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Pyrolysis of poplar bark
P\-ROLYSIS
OF POPLAR BARE
ISTRODUCTIOS
In the pyrolysis of lignocellulosic mater-i& the general spprosch has been to carq- out the reaction at low temperature and crack the vapors evoked. The resulting char x-as subsequently reacted by steam [l-3]. The gaseous products were rich in carbon dio.xidc. Substantisl amounts of liquid znd residuals were also formed. On the other hand. direct pyrolysis of these materials at higher temperatures produced h_\-drogen2nd carbon monxide with considerable amounts of carbon dia.tide and methane [4-6j. In the present studF. the latter approach is used for the pyrolysis of bark to produce gases for energy application. Tran and Rai [7] have reviewed the kinetic studies of several investi_eators on these materials for the design of a suitable pyrol~-sis reactor. These studies were carried out under different expertmental conditions with various materials (bark. wood. cotton, etc.) thereb>- giving widely varying kinetic parameters. e.g. activation energies ran@ns from 14.6 to 227.3 kJ_,*‘mole(7j. Furthermore. most of these studies were based on weight loss data (thcrmogravimetric analysis and fluidized bed) with little or no attention given to the effect of various parameters on the composition of the _gas formed. The weight loss data may or may not correIate accurately with the production of various gases. as the pyolysis of such materials is reported [S] to be 9 complex process involving many parallel and consecutive reactions. Since one of the primaty interests in pyrolysis is the production of gases of higher
c;tlorific value. the determination of the kinetics of the production of these ceases will be more useful for eqineerin_p use. The present paper reports the kinetics of the production of CO. CK, and CO, from poplar bark in the temperature range of 57’3-1023 K and the effect of reaction temperature on the calorific values. yield and composition of cpases alonce with carbon conversion. ESPERI!ilESTAL
Ths hybrid pophr bark i DS-211 was supplied by Forinrck Labs. (Ottawa. Canada! and hr?s the following clcmtnt;ll composition in F (w_,~~“w:C. 43.3 K. 5.9: S. 0.1: 0. 2.3.5: ;?sh. 6.2. The weight perantapes of bark constituents are @\-en be!ow: Sucsessivc S0lvcnt
extrasrives
Summ3tivc r;. . w_.“W
anal_\-& cc. \v_. \v
Constituent
The bark ~-3s shredded in a blender. sieved dried by blow-in_e air wer it at 333 K fcr 20 h.
t ~2-1-12
me=h Tyler)
3x3
.A schematic d&ram of the experimental set-up is &\-en in Fig. 1. The system is designed for operation 3t atmospheric pressure and is flexible with respect to control of experimental variables includiq (1) sample size. (2.1 steam flow-rate. (3) temperature and 14) recovery of ge.s and chars produr‘td after gAfication_ The temperature profile in the reactor ~3s measured e.spcrimcntatly and it 11-3s found that the reactor ~-3s isothermal over 3 4.0 cm zone. The screen was placed in this zone and the samples used in the esperimsnts stayed well within this zone duriq e_xpsrimentation. .A weighed amount of bark sample ( 1.0- 125 ~1)was first held between the two ball ~31~~s and wacustcd for three hours to-remove the trapped air. The reactor was brought to the desired temperature and the system pursed with helium. .A helium flow-rate of 11.6 cm’,JJs (at 29s K) was stablished through a water saturator mtintaiincd at constant temperature. This produced 3 steam flow-rate of 1.4 g/k. The experiment s-35 initiated b_v allowing the sample to fall Gn the screen under gravity and starting the stop-watch. The
RESCL-IS
ASD
Gas produciion
DECX-SSIOS
rmes
The principal gases produced b>- pyrolysis at !o\vtr teaperature I < 73 Kr are CO. CO, and CH, while consider;lbit amounts of hydmgcn zrt pr\+
ductd along with the above gases at higher temperatures. Consequenti_\-. the pas production r ttes were measured only for CO. CO, and CH,. Typica! grr~ production profiles for these gases are shotin in Fig. 2. The zero time in the figure refers to the start of the reaction at the screen placed in the isotherms1 zone of the reactor. Iiates of gas pwduction decrease sharp&- in the beginning and become steady later.
