Lignocellulosic biomass pyrolysis_ A review of ...

Renewable and Sustainable Energy Reviews 57 (2016) 1126–1140

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Lignocellulosic biomass pyrolysis: A review of product properties and effects of pyrolysis parameters Tao Kan n, Vladimir Strezov, Tim J. Evans Department of Environmental Sciences, Faculty of Science and Engineering, Macquarie University, Sydney, NSW 2109, Australia

art ic l e i nf o

a b s t r a c t

Article history: Received 4 February 2015 Received in revised form 23 September 2015 Accepted 17 December 2015

Pyrolysis is one of the thermochemical technologies for converting biomass into energy and chemical products consisting of liquid bio-oil, solid biochar, and pyrolytic gas. Depending on the heating rate and residence time, biomass pyrolysis can be divided into three main categories slow (conventional), fast and flash pyrolysis mainly aiming at maximising either the bio-oil or biochar yields. Synthesis gas or hydrogen-rich gas can also be the target of biomass pyrolysis. Maximised gas rates can be achieved through the catalytic pyrolysis process, which is now increasingly being developed. Biomass pyrolysis generally follows a three-step mechanism comprising of dehydration, primary and secondary reactions. Dehydrogenation, depolymerisation, and fragmentation are the main competitive reactions during the primary decomposition of biomass. A number of parameters affect the biomass pyrolysis process, yields and properties of products. These include the biomass type, biomass pretreatment (physical, chemical, and biological), reaction atmosphere, temperature, heating rate and vapour residence time. This manuscript gives a general summary of the properties of the pyrolytic products and their analysis methods. Also provided are a review of the parameters that affect biomass pyrolysis and a summary of the state of industrial pyrolysis technologies. & 2016 Elsevier Ltd. All rights reserved.

Keywords: Biomass pyrolysis Mechanism Bio-oil Biochar Pretreatment Pyrolysis parameters

Contents 1. 2. 3.

4.

n

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pyrolysis mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Properties and applications of products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Bio-oil. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Biochar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Pyrolytic gas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Analysis of pyrolysis products. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Parameters influencing biomass pyrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Biomass type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Biomass pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1. Physical pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2. Thermal pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.3. Chemical pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.4. Biological pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Effects of reaction conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1. Reaction atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2. Temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.3. Heating rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.4. Vapour residence time. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Corresponding author. Tel.: þ 61 2 9850 7950. E-mail address: [email protected] (T. Kan).

http://dx.doi.org/10.1016/j.rser.2015.12.185 1364-0321/& 2016 Elsevier Ltd. All rights reserved.

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T. Kan et al. / Renewable and Sustainable Energy Reviews 57 (2016) 1126–1140

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5. Biomass pyrolysis reactors and state-of-arts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1135 6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1135 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1136

1. Introduction

2. Pyrolysis mechanism

With the rapid increase in global energy demand and increasing environmental and sustainability challenges, biomass fuels as renewable energy sources have increasingly been considered as a key option to substitute conventional fossil fuels. Currently, biomass and waste contribute to around 10% of the global energy supply [1]. The full estimated potential of annual biomass availability is predicted to be as high as 1.08 " 1011 toe (tons of oil equivalent), which is almost 10 times the world's current energy need [2,3]. The abundant biomass reserves, its renewability, CO2 neutrality, and technical grafting from coal industries have been the main driving forces for research and utilisation of biomass. Thermochemical technologies for converting biomass into energy or chemicals mainly consist of combustion, pyrolysis, gasification, and high-pressure liquefaction [4]. Biomass pyrolysis with a long history of use, initially for the production of charcoal (biochar), has emerged as a frontier research domain. Biomass pyrolysis is generally defined as the thermal decomposition of the biomass organic matrix in non-oxidising atmospheres resulting in liquid bio-oil, solid biochar, and non-condensable gas products. Depending on the heating rate and solid residence time, biomass pyrolysis can be divided into three main types including slow (conventional) pyrolysis, fast pyrolysis and flash pyrolysis [5,6]. Some other pyrolysis processes may also be conducted between these typical pyrolysis types [7]. Slow pyrolysis, termed carbonisation, has been conventionally applied for the production of charcoal. Due to the long residence time (lasting hours to days), relatively low temperature ( # 300–700 °C), and the acceptance of a wide range of particle sizes (5–50 mm) [8], the thermal decomposition of biomass (mostly lignocellulosic types) proceeds under a very low heating rate with sufficient time allowed for repolymerisation reactions to maximise the solid yields. Fast pyrolysis typically involves high heating rates (4 10–200 °C/s) and short residence times (0.5–10 s, typically o2 s) [8]. Bio-oil yield (dry biomass basis) can be as high as 50–70 wt%. The flash pyrolysis process is characterised by higher heating rates of 103– 104 °C/s and shorter residence times (o 0.5 s), resulting in very high bio-oil yields which can achieve up to 75–80 wt% [9–11]. The future of research on biomass pyrolysis toward achievement of high energy efficiencies and tailoring the conditions to produce the desired product types takes into consideration the experience and knowledge of the influences of pyrolysis parameters on the process performance, including reaction rate, product selectivities and yields, product properties and energy efficiency [12]. The pyrolysis parameters for consideration involve feedstock type selection (biomass type, particle size, biomass pretreatment), reaction conditions (pyrolysis temperature, pressure, particle heating rate, residence time), reactor configurations and processes, and miscellaneous variables, such as the addition of catalysts and vapour condensation mechanisms [13]. This paper aims to review the properties and applications of pyrolysis products, as well as the effects of pyrolysis parameters on biomass pyrolysis product yields and properties.

