CO2 gasification rates of char particles from torrefied pine shell, olive stones and straw

Fuel 158 (2015) 753–763

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CO2 gasification rates of char particles from torrefied pine shell, olive stones and straw Oskar Karlström a,⇑, M. Costa b, A. Brink a, M. Hupa a a b

Process Chemistry Centre, Åbo Akademi University, Finland IDMEC, Mechanical Engineering Department, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal

h i g h l i g h t s  Biomass char gasification rates could be predicted with unified activation energies.  Torrefaction of biomass either increases of decreases the reactivity of the char.  At high T, the influence of torrefaction on char reactivity is of minor importance.

a r t i c l e

i n f o

Article history: Received 31 March 2015 Received in revised form 29 May 2015 Accepted 3 June 2015 Available online 10 June 2015 Keywords: Gasification Torrefaction Biomass Char Kinetics

a b s t r a c t Single char particles with a diameter of 8 mm were produced in situ from pellets of raw and torrefied pine shell, olive stones and straw. The single char particles were then gasified at 800, 900 and 1000 °C in two or three CO2 concentrations at each temperature. In the experiments, CO concentrations were measured from the product gases. From the measured CO concentrations, the char conversions versus time were determined. The reactivity of the char from the torrefied olive stones was lower than the reactivity of the char from the raw olive stones, while the reactivity of the char from the torrefied straw was higher than the reactivity of the char from raw straw. For the raw and torrefied pine shell the char reactivities were similar. At 900 °C the influence of torrefaction on char gasification rates was minor, however, since the conversion occurred under Regime II conditions, i.e., the conversion rate is partly limited by mass transfer. A detailed single particle model, taking into consideration mass transfer effects, was used to extract kinetic parameters from the experimentally determined char conversions. For the six chars, activation energies of the adsorption step were in the range 175 and 285 kJ/mol, while activation energies of the desorption step were in the range 145 and 195 kJ/mol. In the study it was also tested whether the char conversion could be computed with unified activation energies for all the chars. The results show that by using unified activation energies – 240 kJ/mol for the adsorption step and 168 kJ/mol for the desorption step – the computed char conversions were in good agreement to the experimental char conversions. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction During the combustion and gasification of solid fuels, the char residue reacts with O2, CO2 and H2O. For coal chars, the carbon (char-C) reaction rates with CO2 and H2O are rapid at high temperatures such as in pulverized fuel combustion [1–3]. Lignocellulosic biomass chars (from e.g. wood, straw) are significantly more reactive than coal chars and the char-C reactions with CO2 and H2O are significant already at 800 °C. Above 800 °C and for particle sizes larger than 1 mm, biomass char-C reactions with CO2 and H2O ⇑ Corresponding author. Tel.: +358 2 215 3275. E-mail address: okarlstr@abo.fi (O. Karlström). http://dx.doi.org/10.1016/j.fuel.2015.06.011 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.

are often so rapid that the conversion rate is limited by the combined effects of chemical kinetics and pore diffusion [4–12], i.e., Regime II conditions. The char reactivity is influenced by various factors such as concentration and type of active sites [13], oxygen concentration in the char (char-O) and catalytic elements such as Na and K [14]. For a given char, the reactivity toward CO2 is generally of comparable magnitude to the reactivity toward H2O, although the latter is slightly higher (often by a factor of 2–5). The char-C reactions with CO2 are frequently described as [15–18]:

C þ CO2 ðgÞ $ CðOÞ þ COðgÞ

ðR1Þ

CðOÞ ! COðgÞ

ðR2Þ

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Nomenclature Aads Ades c c d D E f(X) f H kads kdes K n n_ n_ 000 Nu Q_ r Pr R R Re Sc Sh S t

pre-exp. factor adsorption (1/s) pre-exp. factor desorption (mol/s m3) concentration (mol/m3) specific heat value of the gas mixture (J/kg K) diameter (m) diffusion coefficient (m2/s) activation energy (kJ/mol) surface function (–) objective function specific enthalpy of gas species (J/mol) kinetic rate constant adsorption (1/s) kinetic rate constant desorption (mol/s m3) reaction rate constant (mol/s m3) amount of char (mol) molar flow (mol/s) molar flow per unit volume (mol/s m3) Nusselt number and calculated as Nu = 2 + 0.6Re0.5Pr1/3 heat flow (J/s) distance in radial direction (m) v c q1 Prandtl number and calculated as Pr ¼ 1 kp;1 1 universal gas constant (J/mol K) particle radius (m) ud Reynolds number (–) and calculated as Re ¼ v 1p 1 Schmidt number (–) and calculated as Scj ¼ Dvj;1

