Análisis de Clase 2 Práctica Educativa Tabla

An excellent review on the effect of pressure on pyrolysis is given by Antal and Grønli (2003) in their review on charcoal production. The main studies mentioned in their review are discussed in the following paragraphs.

Klason (1914) discovered that vacuum impacts pyrolysis. He observed that tar yields under vacuum were greatly enhanced (43.66 % (wt/wt) daf) compared to tar yields at atmospheric pressures under slow heating conditions (1.8 % (wt/wt) daf). At the same time the char yield increased from 19.38 % (wt/wt) daf to 39.44 % (wt/wt) daf, which was associated with changes in the elemental composition of the char and tar (ibid), where the vacuum tar distinguishes itself from the atmospheric tar in the way that

it is more translucent, has a firmer consistency, as well as higher oxygen content. Klason also discovered that the decomposition of tar with increasing pressure to char,

2.6 Effect of Pyrolysis Conditions 2-55

water and light volatiles is an exothermic process; indicating that pressure affects pyrolysis and its products in multiple ways.

The effect of elevated pressure in relation to atmospheric pressure on pyrolysis was already observed as early as 1853 by Violette. Antal and Grønli (2003) comment that his “observations are intriguing, and his experiments remain novel even today” (Antal & Grønli, 2003, p. 1628). One of his novel experiments was the pyrolysis of wood in sealed glass tubes. Some of them broke due to the high pressure caused by the evolving pyrolysis gases, but others were analysed and had a charcoal yield and carbon content of 79.1 % (wt/wt) and 77.1 % respectively (ibid). Antal and Grønli note that the

magnitude of these values is implausibly high but the experiments are seminal.

A more subtle increase in charcoal yield with rising pressure was reported by Palmer (1914). He investigated hardwood distillation in an autoclave at 0, 60 and 120/150 psi (0, 0.4 and 0.8/1.0 MPa) pressure. The pressure was set by controlling the gas outlet (ibid), that is, the pressure was caused by the evolving pyrolysis gases, which

is from here on referred to as autogenous pressure. The final temperature in Palmers experiments was limited to about 335 °C. He stated that with increasing pressure the point of destructive distillation occurs at higher temperatures. Palmer also discovered that the yield of charcoal from birch chips increased from 36.61 to 40.48 % (wt/wt) when pressure was raised from 0 psi (0 MPa) to 120 psi (0.8 MPa). Balancing this, the yields of tar, acetic acid and pyroligneous liquor decreased whereas the yield of gas increased. Palmer also noted that in a moderate pressure range the yield of alcohol increases slightly with elevated pressure. In fact, Palmer pointed out that above 60 psi (0.4 MPa) pressure has less effect on the product yields. Also he found that pressure has the most significant effect on tar. Palmer attempted to carry out experiments at 450 psi (3.1 MPa) but the experiments had to be stopped, because of the high intensity of the exothermic reaction, which supports the finding of Klason (1914) that pressure affects the heat of pyrolysis.

The effect of autogenous pressure was also studied by Frolich, Spalding, and Bacon (1928), who applied a maximum pressure of 300 atm (30.4 MPa) and a maximum HTT of 500 °C in their experiments. However, no direct comparison

between the char yields at different pressures was possible, because the temperature was changed between runs. Only at a temperature of 372 °C was a comparison possible between a run at atmospheric pressure and 90 atm (9.1 MPa). In this run the organic residue increased with pressure from 42 to 44 g on a basis of 100 g of dry wood, which

equates to an increase in char yield of about 5 %, which is less than that found by Palmer (1914), mentioned above. However, Frolich et al. frequently tapped the liquid products in this run, which allowed the release of some of the gases, and could have had an impact on the formation of char. Despite this, the overall char yields of Frolich et al. were higher, 42 % (wt/wt), than those observed by Palmer, 36.6 to 38.9 % (wt/wt), at atmospheric pressure, and could be a result of the higher pressure or due to compositional differences (see 2.5). Frolich et al. also studied the effect of hydrogen pressure under gas flow rates but their data did not allow the determination of a relationship between pressure and char yield. Antal and Grønli (2003) argue that it is possible that the whole extent of pressure was unnoticed by Frolich et al. due to the application of high gas flow rates and low HTT’s. Nevertheless, Frolich et al. suggest

that pressure inhibits tar volatilisation, which thus forms char and gases.