The complexity of biomass pyrolysis arises from the difference in decomposition of the biomass components with varying reaction mechanisms and reaction rates which also partly depend on the thermal processing conditions and reactor designs. Interactions between the major constituents of the woody biomass, such as the cellulose, hemicelluloses, and lignin, during pyrolysis have been confirmed previously [14], which makes prediction of biomass pyrolysis characteristics simply based on the thermal behaviour of the three individual components very difficult. For example, the interaction between hemicellulose and lignin promotes production of lignin-derived phenols while hinders the generation of hydrocarbons [15]. Lignin also significantly interacts with cellulose during pyrolysis as lignin hinders the polymerisation of levoglucosan from cellulose thus reducing biochar formation, while the cellulose-hemicellulose interaction has a lower effect on the formation and distribution of pyrolysis products [16]. During biomass pyrolysis, a large number of reactions take place in parallel and series, including dehydration, depolymerisation, isomerization, aromatisation, decarboxylation, and charring [12,17,18]. It is generally accepted that the pyrolysis of biomass consists of three main stages: (i) initial evaporation of free moisture, (ii) primary decomposition followed by (iii) secondary reactions (oil cracking and repolymerisation) [19]. These stages are intermingled, with a possibility to observe their transitional behaviour through thermal analysis. The apparent specific heat of biomass during pyrolysis and the corresponding heats of reactions during different pyrolysis stages have been extensively studied in the past using computer-aided thermal analysis (CATA) under different heating rates [20–26]. Biomass decomposition generally occurs during the primary decomposition to form solid char at 200–400 °C, which is responsible for the largest degradation of biomass [27]. The secondary reactions proceed to take place within the solid matrix with further rising of the temperature [27]. The degradation pathways of the main biomass components have been investigated separately. Decomposition of hemicellulose, generally represented by xylan, mainly takes place between 250 and 350 °C, followed by cellulose decomposition, which primarily occurs between 325 and 400 °C with levoglucosan as the main pyrolysis product [28,29]. Lignin is the most stable component which decomposes at higher temperature range of 300–550 °C [30]. Among the three major biomass constituents of cellulose, hemicelluloses, and lignin, the decomposition of cellulose has been most widely analysed and best comprehended [31]. Fig. 1 illustrates the simplified reaction pathway of cellulose pyrolysis, which is the generally accepted Waterloo-mechanism [32]. Dehydrogenation, depolymerisation and fragmentation are the main competitive reactions dominant at different temperature ranges.

3. Properties and applications of products 3.1. Bio-oil When bio-oil production is the primary product of interest, fast or flash pyrolysis are the conditions of choice to maximise the

1128


k

Char, water, CO , CO, etc.

Dehydration

(dominant at T< 250°C and slow heating rates)

Cellulose

k Low degree of

k

Anhydrosugars (levoglucosan, etc.)

Active cellulose

polymerisation

k Carbonyls, acids, alcohols, etc.

Fig. 1. Waterloo-mechanism of primary decomposition of cellulose [31].

bio-oil yields. The essential principles of fast pyrolysis generally involve moderate pyrolysis temperature (450–650 °C), biomass particle sizes of less than 2 mm, very high heating rates (103– 105 °C/s), very short vapour residence time (o 2 s), and rapid quenching of pyrolytic vapours to suppress secondary reactions [5,33–35]. Fast removal of primary char is a general requirement as it acts as a catalyst for cracking primary organic vapours to form secondary char, gas, and water, leading to lower bio-oil yields [35]. The fast pyrolysis process typically results in bio-oil, gas, and char yields of 60–70 wt%, 13–25 wt%, and 12–15 wt%, based on dry biomass feed weight [36]. Flash pyrolysis requires typical feed particle sizes of not more than 200 mm and higher temperatures of around 800–1000 °C, giving a typical bio-oil yield of 75 wt%, and gas and char yields of 12–13 wt% [5,10]. Bio-oil is also referred to as pyrolysis oil, pyrolysis liquid, pyrolysis tar, bio-crude, wood liquid, wood oil or wood distillate [7,37]. It is a dark brown, free flowing organic liquid mixture, which generally comprises of a great amount of water (usually 15– 35 wt%) and hundreds of organic compounds, such as acids, alcohols, ketones, aldehydes, phenols, ethers, esters, sugars, furans, alkenes, nitrogen compounds and miscellaneous oxygenates [38], as well as solid particles [39]. The final water content of the bio-oils depends on the initial moisture content of the feedstock and water formation during pyrolysis relating to the reaction parameters. It is very hard to achieve chemically accurate identification of some individual components in the bio-oils due to the existence of pyrolytic lignins with molecular weights as high as 5000 amu or even more [40]. The higher heating value (HHV) of the bio-oils typically ranges between 15 and 20 MJ/kg which is only 40–50% of the conventional petroleum fuels' HHV (42–45 MJ/kg) [41]. This is due to the considerable oxygen content which is in the extent of 35-40 wt% on dry basis weight. The HHV of the bio-oils can be approximately calculated from elemental analysis by the following empirical correlation: ! " HHV MJ=kg ¼ 0:3491 " C þ 1:1783 " H þ 0:1005 " S – 0:1034 " O – 0:0151 " N – 0:0211 " A [42], or simplified as: ! " ! " HHV MJ=kg ¼ ð338:2 " C þ 1442:8 " H–O=8 Þ " 0:001

[43] where C, H, S, O, and N are the weight percent of carbon, hydrogen, sulphur, oxygen, and nitrogen, and A is weight percent of ash. The lower heating value (LHV) can be calculated by the proposed equation: LHV ðkJ=kgÞ ¼ HHV ' 218:3 " H%ðwt%Þ [44].

Apart from the high oxygen content, the other undesirable characteristics of the pyrolysis bio-oils are low pH value of 2–3.7 due to the presence of the carboxylic acids. Hence bio-oils are potentially corrosive to common structures, high instability during storage due to the ongoing chemical reactions to form larger molecules (mainly polymerisation, etherification and esterification) [45], and solids retention with a proportion of 0.01–1 wt% of the bio-oils [46]. The detailed comparisons between bio-oil and heavy fuel oil, including moisture content (15–30 wt% vs. 0.1 wt%), specific gravity (1.2 vs. 0.94), elemental composition, and some other fuel indexes, such as pour point ( ' 33 °C vs. ' 18 °C), were presented by Mohan et al. [7] Bio-oils were extensively tested as candidate combustion fuels for electricity and heat production in boilers, furnaces, and combustors [47,48], diesel engines [49,50], and gas turbines [51,52]. Bio-oils were successfully fired in a diesel test engine with limited operation time, whereas long-term operation is not possible due to the poor quality of the bio-oils, such as poor volatility, high viscosity, high corrosiveness and coking [35]. It is generally accepted that further upgrading of the bio-oils is needed prior to their practical application in engines [46,53,54]. Production of transportation liquid fuels from bio-oils has been demonstrated after upgrading through catalytic cracking technologies [55–59] and high-pressure hydroprocessing [60–66]. Combustible syngas and hydrogen can be produced through steam reforming and gasification [67–80]. Butler et al. [81] and Xiu et al. [82] provided comprehensive reviews of bio-oil upgrading through steam reforming and gasification. In addition, bio-oils can be used as a feedstock for production of chemicals, such as phenols for resin production, additives in fertilising and pharmaceutical industries, flavouring agents (such as glycolaldehyde) in food industries and other special chemicals [53,83]. 3.2. Biochar Biochar (also called charcoal) is the major solid product, which contains unconverted organic solids and carbonaceous residues produced from the partial or complete decomposition of biomass components, as well as a mineral fraction. The physical, chemical, and mechanical properties of chars depend on the feedstock type and pyrolysis operating conditions. Slow pyrolysis (typical product yields: bio-oil 30 wt%, biochar 35 wt%, and gas 35 wt%) at pyrolysis temperatures ranging from 300 to 800 °C favours the production of biochar by reducing the yields of bio-oil. Demirbas [84,85] summarised the elemental composition (carbon content ranging from 53% to 96%), HHVs (20–36 MJ/kg), and yields (30–90 wt%) of biochars from pyrolysis of several biomass feedstock, and pyrolysis at different heating rates and temperatures. The high HHV makes chars attractive in some fuel applications as substitutes for coal. The microscopic surface structure of biochars formed during pyrolysis endows their potential for filtration and adsorption of organic and inorganic pollutants [85,86], especially after the chars are physically or chemically activated. The optimal biochar properties for filtration purposes (surface area of 1400 m2/g and micropore volume of 0.7 cm3/g) was obtained from coconut shells with a particle diameter of 1.55 mm, reaction temperature of 850 °C and a retention time of 1.5 h under steam using a fluidised bed reactor [87]. A biochar with similar properties (surface area of 1690 m2/g and micropore volume of 0.7 cm3/g) was achieved by pyrolysing olive seed waste in a fixed bed heated under N2 at 800 °C for 1 h and activating the produced biochar with KOH [88]. More details about the activation conditions (reaction atmosphere, temperature and retention time) and characteristics (BET surface area, micropore volume and ratio of micropore volume to total pore volume) of different activated biochars have been given by Manyà [89].

