Análisis de Clase 4 práctica educativa Tabla

Figure 2-19. Commonly applied pyrolysis mechanism based on Shafizadeh and Chin (1977) and modified by Thurner and Mann (1981) and Di Blasi (1996a). Char is an unspecific term referring to charcoal, coke or soot. The subscript on the pyrolysis products refers to primary (1) and secondary (2) reactions respectively. The primary reactions, depicted in black, are representative of a one-stage semi- global model, and the primary and secondary reactions combined represent a two-stage-semi-global model. k1to k5are reaction rates.

In Figure 2-19 the primary reactions, illustrated in black, represent a one-stage semi- global model, and the primary and secondary reactions combined form a two-stage semi-global model respectively. Both the one-stage semi-global model and the two- stage semi-global model have been applied in the literature for modelling pyrolysis successfully (for instance Ratte et al. (2009) and Fantozzi et al. (2007) respectively). The advantage of semi-global models is that they can be coupled with transport phenomena (Di Blasi, 1996a; Ratte et al., 2009; White et al., 2011). The most preferred is the two-stage semi-global model as it is able to account for the important effect of vapour-phase residence time (see 2.6). The disadvantage of the semi-global models is that the kinetic data cannot be extrapolated to other conditions outside their experimental ranges (Nunn, Howard, Longwell, & Peters, 1985; White et al., 2011). Similarly to the one-step global model, the semi-global model can be turned into a multi-component model, in particular the one-stage semi-global model.

The third major category formmultiple-step models, which aim is to accurately

model pyrolysis under varying process conditions (White et al., 2011). A simple multi- step model is represented by the scheme of Bradbury, Sakai, and Shafizadeh (1979) for cellulose (also known as the Broido-Shafizadeh model (A. L. Brown, Dayton, & Daily, 2001)), which has been extended to include tar cracking in Figure 2-20 (Di Blasi, 2008).

Figure 2-20. Reaction scheme proposed by Bradbury et al. (1979) for cellulose with added tar cracking step by Di Blasi (2008). k is the reaction rate constant. Adapted from Di Blasi (2008).

A more detailed model was proposed by Diebold (1994), which is illustrated in Figure 2-21.

Figure 2-21. Seven-step global reaction scheme proposed by Diebold (1994) for cellulose. k is the reaction rate constant. Adapted from Di Blasi (2008).

The presence of active cellulose is discussed controversially in the literature (Antal & Varhegyi, 1995; A. L. Brown et al., 2001; Lédé, 2012; Varhegyi, Jakab, & Antal, 1994). Nevertheless, the model illustrated in Figure 2-21 is able to accurately predict slow and fast pyrolysis processes (A. L. Brown et al., 2001; Diebold, 1994; White et al., 2011). However, White et al. (2011) state that except for a few simple cases the use of these models remains limited because of the large number of reactions that need to be considered, the incomplete identification of the pyrolysis tar and its intermediate species, as well as the interdependency of serial reactions. In particular, the latter point can lead to model inaccuracies by magnifying small errors in the early part of the serial reaction system (A. L. Brown et al., 2001).

It is important to note that there are other models available like biomass deactivation models and distributed activation energy models. However, since this section is focused on the mechanism and not modelling they are only mentioned here,

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and for more detail the reader is referred to the reviews of Di Blasi (2008) and White et al. (2011).

2.7.2 Complex Multistep Pyrolysis Mechanism

A classic complex multistep pyrolysis mechanism is the one proposed by Mok and Antal (1983b) for cellulose, Figure 2-22.

Figure 2-22. Cellulose pyrolysis mechanism proposed by Mok and Antal (1983b). The numbers above the arrows denote the respective reaction step, and “ENDO” and “EXO” designate the endothermic or exothermic nature of the reaction step respectively. Taken from Mok and Antal (1983b).

The scheme in Figure 2-22 was developed by Mok and Antal to particularly take into account the effects of sweep gas flow rate and pressure on the pyrolysis of cellulose. It also considers the reaction heat effects as discussed earlier in 2.6.

Figure 2-22, similarly to Figure 2-20 and Figure 2-21, includes active cellulose as an intermediate product, which subsequently can produce either anhydrocellulose or levoglucosan. The dehydration path, 1, to form anhydrocellulose is preferred at lower temperatures (ibid). Mok and Antal call the char produced from anhydrocellulose by

reaction 4 primary char, which resembles the feed and is the main char formed. In contrast, char formed by reaction 6, is “soft and fluffy” (Mok & Antal, 1983b, p. 182) and results either from liquid or gaseous volatiles, which they call secondary char (Mok & Antal, 1983b). Mok and Antal explain that high pressure and a low flow rate of sweep gas limits mass transfer and thus inhibits the evaporation of levoglucosan, reaction 5, leading to more secondary char (reaction 6). This is in accordance with the suggestion made by Frolich et al. (1928), that pressure prevents volatilisation. High

pressure and low sweep gas flow favours reaction 4 over 3 (Mok & Antal, 1983b) and results in a decrease in the heat of pyrolysis, i.e., increased exothermicity (see also 2.6), which is due to the suppression of the endothermic reactions 3 and 5 (Mok & Antal, 1983b). However, at low flow rates reaction 7 is actually enhanced making the combination of reaction 3 and 7 more exothermic than reaction 4, as stated by Mok and Antal. Thus, one would anticipate an increase in the overall heat of pyrolysis (increasing endothermicity) due to the absence of the strong exothermic reaction 7 at low flow rates but Mok and Antal argue that the major effect comes from the enhanced role of reaction 6 over 5, and that the contribution of low flow on the exothermicity by reaction 7 with increasing pressure reduces due to the decreased production of volatiles with pressure (reaction 3). Applying the same reasoning, the high heat of pyrolysis (endothermicity) detected during vacuum experiments (see 2.6.4) is attributed to the enhanced evaporation of levoglucosan (reaction 5) and the increased production of intermediate volatiles (reaction 3). In this case reaction 7 plays no role due to the low concentration of the gas molecules and their short residence time (ibid). Mok and Antal

state that the effect of pressure and flow rate (see 2.6) is only due to vapour reactions and not due to reactions in the solid phase (reactions 1 and 2). Mok and Antal (1983b) also mention that high heating rates favour the decomposition path that leads to the formation of levoglucosan (reactions 2 and 5), which explains why higher char yields are observed at lower heating rates (see Table 2-2).

Despite being able to explain the complete effect of pressure and sweep gas rate on the reaction pathway, this mechanism contains the controversial active cellulose step, mentioned in 2.7.1, and the presented scheme is not suitable as a kinetic model due to its complexity (Antal & Grønli, 2003). Pyrolysis of wood is even more complex, because of the presence of the other biomass constituents, leading to more uncertainties and more mechanism “including radical and/or ionic reactions” (Lédé, 2012, p. 28), adding to the complexity.

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