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Fundamentals, kinetics and endothermicity of the biomass pyrolysis reaction
Renewable Energy 35 (2010) 232–242
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Fundamentals, kinetics and endothermicity of the biomass pyrolysis reaction Manon Van de Velden !, Jan Baeyens a, b, *, Anke Brems c, Bart Janssens c, Raf Dewil c, d a
Department of Chemical Engineering, University College London, London, United Kingdom School of Engineering, University of Warwick, Coventry, United Kingdom c Department of Chemical Engineering, Katholieke Universiteit Leuven, Campus De Nayer, Sint-Katelijne-Waver, Belgium d Chemical and Biochemical Process Technology and Control Section, Department of Chemical Engineering, Katholieke Universiteit Leuven, Heverlee, Belgium b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 December 2008 Accepted 22 April 2009 Available online 4 June 2009
The paper reviews the pyrolysis of biomass constituents and possible secondary reactions. Biomass pyrolysis yields mostly liquid and solid fuel, easy to store and transport. Relevant working conditions for experiments and large-scale operation are: (i) biomass particles < 200 mm, (ii) a particle heating rate of at least about 80 K min1 and (iii) a reactor environment where the internal resistance to heat penetration is smaller than the external resistance to heat transfer (Biot-number, Bi < 1). The circumstances of TGA and DSC experiments meet these requirements and fully determine the reaction kinetics and endothermicity of the pyrolysis reaction. The reaction rate constant and the heat of reaction are essential parameters in the design of a pyrolysis reactor. For most of the biomass species tested, the first order reaction rate constant is large and >0.5 s1. The heat of reaction ranges from 207 to 434 kJ kg1. All results tie in with literature data, although the reader is cautioned in using literature data since experiments were not always performed under relevant testing conditions. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Biomass Pyrolysis Kinetics Heat of pyrolysis Experimental and operating conditions
1. Introduction Energy from biomass is recognised as the renewable energy source with the highest potential towards sustainable development in the near future [1–3]. Biomass provides already 14% of the world’s primary energy production [4], but is largely squandered by inefficient use and unsustainable exploitation. To exploit the full potential of this energy source, new approaches and modern technologies are needed. Biomass includes all organic matter that is available on a renewable basis: energy crops and all kinds of organic wastes. Now and in the near future waste products from agriculture and forestry, easy and cheap to collect, will dominate as source for bioenergy. In the European Union wood waste accounts for 94% of the currently used biomass for energy [5]. In the medium and long term, residues from agriculture and forestry, and energy crops will respectively further develop the bio-energy industry [5,6]. Solid biofuels have a low energy density, which limits the commercial applications to locations close to the place of
* Corresponding author at: School of Engineering, University of Warwick, Coventry, CV4.7.AL, United Kingdom. Tel.: þ32 16 532834; fax: þ32 16 538729. E-mail address: [email protected] (J. Baeyens). ! Started the research but died accidentally in 2007. 0960-1481/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2009.04.019
production. One way to solve this problem is the conversion of the feedstock into liquid fuel. These liquids have a higher energy density (Table 1) and are easy to store and to transport. The two most important methods to convert biomass into liquid fuel are (i) the biochemical conversion by the enzymatic activity of micro-organisms (to e.g. ethanol) and (ii) the thermochemical conversion by heat or oxidation. The biochemical conversion requires a feedstock that contains sugars or carbohydrates and a water content in excess of 40%. The thermochemical methods are best suited for dry biomass (moisture content < 10%) which is rich in lignin (this substance is less suited for biochemical conversion since it is hardly broken down by enzymatic activity), such as wood and agricultural wastes. Three main routes exist to thermochemically convert biomass in energy: combustion, gasification and pyrolysis [8] (Fig. 1). This paper targets pyrolysis as conversion technology because of its efficient energy production and the important advantage that mainly liquid fuels and solid char are formed, both easy to store and to transport. Pyrolysis oil moreover contains various chemicals with specific high-quality and added-value applications [2]. The expectations for pyrolysis in general, and for bio-oil in particular, are hence considerable [2,3]. Pyrolysis is the thermal decomposition in the complete absence of an oxidizing agent (air or oxygen), or with such a limited supply
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List of abbreviations A am C cp d D E Ea h DHr k kp kr kht m Q R r
Pre-exponential factor of the Arrhenius equation (s1) Specific surface of a particle per unit weight (m2 kg1) Reaction heat (J kg1) Specific heat capacity (J kg1 K1) Diameter (m) Thermal diffusivity of biomass particle (m2 s1) Energy (J) Activation energy (J mol1) Heat transfer coefficient (W m2 K1) Reaction heat of pyrolysis (J kg1) Global, apparent (action rate constant (s1)) Heat conductivity of the particle (W m1 K1) Intrinsic or chemical reaction rate constant (s1) Rate of heat penetration (s1) Mass (kg) Heat (J) Universal gas constant (8,31 J mol1 K1) Radial position in a particle with radius R (m)
that combustion or gasification do not occur to any appreciable extent. The proportion of products formed is strongly dependent on the reaction temperature and time, and on the heating rate. In comparison to gasification, pyrolysis occurs at relatively low temperatures (673–873 K). High temperatures and long residence times promote the formation of gas, while low temperatures and long residence times promote the formation of char. Optimum to produce oils are medium temperatures and short residence times, and thus high heating rates [9]. Fast pyrolysis occurs in seconds only, so reaction kinetics, phase transitions, heat and mass transfer play important roles. It is therefore critical to subject the biomass particles immediately to the optimum reaction temperature and to limit their exposure to lower temperatures, because this will promote the undesired formation of char. One way to do this is to use small particles in a fluidized bed, as will be described later. Another possibility is to transfer the heat very quickly but only to the surface of the particle, as applied in ablative pyrolysis. The pyrolysis process produces mainly vapours and char. It is essential to separate the char from the vapour as quickly as possible to prevent side reactions, since char acts as a cracking catalyst. Char particles are recovered by a gas–solid separation technique (mostly cyclones). After cooling and condensation, a brown, low viscosity liquid is obtained with a heating value about half that of conventional fuels. To achieve high oil yields the process parameters are to be accurately controlled. The essential parameters include [2,7]: a very fast heating and heat transfer to the reaction surface; a reaction temperature around 773 K and temperatures of the vapour phase of 673–723 K; Table 1 Bulk density, mean heating value and energy density of biomass and thermochemically derived fuels [7]. Energy carrier Straw Sawdust pyrolysis oil char
Bulk density (kg m3)
Heating value (GJ t1)
Energy density (GJ m3)
w100 w400 w1200 w300
20 15 17 30
2 6 20.4 9
t T X Bi
233
Time (s) Temperature (K) Fractional conversion (–) Biot-number (–) Heating rate (K s1) Density (kg m3) Time for total reaction (s)
b r s
Subscripts 0 Initial N End B, b Biomass g Gas phase in Input c Core of the particle min Minimum s Surrounding p Particle out Output
short residence times, for vapours less than 2 s; a fast char separation and cooling of the vapours, to avoid secondary cracking. The main product, bio-oil, is obtained with a maximum yield of 60–70 wt% (dry matter), together with char and gas [9]. These byproducts can be burnt externally (to dry the biomass, in cocombustion etc.) or in the process, to supply the endothermic reaction heat of pyrolysis. Char in itself has a heating value (z30 MJ kg1) [2,3,8] comparable to petroleum cokes (‘‘petcoke’’), and can externally be valorised. The moisture content of the pyrolysis feedstock shall not exceed 10 wt% to limit the water content of the bio-oil. The heart of the fast pyrolysis process is the reactor. Although it probably represents only 10–15% of the total capital cost of an integrated system, almost all research and development are focused on the reactor, since subsequent techniques (separation, condensation etc.) are known as physical and chemical processes. Also the rest of the process (biomass reception, storage, handling, drying and milling, product storage and possible product upgrading) applies known techniques [2]. Within the common pyrolysis reactor designs, mostly fluidized beds are applied, operated in either bubbling or circulating mode: these systems can meet the basic requirements to achieve high oil yields i.e. short residence time, fast heat transfer and fast separation of vapour and char. The technological strength and the market attractiveness (Fig. 2) show that the circulating fluidized bed (CFB) has the highest commercial potential. The expected breakthrough of the CFB is conditioned by a better understanding of the hydrodynamics (flow regimes, residence time) and reaction kinetics of the biomass pyrolysis at different heating rates [10]. pyrolysis
gasification
bio-oil
fuel gas
storage
chemicals turbine engine
combustion
heat
electricity or CHP
boiler
Fig. 1. Alternative thermochemical conversions of biomass [2].
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Fig. 2. Status of the pyrolysis reactors [7].
The production of bio-oil by pyrolysis is a technology on the edge between development and demonstration. The most important technological issues that remain to be solved before large-scale use becomes possible, are related to the complex kinetics and reactor hydrodynamics [10]. The study of the basic kinetics is the priority of the present research, which intends to make an experimental and theoretical contribution to the development of pyrolysis. It therefore concentrates on the determination of (i) the operating conditions required in relevant experimental research and large-scale pyrolysis of biomass; (ii) the kinetic constants of biomass pyrolysis; and (iii) the reaction heat of biomass pyrolysis and other thermal parameters required in the heat balance of the pyrolysis reactor.
The three competitive primary reactions are (i) the fragmentation to hydroxyacetaldehyde and other carbonyls, acids and alcohols; (ii) the depolymerisation to levoglucosan and other primary anhydrosugars; and (iii) the dehydration to char, gases and water. Dehydration dominates at low temperatures (<623 K) and slow heating rates. At higher temperatures, depolymerisation and fragmentation are dominant. Depolymerisation dominates at temperatures between 573 and 723 K, while fragmentation has its optimum at around 873 K. However, inorganic species such as cations, bases, acids and salts can have a major impact on these reactions [15,16]. The selectivity of cellulose for fragmentation and depolymerisation can be as high as 70–80 wt% [17]. 2.2. Hemicellulose Hemicellulose binds the cellulose microfibrils of the cell wall. It varies in structure and composition (mainly xylan) and has a low thermal stability [18,19,20]. It is decomposed in a way similar to cellulose: by dehydration at low temperatures (<553 K) and depolymerisation at higher temperatures [15]. Dehydration yields anhydride fragments, water soluble acids, char, gases and water, while depolymerisation yields volatile organics, levoglucosan and other anhydrohexoses, levoglucosenone and furans [18]. Here too, inorganic impurities cause fragmentation effects, but xylan is specifically sensitive for cations, because of its ion exchange sites.
2. Biomass pyrolysis reactions
2.3. Lignin
Biomass mainly consists of three components: 30–60% cellulose, 20–35% hemicellulose (polysaccharides) and 15–30% lignin (a polymer of methoxy substituted cyclic organics), together with some resins and minerals. Pyrolysis of biomass is very complex due to the diversity, the heterogeneity and the limited thermal stability of some of the components. Pyrolysis of biomass yields about the products expected from the pyrolysis of its three constituent components separately, despite synergetic effects [11]. The study of the individual components thus forms the basis of the expected reaction pathways and determines the occurring primary and secondary reactions. The order of these reactions, the reaction rate and the yields depend on parameters such as heating rate, temperature, pre-treatment, catalytic effects etc. [2,10,12,13]. The study of the reactions and the effect of the parameters are of critical importance to drive the reactions to high yields of the desired products and to avoid side reactions.