Table 1 Nutrient concentration of biochars produced from different lignocellulosic biomass. Feedstock

Reaction conditions

Biochar yield (wt%)

Total concentration of nutrient elements (mg/kg)

Ref.

Macronutrients

Pine wood

Wheat straw

Rice straw

Miscanthus (Miscanthus x giganteus) straw Willow wood (Salix sp.) Switchgrass b Corn stover b Hardwood b Corn stover Birch (Betula pendula) wood Oil palm empty fruit bunch Rice husk

Heating rate (°C/min) or residence time (s)

525

Residence time of 90 s

/

/

450 600 800 450 600 800 300 400 500 300 400 500 700


/

31.3 16.9 11.4 26.6 15.2 9.5 48a 38a 33a 45 a 36 a 32 a /

450 500 500 500 500 450 700 300-350 500

/ Fast pyrolysis in a fluidised bed Fast pyrolysis in a fluidised bed 15 °C/min to 500 °C Fast pyrolysis in a fluidised bed For 4 h For 4 h / /

/ / / / 17.0 28.7 26.7 / /


7–10 °C/min heating rate and 20 min residence time at peak temperature

N

P

K

Ca

815

3622

12800 11300 8600 2300 2300 2600 13800 9400 8500 11500 9800 8500 3400

1963 2400 4046 162 281 439 2600 3000 3400 1100 1300 1400 820

9100 / / / 14700 / / 16300 2300

1560 9202 2582 121 12940 225 230 2100 3600

Mg

S

Na

Mn

Cu

Fe

Se

Mo Zn

4055

2504

498

498

/

/

/

/

/

/

[92]

11309 16452 16734 1684 2889 4237 30000 32000 36000 36000 41000 48000 18330

10439 14611 17513 2318 4158 4915 6300 8300 8700 9100 9800 13300 /

9637 11802 15022 730 1281 1952 4500 5600 6900 8100 9600 11300 1520

1257 1338 1917 153 214 270 / / / / / /

248 208 220 56 104 129 / / / / / / 340

246 341 374 195 324 488 106 117 163 396 554 649 /

14 46 61 ND 6 18 / / / / / / /

457 797 633 202 167 164 158 259 422 195 341 521 /

4.4 5.6 5.1 3.8 7.2 5.7 / / / / / / /

2.3 5.7 6.0 0.5 1.9 4.6 / / / / / / /

102 115 155 49 71 445 47 59 70 67 89 98 /

[93]

5180 27184 16620 1557 23460 750 785 53000 7200

/ 14235 13490 22210 20130 / / 1100 200

1830 6367 7553 502 14240 300 315 1300 800

160 284 737 62 1070 100 75 / /

/ 495 390 697 650 5.5 6.5 / /

/ / / / / 1.7 1.9 / /

/ 4469 10009 3533 15950 8.7 10 / /

/ / / / / / / / /

/ / / / / / / / /

/ / / / / 28 25 / /

2100 600 500 19 19 / /

[94]

[95]

[96]

[97] [98] [99]


Switchgrass (Panicum virgatum L.) Switchgrass

Pyrolysis T (°C)

Micronutrients

ND: not detected. a b

On basis of as received. Concentration values of nutrient elements are calculated from the data in the corresponding references.

1129

1130


Biochars contain a range of plant nutrients, making them valuable as soil amendments [90] and they can also contribute to carbon sequestration to mitigate atmospheric carbon [91]. Table 1 lists the range of nutrient contents (macro- and micronutrients) of biochars produced from different feedstock. The concentration of respective nutrient elements varies in a wide range, depending primarily on the biomass type and pyrolysis conditions. The increase in pyrolysis temperature results in the increased concentrations of nutrient elements in the biochars due to loss of mass of the biomass at higher temperatures. Nitrogen is more

volatile than the other nutrients and the concentrations may change differently, depending on the biomass type and the chemistry of the nitrogen in the feedstock. 3.3. Pyrolytic gas Gases released from biomass pyrolysis may consist of carbon dioxide (CO2), carbon monoxide (CO), hydrogen (H2), low carbon number hydrocarbons such as methane (CH4), ethane (C2H6), and ethylene (C2H4), and small amounts of other gases, such as

Table 2 Analysis methods employed on the biomass and pyrolysis products. Pyrolysis products

Properties

Analysis methods

Ref.

Bio-oil

Water content Carbon residue Qualitative and quantitative identification of bio-oil compounds Molecular weight distributions Gross calorific value (GCV)/ Higher heating value (HHV) Elemental composition (CHN and O by difference) Sulphur content

ASTM E203 or ASTM D-1744 by Karl-Fischer titration ASTM E203 by Destructive distillation method Gas chromatography–mass spectrometry (GC–MS), high performance liquid chromatography (HPLC)

[111,112] [111] [111]

Gel permeation spectroscopy (GPC), Matrix-assisted laser desorption/ionisation (MALDI) mass spectroscopy ASTM D4809 Theoretical calculation ASTM D5373

[113,114] [22,23] [111,115] [116] [111,115]

