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Investigation of Carbon Black Production from Coal Tar via Chemical Looping Pyrolysis Xu Jiang, Longlong Zhang, Fengyin Wang, Yongzhuo Liu, Qingjie Guo, and Cuiping Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02915 • Publication Date (Web): 16 Feb 2016 Downloaded from http://pubs.acs.org on February 22, 2016
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Investigation of Carbon Black Production from Coal Tar via Chemical Looping Pyrolysis
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Xu Jiang1, Longlong Zhang1, Fengyin Wang1, Yongzhuo Liu2, Qingjie Guo2,
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Cuiping Wang1,*
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1. Energy Engineering Research Institute, Qingdao University, Qingdao 266071, China
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2. College of Chemistry and Chemical Engineering, Qingdao University of Science and
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Technology, Qingdao, 266042, China
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Abstract: Chemical looping pyrolysis (CLP) process was used to produce reinforced
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carbon black (CB) in this paper, and the effects on energy conservation and emission
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reduction of the method were investigated. The characteristics of the oxygen carrier (OC)
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type, particle size, and reaction temperature on the CB yield and the morphological
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structure were examined. In addition, the CB morphological structure and flue gas
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emission were analyzed using the TEM, XRD, GC (Gas Chromatography), and flue gas
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analyzer. The results indicated that when the same particle size and reaction temperature
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were used, compared with CaSO4 OC, Fe2O3 OC was more effective at catalyzing coal tar.
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This catalysis occurred via the breaking of C-H bonds to generate CB and H2, and the
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probability of a gasification reaction between the lattice oxygen and the CB was lower.
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Therefore, the CB by Fe2O3 OC is of a higher yield and has a smaller and more uniform
1 2
*Corresponding author. Tel: 860532-85953720. E-mail:
[email protected] (Cuiping Wang)
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particle size, as well as a more developed structure. With temperature increasing, the
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morphological structures of two types of CBs were improved. Two types of OCs have an
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inhibitory effect on NOx production, and the Fe2O3 OC is more effective for emission
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reduction.
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Keywords: chemical looping pyrolysis; oxygen carrier; carbon black; yield; gaseous
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pollutants reduction.
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1. Introduction
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Carbon black (CB) has a variety of applications (e.g., coloring agent,
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conductivity agent, and an ultraviolet screening agent) and is widely used in the
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fields of coatings, printing inks, plastics and others1,2. In addition, CB is also a
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type of widely used carbon material that is primarily used as a rubber reinforcing
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agent3, and its consumption in the rubber industry accounts for approximately
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89.5% of the total production. Experiments have shown that a smaller particle size,
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a larger specific surface area and a more developed dendritic structure could
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effectively improve the reinforcing performance of CB4. Generally, when the CB
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particle size is less than 50 nm, the reinforcing performance is improved5.
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The CB production methods primarily include the contact method (e.g.,
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channel, roller and disk methods), the thermal cracking method, the furnace
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method (gas or oil furnace method), and others. Due to their large energy
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consumption, notable amount of pollution, lower yields and other factors, the first
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two methods are gradually being eliminated. The CB produced via the oil furnace
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method, which is the current primary production method, accounts for more than
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90% of the total output6. The principle of the oil furnace method is as follows:
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coal tar is sprayed into a reactor via compressed air and then comes into contact
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with high temperature flue gas (usually higher than 1800 ℃, which is produced
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from fuel oil combustion); as a result, the coal tar is rapidly gasified and
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pyrolyzed to generate CB. The carbon particle size is always finer with higher
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temperatures. Cold water must be sprayed to decrease the flue gas temperature
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and to terminate the deeper reaction of CB. In this method, the reaction
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temperature is notably high, meaning that the energy consumption is quite large.
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The injected cold water causes rapid increases of the flow rate and the moisture
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content in the exhaust gas; as a result, the heat value of the exhaust gas is reduced.
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In addition, the exhaust gas contains a large number of pollutant gases, such as
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NOx and SO27.
