Microwave-assisted pyrolysis of biomass for liquid biofuels production

Bioresource Technology 120 (2012) 273–284

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Bioresource Technology

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Review

Microwave-assisted pyrolysis of biomass for liquid biofuels production

Chungen Yin ⇑Department of Energy Technology, Aalborg University, Pontoppidanstraede 101, 9220 Aalborg East, Denmark

g r a p h i c a l a b s t r a c t

Pyrolysis is an attractive way to produce

0960-8524/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.biortech.2012.06.016

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liquid biofuels from biomass residues and waste feedstock. Various efforts have been made to improve pyrolysisprocess towards higher yield and quality of liquid biofuels and better energy efficiency. Microwave-assisted pyrolysis is one of the promising attempts,mainly due to efficient heating of feedstock by ‘‘microwave dielectric heating’’ effects. This review starts with a brief overview of the key factors and fea-tures of conventional pyrolysis design and summary of pyrolysis conditions that favor liquid biofuels production. Then the review compares microwavedielectric heating and conventional thermal heating. The key part of this review is that microwave-assisted pyrolysis process (from feedstock pretreat-ment to bio-oil collection, as sketched below) is thoroughly discussed, in which design and operation concerns are highlighted. A handy, up-to-date over-view of the existing research and development efforts and results in microwave-assisted biomass pyrolysis is provided. This review is critical for helpingunderstand the application of microwave heating in bioenergy and developing energy-efficient biomass conversion technologies. The microwave-assistedbiomass pyrolysis process with the main energy flow is sketched below.

a r t i c l e i n f o

Article history:Received 10 May 2012Received in revised form 7 June 2012Accepted 9 June 2012Available online 16 June 2012

Keywords:BiomassPyrolysisBiofuelMicrowaveDielectric heating

a b s t r a c t

Production of 2nd-generation biofuels from biomass residues and waste feedstock is gaining great con-cerns worldwide. Pyrolysis, a thermochemical conversion process involving rapid heating of feedstockunder oxygen-absent condition to moderate temperature and rapid quenching of intermediate products,is an attractive way for bio-oil production. Various efforts have been made to improve pyrolysis processtowards higher yield and quality of liquid biofuels and better energy efficiency. Microwave-assisted pyr-olysis is one of the promising attempts, mainly due to efficient heating of feedstock by ‘‘microwavedielectric heating’’ effects. This paper presents a state-of-the-art review of microwave-assisted pyrolysisof biomass. First, conventional fast pyrolysis and microwave dielectric heating is briefly introduced. Thenmicrowave-assisted pyrolysis process is thoroughly discussed stepwise from biomass pretreatment tobio-oil collection. The existing efforts are summarized in a table, providing a handy overview of the activ-ities (e.g., feedstock and pretreatment, reactor/pyrolysis conditions) and findings (e.g., pyrolysis products)of various investigations.

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Fig. 2. Cellulose pyrolysis chemistry: two major, competitive pathways.

274 C. Yin / Bioresource Technology 120 (2012) 273–284

1. Introduction

Due to environmental and safety of energy supply concerns andnegative implication of 1st-generation biofuels on food resources,production of 2nd-generation biofuels from biomass residues andwaste feedstock is attracting a great interest worldwide. Fast pyr-olysis, a thermochemical conversion process characterized by ra-pid heating of biomass feedstock in oxygen-absent condition tomoderate temperature levels and rapid quenching of the inter-mediate volatile products, is one of the attractive technologiesfor liquid biofuels production (Bridgwater and Peacocke, 2000;Vamvuka, 2011). In spite of the different progresses and advance-ments made in the past decades, pyrolysis still faces some techni-cal challenges in terms of improving yield and quality of theresultant liquid biofuels and increasing energy efficiency of thewhole process. Microwave-assisted pyrolysis is a promising at-tempt to resolve the challenges, because of rapid and efficientheating of materials by ‘‘microwave dielectric heating’’ effects.Controlled microwave heating has been successfully applied inmodern organic synthesis, as reviewed by Kappe (2004). Compara-tively, microwave-enhanced pyrolysis of biomass for biofuels pro-duction has not gained enough concerns or wide applications.

A research project, based on microwave-assisted catalytic pyro-lysis for production of 2nd-generation bio-oil from biomass resi-dues and waste feedstock, was recently granted by the EU underthe seventh framework programme. This project is targeted to op-timize the pyrolysis process for bio-oil production and to developnew technology to upgrade the bio-oil for use in transportationand manufacturing lubricants. In order to better carry out this pro-ject and improve the pyrolysis process, a state-of-the-art review ofthe research and development in microwave-assisted pyrolysis ofbiomass is conducted. In this paper, a brief overview of conven-tional fast pyrolysis and microwave dielectric heating is first pre-sented. Then, microwave-assisted biomass pyrolysis process isdiscussed and the up-to-date research efforts are summarized.

2. Conventional fast pyrolysis of biomass for bio-oil production

Biomass is an environmentally friendly, important energy re-source, constituting 14% of the global primary energy and beingthe fourth largest following coal, oil, and natural gas. Biomass con-sists of three major components (i.e., cellulose, hemicellulose, andlignin) and also contains a variety of minor components (e.g., inor-

Fig. 1. (a) The lignocellulosic biomass structure, components and their degradation condipyrolysis, gasification and combustion, with bio-oil, syngas and heat as the preferred pr

ganic matters), as sketched in Fig. 1a. The crystalline polymers ofglucose in cellulose are held rigidly together as bundles of fibersto provide material strength. The amorphous polymers of varioussugars in hemicelluloses glue the cellulose bundles together. Ligninis the main binder for fibrous cellulosic components, providing notonly rigidity to the structure but also the shield against rapid de-struction of the cellulosic fibers. The basic components and chemi-cal structure of biomass play an important role in development ofbiomass conversion technologies (Yin et al., 2008). Some biomasspretreatment methods can be used to improve biomass conversionprocesses. The general idea is to remove or alter the hemicelluloseor lignin, reduce the crystallinity of cellulose, and increase theoverall porosity and surface area of biomass (Singh et al., 2011).

Pyrolysis, gasification and direct combustion are the threemajor thermochemical conversion technologies that can convertbiomass into fuels, chemicals or electricity, among which pyrolysisplays a central role (Yin et al., 2010). Pyrolysis is not only oftenused as an independent process to produce bio-fuels but also an in-separable initial process of gasification and combustion, assketched in Fig. 1b. Different modes of pyrolysis exist, which leadto different product yields. Nowadays, the term pyrolysis often de-scribes processes in which oils are the preferred products (Mohanet al., 2006). In pyrolysis, biomass is heated to moderate tempera-tures (400–600 �C) in the absence of oxygen and thermally decom-posed to non-condensable gases, condensable species (e.g., waterand organic compounds) and char via different reaction routines.For instance, during pyrolysis of the major biomass component –

tions and products; (b) the major biomass thermochemical conversion technologies:oduct, respectively.

Table 1Different modes of pyrolysis: the reaction conditions and major products. Data compiled from Bridgwater (2006), Bulushev and Ross (2011), Huber et al. (2006), Klass (1998) andVamvuka (2011).

Residence time Heating ratea Temperature Pressure Major products

A qualitative description of the reaction conditions and major products of various modes of pyrolysisSlow (carbonization) Hours – days Very low 300–500 �C �101 kPa CharcoalSlow pyrolysis Hours Low 400–600 �C �101 kPa Charcoal, liquids, gasesSlow pyrolysis 5–30 min Medium 700–900 �C �101 kPa Charcoal, gasesFast pyrolysis 0.1–2 s High 400–650 �C �101 kPa LiquidsFast pyrolysis <1 s High 650–900 �C �101 kPa Liquids, gasesFast pyrolysis <1 s Very high 1000–3000 �C �101 kPa GasesVacuum pyrolysis 2–30 s Medium 350–450 �C �15 kPa LiquidsLiquefaction <10 s High 250–325 �C 50–200 atm Liquids

Temperature Residence time Liquid (%) Char (%) Gas (%)

A quantitative example: Product yields (on dry wood basis) from various modes of pyrolysis of woodFast pyrolysis Moderate, 500 �C Short vapor residence time, �1 s 75 (25% water) 12 13Intermediate Moderate, 500 �C Moderate residence time, 10–20 s 50 (50% water) 20 30Slow pyrolysis Low, 400 �C Very long residence time 30 (70% water) 35 35Gasification High, 800 �C Long residence time 5 10 85

a The heating rates used in pyrolysis experiments vary considerably in literature, most of which may be in the range of 10�1–102 �C/s. However, the boundary between‘‘slow’’ and ‘‘fast’’ is somehow arbitrary. For example, the pyrolysis with heating rate of 0.5 �C/s is named as slow pyrolysis while that with heating rate of 5 �C/s is termed asfast pyrolysis (Onay and Kockar, 2003). The existing experimental data on pyrolysis characteristics of biomass are compiled and reviewed very recently. It is found that inmany investigations only slow or fast pyrolysis is indicated without giving any specific heating rate value. A heating rate threshold of 10 �C/s is adopted to classify the existingexperiments into slow or fast pyrolysis when the data are structured and analyzed to derive a guide of biomass pyrolysis conversion behavior (Neves et al., 2011).

