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Characteristics of cardboard and paper gasification with CO2
Applied Energy 86 (2009) 2626–2634
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Characteristics of cardboard and paper gasification with CO2 I. Ahmed *, A.K. Gupta Combustion Laboratory, Department of Mechanical Engineering, University of Maryland College Park, MD 20742, USA
a r t i c l e
i n f o
Article history: Received 29 January 2009 Received in revised form 28 March 2009 Accepted 1 April 2009 Available online 2 May 2009 Keywords: CO2 gasification Cardboard gasification Paper gasification
a b s t r a c t Evolutionary behavior of syngas chemical composition and yield have been examined for paper and cardboard at three different temperatures of 800, 900 and 1000 °C using CO2 as the gasifying agent at constant flow rate. Specifically the evolution of syngas chemical composition with time has been investigated. Pyrolysis of the sample was dominant at the beginning of the gasification process as observed from the high initial devolatilization of the sample followed by char gasification of material to form syngas for a long period of time. Results provided the role of gasification temperature on kinetics of the CO2 gasification process. Increase in gasification temperature provided increased conversion of the sample material to syngas. Thus the sample conversion to syngas was low at the low temperature of 800 °C while at elevated temperatures of 900 and 1000 °C substantial enhancement of the kinetics process occurred. The evolution of extensive reaction rate of carbon-monoxide was calculated. Results show that increase in temperature increased the extensive reaction rate of carbon-monoxide. The global behavior of syngas chemical composition examined at three different temperatures revealed a peak in concentration of H2 to exhibit after few minutes into the gasification that changed with gasification temperature. At 800 °C gasification temperature peak in H2 was displayed at 3 min into gasification while it decreased to only 2 min, approximately, at gasification temperatures of 900 and 1000 °C. The effect of reactor temperature on CO mole fraction has also been examined. Increase in the gasification temperature enhances the mole fraction of CO yields. This is attributed to the increase in forward reaction rate of the Boudouard reaction (C þ CO2 () 2CO). The results show important role of CO2 gas for the gasification of wastes and low grade fuels to clean syngas. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction The slowest reactions in gasification govern the overall conversion rate of the material being gasified. They include the heterogeneous reactions with carbon, namely the water gas, Boudouard and hydrogenation reactions. The rates of reaction for the water gas and Boudouard reactions with char are comparable and are several orders of magnitude faster than that for the hydrogenation reaction [1]. Gasification of carbon with carbon dioxide is more endothermic than that with steam gasification. Furthermore, the equilibrium constant responds much more favorably to temperature than for steam as the gasification media. CO2 gasification is energy intensive as compared to other methods of fuel reforming. However, the CO2 gasification is useful since it provides direct role on greenhouse gas management. The resulting carbon-monoxide gas is useful in many processes besides its direct use in thermal energy production. Steam gasification and pyrolysis is also considered when providing endothermic heat needed from waste heat gases
for fuel reforming. For example, if hot char is introduced to supply endothermic heat in a steam gasifier (from flue gases of a fuel combustor), some of the char is gasified. The opportunity exists for the remaining char gasification of some of the carbon with CO2 during char heating. The equilibrium constant shows high values at elevated temperatures (K1000 K = 2.617 and K1500 K = 6.081 102) [2]. The CO2 required for gasification can be obtained from the chemical looping wherein the CO2 from the product stream can be removed using CaO (conversion to CaCO3) and then transformed to form CaO and CO2. The concentrated CO2 gas can be used for the gasification. The char–CO2 reaction is often used to test the reactivity of different types of char. It is a relatively slow reaction. For small char particle size (less than 300 lm for coal particles) and low temperatures (below 1000 °C), the char–CO2 reaction is controlled by chemical reaction rate and occurs nearly uniformly throughout the interior surface of the char particles. Several mechanisms have been proposed for the char–CO2 reaction. One of them is given as: K1
Cf þ CO2 ðgÞ () CðOÞ þ COðgÞ K2
* Corresponding author. Tel.: +1 3014055311. E-mail addresses: [email protected] (I. Ahmed), [email protected] (A.K. Gupta). 0306-2619/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2009.04.002
K3
CðOÞ ! COðgÞ
ð1Þ ð2Þ
I. Ahmed, A.K. Gupta / Applied Energy 86 (2009) 2626–2634
The first step involves the dissociation of CO2 in the presence of carbon free active site (Cf), to release carbon-monoxide to form an oxidized carbon surface complex, C(O). In the second step the carbon–oxygen complex produces CO molecule as well as a new free active site. The rate limiting step is desorption of the carbon–oxygen surface complex. The rate expression for this mechanism is given as [2]:
Reaction rate ¼
K 1 ðCO2 Þ 1 þ KK 13 ðCO2 Þ þ KK 23 ðCOÞ
ð3Þ
According to the above Eq. (3), CO would have an inhibition effect. It is to be noted that increased concentration of CO2 does not promote the reaction rate since only a small fraction of CO2 is responsible in the reaction. Alternatively one can view that the order of the reaction with respect to the CO2 concentration would tend to unity at low partial pressure and to zero at high partial pressure of CO2 [2]. This study examines the evolutionary behavior of syngas from a cardboard sample using CO2 as the gasifying agent. The reactor is heated to the desired temperature prior to introducing the sample into the reactor. The sample size of 35 g is introduced into the reactor in a batch mode. During the first few minutes into gasification, the sample first experiences a fast pyrolysis process. However, after the end of the pyrolysis process, the gasification of char produced occurs by the surrounding CO2 gasification media. Evolution of syngas chemical composition was monitored using a Gas Chromatograph (GC) along with the evaluation and monitoring of the flow rate of the syngas produced. Based on the gas flow rate and chemical composition change with time one can distinguish between the gas yield from fast pyrolysis of the sample and the relatively slow char gasification in the presence of carbon dioxide. The experimental results have been obtained at three different reactor temperatures using two different types of cellulosic samples, namely cardboard and paper. However, the gasifying agent flow rate and sample size were kept constant for the test data reported here. 2. Background 2.1. Effect of reactor temperature on CO yield Encinar et al. [3] investigated gasification of agricultural residues by carbon dioxide. They studied the effect of temperature on overall char, liquid and gaseous yield. They observed the increase of temperature leads to a decrease of solid yield and an increase of gas yield. As for the liquid yield the liquid fraction goes through a maximum value, at 600 °C, likely due to strong cracking at this temperature. They suggested the increase observed in gas yield is partially due to the decrease in liquid fraction at elevated temperatures. The temperature exerts a positive effect on the carbon-monoxide yield, which was explained by the growing importance of gasification with respect to pyrolysis when temperature increases. In a similar study by Jaber et al. [4] they found the relation between temperature and CO increase to be linear in the temperature range from 750 to 950 °C. Liu et al. [5] studied the effect of pyrolysis time on the CO production rate during char gasification. Comparing with the case of long pyrolysis time, the CO production rate for short pyrolysis time was, in general, high in the beginning, but decreased rapidly thereafter as gasification reaction continued. This behavior was attributed to structure change of the char. The effect of temperature on CO production rate has been studied as well. Results showed that at high temperatures, the CO production rate was higher than that at low temperatures in the
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beginning, and decreased more rapidly with time, suggesting a shorter period for complete gasification at higher temperatures. 2.2. Effect of reactor temperature on gasification time Sun et al. [6] investigated the CO2-gasification and kinetics of maceral chars it was found that the increase in temperature, time at the conversion of 50% (t50%) is greatly decreased, which suggests that the gasification rate significantly increases with increasing temperature. Zou et al. [7] conducted modeling effort for studying the reaction kinetics of petroleum coke gasification with CO2. They deduced the same conclusion that higher temperature leads to the shorter gasification finishing time and higher gasification rate. The random pore model was used in their study and they found that the gasification rate increases with increasing conversion and then decreases rapidly after reaching the maximal rate around a conversion value, X = 0.3. They attributed the lower gasification rate initially to the poor initial porosity which is also, the main reason of the occurrence of maximal gasification rate. 2.3. CO2 gasification kinetics Tancredi et al. [8] investigated the kinetics of CO2 gasification of chars obtained from Eucalyptus grandis sawdust at different carbonization temperatures. The reactivity (specific rate of gasifica. They concluded that the CO2 tion) was calculated as: r ¼ w1 dw dt gasification reaction is of very low significance up to 700 °C. The weight loss observed for the char below this temperature can be attributed mainly to the completion of devolatilization. They observed a monotonic increase in the reactivity of chars with conversion. For low and intermediate conversion, they attributed this increase in reactivity to the increase in surface area with progress in gasification. However, at high conversion levels they noticed a steeper increase in reactivity, which is unlikely to be explained by the development of surface area. They attributed this steep increase in reactivity to the increase in catalytic effect of the metallic constituents of the inorganic matter (mainly Na and K) present in