The gss yields wee
determined by integrating the gas production profi!es ever a time of 90 s. These art listed in TabIs 1. Gas Fields increase with inaessing temperature. The composition of gases was determined considerin_e complete pyroi_vsis (90 5). The variation in the composition of gases with temperature is shown- in Fig. 3. Although the volume percentage of CO. dc- wi:h increzsine tsmpersture. The steep increase in the smount of hydrogen partI_\- seams t;
?--@ _”
c-8_ __
=
be due to the carbon-stern reaction at higher temperatures_ Cousks ES] and Sls~ynskii [9) obsen-cd a similar trend in rheir work on wood pyrof_\-sis_
The c3lorific value of gss mixtures were c3lculsred component cslorific values rep@rted in the lirsxture [IO]. mixtures produced incxwz with incresing temper3ture cT3ble 1 ! and 3re comparable to thaw rqorred [S] for w& p-r+-sis prxm
Carbon conversion is defined as the ratio of carbon in the product ~2s~ to the carbon present in the bark. Carbon convcrkns increst U-ith increasing temperature (Table i 1.
~rolysis of wood 3nd other cellulosic marerials h= been studied b>various investigators using various technique w_hich zre reiiewcd In detail by Tran and Rai [7]_ Reaction kinetics based on weigh: loss using non-is* thermal and isothermal methods have been studied. Since the decompasition of these material5 is throue complex re3ctioris involving sever31 p3ralls! 3nd consecutive reactions. the production of various gase?; m3y or m3y not accuratei>- be correlated with such data. Furthermore_ since the prim&y- aim of pyrolysis is to produce gases for energv or synhesis. dertrmin~tion of L!X kinetics of the formation of these gases would be more usefu! for enginesting
applications. Simmons and Sanchez [6] determined the kinetics of the ,eas production from sawdust using a reactor system similar to that used in the present study. However. their approach has one major drawback: it does not take into consideration the in.itiaJ time of reaction in calculating the reaction rate constants. In the present stud? an alternative method subwested b? Juntgen and \-an Hctk [ 111 for coal de\-olatihzation has been used for bark p_vrol_vsis for the first time. This chemical reaction model is based on the generation of particular species from bark. The particle size of the bark studied is in the range where heat transfer and diffusions1 effects are reported [It] to be insignificant. This is further confirmed by the effect of different flow-rates in the reactor. ConsequentI_\:. the pFro&-sis was taken to be chemical reaction controlled. It is assumed in the kinetic anaI_\-,cis that no secondrtc- Sas reactions are taking place and that the production of a prrrticulsr sss is a first order rextL.n. The particles are assumed to be iwthermai during the _ea evofution. Thus. the rate of production of a particular gas is given by: JI’ -=kklI;,-
I’)
dr
IE)
where I _ and Ii are volumes of gas produced at any time t and infmitc time respectively and k is an overall reaction rate constant. Integrarins eqn. I and applying boundary conditions one gets:
L-sins a non-linear least square progam (Box modified Gauss. method) the values of 1.; and k are estimated at various temperatures for CO. CH, and CO,. .A t_\pical plot of actua1 and predicted values of the amount of gases produced using this model is presented in Fig. 4. It is clear from the figure that the proposed model predicts the data well. An .Q.rhenius plot for the formation of CO. CH, and CO, is prsented in Fig. 5. The temperature dependence of rate constants is determined to be as follows: k-o = 111.24) e?rp( - 43.50OjR T ) k c-Zi,= (NM.3) exp( -i9_000,‘RT)
(3)
kc,
c3
=
(
1.32) exp( - 27.000,‘RT)
(41
The rate constants for CO and CO, as expressed by the above equations are iou-er than the values reported in earlier fluidized bed [IL 131 and thermogravimetric analysis [ 14.151 u-o&. It seems that the slou-er rates determined in the present stud? are of a subsequent step to the weight loss step and are more representative of the rate controlling step for the formation of various gases. However. caution should be exercised while comparing resalts from
4’C -
2
f
--_
z --_.
= L
_c-
... ; c _-
‘cr
3:
32 . -. .I”
:c ._‘L
‘5 _/1. -a -
-5
other workers as the experiments were conducrcd using different retc~r concepts and wood species. Also. the thermal histork ef she rextOrs c&d be different. The experimental results obtained in the prssnt w\-i>rk can be =td 10 make reasonable predictions for the formation of CO. CH, rs?d CO,. However. more data are needed to determine rhe raw crf formrrtion of hydrrlgen.