Table 2Key factors and features in conventional fast pyrolysis design.

Biomass feedstock and pretreatmentFeedstock Feed composition largely affects the yields of different products, e.g., pyrolysis of sawdust and corn stalk both produce �61 wt.% bio-oil and

18 wt.% char; pyrolysis of rice hull yields �38 wt.% bio-oil and 33 wt.% char (Vamvuka, 2011)Particle size Very fine particles needed to acquire very high heating and heat transfer rates, e.g., <200 lm for rotating cone reactor, <2 mm for fluid beds, <6 mm

for circulating fluid beds. Expensive for size reductionFeed drying Essential to �10% moistureAdditives For chemical production. Use of catalysts can help to reduce problems of the liquid pyrolysis product (e.g., polymerization, high viscosity,

corrosivity) (Balat et al., 2009). Use of catalysts can also change the composition of pyrolysis products, in which the acidic properties and porestructures of the catalysts are believed to determine the products (Bulushev and Ross, 2011)

Fast pyrolysis reactorEssential features (1) very high heating and heat transfer rates; (2) moderate and carefully controlled temperature; (3) very rapid cooling or quenching of the

pyrolysis vaporsHeat supply High heat transfer rate neededHeating rate Limited by convection at particle surface and conduction inside particleReactions

involvedBroadly classified into 4 groups: random main-chain scission (i.e., breaking of the main chain to produce smaller molecules of random sizes),depolymerization (successive removal of monomer units from the chain, leading to formation of free radicals and chain reactions), carbonizationand side-group reactions (including the reactions leading to crosslinking, straight-chain-polymer formation by elimination of side chains,cyclization, and aromatization by dehydrogenation) (Babu, 2008)

Reactiontemperature

Temperature has a great impact on pyrolysis yields. When heated <200 �C for hours, carbohydrates partially depolymerize to short chains of some200 sugar units; upon being heated at around 300 �C, the depolymerization is accompanied by slow dehydration to yield unsaturated polymersand eventually char; when heated at higher temperatures, the depolymerization reactions can be pushed to the point of liberating volatile species,which can again lead to the formation of tars if they are efficiently removed from the reactor; at even higher temperatures, extensive C–C bondbreaking occurs, leading to the formation of various C2–4 oxygenates, and beyond 700 �C to a mixture of CO, CO2, H2 and CH4 (Lange, 2007).Compilation of experimental data indicates two distinct stages: (1) below 500 �C, yield of tar increases to above 50 wt.% (of dry ash-free biomass)at 450–500 �C, whilst yield of pyrolytic water is roughly constant and variations in yield of total non-condensable gases are also small; (2) above500 �C, yield of pyrolytic liquids decreases rapidly with temperature compensated by an increase in yield of total gases, whilst variations in charand pyrolytic water are relatively small (Neves et al., 2011). The trend of literature data indicates liquids yield peaks at about 500 �C. Carefullycontrolled pyrolysis reaction temperature of around 500 �C and vapor phase temperature of 400–450 �C are needed (Bridgwater, 2006)

Reactorconfiguration

The most important configurations are fluidized beds, rotating cones, vacuum and ablative pyrolysis reactors. Others include, e.g., entrained flowand transported bed. The ‘best’ method is not yet established with most processes giving between 65–75% liquids based on dry wood input

Product conditioning and collectionVapor residence

timeA few hundred milliseconds for chemicals; up to 2 s for fuels. Longer residence times result in significant reductions in organic yields due tocracking reactions

Secondarycracking

Reduce bio-oil yield

Char separation Rapid and complete char separation is desirable, but difficult. Char catalyzes secondary cracking in vapor phase; char in cooled liquid productaccelerates slow polymerization processes and contributes to instability problems

Liquids collection Difficult; quenching and electrostatic precipitation seem best

C. Yin / Bioresource Technology 120 (2012) 273–284 275

cellulose, two main, competitive pathways are recognized to be ac-tive: one leading to the formation of levoglucosan as a relativelystable intermediate and the second yielding glycolaldehyde (Antaland Varhegyi, 1995), as sketched in Fig. 2. If quickly swapped out of

the reactor and quenched, the vapors form condensed oils and tars.If held in contact with the solid biomass undergoing devolatiliza-tion within the reactor, the organic vapors degrade further to formchars, non-condensable gases and water. The pyrolysis of hemicel-


lulose gives rise to analogous families of chemical compounds likecellulose.

Research and development activities on biomass pyrolysis havebeen successfully conducted to produce biofuels, as recently sum-marized in a few comprehensive and excellent reviews, e.g., Babu(2008), Balat et al. (2009), Bridgwater (1999), Bridgwater andPeacocke (2000), Bulushev and Ross (2011), Demirbas (2007),Huber et al. (2006), Lange (2007), Mohan et al. (2006), Neves etal. (2011), Vamvuka (2011), and Yaman (2004). For instance,Bridgwater and Peacocke (2000) reviewed the key features of fastpyrolysis and the resultant liquid product and summarized the ma-jor reaction systems and processes that had been developed overthe years from 1980 to 2000 in various countries. Huber et al.(2006) reviewed the current methods and future possibilities forbiomass conversion into transportation fuels. The chemistry, cata-lysts, engineering solutions and challenges were discussed. As oneof the three primary routes to convert biomass into liquid fuels,pyrolysis was also discussed. Among the essential parameters,short vapor residence times (no longer than 2 s for producingbio-oil as a fuel) was recognized as an engineering difficulty, inwhich careful design and temperature control are needed to avoidblockage from differential condensation of the heavy products.Vamvuka (2011) analyzed the modern reactor configurations andthe state-of-the-art systems that have been – or are expected tobe commercialized, and discussed their performance levels, tech-nological status and economics. Neves et al. (2011) collected andanalyzed literature data on pyrolysis characteristics of more than60 different biomass samples (woody and non-woody particles ofvarious shapes and sizes in the range of 0.1–100 mm) in a varietyof reactors and over a wide range of temperatures (200–1000 �C).The general trends of the yields of different pyrolytic products,i.e., total pyrolytic liquids (including pyrolytic water), pyrolyticwater, total gas, major individual gas species (H2, CO, CO2, CH4

and other light hydrocarbons), and char, as a function of pyrolysispeak temperatures were plotted. The influence of heating rate wasalso discussed by classifying the experimental data into slow andfast pyrolysis based on a heating rate threshold of 10 �C/s. The gen-eral trends of pyrolytic product distribution as a function of tem-perature were fitted with an empirical model.

