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Destruction of caking properties of bituminous coal by jetting pre-oxidation in a fluidized bed
Fuel 133 (2014) 45–51
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Destruction of caking properties of bituminous coal by jetting pre-oxidation in a fluidized bed Zhigang Zhao a,b, Juwei Zhang a,⇑, Feixiang Zhao a, Xi Zeng a, Xiaoxing Liu a, Guangwen Xu a,⇑ a b
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China School of Chemistry and Chemical Engineering, Anhui Key Lab of Coal Clean Conversion and Utilization, Anhui University of Technology, Ma’anshan 243002, China
h i g h l i g h t s Destruction of caking propensity of coal by jetting pre-oxidation was tested in a laboratory fluidized bed. Caking destruction was well achieved for a kind of coal with a caking index of 20. Oxidation temperature and oxygen content in jetting gas are essential to the caking destruction. Variations in bed temperature and product gas composition are indicative of caking condition in the bed.
a r t i c l e
i n f o
Article history: Received 25 February 2014 Received in revised form 8 April 2014 Accepted 6 May 2014 Available online 17 May 2014 Keywords: Caking propensity Caking destruction Fluidized bed gasification Coal Jetting pre-oxidation
a b s t r a c t Jetting pre-oxidation was proposed to destroy the caking property of bituminous coal in a fluidized bed (FB) gasifier. By defining an index x to characterize the degree or realized effect of caking destruction, experiments were conducted in an electrically heated laboratory FB reactor under different conditions to investigate the feasibility and required conditions for effectively destroying the caking propensity of a kind of bituminous coal. The tested major parameters were the equivalence air ratio (ER) of jetting gas and the temperature in the jetting zone. At relatively lower temperatures it was hard to completely destroy the caking propensity of the tested coal particles. Without oxygen inside the jetting gas, it was also impossible to fully destroy the caking propensity of the coal even at sufficiently high temperatures. Thus, oxidation of coal particles at suitably high temperatures was essential to the expected good destruction of their caking propensity. For the tested bituminous coal this was achieved at temperatures above 1000 °C in the jetting zone and an equivalent oxygen ratio above 0.1 for the jetting gas (up to 0.3 considering gasification applications). For coal pre-oxidation in a FB, the agglomeration state of char particles due to caking adherence in the reactor could be anticipated via the unsteady variations in the bed temperature and H2/CO concentrations in the effluent gas with reaction time. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction A large part of bituminous coal has certain caking propensity to make the coal particles stick together via colloidal matters formed when heating the coal through its plastic variation stage. Thus, this kind of coal is hard to be treated by the existing commercial fixed bed [1,2] or fluidized bed (FB) [3] gasification technologies. In practice, however, there is a need to gasify the coal with certain caking propensity and even high ash content to produce fuel gas, whereby realizing the value-added utilization of such low-quality or low-value coals. An example is for the coal middlings generated ⇑ Corresponding authors. Tel./fax: +86 10 82544886. E-mail addresses: [email protected] (J. Zhang), [email protected]. cn (G. Xu). http://dx.doi.org/10.1016/j.fuel.2014.05.006 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.
in coking industry, which has ash contents of about 40 wt.%, heating values of 3000–4000 kcal/kg and certainly high caking propensity. Gasifying the middlings to generate the fuel gas required for heating coking oven allows the high-quality coking oven gas (COG) to be replaced and further used for other value-added utilizations such as the production of substitute nature gas (SNG), methanol and hydrogen [4]. In some local areas, there are possibly only coals with caking propensity, and it has also the need to gasify the caking coal to produce not only fuel gas but also syngas for chemical synthesis. Of course, the entrained flow gasifiers can treat caking coal but it requires the feedstock to have low ash contents. Hence, the gasification of high-ash middlings has to rely on FB gasification through incorporating it with a technical means destroying the coal caking propensity.
Fig. 1. Principle diagram of jetting pre-oxidation fluidized bed gasification.
1100
6
1000
5
900
4
800
3
700
2
600
1
3
Many methods have been proposed to destroy the caking property of coals, such as pre-oxidation [5], mixing with coke or non-caking coal [6], mechanical stirring, adding additives [7,8]. However, most of these approaches require complex operations or have limited effects. The coal pre-oxidation has been considered to be effective and simple for eliminating caking propensity of bituminous coal. Gasior et al. [9] developed a pre-oxidation process for caking coal in a fixed bed. The treated coal was heated to its softening temperature and held there for 1–3 h in an inert gas containing 1 vol.% oxygen, after that it passed through the plastic variation stage of the coal in 1 h. The char obtained had 50% of the original volatile in the coal and could be gasified in a fixed bed without agglomeration. Forney et al. [10] found that the caking properties of coal could be eliminated by treating the coal in a FB at 400–425 °C for at least 5 min inside an inert gas or steam atmosphere containing at least 0.2 vol.% oxygen. These pre-oxidation methods for destroying caking propensity of coal, however, were in an isolated reactor so that the coal gasification process is complicated, while this would also lose the combustible matters emitted during caking destruction. Consequently, these pre-oxidation methods are hardly practicable for actual industrial processes. We have devised a novel jetting pre-oxidation fluidized bed gasification (JPFBG) technology for the utilization of caking coal to produce industrial fuel gas [11]. Fig. 1 shows the principle diagram of the JPFBG. The caking coal particles are jetted into the freeboard of the gasifier just above the bottom dense bed with a gas stream containing oxygen (e.g., air). The coal particles are dispersed and pre-oxidized in this gas jet. The formed char particles, which are possibly without caking propensity after the pre-oxidation, fall into the dense bed and are gasified by the gasification agent fed into the dense fluidized bed. Via this way, the char gasification can be also facilitated because of the removal of the inhibition effect of pyrolysis gas on char gasification as its separating the coal pyrolysis from char gasification through adopting the jetting pre-oxidation of coal [12]. In fact, we have built a 1000 t/a pilot JPFBG plant and a coal with the caking index of about 20 was successfully gasified (i.e., without particle agglomeration) in this plant using air as the gasification agent [11]. The typical operating results are shown in Fig. 2. The steady temperature of the dense bed as well as high heating value (HHV) of the produced gas indicates obviously the steady operation of JPFBG and the good destruction of the coal’s caking property. However, we have found in the pilot study that it was vital and difficult to determine the pre-oxidation conditions for caking destruction of the tested coal especially the oxygen content in the jetting gas. Less ER of the jetting gas may lead to insufficient effect on caking destruction, while excess oxygen may decrease the heating value of the product gas [11]. In order
Gas HHV (MJ/mN)
Z. Zhao et al. / Fuel 133 (2014) 45–51
Temperature ( )
46
500
0
50
100
150
200
250
300
350
0
Operating time (min) Fig. 2. Time-series gas temperature in dense fluidized bed and high heating value (HHV) of the produced gas obtained in the JPFBG pilot test [11].
to figure out the critical factors ensuring the caking destruction of the coal, a laboratory FB reactor was adopted in this study to simulate the jetting pre-oxidation conditions in JPFBG and to investigate the effect of major operating parameters on the realized effect for destroying the caking propensity of caking coal.