The authors are grartful to the Satw;Ll Science and En_einecing Research Council of Canada for financial aid (GO 123:,.
REFERESCES
226
OF POPLAR BARE
ISTRODUCTIOS
In the pyrolysis of lignocellulosic mater-i& the general spprosch has been to carq- out the reaction at low temperature and crack the vapors evoked. The resulting char x-as subsequently reacted by steam [l-3]. The gaseous products were rich in carbon dio.xidc. Substantisl amounts of liquid znd residuals were also formed. On the other hand. direct pyrolysis of these materials at higher temperatures produced h_\-drogen2nd carbon monxide with considerable amounts of carbon dia.tide and methane [4-6j. In the present studF. the latter approach is used for the pyrolysis of bark to produce gases for energy application. Tran and Rai [7] have reviewed the kinetic studies of several investi_eators on these materials for the design of a suitable pyrol~-sis reactor. These studies were carried out under different expertmental conditions with various materials (bark. wood. cotton, etc.) thereb>- giving widely varying kinetic parameters. e.g. activation energies ran@ns from 14.6 to 227.3 kJ_,*‘mole(7j. Furthermore. most of these studies were based on weight loss data (thcrmogravimetric analysis and fluidized bed) with little or no attention given to the effect of various parameters on the composition of the _gas formed. The weight loss data may or may not correIate accurately with the production of various gases. as the pyolysis of such materials is reported [S] to be 9 complex process involving many parallel and consecutive reactions. Since one of the primaty interests in pyrolysis is the production of gases of higher
c;tlorific value. the determination of the kinetics of the production of these ceases will be more useful for eqineerin_p use. The present paper reports the kinetics of the production of CO. CK, and CO, from poplar bark in the temperature range of 57’3-1023 K and the effect of reaction temperature on the calorific values. yield and composition of cpases alonce with carbon conversion. ESPERI!ilESTAL
Ths hybrid pophr bark i DS-211 was supplied by Forinrck Labs. (Ottawa. Canada! and hr?s the following clcmtnt;ll composition in F (w_,~~“w:C. 43.3 K. 5.9: S. 0.1: 0. 2.3.5: ;?sh. 6.2. The weight perantapes of bark constituents are @\-en be!ow: Sucsessivc S0lvcnt
extrasrives
Summ3tivc r;. . w_.“W
anal_\-& cc. \v_. \v
Constituent
The bark ~-3s shredded in a blender. sieved dried by blow-in_e air wer it at 333 K fcr 20 h.
t ~2-1-12
me=h Tyler)
3x3
.A schematic d&ram of the experimental set-up is &\-en in Fig. 1. The system is designed for operation 3t atmospheric pressure and is flexible with respect to control of experimental variables includiq (1) sample size. (2.1 steam flow-rate. (3) temperature and 14) recovery of ge.s and chars produr‘td after gAfication_ The temperature profile in the reactor ~3s measured e.spcrimcntatly and it 11-3s found that the reactor ~-3s isothermal over 3 4.0 cm zone. The screen was placed in this zone and the samples used in the esperimsnts stayed well within this zone duriq e_xpsrimentation. .A weighed amount of bark sample ( 1.0- 125 ~1)was first held between the two ball ~31~~s and wacustcd for three hours to-remove the trapped air. The reactor was brought to the desired temperature and the system pursed with helium. .A helium flow-rate of 11.6 cm’,JJs (at 29s K) was stablished through a water saturator mtintaiincd at constant temperature. This produced 3 steam flow-rate of 1.4 g/k. The experiment s-35 initiated b_v allowing the sample to fall Gn the screen under gravity and starting the stop-watch. The
RESCL-IS
ASD
Gas produciion
DECX-SSIOS
rmes
The principal gases produced b>- pyrolysis at !o\vtr teaperature I < 73 Kr are CO. CO, and CH, while consider;lbit amounts of hydmgcn zrt pr\+
ductd along with the above gases at higher temperatures. Consequenti_\-. the pas production r ttes were measured only for CO. CO, and CH,. Typica! grr~ production profiles for these gases are shotin in Fig. 2. The zero time in the figure refers to the start of the reaction at the screen placed in the isotherms1 zone of the reactor. Iiates of gas pwduction decrease sharp&- in the beginning and become steady later.