Sherwood number (–) 1=3 Shj ¼ 2 þ 0:6Re0:5 Scj Surface area of sphere (m2) time (s)

and

calculated

as

where C(O) refers to an occupied site or a carbon–oxygen surface complex. R1 and R2 have been studied for pure carbons (e.g. [19]), coal chars (e.g. [20]), and biomass chars (e.g. [13]). CO may retard the rate of R1 in the forward direction at high temperatures [15,21,22]. It is challenging to determine kinetic parameters of R1 and R2: effects of the backward rate of R1 are difficult to separate when determining parameters for the forward rate; R1 and R2 may consist of several elementary reaction steps; and various elements catalyze the reactions in different ways. Partly because of this, R1 and R2 are often treated as one global reaction: C + CO2 ? 2CO(g) and by modeling the rate using a power law description with a constant reaction order. The limitation of the global approach is evident: the model is unable to describe the change in effective reaction order as functions of temperature and reactant gas concentration. However, the model is useful in the sense that global activation energies among various chars are easily compared. Global activation energies have been determined in the range from 200 to 250 kJ/mol for coal chars [23–25], but also for a large variety of lignocellulosic biomass chars [11]. This may be considered surprising because of the significantly higher reactivity of biomass chars compared to coal chars. This consistency in activation energies between coal and biomass chars indicates that the char-C reactions are catalyzed in the same way, but that a significantly smaller part of the available active sites are catalyzed in coal chars. The present study investigates whether biomass char gasification by CO2 can be modeled with the same adsorption and desorption activation energies for six biomass chars under Regime II conditions. This question is important since (i) there seem to be little difference in activation energies, (ii) under industrially relevant Regime II conditions the conversion is anyway only limited partly by chemical kinetics and (iii) the numbers of experiments to be

T u U v V X d

q e k

r U X

temperature (°C or K) slip velocity between gas and particle (m/s) velocity of released gas species (m/s) kinematic viscosity (m2/s) volume of spherical layer (m3) degree of char conversion (–) boundary layer thickness (m) density (kg/m3) emissivity (–) thermal conductivity (W/m K) Stefan–Boltzmann constant, 5.67  108 W/(m2 K4) porosity (–) stoichiometric coefficient

Subscripts ads adsorption C char carbon des desorption e external h thermal j species (CO2, CO, N2) j species (CO2, CO) m mass rad radiation p particle w denote layer in particle 1 gas bulk phase 0 initial

conducted increase with the number of parameters to be determined. Computed char conversions are compared to experimental measurements from a single particle reactor. The tested single char particles were produced in situ from pellets of raw and torrefied pine shell, olive stones and straw. Experiments were performed at 800, 900 and 1000 °C in two or three CO2 concentrations at each temperature. This study also investigates whether the torrefaction (mild heat treatment of raw biomass) influences the reactivity of the char residue. Recent studies report that the char reactivity can either increase [8] or decrease due to torrefaction [26–29]. To compare the reactivity of the char from torrefied and raw biomass, gasification rates of single char particles from the torrefied biomass are computed using kinetic parameters determined for the chars from the raw (untreated) biomass fuels.

2. Experiments 2.1. Fuels The investigated chars were prepared from raw and torrefied Pine shell, Olive stones and Wheat straw. The raw biomass fuels were sieved to 1 mm size and then torrefied in an insulated stainless steel container with 3 L/min of nitrogen at 310 °C passed during 1 h. During the process the biomass temperature varied between 280 and 300 °C. On dry basis around 25%, 10% and 20% of the mass was lost during torrefaction of the Pine shell, Olive stones and Wheat straw, respectively. Table 1 lists the investigated fuels. The pellets were produced from the sieved biomass with a pressure of 100 bar without the addition of any binder. The initial diameters of the pellets were 8 mm and the heights were approximately 3 mm. In the experiments, the height of the pellets

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2.3. Experimental conditions

Table 1 Characteristics of the raw and torrefied biomass fuels. Parameter

Pine shell Raw

Volatiles (wt.%, db) 74.0 Fixed carbon (wt.%, db) 24.5 Ash (wt.%, db) 1.5 Moisture (wt.%) 13.9 C (wt.%, daf) 47.8 H (wt.%, daf) 5.6 N (wt.%, daf) 0.3 O (wt.%, daf) 46.3 HHV (MJ/kg) 18.8 m0 (g) 0.275

