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Chemosphere 70 (2008) 397–403

www.elsevier.com/locate/chemosphere

Bio-syngas production with low concentrations of CO2 and CH4

from microwave-induced pyrolysis of wet and dried sewage sludge
A. Domı́nguez, Y. Fernández, B. Fidalgo, J.J. Pis, J.A. Menéndez *

Instituto Nacional del Carbón, CSIC, Apartado 73, 33080 Oviedo, Spain

Received 17 April 2007; received in revised form 27 June 2007; accepted 29 June 2007
Available online 9 August 2007

Abstract

This paper assesses the feasibility of producing syngas from sewage sludge via two pyrolysis processes: microwave-induced pyrolysis
(MWP) and conventional pyrolysis (CP). The changes in the composition of the produced gas as a function of the pyrolysis treatment
and the initial moisture content of the sludge were evaluated. It was found that MWP produced a gas with a higher concentration of
syngas than CP, reaching values of up to 94 vol%. Moreover, this gas showed a CO2 and CH4 concentration around 50% and 70%,
respectively, lower than that obtained in the gas from CP. With respect to the effect of moisture on gas composition, this was more pro-
nounced in CP than in MWP. Thus, the presence of moisture increases the concentration of H2 and CO2 and decreases that of CO, espe-
cially when CP was used. In order to elucidate the behaviour of CO2 during the pyrolysis, the CO2 gasification kinetics of the char
obtained from the pyrolysis were investigated. It was established that in microwave heating the gasification reaction is much more
favoured than in conventional heating. Therefore, the low concentration of CO2 and the high concentration of CO in the microwave
pyrolysis gas could be due to the self-gasification of the residue by the CO2 produced during the devolatilization of the sewage sludge
in the pyrolysis process.
 2007 Elsevier Ltd. All rights reserved.

Keywords: Sewage sludge; Pyrolysis; Microwave; Syngas

1. Introduction Specifically, the present paper deals with sewage

sludges, a biomass residue generated in wastewater treat-
Currently the growing demand for energy is largely sat- ment plants. The disposal of these residues is becoming
isfied by fossil fuels. However, the limited reserves of these a serious problem in many industrialized countries due
fuels along with the environmental concern about green- to the constant increase in these wastes, the handling of
house gases have led to the search for new energy sources. which is not easy and inevitably gives rise to collateral
Biomass, which offers a sustainable and renewable energy pollution. At the same time, stricter environmental regula-
system with a net zero CO2 impact, is a clear example of tions are making the treatment and disposal of sewage
alternative energy (Chum and Overend, 2001; McKendry, sludge more and more expensive. There are several meth-
2002). It can be processed to generate syngas (bio-syngas), ods on the market today for treating or disposing sludges,
which could be used as a clean alternative to fossil fuels in such as aerobic or anaerobic digestion, incineration, land-
power generation or for the production of derived liquid filling or agricultural application. However, most of these
fuels such as methanol, dimethyl ether and synthetic diesel methods give rise to subsequent problems, as a result of
(Lv et al., 2007). which secondary treatments are necessary. Consequently,
the search for an economically and environmentally
acceptable means of disposal has become a matter of
*
Corresponding author. increasing importance in recent years (Houillon and
E-mail address: angelmd@incar.csic.es (J.A. Menéndez). Jolliet, 2005).

0045-6535/$ - see front matter  2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.chemosphere.2007.06.075
398 A. Domı́nguez et al. / Chemosphere 70 (2008) 397–403