Lignin is the strengthening component of the cell wall and is mainly present in woody biomass [19]. It is a polymer of hydroxy and methoxy substituted propyl phenol units [18]. The very complex structure depends on the plant species and due to its structural diversity, pyrolysis yields various products (such as catechols, vanillins, aromatic carbonhydrates). At low temperatures (<773 K) dehydration dominates, while at higher temperatures a diversity of lignine monomers is formed. Above 973 K the monomers are decomposed and enter the vapour phase. Lignin is thermally more stable than cellulose and hemicellulose and yields more char and a higher fraction of aromatic compounds [15,19]. In a pyrolysis reactor the biomass is subjected to the whole temperature range and therefore all reactions mentioned above occur to some degree, in function of the heating rate of the whole biomass particle, and thus of the heat transfer and particle diameter.
2.1. Cellulose
2.4. Secondary reactions
Cellulose, (C6H12O5)n, is the main component of plant cell walls and consists of anhydroglucose units, connected by glycoside bonds. Of all lignocellulotic components, the thermal decomposition of cellulose is mostly investigated and best understood. The Waterloo-mechanism (Fig. 3) [14] is widely accepted to represent the reaction pathway, yielding the expected products and defining the effect of temperature and heating rate.
Secondary reactions can occur in the vapour phase or between vapour and solid phase to form mostly gas. This will considerably reduce the desired oil yield [21,22]. Two secondary reactions are of importance for pyrolysis: cracking (>973 K) and the water–gas shift reaction (<1083 K): H2O þ CO 4 H2 þ CO2. These secondary reactions are however limited at temperatures below 923 K, residence times <2 s for the gases and vapours and when char is quickly
fragmentation cellulose
active cellulose (low degree of polymerisation)
depolymerisation dehydration
carbonylcompounds, e.g. hydroxyacetaldehydes, acids and alcohols anhydrosugars, e.g. levoglucosan char, gases and water
Fig. 3. The primary decomposition of cellulose according to the Waterloo-mechanism [14].
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separated. These are the working conditions for commercial pyrolysis. Reforming, carbon–steam and carbon–carbon dioxide conversions are negligible under these conditions [14]. The global pyrolysis mechanism of biomass is mainly based on the pyrolysis of cellulose, although results are similar for wood and other biomass species [10,11]. The Waterloo concept of Fig. 4 is recognised to represent the global mechanism. 3. Materials and methods 3.1. Biomass samples The specific biomass species were selected because of previous literature references, or because of their potential in Europe: spruce, pine, poplar, eucalyptus, sawdust, sewage sludge, straw and the non-harvested stalks and leaves of sunflower and corn. The samples were crushed and dried for over 2 h at 105 C and again 30 min just before thermal processing. 3.2. The kinetic constants of biomass pyrolysis The kinetic constants of the pyrolysis of biomass are determined by thermogravimetric analysis. A Hi-res TGA 2950 Thermogravimetric Analyzer was used at atmospheric pressure with a nitrogen flow of 50 ml min1. The TGA scale was made of platinum and the weight was registered every 0.5 s with an accuracy of 0.001 mg. Following the discussion which is presented in Section 4, all experiments were performed at 100 K min1 and were repeated at least three times. Calculations use the average results, with a standard deviation of maximum 5%. The resulting TGA-profiles plot the weight loss in function of the temperature. 3.3. Reaction heat A DSC Q10 with Refrigerated Cooling System operating within a temperature range from 183 to 823 K was used to measure the heat requirement of the biomass pyrolysis reaction. The experiments were performed from 313 to 823 K, under atmospheric pressure and with nitrogen supplied at 50 ml min1. The DSC scales were aluminium and were not reused. The DSC was calibrated regularly with indium as reference material. The obtained DSC curves give the heat flux (W g1) in function of the temperature or in function of the time (constant DT/Dt ¼ 10 K min1). 3.4. Specific heat of the biomass The specific heat capacity of sawdust, corn residue and poplar were determined at 25 C using a calorimeter (Leybold Heraeus). The temperature increase of n-hexane was measured after addition of preheated biomass.
4. Results and discussion 4.1. The required operating conditions for relevant experimental research and large-scale pyrolysis of biomass 4.1.1. Specific operating conditions The specific working conditions essential to achieve maximal oil yields have already been described in the introduction of this paper and include a reaction temperature around 773 K, temperatures in the vapour phase around 673–723 K; a high heating rate and high heat transfer at the reaction surface; a short residence time, for vapours less than 2 s and a rapid cooling of the vapours and separation of the char, to avoid secondary cracking. Essential for the fast decomposition and high oil yields is the fast heating of the biomass particle. Slow pyrolysis decreases the oil yield considerably in favour of gas and char. Moreover long residence times favour secondary cracking of the oil. Short residence times, rapid cooling of the oil and rapid separation of the char are hence required. This fast heating of the whole biomass particle to a uniform temperature requires a high heat transfer. If the heat supply would be insufficient, important thermal gradients would occur leading to slow pyrolysis and/or incomplete reactions in the core of the particle. Smaller particles have a larger specific surface and are more mobile within the reactor mass, which enhances the heat transfer. Also, temperature gradients in small particles are expected to be negligible, as will be shown below. When choosing the experimental and large-scale parameters, the thermal characteristics of the biomass, the reaction kinetics and the heat transfer will have to be jointly considered. Some basic principles of the reaction kinetics and conversion of solid particles, of heat transfer in powder reactors and of heat penetration inside the particle allow to determine these conditions in anticipation, which makes it possible to program the experiments in a relevant and effective way. This analysis allows at the same time to critically evaluate the literature data: research has not always been performed under these optimum conditions (for example [12]), with results that could lead to wrong conclusions. Recommendations concerning the computational aspects of kinetic analyses were made in a series of articles [24–28], where both one-step/multi-step kinetic schemes and parallel/consecutive reactions are considered. Analysis of the data hereafter by the method of Flynn, Wall and Ozawa reported in [24] demonstrates that the biomass pyrolysis can be considered as a one-step reaction, thus simplifying the treatment, but nevertheless requiring an assessment at different heating rates. The analysis of the effect of heating rate is presented below. 4.1.2. The influence of temperature gradients in the biomass particle Fast pyrolysis and its associated maximum oil yield can only be achieved by fast and uniform heating of the biomass particles. To meet these targets, avoiding thermal gradients in and around the particle is very important. Application of the basic principles of both heat transfer to the particle and of heat conduction within the particle, allows to determine the required working conditions. The temperature uniformity throughout the biomass particles can be determined by the heat conduction law of Fourier, here applied in non-stationary regime and for the simple case of a spherical particle:
kp v2 T 2 vT vT þ ¼ rp cp vr2 r vr vt
Fig. 4. The biomass pyrolysis concept of the University of Waterloo [23].
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!
v2 T 2 vT ¼ D þ vr 2 r vr
! (1)
with kp: heat conductivity of the biomass at temperature T (W m1 K1), cp: specific heat capacity of biomass at temperature T (J kg1 K1), rp: density of the biomass (kg m3), D: thermal diffusivity of the particle (m2 s1).
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The solutions of this unsteady thermal condition are simple when easy boundary conditions can be applied, as will be used further in the text. Solutions are very difficult or impossible for complex boundary conditions and/or bodies of irregular shape. The general solutions are therefore commonly expressed by a dimensional analysis. For a sphere of radius r, initially at temperature T0 and suddenly exposed to surroundings at Ts, the temperature distribution, at any time t and position x, is given by:
Ts T hr t x ¼ f ; D 2; Ts T0 kp r r
(2)
with h: external heat transfer coefficient at surface of sphere (W m2 K1). Since it is important to know the evolution of the temperature in the core of the sphere (Tc at x ¼ 0) and introducing the Biot-number ( ¼ (r/kp)/(1/h)) reduces equation (2) to:
T s Tc t ¼ f Bi; D 2 Ts T0 r
(3)
For large (Bi / N) or very small (Bi / 0) values of the Biotnumber, solutions are graphically given by Carslaw and Jaeger [29]. For applications of biomass particles in a fluidized bed, Biotnumbers are intermediate (Bi ¼ 0.2–1, see further in the text) and extreme conditions do not apply. For these intermediate values of the Biot-number, results are presented by Heisler [30] in the form of charts, expressing (Ts Tc)/(Ts T0) in terms of Dt/r2 (the Fouriernumber) with 1/Bi as parameter. Illustrative application of the chart is given in Table 2, with characteristic properties of sawdust at 773 K, i.e. kp ¼ 0.15 W m1 K1 [31], cp ¼ 1150 J kg1 K1 and rp ¼ 700 kg m3, and for an average fluidized bed heat transfer coefficient at the surface of the sphere i.e. 500 W m2 K1. Since fast pyrolysis requires the reaction to take place within 2– 2.5 s, it is clear that only very small particles will meet the conditions of fast heating to a uniform temperature. In this case, particle sizes above 200 mm diameter (radius 100 mm) already take 0.38 s to warm up, i.e. nearly 15% of the proposed reaction time of 2–2.5 s, thus starting to jeopardize the ideal condition of fast pyrolysis. According to the previous calculations, it can be assumed that the heating rate throughout the particle is uniform when heating very small particles (200 mm) at a rate b (K s1): simplified boundary conditions can hence be applied to Eq. (1), i.e. vT=vt ¼ b for r ¼ 0 to R( ¼ dp/2) and vT=vr ¼ 0 for r ¼ 0 (in the core). The solution is given by Carslaw and Jaeger [29] as:
DTmax ¼ ðTR Tc Þ ¼ bd2p =ð24DÞ
(4)
Eq. (4) can be applied for various values of b and with e.g. the characteristic properties of sawdust mentioned above. The results are a set of curves in function of the diameter as shown in Fig. 5. These calculations show that the temperature differences between the biomass surface and core are very limited, certainly when considering that the surrounding temperature is 773 K and that the heating rate will vary between minimally 1.5 K s1 in a TGA (thermogravimetic analysis) experiment, to a few hundred K s1 in a bubbling or circulating fluidized bed. To obtain a fast heating of the whole particle, it is appropriate thus to use small biomass particles (e.g. <200 mm for saw dust) and no significant thermal gradient will occur in these small particles: the core and the surface of the biomass particle will behave both thermally and kinetically in a similar way. 4.1.3. The influence of heat transfer to the biomass particles Fast pyrolysis requires a fast heat transfer to the biomass. This heat transfer is conditioned by the degree of gas and solid turbulence achieved in the reactor [32,33]. The heat transfer coefficient depends on the gas–solid contacting mode. It ranges from 10 W m2 K1 for a static bed, to 50–100 W m2 K1 in a fixed bed with forced gas circulation (as in TGA), and several hundreds of W m2 K1 for bubbling and circulating fluidized beds. This explains why the fluidized beds are specifically considered as top technologies for biomass conversions. The heat transfer coefficient in a CFB can be estimated from the results of Kobro and Brereton [34]. In a CFB operating with 100 mm sand as bed material at a recommended gas velocity of 5 m s1 and a solid circulation rate of 200 kg m2 s1, the convective heat transfer coefficient is 620 W m2 K1. Since not only external convection but also internal conduction is important, the overall picture is expressed by the Biot-number, which represents the internal resistance to heat penetration divided by external resistance to heat transfer and is written as follows:
Bi ¼
r=kp 1=h
For the 200 mm sawdust particle (r ¼ 100 mm) at 773 K, the values are as follows:
h ¼ 300 W m2 K1
Bi ¼ 100 106 300=0:15 ¼ 0:2
h ¼ 500 W m2 K1
Bi ¼ 100 106 500=0:15 ¼ 0:33
h ¼ 700 W m2 K1
Bi ¼ 100 106 700=0:15 ¼ 0:47
with TR: T at the surface of the particle (at r ¼ R) (K), Tc: T in the core of the particle (at r ¼ 0) (K).
