This thesis comprises 30 ECTS credits and is a compulsory part in the Master of Science

with a Major in Resource Recovery – Sustainable Engineering, 120 ECTS credits

No. 10/2011

Thermal treatment of sludge

Narges Razmjoo

Hamid Sefidari

Thermal treatment of sludge

NARGES RAZMJOO nargesrazmjoo2000@yahoo.com

HAMID SEFIDARI hamidsefidari@ymail.com

Master thesis

Subject Category: Technology

University of Borås

School of Engineering

SE-501 90 BORÅS

Telephone +46 033 435 4640

Examiner: Professor Tobias Richards

Supervisor: Professor Tobias Richards

Supervisor address: Högskolan i Borås, S-501 90 Allégatan 1, Borås

Date: December 22, 2011

Keywords: Pyrolysis, organic sludge, Thermogravimetry, Kinetics, Non-isothermal

methods

Abstract

An experimental study of the thermal decomposition of organic sludge has been carried out.

Both isothermal and non-isothermal experiments at different heating rates have been performed

using the thermogravimetric analysis technique. The objective of this work was to calculate the

kinetic parameters of gasification reactions. Several heating rates were applied simulating

gasification reaction using carbon dioxide as gasifying agent. It was concluded that gasification

characteristics of the sludge samples were moderately dependent on the samples’ properties such

as ash and fixed carbon contents and the components present in the ash. Inherent alkaline and

alkaline earth carbonates and sulphates acted as catalysts.

Keywords: Pyrolysis, organic sludge, Thermogravimetry, Kinetics, Non-isothermal methods

Table of Contents

1. Introduction .............................................................................................................................1

2. Experimental ...........................................................................................................................4

2.1. Materials ...........................................................................................................................4

2.2. Thermogravimetric analysis (TGA)...................................................................................5

2.3. Kinetic methods ................................................................................................................7

3. Results and discussion ........................................................................................................... 10

3.1. Thermal analysis of the sludge heating process ............................................................... 11

3.1.1. Drying ...................................................................................................................... 11

3.1.2. Pyrolysis .................................................................................................................. 11

3.1.3. Non-isothermal gasification ...................................................................................... 11

3.1.4. Isothermal gasification reactions ............................................................................... 17

3.2. Non-isothermal gasification kinetics ............................................................................... 18

3.3. Isothermal gasification kinetics ....................................................................................... 21

4. Conclusion ............................................................................................................................ 23

1

1. Introduction

The ever increasing petroleum price has made many countries to seek clues for producing fuel

from bio-based materials. Accordingly, interest for such an alternative fuel has had an upsurge

during recent years since not only does biomass have the possibility of being generated locally, it

can also make any country self-sufficient and less reliant on foreign petroleum resources [1].

Above and beyond, due to the growing world population and incessantly improving living

standards, energy consumption and the output of wastes have been on the increase in recent

years. Municipal solid wastes (MSWs), sewage sludge and waste paper are being generated by

almost every activity of modern industrial society. Subsequently, the potential environmental and

human health impacts associated with the generation of waste are becoming matter of public

concern. Conventional disposal methods, such as ocean dumping and landfill disposal are not

viable solutions anymore, due to the restrictive environmental regulations, as well as limited

numbers of available sites. Given the above considerations, the thermal treatment of wastes has

attracted growing attention on an increasingly international scale by removing hazardous

components, reducing the size of wastes and providing opportunity for energy recovery which is

of crucial importance considering the global energy crisis [2].

Owing to its accessibility and useable energy content, the organic fraction of municipal solid

waste proves to be a wise choice as prospective feedstock for bioenergy production. Currently,

there are quite a number of processes through which biomass can be converted into fuel, for

instance, biological treatment, thermal treatment and so forth. The former produces ethanol or

methane, whereas the latter targets the production of a syngas which can be either upgraded to

fuels with high octane numbers or used in electricity generation.

As mentioned earlier, the focal point of the research has shifted towards thermal treatments, i.e.,

pyrolysis and gasification. While pyrolysis takes place in an inert atmosphere at moderate

temperatures, high temperature gasification is a thermochemical process by which any

carbonaceous fuel can be converted to gaseous products with useable heating value (above all

carbon monoxide and hydrogen in a controlled oxidizing atmosphere) [3].

