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A novel vacuumed hermetic reactor and its application in coal pyrolysis
Fuel 255 (2019) 115774
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Full Length Article
A novel vacuumed hermetic reactor and its application in coal pyrolysis Bin Zhou, Qingya Liu, Lei Shi, Chong Xiang, Shumiao Quan, Zhenyu Liu
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T
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Pyrolysis Hermetic reactor VH reactor Reaction of volatiles Product collection
A novel vacuumed hermetic (VH) reactor is designed and compared with the common Gray-King (GK) and flowthrough (FT) reactors for lignite pyrolysis under different operation modes. The quantity and quality analyses of pyrolysis products, such as char, tar and its fractions, water and gas show that the VH reactor with pre-evacuation (PE) by a pump and cooling of volatiles by liquid nitrogen (VH-PE-77 K) greatly reduces the volatiles reaction time and collects all of products, and the pyrolysis results are closer to the coal-to-volatiles reaction (or the so called primary pyrolysis) than other reactors commonly used in the literature. Compared with the yields of VH-PE-77 K in the temperature ranges from 50 °C to 500, 550 and 600 °C, the GK reactor allows more volatiles reaction (or the so called secondary reaction) and generally yields about 10% less tar, 7% less water, 3% more char and 10% more gas. It is found that the volatiles reaction contributes little to CO2 yield but significantly to H2, CH4 and CO yields. The water generated from coal-to-volatiles reaction participated in the volatiles reaction. The volatiles reaction reduces the chars’ surface area and the higher heating value (HHV) of gas (on volume basis), but increases the chars’ HHV (on mass basis).
1. Introduction
of withdrawing the volatiles as soon as their formation and collecting the products completely. The idea of a standard laboratory pyrolysis reactor is not new and such reactors had been designed and used. The most popular one is the Gray-King (GK) assay [4] and another one probably is the Fischer assay [5]. These reactors pyrolyze a fixed amount of coal under a default heating program without purging and collect the condensable products by a flask cooled by ice water before venting the incondensable gases. The quality and quantity of products of these reactors have been deemed as the indexes for other reactors, laboratory scale or large scale [4,5]. A characteristic of these reactors is withdrawing the volatiles naturally by the volume expansion caused by the coal-to-volatiles (solid-to-gas) reaction at the atmospheric pressure, which is temperature dependent. It was estimated that the residence time of volatiles at the pyrolysis zone of a standard GK reactor is approximately 9 s for volatiles generated at 450 °C and approximately 28 s for volatiles generated at 600 °C [1]. These volatiles’ withdrawing rates are slow and their reaction time is long because it has been shown that the volatiles reaction occurred extensively in a few seconds at these temperatures [6]. To reduce the extent of volatiles reaction, inert sweeping gases were used in many studies, which shortened the volatiles residence time, to less than 2 s in some cases [1], but inevitably increased the burden of product collection system leading to increased loss of condensable products. To collect all the condensable products multi-stage
Pyrolysis plays an important role in many fuel and chemical production processes from coal, oil shale, biomass and many other organic resources. It has been studied extensively in laboratories and its product yields were found to be dependent on reactor type and operating conditions, such as temperature, heating rate and pressure. It is also frequently observed that the pyrolysis results of small laboratory reactors differ significantly from those of large-scale reactors even though the reactor type and pyrolysis conditions are nominally the same. This difference in pyrolysis result between reactors has been indiscriminately termed as the scaling-up effect in many studies. However, some studies showed that the different pyrolysis results can be attributed to the differences in reaction of volatiles inside these reactors, due to different temperature-time profiles of the gas phase in different reactors, as well as the difference in products collection systems [1–3]. Since it is not easy to simulate a large-scale reactor exactly by a small laboratory reactor, it is logical to obtain the “intrinsic pyrolysis behavior” from a small laboratory reactor, which can be used to simulate the performance of a large-scale reactor by combining with the kinetics of volatiles reaction and the transport processes. The meaning of “intrinsic” is free, as much as possible, from the influences of the volatiles reaction, the transport effects and the efficiency of products collection method. In other words, an ideal laboratory reactor should be capable
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Corresponding author. E-mail address: [email protected] (Z. Liu).
https://doi.org/10.1016/j.fuel.2019.115774 Received 8 October 2018; Received in revised form 24 June 2019; Accepted 5 July 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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tube has two ports on the top, one for a pressure transducer and one for a vacuum pump. Fig. 1(b) shows that the pyrolysis tube is preloaded with a sample boat containing a predetermined amount of sample, coal in this case. After been connected to the volatiles collection tube, the pyrolysis tube is placed in a tubular furnace at room temperature and the whole system is vacuumed by a pump to a low pressure. At the steady state the vacuum port is closed and the volatiles collection tube is immerged into a liquid nitrogen dewar. The pyrolysis experiment starts by heating the pyrolysis tube at a designated rate to various temperatures. The volatiles generated from coal diffuse through the coal bed and then flow to the volatiles collection tube where their volume is reduced by condensing into liquid and solid under the low temperature. This volatiles’ volume reduction in the volatiles collection tube results in a pressure drop from the pyrolysis tube to volatiles collection tube that drives the volatiles to flow in the same direction. This operation mode is termed VH-PE-77 K, where VH represents the reactor, PE stands for pre-evacuation by a pump and 77 K stands for cooling the volatiles collection tube to 77 K by liquid nitrogen. Of course, as discussed earlier, the VH reactor may not be preevacuated and the vacuum is solely achieved by cooling the volatiles collection tube with liquid nitrogen, after the whole reactor been swept by nitrogen to exclude the influence of oxygen. In this case the operation mode is termed VH-77 K.
cooling system were used [7–9], which complicates the products analysis and introduces large experimental errors. Clearly the experimental reactors developed so far cannot simultaneously reduce the volatiles reaction time and collect all the products with a simple reactor configuration and convenient operation. This article reports an invention of a vacuumed hermetic reactor (VH) or LZ (the initials of the main inventors, Liu and Zhou) reactor, which is capable of quickly withdrawing the volatiles from the sample bed and pyrolysis zone, and at the same time collecting all the volatile products. These characteristics are demonstrated by comparing the reactor’s pyrolysis results with those obtained from other reactors commonly used in the literature.
2. Methods 2.1. Invention of the VH reactor The hermetic reactor under vacuum is an ideal solution to simultaneously achieve fast withdrawing the volatiles and fully collecting of all the volatile products at the same time. Hermetic and vacuum, however, seem contradictive with each other, because the constant generation of volatiles from coal in pyrolysis raises the pressure of a hermetic reactor and increases the reaction time of volatile, while the use of a vacuum pump to withdraw the volatiles requires the reactor to be open which makes full collection of volatiles difficult. These two contradictive targets can be simultaneously achieved or closely simulated in a hermetic reactor by cooling the product collection system to the liquid nitrogen temperature (−196 °C) which lowers the reactor pressure before the pyrolysis experiment by approximately 3/5 in comparison to that at the room temperature, condenses most of the volatiles into liquid and solid, and reduces the volume of incondensable gas products during pyrolysis. This volume reduction effect is more pronounced when the volume of the product collection system is much larger than the volume of pyrolysis zone, and is further intensified when the reactor is pre-evacuated by a vacuum pump. With such a principle in mind the vacuumed hermetic reactor (VH reactor) is designed as shown in Fig. 1. Fig. 1(a) shows that the VH reactor is mainly composed of two parts, a horizontal pyrolysis tube and a volatiles collection tube, both with one open end. The volume of pyrolysis tube is about 78 ml while that of the volatiles collection tube is approximately 2 times larger, about 154 ml. These two tubes are coupled by the open ends by a ground joint that is sealed by silicon grease and fastened by a string on the hooks on each part. The open end of the volatiles collection tube is inserted deeper into the open end of the pyrolysis tube to reduce the volatiles condensation and coking on the pyrolysis tube. The volatiles collection
2.2. Common reactors used to compare with the VH reactor Fig. 2(a) and (b) show two reactors that simulate the open reactors commonly reported in the literature. The reactor in Fig. 2(a) is similar to the VH reactor in Fig. 1(a) but has a discharge port on the lower side of the volatiles collection tube to vent the incondensable volatiles at the ice-water temperature, 273 K. Before the pyrolysis experiment, the whole system is swept with nitrogen to eliminate oxygen. This operation mode simulates the GK assay and the Fischer assay, and is termed GK-273 K. The reactor in Fig. 2(b) has not only a discharge port on the volatiles collection tube as that in Fig. 2(a) but also a purging gas feeding port on the pyrolysis tube, simulating the flow through (FT) reactors. This reactor can be operated at the volatiles collection tube temperature of 77 K with liquid nitrogen or 273 K with ice-water, and is termed as FT77 K or FT-273 K, respectively. The purging gas is N2 with a flow rate of 150 ml/min. The main dimensions of the pyrolysis tubes and the volatiles collection tubes of GK reactor and FT reactor are the same as those of VH reactor, 20 mm in diameter and 150 mm in length for the pyrolysis tube, and 40 mm in diameter and 150 mm in length for the volatiles
Fig. 1. The VH reactor (a) and the pyrolysis system (b). 2
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Fig. 2. The GK reactor (a) and the FT reactor (b).
collection tube. The tubular furnace and the dewar used in these reactors are also the same. This consistence in equipment dimension warrants a rational comparison of coal pyrolysis behavior in all the reactors.
