product composition and kinetics of flash pyrolysis of erica arborea (biomass)

Journal of Analytical and Applied Pyrolysis, 29 (1994) 73-87 Elsevier Science B.V.

73

Product composition and kinetics of flash pyrolysis of Erica Arboreu (biomass)

A.A. Zabaniotou *, D. Gogotsis and A.J. Karabelas

Department of Chemical Engineering and Chemical Process Engineering, Research Institute, P.O. Box 1517, 54006 Thessaloniki, Greece

(Received June 21, 1993; accepted November 8, 1993)

ABSTRACT

Erica Arborea flash pyrolysis experiments were performed in a captive sample reactor. The effects of temperature and moisture content on the yields and compositions of the products were investigated. The particle size of Erica Arborea, the temperature of pyrolysis, the residence time and the heating rates were varied in the ranges 100-500 pm, lOO-7OO”C, 0.1 - 1 .O s and 100- lSo”C/s respectively. The experiments were carried out at atmospheric pressure under nitrogen. Elemental analysis was carried out on the char, and the heat content of the char was estimated. Char yield decreases with temperature, approaching the asymptotic value of 23 wt.% (dry wood) at 5Oo”C, while the total gas yield increases with tem~rature. Tar yield shows a maximum of 45 wt.% (dry wood) at approx. 400°C. The gases produced from the pyrolysis of Erica Arborea consist mainly of H,, CO, COz, CH4, C2H, and C,H,. The main products are CO and CO,. Both increase with temperature, the latter increasing at a lower rate. The heat content of the char varies from 24.500 to 30.200 kJ/kg. Experiments combined with the Anthony and Howard model allowed us to estimate the kinetic parameters of the total weight loss and of the CO, CO,, CH,, C,H, yields.

Biomass; captive sample reactor; Erica Arborea; kinetics; pyrolysis.

INTRODUCTION

The Pyrolysis of biomass to produce pyrolytic oil (a mixture of organic chemicals with water), low heat content gas, and charcoal is an ancient art. Biomass consists of cellulose, lignin and carbohydrates. In the literature, studies have been reported on the pyrolysis of cellulose and lignin sepa- rately, as well as for various types of wood and other biomass. Sys- tematic studies on the rapid pyrolysis of cellulose in a captive sample reactor have been performed by ~ajaligol et al. [l] for the determination of the effects of temperature, residence time, and heating rates on yield and product composition.

’ Corresponding author.

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74 A.A. Zabaniotou et al. / f. Anal. Appl. Pyrolysb 29 (11994) 13-87

Mok and Antal [2] have studied the effects of pressure on biomass pyrolysis in a tubular, laminar flow, microreactor system. Knight et al. [3] have undertaken research to determine the conditions for optimum oil production in the entrained-flow pyrolysis of biomass. The effects of moisture and ash content on the pyrolysis of wood waste were investigated in a batch fluid-bed reactor under helium by Gray et al. [4]. A continuous fluidized bed bench scale atmospheric pressure flash pyrolysis process of Aspen-Poplar wood has been demonstrated at a fluid rate of 15gjh by Scott and Piskorz [5].

European researchers, such as Lede et al. [6], Font et al. [7], Maniatis and Buekens [ 81, Zaror et al. [ 91, Vasalos et al. [lo], and Beaumont and Schwab [ll], have shown special interest in wood pyrolysis as an alternative energy supply method. Funazukuri et al. [ 12] have used a Pyroprobe 100 solid pyrolyser to determine the composition of volatile products from fast cellulose pyrolysis.

This paper presents recent results from a research programme involving experimental measurements and associated modelling efforts to determine how reaction conditions influence the pyrolysis of biomass produced in Central Greece. The objective is to produce charcoal and tar.

Biomass characteristics

Samples of Erica Arborea were prepared by cutting and crushing pieces of a tree trunk supplied from Central Greece, followed by separation of fractions of desired particle size (18-80 U.S. standard mesh). The elemental analysis of the samples was carried out using a LECO COR Analyzer CHN-800. Some characteristics of the Erica Arborea samples are presented in Table 1. Determination of ash and moisture in feed samples was performed according to ASTM standards (D-l 102-84 for ash in wood and

TABLE 1

Characteristics of Erica Arborea

Moisture (wt.%) Ash (wt.% mf)

Elemental analysis (% mt) C H N 0 (by diff.)

Heating value (kJ/kg)

26 0.6

51.0 6.2 1.0

41.9

20.580

A.A. Zabaniotou et al. 1 J. Anal. Appl. Pyrolysis 29 (1994) 73-87 15

D-2016-74 for moisture content of wood), in our laboratory. The density of Erica Arborea particles was determined to be approx. 0.9 g/ml.

