Bachelor Thesis
Investigation of activated carbon with regard to
the first experiments of an
activated carbon test plant for flue gas cleaning
for the academic degree of Bachelor of Science (BSc)
under the guidance of
Univ.Prof. Dipl.-Ing. Dr.techn. Hermann Hofbauer
at the
Institute of Chemical, Environmental and Bioscience Engineering (ICEBE)
supervised by
Univ.Ass. Dipl.-Ing. Josef Fuchs
submitted at
Technische Universität Wien,
Fakultät für Maschinenwesen und Betriebswissenschaften
by
Amr Khaled Mohamed Ahmed Abdelbaky
Matriculation number 1476103
Vienna, 14.11.2019
Abstract
This thesis provides an overview of the usages of activated carbon (AC) in its various
applications in the liquid and solid phases in both daily life and an industrial point of view. The
adsorption behaviour of SO2 under different conditions is clarified. Similarly, the adsorption
behaviour of NO under different experimental conditions is discussed. Required preliminary
experiments are executed as a guideline for further research on adsorption of flue gas in an
activated carbon adsorption test plant. These experiments allow us to know the adsorption
capacity of ammonia and its deposition on the surface of activated carbon under different
partial pressures. Additionally, experiments are executed to determine the pressure loss of an
activated carbon fixed bed over superficial velocity. These experimental results are compared
to a correlation from literature. In general, a satisfactory agreement between the experimental
and theoretical results can be found. Moreover, a detailed flowchart for the activated carbon
test plant is provided and all units are discussed.
2
Table of contents
1
Introduction ..................................................................................................................................... 4
2
General applications of AC ............................................................................................................. 5
3
4
5
2.1
AC applications in the liquid phase ........................................................................................ 5
2.2
AC applications in the gas phase ............................................................................................ 9
Purification of flue gas .................................................................................................................. 13
3.1
Volatile organic compounds (VOC) removal ....................................................................... 14
3.2
SO2 adsorption ...................................................................................................................... 14
3.3
NOx adsorption [86] .............................................................................................................. 18
Preliminary experiment and test plant........................................................................................... 23
4.1
Investigation of NH3 adsorption capacity of AC at different partial pressures ..................... 23
4.2
Pressure loss of the AC fixed bed ......................................................................................... 26
4.3
Test plant for adsorption on AC ............................................................................................ 31
Conclusion .................................................................................................................................... 34
References ............................................................................................................................................. 35
List of Figures ....................................................................................................................................... 42
List of Tables ........................................................................................................................................ 42
List of abbreviations ............................................................................................................................. 44
3
1 Introduction
Activated carbon (AC), also called activated charcoal, is a porous and highly adsorptive form
of carbon used to remove colour or impurities from liquids and gases. It is used for the
separation and extraction of chemical compounds, and in the recovery of solvents [1].
An overview of the fields of application and characterization of AC in the liquid and gas phases
is provided in this work. Adsorption of acidic gases and specifically flue gas on AC will be the
focus in this thesis.
The experimental section of this work contains preliminary experiments required for adsorption
tests executed with an AC test plant. All evaluations are illustrated and comparisons between
actual measurements and theoretical results will be shown in forms of tables and graphs.
The company “Integral” which is specialized in the field of environmental protection systems,
successfully develops technologies to remove sulphur and nitrogen compounds from exhaust
gases in e.g. steel industry. A test plant has been provided by the company for further research
on the usability and properties of different AC. A detailed description of this test plant is
provided in this work.
4
2 General applications of AC
From a general sense of speaking the AC applications are divided into two main categories, the
first deals with adsorption of gases and vapours whereas the second deals with purification of
liquids. Within this work the adsorptive abilities of AC towards gases and specifically flue gas
will be discussed.
Common types of AC are powdered AC (PAC), granular AC (GAC), AC fibres [2], carbon
molecular sieve, biological AC and AC pellets of various shapes [3, 4]. Granular AC is
classified commercially according to their adsorptive capabilities [5, 6]. Further, it is worth
mentioning that the most commonly used carbon filtration system is the fixed bed with granular
AC.
2.1 AC applications in the liquid phase
The applications of AC in liquids are purification processes that comprise the removal of
contaminating chemicals causing odour, colour or taste [7]. The AC used for liquid phase
applications shows a relatively larger pore size (compared to those used for gas phase
applications). In Table 1 the typical application and parameters of AC adsorbents are illustrated
for the liquid phase.
Table 1: Typical parameter of carbon adsorbents for liquid-phase applications [6]
Adsorbent
Unit
Typical
applications
d<20nm
d>20 nm
Specific surface
area
3 -1
cm g
cm3g-1
m2g-1
Fine-pore
AC
dechlorination,
removal
of micropollutants,
decaffeination
0.5-0.7
0.3-0.5
800-1200
Medium-pore
AC
Potable and waste
water purification
Pore volume
0.4-0.6
0.5-0.7
800-1300
Wide-pore
AC
decolorization,
waste water
purification
0.3-0.5
0.5-1.1
800-1400
Potable water treatment
Due to the AC’s relatively large surface area, it can be utilized as an effective adsorbent for
pollutants and contaminants [8, 9, 10]. Water treatment can be categorized into three main
categories: drinking water treatment, industrial water treatment and municipal water treatment.
5
Mainly GAC or PAC are used for such treatment. The purification system typically consists of
a fixed bed filled with GAC. Depending on both quantity and quality of dissolved or suspended
contaminants, contact time during filtration can range from minutes to over an hour. In Table 2
a typical design [6] for an adsorber for water purification is illustrated.
Table 2: Typical design data of adsorbers for water purification [6]
Parameter
Unit
Value
Granular AC size
Depth of granular AC bed
Mass transfer zone
Superficial velocity
Residence time
mm
m
m
cm s-1
min
0.5-2.5
2.0-15
0.5-5.0
0.03-0.4
30-120
Apart from odour removal there are various other roles to AC [11, 12]. However, generally
organic and inorganic contaminant’s removal from water is currently considered to be an
attractive as well as cheap purification option [13, 14]. Efficiency of the assigned GAC is
dependent on various factors such as the characteristics of carbon, its age, the filter’s design
and methods of usage [15]. As for short to medium term or seasonal problems, PAC is used in
drinkable water treatment [16]. Before allowing water to get in contact with AC, some
processes are applied such as passing water through rapid gravity filter, ozonation and
coagulation. Raid gravity filter is a filtration process where wastewater passes through granular
materials to remove suspended particles and other impurities, whereas the process of
coagulation involves adding chemical and then mixing to dissolve the chemical and evenly
distribute it throughout the water [17]. Then when directing water to the AC bed a period of
contact ranging from several to 50 minutes is considered and velocities between 5 to 20 m/h
corresponding to a bed’s depth of 2-4 m [6]. Afterwards, a separation process takes place,
where PAC along with flocculant gets separated from water by means of sedimentation and
subsequent filtration.
As much as the previously mentioned parameters in Table 2 affect the efficiency of the AC
bed, its lifetime is additionally affected by the quality and quantity of the water source as well
as GAC particle size and contact time. Typically, the lifetime of the AC is limited to a range
from six months to two years [6]. In many cases the AC bed can be reactivated afterwards.