As discussed previously, pyrolysis is a thermochemical decomposition of

a material or compound due to heat without the participation of oxygen or any other reagents. It

2

is worth mentioning that pyrolysis is most often used for organic materials. Furthermore,

Pyrolysis is an irreversible process involving the concurrent change of chemical composition and

physical phase. The term originates from the Greek-derived words "pyr" along with "lysis"

meaning "fire" and "breaking" respectively. On the whole, pyrolysis of organic matters produces

gaseous and liquid products and leaves a solid substance with a higher carbon content that is

char. Carbonization or extreme pyrolysis is a process where mostly carbon is left as the residue

of the process.

Pyrolysis is used far and wide in chemical industry, for instance, to produce charcoal, activated

carbon and other chemicals. Moreover, pyrolysis is employed in the process of

converting ethylene dichloride into vinyl chloride in order to produce PVC.

Aside from the abovementioned applications , there is a wide range of other specialized uses of

pyrolysis, e.g., producing coke from coal, converting biomass into syngas and biochar and

turning waste into safe-and-sound substances which can be safely disposed of [4].

Even though Pyrolysis seems to resemble other high-temperature processes such

as combustion and hydrolysis, it appears to be different from the foregoing processes since it

does not involve reactions with oxygen, steam, or any other reagents. Hypothetically, the

presumption of a completely oxygen-free atmosphere is possible whereas in reality since there is

some oxygen in any pyrolysis system, a small amount of oxidation is inevitable. In practice,

specifically, in industrial applications, pyrolysis is carried out under pressure and at temperatures

above 400 °C. For instance, for agricultural waste, usual temperatures are 450 to 550 °C.

Products might be addressed in two groups, intermediate and main products. For intermediate

products syngas and charcoal could be listed whereas final products (main products) include bio-

oil. Besides, electricity and thermal energy are perceived as byproducts for pyrolysis.

Gasification, however, is a process through which carbon-containing materials (either organic or

fossil based carbonaceous fuels) are converted into hydrogen, carbon dioxide, methane and

above all, carbon monoxide. Gasification is achievable at temperatures over 700°C while

subjecting the material to a controlled amount of oxygen and/or steam as reagents or gasifying

agents.

The resultant is a valuable gaseous mixture referred to as syngas or producer gas with

considerable energy content. In view of the foregoing, the power derived from gasification and

the following combustion of the resulting gas is regarded as a source of renewable energy. On

3

the contrary, the gasification of fossil-fuel derivatives, for example, plastic, is not considered to

be renewable energy. Gasification is gainful since it allows for producing a fuel with a higher

heating value than that of the original fuel. The aforesaid advantage makes gasification a better

option when compared to direct combustion. Another positive point attributed to gasification is

the possibility of using the resultant syngas in gas turbines which are of higher efficiency

compared to steam turbines. Moreover, gasification can be employed using material which

would otherwise have been disposed, for instance, biodegradable waste. Added to the above-

mentioned benefits, the high-temperature process purifies the system from corrosive ash particles

such as chloride and potassium, allowing clean gas production. Gasification of fossil fuels,

however, is most commonly used on industrial scales in order to generate electricity [5-10]

Thermogravimetric analysis (TGA) is one of the most frequently used techniques to study the

primary reactions of the decomposition of solids. TGA provides valuable information about char

reactivity under active atmospheres which makes it a useful method to study devolatilization of

biomass gasification. Moreover, quantitative methods can be applied to TGA curves in order to

find kinetic parameters of the thermal events. Knowledge of the kinetics for the thermal

decomposition of the material under study is required for the design of gasifiers and pyrolysis

reactors [11]. During the past few decades many researchers have studied pyrolysis and

gasification characteristics of various materials and established significant information on the

thermal behavior and reaction kinetics. Kumar et al. [1] studied the thermal characteristics of

corn stover using TGA and found that the second phase of pyrolysis was close to first order

reaction. Kasaoka et al. [12] also concluded that in an isothermal experiment, a long-winded

repetition of experimental runs is needed to obtain the kinetic parameters of the Arrhenius

equation. As discussed above, a particular knowledge of the kinetic characteristics of the

gasification process is necessary for comprehending and modeling gasification at industrial scale.

There are several studies on coal gasification kinetics and some on biomass gasification kinetics.

However, the gasification of the organic fraction of municipal solid waste has hardly been

studied at all. The aim of the present work was to study the CO2 gasification reactivity and

kinetic behavior of the so-called organic sludge.