In summary, the yields and compositions of the products of bio-mass pyrolysis are greatly dependent on feedstock properties andpyrolysis conditions. The major products of several different modesof pyrolysis are listed in Table 1. Long residence times at low tem-peratures produce primarily charcoal, and high temperatures re-sult in mainly gas product. Process conditions that favor liquid

Fig. 3. Two main mechanisms of microwave hea

product are fast heating rates, moderate temperature (around500 �C), and short residence times of the vapor-phase products(Babu, 2008; Bridgwater and Peacocke, 2000; Ciacci et al., 2010;Demirbas, 2007; Huber et al., 2006; Mohan et al., 2006; Neves etal., 2011; Shen et al., 2009; Vamvuka, 2011; Yaman, 2004). Higherheating rates favor bond-scission reactions to form tar fragmentswhile lower heating rates favor recombination of tar fragmentson biomass matrix (charring reactions to form coke). The primarycondensable species produced by pyrolysis are non-thermodyna-mically controlled products, among which the organic compoundsare particularly vulnerable to further conversion into light gases,water, secondary char and refractory tars via secondary reactions(e.g., cracking, dehydration, polymerization). Gas-phase crackingof the primary organic vapors strongly depends on temperatures:below around 500 �C they are not easily cracked in gas-phase;while at higher temperatures they can be readily converted evenif they are highly diluted and the residence time is short (e.g., hun-dreds of milliseconds). However, hot char-catalyzed heterogeneouscracking of the primary organic vapors can occur at comparativelylower temperatures (<500 �C) (Di Blasi, 2008; Mohan et al., 2006;Neves et al., 2011). Therefore, optimal temperatures and residencetimes and rapid quenching are necessary to freeze the desired in-termediates and terminate the secondary conversion of the pro-ducts. To gain a quantitative impression about the effect ofpyrolysis conditions, Table 1 also lists the typical product yieldsobtained from different modes of pyrolysis of wood. The key fac-tors and their impacts in fast pyrolysis design are summarized inTable 2, enriched mainly based on Bridgwater and Peacocke(2000). Special care must be taken in feedstock and pretreatment,pyrolysis reactor design, and product conditioning and collections,in order to maximize the yield of liquid product.

3. Fundamentals of microwave dielectric heating

All domestic microwave ovens and dedicated microwave reac-tors for chemical reactions operate at a frequency of 2450 MHz, afrequency region assigned for heating applications. The energy ofthe microwave photon at this frequency is 0.0016 eV, which is low-er than the energy of Brownian motion and is too low to break che-mical bonds. For instance, the chemical bond energies of H–OH andCH3–CH3 are 5.2 and 3.8 eV, respectively. As a result, microwavesare not ionizing radiation; they cannot induce chemical reactions(Gabriel et al., 1998; Kappe, 2004; Mingos and Baghurst, 1991).

Microwave-assisted chemical conversion is based on efficientheating of feedstock by ‘‘microwave dielectric heating’’ effects, in

ting: (a) dipole rotation; (b) ionic migration.

Table 3Loss tangent, tan d , of different solvents and materials at 2450 MHz and 20 �C. Datacompiled from Bogdal and Prociak (2007), Gabriel et al. (1998), Kappe (2004, 2008),Larhed et al. (2002), Lidström et al. (2001), Omar et al. (2011).

Material tan d Material tan d

Ethylene glycol 1.350 Coconut activated carbon 1.646Ethanol 0.941 Dried EFB (oil palm empty fruit

bunch) char0.134

2-propanol 0.799 EFB sample with 18 wt.% moisture 0.297Formic acid 0.722 EFB sample with 45 wt.% moisture 0.535Methanol 0.659 EFB sample with 64 wt.% moisture 0.3241-butanol 0.571 Plexiglass 5.7 � 10�3

2-butanol 0.447 Porcelain No 4462 1.1 � 10�3

Acetic acid 0.174 Borosilicate glass 1.06 � 10�3

Water 0.123 Ceramic F-66 5.5 � 10�4

Acetone 0.054 Polyethylene 3.1 � 10�4

Dichloromethane 0.042 Teflon PFA 1.5 � 10�4

Toluene 0.040 Fused quartz 6 � 10�5

Table 4Comparison between microwave and conventional heating.

Microwave dielectric heating Conventional thermal heating

Conversion of energy Transfer of energyIn-core volumetric and uniform heating:

the whole material heatedsimultaneously, energetic coupling atmolecular level

Superficial heating: viaconvection/conduction

Rapid and efficient, e.g., methanol can berapidly super-heated to above 100 �C;ionic liquids can gain temperature jumpsof 200 �C within seconds

Slow, inefficient, limited bymaterial thermal conductivity

Selective: rapid intense heating for polarsubstances while ineffective for apolarsubstances

Non-selective

Hot spots: an effect due to inhomogeneitiesof microwave field or dielectricproperties within a material, resulting inlocal temperatures in the material beingmuch higher than the temperaturemeasured in the bulk

No hot spot

Dependent on material’s properties Less dependentPrecise and controlled heating: the energy

input starts and stops immediately whenthe power is turned on or off,respectively

Less controllable


which dipolar polarization and ionic conduction are the two mainmechanisms (Gabriel et al., 1998; Kappe, 2004; Mingos andBaghurst, 1991). Irradiation of a material at microwave frequenciesresults in the dipoles or ions aligning in the applied electric field.As the applied field oscillates, the dipole or ion field attempts torealign itself with the alternating electric field, as sketched in Fig.3. In this process, energy is lost in the form of heat through mole-cular friction and dielectric loss. If the dipoles or ions do not haveenough time to realign or reorient too quickly with the appliedfield, no heating will occur. The frequency (2450 MHz) lies be-tween these two extremes and gives the dipoles or ions time toalign in the field, but not to follow the alternating field precisely.

The ability of a specific material to absorb microwave energyand convert it into heat plays a vital role in microwave heating.It depends on the dielectric properties of the material, i.e., dielec-tric constant (e0) and dielectric loss (e00). The former describes theability of molecules to be polarized by an electric field, whereasthe latter measures the efficiency with which the energy of theelectromagnetic radiation can be converted into heat. The word‘‘loss’’ is used to indicate the amount of input microwave energythat is lost to the material by being dissipated into heat. The ratioof the dielectric loss to dielectric constant gives rise to the loss

tangent, tan d = e00/e 0, which is a parameter used to describe theoverall efficiency of a material to utilize energy from microwaveradiation. Materials can be classified into three types accordingto their interactions with microwave: conductors (reflective), insu-lators (transparent), and dielectrics (absorptive). A reaction med-ium with a high tan d is needed for rapid heating (Kappe, 2004;Larhed et al., 2002). The loss tangents of materials, e.g., commonorganic solvents, materials for reaction vessels or for separationbetween microwave generator and reaction medium, raw biomass,and biomass char, are summarized in Table 3. In general, materialscan be classified as high (tan d > 0.5), medium (0.1–0.5), and lowmicrowave absorbing (<0.1). A low tan d value does not precludea material from being used in a microwave-heated reaction: addi-tives can be used to achieve rapid heating of the material of lowtan d value. For example, raw biomass can be blended with acti-vated carbon or char before being fed into a microwave-assistedpyrolysis reactor.

Remarkable rate enhancements in chemical reactions undermicrowave irradiation are often observed. Because some of themcannot be easily reproduced by conventional thermal heating,there are speculations on existence of non-thermal microwave ef-fects. For instance, microwave is reported to reduce activation en-ergy for decomposition of sodium bicarbonate (Shibata et al.,1996), increase Arrhenius pre-exponential factor by a factor of3.3 in synthesis reaction of titanium carbide (Binner et al.,1995), reduce reaction time of synthesis of aromatic esters inthe absence of microwave thermal effects where microwave irra-diation at 1000 MHz were used (Zhang et al., 2001), or diminishcatalyst deactivation. Microwave-induced spin dynamics is specu-lated to be an archetype of the non-thermal effects. To reliably in-terpret the unexpected observations, one has to be aware of thesignificant differences between microwave and conventional heat-ing, as listed in Table 4, which is enriched based on de la Hoz et al.(2005).

Conventional thermal heating transfers heat from the surface tocenter of a material, which is comparatively slow and inefficient. Inconventional heating, the reaction vessel (or material surface) hashigher temperatures than the reaction mixture (or material inter-ior). In contrast, microwave irradiation produces rapid, efficientin-core volumetric heating by direct coupling of microwave energywith the molecules that are present in the reactants. Since the re-action vessels are typically made by microwave-transparent mate-rials (e.g., quartz or borosilicate glass), an inverted temperaturegradient results, i.e., lower temperatures for the reaction vessel(or material surface) and higher temperatures for the reaction mix-ture (or material interior). The inverted heat transfer may inducesuperheating effects. For instance, polar liquids can be superheatedto temperatures tens degrees above their normal boiling points be-cause the boiling nuclei are formed at the surface of the liquids(Baghurst and Mingos, 1992). The unique features of microwaveheating are very difficult to be attained by conventional heating.Moreover, accurate temperature measurements in a microwave-assisted reactor may not be easy, due to, e.g., the interference ofthe electromagnetic irradiation with thermocouple sensor. Thesefactors may sometimes give rise to unexpected results. So a simple,direct comparison with conventional heating processes could leadto incorrect interpretations. Today, it is widely accepted that theinherent reason for the observed rate enhancements under micro-wave irradiation is purely due to thermal effects. Even if non-ther-mal effects may not be completely ruled out, they would be muchless important (de la Hoz et al., 2005; Kappe, 2004, 2008; Larhed etal., 2002; Lidström et al., 2001).