2. Experimental A kind of bituminous coal with diameters of 0.5–1.0 mm and a particle density of 1300 kg/m3 was used for the bench-scale test. The characteristics of the coal are shown in Table 1. The coal has a caking index (GR.I.) of 20 and cannot be gasified in the conventional fluidized bed reactor because of its caking propensity. Fig. 3 presents a schematic diagram of the experimental system and its main body of a bench-scale FB reactor. The reactor was made of stainless steel and included a reaction section of 600 mm in height, 50 mm in inner diameter (i.d.), and an upper enlarged section of 120 mm in height, 80 mm in i.d. The coal particles were jetted into the reactor using an O2-containing gas (gas jet) via a vertical feeding tube of 6 mm in i.d. mounted at the central axis of the porous gas distributor of the FB. Inert gas nitrogen was fed into the bed through the gas distributor as the fluidizing gas. An overflow outlet of particles was made on the wall of the fluidized bed reactor at a position 300 mm above the gas distributor. The overflowed char particles from the reactor were collected into a bottle through a pipe connected to the overflow outlet. The effluent gas from the top of the fluidized bed reactor was, in succession, cooled, purified and finally analyzed with a three-channel micro-gas chromatograph (GC, Agilent 3000A). Before each experiment, a certain amount of aluminum oxide pellets of 5.0 mm in diameter and quartz sand with a Sauter mean diameter of 0.156 mm were loaded into the reactor above the gas distributor but the formed particle bed height was limited to the outlet of the central gas jet pipe. Then, the reactor was heated to its designated temperature by an electrical furnace. After the temperature was steady, both the jetting gas for feeding coal particles and the fluidizing gas through the gas distributor were introduced into the reactor for several minutes to stabilize the fluidization state of sand particles. In turn, coal particles were fed into the reactor for 20 min by letting the screw feeder work and supply coal particles into the jetting gas stream. Bed temperatures at different positions of the reactor were recorded, while the product gas from the reactor was collected for analysis by GC. After finishing the coal feed, the heating of the reactor was stopped and the reactor was cooled to room temperature in N2 (fluidizing gas). Finally, size sieving was performed for the particles from the reactor and the
47
Z. Zhao et al. / Fuel 133 (2014) 45–51 Table 1 Analysis of the tested bituminous coal. Proximate analysis (wt.%, ad) Ultimate analysis (wt.%, ad) HHV (MJ/Kg) GR.I. M
A
V
FC
C
2.08
20.31
25.74
51.87
65.22 3.84 6.84 1.15 0.56 26.18
T3 T1
Thermocouple
H
O
N
S 20
T4 T2
Condenser
A good destruction of the caking propensity for the tested coal was identified if the resulting char particles fluently flowed out through the overflow pipe from the reactor. Otherwise, the overflow of the char particles should be unsteady, even stopped, if a serious agglomeration occurred to the char particles in the reactor. It was found that for the char particles collected after each test, the caking property cannot be well characterized with the standard caking index GR.I. because the measured GR.I for all the collected chars were close to zero after the tests, even if serious caking agglomeration actually occurred in the tests. It is generally known that the bituminous coal particles would swell during pyrolysis, and the swelling ratio was usually lower than 2 [13]. If there is no caking occurred, the size of a single char particle should thus be smaller than 2 mm considering all of the tested coals were smaller than 1 mm. Thus, a new caking index x was defined by the following equation to quantify the actually realized effect of caking destruction for the tested coal:
x¼ Coal
GC Pump
N2
O2
Char
Ice-water bath
Fig. 3. A schematic diagram of the adopted experimental fluidized bed apparatus.
particle collecting bottle to separate char from their mixed aluminum oxide pellets and quartz sand. Table 2 summarizes the experimental conditions of the performed tests. The examined major parameters for coal preoxidation were the equivalence ratio (ER) of oxygen of jetting gas and the temperatures in the jetting zone of the reactor. The heating temperature for the FB reactor varied from 750 °C to 950 °C in the pre-oxidation tests 4–7 shown in Table 2. The superficial velocity of the fluidizing gas N2 was several times higher than the minimum fluidization velocity (Umf) of the tested coal particles, while the adopted jetting gas velocity was usually much higher than the terminal velocity (Ut) of the fed coal particles for ensuring a smooth jetting fed of the coal. The feeding rate of coal particles were adjusted from 6.7 g/min to 20 g/min to ensure the same jetting gas velocity in different cases. The variation in ER of the jetting gas was from 0.1 to 0.3 as shown for tests 1–4 in Table 2. The O2 concentrations in the O2-containing jetting gas were all 21 vol.% (air) except for the case 1 of ER = 0.
Table 2 Experimental conditions of the performed tests. Experimental case
1
Setting temperature of furnace Ts (°C) 950 ER of jetting gas for coal feed 0 Oxygen content in jet gas (vol.%) 0 Coal feed rate (g/min) 10 Jetting gas velocitya (m/s) 7.84 Fluidizing gas velocityb (m/s) 0.45 b Umf of coal (m/s) 0.07 Ut of coala (m/s) 2.76 a b
2
3
950 950 0.1 0.3 21 21 20 6.7 (the same
The velocity is at 20 °C based on the jetting tube. The velocity is at 900 °C based on bed diameter.
4 950 0.2 21 10 for all
5
6
750 850 0.2 0.2 21 21 10 10 the tests)
7 900 0.2 21 10
m2 100% mt
ð1Þ
where m2 is the mass of char particles larger than 2 mm in the char from the reactor (for serious caking cases) or from the char collecting bottle (for non-caking cases), and mt represents the total mass of all char particles from the reactor or the char collecting bottle. The lower x means the better caking destruction realized by the jetting pre-oxidation. By using x, this study quantitatively analyzed the realized effects of caking destruction under different conditions. Experiments were usually repeated three times to calculate the averages of the analyzed parameters in this article. Table 3 shows an error analysis for the caking index x tested under the experimental conditions having the setting temperature of the heating furnace (Ts) at 950 °C and using air as the jetting gas according to an equivalence ratio (ER) of 0.2. We can see that the relative error of x was less than 4%, thus verifying the reliability of the testing system and the accuracy of the results reported herein. 3. Results and discussion 3.1. Caking destruction Table 4(a) shows the effect of the ER of the jetting gas on the realized caking destruction for the tested coal (cases 1–4). The tests used air as the oxidizing gas and the setting temperature Ts for the heating furnace was 950 °C. It can be seen that the ER of the feeding gas significantly influenced the effect of caking destruction characterized in terms of the defined parameters x. The caking destruction was obviously improved by increasing ER. When the coal was blown into the reactor using only N2 at ER = 0, x was high as 60%. In this experiment serious agglomeration of particles occurred so that char particles did not smoothly flow out from the reactor. When the ER was higher than 0.1, the values of x were all below 15% and decreased with increasing ER. Under these conditions there was almost no agglomeration of particles occurred in the reactor and the char particles could smoothly flow out from the
Table 3 Demonstration of experimental repeatability via the parameter x (Ts = 950 °C, ER = 0.1). Test no.
mt (g)
m2 (g)
x (%)
Relative error (%)
1 2 3 Average
119.12 125.73 110.56 118.47
6.32 6.91 5.68 6.30
5.31 5.50 5.14 5.32
0.19 3.38 3.38 –
Note: mt – total mass of all char particles from the char collecting bottle, m2 – mass of char particles larger than 2 mm, x – caking index.
Z. Zhao et al. / Fuel 133 (2014) 45–51
(a) Ts = 950 °C
(b) ER = 0.2
Experimental case
ER
1 2 4 3
0 0.1 0.2 0.3
Caking index x (%)
Experimental case
Ts (°C)
x (%)
62.9 15.7 5.8 3.3
5 6 7 4
750 850 900 950
44 25.2 18.3 5.8
Caking index
1100 1000 900 800 700 600 500
1000
1000
900
( )
1100
900
800
800
700 600
, case 1, ω=62.9 %
(a) ER=0, Ts=950
400
1100
1100
1000
1000
900
900
600
(b) ER=0.2, Ts=950
500 400
0
2
4
6
8
10
, case 4, ω=5.8 % 12
14
16
18
22
Operating time (min) Fig. 4. Time-series bed temperature at different central axial locations for the test cases 1 and 4 with and without occurrence of agglomeration (j T1, s T2, N T3, rT4).
, case 6, ω =25.2 %
(c) ER=0.2, Ts=950
, case 4, ω =5.8 %
800 700 600 500
20
(b) ER=0.2, Ts=850
600
400
700
, case 5, ω =44 %
700
500
800
(a) ER=0.2, Ts=750
400
Temperature ( )
)
The temperature in the pre-oxidation zone of the reactor should work critically on the realized effect of caking destruction.
