Análisis de Clase 3 Práctica Educativa Tabla

The results in 2.6.4 showed that increased vapour-phase residence time of the volatile pyrolysis products in the pyrolysing solid enhances secondary reactions and produces higher char yields.

This relationship was further investigated in a recent study by L. Wang et al. (2011). They used corncobs prepared in different ways and found that samples pyrolysed by conventional pyrolysis, that is proximate analysis according to ASTM E 871 and 872, resulted in fixed carbon yields of only 49 to 54 % of the theoretical value. Higher yields were obtained when corncob powders were pyrolysed in deep crucibles that prolonged the vapour-phase residence time by limiting the access of the nitrogen purge gas. These yields were further increased when the crucibles were closed with a lid that had a small pin hole. Compared to the powder, single particles with similar weights had higher yields, which are believed to be partly due to compositional differences and changed sample dimensions that affected the internal pressure profiles. In agreement with the powder experiments the yield increased with increasing sample size/ mass. The next highest yields were obtained by pyrolysing whole corncobs in closed crucibles in a muffle furnace purged with nitrogen or when pyrolysing corncob cross-sections in a micro-TGA. The highest yields of fixed carbon, 70 to 85 % of the theoretical value at 950 °C, were obtained at elevated pressure, 0.8 MPa, in a flash carbonisation reactor. These findings illustrate the close relationship between pressure, vapour phase residence time, sample mass and sample dimension.

2.6 Effect of Pyrolysis Conditions 2-61

That is, any method that increases the vapour-phase residence time of the volatile pyrolysis products inside or on the surface of the pyrolysing solid (e.g. bed of particles, large sample size or mass, low gas flow rates and high pressure) increases the fixed carbon yield by enhancing secondary reactions. Interestingly L. Wang et al. (2011) observed the highest yields at pressure even when employing an air atmosphere compared to nitrogen experiments at atmospheric pressure revealing the potential of pressure during pyrolysis. These results were confirmed for wood by L. Wang et al. (2013), who concluded that any pyrolysis condition that enhances secondary reactions, by prolonging the vapour-phase residence time, might be more important than the heating rate for the charcoal yield.

In the studies of L. Wang et al. above, pressure was found to be the main parameter for obtaining near theoretical yields, and in 2.6.4 it was proposed that the vapour-phase concentration is more important than the absolute pressure. Thus, it is expected that the char yield is also a function of the sample loading, which is the amount of sample relative to the volume of the reactor and directly impacts on the achievable autogenous pressure. Mok et al. (1992) investigated sample loading and found that the exothermic reaction heat and the yield of char increased with loading while at the same time the onset temperature of the reaction decreased and the reaction kinetics were faster. In the case of cellulose, the char yield increased from 36 to 40 % (wt/wt) with increasing mass loading, corresponding to an autogenous pressure change from 3 to 14 MPa. They observed similar trends for hemicellulose pyrolysis but for lignin no trends could be observed due to the fact that its decomposition occurs over a wide temperature range (2.5.4).

2.6.6 Moisture

Antal et al. (1996) state that “there is a long history of confusing and contradictory results concerning the influence of moisture content and steam on the pyrolysis chemistry of biomass materials” (p. 655), which they outline in their short review. Antal et al. argue that these seemingly contradictory results might be a consequence of the fact that water affects pyrolysis only in a stagnant environment at elevated pressure, that is, under autogenous pressure conditions. One of the main studies supporting this hypothesis is the work of Mok et al. (1992), where they discovered that the addition of water, 6.6 % to 27.3 %, in sealed crucibles with a cellulose loading of у67 mg/ml

improved the charcoal yield from 36 to 40 % (wt/wt) on a dry basis and decreased the pyrolysis onset temperature from above 275 °C to 250 °C. No correlation was found by them between moisture and the reaction heat. Mok et al. state that similar trends were discovered for hemicellulose but for lignin no trends could be observed due to the fact that lignin decomposition occurs over a wide temperature range, as mentioned in 2.6.5. Thus, water can act as an autocatalyst during pyrolysis (Mok et al., 1992). These findings are supported by research in the field of the geological formation of fossil fuels (Pennisi, 1993), which is discussed in more detail in 2.8.3, although some uncertainties remain (see 2.8.4). A more recent review on the effect of water is given by Antal and Grønli (2003), who come to the same conclusion that autogenous pressure is the critical parameter, but also state that the underlying mechanisms are not resolved yet.

Few studies are available on the impact of moisture on the structure of the char, except that steam pyrolysis removes labile carbon that blocks the internal pore structure (Antal & Grønli, 2003), and thus is used to produce activated carbon (Downie et al., 2009). Also Kantarelis et al. (2013) showed that steam affects the quantity and quality of all the pyrolysis products, and that it even causes reduced oxygen content, which is one of the major problems associated with bio-oil as discussed in 2.3.2, in the liquid product.