A possible alternative to the traditional methods of dis- microwave heating with respect to the conventional process
posal, just mentioned, is the conversion of sewage sludge would be: a higher heating efficiency and heating rate, and
into fuels by means of pyrolysis (Shen and Zhang, 2005; therefore a greater saving of time (Mujundar, 1995; Jones
Karayildirim et al., 2006). Pyrolysis involves heating the et al., 2002). Furthermore, this form of heating might
sludge in an inert atmosphere. This process leads to the favour the ‘‘in situ’’ heterogeneous catalytic reaction
production of a volatile fraction consisting of gases and between the volatiles evolved during the pyrolysis, the car-
tar components, and a carbon rich solid residue. The yields bonaceous residue formed and the mineral content of the
of the pyrolysis products depend on the operating condi- raw material (Zhang and Hayward, 2006).
tions (fundamentally temperature, heating rate and resi- The purpose of this paper was to compare the micro-
dence time of the volatiles in the hot zone), as well as the wave and conventional pyrolysis of sewage sludge in the
experimental equipment employed (Horne and Williams, production of syngas (H2+CO) paying special attention
1996; Inguanzo et al., 2002). In a previous work the to the generation of greenhouse gases such as CO2 and
authors showed that the pyrolysis of sewage sludge at high CH4. The influence of the raw material and moisture con-
temperature favoured the formation of the gas fraction tent on the gas composition was also evaluated. An addi-
with a high hydrogen content (Domı́nguez et al., 2006). tional objective of this work was to demonstrate that the
Most of the pyrolysis process has been mainly carried CO2 gasification of the char obtained from the pyrolysis
out by means of conventional heating. However, in the is more favoured in microwave than in conventional
recent years microwave heating has been considered as an heating.
alternative for the pyrolysis of biomass (Kriegerbrockett,
1994; Miura et al., 2000; Miura et al., 2004; Domı́nguez 2. Materials and methods
et al., 2007), coal (Monsef-Mirzai et al., 1992; Monsef-
Mirzai et al., 1995), oil shales (Chanaa et al., 1994; El harfi 2.1. Materials
et al., 2000), and various organic wastes (Kenneth, 1995;
Appleton et al., 2005). Compared to conventional technol- The starting material consisted of two sewage sludges
ogy, microwave devices can eliminate or severely reduce from urban wastewater treatment plants. Sludge V was
heat transfer resistances that are common in conventional subjected to aerobic digestion, while sludge L was digested
chemical processes. This is due to the potential of micro- anaerobically. The sludges were dewatered by centrifuga-
waves to heat the material directly, since the transfer of tion, after which the samples were collected. Table 1 shows
energy occurs through the interaction of the molecules or the chemical characteristics of the sewage sludges, whereas
atoms within the material. Thus, the main advantages of Tables 2 and 3 show the main inorganic elements and the

Table 1
Chemical characteristics of the sewage sludges
Proximate analysis (wt%) Ultimate analysisa,b (wt%)
M Aa
VM a
FC a,c
C H N S O H/C HHVa (MJ kg1)
V 71.0 31.2 62.3 6.5 52.3 8.0 6.7 0.7 32.3 1.83 16.7
L 81.0 38.1 54.7 7.2 49.1 7.3 8.1 1.5 34.0 1.78 14.0
M: moisture content; A: ash content; VM: volatile matter content; FC: fixed carbon.
a
Dry base.
b
Ash free basis.
c
Calculated by difference.

Table 2
Main inorganic element composition of the sewage sludges (expressed as wt% of metal oxides on a dry basis)
Na2O MgO Al2O3 CaO Fe2O3 K2O MnO SiO2 TiO2
V 0.28 1.15 4.71 5.97 1.75 0.76 0.04 15.7 0.25
L 0.42 1.49 7.34 5.29 2.98 0.97 0.05 18.9 0.36

Table 3
Trace elements of the sewage sludge
Co Cr Pb Mn Ni Cu Zn Fe Cd Hg
(ppm) (ppb)
V 3.5 121 246 214 13.2 143 662 9900 2906 919
L 5.0 163 117 239 42.7 192 1020 17400 1355 1446
 A. Domı́nguez et al. / Chemosphere 70 (2008) 397–403 399