Table 2 Time required for the core of a spherical saw dust particle to nearly (99%) reach the temperature of the surroundings (773 K) where the particle is suddenly submerged in. r (mm)
Bi-number
Dt/r2
t (s)
50 100 200 400
0.17 0.34 0.67 1.33
13 7 5 2.2
0.017 0.38 1.08 2.2
It is assumed that a condition where the core temperature has reached 99% of the temperature of the surroundings (Ts z 773 K) meets conditions of uniform temperature T throughout the sphere (i.e. Tc z 0.99 Ts). The initial T of the sphere is 293 K and D z 1.86 107 m2 s1.
Fig. 5. Maximum temperature difference (DTmax) between the surface and core of the sawdust particle in function of the particle diameter at different heating rates (b).
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The Biot-number is <1 in all cases, implying that the external resistance associated with convectional heat transfer largely dominates. Bi is only >1 for coarse particles and/or too high values of h. This result confirms Fig. 5 where increasing DTmax values are noticed with increasing b values (achieved at high convection heat transfer rates). 4.2. The selection parameters for the conversion of biomass in a powder reactor 4.2.1. The kinetic expression for the conversion of an individual biomass particle Occurring reactions during pyrolysis are endothermic, as will be further determined from DSC (differential scanning calorimetry) measurements. As illustrated in Fig. 4, pyrolysis of biomass progresses along a reaction path with primary and secondary reactions, each reaction with its specific expression for the rate of product formation [35,36]. The present paper considers the overall decomposition rate of the biomass particle only, expressed by a first order reaction. The overall reaction rate constant, k, is however the sum of the reaction rate constants of the parallel primary reactions. The experimental data moreover enable the reader to apply a more complex kinetic assessment, if so desired. In the present
237
assessment, the kinetics are expressed by a continuous reaction mechanism for the small particles under consideration: the temperature distribution across the small particle is quasi-uniform so that the pyrolysis reaction proceeds at the same rate everywhere throughout. The slowest step in the process is the heat transfer through the external film around the particle. For small particles, the time needed to reach a certain conversion (including complete conversion) increases therefore in a way that is approximately linear with the particle size dp [33]. For relatively large particles, the internal heat conduction resistance is larger, leading to temperature gradients inside the particle, as mentioned above. The rate of pyrolysis is now limited by the slower heat conduction process, whereby the time needed to reach a particular (or full) conversion increases as d2p. This distinction is demonstrated in Fig. 6 where the time of complete conversion, as determined from TGA experiments, is illustrated for sieved biomass fractions of sawdust (0–200 mm) and eucalyptus (0–600 mm). The continuous reaction model can hence certainly be used for the description of the conversion of smaller biomass particles, expressed for a first order reaction as:
1 X ¼ expðktÞ
Fig. 6. Time for complete conversion as function of the biomass particle size for (a) sawdust, (b) eucalyptus.
(5)
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The apparent and measured rate constant, k, should be expressed in terms of the rate of heat penetration (kht) and the intrinsic or chemical reaction rate constant (kr) [33,37–39]:
1=k ¼ 1=kr þ 1=kht
(6)
Table 4 The total heat requirements for pyrolysis. The total heat requirement for pyrolysis sample
Heat requirement for pyrolysis (J g1)
pruce eucalyptus poplar sawdust corn sunflower straw sewage sludge
342 400 207 434 267 292 375 385
with kht: rate of heat penetration, equal to ham/cp (s1), h: heat transfer coefficient from reactor bulk to the particle (W m2 K1), am: specific surface area of the particle per unit weight (m2 kg1), cp: specific heat capacity of the particle (J kg1 K1). Heat transfer limitations are negligible in operations achieving a high value of kht and reaction rates are then kinetically controlled only (k z kr) if additionally the internal heat conduction resistance is negligible (small particles). Experiments should therefore be preferably performed under such conditions. Such high values of kht and thus of h can only be obtained in bubbling or circulating fluidized beds. Since the use of small particles is required and the residence time needs to accurately controllable, the circulating fluidized bed is the most appropriate reactor for biomass pyrolysis. As will be shown in the experimental results in further sections, the reaction rate constant is a strong function of the temperature, and follows the Arrhenius equation (7). The activation energy has a constant but different value for each specific biomass.
(see Section 4.2.4). The required heat of reaction is hence C.m0 (J). The heat balance then results in equation (8a).
k ¼ A expðEa =RTÞ
k cp b lnð1 XÞ=ð0:7CXÞ
(7)
Table 3 compares kht and k for a few target samples, whereby equation (6) is used to determine the error on k if no heat transfer limitations are accounted for. The error is insignificant, even at high temperatures, indicating that virtually no external heat transfer limitations are present under the experimental conditions used. 4.2.2. The influence of the heating rate in TGA experiments The external heating rate b (K s1) can be programmed in TGA experiments, and literature data are available for b values between 0.033 K s1 and 1.67 K s1. Experimental values of k vary significantly, even for a single biomass species. The influence of the heating rate is of paramount importance since the heat transferred must be sufficient to supply the total endothermal heat of reaction (C, J kg1). A too low heating rate (low value of b) will retard the pyrolysis reaction and will yield an apparently lower value of the reaction rate constant. The anticipated definition of the required minimum b value can prevent faulty experiments and related experimental data. During TGA-pyrolysis, reactions take place between approx. 473 and 673 K (average 573 K). The relationship between sufficient heat supply and reaction yield can be expressed in terms of kinetic and thermal data, as determined below. The first order kinetics relate conversion (X), reaction rate constant (k) and time (t), according to expression (5). Since the heat supplied should at any time and for any conversion X exceed the heat consumed, a heat balance can be made. The heat supplied during the reaction equals m0cpbt (J), with m0 the initial amount of biomass (kg). The char yield is on average 30 wt%. Heat requirements (C) were measured by DSC
m0 cp bt 0:7m0 XC
(8a)
Inserting the value of t from Eq. (5), the general equation (8b) is obtained.
cp b lnð1 XÞ=k 0:7XC
(8b)
or
(8c)
This equation can thus be applied to any values of conversion (X). In the present research, overall values of k are measured, i.e. at nearly complete conversion of the biomass (Table 4). Equation (8c) is hence applied using these experimental k values and a conversion of 95% is assumed to be the measure of complete pyrolysis. Equation (8c) is then reduced to:
k 4:5cp b=C
(8d)
It should be noted that for very low conversions (X / 0) a more restrictive condition on k is obtained since the initial reaction rate is highest. This condition of very small conversions will however not be applied in the present treatment since only overall k values are known. This approach was tested using the TGA results of Section 4.2.3 below, obtained at different heat supply rates and for various biomass species. Illustration for spruce is given in Fig. 7, which clearly indicates that relevant k values can only be obtained if b exceeds 80 K min1. Application of Eq. (8d) at an average 573 K, with a cp of 1.08 kJ kg1 K1 (at 573 K) and with the experimentally determined C values (Table 4) demonstrates that the theoretical value of b should exceed 79 K min1, in line with the experimental findings. Similar calculations were performed for the different biomass species, using the experimentally determined char yield and C
0,025
k (s-1)
0,02
Table 3 Effect of the heat penetration upon the reaction rate constant (k). T (K)
573 673 773
Saw dust (85 mm)
Eucalyptus (37.5 mm)
Corn (50 mm)
k
kht
error on kr (%)
k
kht
error on kr (%)
k
kht
error on kr (%)
0.01 0.12 0.68
5.88 5.47 5.12
0.2 2.1 11.8
0.01 0.21 1.52
26.67 24.81 23.19
0.05 0.8 6.1
0.02 0.27 1.59
24.00 22.33 20.87
0.1 1.2 7.1
0,015 0,01 0,005 0 0
20
40
60
80
100
120
140
heating rate (K min-1) Fig. 7. The effect of the heating rate upon the reaction rate constant for spruce.
M. Van de Velden et al. / Renewable Energy 35 (2010) 232–242 Table 5 Required minimum heating rates for an accurate measurement of the reaction rate constant. Biomass
k at 573 K (s1)
bmin (K min1)
spruce eucalyptus poplar sawdust corn sunflower straw sewage sludge
0.0200 0.0138 0.0116 0.0111 0.0242 0.0856 0.0350 0.0067
79 64 28 56 75 289 152 30
239
biomass and represents 25–35 wt%, with the exception of corn with only 10 wt% residue, and sunflower and sewage sludge, where the char fraction is significantly higher. The high ash content of sewage sludge is related to its well-known high content of minerals. The results correspond fairly with previous literature data, i.e. 21% char for spruce [40], 30% for straw [40,41] and on average between 19 and 26% [42]. From the dynamic TGA experiments, the reaction rate constant k can be determined by Eqs. (9)–(11), as used in e.g. [10–12,43].
dXðtÞ=dt ¼ k Xp XðtÞ
(9)
with values. A heating rate b of 80 K min1 is sufficient to meet the required reaction heat (Table 5) for most biomass species. All experiments were therefore performed at 100 K min1. To accurately measure the kinetics, heating rates should exceed 100 K min1 for sunflower residue and straw. The previous calculations demonstrate that literature data obtained at lower heating rates (e.g. [12]) should be considered with caution and should not be compared to results obtained under relevant conditions. 4.2.3. Kinetic analysis The obtained TGA-profile plots, shown in Fig. 8, are similar for all tested biomass samples. The results show that pyrolysis occurs in the temperature range between 473 and 673 K, depending on the type of biomass. This observed temperature range was also found in literature data [10,12,40]. It is obvious that the pyrolysis reaction produces a solid residue or char, noticed by a final limited loss of weight per unit of time. This char consists of minerals and the organic coking-residue of the
XðtÞ ¼ ðm0 mðtÞÞ=m0
(10)
Xp ¼ ðm0 mN Þ=m0
(11)
with m0: initial weight of biomass (at t ¼ 0) (mg), mN : residual weight of biomass after the reaction (mg), m(t): weight of biomass at time t during the experiment (mg). As the weight of the sample is continuously registered, X and Xp are known parameters and the reaction rate constant can be derived from Eq. (9) at every moment. Because of the known temperature profiles of the TGA (known DT/Dt), the reaction rate constant is also known as a function of the temperature. The Arrhenius Equation (7) can be applied after transformation and plotting of the results as ln(k) versus 1/T. The activation energy and the pre-exponential factor can be derived from respectively the slope and the intersection of the obtained straight line with the ln(k)-axis. Results for the different biomass species at b ¼ 100 K min1 are illustrated in Fig. 9 and summarised in Table 6, together with some literature data.