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Thus, a new method to produce reinforcing CB using chemical looping
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pyrolysis (CLP) is proposed in this paper. The principle was that the oxygen
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carriers (OCs) circulated between the air reactor (AR) and the pyrolysis reactor
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(PR) to achieve oxygen transfer. In the ideal PR, the OC lattice oxygen primarily
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reacted with the active H in coal tar, thereby promoting the generation of CB and
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reducing the OC. Next, the reduced OC returned to the AR, which was oxidized
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with air, realizing the OC regeneration and abundant heat release to maintain the
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whole cycle process8. The chemical looping reaction temperature (approximately
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1000 °C) is lower in a fluidized bed reactor9. From the theoretical deduction, the
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coal tar is evenly dispersed and pyrolyzed by fluidized OCs. The OC particles not
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only enhanced the heat transfer but also catalyzed the breaking of the hydrocarbon
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bonds and promoted the aggregation of carbon atoms into microcrystals. The
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residence time of the flow reaction passing through the high temperature region
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was controlled to adjust the process in which the microcrystals CB particles
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form10. Due to the lower reaction temperature, the energy consumption and
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irreversible loss were both reduced, and the thermal NOx also decreased.
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Generally, the OCs has an inhibitory effect on gaseous pollutants11. Therefore, this
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research is aimed to discover the new CB structure characteristics, energy
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conservation and emissions reduction using the CLP method for CB production.
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2. Experimental
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2.1 Materials
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The coal tar used in the experiments was obtained from the coking plant of
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Jinan Iron and Steel Company in Shandong Province, it is the byproduct of the
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coke furnace gas. At room temperature, the coal tar was dark brown with a
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pungent smell, and its liquidity was good after heating to 50 ℃. The proximate
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and ultimate analysis of the coal tar is shown in Table 1.
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Table 1 Proximate and ultimate analysis of the coal tar Proximate analysisa (wt%)
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Ultimate analysisa (wt%)
LHVb
M
V
FC*
A
C
H
O*
N
S
(MJ/kg)
3.22
66.47
30.22
0.09
91.34
5.43
1.25
1.06
0.92
38.191
Note:M-Moisture,V-Volatile,A-Ash,FC-Fixed Carbon,LHV-Low heat value;
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* The value was obtained via subtraction;
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a
As received basis;
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b
Dry basis.
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Due to their excellent performance of carrying oxygen and low price, two
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types of OCs were used in the experiments and they are also commonly used in
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many chemical-looping study works8-9,11. One was natural hematite and crushed in
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a crusher. The particles were screened into three groups, with particle sizes of
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approximately 0.1 mm, 0.5 mm and 1 mm. Each group was calcined for 2 h in a
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950 ℃ muffle furnace. The composition analysis of the calcined hematite is listed
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in Table 2. The second OC was analytic pure dihydrate calcium sulfate. The
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particles were also crushed and screened into three groups of the same sizes. The
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crystal water was removed after calcination for 2 h at 163 ℃. Its composition
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analysis is provided in Table 3.
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96
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Table 2 Chemical composition analysis of hematite Component
Fe2O3
Al2O3
SiO2
CaO
TiO2
K2O
Na2O
P2O5
Other
Content (%)
87.85
3.25
7.92
0.15
0.1
0.15
0.15
0.28
0.15
Table 3 Chemical composition analysis of gypsum Component
CaSO4
MgO
Al2O3
Fe2O3
SiO2
TiO2
Na2O
K2O
Other
Content (%)
93.26
2.39
0.31
0.13
0.58
0.01
0.06
0.05
3.21
2.2 Facility and method
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The testing apparatus is shown in Fig. 1. The apparatus primarily includes an
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intake pipe, a feeding device (for OC, coal tar), a discharge pipe, a two-stage
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heating furnace, a nitrogen cooling coil, a cyclone separator, and a double filter
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screen. The furnace is the PR in a CLP process, mainly used in our work for CB
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production; the OC regeneration process in AR is not involved in this apparatus,
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but in the Muffle furnace. The diameter of heating furnace is 32 mm, and the
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height is 700 mm. The electric heating element twines around the outer surface of
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the furnace, and the furnace and the heating element were coated using refractory
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material. The two-stage heating furnace is conducive to maintaining the operating
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temperature, and it even helps to realize different temperatures at two stages; the
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furnace temperatures at the two sections are the same in the current experiments.
