particle sizes can be processed System is more intensive Moderate temperature required	Reactor is costly Low reaction rate Low reaction rate	70%
Auger	Compact No carrier gas required Lower process temperature	Moving parts in hot zone Heat transfer in large scale is not suitable	30–50%
PyRos	Compact and low cost High heat transfer Short gas residence time	Complex design Solids in the oil Alkali dissolved in the oil High temperature required	70–75%
Plasma	High energy density High heat transfer High temperature Very good control	High electrical power consumption High operating costs Small particle sizes required	30–40%
Microwave	Efficient heat transfer Exponential control Compact High heating rate Large size biomass can be processed Uniform temperature distribution Moderate temperature	Economics of the process scaling-up	60–70%
Solar	Use renewable energy High heating rate High temperature	High costs Weather dependant	40–60%

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