6
Industrial and municipal waste water treatment
Suspended solids, organic and inorganic toxic chemicals as well as other hazardous
microorganisms are all common components of waste water, which must be either destroyed
or removed before its discharging. Due to the complexity and variability of industrial water
composition, several different purification methods must be combined and applied aiming for
a required purity while being cost effective. Apart from some low molecular weight compounds
like sugars and alcohol, AC filters are frequently used to remove toxic compounds and other
organic matter. Its use significantly reduces chemical oxygen demand in wastewater [18].
Moreover, heavy metal can possibly be adsorbed on AC [19]. In Table 3 the effects of AC in
some applications and its reduction efficiency of total organic carbon are illustrated.
Table 3: Total organic carbon reduction by GAC in some industrial waste water [20]
Industry
Waste water total
organic carbon
Carbon usage
Average
removal
Unit
Food
Textiles
Paper
Printing
Chemicals
Rubber
Stone, glass, clay
mg C dm-3
25-5300
9.0-4670
100-3500
34-170
36-4400
120-8350
12.0-8350
kg m-3
0.1-41
0.12-29
0.4-19
0.52-0.55
0.13-17
0.62-20
0.34-36
%
90
93
90
98
92
95
87
The effectiveness of AC differs when it acts as an adsorptive material for dyes: Cationic,
mordant and acid dyes can be removed easily, while for dispersed, direct pigments and reactive
dyes it is less effective. Nevertheless, AC is one of the most commonly used mediums for dye
removal [21].
Easily adsorbed contaminants by AC are typically stubborn against biodegradation. While
those organics which are barely adsorbed by carbon, have a higher tendency to be biodegraded.
From that perspective, removal of non-biodegradable species from aqueous steam by means of
AC adsorption is considered as the best available technology [22].
In order to acquire drinking water from waste water, a bacteria immobilization-based process
using GAC is used [23]. This process is called filtration by biological AC. Main usages of
biological AC cover biodegradation of dissolved organic carbon [24, 25, 26] and oxidation of
ammonia [27, 28, 29]. It is also more effective for the removal of dyes than a combination of
7
single carbon adsorption and biological processes [23]. Furthermore, the two-step process of
biological nitrification is achieved with biological AC. These two steps are oxidation of
ammonia into nitrite, and nitrite into nitrate by Nitrosomonas and Nitrobacter bacteria [30].
According to equations (1) and (2) the aim of the nitrification process is to increase the
chemical and biological stability of water distribution systems and decrease unwanted
formation of chlorinated by-products. A significant reduction of nitrite presence, taste, odours
and bacterial regrowth in water distribution systems can be achieved.
NH4+ + 3/2O2 + H2O ⟶ NO2- +2H3O+
(1)
NO2- + 1/2 O2 ⟶ NO3-
(2)
Application of AC filtration in food industry
There is a variety of applications of AC within the food industry. It is one of the most common
methods for decontaminating different sorts of food, drinks and cooking oils, where the main
role remains within decolourization [31]. Examples are colour removal from sucrose sugar,
and corn syrup [32] as well as glucose, and vinegar decolourization by AC adsorption [33].
Moreover, as mentioned previously, AC is efficient at treating water and removing or
decreasing non-desired contaminants causing odour or taste as well as chlorine remaining.
Therefore, enhancing the characteristics of the feed water for the production of beverages.
Additionally, in order to enhance the value of edible products, fats within foods are
decontaminated on AC as a method of refinement from natural contaminants. In the caffeine
industry, seeking a decaffeinate product needs the application of extraction and adsorption
processes. The extraction of caffeine from beans will be prior to roasting. Then by using AC
the extracted caffeine is removed from the solvents [34].
Further liquid-phase applications
Domestic use of carbon filter falls under this category as they are being utilized to even further
purify tap water, as well as their usage for removal of different contaminants as a section of
water treatment technology.
In order to produce ultra-pure water, water with a significantly low
•
carbon content
•
conductivity
8
•
silica levels and
•
heavy metals
is required.
Ultra-pure water is needed for semiconductor industry. This production is one of the important
lately developed applications of AC [35, 36].
Oils and hydrocarbons separation from steam condensate is achieved by oil refinery and
petrochemical industries by means of carbon filtration [5, 37]. Carbon combined with zeolites
has the ability of removing sulphur from naphtha. Therefore, carbon is applied with oil refinery
steams for the purpose of deep desulphurization [38, 39].
Further, purification of enzymes, which is one of the non-agricultural ingredients, is another
application for AC filters [40]. AC filters are also very effective at removing radioactive
uranium and caesium from nuclear waste waters [41]. Further applications of AC filters are
•
the adsorption of impurities present in solvents used in dry cleaning or to recover the
cleaning solvent
•
blood detoxification of patients having an artificial kidney and
•
the recovery of some volatile aroma from water streams [42, 43].
Moreover, the applications of AC in the pharmaceutical and chemical industries field are
various and mainly focusing on purifying pharmaceuticals like for example glycerine.
2.2 AC applications in the gas phase
Air pollution and its reduction and controlling is a prominent problem. Therefore, AC is a
possible gas phase adsorbent. In these applications, AC is primarily used as a final purification
unit from e.g. organic species, mercury, lead, acidic gases released within incineration systems.
There are various parameters, factors and physical properties that affect the usability and
suitability of AC when applied in the gas phase. Some of these are the AC’s porosity, surface
area, size as well as the processed material and the manufacturing. Manufacturing of AC affects
the strength and the degree of reactivity. Depending on these parameters the usage of AC can
vary widely. Examples of such applications are solving environmental problems and health
hazards as in ventilation and air conditioning systems [44].
9
Influencing factors on adsorption are pressure, adsorbate’s concentration, AC’s working
capacity and temperature. In Table 4 the typical application and parameters of carbon
adsorbents are illustrated.
Table 4: Typical parameter of carbon adsorbents for gas-phase applications [6]
Adsorbent
Unit
Fine-pore
AC
Air clean up, odour,
control, adsorption of
low-boiling
hydrocarbons
cm3g-1
cm3g-1
m2g-1
0.5-0.7
0.3-0.5
1000-1200
Typical
applications
d<20nm,
d>20 nm,
Specific surface
area,
Medium-pore
AC
Solvent recovery,
adsorption of
medium-boiling
hydrocarbons
Pore volume for pore size
0.4-0.6
0.5-0.7
1200-1400
Wide-pore
AC
Adsorption
recovery of
high-boiling
hydrocarbons
0.3-0.5
0.5-1.1
1000-1500
In the following Table typical design data of adsorbers for gas purification [6] is illustrated.
Table 5: Typical design data of adsorbers for adsorptive gas purification [6]
Parameter
Unit
Value
Carbon particle size,
Depth of adsorbent bed,
mm
m
3-5
0.5-1.5
Mass transfer zone,
m
0.05-0.3
Superficial velocity,
cm s-1
10-50
Residence time,
s
1-15
Application of additive-loaded AC
Using additives, AC can be supplied with a variety of special properties according to the need.