The unique features of microwave heating, as summarized inTable 4, can be used to improve processes, modify selectivities oreven to perform reactions that do not occur under conventionalthermal heating conditions.


4. Microwave-assisted pyrolysis of biomass

Considering the pyrolysis conditions required for liquid biofuelsproduction and the unique features of microwave dielectric heat-ing, microwave is expected to enhance biomass pyrolysis and im-prove process efficiency. As depicted in, e.g., Bridgwater andPeacocke (2000) and Vamvuka (2011), a conventional fast pyroly-sis process consists of the following main stages/elements: (1)reception and storage of a roughly prepared feedstock; (2) pre-treatment of the feedstock, e.g., feed drying, grinding or milling;(3) a pyrolysis reactor, essentially characterized by very high heat-ing and heat transfer rates, moderate and carefully controlled tem-perature, and rapid cooling or quenching of pyrolytic vapors; (4) asolid separator for char and ash removal; (5) a liquid collector, e.g.,a condenser for quenching the vapors and collecting liquid bio-fuels. Before being condensed, pyrolytic vapors may be subjectedto catalytic reforming for a preferred product selectivity. In thelight of such a conventional pyrolysis process, a microwave-as-sisted pyrolysis process can be sketched in Fig. 4, in which the cat-alytic reformer before the condenser is optional. In this process,part of the char product can be recycled into the microwave reac-tor, to serve as a microwave absorber and to recover some sensibleheat. Part of the gas product can be burned to provide the heat forfeedstock pretreatment (e.g., drying). The whole process is dis-cussed step by step here.

During biomass pretreatment in conventional pyrolysis, feed-stock particles have to be very fine (as seen in Table 2) to fulfillthe requirement for rapid heating. Such strict size requirementscan be significantly mitigated in microwave-assisted pyrolysisdue to the rapid, volumetric heating, greatly lowering the energyconsumption in size reduction. The 10% moisture as required byconventional pyrolysis during feed drying may also be mitigatedto some extent in microwave-assisted pyrolysis. A certain amountof moisture in biomass can improve the overall efficiency of thefeedstock to utilize energy from microwave radiation. For instance,the overall microwave energy utilization efficiency of the EFBbiomass sample first increases with moisture content, peaking at45 wt.% moisture, and then decreases with further increase inmoisture (as seen in Table 3). However, microwave-assisted pyro-lysis often needs other pretreatments because biomass residuesand waste feedstock are often low-loss poorly absorbing materials.For instance, microwave absorbers may need to be added andhomogeneously blended with biomass before being fed into amicrowave reactor, to achieve the required rapid heating rate. Mi-crowave-assisted pyrolysis often benefits from chemical pretreat-ments, e.g., by using ionic liquids or acids. Chemical additives areput into water to make a solution of a certain concentration, intowhich a certain amount of biomass is poured (or the solution issprayed on the feedstock). After sufficient interaction, the feed-

Fig. 4. Sketch of microwave-assisted biomass pyrolys

stock can be subjected to drying at 100–105 �C to reduce moisture.Catalysts can also be added to the feedstock, serving as microwaveabsorbers and in situ upgrading pyrolytic oil vapors in the reactor.

The reactor itself makes the most distinct difference betweenconventional and microwave-assisted pyrolysis systems. Theconditions required in a conventional pyrolysis reactor for liquidbiofuels production most likely still need to be fulfilled in a micro-wave-assisted reactor, among which a rapid heating rate and uni-form temperature distribution throughout the fuel bed may bemore readily achieved in a microwave reactor. The rapid, in-corevolumetric heating also results in a very rapid moisture releasefrom the feedstock, increasing the surface area and improvingthe surface/pore structure, which favors a quick release of volatilesduring biomass pyrolysis. This, together with the inverted heattransfer feature of microwave heating (i.e., lower temperatures atmaterial surface and higher temperatures at material interior), re-duces the contact between the released pyrolytic vapors and thehot char and therefore, reduces char-catalyzed secondary crackingin the fuel bed. Moreover, microwave heating is easily controlledand therefore, the reaction temperature may be easily controlledand maintained at a desired moderate level. All these factors im-prove the fast pyrolysis process. However, care still must be takenin design, construction and operation of a microwave reactor. Com-patibility of microwave process with the rest of the reactor processneeds to be considered in order to better integrate microwave pro-cess into the reactor system and to more efficiently introduce mi-crowave energy into the reaction medium. The requirement on theresidence time of volatile vapors in a microwave reactor may bestricter than what is required in a conventional pyrolysis reactor.In a microwave reactor, the volatile vapors (including small polarmolecules), laden by solids (mainly fine char particles), can bereadily heated up by microwave to higher temperatures. Therefore,the solids-laden vapors have to be swept out of a microwave reac-tor very rapidly to reduce secondary cracking of organic vapors inthe freeboard and to drive water vapor and water-soluble smallpolar molecules out of the reactor. Efficient transfer of microwaveenergy to the feedstock in the fuel bed is also a vital issue in amicrowave reactor. The reaction vessel, made by microwave-trans-parent materials (e.g., quartz) and used to separate the reaction vo-lume from microwave generators (magnetrons), needs to becarefully and properly purged, especially in continuous operation.Deposits of volatile vapors and fine carbon particles on the vesselwalls will impose huge difficulties in introducing microwave intothe reaction medium and even cause localized burning of the ves-sel. Moreover, the homogeneities of the microwave field and thefeedstock/absorber blends in the reactor need to be improved toavoid hotspots. Hotspots can easily make local pyrolysis tempera-tures and thus reactions out of control. The determination and useof appropriate microwave power is also important. Lower power

is process, with the main energy flow indicated.

Table 5Summary of microwave-assisted pyrolysis of biomass.

Biomass (e.g., properties) and pretreatment (e.g.,chemical treatment, use of additives/catalysts/microwave absorbers)

Reactor and key pyrolysis conditions (e.g.,microwave power, reaction temperature, heatingrate, reaction time, vapor residence time)

Pyrolysis products (e.g., yields, quality,composition)

References

Wheat straw: pelletized to large cylindricalparticle (1 cm diameter, 1 cm high), pre-dried; 46.48% C; 6.7% H, 33.71% O, 5.52% ash;density 930 kg/m3

No pretreatment, because wheat straw withhigh inorganic ash contents was believed toabsorb microwave readilyOther pellets (e.g., cellulose, wood, lignin)also tested

A batch-type microwave reactor containing asingle large pellet particle; Helium as sweepinggas; total batch reaction time 1–5 min; veryshort induction period (of low absorption andslow temperature rise due to temperature-dependent absorption) for heating strawparticle; particle fractured due to extremelyrapid water loss; high power levels (1–2 kW)needed to accomplish biomass pyrolysis

Yields of products from straw pyrolysis (wt.%):char 17.8, tar 27.9 (incl. 8.2 in cold trap), water8.2, gases 46.1 (H2/CO/CO2/CH4/C2H2/C2H4/C2H6 = 1.3/24.4/14.5/2.6/1.2/1.5/0.6), which aresimilar to those from conventional pyrolysisusing much smaller particles at hightemperatures and rapid heating rates. Tarcontains phenols, guaiacols, cresols, condensedaromatics

Krieger-Brockett(1994)

Aerobically digested sewage sludge: from anurban waste treatment plant; 71% moisture(as received); 31.2% ash, 62.3% volatiles, 6.5%fixed carbon, heating value 16.68 MJ/kg (dry);C/H/O/N/S = 52.3/8.0/32.3/6.7/0.7 (wt.%, ondry ash-free (DAF) basis)Pretreatment: two different microwaveabsorbers tested, (1) pure graphite, 3 � 3 mm,and (2) char from the pyrolysis of the sludgeobtained in previous experiments. Withoutmicrowave absorber, the sludge cannot beheated >200 �COther sewage sludges from urban wastetreatment plants or milk-derivative factoryalso tested