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Two thermocouples are respectively arranged at the surface centers of two stages
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to measure the operating temperatures. jacket cooler feeding device N 2
coil
filter screen water
cyclone separator
second-stage heating
first-stage heating
intake pipe
discharge pipe
N2
111 112
Fig. 1. Schematic diagram of the experimental apparatus
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First, some of the OCs were added to the furnace through the feeding device.
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During the heating process, an appropriate amount of high purity nitrogen was
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preheated in the top coil; next, the N2 as fluidization gas was blown into the
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furnace through the bottom intake pipe, and then the air in the furnace was
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drained. When the temperature reached the set value, the coal tar and another
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portion of the OC particles were added to the furnace. According to the critical
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fluidization velocity of the OC (0.36 m/s for 0.5-mm-Fe2O3 OC, and 0.21 m/s for
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0.5-mm-CaSO4 OC), the fluidization gas flow was properly adjusted. The coal tar
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was pyrolyzed, resulting in formation of a carbon crystallite. The crystallites
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agglomerated to form CB particles. The CBs carried by the flue gas flow were
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cooled when passed through a N2 cooling coil and a water jacket cooler, and then
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a small amount of CB and most of the suspension of OCs were captured by a
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cyclone separator. Most CBs were collected with the double filter screen.
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Afterward, the flue gas was collected with a gas collecting bag periodically. When
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each experiment finished, the reduced OC was discharged through the bottom
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pipe and regenerated in the muffle furnace.
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Prior to the experimental studies, according to the principle of Gibbs free
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energy minimization, the reaction process was simulated using HSC chemistry
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software. When the mass ratios of hematite/tar and gypsum/tar are 1.3 and 1.6,
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respectively, the CB yield is higher. In this case, the oxygen supply from two
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types of OCs are approximately equivalent, which prevents the error caused by
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different oxygen supplies. Therefore, the actual experiments were conducted
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according to the two types of mass ratios. To guarantee coal tar pyrolysis, the
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design consumption of coal tar is approximately 0.3 kg/h. The mass flow rate of
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coal tar used in each experiment was 5 g/min. To prevent CaSO4 pyrolysis, the
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temperature in furnace was set to 700-900 °C as three experimental temperatures
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with a 100 °C interval.
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3. Results and discussion
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3.1 The influence of the OC particle size
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The OC particle size has an obvious effect on the yield and impurity content
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of the CB. The yield is equal to the collected CB mass divided by the carbon
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content of the coal tar. And the content of impurity in CB, including the mingled
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reduced OC powder and trace ash, is determined by calcining the CB sample at
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high temperature. When the CB burns out and the remaining mass is mainly the
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oxidized OC and ash, and the mass of reduced OC could be calculated, whose
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content is its mass divided by the sample mass.
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According to the HSC theoretical calculation and previous experimental
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verification, the OC activity increased with the reaction temperature increasing,
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and the CB yield also improved. So the compared effect of particle size is chosen
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the results when maximum reaction temperature of 900°C, the effect of the OC
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particle size is demonstrated as shown in Fig. 2. Fe2O3 impurity
100
Fe2O3 yield CaSO4 impurity
80
Mass fraction /%
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CaSO4 yield 60 40 20 0
0.1
0.5
1
Particle size of oxygen carrier / mm
Fig. 2. Effect of the OC particle size on the CB yield and purity
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"Fe2O3 impurity" denotes the impurity content in CB when Fe2O3 is used as the OC. Similarly, "Fe2O3 yield" denotes the CB yield when the OC is Fe2O3.
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Fig. 2 shows that with an increasing particle size, the yields of the two types
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of CB decreased. The CB yields below the particle sizes of 0.1 mm and 0.5 mm
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are approximately the same; however, the yield for particle sizes of 1 mm
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decreases more than the former two. This result indicates that when the particle
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size is smaller, the OC reactivity is higher, which is beneficial to increase the
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effective contact area between the coal tar and the OC and to promote CB
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generation. For the similar particle size range used in this paper, the reaction rate
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is similar and the fluctuation in the CB yield is small. However, when the particle
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size is too large, e.g., 1 mm, the reaction rate decreases and the CB yield is lower.
