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Experimental measurement of gas concentration distribution in an impinging entrained-flow gasifier
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a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m
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Experimental measurement of gas concentration distribution in an impinging entrained-flow gasifier Miaoren Niu, Zhuoyong Yan, Qinghua Guo, Qinfeng Liang, Guangsuo Yu, Fuchen Wang⁎, Zunhong Yu Institute of Clean Coal Technology, East China University of Science and Technology, Mei Long Road No. 130, Shanghai, 200237, PR China Key Laboratory of Coal Gasification of Ministry of Education, East China University of Science and Technology, Shanghai 200237, China
AR TIC LE I N FO
ABS TR ACT
Article history:
On a laboratory-scale testing platform of impinging entrained-flow gasifier with two
Received 29 May 2007
opposed burners, the detailed measurements of gas concentration distribution have been
Received in revised form 8 April 2008
performed for carbonaceous compound (diesel oil) at atmospheric pressure. Under the
Accepted 22 April 2008
condition of 1.48–2.36 O/C ratios (kg/kg), radial gas samples are collected at three axial positions and the syngas exit position with stainless steel water-cooled probes, the
Keywords:
concentration distribution of the major gases (H2, CO, CO2, CH4 and O2) under stable
Gasification
operating state was determined with a mass spectrometry. These data are used to clarify
Entrained-flow gasifier
mixing and reaction characteristics within the reactor, to give insight into the combustion
Gas concentration distribution
process and provide a database for evaluating predictive mathematical models. © 2008 Published by Elsevier B.V.
1.
Introduction
Gasification is a very versatile process to convert a variety of carbon-containing feedstocks, such as coal, petroleum coke, lignite, heavy oils, residues and natural gas, into syngas. The entrained-flow gasification technology has been extensively applied to the production of ammonia, methanol, acetic acid, other chemicals and power generation through Integrated Gasification Combined Cycle (IGCC). The gasification process of an entrained-flow gasifier is very complicated, because it relates to the fluid flow under the condition of high temperature, high pressure and heterogeneous state. Impinging stream flow configurations are characterized by streams of fluid jets impinging against each other in a confined vessel, which have proved useful in conducting a wide array of chemical engineering unit operations and enhancing heat and mass transfer between phases due to the high transfer coefficients [1]. The opposing jet technique has been applied in many fields and extensively studied practically [2–7] and theoretically [8,9].
It is necessary to understand the gas concentration distribution inside the gasifier in detail for gasifier's optimum design and characteristic evaluation. Numerical simulation is an effective technique for predicting the gasification characteristics, there are several mathematical models developed specially for entrained-flow gasifiers in the literature. Wen and Chaung [10], and Govind and Shah [11] developed models for Texaco downflow, slurry-fed entrained gasifiers. Ni and Williams [12] developed a multivariable model for Shell coal gasifiers on the basis of equilibrium, mass and energy balances. Liu et al. [13] presented a model for a pressured entrained-flow coal gasifier to determine the effect of pressure, reaction kinetics and char structure on the gasification reactions. Chen et al. [14,15] developed a comprehensive three-dimensional simulation model for entrained-flow coal gasifiers, in their model, the numerical methods and the submodels were used to a two-stage air blown entrained-flow gasifier. Chen et al. [16] used a Multi Solid Progress Variables (MSPV) method to simulate the gasification reaction and reactant mixing process for a entrained-flow coal gasifier. Choi et al [17] predicted a slurry feed type, entrained-flow coal gasifier.
⁎ Corresponding author. Presently at Key Laboratory of Coal Gasification of Ministry of Education, East China University of Science and Technology, Shanghai 200237, China. Tel.: +86 21 64252521; fax: +86 21 64251312. E-mail address: [email protected] (F. Wang). 0378-3820/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.fuproc.2008.04.009
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Fig. 1 – Schematic diagram of the experimental apparatus and top view of gasifier (all dimensions in mm). (1) Liquid tank, (2) O2 steel cylinder, (3) Argon steel cylinder, (4) pump, (5) gas mass flow meter, (6) burner, (7) vortex flowmeter, (8) slag discharge, (9) gas pretreatment system, (10) mass spectrometry, (11) computer, (12) flame monitoring system.
Watanabe and Otaka [18] modeled the gasification reaction in the entrained-flow coal gasifier introducing the coal reactivity and predicted the change in basic gasification characteristics under different operating conditions. However, such models require thorough validation for each application, the ability to determine the precision and accuracy of a particular modeling approach is limited, because the detailed experimental information that can be used for data model comparisons is nearly nonexistent, full-scale plants are not entirely suited to the collection of the data, being very expensive to operate and also very difficult to access. These
drawbacks reinforce the importance of gasifier as a valuable means of obtaining data. The major objective of present work is to obtain detailed local measurements of the major syngas species (H2, CO, CO2, CH4 and O2) concentration, to clarify mixing and reaction characteristics and provide further insight into mechanisms which govern the gasification process.
Fig. 2 – General view of the burner.
Fig. 3 – The typical two-burner impinging flame image.
2.
Experimental equipment and procedure
The schematic drawing of the experimental apparatus and top view of gasifier were shown in Fig. 1. The gasifier was
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Fig. 4 – The stainless steel, water-cooled probe (all dimensions in mm).