1 g additive homogeneously blended with 15 gwet sludge sampleThe sample exposed to microwave irradiation of1000 W for 6 min (average heating rate of200 �C/min, max. temperature 1000�1040 �C;rapid temperature rise to ca. 135 �C andremaining at this temperature for 2 min until allmoisture evaporated, then another rapidtemperature increase up to 1000 �C), 15 mL/minHelium used as carrier gasPyrolysis in electrical oven also conducted:heating rate 72 �C/min, max. temperature1000 �C, reaction time 24 min (incl. 14 minheating)Recovery of bio-oil: the organic fractionseparated from the aqueous product (mostlywater), then dissolved in CH2Cl2, dried usinganhydrous Na2SO4 and filtered

When char used as absorber, yields of char/aqueous/oil/gas (wt.%): 10.7/68.8/4.0/16.5 formultimode microwave; 10.1/62.7/3.0/24.2 forsingle-mode; 10.4/58.6/0.9/30.1 for electricalovenQuality of bio-oils from multi- and single-modeis similar. Elements (wt.%, DAF): C/H/N/S/O = 72.5/9.2/5.8/0.6/11.9; 35.7 MJ/kg. Comparingto electrical oven, microwave pyrolysis oils aremore aliphatic and oxygenated and contain noheavy PAHsChar from multi- and single-mode: 6.7 MJ/kgand 5.8 MJ/kg. Gas from multimode: 66.4 vol.%H2 + CO, 6.9 vol.% CxHy, 6.7 vol.% CO2, 7.4 MJ/Nm3. Gas from single-mode: 60.3 vol.% H2 + CO,7.8 vol.% CxHy, 9.7 vol.% CO2, 8.4 MJ/Nm3

Domínguezet al. (2003,2005, 2006)

Coffee hulls (rich in cellulose): cylindricalpellets, 3 mm (diameter) � 20 mm (height);8.2% moisture (as received); 5.6% ash, 77.0%volatiles, 17.4% fixed carbon (dry); HHV(higher heating value) 17.9 MJ/kg. C/H/N/S/O = 47.3/6.4/ 2.7/0.3/37.7 (wt.%, on dry basis)Pretreatment: Char (obtained from previouspyrolysis experiments at 1000 �C) used asmicrowave absorber

15 g sample (mixed with 3 g char) placed in aquartz tube (40 cm length � 3 cm i.d.) in amicrowave oven, 60 mL/min N2 as carrier gas,reaction temperature 500, 800 and 1000 �C, totalreaction time 20 min (incl. 5 min heating and15 min under the target temperature). Sampletemperature monitored by an infrared opticalpyrometer. Pyrolysis in electrical oven under500, 800 and 1000 �C also performed, reactiontime 25 min (incl. 10 min needed for heating tothe target temperature)Oil: the organic fractions being dissolved inCH2Cl2 and then evaporating solvent at 40 �C.Char separated from absorber by sieving. Oil andchar yields calculated on a dry basis from theirweights

Gas is the major product and oil is the minorproduct in all tests. Microwave pyrolysisproduces less oil and more gas than electricalpyrolysis for all the tested conditionsThe products (dry basis) from microwavepyrolysis under 500 �C: 7.9% oil (34.4 MJ/kg),30.2% char (24.3 MJ/kg), and 61.9% gas (incl.61.4 vol.% H2 + CO, 28.4 vol.% CO2; 12.5 MJ/kg)The products (dry basis) from electrical pyrolysisunder 500 �C: 13.6% oil (31.2 MJ/kg), 29.2% char(24.2 MJ/kg), and 57.2% (incl. 29.9 vol.% H2 + CO,56.58 vol.% CO2; 6.6 MJ/kg)

Domínguezet al. (2007)

Larch: cylindrical blocks of different sizes (60,80, 100 and 300 mm in diameter,respectively; height same as diameter);moisture adjusted to 8–12%; maincomponents 40% a-cellulose, 31% lignin, 12%pentosanNo pretreatment

Microwave power 1.5 and 3 kW, irradiation timefrom a couple of minutes to 90 min. Reactorpurged with 5L/min N2 and evacuated with anaspirator, vapor residence time >10 min. Thevolatiles cooled and condensed to collectpyrolysis liquids and tar; tar adhering to reactorinterior wall also washed down and collected.The viscous pyrolysis liquids with moistureremoved are termed as tars in this study

For the 80 mm block (microwave power1.5 kW): all moisture released in 3 min; yield ofchar decreases monotonically with time, �20%after 15 min; yield of tar peaks at �25% after12 min (5% levoglucosan based on tar weight)and decreases with further irradiation. Tarincludes carboxylic acids, furfural, cresol,guaiacol, eugenol, arabinoses, xylitol,levoglucosan, etc.

Miura et al.(2004)

Pine wood sawdust (dry basis): 81.98% volatilematter, 15.86% fixed carbon, 2.16% ash;19.92 MJ/kg; 40.32% cellulose, 26.73%hemicellulose, 30.97% ligninPretreatment: put 6 g certain chemicaladditive into 400 mL water to make solution,pour 60 g dry sawdust into it, mix them for5 min, then dry the mixture at 105 �C. SiCadded as microwave absorber

15 g untreated sawdust or 16.5 g treatedsawdust (incl. 1.5 g additive), blendedhomogeneously with 25 g SiC, are put in a quartzbeaker-shaped reactor (12 cm i.d. � 13 cmheight) in a microwave cavity. Actual pyrolysistemperature 470 �C, heating rate 5 �C/s (lower,favoring coke formation), run time 12 min. Purgegas 0.2 m3/h N2, vapor residence time ca. 2.7 s(relatively long, adequate for 2nd decompositionof some heavy components into light gases)

For untreated sawdust: 22.7% liquid (in which30.1% water), 17.3% solid, 60% gas. Max. oil yieldis from Fe2(SO4)3-treated sawdust: 26% liquid(27.7% water), 36.7% solid, 37.3% gas. Liquid fromuntreated sawdust (area%): acetol 26.6, furfural5.72, 2-furanmethanol 4.81, 4-methyl-guaiacol5.05, guaiacol 5.12, levoglucosan 1.04. Longresidence time and low heating rate concludedto be the main reasons for the low oil yield

Chen et al.(2008)

Pine wood sawdust, peanut shell, and maizestalk: their moisture /volatiles/fixed carbon/ash (air-dried, wt.%) and LHV (lower heatingvalue, MJ/kg) are 8.1/77.3/14.3/0.3 and 17.2for pine wood sawdust, 8.8/68.5/18/4.7 and16.1 for peanut shell; 8.5/68.1/16.3/7.1 and15.5 for maize stalk. All particle size0.5�1.0 mm.Pretreatment: Microwave (600 W, 6 min)used only for drying.

The microwave-dried biomass undergoespyrolysis in a bench-scale, conventional,electrically-heated fluidized-bed reactor.Pyrolysis temperature 500 �C. Pure N2 as carriergas, <1 s vapor residence timeQuick moisture release induced by microwavedrying increases surface area and modifiessurface/pore structure, leading to quick releaseof volatiles during biomass pyrolysis, reducingsecondary cracking, yielding more liquid oil and

Pine sawdust: 70% oil, 15% gas, 15% char; oilquality: 26% water, pH 2.5, viscosity 15.2 mPa sat 40 �C, LHV 15 MJ/kgPeanut shell: 52% oil, 17% gas, 31% char; oilquality: 34% water, pH 2.6, viscosity 12.6 mPa sat 40 �C, LHV 14 MJ/kgMaize stalk: 51% oil, 19% gas, 30% char. Oilquality: 35% water, pH 2.7, viscosity 9.8 mPa s at40 �C, LHV 12 MJ/kgMain oil compounds: alcohols, ketones, furfural,

Wang et al.(2008)

(continued on next page)


Table 5 (continued)




References

solid char. Water content is reduced due toweakened secondary pyrolysis

phenol, alkylated phenols, furan derivatives, etc.