1100
500
Temperature (
3.2. Temperature distribution in reactor
Temperature
Temperature (
)
reactor. These results indicate that high ER of the jetting gas enhanced the pre-oxidation of coal so that good caking destruction was achieved. Nonetheless, the improvement on caking destruction was not evident when further increasing ER from 0.1 to 0.3. This means that at the tested Ts of 950 °C, a small amount of oxygen in the jetting gas was enough to effectively oxidize the coal particles and suppress their caking propensity. Table 4(b) shows the variation of the realized caking destruction with the setting temperature Ts for heating furnace under specified ER of 0.2 and O2 concentration of 21 vol.% in the jetting gas (cases 4–7). The Ts varied in 750–950 °C and the corresponding temperatures T1 to T4 in the jetting zone of the reactor, as will be shown in Fig. 5, was approximately 850–1050 °C. Table 4 clarifies that the caking index x sharply decreased with increasing the setting temperature for heating furnace. The caking propensity of the tested coal was completely destroyed at the furnace setting temperature of 950 °C which corresponded to a reaction temperature of about 1050 °C in the gas jetting or pre-oxidation zone because its caking index x was about 5%. In this case, char particles smoothly overflowed from the reactor. At rather lower heating temperatures, the blockage of overflowing pipe occurred by some
large char particles formed through caking agglomeration in the reactor. Indeed, high furnace temperature can greatly accelerate the consumption of viscous organic matters or metaplast on the surface of char particles through reactions like oxidation and polymerization that make the char surface rather steady and hard to weaken thus the caking propensity of the char particles [14]. The effective experimental conditions for caking destruction of the tested coal could then be determined from the tests shown above, which are the temperature of about 1000 °C in the preoxidation zone and the ER higher than 0.1 for the jetting gas when air is used. The clarification agrees with the results of our pilot plant tests [11], and thus it can provide a reliable guidance for optimization of the operating conditions and the scale-up of the JPFBG technology.
( )
Table 4 Caking destruction varying with ER of jetting gas (a) and temperatures of heating furnace (b).
Temperature
48
400
0
2
4
6
8
10
12
14
16
18
20
22
Operating time (min) Fig. 5. Time-series bed temperature at different central axial locations for the test cases 4–6 at different setting temperature for furnace (j T1, s T2, N T3, r T4).
49
Z. Zhao et al. / Fuel 133 (2014) 45–51
for T4 in the tested period because the accumulated char particles did not reach the height of the thermocouple for T4. Thus, the onset of the sharp temperature decrease in Fig. 4(a) can be judged to be the time when the char particles started to agglomerate, and the corresponding thermocouple can be used to determine the height of the accumulated char particle bed. Also, the temperatures in Fig. 4(a) seriously fluctuated with operation time, showing the unsteady reaction conditions inside the reactor. At about 11 min, the agglomerated char particles blocked the jetting tube and the test was stopped. The time-series temperature distribution characteristics were very similar for ERs varying from 0.1 to 0.3 (case 2–4), and Fig. 4(b) shows thus only the data for the case 4. The temperatures T1–T4 nearly constantly remained after the onset of the reaction, indicating that there was no agglomeration of char particles in the reactor and the caking propensity of the tested coal was effectively destroyed by the jetting pre-oxidation. The temperatures T1
10
Gas composition (%)
Nonetheless, the temperature inside the FB is somehow nonuniform and it is thus necessary to ascertain the temperature distribution characteristics in the reactor to understand the coal pre-oxidation process realizing caking destruction. Four thermocouples were mounted along the axial central line of the reactor to measure the bed temperatures. As shown in Fig. 3, the distances of these thermocouples were 60 mm (T1), 120 mm (T2), 180 mm (T3) and 240 mm (T4) above the outlet of the jetting tube, respectively,. Fig. 4 shows the time series of the measured bed temperatures for the cases 1 and 4 with and without occurrence of particle agglomeration, respectively. In the figure the 0 min means the time when the coal particles were fed into the reactor. As shown in Fig. 4(a) for the test case 1, the measured bed temperatures were lower than the setting temperature of 950 °C because there was no oxygen in the jetting gas (ER = 0). This reflected the occurrence of endothermic pyrolysis reactions in the N2 atmosphere. In this case, no pre-oxidation occurred in the gas jetting area, and the char particles resulting from pyrolysis bonded together by the surface colloidal matters to form large char agglomerates that hardly flowed out from the reactor. This causes the char particles to gradually accumulate inside the reactor and makes the agglomerates gradually large until they finally occupied the entire cross section of the reactor. The temperatures T1–T4 firstly slowly deceased, and then the temperatures T1–T3 sharply decreased but T4 did not. The slow decease of all the measured temperatures was attributed to the occurrence of pyrolysis reactions, and the sharp decrease in temperature for T1–T3 should be attributed to the fact that the char agglomerates covered the tip of the thermocouples to cause a large temperature difference between the tips of thermocouples and the bed circumstance. There was no sharp decrease
(a) ER=0.2, Ts=750
, case 5, ω =44 %
(b) ER=0.2, Ts=850
, case 6, ω =25.2 %
(c) ER=0.2, Ts=950
, case 4, ω =5.8 %
8
6
4
2
0 10
10
, case 1, ω=62.9 %
(a) ER=0, Ts=950
8
Gas composition (%)
Gas composition (%)
8
6
4
2
0
10
10
(b) ER=0.2, Ts=950
, case 4, ω=5.8 %
8
8
Gas composition (%)
Gas composition (%)
4
2
0
6
4
6
4
2
2
0
6
0
2
4
6
8
10
12
14
16
18
20
Operating time (min) Fig. 6. Composition of product gas in pre-oxidation corresponding to the tests in Fig. 6 (h H2, N CH4, r CO, d CO2).
0
0
2
4
6
8
10
12
14
16
18
20
Operationg time (min) Fig. 7. Composition of product gas in pre-oxidation corresponding to the tests in Fig. 7 (h H2, N CH4, r CO, d CO2).
50
Z. Zhao et al. / Fuel 133 (2014) 45–51
and T3 were very close and T2 was the highest, showing that the strongest pre-oxidation occurred at the axial position of 120 mm (the position of thermocouple T2) above the outlet of the jetting tube. In this case the temperatures in the bed were higher than the setting temperature of 950 °C because of the exothermic effect of the partial combustion of coal particles, except for T4 which was close to the outlet of the reactor. Fig. 5 shows the variation of temperature with operation time at different setting temperatures Ts (cases 4–6). It can be seen that the temperatures in the FB were not steady when the setting temperatures were relatively low (750 °C) in Fig. 5(a). This is because at this temperature the oxidation and polymerization reactions were slow so that the caking tendency could not be fully suppressed, although ER was 0.2. Increasing the setting temperature for heating furnace to 850 °C (5(b)) and 950 °C (5(c)) caused the measured temperatures T1–T4 in the reactor to vary smoothly with operation time. These temperatures showed in fact the steady jetting pre-oxidation reactions of coal and char particles inside the reactor. The demonstration from Fig. 5 are also fully consistent with the effect of temperature on caking destruction clarified with the caking index x in Table 4, where the particle agglomeration in the reactor occurred to a certain degree at the setting temperature of 750 °C but it was fully suppressed at setting temperatures above 950 °C. Therefore, the variation with operating time of the temperatures in the jetting pre-oxidation zone could be a criterion for judging the implementation degree of caking destruction by jetting pre-oxidation. The results shown above demonstrate that a certain amount of oxygen in the jetting gas (ER > 0.1) and an enough high oxidation temperature (>1000 °C) are both necessary and essential for realizing the full caking destruction of coals having certain caking propensity by the proposed jetting pre-oxidation in a fluidized bed. 3.3. Further analysis and characterization Gaseous product has to be generated in the jetting preoxidation of caking coal. Fig. 6 shows the time-series composition
(a) Raw coal
(c) Ts=750
, ER=0.2, ω=44%
of product gas corresponding to the temperature profiles in Fig. 4 (cases 1 and 4). The product gas contained mainly H2, CH4, CO and CO2, while the major component N2 from the fluidizing and jetting gas was not shown in Fig. 6. For the test case 1 of ER = 0 in Fig. 6(a), the formed major gas was H2 and CH4 was next to it because of the pure pyrolysis. In Fig. 6(b) for the test case 4 the pre-oxidation or partial combustion in the gas jetting zone made CO2 be the main gas component, while there were also some CO and H2. Fig. 7 shows the time-series product gas compositions corresponding to the tests in Fig. 5 at the same ER but different setting temperatures Ts for the heating surface (test cases 4–6). Table 4 clarified that particle agglomeration more or less occurred for the cases 5 and 6 while effective caking suppression was realized for the case 4. At the low setting temperature of 750 °C (case 5), the amount of gaseous product was low due to the low reaction temperature. In all the test cases of Fig. 7, the presence of O2 in the jetting gas caused CO2 to be the major gas component in the product gas. Comparing Fig. 7(b) (case 6) and Fig. 7(c) (case 4) showed that the H2 and CO concentrations in case 6 were obviously higher than that in case 4 as well as case 5, although the setting temperature in case 6 was lower. This phenomenon should be attributed to the more accumulated char particles in the reactor and the longer contacting time between char and gas in case 6 than in case 4. At the higher temperature of case 6, the product gas was more so that the residence time in the reactor was shorter. For the case 5 in Fig. 7(a), the reaction temperature was too low so that the formed H2 and CO were also low. The results suggest that for testing the caking destruction by pre-oxidation in a fluidized bed, as done in this study, the appearance of unusually high H2 and CO concentrations in the product gas may a sign for the occurrence of particle agglomeration due to caking in the reactor. Fig. 8 compares the morphologies of raw coal and the char particles from the collecting bottle under different conditions. The raw coal was less porous and had few pores on the particle surfaces (Fig. 8(a)). At the high in-bed temperature above 1000 °C (setting temperature being 950 °C) and in the inert atmosphere (no O2 in
(b) Ts=950
(d) Ts=950
, ER=0, ω =62.9%
, ER=0.1, ω =15.7%
Fig. 8. Typical SEM photographs of raw coal and chars obtained from tests under different jetting pre-oxidation conditions.