the chars. The relative proportion of this inorganic matter in the remaining solid increases with burnoff of the char and eventually the Na and K contents reach levels at which an increasingly important catalytic activity is exerted. They calculated the reactivity values per unit BET (Brunauer, Emmett and Teller) surface area, for which these reactivity values remain essentially constant. A slight but monotonic increase is observed as burnoff increases which support the point of increased catalytic effect of metallic constitutes at high conversion values. Montesinos et al. [9] investigated the kinetics of CO2 and steam gasification of char made from grapefruit skin. They found that this agriculture waste shows a comparatively high reactivity and they attributed this to the catalytic effect of inorganic matter present in the sample. The reactivity of both steam and CO2 gasification was found to increase with the increase in conversion which also increased the reactivity per unit area. These findings support the conclusion about the catalytic effect of inorganic matter present in the sample char. However, lowering the catalytic effect by washing the sample with an acid leads to decreased reactivity thus further supporting the catalytic activity of inorganic matter. Ash content in the tested char was 14.6%, with potassium being the major metallic constituent. As the gasification precedes the potassium to carbon ratio increases with the consequent result of increased catalytic effect of the potassium. They also concluded that the reactivity and conversion data did not show any saturation effect of the catalyst even at high conversion values of 0.9. Cetin et al. [10] studied the CO2 gasification kinetics of chars from biomass species within the temperature range of 800–950 °C and
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pressures between 1 and 20 bar using thermogravimetric analysis (TGA). Pressure has been found to have no effect on reactivity during char conversion while it has a dramatic effect on the chemical and physical structure during the pyrolysis process. They found that increase in total pressure decreases the average reactivity of gasification of pine chars. During these experiments, partial pressure of CO2 was kept constant while the total pressure was varied. The total pressures used in these experiments were identical to those used during pyrolysis to generate the char samples. They concluded that the difference shown in reactivities could be due to the role of pyrolysis pressure on the intrinsic reactivity and/or the effect of total gasification pressure on the apparent reactivity. To distinguish the effect of pyrolysis pressure on the intrinsic reactivity, radiata pine chars generated at different pressures were gasified at 850 °C and 1 bar for comparison with the biomass char. They suggested that the difference in intrinsic reactivity can only be assigned to the effect of pyrolysis pressure rather than the total surface area effect. They used the X-ray diffraction (XRD) technique to quantify the effect of pyrolysis pressure by characterizing the atomic structure of the char samples. Results showed that the reactivity difference can be linked to the graphitic structure found in chars generated at pressures greater than atmospheric pressure. Consequently, they concluded that the difference in gasification reactivities under different gasification pressures is mainly due to graphitization in biomass char structure at higher pressures. Everson et al. [11] investigated the effect of CO presence in the reactor on the reaction kinetics of pulverized coal–chars. To evaluate the parameters for the intrinsic reaction rate they conducted two sets of experiments, (1) with carbon dioxide and an inert (nitrogen), and (2) with carbon dioxide and carbon-monoxide. A comparison between char reactivities for the two experiments, CO was found to have an inhibiting effect of carbon-monoxide. Research in the area of biomaterial gasification and pyrolysis is fairly rich in literature. However, not enough attention is given to gasification of municipal wastes that includes significant portions of paper and cardboard. This paper is concerned about the behavior of syngas during paper and cardboard gasification. Effect of operational conditions on gasification period and overall CO yield can be found in the literature. However, not enough focus is given to the evolution of syngas chemical composition with time, especially the temporal evolution of carbon-monoxide (CO). This paper investigates the evolution of syngas composition, evolution of CO mole fraction, and the evolution of CO2 consumption during the process. Also, the progress of sample conversion to syngas with time is investigated. The effect of reactor temperature on this evolutionary behavior is also presented.
3. Experimental Fig. 1 shows the experimental setup used here to examine gasification of paper and cardboard using CO2 as the gasifying agent. The gasifying agent (CO2) is heated to the main reactor nominal temperature using an electrically heated furnace tube. The gasifying agent is introduced into the main reactor maintained at the desired temperature of gasification. An inert tracer gas was introduced with the gas flow in order to calculate the flow rates of main gaseous species evolved during gasification. Nitrogen is assumed to be inert in the range of temperatures examined here. Flow rate of syngas evolved was calculated by measuring the species molar ratio to nitrogen and by knowing nitrogen flow rate. Mole fractions of main species and nitrogen were measured using a Gas Chromatograph (GC). The exit flow after the main reactor was split into two streams. The first stream was a stream to the sampling system, while the second stream is exhausted to the atmosphere. The bypass line incorporated a non-return valve and
flow non return valve indicator CO2 heating furnace
Electronically controlled reactor Ar Reactor
Pressure gauge
Condenser
Flow controller
PC N2 CO2 Micro GC
Condensate removal suction pump sampling bottles
Fig. 1. Experimental setup.
a flow indicator (rotameter) to ensure the flow direction to the exit (exhaust) direction. The bypass line with this configuration assured that the sample is not diluted by any surrounding air. More details about the experimental setup and experimental procedures are provided elsewhere [12]. Gasification conditions: Sample: 35 g of cardboard or paper. Nominal gasification reactor temperature: 800, 900 and 1000 °C. Gasifying agent: 2.54 L/min of carbon dioxide. Tracer gas: 2.38 L/min of nitrogen. Sampling time from the start of gasification: 1, 2, 3, 5, 8, 12, 17, 23 and 30 min. 4. Results and discussion Evolutionary behavior of syngas chemical composition and yield have been examined for paper and cardboard using a constant flow rate of CO2 gasifying agent at three different temperatures of 800, 900 and 1000 °C. The results have been obtained on the syngas composition evolved at different times from the start of the gasification. 4.1. Evolution of syngas chemical composition Figs. 2–4 show evolution of syngas chemical composition with time for cardboard and paper at gasification temperatures of 800, 900 and 1000 °C. Data have been obtained on the chemical behavior of the syngas composition at the three different temperatures examined here. Hydrogen mole fraction is shown to exhibit a peak in concentration after few minutes into the gasification process. This peak value decreases with increase in gasification temperature. As an example at 800 °C gasification temperature the peak in H2 was obtained at 3 min into the gasification while at 1000 °C this time was reduced to 2 min. The hydrogen yield is attributed to the pyrolysis process; devolatilization of the material at elevated temperatures. The peak in hydrogen concentration indicates the time taken by the sample to reach the desired gasification temperature in the furnace. Thus the shift in hydrogen peak to lower time at higher temperatures is attributed to higher heating rate at elevated temperatures. After about 8 min into gasification one can barley see any hydrogen mole fraction which indicates that pyrolysis of the material to be complete and contribution of hydrogen in the syngas is also complete by this time. The
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a 100
100
Mole fraction (%)
80
H2 CO
60
CO2 40
CH4 CnHm
20
Mole fraction (%)
a
0
80
H2 CO
60
CO2 40
CH4 CnHm
20 0
0
3
6
9
12 15 18 21 24 27 30 33
0
3
6
9
Time (min) 100
b 100
80 Mole fraction (%)
Time (min)
H2 CO
60
CO2 40
CH4 CnHm
20
H2
80 Mole fraction (%)
b
12 15 18 21 24 27 30 33
CO 60 CO2 40
CH4
20
CnHm
0 0
3
6
9
12 15 18 21 24 27 30 33 Time (min)
Fig. 2. Evolution of syngas chemical composition with time for (a) cardboard and (b) paper at 800 °C.
figures show also behavior of the gasifying agent, CO2 mole fraction, for both cardboard and paper at the three temperatures examined here. CO2 mole fraction increases with time. The reason varies with time; initially in the first few minutes CO2 mole fraction is reduced by the effect of other gases from sample pyrolysis. However, the linear increase in CO2 mole fraction is attributed to the char gasification process which decays monotonically with time resulting in CO2 mole fraction increase. CO2 mole fraction should reach an asymptotic value of 100% as the sample vanishes in the reactor. At the beginning of the process CO2 presence is attributed to the pyrolysis process and the unreacted CO2 introduced into the reactor. Note that the CO2 mole fraction is still not high as compared to later times into gasification. The local minimum in CO2 mole fraction at 800 °C is attributed to the longer time taken by the sample to reach a suitable temperature to start pyrolysis. On the contrary, observing the CO mole fraction starting from time duration between 8 and 30 min reveals that CO mole fraction decrease monotonically with time. This decrease in mole fraction of CO is attributed to the sample decay in the reactor. The presence of CO in the first 5 min is mainly attributed to pyrolysis of the material. This point will be discussed further later. Observing CO2 mole fractions and CO mole fractions in Figs. 2–4 one can observe that the CO mole fraction is getting closer to the CO2 mole fraction with increase in gasification temperature. This emphasizes the role of reactor temperature in CO2–char reaction. 4.2. Effect of reactor temperature on CO mole fraction Fig. 5 shows the effect of reactor temperature on CO mole fraction. As expected, increase in gasification temperature enhances
0 0
3
6
9
12 15 18 21 24 27 30 33 Time (min)
Fig. 3. Evolution of syngas chemical composition with time for (a) cardboard and (b) paper at 900 °C.