T are pressure (Pa), volume (m3) and temperature (K), respectively, which were measured directly, R is the gas constant (8.314 J·K−1·mol−1) and M is the mean molecular weight of gas based on the gas composition determined by GC.
2.3. Coal and pyrolysis conditions
ygas =
The coal used is Hulunbeier (HLBE) lignite from China. It was ground to 80–100 mesh in size and dried at 110 °C in a vacuum for 12 h. Its proximate and ultimate analyses are shown in Table 1. All the pyrolysis experiments used 3 g HLBE lignite, started at 50 °C and ended at 500, 550 and 600 °C under the same heating rate of 10 °C/min.
mgas =
After the pyrolysis experiments using VH reactor, when the reactor and the volatiles collection tube return to the room temperature, the gas was sampled by an injector through the gas port, and the char in sample boat and the liquid in volatiles collection tube were then weighed. The char and liquid yields were determined by Eqs. (1) and (2), and the gas yield was determined by difference (Eq. (3)). The liquid in volatiles collection tube was then dissolved in 5 ml tetrahydrofuran (THF) and the solution’s water content was determined by the Karl Fischer method. The water yield was determined by Eq. (4). The tar yield, i.e. the liquid excluding water, was determined by Eq. (5). In these equations, y and m stand for the yield and mass, respectively, while cwater is the water content of the THF solution.
yliquid =
mchar × 100% mcoal mliquid mcoal
PVM RT
(6) (7)
2.5. Char characterization The chars were characterized by surface analysis, higher heating value (HHV) and the ultimate analysis. The surface analysis was performed using CO2 at 0 °C on an Autosorb IQ (Quantachrome) after drying at 110 °C for 3 h and degassing at 300 °C for 8 h. The surface area was determined by Brunauer-Emmett-Teller (BET) methods. The chars’ HHV was determined using a bomb calorimeter, IKA C7000 (IKA-Werke GmbH & Co. KG), and calibrated by benzoic acid. 2.6. Tar characterization (1) Simulated distillation (SIMDIST)
(1)
× 100%
The tar dissolved in THF was analyzed by SIMDIST on a GC (Agilent 7890B) with a stainless steel capillary column (10 m × 0.53 mm × 0.5 μm), a FID detector, and automatic sampling. The sample size was 2 μL. The carrier gas was N2. The injection port was at 420 °C. The column temperature was 1 min at 35 °C, 10 °C/min to 400 °C and 5 min at 400 °C. The GC peak area was calibrated by various standards including p-xylene, n-nonane and n-decane. The components with boiling points lower than 360 °C is defined as the light fraction while those with boiling points higher than 360 °C and the residue in the volatiles collection tube and in the GC column are defined collectively as the heavy fraction [7].
(2)
ygas = 100% − ychar − yliquid
ywater
× 100%
The yields of char, liquid, gas, water and tar for pyrolysis in the GK and FT reactors were determined with the same method as discussed above for the VH reactor. The gas products during pyrolysis in GK reactor were collected continuously using a gas bag of 200 ml while those of FT reactor were collected continuously using a gas bag of 10 L.
2.4. Determination of the yields of pyrolysis products
ychar =
mgas mcoal
(3)
m × c water = solution × 100% mcoal
(4)
ytar = yliquid − ywater
(5)
The gas yield was also determined by Eq. (6) where mgas is the gas mass determined by the ideal gas equation (Eq. (7)). In Eq. (7), P, V and
(2) Gel permeation chromatography (GPC)
Table 1 Proximate and ultimate analyses of HLBE lignite. Proximate analysis (wt%)
The molecular weight (MW) distribution of tar was determined by GPC with a Waters e2695 solvent delivery pump and a Waters 2414 RI Detector, using the sequentially connected Waters styragel 0.5-THF, 1THF and 3-THF (7.8 mm × 300 mm) columns packed with polystyrenedivinylbenzene polymer. Mono-disperse polystyrene was used as the standard for calibration. The carrier was THF at a flow rate of 1 ml/min. The sample size was 50 μL. The temperature of columns and detector was 35 °C. According to Qin et al. [10,11], the MW of 100–134 amu was
Ultimate analysis (wt%, daf)
Mad
Ad
Vdaf
C
H
N
S
O*
30.99
9.1
29.94
73.93
5.12
1.14
0.38
19.43
ad: air-dry basis; d: dry basis; daf: dry-and-ash-free basis. M: moisture; A: ash; V: volatile matter content. *: By difference. 3
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defined as 1-ring aromatics (with hetero-atom substitution) and termed as F1; the MW of 134–198 amu was defined as 2–3-ring aromatics and termed as F2; the MW of 198–276 amu was defined as 4–5-ring aromatics (with conjugated side chains) and termed as F3; the MW of 276–433 amu was defined as the saturated long chain alkane and > 5ring aromatics with conjugated side chains and termed as F4. The aromatics with conjugated side chains may dominate F4 because the aliphatic CeC bonds in long chain alkane are easy to break during pyrolysis [12]. The MW beyond 433 amu was defined as asphaltenene and pre-asphaltenene and termed as F5. 2.7. Gas characterization The gas samples were analyzed by a GC (Agilent 7890A) equipped with an Al2O3 capillary column (30 m × 0.53 mm × 25 μm) and a FID for light hydrocarbons and a porapak Q column (3 m × 2 mm) and a TCD detector for permanent gases. The carrier gas was N2. The temperatures of columns and detectors were 95 and 140 °C, respectively. These two sets of GC data were calibrated by CH4 and normalized to yield the gas composition. The error of GC analysis is within 5%.
Fig. 4. The volatiles residence time in various reactors during pyrolysis in different operation modes.
3. Results and discussion residence time in the FT reactor is also determined based on the N2 purging rate and is shown in Fig. 4. It is clear that the volatiles residence time in the GK reactor is the longest, from around 48 s at 400 °C to about 3 s at 500 °C, and to about 20 s at 600 °C. The volatiles residence time in VH-77 K mode shows a similar trend as that in the GK reactor but is generally 60–70% shorter, around 30 s at 400 °C, 2 s at 500 °C and 14 s at 600 °C. Although the trend of volatiles residence time in VH-PE-77 K mode is also similar to that in the GK mode, it is very short, around 2, 0.5, 6.5 s at 400, 500 and 600 °C, respectively. The volatiles residence time in FT reactor is mainly affected by the N2 purging rate, it is similar to that in VH-PE-77 K in the temperature range of 400–550 °C, the range for the generation of most volatiles.