heavy metal content, respectively. In order to study the The temperature of the sample in the microwave exper-
influence of the moisture content on the characteristics of iments was monitored by an infrared optical pyrometer.
the gases produced, the experiments were carried out, Accurate measurement of the evolution of temperature
using: (i) the as received wet sewage sludge, with a moisture during the process was very difficult due to inherent diffi-
content of 81 and 71 wt% (L81 and V71, respectively) and culties in measuring this parameter in microwave devices.
(ii) a totally dried aliquot of the sludge (L0 and V0). Nevertheless, for the temperature of the bulk sample, the
optical pyrometer was calibrated for different temperatures
2.2. Pyrolysis experiments (in separate experiments) by switching off the microwaves
and immediately introducing a thermocouple in the centre
The sewage sludges used in this study were pyrolysed in of the bulk sample. The emissivity parameter was set in the
an electrical furnace and in a single mode microwave oven pyrometer in such a way that the temperature measured by
at 1000 C and were kept at this temperature for 20 min. both the optical pyrometer and thermocouple was the
The sample was placed in a quartz reactor (40 cm same. Once the steady state temperature was reached, the
length · 3 cm i.d.) and N2 was used as inert gas at a flow temperature shown by the optical pyrometer could be
rate of 60 cm3 min1. The reactor with the sample was expected to represent the average temperature of the bulk
introduced in the electrical oven, which had been previ- sample quite accurately.
ously heated to the required pyrolysis temperature; conse- The volatiles evolved from the pyrolysis of the sample
quently the temperature of the sample rose rapidly. In passed through five consecutive condensers placed in an
the case of microwave heating, the sample was placed in ice bath, the last three of which contained dichlorometh-
the same quartz reactor, which in turn was placed in the ane. The liquids obtained in the pyrolysis were divided by
centre of the microwave guide. Details of this experimental centrifugation into an aqueous and an organic fraction.
set-up have been described previously (Domı́nguez et al., Both phases were separated and the organic fraction dis-
2005). Sewage sludge has a very high transparency to solved in the dichloromethane was obtained by evaporat-
microwaves. It was therefore necessary to mix it with an ing the solvent at 40 C. The carbonaceous residue from
appropriate microwave receptor to achieve the high tem- the microwave and conventional pyrolysis consisted of
peratures required for pyrolysis. The char obtained in pre- the char formed from the original sample and the char that
vious experiments from the pyrolysis of the samples at was added. Once the pyrolysis was over, the solid carbona-
1000 C, was used as microwave receptor. In order to ceous residue was recovered from inside the quartz reactor
explore the possible influence of the addition of char on for subsequent characterizations. The non-condensable
the pyrolysis, the experiments in the electrical furnace were gases were collected in 12 l Tedlar sample bags with a poly-
also conducted in the presence of char. The amounts of propylene fitting for sampling. The solid and oil fraction
sewage sludge and absorber used in each pyrolysis experi- yields were calculated on a dry basis from the weight of
ment are given in Table 4. The experiments were labelled each fraction, while the gas yield was evaluated by differ-
XY, where X is the sludge and Y is the moisture content. ence. The non-condensable gases were analyzed in a gas-
In the microwave oven the required pyrolysis tempera- chromatograph Perkin–Elmer Sigma 15 fitted with a
ture was reached by varying the microwave power. To TCD (thermal conductivity detector). A Teknokroma
reach (and maintain) the temperature of 1000 C the 10FT Porapak N, 60/80, a Teknokroma 3FT Molecular
average power used was 530 W for the dry sludges and Sieve 13X and 80/100 columns were used. The oven tem-
911 W for the wet sludges. In the case of the electric fur- perature was set at 50 C and the carrier gas flow rate
nace the mean time required to reach the pyrolysis temper- (He) was 20 ml min1. The injector and detector tempera-
ature was 9 min, compared to only 5 min in microwave tures were 100 and 150 C, respectively. The TCD was cal-
heating. ibrated with a standard gas mixture at periodic intervals.

Table 4
Amounts (g) of sewage sludges and absorber used in the pyrolysis experiments and product distribution
MWP CP
L81 L0 V71 V0 L81 L0 V71 V0
Sewage sludge 21.09 4.09 14.03 4.06 21.0 4.02 14.08 4.01
Sewage sludge (db) 4.01 4.09 4.07 4.06 3.99 4.02 4.08 4.01
Absorber 5.19 5.02 5.01 5.03 5.01 5.01 5.01 5.01
Total (db) 9.19 9.10 9.08 9.09 8.10 9.04 9.09 9.02
Product distribution (wt%, db)
Residue 59.9 57.8 57.0 55.3 69.3 74.1 67.7 68.7
Oil 3.7 1.8 4.0 2.1 2.4 1.6 2.1 2.3
Gasa 36.4 40.4 39.0 42.6 28.3 24.3 30.2 29.0
db: dry basis; MWP: microwave pyrolysis; CP: conventional pyrolysis.
a
Calculated by difference.
400 A. Domı́nguez et al. / Chemosphere 70 (2008) 397–403