Olive stones

Wheat straw

Torrefied Raw

Torrefied Raw

Torrefied

70.1 27.9 2.0 1.0 54.4 5.5 0.4 39.7 23.5 0.220

60.8 23.2 16.0 0.3 47.8 5.1 2.3 44.8 20.7 0.220

55.5 24.2 20.3 0.9 44 4.3 0.8 50.9 19.4 0.220

63.8 21.7 14.5 9.4 43.2 5.6 1.9 49.3 17.5 0.260

71.2 12.6 16.2 8.9 39.4 5.2 0.5 54.9 19.0 0.225

increased during devolatilization. For all the investigated biomasses the particles swelled so that the height was approximately 8 mm after the devolatilization. The masses of the raw biomass pellets were chosen so that the energy contents of each fuel pair – raw fuel/torrefied fuel – were similar. In all cases, the initial masses of the raw fuel pellets were slightly higher than those of the corresponding torrefied fuel pellets. 2.2. Experimental setup The experiments were conducted in the Åbo Akademi Single Particle Reactor (see Fig. 1). A detailed description of the reactor and pictures regarding the particle swelling can be found elsewhere [12,30]. The gas was inserted in the bottom of the reactor system and the product gases left the reactor at the top of the reactor system. The sample was inserted into the reactor using a movable probe that could be inserted from room temperature into the hot reactor within a fraction of a second. The movable probe with the sample holder was horizontally inserted into the reactor (see Fig. 1). The sample holder consisted of a thin net on which a single fuel pellet was placed. It is plausible that the net influenced the heat and mass transfer to and from the char particle, but considering that only a part of the particle was in contact with the net, effects due to the net were not believed to play a significant role. CO concentrations of the product gases were measured continuously using an ABB AO2020 analyzer.

The experiments were conducted at 800, 900 and 1000 °C. At 900 and 1000 °C experiments were performed with 13, 34 and 68 vol.% CO2 (hereafter %) in the surrounding gas with N2 as the balance gas. At 800 °C experiments were conducted with 34 and 68% CO2. In general the experiments were repeated three times. Fig. 2 shows a typical example of three char gasification tests of a single raw wheat straw pellet at 900 °C and 68% CO2. In the experiments, the devolatilization lasted for approximately 20– 40 s. In the example given in Fig. 2, the devolatilization is defined by the peak close to the vertical axis (concentration of CO) on the left hand side. After devolatilization, gasification of the char residue started. In this study only the char gasification is considered. By integrating (and normalizing) the char gasification part from the left-hand side of Fig. 2 an experimentally-derived char gasification curve is obtained. This experimentally-derived char gasification profile (hereafter char conversion) is shown on the right-hand side of Fig. 2. Note that the char gasification is defined to start at 0 s on the right-hand side. The carbon balance, i.e., the mass of C calculated based on the measured CO divided by the mass of solid C, is typically more than 0.9. In the end of the devolatilization, the time derivatives of the measured CO signal decreased rapidly, followed by a steep increase in the derivative. The local minimum of the derivatives is taken as the starting point of the char gasification. The end point of the char gasification is defined as when CO is not released anymore from the particle. In the results section the term char gasification time is used. This is the time between when the char gasification starts and when 95% of the char has been consumed. After the devolatilization the particle shape had changed from cylindrical to a cylinder with rounded edges. During the conversion the shape changed and became more and more spherical. Examples of the different char particles inside the reactor are shown in Fig. 1. For simplicity the particle is assumed spherical in the modeling.

2.4. Char-C yields The initial carbon amounts of the char particles were estimated from the measured CO concentrations during the char gasification.

Char parcle from:

Olive stones

Pine shell

Straw

Torrefied O.s

Torrefied P.s.

Torrefied straw

Fig. 1. Single particle reactor and snapshots of particles during char gasification.

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1

3.5

900 °C 68 % CO2

cCO (vol.%)

2.5 2

0.8

run 1

Char conversion

3

run 2 run 3

1.5 1 0.5

0.6

900 °C 68 % CO2

0.4 0.2 0

0 0

200

400

600

0

200

400

600

time (s)

time (s)

Fig. 2. Left: measured CO concentrations of three experimental runs at 900 °C and 68 vol.% CO2. Right: char conversion versus time derived from the measured data on the left-hand side.