4

2. Experimental

2.1. Materials

The samples used in the present study were obtained from Sobacken; a biogas plant located 8 km

west of Borås. Samples of organic sludge before the biological degradation process in the

fermentation chambers, were taken daily during a week since each day the incoming waste to the

plant was of a different composition. Source of organic waste is given by Sobacken and is

reported in Table 1. Samples of digested sludge, after degradation tank, were also collected three

weeks after the first sampling. Both samples contained a large amount of volatile material, a

significant quantity of ash and only a small amount of fixed carbon. Received materials were put

in to the oven for 48 hours at 105 °C to be dried due to their high water content. Subsequently,

the dried samples were broken into small particles, ground in a cutting mill and then passed

through a sieve with a mesh size of 0.5 mm to minimize mass or heat transfer effects during

pyrolysis and gasification processes. Sludge mixtures were prepared by weighing equal ratios of

samples taken in a week and mixed properly to obtain as homogenous sample as possible.

Therefore, the two sludge samples used in the current study were: mixture of organic sludge

collected during one week from Sobacken waste treatment plant before and after biological

treatment. Prior to the thermochemical characteristics analyses and TGA tests, the two prepared

samples were closely stored in bottles.

Table 1

Sobacken waste source

Waste (tones) Monday Tuesday Wednesday Thursday Friday

House hold 80 80 80 50 80

House hold waste-like, supermarket, large kitchen (schools, hospital)

25 25 - 30 -

Fish sludge 80 - 40 - -

Meat-industry (slaughter, animal foods) 10 25 20 15 20

Dry animal foods (pellet) 4 - - - 5

Representative samples were characterized in terms of proximate and ultimate analysis by ALS

Scandinavia AB. The higher heating values (HHV) of the two sludges were measured by C 200

bombcalorimeter according to the ASTM standards (ASTM D 240, ASTM D 5865).

Decomposition vessel was filled with pure oxygen at a pressure of 3 bar and 1 gram of the sludge

sample placed in a crucible to be ignited. Combustion was carried out inside the vessel and the

5

temperature increase in the calorimeter was evaluated. It performed the measurement

automatically in10 minutes and calculated the calorific value of the fuels. In order to evaluate the

precision of the measurements; each experiment was conducted in duplicate. The proximate and

ultimate analysis results, as well as the heating values of sludges are listed in Table 2.

ALS Scandinavia AB analyzed the inorganic materials of the sludge samples by inductively

coupled plasma atomic emission spectrometry and inductively coupled plasma mass

spectrometry in accordance with Swedish standard (SS 02 81 13-1). The results are summarized

in Table 3.

Table 2

Proximate and ultimate analysis of organic sludges

Material Ultimate analysis (wt % dry basis) Proximate analysis (wt % dry basis) Heating value (MJ/kg)

C H O N S Volatile matter Fixed carbon Ash High heating value

Undigested sludge 44.9 6.0 22.8 4.0 0.37 65 13.1 21.9 20.108

Digested sludge 35.3 4.6 17.8 3.6 1.01 53 9.4 37.6 15.376

Table 3

Major and trace elements in ash

Material Major elements in ash(wt % TS)

Na2O CaO Fe2O3 Al2O3 K2O SiO2 TiO2 MgO MnO P2O5

Undigested sludge 5.89 2.34 0.32 0.88 1.53 1.55 0.040 0.367 0.0154 1.97

Digested sludge 0.83 9.41 2.65 3.09 0.73 6.04 0.155 0.924 0.0468 6.10

Material Trace elements in ash (mg/kg TS)

Ba Cd Co Cr Cu Hg Ni Pb S Sr V Zn

Undigested sludge 29.0 0.160 0.72 21.7 24.8 0.04 7.06 3.3 3730.0 38.0 2.4 133.0

Digested sludge 105.0 0.635 3.35 52.7 86.7 0.073 6.34 15.3 10100.0 136.0 10.6 498.0

2.2. Thermogravimetric analysis (TGA)

The pyrolysis and gasification experiments were performed by a thermogravimetric analyzer

(TGA, Rubotherm) coupled with a magnetic suspension balance and gas &vapor dosing and

mixing devices. Wide range of operating conditions from ambient temperature to 1100 °C is

available for this instrument. In each test 150 mg of the sludge sample was weighted directly into

a cylindrical ceramic crucible and was spread uniformly. Afterward, the loaded crucible was

mounted in the heating chamber of TGA under atmospheric pressure. Control of temperature was