the CO mole fraction. This is attributed to the increase in forward reaction rate of the Boudouard’s reaction (C þ CO2 () 2CO). 4.3. CO2 consumption during gasification Fig. 6 shows the difference between the outlet molar flow rate of CO2 and the inlet flow rate (CO2 (out)–CO2 (in)). The results show positive values at the beginning of the process and negative values after about 2–3 min. The high positive values at the beginning support the dominance of initial pyrolysis of the material. The decrease in the difference value is due to the exertion between pyrolysis and gasification processes. The pyrolysis trends to increase the CO2 yield while the gasification tends to decrease the CO2 yield or the increase in CO2 consumption. Negative values appear after the third minute which suggests domination of gasification process. Asymptotic behavior of the curves to zero value is as expected since the sample remaining in the reactor decreases with time. Hence, neither gasification nor pyrolysis reactions will prevail at later times. Carbon dioxide seems to be consumed during the pyrolysis phase because there is no strict distinction between the pyrolysis period and the char gasification period. In other words, there is an overlap between the pyrolysis and char gasification periods. Although pyrolysis is dominant initially, char gasification may have a partial contribution to the process. The overlap between char gasification and pyrolysis is more pronounced at high sample heating rate. In the experiments conducted here the reactor is heated to the desired operational temperature and then the sample is introduced into the reactor. This introduces high heating rate to the sample to cause an overlap between pyrolysis and char gasifi-
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a
80
H2 CO
60
CO2 40
CH4
20
CnHm
0 0
3
6
9
12 15 18 21 24 27 30 33
CO2 (out) - CO 2 (in) moles/min
Mole fraction (%)
a 100
0.11
800 °C
0.09
900 °C
0.07
1000 °C
0.05 0.03 0.01 -0.01 0
3
6
9
12
15
18 21
24 27
30
Time (min)
-0.03
b
80
H2
60
CO CO2
40
CH4 CnHm
20 0 0
3
6
9
12 15 18 21 24 27 30 33 Time (min)
Fig. 4. Evolution of syngas chemical composition with time for (a) cardboard and (b) paper at 1000 °C.
CO2 (out) - CO 2 (in) moles/min
Mole fraction (%)
b 100
Time (min)
0.075
800 °C
0.06
900 °C
0.045 1000 °C 0.03 0.015 0 -0.015 -0.03
0
3
6
9
12
15
18 21
24 27
30
Time (min)
Fig. 6. Effect of reactor temperature on evolution of CO2 consumption [CO2 (out)– CO2 (in)] during gasification for (a) cardboard and (b) paper.
CO mole fraction (%)
a
50
800 °C
40
900 °C
30
1000 °C
cation. Increase in temperature increases the overlap between pyrolysis and char gasification. This has been supported by the experimental data by Ahmed and Gupta [13] in which they reported a direct comparison between pyrolysis and gasification of paper. The results showed 27%, 50% and 95% overlap between pyrolysis and char gasification at reactor temperature 800, 900 and 1000 °C, respectively.
20 10
4.4. Effect of reactor temperature on temporal evolution of sample conversion to syngas
0 0
3
6
9
12 15 18 21 24 27 30 33 Time (min)
CO mole fraction (%)
b
50
800 °C 900 °C
40
1000 °C 30 20 10 0 0
3
6
9
12 15 18 21 24 27 30 33 Time (min)
Fig. 5. Effect of reactor temperature on the evolution of CO mole fraction with time from (a) cardboard and (b) paper.
Figs. 7 and 8 show the evolution of sample to syngas conversion from cardboard and paper, respectively, at 800, 900 and 1000 °C temperature reactor. The sample to syngas conversion represents the mass converted from the sample to gaseous form. In other word it’s the portion of syngas flow rate resulting form the sample conversion to gaseous form. The curve shows a steep decrease in slope at the beginning indicating a relatively high flow rate at the beginning as compared to later times in gasification. The data reveals two distinct regimes of flow rates. The first part has high flow rate while the second part possess low flow rate. At the end of first part, which possesses a sharp decrease in flow rate, the second regime start, which is characterized by a small monotonically decrease in flow rate. Flow rate in this region is relatively small as compared to the first regime. This behavior is attributed to the pyrolysis process existing at the beginning of the process which causes the charring of the sample, while after the fifth minute the syngas yield is attributed mainly to the CO2 gasification of the sample char which is known by its slower reaction rates as compared to the pyrolysis kinetics. The results show that charring
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a 14
a
800 °C
1.2
800 °C
12
0.9 1000 °C
1000 °C
8
g/min
g/min
900 °C
900 °C
10
6
0.6
4
0.3
2 0 0
b
3
6
9
12 15 18 Time (min)
21
24
27
30
5
8
800 °C
b
7
10
15 20 Time (min)
25
1.25
800 °C
1
900 °C
30
900 °C
6 5
1000 °C
4
g/min
g/min
0
3 2
1000 °C
0.75 0.5
1
0.25
0 0
5
10 15 Time (min)
20
25
30
Fig. 7. Effect of reactor temperature on evolution of sample conversion to syngas for (a) cardboard and (b) paper.
0
5
10
15 20 Time (min)
25
30
Fig. 8. Effect of reactor temperature on evolution of sample conversion to syngas for (a) cardboard and (b) paper.
of the sample ends at about the eighth minutes into the gasification so that at this time the sample to gas yield is totally from char gasification. As the temperature increases sample to syngas conversion increases in both regimes (pyrolysis and char gasification) of the process. In Fig. 8 the char gasification part is magnified to show the role of temperature on CO2 assisted gasification. Increase in temperature causes increase in the sample to gas conversion. This finding is also supported from the increase in negative slope of the curves with the increase in reactor temperature which is attributed to the acceleration in Boudouard reaction rates at higher temperatures. The results clearly show that low temperatures (e.g., 800 °C) results in poor sample to syngas yield while at elevated temperatures of 900 and 1000 °C substantial increase in the kinetics of the process promotes higher gas yield. 4.5. (C), (O) and (H) molar yield from the sample Molar yield of main elemental components from the sample have also been investigated. Molar yield of elemental carbon, oxygen and hydrogen can be calculated from the following mole balance:
xðCO2 Þ þ yðN2 Þ þ aðCÞ þ bðOÞeðHÞ ! f ðCO2 Þ þ gðCOÞ þ iðH2 Þ þ yðN2 Þ þ jðCH4 Þ þ kðCn Hm Þ where the calculated values of carbon, oxygen and hydrogen yield represents the amount of these elements that are converted to the gas phase only. An important assumptions made here are that no CO2 reacts with the sample to form products in liquid phase and tar is totally due to sample pyrolysis. In the above reaction, (x) and (y) are initially known and (f), (g), (i), (j) and (k) are calculated by proportionality to (y). The nitrogen mole fraction in syngas
is measured by the Gas Chromatograph. Now after performing chemical balance for oxygen, hydrogen and carbon, one can calculate values of (a), (b) and (e) in the above equation. Figs. 9 and 10 shows the molar flow rate converted from the sample to the syngas having main elements of (C), (O) and (H). The results shown in Fig. 9 are for reactor temperature of 1000 °C. Results for reactor temperatures 800 and 900 °C are qualitatively the same but quantitatively lower in values than those obtained at 1000 °C. Molar yield of main elements is consistent with the general behavior of sample to syngas yield. The molar flow rates of (C), (O) and (H) starts at high flow rate then experience a steep decrease in flow rate from the first to the fifth minute into gasification. At the end of fifth minute a monotonic decrease in flow rate occurs until the end of the process. Contribution of pyrolysis is at the beginning of the process which is characterized by high devolatilization of the sample for a short time. The char gasification occurs in the second part which is characterized by slow flow rate for a long period of time. This means that the char gasification reactions are the controlling reaction in the process. Fig. 10 shows magnified behavior of the char gasification process. The results show almost zero yield of hydrogen from the sample after the 8 min into the gasification, which indicates the end of the pyrolysis process. Fig. 11 shows the detailed contribution of char gasification and sample pyrolysis to the CO yield. The individual yields are compared with the total CO yield to show the change in their share in the total with time. One can see that CO yield at the beginning is totally attributed to sample pyrolysis. However near the end of the process the CO yield is totally attributed to char gasification. One can also, see the decay of pyrolysis contribution near the
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Moles/min
a
I. Ahmed, A.K. Gupta / Applied Energy 86 (2009) 2626–2634
0.6
(C)
0.5
(O)
0.4
(H)
0.3 0.2
eighth minute at which CO yield from char gasification is peaking. One can conclude how fast the pyrolysis process is compared to char gasification. For simplicity one can model the whole process as two process in series at low and moderate temperatures; first a fast pyrolysis process followed by the slow char gasification reactions. On the other hand Fig. 12 shows the overall contribution from both pyrolysis and char gasification compared to the total yield. One can notice the increase in char gasification share when the reactor temperature increases. This indicates that char gasifica-
0
0
Moles/min
b
5
10
15 20 Time (min)
25
30
0.35
C
0.3
O
0.25
H
0.2 0.15
a
0.128
CO flow rate(Moles/min)
0.1
0.112
Total Char gasification
0.096 Pyrolysis 0.08 0.064 0.048 0.032 0.016
0.1
0
0.05
0
3
6
9
0 5
10
15 20 Time (min)
25
b
Fig. 9. Evolution of (C), (O) and (H) molar yield from (a) cardboard and (b) paper at 1000 °C.