Here nC is the amount of carbon on molar basis; subscript 0 refers to initial; and total refers to the total amount of carbon in all layers. As the degree of conversion in one layer is >0.999, that layer disappears and, as a result, the particle shrinks. In a layer w the amount of carbon at a given time is given by

35%

char-C yield

800 °C 30%

900 °C

25%

1000 °C

nC;w ðtÞ ¼ nC;w ðt  DtÞ þ Dt  n_ C;w

20%

The char gasification rate is given by

15%

n_ C;w ¼ V w n_ 000 C;w

10%

0%

n_ 000 C;w ¼ Xf ðX w ÞK CO2

Fig. 3. Char-C yields of the chars.

Fig. 3 shows the char-C yields as a function of gas temperature of the reactor. In general the char-C yields decrease slightly as the temperature increases. 3. Model To model the char-C reactions with CO2, the simplified reaction mechanism R1 and R2 is used. Based on R1 and R2, the reaction rate constants K CO2 (mol/m3 s) for char consumption by CO2 can be expressed in a Langmuir–Hinshelwood form as

1 kads;CO

1þk

ð1Þ 2

des;CO2

cCO2

where k = AeE/RT is an Arrhenius rate; E is the activation energy (kJ/mol); and A is the pre-exponential factor (1/s for Aads, mol/(s m3) for Ades). R1 is modeled as a lumped reaction in the forward direction. The char particle is assumed spherical and divided into 40 layers. In a discretized layer w in the particle, the degree of char-C conversion, Xw, at a given time is given as

Xw ¼ 1 

nC;w nC;0

ð2Þ

and the total char-C conversion for the particle at a given time is given by

X ¼1

nC;total nC;total;0

ð5Þ

Here V is the volume of layer w (m3); and n_ 000 C;w refers to char-C consumption rate per unit volume (mol/m3 s) in layer w and can be expressed as

5%

K CO2 ¼ cCO2 kads;CO2

ð4Þ

ð3Þ

ð6Þ

Here X is a stoichiometric ratio: 1 for CO2; K CO2 is the reaction rate constant for char consumption by CO2 and f(X) is the surface function [12]. The surface function is defined in the following way: at any degree of conversion, X, the surface function is defined as the rate occurring under kinetically limited conditions divided by the initial rate (X = 0) occurring under kinetically limited conditions. Thus for X = 0, f(X) = 1 and for 0 < X < 1, f(X) > 0. To calculate the char consumption rate in Eq. (6), concentrations of species and temperature as a function of particle radius are needed. The concentrations of species and temperature in the particle and outside the particle are calculated from

  dcj 1 d SðrÞDj þ Ucj SðrÞ ¼ n_ 000 j SðrÞ dr dr

ð7Þ

and

! j   imax max X 1 d dT 1 d X þ n_ i Hi ¼ n_ 000 SðrÞk j Hj SðrÞ dr dr SðrÞ dr i¼1 j¼1

ð8Þ

Here S(r) is the surface area of a sphere with the radius r; c is the concentration (mol/m3); U is the velocity of gases flowing; inside the particle k is the effective thermal conductivity of the solid particle and outside the particle k is the mixture-averaged gas thermal conductivity (W/m K); and Hj is the specific enthalpy of a gas species (J/mol). The initial porosity was around 90% and the effective thermal conductivity was estimated following Thunman et al. [31]. The heterogeneous and homogeneous reactions are taken into account by n_ 000 j which is the consumption or production rate of species per unit volume (mol/m3 s). In Eq. (7), the second term on the left-hand side accounts for effects of the Stefan flow. The velocity, U, is calculated based on the boundary condition that the total pressure in the entire particle equals to 1 atm. Outside the particle, Dj (m2/s) equals the diffusion coefficient for species j in a

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multicomponent gas mixture [32]. Inside the particle the diffusion coefficient is calculated as [16]:

Dj ¼

U 2

Dj;1

ð9Þ

where U is the porosity of the char. The initial porosity is calculated based on the size of the particle and by assuming that the true density is 1800 kg/m3. By using this assumption, the initial porosity is between 0.83 and 0.93 of the different chars. In the modeling, the diffusion in the particle is proportional to U/2. As a result the diffusion coefficients are similar for a given gas composition but inside the different chars. At the external surface of the particle, the heat transfer is calculated by conduction and convection according to Eq. (8) and by radiation. Radiation heat transfer (J/s) is calculated as

