3.1 Effect of different particle sizes of coal on the temperaturefield distribution of furnace’s longitudinal and transverse center sections
When the as-fired coal particle size was set at 40, 35, 30, 25, 20, 15, 10, and 5 mm, the temperature field distribution of y-direction and x-direction center sections are shown in Figure 3 and 4, which is for the numerical simulation results of the SJ carbonization furnace. The temperature field distribution in the furnace cavity was symmetric, chiefly from Figure 3 and 4, because the structure of the SJ furnace was symmetric and materials basically entered various feeding inlets in a uniform manner. When the coal with the same particle size entered the SJ furnace, the furnace cavity as a whole presented an increasing trend first and decreasing afterwards from the peripheral zone of the inlet to the furnace top. However, the temperature at the inlet of the gas mixture of the coal gas and air was relatively low, and the maximum temperature in the furnace emerged at the joints between the grid walls and furnace cavity (red in the zone) while the minimum temperature occurred near the wall surface. The reason was that the study sections y = 1.4 m and x = 2.75 m selected in Figure 3 and 4, respectively, were the center sections of the as-fired gas inlets, and the gas mixture of the coal gas and air fed into the grid walls after mixing via a branch mixer had a relatively low temperature, resulting in the relatively low temperature at the gas mixture inlet. The gas mixture gradually increased in the process of upward motion to the kindling points of various components of coal gas with its spreading, and carbon monoxide, hydrogen, methane, ethylene, acetylene, and other gases engaged in the combustion reaction with oxygen in the air at the joints between the grid walls and furnace cavity, releasing a large amount of heat and gradually elevated the furnace temperature (to the maximum temperature upon complete combustion). After that, these high temperature gases were used as heat-carriers, which engaged in heat transfer with coal seams from bottom up, gradually declining the temperature in the furnace cavity. The wall surface was far away from high temperature heat sources and ceaselessly emitted heat to the outside area; therefore, it had the minimum temperature [28]. When different particle sizes of coal entered the SJ furnace, the bulk temperature in the furnace cavity decreased with decreasing particle size, but the overall temperature distribution trends were similar. This was probably because the decrease in the as-fired coal particle size increased the bulk density of coal seams in the furnace, narrowed the gas channels, intensified resistance, and lowered the heat transfer efficiency, and thus decreased the temperature [2, 3, 29, 30].
D = 20 mm was taken as an example to analyze the flow field distribution in the high temperature zones. The partial enlarged drawings of x-direction high-temperature zone section x = 2.75 m, y-directionhigh-temperature zone section y = 1.4 m and z-direction high temperature zone section z = 0 m (D = 20 mm) are shown in Figure 5. The high-temperature zone took the combustion of coal in two opposite feed inlets as a combustion point and regarded the boundary of the two semi-grid walls as a combustion unit for the convenience of study. As shown in Figure 5(a), the high temperature zone of the central combustion unit was approximately “tripod-shaped”, that is the temperatures of the combustion points on the two sides were basically equal (about 50 K) and lower than the temperature of the combustion point in the center. This was probably because the center released less heat and had slightly more combustion gas within a combustion unit [31]. Figure 5(b) shows that the maximum temperatures of the five combustion units were close to each other; however, the area of high-temperature zone continuously expanded from the furnace center to its two sides and diffused combustion zones emerged. This was probably because the high-temperature point at the center of the furnace cavity directly faced the gas outlet of the furnace, and the furnace pressure ascended progressively at small amplitude from the center to the two sides, gradually decreasing the gas flow rate from the center to both the sides [32]. The return coal gas with the assistance of oxygen combusted at a higher rate at the furnace center and was completely combusted in a relatively small zone at an earlier point.As shown in Figure 5(c), the maximum temperature of each combustion point emerged at the symmetric center of the two semi-grid walls, the maximum temperatures of the five combustion units all occurred at the y-direction of the SJ furnace, and the wall surface and combustion unit interface exhibited the minimum temperatures. This was because the feed inlets were in paired layout, the gas mixture began to move upwards, such intense mixing increased the turbulence intensity of updraft and facilitated the combustion when it entered the furnace from two opposite channels and experienced intense mixing at the center of the combustion unit [33]. On section z = 0, it reached its kindling point and received complete combustion. The combustion unit interface and wall surface were far away from the combustion points; therefore, their temperatures were relatively low.