cylindrical, vertically oriented, the inner diameter and length of the combustion chamber composed of a 15 mm thick cast refractory shell, were 300 and 2250 mm, respectively. The cast refractory shell, wrapped with a 235 mm thick, low thermal conductivity fiber blanket to reduce the heat transfer, was protected by a stainless-steel column shell of 0.8 m in diameter and 2.5 m in height. Ports were located at sides of the gasifier for viewing, temperature measurement and insertion of the water-cooled probes. Opposed turbulent flow fields were obtained by two opposed round burners composed of inner and outer channels. The O2 was fed into the burner outer channel by steel cylinder, with a pressure-reducing valve to avoid pressure oscillations in order to achieve steady flow. The gas flow rates were measured by mass flow meters (D07-9C/ZM, Beijing Sevenstar Huachuang Electronic Co., Ltd). The diesel oil was fed into the burner inner channel by a gear pump (A-73004-00#, America Cole-Parmer Company), the flow rate was determined gravimetrically with an elapsed timer and an electronic weight scale. In the gasification process, two burners were used to produce opposite jets of fuel that impinge on the center of the combustion chamber. The size of the burners is shown in Fig. 2. High relative velocities between the particulate matter and the gaseous phase in the central area provided good conditions for active diffusion and convection at the particle surface, and the high temperature together resulted in fast burning and gasification reaction under highly reducing conditions to produce raw syngas. High-temperature gaskets interfaced the furnace segments and eliminated all leakage. In addition, the pressure inside the furnace was maintained near the atmospheric or slightly above, to ensure that measurements would not suffer bias due
to contamination from the in-leakage of room air. From the reaction chamber, the raw syngas flowed into the quench chamber, where the raw syngas was cooled and partially scrubbed by the water, then the syngas was discharged. A flame image detector is fixed on the top refractory wall of the gasifier, and the whole flames are visible during the experimental process. The flame monitoring system consists of a lens, an optical probe, a CCD camera (Panasonic WV-CP470), and a microcomputer. The CCD camera and its accessories are cooled by water to avoid overheating. Argon was fed into the gasifier from the top of the gasifier to protect the flame monitoring lens. A typical flame image is illustrated in Fig. 3. The gas analysis was performed by a mass spectrometry. The gas sampling and analyzing systems were composed of two parts, (a) The water-cooled probe. In order to take gas samples continuously, stainless steel, water-cooled probes were designed to cool the extracted gas mixture and to prevent the oxidation of carbon monoxide, shown in Fig. 4. The probe temperature was controlled by the cooling water flow rate and was adjusted to avoid condensation of water at the probe surface. The inlet cooling water temperature is about 25 °C, the outlet cooling water temperature is about 26 °C~30 °C. Each analyzing apparatus required a certain flow rate. Therefore, it was necessary to provide enough test gas, but on the other hand, in order not to disturb the flow field, the suction of the probe was adjusted to the lowest flow velocity where measurements were taken. We got samples at three axial positions B, C, D and syngas exit position E, see Fig. 1, at each axial position, probe was traversed radially from the wall to reactor centerline (spaced 15 cm apart) for six radial positions (0, 3, 6, 9, 12, and 15 cm) to collect samples. (b) The sample pretreatment system. The gas pretreatment apparatus was shown in Fig. 5. The sample pretreatment system used a small vacuum pump to draw the gas, liquid, and solid samples through the probe, firstly, the sampled gas was filtered where most of the soot carbon was eliminated from the test gas, then gas was dried and fine filtered, and passed through a condenser to remove most of the water vapor and next passed through a desiccator again to remove the residual water,
Fig. 5 – The gas pretreatment system.
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Table 1 – The physical properties of diesel oil Density kg m− 3 Viscosity Pa s Heat value MJ kg− 1 C element wt.% H element wt.% N element wt.% Inlet temperature °C 5 × 10− 3
840
44.2
86.18
a condition required by the gas analyzer, and then passed through a flowmeter, at last the gas entered into the mass spectrometry Model HPR20QIC, where the major gases (H2, CO, CO2, CH4 and O2) were analyzed.
13.72
3.
Results and discussion
Due to the high thermal capacity of refractory walls, from the ignition starting point, about 4 h are needed to achieve steady state as monitored by steady readings of wall temperatures. Among the important operating parameters, oxygen feeding amount is the most important. Flame stability is strongly affected by the atomized droplet size, droplet velocity and spray distribution which are controlled by the O/C ratio. In order to determine how to affect the overall gasifier performance with the change of oxygen feed, O/C ratios (kg/kg) are changed from 1.48 to 2.36 while diesel oil amount is maintained at about 2.0 kg/h. A summary of experimental conditions is shown in Table 2. The reactor is operated at quasisteady conditions in which the wall temperatures are stable during the sampling period. In order to help understand the gasification characteristics, the wall temperature distribution along the centerline was shown in Fig. 6. The O–O′ plane, shown in Fig. 1, was set to reference plane, downward direction is the positive. We can see that, the maximum temperature appears in the burner plane,
25
the farther away from the burner, the lower the temperature, the gasifier heat load from the impact area to top and bottom transfer.
3.1. All components, lines, and fittings of the sampling system were made of Teflon to diminish catalytic influences. Time interval that analyzes a set of data was 16–20 s. To obtain sufficient quantities of gas sample for analysis, the sampling time at each position was maintained at 5–10 min, so the time was sufficient to average out the random fluctuations, each analyzing apparatus required a certain flow rate, therefore, it was necessary to provide enough test gas. The combustor wall temperature was measured by a platinum–rhodium thermocouple installed to the inside wall surface at the axial position A. The instrument errors associated with the concentration and temperature measurements were generally less than 1% of the measured value. The physical properties of diesel oil were shown in Table 1. The inlet temperature for both oxygen and argon is 20 °C.
0.10
The syngas concentration radial distribution at position B
Under different O/C ratios, the syngas concentration radial distributions (all on the dry basis) at axial position B are shown in Figs. 7–10. Where, ‘Position A temp’ represents the wall temperature of position A. Here, we are not trying to describe a specific figure or a specific O/C ratio condition, our main objective is to provide the detailed experimental data about syngas concentration radial distribution at position B, at the same time, we tried to give some explanation about the effect of O/C ratios. It can be seen that, with the O/C ratios increase, CO2 concentration increases, CO concentration remains basically a constant when O/C ratio larger than 1.48, H2 and CH4 concentrations decrease, and O2 concentration approaches to zero. And for each fixed O/C ratio, (1) The H2 concentration gradually decreased from the wall to the centerline, while CO had the opposite trend. (2) The CO2 concentration firstly decreased and then increased. At different radial positions, O2 was rapidly depleted near zero, which showed that the gasification reaction occurred quickly at the higher temperature. For the entrained-flow type gasification system, reaction time is normally less than 4–5 s that requires fast monitoring of reaction. (3) The nearer gasifier centerline, the higher CH4 concentration, but CH4 concentration had the least value at gasifier centerline. (4) Argon concentration had the maximum value at radial 6 cm position, then decreased slowly. Because argon was fed into the gasifier from the top of the gasifier and to protect the flame monitoring lens. In the gasification
Table 2 – Experimental conditions Condition
1 2 3 4 5 6 7
1# 2# 1# O2 diesel diesel Nm3/h kg/h kg/h 1.95 1.95 2.01 2.08 2.00 2.00 2.00
2.04 2.04 1.98 2.04 1.98 2.00 2.00
1.75 1.89 2.04 2.21 2.32 2.49 2.79
2# O2 Nm3/h 1.81 2.04 2.19 2.39 2.50 2.62 2.91
Ar O/C Nm3/h kg/kg 0.82 0.72 0.67 0.12 – 0.72 0.81
1.48 1.63 1.76 1.85 2.01 2.12 2.36
Fig. 6 – The wall temperature distribution along the centerline (O/C = 1.76).