The gss yields wee
determined by integrating the gas production profi!es ever a time of 90 s. These art listed in TabIs 1. Gas Fields increase with inaessing temperature. The composition of gases was determined considerin_e complete pyroi_vsis (90 5). The variation in the composition of gases with temperature is shown- in Fig. 3. Although the volume percentage of CO. dc
?--@ _”
c-8_ __
=
be due to the carbon-stern reaction at higher temperatures_ Cousks ES] and Sls~ynskii [9) obsen-cd a similar trend in rheir work on wood pyrof_\-sis_
The c3lorific value of gss mixtures were c3lculsred component cslorific values rep@rted in the lirsxture [IO]. mixtures produced incxwz with incresing temper3ture cT3ble 1 ! and 3re comparable to thaw rqorred [S] for w& p-r+-sis prxm
Carbon conversion is defined as the ratio of carbon in the product ~2s~ to the carbon present in the bark. Carbon convcrkns increst U-ith increasing temperature (Table i 1.
~rolysis of wood 3nd other cellulosic marerials h= been studied b>various investigators using various technique w_hich zre reiiewcd In detail by Tran and Rai [7]_ Reaction kinetics based on weigh: loss using non-is* thermal and isothermal methods have been studied. Since the decompasition of these material5 is throue complex re3ctioris involving sever31 p3ralls! 3nd consecutive reactions. the production of various gase?; m3y or m3y not accuratei>- be correlated with such data. Furthermore_ since the prim&y- aim of pyrolysis is to produce gases for energv or synhesis. dertrmin~tion of L!X kinetics of the formation of these gases would be more usefu! for enginesting
applications. Simmons and Sanchez [6] determined the kinetics of the ,eas production from sawdust using a reactor system similar to that used in the present study. However. their approach has one major drawback: it does not take into consideration the in.itiaJ time of reaction in calculating the reaction rate constants. In the present stud? an alternative method subwested b? Juntgen and \-an Hctk [ 111 for coal de\-olatihzation has been used for bark p_vrol_vsis for the first time. This chemical reaction model is based on the generation of particular species from bark. The particle size of the bark studied is in the range where heat transfer and diffusions1 effects are reported [It] to be insignificant. This is further confirmed by the effect of different flow-rates in the reactor. ConsequentI_\:. the pFro&-sis was taken to be chemical reaction controlled. It is assumed in the kinetic anaI_\-,cis that no secondrtc- Sas reactions are taking place and that the production of a prrrticulsr sss is a first order rextL.n. The particles are assumed to be iwthermai during the _ea evofution. Thus. the rate of production of a particular gas is given by: JI’ -=kklI;,-
I’)
dr
IE)
where I _ and Ii are volumes of gas produced at any time t and infmitc time respectively and k is an overall reaction rate constant. Integrarins eqn. I and applying boundary conditions one gets:
L-sins a non-linear least square progam (Box modified Gauss. method) the values of 1.; and k are estimated at various temperatures for CO. CH, and CO,. .A t_\pical plot of actua1 and predicted values of the amount of gases produced using this model is presented in Fig. 4. It is clear from the figure that the proposed model predicts the data well. An .Q.rhenius plot for the formation of CO. CH, and CO, is prsented in Fig. 5. The temperature dependence of rate constants is determined to be as follows: k-o = 111.24) e?rp( - 43.50OjR T ) k c-Zi,= (NM.3) exp( -i9_000,‘RT)
(3)
kc,
c3
=
(
1.32) exp( - 27.000,‘RT)
(41
The rate constants for CO and CO, as expressed by the above equations are iou-er than the values reported in earlier fluidized bed [IL 131 and thermogravimetric analysis [ 14.151 u-o&. It seems that the slou-er rates determined in the present stud? are of a subsequent step to the weight loss step and are more representative of the rate controlling step for the formation of various gases. However. caution should be exercised while comparing resalts from
4’C -
2
f
--_
z --_.
= L
_c-
... ; c _-
‘cr
3:
32 . -. .I”
:c ._‘L
‘5 _/1. -a -
-5
other workers as the experiments were conducrcd using different retc~r concepts and wood species. Also. the thermal histork ef she rextOrs c&d be different. The experimental results obtained in the prssnt w\-i>rk can be =td 10 make reasonable predictions for the formation of CO. CH, rs?d CO,. However. more data are needed to determine rhe raw crf formrrtion of hydrrlgen.
The authors are grartful to the Satw;Ll Science and En_einecing Research Council of Canada for financial aid (GO 123:,.
REFERESCES
226