6

done by a thermocouple in contact with the sample bed. The weight loss of the fuel was recorded

by the highly sensitive balance continuously as function of time, and was monitored by a

computer working in line with the furnace. The reactor was completely closed and only the

controlled reaction gas atmosphere was in direct contact with the sample. The furnace was

purged by Nitrogen and Argon prior to the start of the experiments for 5 minutes. High purity

Nitrogen was used as the carrier gas during devolatilization to ensure an inert atmosphere around

the sample and bring out the pyrolysis products from the reaction chamber. Total flow rate of the

inlet gases introduced into the thermobalance was measured by a volumetric flow meter and

maintained at 100 ml/min. Schematic setup of the apparatus used for TGA studies is depicted in

Fig.1[13].

Fig.1. Schematic setup of Rubotherm TGA

7

The fuel sample underwent different sets of experiments. During pyrolysis, Nitrogen at a flow

rate of 100 ml/min was applied with different heating rates till 800 °C when it was switched off.

Isothermal and non-isothermal runs were performed with CO2/N flow (P CO2= 50 kPa) as

gasifying agent. The isothermal gasification reaction of the produced char at three different

temperatures (700, 750 and 800 °C) was initiated after the pyrolysis temperature achieved its set

value, by switching on a flow of carbon dioxide. In non-isothermal runs, gasification of the

obtained chars was carried out up to 1050 °C subjected to heating rates of 5, 10 and 20°C/min.

Several pyrolysis tests were conducted at heating rate of 10 °C/min to obtain enough char

material for analysis. The experiments were replicated twice to determine their reproducibility

and average results are reported hereafter. Summary of different sets of experiments and their

main experimental conditions, performed in the current study is presented in Table 4.

Table 4

Main experimental conditions

2.3. Kinetic methods

The kinetics study involves the measurement of material conversion as a function of temperature.

The role of kinetics analysis is to relate the experimental data to predicted ones by using a model

fitting approach [14-15]. So many investigations have been done on the kinetics of biomass

pyrolysis and gasification, and different methods and assumptions have been illustrated.

Isothermal and non-isothermal gasification kinetics was analyzed in the present work. Overall

reaction rate in gas-solid phase depends on different factors and is expressed as follows [9, 16]:

Where is the reaction rate constant which includes the effect of temperature ( ) and partial

pressure of the gas agent ( ), is time and describes the change of surface reactivity and

physical or chemical properties of the sample as a function of conversion. The conversion of the

fuel material ( , and are defined by following equations [17-19].

Experiment Feedstock Final temperature

(°C)

Pressure Heating rate

(°C/min)

Gas flow

(ml/min)

Non-isothermal pyrolysis Raw sludge sample 800 atmospheric 5,10,20 100 (N2)

Isothermal gasification Fresh char 700,750,800 atmospheric - 100 (N2/CO2)

Non-isothermal gasification Prepared char 1050 atmospheric 5,10,20 100 (N2/CO2)

8

In the forgoing equation, is the initial weight of the sample, is the weight of the sample at

time and is its final weight. In Eq. is the reaction order with respect to fuel

conversion.

Assuming a constant partial pressure for the agent gas, the dependence of the rate constant

( will be on temperature and can be expressed by Arrhenius equation.

Where is the pre-exponential factor, is the activation energy and R is the universal gas

constant. To simplify the calculations, the order of reaction ( is assumed unity [20].

The logarithmic form of Eq. gives

The integration of Eq. gives

is the conversion of the fuel at the beginning of the gasification reaction, where . The

gasification reactivity of the fresh char at isothermal condition was analyzed. In order to evaluate

the kinetic parameters of the isothermal gasification of sludge samples, according to Eq. , the

plot of values versus the gasification time is depicted. It gives a straight

line with the slope which is equal to the rate constant of the gasification reaction. A set of

rate constants at different gasification temperatures has been calculated for the present study. If

the plot for the measured rate constants versus featured by a straight line, the kinetic

parameters can be obtained subsequently. The line intercept and slope are equal to and

respectively [21].