51
Z. Zhao et al. / Fuel 133 (2014) 45–51 Table 5 Analysis of non-agglomerating char particles. ER
Proximate analysis (wt.%, ad)
Ultimate analysis (wt.%, ad)
M
A
V
FC
C
H
O
N
S
0.1 0.2 0.3
1.17 0.69 0.49
24.51 28.20 32.14
5.68 4.44 3.41
68.65 66.67 63.96
70.66 68.27 64.83
0.42 0.39 0.40
1.44 0.72 0.52
1.34 1.28 1.17
0.47 0.46 0.45
the jetting gas), the obtained char particles greatly swelled to cause severe distortion and some pores on the smooth surface, while the particles themselves adhered each other by metaplastic matters to form large agglomerates (Fig. 8(b)). At ER of 0.2, for the pre-oxidation temperature, such as at 850 °C corresponding to the setting temperature of 750 °C, the oxidation and polymerization reactions occurring to metaplast was slow so that the particles in the bed had still certain caking propensity to adhere some quartz sand and also some other char particles (Fig. 8(c)). For Fig. 8(d) at rather high temperature (above 1000 °C) and an ER of 0.1, the obtained char particles were highly scattered and no fine particle was attached to the char particles. The appearance of char particles exhibited also a high degree of carbonization. Consequently, all of these SEM pictures directly verified the fact that the jetting pre-oxidation in a FB reactor can effectively destroy the caking property of bituminous coals at appropriately high oxidation temperatures (e.g., >1000 °C) and suitable ER such as above 0.1 for the jetting gas. The results analyzing the char particles from non-agglomerating conditions are shown in Table 5. Increasing the equivalence ratio of jetting gas decreased the volatile and fixed carbon contents but increased the ash content of the char particles. This indicates the increase in carbon loss from the char particles during jetting pre-oxidation when raising the oxygen amount fed through the jetting gas. Most of the carbon, however, remained in the char particles after jetting pre-oxidation and requires for the subsequent gasification. From the aspect of char gasification, the use of higher ER for pre-oxidation is thus plausible, but this would much reduce the oxygen amount fed into the char gasification zone in the bottom of the fluidized bed. Overall, the ER for the jetting gas should thus be as low as possible to allow the possibly highest oxygen fed into the char gasification zone from the bed bottom. 4. Conclusions The capability for destroying the caking propensity of a kind of bituminous coal by jetting pre-oxidation was tested in a benchscale fluidized bed (FB) reactor. The tested parameters included temperature in jetting or pre-oxidation zone and equivalent ratio (ER) of oxygen for jetting gas to feed coal. A new caking index x defined to be the mass fraction of particles larger than 2 mm in the collected char particles was used to characterize the caking extent of particles under various conditions. The study clarified the following major conclusions. (1) Increasing temperature in the gas jetting or pre-oxidation zone and O2 content in the jetting gas both facilitated the realized effect of caking destruction for the tested coal. Full destruction of the caking propensity for the examined bituminous coal was successfully achieved under the conditions of pre-oxidation temperature above 1000 °C and with ER higher than 0.1 for the jetting gas. (2) It was shown that the occurrence of serious agglomeration of char particles due to caking adherence in the reactor corresponded not only to a slight increase in the H2 and CO concentrations of the product gas but also a sharp decrease in
Carbon loss (wt.%)
10.22 24.62 37.18
the in-bed temperatures for the pre-oxidation zone at different axial positions, especially nearby the outlet of the jetting gas. (3) SEM images of the resulting char particles from the reactor demonstrated that the caking agglomeration of char particles in the reactor occurred by adherence of a few mono-char particles through metaplastic matters among them, while the complete destruction of caking propensity was indicated by highly scattered and deeply pyrolyzed mono-char particles. Conflict of interest The authors declare no competing financial interest. Acknowledgements The work was conducted under the research programs supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA07050400), the National Natural Science Foundation of China (21306209), the National Basic Research Program of China (2011CB201304), the International Science & Technology Cooperation Program of China (2013DFG60060), the National Natural Science Foundation of China (21376250) and the National Natural Science Foundation of China (U1302273). References [1] Bunt JR, Waanders FB. An understanding of the behaviour of a number of element phases impacting on a commercial-scale Sasol–Lurgi FBDB gasifier. Fuel 2008;87:1751–2. [2] He C, Feng X, Chu KH. Process modeling and thermodynamic analysis of Lurgi fixed-bed coal gasifier in an SNG plant. Appl Energy 2013;111:742–7. [3] Shen DY inventor, Li ZN, assignee. Atmospheric powder coal gasifier. Chinese Patent 200720011795.1. 2007 Apr 19. [4] Dong L, Zhang JW, Wang Y, Tian YJ, Gao SQ, Xu GW inventor, Li B, assignee. Jetting pre-oxidation fluidized bed for gasification and pyrolysis of solid fuels containing carbon. Chinese Patent 102120936 A. 2011 Jul 13. [5] Gasior SJ, Forney AJ, Field JH. Decaking of coal in free fall. In: Schora F, editor. Fuel gasification. Washington DC: ACS; 1967. p. 1–7. [6] Van Dyk JC, Keyser MJ, Van Zyl JW. Suitability of feedstocks for the Sasol–Lurgi. fixed bed dry bottom gasification process. Gasification technologies 2001. San Francisco, California; 2001 Oct 7-10. [7] Crewe GF, Gat U, Dhir VK. Decaking of bituminous coals by alkaline solutions. Fuel 1975;54:20–3. [8] Rashid KM inventor. Dept. of Energy, assignee. Decaking of coal or oil shale during pyrolysis in the presence of iron oxides. United States Patent A7190429. 1988, May 5. [9] Gasior SJ, Forney AJ, Field JH. Destruction of caking quality of bituminous coal in a fixed bed. Ind Eng Chem Prod Res Dev 1964;3(1):43–7. [10] Forney AJ, Kenny RF, Gasior SJ. Destruction of caking properties of coal by pretreatment in a fluidized bed. Ind Eng Chem Prod Res Dev 1964;3(1):48–53. [11] Zhang JW, Zhao ZG, Zhang GY, Zeng X, Zhao FX, Dong L, et al. Pilot study on jetting pre-oxidation fluidized bed gasification adapting to caking coal. Appl Energy 2013;110:276–84. [12] Huang ZM, Zhang JS, Zhao Y, Zhang H, Yue GX, Suda T. Kinetic studies of char gasification by steam and CO2 in the presence of H2 and CO. Fuel Process Technol 2010;91(8):843–7. [13] Yu JL, Lucas JA, Wall TF. Formation of the structure of chars during devolatilization of pulverized coal and its thermoproperties: a review. Prog Energy Combust 2007;33:135–70. [14] Kam AY, Hixson AN, Perimutter DD. The oxidation of bituminous Coal. 3. Effect on caking properties. Ind Eng Chem Process Des Dev 1976;15(3):416–22.