  Q_ rad ¼ Se re T 4rad  T 4p

ð10Þ

Here Se is external surface area of the particle; r is the Stefan– Boltzmann constant (5.67  108 W m2 K4); e is surface emissivity, here set to 0.6 [33]; and Trad is the radiation temperature. The radiation temperature is assumed to be the same as the temperature of the furnace walls. The radiation is modeled considering a small convex object in a large enclosure [34]. The apparent one-dimensional boundary layer thicknesses for the concentration, dm,j, and temperature dh, are defined as

dm;j ¼

dp Shj  2

ð11Þ

particle temperature was set to 500 °C and the term d(mcpT)/dt was added to the left hand side of Eq. (8). With this model, the time to reach the steady state temperature profile was around 5 s. Since this time is always short in comparison to the total char gasification time, the quasi steady state assumptions of Eq. (8) is justified. The kinetic parameters needed in Eq. (1) were determined by minimizing the residuals between the computed and experimental char conversions according to the least-squares objective function:

f min ¼

XX 2 1 f v max zmax v z v ;z

!1=2 ð17Þ

;

where

f v ;w ¼ ðX mod  X expt Þv ;z :

ð18Þ

Here, the subscript mod stands for modeled and expt for measured; and v, z refers to the vth point of the experimentally-derived char conversion for the zth experimental test condition. In the procedure, parameters were determined according to two hypotheses as follows. Hypothesis 1. All the kinetic parameters differ for the six chars.

Hypothesis 2. The adsorption activation energy is the same for the six chars and the desorption activation energy is the same for the six chars. These activation energies are called unified activation energies. The pre-exponential factors of the six chars differ.

and

dh ¼

dp Nu  2

4. Results

ð12Þ

where dp is particle diameter (m); Sh is the Sherwood number; and Nu is the Nusselt number. At the investigated temperatures, the superficial gas velocity in the reactor is between 0.14 and 0.17 m/s. The Reynolds number for the flow in the reactor is around 40 and the Reynolds number for the flow around the initial particle is around 10. Note that the temperature and each species have different apparent boundary layer thicknesses: species with a higher diffusivity have a thicker apparent boundary layer. The boundary conditions for Eqs. (7) and (8) are at r = 0:

dci ¼0 dr

ð13Þ

dT ¼0 dr

ð14Þ

at r = R + dm,j

cj ¼ cj;1

4.1. Fuel specific vs unified kinetic parameters – comparing model and experiments Table 2 shows the fuel specific and the unified kinetic parameters. The fuel specific activation energies of the adsorption step (Hypothesis 1) lie between 175 and 285 kJ/mol, while those of the desorption step lie between 145 and 195 kJ/mol. Thus, the activation energies of the desorption step are lower than the activation energies of the adsorption step. This implies that the rate of the adsorption step increases more rapidly than that of the desorption step as the temperature increases. As a result the effective reaction order decreases as temperature increases: the reaction order of the desorption step is zero, while the reaction order of the adsorption step is one, and the effective reaction order is closer to the reaction

Table 2 Kinetic parameters.

ð15Þ

and at r = R + dh

T ¼ T1

ð16Þ

In Eq. (15), the concentration of the species in the bulk gas at the reactor inlet is used. This can be justified by the fact that the production rates of gases during the char conversion were always low compared to the total gas flow. In the case the influence of the released gases on the bulk gas composition would be significant, another modeling approach would be required, resolving the concentration and velocity field in the reactor. In the experiments, when the char gasification begins it is likely that the particle temperature is above 500 °C since the devolatilization has completed. To evaluate effects of the heating, the following separate modeling exercise was done: the initial

a b c

Eadsa

Edes

Aadsb

Adesc

Fuel specific E Olive stones Torrefied olive stones Pine shell Torrefied pine shell Straw Torrefied straw

255 260 285 269 175 196

145 170 185 195 163 150

2.34E+13 4.73E+13 2.99E+14 6.34E+13 3.13E+09 4.11E+10

2.24E+08 4.24E+09 1.37E+10 4.64E+10 1.60E+09 5.69E+08

Unified E Olive stones Torrefied olive stones Pine shell Torrefied pine shell Straw Torrefied straw

240 240 240 240 240 240

168 168 168 168 168 168

4.34E+12 5.05E+12 1.93E+12 2.46E+12 4.57E+12 5.75E+12

3.69E+09 4.25E+09 2.53E+09 2.79E+09 3.51E+09 5.35E+09

E (kJ/mol). Aads (1/s). Ades (mol/s m3).

