4.1 Investigation of the effect of particle size variation
The temperature distribution in the coal bed for the reactions of coals A, B, and C can be seen in Figure 7. This shows the pattern of temperature profile recorded by the thermocouples. The maximum temperature reached by each coal bed was 624oC, 582oC, and 569oC, for A, B, and C respectively. For this parameter, coal A had the highest bed temperature, while the lowest occurred with coal C. This indicates that the bed with smaller particles obtained the higher temperature. The different particle size in the coal bed caused the difference in porosity for each coal bed. The smaller particle forms less porosity than the bigger particle size. The less porosity causes the heat transfer to take place more through the particle (conduction) than the porous material (convection). With the property of the heat capacity higher than gas, the particle reserves more heat than the gas. As a result, the coal bed with less porosity has a higher temperature than the bed with higher porosity.
Figure 7. Temperature profile for each channel in bed of coals A, B and C
From Figure 7, it can also be seen that the temperature gradient (dT/dt) of the bed with smaller particle size was higher – as can be seen in each channel of temperature measurement. The bed with coal A achieves the maximum temperature at channels 1, 2 and 5 earlier than the bed with coals B and C; and the bed with coal B was earlier than the bed with coal C. Another indicator was the heat propagation rate, which can be identified by measuring the time interval of maximum temperature (peak temperature) between the two sensor temperature channels along the gas flow. One sample case was the time interval of heat propagation from channel 1 to 2, at each coal bed. Figure 7 shows that the time needed for the heat to propagate (reach peak temperature) from channel 1 to 2 was ~4600s, ~4700s, and 8300s, for coal A, B and C, respectively. This indicates that heat propagation was faster in the bed with a smaller size of coal particle. Thus, this clarifies that the porosity has an important role in heat propagation in the coal block.
Figure 8.The reaction propagation over a certain time period for coals A, B, and C
Visual observation can be used to evaluate and compare the gasification process under different operating conditions. Figure 8 illustrates the patterns of the coal particles packed bed at different times over the gasification procedures. The captures were taken from the top of the reactor or particle bed. This figure shows that the reaction process started with the same condition at minute zero. Over this time, the reaction fronts propagate and were indicated by the ash formation (white colour). The ash zone gets wider over the time of reactions in line with the gas flow direction. The final length of ash formation was compared for each coal after 180minutes. The results show that the distance was approximately 5cm, 4.5cm and 4cm for the bed with coal A, B, and C, respectively. This indicates that the smaller particle size has a longer distance from the inlet side of the coal bed. Therefore, the picture presented for observing the reactions’ propagation was the surface area of the coal bed. This area has direct contact with the transparent lid and there was a gap between the coal bed surface and the lid. This notice was important in order to develop an understanding about the process observation of coal reactions.
The composition of gas products is an important index for the gasification process. As introduced in Section 2, four gas sensors were used to analysis CO2, CO, CH4 and O2 in the exhaust during the experiments. The results of gas CO2, CO and CH4 from the gasification of coals A, B, and C are reported in Figure 9. The experiments were performed for approximately 10,000s (~180 minutes) at (temperature) and the mass flowrate of the injected air was increased gradually from 2slpm to 3.5slpm.
Figure 9(a) illustrate the concentration of CO2 in the exhaust gas during the gasification process of coal A, B and C, respectively. It can be clearly seen that, the different coal sizes has the amount and the trend for CO2 formation are almost the same during the process. The CO2 concentrations increase from the beginning when the reactions start. The growth lasts about for 30 mins and then the concentrations become stable, which indicates the stability of the reactions.
The formation of CO in the gasification procedure is shown in Figure 9(b). Similar to that of CO2, at the initial stage, they increase and then remain nearly constant. However, the differences among coal A, B and C are quite significant. It indicates that the concentration of CO is the lowest during the gasification of the coal packed bed formed by the smallest particle size A generates. The results of coal B and C are quite close, but the larger size C has a greater value.
The gas production of CH4 reported in Figure 9(c) has a different changing trend compared with that of CO2 and CO. It is shown that, they initially increase and then decrease, after sometimes they finally dropped. This indicates an unstable supply of element to support of CH4 formation. The obtained results had a similar trend with the conditions of CH4 formation in the case of a single particle model [24]. However, Figure 9 shows that the gas products (CO2, CO and CH4) obtained were higher with the bigger size of coal particle. These results need to be clarified to develop a strong understanding with the results shown in Figure 7 and Figure 8. It is provided after all measurement data observed.
Figure 9. Gas products of coal bed reactions for (a) CO2, (b) CO, and (c) CH4
Figure 10. Excess oxygen in gas products
The excess oxygen measured by the oxygen sensor is used to find the correlation between the results of temperature and gas products. The result is shown in Figure 10. It can be found that, the concentration of oxygen in the exhaust gas was slightly different from each test. The amount of oxygen left with the most after the air went through and reacted with the packed bed formed by the smallest size coal A. This is consistent with the findings in Ref [16] where says the bed reactions with smaller coal size have more excess oxygen. This indicates that less oxygen reacts with charcoal, and therefore fewer products of CO2 and CO occurred in that case.
The packed bed piling with coal particles has different porosity when the particle size varies. The smaller the particle size is, the smaller the porosity of packed bed is. In this study, the air flows through the porous packed bed to reacts with the coal surface and then produce the gas products. Therefore, the coal packed bed that has a smaller porosity where the air has less access to have the chemical reactions results in less gas products. The coal packed bed with a bigger porosity provides more space for the char and oxygen reactions to generate more products. This confirms the results obtained in Figure 9 where there are the least gas products CO and CO2 and most excess oxygen in coal A experiment.