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Fig. 9 – O/C = 2.01 (Position A temp 1360 °C).
Fig. 7 – O/C = 1.48 (Position A temp 1290 °C). process two burners were used to produce opposite jets of fuel that impinged on the center of the combustion chamber and forming an impinging plane, so argon was difficult to enter into gasifier center. Diesel oil gasification occurs two different types of chemical reaction: exothermic primary reaction (combustion) and endothermic secondary reaction. Which types of reaction actually occur in an area in the gasifier depend on flow characteristics and mixing process in the area. At position B, the main chemical reaction is combustion of pyrolysis components with oxygen, accompanied with combustion of CO and H2 from reflux region. Lefebvre [19] and Smoot et al. [20] reported that velocity of combustion reaction is very fast with time scale between 2 and 4 ms that is far less than material mixing time scale (~0.1 s reported by Wang [21]). All the important reactions occurring in primary reactions region are: Pyrolysis þ O2 →CO2 þ H2 O
ð1Þ
2CO þ O2 ¼ 2CO2
ð2Þ
2H2 þ O2 ¼ 2H2 O
ð3Þ
CH4 þ 2O2 ¼ CO2 þ 2H2 O
ð4Þ
which has a very high temperature (N1000 °C), it takes place chemical decomposition quickly, and produces a variety of species, including C, CH4, H2, and other pyrolysis products. These species quickly react with oxygen to produce the hot syngas. Secondly, In this reactor, two equal suspension streams flow against one another at high velocity and impinge at their midpoint, resulting in a highly turbulence zone at position B, which can enhance mix quickly.
3.2.
The syngas concentration radial distribution at position C
Under different O/C ratios, the radial distributions of syngas concentration (all on the dry basis) at axial position C are shown in Figs. 11–14. Where, ‘Position A temp’ represents the wall temperature of position A. It is seen that, the concentration and radial distributions of each component are shown nearly as a horizontal line, which means that there are equal values at different radial positions. Radial profiles of gas concentration rapidly become uniform, which essentially indicates that the completeness of mixing composition is within 20 cm downstream the burner plane, only a slight overall change in gas composition is observed beyond that. The average composition of syngas is shown in Table 3.
We think that CO, H2 and CH4 appear in this area for two reasons: Firstly, when diesel oil was fed into the gasifier,
Fig. 8 – O/C = 1.76 (Position A temp 1308 °C).
Fig. 10 – O/C = 2.36 (Position A temp 1380 °C).
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Fig. 11 – O/C = 1.48 (Position A temp 1282 °C).
Fig. 13 – O/C = 2.01 (Position A temp 1360 °C).
Fig. 15 shows the effect of O/C ratio on syngas composition at position C, it can be seen that, with the O/C ratios increase, CO2 concentration increases, CO concentration remains basically a constant, H2 and CH4 concentrations decrease, and O2 concentration approaches to zero. In these reactions region, products from primary reactions region are reacted further as follows:
about 10 s calculated by Sun et al. [22]. It is a common knowledge that reaction (6) is faster than reaction (5). Reactions from (7) to (9) are homogenous reactions with higher velocities than reactions (5) and (6) at high temperature. Time scale of reaction between residue carbon with steam or carbon dioxide is higher than that of micro mixing process. Hence, chemical reaction is the controlling step of residue carbon gasification process.
C þ CO2 ¼ 2CO
ð5Þ
C þ H2 O ¼ CO þ H2
ð6Þ
CH4 þ H2 O ¼ CO þ 3H2
ð7Þ
CH4 þ CO2 ¼ 2CO þ 2H2
ð8Þ
CO2 þ H2 ¼ CO þ H2 O
ð9Þ
3.3.
When the temperature lies between 900 and 1500 °C, the rate of char–H2 reactions is one to two orders of magnitude slower than the rates of char reactions with CO2 or H2O, which is similar over that temperature range. Generally, a reciprocal of reaction velocity constant 1/k is used to characterize reaction time scale, reaction time scale of reaction (5) is
Fig. 12 – O/C = 1.76 (Position A temp 1290 °C).
The syngas concentration radial distribution at position D
The average radial composition of syngas at position D is shown in Table 4. Fig. 16 shows the effect of O/C ratio on syngas composition at position D, it can be seen that, with the O/C ratios increase, CO2 concentration increases, CO concentration remains basically a constant, H2 and CH4 concentrations decrease, and O2 concentration approaches to zero.
3.4.
The syngas concentration at position E
The average radial composition of syngas at position E is shown in Table 5. Fig. 17 shows the effect of O/C ratio on syngas composition at position E, it can be seen that, with the O/C ratios increase, CO 2 concentration increases, CO
Fig. 14 – O/C = 2.36 (Position A temp 1390 °C).
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Table 3 – The average radial composition by dry vol.% at position C O/C kg/kg
CO2
CO
H2
CH4
O2
1.48 1.63 1.76 1.85 2.01 2.12 2.36
7.60 10.38 10.24 14.15 13.87 15.78 21.76
45.77 49.46 49.52 48.35 51.67 51.50 51.47
44.42 38.75 38.57 36.84 34.15 32.52 26.62
2.06 1.23 1.51 0.49 0.16 0.03 0
0.03 0.03 0.03 0.04 0.04 0.02 0.02
concentration remains basically a constant, H2 and CH4 concentrations decrease, and O2 concentration approaches to zero. There are a number of different criteria that are frequently quoted for gasification process. The two most commonly encountered are carbon conversion efficiency and cold gas efficiency, which have been indicated for all cases in Table 6. We can see that the optimal O/C ratio range is 1.70–2.00.