Low ash AC e.g. can be obtained by simple washing with water or minimal acids. Such AC is
utilized to produce certain chemicals or used in different processes in the pharmaceutical
industry.
A critical factor on the adsorption of specific adsorbents is the surface chemistry of AC [45,
46]. By treating carbon adsorbents with liquid oxidizing agents including nitric acid, hydrogen
peroxide, air or gas mixtures containing ozone and other chemicals, post oxidation of these
carbon adsorbents take place and reveals additional surface oxides [47, 48].
10
A hydrophilic surface grants an improved accessibility of micropores to polar adsorbates by
creating bonds with hydrogen and dipole-dipole interaction. Such a hydrophilic surface is
characterized by a high content of surface oxygen [49]. On the other hand, AC is exposed to
inert gas or hydrogen on high temperatures to remove chemically bound oxygen [48].
Treating AC with ammonia will introduce alkaline functionalities [50]. Addition of additives
is usually achieved by impregnating the AC with suitable organic or inorganic compounds. For
industrial AC-based filtration media, subsequent drying or other treatments are required
(chemical filters) [51]. Furthermore, the blending method can be applied [52, 53], in which
additives are mixed with an AC precursor then treated to produce AC loaded with additive.
Using Ag, Pt, and CO oxides as catalysts loading the AC, volatile organic compounds (VOCs)
can be effectively removed from air on the AC surface [54, 55]. Potassium Iodide (KI)
impregnated AC achieves the removal of radioactive methyl iodide [56]. Triethylenediamine
impregnated AC can also be used for the same purpose [57]. Manganese oxides as oxidation
catalyst are deposited on the AC surface performing catalytic decomposition of formaldehyde
[58]. Effective filtering mediums to remove Hg vapours from gases can be achieved by loading
the AC with elemental sulphur [59]. Using solid amine sorbents, AC can be used to capture
CO2 in aircrafts, submarines and spacecrafts [60, 61, 62].
Table 6 shows the gas phase applications of commercial grades of impregnated AC regarding
the target substances and the weight percentages of the chemicals used for the impregnation of
AC.
Table 6: Commercial grades of impregnated ACs for gas-phase applications [63, 64]
Chemicals used for
impregnation
Quantity in
wt.%
Target substances to be removed from gas
phase
Sulfuric acid
Phosphoric acid
Potassium hydroxide or
sodium hydroxide
Potassium carbonate
Iron oxide
Potassium iodide
2-25
5-30
3-12
Ammonia, amine, mercury
Ammonia, amine
Acid gases (HCl, SO2, H2S, Cl2)
10-20
10
1-5
Triethylenediamine
Sulphur
Potassium permanganate
2-5
10-20
5
Acid gases (HCl, HF, SO2, H2S, NO2), CS2
H2S, thiols, COS
H2S, PH3, Hg, ASH3, radioactive methyl
iodide
Radioactive methyl iodide
Mercury
H2S from oxygen-lacking gases
11
Silver nitrate or copper
nitrate
Zinc oxide
Cr-Cu-Ag-Mo salts
0.1-3
Phosphine, arsine
10
10-20
Hydrogen cyanide
Civil and military gas protection (phosgene,
chlorine, arsine, chloropicrin, sarin, other
nerve gases)
For gas purification, the most common particle size for either granular or pelletized AC ranges
between 0.8-4 mm. An appropriate impregnation method must be carried out to obtain a highquality result, while the product’s specific surface area and pore size are usually affected by
the deposition of additives. It is important to avoid the pores blocking by impregnates, which
helps to distribute finely dispersed chemicals on the surface of AC in a homogeneous manner
[65, 66]. Occurrence of pores blocking negatively affects the performance of additive-loaded
AC as it depends on its porous structure.
Another factor affecting the effects of additives is the preparation technique as shown in the
Table 7 [55, 67]. It can be seen, that the surface area of the AC is dependent on chronological
order of the preparation technique.
Table 7: Influence of CoO loading on BET specific area and porous structure of AC [55]
CoO loading in
BET
AC
Units wt.%
m2g-1
0
1190
1.5a
1100
3.0a
1060
1.5b
789
3.0b
440
a- impregnation after activation
Micropore
volume
cm3g-1
0.582
0.509
0.487
0.405
0.203
Mesopore
volume
cm3g-1
0.032
0.03
0.031
0.13
0.276
b- impregnation before activation
12
3 Purification of flue gas
Combustion results in an exhaust gas including oxides of carbon, sulphur, nitrogen, as well as
other pollutants. Therefore, AC can be used for the removal of such impurities. Despite its
lower efficiency when removing sulphur and nitrogen oxides from flue gas compared to its
adsorptive capabilities of VOC, it is frequently used [51, 68, 69, 70] for these impurities. AC
or cokes manufactured from lignite are mainly used to separate SO2 and dioxins from the flue
gas.
A common separation process of SO2 from flue gas includes further oxidation of SO2. The
oxidation is illustrated in the following reaction, where SO2 reacts with O2 and H2O. When SO2
contacts the AC, the oxidation is promoted and results in H2SO4.
2𝑆𝑂2 + 𝑂2 + 2𝐻2 𝑂 → 2𝐻2 𝑆𝑂4
(3)
Afterwards, thermal regeneration of sulphuric acid loaded coke takes place from 400-500 o C.
The desorption of sulphuric acid occurs through the following reaction:
2𝐻2 𝑆𝑂4 + 𝐶 → 2𝑆𝑂2 + 𝐶𝑂2 + 2𝐻2 𝑂
(4)
Through the regeneration process, AC with higher catalytic activity and enhanced specific
surface area is produced as a result of the partial combustion of carbon.
In order to remove nitrogen oxides (NOx) from flue gases, the NOx is reduced through the
reaction with ammonia gas. AC acts as a catalyst within a temperature ranging from 100150 oC, resulting in the following reactions [71]:
4𝑁𝑂 + 4𝑁𝐻3 + 𝑂2 → 4𝑁2 + 6𝐻2 𝑂
(5)
2𝑁𝑂2 + 4𝑁𝐻3 + 𝑂2 → 3𝑁2 + 6𝐻2 𝑂
(6)
PAC is commonly utilized for the treatment of gases from waste incineration plants. The
contact between the gases and the AC is achieved by injecting the PAC into the stream of flue
gas. Afterwards, separation of PAC from the gas takes place on a fabric filter. Further
purification of gas takes place on a fixed AC bed [72]. The injection or spraying of AC and/or
fixed bed columns is considered as state-of-the-art gas cleaning method for dioxins in plants
with various sizes. Table 8 shows the operational conditions of two examples for the removal
of dioxins and furans.
13
Table 8: Operational conditions used for removal of dioxins and furans on activated coke [72].