In non-isothermal TGA experiments, the weight of the fuel is monitored as a function of

temperature while there is a fixed temperature ramp (K/min) during reaction progress. Integral

method is applied by many researchers to calculate the Arrhenius parameters for the non-

9

isothermal pyrolysis or gasification of the fuels [22-27]. For a constant heating rate, , we can

write [28]

Conversion can be written as a function of temperature as follows [3]

Now, substituting Eq. and into , rearranging gives

Integration of Eq. with temperature limits from to and conversion limits from to

results in the following expression

Integration of left side of Eq. becomes

The right hand side of Eq. cannot be integrated directly. Several assumptions and

approximation solutions to the integral temperature term have been applied by Singh[29]. The

first assumption is to take therefore the right hand side of Eq. can be expressed as

following

Supposing

Integral method is based on the approximation of temperature integral made by Coats and

Redfern [30] which is widely accepted and used. They have used series of expansion to solve the

right hand side of the above integral. Taking only the first three terms of the approximation

results in the following equation

10

Substituting Eq. and in

Taking logarithm on both sides gives

By a reasonable assumption, , Eq. can be more simplified as follows

A graph of the left hand side of above equation versus

can be plotted by applying the values

from the results of TGA experiments. The plot shall result in a straight line if the assumption for

the order of the reaction as unity is correct. Non-isothermal kinetic parameters can be calculated

since the line has a slope and an intercept equal to

and

, respectively.

3. Results and discussion

Digested sample has a higher ash content compared to the undigested sample on account of a

complete digestion process which the digested sample has undergone, meaning that the

maximum heating value of the digested is lower than that of the undigested. Any differences in

analysis, composition and thermal behavior most likely stem from the digestion process.

Non-isothermal experiments performed with constant heating rates were found to be more

advantageous than isothermal experiments. One of these rewards is the possibility of obtaining

results in a larger temperature range. Moreover, these experiments make it possible to study the

influence of the heating rate on the thermal decomposition process. Considering the foregoing,

experiments at different heating rates between 5 and 20 °C have been conducted. The most

important drawback associated with isothermal experiments is that they are sternly valid for the

temperatures for which they have been attained. The results achieved from non-isothermal

experiments cover a larger range of temperatures of decomposition, up to 1100 °C [4, 31].

11

3.1. Thermal analysis of the sludge heating process

3.1.1. Drying

This step can be observed in burning profiles representing a loss of mass between ambient

temperature at the outset of the experiment and about 140°C, corresponding to a loss of moisture

and the very light volatile matters. In most thermogravimetry analyses researchers often neglect

this stage since the moisture content of the sample is too low, so this stage will not be addressed

in this work. Samples in the present study had the moisture content of 2-3 wt%.

3.1.2. Pyrolysis

This stage starts at about 150 °C for the digested sample and about 180 °C for the undigested.

The digested sludge seems to decompose more easily compared to the other sample. TG curves

show that the decomposition rate starts to decrease at about 340°C for the undigested and about

310 °C for the digested. The aforementioned phenomenon can be justified considering the fact

that within the temperature ranges of 150-310 °C (for the digested) and 180-340 °C (for the

undigested) semi-volatile materials start to decompose and as decomposition proceeds organic

polymers begin to devolatilize as they need higher temperatures. Decomposition stops at 490 °C

for the digested and at 460 °C for the digested. As discussed earlier, the samples used in this

work are of high volatile materials constituting 53 wt% (dry basis) of the digested sample and

65 wt% (dry basis) of the undigested.

3.1.3. Non-isothermal gasification

The starting temperature for the gasification of the samples with carbon dioxide was determined

by non-isothermal thermogravimetric measurements. Gasification is the result of chemical

reaction between carbon in the char, carbon dioxide in the gasifier vessel as well as chemical

reaction between the resulting gases. In this study gasification reaction is carried out in the

absence of oxygen. By the above, the mass loss detected in non-isothermal thermogravimetric

curve is attributed to the reaction of fixed carbon present in the char with CO2 (g) to produce

CO (g) according to Boudouard reaction as follows [32]

12

The effect of heating rate on the thermal behavior of the two sludges, digested and undigested,

during gasification process is illustrated in Table 5 and Fig. 2.

Fig.2 (a) which plots the mass loss of the digested sample versus temperature shows the mass

loss for both samples at different heating rates up to 1100°C. As illustrated in the figure, the total

mass is almost linearly decreasing as temperature increases. Moreover, it can also be found from

the figure that the main mass loss starts at around 630°C whereas we can hardly see any further

loss of mass at 780°C (for 5 and 10°C/min heating rates) and 850°C ( for 20°C/min heating rate).