a
-2 0,0015
0,0016
0,0017
0,0018
0,0019
0,002
lnk
-3 -4 -5 -6 -7
1/T (K-1) spruce
lnk
b
-2 0,0015 -3
poplar
eucalyptus
0,0016
0,0017
corn
sun flower
0,0018
sawdust
0,0019
0,002
-4 -5 -6 -7
1/T (K-1)
Fig. 8. TGA plots for (a) spruce, eucalyptus, poplar and sawdust, and (b) corn, sunflower, straw and sewage sludge.
straw
sewage sludge
Fig. 9. Temperature dependency of the rate constant of biomass pyrolysis at a heating rate of 100 K min1 for (a) spruce, eucalyptus, poplar and sawdust, and (b) corn, sunflower, straw and sewage sludge.
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Table 6 Kinetic constants from own experiments and literature. Ea (kJ mol1)
Sample Own experiments
Thurner and Mann [43] Gorton and Knight [44] Nunn et al. [45] Reina et al. [12] Di Blasi and Branca [46]
Spruce Eucalyptus Poplar Sawdust Corn Sunflower Straw Sewage sludge Oak Hard wood
A (s1) at 100 K min1
k (s1) at 773 K
68.4 86.4 54.1 75.8 77.0 63.9 76.3 45.3
3.47 104 1.06 106 1.00 103 9.12 104 2.55 105 2.48 104 3.16 105 8.95 101
0.824 1.52 0.219 0.684 1.59 1.19 2.21 0.078
106.5
2.47 106
0.156
1.48 10
6
1.31
69.1
3.64 10
4
0.775
95.4 95.4
2.40 105 2.4 105
89.52
Gum tree, hard wood Forest wood Beech
Table 7 Specific heat at 298 K.
0.0852 0.156
The activation energy of a specific biomass is nearly constant (and representative for the biomass type) at different heating rates (Fig. 10), as can be seen from the parallel ln(k) versus 1/T lines. Fig. 10 shows however that the pre-exponential factors, and thus the reaction rate constant depend on the heating rate. When the heating rate increases, the pre-exponential factor A (and thus k) increases first quickly to then reach a maximum value: at low heating rates insufficient reaction heat is supplied for the endothermic reaction, as was previously discussed. Although the obtained k values are nearly constant when obtained at high heating rates (b) during the TGA experiments, the b values are still of the order of 1–2 K s1 only (due to TGA limitations). In fast pyrolysis, biomass particles are subjected to heating rates in excess of 100 K s1. Section 4.2.2. discussed the influence of the heating rate on the biomass conversion in view of balancing the heat supply rate with both the heat required for pyrolysis and progress of conversion. This balance is met at b w 1–2 K s1. Heating rates in excess of 100 K s1 can only be examined in pyrolysis reactors such as fluidized beds. This study is ongoing at present, where the batch pyrolysis of biomass is studied. Results will be reported in the near future and are essential to evaluate the possible extrapolation of the current data and kinetic expressions to systems with a fast heating rate. From these A and Ea values, the rate constant of the pyrolysis reaction can be calculated at any temperature, and values at 500 C
-2 0,0015 -2,5
0,0017
0,0016
Sample
cp (kJ kg1 K1)
Poplar Sawdust Corn residue
1.2 1 0.9
(the optimum T in pyrolysis reactors) are included in Table 6. Literature data should be used with caution: although data for k, A and Ea are given, both the method and procedure used, and the heating rates are generally not included. It is however clear that the Ea values are in the range of those of the biomass components at their respective wt%: 195–213 kJ mol1 for cellulose, 105– 111 kJ mol1 for hemicellulose and 34–65 kJ mol1 for lignin [47], each present in biomass for around respectively 45, 30 and 25%. Moreover, it should be mentioned that the k values, with the exception of poplar and sludge, exceed 0.5 s1 corresponding to a fast reaction of the biomass. A high conversion can thus be achieved in short reaction times so that the possible occurrence of side reactions is limited. 4.2.4. Thermal analysis The thermal parameters are necessary to formulate the heat balance of the pyrolysis reaction. Since pyrolysis, as every cracking reaction, is an endothermic reaction, the reaction heat is to be supplied indirectly (heat exchangers) or directly (e.g. by preheating the bed material or the fluidization gas). Usually the combustion of pyrolysis gas supplies the required heat (C, J g1) and the combustion gases form the oxygen-free carrier gas. The heat required to pyrolyse biomass contains two components (Eq. (2)): the heat required to heat the biomass to the temperature at which
0,0018
-3
lnk
-3,5 -4 -4,5 -5 -5,5 -6 -6,5
1/T (K-1) 50 K min
-1
80 K min-1
100 K min-1
120 K min-1
Fig. 10. Temperature dependency of the reaction rate of the pyrolysis of spruce at different heating rates.
Fig. 11. DSC curves (10 K min1) for (a) spruce, eucalyptus, poplar and sawdust, and (b) corn, sunflower, straw, sewage sludge.
M. Van de Velden et al. / Renewable Energy 35 (2010) 232–242
241
The calculated heat requirements for pyrolysis of the tested biomass species are presented in Table 4. These values are the average of three experiments, with a deviation of approximately 6%. The end temperature of pyrolysis was obtained from the TGA experiments. The results are in the same order of magnitude of literature data: 500 kJ kg1 for straw [38], 400–450 kJ kg1 for cellulose [41,50], 235 kJ kg1 [51] and 418 kJ kg1 [52] for unspecified wood and 120– 360 kJ kg1 for various wood species [42]. 5. Conclusions
Fig. 12. Heat requirement for pyrolysis in function of the temperature for (a) spruce, eucalyptus, sawdust, poplar, and (b) corn, sunflower, straw, sewage sludge.
pyrolysis occurs and the heat required for the endothermic pyrolysis as such. Hence the required heat can be written as follows:
C ¼ mb cp;b ðTr T0 Þ þ mb DHr
(12)
The temperature at which pyrolysis occurs, the specific heat capacity cp and the reaction heat DHr (J g1) required for the pyrolysis itself were experimentally investigated and will be discussed hereafter. The specific heat capacity cp (J g1 K1) of the biomass samples under scrutiny is presented in Table 7. Since cp values increase with increasing temperature, the value at 500 C will be about 15% higher than the experimental value at 298 K. Results are close to literature data, which at 773 K vary between 1.1 kJ kg1 K1 [48] to 1.2 kJ kg1 K1 [49]. The obtained DSC curves, giving the heat flux (W g1) in function of the temperature or in function of the time (constant DT/ Dt ¼ 10 K min1) are shown in Fig. 11. The DSC curves show that the heating process is clearly in the domain of heat requirement, which confirms the endothermic character of pyrolysis. Integration of these heat fluxes over time (Eq. (13)) gives the total heat requirement C (J g1) as a function of the temperature (Fig. 12). This includes the heat required for heating the biomass as well as the heat for the pyrolysis reaction. The endothermic peak at about 100 C, due to the evaporation of residual water, was eliminated before integration. When the pyrolysis is completed, the heat requirement decreases. Declining values before the end of the reaction and beyond the normal temperature range of pyrolysis can be explained by the occurrence of secondary reactions (e.g. the aggregation of the char).
dC dT ¼ cp þ DHr /C ¼ dt dt
Zt
cp
0
dT þ DHr dt dt
(13)
Energy from biomass is recognised as the renewable energy source with the highest potential in the near future. Within the thermochemical conversion techniques, pyrolysis offers major advantages through yielding mostly liquid and solid fuel, easy to store and transport. Although the operating conditions for a maximum oil yield are well-known, the kinetics of the biomass conversion need to be determined. The paper considered (i) the required working conditions for relevant experimental research; (ii) the thermogravimetric measurement of the kinetic constants and (iii) the reaction heat of the biomass pyrolysis. A review of the pyrolysis of the main constituents of biomass i.e. cellulose, hemicellulose and lignin, and of the possible secondary reactions stresses the value of the biomass pyrolysis concept of the University of Waterloo. To program experiments in a relevant and effective way, and to critically assess previous literature results, it was demonstrated that tests should be carried out (i) with biomass particles smaller than approx. 200 mm and (ii) in a reaction environment where the particle heating rate is about 80 K min1 and where the internal resistance to heat penetration is smaller than the external resistance to heat transfer (expressed in the condition of Biot-number, Bi < 1). TGA and DSC experiments on a wide variety of biomass particles determine respectively the reaction kinetics and the Arrhenius dependency, and the endothermicity of the reaction. For most of the biomass species, the reaction rate constant is >0.5 s1, corresponding with a fast reaction. The endothermic heat of reaction ranges from 207 to 434 kJ kg1. These results tie in with literature data, although the reader is cautioned in the use of literature data since experiments were often performed at non-representative testing conditions. Both kinetic and thermal data are essential in the design of pyrolysis reactor, determining respectively the required residence time and the amount of heat to be supplied to the reactor. Their use will be illustrated in a further paper where the design strategy of a 10 MW (bio-oil) pyrolyser is presented. References [1] Maniatis K, Guiu G, Reisgo J. The European commission perspective in biomass and waste thermochemical conversion. In: Bridgwater AV, editor. Pyrolysis and gasification of biomass and waste. Newbury: CPL Press; 2003. p. 1–18. [2] Bridgwater AV. Renewable fuels and chemicals by thermal processing of biomass. Chem Eng J 2003;91:87–102. [3] Faaij APC. Bio-energy in Europe: changing technology choices. Energy Policy 2006;34:322–42. [4] Iea – International Energy Agency. www.ieabioenergy.com; 2006 [accessed 10.12.08]. [5] Eurostat. http://epp.eurostat.cec.eu.int; 2003 [accessed 10.12.08]. [6] Nan L, Best G, Coelho C, De Carvalho N. The research progress of biomass pyrolysis processes, integrated energy systems in China – The cold northeastern region experience. FAO – Food and Agricultural Organization of the United Nations, http://www.fao.org/docrep/T4470E/T4470E00.htm; 1994. [7] PyNe – Pyrolysis Network of IEA Bioenergy. www.pyne.co.uk; 2006 [accessed 10.12.08].