The effect of absolute pressure on cellulose pyrolysis was studied by Mok and Antal (1983a, 1983b). In their studies the pressure was regulated by a carrier gas rather than by autogenous pressure generation. By using steam as the carrier gas Mok and Antal (1983a) discovered that the yield of char and CO2 increased with pressure

whereas the amount of CO and hydrocarbons decreased. For measuring the heat of pyrolysis Mok and Antal (1983b) applied argon as carrier gas and found that the heat of pyrolysis declines (becomes less endothermic) with increasing pressure. To demonstrate that the vapour-phase residence is a major factor Mok and Antal (1983b) varied the volumetric gas flow rates. In the case of the lowest flow rate applied, they recorded an increase in char yield from about 12 % (wt/wt) at 1 atm (0.1 MPa) to about 18.5 % (wt/wt) at 10 atm (1 MPa) pressure respectively. The yield increased even further to 22 % (wt/wt) at a pressure of 25 atm (2.5 MPa). While these increases are significantly higher than found by Palmer (1914) and Frolich et al. (1928), these values cannot be compared directly, as the effect of pressure on pure cellulose pyrolysis is expected to be different from wood due to the compositional dissimilarities. Much more radical (at the time) is the conclusion of Antal and Varhegyi (1995), who argue that the formation of char from cellulose is only a result of secondary vapour-solid interactions ranging from 0 to 40 % (wt/wt) for cellulose char. Returning to the work of Mok and Antal (1983b), when volumetric carrier gas flowrates were higher, char yield was lowered and heat of pyrolysis was raised (more endothermic). The heat of pyrolysis became even more endothermic under vacuum (ibid).

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The intricacies of the relationship between pressure and product composition were investigated in more detail by Mok et al. (1992), who carried out experiments in sealed reactors. The autogenous pressures were changed by varying the sample loading of cellulose, and ranged from 3 to 14 MPa at 450 °C. They discovered that with increasing autogenous pressure the reaction becomes more exothermic and the yield of char increased, whereas the onset temperature of the reaction decreased. The latter point contradicts Palmer (1914) who found that destructive distillation occurs at higher temperatures when pressure is increased. A possible explanation for this is that Palmer based the destructive distillation point on the occurrence of tar formation—percent total distillate as a function of temperature—whereas Mok et al. obtained the onset temperature from the analysis of the Differential Scanning Calorimetry, DSC, curves. Thus, in Palmers data a lag is included between the collection of the tar product and the starting point of the actual reaction. Furthermore, the DSC data includes any reaction, not only tar formation reactions. As Mok et al. pointed out, the effect of pressure is evident when the yields obtained in the sealed reactors, 36 to 40 % (wt/wt), are compared to the yield of a cellulose sample, 22 % (wt/wt), that was pyrolysed in an open crucible, exposed to argon, at 1 atm (0.1 MPa) pressure. When the argon sweep gas flowrate was increased the yield of char even decreased further to 6 % (wt/wt). To see what role the vapour-phase concentration and absolute system pressure play, Mok et al. (1992) then carried out an experiment, where they added dry ice to increase the total pressure independently of the concentration of the released volatiles apart from CO2.

They found that the increased pressure, due to the addition of CO2, at comparable mass

loadings caused a decrease in the exothermic reaction heat, and the charcoal yield from cellulose was reduced by circa 10 %. However, this yield is still substantially higher than under atmospheric conditions. They concluded that the vapour-phase concentration has a stronger impact on the yield of char and the heat of reaction than the absolute pressure. Mok et al. (1992) also carried out experiments for hemicellulose and lignin, which are briefly mentioned in 2.6.5. Lastly, they further performed sealed reactor experiments for six woody and three herbaceous biomass samples. All samples produced very high charcoal yields close to theoretical yields, 40 % (wt/wt) for cellulose and 48 % (wt/wt) for Eucalyptus gummifera (cf. the proposed theoretical

yields in Table 2-1).

Antal et al. (1996) tried to investigate at what pressures high yields can be obtained. They discovered that high char yields of 40.5 % (wt/wt) from Macadamia nut

shells are already achievable at 0.4 MPa in a stagnant gas environment (autogenous pressure). Their results also showed that the yield increases further with pressure to 51 % (wt/wt) at a pressure of 3.3 MPa. However, they mention that this increase is partly a result of the higher volatile matter content of their high pressure chars and would have been less if the sample was heated longer. Antal et al. further note that the pressure enhances the heat transfer in the reactor leading to a more constant product, while decreasing the heating time.

The findings of Antal et al. (1996) were confirmed by Antal et al. (2000), who discovered that near theoretical fixed carbon yields can be obtained by pyrolysis in a stagnant gas environment at elevated pressure (autogenous pressure). They also showed that their method reduced the commonly experienced pyrolysis time of oak wood in Missouri kilns from about 10,000 min (у7 days) to approximately 70 min, which can provide economic incentives as discussed by Antal and Grønli (2003).