Apparatus and procedure

A captive sample technique, similar to that previously employed in our laboratory for the pyrolysis of Greek lignite, was used (Fig. 1). The procedure followed is described elsewhere [ 131.

A set of experiments was carried out for the determination of total weight loss in the temperature range lOO-7OO”C, at a heating rate of 15O”C/s and residence time of approx. 1 s, using a dry feed. Another set of experiments was performed in the temperature range 400-600°C in order to investigate the effect of temperature and moisture on the yields and compositions of products. For all experiments, particles of size 100-500 pm were used.

Elemental analysis of biomass and charcoal was carried out using a LECO COR Analyser CHN-800 for various pyrolysis temperatures. The heat content of charcoal (Table 2) was estimated using the following equation [ 141:

Q = 146.58(C) +568.78(H) -51.53(O)

Fig. 1. Schematic diagram of flash pyrolysis reactor.

76 A.A. Zabuniotau et al. / J. Anal. Appl. Pyrolysis 29 (1994) 73-87

where Q = gross heat content (BTU/lb), and C, H, 0 are, respectively, the amount of carbon, hydrogen and oxygen in the sample, in weight percent.

For the calculation of the heat content of the gaseous products, the heats of combustion of the individual gaseous products were taken into consider- ation (Table 3).

RESULTS AND DISCUSSION

Temperature eflect

Data showing the effect of temperature on total weight loss, and on the yields of gases, tar, liquid hydrocarbons and char, are presented in Figs. 2 and 3, respectively.

Figure 2 shows that pyrolysis starts at approx. 200°C with the appearance of the first products, presented in the Figure as total weight loss. A dramatic increase in weight loss is observed with tem~rature, approaching the asymptotic value of 80 wt.% at approx. 350°C. The yield of char decreases with temperature as Fig. 3 shows, and approaches the asymptotic value of approx. 23 wt.% at 350°C. The total amount of gases increases rapidly with

a

60 - w l @@--- 70 -

60 -

50 -

0 100 200 300 400 500 600 700 800 900

Peak Temperature (OC)

Fig. 2. Effect of temperature on flash pyrolysis total weight loss from dry Erica Arboreu (biomass) : ( --) simulated data; ( 0) experimental data.


80 0

60

1

40 -

30-

20-

lo-

0 1 , , , , , ? ,oqo “I “popo~0 . 0 100 200 300 400 500 600

PeakTemperature (OC)

Fig. 3. Effect of temperature on flash pyrolysis yields (biomass): (0) char; (A) tar; (0) liquids; (A) gases.

TABLE 2

Elemental analysis and heat content of charcoal

700 800 900

of products from Erica Arborea

T W> c (%) H (%) 0 (%) Q(kJ/kg x 103)

270 79.25 1.46 17.79 26.5 295 70.20 2.92 25.32 24.5 320 71.79 3.30 23.76 25.7 364 83.95 1.57 13.24 28.8 380 82.31 2.47 14.26 29.3 396 86.35 1.12 10.39 29.3 435 83.42 0.62 13.31 27.3 447 86.50 1.23 10.91 28.2 460 85.50 0.67 12.26 27.4 472 84.87 0.101 12.81 38.4 508 81.11 2.17 15.35 28.4

532 82.58 1.71 14.36 28.4

556 84.13 1.80 12.83 26.8

578 84.79 1.02 12.90 28.4

604 85.98 0.73 11.68 28.6

620 88.13 0.77 9.51 29.6

640 89.73 0.46 8.31 29.9

652 84.86 0.76 12.85 28.1

660 89.27 0.85 8.74 30.2

78 A.A. Zabaniotou et al. 1 J. Anal. Appl. Pyrolysis 29 (1994) 73-87

100

90 -

80 -

60 -

50 -

40 - _ model

300 400 500 600 700 600 900


Fig. 4. Effect of temperature on CO flash pyrolysis yeilds from Erica Arborea (biomass).

(- ) simulated data; (0) experimental data.