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Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Fundamentals, kinetics and endothermicity of the biomass pyrolysis reaction Manon Van de Velden !, Jan Baeyens a, b, *, Anke Brems c, Bart Janssens c, Raf Dewil c, d a
Department of Chemical Engineering, University College London, London, United Kingdom School of Engineering, University of Warwick, Coventry, United Kingdom c Department of Chemical Engineering, Katholieke Universiteit Leuven, Campus De Nayer, Sint-Katelijne-Waver, Belgium d Chemical and Biochemical Process Technology and Control Section, Department of Chemical Engineering, Katholieke Universiteit Leuven, Heverlee, Belgium b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 December 2008 Accepted 22 April 2009 Available online 4 June 2009
The paper reviews the pyrolysis of biomass constituents and possible secondary reactions. Biomass pyrolysis yields mostly liquid and solid fuel, easy to store and transport. Relevant working conditions for experiments and large-scale operation are: (i) biomass particles < 200 mm, (ii) a particle heating rate of at least about 80 K min1 and (iii) a reactor environment where the internal resistance to heat penetration is smaller than the external resistance to heat transfer (Biot-number, Bi < 1). The circumstances of TGA and DSC experiments meet these requirements and fully determine the reaction kinetics and endothermicity of the pyrolysis reaction. The reaction rate constant and the heat of reaction are essential parameters in the design of a pyrolysis reactor. For most of the biomass species tested, the first order reaction rate constant is large and >0.5 s1. The heat of reaction ranges from 207 to 434 kJ kg1. All results tie in with literature data, although the reader is cautioned in using literature data since experiments were not always performed under relevant testing conditions. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Biomass Pyrolysis Kinetics Heat of pyrolysis Experimental and operating conditions
1. Introduction Energy from biomass is recognised as the renewable energy source with the highest potential towards sustainable development in the near future [1–3]. Biomass provides already 14% of the world’s primary energy production [4], but is largely squandered by inefficient use and unsustainable exploitation. To exploit the full potential of this energy source, new approaches and modern technologies are needed. Biomass includes all organic matter that is available on a renewable basis: energy crops and all kinds of organic wastes. Now and in the near future waste products from agriculture and forestry, easy and cheap to collect, will dominate as source for bioenergy. In the European Union wood waste accounts for 94% of the currently used biomass for energy [5]. In the medium and long term, residues from agriculture and forestry, and energy crops will respectively further develop the bio-energy industry [5,6]. Solid biofuels have a low energy density, which limits the commercial applications to locations close to the place of
* Corresponding author at: School of Engineering, University of Warwick, Coventry, CV4.7.AL, United Kingdom. Tel.: þ32 16 532834; fax: þ32 16 538729. E-mail address: [email protected] (J. Baeyens). ! Started the research but died accidentally in 2007. 0960-1481/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2009.04.019
production. One way to solve this problem is the conversion of the feedstock into liquid fuel. These liquids have a higher energy density (Table 1) and are easy to store and to transport. The two most important methods to convert biomass into liquid fuel are (i) the biochemical conversion by the enzymatic activity of micro-organisms (to e.g. ethanol) and (ii) the thermochemical conversion by heat or oxidation. The biochemical conversion requires a feedstock that contains sugars or carbohydrates and a water content in excess of 40%. The thermochemical methods are best suited for dry biomass (moisture content < 10%) which is rich in lignin (this substance is less suited for biochemical conversion since it is hardly broken down by enzymatic activity), such as wood and agricultural wastes. Three main routes exist to thermochemically convert biomass in energy: combustion, gasification and pyrolysis [8] (Fig. 1). This paper targets pyrolysis as conversion technology because of its efficient energy production and the important advantage that mainly liquid fuels and solid char are formed, both easy to store and to transport. Pyrolysis oil moreover contains various chemicals with specific high-quality and added-value applications [2]. The expectations for pyrolysis in general, and for bio-oil in particular, are hence considerable [2,3]. Pyrolysis is the thermal decomposition in the complete absence of an oxidizing agent (air or oxygen), or with such a limited supply
M. Van de Velden et al. / Renewable Energy 35 (2010) 232–242
List of abbreviations A am C cp d D E Ea h DHr k kp kr kht m Q R r
Pre-exponential factor of the Arrhenius equation (s1) Specific surface of a particle per unit weight (m2 kg1) Reaction heat (J kg1) Specific heat capacity (J kg1 K1) Diameter (m) Thermal diffusivity of biomass particle (m2 s1) Energy (J) Activation energy (J mol1) Heat transfer coefficient (W m2 K1) Reaction heat of pyrolysis (J kg1) Global, apparent (action rate constant (s1)) Heat conductivity of the particle (W m1 K1) Intrinsic or chemical reaction rate constant (s1) Rate of heat penetration (s1) Mass (kg) Heat (J) Universal gas constant (8,31 J mol1 K1) Radial position in a particle with radius R (m)
that combustion or gasification do not occur to any appreciable extent. The proportion of products formed is strongly dependent on the reaction temperature and time, and on the heating rate. In comparison to gasification, pyrolysis occurs at relatively low temperatures (673–873 K). High temperatures and long residence times promote the formation of gas, while low temperatures and long residence times promote the formation of char. Optimum to produce oils are medium temperatures and short residence times, and thus high heating rates [9]. Fast pyrolysis occurs in seconds only, so reaction kinetics, phase transitions, heat and mass transfer play important roles. It is therefore critical to subject the biomass particles immediately to the optimum reaction temperature and to limit their exposure to lower temperatures, because this will promote the undesired formation of char. One way to do this is to use small particles in a fluidized bed, as will be described later. Another possibility is to transfer the heat very quickly but only to the surface of the particle, as applied in ablative pyrolysis. The pyrolysis process produces mainly vapours and char. It is essential to separate the char from the vapour as quickly as possible to prevent side reactions, since char acts as a cracking catalyst. Char particles are recovered by a gas–solid separation technique (mostly cyclones). After cooling and condensation, a brown, low viscosity liquid is obtained with a heating value about half that of conventional fuels. To achieve high oil yields the process parameters are to be accurately controlled. The essential parameters include [2,7]: a very fast heating and heat transfer to the reaction surface; a reaction temperature around 773 K and temperatures of the vapour phase of 673–723 K; Table 1 Bulk density, mean heating value and energy density of biomass and thermochemically derived fuels [7]. Energy carrier Straw Sawdust pyrolysis oil char
Bulk density (kg m3)
Heating value (GJ t1)
Energy density (GJ m3)
w100 w400 w1200 w300
20 15 17 30
2 6 20.4 9
t T X Bi
233
Time (s) Temperature (K) Fractional conversion (–) Biot-number (–) Heating rate (K s1) Density (kg m3) Time for total reaction (s)
b r s
Subscripts 0 Initial N End B, b Biomass g Gas phase in Input c Core of the particle min Minimum s Surrounding p Particle out Output
short residence times, for vapours less than 2 s; a fast char separation and cooling of the vapours, to avoid secondary cracking. The main product, bio-oil, is obtained with a maximum yield of 60–70 wt% (dry matter), together with char and gas [9]. These byproducts can be burnt externally (to dry the biomass, in cocombustion etc.) or in the process, to supply the endothermic reaction heat of pyrolysis. Char in itself has a heating value (z30 MJ kg1) [2,3,8] comparable to petroleum cokes (‘‘petcoke’’), and can externally be valorised. The moisture content of the pyrolysis feedstock shall not exceed 10 wt% to limit the water content of the bio-oil. The heart of the fast pyrolysis process is the reactor. Although it probably represents only 10–15% of the total capital cost of an integrated system, almost all research and development are focused on the reactor, since subsequent techniques (separation, condensation etc.) are known as physical and chemical processes. Also the rest of the process (biomass reception, storage, handling, drying and milling, product storage and possible product upgrading) applies known techniques [2]. Within the common pyrolysis reactor designs, mostly fluidized beds are applied, operated in either bubbling or circulating mode: these systems can meet the basic requirements to achieve high oil yields i.e. short residence time, fast heat transfer and fast separation of vapour and char. The technological strength and the market attractiveness (Fig. 2) show that the circulating fluidized bed (CFB) has the highest commercial potential. The expected breakthrough of the CFB is conditioned by a better understanding of the hydrodynamics (flow regimes, residence time) and reaction kinetics of the biomass pyrolysis at different heating rates [10]. pyrolysis
gasification
bio-oil
fuel gas
storage
chemicals turbine engine
combustion
heat
electricity or CHP
boiler
Fig. 1. Alternative thermochemical conversions of biomass [2].
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Fig. 2. Status of the pyrolysis reactors [7].
The production of bio-oil by pyrolysis is a technology on the edge between development and demonstration. The most important technological issues that remain to be solved before large-scale use becomes possible, are related to the complex kinetics and reactor hydrodynamics [10]. The study of the basic kinetics is the priority of the present research, which intends to make an experimental and theoretical contribution to the development of pyrolysis. It therefore concentrates on the determination of (i) the operating conditions required in relevant experimental research and large-scale pyrolysis of biomass; (ii) the kinetic constants of biomass pyrolysis; and (iii) the reaction heat of biomass pyrolysis and other thermal parameters required in the heat balance of the pyrolysis reactor.
The three competitive primary reactions are (i) the fragmentation to hydroxyacetaldehyde and other carbonyls, acids and alcohols; (ii) the depolymerisation to levoglucosan and other primary anhydrosugars; and (iii) the dehydration to char, gases and water. Dehydration dominates at low temperatures (<623 K) and slow heating rates. At higher temperatures, depolymerisation and fragmentation are dominant. Depolymerisation dominates at temperatures between 573 and 723 K, while fragmentation has its optimum at around 873 K. However, inorganic species such as cations, bases, acids and salts can have a major impact on these reactions [15,16]. The selectivity of cellulose for fragmentation and depolymerisation can be as high as 70–80 wt% [17]. 2.2. Hemicellulose Hemicellulose binds the cellulose microfibrils of the cell wall. It varies in structure and composition (mainly xylan) and has a low thermal stability [18,19,20]. It is decomposed in a way similar to cellulose: by dehydration at low temperatures (<553 K) and depolymerisation at higher temperatures [15]. Dehydration yields anhydride fragments, water soluble acids, char, gases and water, while depolymerisation yields volatile organics, levoglucosan and other anhydrohexoses, levoglucosenone and furans [18]. Here too, inorganic impurities cause fragmentation effects, but xylan is specifically sensitive for cations, because of its ion exchange sites.
2. Biomass pyrolysis reactions
2.3. Lignin
Biomass mainly consists of three components: 30–60% cellulose, 20–35% hemicellulose (polysaccharides) and 15–30% lignin (a polymer of methoxy substituted cyclic organics), together with some resins and minerals. Pyrolysis of biomass is very complex due to the diversity, the heterogeneity and the limited thermal stability of some of the components. Pyrolysis of biomass yields about the products expected from the pyrolysis of its three constituent components separately, despite synergetic effects [11]. The study of the individual components thus forms the basis of the expected reaction pathways and determines the occurring primary and secondary reactions. The order of these reactions, the reaction rate and the yields depend on parameters such as heating rate, temperature, pre-treatment, catalytic effects etc. [2,10,12,13]. The study of the reactions and the effect of the parameters are of critical importance to drive the reactions to high yields of the desired products and to avoid side reactions.