Biomass/ Char

Gas

Functional groups Types of hydrogens or carbons in specific structures Acid number Density at 15 °C (kg/m3) Kinematic viscosity (mm2/s) Flash point, pour point, and boiling range (°C) Water insolubles Proximate analysis (moisture, ash, volatile matter, and fixed carbon contents) Ultimate analysis (elemental analysis of CHN, and O by difference) Metal content Elemental surface distribution Functional groups Aromaticity Mass/heat change during heating in different atmospheres (N2, O2, air, etc.) Crystalline phases, and their qualitative and semi-quantitative data Brunauer–Emmet–Teller (BET) surface area, porous structure Surface morphology Particle size distribution Electrical conductivity (EC) Particle size distribution Surface acidity and alkalinity Organic carbon Cation exchange capacity [136] Gases species and concentrations Functional groups Lower heating value (LHV)

H2S content Tar content and composition Size distribution of particles

ASTM D4294 by Energy dispersive X-ray fluorescence (EDXRF) spectrometry, or ASTM D4239 by Infrared [111] measurement of SOx after combustion of bio-oils Infrared techniques including near-IR (NIR) and Fourier transform infrared (FT-IR) spectroscopy [117] Nuclear magnetic resonance (NMR) spectroscopy [118,119] ASTM ASTM ASTM ASTM

D664 by potentiometric titration method, or ASTM D974 by colour-indicator titration D4052 by digital density metre D445–03 D93, D97, and D2887, respectively

[111] [111] [111] [111]

FT-IR, 13C NMR, and column chromatography ASTM D1762-84, 2007

[117] [92,96,120]

ASTM standard D3176-89, 2002

[120,121]

X-ray fluorescence (XRF), inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma atomic emission spectroscopy (ICP-AES) Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX)

[96,122,123]

FT-IR 13 C NMR Thermogravimetric analysis with differential scanning calorimetry (TGA/DSC)

[89,96] [96,125,126] [121,127–129]

X-ray diffraction (XRD)

[128,130]

Nitrogen gas sorption

[96,121]

SEM Laser sizing equipment Conductivity metre Microscopy, particle counter laser sizing equipment Boehm titration DPI in-house method 236 Ion chromatography (IC)

[99,128,131] [132] [128,133] [132] [134] [134,135] [137]

Gas chromatography (GC); mass spectrometry (MS); non-dispersive infrared (NDIR) analysis

[23,73,137,138]

FT-IR Calculation from gas composition: LHV(MJ/m3) ¼ (107.98 " H2 þ126.36 " COþ358.18 " CH4 þ59.036 " C2H4 þ 63.772 " C2H6)/1000, where gas species represent the respective volumetric fractions Lead sulphur precipitate through reaction between H2S and lead nitrate Cold trapping; solid phase adsorption (SPA) Scanning mobility particle sizer (SMPS)

[139,140] [141]

[121,124]

[142] [143,144] [145]

Table 3 Cellulose, hemicelluose, lignin, and extractives in selected groups of biomass, and pyrolysis behaviour of biomass [146–149]. Biomass groups

Cellulose (%) Hemicellulose (%) Lignin (%) Extractives (%) Pyrolysis behaviour Species

Hardwood

Lignin containing crop residues (corn stover, wheat straw)

Lignin free crop residues (soybean, rye straw)

Lignin containing warm season grasses (switchgrass, mischantus, big bluestern) Lignin free warm season grasses Cool season grasses

35–40

30–33

–

0–3

Two-step vacuum pyrolysis (200 –275 °C for 30–45 min, and then 450 °C for 1 h) Hardwood shavings Fast pyrolysis with τ o 5 s, T ¼ 500 °C Aspen poplar þwhite birch Vacuum pyrolysis Sweet Gum Heating rate at 1000 °C/s, # 620 °C Two-step vacuum pyrDe-barked Lodgepole pine (Pinus contorta) and Douglas Fir olysis (200–275 °C for 30–45 min, and then (Pseudotsuga menziesii)wood 450 °C for 1 h) White spruceþ balsam Vacuum pyrolysis fir þ larch Softwood Flash pyrolysis Spruce wood (Picea orientalis) Final T of 750 °C in a horizontal cylindrical reactor Single-pass corn stover τ ¼ 1.5 s, 500 °C in a free(ensiled) fall pyrolyzer Corn stover 400 °C for 20 min in in a batch pressure reactor Corn stover Fast pyrolysis at 500 °C in a fluidized bed Wheat straw 400 °C and τ ¼ 1 s in a CFB reactor Acid-treated wheat straw 400 °C and τ ¼ 1 s in a CFB reactor Soybean cake 5 °C/min to 550 °C in sweeping N2 Soybean (Glycine max L.) 400 °C with a heating rate of 50 °C/min Soybean cake 300 °C/min to 550 °C in sweeping N2 Switchgrass (Panicum virgatum) 6 °C/min to 600 °C and then 20 min at 600 °C Switchgrass (Cave-in-Rock 480 °C in fluidized bed variety) / /

25–35

30–35

–

3–15

/

35–40

38

31–42

37–43

20–25

20–25

26–29

15–25

24–29

20–25

27–30

15–19

–

18–19

2–4

1–10

5

3–7

3–6

Mixture of various Eastern tree species

/

Product yields (wt%)

Oil composition

Ref.

Liquid

Char

Gas

50–55

25–27 Others

C 36–38%, H 8%, O [160] 53–55%

63.3

12.7

24.0

53.9 52.5

26.2 /

19.9 /

50–55

25–27 Others

C 55.2%, H 6.5%, N o 0.5%, O 37.6% / C 53.9%, H 5.9%, O 37.1% C 33–36%, H 8%, O 55–60%

45

27.6

27.4

75 39.7

/ 32.4

/ 28.9

55

25.5

16.2

31

37

15

61.6

17

21.9

46

47

7

57

38

5

30 [168] þ 20 (water) 25.81

25

25

23.56

Others

39 [168] þ 22 (water) 37

21

18

25

26 (syn-gas)

60.7

12.9

/

/

11.3 (noncondensable) /

/

/

/

[161] [112] [162] [160]

C 62.6%, H 7.0%, N [112] 1.05%, O 29.0% C 55%, H 7% [163] C 69.3%, H 8.6%, N [164] 0.7%, O 21.4% C 46.7%, H 14.7%, N 0.5%, O 38.1% C 78%, H 9%, N 1.9%, O 10.6% C 54%, H 6.9%, N 1.2%, O 37.9% C 34.5%, H 8.2%, N 0.8%, O 56.5% C 41.3%, H 7.5%, N 0.9%, O 50.3% C 62.2%, H 8.3%, N 7.5%, O 22.0% C 67.9%, H 7.8%, N 10.8%, O 13.5% C 67.2%, H 9.0%, N 10.8%, O 13.0% C 50%, H 9.3%, N 1.5%, S 0.6%, O 37% /

[165] [166] [97] [167] [167] [169]


Softwood

45–50

Pyrolysis conditions

[170] [171] [172] [173]