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Destruction of caking properties of bituminous coal by jetting pre-oxidation in a fluidized bed Zhigang Zhao a,b, Juwei Zhang a,⇑, Feixiang Zhao a, Xi Zeng a, Xiaoxing Liu a, Guangwen Xu a,⇑ a b
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China School of Chemistry and Chemical Engineering, Anhui Key Lab of Coal Clean Conversion and Utilization, Anhui University of Technology, Ma’anshan 243002, China
h i g h l i g h t s Destruction of caking propensity of coal by jetting pre-oxidation was tested in a laboratory fluidized bed. Caking destruction was well achieved for a kind of coal with a caking index of 20. Oxidation temperature and oxygen content in jetting gas are essential to the caking destruction. Variations in bed temperature and product gas composition are indicative of caking condition in the bed.
a r t i c l e
i n f o
Article history: Received 25 February 2014 Received in revised form 8 April 2014 Accepted 6 May 2014 Available online 17 May 2014 Keywords: Caking propensity Caking destruction Fluidized bed gasification Coal Jetting pre-oxidation
a b s t r a c t Jetting pre-oxidation was proposed to destroy the caking property of bituminous coal in a fluidized bed (FB) gasifier. By defining an index x to characterize the degree or realized effect of caking destruction, experiments were conducted in an electrically heated laboratory FB reactor under different conditions to investigate the feasibility and required conditions for effectively destroying the caking propensity of a kind of bituminous coal. The tested major parameters were the equivalence air ratio (ER) of jetting gas and the temperature in the jetting zone. At relatively lower temperatures it was hard to completely destroy the caking propensity of the tested coal particles. Without oxygen inside the jetting gas, it was also impossible to fully destroy the caking propensity of the coal even at sufficiently high temperatures. Thus, oxidation of coal particles at suitably high temperatures was essential to the expected good destruction of their caking propensity. For the tested bituminous coal this was achieved at temperatures above 1000 °C in the jetting zone and an equivalent oxygen ratio above 0.1 for the jetting gas (up to 0.3 considering gasification applications). For coal pre-oxidation in a FB, the agglomeration state of char particles due to caking adherence in the reactor could be anticipated via the unsteady variations in the bed temperature and H2/CO concentrations in the effluent gas with reaction time. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction A large part of bituminous coal has certain caking propensity to make the coal particles stick together via colloidal matters formed when heating the coal through its plastic variation stage. Thus, this kind of coal is hard to be treated by the existing commercial fixed bed [1,2] or fluidized bed (FB) [3] gasification technologies. In practice, however, there is a need to gasify the coal with certain caking propensity and even high ash content to produce fuel gas, whereby realizing the value-added utilization of such low-quality or low-value coals. An example is for the coal middlings generated ⇑ Corresponding authors. Tel./fax: +86 10 82544886. E-mail addresses: [email protected] (J. Zhang), [email protected]. cn (G. Xu). http://dx.doi.org/10.1016/j.fuel.2014.05.006 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.
in coking industry, which has ash contents of about 40 wt.%, heating values of 3000–4000 kcal/kg and certainly high caking propensity. Gasifying the middlings to generate the fuel gas required for heating coking oven allows the high-quality coking oven gas (COG) to be replaced and further used for other value-added utilizations such as the production of substitute nature gas (SNG), methanol and hydrogen [4]. In some local areas, there are possibly only coals with caking propensity, and it has also the need to gasify the caking coal to produce not only fuel gas but also syngas for chemical synthesis. Of course, the entrained flow gasifiers can treat caking coal but it requires the feedstock to have low ash contents. Hence, the gasification of high-ash middlings has to rely on FB gasification through incorporating it with a technical means destroying the coal caking propensity.
Fig. 1. Principle diagram of jetting pre-oxidation fluidized bed gasification.
1100
6
1000
5
900
4
800
3
700
2
600
1
3
Many methods have been proposed to destroy the caking property of coals, such as pre-oxidation [5], mixing with coke or non-caking coal [6], mechanical stirring, adding additives [7,8]. However, most of these approaches require complex operations or have limited effects. The coal pre-oxidation has been considered to be effective and simple for eliminating caking propensity of bituminous coal. Gasior et al. [9] developed a pre-oxidation process for caking coal in a fixed bed. The treated coal was heated to its softening temperature and held there for 1–3 h in an inert gas containing 1 vol.% oxygen, after that it passed through the plastic variation stage of the coal in 1 h. The char obtained had 50% of the original volatile in the coal and could be gasified in a fixed bed without agglomeration. Forney et al. [10] found that the caking properties of coal could be eliminated by treating the coal in a FB at 400–425 °C for at least 5 min inside an inert gas or steam atmosphere containing at least 0.2 vol.% oxygen. These pre-oxidation methods for destroying caking propensity of coal, however, were in an isolated reactor so that the coal gasification process is complicated, while this would also lose the combustible matters emitted during caking destruction. Consequently, these pre-oxidation methods are hardly practicable for actual industrial processes. We have devised a novel jetting pre-oxidation fluidized bed gasification (JPFBG) technology for the utilization of caking coal to produce industrial fuel gas [11]. Fig. 1 shows the principle diagram of the JPFBG. The caking coal particles are jetted into the freeboard of the gasifier just above the bottom dense bed with a gas stream containing oxygen (e.g., air). The coal particles are dispersed and pre-oxidized in this gas jet. The formed char particles, which are possibly without caking propensity after the pre-oxidation, fall into the dense bed and are gasified by the gasification agent fed into the dense fluidized bed. Via this way, the char gasification can be also facilitated because of the removal of the inhibition effect of pyrolysis gas on char gasification as its separating the coal pyrolysis from char gasification through adopting the jetting pre-oxidation of coal [12]. In fact, we have built a 1000 t/a pilot JPFBG plant and a coal with the caking index of about 20 was successfully gasified (i.e., without particle agglomeration) in this plant using air as the gasification agent [11]. The typical operating results are shown in Fig. 2. The steady temperature of the dense bed as well as high heating value (HHV) of the produced gas indicates obviously the steady operation of JPFBG and the good destruction of the coal’s caking property. However, we have found in the pilot study that it was vital and difficult to determine the pre-oxidation conditions for caking destruction of the tested coal especially the oxygen content in the jetting gas. Less ER of the jetting gas may lead to insufficient effect on caking destruction, while excess oxygen may decrease the heating value of the product gas [11]. In order
Gas HHV (MJ/mN)
Z. Zhao et al. / Fuel 133 (2014) 45–51
Temperature ( )
46
500
0
50
100
150
200
250
300
350
0
Operating time (min) Fig. 2. Time-series gas temperature in dense fluidized bed and high heating value (HHV) of the produced gas obtained in the JPFBG pilot test [11].
to figure out the critical factors ensuring the caking destruction of the coal, a laboratory FB reactor was adopted in this study to simulate the jetting pre-oxidation conditions in JPFBG and to investigate the effect of major operating parameters on the realized effect for destroying the caking propensity of caking coal.
