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This section discusses the most relevant theory on the pyrolysis process and the process conditions with special attention to design implications. Therefore the focus is on fast pyrolysis (FP) but the principles can be applied to any of the pyrolysis technologies. This theory will clarify why different pyrolysis conditions lead to different product yields and quantities.
2.4.1 Temperature
Temperature can be said to be the most dominant process variable with regard to pyrolysis product yields. For most types of woody biomass, the liquid yields in FP are optimized in the temperature range 500- 520°C (Bridgwater et al., 1999). If the reaction temperature is too low, char formation increases. At lower temperatures only certain lignocellulosic components react. Figure 6 shows a typical temperature-yield
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curve for pyrolysis of wood (Bridgwater et al., 1999). Similar results have been published (Gerdes et al., 1999; and Asdullah et al., 2007). Clearly the liquid yield is optimized around 500°C, which favours the depolymerisation reaction described in Figure 4. If the temperature is increased further, the liquid yield will decrease as a result of secondary reactions.
Figure 6: Typical products from FP of wood (Bridgwater et al., 1999)
2.4.2 Heating rate
A higher heating rate produces a higher liquid product yield (Bridgewater et al., 1999). This is can be seen from flash processes which are optimized for liquid production and uses high heating rates, up to 104 C/s (Horne et al., 1996). Bahng et al. (2010) differentiated between fast and flash pyrolysis at their respective heating rates of 200°C/s and >1000°C/s, which is dependent on their respective particle size, <2mm and <200μm (Van de Velden et al., 2010). Producing powdered biomass <200μm for pyrolysis is expensive and therefore unrealistic to run at large scale.
It is difficult to control or accurately measure the heating rate of FP. Instead the heating rate is simply maximized for FP. For slow pyrolysis heating rates are much lower and are mostly operated between 10-
0 10 20 30 40 50 60 70 450 500 550 600 Y ie ld , w t% o n d ry f ee d Reaction temperature ( C) Organic liquid Gas Water Char
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50 °C/min. When higher heating rates are used, the emphasis will typically shift to that of oil or vapour yield, and not char production. A slower heating rate will cause an increase in char yield, and higher temperature will reduce the yield but increase the Higher Heating Value (HHV). Heating rate had a less significant effect on the BET surface area of the chars, than hold time or temperature (Lua et al., 2006).
Studies on heating rates inside FP reactors have not been reported. This is because the heating rate and flux are dependent on local condition inside a continuously fluidized-bed reactor. Heating rate effects have only been studied on batch and fixed bed scale where the heating rate is controllable. Thermogravimetric Analysis (TGA) equipment is typically used for these studies. From these test it was concluded that a higher heating rate produces an increased oil yield (Garcia-Perez et al., 2002). Typical TGA experiments will study low heating rates of up to 50°C/min. However, it is not clear how experiments under controlled heating conditions with small samples can be translated to larger scale continuous reactors with high heating rates (Kersten et al., 2005). Modelling devolatilization kinetics might be the most accurate method for understanding the relation.
2.4.3 Feed-particle size
The feed particle size is determined by the desired heat transfer rate to the particle. The thermal conductivity of biomass is very low: 0.1 W/mK along the grain and 0.05 W/mK across the grain (Bridgwater et al., 1999). Therefore if the particles are too large, char formation will increase because of slow heating of the core, and secondary reactions become increasingly significant (Scott et al., 1984). Particle size therefore has a direct affect on heat transfer. Scott et al. (1982) found that particle sizes smaller that 2 mm do not significantly affect FP product yields. Van de Velden et al. (2010) modelled heat transfer in small particles and found that thermal gradients only become insignificant for particles smaller than 200 μm. The generally accepted particle size for fast pyrolysis is 2 mm or smaller according to Bridgwater et al. (1999). The gas velocity in fluidized-bed reactors is limited to the sand blow-out velocity and maximum particle size capable of being fed through the feeder.
2.4.4 Vapour residence time and secondary reactions
The vapour residence time is defined as the average time a molecule spends inside the reactor, and is a function of reactor volume and sweep gas flow rate (Equation 1).
16 ] / [ ] [ 3 3 s m Q m V =
τ
Equation 1 Scott el al. (1999) measured the effect of vapour residence time on liquid yield. An increased residence time caused a rapid decrease in oil yield. It was concluded that the decrease is due to secondary cracking reactions, which reduce specific chemicals and overall liquid yield. At lower temperatures (lower than 400°C), secondary condensation occurs, which lowers the molecular weight of the liquid product. In essence the vapour residence time should be short, less than 2 seconds secondary reactions (Yaman et al., 2004; Bridgwater et al., 1999). It was also reported that the amount of char in the reactor had a significant effect on the rate of the secondary reaction. The composition of oil is also affected by the residence time. A mechanism was proposed by Antal et al. (1995) that suggests that primary tar can be rapidly converted into gasses and refractory tar (less reactive), after which the two tars form a single solution upon condensation. Ash, and char components carried over from the reactor acts as a catalyst for these secondary reactions, which is also unfavourable (Das et al., 2004). A summary of the process conditions is given in Table 6.Table 6: Summary of process conditions, effects and modelling
Parameter Condition Optimal condition for fast pyrolysis
Reaction temperature 500 – 520°C Constant Vapour residence time < 2s Shorter is better
Secondary cracking Avoid Bad for product quality and yield Heat transfer rate 200°C/s High as possible to increase liquid yield. Particle size Typically <2mm Large particles limit heat transfer, feeding and
fluidization. Kinetic modelling Batch wise for
low hearing rates
Difficult to relate to fast pyrolysis with high heating rate and large sample sizes.