Table 4 – The average radial composition by dry vol.% at position D O/C kg/kg
CO2
CO
H2
CH4
O2
1.48 1.63 1.76 1.85 2.01 2.12 2.36
8.05 11.65 11.30 15.77 14.73 18.41 24.35
46.79 47.80 48.77 47.32 50.18 48.92 47.06
42.91 39.12 38.96 36.27 34.77 32.47 28.41
2.11 1.19 0.77 0.43 0.15 0.02 0
0.01 0.05 0.04 0.04 0.04 0.02 0.02
On a laboratory-scale testing platform of impinging entrainedflow gasifier with two opposed burners, the detailed measurements of gas concentration distribution have been performed
for carbonaceous compound (diesel oil) at atmospheric pressure with stainless steel water-cooled probes. The dearth of these experimental data is to a great extent a direct result of the physical difficulties involved in its retrieval, we can say that the magnitude of the task of obtaining repeatable and consistent data exceeded all our estimates, with numerous irritating problems being encountered in what we had in advance supposed to be a fairly conventional experimental undertaking. This work has attempted to overcome such difficulties and presents some detailed experimental data for diesel oil gasification. We have a high degree of confidence in our measurements, however, it should be noted that measured concentrations are falsified by ongoing reactions in the sampling probe, the cooling of the test gas is not rapid enough to freeze the samples of the reaction zone. Although the
Fig. 15 – The effect of O/C ratio on syngas composition at position C.
Fig. 16 – The effect of O/C ratio on syngas composition at position D.
4.
Conclusions
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Table 5 – The composition by dry vol.% at position E O/C kg/kg
CO2
CO
H2
CH4
O2
1.48 1.63 1.76 1.85 2.01 2.12 2.36
9.79 10.74 11.50 16.02 15.01 19.10 24.48
43.89 49.14 48.89 47.96 49.72 48.01 46.46
43.89 38.81 38.41 35.55 34.92 32.62 28.83
2.30 1.15 0.94 0.31 0.15 0.02 0
0.05 0.04 0.05 0.05 0.06 0.05 0.06
experimental technique used here results in certain errors due to the probe sampling and the analyzing procedures, the special resolution of the measurements is sufficient to obtain radial profiles of the major gas species. The main conclusions obtained are as follows, (1) As a whole, for a fixed O/C ratio, each component had some fluctuations near the burner zone. From the wall to radial 6 cm position, each component had a small fluctuation. From radial 6 cm to 15 cm position, CO and CO2 concentrations increased, both of them had the maximum value at gasifier centerline. H2 and argon concentrations decreased, both had the minimum at gasifier centerline. The nearer the gasifier centerline, the higher the CH4 concentration, but CH4 concentration
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Table 6 – Carbon conversion efficiency and cold gas efficiency O/C kg/kg
1.48 1.63 1.76 1.85 2.01 2.12 2.36
Carbon conversion 93.13 95.82 96.94 97.51 98.17 98.50 99.02 efficiency Cold gas efficiency 77.66 79.72 80.12 77.78 79.10 76.95 74.60
had the least value at gasifier centerline. With the increase of O/C ratio, CH4 concentration decreases. At different radial positions, O2 was rapidly depleted near zero, which showed that the gasification reaction occurred quickly at the higher temperature. (2) With the O/C ratios increase, CO 2 concentration increases, CO concentration remains basically a constant, H2 concentration decreases, and O2 concentration approaches to zero. Optimal O/C ratio range is 1.70–2.00. (3) Studies give insight into the combustion process and provide evaluation data for predictive mathematical models that can be used to assist in the design and scale-up of industrial gasifiers. Coal–water slurries have been regarded as a potential substitute for heavy fuel oil, various demonstrations of coal–water slurry combustion have been performed. In future work we intend to include coal–water slurries feed-in and several other effects which are neglected in the present work.
Acknowledgements We are grateful to Prof. Jie Wang (School of Resource and Environmental Engineering, ECUST), who read the manuscript and gave many valuable suggestions for improvement. This work was partially supported by the National Basic Research Program of China (No. 2004CB217703), Shanghai Shuguang Training Program for the Talents (06SG34), the Program for New Century Excellent Talents in University (NCET-05-o413), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT0620).
REFERENCES
Fig. 17 – The effect of O/C ratio on syngas composition at position E.
[1] A. Tamir, Impinging-stream Reactors: Fundamentals and Applications, Elsevier, Amsterdam, 1994. [2] A. Tamir, I. Elperin, K. Luzzatto, Drying in a new two impinging streams reactor, Chemical Engineering Science 39 (1984) 139–146. [3] N.S. Nosseir, S. Behar, Characteristics of jet impingement in a side-dump combustor, AIAA Journal 24 (1986) 1752–1757. [4] Y. Berman, A. Tamir, Experimental investigation of phosphate dust collection in impinging streams (IS), Canadian Journal of Chemical Engineering 74 (1996) 817–821. [5] Y. Berman, A. Tanklevsky, Y. Oren, A. Tamir, Modeling and experimental studies of SO2 absorption in coaxial cylinders with impinging streams: part I, Chemical Engineering Science 55 (2000) 1009–1021. [6] Y. Berman, A. Tanklevsky, Y. Oren, A. Tamir, Modeling and experimental studies of SO2 absorption in coaxial cylinders with impinging streams: part II, Chemical Engineering Science 55 (2000) 1023–1028.
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[7] A.M. Dehkordi, Application of a novel-opposed-jets contacting device in liquid–liquid extraction, Chemical Engineering and Processing 41 (2002) 251–258. [8] L.W. Kostiuk, P.A. Libby, Comparison between theory and experiments for turbulence in opposed streams, Physics of Fluids. A, Fluid Dynamics 5 (1993) 2301–2303. [9] M. Champion, P.A. Libby, Reynolds stress description of opposed and impinging turbulent jets. Part I: closely spaced opposed jets, Physics of Fluids. A, Fluid Dynamics 5 (1993) 203–216. [10] C.Y. Wen, T.Z. Chaung, Entrainment coal gasification modeling, Industrial and Engineering Chemistry, Process Design and Development 18 (1979) 684–694. [11] R. Govind, J. Shah, Modeling and simulation of an entrained flow coal gasifier, AIChE Journal 30 (1984) 79–91. [12] Q.Z. Ni, A. Williams, A simulation study on the performance of an entrained-flow coal gasifier, Fuel 74 (1995) 102–110. [13] G.S. Liu, H.R. Rezaei, J.A. Lucas, D.J. Harris, T.F. Wall, Modeling of a pressurized entrained flow coal gasifier: the effect of reaction kinetics and char structure, Fuel 79 (2000) 1767–1779. [14] C.X. Chen, M. Horio, T. Kojima, Numerical simulation of entrained flow coal gasifiers. Part I: modeling of coal gasification in an entrained flow gasifier, Chemical Engineering Science 55 (2000) 3861–3874.