a
0.07
(C)
0.06
(O)
Moles/min
0.05 (H) 0.04
21
24
27
30
0.03
Total
0.12
Char gasification
0.1 Pyrolysis 0.08 0.06 0.04 0.02 0 0
3
6
9
12
15
18
21
24
27
30
Time (min)
0.01 0 5
10
15 20 Time (min)
25
0.07
30
Fig. 11. Evolution of CO yield from paper char gasification and paper pyrolysis at reactor temperature (a) 900 °C and (b) 1000 °C.
C
0.06
O
1.4
CO(totat)
1.2
CO(char gasification)
1
H
0.04 0.03
Moles
0.05 Moles/min
18
0.14
0.02
b
15
Time (min)
30
CO flow rate (Moles/min)
0
12
CO(pyrolysis)
0.8 0.6
0.02
0.4
0.01
0.2 0
0 5
10
15 20 Time (min)
25
30
Fig. 10. Evolution of (C), (O) and (H) molar yield from (a) cardboard and (b) paper at 1000 °C.
800
900 o Temperature ( C)
1000
Fig. 12. Contribution to overall CO yield from paper char gasification and paper pyrolysis.
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I. Ahmed, A.K. Gupta / Applied Energy 86 (2009) 2626–2634 Table 1 Extensive rate of reaction of CO (RCOmax) and percentage increase in extensive rate of reaction (R(CO)5 and {[(R(CO)5. R(CO)5)800)/R(CO)5)800] 100}) for cardboard.
Table 2 Extensive rate of reaction of CO (RCOmax) and percentage increase in extensive rate of reaction (R(CO)5 and {[(R(CO)5. R(CO)5)800)/R(CO)5)800] 100}) for paper.
Temperature (°C)
Extensive rate of reaction R(CO)5 (mol/min)
Percentage increase in extensive rate of reaction [(R(CO)5. R(CO)5)800)/ R(CO)5)800] 100 (%)
Temperature (°C)
Extensive rate of reaction R(CO)5 (mol/min)
Percentage increase in extensive rate of reaction [(R(CO)5. R(CO)5)800)/ R(CO)5)800] 100 (%)
800 900 1000
0.023 0.046 0.079
Reference condition 100 243
800 900 1000
0.016 0.052 0.08
Reference condition 225 400
tion is more sensitive to reactor temperature than pyrolysis in the examined range of temperatures. 4.6. Extensive rate of reaction of carbon-monoxide Gasification of paper or cardboard incorporates some complexity which arises from the fact that it is a combined pyrolysis-char gasification process with the possibility of overlapping periods of pyrolysis and gasification. Because of the complexity of the process in hand, kinetics of the process will be indicated using a general expression of reaction rate, namely extensive rate of reaction of carbon-monoxide, which is defined as [14]:
RCO ¼
@nCO @t
where, RCO is the extensive rate of reaction with respect to (CO), nCO is number of (CO) moles formed and t is time. Quantification of the above expression for reactor temperatures, 800, 900 and 1000 °C, will show the effect of temperature on the process kinetics. Carbon-monoxide was chosen because (CO) is a main product of both pyrolysis and gasification.
Table 1 shows the maximum extensive rate of reaction of CO (RCOmax.), the time of maximum extensive reaction rate (tmax) and percentage increase in extensive rate of reaction {[(RCOmax. RCOmax.800)/RCOmax.800] 100} for temperatures 800, 900 and 1000 °C. Fig. 13a and b shows the evolution of extensive reaction rate of (CO) with time. One can clearly see the reaction rate with respect to (CO) production rate at higher temperatures. The increase in CO extensive reaction with temperature is more obvious in the first part of the plot, from 1 to 12 min time duration in the process. This is attributed to the diminishing sample inside the reactor, which forces an asymptotic value of RCO of zero. For more quantitative evaluation, a comparison between extensive reaction rates (R(CO)5) and percentages of (R(CO)5) increase for the three temperatures at the fifth minute is shown in Tables 1 and 2. 4.7. Error analysis The GC accuracy is estimated to be ±0.1%. Consequently, the uncertainty in the syngas chemical composition and flow rates is estimated to lie within the symbol sizes given in the figure. 5. Conclusions
a
800 °C
0.2
RCO (moles/min)
900 °C 1000 °C
0.16 0.12 0.08 0.04 0 0
RCO (moles/min)
b
3
6
9
12 15 18 Time (min)
21
24
27
30
0.12 800 °C
0.1
900 °C
0.08
1000 °C
0.06 0.04 0.02 0 0
3
6
9
12
15
18
21
24
27
Time resolved behavior of syngas mole fraction has been examined using CO2 as the gasification media. The results showed that hydrogen mole fraction peaks at early stages of the process which is attributed mainly to the sample pyrolysis. However, CO2 mole fraction keeps on increasing with time as a result of sample decay with time in the reactor. On the other hand CO mole fraction decreases monotonically with time and its formation is attributed to the gasification of char with CO2 to produce CO. Kinetics of char gasification by CO2 as gasifying agent was found to be of much slower than kinetics of the pyrolysis process. This is directly evidenced from the slopes of sample to syngas conversion data as well as the elemental yields of; (C), (H) and (O) in the sample constituents. Sample to syngas conversion data as well as the elemental yields from sample shows high values at the beginning of the process followed by a steep decrease in their value until the fifth minute. After this time the data showed a monotonic decrease in vales after the eighth minute. This data as well as the differences in CO2 out and CO2 at the inlet (CO2 (out)–CO2 (in)) data supported the dominance of pyrolysis at the beginning. The end of the pyrolysis occurs at about 8 min into the gasification while the actual start of the gasification occurs from about five minutes into the gasification process. Positive values of CO2 (out)–CO2 (in) indicates production of CO2 due to pyrolysis. However, negative values after the 5 min into the gasification indicated the consumption of CO2 in the gasification process. Therefore, studying CO2 as a gasification agent is important for gasification of wastes and biomass.
30
Time (min) Fig. 13. Evolution of extensive reaction rate of (CO) at reactor temperatures of 800, 900 and 1000 °C for (a) cardboard and (b) paper.
Acknowledgment This research was supported by ONR, Program Manager Dr. Steve McElvany. This support is gratefully acknowledged.
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References [1] Higman C, van der Burgt M. Gasification. Elsevier Science; 2003. [2] Wen Y, Lee S. Coal Conversion Technology. Energy Sci Technol. Massachusetts: Addison–Wesley Publishing Company; 1979. [3] Encinar JM, Beltran FJ, Ramiro A, Gonzalez JF. Pyrolysis/gasification of agricultural residues by carbon dioxide in the presence of different additives: influence of variables. Fuel Process Technol 1998;55:219–33. [4] Jaber JO, Probert SD, Williams PT, Tahat M. Gasification potential and kinetics of Jordanian oil shales using CO2 the reactant gas. Energy Sources 2000;22:573–85. [5] Liu H, Kaneko M, Luo C, Kato S, Kojima T. Effect of pyrolysis time on the gasification reactivity of char with CO2 at elevated temperatures. Fuel 2004;83:1055–61. [6] Sun Q, Li W, Chen H, Li B. The CO2-gasification and kinetics of Shenmu Maceral chars with and without catalyst. Fuel 2004;83:1787–93. [7] Zou JH, Zhou ZJ, Wang FC, Zhang W, Dai ZH, Liu HF, et al. Modeling reaction kinetics of petroleum coke gasification with CO2. Chem Eng Process 2007;46:630–6.