3.1. Pressure characteristics of different reactors Fig. 3 shows the pressure change during pyrolysis in the temperature range of 50–600 °C in the reactors under different operation modes. As expected, the pressures of GK-273 K, FT-77 K and FT-273 K are always 0.1 MPa because they were open to the atmosphere. The pressure of VH-PE-77 K was initially very low, approximately 0.0006 MPa due to the pre-evacuation by a vacuum pump and the subsequent cooling of the volatiles collection tube by liquid nitrogen, and increased over time especially at temperatures higher than 350 °C, and reached only 0.032 MPa at 600 °C. The pressure of VH-77 K was also initially low, approximately 0.044 MPa due to the cooling of volatiles collection tube by liquid nitrogen, and increased over time slowly in a somewhat linear way to reach 0.071 MPa at 600 °C. The sub-atmosphere pressures in the VH reactor during the whole pyrolysis experiments under the two operation modes indicate that the volatiles residence time in pyrolysis tube is short because the evaporation and diffusion rates of volatiles are reversely proportional to the pressure [8]. Based on the volatiles residence time in the GK reactor reported in the literature [1] the volatiles residence time in the VH reactor in two operation modes is estimated in Fig. 4. The volatiles
3.2. Comparison of products’ yields The different volatiles residence time and collection methods suggest that the products collected from pyrolysis in different operation modes are different in yield and in composition. To understand the certainty of these differences, mass balance of the experiments under different operation modes is evaluated. Since the mass of char and liquid was directly measured with small errors the main experimental error is in the gas yield which is determined either by mass difference or by gas analysis. Fig. 5 shows the mass balance based on the gas analysis and the mean molecular weight of gas used to determine the mass of gas. It is seen that the mean molecular weight of gas is in the range of 10–30 g/mol, which is smaller at a higher final pyrolysis temperature due to the formation of more lighter products, such as H2 and CH4, and larger for FT-77 K and FT-273 K than other reactors due to difficulty in quantifying the diluted H2 by GC. The mass balance is 100.0 ± 1.0 wt % for VH-PE-77 K, VH-77 K and GK-273 K, and 100.0 + 10.0 wt% for FT-77 K and FT-273 K. It seems that the gas yields determined by the mass difference are more accurate than those determined by the gas analysis, and therefore the gas yields by the mass difference are used in the following study. Fig. 6 compares the product yields generated in these reactors under different operation modes. The error bars were determined from the differences of triple experiments. Clearly, the errors of VH reactor in both modes are generally smaller than those of GK and FT reactors. To more clearly compare the yields, the yields of VH-PE-77 K are set as the baselines and labeled 100% while the yields of other reactors and operation modes are labeled by the percentage deviation from the corresponding baselines. For example, in Fig. 6(a) the +1.5% in char of VH77 K means 1.5% more char been formed than that of VH-PE-77 K in the
Fig. 3. The pressure in various reactors during pyrolysis in different operation modes. 4
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Compared to the VH modes, GK-273 K generated more char and gas while less tar and water in three temperature ranges. These differences are attributed to the slow flow rate of volatiles under the atmospheric pressure in GK reactor, which leads to more volatiles reactions in the reactor than those in the VH reactor. It is seen in Fig. 6 that the char and gas yields of FT-77 K and FT273 K are higher than those of VH-PE-77 K although the volatiles residence time in these reactors are similar. This phenomenon is very important because it indicates that the N2 purging in FT-77 K and FT273 K decreases only the volatiles’ residence time in the gas phase [15] while the sub-atmosphere pressure in VH-PE-77 K decreases the volatiles’ residence time in the gas phase as well as in the sample bed [8]. In other words, the extent of V + V reaction in FT-77 K, FT-273 K and VHPE-77 K are similar as indicated in Fig. 4, but the V + C reaction in FT77 K and FT-273 K is more extensive than that in VH-PE-77 K. The lower tar and water yields and the higher gas yields of the two FT modes, especially for FT-273 K, than those of VH-PE-77 K also demonstrate the loss of condensable volatiles in the FT reactor even though the volatiles collection tube was cooled to the liquid nitrogen temperature. It is interesting to note that the water yields of VH-PE-77 K are higher than those of other reactors and operation modes, such as that of VH-77 K. This seems suggesting that the water generated from the coalto-volatiles reaction participated in the volatiles reaction, such as V + C, leading to increased gas yield. Similar phenomenon was reported by Cheng et al., who pyrolyzed a coal with 82.3% carbon in a fixed bed reactor from the room temperature to 550 °C and found that the water yields increased when using reactor’s internals to reduce volatiles reaction, and proposed the reaction of water with char and tar [8].
Fig. 5. The mean molecular weights of pyrolysis gas and mass balance in different operation modes.
3.3. Comparison of char Table 2 lists the surface areas of chars from different operation modes at the final pyrolysis temperatures of 500 and 600 °C. The data of chars from FT-273 K are not listed because they are the same as those from FT-77 K. It is seen that the surface area of char from any operation mode increases from around 200 to around 300 m2/g when the final pyrolysis temperature increases from 500 to 600 °C, suggesting more pores been formed at a higher pyrolysis temperature due to more volatiles release. It is interesting to see that the surface areas of chars from VH-PE-77 K are larger than those of other chars. Furthermore, the surface areas of chars from FT-77 K are larger than those from the corresponding GK-273 K although both modes were operated under the atmospheric pressure. Since the V + C reaction in VH-PE-77 K is less than that in other operation modes and the V + C reaction in FT-77 K is less than that in GK-273 K, these behaviors indicate that the V + C reaction promoted coke deposition from volatiles and the deposited coke reduced the char’s surface area probably by blocking the char’s micro-pores. Table 3 lists the HHV of chars from different operation modes at the final pyrolysis temperatures of 500 and 600 °C. It is seen that the HHV of chars increases with an increase in final pyrolysis temperature, suggesting reduction in oxygen content of chars at a higher temperature. The lower HHV of chars from VH-PE-77 K than those of other chars suggests that the coke deposited on chars through the V + C reaction probably is of lower oxygen content and higher carbon as well as
Fig. 6. The yields of pyrolysis products in different operation modes.
temperature range of 50–500 °C, while the −11.8% in tar of VH-77 K means 11.8% less tar been formed than that of VH-PE-77 K in the temperature range of 50–500 °C. It is seen that VH-PE-77 K always yielded less char and gas, but more tar and water than other reactors and operation modes, indicating that VH-PE-77 K allowed the least volatiles reaction, including condensation and cracking that convert the condensable volatile products to char and incondensable gas, and collected all the volatiles. It has been reported that the pyrolysis of coal involves three types of reactions, coal-to-volatiles, volatiles-with-coal or -char (V + C), and volatiles-with-volatiles (V + V), the latter two can be addressed collectively as the volatiles reactions [13,14]. In principle, the coal-tovolatiles reaction are the same in all the reactors under all the operation modes, so the different product yields in Fig. 6 should be partly attributed to the different volatiles reactions. The different yields of VHPE-77 K and VH-77 K, therefore, indicate that the pre-evacuation is effective to reduce the V + C and V + V reactions. Furthermore, the decreasing difference in tar yield between these two modes with increasing final pyrolysis temperature, i.e. from −11.8% at 500 °C to −10.5% at 550 °C and then to −8.8% at 600 °C, indicates that the volatiles generated at temperatures below 500 °C contain more reactive and condensable fractions than those generated at temperatures higher than 500 °C. In other words, the content of incondensable gases in volatiles is more at a higher temperature than that at a lower temperature.
Table 2 Surface areas of chars from different operation modes. Temperature (°C)
50–500 50–600
Relative error = 2%. 5
Surface Area (m2/g) VH-PE-77 K
VH-77 K
GK-273 K
FT-77 K
204 319
174 275
173 263
186 287
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Table 3 HHV of chars from different operation modes. Temperature (°C)
50–500 50–600
HHV (MJ/kg, af) VH-PE-77 K
VH-77 K
GK-273 K
FT-77 K
18.5 25.5
25.0 30.1
25.2 34.5
25.5 29.5
Relative error = 5%.
Fig. 8. Chemical composition (GPC) of the tars obtained in the pyrolysis temperature range of 50–600 °C from different operation modes.
range, the yields of light and heavy tar fractions of VH-PE-77 K are higher than those of FT-77 K, while those of the latter are higher than the corresponding yields of FT-273 K, although the volatiles residence time in these reactors is similar. This trend is attributed mainly to the volatiles loss in the FT reactor especially at the product collection temperature of 273 K. Fig. 8 compares chemical composition of tars from different operation modes under the pyrolysis temperature range of 50–600 °C. The tars were dissolved in THF and the chemical composition was determined by GPC with experimental errors of within 3%. It is seen that the dominant component of tars from all the operation modes is F5 (around 45–55%), which is followed by F4 (around 17%) and then by F3 and F2 (both around 10–14%). With increasing volatiles residence time from VH-PE-77 K to VH-77 K to GK-273 K, as shown in Fig. 4, the content of F5 decreases notably, the contents of F1 and F2 increase notably, but the contents of F3 and F4 increase slightly. These behaviors indicate that the cracking of asphaltene and pre-asphaltene yields mainly small molecules such as those containing a few aromatic rings rather than those with more than 5 aromatic rings. Apparently, the tar from VH-PE-77 K is closer to the primary tar than the tar from GK-273, and therefore VH-PE-77 K is a better method to characterize the coal-tovolatiles reaction. In principle, the FT reactor with N2-purging, FT-77 K and FT-273 K, cannot collect all the products especially the lighter ones, which would result in tars with more heavy fractions, so it is not surprise to see that the tars from the FT reactor are heavier than that from GK-273 K and the tar from FT-273 K is heavier than that from FT-77 K.
Fig. 7. Yields and simulated distillation fractions of tars in different operation modes.
hydrogen content. For example, the chars been subjected to the most severe V + C reaction in GK-273 mode and under 50–600 °C were 2.5% more in carbon content and 0.3% more in hydrogen content whereas 2.8% less in oxygen content, than those been subjected to the limited V + C reaction in VH-PE-77 K and in the same temperature range.