Lignin is the strengthening component of the cell wall and is mainly present in woody biomass [19]. It is a polymer of hydroxy and methoxy substituted propyl phenol units [18]. The very complex structure depends on the plant species and due to its structural diversity, pyrolysis yields various products (such as catechols, vanillins, aromatic carbonhydrates). At low temperatures (<773 K) dehydration dominates, while at higher temperatures a diversity of lignine monomers is formed. Above 973 K the monomers are decomposed and enter the vapour phase. Lignin is thermally more stable than cellulose and hemicellulose and yields more char and a higher fraction of aromatic compounds [15,19]. In a pyrolysis reactor the biomass is subjected to the whole temperature range and therefore all reactions mentioned above occur to some degree, in function of the heating rate of the whole biomass particle, and thus of the heat transfer and particle diameter.
2.1. Cellulose
2.4. Secondary reactions
Cellulose, (C6H12O5)n, is the main component of plant cell walls and consists of anhydroglucose units, connected by glycoside bonds. Of all lignocellulotic components, the thermal decomposition of cellulose is mostly investigated and best understood. The Waterloo-mechanism (Fig. 3) [14] is widely accepted to represent the reaction pathway, yielding the expected products and defining the effect of temperature and heating rate.
Secondary reactions can occur in the vapour phase or between vapour and solid phase to form mostly gas. This will considerably reduce the desired oil yield [21,22]. Two secondary reactions are of importance for pyrolysis: cracking (>973 K) and the water–gas shift reaction (<1083 K): H2O þ CO 4 H2 þ CO2. These secondary reactions are however limited at temperatures below 923 K, residence times <2 s for the gases and vapours and when char is quickly
fragmentation cellulose
active cellulose (low degree of polymerisation)
depolymerisation dehydration
carbonylcompounds, e.g. hydroxyacetaldehydes, acids and alcohols anhydrosugars, e.g. levoglucosan char, gases and water
Fig. 3. The primary decomposition of cellulose according to the Waterloo-mechanism [14].
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separated. These are the working conditions for commercial pyrolysis. Reforming, carbon–steam and carbon–carbon dioxide conversions are negligible under these conditions [14]. The global pyrolysis mechanism of biomass is mainly based on the pyrolysis of cellulose, although results are similar for wood and other biomass species [10,11]. The Waterloo concept of Fig. 4 is recognised to represent the global mechanism. 3. Materials and methods 3.1. Biomass samples The specific biomass species were selected because of previous literature references, or because of their potential in Europe: spruce, pine, poplar, eucalyptus, sawdust, sewage sludge, straw and the non-harvested stalks and leaves of sunflower and corn. The samples were crushed and dried for over 2 h at 105 C and again 30 min just before thermal processing. 3.2. The kinetic constants of biomass pyrolysis The kinetic constants of the pyrolysis of biomass are determined by thermogravimetric analysis. A Hi-res TGA 2950 Thermogravimetric Analyzer was used at atmospheric pressure with a nitrogen flow of 50 ml min1. The TGA scale was made of platinum and the weight was registered every 0.5 s with an accuracy of 0.001 mg. Following the discussion which is presented in Section 4, all experiments were performed at 100 K min1 and were repeated at least three times. Calculations use the average results, with a standard deviation of maximum 5%. The resulting TGA-profiles plot the weight loss in function of the temperature. 3.3. Reaction heat A DSC Q10 with Refrigerated Cooling System operating within a temperature range from 183 to 823 K was used to measure the heat requirement of the biomass pyrolysis reaction. The experiments were performed from 313 to 823 K, under atmospheric pressure and with nitrogen supplied at 50 ml min1. The DSC scales were aluminium and were not reused. The DSC was calibrated regularly with indium as reference material. The obtained DSC curves give the heat flux (W g1) in function of the temperature or in function of the time (constant DT/Dt ¼ 10 K min1). 3.4. Specific heat of the biomass The specific heat capacity of sawdust, corn residue and poplar were determined at 25 C using a calorimeter (Leybold Heraeus). The temperature increase of n-hexane was measured after addition of preheated biomass.
4. Results and discussion 4.1. The required operating conditions for relevant experimental research and large-scale pyrolysis of biomass 4.1.1. Specific operating conditions The specific working conditions essential to achieve maximal oil yields have already been described in the introduction of this paper and include a reaction temperature around 773 K, temperatures in the vapour phase around 673–723 K; a high heating rate and high heat transfer at the reaction surface; a short residence time, for vapours less than 2 s and a rapid cooling of the vapours and separation of the char, to avoid secondary cracking. Essential for the fast decomposition and high oil yields is the fast heating of the biomass particle. Slow pyrolysis decreases the oil yield considerably in favour of gas and char. Moreover long residence times favour secondary cracking of the oil. Short residence times, rapid cooling of the oil and rapid separation of the char are hence required. This fast heating of the whole biomass particle to a uniform temperature requires a high heat transfer. If the heat supply would be insufficient, important thermal gradients would occur leading to slow pyrolysis and/or incomplete reactions in the core of the particle. Smaller particles have a larger specific surface and are more mobile within the reactor mass, which enhances the heat transfer. Also, temperature gradients in small particles are expected to be negligible, as will be shown below. When choosing the experimental and large-scale parameters, the thermal characteristics of the biomass, the reaction kinetics and the heat transfer will have to be jointly considered. Some basic principles of the reaction kinetics and conversion of solid particles, of heat transfer in powder reactors and of heat penetration inside the particle allow to determine these conditions in anticipation, which makes it possible to program the experiments in a relevant and effective way. This analysis allows at the same time to critically evaluate the literature data: research has not always been performed under these optimum conditions (for example [12]), with results that could lead to wrong conclusions. Recommendations concerning the computational aspects of kinetic analyses were made in a series of articles [24–28], where both one-step/multi-step kinetic schemes and parallel/consecutive reactions are considered. Analysis of the data hereafter by the method of Flynn, Wall and Ozawa reported in [24] demonstrates that the biomass pyrolysis can be considered as a one-step reaction, thus simplifying the treatment, but nevertheless requiring an assessment at different heating rates. The analysis of the effect of heating rate is presented below. 4.1.2. The influence of temperature gradients in the biomass particle Fast pyrolysis and its associated maximum oil yield can only be achieved by fast and uniform heating of the biomass particles. To meet these targets, avoiding thermal gradients in and around the particle is very important. Application of the basic principles of both heat transfer to the particle and of heat conduction within the particle, allows to determine the required working conditions. The temperature uniformity throughout the biomass particles can be determined by the heat conduction law of Fourier, here applied in non-stationary regime and for the simple case of a spherical particle:
kp v2 T 2 vT vT þ ¼ rp cp vr2 r vr vt
Fig. 4. The biomass pyrolysis concept of the University of Waterloo [23].
235
!
v2 T 2 vT ¼ D þ vr 2 r vr
! (1)
with kp: heat conductivity of the biomass at temperature T (W m1 K1), cp: specific heat capacity of biomass at temperature T (J kg1 K1), rp: density of the biomass (kg m3), D: thermal diffusivity of the particle (m2 s1).
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The solutions of this unsteady thermal condition are simple when easy boundary conditions can be applied, as will be used further in the text. Solutions are very difficult or impossible for complex boundary conditions and/or bodies of irregular shape. The general solutions are therefore commonly expressed by a dimensional analysis. For a sphere of radius r, initially at temperature T0 and suddenly exposed to surroundings at Ts, the temperature distribution, at any time t and position x, is given by:
Ts T hr t x ¼ f ; D 2; Ts T0 kp r r
(2)
with h: external heat transfer coefficient at surface of sphere (W m2 K1). Since it is important to know the evolution of the temperature in the core of the sphere (Tc at x ¼ 0) and introducing the Biot-number ( ¼ (r/kp)/(1/h)) reduces equation (2) to:
T s Tc t ¼ f Bi; D 2 Ts T0 r
(3)
For large (Bi / N) or very small (Bi / 0) values of the Biotnumber, solutions are graphically given by Carslaw and Jaeger [29]. For applications of biomass particles in a fluidized bed, Biotnumbers are intermediate (Bi ¼ 0.2–1, see further in the text) and extreme conditions do not apply. For these intermediate values of the Biot-number, results are presented by Heisler [30] in the form of charts, expressing (Ts Tc)/(Ts T0) in terms of Dt/r2 (the Fouriernumber) with 1/Bi as parameter. Illustrative application of the chart is given in Table 2, with characteristic properties of sawdust at 773 K, i.e. kp ¼ 0.15 W m1 K1 [31], cp ¼ 1150 J kg1 K1 and rp ¼ 700 kg m3, and for an average fluidized bed heat transfer coefficient at the surface of the sphere i.e. 500 W m2 K1. Since fast pyrolysis requires the reaction to take place within 2– 2.5 s, it is clear that only very small particles will meet the conditions of fast heating to a uniform temperature. In this case, particle sizes above 200 mm diameter (radius 100 mm) already take 0.38 s to warm up, i.e. nearly 15% of the proposed reaction time of 2–2.5 s, thus starting to jeopardize the ideal condition of fast pyrolysis. According to the previous calculations, it can be assumed that the heating rate throughout the particle is uniform when heating very small particles (200 mm) at a rate b (K s1): simplified boundary conditions can hence be applied to Eq. (1), i.e. vT=vt ¼ b for r ¼ 0 to R( ¼ dp/2) and vT=vr ¼ 0 for r ¼ 0 (in the core). The solution is given by Carslaw and Jaeger [29] as:
DTmax ¼ ðTR Tc Þ ¼ bd2p =ð24DÞ
(4)
Eq. (4) can be applied for various values of b and with e.g. the characteristic properties of sawdust mentioned above. The results are a set of curves in function of the diameter as shown in Fig. 5. These calculations show that the temperature differences between the biomass surface and core are very limited, certainly when considering that the surrounding temperature is 773 K and that the heating rate will vary between minimally 1.5 K s1 in a TGA (thermogravimetic analysis) experiment, to a few hundred K s1 in a bubbling or circulating fluidized bed. To obtain a fast heating of the whole particle, it is appropriate thus to use small biomass particles (e.g. <200 mm for saw dust) and no significant thermal gradient will occur in these small particles: the core and the surface of the biomass particle will behave both thermally and kinetically in a similar way. 4.1.3. The influence of heat transfer to the biomass particles Fast pyrolysis requires a fast heat transfer to the biomass. This heat transfer is conditioned by the degree of gas and solid turbulence achieved in the reactor [32,33]. The heat transfer coefficient depends on the gas–solid contacting mode. It ranges from 10 W m2 K1 for a static bed, to 50–100 W m2 K1 in a fixed bed with forced gas circulation (as in TGA), and several hundreds of W m2 K1 for bubbling and circulating fluidized beds. This explains why the fluidized beds are specifically considered as top technologies for biomass conversions. The heat transfer coefficient in a CFB can be estimated from the results of Kobro and Brereton [34]. In a CFB operating with 100 mm sand as bed material at a recommended gas velocity of 5 m s1 and a solid circulation rate of 200 kg m2 s1, the convective heat transfer coefficient is 620 W m2 K1. Since not only external convection but also internal conduction is important, the overall picture is expressed by the Biot-number, which represents the internal resistance to heat penetration divided by external resistance to heat transfer and is written as follows:
Bi ¼
r=kp 1=h
For the 200 mm sawdust particle (r ¼ 100 mm) at 773 K, the values are as follows:
h ¼ 300 W m2 K1
Bi ¼ 100 106 300=0:15 ¼ 0:2
h ¼ 500 W m2 K1
Bi ¼ 100 106 500=0:15 ¼ 0:33
h ¼ 700 W m2 K1
Bi ¼ 100 106 700=0:15 ¼ 0:47
with TR: T at the surface of the particle (at r ¼ R) (K), Tc: T in the core of the particle (at r ¼ 0) (K).