Absorber type
Unit
One moving bed,
counter current
Two moving beds,
counter current
Inlet
conc.
ng m-3
Outlet
conc.
ng m-3
Flow rate
Temp.
m3 h-1
K
Bed height Specific
surface area,
m
m2 g-1
54-100
0.65-9
1300-3000
403-423
1.5
150
26-86
<0.05
45000-60000
333-353
0.5
150-300
3.1 Volatile organic compounds (VOC) removal
AC proves an astonishing efficiency when dealing with the removal of VOC. Due to the AC’s
high porosity and relatively big surface area, it is suitable and easy for the adsorption of
hydrocarbon and organics. Due to AC’s very high adsorption capacity, it is elected to be the
most suitable material for separating air from VOC and other gases [73, 74, 75, 76, 77] to reach
a 99% removal. The general efficiency of the adsorption process by AC is dependent on some
factors during its life cycle which ranges from 12-18 months before being replaced. Whereas
the higher the concentration of VOCs within the gas mixture, the higher the uptake of such
organics.
3.2 SO2 adsorption
SO2 removal by AC is a process applied to separate SO2 from flue gas. SO2 adsorption duration
and efficiency is dependent on various parameters such as the presence of O2 and H2O in the
flue gas and their concentration, the temperature under which the adsorption takes place, the
concentration of SO2 itself within flue gas and the mechanism of SO2 removal by the AC.
Effects of different parameters on the SO2 adsorption by AC
Under presence of oxygen and water vapour, oxidization of SO2 into sulphuric acid (H2SO4)
takes place at the AC. Depending on the amount of O2 and H2O, the period in which SO2 is
adsorbed on AC differs significantly. The pores of AC then act as hosts to store the formed
sulphuric acid [78, 79]. Figure 1 demonstrates this influence.
Under the absence of O2 and H2O, the adsorption saturation of the SO2 on the AC is reached
after 24 minutes. Due to the absence of O2 and H2O the adsorption is not promoted, and
14
saturation is reached in a relatively short time with a low SO2 adsorption capacity of
approximately 16 mg/g in this case.
With the same amount of SO2 and the presence of 6 % O2 in the flue gas but the absence of
H2O, the adsorption saturation of SO2 on the AC is reached after 85 minutes. Due to the
presence of O2 the adsorption is promoted, and the saturation is reached with an SO2 adsorption
capacity of approximately 24 mg/g.
Again, using the same amount of SO2 while having 6 % O2 and 8 % H2O in the flue gas, the
adsorption saturation of SO2 on the AC is not finally reached after 120 minutes. Due to the
presence of both O2 and H2O the adsorption is further promoted, and the saturation turns out to
be approximately 53 mg/g after 120 min.
Figure 1: Effect of O2 and H2O on SO2 adsorption
SO2 undergoes catalytic oxidation and turns into sulphuric acid (H2SO4), which facilitates the
removal of SO2 from the flue gas. The reaction temperature affects both, SO2 adsorption onto
AC as well as the prementioned catalytic oxidation. With increasing temperature the SO2 initial
adsorption rate and capacity decreases. However, a certain temperature should be maintained
to break and recreate bonds. The next Figure illustrates the effect of temperature on SO2
adsorption with an SO2 concentration of 0.3 %, O2 concentration of 6 % and H2O concentration
of 8 % [80].
15
Figure 2: Effect of temperature on SO2 adsorption
Last, but not least, the SO2 concentration influences the adsorption capacity as well. The SO2
initial adsorption rate is the average amount of SO2 adsorbed in the timeframe of 3 minutes
from the beginning of the adsorption process. That rate is directly proportional to the
concentration of SO2 within the flue gas. Increasing SO2 concentration leads to an increase in
the adsorption’s driving force of SO2 [81]. Table 9 shows the influence of SO2 concentration
on SO2 initial adsorption rate.
Table 9: Effect of temperature and SO2 concentration on SO2 initial adsorption rate
Sample
size
in mm
0.75
SO2 concentration
in %
SO2 initial adsorption rate in mg/(g min)
65 oC
80 oC
90 oC
100 oC
0.02
0.458
0.458
0.458
0.458
0.1
2.218
2.134
2.053
1.928
0.2
3.592
3.278
2.941
2.82
0.3
4.703
4.288
3.693
3.439
16
The mechanism of SO2 adsorption by AC
The SO2 adsorption on AC comprises two stages. The first is physically adsorbed SO2 known
as physisorption. The second is oxidizing SO2 to SO3 and further formation of sulphuric acid
(H2SO4). The physisorbed SO2 desorbs at a temperature of about 150oC while the strongly
adsorbed SO2 desorbs within the temperature range of 200-500oC [82]. Using the same values
used for the experiments in Figure 1, a thermogravimetric analysis of AC after SO2 adsorption
is illustrated in Figure 3.
Figure 3: Thermogravimetric analysis of the samples after SO2 adsorption
AC without SO2
Analysing Figure 3, a mass reduction of 2.5% at the temperature range of 30-430oC is noticed.
This reduction has taken place as a result of moisture release. The mass continues to decrease
slightly at 430-1000oC and that could be explained as a result of decomposition of surface
oxygen functional groups.
Presence of SO2 without O2 and H2O (red line)
Within the temperature range of 30-430oC a 4.2% of mass reduction takes place. When
comparing this to the case of untreated AC, a slight increase in mass reduction is noticed. As a
result of water and physically adsorbed (physisorbed) SO2 desorption at 70oC, a peak in the
derivative thermogravimetric curve (DTG) is observed. Due to the absence of O2, oxidation of
SO2 to SO3 does not take place and only one state of adsorption, which is physical adsorption
(physisorption) of SO2, occurs. This can be deduced from the fact that there is no weight loss
due to desorption of strongly adsorbed SO2 [83].
17
Presence of SO2 with O2 and without H2O (blue line)
In this case a peak is reached within the temperature range of 230-430oC in the DTG curve
after SO2 adsorption due to desorption of SO3 [83].
Presence of SO2 with O2 and H2O (green line)
In this case a large peak is present within the temperature range of 230-430oC in the DTG curve
for AC after SO2 adsorption. This indicates the increase in the amount of SO3 and hence H2SO4
generated in the presence of O2 as well as H2O in comparison to the presence of O2 only.
3.3 NOx adsorption [84]
Nitrogen oxides are side products of flue gas during combustion processes. They are
characterised by their high toxicity and are indicated to as NOx. The adsorption method of NOx
on AC is physisorption. Generally, the bigger the surface area of AC the higher the adsorption
capacity of the NOx. However, NOx removal efficiency is low even with high surface area and
pore volume [85]. Research for reducing NO through carbon-based catalysts using various
metal oxides has yielded very promising results. These metal oxides are oxides of Mn, Fe, V,
Cu and Ni [86].
Effects of accompanying gases on NOx adsorption
Due to the chemisorption of O2 and the conversion of the OH-groups on the surface of the AC,
the presence of the O2 during the adsorption of the NOx has a positive effect. The OH groups
have affinity to the NOx. Moreover, the O2 acts as an oxidizing agent to convert NO to NO2 for
the subsequent adsorption. AC impregnated with KOH is also used for the removal of NOx and
SO2 due to the ability of potassium ions to act as catalyst for this process. This kind of removal
takes place in an oxygen-rich environment. In general, O2 favours the removal of NOx more
than SO2. When a simultaneous removal takes place SO2 gets chemisorbed quicker due to its
high affinity to the AC compared to NOx.
