7.5.1
Effect of particle size
The effect of particle size on the extent of pyrolysis was studied for sycamore particles and the results are listed in Table 7-1. As can be seen in Table 7-1, the solid pyrolysed increases with increasing particle size at a constant energy input. Table 7-1: Effect of the particle size on the degree of pyrolysis at 3.5 kJ·g-1 specific energy
Particle size range Gas velocity (m·s-1) Solid pyrolysed (%) 0.6 – 0.85 mm 0.38 42.15 ± 2.21 0.85 – 1.18 mm 0.38 53.61 ± 0.62 1.18 – 1.70 mm 0.38 59.08 ± 0.68 1.70 – 2.36 mm 0.59 60.13 ± 0.69
Increasing the particle size decreases the gas-particle contact area leading to a reduction in the heat loss to the fluidising gas. This results in an increase in the temperature at the centre of the particles allowing for more solid to be pyrolysed. The relationship between the particle size and the solid pyrolysed obtained from the batch experiments is in line with the results from the numerical model discussed in Section 6.3. It was shown through numerical modelling that increasing the particle size of the biomass material leads to reduction in the power density needed to reach the pyrolysis temperature, meaning that increasing the particle size at a given
166 power density would result in an increase in the particle temperature and therefore an increase the degree of pyrolysis.
As can be seen in Table 7-1, there is no significant increase in the solid pyrolysed when the particle size was increased from 1.18 - 1.70 mm to 1.70 - 2.36 mm compared to the changes between the other particle size groups. This can be attributed to the increase in the gas velocity for the 1.70 – 2.36 mm particle size group which required higher gas velocity than the other groups to be pyrolysed without thermal runaway.
The relationship between the particle size and the product yield in conventionally heated fluidised bed systems is different. Previously, Shen et al. (2009) studied the effect of the particle size on the pyrolysis of wood particles in a fluidised bed reactor heated in an electric furnace at 500 oC using preheated nitrogen as the fluidising gas. They found that the bio-oil yield decreased when the particle size was increased from 0.3 to 1.5 mm. This drop in the yield with the particle size was regarded to the reduction in the heat transfer rate for the larger particles. A similar relationship between oil yield and particle size was reported by other authors (Choi et al., 2012; Montoya et al., 2015).
This difference in the effect of particle size on the product yield between the microwave and conventionally heated fluidised bed systems is mainly because of the difference in the direction of heat transfer. In conventionally heated systems, the heat is transferred from the fluidising gas to the particles. The extent of pyrolysis in this case is improved by reducing the particle size as it increases the specific surface area and, therefore, increases the heat transfer rate to the particles. While in the microwave heated system, where cold fluidising gas is used, the heat is transferred from the biomass particles to the gas. Using larger particles is, therefore, favoured as it reduces the heat losses to the fluidising gas resulting in an improvement in the extent of pyrolysis at a cetrain energy input.
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7.5.2
Effect of gas velocity
Figure 7-6 shows the effect of the gas velocity on the degree of pyrolysis for 1.18 - 1.70 mm sycamore particles at 3.5 kJ·g-1 specific energy. It can be seen that increasing the gas velocity from 0.38 m·s-1 to 0.64 m·s-1 results in a limited drop in the solid pyrolysed.
Figure 7-6: Effect of the fluidising gas velocity on the solid pyrolysed for 1.18 – 1.70 mm sycamore particles at 3.5 kJ·g-1 specific energy.
The reduction in the solid pyrolysed with the gas velocity was expected because the fluidising gas is fed at room temperature, and increasing the gas velocity increases the heat losses to the fluidising gas. However, it can be seen that only 12 % reduction in the solid pyrolysed for about 68 % increases in the gas velocity. This limited reduction in the solid pyrolysed compared to the large increase in the gas velocity provides flexibility for using the gas velocity for controlling the other processing parameters including the bed temperature and the solids residence time in the case of continuous processing.
In conventionally heated fluidised bed systems, where the gas is preheated to provide all or part of the energy required for pyrolysis, the relationship between the gas velocity and the product yield is different to that shown in Figure 7-6. Choi et al. (2012) showed that the bio-oil yield in a fluidised bed reactor heated in an electric furnace increases when the preheated gas velocity is increased up to a certain value
0 10 20 30 40 50 60 70 80 0.3 0.4 0.5 0.6 0.7 So lid p yr o ly sed ( %) Gas velocity (m·s-1)
168 beyond which the oil yield starts to decrease. The initial increase in the product yield with the gas velocity was explained by the improvement in the heat transfer rate due to the better mixing provided by the faster-moving bubbles. The following reduction in the bio-oil yield at higher gas velocities was attributed to the formation of large bubbles (slugging) leading to poorer heat transfer. The explanation provided by Choi et al. (2012) for the deterioration in the heat transfer at high gas velocities could be used understand the limited drop in the solid pyrolysed with increasing the gas velocity in the present study especially at higher gas velocities as can be seen in Figure 7-6.
7.5.3
Effect of energy input
The effect of the specific energy input on the degree of pyrolysis was studied for various particle size groups of sycamore. Figure 7-7 shows that the solid pyrolysed increases steadily with the specific energy up to nearly 70 % depending on the particle size.
Figure 7-7: Increase in the degree of pyrolysis with the specific energy for sycamore of different particle size at 5 kW incident power. The gas velocity is indicated between brackets. 0 10 20 30 40 50 60 70 80 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 So lid p yr o ly sed ( %) Specific energy (kJ·g-1) 0.85 - 1.18 mm (0.38m/s) 1.18 - 1.70 mm (0.38m/s) 1.70 - 2.36 mm (0.59m/s)
169 The extent to which the solid pyrolysed increases with the specific energy is restricted by the drop in the absorber power as shown in Figure 7-5 which was explained by the reduction in the loss factor of the particle and the increase in the bed porosity.
Figure 7-7 shows that around 3.5 to 4.2 kJ·g-1 energy input is needed to achieve 60 to 70 % solid pyrolysis. Previously, Robinson et al. (2015) showed that the energy required to reach the same degree of pyrolysis in a microwave heated fixed bed reactor is around 2.2 to 2.5 kJ·g-1.
The high specific energy in the fluidised bed system compared to the fixed bed system is mainly because of the heat loss to the fluidising gas, which is fed at room temperature. It was shown in Section 6.3 that the enthalpy for pyrolysis, which excludes any heat losses, of sycamore at 400 oC is about 0.88 kJ·g-1 (Table 5-4). However, it was shown through the numerical modelling in the same section that the specific energy required to pyrolysis 600 µm sycamore particles in the fluidised bed system can range from 1.07 kJ·g-1 to 4.85 kJ·g-1 (Table 5-5) depending on the power density. The high specific energy in the fluidised bed system compared to the enthalpy for pyrolysis was attributed to the heat losses to the fluidising gas.