TABLE 3

Element analysis and heating values of gases

T (“C) c (%) H (%) 0 (%) Q @J/W

320 32.08 21.81 46.11 1228 364 39.78 11.07 49.15 858.9 380 41.93 8.36 49.71 756.8 396 44.83 9.69 45.48 856 435 43.53 6.48 49.99 755.6 447 41.79 7.51 50.70 849.7 460 42.31 6.74 50.95 825 472 41.07 9.71 49.22 1022 508 41.31 7.17 51.52 966 532 42.33 6.00 51.67 915 556 41.86 6.39 51.75 998 578 42.53 7.75 49.72 1140 604 42.38 6.39 51.23 1091 620 42.58 6.94 50.48 1163 640 42.11 8.17 47.92 1318 652 41.73 8.46 49.81 1363 660 43.67 6.30 50.03 1189 680 43.68 6.24 50.08 1225 705 53.61 6.30 50.09 1283


temperature at low temperatures ( -350C), the rate becoming slower at higher temperatures (350-799°C). The tar yield of Erica Arborea, as shown in Fig. 3, exhibits a maximum of 45 wt.% at about 500°C and then decreases at higher temperatures, indicating the start of secondary reactions. Liquid HC increase slowly with temperature to approx. 3 wt.%.

Gas chromatographic analysis shows that the gaseous products consist of CO, HZ, CO*, CH4, C2H4 and C2H6. The main component is CO, the yield of which increases sharply with temperature above 400°C (Fig. 4). CO2 increases with temperature to a maximum of approx. 10% at 600°C (Fig. 5). CH, also tends to increase with temperature to a value of 2% at approx. 450°C (Fig. 6), while C,H, increases to an asymptotic value of approx. 2 wt.% at 450°C (Fig. 7).

E,fect of moisture content

Pyrolysis of a wood particle is a complex phenomenon. It involves inter- and intraparticle chemical reactions, and the escape of volatiles through the particle.

In order to contrast the behaviour of dry and wet biomass, a set of experiments was carried out using wet feed. Data concerning the effect of

500 600 700

PeakTemperature (OC)

800 900

Fig. 5. Effect of temperature on CO2 flash pyrolysis yields from Erica Arboreu (biomass).

(- ) simulated data; ( l ) experimental data.

80 A.A. Zabaniotou et al. / J. Anal. Appl. Pyrolysis 29 (1994) 73-87

model

300 400 500 600 700 800


Fig. 6. Effect of tem~rature on CH, flash pyrolysis yields from Erica Arborea (biomass).

f -) simulated data; ( l ) ex~rimental data.

In order to contrast the behaviour of dry and wet biomass, a set of experiments was carried out using wet feed. Data concerning the effect of moisture on the yields of char, gaseous products and tar are plotted in Figs. 8-10.

The data show that dry biomass gives approx. 25% yield of char at 4OO”C, while “as received” (wet) biomass gives, at higher temperature (65O”C), a lower yield of approx. 15%. The same trend is observed for the gaseous products; dry biomass gives approx. 35% of gaseous product at 6OO”C, while wet biomass gives a lower yield of approx loo/o. The opposite trend is found in the case of liquid products (tar and water); dry biomass gives a yield of approx. 42% at about 650-7OO”C, while wet biomass gives approx 65%.

To explain the influence of moisture content on product yield, one may first recall that the pyrolysis process is delayed due to the additional heat (as compared to dry biomass) required for water evaporation before reaction temperatures are reached. Additionally, as described by Alves and Figueiredo. [ 151, some complex processes may take place during the pyrolysis of wet biomass, related ot the presence of free and bound water within each particle. Such processes may include the following steps: (1) conduc-


600


Fig. 7. Effect of temperature on C&H6 flash pyrolysis yields from Ericu Arboreu (biomass). (-) simulated data; ( l ) experimental data.

boundary (higher temperature), (3) bound and free water movement outwards by diffusion and capilary action; (4) water varpour movement by convection and diffusion. After pyrolysis starts at temperatures greater than 200-25O”C, volatile products tend to move outwards, while water vapour (step 4) may move towards the inner part of the particle, enhancing intraparticle transport of volatiles and leading to an increase of secondary reactions. The latter may be responsible for the reduction of gaseous product yield and for the increase of pyrolysis liquids, in the case of wet biomass.

Kinetic modelling

The pyrolysis process is quite complex because biomass consists of two main types of compound, i.e. cellulose and lignin. Different kinetic models have been suggested for each of these over the years; this adds complications to the interpretation of data from biomass pyrolysis experiments.

A.A. Zabaniotou et al. 1 J. Anal. Appl. Pyrolysis 29 (1994) 73-87

80 -

70 -

60 -

50 -

40 -

0 dry biomass

0

0 0

0 0

wet biomass

J

30 - 0

.a. 0

20 - - ti l .wm,

IO- 0°0*

0 100 200 300 400 500 600 700 800 900


Fig. 8. Effect of moisture on char yield of flash pyrolysis from Ericu Arborea (biomass).