In summary, the breakthrough for NOx takes place faster and its adsorption capacity decreases,
the more SO2 gets adsorbed. Further studies illustrate that for flue gases with SO2, CO2, H2O,
O2 and N2, the selective adsorption of NOx is most effective when the O2 concentration is high,
the SO2 concentration is low and the temperature is between 35 and 120 oC. Within that
temperature range the NO2 is favoured over the CO2. Due to the high adsorption rate and
18
capacity of SO2, it is recommended for the SO2 adsorption to take place before the NOx
adsorption [85]. The following results are represented in a study dealing with simultaneous
adsorption of SO2, NO, H2O, O2 and N2 at 120 oC [87].
I.
The chemical adsorption spots for SO2 and NO are identical, whereas SO2 is more
tightly bound to these spots than NO.
II.
III.
The amount of adsorbed SO2 is tripled in the presence of H2O and NO in the exhaust.
SO2 reduces the NO adsorption up to 93 %.
In summary, NO promotes the adsorption of SO2, while the SO2 impedes the NO adsorption.
This inhibition effect of SO2 on NO is caused due to the weaker Van-der-Waals forces of NO
[87].
Adsorption of NOx by means of NO oxidation
Due to the difference in behaviour between NO and NO2 while being adsorbed, the adsorption
of NOx through the oxidation of NO is useful. This difference in behaviour includes that NO
and NO2 get adsorbed on different adsorbents and within different adsorption conditions. For
a low NO concentration, a poor NO removal is noticeable due to the low adsorption capacity.
Table 10 shows the complexity of the chemical mechanism for NO adsorption and desorption
at 100 – 150 oC. These chemical equations proof the system to be complex, where NO
adsorption, NO reduction, NO oxidation, NO2 adsorption, reductive NO2 desorption, H2O
adsorption and O2 gasification take place simultaneously [88]. Further information about the
detailed mechanism can be found in [84].
Table 10: Equation of the adsorption and desorption of NO [88]
Dimer- Formation and NO-Reduction
NO catalytic oxidation
NO2 reductive desorption
2(𝑁𝑂) → (𝑁𝑂)2
(12)
2𝐶𝑓 + (𝑁𝑂)2 → 2𝐶(𝑂) + 𝑁2
(13)
𝐶𝑓 + (𝑁𝑂)2 → 2𝐶(𝑂) + 𝑁2
(14)
𝑁𝑂 → (𝑁𝑂)
(15)
2(𝑁𝑂) + 𝑂2 → 2𝑁𝑂2
(16)
(𝑁𝑂2 ) + 𝐶𝑓 → 𝑁𝑂 + 𝐶𝑂
(17)
2(𝑁𝑂2 ) + 𝐶𝑓 → 2𝑁𝑂 + 𝐶𝑂2
(18)
19
O2 gasification
2𝐶𝑓 + 𝑂2 → 2𝐶(𝑂)
(19)
2𝐶(𝑂) → 2𝐶𝑂2 + 𝑛𝐶𝑓
(20)
𝐶(𝑂) → 𝐶𝑂 + 𝑛𝐶𝑓
(21)
𝐶(𝑂) ↔ 𝐶 − 𝑂
(22)
In total four different NO species are adsorbed on AC during the previously described process
[89]:
I.
Weekly adsorbed NO, desorbing at testing temperatures.
II.
Strongly adsorbed NO2, desorbing at low temperatures.
III.
Strongly adsorbed NO, desorbing at middle temperatures.
IV.
Strongly adsorbed (NO)2-dimer, desorbing at high temperatures.
However, the adsorption velocity increases with an increase in O2 in the flue gas and decreases
with increasing H2O concentration [88]. Flue gases used in the industry mostly have an amount
of air humidity, which plays an important role in the oxidation conversion of NO. The
efficiency of oxidation is decreased in the presence of air humidity as it occupies the
oxidization areas, and therefore the catalytic adsorption of NO on AC is decreased. In case of
dry conditions, NO conversion and adsorption take place noticeably better. Nevertheless,
modifications on AC through heat treatment at 800oC can be applied to improve the NO
conversion under humid conditions [85].
Selective catalytic reduction (SCR) of NO by means of NH3
Conventional SCR is associated with high catalyst cost, high temperatures and ammonia slip
[85]. N2 gas results out of the reduction of the toxic NO gas. Through SO2 poisoning a catalytic
deactivation may take place [90]. To overcome these problems SCR utilizes AC based catalysts
which are doped with reduction agents like NH3 and urea [85]. As N2 is formed, NO and NO2
are reduced by means of NH3 on the AC as shown in the following equations (23) and (24)
[91].
4𝑁𝑂 + 4𝑁𝐻3 + 𝑂2 → 4𝑁2 + 6𝐻2 𝑂
(23)
6𝑁𝑂2 + 8𝑁𝐻3 → 7𝑁2 + 12𝐻2 𝑂
(24)
20
The SCR process is executed through the following three steps [92]:
I.
II.
NH3 gets adsorbed on the AC centres.
NO gets adsorbed on the AC centres and is oxidized to NO2, while surficial carbon
atoms are oxidized through adsorbed O2 to CO.
III.
Gaseous NO and the adsorbed NOx are reduced through the adsorbed NH3 and CO.
Formation of N2, H2O and CO2.
In the presence of O2 and especially at low temperatures, the reduction from NO with NH3 is
improved [85]. Despite that NO and NH3 compete for the selective adsorption on the AC’s
surface area, but the main rate determining step is the oxidation of NO [85, 92].
Figure 4 shows the complexity of a possible surface mechanism of the formed nitrogen
compounds [85].
Figure 4: Surface area mechanism during the SCR [87]
Through many studies regarding SCR method on AC and the effects of different parameters, it
has been shown that the presence of O2, metal oxides and the pre-treatment of AC encourage
SCR activity. Temperature has the most significant effect on the SCR compared to feed’s
superficial velocity and NH3/NO ratio. When the temperature rises in the range of 30 – 250 oC
along with an increased humidity the NO reduction decreases. Increased humidity leads to its
condensation in the pores, which hinders the reduction due to its limiting effect on the reaction
between NH3 and NO [85]. Furthermore, the AC’s surface area can be modified with functional
groups containing oxygen, acidic bonding sites as well as oxygen bound by metal oxides
21
improving the SCR [85, 92]. On the other hand, the structural characteristics (e.g. BET-surface
area or the pore volume) do not affect the catalytic activity [93].
The following Table summarizes factors affecting the adsorption of NO on AC:
Table 11: Factors affecting NO adsorption
Material separated
Positive effect
Negative effect
No effect
NOx through oxidation
O2 [85, 94], O3 [85];
surface area modification
with metal oxides [86, 94],
KOH [85]
O2, SO2; surface area
modification with metal
oxides [85] and functional
groups containing oxygen
[85, 92]
SO2 [85],
H2O [85];
high temperature
[88, 89]
H2O [85];
high temperature
[85]
Structural
characteristics
of AC [93]
NOx through NH3
22
4 Preliminary experiment and test plant
Preliminary experiments have been conducted to gain valuable information for the operation
of the test plant and the assessment of innovative processes regarding to NOx reduction.