[15] C.X. Chen, M. Horio, T. Kojima, Numerical simulation of entrained flow coal gasifiers. Part II: effects of operating conditions on gasifier performance, Chemical Engineering Science 55 (2000) 3875–3883. [16] C.X. Chen, M. Horio, T. Kojima, Use of numerical modeling in the design and scale-up of entrained flow coal gasifiers, Fuel 80 (2001) 1513–1523. [17] Y.C. Choi, X.Y. Li, T.J. Park, J.H. Kim, J.G. Lee, Numerical study on the coal gasification characteristics in an entrained flow coal gasifier, Fuel 80 (2001) 2193–2201. [18] H. Watanabe, M. Otaka, Numerical simulation of coal gasification in entrained flow coal gasifier, Fuel 85 (2006) 1935–1943. [19] A.H. Lefebvre, Atomization and Sprays, Hemisphere Publishing Corporation, New York, 1989. [20] L.D. Smoot, P.J. Smith, Coal Combustion and Gasification, Plenum Press, New York, 1985. [21] F.C. Wang, Investigation of Gasification Process for Jet-entrained Gasifier, East China University of Science and Technology, Shanghai, China, 1995. [22] X.X. Sun, J.Y. Chen, Physical and Chemistry Basis for Pulverized Coal Combustion, Huazhong University of Science and Technology Press, Wuhan China, 1991.
a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m
w w w. e l s e v i e r. c o m / l o c a t e / f u p r o c
Experimental measurement of gas concentration distribution in an impinging entrained-flow gasifier Miaoren Niu, Zhuoyong Yan, Qinghua Guo, Qinfeng Liang, Guangsuo Yu, Fuchen Wang⁎, Zunhong Yu Institute of Clean Coal Technology, East China University of Science and Technology, Mei Long Road No. 130, Shanghai, 200237, PR China Key Laboratory of Coal Gasification of Ministry of Education, East China University of Science and Technology, Shanghai 200237, China
AR TIC LE I N FO
ABS TR ACT
Article history:
On a laboratory-scale testing platform of impinging entrained-flow gasifier with two
Received 29 May 2007
opposed burners, the detailed measurements of gas concentration distribution have been
Received in revised form 8 April 2008
performed for carbonaceous compound (diesel oil) at atmospheric pressure. Under the
Accepted 22 April 2008
condition of 1.48–2.36 O/C ratios (kg/kg), radial gas samples are collected at three axial positions and the syngas exit position with stainless steel water-cooled probes, the
Keywords:
concentration distribution of the major gases (H2, CO, CO2, CH4 and O2) under stable
Gasification
operating state was determined with a mass spectrometry. These data are used to clarify
Entrained-flow gasifier
mixing and reaction characteristics within the reactor, to give insight into the combustion
Gas concentration distribution
process and provide a database for evaluating predictive mathematical models. © 2008 Published by Elsevier B.V.
1.
Introduction
Gasification is a very versatile process to convert a variety of carbon-containing feedstocks, such as coal, petroleum coke, lignite, heavy oils, residues and natural gas, into syngas. The entrained-flow gasification technology has been extensively applied to the production of ammonia, methanol, acetic acid, other chemicals and power generation through Integrated Gasification Combined Cycle (IGCC). The gasification process of an entrained-flow gasifier is very complicated, because it relates to the fluid flow under the condition of high temperature, high pressure and heterogeneous state. Impinging stream flow configurations are characterized by streams of fluid jets impinging against each other in a confined vessel, which have proved useful in conducting a wide array of chemical engineering unit operations and enhancing heat and mass transfer between phases due to the high transfer coefficients [1]. The opposing jet technique has been applied in many fields and extensively studied practically [2–7] and theoretically [8,9].
It is necessary to understand the gas concentration distribution inside the gasifier in detail for gasifier's optimum design and characteristic evaluation. Numerical simulation is an effective technique for predicting the gasification characteristics, there are several mathematical models developed specially for entrained-flow gasifiers in the literature. Wen and Chaung [10], and Govind and Shah [11] developed models for Texaco downflow, slurry-fed entrained gasifiers. Ni and Williams [12] developed a multivariable model for Shell coal gasifiers on the basis of equilibrium, mass and energy balances. Liu et al. [13] presented a model for a pressured entrained-flow coal gasifier to determine the effect of pressure, reaction kinetics and char structure on the gasification reactions. Chen et al. [14,15] developed a comprehensive three-dimensional simulation model for entrained-flow coal gasifiers, in their model, the numerical methods and the submodels were used to a two-stage air blown entrained-flow gasifier. Chen et al. [16] used a Multi Solid Progress Variables (MSPV) method to simulate the gasification reaction and reactant mixing process for a entrained-flow coal gasifier. Choi et al [17] predicted a slurry feed type, entrained-flow coal gasifier.
⁎ Corresponding author. Presently at Key Laboratory of Coal Gasification of Ministry of Education, East China University of Science and Technology, Shanghai 200237, China. Tel.: +86 21 64252521; fax: +86 21 64251312. E-mail address: [email protected] (F. Wang). 0378-3820/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.fuproc.2008.04.009
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Fig. 1 – Schematic diagram of the experimental apparatus and top view of gasifier (all dimensions in mm). (1) Liquid tank, (2) O2 steel cylinder, (3) Argon steel cylinder, (4) pump, (5) gas mass flow meter, (6) burner, (7) vortex flowmeter, (8) slag discharge, (9) gas pretreatment system, (10) mass spectrometry, (11) computer, (12) flame monitoring system.