3.4. Comparison of tar Fig. 7 shows the yields of tars from different operation modes and the tar fractions based on SIMDIST. The overall experimental errors are within 5%, corresponding to 0.15–0.45% for tar yields of 3–9%. The percentages noted in the bars are the contents of light and heavy fractions. It is seen that the yields of light and heavy fractions under all operation modes increase with an increase in final pyrolysis temperature. VH-PE-77 K generates the highest light and heavy tar yields in the same temperature range, indicating its tar underwent the least reactions due to the fast withdraw of volatiles from the char bed and from the pyrolysis tube, and also the full collection of volatiles. It is also seen in Fig. 7 that the yields of heavy tar fraction of VH-PE77 K in each temperature range are significantly higher than those of VH-77 K, while the yields of light tar fraction of VH-PE-77 K are only slightly higher than those of VH-77 K. This behavior suggests that the reaction of heavy tar fractions is more extensive than that of the light tar fractions, and the formation of light tar fractions from the heavy tar fractions overwhelms the conversion of light tar fractions. This suggestion agrees with that reported by Wu et al. in a pyrolysis study of coal tars [16] and that reported by Katheklakis et al. in a study of molecular mass distribution of the tars under different extent of secondary reactions [17]. The lower tar yields and the higher light tar contents of GK-273 K than those of VH-77 K is also ascribed to the increased volatiles reaction in GK-273 K due to the longer residence time of volatiles. It is not surprise to see that, in the same pyrolysis temperature
3.5. Comparison of gas Fig. 9 shows the yields of H2, CH4, C2-C4 HC (hydrocarbons), CO and CO2 of VH-PE-77 K, VH-77 K and GK-273 K. The yields of FT-77 K and FT-273 K are not shown due to large experimental errors caused by the N2-purging, which greatly lowered the products concentration in the gas bag. It is apparent that the yields of H2, CH4 and CO increase with the volatiles residence time in the pyrolysis tube, from VH-PE-77 K to VH-77 K and to GK-273 K, and increase with the final temperature, from 500 to 550 and to 600 °C. These behaviors indicate that in the temperature range studied these compounds are generated not only from the coal-to-volatiles reaction but also from the volatiles reactions. The 6
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designed. The reactor was compared with common laboratory reactors, including a simulated GK reactor and a flowed through reactor for lignite pyrolysis under various operation modes differing in pressure, purging and cooling methods. The VH reactor with pre-evacuation (PE) by a pump and cooling of volatiles by liquid nitrogen (VH-PE-77 K) is characterized by the shortest volatiles residence time and full collection of the products. The commonly used GK reactor with the standard volatiles’ cooling method (at 273 K) has the longest volatiles residence time and inefficient tar’s trapping. Compared with VH-PE-77 K, the GK-273 K yielded about 10% less tar, 7% less water, 3% more char and 10% more gas. The VH reactor can also be operated under VH-77 K mode, without pre-evacuation, to yield more tar, less char and gas than GK-273 K. The tars generated from VH-PE-77 K are slightly heavier than those from GK-273 K due to the least volatiles cracking. The VH-PE-77 K reduces the volatiles residence time not only in the gas phase but also in the solid sample bed, while the commonly practiced flow-through (FT) reactor reduces only the volatiles residence time in the gas phase. The CO2 yield varied little with the reactor type and operation mode, and is attributed to the decomposition of carboxylic groups in coal, not in the volatiles reactions. CO2 did not react with the volatiles while water was consumed in the volatiles reaction. H2, CH4 and CO were generated from the coal-to-volatiles reaction as well as from the volatiles reaction.
Fig. 9. The yield and main components of gas from different operation modes.
Table 4 HHV of gas from different operation modes. Temperature (°C)
50–500 °C 50–550 °C 50–600 °C
HHV of gas (MJ/Nm3) VH-PE-77 K
VH-77 K
GK-273 K
46.14 37.71 25.01
45.59 35.31 24.32
42.49 33.12 21.53
Acknowledgments We thank the National Key Research and Development Program of China (2017YFB0602401) and Joint Funds of Natural Science Foundation of China (U1610107) for financial support.
contribution of these volatiles reactions in GK-273 K is much higher than that in VH-PE-77 K, producing 0.7% more in H2 yield, 0.5% more in CH4 yield and 1.0% more in CO yield for pyrolysis in the temperature range of 50–600 °C. It is interesting to note that the CO2 yields vary little with the reactor type, the operation mode, and even the pyrolysis temperature range. This behavior suggests that the oxygen-containing moieties responsible for CO2 formation are less stable than those responsible for CO formation and decompose mainly at temperatures lower than 500 °C. The oxygen-containing moieties for CO2 generation are likely carboxylic groups that decompose in the coal-to-volatiles reaction and present little in the volatiles [12,18,19]. The data further suggests that CO2 does not participate significantly in the volatiles reactions otherwise its quantity would decrease with increasing volatiles residence time. In a similar way, the formation of CO may be attributed, at least partially, to the decomposition carbonyl and ether groups [20] in the volatiles generated from the coal-to-volatiles reaction. The insignificant variation of C2–C4 HC yields, within experimental errors, with the reactor type and pyrolysis temperature range suggests that these gases were mainly generated from the coal-to-volatiles reaction and their reactivity are low under the conditions used. It is also possible that these compounds participated in the volatiles reaction but their consumption was roughly balanced by their generation. The HHV of the gas products is estimated based on the GC results, excluding the gases from FT-77 K and FT-273 K due to the large sampling and GC errors discussed earlier. It is seen in Table 4 that the gases from VH-PE-77 K are the highest while those from GK-273 K are the lowest in HHV. This phenomenon may be attributed to the increased H2 contents in the gas with increasing volatiles reactions, because H2 is low HHV on volumetric basis.
References [1] Liu Z, Guo X, Shi L, He W, Wu J, Liu Q, et al. Reaction of volatiles – a crucial step in pyrolysis of coals. Fuel 2015;154:361–9. [2] Gonenc ZS, Gibbinst JR, Katheklakis LE, Kandiyoti R. Comparison of coal pyrolysis product distributions from three captive sample techniques. Fuel 1990;69:383–90. [3] Khan MR. A literature survey and an experimental study of coal devolatilization at mild and severe conditions: influences of heating rate, temperature, and reactor type on products yield and composition. Fuel 1989;68(12):1522–31. [4] ISO 502:2015 Coal – Determination of caking power – Gray-King coke test; 2015. [5] 27 TCIT. ISO 647:2017 Brown coals and lignites – Determination of the yields of tar, water, gas and coke residue by low temperature distillation; 2017. [6] Zhou Q, Liu Q, Shi L, Yan Y, Wu J, Xiang C, et al. Effect of volatiles’ reaction on composition of tars derived from pyrolysis of a lignite and a bituminous coal. Fuel 2019;242:140–8. [7] Zhang C, Wu R, Xu G. Coal pyrolysis for high-quality tar in a fixed-bed pyrolyzer enhanced with internals. Energy Fuels 2013;28(1):236–44. [8] Cheng S, Lai D, Shi Z, Hong L, Zhang J, Zeng X, et al. Suppressing secondary reactions of coal pyrolysis by reducing pressure and mounting internals in fixed-bed reactor. Chin J Chem Eng 2017;25(4):507–15. [9] Zhou Q, Liu Q, Shi L, Yan Y, Liu Z. Behaviors of coking and radicals during reaction of volatiles generated from fixed-bed pyrolysis of a lignite and a subbituminous coal. Fuel Process Technol 2017;161:304–10. [10] Qin Y. Thermochemical model of tar formation during biomass gasification (Doctoral dissertation). Taiyuan University of Technology; 2009. (in Chinese). [11] Qin Y, Huang H, Wu Z, Feng J, Li W, Xie K. Characterization of tar from sawdust gasified in the pressurized fluidized bed. Biomass Bioenergy 2007;31(4):243–9. [12] Shi L, Liu Q, Guo X, Wu W, Liu Z. Pyrolysis behavior and bonding information of coal – a TGA study. Fuel Process Technol 2013;108:125–32. [13] Shi L, Cheng X, Liu Q, Liu Z. Reaction of volatiles from a coal and various organic compounds during co-pyrolysis in a TG-MS system. Part 1. Reaction of volatiles in the void space between particles. Fuel 2018;213:37–47. [14] Shi L, Cheng X, Liu Q, Liu Z. Reaction of volatiles from a coal and various organic compounds during co-pyrolysis in a TG-MS system. Part 2. Reaction of volatiles in the free gas phase in crucibles. Fuel 2018;213:22–36. [15] Griffiths DML, Mainhood JSR. Cracking of tar vapor and aromatic compounds on activated carbon. Fuel 1967;46:167–76. [16] Wu J, Liu Q, Wang R, He W, Shi L, Guo X, et al. Coke formation during thermal reaction of tar from pyrolysis of a subbituminous coal. Fuel Process Technol 2017;155:68–73. [17] Katheklakis LE, Lu S-L, Bartle KD, Kandiyoti R. Effect of freeboard residence time on the molecular mass distributions of fluidized bed pyrolysis tars. Fuel 1990;69(2):172–6. [18] McKee DW. Fundamental issues in control of carbon gasification reactivity. France:
4. Conclusions To solve the problems of common laboratory reactors, such as significant volatiles reactions and loss of light products, the VH reactor is 7
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[20] Liu J, Jiang X, Shen J, Zhang H. Pyrolysis of superfine pulverized coal. Part 2. Mechanisms of carbon monoxide formation. Energy Convers Manage 2014;87(5):1039–49.
Kluwer Academic Publishers; 1991. p. 483–8. [19] Zhang K, Li Y, He Y, Wang Z, Li Q, Kuang M, et al. Volatile gas release characteristics of three typical Chinese coals under various pyrolysis conditions. J Energy Inst 2017;91:1045–56.