4.1 Investigation of NH3 adsorption capacity of AC at different
partial pressures
Investigations about NH3 preloading capacity for subsequent NOx reduction are presented in
this chapter. The principle behind the experiments is that the ammonia dissolved in water will
start evaporating then find its way to reach the other bottle where the AC is placed. The AC
will then adsorb part of the ammonia present in vapour, allowing us to observe a change in the
AC’s weight as a reason of the adsorption process taking place. However, the AC adsorbs
steam as well. Thus, a gravimetric determination of the adsorbed NH3 is not possible. For this
reason, the NH3 of the preloaded AC is dissolved and analysed in diluted sulphuric acid to
calculate the amount of NH3 adsorbed on the AC.
NH3 preloading with 0.26 bar, 0.09 bar and 0.84 bar
In the first experiment 18% and 9% ammonia solutions were prepared. Thus, the partial
pressure in the atmosphere over the solution is 0.26 bar and 0.09 bar, respectively. The solution
was then put in a 0.5 l bottle. In the other 0.25 l bottle, a sample of approx. 12 g of our AC has
been placed. The two bottles were then connected through a series of tubes. At the upper part
of the top tube a hole has been opened in order to seek pressure equilibrium within the vapour
pressure of NH3 in the system.This set-up has been left for 24 hours to allow some time for the
adsorption to take place. Afterwards, the NH3 loading of AC was checked twice. Firstly, the
AC is left under atmospheric conditions for about 16 hours. Secondly, half of the NH3
preloaded AC was heated up to 140oC for about 2 hours. In the second experiment the 18%
ammonia solution was used. The difference is that the ammonia solution was heated to 50oC
and left for the 24 hours to reach adsorption of the NH3 on the AC.
23
The following Figure is a sketched illustration of the experimental setup.
Öffnung zum
Druckausgleich
Öffnung zum
Druckausgleich
18%ige
Ammoniaklösung
(100g 25%ig
39g H2O)
Wasserbad 50 °C
Aktivkohle approx. 12 g
Aktivkohle approx. 12 g
18%ige
Ammoniaklösung
(100g 25%ig
39g H 2O)
Figure 5: Experimental setup with and without heating
Ammonia analysis using sulphuric acid
The NH3 preloaded samples were left 16 hours under atmospheric conditions and half of the
sample was heated to 140 °C for two hours. Afterwards, the samples have been taken and left
in 250 ml of sulphuric acid for more than 48 hours (Figure 6). Then, the content of ammonium
within the AC has been analysed. The obtained values from ammonium have been recalculated
to finally obtain the loading values of ammonia.
250 ml
Schwefelsäure
Aktivkohle approx. 5 g
Figure 6:NH3 loaded AC in sulphuric acid
24
For 0.26 bar NH3 partial pressure the NH3 loading of the AC was analysed to be 0.35 gNH3/kg.
After the heat up to 140 oC it further dropped to 0.18 gNH3/kg. In the second analysis (0.84 NH3
partial pressure) the loading was 0.73 gNH3/kg after 16 h under atmospheric conditions and 0.45
gNH3/kg after heating up procedure. In the third analysis with 0.09 bar NH3 partial pressure 0.19
gNH3/kg and 0.04 gNH3/kg, respectively were measured.
The following table is an illustration of the results of the trials and the NH3 analysis:
Table 12: Results of trials and NH3 analysis
bar
1st Trial
(18% ammonia
solution)
0.26
2nd Trial
(18% ammonia
solution, 50 oC)
0.84
3rd Trial
(9% ammonia
solution)
0.09
gNH3/kg
gNH3/kg
0.45
0.18
0.73
0.35
0.19
0.04
Unit
NH3 partial pressure
Loading (20 °C)
Loading (140 °C)
The following Figure shows the NH3 loading on the surface of the AC under two conditions
for different NH3 partial pressures. The first is at 20 oC and the second is after heating the AC
to a temperature of 140oC for 2 hours.
NH3 loading in mg/kg
800
Under atmospheric
conditions
After 2 hrs at 140 °C
600
727
447
400
347
200
186
183
0
0
0.00
40
0.10
0.20
0.30 0.40 0.50 0.60 0.70
Partial pressure of NH3 in bar
0.80
0.90
Figure 7: Variation of NH3 deposited on the surface of AC
It facilitates deduction of the value of the weight of ammonia deposited on the surface of AC
at any point along the variation of the NH3 partial pressure within the two given temperatures.
25
The NH3 deposited on the AC’s surface and within its pores is constantly increasing with the
increase of the NH3 partial pressure. By heating up the AC to temperature of 140 oC for two
hours, approximately 50-80% of NH3 was desorbed.
4.2 Pressure loss of the AC fixed bed
When a fluid passes through a fixed bed, friction between that fluid and the particles in the
fixed bed takes place. Due to the occurrence of such friction, the fluid experiences a pressure
loss. The behaviour has been explained and described through the usage of the Ergun equation
(1952) [95], which also takes into account turbulent conditions.
∆𝑃
𝐻
•
•
•
•
•
•
•
= 150.
(1−𝜀)2
𝜀3
.
𝜇.𝑈
𝑑𝑠𝑣 2
+
(1−𝜀) 𝜌𝑓 .𝑈 2
1.75. 3 .
𝜀
𝑑𝑠𝑣
(25)
𝐻: height of packed bed in m
∆𝑃: pressure drop through the packed bed in Pa
𝑈: superficial velocity in m/s
𝑑𝑠𝑣 : Sauter mean diatmeter
𝜀: bed’s porosity
𝜌𝑓 : density of fluid through the packed bed in kg/m3
𝜇: viscosity of the fluid flowing through the packed bed in Pa.s
Ergun’s equation is mainly dealing with both the laminar and the turbulent behaviour and their
pressure loss. In the first part, the laminar behaviour, in which Re is less than 1, has been
represented relaying on the Carman-Kozeny equation. In contrast to the turbulent regime, the
pressure drop of the laminar regime is independent from the density of the fluid while in the
turbulent regime the pressure drop is directly proportional to the fluid’s density.
Moreover, within the laminar regime the pressure drop has a linear correlation to the superficial
velocity and in the turbulent regime the pressure is increasing in a squared manner with the
superficial velocity.
In case of a very turbulent flow, represented by Re > 1000, the second term of the Ergun
equation is the dominating factor [96].
𝜌𝑓 . 𝑈 2
∆𝑃
𝑝𝑟𝑜𝑝
𝐻
𝑑𝑠𝑣
26
While dealing specially with non-spherical particles packing of the bed, the parameter Sauter
mean diameter 𝑑𝑠𝑣 will substitute 𝑑𝑝 , which resembles the spherical equivalent particle
diameter. In case of unsimilarity of particle size the parameter 𝑑̅𝑠𝑣 , representing the average
Sauter mean diameter, will substitute 𝑑𝑠𝑣 . The Sauter mean diameter, originally introduced in
the late 1920s by the German scientist ‘Josef Sauter’ [97], deals with the diameters of the
shaped particles. It is defined as the diameter of a sphere that has the same volume to surface
area ratio as a particle of interest.