Watanabe and Otaka [18] modeled the gasification reaction in the entrained-flow coal gasifier introducing the coal reactivity and predicted the change in basic gasification characteristics under different operating conditions. However, such models require thorough validation for each application, the ability to determine the precision and accuracy of a particular modeling approach is limited, because the detailed experimental information that can be used for data model comparisons is nearly nonexistent, full-scale plants are not entirely suited to the collection of the data, being very expensive to operate and also very difficult to access. These
drawbacks reinforce the importance of gasifier as a valuable means of obtaining data. The major objective of present work is to obtain detailed local measurements of the major syngas species (H2, CO, CO2, CH4 and O2) concentration, to clarify mixing and reaction characteristics and provide further insight into mechanisms which govern the gasification process.
Fig. 2 – General view of the burner.
Fig. 3 – The typical two-burner impinging flame image.
2.
Experimental equipment and procedure
The schematic drawing of the experimental apparatus and top view of gasifier were shown in Fig. 1. The gasifier was
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Fig. 4 – The stainless steel, water-cooled probe (all dimensions in mm).
cylindrical, vertically oriented, the inner diameter and length of the combustion chamber composed of a 15 mm thick cast refractory shell, were 300 and 2250 mm, respectively. The cast refractory shell, wrapped with a 235 mm thick, low thermal conductivity fiber blanket to reduce the heat transfer, was protected by a stainless-steel column shell of 0.8 m in diameter and 2.5 m in height. Ports were located at sides of the gasifier for viewing, temperature measurement and insertion of the water-cooled probes. Opposed turbulent flow fields were obtained by two opposed round burners composed of inner and outer channels. The O2 was fed into the burner outer channel by steel cylinder, with a pressure-reducing valve to avoid pressure oscillations in order to achieve steady flow. The gas flow rates were measured by mass flow meters (D07-9C/ZM, Beijing Sevenstar Huachuang Electronic Co., Ltd). The diesel oil was fed into the burner inner channel by a gear pump (A-73004-00#, America Cole-Parmer Company), the flow rate was determined gravimetrically with an elapsed timer and an electronic weight scale. In the gasification process, two burners were used to produce opposite jets of fuel that impinge on the center of the combustion chamber. The size of the burners is shown in Fig. 2. High relative velocities between the particulate matter and the gaseous phase in the central area provided good conditions for active diffusion and convection at the particle surface, and the high temperature together resulted in fast burning and gasification reaction under highly reducing conditions to produce raw syngas. High-temperature gaskets interfaced the furnace segments and eliminated all leakage. In addition, the pressure inside the furnace was maintained near the atmospheric or slightly above, to ensure that measurements would not suffer bias due
to contamination from the in-leakage of room air. From the reaction chamber, the raw syngas flowed into the quench chamber, where the raw syngas was cooled and partially scrubbed by the water, then the syngas was discharged. A flame image detector is fixed on the top refractory wall of the gasifier, and the whole flames are visible during the experimental process. The flame monitoring system consists of a lens, an optical probe, a CCD camera (Panasonic WV-CP470), and a microcomputer. The CCD camera and its accessories are cooled by water to avoid overheating. Argon was fed into the gasifier from the top of the gasifier to protect the flame monitoring lens. A typical flame image is illustrated in Fig. 3. The gas analysis was performed by a mass spectrometry. The gas sampling and analyzing systems were composed of two parts, (a) The water-cooled probe. In order to take gas samples continuously, stainless steel, water-cooled probes were designed to cool the extracted gas mixture and to prevent the oxidation of carbon monoxide, shown in Fig. 4. The probe temperature was controlled by the cooling water flow rate and was adjusted to avoid condensation of water at the probe surface. The inlet cooling water temperature is about 25 °C, the outlet cooling water temperature is about 26 °C~30 °C. Each analyzing apparatus required a certain flow rate. Therefore, it was necessary to provide enough test gas, but on the other hand, in order not to disturb the flow field, the suction of the probe was adjusted to the lowest flow velocity where measurements were taken. We got samples at three axial positions B, C, D and syngas exit position E, see Fig. 1, at each axial position, probe was traversed radially from the wall to reactor centerline (spaced 15 cm apart) for six radial positions (0, 3, 6, 9, 12, and 15 cm) to collect samples. (b) The sample pretreatment system. The gas pretreatment apparatus was shown in Fig. 5. The sample pretreatment system used a small vacuum pump to draw the gas, liquid, and solid samples through the probe, firstly, the sampled gas was filtered where most of the soot carbon was eliminated from the test gas, then gas was dried and fine filtered, and passed through a condenser to remove most of the water vapor and next passed through a desiccator again to remove the residual water,
Fig. 5 – The gas pretreatment system.
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Table 1 – The physical properties of diesel oil Density kg m− 3 Viscosity Pa s Heat value MJ kg− 1 C element wt.% H element wt.% N element wt.% Inlet temperature °C 5 × 10− 3
840
44.2
86.18
a condition required by the gas analyzer, and then passed through a flowmeter, at last the gas entered into the mass spectrometry Model HPR20QIC, where the major gases (H2, CO, CO2, CH4 and O2) were analyzed.
13.72
3.
Results and discussion
Due to the high thermal capacity of refractory walls, from the ignition starting point, about 4 h are needed to achieve steady state as monitored by steady readings of wall temperatures. Among the important operating parameters, oxygen feeding amount is the most important. Flame stability is strongly affected by the atomized droplet size, droplet velocity and spray distribution which are controlled by the O/C ratio. In order to determine how to affect the overall gasifier performance with the change of oxygen feed, O/C ratios (kg/kg) are changed from 1.48 to 2.36 while diesel oil amount is maintained at about 2.0 kg/h. A summary of experimental conditions is shown in Table 2. The reactor is operated at quasisteady conditions in which the wall temperatures are stable during the sampling period. In order to help understand the gasification characteristics, the wall temperature distribution along the centerline was shown in Fig. 6. The O–O′ plane, shown in Fig. 1, was set to reference plane, downward direction is the positive. We can see that, the maximum temperature appears in the burner plane,
25
the farther away from the burner, the lower the temperature, the gasifier heat load from the impact area to top and bottom transfer.