The most widely accepted kinetic model for the thermal decomposition of cellulose at low to intermediate temperatures considers a simple competition between two parallel pathways [ 161:

Cellulose c

Char, H20, CO, CO;?, Tar

Shafizadeh et al. [ 171 earlier proposed that three pathways described the pyrolysis of cellulose:

/ Char, H20, CO, CO2

Cellulose + Tar \ Gases low-molecular-weight volatiles.

Anthony and Howard [ 181 have based a model on a single first-order decomposit$m:

Cellulose A product i

If i denotes one of these reactions then the rate of volatile production is

dV --! =K,(V,” - Vi) dt

Ki = I& exp( - Ei /RT) (2)

A.A. Zabaniotou et al. /J. Anal. Appl. Pyrolysis 29 (1994) 73-87 83

100 1

90 1 1

80 -

t/l % 70 -

‘5, 5 60 - -z e a 50 -

2 $ 40 - 3 dry biomass 0 00

/0..

c_!l 00

30 - 0.. Ho

0

20 - 0

lo- 0

0

o 0 wet biomass

0 I . 1 I . I * I 300 400

Fig. 9. Effect of moisture on gas yield of flash pyrolysis from Ericu Arboreu (biomass).

where Vi is the amount of volatiles at time t of the reaction i, V” is the maximum amount of volatile due to reaction i, Kj is an Arrhenius type kinetic constant, Ki, is the pre-exponential factor of reaction i, and Ei is the activation energy of reaction i.

500 600 700


800 900

In order to integrate eqn. (l), the temperature-time history of a particle during pyrolysis must be known. Our experimental system can attain a nearly constant heating rate until a final temperature is reached; thus we consider an average value corresponding to the experiments reported here:

dT - = m = constant = 122S”C/s dt

(3)

The experimental data for yields of products were fitted to this model by employing a non-linear least-squares curve fitting. The same model was used for modelling the flash pyrolysis of lignite carried out in this Laboratory

[131. Following the above procedure, kinetic parameters were obtained con-

cerning Erica Arborea (E, K,, P) listed in Table 4. These parameters were also used to calculate yields of products as a function of temperature. The estimates concerning total weight loss of CO, CO*, CH4 and C2H6 (compared to experimental data) are given in Figs. 4-7.


100

90 -

80-

70- wet biomass

60 - 0.

0

50- 0

40- dry biomass

00°

0@ 0o OOO

00

30- 0 0

0 0 20-

lo-

0 i I I I 1 300 400 500 600 700 800 900

Peak Temperature (“C)

Fig. 10. Effect of moisture on tar yield of flash pyrolysis from Ericu Arborea (biomass).

TABLE 4

Kinetic parameters

Product E (kJ/g mol) log k. (s-1) V” (wt.%) Correlation factor

Total weight loss 57.5 4.01 82.40 0.969 co 95.8 5.59 18.95 0.688 CO* 53.8 2.87 10.31 0.819 CH, 197 11.60 2.63 0.912 C,H, 189.8 10.80 2.11 0.925

Comparison of experimental data and model predictions

The calculated curves fit the experimental data well, suggesting that the single step, first-order kinetic model may be a satisfactory first approximation. Table 5 includes sets of kinetic parameters obtained in this study for Erica Arborea, cellulose and other type of biomass available in the literature __ [1,141.


TABLE 5

Comparison of kinetic parameters of: (1) cellulose rapid pyrolysis [l]; (2) sweet gum pyrolysis [ 141; (3) Erica Arboreu

Product E (kJ/g mol) log k, (s-1) V” (wt.%)

(1) Cellulose (T = 300- 1000°C; heating rate = lOOO”C/s; P = 1.34 atm.; sample size 6 x 2 x 0.0101 cm)

Total weight loss 133.5 8.30 94.08 co 221.5 11.75 21.64 CO* 98.4 5.39 3.08 CI-L 252.2 13.00 2.41 C& 207.1 10.32 2.07

(2) Sweet gum (T = 327- 1127 K; heating rate = 1000 K/s; P = 1.34 atm.; sample diame- ter = 45-88 pm)

Total weight loss 69.3 4.53 92.97 co 61.3 3.36 17.05 CO, 60.1 3.79 1.91 CH, 69.7 11.60 2.63 CI-W, 80.6 4.41 1.17

(3) Ericu Arborea (T = 400-700°C; heating rate = lOO-15O”C/s; P = atmospheric; sample idameter = 100-500 pm)

Total weight loss 57.5 4.01 82.40 co 95.8 5.59 18.95 CO* 53.8 2.87 10.31 CH, 197 11.60 2.63 WI, 189.8 10.80 2.11

Among the products studied here, CO2 has the smallest values of kinetic parameters. The same observation is made for cellulose and sweep gum. Kinetics parameters for total weight loss are lower than those reported by other investigators [ 1,141 for other types of wood, probably because of differences in wood composition. It appears [ lo- 131 that an increased lignin content is associated with reduced values of kinetic parameters. Forest biomass, e.g. Ericu Arborea, has a relatively high content of lignin and may result in the smaller parameters observed here.