Calculation of the Ergun equation for cylindrical shaped AC
The Ergun equation is used to calculate the pressure drop of the AC fixed bed. In order to
obtain the diameter of the cylindrical shaped AC particles in Figure 8, ten randomly selected
carbon particles have been taken out and measured, then from the results a mean average
diameter has been calculated. The result is 0.0084 m.
Figure 8: Activated carbon particles used for the experiments
Further, the porosity of the fixed bed needs to be calculated. Given the fact that the porosity of
a fixed bed filled with AC equals to 1 minus its bulk density divided by density of a single
particle, a small experiment has been executed to obtain this value.
To determine the bulk density, 3 samples of 800 ml of AC have been taken and weighed
excluding the weight of the beaker including them. Dividing their weight by their volume, an
average bulk density of 653 kg/m3 has been obtained. The experimental data are displayed in
Table 13 and the setup is displayed in Figure 9.
27
Figure 9: AC used for the preliminary experiments
After determining the diameters of the AC, its heights and weights have been similarly
determined. Due to the cylindrical shape of the particles, an assumption of calculating their
volume as cylinders has been applied. From all the calculated densities of the ten samples an
average has been determined to be 976 kg/m3. Table 14 shows the experimental data of this
experiment.
Table 13: Values for obtaining the average bulk density
Unit
Weight of
samples
Volume
Bulk density of
the samples
I
II
III
kg
0.53
0.52
0.51
L
0.0008
0.0008
0.0008
kg/m3
665.87
650.12
642
Average bulk
density
653
Table 14: Values for obtaining the average particle density
Particle
weight
Units [kg]
0.00103
0.00104
0.00068
0.00078
0.00086
0.00074
0.00058
Particle
diameter
[m]
0.0083
0.0084
0.0081
0.0083
0.0087
0.0087
0.0081
Particle
length
[m]
0.018
0.019
0.015
0.014
0.014
0.012
0.012
Particle
volume
[m3]
9.69*E-07
1.03*E-06
7.73*E-07
7.79*E-07
8.47*E-07
7.37*E-07
6.18*E-07
Particle
density
[kg/m3]
1063.12
1010.31
885.96
1002.79
1012.97
1011.07
941.52
Average particle
density
[kg/m3]
28
0.00058 0.0083
0.00059 0.0082
0.00066 0.0086
0.012
0.012
0.012
6.22*E-07
6.07*E-07
7.14*E-07
936.81
968.19
925.84
976
Dividing the average bulk density by the average particle density, the porosity of the AC bed
has been determined:
653
𝜀 = 1 − 976 = 0.331
(26)
Since 𝑑𝑠𝑣 ≈ 𝑑𝑝 for elliptically shaped particles [96], it is assumed, that this is approximately
valid for the cylindrically shaped particles displayed in Figure 8 as well. The average diameter
of the ten chosen samples does not need an adjustment factor and can be directly used in the
Ergun equation. Finally, Table 15 summarizes all values necessary for the calculation of the
pressure drop with the Ergun equation.
Table 15: Values used for the Ergun equation
Height Superficial
of bed (𝐻) velocity (𝑈)
Units
m
0.75
Averaged particle
size (𝑑̅𝑠𝑣 )
Bed’s
porosity (𝜀)
Density of
the fluid (𝜌𝑓 )
Viscosity of
the fluid (𝜇)
m
0.0083
0.33
kg/m3
1.204
Pa.s
18.13*10-6
m/s
0 – 0.48
Table 16 shows a summary of the experimental results of the pressure measurement for a bed
height of 3 m and a superficial velocity ranging from 0 to 0.48 m/s. Additionally, the calculated
(Ergun equation) pressure gradients are given as well.
Table 16: Measurements for obtaining the calculated pressure gradient
-
I
II
III
IV
V
VI
VII
Column temp.
Flow
Press. loss
°C
Nm3/h
mbar
0
0
2
2.8
3
4.8
4
7.6
5
10.8
6
14.4
8
23.6
10
35.2
Bed height
m
3
3
3
3
3
3
3
3
mbar/m
0
0.9
1.6
2.5
3.6
4.8
7.9
11.7
mbar/m
0
0.9
1.7
2.4
3.8
5.2
8.7
13.0
0.29
0.38
0.48
Measured press.
gradient
Calculated press.
gradient
Cross sec.
column
Superficial
velocity
~ 20
m2
m/s
~ 5.8*E-3
0
0.1
0.14
0.19
0.24
29
The following Figure shows the pressure drop in dependency on the superficial velocity based
on the results obtained in Table 16.
14
Pressure gradient in mbar/m
12
10
Measured pressure
gradient
8
6
Calculated
pressure gradient
4
2
0
0
0.1
0.2
0.3
0.4
0.5
0.6
Superficial velocity in m/s
Figure 10: Relation between pressure gradient and superficial velocity at 20 oC
The measured pressure gradient and the calculated pressure gradient are in a good agreement
for low superficial velocities (up to 0.25). For an increasing superficial velocity, and therefore
increasing turbulent flow, the accuracy of the calculated values decreases noticeably. At a
superficial velocity of 0.48 m/s the actual pressure gradient shows a value of 11.7 mbar/m,
whereas the calculated pressure gradient is 13 mbar/m. Eventually, the difference can be traced
back to the assumption, that the cylindrical particles are treated like elliptical ones for the
calculation of the Sauter mean diameter 𝑑𝑠𝑣 . However, in general a satisfactory agreement
between the measured pressure gradient and the one calculated by the Ergun equation can be
observed for the investigated range.
30
4.3 Test plant for adsorption on AC
Figure 11 shows a picture and Figure 12 shows the flow chart of the test plant, which can be
used for tests of the AC regarding to NO reduction efficiency and SO2 adsorption capacity. The
purpose of such experiments is to test for the AC adsorption capabilities under specific
conditions and selected dosed gases.
SO2, NO and NH3 can be dosed through three different tubes into the unit. NO and NH3 can be
3 ⁄
dosed within the range of 100-1000 ppm and with a volumetric air flow of 10 𝑚𝑠𝑡𝑝
ℎ. The
flow rates of NO, NH3 and SO2 are controlled through a float-type flowmeter from the company
“INFLUX”.
Through a dosing pump, shown on top of the flow chart, water can be added to the gas stream
and guarantees a defined moisturizing of the gas. Further, the gases are heated through
electrical heating units, afterwards mixed in a gas mixer and dosed to the columns where the
AC is located. The inner diameter of the columns is 86 mm and filled with AC to a filling level
of approximately 0.75 m per column. Altogether, the plant comprises of four columns. Thus, a
total filling level of approximately 3 m can be reached. Within these columns adsorption by
means of AC takes place. The columns are heated externally and thus temperatures up to
140 °C can be set. For controlling purposes, before and after each column a gas sample can be
taken and analysed. Moreover, temperatures at the top and bottom of each column are tracked.