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order of the slowest step. In recent works, desorption activation energies for biomass char gasification by CO2 have been determined in the range 120–421 kJ/mol [21]. Thus, the desorption activation energies of the present study are within the range of the reported desorption activation energies. The adsorption activation energies of the present study are not directly comparable to reported values, since in this study the adsorption step is modeled as a lumped step in the forward direction, while both steps (forward and backward) generally have been considered. Adsorption activation energies in the forward direction have been determined in the range 100–245 kJ/mol [21]. Activation energies of the backward direction have proven to be difficult to determine – several times negative activation energies have been determined [11,21]. In the present study, the unified adsorption and desorption activation energies (Hypothesis 2) were determined to be 240 and 168 kJ/mol, respectively. Fig. 4 shows computed and experimental char gasification times using the fuel specific activation energies and the unified activation energies for the six investigated chars at 800, 900 and

15000

1000 °C. There is a good general agreement between the unified model and the experimental data, implying that the unified activation energy can be used to predict the char conversions under the investigated conditions. It can be seen that the chars of the raw and torrefied pine are the least reactive, while the chars of the raw and torrefied straw are the most reactive. To demonstrate the conversion in detail, Fig. 5 plots computed and experimentally derived char conversions at the investigated conditions for the raw olive stones char using fuel specific kinetic parameters on the left hand side (Hypothesis 1) and using the unified activation energies on the right hand side (Hypothesis 2). In both cases there is good agreement between computed and experimental char conversions. This is not only because the unified activation energies are relatively similar to the fuel specific activation energies, but also because the conversion occurs under Regime II conditions – the conversion is not solely limited by chemical kinetics. For all the chars, the observed reaction orders are low: the char gasification rate is weakly/moderately dependent on the gas concentration. Generally the effective reaction order in the interval

34

15000

34 68

68

13

34 68

time (s)

time (s)

Pine shell char

5000

800 °C

1000 °C 5000

4000

4000

Olive stones char

34

2000

900 °C

13 34 68 1000 °C

13

Torrefied olive stones char

68

68

1000

3000 2000

13 34 68 13

1000

34 68 13 34 68

34 68

0

0 900 °C

800 °C

800 °C

1000 °C

900 °C

1000 °C

3000

3000

34 68

2500

Straw char

1500

13

1000

34

500

34

68 13 34 68

Torrefied straw char

68

2000

time (s)

2000

34 68

34 time (s)

time (s)

900 °C

5000

2500

13

0 800 °C

time (s)

5000

13 34 68

0

3000

Torrefied pine shell char

10000

10000

1500

13

1000

34 68 13

500

34 68

0

0 800 °C

900 °C

1000 °C

800 °C

900 °C

1000 °C

Experiments with 13, 34 or 68 vol.% CO2 Model Model using unified activation energies Fig. 4. Computed and experimental char gasification times when 99% of the char has reacted for the six investigated chars at 800, 900 and 1000 °C with 13, 34 or 68 vol.% CO2.

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1

0.8

0.8

0.6

0.6

X

X

1

13 % CO2 34% CO2 68% CO2

0.4

0.4 0.2

0.2

800 °C

0

0 0

1000

2000

0

3000

1000

2000

3000

time (s)

time (s)

0.8

0.8

0.6

0.6

X

1

X

1

0.4

0.4

0.2

Expt Model

0.2

900 °C

0

0 0

500

1000

0

500

1000

time (s)

time (s)

0.8

0.8

0.6

0.6

X

1

X

1

0.4

0.4

0.2

0.2

1000 °C

0

0 0

500

1000

time (s)

0

500

1000

time (s)

Fig. 5. Computed and experimental char conversion versus time at 800, 900 and 1000 °C. Left: adsorption and desorption activation energies determined for olive stones char. Right: unified adsorption and desorption activation energies.

between 13% and 34% CO2 is higher than the reaction order in the interval between 34% and 68% CO2 as for example can be seen in Fig. 5. This cannot be predicted by simple global kinetic expressions that are frequently used, but can be explained by Langmuir–Hinshelwood type of kinetic expressions such as the one used in the present study. As the CO2 concentration of the gas increases, more of the char surface becomes occupied by surface complexes and the effective reaction order with respect to the gaseous reactant decreases. 4.2. Influence of torrefaction on char reactivity The char yields of the pairs – raw fuel char vs. torrefied fuel char – differ and, therefore, the char gasification times are not directly comparable. To evaluate the effect of torrefaction on char reactivity the following was done: char gasification rates of torrefied fuels were computed using parameters of the raw fuel, i.e., pre-exponential factors and surface function, but with the unified activation energies. Fig. 6 shows computed and experimental char gasification times for the torrefied fuels. It can be seen that the computed char gasification times for the torrefied pine shell are very similar whether using parameters of the raw pine shell char