3.1. All components, lines, and fittings of the sampling system were made of Teflon to diminish catalytic influences. Time interval that analyzes a set of data was 16–20 s. To obtain sufficient quantities of gas sample for analysis, the sampling time at each position was maintained at 5–10 min, so the time was sufficient to average out the random fluctuations, each analyzing apparatus required a certain flow rate, therefore, it was necessary to provide enough test gas. The combustor wall temperature was measured by a platinum–rhodium thermocouple installed to the inside wall surface at the axial position A. The instrument errors associated with the concentration and temperature measurements were generally less than 1% of the measured value. The physical properties of diesel oil were shown in Table 1. The inlet temperature for both oxygen and argon is 20 °C.
0.10
The syngas concentration radial distribution at position B
Under different O/C ratios, the syngas concentration radial distributions (all on the dry basis) at axial position B are shown in Figs. 7–10. Where, ‘Position A temp’ represents the wall temperature of position A. Here, we are not trying to describe a specific figure or a specific O/C ratio condition, our main objective is to provide the detailed experimental data about syngas concentration radial distribution at position B, at the same time, we tried to give some explanation about the effect of O/C ratios. It can be seen that, with the O/C ratios increase, CO2 concentration increases, CO concentration remains basically a constant when O/C ratio larger than 1.48, H2 and CH4 concentrations decrease, and O2 concentration approaches to zero. And for each fixed O/C ratio, (1) The H2 concentration gradually decreased from the wall to the centerline, while CO had the opposite trend. (2) The CO2 concentration firstly decreased and then increased. At different radial positions, O2 was rapidly depleted near zero, which showed that the gasification reaction occurred quickly at the higher temperature. For the entrained-flow type gasification system, reaction time is normally less than 4–5 s that requires fast monitoring of reaction. (3) The nearer gasifier centerline, the higher CH4 concentration, but CH4 concentration had the least value at gasifier centerline. (4) Argon concentration had the maximum value at radial 6 cm position, then decreased slowly. Because argon was fed into the gasifier from the top of the gasifier and to protect the flame monitoring lens. In the gasification
Table 2 – Experimental conditions Condition
1 2 3 4 5 6 7
1# 2# 1# O2 diesel diesel Nm3/h kg/h kg/h 1.95 1.95 2.01 2.08 2.00 2.00 2.00
2.04 2.04 1.98 2.04 1.98 2.00 2.00
1.75 1.89 2.04 2.21 2.32 2.49 2.79
2# O2 Nm3/h 1.81 2.04 2.19 2.39 2.50 2.62 2.91
Ar O/C Nm3/h kg/kg 0.82 0.72 0.67 0.12 – 0.72 0.81
1.48 1.63 1.76 1.85 2.01 2.12 2.36
Fig. 6 – The wall temperature distribution along the centerline (O/C = 1.76).
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Fig. 9 – O/C = 2.01 (Position A temp 1360 °C).
Fig. 7 – O/C = 1.48 (Position A temp 1290 °C). process two burners were used to produce opposite jets of fuel that impinged on the center of the combustion chamber and forming an impinging plane, so argon was difficult to enter into gasifier center. Diesel oil gasification occurs two different types of chemical reaction: exothermic primary reaction (combustion) and endothermic secondary reaction. Which types of reaction actually occur in an area in the gasifier depend on flow characteristics and mixing process in the area. At position B, the main chemical reaction is combustion of pyrolysis components with oxygen, accompanied with combustion of CO and H2 from reflux region. Lefebvre [19] and Smoot et al. [20] reported that velocity of combustion reaction is very fast with time scale between 2 and 4 ms that is far less than material mixing time scale (~0.1 s reported by Wang [21]). All the important reactions occurring in primary reactions region are: Pyrolysis þ O2 →CO2 þ H2 O
ð1Þ
2CO þ O2 ¼ 2CO2
ð2Þ
2H2 þ O2 ¼ 2H2 O
ð3Þ
CH4 þ 2O2 ¼ CO2 þ 2H2 O
ð4Þ
which has a very high temperature (N1000 °C), it takes place chemical decomposition quickly, and produces a variety of species, including C, CH4, H2, and other pyrolysis products. These species quickly react with oxygen to produce the hot syngas. Secondly, In this reactor, two equal suspension streams flow against one another at high velocity and impinge at their midpoint, resulting in a highly turbulence zone at position B, which can enhance mix quickly.
3.2.
The syngas concentration radial distribution at position C
Under different O/C ratios, the radial distributions of syngas concentration (all on the dry basis) at axial position C are shown in Figs. 11–14. Where, ‘Position A temp’ represents the wall temperature of position A. It is seen that, the concentration and radial distributions of each component are shown nearly as a horizontal line, which means that there are equal values at different radial positions. Radial profiles of gas concentration rapidly become uniform, which essentially indicates that the completeness of mixing composition is within 20 cm downstream the burner plane, only a slight overall change in gas composition is observed beyond that. The average composition of syngas is shown in Table 3.
We think that CO, H2 and CH4 appear in this area for two reasons: Firstly, when diesel oil was fed into the gasifier,
Fig. 8 – O/C = 1.76 (Position A temp 1308 °C).
Fig. 10 – O/C = 2.36 (Position A temp 1380 °C).
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Fig. 11 – O/C = 1.48 (Position A temp 1282 °C).
Fig. 13 – O/C = 2.01 (Position A temp 1360 °C).
Fig. 15 shows the effect of O/C ratio on syngas composition at position C, it can be seen that, with the O/C ratios increase, CO2 concentration increases, CO concentration remains basically a constant, H2 and CH4 concentrations decrease, and O2 concentration approaches to zero. In these reactions region, products from primary reactions region are reacted further as follows:
about 10 s calculated by Sun et al. [22]. It is a common knowledge that reaction (6) is faster than reaction (5). Reactions from (7) to (9) are homogenous reactions with higher velocities than reactions (5) and (6) at high temperature. Time scale of reaction between residue carbon with steam or carbon dioxide is higher than that of micro mixing process. Hence, chemical reaction is the controlling step of residue carbon gasification process.
C þ CO2 ¼ 2CO
ð5Þ
C þ H2 O ¼ CO þ H2
ð6Þ
CH4 þ H2 O ¼ CO þ 3H2
ð7Þ
CH4 þ CO2 ¼ 2CO þ 2H2
ð8Þ
CO2 þ H2 ¼ CO þ H2 O
ð9Þ
3.3.