3.2 Temperature distribution characteristics of longitudinal center section at different coal seam heights
Coal seams with the heights z = 0, 0.5, 1, 2, and 4 m were taken as the study objects. The temperature distribution at each height is shown in Figure 6. When the boundary between two of semi-grid walls was taken as the study object, the temperature distribution of the longitudinal center section at different coal seam heights could be roughly divided into three stages with increasing coal seam height. Stage I: At the height z = 0 m, coal seam temperatures presented a symmetric distribution on the whole, and the coal seam temperatures of the five combustion units all showed an “inverted v-shaped” distribution. The maximum temperatures were approximately equal (1,030 K) and located at the symmetric center of two semi-grid walls. The temperatures at the symmetric centers of the grid walls and the wall surface on both sides were relatively low, and the temperature difference between the coal seams at the same height reached the maximum level in this case. Stage II: The wave peaks at the coal seam heights of z = 0.5 and 1 m gradually became gentle with increasing coal seam height, and the high temperature point positions moved towards the furnace center step by step. In addition, high temperature points were reduced into four high temperature wave peaks in a symmetric distribution, and the maximum temperature was close to the symmetric centers of the grid walls on the both sides of the furnace. When z = 1 m, the maximum temperature difference between the high temperaturepoints was approximately 100 K. Stage III: When the coal seam height exceeded z = 2 m, the temperature at the furnace center (except the two sides of furnace close to wall surface) was relatively uniform and gradually declined with rising coal seam. This was because the gas mixture of the coal gas and air began to combust at the height z of approximately 0 m when it was fed into the furnace. During the rise of the extremely hot heat-carriers, the gas at the center was moving at a faster pace than that on both sides. As a result, coal seams presented a symmetric temperature distribution, and the temperature at the center was higher than that on each side in stage I. To be specific, the temperatures at the symmetric centers of grid walls were low, primarily because they were far away from the heat sources and the unsmooth cusp-shaped grid walls blocked the transverse flow of gas [14]. In stage II, the temperature difference between the coal seams presented a gradually narrowing trend in the process of coal seam height gradually increasing to 0.5 m and 1 m. This was because the heat transfer between the coal seams and gas was dominated by radiative heat transfer in the high-temperature zone of the furnace, and higher temperature led to quickerheat transfer, resulting in a relatively significant temperature difference [34]. The reduction in the high temperature points into four high temperature wave peaks in symmetric distribution resulted from the rising high temperature gases converging on four intact grid walls, and the maximum temperatures were close to the symmetric centers of the grid walls on both sides due to the emergence of diffused combustion zones on the two sides. The height z = 1 m was at the centers of the diffused combustion points, creating a temperature difference between the high temperature points to be the maximum. In stage III, the temperature difference at the furnace center was relatively uniform principally because higher coal seams decreased the temperature in the SJ furnace. The heat transfer between relatively low thermal conductivity of coal and gas was dominated by heat transmission, and the rate of heat transfer was relatively balanced [14]. It is also clear that the smaller the distance between the coal seam height and wall surface, the lower the temperature. The wall surface constantly emitted heat, resulting in a relatively low temperature.
3.3 Relationship between the maximum temperature in furnace and particle size
According to the temperature field simulation results of different particle sizes of the as-fired coal, the temperature in the furnace cavity decreased with decreasing particle size. The correspondence between the maximum temperature in the furnace and as-fired coal particle size is shown in Table 3, indicating that the maximum temperature Tmax gradually decreased with decreasing as-fired coal particle size. When particle size was 40 mm, the maximum temperature was 1,148 K; when particle size was 5 mm, the maximum temperature decreased to 921 K.
Table 3 Maximum temperatures of different particle sizes of as-fired coal.
D/mm
|
40
|
35
|
30
|
25
|
20
|
15
|
10
|
5
|
Tmax/K
|
1148
|
1115
|
1092
|
1063
|
1006
|
976
|
950
|
921
|
The relationship between the maximum temperature in the furnace and particle size is shown in Figure 7. Clearly, the maximum temperature in the furnace decreased linearly with decreasing coal particle size, mainly due to the variations in the porosity of coal seams. When the as-fired coal particle size decreased from 40 m to 5 mmthe true density and bulk density of different particle sizes of coal were experimentally measured and substituted into the computational Eq. (1) to calculate the porosity of coal of different particle sizes [35, 36], and the results are listed in Table 4:
where P is the porosity of the coal, ρ0 is the true density of the coal, and ρ is the bulk density of the coal.
As listed in Table 4, the porosity of coal seams in the furnace gradually decreased from 0.395 to 0.263 with decreasing coal particle size, increasing the resistance coefficient of coal seams, which further increased the resistance to the passage of gas, reduced the flow rate of rising gases, quickly combusted the coal and ultimately lowered the maximum temperature[37].