8
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Full Length Article
A novel vacuumed hermetic reactor and its application in coal pyrolysis Bin Zhou, Qingya Liu, Lei Shi, Chong Xiang, Shumiao Quan, Zhenyu Liu
⁎
T
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Pyrolysis Hermetic reactor VH reactor Reaction of volatiles Product collection
A novel vacuumed hermetic (VH) reactor is designed and compared with the common Gray-King (GK) and flowthrough (FT) reactors for lignite pyrolysis under different operation modes. The quantity and quality analyses of pyrolysis products, such as char, tar and its fractions, water and gas show that the VH reactor with pre-evacuation (PE) by a pump and cooling of volatiles by liquid nitrogen (VH-PE-77 K) greatly reduces the volatiles reaction time and collects all of products, and the pyrolysis results are closer to the coal-to-volatiles reaction (or the so called primary pyrolysis) than other reactors commonly used in the literature. Compared with the yields of VH-PE-77 K in the temperature ranges from 50 °C to 500, 550 and 600 °C, the GK reactor allows more volatiles reaction (or the so called secondary reaction) and generally yields about 10% less tar, 7% less water, 3% more char and 10% more gas. It is found that the volatiles reaction contributes little to CO2 yield but significantly to H2, CH4 and CO yields. The water generated from coal-to-volatiles reaction participated in the volatiles reaction. The volatiles reaction reduces the chars’ surface area and the higher heating value (HHV) of gas (on volume basis), but increases the chars’ HHV (on mass basis).
1. Introduction
of withdrawing the volatiles as soon as their formation and collecting the products completely. The idea of a standard laboratory pyrolysis reactor is not new and such reactors had been designed and used. The most popular one is the Gray-King (GK) assay [4] and another one probably is the Fischer assay [5]. These reactors pyrolyze a fixed amount of coal under a default heating program without purging and collect the condensable products by a flask cooled by ice water before venting the incondensable gases. The quality and quantity of products of these reactors have been deemed as the indexes for other reactors, laboratory scale or large scale [4,5]. A characteristic of these reactors is withdrawing the volatiles naturally by the volume expansion caused by the coal-to-volatiles (solid-to-gas) reaction at the atmospheric pressure, which is temperature dependent. It was estimated that the residence time of volatiles at the pyrolysis zone of a standard GK reactor is approximately 9 s for volatiles generated at 450 °C and approximately 28 s for volatiles generated at 600 °C [1]. These volatiles’ withdrawing rates are slow and their reaction time is long because it has been shown that the volatiles reaction occurred extensively in a few seconds at these temperatures [6]. To reduce the extent of volatiles reaction, inert sweeping gases were used in many studies, which shortened the volatiles residence time, to less than 2 s in some cases [1], but inevitably increased the burden of product collection system leading to increased loss of condensable products. To collect all the condensable products multi-stage
Pyrolysis plays an important role in many fuel and chemical production processes from coal, oil shale, biomass and many other organic resources. It has been studied extensively in laboratories and its product yields were found to be dependent on reactor type and operating conditions, such as temperature, heating rate and pressure. It is also frequently observed that the pyrolysis results of small laboratory reactors differ significantly from those of large-scale reactors even though the reactor type and pyrolysis conditions are nominally the same. This difference in pyrolysis result between reactors has been indiscriminately termed as the scaling-up effect in many studies. However, some studies showed that the different pyrolysis results can be attributed to the differences in reaction of volatiles inside these reactors, due to different temperature-time profiles of the gas phase in different reactors, as well as the difference in products collection systems [1–3]. Since it is not easy to simulate a large-scale reactor exactly by a small laboratory reactor, it is logical to obtain the “intrinsic pyrolysis behavior” from a small laboratory reactor, which can be used to simulate the performance of a large-scale reactor by combining with the kinetics of volatiles reaction and the transport processes. The meaning of “intrinsic” is free, as much as possible, from the influences of the volatiles reaction, the transport effects and the efficiency of products collection method. In other words, an ideal laboratory reactor should be capable
⁎
Corresponding author. E-mail address: [email protected] (Z. Liu).
https://doi.org/10.1016/j.fuel.2019.115774 Received 8 October 2018; Received in revised form 24 June 2019; Accepted 5 July 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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tube has two ports on the top, one for a pressure transducer and one for a vacuum pump. Fig. 1(b) shows that the pyrolysis tube is preloaded with a sample boat containing a predetermined amount of sample, coal in this case. After been connected to the volatiles collection tube, the pyrolysis tube is placed in a tubular furnace at room temperature and the whole system is vacuumed by a pump to a low pressure. At the steady state the vacuum port is closed and the volatiles collection tube is immerged into a liquid nitrogen dewar. The pyrolysis experiment starts by heating the pyrolysis tube at a designated rate to various temperatures. The volatiles generated from coal diffuse through the coal bed and then flow to the volatiles collection tube where their volume is reduced by condensing into liquid and solid under the low temperature. This volatiles’ volume reduction in the volatiles collection tube results in a pressure drop from the pyrolysis tube to volatiles collection tube that drives the volatiles to flow in the same direction. This operation mode is termed VH-PE-77 K, where VH represents the reactor, PE stands for pre-evacuation by a pump and 77 K stands for cooling the volatiles collection tube to 77 K by liquid nitrogen. Of course, as discussed earlier, the VH reactor may not be preevacuated and the vacuum is solely achieved by cooling the volatiles collection tube with liquid nitrogen, after the whole reactor been swept by nitrogen to exclude the influence of oxygen. In this case the operation mode is termed VH-77 K.
cooling system were used [7–9], which complicates the products analysis and introduces large experimental errors. Clearly the experimental reactors developed so far cannot simultaneously reduce the volatiles reaction time and collect all the products with a simple reactor configuration and convenient operation. This article reports an invention of a vacuumed hermetic reactor (VH) or LZ (the initials of the main inventors, Liu and Zhou) reactor, which is capable of quickly withdrawing the volatiles from the sample bed and pyrolysis zone, and at the same time collecting all the volatile products. These characteristics are demonstrated by comparing the reactor’s pyrolysis results with those obtained from other reactors commonly used in the literature.
2. Methods 2.1. Invention of the VH reactor The hermetic reactor under vacuum is an ideal solution to simultaneously achieve fast withdrawing the volatiles and fully collecting of all the volatile products at the same time. Hermetic and vacuum, however, seem contradictive with each other, because the constant generation of volatiles from coal in pyrolysis raises the pressure of a hermetic reactor and increases the reaction time of volatile, while the use of a vacuum pump to withdraw the volatiles requires the reactor to be open which makes full collection of volatiles difficult. These two contradictive targets can be simultaneously achieved or closely simulated in a hermetic reactor by cooling the product collection system to the liquid nitrogen temperature (−196 °C) which lowers the reactor pressure before the pyrolysis experiment by approximately 3/5 in comparison to that at the room temperature, condenses most of the volatiles into liquid and solid, and reduces the volume of incondensable gas products during pyrolysis. This volume reduction effect is more pronounced when the volume of the product collection system is much larger than the volume of pyrolysis zone, and is further intensified when the reactor is pre-evacuated by a vacuum pump. With such a principle in mind the vacuumed hermetic reactor (VH reactor) is designed as shown in Fig. 1. Fig. 1(a) shows that the VH reactor is mainly composed of two parts, a horizontal pyrolysis tube and a volatiles collection tube, both with one open end. The volume of pyrolysis tube is about 78 ml while that of the volatiles collection tube is approximately 2 times larger, about 154 ml. These two tubes are coupled by the open ends by a ground joint that is sealed by silicon grease and fastened by a string on the hooks on each part. The open end of the volatiles collection tube is inserted deeper into the open end of the pyrolysis tube to reduce the volatiles condensation and coking on the pyrolysis tube. The volatiles collection
2.2. Common reactors used to compare with the VH reactor Fig. 2(a) and (b) show two reactors that simulate the open reactors commonly reported in the literature. The reactor in Fig. 2(a) is similar to the VH reactor in Fig. 1(a) but has a discharge port on the lower side of the volatiles collection tube to vent the incondensable volatiles at the ice-water temperature, 273 K. Before the pyrolysis experiment, the whole system is swept with nitrogen to eliminate oxygen. This operation mode simulates the GK assay and the Fischer assay, and is termed GK-273 K. The reactor in Fig. 2(b) has not only a discharge port on the volatiles collection tube as that in Fig. 2(a) but also a purging gas feeding port on the pyrolysis tube, simulating the flow through (FT) reactors. This reactor can be operated at the volatiles collection tube temperature of 77 K with liquid nitrogen or 273 K with ice-water, and is termed as FT77 K or FT-273 K, respectively. The purging gas is N2 with a flow rate of 150 ml/min. The main dimensions of the pyrolysis tubes and the volatiles collection tubes of GK reactor and FT reactor are the same as those of VH reactor, 20 mm in diameter and 150 mm in length for the pyrolysis tube, and 40 mm in diameter and 150 mm in length for the volatiles
Fig. 1. The VH reactor (a) and the pyrolysis system (b). 2
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Fig. 2. The GK reactor (a) and the FT reactor (b).
collection tube. The tubular furnace and the dewar used in these reactors are also the same. This consistence in equipment dimension warrants a rational comparison of coal pyrolysis behavior in all the reactors.