or of the torrefied pine shell char. For the torrefied olive stones the char gasification times are slightly underpredicted by using the parameters for the raw olive stones char, while for the torrefied straw the predicted char gasification times are slightly overpredicted by using the parameters of the raw straw char. Thus, the char reactivity increases due to the torrefaction of the straw, while the char reactivity decreases due to the torrefaction of the olive stones. As a result, it seems that the char reactivity can either decrease or increase due to torrefaction. This can be explained by that several competing processes influence the char reactivity. For example, the heating rate of the torrefied fuel may be slightly higher than that of the raw fuel since less material undergoes devolatilization, and it is well known that the heating rate influences the char reactivity. Moreover, more volatiles are released within a very short time for the raw fuel compared to the torrefied fuel, and this might open up the char structure differently. Other possible factors influencing the char reactivity are oxygen content of the char and concentration of Na and K [35]. In the literature no data on reactivity of char from torrefied Olive stones, Straw or Pine Shells was found. As pointed out, the gasification reactivity of char residues from torrefied fuels can differ from the reactivity of chars from

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12000

time (s)

Torrefied pine shell char

34

4000

68

9000 6000 13

3000

34

68

34

3000 2000 13

34

1000 13

34

900 °C

1000 °C

800 °C

3500

time (s)

13

34

68

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Fig. 6. Computed and experimental char gasification times of torrefied biomass chars when 95% of the char has reacted at 800, 900 and 1000 °C. The same adsorption and desorption activation energies are used for all chars. In the middle bar, pre-exponential factors determined for the torrefied chars are used. In the right hand side bar, preexponential factors of the raw chars are used.

corresponding raw fuels. However, Fig. 6 shows that the predicted char gasification rates are very similar at 900 °C regardless of whether kinetic parameters from the raw fuel char or from the torrefied fuel char are used. This can be explained by that the torrefaction only influences the char reactivity slightly and by that the conversion occurs under Regime II conditions, i.e., the conversion is partly limited by mass transfer. An implication from this is that in the design of biomass gasification processes for torrefied biomass, the influence of torrefaction on char gasification rates may be neglected, provided that the conversion is partly limited by mass transfer.

4.3. Occurrence of char gasification in Regime II conditions Fig. 7 shows modeled concentration profiles of CO2 inside and outside the olive stones char particle at 800 and 1000 °C when 10% of the char has reacted. At 1000 °C, concentration gradients inside the particle are steep, but also at 800 °C small concentration gradients exist. Thus, the char gasification occurs under Regime II conditions. Fig. 7 also shows the particle temperature and the gas temperature outside the particle. Note, that these temperature profiles are quasi-steady state solution. As expected, the particle temperature is lower than the gas temperature because of the

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Fig. 7. Computed concentration and temperature profiles for the olive stones char at 800 and 1000 °C when 10% of char particle has reacted in the conditions of the single particle reactor.

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Fig. 9. Global kinetic char gasification rates of biomass chars (left) and Langmuir–Hinshelwood char gasification rates of this study. The gray area in the figure on the right hand side represents the range of kinetic rates of the global rates on the left hand side.

endothermic reactions. Further, the temperature difference between the particle and the gas bulk phase is more significant at 1000 °C than at 800 °C due to the higher char gasification rate at the higher temperature. The figure also illustrates that significant concentrations of CO exist inside the char particle, while in the used model inhibition of CO on the adsorption rate is not explicitly included; the CO inhibition effects are incorporated in the lumped description of the adsorption step. To evaluate the influence of diffusion, the following additional computations were done: char gasification times were calculated assuming that the process is kinetically limited. This was done by giving the diffusion coefficients arbitrarily high values. Fig. 8 shows experimental and modeled char gasification times assuming kinetic control for the Olive stone char. The figure shows that the gasification rates are overestimated in all cases assuming kinetic control. This implies that the char gasification occurs under Regime II conditions. The differences in gasification times are smallest for the lowest temperatures, as expected since the mass transfer becomes more important at higher temperatures. Surprisingly the diagram shows that the assumed kinetic and experimental char gasification times are similar at 1000 °C and 68% CO2. The reason for this is that the char gasification reactions are endothermic, i.e., the higher the gasification rate, the lower the particle temperature. At sufficiently high char gasification rates and sufficiently low particle temperatures, the char gasification rate is not strongly dependent on the diffusion. Conclusively, under some conditions when the mass transfer is expected to play an important role, it may be sufficient to assume kinetic control. In many engineering applications the char particle temperature is unknown. Fig. 8 also shows char gasification times by assuming