Table 2 Time required for the core of a spherical saw dust particle to nearly (99%) reach the temperature of the surroundings (773 K) where the particle is suddenly submerged in. r (mm)
Bi-number
Dt/r2
t (s)
50 100 200 400
0.17 0.34 0.67 1.33
13 7 5 2.2
0.017 0.38 1.08 2.2
It is assumed that a condition where the core temperature has reached 99% of the temperature of the surroundings (Ts z 773 K) meets conditions of uniform temperature T throughout the sphere (i.e. Tc z 0.99 Ts). The initial T of the sphere is 293 K and D z 1.86 107 m2 s1.
Fig. 5. Maximum temperature difference (DTmax) between the surface and core of the sawdust particle in function of the particle diameter at different heating rates (b).
M. Van de Velden et al. / Renewable Energy 35 (2010) 232–242
The Biot-number is <1 in all cases, implying that the external resistance associated with convectional heat transfer largely dominates. Bi is only >1 for coarse particles and/or too high values of h. This result confirms Fig. 5 where increasing DTmax values are noticed with increasing b values (achieved at high convection heat transfer rates). 4.2. The selection parameters for the conversion of biomass in a powder reactor 4.2.1. The kinetic expression for the conversion of an individual biomass particle Occurring reactions during pyrolysis are endothermic, as will be further determined from DSC (differential scanning calorimetry) measurements. As illustrated in Fig. 4, pyrolysis of biomass progresses along a reaction path with primary and secondary reactions, each reaction with its specific expression for the rate of product formation [35,36]. The present paper considers the overall decomposition rate of the biomass particle only, expressed by a first order reaction. The overall reaction rate constant, k, is however the sum of the reaction rate constants of the parallel primary reactions. The experimental data moreover enable the reader to apply a more complex kinetic assessment, if so desired. In the present
237
assessment, the kinetics are expressed by a continuous reaction mechanism for the small particles under consideration: the temperature distribution across the small particle is quasi-uniform so that the pyrolysis reaction proceeds at the same rate everywhere throughout. The slowest step in the process is the heat transfer through the external film around the particle. For small particles, the time needed to reach a certain conversion (including complete conversion) increases therefore in a way that is approximately linear with the particle size dp [33]. For relatively large particles, the internal heat conduction resistance is larger, leading to temperature gradients inside the particle, as mentioned above. The rate of pyrolysis is now limited by the slower heat conduction process, whereby the time needed to reach a particular (or full) conversion increases as d2p. This distinction is demonstrated in Fig. 6 where the time of complete conversion, as determined from TGA experiments, is illustrated for sieved biomass fractions of sawdust (0–200 mm) and eucalyptus (0–600 mm). The continuous reaction model can hence certainly be used for the description of the conversion of smaller biomass particles, expressed for a first order reaction as:
1 X ¼ expðktÞ
Fig. 6. Time for complete conversion as function of the biomass particle size for (a) sawdust, (b) eucalyptus.
(5)
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The apparent and measured rate constant, k, should be expressed in terms of the rate of heat penetration (kht) and the intrinsic or chemical reaction rate constant (kr) [33,37–39]:
1=k ¼ 1=kr þ 1=kht
(6)
Table 4 The total heat requirements for pyrolysis. The total heat requirement for pyrolysis sample
Heat requirement for pyrolysis (J g1)
pruce eucalyptus poplar sawdust corn sunflower straw sewage sludge
342 400 207 434 267 292 375 385
with kht: rate of heat penetration, equal to ham/cp (s1), h: heat transfer coefficient from reactor bulk to the particle (W m2 K1), am: specific surface area of the particle per unit weight (m2 kg1), cp: specific heat capacity of the particle (J kg1 K1). Heat transfer limitations are negligible in operations achieving a high value of kht and reaction rates are then kinetically controlled only (k z kr) if additionally the internal heat conduction resistance is negligible (small particles). Experiments should therefore be preferably performed under such conditions. Such high values of kht and thus of h can only be obtained in bubbling or circulating fluidized beds. Since the use of small particles is required and the residence time needs to accurately controllable, the circulating fluidized bed is the most appropriate reactor for biomass pyrolysis. As will be shown in the experimental results in further sections, the reaction rate constant is a strong function of the temperature, and follows the Arrhenius equation (7). The activation energy has a constant but different value for each specific biomass.
(see Section 4.2.4). The required heat of reaction is hence C.m0 (J). The heat balance then results in equation (8a).
k ¼ A expðEa =RTÞ
k cp b lnð1 XÞ=ð0:7CXÞ
(7)
Table 3 compares kht and k for a few target samples, whereby equation (6) is used to determine the error on k if no heat transfer limitations are accounted for. The error is insignificant, even at high temperatures, indicating that virtually no external heat transfer limitations are present under the experimental conditions used. 4.2.2. The influence of the heating rate in TGA experiments The external heating rate b (K s1) can be programmed in TGA experiments, and literature data are available for b values between 0.033 K s1 and 1.67 K s1. Experimental values of k vary significantly, even for a single biomass species. The influence of the heating rate is of paramount importance since the heat transferred must be sufficient to supply the total endothermal heat of reaction (C, J kg1). A too low heating rate (low value of b) will retard the pyrolysis reaction and will yield an apparently lower value of the reaction rate constant. The anticipated definition of the required minimum b value can prevent faulty experiments and related experimental data. During TGA-pyrolysis, reactions take place between approx. 473 and 673 K (average 573 K). The relationship between sufficient heat supply and reaction yield can be expressed in terms of kinetic and thermal data, as determined below. The first order kinetics relate conversion (X), reaction rate constant (k) and time (t), according to expression (5). Since the heat supplied should at any time and for any conversion X exceed the heat consumed, a heat balance can be made. The heat supplied during the reaction equals m0cpbt (J), with m0 the initial amount of biomass (kg). The char yield is on average 30 wt%. Heat requirements (C) were measured by DSC
m0 cp bt 0:7m0 XC
(8a)
Inserting the value of t from Eq. (5), the general equation (8b) is obtained.
cp b lnð1 XÞ=k 0:7XC
(8b)
or
(8c)
This equation can thus be applied to any values of conversion (X). In the present research, overall values of k are measured, i.e. at nearly complete conversion of the biomass (Table 4). Equation (8c) is hence applied using these experimental k values and a conversion of 95% is assumed to be the measure of complete pyrolysis. Equation (8c) is then reduced to:
k 4:5cp b=C
(8d)
It should be noted that for very low conversions (X / 0) a more restrictive condition on k is obtained since the initial reaction rate is highest. This condition of very small conversions will however not be applied in the present treatment since only overall k values are known. This approach was tested using the TGA results of Section 4.2.3 below, obtained at different heat supply rates and for various biomass species. Illustration for spruce is given in Fig. 7, which clearly indicates that relevant k values can only be obtained if b exceeds 80 K min1. Application of Eq. (8d) at an average 573 K, with a cp of 1.08 kJ kg1 K1 (at 573 K) and with the experimentally determined C values (Table 4) demonstrates that the theoretical value of b should exceed 79 K min1, in line with the experimental findings. Similar calculations were performed for the different biomass species, using the experimentally determined char yield and C
0,025
k (s-1)
0,02
Table 3 Effect of the heat penetration upon the reaction rate constant (k). T (K)
573 673 773
Saw dust (85 mm)
Eucalyptus (37.5 mm)
Corn (50 mm)
k
kht
error on kr (%)
k
kht
error on kr (%)
k
kht
error on kr (%)
0.01 0.12 0.68
5.88 5.47 5.12
0.2 2.1 11.8
0.01 0.21 1.52
26.67 24.81 23.19
0.05 0.8 6.1
0.02 0.27 1.59
24.00 22.33 20.87
0.1 1.2 7.1
0,015 0,01 0,005 0 0
20
40
60
80
100
120
140
heating rate (K min-1) Fig. 7. The effect of the heating rate upon the reaction rate constant for spruce.
M. Van de Velden et al. / Renewable Energy 35 (2010) 232–242 Table 5 Required minimum heating rates for an accurate measurement of the reaction rate constant. Biomass
k at 573 K (s1)
bmin (K min1)
spruce eucalyptus poplar sawdust corn sunflower straw sewage sludge
0.0200 0.0138 0.0116 0.0111 0.0242 0.0856 0.0350 0.0067
79 64 28 56 75 289 152 30
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biomass and represents 25–35 wt%, with the exception of corn with only 10 wt% residue, and sunflower and sewage sludge, where the char fraction is significantly higher. The high ash content of sewage sludge is related to its well-known high content of minerals. The results correspond fairly with previous literature data, i.e. 21% char for spruce [40], 30% for straw [40,41] and on average between 19 and 26% [42]. From the dynamic TGA experiments, the reaction rate constant k can be determined by Eqs. (9)–(11), as used in e.g. [10–12,43].