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The CB yield of Fe2O3 is slightly higher than that of CaSO4 in the current particle
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size range, which suggests that the reactivity between the Fe2O3 and coal tar is
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higher. Therefore, the Fe2O3 can more effectively catalyze the rupturing of the
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hydrocarbon bonds and promote the reaction between a lattice oxygen and
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hydrogen, resulting in the remaining carbon. As a result, the CB yield is higher, as
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will be confirmed in the following chromatographic analysis.
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Although the yields for the CBs with a particle size of less than 0.1 mm are
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the highest, both types of CBs contain fewer OC impurities, and the impurity
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content of the CaSO4 CB is the highest. The other two groups have almost no
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impurities, indicating that the OC that has a smaller particle size is easier to mix
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into the CB gas flow. This mixing increases the impurity contents of the CB.
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Because the mechanical strength of CaSO4 is lower at a high temperature, it is
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easy to be broken into fine particles, thereby introducing more impurities in this
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CB. Therefore, to ensure a high yield and to avoid the introduction of impurities,
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the 0.5-mm OC is more appropriate for the following experiments.
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3.2 Analysis of the CB quality as a function of the OC type and reaction
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temperature
(a) 800℃
(b) 900℃
Fig. 3. TEM images of the CB produced using Fe2O3 OC: (a) 800℃ and (b) 900℃ 182
Two types of CBs that were produced using different OCs were characterized
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using TEM (Transmission Electron Microscopy). The images are shown in Fig. 3
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and Fig. 4. The CBs were both produced at 800℃ and 900℃. The effects of the
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OC types and the reaction temperature on the CB quality are analyzed via the
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differences in such factors as CB morphology and particle size.
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As shown in Fig. 3(a), the particle size of the CB produced using the Fe2O3
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OC at 800℃ is approximately 40 nm. The particle sizes are relatively uniform,
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their shapes are nearly spherical, and there are few complete single particles. Most
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of the CB particles aggregate to form a dendritic aggregate, and a small amount of
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the CB particles are aggregated more closely to form a larger CB block. The
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particle sizes of the CB produced at 900℃ are smaller at approximately 20 nm.
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The particle shape is similar to that of the CB produced at 800℃. However, these
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CB particles are almost linearly connected, and they almost form a ring-shaped
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aggregate. The overall size of the aggregate is slightly reduced, its spatial
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structure is more developed, and the specific surface area is also larger. Hence, an
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increase in the reaction temperature is beneficial to the decrease of CB particle
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size and to the improvement of the spatial structure, which can enhance the
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reinforcing performance, according to the reference2. The reason for the decrease
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of the CB particle size may be due to the increase in the Fe2O3 reactivity at high
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temperatures, which can more effectively catalyze the breaking of the
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hydrocarbon bonds. While the lattice oxygen primarily reacts with hydrogen,
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there is also a small number of CBs involved in the carbon gasification reaction.
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This process decreases the aggregation of the remaining carbon chains, and
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smaller crystallites appear. Thus, eventually, smaller CB particles form.
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(a) 800℃
(b) 900℃
Fig. 4. TEM images of the CB produced using the CaSO4 OC: (a) 800℃ and (b) 900℃
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Fig. 4 shows the comparison of the CBs particle sizes of approximately 60
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nm and 50 nm produced with CaSO4 under 800℃ and 900℃, respectively. These
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CB particles are obviously larger than those produced with Fe2O3 OC at the
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corresponding temperatures, and their size distributions are not uniform. For
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example, for CBs produced at 800℃, the minimum particle size is less than 10 nm,
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but the maximum is greater than 100 nm. As shown in Fig. 4(b), with an
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increasing temperature, the uniformity of the CB particle sizes is slightly
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improved, and the average particle size is slightly decreased. At the two
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temperatures, the CBs produced using CaSO4 are spherical, and more complete
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single particles are found. This phenomenon is more obvious at 900°C, in which
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the particles are less connected with one another at the edges and the dendritic
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aggregate formed. However, the aggregate’s spatial structure is undeveloped, the
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overall size is smaller, and there are fewer particles in it. At 800°C, the CB
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particle gathers together, and the dendritic aggregate is almost not formed.