It is noteworthy that the total mass loss for all heating rates is almost the same but distributed

over a different temperature range. (The highest mass loss is observed when the heating rate is

5 °C/min)

20

40

60

80

100

600 700 800 900 1000 1100

Wei

gh

t%

Temperature/°C

β= 5 °C/min

β= 10 °C/min

β= 20 °C/min

(a)

13

Fig. 2. Weight loss (%) of (a) digested and (b) undigested sludge.

As it can be seen from Fig.2 (b) there is a noticeable mass loss after the gasification region

which is equivalent to about 30 wt% of the original fresh char produced after pyrolysis. In order

to figure out what was happening, the ash samples taken at 850°C and 1050 °C, i.e. before and

after the phenomenon, were subjected to SEM analysis (area, spot and mapping). The results

showed that the amount of some elements, such as Na, K, Ca and Cl had decreased to a great

extent.

According to Fig.3 the conversion of char into gaseous products always tends to increase as long

as the temperature is escalating. Besides, it is clear from the figure that conversion is

approximately a linear function of the temperature. Not to mention that such a trend was

observed for all the three different heating rates. There is a vivid difference between the two

figures (a) and (b) which is due to the inherent properties of the samples together with the

porosity and surface area characteristics of the chars obtained after pyrolysis. As it can be

inferred from the figures, the increase of the heating rate delays the conversion, so the same

value of conversion is achieved for higher temperature of the experimental system [33].

20

40

60

80

100

600 700 800 900 1000 1100

Wei

gh

t %

Temperature/°C

β= 5 °C/min

β= 10 °C/min

β= 20 °C/min

(b)

14

Fig.3. Conversion of samples as a function of temperature at different heating rates for (a): digested, (b) undigested

sludge in non-isothermal gasification.

0

0.2

0.4

0.6

0.8

1

600 700 800 900 1000 1100

Con

ver

sion

%

Temperature/°C

β= 5 °C/min

β= 10 °C/min

β= 20 °C/min

0

0.2

0.4

0.6

0.8

1

600 700 800 900 1000 1100

Con

ver

sion

%

Temperature/°C

β= 5 °C/min

β= 10 °C/min

β= 20 °C/min

(a)

(b)

15

Fig.4 plots data (or conversion rate, calculated using the parameters obtained from the

data at the three heating rates), versus temperature for both samples. According to Fig.4.(a)

which shows the conversion rate for the digested sample, the temperature at which maximum

mass loss rate occurs (Tmax) is shifted upwards as a result of increased heating rates. 705, 730 and

760°C are Tmax’s corresponding to 5, 10 and 20°C/min heating rates respectively. The maximum

rate of mass loss for the digested sample increased by increasing the heating rate. The values of

for β = 5, 10 and 20 °C/min are 0.0008, 0.0014 and 0.0022 s-1

respectively.

For the undigested sample, however, a maximum of 0.002 s-1

is observed in for β = 20

°C/min and a minimum of 0.001 s-1

for β = 5 °C/min. Agrawal [34-35] believes that the values of

versus T calculated at different β’s can show if the process can be described as a simple

reaction. The width of the peak specifies the range of temperatures for which the reaction occurs,

and an increase of this range therefore shows a decrease in the activation energy [33].

0

0.0005

0.001

0.0015

0.002

0.0025

600 700 800 900 1000 1100

dX

/dt

1⁄(s)

Temperature/°C

β= 20 °C/min

β= 10 °C/min

β= 5 °C/min

(a)

16

Fig.4. Experimental reaction rate curves during the non-isothermal gasification of (a) digested and (b) undigested

sludge.

Table 5

Initial, peak and final temperature of the reactivity plots.

Sample Temperature ( °C )

Initial Peak Final

5°C min-1

Digested sludge 602 695 806

Undigested sludge

610 722 762

10°C min-1

Digested sludge 620 732 825

Undigested sludge 625 795 853

20°C min-1

Digested sludge

642

763

891

Undigested sludge

640 813 910

0

0.0005

0.001

0.0015

0.002

0.0025

600 700 800 900 1000 1100

dX

/dt

1⁄(

s)

Temperature/°C

β= 5 °C/min

β= 10 °C/min

β= 20 °C/min

(b)

17

3.1.4. Isothermal gasification reactions

Isothermal experiments were carried out at different temperatures (700, 750 and 800 °C),

employing carbon dioxide as gasifying agent. Fig.5 indicates the solid mass loss with time in the

isothermal experiments carried out with sludge samples.