dXðtÞ=dt ¼ k Xp XðtÞ
(9)
with values. A heating rate b of 80 K min1 is sufficient to meet the required reaction heat (Table 5) for most biomass species. All experiments were therefore performed at 100 K min1. To accurately measure the kinetics, heating rates should exceed 100 K min1 for sunflower residue and straw. The previous calculations demonstrate that literature data obtained at lower heating rates (e.g. [12]) should be considered with caution and should not be compared to results obtained under relevant conditions. 4.2.3. Kinetic analysis The obtained TGA-profile plots, shown in Fig. 8, are similar for all tested biomass samples. The results show that pyrolysis occurs in the temperature range between 473 and 673 K, depending on the type of biomass. This observed temperature range was also found in literature data [10,12,40]. It is obvious that the pyrolysis reaction produces a solid residue or char, noticed by a final limited loss of weight per unit of time. This char consists of minerals and the organic coking-residue of the
XðtÞ ¼ ðm0 mðtÞÞ=m0
(10)
Xp ¼ ðm0 mN Þ=m0
(11)
with m0: initial weight of biomass (at t ¼ 0) (mg), mN : residual weight of biomass after the reaction (mg), m(t): weight of biomass at time t during the experiment (mg). As the weight of the sample is continuously registered, X and Xp are known parameters and the reaction rate constant can be derived from Eq. (9) at every moment. Because of the known temperature profiles of the TGA (known DT/Dt), the reaction rate constant is also known as a function of the temperature. The Arrhenius Equation (7) can be applied after transformation and plotting of the results as ln(k) versus 1/T. The activation energy and the pre-exponential factor can be derived from respectively the slope and the intersection of the obtained straight line with the ln(k)-axis. Results for the different biomass species at b ¼ 100 K min1 are illustrated in Fig. 9 and summarised in Table 6, together with some literature data.
a
-2 0,0015
0,0016
0,0017
0,0018
0,0019
0,002
lnk
-3 -4 -5 -6 -7
1/T (K-1) spruce
lnk
b
-2 0,0015 -3
poplar
eucalyptus
0,0016
0,0017
corn
sun flower
0,0018
sawdust
0,0019
0,002
-4 -5 -6 -7
1/T (K-1)
Fig. 8. TGA plots for (a) spruce, eucalyptus, poplar and sawdust, and (b) corn, sunflower, straw and sewage sludge.
straw
sewage sludge
Fig. 9. Temperature dependency of the rate constant of biomass pyrolysis at a heating rate of 100 K min1 for (a) spruce, eucalyptus, poplar and sawdust, and (b) corn, sunflower, straw and sewage sludge.
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Table 6 Kinetic constants from own experiments and literature. Ea (kJ mol1)
Sample Own experiments
Thurner and Mann [43] Gorton and Knight [44] Nunn et al. [45] Reina et al. [12] Di Blasi and Branca [46]
Spruce Eucalyptus Poplar Sawdust Corn Sunflower Straw Sewage sludge Oak Hard wood
A (s1) at 100 K min1
k (s1) at 773 K
68.4 86.4 54.1 75.8 77.0 63.9 76.3 45.3
3.47 104 1.06 106 1.00 103 9.12 104 2.55 105 2.48 104 3.16 105 8.95 101
0.824 1.52 0.219 0.684 1.59 1.19 2.21 0.078
106.5
2.47 106
0.156
1.48 10
6
1.31
69.1
3.64 10
4
0.775
95.4 95.4
2.40 105 2.4 105
89.52
Gum tree, hard wood Forest wood Beech
Table 7 Specific heat at 298 K.
0.0852 0.156
The activation energy of a specific biomass is nearly constant (and representative for the biomass type) at different heating rates (Fig. 10), as can be seen from the parallel ln(k) versus 1/T lines. Fig. 10 shows however that the pre-exponential factors, and thus the reaction rate constant depend on the heating rate. When the heating rate increases, the pre-exponential factor A (and thus k) increases first quickly to then reach a maximum value: at low heating rates insufficient reaction heat is supplied for the endothermic reaction, as was previously discussed. Although the obtained k values are nearly constant when obtained at high heating rates (b) during the TGA experiments, the b values are still of the order of 1–2 K s1 only (due to TGA limitations). In fast pyrolysis, biomass particles are subjected to heating rates in excess of 100 K s1. Section 4.2.2. discussed the influence of the heating rate on the biomass conversion in view of balancing the heat supply rate with both the heat required for pyrolysis and progress of conversion. This balance is met at b w 1–2 K s1. Heating rates in excess of 100 K s1 can only be examined in pyrolysis reactors such as fluidized beds. This study is ongoing at present, where the batch pyrolysis of biomass is studied. Results will be reported in the near future and are essential to evaluate the possible extrapolation of the current data and kinetic expressions to systems with a fast heating rate. From these A and Ea values, the rate constant of the pyrolysis reaction can be calculated at any temperature, and values at 500 C
-2 0,0015 -2,5
0,0017
0,0016
Sample
cp (kJ kg1 K1)
Poplar Sawdust Corn residue
1.2 1 0.9
(the optimum T in pyrolysis reactors) are included in Table 6. Literature data should be used with caution: although data for k, A and Ea are given, both the method and procedure used, and the heating rates are generally not included. It is however clear that the Ea values are in the range of those of the biomass components at their respective wt%: 195–213 kJ mol1 for cellulose, 105– 111 kJ mol1 for hemicellulose and 34–65 kJ mol1 for lignin [47], each present in biomass for around respectively 45, 30 and 25%. Moreover, it should be mentioned that the k values, with the exception of poplar and sludge, exceed 0.5 s1 corresponding to a fast reaction of the biomass. A high conversion can thus be achieved in short reaction times so that the possible occurrence of side reactions is limited. 4.2.4. Thermal analysis The thermal parameters are necessary to formulate the heat balance of the pyrolysis reaction. Since pyrolysis, as every cracking reaction, is an endothermic reaction, the reaction heat is to be supplied indirectly (heat exchangers) or directly (e.g. by preheating the bed material or the fluidization gas). Usually the combustion of pyrolysis gas supplies the required heat (C, J g1) and the combustion gases form the oxygen-free carrier gas. The heat required to pyrolyse biomass contains two components (Eq. (2)): the heat required to heat the biomass to the temperature at which
0,0018
-3
lnk
-3,5 -4 -4,5 -5 -5,5 -6 -6,5
1/T (K-1) 50 K min
-1
80 K min-1
100 K min-1
120 K min-1
Fig. 10. Temperature dependency of the reaction rate of the pyrolysis of spruce at different heating rates.
Fig. 11. DSC curves (10 K min1) for (a) spruce, eucalyptus, poplar and sawdust, and (b) corn, sunflower, straw, sewage sludge.
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The calculated heat requirements for pyrolysis of the tested biomass species are presented in Table 4. These values are the average of three experiments, with a deviation of approximately 6%. The end temperature of pyrolysis was obtained from the TGA experiments. The results are in the same order of magnitude of literature data: 500 kJ kg1 for straw [38], 400–450 kJ kg1 for cellulose [41,50], 235 kJ kg1 [51] and 418 kJ kg1 [52] for unspecified wood and 120– 360 kJ kg1 for various wood species [42]. 5. Conclusions
Fig. 12. Heat requirement for pyrolysis in function of the temperature for (a) spruce, eucalyptus, sawdust, poplar, and (b) corn, sunflower, straw, sewage sludge.
pyrolysis occurs and the heat required for the endothermic pyrolysis as such. Hence the required heat can be written as follows:
C ¼ mb cp;b ðTr T0 Þ þ mb DHr
(12)
The temperature at which pyrolysis occurs, the specific heat capacity cp and the reaction heat DHr (J g1) required for the pyrolysis itself were experimentally investigated and will be discussed hereafter. The specific heat capacity cp (J g1 K1) of the biomass samples under scrutiny is presented in Table 7. Since cp values increase with increasing temperature, the value at 500 C will be about 15% higher than the experimental value at 298 K. Results are close to literature data, which at 773 K vary between 1.1 kJ kg1 K1 [48] to 1.2 kJ kg1 K1 [49]. The obtained DSC curves, giving the heat flux (W g1) in function of the temperature or in function of the time (constant DT/ Dt ¼ 10 K min1) are shown in Fig. 11. The DSC curves show that the heating process is clearly in the domain of heat requirement, which confirms the endothermic character of pyrolysis. Integration of these heat fluxes over time (Eq. (13)) gives the total heat requirement C (J g1) as a function of the temperature (Fig. 12). This includes the heat required for heating the biomass as well as the heat for the pyrolysis reaction. The endothermic peak at about 100 C, due to the evaporation of residual water, was eliminated before integration. When the pyrolysis is completed, the heat requirement decreases. Declining values before the end of the reaction and beyond the normal temperature range of pyrolysis can be explained by the occurrence of secondary reactions (e.g. the aggregation of the char).
dC dT ¼ cp þ DHr /C ¼ dt dt
Zt
cp
0
dT þ DHr dt dt
(13)
Energy from biomass is recognised as the renewable energy source with the highest potential in the near future. Within the thermochemical conversion techniques, pyrolysis offers major advantages through yielding mostly liquid and solid fuel, easy to store and transport. Although the operating conditions for a maximum oil yield are well-known, the kinetics of the biomass conversion need to be determined. The paper considered (i) the required working conditions for relevant experimental research; (ii) the thermogravimetric measurement of the kinetic constants and (iii) the reaction heat of the biomass pyrolysis. A review of the pyrolysis of the main constituents of biomass i.e. cellulose, hemicellulose and lignin, and of the possible secondary reactions stresses the value of the biomass pyrolysis concept of the University of Waterloo. To program experiments in a relevant and effective way, and to critically assess previous literature results, it was demonstrated that tests should be carried out (i) with biomass particles smaller than approx. 200 mm and (ii) in a reaction environment where the particle heating rate is about 80 K min1 and where the internal resistance to heat penetration is smaller than the external resistance to heat transfer (expressed in the condition of Biot-number, Bi < 1). TGA and DSC experiments on a wide variety of biomass particles determine respectively the reaction kinetics and the Arrhenius dependency, and the endothermicity of the reaction. For most of the biomass species, the reaction rate constant is >0.5 s1, corresponding with a fast reaction. The endothermic heat of reaction ranges from 207 to 434 kJ kg1. These results tie in with literature data, although the reader is cautioned in the use of literature data since experiments were often performed at non-representative testing conditions. Both kinetic and thermal data are essential in the design of pyrolysis reactor, determining respectively the required residence time and the amount of heat to be supplied to the reactor. Their use will be illustrated in a further paper where the design strategy of a 10 MW (bio-oil) pyrolyser is presented. References [1] Maniatis K, Guiu G, Reisgo J. The European commission perspective in biomass and waste thermochemical conversion. In: Bridgwater AV, editor. Pyrolysis and gasification of biomass and waste. Newbury: CPL Press; 2003. p. 1–18. [2] Bridgwater AV. Renewable fuels and chemicals by thermal processing of biomass. Chem Eng J 2003;91:87–102. [3] Faaij APC. Bio-energy in Europe: changing technology choices. Energy Policy 2006;34:322–42. [4] Iea – International Energy Agency. www.ieabioenergy.com; 2006 [accessed 10.12.08]. [5] Eurostat. http://epp.eurostat.cec.eu.int; 2003 [accessed 10.12.08]. [6] Nan L, Best G, Coelho C, De Carvalho N. The research progress of biomass pyrolysis processes, integrated energy systems in China – The cold northeastern region experience. FAO – Food and Agricultural Organization of the United Nations, http://www.fao.org/docrep/T4470E/T4470E00.htm; 1994. [7] PyNe – Pyrolysis Network of IEA Bioenergy. www.pyne.co.uk; 2006 [accessed 10.12.08].
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