This work has so far only looked at the effect of pressure on the yield of char and not on its quality as measured by its properties as defined by the International Biochar Initiative (2014) (IBI) or European Biochar Foundation (2012) (EBF). Unfortunately, many studies did not report fixed carbon contents or other char properties due to the small amount of sample available for analysis, for example in the study of Mok et al. (1992). In the low temperature experiments of Palmer (1914) and Frolich et al. (1928) it is possible that the produced char was of inferior quality than the one produced by Mok et al. (1992). The properties of char will vary not only with degree of pyrolysis, as outlined in 2.3.1 and 2.6.2, but also expected with the mechanism by which it is formed; that is, primary or secondary char (see 2.6.1). In the work of Violette (1853, as cited in Antal & Grønli, 2003) the char formed under high pressure from wood in sealed glass tubes was reported to be “shiny and brittle and had undergone fusion similar to coking coal” (p. 1628). Mok and Antal (1983b) report the secondary char from cellulose pyrolysis as being “soft and fluffy” (p. 182), whereas they describe the primary char as resembling the feed. Mok et al. (1992) used Fourier transform infrared spectroscopy, FTIR, to analyse the formation of charcoal from cellulose in sealed and open reactors. They compared spectra obtained at different stages of carbonisation and found hardly any difference between runs in sealed and open reactors. Chemical changes they analysed were dehydration, carbonyl group formation and elimination, pyranose ring opening, decomposition of aliphatic char units and formation of aromatic char units. They cross-checked their results with literature

2.6 Effect of Pyrolysis Conditions 2-59

data on atmospheric cellulose pyrolysis and found it to be consistent. Thus, they concluded that the chars produced in sealed and open reactors are chemically identical. Mok et al. (1992) also state that their spectra were similar to the one obtained by Pakdel et al. (1989, as cited in Mok et al., 1992) for the char of Populus deltoids produced by

vacuum pyrolysis, apart from the fact that the carbonyl content seemed to be higher. This again indicates that there is little difference chemically between primary and secondary char from cellulose pyrolysis. These findings are contrary to the physically observed difference by Mok and Antal (1983b). When comparing the appearance of char formed at atmospheric pressure to that formed at elevated pressure, either autogenously or with a regulated hydrogen environment, Frolich et al. (1928) found that the physical appearance varied only as a function of the pressure and not by the mode of pressure application.

Performing a more morphology detailed char analysis Cetin et al. (2004) discovered that chars produced at increasing pressures have bigger cavities, thinner cell walls, slightly decreased surface areas, are generally bigger, and have a perforated surface. They also report that the effect of disappearing cell wall structures with high heating rates, as discussed in 2.6.1, is enhanced at elevated pressures. Cetin et al. mention that these effects were less pronounced in bagasse compared to the soft and hardwood species indicating some compositional effects. They observed that particles pyrolysed at atmospheric pressure at high heating rates undergo the following steps: (a) swell, (b) melt, (c) form a droplet, and (d) rupture due to the evolving volatiles. In contrast, at high pressure they report that step (a) is not evident and the rupturing of step (d) is missing. This is more likely to be as the high system pressure means the gradient across the particles is reduced. This results in the formation of bubbles, which explain the larger cavities formed under pressure (Cetin et al., 2004). This reveals that pressure increases the resistance or reduces the gradient for mass transfer and so becomes a major factor in determining the physical properties of the char. This is supported by the findings of L. Wang et al. (2011) and L. Wang, Skreiberg, Grønli, Specht, and Antal (2013).

Two mechanisms have to be distinguished when it comes to the effect of pressure. There is; a), autogenous pressure, which produces long vapour-phase residence times and high concentrations of the primary volatile pyrolysis products while concurrently providing intimate contact with the pyrolysing solid; and b), absolute pressure, which produces similarly long residence times but at lower vapour pressure of

tarry compounds due to the presence of carrier gas (Antal & Grønli, 2003). Thus, both mechanisms result in increased secondary reactions (see Figure 2-1) with associated increased char yield and reaction exothermicity. Further, Antal and Grønli (2003) outline in their review that under both conditions, a) and b), char can act as a catalyst (see also 2.6.2) due to the close contact between volatiles and the charring solid, water, vapour or chemisorbed moisture, can have an autocatalytic effect (2.6.6), as well as the presence of early pyrolysis products, formic acid and acetic acid, in combination with water can change the reaction chemistry of the holocellulose to acid catalysed hydrolysis, which creates a unique high pressure pyrolysis chemistry. Autogenous pressure will produce higher yields than achieving the same absolute pressure with a carrier gas, because the tarry vapour and catalyst molecules are in closer proximity.

2.6.5 Vapour-phase Residence Time, Particle Size and Sample