/

/

/

/

1131

1132


propane (C3H8), ammonia (NH3), nitrogen oxides (NOx), sulphur oxides (SOx) and alcohols of low carbon numbers. The typical LHVs of the pyrolytic gases range between 10 and 20 MJ/N m3, depending on their practical composition. As the primary products of biomass pyrolysis, CO2 and CO mainly originate from the decomposition and reforming of carbonyl (C ¼ O) and carboxyl (COO) groups [23,100]. Light hydrocarbons (primarily CH4) are primarily attributed to the decomposition of weakly bonded methoxyl ( 'O 'CH3) and methylene ( ' CH2 ') groups as well as the secondary decomposition of the oxygenated compounds, while H2 results from secondary decomposition and reforming of the aromatic C ¼C and C–H groups at high temperatures [101,102]. Synthesis gas or hydrogen-rich gas, can be produced from biomass pyrolysis. Wet biomass could give up to 40% higher H2 yield and content in the gas, compared to the dried biomass [103]. Temperature and catalysts can further enhance the hydrogen production from biomass [104]. Catalysts which can promote H2 production and adjust the gas composition for downstream applications (e.g., Fischer–Tropsch synthesis) include ZnCl2, dolomite, K2CO3, Na2CO3, Ni/Al, Ni/Fe, CaO, Fe2O3, Cr2O3 and Rh/CeO2 [105–109]. Prior to practical use of the pyrolytic gas, some treatments are required to reduce or eliminate the undesired constituents which may include tars, dust/aerosols, evaporated heavy metals, steam, HCN, NH3, and H2S. The pyrolysis gas has multiple potential applications, such as direct use for production of heat or electricity (e.g., gas combustion in spark ignition and compression ignition engines [110]), either directly or co-fired with coal, production of individual gas components, including CH4, H2 or other volatiles, or in production of liquid bio-fuels through synthesis. In some applications, the hot pyrolytic gas can be used to preheat the inert sweeping gas or can be returned to the pyrolysis reactor as a carrier gas. 3.4. Analysis of pyrolysis products A number of analysis methods have been employed to chemically and physically characterise the bio-oil, char, and bio-gas products of pyrolysis. Table 2 summarises the properties of these products as well as the corresponding analysis methods.

growth environment and harvesting time. Pyrolysis of each constituent features unique reaction pathways and thermochemical characteristics, and produces different products [29,140,150–152]. Cellulose and hemicelluloses contribute to the bio-oil production yield, while lignin yields larger proportion of solid char [153]. Higher lignin content may increase the average molecular weight and viscosity but decrease the water concentration of the bio-oils [154]. Extractives in lignocellulosic biomass refer to the nonstructural materials that can be extracted by solvents (e.g., water, ethanol, acetone, benzene and toluene), such as fatty acids, simple sugars, waxes and sterols [155,156]. Wang et al. [157] found that the extractives could benefit the bio-oil yield and suppress the char and gas production when using corn stalk and wheat straw as the feedstocks for pyrolysis. The bio-oils from the extracted samples with reduced extractives also exhibited higher oxygen and lower alkane contents than those from the original samples. In another study, the extractives were confirmed to lower the activation energy and the yields of CO2, CO and aldehydes while enhance and acid generation during the pyrolysis of Mongolian pine and manchurian ash [158]. The structural combination of the components generally differ from biomass to biomass (Table 3), which makes the interactions among components change with biomass types, and subsequently affects the pyrolysis performance. Additionally, the mineral matter composition and content in the biomass types can also be factors that influence distribution and properties of products due to its catalytic effect during biomass pyrolysis [154,159]. 4.2. Biomass pretreatment The biomass feedstock usually requires some form of pretreatment before its pyrolysis. The aim of the pretreatment is to change or even destruct the lignocellulosic structure so that the pyrolysis efficiency can be enhanced. The technologies for biomass pretreatment can be divided into five main categories, including 1) physical (e.g., milling/grinding and extrusion); 2) thermal (e.g., torrefaction, steam explosion/liquid hot water pretreatment and ultrasound/microwave irradiation); 3) chemical (e.g., treatment with acids, bases and ionic liquids); 4) biological (e.g., fungal, microbial consortium and enzymatic); and 5) above combined pretreatments [174].

4. Parameters influencing biomass pyrolysis 4.1. Biomass type Lignocellulosic biomass is composed of cellulose (25–50 wt%), hemicellulose (15–40 wt%), lignin (10–40 wt%), extractives (0– 15 wt%), and generally a small fraction of inorganic mineral matter [146–149]. The biomass type influences the pyrolysis process and products in several ways. Firstly, the relative mass ratios of the organic and inorganic components vary with biomass types,

4.2.1. Physical pretreatment Milling or grinding of biomass to smaller particles is a conventional treatment to facilitate the biomass feeding into reactors and improve the pyrolysis performance. As biomass is generally a poor conductor of heat, the temperature gradient across the particle will influence the biomass pyrolysis mechanism [175,176]. Generally, smaller particles promote the heat and mass transfer to form uniform temperature within particles during pyrolysis, thereby enhancing the bio-oil production by restraining the char

Table 4 Effect of biomass particle size on pyrolysis products distribution. Feedstock Cotton stalk Hazelnut bagasse Rapeseed Oil mallee Cylindrical wood particles

Pyrolysis conditions

Particle size range, in mm (liquid, biochar and gas yields, wt%) '1

550 °C, heating rate: 7 °C min in 0.225–0.425 (21, a fixed bed 27.5, 31.1) '1 10 °C min to 500 °C in a fixed bed 0.224–0.425 (30.7, 27.8, 28.5) 30 °C min ' 1 to 550 °C in a fixed bed o 0.425 (42.7, 23, without sweeping gas 26) 500 °C in a fluidised bed 1–2 (52.0, 18.8, 19.4) 500 °C in a fluidised bed

4 (64.2, /, /)

0.425–0.850 (22, 27.8, 30.5) 0.425–0.6 (33.2, 27.9, 29) 0.6–0.85 (48, 17.5, 27.5) 2.0–3.35 (47.5, 18.8, 28.4) 6 (64.6, /, /)

0.850–1.80 (23.8, 27.1, 28.6) 0.6–0.85 (32.5, 27.5, 29.7) 0.85–1.25 (49, 16, 29) 4.75–5.6 (51.0, 20.2, 24.0) 9 (64.0, /, /)

Ref. 41.80 (22.5, 26.6, 28.5) 0.85–1.8 (31.9, 27.4, 31) 1.25–1.8 (48, 17.5, 27.5) /

/

[177]

/

[178]

41.8 (46, 21, 26.5) /

[179] [180]

12 (63.0, /, /)

15 (61.2, /, /)