T are pressure (Pa), volume (m3) and temperature (K), respectively, which were measured directly, R is the gas constant (8.314 J·K−1·mol−1) and M is the mean molecular weight of gas based on the gas composition determined by GC.
2.3. Coal and pyrolysis conditions
ygas =
The coal used is Hulunbeier (HLBE) lignite from China. It was ground to 80–100 mesh in size and dried at 110 °C in a vacuum for 12 h. Its proximate and ultimate analyses are shown in Table 1. All the pyrolysis experiments used 3 g HLBE lignite, started at 50 °C and ended at 500, 550 and 600 °C under the same heating rate of 10 °C/min.
mgas =
After the pyrolysis experiments using VH reactor, when the reactor and the volatiles collection tube return to the room temperature, the gas was sampled by an injector through the gas port, and the char in sample boat and the liquid in volatiles collection tube were then weighed. The char and liquid yields were determined by Eqs. (1) and (2), and the gas yield was determined by difference (Eq. (3)). The liquid in volatiles collection tube was then dissolved in 5 ml tetrahydrofuran (THF) and the solution’s water content was determined by the Karl Fischer method. The water yield was determined by Eq. (4). The tar yield, i.e. the liquid excluding water, was determined by Eq. (5). In these equations, y and m stand for the yield and mass, respectively, while cwater is the water content of the THF solution.
yliquid =
mchar × 100% mcoal mliquid mcoal
PVM RT
(6) (7)
2.5. Char characterization The chars were characterized by surface analysis, higher heating value (HHV) and the ultimate analysis. The surface analysis was performed using CO2 at 0 °C on an Autosorb IQ (Quantachrome) after drying at 110 °C for 3 h and degassing at 300 °C for 8 h. The surface area was determined by Brunauer-Emmett-Teller (BET) methods. The chars’ HHV was determined using a bomb calorimeter, IKA C7000 (IKA-Werke GmbH & Co. KG), and calibrated by benzoic acid. 2.6. Tar characterization (1) Simulated distillation (SIMDIST)
(1)
× 100%
The tar dissolved in THF was analyzed by SIMDIST on a GC (Agilent 7890B) with a stainless steel capillary column (10 m × 0.53 mm × 0.5 μm), a FID detector, and automatic sampling. The sample size was 2 μL. The carrier gas was N2. The injection port was at 420 °C. The column temperature was 1 min at 35 °C, 10 °C/min to 400 °C and 5 min at 400 °C. The GC peak area was calibrated by various standards including p-xylene, n-nonane and n-decane. The components with boiling points lower than 360 °C is defined as the light fraction while those with boiling points higher than 360 °C and the residue in the volatiles collection tube and in the GC column are defined collectively as the heavy fraction [7].
(2)
ygas = 100% − ychar − yliquid
ywater
× 100%
The yields of char, liquid, gas, water and tar for pyrolysis in the GK and FT reactors were determined with the same method as discussed above for the VH reactor. The gas products during pyrolysis in GK reactor were collected continuously using a gas bag of 200 ml while those of FT reactor were collected continuously using a gas bag of 10 L.
2.4. Determination of the yields of pyrolysis products
ychar =
mgas mcoal
(3)
m × c water = solution × 100% mcoal
(4)
ytar = yliquid − ywater
(5)
The gas yield was also determined by Eq. (6) where mgas is the gas mass determined by the ideal gas equation (Eq. (7)). In Eq. (7), P, V and
(2) Gel permeation chromatography (GPC)
Table 1 Proximate and ultimate analyses of HLBE lignite. Proximate analysis (wt%)
The molecular weight (MW) distribution of tar was determined by GPC with a Waters e2695 solvent delivery pump and a Waters 2414 RI Detector, using the sequentially connected Waters styragel 0.5-THF, 1THF and 3-THF (7.8 mm × 300 mm) columns packed with polystyrenedivinylbenzene polymer. Mono-disperse polystyrene was used as the standard for calibration. The carrier was THF at a flow rate of 1 ml/min. The sample size was 50 μL. The temperature of columns and detector was 35 °C. According to Qin et al. [10,11], the MW of 100–134 amu was
Ultimate analysis (wt%, daf)
Mad
Ad
Vdaf
C
H
N
S
O*
30.99
9.1
29.94
73.93
5.12
1.14
0.38
19.43
ad: air-dry basis; d: dry basis; daf: dry-and-ash-free basis. M: moisture; A: ash; V: volatile matter content. *: By difference. 3
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defined as 1-ring aromatics (with hetero-atom substitution) and termed as F1; the MW of 134–198 amu was defined as 2–3-ring aromatics and termed as F2; the MW of 198–276 amu was defined as 4–5-ring aromatics (with conjugated side chains) and termed as F3; the MW of 276–433 amu was defined as the saturated long chain alkane and > 5ring aromatics with conjugated side chains and termed as F4. The aromatics with conjugated side chains may dominate F4 because the aliphatic CeC bonds in long chain alkane are easy to break during pyrolysis [12]. The MW beyond 433 amu was defined as asphaltenene and pre-asphaltenene and termed as F5. 2.7. Gas characterization The gas samples were analyzed by a GC (Agilent 7890A) equipped with an Al2O3 capillary column (30 m × 0.53 mm × 25 μm) and a FID for light hydrocarbons and a porapak Q column (3 m × 2 mm) and a TCD detector for permanent gases. The carrier gas was N2. The temperatures of columns and detectors were 95 and 140 °C, respectively. These two sets of GC data were calibrated by CH4 and normalized to yield the gas composition. The error of GC analysis is within 5%.
Fig. 4. The volatiles residence time in various reactors during pyrolysis in different operation modes.
3. Results and discussion residence time in the FT reactor is also determined based on the N2 purging rate and is shown in Fig. 4. It is clear that the volatiles residence time in the GK reactor is the longest, from around 48 s at 400 °C to about 3 s at 500 °C, and to about 20 s at 600 °C. The volatiles residence time in VH-77 K mode shows a similar trend as that in the GK reactor but is generally 60–70% shorter, around 30 s at 400 °C, 2 s at 500 °C and 14 s at 600 °C. Although the trend of volatiles residence time in VH-PE-77 K mode is also similar to that in the GK mode, it is very short, around 2, 0.5, 6.5 s at 400, 500 and 600 °C, respectively. The volatiles residence time in FT reactor is mainly affected by the N2 purging rate, it is similar to that in VH-PE-77 K in the temperature range of 400–550 °C, the range for the generation of most volatiles.