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Comparing with CaSO4 OC, the Fe2O3 OC can more effectively catalyze the
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breaking of the hydrocarbon bonds, then shorter carbon chains aggregate into
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smaller crystallites. Therefore, eventually, the crystal order and crystallinity of
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Fe2O3 CB is lower, and the CB particles are also smaller. Under the same
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fluidization, the smaller CB particles are not easy to deposite and aggregate
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together, thus the spatial structure of Fe2O3 CB aggregate is more developed.
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Therefore, from the morphological structure comparison of two types of CBs,
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under the same conditions, the Fe2O3 OC is better than the CaSO4 OC, and the
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former is more suitable for the production of CBs.
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Table 4 Some technical indices of the CBs Average particle size(nm)
Iodine absorption (g/kg)
DBP (10-5 m3/kg)
Standard / Experiment
Standard / Experiment
Standard / Experiment
Fe2O3 CB
26-30 / 30
82±7 / 78.5
102±7 / 99
CaSO4 CB
49-60 / 50
36±6 / 30.5
90±7 / 86
Sample
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The DBP absorption, iodine absorption and average particle size of the CBs
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produced from the two types of OCs at 900°C are listed in Table 4. The iodine
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absorption of the CB produced with Fe2O3 is found to be higher, which indicates
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that its specific surface area is larger. In addition, the DBP absorption of this CB is
234
also higher, suggesting that the number of aggregates of these CB particles is
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higher and that a large number of void volumes are formed; thus, the spatial
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structure of this aggregate is more developed. This result is consistent with the
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TEM analysis, which indicates that the test results are reliable. According to the
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results of various technical indexes listed in national standard of the People's
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Republic of China (GB 3778-2011), the Fe2O3 CB coincides with the hard CB
240
N330 and the CaSO4 CB coincides with the general purpose CB N660.
241
3.3 X-ray diffraction analysis 26°
2400
0.5mm Fe2O3 OC
2000
Intensity /(a.u.)
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Energy & Fuels
0.5mm CaSO4 OC
1600 1200
43.7°
800 400 0 10
20
30
40
50
60
70
80
90
2Theta / (degrees)
Fig. 5. X-ray diffraction of CBs respectively obtained by Fe2O3 OC and CaSO4 OC 242
The XRD patterns of the CB samples produced at 900°C are shown in Fig. 5.
243
The figure shows that both diffractograms display two broad peaks, implying that
244
the two samples are amorphous carbon materials with small regions of
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crystallinity. The peaks at 2θ ≈ 26° for the two samples correspond to the (002)
246
planes of graphitic structures, and the second peak at approximately 43.7° in both
247
samples corresponds to the (100) planes of the graphitic structure. Both of these
248
peaks serve as validation of the successful transformation of the CBs to graphitic
249
nanostructures at the high temperature.
250
The diffraction peak intensity (peak height) of the CaSO4 CB is generally
251
higher than that of the Fe2O3 CB. In addition, the peak intensity can
252
approximately reflect the degree of crystallinity and crystal order. The narrower
253
and more intense diffraction peak indicates that the crystallinity and crystal order
254
of the mini-crystals improved, but that the size is larger. Therefore, the XRD
255
results show that the crystal order and crystallinity of the Fe2O3 CB are lower and
256
that the minicrystal particle size became smaller. Thus, the CB particles composed
257
of these minicrystals are also smaller and irregular in shape. However, the
258
crystallinity of the CaSO4 CB is higher. Specifically, there are more of the larger
259
graphitic minicrystals, and its crystal order is also higher. Thus, the CB particles
260
are larger and nearly spherical. This result is consistent with the above TEM
261
analysis. In addition, we did not observe a sharp peak resulting from impurities in
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the XRD patterns, indicating that the purity of the produced CBs in this
263
experiment is higher, which is in agreement with the results shown in Fig. 2.
264
3.4. Flue gas analysis
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Four gas components, specifically, H2, CO, CH4 and CO2, from the flue gas
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were tested using gas chromatography and a flue gas analyzer. The volume
267
fraction of each component was calculated using the normalization method, and
268
the results are shown in Fig. 6. When Fe2O3 is used as the OC, the volume fraction
269
of CO2 in flue gas is significantly lower than that when CaSO4 is used as the OC.