For the experiment in which the gasification temperature was fixed at 700 °C, the solid

conversion rate observed was very low whereas for 800 °C the aforesaid value was high and for

750 °C reasonable. For instance, for 800 °C, the maximum conversion rate is attainable after

21min while for 750 and 700 °C the maximum conversion rate is reached after 36 and 170 min

respectively.

Fig. 5. Weight loss % of samples as a function of temperature at different heating rates for (a): digested, (b)

undigested sludge in isothermal gasification

60

70

80

90

100

0 20 40 60 80

Wei

gh

t (%

)

Time (min)

T= 700 °C

T= 750 °C

T= 800 °C

60

70

80

90

100

0 20 40 60 80

Wei

gh

t %

Time (min)

700 °C

750 °C

800 °C

(a)

(b)

18

3.2. Non-isothermal gasification kinetics

The kinetic data of sludge samples for non-isothermal gasification were analyzed assuming first

order kinetics. Different temperature ranges were applied depending on the heating rate. Fig.6.

shows the plot of versus which indicates satisfactory correlation with

R2 coefficient ranging from 0.9903 to 0.9958 for undigested sludge and 0.9784 to 0.9917 for

digested sludge over the temperature range used, which supports the assumption of first order

kinetics mentioned earlier. It was found that each TG curve could be well described by the

integral method resulting in correlation coefficients close to the unity. The pre-exponential

factors and the corresponding activation energy values from the application of non-isothermal

gasification in the 10-90% conversion range are presented in Table 6. The activation energy

depends on the heating rate as shown in Table. 6. There is a negligible difference between the

activation energy values 147 kJ/mol and 150 kJ/mol (calculated for the digested sample)

corresponding to the heating rates 5 and 10°C/min, whereas the activation energy increases to

value 172 kJ/mol when the heating rate is 20°C/min. According to many other studies [36-37] it

is sensible to consider the following conclusion as a rule of thumb: The activation energy always

increases as the heating rate increases. The same rule also applies to pre-exponential factors

meaning that the pre-exponential factors will increase when the heating rate increases. This is

assigned to the effect of heat transfer at the different heating rates, resulting in delayed reaction.

To the authors’ best knowledge, no studies have been published regarding the gasification of the

sample under study (organic fraction of the municipal solid waste). Activation energy values for

the high temperature region varying between 180 and 370 kJ/ mol are typical of CO2 gasification

of biomass chars and show a chemically controlled process. However, the results of the present

work can be compared with available data for different types of comparable carbonaceous fuels.

Zhongsuo Liu et al [38] reported the activation energy values of 153.06 and 157.95 kJ/mol at 10

and 20 °C/min respectively for graphite non-isothermal gasification with CO2 as gasifying agent.

The values obtained in the present study are all in close agreement with those mentioned above.

The obtained activation energy values for undigested sludge are higher than those of the digested

one. In accordance with SEM analysis of the undigested sludge, the results imply that alkaline

(Na, K) and alkaline earth (Ca, Mg) metals present in carbonate and sulphate minerals, acted as

catalysts throughout the gasification process by enhancing the reactivity of undigested sludge

19

and causing the gasification of the biomass materials to start at lower temperatures. Vamvuka et

al [17] determined the kinetic parameters of raw and acid-washed MSW, sewage sludge and

waste paper at different heating rates. The kinetic parameters corresponding to the acid-washed

chars are lower than those calculated for the original chars, causing a significant drop in the

reaction rates. This can be considered as a result of the presence of alkaline metals, resulting in

catalytic reactions. In effect, the results of the present study testify to the presence of such

alkaline metals when the activation energy value of the undigested sample is compared to that of

the digested one [39].

-20

-18

-16

-14

-12

-10

0.0008 0.0009 0.001 0.0011 0.0012

ln [

-ln

(1-x

)/T

2]

1/T (1/K)

β= 20 °C/min

β= 10 °C/min

β= 5 °C/min

(a)

20

Fig .6. Arrhenius plot for non-isothermal gasification of (a) digested sludge and (b) undigested sludge.

Table 6

Calculated kinetic parameters of digested and undigested sludge for non-isothermal gasification.