CONCLUDING REMARKS

The yield of biomass products depends on experimental conditions, such as temperature, heating rate and moisture content, as well as composition of the biomass.


Several investigators [ 1,10,14] observed a maximum yield of liquids, but the temperature at which this maximum occurs as well as its value vary, depending on the type of biomass and especially on the lignin content. Forest biomass (such as Erica Arborea) is reported to have a relatively high lignin content, which may to some extent account for the increased production of pyrolysis liquids obtained in this study.

In general, high-temperature pyrolysis favours the production of gaseous products, whereas low temperatures favour increased char yields. The experiments reported here show that flash pyrolysis of wet (“as received”) biomass at relatively high temperatures results in higher yields of liquid products and in lower yields of char and gaseous products, as compared to these obtained with dry biomass. The complex intraparticle processes that involve bound and free water (moisture) promote secondary reactions, and may be responsible for the observed yield maximization of biomass pyrolysis liquids.

A simple (single step) first-order kinetic model appears to provide a satisfactory first approximation for the flash pyrolysis of Erica Arborea. The kinetic parameters, obtained by using this model, are generally lower than those corresponding to the literature data for cellulose, probably because of the increased lignin content of forest biomass.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the financial support of ETVA (Greek Bank for Industrial Development) and the General Secretariat for Research and Technology of Greece.

REFERENCES

1 M.R. Hajaligol, W.A. Peters J.B. Howard and J.P. Longwell, Ind. Eng. Chem., Prod. Res. Dev., 21 (1982) 447.

2 W.S. Mok and M.J. Antal, Thermochim. Acta, 68 (1983) 155. 3 J.A. Knight, C.W. Gordon and R.J. Kovac, Biomass, 6 (1984) 69. 4 M.R. Gray, W, H. Concoran and G.R. Gavalas, Ind. Eng. Chem., Process Des. Dev., 24

(1985) 646. 5 D.S. Scott and J. Piskorz, Can. J. Chem. Eng., 60 (1982) 666. 6 J. Lede, J. Panagopoulos, H.Z. Li and J. Villernaux, Fuel, 64 (1985) 1514. 7 R. Font, A. Marcilla, J. Deresa and E. Verdu, Ind. Eng. Chem. Res., 27 (1988)

1143. 8 K. Maniatis and A. Buekens, Research in Thermochemical Biomass Conversion, Elsevier,

London, 1987, 179. 9 C.A. Zaror, IS. Hutchings, D.L. Pyle and H.N. Stiles, Fuel, 64 ( 1985) 990.

10 I.A. Vasalos, M.C. Samolada and G.E. Achladas, Biomass Pyrolysis for maximizing Phenolic Liquids, in A.V. Bridgewater and J:L. Kuester (Eds.), Proc. International Conference, Phoenix, AZ, 2-6 May 1988, Elsevier, Amsterdam, 1988, p. 251.

11 0. Beaumont and Y. Schwab, Ind. Eng. Chem., Process Des. Dev., 23 (1984) 637.


12 T. Funazukuri, R.R. Hudgins and R.I. Silveston, Ind. Eng. Chem., Process Des. Dev., 25 (1986) 172.

13 A.A. Zabaniotou, A.A. Lappas, and N. Kousidis, J. Anal. Appl. Pyrolysis., 21 (1991) 293.

14 T.R. Nunn, J.B. Howard, J.P. Longwell and W.A. Peters, Fundamentals of Thermo- chemical Biomass Conversion, Elsevier, London, 1985, p. 293.

15 S.S. Alves and J.L. Figueiredo, Chem. Eng. Sci., 12 (1989) 2861. 16 M.J. Antal, Jr., in K.W. Boer and J.A. Duffie (Eds.), Advances in Solar Energy,

American Energy Society, Golden, CO, 1983. 17 F. Shafizadeh and A.G.W. Bradbury, J. Appl., Polym. Sci., 23 (1979) 1431. 18 B.D. Anthony and J.B. Howard, AIChE J., 22 (1976) 625.

product composition and kinetics of flash pyrolysis of erica arborea (biomass)

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