2. Experimental A kind of bituminous coal with diameters of 0.5–1.0 mm and a particle density of 1300 kg/m3 was used for the bench-scale test. The characteristics of the coal are shown in Table 1. The coal has a caking index (GR.I.) of 20 and cannot be gasified in the conventional fluidized bed reactor because of its caking propensity. Fig. 3 presents a schematic diagram of the experimental system and its main body of a bench-scale FB reactor. The reactor was made of stainless steel and included a reaction section of 600 mm in height, 50 mm in inner diameter (i.d.), and an upper enlarged section of 120 mm in height, 80 mm in i.d. The coal particles were jetted into the reactor using an O2-containing gas (gas jet) via a vertical feeding tube of 6 mm in i.d. mounted at the central axis of the porous gas distributor of the FB. Inert gas nitrogen was fed into the bed through the gas distributor as the fluidizing gas. An overflow outlet of particles was made on the wall of the fluidized bed reactor at a position 300 mm above the gas distributor. The overflowed char particles from the reactor were collected into a bottle through a pipe connected to the overflow outlet. The effluent gas from the top of the fluidized bed reactor was, in succession, cooled, purified and finally analyzed with a three-channel micro-gas chromatograph (GC, Agilent 3000A). Before each experiment, a certain amount of aluminum oxide pellets of 5.0 mm in diameter and quartz sand with a Sauter mean diameter of 0.156 mm were loaded into the reactor above the gas distributor but the formed particle bed height was limited to the outlet of the central gas jet pipe. Then, the reactor was heated to its designated temperature by an electrical furnace. After the temperature was steady, both the jetting gas for feeding coal particles and the fluidizing gas through the gas distributor were introduced into the reactor for several minutes to stabilize the fluidization state of sand particles. In turn, coal particles were fed into the reactor for 20 min by letting the screw feeder work and supply coal particles into the jetting gas stream. Bed temperatures at different positions of the reactor were recorded, while the product gas from the reactor was collected for analysis by GC. After finishing the coal feed, the heating of the reactor was stopped and the reactor was cooled to room temperature in N2 (fluidizing gas). Finally, size sieving was performed for the particles from the reactor and the
47
Z. Zhao et al. / Fuel 133 (2014) 45–51 Table 1 Analysis of the tested bituminous coal. Proximate analysis (wt.%, ad) Ultimate analysis (wt.%, ad) HHV (MJ/Kg) GR.I. M
A
V
FC
C
2.08
20.31
25.74
51.87
65.22 3.84 6.84 1.15 0.56 26.18
T3 T1
Thermocouple
H
O
N
S 20
T4 T2
Condenser
A good destruction of the caking propensity for the tested coal was identified if the resulting char particles fluently flowed out through the overflow pipe from the reactor. Otherwise, the overflow of the char particles should be unsteady, even stopped, if a serious agglomeration occurred to the char particles in the reactor. It was found that for the char particles collected after each test, the caking property cannot be well characterized with the standard caking index GR.I. because the measured GR.I for all the collected chars were close to zero after the tests, even if serious caking agglomeration actually occurred in the tests. It is generally known that the bituminous coal particles would swell during pyrolysis, and the swelling ratio was usually lower than 2 [13]. If there is no caking occurred, the size of a single char particle should thus be smaller than 2 mm considering all of the tested coals were smaller than 1 mm. Thus, a new caking index x was defined by the following equation to quantify the actually realized effect of caking destruction for the tested coal:
x¼ Coal
GC Pump
N2
O2
Char
Ice-water bath
Fig. 3. A schematic diagram of the adopted experimental fluidized bed apparatus.
particle collecting bottle to separate char from their mixed aluminum oxide pellets and quartz sand. Table 2 summarizes the experimental conditions of the performed tests. The examined major parameters for coal preoxidation were the equivalence ratio (ER) of oxygen of jetting gas and the temperatures in the jetting zone of the reactor. The heating temperature for the FB reactor varied from 750 °C to 950 °C in the pre-oxidation tests 4–7 shown in Table 2. The superficial velocity of the fluidizing gas N2 was several times higher than the minimum fluidization velocity (Umf) of the tested coal particles, while the adopted jetting gas velocity was usually much higher than the terminal velocity (Ut) of the fed coal particles for ensuring a smooth jetting fed of the coal. The feeding rate of coal particles were adjusted from 6.7 g/min to 20 g/min to ensure the same jetting gas velocity in different cases. The variation in ER of the jetting gas was from 0.1 to 0.3 as shown for tests 1–4 in Table 2. The O2 concentrations in the O2-containing jetting gas were all 21 vol.% (air) except for the case 1 of ER = 0.
Table 2 Experimental conditions of the performed tests. Experimental case
1
Setting temperature of furnace Ts (°C) 950 ER of jetting gas for coal feed 0 Oxygen content in jet gas (vol.%) 0 Coal feed rate (g/min) 10 Jetting gas velocitya (m/s) 7.84 Fluidizing gas velocityb (m/s) 0.45 b Umf of coal (m/s) 0.07 Ut of coala (m/s) 2.76 a b
2
3
950 950 0.1 0.3 21 21 20 6.7 (the same
The velocity is at 20 °C based on the jetting tube. The velocity is at 900 °C based on bed diameter.
4 950 0.2 21 10 for all
5
6
750 850 0.2 0.2 21 21 10 10 the tests)
7 900 0.2 21 10
m2 100% mt
ð1Þ
where m2 is the mass of char particles larger than 2 mm in the char from the reactor (for serious caking cases) or from the char collecting bottle (for non-caking cases), and mt represents the total mass of all char particles from the reactor or the char collecting bottle. The lower x means the better caking destruction realized by the jetting pre-oxidation. By using x, this study quantitatively analyzed the realized effects of caking destruction under different conditions. Experiments were usually repeated three times to calculate the averages of the analyzed parameters in this article. Table 3 shows an error analysis for the caking index x tested under the experimental conditions having the setting temperature of the heating furnace (Ts) at 950 °C and using air as the jetting gas according to an equivalence ratio (ER) of 0.2. We can see that the relative error of x was less than 4%, thus verifying the reliability of the testing system and the accuracy of the results reported herein. 3. Results and discussion 3.1. Caking destruction Table 4(a) shows the effect of the ER of the jetting gas on the realized caking destruction for the tested coal (cases 1–4). The tests used air as the oxidizing gas and the setting temperature Ts for the heating furnace was 950 °C. It can be seen that the ER of the feeding gas significantly influenced the effect of caking destruction characterized in terms of the defined parameters x. The caking destruction was obviously improved by increasing ER. When the coal was blown into the reactor using only N2 at ER = 0, x was high as 60%. In this experiment serious agglomeration of particles occurred so that char particles did not smoothly flow out from the reactor. When the ER was higher than 0.1, the values of x were all below 15% and decreased with increasing ER. Under these conditions there was almost no agglomeration of particles occurred in the reactor and the char particles could smoothly flow out from the
Table 3 Demonstration of experimental repeatability via the parameter x (Ts = 950 °C, ER = 0.1). Test no.
mt (g)
m2 (g)
x (%)
Relative error (%)
1 2 3 Average
119.12 125.73 110.56 118.47
6.32 6.91 5.68 6.30
5.31 5.50 5.14 5.32
0.19 3.38 3.38 –
Note: mt – total mass of all char particles from the char collecting bottle, m2 – mass of char particles larger than 2 mm, x – caking index.
Z. Zhao et al. / Fuel 133 (2014) 45–51
(a) Ts = 950 °C
(b) ER = 0.2
Experimental case
ER
1 2 4 3
0 0.1 0.2 0.3
Caking index x (%)
Experimental case
Ts (°C)
x (%)
62.9 15.7 5.8 3.3
5 6 7 4
750 850 900 950
44 25.2 18.3 5.8
Caking index
1100 1000 900 800 700 600 500
1000
1000
900
( )
1100
900
800
800
700 600
, case 1, ω=62.9 %
(a) ER=0, Ts=950
400
1100
1100
1000
1000
900
900
600
(b) ER=0.2, Ts=950
500 400
0
2
4
6
8
10
, case 4, ω=5.8 % 12
14
16
18
22
Operating time (min) Fig. 4. Time-series bed temperature at different central axial locations for the test cases 1 and 4 with and without occurrence of agglomeration (j T1, s T2, N T3, rT4).
, case 6, ω =25.2 %
(c) ER=0.2, Ts=950
, case 4, ω =5.8 %
800 700 600 500
20
(b) ER=0.2, Ts=850
600
400
700
, case 5, ω =44 %
700
500
800
(a) ER=0.2, Ts=750
400
Temperature ( )
)
The temperature in the pre-oxidation zone of the reactor should work critically on the realized effect of caking destruction.