Rice straw: 8.25% moisture (as received);volatiles/fixed carbon/ash (wt.%, dry): 72.2/14.4/13.4. C:H:O = 1:1.66:0.78 (molar ratio,dry ash-free). Heating value 15.26 MJ/kg. 30–35% hemicellulose, 21–31% cellulose, 4–19%lignin; particle size <0.85 mmNo pretreatment

Ca. 3–5 g sample in a quartz holder (3 cmheight � 4 cm o.d.) placed in a reaction tube(40 cm height � 5 cm o.d.; quartz); 50 mL/min N2 as carrier gas; single-mode microwave,power (P) 100–500 W. Heating rate fit as 0.32P-27.6 �C/min; max. temperature fit as1.07P + 47.2 �C300 W (55 �C/min heating rate, 407 �C max.temperature) was concluded as the mostacceptable microwave power for their studies

At P = 300 W: 28.07% solids (�19 MJ/kg); 49.37%gases (incl. H2/CO2/CO/CH4 = 53/20/15/9 vol.%;�11 MJ/Nm3); 22.56% liquids (43.68 area%alkanes from C12 to C32; 25.84 area% polars incl.phenol and its derivatives; 8.94 area% polycyclicaromatic hydrocarbons, 2–3 rings and theirderivatives). The commonly-referred, toxic PAHsare not detected in liquids

Huang et al.(2008,2010)

Wheat straw pellets: 10% moisture (as received);C/H/N = 40.21/5.34/0.7 wt.% in the startingmaterialsPretreatment: 3 chemical pretreatmentsused. The biomass sample treated by: (1)H2SO4 solution, 10%w/w, then dried; (2) NH3

(3% v/v) into the sample as a gas; (3) HCl (3%v/v) into the sample as a gas

150–200 g samples exposed to a max.microwave power of 1200 W, heated up to amax. temperature of 180 �C at a heating rate of17 �C/min; process pressure monitored at alltimesThis is one of the very few efforts on biomasspyrolysis at very low temperatures for bio-oilproduction. Biomass is believed to start to‘‘soften’’ at around 180 �C. Microwaves can thenget in and trigger the formation of protons andthe protons rapidly decompose the cellulose

H2SO4 and NH3 result in a sacrifice of oil yield forgreater quantities of char; HCl has minimaleffect on the oil yieldPyrolysis products (1000 W, 130 �C, no additive):29% char, 21% oil, 36% water, 14% gas. Oilcomposition: 11 area% 4-vinylguaiacol/dihydrobenzofuran, 28 area% levoglucosan, 10area% phenols, 1.3 area% desaspidinol. Oilheating value: 16–22 MJ/kg

Budarin etal. (2009)

Corn stover pellets: 6.2 mm in diameter � (10–20)mm in length (D � L); 6.4% moisture,24.5 MJ/kg; 18% lignin, 37% cellulose, 27%hemicelluloseAspen pellets: 4.8�(4–8)mm (D � L); 5.9%moisture, 17.9 MJ/kg; 19% lignin, 53%cellulose, 27% hemicellulosePretreatment: Inorganic compounds (e.g.,MgCl2, Na2HPO4) added as microwaveabsorber for speeding up heating and/orchemical catalyst for in situ upgrading oforganic vapors

100 g biomass samples (mixed with 0/2/4/8 gcatalyst, respectively) placed in a quartz flaskinside a microwave cavity; microwave (of aconstant power input 875 W) treatment: 20 minfor each batch; temperatures of the biomassmixture (measured immediately after heatingwas terminated): 450–550 �CBoth the condensable vapors and thecondensates adhering to the interior wall of theflask collected and weighted as liquid (bio-oil)

KAc, Al2O3, MgCl2, H3BO3, Na2HPO4 all increasebio-oil yield. Increase in MgCl2 dosage (bymixing 0/2/4/8 g MgCl2 with 100 g biomass)yields more bio-oil from corn stover and aspen:both produce ca. 42% bio-oil when 8 g MgCl2 isused; charcoal yield also increases for aspen butdoes not change much for corn stover. MgCl2

also effective in improving product selectivity:2–3 major peaks on bio-oil GC–MS spectra atlower levels of MgCl2, while furfural peakaccounting for 80 area% when 8 g MgCl2 per100 g biomass is used

Wan et al.(2009)

Aspen pellets: 5 � 20 mm (D � L); 240 kg/m3

bulk density; 5.9% moisture; 17.9 MJ/kg; C/H/N/S/O = 45.4/4.8/0.5/0/49.3 (wt.%, DAF basis)No pretreatment

50–250 g samples placed in a quartz flask insidea microwave reactor: constant microwave powerinput of 700 W, 20 min irradiation. The evolvedvolatiles pass through a catalyst column (undercontrolled temperature 300–600 �C) beforebeing condensed on water-cooling columns andcollected

Solid acids are effective catalysts to decomposepyrolysis vapors, while solid alkaline and othercatalysts do not affect the composition of theliquid product. Increasing the temperature of thecatalyst bed and the ratio of catalysts to biomassadversely affected the liquid yield

Zhang et al.(2010)

Chlorella sp. (a wide-type algae strain):naturally-dried; 13.7% moisture, 68.4%volatiles, 10.1% fixed carbon, 7.8% ash; C/H/N/O = 49.7/6.98/10.92/24.6 (wt.%, dry basis)Pretreatment: Char (produced from pyrolysisof biomass) used as a microwave absorber

30 g algae (blended with 6 g solid char) placed ina 500 ml quartz flask in a microwave cavity,500 ml/min N2 as carrier gas, 20 min reactiontime, reaction temperature monitored using aninfrared optical pyrometer, final temperaturemeasured by inserting a thermocoupleimmediately at the end of reaction. Thecondensable volatiles continuously collected incondensers (0–2 �C)Temperature increases in the first 8–9 min(�60 �C/min), and then stays relatively stable.The final temperature measured: ca. 462 �C at500 W, 569 �C at 750 W, 600 �C at 1000 W, and627 �C at 1250 W

Water yield is ca. 21–22%, independent of inputpower. Oil increases from 26% at 500 W to a max.yield of 28.6% at 750 W and then decreasesgradually (to 18% at 1250 W). Gas yield increasesover the entire range of power (from 24% at500 W to 35% at 1250 W). Char yield decreasesfrom 28% at 500 W to 25% at 750 W and thenremains constant. Bio-oils compose of aliphatichydrocarbons, aromatic hydrocarbons, phenols,long chain fatty acids, nitrogenated compounds,among which aliphatic and aromatichydrocarbons account for 22.18 area%

Du et al.(2011)

Corn stover (from South Dakota, USA): air-driedat room temperature, mechanicallypulverized into 0.5–4 mmNo pretreatment

Biomass sample placed in a 1L quartz flask in amicrowave oven, under 700 W microwaveirradiation, which results in a stage heating rate(50 �C/min in the first 3 min, 160 �C/minbetween 3 and 5 min, reaching desiredtemperatures after 6 min); N2 as purge gas;heavier volatiles condensed and collected as bio-oils, lighter volatiles escaped as syngas, char leftin the quartz flask. Effects of pyrolysis conditions(reaction temperatures 515–685 �C, reactiontime 4–22 min, biomass particle size 0.5–4 mm)on yields of different products studied

Maximum volatile yield of 76.08% (33.72% bio-oil, 42.36% syngas) occurs for 8 min reactiontime, 650 �C temperature, and 3 mm particlesize. Maximum bio-oil yield (36.98%) for 18 minreaction time, 650 �C temperature and 1 mmparticle size. The effect of particle size (0.5–4 mm) is insignificant in microwave-assistedpyrolysis of corn stover. The bio-oil mainlyincludes phenols (28–40 area%), aliphatichydrocarbons (11–24 area%), aromatichydrocarbons, furan derivatives, some acids, etc.