270
However, the H2 volume fraction displays the opposite trend. This behavior may
271
be due to more effective catalysis of the Fe2O3 via the rupturing of the tar
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hydrocarbon bond than CaSO4 to promote the generation of H2. The gasification
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reactions between the remaining carbon and the lattice oxygen of Fe2O3 also
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became difficult; this is beneficial to improve the yield of the Fe2O3 CBs. When
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CaSO4 is used as the OC, the CO2 content is higher. However, the CO content is
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lower, which indicates that the lattice oxygen of CaSO4 is more likely to react
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with the remaining carbon for complete oxidation to the gas phase component,
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namely, CO2. This lower CO content is also the reason for the low yield of CaSO4
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CB. From the standpoint of carbon conversion, in the Fe2O3 CB production
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process, the carbon converted to the gas phase is less than that when CaSO4 was
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used. Therefore, Fe2O3 OCs are more suitable for the production of CBs.
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25
CaSO4
CaSO4 Fe2O3
250 Gas concentration /ppm
Fe2O3
20 Volume fraction /%
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15 10 5
Emission limit 200 150 100 50 0
0 H2
CO CH4 Gas component
CO2
NOx SO2 Gas pollutant component
Fig. 6. Gas chromatographic analysis of the
Fig. 7. Analysis of the gas pollutant
gas component
concentration
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The SO2 and NOx in the flue gas were tested using the flue gas analyzer, and
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the results are compared with the emission limits (SO2 ≤ 105 ppm and NOx ≤ 195
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ppm) in the 2015 CB industry access standards of China, which is displayed in
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Fig. 7. The NOx concentrations in the flue gas are both lower than the emission
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limit without any denitrification process because the temperature of chemical
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looping pyrolysis is low, generating almost no thermal NOx. The probability of
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nitrogen in the coal tar reacting with lattice oxygen of Fe2O3 is lower. Thus, the
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NOx concentration in flue gas is also lower. However, the SO2 content in the flue
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gas is both higher than the emission limits, hence the OCs both have less
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inhibitory effect on the SO2 generation. Comparing with oil furnace method, the
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reaction temperature in CLP process is much lower due to the catalytic effect of
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OCs, which makes thermal NOx concentrations declined, thus the CLP method
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has a certain synergistic effect to reduce the NOx emission. The SO2 concentration
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in the flue gas is higher when CaSO4 is used as the OC, which may be due to the
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high temperature decomposition of CaSO4. This decomposition is also one of the
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reasons for the limited use of CaSO4.
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4. Conclusions
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In view of the high energy consumption and pollution problems that result
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from the production process of the oil furnace method, the method of chemical
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looping pyrolysis to produce reinforcing CB was studied. The effects of the OC
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type, particle size, and reaction temperature on the CB yield, quality and gas
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emission were studied. The primary conclusions of the study are as follows:
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(1) The CB yield increases as the OC particle size decreases, but there are
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fewer OC impurities in the CBs when the particle size is too small. Therefore, the
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size of the OC particles chosen in this experiment was 0.5 mm. With an increasing
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temperature, the qualities of the two types of CBs both improved.
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(2) Under the same conditions (the reaction temperature and particle size),
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the activity of Fe2O3 OC was found to be higher than that of CaSO4 OC. Fe2O3
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can more effectively catalyze the breaking of the hydrocarbon bonds in tar,
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thereby promoting the generation of CB and H2. In addition, the gasification
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reactions between the remaining carbon and lattice oxygens of Fe2O3 proceeded
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with difficulty. Hence, the yield of Fe2O3 CB is higher. The particle size of the
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Fe2O3 CB is smaller and uniform, and the spatial structure of the aggregate is
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more developed; thus, the reinforcing performance improved. However, the
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particle size of the CaSO4 CB is larger and non-uniform, and the dendritic
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structure is undeveloped, whereas the particle shape is more regular and nearly
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spherical, making it a general purpose CB.
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(3) The two types of OCs both have an inhibitory effect on NOx, and the
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Fe2O3 OC is more effective to reduce emissions. In conclusion, the Fe2O3 OC is
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more suitable for CB production via chemical looping pyrolysis.
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Acknowledgments
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The financial support was received from the Natural Science Foundation of
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Shandong Province (Project No.ZR2015EM004).
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References
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