Table 4 True density, bulk density, and porosity of different particle sizes of coal.
D/mm
|
40
|
35
|
30
|
25
|
20
|
15
|
10
|
5
|
/(kg/m3)
|
1227
|
1227
|
1227
|
1227
|
1227
|
1227
|
1227
|
1227
|
(kg/m3)
|
742
|
763.2
|
785.3
|
802.5
|
822.1
|
838
|
862.6
|
904.3
|
P
|
0.395
|
0.378
|
0.36
|
0.346
|
0.33
|
0.317
|
0.297
|
0.263
|
After linearly fitting the relationship between the maximum temperature in the furnace and as-fired coal particle size, the fitting curve showed a relatively high correlation coefficient, i.e., 0.9937. The fitting equation is as follows:
3.4 Relationship between the temperature range percentage in the furnace and particle size
The medium and low temperature carbonization of coal refers to the thermal decomposition of coal into coal gas, coal tar and semi-coke in an air-free environment after being heated in the range 773~1,023 K [1, 38, 39]. The pyrolysis process of coal has been extensively investigated, and carbonization is divided into three stages according to temperature, including normal temperature to 573 K, 573~773 K, and 773~1,023 K. The stage of normal temperature to 573 K mainly provides drying and desorption to remove free water and adsorbed gases. The second stage mainly generates and discharges a large amount of volatiles through decomposition and depolymerization. To be specific, the precipitates amount of coal tar can reach the largest when the temperature is about 723 K [3, 39, 40]. In the range 773~1,023 K, mainly plastic mass experiences polycondensation and curing reactions and generates semi-coke [3, 39, 40]. Figure 8 shows the volume percentage distribution in various temperature ranges when the as-fired coal particle size varied in the range 40~ 5 mm.
When D = 40 mm, the volume percentages of temperature ranges a, b, c and d in the temperature field were 45.21%, 34.01%, 18.68% and 2.1%, respectively. As shown in Figure 11, when the as-fired coal particle size decreased from 40 mm to 5 mm, the percentage of temperature range gradually increased from 45.21% to 62.23% at an increasingly faster pace. Specifically, in the sub-range 10~5 mm, the increasing rate was the highest (~6%) and the area percentage of temperature range exceeded 60%. In the range 40~10 mm, the volume percentage of temperature range b in the temperature field approximately decreased linearly at a rate of 1%, and the volume percentage of temperature range b dropped by approximately 2% when particle size decreased from 10 mm to 5 mm, in which case it reached 25.92%. Temperature range c served as an important reference index of the normal efficient running of SJ low temperature carbonization furnace and is further analyzed as follows. The volume percentage of temperature range d was relatively low and continuously narrowed. It dropped to zero when particle size fell below 20 mm, because the induced variations in the volume percentages of various temperature ranges was the same that gave rise to the relationship between the maximum temperature in the furnace and particle size [41].
Figure 9 shows the variation trend of the volume percentages of the carbonization zone (773~1,023 K) for different particle sizes of the as-fired coal, indicating that the variation trend in the volume percentage of the carbonization zone could be divided into three stages when the as-fired coal particle size decreased from 40 mm to 5 mm. Stage I (40~20 mm): In this stage, the volume percentage of the carbonization zone decreased, but only by a very small amplitude, mainly because the varied particle size was still relatively large and the pressure difference in furnace changed slightly. As a result, the effect on the flow of gas in the furnace was not obvious and combustion was basically unaffected; therefore, the carbonization zone did not experience any obvious variation in the area. In addition, a high-temperature zone higher than 1,023 K emerged at this point, and radiation also exerted a vital effect on the carbonization zone. Stage II (20~10 mm): In this stage, the area percentage of the carbonization zone basically decreased linearly at a rate of approximately 3%, because the pressure difference in the furnace cavity relatively increased and gas flow rate declined to some extent with decreasing particle size, accompanied by weakened combustion reaction, constantly lowered the maximum temperature, and gradually diminished the radiation capacity [41]. Stage III (10~5 mm): This stage saw a significant decrease in the volume percentage of the carbonization zone approximately in the range 4~11.85%. For the small particle size, the porosity of coal seams declined substantially, while the gas flow resistance increased significantly, thus the combustion was incomplete, and blocking might have taken place at 5 mm [41].