In column 3 the temperature can be tracked in the middle of the column additionally.
The gas analysis of SO2 and NO components is executed online through the measurement
apparatus “NGA-2000” of the company “Rosemount”. NH3 can be analysed through analysis
tubes of the company “Dräger”.
31
Figure 11: Test plant for characterization of activated carbon
32
M
Demiwater
NH3
10 L
SO2
NOx
QI
QI
QI
FI
Gas-Cooler
SO2
FI
NO
TC
FI
NH3
TI
TI
TI
TI
DN80
DN80
DN80
DN80
TI
TIC
6
bar
1.2
bar
FI
Mixer
Clean Gas to
Atmosphere
Fresh Air
PI
N2
Figure 12: Flowchart of the AC test plant
TI
TI
TI
TI
5 Conclusion
A broad literature review shows the widespread application of AC in different fields. The
application of AC can be divided into liquid phase application and gas phase application. The
AC used for liquid phase applications shows a relatively larger pore size compared to those
used for gas phase applications. Typical liquid phase applications are potable water cleaning
and waste water cleaning, whereas gas phase applications aim at e.g. gas cleaning from
incineration plants to remove organic species, mercury, lead and acidic gases like SO2 and NOx.
The SO2 adsorption duration and efficiency of AC is dependent on various parameters such as
the presence of O2 and H2O in the flue gas and their concentration, the temperature under which
the adsorption takes place, the concentration of SO2 itself within flue gas and the mechanism
of the AC regeneration. NOx removal efficiency is low even with high surface area and pore
volume of the AC. Nevertheless, the AC can act as a catalyst to promote NOx reduction in
presence of NH3. However, the adsorption capacity increases with an increase in O2 in the flue
gas and decreases with increasing H2O concentration.
Preliminary experiments have been conducted to gain valuable information for the operation
of a test plant and the assessment of innovative processes regarding to NOx reduction. Thus,
the NH3 adsorption capacity of AC was investigated under ambient conditions on the one hand
and at 140 °C on the other hand. Different partial pressures from 0.09 bar to 0.84 bar were
investigated. A constant increase in the NH3 adsorbed on the AC’s surface with the increase of
NH3 partial pressure was found. Further, it turned out, that between 50% and 80% of the
adsorbed NH3 is desorbed by heating the samples to 140 °C. Finally pressure loss, calculations
have been made using the Ergun equation. A satisfactory agreement between the measured
pressure gradient and the one calculated by the Ergun equation could be observed for the
investigated range.
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List of Figures
Figure 1: Effect of O2 and H2O on SO2 adsorption ............................................................................... 15
Figure 2: Effect of temperature on SO2 adsorption ............................................................................... 16
Figure 3: Thermogravimetric analysis of the samples after SO2 adsorption ......................................... 17
Figure 4: Surface area mechanism during the SCR [87]....................................................................... 21
Figure 5: Experimental setup with and without heating ....................................................................... 24
Figure 6: Loaded AC in sulphuric acid ................................................................................................. 24
Figure 7: Variation of NH3 deposited on the surface of AC ................................................................. 25
Figure 8: Activated carbon particles used for the experiments ............................................................. 27
Figure 9: AC used for the preliminary experiments.............................................................................. 28
Figure 10: Relation between pressure gradient and superficial velocity at 20 oC................................. 30
Figure 11: Test plant for characterization of activated carbon ............................................................. 32
Figure 12: Flowchart of the AC test plant............................................................................................. 33
List of Tables
Table 1: Typical parameter of carbon adsorbents for liquid-phase applications [6] ............................... 5
Table 2: Typical design data of adsorbers for water purification [6] ...................................................... 6
Table 3: Total organic carbon reduction by GAC in some industrial waste water [20] ........................ 7
Table 4: Typical parameter of carbon adsorbents for gas-phase applications [6] ................................. 10
Table 5: Typical design data of adsorbers for adsorptive gas purification [6] ...................................... 10
Table 6: Commercial grades of impregnated ACs for gas-phase applications [63, 64] ........................ 11
Table 7: Influence of CoO loading on BET specific area and porous structure of AC [55] ................. 12
Table 8: Operational conditions used for removal of dioxins and furans on activated coke [74]......... 14
Table 9: Effect of temperature and SO2 concentration on SO2 initial adsorption rate .......................... 16
Table 10: Equation of the adsorption and desorption of NO [92]......................................................... 19
Table 11: Factors affecting NO adsorption ........................................................................................... 22
Table 12: Results of trials and NH3 analysis......................................................................................... 25
Table 13: Values for obtaining the average bulk density ...................................................................... 28
42
Table 14: Values for obtaining the average particle density ................................................................. 28
Table 15: Values used for the Ergun equation ...................................................................................... 29
Table 16: Measurements for obtaining the calculated pressure gradient .............................................. 29
43
List of abbreviations
Abbreviation
Explanation
AC
Activated carbon
(ad)
Adsorption state
Ag
Silver
approx..
Approximately
ASH3
Arsine
BET-method
Brunauer-Emmert-Teller method
C
Carbon
Cf
Free active spots on the activated carbon’s surface
Cl2
Chlorine
cm
Centimetre
cm3
Cubic centimetre
Conc.
Concentration
CoO
Cobalt oxide
CO2
Carbon dioxide
COS
Carbonyl sulphide
Cr
Chromium
CS2
Carbon disulphide
Cu
Copper
d
Diameter
dm
Decimetre
dp
Particle diameter
dsv
Sauter diameter
DTG
Derivative thermogravimetric curve
e.g.
(Exempli gratia) meaning for example
g
Gram
(g)
Gas phase
GAC
Granular activated carbon
H
Height
H2
Hydrogen
44
H2O
Water
H2S
Hydrogen sulphide
HCl
Hydrogen chloride
HF
Hydrogen fluoride
Hg
Mercury
hr
Hour
kg
Kilogram
KI
Potassium iodide
l
Litre
(liq)
Liquid phase
m
Metre
m2
Squared metre
m3
Cubic metre
mbar
Millibar
mg
Milligram
min
Minute
mm
Millimetre
Mn
Manganese
MO
Molybdenum
NH3
Ammonia
NH4NO3
Ammonium nitrate
nm
Nanometre
Nm3/hr
Newton cubic metre per hour
NO
Nitric oxide or Nitrogen monoxide
NO2
Nitrogen dioxide
(NO)2
NO-dimer
NOx
Nitrogen oxide
O2
Oxygen
PAC
Powdered activated carbon
Pa
Pascal
PH3
Phosphine or Phosphane
ppm
Part per million
s
Second
45
S
Sulphur
SCR
Selective catalytic reduction
SO2
Sulphur dioxide
stp
Standard temperature and pressure
t
Time
T
Temperature
TEDA
Triethylenediamine
TG
Thermogravimetric curve
U
Superficial velocity
v
Volume
VOC
Volatile organic compound
wt.
Weight
X
Loading
−∆𝑃
Pressure drop
𝜀
Porosity
𝜌𝑓
Fluid density
𝜇
Dynamic viscosity
o
Degree Celsius
C
46