[181]


formation and secondary cracking of vapours. However, particle size reduction can be costly and significantly increase the overall cost of the biomass pyrolysis operation. Table 4 summarises the effect of biomass particle size on production of liquid, biochar and gas yields in fixed and fluidised bed reactors. Extrusion of biomass under higher pressure produces biomass pellets which generally take the shape of small cylinders, increasing the volumetric energy density of biomass, while decreasing the moisture content [182]. Xue et al. [183] found that larger pellet diameters increased the char and gas yields as well as the char density, but lowered the tar yield. Similar phenomena were also observed by Erlich et al. [182]. Pellets of mixed biomass materials may also be used in the pyrolysis practices. Zabaniotou et al. [184] mixed pine (25%), fir (25%) wood and cotton (50%) or corn (50%) to form pellets, and the results of pyrolysis at 400– 750 °C showed that gas products with high CO and H2 and low CO2 contents were generated and the gas heating value was around 14–15 MJ/m3. 4.2.2. Thermal pretreatment Biomass drying prior to pyrolysis increases the energy efficiency of the pyrolysis process and improves the quality of the biooil products. There are various different industrial dryers applicable for biomass drying [185] which are designed to reuse the fugitive heat released during the heating pyrolysis process and remove the moisture from the biomass. When the thermal pretreatment is conducted at temperatures between 200 and 300 °C, process termed as torrefaction, the water content is fully removed and the oxygen content is partially reduced from the biomass [186]. Compared to untreated biomass, torrefied biomass possesses several advantages. It is higher in energy density, it has improved grindability, lower hygroscopicity when stored in open air, lowers the risk of biological degradation and self-ignition, and improves feeding in the reactors [187,188]. During torrefaction, some decomposition reactions begin to take place, forming CO2, CO, acetic acid and levoglucosan [189,190]. Boateng et al. [190] found that the fast pyrolysis of torrefied hardwood and switchgrass pellets in a fluidized bed produced bio-oils with lower acidity and higher energy density but lower liquid yield and carbon conversion than the oils from untreated biomass. Higher quality and lower yield of bio-oils from pyrolysis of torrefied biomass were also confirmed by others [191,192]. Torrefaction was found to improve the quality of produced syngas via reduction of its CO2 content and increase in H2 and CH4 contents [193]. Steam explosion (SE) is a process consisting of exposure of biomass to saturated steam at generally 1.5–5 MPa and 150–260 °C for seconds to minutes in a sealed vessel followed by a sudden depressurisation to ‘explode’ the biomass structure [194–197]. SE causes the breakage of carbohydrate linkages and also alters the physical properties of lignocellulose [198], thus changing the behaviour of biomass pyrolysis and product properties. Biswas et al [194] investigated the pyrolysis of willow chips after SE pretreatment at 205 °C through thermogravimetric analysis at 10 °C/ min, and observed increased cellulose crystallinity compared to the untreated material. During this process, the degradation of hemicellulose shifted to lower temperature region and became more active but the thermal stability of cellulose and lignin increased. Wang et al [199] pretreated loblolly pine chips by SE (1.3 MPa and 173–193 °C) and the untreated and pretreated materials were then separately pyrolysed in a proprietary auger reactor. Results showed that, in comparison to the untreated feedstock, the chips after SE pretreatment had increased cellulose and lignin contents while reduced hemicellulose content. The SE pretreatment also resulted in a bio-oil product with different acid value from 90.1 to 64.2, viscosity (cSt at 40 °C) from 6.5 to 3.9, and water content (%) from 20.8 to 29.3. Similarly, hot liquid water is

1133

sometimes ultilised to partially remove the hemicellulose in the biomass feedstock, which is beneficial to the decrease in acetic acid content and the stabilisation of bio-oils [188]. Ultrasound and microwave irradiation are unconventional thermal methods for biomass pretreatment. The ultrasound is mostly used to enhance the anaerobic digestion and biogas (mainly methane) production from sludge [200]. Several studies have also been performed on lignocellulosic biomass. Yachmenev et al. [201] applied ultrasound to efficiently accelerate the enzymatic hydrolysis of cellulose in corn stover and sugar cane bagasse to generate sugars due to the cavitation effects which could enhance the movement of enzyme molecules and the opening up of the substrates surface [201]. Sun et al. [202] used KOH solution and ultrasonic irradiation to extract hemicellulose from the wheat straw and found that the ultrasound assistance produced hemicellulose with larger linearity and less acidity than those obtained from conventional KOH extraction. The effect of ultrasound pretreatment on the biomass pyrolysis needs further investigation. Microwave irradiation is nowadays a common alternative to traditional heating of lignocellulosic biomass, and it is able to generate ‘hot spots’ in the biomass [200,203]. Although microwave-assisted pyrolysis of biomass has been widely studied [204], biomass pretreatment by microwave irradiation prior to the pyrolysis is not well investigated [205]. Microwave drying at 600 W and 6 min was proposed to improve the bio-oil and char yields with performance showing better yields than conventional electrical oven drying due to the suppression of secondary reactions during pyrolysis after biomass drying in a microwave oven [205]. 4.2.3. Chemical pretreatment The presence of inorganic minerals, especially the alkali (K, Na, etc.) and alkaline-earth (Mg, Ca, etc.) metal salts, is believed to affect the mechanism of biomass pyrolysis [206,207]. For example, K in the biomass mineral matter catalytically promotes the formation of lower molecular weight compounds and suppresses the formation of levoglucosan during the primary pyrolysis of cellulose [208]. The cations, acting as catalysts, induce the fragmentation of the biomass monomers rather than the depolymerisation, which favours char formation and lowers bio-oil yields [208–211]. The deposit of salts onto the reactor and pipeline inner walls also causes corrosion and engineering difficulties [212]. Besides, the presence of ash in bio-oils affects the subsequent applications of bio-oils and accelerates their aging. The above shortcomings can be improved by reducing the ash content through water or acid washing. Water washing is used to remove the dirt and minerals on the surface of biomass particles during biomass harvesting, transport and storage. However, the structural minerals will still remain within the biomass matrix. Washing by acids, such as HNO3 and HF, can further reduce the ash content [213]. Blasi et al. [214] examined the effect of water washing on pyrolysis characteristics of straw, and found that water washing increases the bio-oil yields while char formation is decreased. Phosphoric acid was applied to pretreat cellulose feedstock to obtain higher production of levoglucosan and levoglucosenone in the bio-oils [215]. In some cases, concentrated acids (e.g., H2SO4) were applied to hydrolyse and solubilise carbohydrates in biomass to extract lignin [216], and alkaline solutions (e.g., NaOH) were used to remove lignin, hemicellulose, and/ or cellulose [174]. Ionic liquids are a series of recently emerging compounds mainly consisting of organic cations and inorganic/organic anions and can take the form of/turn into liquids at temperatures below 100 °C (especially room temperature) [217]. They are regarded as green solvents with unique physical and chemical characteristics, such as low vapour pressure, strong chemical stability, and nonflammability [218–220]. Ionic liquids have found applications in