2.3. Reactivity measurement of char than those obtained with wet sludges. As regards
oil, the presence of water seemed to increase the yield of
The chars (4 g and particle size 1–3 mm) used for the this product, above all when microwave pyrolysis was used.
reactivity experiments with CO2 were obtained by pyrolyz- A comparison of the sewage sludges showed that the
ing the L and V sewage sludges at 1000 C in the electric highest char yield was obtained from the pyrolysis of the
furnace. The temperatures of the experiments were: 500, anaerobically digested sludge (L) while the highest gas
650, 800 and 1000 C for conventional heating and 350, and oil yields corresponded to the sludge obtained aerobi-
400, 500 and 800 C for microwave heating. Before the cally (V).
reactant gas (CO2) was introduced, the system was heated
up to the reaction temperature under N2 atmosphere at a 3.2. Gaseous components
flow rate of 40 ml min1. The CO2 flow rate used in all
the experiments was 60 ml min1. The only gaseous com- Fig. 1 shows the composition (N2 free-vol%) of the gases
ponents detected in the effluent gas were N2, CO and produced from the microwave and conventional pyrolysis
CO2 which were collected in nine 0.5 l Tedlar bags. The (MWP and CP, respectively) of dry and wet sewage
first sample of gases was taken 2 min after the CO2 was sludges. These results showed the influence of the pyrolysis
switched on to flush out the maximum amount of N2. Each treatment and moisture content on the production of H2,
bag was filled with the produced gases for 3 min, so the CO, CO2 and light hydrocarbons. It was observed that
total reaction time was 29 min. the gas obtained from MWP shows a higher concentration
of CO and lower concentrations of CO2 and hydrocarbons
3. Results and discussion than that produced by CP. Thus, MWP produced a gas
with a concentration of CO2 and CH4 which was around
Table 1 summarizes the main chemical characteristics of 50% and 70%, respectively, lower than that obtained in
the sewage sludges used. As can be seen, the sludge treated CP. On the other hand, from Fig. 1 it can be also deduced
aerobically (V) has a higher volatile matter content and that in the pyrolysis of wet sludge the concentration of H2
therefore a higher heating value than the one treaded increased, whereas the concentration of CO decreased in
anaerobically (L). This can be attributed to the fact that the product gas. The effect that the moisture had on H2
aerobic treatment produces sludges with highly degraded and CO concentration was more pronounced in CP than
organic compounds (Conesa et al., 1998) which are easy in MWP. As a result, the highest production of H2 and
to volatilize. A high concentration of ash and oxygen and the lowest production of CO were reached by the CP of
a high value for the H/C atomic ratio were observed. The wet sewage sludge.
high heating values (HHV) of the sewage sludge (dry basis)
were found to be between 14 and 16 MJ kg1. Other wastes
such as plastics, wood, paper, rags and garbage present
HHVs in the 17.6–20.0 MJ kg1 range while coal has a
HHV between 14.6 and 26.7 MJ kg1 (Perry, 1984). As
regards mineral content, Table 2 shows that the main con-
stituents of the sludges studied were Si, Ca and Al,
although other metals such as Fe, Ni, Cu, Cr and Zn were
also observed in significant amounts, particularly in the L
sludge (see Table 3).

3.1. Product yields

The effect of both microwave and conventional pyrolysis

(MWP and CP, respectively) on the distribution of the
products during the pyrolysis of wet and dried sewage
sludge is shown in Table 4. It was found that the gas and
oil yields increased and the char yield decreased when the
microwave was used instead of conventional heating. In
addition, it was proved that the effect of initial moisture
content of the sludge on the product yields also depended
on the type of heating. Thus, in CP the presence of mois-
ture favoured the formation of gases and caused a reduc-
tion in the production of char, especially in the case of
the L sludge. However, the opposite result was found in Fig. 1. Composition (N2 free-vol%) of the gases produced by microwave
the case of microwave heating i.e. the pyrolysis of dried and conventional pyrolysis (MWP and CP, respectively) of dry and wet
sludges produced a higher yield of gases and a lower yield sewage sludges.
 A. Domı́nguez et al. / Chemosphere 70 (2008) 397–403 401