1100
500
Temperature (
3.2. Temperature distribution in reactor
Temperature
Temperature (
)
reactor. These results indicate that high ER of the jetting gas enhanced the pre-oxidation of coal so that good caking destruction was achieved. Nonetheless, the improvement on caking destruction was not evident when further increasing ER from 0.1 to 0.3. This means that at the tested Ts of 950 °C, a small amount of oxygen in the jetting gas was enough to effectively oxidize the coal particles and suppress their caking propensity. Table 4(b) shows the variation of the realized caking destruction with the setting temperature Ts for heating furnace under specified ER of 0.2 and O2 concentration of 21 vol.% in the jetting gas (cases 4–7). The Ts varied in 750–950 °C and the corresponding temperatures T1 to T4 in the jetting zone of the reactor, as will be shown in Fig. 5, was approximately 850–1050 °C. Table 4 clarifies that the caking index x sharply decreased with increasing the setting temperature for heating furnace. The caking propensity of the tested coal was completely destroyed at the furnace setting temperature of 950 °C which corresponded to a reaction temperature of about 1050 °C in the gas jetting or pre-oxidation zone because its caking index x was about 5%. In this case, char particles smoothly overflowed from the reactor. At rather lower heating temperatures, the blockage of overflowing pipe occurred by some
large char particles formed through caking agglomeration in the reactor. Indeed, high furnace temperature can greatly accelerate the consumption of viscous organic matters or metaplast on the surface of char particles through reactions like oxidation and polymerization that make the char surface rather steady and hard to weaken thus the caking propensity of the char particles [14]. The effective experimental conditions for caking destruction of the tested coal could then be determined from the tests shown above, which are the temperature of about 1000 °C in the preoxidation zone and the ER higher than 0.1 for the jetting gas when air is used. The clarification agrees with the results of our pilot plant tests [11], and thus it can provide a reliable guidance for optimization of the operating conditions and the scale-up of the JPFBG technology.
( )
Table 4 Caking destruction varying with ER of jetting gas (a) and temperatures of heating furnace (b).
Temperature
48
400
0
2
4
6
8
10
12
14
16
18
20
22
Operating time (min) Fig. 5. Time-series bed temperature at different central axial locations for the test cases 4–6 at different setting temperature for furnace (j T1, s T2, N T3, r T4).
49
Z. Zhao et al. / Fuel 133 (2014) 45–51
for T4 in the tested period because the accumulated char particles did not reach the height of the thermocouple for T4. Thus, the onset of the sharp temperature decrease in Fig. 4(a) can be judged to be the time when the char particles started to agglomerate, and the corresponding thermocouple can be used to determine the height of the accumulated char particle bed. Also, the temperatures in Fig. 4(a) seriously fluctuated with operation time, showing the unsteady reaction conditions inside the reactor. At about 11 min, the agglomerated char particles blocked the jetting tube and the test was stopped. The time-series temperature distribution characteristics were very similar for ERs varying from 0.1 to 0.3 (case 2–4), and Fig. 4(b) shows thus only the data for the case 4. The temperatures T1–T4 nearly constantly remained after the onset of the reaction, indicating that there was no agglomeration of char particles in the reactor and the caking propensity of the tested coal was effectively destroyed by the jetting pre-oxidation. The temperatures T1
10
Gas composition (%)
Nonetheless, the temperature inside the FB is somehow nonuniform and it is thus necessary to ascertain the temperature distribution characteristics in the reactor to understand the coal pre-oxidation process realizing caking destruction. Four thermocouples were mounted along the axial central line of the reactor to measure the bed temperatures. As shown in Fig. 3, the distances of these thermocouples were 60 mm (T1), 120 mm (T2), 180 mm (T3) and 240 mm (T4) above the outlet of the jetting tube, respectively,. Fig. 4 shows the time series of the measured bed temperatures for the cases 1 and 4 with and without occurrence of particle agglomeration, respectively. In the figure the 0 min means the time when the coal particles were fed into the reactor. As shown in Fig. 4(a) for the test case 1, the measured bed temperatures were lower than the setting temperature of 950 °C because there was no oxygen in the jetting gas (ER = 0). This reflected the occurrence of endothermic pyrolysis reactions in the N2 atmosphere. In this case, no pre-oxidation occurred in the gas jetting area, and the char particles resulting from pyrolysis bonded together by the surface colloidal matters to form large char agglomerates that hardly flowed out from the reactor. This causes the char particles to gradually accumulate inside the reactor and makes the agglomerates gradually large until they finally occupied the entire cross section of the reactor. The temperatures T1–T4 firstly slowly deceased, and then the temperatures T1–T3 sharply decreased but T4 did not. The slow decease of all the measured temperatures was attributed to the occurrence of pyrolysis reactions, and the sharp decrease in temperature for T1–T3 should be attributed to the fact that the char agglomerates covered the tip of the thermocouples to cause a large temperature difference between the tips of thermocouples and the bed circumstance. There was no sharp decrease
(a) ER=0.2, Ts=750
, case 5, ω =44 %
(b) ER=0.2, Ts=850
, case 6, ω =25.2 %
(c) ER=0.2, Ts=950
, case 4, ω =5.8 %
8
6
4
2
0 10
10
, case 1, ω=62.9 %
(a) ER=0, Ts=950
8
Gas composition (%)
Gas composition (%)
8
6
4
2
0
10
10
(b) ER=0.2, Ts=950
, case 4, ω=5.8 %
8
8
Gas composition (%)
Gas composition (%)
4
2
0
6
4
6
4
2
2
0
6
0
2
4
6
8
10
12
14
16
18
20
Operating time (min) Fig. 6. Composition of product gas in pre-oxidation corresponding to the tests in Fig. 6 (h H2, N CH4, r CO, d CO2).
0
0
2
4
6
8
10
12
14
16
18
20
Operationg time (min) Fig. 7. Composition of product gas in pre-oxidation corresponding to the tests in Fig. 7 (h H2, N CH4, r CO, d CO2).
50
Z. Zhao et al. / Fuel 133 (2014) 45–51
and T3 were very close and T2 was the highest, showing that the strongest pre-oxidation occurred at the axial position of 120 mm (the position of thermocouple T2) above the outlet of the jetting tube. In this case the temperatures in the bed were higher than the setting temperature of 950 °C because of the exothermic effect of the partial combustion of coal particles, except for T4 which was close to the outlet of the reactor. Fig. 5 shows the variation of temperature with operation time at different setting temperatures Ts (cases 4–6). It can be seen that the temperatures in the FB were not steady when the setting temperatures were relatively low (750 °C) in Fig. 5(a). This is because at this temperature the oxidation and polymerization reactions were slow so that the caking tendency could not be fully suppressed, although ER was 0.2. Increasing the setting temperature for heating furnace to 850 °C (5(b)) and 950 °C (5(c)) caused the measured temperatures T1–T4 in the reactor to vary smoothly with operation time. These temperatures showed in fact the steady jetting pre-oxidation reactions of coal and char particles inside the reactor. The demonstration from Fig. 5 are also fully consistent with the effect of temperature on caking destruction clarified with the caking index x in Table 4, where the particle agglomeration in the reactor occurred to a certain degree at the setting temperature of 750 °C but it was fully suppressed at setting temperatures above 950 °C. Therefore, the variation with operating time of the temperatures in the jetting pre-oxidation zone could be a criterion for judging the implementation degree of caking destruction by jetting pre-oxidation. The results shown above demonstrate that a certain amount of oxygen in the jetting gas (ER > 0.1) and an enough high oxidation temperature (>1000 °C) are both necessary and essential for realizing the full caking destruction of coals having certain caking propensity by the proposed jetting pre-oxidation in a fluidized bed. 3.3. Further analysis and characterization Gaseous product has to be generated in the jetting preoxidation of caking coal. Fig. 6 shows the time-series composition
(a) Raw coal
(c) Ts=750
, ER=0.2, ω=44%
of product gas corresponding to the temperature profiles in Fig. 4 (cases 1 and 4). The product gas contained mainly H2, CH4, CO and CO2, while the major component N2 from the fluidizing and jetting gas was not shown in Fig. 6. For the test case 1 of ER = 0 in Fig. 6(a), the formed major gas was H2 and CH4 was next to it because of the pure pyrolysis. In Fig. 6(b) for the test case 4 the pre-oxidation or partial combustion in the gas jetting zone made CO2 be the main gas component, while there were also some CO and H2. Fig. 7 shows the time-series product gas compositions corresponding to the tests in Fig. 5 at the same ER but different setting temperatures Ts for the heating surface (test cases 4–6). Table 4 clarified that particle agglomeration more or less occurred for the cases 5 and 6 while effective caking suppression was realized for the case 4. At the low setting temperature of 750 °C (case 5), the amount of gaseous product was low due to the low reaction temperature. In all the test cases of Fig. 7, the presence of O2 in the jetting gas caused CO2 to be the major gas component in the product gas. Comparing Fig. 7(b) (case 6) and Fig. 7(c) (case 4) showed that the H2 and CO concentrations in case 6 were obviously higher than that in case 4 as well as case 5, although the setting temperature in case 6 was lower. This phenomenon should be attributed to the more accumulated char particles in the reactor and the longer contacting time between char and gas in case 6 than in case 4. At the higher temperature of case 6, the product gas was more so that the residence time in the reactor was shorter. For the case 5 in Fig. 7(a), the reaction temperature was too low so that the formed H2 and CO were also low. The results suggest that for testing the caking destruction by pre-oxidation in a fluidized bed, as done in this study, the appearance of unusually high H2 and CO concentrations in the product gas may a sign for the occurrence of particle agglomeration due to caking in the reactor. Fig. 8 compares the morphologies of raw coal and the char particles from the collecting bottle under different conditions. The raw coal was less porous and had few pores on the particle surfaces (Fig. 8(a)). At the high in-bed temperature above 1000 °C (setting temperature being 950 °C) and in the inert atmosphere (no O2 in
(b) Ts=950
(d) Ts=950
, ER=0, ω =62.9%
, ER=0.1, ω =15.7%
Fig. 8. Typical SEM photographs of raw coal and chars obtained from tests under different jetting pre-oxidation conditions.