1134


not only various industrial areas applications, such as catalysis, chemical synthesis and engineering fluids, but also the deconstruction and dissolution of cellulose, hemicellulose and lignin [221]. Ionic liquids have been used to pretreat lignocellulosic biomass for the production of sugars from enhanced enzymatic hydrolysis of oil palm fronds [222], renewable chemicals of vanillin, syringyl and allyl guaiacol from eucalyptus, switchgrass and pine respectively [223], levulinic acid from cellulose [224], and biogas from improved anaerobic digestion of water hyacinth, rice straw, mango leaves and spruce[225]. The thermal behaviour of biomass materials can also be changed after their pretreatment by ionic liquids. Zhang et al. [226] found that after the pretreatment by 1-butyl-3-methylimidazolium acetate ([C4mim][OAc]), the Avicel and switchgrass samples showed higher thermal resistance due to the crystal change of cellulose and the removal of minerals, respectively [226]. 4.2.4. Biological pretreatment Comparing physical and chemical pretreatments, biological methods are slower but less energy-consuming and with better environmental footprint [227]. Fungal pretreatment of lignocellulose prior to pyrolysis has been proved to improve the efficiency of the pyrolysis performance. White-rot fungus has been selected to pretreat the natural lignocellulose as it could selectively decompose the refractory lignin component during pyrolysis [228,229]. Yang et al. [230] employed three different species of white-rot fungus (Pleurotus ostreatus BP2, Echinodontium taxodii 2538, and Irpex lacteus CD2) to biopretreat corn stover and then studied its thermal characteristics during pyrolysis in a TGA instrument. Results showed that this biopretreatment could effectively lower the pyrolysis temperature by 1–35 °C and decrease the emission of toxic SOx through reduction of the sulphur content in the feedstock by 30–45%. Yu et al. [227] investigated fast pyrolysis of corn stover pretreated by white-rot fungus I. lacteus CD2 in the presence of ZSM-5 zeolite, and observed 10% improvement in valuable aromatics product yields and 20% reduction in deposition of undesired coke on catalysts. Microbial consortium has been generally used to pretreat lignocellulosic biomass to enhance the biogas production. It employs certain microbes selected from the natural environment which mainly degrade the cellulose and hemicellulose components [174]. The process lasts for several hours to several days with the ability of increasing the methane yield by 25% to nearly 100% [174]. Enzymes have been proposed as a pre-treatment for hydrolysis of lignin prior to its pyrolysis in order to enhance production of aromatic phenols and hydrocarbons [231]. The produced chars also appeared to be highly porous with vesicles. 4.3. Effects of reaction conditions 4.3.1. Reaction atmosphere Biomass pyrolysis is typically carried out under inert atmosphere. Other gases can be also introduced to modify the pyrolysis process. For instance, steam can weakly oxidise the biomass and provide partial gasification. In a recently developed pyrolysis process, referred to as steam pyrolysis, the steam is used as the carrier gas, which may also take part in the reactions. The steam for biomass pyrolysis has several advantages. Primarily, it can upgrade the yield of organic oxygenated products by preventing to some extent the secondary cracking reactions in the gas phase [232]. Zhang et al. [233] studied the effects of N2, CO2, CO, CH4 and H2 atmospheres on biomass pyrolysis in a fluidised bed reactor. It was found that CH4 atmosphere gave the highest bio-oil yields (58.7%) with the lowest yield in CO (49.6%). Lower methoxycontaining compounds and larger monofunctional phenols in the bio-oils were detected when applying CO and CO2 as the sweeping

gas. Under H2 atmosphere, the HHV of the bio-oils could reach the highest value of 24.4 MJ/kg and more oxygen in biomass was converted into H2O comparing to other atmospheres. The char obtained in the CO2-containing atmosphere had increased surface area and different chemical composition [234] compared to the chars produced under inert atmosphere. 4.3.2. Temperature Pyrolysis temperature significantly influences the distribution and properties of products [235–237]. Generally, the bio-oil yields reach their peak concentrations at temperatures between 400 and 550 °C, and then decline after proceeding with heating. At temperatures higher than 600 °C, the bio-oils and char products are converted into gas due to the dominant secondary cracking reactions [238]. The polar, aliphatic and aromatic fractions in the biooils enhance with increased temperatures from 300–500 °C to 600–800 °C [239]. Generally, temperatures exceeding 700 °C increase the carbon content of the bio-oils in the form of polycyclic aromatic hydrocarbons (PAHs), such as pyrene and phenanthrene, due to the decarboxylation and dehydration reactions [153]. The variation of gas yields and composition (CO, CO2, CH4, H2, etc.) with pyrolysis temperature has been reviewed by Uddin et al. [101]. The physicochemical characteristics (e.g., surface area, electrical conductivity, concentration of inorganic elements, carbon content, aromatic structure, and HHV) of biochars from biomass fast pyrolysis at different temperatures were also investigated in the past [128,240–242]. 4.3.3. Heating rate Heating rate is a fundamental parameter that defines the type of biomass pyrolysis, i.e., flash, fast, and slow pyrolysis. Fast heating rates favour quick fragmentation of the biomass and yield more gases and produce less char. Bio-oil production is also enhanced at fast heating rates due to the reduction in mass and heat transfer limitations, and short time available for secondary reactions [153]. The effect of heating rate on yields and properties of products has been determined previously [176,243,244]. Salehi et al. [245] observed that an increase in heating rate from 500 °C/ min to 700 °C/min increased the bio-oil yields from sawdust by 8%, however, no obvious change in the bio-oil yields was detected when further increasing the heating from 700 to 1000 °C/min due to the overcoming of mass and heat transfer limitations. Similarly, rapid increase in liquid yield for pyrolysis of cottonseed cake was observed when elevating the heating rate from 5 °C/min (26 wt%) to 300 °C/min (35 wt%), where as no obvious change of liquid yield was found when further increasing the heating rate from 300 °C/ min to 700 °C/min [246]. 4.3.4. Vapour residence time Shorter residence time favours bio-oil production due to the quick removal of organic vapours from reactors which minimises the secondary reactions [247]. For the pyrolysis of raw sorghum bagasse at 525 °C Scott et al. [248] observed that an increase in vapour residence time from 0.2 to 0.9 s resulted in a decrease in the bio-oil yields from 75% to 57%, while the char and gas yields increased. Similarly, during the pyrolysis of sweet gum hardwood at 700 °C, the oil yield dropped from 22 wt% to 15 wt% by increasing the vapour residence time from 0.7 s to 1.7 s [247]. Although the effect of vapour residence time on product distribution is well studied, the interaction between vapour residence time and pyrolysis temperature on not only product yields but also product quality needs further clarification [101].