that the particle temperature equals the gas temperature and assuming kinetic control. With these assumptions it can clearly be seen that char gasification times always decreases significantly compared to the experiments. As a result, in predicting char gasification times it is extremely important to account for effects of reaction enthalpies in order to obtain reasonable particle temperatures. The same computations as performed for the Olive Stone in Fig. 8 were also performed for the other biomass chars. The conclusions regarding the role of diffusion for these chars are the same as for the Olive Stone char. 4.4. Comparing the reactivity of the chars to other studies To compare the char reactivities of the present study to reported char reactivities, the following is done: the kinetic contribution of the Langmuir–Hinshelwood mechanism of this study is compared to global kinetic char gasification rates taken from the review study by Di Blasi [11]. Those expressions are available elsewhere [5,21,36–40]. Fig. 9 shows kinetic rates of this study and of those studies for temperatures below 1000 °C assuming 1 atm CO2 in the surrounding gas following Di Blasi [11]. In this figure all the global char gasification rates are based on parameters determined at similar temperatures as those in the present study and the global rates are of the type r = f(pCO2n)  f(k). The figure shows that rates of this study are within the range of rates of the global kinetic expressions. In this study, the heating rate during the char preparation is relatively high, and it is surprising to see that the kinetic char gasification rates according to some of the global expressions are several magnitudes higher than those of the present study. One explanation for this scatter is that some of the global expressions

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were derived at conditions with CO2 pressures at almost two orders of magnitudes lower than 1 atm. It is possible that those global expressions strongly overestimate char gasification rates with CO2 pressures of 1 atm. Fig. 9 also shows that the slopes of the global expressions are linear, while the slopes of the Langmuir–Hinshelwood kinetics are non-linear. This is natural due to the different activation energies of the adsorption and desorption steps in the Langmuir–Hinshelwood mechanism, in combination with similar magnitudes of adsorption and desorption rates. Frequently it is pointed out that global kinetic expressions should not be used at other concentrations that those used when deriving the kinetic data. In addition to this we argue, based on the right hand side of Fig. 9, that the global kinetic expressions should neither be used for large temperature ranges. 5. Conclusions The present study investigated char gasification kinetics at Regime II conditions for raw and torrefied pine shell, olive stones and straw. Computed char conversions were compared to experiments at 800, 900 and 1000 °C in 2–3 CO2 concentrations at each temperature. The following conclusions can be drawn: (1) The char conversions could be computed by using unified adsorption and desorption activation energies. The unified activation energy of the adsorption step was determined to be 240 kJ/mol and the unified activation energy of the desorption step was determined to be 168 kJ/mol. (2) The char reactivity of the torrefied olive stones was lower than the char reactivity of the raw olive stones, while the char reactivity of the torrefied straw was higher than the char reactivity of the raw straw. For the raw and torrefied pine shell the char reactivities were similar. Thus, the torrefaction did not influence the reactivity of the chars in any systematic way. (3) Although the torrefaction influenced the char reactivity, gasification rates of 8 mm char particles from torrefied biomass could be predicted by using kinetic parameters of the char from the raw biomass at 900 °C. This can be explained by that the char conversion was partly limited by mass transfer. An implication from this is that in the design of a biomass gasification process, the change in char reactivity due to torrefaction may not be of importance, provided that temperatures are sufficiently high and that the external surface area of the particle is sufficiently large. (4) The char reactivities were in good agreement to reported biomass char reactivities. However, while reported char reactivities generally are proposed so that the logarithmic rate is proportional to the inverse temperature, it is here argued that this extrapolation is unreasonable. Since activation energies of adsorption and desorption steps differ, while adsorption and desorption rates are of comparable magnitude, the logarithmic kinetic rate cannot vary linearly as a function of 1/T. (5) At 800 °C the char gasification occurred already in Regime II. In this case, the computed concentration gradients of the gaseous products and reactants were small, indicating that the conversion was close to Regime I conditions. At 1000 °C, steep concentration gradients existed inside the particles, strongly implying Regime II conditions. Acknowledgements This work has been partly carried out within CLIFF (2014–2017) as part of the activities of Åbo Akademi University. Other research partners are VTT Technical Research Centre of Finland Ltd,

Lappeenranta University of Technology, Aalto University and Tampere University of Technology. Support from the National Technology Agency of Finland (Tekes), Andritz Oy, Valmet Technologies Oy, Foster Wheeler Energia Oy, UPM-Kymmene Oyj, Clyde Bergemann GmbH, International Paper Inc., and Top Analytica Oy Ab is gratefully acknowledged. As part of the BRISK project (Biofuels Research Infrastructure for Sharing Knowledge) the work at the SPR were conducted during a research visit at Åbo Akademi University.

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