Heating rate, °C/min Temp. range, °C E, kJ/mol A,min-1 R2

Undigested sludge

5 630-750 141 3.23E+05 0.9903

10 680-720 148 8.97E+05 0.9906

20 700-860 168 19.26E+05 0.9958

Digested sludge

5 625-790 147 1.05E+05 0.9917

10 645-795 150 5.52E+05 0.9802

20 660-710 172 2.65E+06 0.9784

-20

-18

-16

-14

-12

-10

0.0008 0.0009 0.001 0.0011 0.0012

ln [

-ln

(1-x

)/T

2]

1/T (1/K)

β= 20 °C/min

β= 5 °C/min

β= 10 °C/min

(a)

(b)

21

3.3. Isothermal gasification kinetics

A kinetic analysis of the CO2 gasification of the two samples was carried out at constant

temperature. The values of were calculated from the TG experiments and plotted against

for different temperatures (700,750 and 800°C). The plots gave a straight line with the slope

equal to the rate constant . The distributions indicate that the values of R2

of the regression

lines are always larger than 0.97, reflecting that n=1 is an appropriate value to approach the rate

constant of organic sludge. Three rate constants corresponding to the three gasification

temperatures were obtained from Fig. 7. Consequently, the activation energy values of the

isothermal gasification of digested and undigested sludge were 217 and 119 kJ/mol respectively.

The values of activation energy, pre-exponential factor and the correlation coefficient in the

chemical kinetics of the two samples from the present study are tabulated in Table7. Similar to

what was discussed earlier regarding the non-isothermal gasification; the activation energy for

the digested sample turns out to be higher than that of the undigested sample which is due to the

catalytic effect of mineral content incorporated in the undigested sludge.

y = -26136x + 16.889

R² = 0.9609

-11

-10

-9

-8

-7

-6

0.0009 0.00095 0.001 0.00105

ln (

K)

1/T (1/K)

(a)

22

Fig. 7. Plot of reaction rate constant with respect to gasification temperature of (a) digested sludge and (b)

undigested sludge.

Table 7 Calculated kinetic parameters of digested and undigested sludge for isothermal gasification.

Sample E, kJ/mol A,1/min R2

Undigested sludge 119 4.20E+02 0.9709

Digested sludge 217 2.16E+07 0.9609

y = -14317x + 6.0413 R² = 0.9709

-11

-10

-9

-8

-7

-6

0.0009 0.00095 0.001 0.00105

ln (

K)

1/T (1/K)

(b)

23

4. Conclusion

The thermogravimetry experiments were interpreted regarding the reactivity of the samples.

Isothermal and non-isothermal data were used to evaluate the kinetic parameters. The

determination of the Arrhenius parameters (activation energy and pre-exponential factor) of the

gasification of two types of sludge, namely digested and undigested, was analyzed by isothermal

and non-isothermal thermogravimetry. The activation energy values of the gasification of both

samples by carbon dioxide were determined by integral method, which required analytical data

taken from a sequence of experiments at different heating rates. The calculated activation

energies were found to increase with the increase of heating rate due to heat and mass transfer

effects. The obtained values of the activation energy are in close agreement with a first order

reaction. In reality, deviations of the experimental data from the straight line trend indicate the

degree of inaccuracy of the assumption of a first-order reaction. In this study, plots of

were prepared for the samples, and the activation energies were

determined from the best-fit lines. The assumption of a first-order reaction appears reasonable:

the regression correlation factor exceeded 0.97.

There is a raise in reaction temperatures corresponding with the maximum mass loss with

increasing heating rate. The temperature at which the maximum mass rate occurs (in non-

isothermal gasification) is higher for the undigested sludge in about 40°C, compared to that of

the digested sample, which confirms the catalytic effect of mineral content. It is apparent that

mineral content of the undigested sludge plays a fundamental role in carbon evolution and

reactions. The activation energies of the non-isothermal gasification of digested and undigested

sludge were 147-172 and 141-168kJ/mol, respectively.

24

Acknowledgement

The special thank goes to our supportive supervisor Prof. Dr. T. Richards whose help,

stimulating suggestions and encouragement helped us throughout the thesis whilst allowing us

the room to work in our own way. One simply could not wish for a better or friendlier

supervisor. Our deepest appreciation is to Dr. Anita Pettersson and our program director Peter

Therning for their support and guidance during the project. In our daily work we have been

blessed with a friendly and cheerful group of fellow students, Abas Mohsenzadeh and Farzad

Moradian who indeed helped us.

We also offer our sincerest gratitude to our parents for their never-ending support and the fact

that they have always had to bear the burden of our frustration and rages against the world and

intractable TGA apparatus with composure and friendship.

25

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