3.5 Effect of different particle sizes of coal on the pressure field distribution of furnace’s longitudinal center section
Figure 10 shows the pressure field distribution of y-direction center section for the numerical simulation results of the SJ carbonization furnace when the as-fired coal particle size was set at 40, 35, 30, 25, 20, 15, 10, and 5 mm. As shown in Figure 10, the pressure in the furnace increased with decreasing particle size when different particle sizes coal entered the furnace, but the distribution trends were similar. At the same coal seam height, pressures were basically equal, because the porosity of porous medium declined with decreasing particle size, and the permeation property of gases weakened [41]. Considering the similar trends of pressure distribution in the furnace, D = 20 mm was taken as a typical case to analyze the distribution law of the pressure in the SJ furnace along the coal seam height direction.
Figure 11 shows the relationship between the coal seam heights and the pressure in the furnace when D = 20 mm. It is evident that the pressure in the furnace presented an overall declining trend with increasing coal seam height when the same particle size of coal entered the furnace; however, the pressure showed a small-amplitude increase with increasing height in the vicinity of the coal seam height h = 1.05 m, largely because the coal seam height of 1.05 m was the gas inlet of SJ carbonization furnace, resulting in a relatively high temperature around the inlet. When the coal seam height h reached 5.5 m, the pressure in the SJ furnace dropped quickly, because the coal seam served as the gas outlet on the furnace top after its height h exceeded 5 m, and gas resistance was particularly low. Besides, a negative-pressure device (whose pressure was slightly lower than atmospheric pressure) was used to collect gases at the furnace outlet, resulting in a quick pressure drop and causing the minimum pressure to drop below atmospheric pressure. The pressure in the furnace cavity (except at the outlet of the furnace top) presented an approximately linear decline, primarily because the gas flow rate u across the porous medium zone remained approximately constant, and the pressure drop △P was directly proportional to the gas flow rate u according to the porous medium model [21, 42].
The pressure in the furnace cavity increased with decreasing particle size according to the pressure field simulation results of different particle sizes of the as-fired coal. Table 5 lists the correspondence between the maximum pressure in the furnace and as-fired coal particle size, indicating that the maximum pressure in the furnace Pmax gradually increased with decreasing as-fired coal particle size. The maximum pressure was 703 Pa when the particle size was 40 mm, and it rose up to 4,184 Pa at 5 mm particle size.
Table 5 Maximum pressures of different particle sizes of as-fired coal.
D/mm
|
40
|
35
|
30
|
25
|
20
|
15
|
10
|
5
|
Pmax/Pa
|
703
|
968
|
1200
|
1498
|
1816
|
2308
|
3178
|
4184
|
Figure 12 shows the relationship between the maximum pressure in the furnace and particle size. It is obvious that the maximum pressure in the furnace approximately increased exponentially from 703 Pa to 4,184 Pa when particle size decreased from 40 mm to 5 mm. The exponential fitting of the relationship between the maximum pressure in the furnace and particle size showed a correlation coefficient of 0.9970. The fitting equation is as follows:
3.6 Comparative analysis between simulation data and factory test data
Tables 6 and 7 provide the temperatures and pressures of various factory measuring points. Figure 13 show the data based fitting curves.
Table 6 Temperature statistics of various measuring points.
/mm
|
32.44
|
16.97
|
14.76
|
12.55
|
8.13
|
TZ=1/K
|
842
|
738
|
710
|
682
|
640
|
TZ=2.5/K
|
1003
|
937
|
929
|
910
|
879
|
Tmax/K
|
1114
|
1023
|
1008
|
977
|
926
|
Table 7Pressure statistics of various measuring points.
/mm
|
32.44
|
16.97
|
14.76
|
12.55
|
8.13
|
PZ=2/Pa
|
893
|
2021
|
2274
|
2549
|
3168
|
PZ=4.5/P
|
855
|
1829
|
2043
|
2276
|
2799
|
Pmax/Pa
|
970
|
2225
|
2509
|
2820
|
3519
|
Figure13 provide the polynomial temperature curves at the maximum temperature in the furnace (Tmax) and at the heights of z = 1 and z = 2.5, respectively, and the polynomial pressure curves at the maximum pressure (Pmax) and at the heights of z = 2 m and z = 4.5 m, fitted according to the eight groups of temperature and pressure data measured at the corresponding positions in the furnace in the factory when particle size decreased from 40 mm to 5 mm. As shown in Figure 13(a), the simulation values basically matched with the experimental values. According to the computational data, the maximum temperature error was 6.25% when the average particle size and height were 8.13 and 1 m, respectively. As shown in Figure 13(b), the simulation values also basically matched with the experimental values. The maximum pressure error was 12.8% when the average particle size and height were 32.44 and 4.5 m, respectively according to the computational data.