51
Z. Zhao et al. / Fuel 133 (2014) 45–51 Table 5 Analysis of non-agglomerating char particles. ER
Proximate analysis (wt.%, ad)
Ultimate analysis (wt.%, ad)
M
A
V
FC
C
H
O
N
S
0.1 0.2 0.3
1.17 0.69 0.49
24.51 28.20 32.14
5.68 4.44 3.41
68.65 66.67 63.96
70.66 68.27 64.83
0.42 0.39 0.40
1.44 0.72 0.52
1.34 1.28 1.17
0.47 0.46 0.45
the jetting gas), the obtained char particles greatly swelled to cause severe distortion and some pores on the smooth surface, while the particles themselves adhered each other by metaplastic matters to form large agglomerates (Fig. 8(b)). At ER of 0.2, for the pre-oxidation temperature, such as at 850 °C corresponding to the setting temperature of 750 °C, the oxidation and polymerization reactions occurring to metaplast was slow so that the particles in the bed had still certain caking propensity to adhere some quartz sand and also some other char particles (Fig. 8(c)). For Fig. 8(d) at rather high temperature (above 1000 °C) and an ER of 0.1, the obtained char particles were highly scattered and no fine particle was attached to the char particles. The appearance of char particles exhibited also a high degree of carbonization. Consequently, all of these SEM pictures directly verified the fact that the jetting pre-oxidation in a FB reactor can effectively destroy the caking property of bituminous coals at appropriately high oxidation temperatures (e.g., >1000 °C) and suitable ER such as above 0.1 for the jetting gas. The results analyzing the char particles from non-agglomerating conditions are shown in Table 5. Increasing the equivalence ratio of jetting gas decreased the volatile and fixed carbon contents but increased the ash content of the char particles. This indicates the increase in carbon loss from the char particles during jetting pre-oxidation when raising the oxygen amount fed through the jetting gas. Most of the carbon, however, remained in the char particles after jetting pre-oxidation and requires for the subsequent gasification. From the aspect of char gasification, the use of higher ER for pre-oxidation is thus plausible, but this would much reduce the oxygen amount fed into the char gasification zone in the bottom of the fluidized bed. Overall, the ER for the jetting gas should thus be as low as possible to allow the possibly highest oxygen fed into the char gasification zone from the bed bottom. 4. Conclusions The capability for destroying the caking propensity of a kind of bituminous coal by jetting pre-oxidation was tested in a benchscale fluidized bed (FB) reactor. The tested parameters included temperature in jetting or pre-oxidation zone and equivalent ratio (ER) of oxygen for jetting gas to feed coal. A new caking index x defined to be the mass fraction of particles larger than 2 mm in the collected char particles was used to characterize the caking extent of particles under various conditions. The study clarified the following major conclusions. (1) Increasing temperature in the gas jetting or pre-oxidation zone and O2 content in the jetting gas both facilitated the realized effect of caking destruction for the tested coal. Full destruction of the caking propensity for the examined bituminous coal was successfully achieved under the conditions of pre-oxidation temperature above 1000 °C and with ER higher than 0.1 for the jetting gas. (2) It was shown that the occurrence of serious agglomeration of char particles due to caking adherence in the reactor corresponded not only to a slight increase in the H2 and CO concentrations of the product gas but also a sharp decrease in
Carbon loss (wt.%)
10.22 24.62 37.18
the in-bed temperatures for the pre-oxidation zone at different axial positions, especially nearby the outlet of the jetting gas. (3) SEM images of the resulting char particles from the reactor demonstrated that the caking agglomeration of char particles in the reactor occurred by adherence of a few mono-char particles through metaplastic matters among them, while the complete destruction of caking propensity was indicated by highly scattered and deeply pyrolyzed mono-char particles. Conflict of interest The authors declare no competing financial interest. Acknowledgements The work was conducted under the research programs supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA07050400), the National Natural Science Foundation of China (21306209), the National Basic Research Program of China (2011CB201304), the International Science & Technology Cooperation Program of China (2013DFG60060), the National Natural Science Foundation of China (21376250) and the National Natural Science Foundation of China (U1302273). References [1] Bunt JR, Waanders FB. An understanding of the behaviour of a number of element phases impacting on a commercial-scale Sasol–Lurgi FBDB gasifier. Fuel 2008;87:1751–2. [2] He C, Feng X, Chu KH. Process modeling and thermodynamic analysis of Lurgi fixed-bed coal gasifier in an SNG plant. Appl Energy 2013;111:742–7. [3] Shen DY inventor, Li ZN, assignee. Atmospheric powder coal gasifier. Chinese Patent 200720011795.1. 2007 Apr 19. [4] Dong L, Zhang JW, Wang Y, Tian YJ, Gao SQ, Xu GW inventor, Li B, assignee. Jetting pre-oxidation fluidized bed for gasification and pyrolysis of solid fuels containing carbon. Chinese Patent 102120936 A. 2011 Jul 13. [5] Gasior SJ, Forney AJ, Field JH. Decaking of coal in free fall. In: Schora F, editor. Fuel gasification. Washington DC: ACS; 1967. p. 1–7. [6] Van Dyk JC, Keyser MJ, Van Zyl JW. Suitability of feedstocks for the Sasol–Lurgi. fixed bed dry bottom gasification process. Gasification technologies 2001. San Francisco, California; 2001 Oct 7-10. [7] Crewe GF, Gat U, Dhir VK. Decaking of bituminous coals by alkaline solutions. Fuel 1975;54:20–3. [8] Rashid KM inventor. Dept. of Energy, assignee. Decaking of coal or oil shale during pyrolysis in the presence of iron oxides. United States Patent A7190429. 1988, May 5. [9] Gasior SJ, Forney AJ, Field JH. Destruction of caking quality of bituminous coal in a fixed bed. Ind Eng Chem Prod Res Dev 1964;3(1):43–7. [10] Forney AJ, Kenny RF, Gasior SJ. Destruction of caking properties of coal by pretreatment in a fluidized bed. Ind Eng Chem Prod Res Dev 1964;3(1):48–53. [11] Zhang JW, Zhao ZG, Zhang GY, Zeng X, Zhao FX, Dong L, et al. Pilot study on jetting pre-oxidation fluidized bed gasification adapting to caking coal. Appl Energy 2013;110:276–84. [12] Huang ZM, Zhang JS, Zhao Y, Zhang H, Yue GX, Suda T. Kinetic studies of char gasification by steam and CO2 in the presence of H2 and CO. Fuel Process Technol 2010;91(8):843–7. [13] Yu JL, Lucas JA, Wall TF. Formation of the structure of chars during devolatilization of pulverized coal and its thermoproperties: a review. Prog Energy Combust 2007;33:135–70. [14] Kam AY, Hixson AN, Perimutter DD. The oxidation of bituminous Coal. 3. Effect on caking properties. Ind Eng Chem Process Des Dev 1976;15(3):416–22.