[8] Tancredi N, Cordero T, Mirasol JR, Rodriguez JJ. CO2 gasification of eucalyptus wood chars. Fuel 1996;75(13):1505–8. [9] Montesinos FM, Cordero T, Mirasol JR, Rodriguez JJ. CO2 and steam gasification of a grapefruit skin char. Fuel 2002;81:423–9. [10] Cetin E, Moghtaderi B, Gupta R, Wall TF. Biomass gasification kinetics: influence of pressure and char structure. Combust Sci Technol 2005;177:765–91. [11] Everson RC, Neomagus HW, Kasaini H, Njapha D. Reaction kinetics of pulverized coal chars derived from inertinite-rich coal discards: gasification with carbon dioxide and steam. Fuel 2006;85:1076–82. [12] Ahmed I, Gupta AK. Evolution of syngas from cardboard gasification. Appl Energy 2009;86:1732–40. [13] Ahmed I, Gupta AK. Syngas yield during pyrolysis and steam gasification of paper. Appl Energy 2009;86:1813–21. [14] Missen RW, Mims CA, Saville BA. Introduction to chemical reaction engineering and kinetics. John Wiley & Sons Inc.; 1999, ISBN 0-47116339-2.
Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Characteristics of cardboard and paper gasification with CO2 I. Ahmed *, A.K. Gupta Combustion Laboratory, Department of Mechanical Engineering, University of Maryland College Park, MD 20742, USA
a r t i c l e
i n f o
Article history: Received 29 January 2009 Received in revised form 28 March 2009 Accepted 1 April 2009 Available online 2 May 2009 Keywords: CO2 gasification Cardboard gasification Paper gasification
a b s t r a c t Evolutionary behavior of syngas chemical composition and yield have been examined for paper and cardboard at three different temperatures of 800, 900 and 1000 °C using CO2 as the gasifying agent at constant flow rate. Specifically the evolution of syngas chemical composition with time has been investigated. Pyrolysis of the sample was dominant at the beginning of the gasification process as observed from the high initial devolatilization of the sample followed by char gasification of material to form syngas for a long period of time. Results provided the role of gasification temperature on kinetics of the CO2 gasification process. Increase in gasification temperature provided increased conversion of the sample material to syngas. Thus the sample conversion to syngas was low at the low temperature of 800 °C while at elevated temperatures of 900 and 1000 °C substantial enhancement of the kinetics process occurred. The evolution of extensive reaction rate of carbon-monoxide was calculated. Results show that increase in temperature increased the extensive reaction rate of carbon-monoxide. The global behavior of syngas chemical composition examined at three different temperatures revealed a peak in concentration of H2 to exhibit after few minutes into the gasification that changed with gasification temperature. At 800 °C gasification temperature peak in H2 was displayed at 3 min into gasification while it decreased to only 2 min, approximately, at gasification temperatures of 900 and 1000 °C. The effect of reactor temperature on CO mole fraction has also been examined. Increase in the gasification temperature enhances the mole fraction of CO yields. This is attributed to the increase in forward reaction rate of the Boudouard reaction (C þ CO2 () 2CO). The results show important role of CO2 gas for the gasification of wastes and low grade fuels to clean syngas. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction The slowest reactions in gasification govern the overall conversion rate of the material being gasified. They include the heterogeneous reactions with carbon, namely the water gas, Boudouard and hydrogenation reactions. The rates of reaction for the water gas and Boudouard reactions with char are comparable and are several orders of magnitude faster than that for the hydrogenation reaction [1]. Gasification of carbon with carbon dioxide is more endothermic than that with steam gasification. Furthermore, the equilibrium constant responds much more favorably to temperature than for steam as the gasification media. CO2 gasification is energy intensive as compared to other methods of fuel reforming. However, the CO2 gasification is useful since it provides direct role on greenhouse gas management. The resulting carbon-monoxide gas is useful in many processes besides its direct use in thermal energy production. Steam gasification and pyrolysis is also considered when providing endothermic heat needed from waste heat gases
for fuel reforming. For example, if hot char is introduced to supply endothermic heat in a steam gasifier (from flue gases of a fuel combustor), some of the char is gasified. The opportunity exists for the remaining char gasification of some of the carbon with CO2 during char heating. The equilibrium constant shows high values at elevated temperatures (K1000 K = 2.617 and K1500 K = 6.081 102) [2]. The CO2 required for gasification can be obtained from the chemical looping wherein the CO2 from the product stream can be removed using CaO (conversion to CaCO3) and then transformed to form CaO and CO2. The concentrated CO2 gas can be used for the gasification. The char–CO2 reaction is often used to test the reactivity of different types of char. It is a relatively slow reaction. For small char particle size (less than 300 lm for coal particles) and low temperatures (below 1000 °C), the char–CO2 reaction is controlled by chemical reaction rate and occurs nearly uniformly throughout the interior surface of the char particles. Several mechanisms have been proposed for the char–CO2 reaction. One of them is given as: K1
Cf þ CO2 ðgÞ () CðOÞ þ COðgÞ K2
* Corresponding author. Tel.: +1 3014055311. E-mail addresses: [email protected] (I. Ahmed), [email protected] (A.K. Gupta). 0306-2619/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2009.04.002
K3
CðOÞ ! COðgÞ
ð1Þ ð2Þ
I. Ahmed, A.K. Gupta / Applied Energy 86 (2009) 2626–2634
The first step involves the dissociation of CO2 in the presence of carbon free active site (Cf), to release carbon-monoxide to form an oxidized carbon surface complex, C(O). In the second step the carbon–oxygen complex produces CO molecule as well as a new free active site. The rate limiting step is desorption of the carbon–oxygen surface complex. The rate expression for this mechanism is given as [2]:
Reaction rate ¼
K 1 ðCO2 Þ 1 þ KK 13 ðCO2 Þ þ KK 23 ðCOÞ
ð3Þ
According to the above Eq. (3), CO would have an inhibition effect. It is to be noted that increased concentration of CO2 does not promote the reaction rate since only a small fraction of CO2 is responsible in the reaction. Alternatively one can view that the order of the reaction with respect to the CO2 concentration would tend to unity at low partial pressure and to zero at high partial pressure of CO2 [2]. This study examines the evolutionary behavior of syngas from a cardboard sample using CO2 as the gasifying agent. The reactor is heated to the desired temperature prior to introducing the sample into the reactor. The sample size of 35 g is introduced into the reactor in a batch mode. During the first few minutes into gasification, the sample first experiences a fast pyrolysis process. However, after the end of the pyrolysis process, the gasification of char produced occurs by the surrounding CO2 gasification media. Evolution of syngas chemical composition was monitored using a Gas Chromatograph (GC) along with the evaluation and monitoring of the flow rate of the syngas produced. Based on the gas flow rate and chemical composition change with time one can distinguish between the gas yield from fast pyrolysis of the sample and the relatively slow char gasification in the presence of carbon dioxide. The experimental results have been obtained at three different reactor temperatures using two different types of cellulosic samples, namely cardboard and paper. However, the gasifying agent flow rate and sample size were kept constant for the test data reported here. 2. Background 2.1. Effect of reactor temperature on CO yield Encinar et al. [3] investigated gasification of agricultural residues by carbon dioxide. They studied the effect of temperature on overall char, liquid and gaseous yield. They observed the increase of temperature leads to a decrease of solid yield and an increase of gas yield. As for the liquid yield the liquid fraction goes through a maximum value, at 600 °C, likely due to strong cracking at this temperature. They suggested the increase observed in gas yield is partially due to the decrease in liquid fraction at elevated temperatures. The temperature exerts a positive effect on the carbon-monoxide yield, which was explained by the growing importance of gasification with respect to pyrolysis when temperature increases. In a similar study by Jaber et al. [4] they found the relation between temperature and CO increase to be linear in the temperature range from 750 to 950 °C. Liu et al. [5] studied the effect of pyrolysis time on the CO production rate during char gasification. Comparing with the case of long pyrolysis time, the CO production rate for short pyrolysis time was, in general, high in the beginning, but decreased rapidly thereafter as gasification reaction continued. This behavior was attributed to structure change of the char. The effect of temperature on CO production rate has been studied as well. Results showed that at high temperatures, the CO production rate was higher than that at low temperatures in the
2627