Quick char removal is essential; special designs are required to minimise temperature and concentration gradients Commercial 2–20 t/h Water and high level of ash and charcoal Ash formation in circulating solids leads to particles in oils. loss of bio-oil yield; acceptance of very large throughputs Commercial o 4000 kg/h / Simple reactor design; clogging and fouling of gas handling system due to tars Commercial 200–2000 kg/h Water, ash and charcoal particles in oils. Heat transfer to bed has yet to be proven at large scales Laboratory 1–20 kg/h Water and some ash and charcoal particles High heat transfer and high pressure of in oils. biomass particles on the hot reactor wall; rapid removal of volatiles Pilot 20–200 kg/h Gas can contain particulates and tar, may be Lower process temperatures (400 °C); heat transfer at large scales may be a problem very acidic. High moisture content in gas and/or oil if feed is not dried before pyrolysis 200–2000 kg/h Hard Demonstration /

Medium

Hard

Medium

Low

High

High

Medium

High

Easy High

Water and high level of ash and charcoal particles in oils. Commercial 2–20 t/h Easy Medium

60% Low o 10% 5–50 mm Vacuum

60% Low o 10% Auger/screw feed 5–50 mm

75% Low o 10% o 20 mm Ablative

70% Low o 10% o 0.2–6 mm Rotating cone

/ / o 10% 5–50 mm Heated kiln

75% High o 10% o 6 mm Circulating fluidised bed

75% o 10% o 2 mm Bubbling fluidised bed

High

Feed moisture Feed size requirement

Carrier gas need

1135

5. Biomass pyrolysis reactors and state-of-arts Modern reactor configurations for biomass pyrolysis mainly comprise of fixed bed, fluidised bed, heated kiln, rotating cone, ablative, screw feeder/auger and vacuum pyrolysers [33,249]. Typical industrial pyrolysis processes for various reactor configurations have been well reviewed in the past [33,250]. Mohan et al. [7] also reviewed the properties of bio-oils produced from pyrolysis of various biomass feedstock in different reactor designs. Table 5 summarises the characteristics and current status of common pyrolysis reactors. Three reactor types, including bubbling and circulating fluidised beds and rotating cone, have been commercialised in the market while others still remain at the stage of demonstration, pilot or even laboratory research. Table 6 lists the typical examples of pyrolysis facilities at commercial scales. During the period of 1994–2002 the cost of production of bio-oil was estimated to range between US$0.62 and US$1.40 per gallon. [251]. Beside the above-mentioned pyrolysis processes, there is also a range of other technologies at research and development stage which are being considered for industrial biomass pyrolysis operations, including microwave-assisted pyrolysis [204,253–256], hydrothermal pyrolysis [257–259], catalytic pyrolysis [260–263], and integration of biomass pyrolysis with other processes, such as iron ore reduction [264] and NOx reduction systems [265]. Biomass pyrolysis in now a promising technology for generating energy, fuels and chemicals. Although rapid progress in the biomass pyrolysis field has been achieved, a number of barriers and challenges are still to be overcome to unleash the full potential of various pyrolysis processes, including limitations in unsatisfying overall energy efficiency relating to feedstock pretreatment, reliability of reactors and processes, poor product quality, complete product standards for producers and customers, and reactor scalability [10,266]. Additional technology development is required on the integration of biomass pyrolysis process and downstream upgrading of pyrolytic products aiming at final products with high added value. The science and engineering development of biomass pyrolysis is expected to address the production of designed fuels or chemicals from specific biomass materials with finer control of the pyrolysis process (e.g., pyrolysis parameters and addition of catalysts), simpler pyrolysis procedures and reduction of the production cost.

6. Conclusions

Reactor type

Table 5 Characteristics and current status of pyrolysis reactors [7,250,252].

Bio-oil yield

Scale Status Complexity Scale-up difficulty

Product quality

Advantages and disadvantages


This paper reviews the parameters and pre-treatment processes that influence biomass pyrolysis. Heating rate and temperature are the two the most studied process conditions. Higher heating rates promote production of higher liquid yields, while lower heating rates are applied for enhanced biochar yields. The optimum temperature for maximised liquid and solid product yields is in the range of 400–550 °C. Bio-gas is now increasingly being considered as a product of choice from biomass pyrolysis, mainly for production of hydrogen rich synthetic gas. High temperature catalytic pyrolysis is proposed for achieving high yield and quality of bio-gas. Reaction atmosphere and residence time both significantly affect the products of biomass pyrolysis. Slightly oxidising atmospheres and increased residence times promote gasification and reduce the bio-oil yields. Significant improvements to the pyrolysis process can be achieved through pretreatment of the biomass prior to pyrolysis. Physical, thermal, chemical and biological pre-treatments have been developed to tailor for the desired properties of the pyrolysis products. Physical pre-treatment involves reduction of particle size to promote intraparticle heat and mass transfer and enhance bio-oil

1136


Table 6 Examples of pyrolysis facilities at commercial scales [250]. Company name

Location

Pyrolysis type

Reactor type

Capacity (t/ day)

Feed

Primary Product

Dynamotive

West Lorne, Ontario, Canada Guelph, Ontario, Canada Mie Prefecture, Japan

Flash pyrolysis

Bubbling fluidised bed

100–130 (dry)

Bio-oil

Flash pyrolysis

Bubbling fluidised bed

200 (dry)

Waste sawdust and woodchips Wood waste

Slow pyrolysis

Indirect heating rotary kiln 100

Woodchips

Gas

Fast pyrolysis

Circulating fluidised bed

40 (dry)

Hardwood wastes

Bio-oil

Fast Pyrolysis Fast pyrolysis Slow pyrolysis and gasification

Circulating fluidised bed Rotating cone Heated kiln pyrolysis followed by gasification

100 (dry) 50 (dry) 180 (dry)

Wood residues Palm oil waste Woody biomass and agricultural residue

Bio-oil Syngas

Mitsubishi Heavy Industries/ Mie Chuo Kaihatsu Ensyn

BTG Choren

Rhinelander, Wisconsin Renfrew, Canada Malaysia Germany

production yields. Thermal pre-treatment involves reduction in the intrinsic moisture and oxygen content in the biomass, generally utilising the waste heat from the pyrolysis process, in order to improve the energy efficiency and product quality. Chemical pre-treatment reduces the mineral matter content, while biological pre-treatment has been trialled to reduce the lignin content, generally the most thermally resistant compound in the lignocellulosic biomass, so that the pyrolysis bio-oil yields can be enhanced. Generally, biomass pyrolysis is currently at the mature stage of development with several technologies already achieving full commercialization stage. Their input into the global energy production is expected to achieve the full potential in the near future.

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