When the temperature lies between 900 and 1500 °C, the rate of char–H2 reactions is one to two orders of magnitude slower than the rates of char reactions with CO2 or H2O, which is similar over that temperature range. Generally, a reciprocal of reaction velocity constant 1/k is used to characterize reaction time scale, reaction time scale of reaction (5) is
Fig. 12 – O/C = 1.76 (Position A temp 1290 °C).
The syngas concentration radial distribution at position D
The average radial composition of syngas at position D is shown in Table 4. Fig. 16 shows the effect of O/C ratio on syngas composition at position D, it can be seen that, with the O/C ratios increase, CO2 concentration increases, CO concentration remains basically a constant, H2 and CH4 concentrations decrease, and O2 concentration approaches to zero.
3.4.
The syngas concentration at position E
The average radial composition of syngas at position E is shown in Table 5. Fig. 17 shows the effect of O/C ratio on syngas composition at position E, it can be seen that, with the O/C ratios increase, CO 2 concentration increases, CO
Fig. 14 – O/C = 2.36 (Position A temp 1390 °C).
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Table 3 – The average radial composition by dry vol.% at position C O/C kg/kg
CO2
CO
H2
CH4
O2
1.48 1.63 1.76 1.85 2.01 2.12 2.36
7.60 10.38 10.24 14.15 13.87 15.78 21.76
45.77 49.46 49.52 48.35 51.67 51.50 51.47
44.42 38.75 38.57 36.84 34.15 32.52 26.62
2.06 1.23 1.51 0.49 0.16 0.03 0
0.03 0.03 0.03 0.04 0.04 0.02 0.02
concentration remains basically a constant, H2 and CH4 concentrations decrease, and O2 concentration approaches to zero. There are a number of different criteria that are frequently quoted for gasification process. The two most commonly encountered are carbon conversion efficiency and cold gas efficiency, which have been indicated for all cases in Table 6. We can see that the optimal O/C ratio range is 1.70–2.00.
Table 4 – The average radial composition by dry vol.% at position D O/C kg/kg
CO2
CO
H2
CH4
O2
1.48 1.63 1.76 1.85 2.01 2.12 2.36
8.05 11.65 11.30 15.77 14.73 18.41 24.35
46.79 47.80 48.77 47.32 50.18 48.92 47.06
42.91 39.12 38.96 36.27 34.77 32.47 28.41
2.11 1.19 0.77 0.43 0.15 0.02 0
0.01 0.05 0.04 0.04 0.04 0.02 0.02
On a laboratory-scale testing platform of impinging entrainedflow gasifier with two opposed burners, the detailed measurements of gas concentration distribution have been performed
for carbonaceous compound (diesel oil) at atmospheric pressure with stainless steel water-cooled probes. The dearth of these experimental data is to a great extent a direct result of the physical difficulties involved in its retrieval, we can say that the magnitude of the task of obtaining repeatable and consistent data exceeded all our estimates, with numerous irritating problems being encountered in what we had in advance supposed to be a fairly conventional experimental undertaking. This work has attempted to overcome such difficulties and presents some detailed experimental data for diesel oil gasification. We have a high degree of confidence in our measurements, however, it should be noted that measured concentrations are falsified by ongoing reactions in the sampling probe, the cooling of the test gas is not rapid enough to freeze the samples of the reaction zone. Although the
Fig. 15 – The effect of O/C ratio on syngas composition at position C.
Fig. 16 – The effect of O/C ratio on syngas composition at position D.
4.
Conclusions
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Table 5 – The composition by dry vol.% at position E O/C kg/kg
CO2
CO
H2
CH4
O2
1.48 1.63 1.76 1.85 2.01 2.12 2.36
9.79 10.74 11.50 16.02 15.01 19.10 24.48
43.89 49.14 48.89 47.96 49.72 48.01 46.46
43.89 38.81 38.41 35.55 34.92 32.62 28.83
2.30 1.15 0.94 0.31 0.15 0.02 0
0.05 0.04 0.05 0.05 0.06 0.05 0.06
experimental technique used here results in certain errors due to the probe sampling and the analyzing procedures, the special resolution of the measurements is sufficient to obtain radial profiles of the major gas species. The main conclusions obtained are as follows, (1) As a whole, for a fixed O/C ratio, each component had some fluctuations near the burner zone. From the wall to radial 6 cm position, each component had a small fluctuation. From radial 6 cm to 15 cm position, CO and CO2 concentrations increased, both of them had the maximum value at gasifier centerline. H2 and argon concentrations decreased, both had the minimum at gasifier centerline. The nearer the gasifier centerline, the higher the CH4 concentration, but CH4 concentration
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Table 6 – Carbon conversion efficiency and cold gas efficiency O/C kg/kg
1.48 1.63 1.76 1.85 2.01 2.12 2.36
Carbon conversion 93.13 95.82 96.94 97.51 98.17 98.50 99.02 efficiency Cold gas efficiency 77.66 79.72 80.12 77.78 79.10 76.95 74.60
had the least value at gasifier centerline. With the increase of O/C ratio, CH4 concentration decreases. At different radial positions, O2 was rapidly depleted near zero, which showed that the gasification reaction occurred quickly at the higher temperature. (2) With the O/C ratios increase, CO 2 concentration increases, CO concentration remains basically a constant, H2 concentration decreases, and O2 concentration approaches to zero. Optimal O/C ratio range is 1.70–2.00. (3) Studies give insight into the combustion process and provide evaluation data for predictive mathematical models that can be used to assist in the design and scale-up of industrial gasifiers. Coal–water slurries have been regarded as a potential substitute for heavy fuel oil, various demonstrations of coal–water slurry combustion have been performed. In future work we intend to include coal–water slurries feed-in and several other effects which are neglected in the present work.
Acknowledgements We are grateful to Prof. Jie Wang (School of Resource and Environmental Engineering, ECUST), who read the manuscript and gave many valuable suggestions for improvement. This work was partially supported by the National Basic Research Program of China (No. 2004CB217703), Shanghai Shuguang Training Program for the Talents (06SG34), the Program for New Century Excellent Talents in University (NCET-05-o413), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT0620).
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Fig. 17 – The effect of O/C ratio on syngas composition at position E.
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