The low CO yield and the enhancement of the CO2 and decrease in both the concentration of light hydrocarbons
H2 concentrations in CP could be due to the fact that the and in the CO/CO2 ratio, and an increase in the H2/CO
water–gas shift reaction (reaction 1) is more favored in ratio. However, in MWP the influence of the moisture on
CP than in MWP, especially in the case of the pyrolysis these parameters seems to be negligible. From Table 5 it
of wet sludge. is also observed that the calorific value of the product
gas increased from 12–13 MJ m3 in MWP to 15–
CO + H2 O = H2 + CO2 DH298 =  41 kJ mol1 ð1Þ
17 MJ m3 in CP, due to the high hydrocarbon concentra-
This behavior may be due to the fact that the stages of tion in the gas from CP.
drying and devolatilization occur simultaneously during Depending on the industrial applications in which raw
the pyrolysis of wet sludge in CP. On the other hand, in syngas are to be used, the separation of CH4 and CO2
MWP the water is removed very quickly from the reactor may or may not be necessary. Thus, the residual CH4 can
and therefore the extent of the reactions between water be reformed with steam to produce more CO and H2 while
and the produced volatiles is not as great as in the case an amine treatment process can be used for CO2 removal
of CP. (Tijmensen et al., 2002).
The low CO2 and hydrocarbon values observed in the A comparison of the results obtained from the pyrolysis
gas from MWP might be due to heterogeneous reactions of each sewage sludge, showed that the anaerobically trea-
2 and 3: ted sludge (L) produces a gas with a lower calorific value, a
higher concentration of H2 and syngas and a lower concen-
C + CO2 $ 2CO DH298 = 173 kJ mol1 ð2Þ tration of hydrocarbons and CO2, than the aerobically
treated sludge (V). This may be partly due to the high con-
CH4 $ C + 2H2 DH298 = 75.6 kJ mol1 ð3Þ
centration of specific metals in the L sludge, i.e. Ni which
These reactions would be favoured in MWP, due to the catalyzes the decomposition of methane, K and Na which
high temperatures inside the particles of the char (micro- catalyze the gasification reaction with CO2 and Cr, Cu,
wave absorber) even at the commencement of devolatiliza- Fe or Zn which catalyze the water–gas shift reaction.
tion. Thus in MWP, hot spots, which might be considered
as ‘‘micro-plasmas’’ located inside the dielectric solid, were 3.3. Gasification experiments and kinetic study
formed. Here the temperature is much higher than the
average temperature of the bed as measured by the optical In order to corroborate that MWH is able to cause the
pyrometer (Menéndez et al., 2007). ‘‘in situ’’ gasification of the char with CO2, a set of gasifi-
Table 5 shows the effect of pyrolysis treatment and mois- cation experiments were carried out at different tempera-
ture content on the syngas and hydrocarbon content tures using both MWH and CH. Considering that CO
(H2+CO and HC, respectively), on the H2/CO and CO/ and CO2 were the only gases produced from the reaction,
CO2 ratio and on the HHV of the gas. As can be seen, reaction 2 can be said to occur exclusively during the experi-
the total concentration of CO+H2 (syngas) is higher in ments. The conversion of CO2 can therefore be calculated
MWP than in CP, with values as high as 94 vol%. Regard- from the concentrations of CO and CO2 in the effluent gas
ing the light hydrocarbons, it was observed that their con- by using the following equation (Bai et al., 2006):
centration ranged from 12 to 18 vol% in CP and from 2 to CO ðmol%Þ
5 vol% in MWP. The CO/CO2 ratio is much higher in X CO2 ðmol%Þ ¼ 100  ð4Þ
CO ðmol%Þ þ 2CO2 ðmol%Þ
MWP than in CP, their values being between 6 and 11.6
and between 1.3 and 2.8, respectively. On the other hand, where X CO2 (mol%) is the CO2 conversion at a given time
the H2/CO ratio increased in CP. Thus, in CP the values and CO (mol%) and CO2 (mol%) are the concentrations
of this ratio ranged between 1.0 and 3.8, whereas values of both gases in the effluent gas determined by gas
close to 1 were obtained in the gas from MWP. It must also chromatography.
be noted that in CP an increase in the moisture produces a Fig. 2 shows the results for CO2 conversion (XCO2) at
different temperatures with time using MWH and CH.
Only the results obtained using the char from sludge L
Table 5
Effect of pyrolysis treatment and moisture content on the H2 + CO, HC,
are shown, although a similar profile was also found for
H2/CO ratio, CO/CO2 ratio and HHV of the gas the char from sludge V. A comparison of the conversions
MWP CP
obtained when the reaction was performed under CH with
those obtained under MWH reveals significant differences.
L81 L0 V71 V0 L81 L0 V71 V0
For example, the CO2 conversion at 1000 C in CH is sim-
H2+ CO (vol%) 94.1 92.5 87.9 87.9 76.2 73.3 68.8 69.6 ilar to that at 800 C in MWH, i.e. the conversion is prac-
H2/CO 1.1 0.9 1.1 1.0 3.8 1.6 2.5 1.0
CO/CO2 11.6 11.5 6.3 6.0 1.4 2.4 1.3 2.8
tically complete. Conversely, in CH the CO2 conversion at
HC (vol%) 2.2 3.4 5.3 4.9 12.7 15.2 15.9 17.7 800 C is around 40% at the initial stage but then it quickly
HHV (MJ m3) 12.8 13.1 13.6 13.4 15.2 16.3 16.3 17.4 decreases to about 5%. However, in MWH at 500 C the
MWP: microwave values of CO2 conversion are even higher than in CH at
P pyrolysis; CP: conventional pyrolysis; HC (hydrocar-
bon content)= C1  C2; HHV = high heating value. 800 C.
402 A. Domı́nguez et al. / Chemosphere 70 (2008) 397–403