Lei et al.(2009)

DDGS (distillers dried grain with solubles):36.74% neutral detergent fiber (composed ofcellulose, lignin and hemicellulose), 29.93%crude protein, 11.07% starch, 10.5% fat, and12.82% crude ash (all on dry basis). Ca. 6%

100 g DDGS placed in a 500 ml quartz flaskinside a microwave oven with a rated power of1000 W. Heating rate of 100 g DDGS at variouspower inputs: 105, 130, 255, 500, and 570 �C/min at 600, 700, 800, 900, and 1000 W,

Power input greatly affects DDGS biofuel yields.At same reaction time and lower temperature,higher power inputs result in increased yields ofbio-oil and syngas. At same reaction time andhigher temperature, higher power inputs reduce

Lei et al.(2011)


Table 5 (continued)




References

moisture in DDGSNo pretreatment

respectively. N2 as the reactor purge gas; heaviervolatiles condensed and collected as bio-oils,char left in the quartz flask. Effects of reactiontemperatures (516–684 �C), reaction time (thetime under the desired temperature, 4.6–21.4 min), and power input (600–1000 W) arestudied

bio-oil yield and increase syngas yield. Bio-oilsare phase-separated to two levels at roomtemperature: the lower level as aqueous phase(2–5 MJ/kg); the upper level as oil phase (20–28 MJ/kg). C6–14 chemical compounds are up to95% of DDGS bio-oils

Douglas fir: 7 mm � 15 mm (D � L)Pretreatment: activated carbon used asmicrowave absorber and different carbon-to-fir ratios (0, 2:1, 3:1 and 4:1) tested andcompared

A fixed total loading of 120 g for differentadditive/biomass blends. 60 K/min heating ratebefore reaching a desired reaction temperature;after that automatic temperature/power controlto maintain the temperature. Tests conductedunder different additives-biomass ratios (0, 2, 3,and 4), reaction temperatures (316, 350, 400,450 and 484 �C, measured by an infrared opticalpyrometer), and reaction times under thedesired temperature (4, 8, 12 and 15 min)

Liquid yields range from 6.8 wt.% (12 min under350 �C, additive/biomass = 4/1) to 48.1 wt.% ofbiomass (12 min under 350 �C, additive/biomass = 2/1), with the majority in 25–35%.Reaction temperatures and additive-biomassratios greatly affect liquid yields, and also affectliquid compositions (e.g., phenol, phenolics,furfural and its derivatives). The reaction timehas less important effect on product distribution

Bu et al.(2011)

Sewage sludge: from urban wastewatertreatment plant; ca. 80% moisture in sludgesample; 75.5% volatiles and 24.5% ash (on drybasis). Element mass fraction (%, DAF basis):C/H/N/S/O = 39.4/5.7/4.75/1.18/24.4Pretreatment: Graphite (1 � 1 mm) used asmicrowave absorber

100 g sludge (homogenously blended with 25 ggraphite) placed in a quartz tube in a multimodemicrowave cavity; sample temperaturemeasured by an infrared optical pyrometer;100 cm3/min N2 as carrier gas; microwavepower 200–1200 W tested, 400 W (max.temperature 490 �C) leading to max. oil yield;10 min reaction timePyrolysis in electric furnace at 500 �C (for30 min) also performed

Microwave pyrolysis under 400 W yields 49.79%bio-oil (35 MJ/kg, 929 kg/m3, viscosity 45cSt atroom temperature, flash point 45 �C, 29.5 wt%monoaromatics), 10.84% gases, 39.37% solids.800 W (800 �C) results in a reduced oil yield(11.71%) due to cracking into gases. Electricalpyrolysis yields 37% oils (30 MJ/kg, with higheroxygen content), 23% gases, 40% solids

Tian et al.(2011)

OPF (oil palm fiber): 300�600 lm (<1 mm inwidth)OPS (oil palm shell): 1–100 mm (0.5–4 mm inwidth)On as received basis, OPF has 10% moistureand OPS has 8% moisture. On dry basis, bothcontain �75% volatile matters, 20% fixedcarbon, and 5% ashPretreatment: char from conventionalpyrolysis of OPS used as microwave absorber,100–300 lm in size

Fluidized bed quartz glass reactor (0.1 mi.d. � 0.15 m height), with perforated steeldistributor plate of 1 mm holes. Purge gas �10 L/min N2 (ca. 7 s of vapor residence time);multimode microwave of 450 W, irradiationtime 25 minTemperatures in bed and just above the bedsurface both measured via thermocouples. Inpyrolysis of OPF (1:0.5 blended with char):maximum in-bed temperature 1460 �C, veryhigh heating rate (�0.1 s to reach 1000 �C). Inpyrolysis of OPS (also 1:0.5 blended with char):maximum in-bed temperature �237 �C, lowheating rate (�18 �C/min)

The 1:0.5 biomass-char blend results in thehighest oil yield and lowest char yield amongvarious blend ratios (e.g., 1:0.25, 1:1) for bothOPF and OPS. For OPF, pyrolysis products include22 wt.% bio-oil (incl. water), 29 wt.% gas, 49 wt.%char. For OPS, the products include 25 wt.% bio-oil (incl. water), 30 wt.% gas, 45 wt.% char (forlarge OPS particles, the particle centertemperature is believed to be much higher thanthe measured value). Bio-oils: pH 2.7–3.0,viscosity �0.015 cm2/s at room temperatures

Salema andAni (2011)

Wheat straw: 7.4% moisture, 69.3% volatiles,17.4% fixed carbon, 5.9% ash (air-dried)Corn straw: 7.5%, 70.2%, 17.5%, 4.7%,respectivelyBoth straws are baled after air-drying: balesize 1 � 0.6 � 0.6 m3, bulk density 80 kg/m3

No pretreatment

Microwave-assisted pyrolysis of 1 � 0.6 � 0.6 m3

straw bale (20 sets of magnetron, total power18 kW), temperatures in bale measured by nineK-type thermocouples of 4 mm in diameter (5horizontally, 4 vertically), bale mass measuredcontinuously as a function of time (accuracy50 g). 1L/min N2 as carrier gas. Reaction time85 min when 0.334 kW/kg-straw microwavepower used; ca. 46 min when 0.668 kW/kg-straw used. Final temperature ca. 600 �C

Pyrolysis products: close to 1:1:1 for gas, liquidand char (due to medium pyrolysis temperatureand slow heating rate), in which solids are theslightly highestThe pure pyrolysis gas from both kinds of strawbales: �18 vol.% CO, 36 vol.% H2, 22 vol.% CH4,19 vol.% CO2; LHV 15 MJ/Nm3

Electricity consumption: 0.58–0.65 kWh/kg-straw

Zhao et al.(2011)

Wheat straw: 7.4% moisture, 69.3% volatiles,17.4% fixed carbon, 5.9% ash (air-dried); C/H/O/N/S = 41.41/5.86/45.22/1.44/0.17 (wt.%,DAF). The straw is shredded and sieved toparticles with sizes <0.09 mmPretreatment: Pyrolysis residue used asmicrowave absorber

Microwave-assisted pyrolysis of shredded wheatstraw. 5–30 g shredded straw mixed at 2:1 inmass ratio with additives, placed in a quartzreaction tube (18 cm length, 8 cm outerdiameter) in a modified 3000 W domesticmicrowave oven cavity. N2 (3L/min) used aspurge gas. In case of need, hot air (up to 400 �C)can flush the microwave cavity to prevent thecondensation of liquids on walls of the reactiontube and pipelines. Low heating rate (<60 �C/mineven using SiC as the sample in the reactor);10 min reaction time under target pyrolysistemperatures (400, 500, 600 �C)

Yield of gas/liquid/solid (wt.%): 18/25/57 at400 �C; 21/31/48 at 500 �C; 23/31/46 at 600 �CGas product: heating value 11–12 MJ/Nm3; CO/CO2/H2/CH4 (vol.%): 35/34/22/8 at 400 �C; 33/33/25/8 at 500 �C; 27/24/44/6 at 600 �CSolid product: C/H/O/N/S = 53/2.5/36.5/1/1 wt.%;temperatures have no big effect on thecompositions. Increase in temperatures greatlyincreases specific surface area and pore volume,and decreases average pore size

Zhao et al.(2012)


with lower heating rates always favors char formation, while high-er power with higher pyrolysis temperatures favors gasification re-actions, both of which will reduce the yield of liquid product. Thefollowing correlations may be used to quantitatively estimate thepower absorbed, feedstock heating rate, and microwave penetra-tion depth (Clark et al., 2000), in design of a microwave reactor,

P ¼ 2p � f � e0 � e0r � tan d � jEj2 ð1ÞDTDt¼ P

q � Cpð2Þ

D ¼ 3k0

8:686p � tan d �ffiffiffiffiffiffiffiffiffiffiffie0r=e0

p ð3Þ


where P , DTDt , and D represent the power absorbed per unit volume,

heating rate of the feedstock assuming the majority of the absorbedmicrowave power is converted to heat within the reaction medium,and microwave penetration depth (i.e., the depth at which the inci-dent power is reduced by one half), respectively. f, e0, e0r , tan d, E, q,Cp, and k0 denote the microwave frequency, permittivity of freespace, relative dielectric constant, loss tangent, magnitude of the in-ternal electric field, density of feedstock, specific heat of feedstock,and incident wavelength, respectively. As can be seen from the cor-relations, the power absorbed and depth of microwave penetrationare highly dependent on the dielectric properties of the material.One of the limitations in designing a microwave reactor is the lackof accurate dielectric data in the microwave frequency range as afunction of temperature. Given all the physical properties (includingdielectric data) of a feedstock, one may estimate the appropriatemicrowave power required to process the feedstock of a certainmass flow rate and to achieve a desired heating rate. The cross-sec-tion area and stirring demand of the fuel bed can also be estimatedfrom the microwave penetration depth, in order to heat up thewhole fuel bed uniformly and efficiently. If the penetration depthD is much smaller than the height of the fuel bed, surface heatingwill occur. If D is much greater than the fuel bed height, the fuelbed is almost transparent and heating will be negligible.