beginning, and decreased more rapidly with time, suggesting a shorter period for complete gasification at higher temperatures. 2.2. Effect of reactor temperature on gasification time Sun et al. [6] investigated the CO2-gasification and kinetics of maceral chars it was found that the increase in temperature, time at the conversion of 50% (t50%) is greatly decreased, which suggests that the gasification rate significantly increases with increasing temperature. Zou et al. [7] conducted modeling effort for studying the reaction kinetics of petroleum coke gasification with CO2. They deduced the same conclusion that higher temperature leads to the shorter gasification finishing time and higher gasification rate. The random pore model was used in their study and they found that the gasification rate increases with increasing conversion and then decreases rapidly after reaching the maximal rate around a conversion value, X = 0.3. They attributed the lower gasification rate initially to the poor initial porosity which is also, the main reason of the occurrence of maximal gasification rate. 2.3. CO2 gasification kinetics Tancredi et al. [8] investigated the kinetics of CO2 gasification of chars obtained from Eucalyptus grandis sawdust at different carbonization temperatures. The reactivity (specific rate of gasifica. They concluded that the CO2 tion) was calculated as: r ¼ w1 dw dt gasification reaction is of very low significance up to 700 °C. The weight loss observed for the char below this temperature can be attributed mainly to the completion of devolatilization. They observed a monotonic increase in the reactivity of chars with conversion. For low and intermediate conversion, they attributed this increase in reactivity to the increase in surface area with progress in gasification. However, at high conversion levels they noticed a steeper increase in reactivity, which is unlikely to be explained by the development of surface area. They attributed this steep increase in reactivity to the increase in catalytic effect of the metallic constituents of the inorganic matter (mainly Na and K) present in the chars. The relative proportion of this inorganic matter in the remaining solid increases with burnoff of the char and eventually the Na and K contents reach levels at which an increasingly important catalytic activity is exerted. They calculated the reactivity values per unit BET (Brunauer, Emmett and Teller) surface area, for which these reactivity values remain essentially constant. A slight but monotonic increase is observed as burnoff increases which support the point of increased catalytic effect of metallic constitutes at high conversion values. Montesinos et al. [9] investigated the kinetics of CO2 and steam gasification of char made from grapefruit skin. They found that this agriculture waste shows a comparatively high reactivity and they attributed this to the catalytic effect of inorganic matter present in the sample. The reactivity of both steam and CO2 gasification was found to increase with the increase in conversion which also increased the reactivity per unit area. These findings support the conclusion about the catalytic effect of inorganic matter present in the sample char. However, lowering the catalytic effect by washing the sample with an acid leads to decreased reactivity thus further supporting the catalytic activity of inorganic matter. Ash content in the tested char was 14.6%, with potassium being the major metallic constituent. As the gasification precedes the potassium to carbon ratio increases with the consequent result of increased catalytic effect of the potassium. They also concluded that the reactivity and conversion data did not show any saturation effect of the catalyst even at high conversion values of 0.9. Cetin et al. [10] studied the CO2 gasification kinetics of chars from biomass species within the temperature range of 800–950 °C and
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pressures between 1 and 20 bar using thermogravimetric analysis (TGA). Pressure has been found to have no effect on reactivity during char conversion while it has a dramatic effect on the chemical and physical structure during the pyrolysis process. They found that increase in total pressure decreases the average reactivity of gasification of pine chars. During these experiments, partial pressure of CO2 was kept constant while the total pressure was varied. The total pressures used in these experiments were identical to those used during pyrolysis to generate the char samples. They concluded that the difference shown in reactivities could be due to the role of pyrolysis pressure on the intrinsic reactivity and/or the effect of total gasification pressure on the apparent reactivity. To distinguish the effect of pyrolysis pressure on the intrinsic reactivity, radiata pine chars generated at different pressures were gasified at 850 °C and 1 bar for comparison with the biomass char. They suggested that the difference in intrinsic reactivity can only be assigned to the effect of pyrolysis pressure rather than the total surface area effect. They used the X-ray diffraction (XRD) technique to quantify the effect of pyrolysis pressure by characterizing the atomic structure of the char samples. Results showed that the reactivity difference can be linked to the graphitic structure found in chars generated at pressures greater than atmospheric pressure. Consequently, they concluded that the difference in gasification reactivities under different gasification pressures is mainly due to graphitization in biomass char structure at higher pressures. Everson et al. [11] investigated the effect of CO presence in the reactor on the reaction kinetics of pulverized coal–chars. To evaluate the parameters for the intrinsic reaction rate they conducted two sets of experiments, (1) with carbon dioxide and an inert (nitrogen), and (2) with carbon dioxide and carbon-monoxide. A comparison between char reactivities for the two experiments, CO was found to have an inhibiting effect of carbon-monoxide. Research in the area of biomaterial gasification and pyrolysis is fairly rich in literature. However, not enough attention is given to gasification of municipal wastes that includes significant portions of paper and cardboard. This paper is concerned about the behavior of syngas during paper and cardboard gasification. Effect of operational conditions on gasification period and overall CO yield can be found in the literature. However, not enough focus is given to the evolution of syngas chemical composition with time, especially the temporal evolution of carbon-monoxide (CO). This paper investigates the evolution of syngas composition, evolution of CO mole fraction, and the evolution of CO2 consumption during the process. Also, the progress of sample conversion to syngas with time is investigated. The effect of reactor temperature on this evolutionary behavior is also presented.
3. Experimental Fig. 1 shows the experimental setup used here to examine gasification of paper and cardboard using CO2 as the gasifying agent. The gasifying agent (CO2) is heated to the main reactor nominal temperature using an electrically heated furnace tube. The gasifying agent is introduced into the main reactor maintained at the desired temperature of gasification. An inert tracer gas was introduced with the gas flow in order to calculate the flow rates of main gaseous species evolved during gasification. Nitrogen is assumed to be inert in the range of temperatures examined here. Flow rate of syngas evolved was calculated by measuring the species molar ratio to nitrogen and by knowing nitrogen flow rate. Mole fractions of main species and nitrogen were measured using a Gas Chromatograph (GC). The exit flow after the main reactor was split into two streams. The first stream was a stream to the sampling system, while the second stream is exhausted to the atmosphere. The bypass line incorporated a non-return valve and
flow non return valve indicator CO2 heating furnace
Electronically controlled reactor Ar Reactor
Pressure gauge
Condenser
Flow controller
PC N2 CO2 Micro GC
Condensate removal suction pump sampling bottles
Fig. 1. Experimental setup.
a flow indicator (rotameter) to ensure the flow direction to the exit (exhaust) direction. The bypass line with this configuration assured that the sample is not diluted by any surrounding air. More details about the experimental setup and experimental procedures are provided elsewhere [12]. Gasification conditions: Sample: 35 g of cardboard or paper. Nominal gasification reactor temperature: 800, 900 and 1000 °C. Gasifying agent: 2.54 L/min of carbon dioxide. Tracer gas: 2.38 L/min of nitrogen. Sampling time from the start of gasification: 1, 2, 3, 5, 8, 12, 17, 23 and 30 min. 4. Results and discussion Evolutionary behavior of syngas chemical composition and yield have been examined for paper and cardboard using a constant flow rate of CO2 gasifying agent at three different temperatures of 800, 900 and 1000 °C. The results have been obtained on the syngas composition evolved at different times from the start of the gasification. 4.1. Evolution of syngas chemical composition Figs. 2–4 show evolution of syngas chemical composition with time for cardboard and paper at gasification temperatures of 800, 900 and 1000 °C. Data have been obtained on the chemical behavior of the syngas composition at the three different temperatures examined here. Hydrogen mole fraction is shown to exhibit a peak in concentration after few minutes into the gasification process. This peak value decreases with increase in gasification temperature. As an example at 800 °C gasification temperature the peak in H2 was obtained at 3 min into the gasification while at 1000 °C this time was reduced to 2 min. The hydrogen yield is attributed to the pyrolysis process; devolatilization of the material at elevated temperatures. The peak in hydrogen concentration indicates the time taken by the sample to reach the desired gasification temperature in the furnace. Thus the shift in hydrogen peak to lower time at higher temperatures is attributed to higher heating rate at elevated temperatures. After about 8 min into gasification one can barley see any hydrogen mole fraction which indicates that pyrolysis of the material to be complete and contribution of hydrogen in the syngas is also complete by this time. The
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a 100
100
Mole fraction (%)
80
H2 CO
60
CO2 40
CH4 CnHm
20
Mole fraction (%)
a
0
80
H2 CO
60
CO2 40
CH4 CnHm
20 0
0
3
6
9
12 15 18 21 24 27 30 33
0
3
6
9
Time (min) 100
b 100
80 Mole fraction (%)
Time (min)
H2 CO
60
CO2 40
CH4 CnHm
20
H2
80 Mole fraction (%)
b
12 15 18 21 24 27 30 33
CO 60 CO2 40
CH4
20
CnHm
0 0
3
6
9
12 15 18 21 24 27 30 33 Time (min)
Fig. 2. Evolution of syngas chemical composition with time for (a) cardboard and (b) paper at 800 °C.