3.1. Pressure characteristics of different reactors Fig. 3 shows the pressure change during pyrolysis in the temperature range of 50–600 °C in the reactors under different operation modes. As expected, the pressures of GK-273 K, FT-77 K and FT-273 K are always 0.1 MPa because they were open to the atmosphere. The pressure of VH-PE-77 K was initially very low, approximately 0.0006 MPa due to the pre-evacuation by a vacuum pump and the subsequent cooling of the volatiles collection tube by liquid nitrogen, and increased over time especially at temperatures higher than 350 °C, and reached only 0.032 MPa at 600 °C. The pressure of VH-77 K was also initially low, approximately 0.044 MPa due to the cooling of volatiles collection tube by liquid nitrogen, and increased over time slowly in a somewhat linear way to reach 0.071 MPa at 600 °C. The sub-atmosphere pressures in the VH reactor during the whole pyrolysis experiments under the two operation modes indicate that the volatiles residence time in pyrolysis tube is short because the evaporation and diffusion rates of volatiles are reversely proportional to the pressure [8]. Based on the volatiles residence time in the GK reactor reported in the literature [1] the volatiles residence time in the VH reactor in two operation modes is estimated in Fig. 4. The volatiles
3.2. Comparison of products’ yields The different volatiles residence time and collection methods suggest that the products collected from pyrolysis in different operation modes are different in yield and in composition. To understand the certainty of these differences, mass balance of the experiments under different operation modes is evaluated. Since the mass of char and liquid was directly measured with small errors the main experimental error is in the gas yield which is determined either by mass difference or by gas analysis. Fig. 5 shows the mass balance based on the gas analysis and the mean molecular weight of gas used to determine the mass of gas. It is seen that the mean molecular weight of gas is in the range of 10–30 g/mol, which is smaller at a higher final pyrolysis temperature due to the formation of more lighter products, such as H2 and CH4, and larger for FT-77 K and FT-273 K than other reactors due to difficulty in quantifying the diluted H2 by GC. The mass balance is 100.0 ± 1.0 wt % for VH-PE-77 K, VH-77 K and GK-273 K, and 100.0 + 10.0 wt% for FT-77 K and FT-273 K. It seems that the gas yields determined by the mass difference are more accurate than those determined by the gas analysis, and therefore the gas yields by the mass difference are used in the following study. Fig. 6 compares the product yields generated in these reactors under different operation modes. The error bars were determined from the differences of triple experiments. Clearly, the errors of VH reactor in both modes are generally smaller than those of GK and FT reactors. To more clearly compare the yields, the yields of VH-PE-77 K are set as the baselines and labeled 100% while the yields of other reactors and operation modes are labeled by the percentage deviation from the corresponding baselines. For example, in Fig. 6(a) the +1.5% in char of VH77 K means 1.5% more char been formed than that of VH-PE-77 K in the
Fig. 3. The pressure in various reactors during pyrolysis in different operation modes. 4
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Compared to the VH modes, GK-273 K generated more char and gas while less tar and water in three temperature ranges. These differences are attributed to the slow flow rate of volatiles under the atmospheric pressure in GK reactor, which leads to more volatiles reactions in the reactor than those in the VH reactor. It is seen in Fig. 6 that the char and gas yields of FT-77 K and FT273 K are higher than those of VH-PE-77 K although the volatiles residence time in these reactors are similar. This phenomenon is very important because it indicates that the N2 purging in FT-77 K and FT273 K decreases only the volatiles’ residence time in the gas phase [15] while the sub-atmosphere pressure in VH-PE-77 K decreases the volatiles’ residence time in the gas phase as well as in the sample bed [8]. In other words, the extent of V + V reaction in FT-77 K, FT-273 K and VHPE-77 K are similar as indicated in Fig. 4, but the V + C reaction in FT77 K and FT-273 K is more extensive than that in VH-PE-77 K. The lower tar and water yields and the higher gas yields of the two FT modes, especially for FT-273 K, than those of VH-PE-77 K also demonstrate the loss of condensable volatiles in the FT reactor even though the volatiles collection tube was cooled to the liquid nitrogen temperature. It is interesting to note that the water yields of VH-PE-77 K are higher than those of other reactors and operation modes, such as that of VH-77 K. This seems suggesting that the water generated from the coalto-volatiles reaction participated in the volatiles reaction, such as V + C, leading to increased gas yield. Similar phenomenon was reported by Cheng et al., who pyrolyzed a coal with 82.3% carbon in a fixed bed reactor from the room temperature to 550 °C and found that the water yields increased when using reactor’s internals to reduce volatiles reaction, and proposed the reaction of water with char and tar [8].
Fig. 5. The mean molecular weights of pyrolysis gas and mass balance in different operation modes.
3.3. Comparison of char Table 2 lists the surface areas of chars from different operation modes at the final pyrolysis temperatures of 500 and 600 °C. The data of chars from FT-273 K are not listed because they are the same as those from FT-77 K. It is seen that the surface area of char from any operation mode increases from around 200 to around 300 m2/g when the final pyrolysis temperature increases from 500 to 600 °C, suggesting more pores been formed at a higher pyrolysis temperature due to more volatiles release. It is interesting to see that the surface areas of chars from VH-PE-77 K are larger than those of other chars. Furthermore, the surface areas of chars from FT-77 K are larger than those from the corresponding GK-273 K although both modes were operated under the atmospheric pressure. Since the V + C reaction in VH-PE-77 K is less than that in other operation modes and the V + C reaction in FT-77 K is less than that in GK-273 K, these behaviors indicate that the V + C reaction promoted coke deposition from volatiles and the deposited coke reduced the char’s surface area probably by blocking the char’s micro-pores. Table 3 lists the HHV of chars from different operation modes at the final pyrolysis temperatures of 500 and 600 °C. It is seen that the HHV of chars increases with an increase in final pyrolysis temperature, suggesting reduction in oxygen content of chars at a higher temperature. The lower HHV of chars from VH-PE-77 K than those of other chars suggests that the coke deposited on chars through the V + C reaction probably is of lower oxygen content and higher carbon as well as
Fig. 6. The yields of pyrolysis products in different operation modes.
temperature range of 50–500 °C, while the −11.8% in tar of VH-77 K means 11.8% less tar been formed than that of VH-PE-77 K in the temperature range of 50–500 °C. It is seen that VH-PE-77 K always yielded less char and gas, but more tar and water than other reactors and operation modes, indicating that VH-PE-77 K allowed the least volatiles reaction, including condensation and cracking that convert the condensable volatile products to char and incondensable gas, and collected all the volatiles. It has been reported that the pyrolysis of coal involves three types of reactions, coal-to-volatiles, volatiles-with-coal or -char (V + C), and volatiles-with-volatiles (V + V), the latter two can be addressed collectively as the volatiles reactions [13,14]. In principle, the coal-tovolatiles reaction are the same in all the reactors under all the operation modes, so the different product yields in Fig. 6 should be partly attributed to the different volatiles reactions. The different yields of VHPE-77 K and VH-77 K, therefore, indicate that the pre-evacuation is effective to reduce the V + C and V + V reactions. Furthermore, the decreasing difference in tar yield between these two modes with increasing final pyrolysis temperature, i.e. from −11.8% at 500 °C to −10.5% at 550 °C and then to −8.8% at 600 °C, indicates that the volatiles generated at temperatures below 500 °C contain more reactive and condensable fractions than those generated at temperatures higher than 500 °C. In other words, the content of incondensable gases in volatiles is more at a higher temperature than that at a lower temperature.
Table 2 Surface areas of chars from different operation modes. Temperature (°C)
50–500 50–600
Relative error = 2%. 5
Surface Area (m2/g) VH-PE-77 K
VH-77 K
GK-273 K
FT-77 K
204 319
174 275
173 263
186 287
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Table 3 HHV of chars from different operation modes. Temperature (°C)
50–500 50–600
HHV (MJ/kg, af) VH-PE-77 K
VH-77 K
GK-273 K
FT-77 K
18.5 25.5
25.0 30.1
25.2 34.5
25.5 29.5
Relative error = 5%.
Fig. 8. Chemical composition (GPC) of the tars obtained in the pyrolysis temperature range of 50–600 °C from different operation modes.
range, the yields of light and heavy tar fractions of VH-PE-77 K are higher than those of FT-77 K, while those of the latter are higher than the corresponding yields of FT-273 K, although the volatiles residence time in these reactors is similar. This trend is attributed mainly to the volatiles loss in the FT reactor especially at the product collection temperature of 273 K. Fig. 8 compares chemical composition of tars from different operation modes under the pyrolysis temperature range of 50–600 °C. The tars were dissolved in THF and the chemical composition was determined by GPC with experimental errors of within 3%. It is seen that the dominant component of tars from all the operation modes is F5 (around 45–55%), which is followed by F4 (around 17%) and then by F3 and F2 (both around 10–14%). With increasing volatiles residence time from VH-PE-77 K to VH-77 K to GK-273 K, as shown in Fig. 4, the content of F5 decreases notably, the contents of F1 and F2 increase notably, but the contents of F3 and F4 increase slightly. These behaviors indicate that the cracking of asphaltene and pre-asphaltene yields mainly small molecules such as those containing a few aromatic rings rather than those with more than 5 aromatic rings. Apparently, the tar from VH-PE-77 K is closer to the primary tar than the tar from GK-273, and therefore VH-PE-77 K is a better method to characterize the coal-tovolatiles reaction. In principle, the FT reactor with N2-purging, FT-77 K and FT-273 K, cannot collect all the products especially the lighter ones, which would result in tars with more heavy fractions, so it is not surprise to see that the tars from the FT reactor are heavier than that from GK-273 K and the tar from FT-273 K is heavier than that from FT-77 K.
Fig. 7. Yields and simulated distillation fractions of tars in different operation modes.
hydrogen content. For example, the chars been subjected to the most severe V + C reaction in GK-273 mode and under 50–600 °C were 2.5% more in carbon content and 0.3% more in hydrogen content whereas 2.8% less in oxygen content, than those been subjected to the limited V + C reaction in VH-PE-77 K and in the same temperature range.