Solids separator is also desirable in a microwave-assisted reac-tor. Char can catalyze secondary cracking in the vapor phase. Evenin the cooled liquid product, char contributes to the instabilityproblems, accelerating slow polymerization processes and increas-ing viscosity. As a result, rapid and complete char separation fromvolatile vapors is needed. Char separation is not easy to implement,especially for fine char particles. Microwave-assisted processmakes no difference with conventional pyrolysis in terms of solidsseparation and therefore, solids separators successfully used in thelatter can also be employed in the former.

Catalytic reforming is an optional stage in a pyrolysis process.Sometimes, it may be integrated to help to improve the selectivityof liquid product towards the preferred composition profiles. How-ever, care must be taken in temperature condition and residencetime of the volatile vapors in the reformer to reduce secondarycracking.

Table 6Key properties of various microwave-assisted pyrolysis bio-oils, No. 2 diesel fuel, and con

Properties Microwave-assisted pyrolysis bio-oil

Sewagesludgea

Coffeehullsb

Cornstoverc

Pine woodsawdustd

Elemental compositions (wt.%, dehydrate base)C 72.3 74 60.7 48.8H 9.1 8.4 7.7 6.8N 5.8 8.1 2.0 0.9S 0.9 0.8 0.2 0O 11.9 8.7 29.4 43.5Water (%) – – 15.2 26.2Density (kg/m3) – – 1250 1145pH value – – 2.87 2.5Viscosity (mPa s, 40 �C) – – 185 15.2HHV (MJ/kg) 35.7 34.4 17.5 15k

a From the reference Domínguez et al. (2006).b From the reference Domínguez et al. (2007).c From the reference Yu et al. (2007).d From the reference Wang et al. (2008). Among the microwave-assisted pyrolysis bio

then the dried sample undergoes pyrolysis in a conventional pyrolysis reactor. All othere From the reference Budarin et al. (2009).f From the reference Du et al. (2011).g From the reference Tat and van Gerpen (1999).h From the reference Bridgwater and Peacocke (2000).i From the reference Nexant Ltd. (2008).j Acid number.k LHV (Lower heating value in MJ/kg).

Just same as conventional pyrolysis, a liquid collector is neededin microwave-assisted pyrolysis to condense the vapors and collectthe liquid product. Careful design and temperature control in thecondenser is required to avoid blockage from differential conden-sation. The collected liquid biofuel typically consists of two frac-tions: a water-soluble phase and an insoluble oil phase. Theformer comprises a mixture of compounds (e.g., furfural), and thelatter contains a mixture of hydrocarbon-based heavy oils whichneed further downstream refinements.

The major energy fluxes are also indicated in the process in Fig.4, where QB, QE, and W represent biomass chemical energy, inputelectricity energy, and mechanical work of feeder/stirrer for bio-mass (and blower/fan for carrier gas), respectively; QC, QG, QL, QW,and Qloss denote total (including chemical and sensible) energy ofthe solid product (char), chemical energy of gas product (syn-gas), chemical energy of liquid product (bio-oil), heat taken outby cooling water in condenser, and conversion loss from electricityto thermal energy and other heat losses, respectively. An energybalance, which includes a mass balance, needs to be verified in mi-crowave-assisted pyrolysis tests in order to gradually improve theprocess for better efficiency,

QB þ QE þW ffi QC þ QG þ QL þ QW þ Q loss: ð4Þ

By following the above stepwise discussion, the research anddevelopment efforts in the literature on microwave-assisted bio-mass pyrolysis are reviewed and summarized in Table 5. Most ofthe reported efforts are on lab-scale, batch-mode tests. In the tabu-lated summary, the pretreatment and reactor/pyrolysis conditionsare highlighted, mainly because they represent the most distinctdifference with conventional pyrolysis. The resultant productsand key findings are also provided. Among various contributors,some groups, e.g., Clark and his co-workers (Budarin et al., 2009,2010, 2011; Clark et al., 2009), Lei and Ruan and their co-workers(Bu et al., 2011; Du et al., 2011; Lei et al., 2009, 2011; Wan et al.,2009; Yu et al., 2007; Zhang et al., 2010), Lo and his co-workers(Huang et al., 2008, 2010), Menéndez and his co-workers(Domínguez et al., 2003, 2005, 2006, 2007, 2008; Fernández andMenéndez, 2011; Menéndez et al., 2004), Ma and Song and theirco-workers (Zhao et al., 2010, 2011, 2012), have made consistent,

ventional fast pyrolysis bio-oils.

2# Diesel fuelg Fast pyrolysis bio-oil

Wheatstrawe

Chlorella sp.f Woodh Wheatstrawi

58.9 65.4 86.3 56.4 45–556.85 7.84 13.3 6.2 6–71.15 10.28 – 0.1 0.30.02 0 – - 0.5–533.2 16.48 – 37.3 40–50<1 – 15–30 –1200 980 830 1200 12501.4j 9.7 – 2.5 70–150j

– 60 2–2.7 40–100 –16–22k 30.7 43 16–19 16–21k

-oils, this is the only case in which microwave is used only for drying purpose andmicrowave-assisted pyrolysis bio-oils are produced in microwave reactors.


successful attempts in this attractive area, as summarized in Table5. Here, the low-temperature microwave-assisted pyrolysis beinginvestigated by Clark and his co-workers are highlighted a littlebit. Their pyrolysis process begins at around 180 �C, which is muchlower than all other reported attempts. Once the biomass hasbegun to soften, there is some mobility so that the microwavecan get in and trigger the formation some reactive intermediates,which are believed to kick the start of the decomposition of every-thing else in biomass feedstock.

In Table 5, works from a same group are either merged into onerow, or listed in separate but successive rows in the chronologicalorder. Some of the resultant microwave-assisted pyrolysis bio-oilsare given in Table 6, in which No. 2 diesel fuel and some conven-tional fast pyrolysis bio-oils are also listed for comparison. Thelab-scale, batch-mode efforts indicate that microwave-assistedpyrolysis is a promising technology for liquid biofuels productionfrom biomass residues and waste feedstock. With careful designand operation as discussed earlier, continuous microwave-assistedbiomass pyrolysis process can be developed and scaled up for in-dustrial applications.

5. Conclusions

Due to the unique features of microwave heating (e.g., rapid,volumetric heating, inverted heat transfer, easy control) and therequired pyrolysis conditions for liquid biofuels production (e.g.,rapid heating rate, fine control of temperature at a moderate level),microwave heating is expected to enhance pyrolysis for bio-oilproduction. This paper thoroughly discusses microwave-assistedpyrolysis of biomass stepwise from feedstock pretreatment tobio-oil collection, and provides a handy, up-to-date overview ofthe research and development efforts on microwave-assistedbiomass pyrolysis in the literature and their major findings.

Acknowledgements

The research leading to these results has received funding fromthe European Union’s Seventh Framework Programme managed byREA – Research Executive Agency http://ec.europa.eu/research/rea(FP7/2007-2013) under Grant Agreement Number 286674.

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Microwave-assisted pyrolysis of biomass for liquid biofuels production

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