CH1000 CH800 CH650 CH500 MWH-L MWH-V CH-L CH-V

-4.00
100 -5.00
-6.00
80 Ea = 59 kJ mol-1
-7.00
X CO2 (Vol%)

-8.00
60

LK
-9.00
Ea =
-10.00 109 kJ mol-1
40 Ea = 125 kJ mol-1 Ea = 101 kJ mol-1
-11.00
-12.00
20
-13.00
-14.00
0 0.0007 0.0009 0.0011 0.0013 0.0015 0.0017
MWH800 MWH500 MWH400 MWH350 1/T

100 Fig. 3. Arrhenius plot of ln k as a function of 1/T for the CO2 gasification
of the chars (from the L and V sewage sludges) under microwave (MWH)
80 and conventional heating (CH).
X CO2 (Vol%)

60
Arrhenius plot of ln k against 1/T in which the slope of
40 the linear plot is Ea/R. The results are shown in Fig. 3.
It can be seen that the Ea of the gasification reaction was
20 higher in CH than in MWH. Moreover, it was also
observed that the char from the sludge L was more reactive
0 than the char from sludge V. These results are in agreement
2 5 8 11 14 17 20 23 26 to the higher temperatures reached by the char in MWH
time (min) and with the higher concentration of Na and K in sludge
L than in sludge V, which favoured the gasification reac-
Fig. 2. Effect of the temperature on CO2 gasification over the char from
tion with CO2. Furthermore, the differences in the Ea val-
sludge L, using conventional and microwave heating (CH and MWH,
respectively). ues of L and V are greater in MWH than in CH. This
may be due to the increase in the catalytic effect of the met-
als present in sludge L when MWH is used instead of CH,
The CO2 conversion data were used to determine kinetic corroborating the capacity of MWH to promote heteroge-
parameters such as the constant rate or the activation neous catalytic reactions.
energy. Assuming a first order reaction with respect to
the carbon concentration, reactivity (R) is defined as
(Radovic et al., 1983): 4. Conclusions
  
1 1 dX
Rðs Þ ¼ ¼k ð5Þ From the results of this paper it was shown that MWP
1X dt
at high temperature offers an environmentally disposal
where k is the constant rate (k = A exp(Ea/RT)) and X is option for sewage sludge. Compared to CP, MWP pro-
the carbon conversion (X=(mc,0–mc)/mc,0, mc,0 and mc are duces more gas and less char. Moreover, the gas from
the initial mass of carbon and mass of carbon at time t, MWP contains a higher concentration of syngas and lower
respectively). The integration Eq. (5) gives rise to the concentrations of CO2 and CH4 (around 50% and 70%,
expression: respectively) than in the gas obtained from CP. The
lnð1  X Þ ¼ kt ð6Þ ‘‘in situ’’ gasification of the char by the CO2 produced
and the decomposition of CH4 may explain the results
The weight loss of carbon during gasification is caused achieved by MWP. With respect to the effect of moisture
by reaction (2). Therefore, it is possible to calculate the on gas composition, this was more pronounced in CP than
conversion of carbon (X) from XCO2 (see Eq. (4)) (Menén- in MWP. Thus, the presence of moisture increases the con-
dez et al., 2007): centration of H2 and CO2 and reduces that of CO, espe-
X CO2  M 0;CO2  12 cially when CP is used. This seems to indicate that in CP
X ¼ ð7Þ the stages of drying and devolatilization occur simulta-
m0;c
neously during the pyrolysis of wet sludge, so that water–
where M0,CO2 are the initial moles of CO2 which were made gas shift reaction is favored. In order to elucidate the
to pass through the char bed for 3 min. behaviour of CO2 during the pyrolysis, CO2 gasification
The first order reaction rate constant (k) can be calcu- kinetic of the pyrolysis char were investigated. The activa-
lated for each temperature from the plot of ln (1-X) vs. t. tion energy values of the gasification reaction were higher
The activation energy (Ea) was determined by using the in conventional heating than in microwave heating which
 A. Domı́nguez et al. / Chemosphere 70 (2008) 397–403 403

confirms that the self-gasification of the residue with the conditions on solid, liquid and gas fractions. J. Anal. Appl. Pyrol. 63,
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