3.4. Comparison of tar Fig. 7 shows the yields of tars from different operation modes and the tar fractions based on SIMDIST. The overall experimental errors are within 5%, corresponding to 0.15–0.45% for tar yields of 3–9%. The percentages noted in the bars are the contents of light and heavy fractions. It is seen that the yields of light and heavy fractions under all operation modes increase with an increase in final pyrolysis temperature. VH-PE-77 K generates the highest light and heavy tar yields in the same temperature range, indicating its tar underwent the least reactions due to the fast withdraw of volatiles from the char bed and from the pyrolysis tube, and also the full collection of volatiles. It is also seen in Fig. 7 that the yields of heavy tar fraction of VH-PE77 K in each temperature range are significantly higher than those of VH-77 K, while the yields of light tar fraction of VH-PE-77 K are only slightly higher than those of VH-77 K. This behavior suggests that the reaction of heavy tar fractions is more extensive than that of the light tar fractions, and the formation of light tar fractions from the heavy tar fractions overwhelms the conversion of light tar fractions. This suggestion agrees with that reported by Wu et al. in a pyrolysis study of coal tars [16] and that reported by Katheklakis et al. in a study of molecular mass distribution of the tars under different extent of secondary reactions [17]. The lower tar yields and the higher light tar contents of GK-273 K than those of VH-77 K is also ascribed to the increased volatiles reaction in GK-273 K due to the longer residence time of volatiles. It is not surprise to see that, in the same pyrolysis temperature
3.5. Comparison of gas Fig. 9 shows the yields of H2, CH4, C2-C4 HC (hydrocarbons), CO and CO2 of VH-PE-77 K, VH-77 K and GK-273 K. The yields of FT-77 K and FT-273 K are not shown due to large experimental errors caused by the N2-purging, which greatly lowered the products concentration in the gas bag. It is apparent that the yields of H2, CH4 and CO increase with the volatiles residence time in the pyrolysis tube, from VH-PE-77 K to VH-77 K and to GK-273 K, and increase with the final temperature, from 500 to 550 and to 600 °C. These behaviors indicate that in the temperature range studied these compounds are generated not only from the coal-to-volatiles reaction but also from the volatiles reactions. The 6
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designed. The reactor was compared with common laboratory reactors, including a simulated GK reactor and a flowed through reactor for lignite pyrolysis under various operation modes differing in pressure, purging and cooling methods. The VH reactor with pre-evacuation (PE) by a pump and cooling of volatiles by liquid nitrogen (VH-PE-77 K) is characterized by the shortest volatiles residence time and full collection of the products. The commonly used GK reactor with the standard volatiles’ cooling method (at 273 K) has the longest volatiles residence time and inefficient tar’s trapping. Compared with VH-PE-77 K, the GK-273 K yielded about 10% less tar, 7% less water, 3% more char and 10% more gas. The VH reactor can also be operated under VH-77 K mode, without pre-evacuation, to yield more tar, less char and gas than GK-273 K. The tars generated from VH-PE-77 K are slightly heavier than those from GK-273 K due to the least volatiles cracking. The VH-PE-77 K reduces the volatiles residence time not only in the gas phase but also in the solid sample bed, while the commonly practiced flow-through (FT) reactor reduces only the volatiles residence time in the gas phase. The CO2 yield varied little with the reactor type and operation mode, and is attributed to the decomposition of carboxylic groups in coal, not in the volatiles reactions. CO2 did not react with the volatiles while water was consumed in the volatiles reaction. H2, CH4 and CO were generated from the coal-to-volatiles reaction as well as from the volatiles reaction.
Fig. 9. The yield and main components of gas from different operation modes.
Table 4 HHV of gas from different operation modes. Temperature (°C)
50–500 °C 50–550 °C 50–600 °C
HHV of gas (MJ/Nm3) VH-PE-77 K
VH-77 K
GK-273 K
46.14 37.71 25.01
45.59 35.31 24.32
42.49 33.12 21.53
Acknowledgments We thank the National Key Research and Development Program of China (2017YFB0602401) and Joint Funds of Natural Science Foundation of China (U1610107) for financial support.
contribution of these volatiles reactions in GK-273 K is much higher than that in VH-PE-77 K, producing 0.7% more in H2 yield, 0.5% more in CH4 yield and 1.0% more in CO yield for pyrolysis in the temperature range of 50–600 °C. It is interesting to note that the CO2 yields vary little with the reactor type, the operation mode, and even the pyrolysis temperature range. This behavior suggests that the oxygen-containing moieties responsible for CO2 formation are less stable than those responsible for CO formation and decompose mainly at temperatures lower than 500 °C. The oxygen-containing moieties for CO2 generation are likely carboxylic groups that decompose in the coal-to-volatiles reaction and present little in the volatiles [12,18,19]. The data further suggests that CO2 does not participate significantly in the volatiles reactions otherwise its quantity would decrease with increasing volatiles residence time. In a similar way, the formation of CO may be attributed, at least partially, to the decomposition carbonyl and ether groups [20] in the volatiles generated from the coal-to-volatiles reaction. The insignificant variation of C2–C4 HC yields, within experimental errors, with the reactor type and pyrolysis temperature range suggests that these gases were mainly generated from the coal-to-volatiles reaction and their reactivity are low under the conditions used. It is also possible that these compounds participated in the volatiles reaction but their consumption was roughly balanced by their generation. The HHV of the gas products is estimated based on the GC results, excluding the gases from FT-77 K and FT-273 K due to the large sampling and GC errors discussed earlier. It is seen in Table 4 that the gases from VH-PE-77 K are the highest while those from GK-273 K are the lowest in HHV. This phenomenon may be attributed to the increased H2 contents in the gas with increasing volatiles reactions, because H2 is low HHV on volumetric basis.
References [1] Liu Z, Guo X, Shi L, He W, Wu J, Liu Q, et al. Reaction of volatiles – a crucial step in pyrolysis of coals. Fuel 2015;154:361–9. [2] Gonenc ZS, Gibbinst JR, Katheklakis LE, Kandiyoti R. Comparison of coal pyrolysis product distributions from three captive sample techniques. Fuel 1990;69:383–90. [3] Khan MR. A literature survey and an experimental study of coal devolatilization at mild and severe conditions: influences of heating rate, temperature, and reactor type on products yield and composition. Fuel 1989;68(12):1522–31. [4] ISO 502:2015 Coal – Determination of caking power – Gray-King coke test; 2015. [5] 27 TCIT. ISO 647:2017 Brown coals and lignites – Determination of the yields of tar, water, gas and coke residue by low temperature distillation; 2017. [6] Zhou Q, Liu Q, Shi L, Yan Y, Wu J, Xiang C, et al. Effect of volatiles’ reaction on composition of tars derived from pyrolysis of a lignite and a bituminous coal. Fuel 2019;242:140–8. [7] Zhang C, Wu R, Xu G. Coal pyrolysis for high-quality tar in a fixed-bed pyrolyzer enhanced with internals. Energy Fuels 2013;28(1):236–44. [8] Cheng S, Lai D, Shi Z, Hong L, Zhang J, Zeng X, et al. Suppressing secondary reactions of coal pyrolysis by reducing pressure and mounting internals in fixed-bed reactor. Chin J Chem Eng 2017;25(4):507–15. [9] Zhou Q, Liu Q, Shi L, Yan Y, Liu Z. Behaviors of coking and radicals during reaction of volatiles generated from fixed-bed pyrolysis of a lignite and a subbituminous coal. Fuel Process Technol 2017;161:304–10. [10] Qin Y. Thermochemical model of tar formation during biomass gasification (Doctoral dissertation). Taiyuan University of Technology; 2009. (in Chinese). [11] Qin Y, Huang H, Wu Z, Feng J, Li W, Xie K. Characterization of tar from sawdust gasified in the pressurized fluidized bed. Biomass Bioenergy 2007;31(4):243–9. [12] Shi L, Liu Q, Guo X, Wu W, Liu Z. Pyrolysis behavior and bonding information of coal – a TGA study. Fuel Process Technol 2013;108:125–32. [13] Shi L, Cheng X, Liu Q, Liu Z. Reaction of volatiles from a coal and various organic compounds during co-pyrolysis in a TG-MS system. Part 1. Reaction of volatiles in the void space between particles. Fuel 2018;213:37–47. [14] Shi L, Cheng X, Liu Q, Liu Z. Reaction of volatiles from a coal and various organic compounds during co-pyrolysis in a TG-MS system. Part 2. Reaction of volatiles in the free gas phase in crucibles. Fuel 2018;213:22–36. [15] Griffiths DML, Mainhood JSR. Cracking of tar vapor and aromatic compounds on activated carbon. Fuel 1967;46:167–76. [16] Wu J, Liu Q, Wang R, He W, Shi L, Guo X, et al. Coke formation during thermal reaction of tar from pyrolysis of a subbituminous coal. Fuel Process Technol 2017;155:68–73. [17] Katheklakis LE, Lu S-L, Bartle KD, Kandiyoti R. Effect of freeboard residence time on the molecular mass distributions of fluidized bed pyrolysis tars. Fuel 1990;69(2):172–6. [18] McKee DW. Fundamental issues in control of carbon gasification reactivity. France:
4. Conclusions To solve the problems of common laboratory reactors, such as significant volatiles reactions and loss of light products, the VH reactor is 7
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[20] Liu J, Jiang X, Shen J, Zhang H. Pyrolysis of superfine pulverized coal. Part 2. Mechanisms of carbon monoxide formation. Energy Convers Manage 2014;87(5):1039–49.
Kluwer Academic Publishers; 1991. p. 483–8. [19] Zhang K, Li Y, He Y, Wang Z, Li Q, Kuang M, et al. Volatile gas release characteristics of three typical Chinese coals under various pyrolysis conditions. J Energy Inst 2017;91:1045–56.
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