figures show also behavior of the gasifying agent, CO2 mole fraction, for both cardboard and paper at the three temperatures examined here. CO2 mole fraction increases with time. The reason varies with time; initially in the first few minutes CO2 mole fraction is reduced by the effect of other gases from sample pyrolysis. However, the linear increase in CO2 mole fraction is attributed to the char gasification process which decays monotonically with time resulting in CO2 mole fraction increase. CO2 mole fraction should reach an asymptotic value of 100% as the sample vanishes in the reactor. At the beginning of the process CO2 presence is attributed to the pyrolysis process and the unreacted CO2 introduced into the reactor. Note that the CO2 mole fraction is still not high as compared to later times into gasification. The local minimum in CO2 mole fraction at 800 °C is attributed to the longer time taken by the sample to reach a suitable temperature to start pyrolysis. On the contrary, observing the CO mole fraction starting from time duration between 8 and 30 min reveals that CO mole fraction decrease monotonically with time. This decrease in mole fraction of CO is attributed to the sample decay in the reactor. The presence of CO in the first 5 min is mainly attributed to pyrolysis of the material. This point will be discussed further later. Observing CO2 mole fractions and CO mole fractions in Figs. 2–4 one can observe that the CO mole fraction is getting closer to the CO2 mole fraction with increase in gasification temperature. This emphasizes the role of reactor temperature in CO2–char reaction. 4.2. Effect of reactor temperature on CO mole fraction Fig. 5 shows the effect of reactor temperature on CO mole fraction. As expected, increase in gasification temperature enhances
0 0
3
6
9
12 15 18 21 24 27 30 33 Time (min)
Fig. 3. Evolution of syngas chemical composition with time for (a) cardboard and (b) paper at 900 °C.
the CO mole fraction. This is attributed to the increase in forward reaction rate of the Boudouard’s reaction (C þ CO2 () 2CO). 4.3. CO2 consumption during gasification Fig. 6 shows the difference between the outlet molar flow rate of CO2 and the inlet flow rate (CO2 (out)–CO2 (in)). The results show positive values at the beginning of the process and negative values after about 2–3 min. The high positive values at the beginning support the dominance of initial pyrolysis of the material. The decrease in the difference value is due to the exertion between pyrolysis and gasification processes. The pyrolysis trends to increase the CO2 yield while the gasification tends to decrease the CO2 yield or the increase in CO2 consumption. Negative values appear after the third minute which suggests domination of gasification process. Asymptotic behavior of the curves to zero value is as expected since the sample remaining in the reactor decreases with time. Hence, neither gasification nor pyrolysis reactions will prevail at later times. Carbon dioxide seems to be consumed during the pyrolysis phase because there is no strict distinction between the pyrolysis period and the char gasification period. In other words, there is an overlap between the pyrolysis and char gasification periods. Although pyrolysis is dominant initially, char gasification may have a partial contribution to the process. The overlap between char gasification and pyrolysis is more pronounced at high sample heating rate. In the experiments conducted here the reactor is heated to the desired operational temperature and then the sample is introduced into the reactor. This introduces high heating rate to the sample to cause an overlap between pyrolysis and char gasifi-
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a
80
H2 CO
60
CO2 40
CH4
20
CnHm
0 0
3
6
9
12 15 18 21 24 27 30 33
CO2 (out) - CO 2 (in) moles/min
Mole fraction (%)
a 100
0.11
800 °C
0.09
900 °C
0.07
1000 °C
0.05 0.03 0.01 -0.01 0
3
6
9
12
15
18 21
24 27
30
Time (min)
-0.03
b
80
H2
60
CO CO2
40
CH4 CnHm
20 0 0
3
6
9
12 15 18 21 24 27 30 33 Time (min)
Fig. 4. Evolution of syngas chemical composition with time for (a) cardboard and (b) paper at 1000 °C.
CO2 (out) - CO 2 (in) moles/min
Mole fraction (%)
b 100
Time (min)
0.075
800 °C
0.06
900 °C
0.045 1000 °C 0.03 0.015 0 -0.015 -0.03
0
3
6
9
12
15
18 21
24 27
30
Time (min)
Fig. 6. Effect of reactor temperature on evolution of CO2 consumption [CO2 (out)– CO2 (in)] during gasification for (a) cardboard and (b) paper.
CO mole fraction (%)
a
50
800 °C
40
900 °C
30
1000 °C
cation. Increase in temperature increases the overlap between pyrolysis and char gasification. This has been supported by the experimental data by Ahmed and Gupta [13] in which they reported a direct comparison between pyrolysis and gasification of paper. The results showed 27%, 50% and 95% overlap between pyrolysis and char gasification at reactor temperature 800, 900 and 1000 °C, respectively.
20 10
4.4. Effect of reactor temperature on temporal evolution of sample conversion to syngas
0 0
3
6
9
12 15 18 21 24 27 30 33 Time (min)
CO mole fraction (%)
b
50
800 °C 900 °C
40
1000 °C 30 20 10 0 0
3
6
9
12 15 18 21 24 27 30 33 Time (min)
Fig. 5. Effect of reactor temperature on the evolution of CO mole fraction with time from (a) cardboard and (b) paper.
Figs. 7 and 8 show the evolution of sample to syngas conversion from cardboard and paper, respectively, at 800, 900 and 1000 °C temperature reactor. The sample to syngas conversion represents the mass converted from the sample to gaseous form. In other word it’s the portion of syngas flow rate resulting form the sample conversion to gaseous form. The curve shows a steep decrease in slope at the beginning indicating a relatively high flow rate at the beginning as compared to later times in gasification. The data reveals two distinct regimes of flow rates. The first part has high flow rate while the second part possess low flow rate. At the end of first part, which possesses a sharp decrease in flow rate, the second regime start, which is characterized by a small monotonically decrease in flow rate. Flow rate in this region is relatively small as compared to the first regime. This behavior is attributed to the pyrolysis process existing at the beginning of the process which causes the charring of the sample, while after the fifth minute the syngas yield is attributed mainly to the CO2 gasification of the sample char which is known by its slower reaction rates as compared to the pyrolysis kinetics. The results show that charring
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a 14
a
800 °C
1.2
800 °C
12
0.9 1000 °C
1000 °C
8
g/min
g/min
900 °C
900 °C
10
6
0.6
4
0.3
2 0 0
b
3
6
9
12 15 18 Time (min)
21
24
27
30
5
8
800 °C
b
7
10
15 20 Time (min)
25
1.25
800 °C
1
900 °C
30
900 °C
6 5
1000 °C
4
g/min
g/min
0
3 2
1000 °C
0.75 0.5
1
0.25
0 0
5
10 15 Time (min)
20
25
30
Fig. 7. Effect of reactor temperature on evolution of sample conversion to syngas for (a) cardboard and (b) paper.
0
5
10
15 20 Time (min)
25
30
Fig. 8. Effect of reactor temperature on evolution of sample conversion to syngas for (a) cardboard and (b) paper.