This looks slightly contradictory to the results explained in Figure 8 and Figure 9 about the effects of particle size on the reaction rate. It should be pointed out that the main factor causing the reaction propagation is the interaction between the coal and air. In theory, there are more air potentially reacts with coal if the porosity of the coal packed bed is greater. In the case taken from Figure 8, the smaller particles exist on the surface of the coal bed and there was a gap between the bed and the transparent lid. This gap possibly provides more air on the surface, and therefore the heat propagation was faster in the smaller coal particle on this case.
4.2 Investigation of the effect of temperature variation
An investigation of the effect of temperature on the coal particle reactions was performed in the modelling and now the study continues through the experimental test. The aim is to identify the reaction behaviour by developing an understandable correlation between the modelling and the experiment. The experiments were conducted at three conditions: (1) external heater off, (2) external heater set at 200oC, (3) external heater set at 350oC. The mass flow rate of the injected air was 2slpm. The test was conducted for 4000s or about 60minutes and the capture was taken every 15 minutes.
Figure 11. Reaction front propagation of surface coal packed bed for coal A
Figure 11 shows the reaction front propagation identified with the ash products for coal A packed bed reactions. The initial results were shown at minute zero and, over time, the length of ash formed by the reaction got longer. After 60 minutes, the distance between the inlet bed and the boundary of coal and ash was measured. The maximum distances obtained were 2cm, 2.8cm, and 3.2cm, for the condition of the heater at off, 200oC, and 350oC, respectively, as shown in the figure. The result indicates that a particle bed with a higher temperature has a longer distance or a faster rate of reaction propagation. This is because with the same boundary condition the charcoal at higher reactor temperature will achieve their ignition temperature faster. The ignited charcoal produces heat and transfer to another spot, therefore the propagation of reaction front occurred faster.
The repetition scheme was conducted on the bed with coal B and C to confirm the results obtained. The same procedures, and boundary conditions as in Table 4 were applied. The pictures were captured on the test performance of coal A and B at the same time intervals under various temperatures, and these can be seen in Figure 12 and Figure 13, respectively.
Figure 12 presents the reaction front propagation presented with ash products on the bed with coal B. It has a similar trend to that of coal A. They were initiated at time zero and over time the length of ash formed by the reaction got longer. After 60 minutes, the distance between the inlet bed and the boundary of reaction front was measured. It showed that the maximum distances were 1.9cm, 2.6cm, and 2.8cm, for the temperature heater at off, 200oC, and 275oC, respectively.
Figure 12. Reaction front propagation of surface coal packed bed for coal B
The reaction front propagation on the bed with coal C is reported on Figure 13. The test performed, and the bed reactor was captured every 15 minutes. After 60 minutes, it showed that the maximum distances were 1.8cm, 2.3cm, and 2.8cm, for the temperature of external heater at off, 200oC, and 350oC, respectively.
Figure 13. Reaction propagation of surface coal packed bed for coal C
Figures 11-13 indicate that on the parameter of reaction front propagation, the coal A, B, and C have the same trend. The particle bed with the higher temperature has a longer distance of reaction front propagation. However, further observation can be conducted to identify the combination between coal particle size and temperature effect. At the same level of temperature heater set, for example at heater off, the length of reaction front propagation for coal A, B, and C, were 2cm, 1.9cm, and 1.8cm, respectively. And, when temperature heater set up to 275oC, the length of reaction front for coal A, B, and C, were 2.8cm, 2.6cm, and 2.3cm, respectively. All results identify that coal A had a maximum length of reaction front propagation compared with the results of coal B and C, at the same level of temperature heater. And, coal B had greater of reaction front length than coal C. It again affirms of the effects coal particle size as described in section 4.1.
The coal reaction behaviour was also investigated through the monitoring of gas products. The results of the measurement of gaseous CO2, CO and CH4 in the exhaust can be seen in Figure 14. This figure shows the concentrations of these gas products during the coal C packed bed reactions at various heater temperatures. The test was performed at five different temperature levels in order to identify the effects. For gas products of CO2 and CO, they had a similar trend. Initially they increase then become stable at some point, while CH4 had initially increased and then dropped. The gas CH4 dropped possibly caused by the lack supply of hydrogen element. However, all gas products indicated have more gas products at higher reactor temperatures.
More tests were conducted for coals A and B, but only at three temperature levels to confirm obtained results. The results for gas CO2 and CO can be seen in Figure 15 and Figure 16 for coals A and B, respectively.
Figure 14. Gas products of coal C in various temperature (a) CO2, (b) CO, and (d) CH4.
Figure 15. Gas products of coal A at various temperatures (a) CO2, and (b) CO.
Figure 15 and Figure 16 show a similar trend, during which they initially increase and stabilise after a certain period of time. The gas products’ level was higher for the coal reactions at a higher temperature. Again, these results confirm the behaviour that the higher temperature could affect the coal reaction for producing more gas products.
However, the results of gas production in the experiments show a similar trend to the modelling in ref-[7] simulation performance in various temperature. This was a good indication for an initial development of a coal particle model for gasification reactions.
Figure 16. Gas products of coal B at various temperatures (a) CO2, and (b) CO.