of the sample ends at about the eighth minutes into the gasification so that at this time the sample to gas yield is totally from char gasification. As the temperature increases sample to syngas conversion increases in both regimes (pyrolysis and char gasification) of the process. In Fig. 8 the char gasification part is magnified to show the role of temperature on CO2 assisted gasification. Increase in temperature causes increase in the sample to gas conversion. This finding is also supported from the increase in negative slope of the curves with the increase in reactor temperature which is attributed to the acceleration in Boudouard reaction rates at higher temperatures. The results clearly show that low temperatures (e.g., 800 °C) results in poor sample to syngas yield while at elevated temperatures of 900 and 1000 °C substantial increase in the kinetics of the process promotes higher gas yield. 4.5. (C), (O) and (H) molar yield from the sample Molar yield of main elemental components from the sample have also been investigated. Molar yield of elemental carbon, oxygen and hydrogen can be calculated from the following mole balance:
xðCO2 Þ þ yðN2 Þ þ aðCÞ þ bðOÞeðHÞ ! f ðCO2 Þ þ gðCOÞ þ iðH2 Þ þ yðN2 Þ þ jðCH4 Þ þ kðCn Hm Þ where the calculated values of carbon, oxygen and hydrogen yield represents the amount of these elements that are converted to the gas phase only. An important assumptions made here are that no CO2 reacts with the sample to form products in liquid phase and tar is totally due to sample pyrolysis. In the above reaction, (x) and (y) are initially known and (f), (g), (i), (j) and (k) are calculated by proportionality to (y). The nitrogen mole fraction in syngas
is measured by the Gas Chromatograph. Now after performing chemical balance for oxygen, hydrogen and carbon, one can calculate values of (a), (b) and (e) in the above equation. Figs. 9 and 10 shows the molar flow rate converted from the sample to the syngas having main elements of (C), (O) and (H). The results shown in Fig. 9 are for reactor temperature of 1000 °C. Results for reactor temperatures 800 and 900 °C are qualitatively the same but quantitatively lower in values than those obtained at 1000 °C. Molar yield of main elements is consistent with the general behavior of sample to syngas yield. The molar flow rates of (C), (O) and (H) starts at high flow rate then experience a steep decrease in flow rate from the first to the fifth minute into gasification. At the end of fifth minute a monotonic decrease in flow rate occurs until the end of the process. Contribution of pyrolysis is at the beginning of the process which is characterized by high devolatilization of the sample for a short time. The char gasification occurs in the second part which is characterized by slow flow rate for a long period of time. This means that the char gasification reactions are the controlling reaction in the process. Fig. 10 shows magnified behavior of the char gasification process. The results show almost zero yield of hydrogen from the sample after the 8 min into the gasification, which indicates the end of the pyrolysis process. Fig. 11 shows the detailed contribution of char gasification and sample pyrolysis to the CO yield. The individual yields are compared with the total CO yield to show the change in their share in the total with time. One can see that CO yield at the beginning is totally attributed to sample pyrolysis. However near the end of the process the CO yield is totally attributed to char gasification. One can also, see the decay of pyrolysis contribution near the
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I. Ahmed, A.K. Gupta / Applied Energy 86 (2009) 2626–2634
0.6
(C)
0.5
(O)
0.4
(H)
0.3 0.2
eighth minute at which CO yield from char gasification is peaking. One can conclude how fast the pyrolysis process is compared to char gasification. For simplicity one can model the whole process as two process in series at low and moderate temperatures; first a fast pyrolysis process followed by the slow char gasification reactions. On the other hand Fig. 12 shows the overall contribution from both pyrolysis and char gasification compared to the total yield. One can notice the increase in char gasification share when the reactor temperature increases. This indicates that char gasifica-
0
0
Moles/min
b
5
10
15 20 Time (min)
25
30
0.35
C
0.3
O
0.25
H
0.2 0.15
a
0.128
CO flow rate(Moles/min)
0.1
0.112
Total Char gasification
0.096 Pyrolysis 0.08 0.064 0.048 0.032 0.016
0.1
0
0.05
0
3
6
9
0 5
10
15 20 Time (min)
25
b
Fig. 9. Evolution of (C), (O) and (H) molar yield from (a) cardboard and (b) paper at 1000 °C.
a
0.07
(C)
0.06
(O)
Moles/min
0.05 (H) 0.04
21
24
27
30
0.03
Total
0.12
Char gasification
0.1 Pyrolysis 0.08 0.06 0.04 0.02 0 0
3
6
9
12
15
18
21
24
27
30
Time (min)
0.01 0 5
10
15 20 Time (min)
25
0.07
30
Fig. 11. Evolution of CO yield from paper char gasification and paper pyrolysis at reactor temperature (a) 900 °C and (b) 1000 °C.
C
0.06
O
1.4
CO(totat)
1.2
CO(char gasification)
1
H
0.04 0.03
Moles
0.05 Moles/min
18
0.14
0.02
b
15
Time (min)
30
CO flow rate (Moles/min)
0
12
CO(pyrolysis)
0.8 0.6
0.02
0.4
0.01
0.2 0
0 5
10
15 20 Time (min)
25
30
Fig. 10. Evolution of (C), (O) and (H) molar yield from (a) cardboard and (b) paper at 1000 °C.
800
900 o Temperature ( C)
1000
Fig. 12. Contribution to overall CO yield from paper char gasification and paper pyrolysis.
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I. Ahmed, A.K. Gupta / Applied Energy 86 (2009) 2626–2634 Table 1 Extensive rate of reaction of CO (RCOmax) and percentage increase in extensive rate of reaction (R(CO)5 and {[(R(CO)5. R(CO)5)800)/R(CO)5)800] 100}) for cardboard.
Table 2 Extensive rate of reaction of CO (RCOmax) and percentage increase in extensive rate of reaction (R(CO)5 and {[(R(CO)5. R(CO)5)800)/R(CO)5)800] 100}) for paper.
Temperature (°C)
Extensive rate of reaction R(CO)5 (mol/min)
Percentage increase in extensive rate of reaction [(R(CO)5. R(CO)5)800)/ R(CO)5)800] 100 (%)
Temperature (°C)
Extensive rate of reaction R(CO)5 (mol/min)
Percentage increase in extensive rate of reaction [(R(CO)5. R(CO)5)800)/ R(CO)5)800] 100 (%)
800 900 1000
0.023 0.046 0.079
Reference condition 100 243
800 900 1000
0.016 0.052 0.08
Reference condition 225 400
tion is more sensitive to reactor temperature than pyrolysis in the examined range of temperatures. 4.6. Extensive rate of reaction of carbon-monoxide Gasification of paper or cardboard incorporates some complexity which arises from the fact that it is a combined pyrolysis-char gasification process with the possibility of overlapping periods of pyrolysis and gasification. Because of the complexity of the process in hand, kinetics of the process will be indicated using a general expression of reaction rate, namely extensive rate of reaction of carbon-monoxide, which is defined as [14]:
RCO ¼
@nCO @t
where, RCO is the extensive rate of reaction with respect to (CO), nCO is number of (CO) moles formed and t is time. Quantification of the above expression for reactor temperatures, 800, 900 and 1000 °C, will show the effect of temperature on the process kinetics. Carbon-monoxide was chosen because (CO) is a main product of both pyrolysis and gasification.
Table 1 shows the maximum extensive rate of reaction of CO (RCOmax.), the time of maximum extensive reaction rate (tmax) and percentage increase in extensive rate of reaction {[(RCOmax. RCOmax.800)/RCOmax.800] 100} for temperatures 800, 900 and 1000 °C. Fig. 13a and b shows the evolution of extensive reaction rate of (CO) with time. One can clearly see the reaction rate with respect to (CO) production rate at higher temperatures. The increase in CO extensive reaction with temperature is more obvious in the first part of the plot, from 1 to 12 min time duration in the process. This is attributed to the diminishing sample inside the reactor, which forces an asymptotic value of RCO of zero. For more quantitative evaluation, a comparison between extensive reaction rates (R(CO)5) and percentages of (R(CO)5) increase for the three temperatures at the fifth minute is shown in Tables 1 and 2. 4.7. Error analysis The GC accuracy is estimated to be ±0.1%. Consequently, the uncertainty in the syngas chemical composition and flow rates is estimated to lie within the symbol sizes given in the figure. 5. Conclusions
a
800 °C
0.2
RCO (moles/min)
900 °C 1000 °C
0.16 0.12 0.08 0.04 0 0
RCO (moles/min)
b
3
6
9
12 15 18 Time (min)
21
24
27
30
0.12 800 °C
0.1
900 °C
0.08
1000 °C
0.06 0.04 0.02 0 0
3
6
9
12
15
18
21
24
27
Time resolved behavior of syngas mole fraction has been examined using CO2 as the gasification media. The results showed that hydrogen mole fraction peaks at early stages of the process which is attributed mainly to the sample pyrolysis. However, CO2 mole fraction keeps on increasing with time as a result of sample decay with time in the reactor. On the other hand CO mole fraction decreases monotonically with time and its formation is attributed to the gasification of char with CO2 to produce CO. Kinetics of char gasification by CO2 as gasifying agent was found to be of much slower than kinetics of the pyrolysis process. This is directly evidenced from the slopes of sample to syngas conversion data as well as the elemental yields of; (C), (H) and (O) in the sample constituents. Sample to syngas conversion data as well as the elemental yields from sample shows high values at the beginning of the process followed by a steep decrease in their value until the fifth minute. After this time the data showed a monotonic decrease in vales after the eighth minute. This data as well as the differences in CO2 out and CO2 at the inlet (CO2 (out)–CO2 (in)) data supported the dominance of pyrolysis at the beginning. The end of the pyrolysis occurs at about 8 min into the gasification while the actual start of the gasification occurs from about five minutes into the gasification process. Positive values of CO2 (out)–CO2 (in) indicates production of CO2 due to pyrolysis. However, negative values after the 5 min into the gasification indicated the consumption of CO2 in the gasification process. Therefore, studying CO2 as a gasification agent is important for gasification of wastes and biomass.
30
Time (min) Fig. 13. Evolution of extensive reaction rate of (CO) at reactor temperatures of 800, 900 and 1000 °C for (a) cardboard and (b) paper.
Acknowledgment This research was supported by ONR, Program Manager Dr. Steve McElvany. This support is gratefully acknowledged.
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