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Effect of the Temperature on Yields and Characteristics of the Pyrolysis Products
The effect of the temperature on yields of the bio-char (YS), bio-oil (YL), and the gases (YG) generated from the pyrolysis of the yerba mate twigs, on a dry-basis, is illustrated in Figure 2. As may be noticed, the results show that the temperature markedly affected the distribution of the pyrolysis products, especially at temperatures higher than 500 °C, in agreement with other reported data (Garcia Perez et al., 2008; Goyal et al., 2008).
Figure 2. Effect of the temperature on yields of the three kinds of pyrolysis products: solid (YS), liquid (YL), and gas (YG).
Increasing the temperature led to enhance gas formation and to reduce the solid fraction, even though the effect for the latter was less pronounced. Maximum yield for the gaseous products was attained at the highest temperature within the investigated range, whereas formation of the bio-char achieved the minimum value at 700 °C. These trends may be due to a competition between devolatilization reactions promoting gas production, which are progressively favored with temperature increase, and reactions which induce char formation (Di Blasi, 2008). Dehydration, slow decarboxylation, depolymerization, and recombination of the decomposition products taking place during pyrolysis at relatively low temperatures have been ascribed as the main reactions responsible for the formation and higher yield of the biochar. Instead, at higher temperatures, cleavage of macromolecules and subsequent degradation to volatiles of low-molecular weight would take place.
In turn, bio-oil yield increased with temperature, attaining a maximum at around 500 °C, and decreased at temperatures higher than 500 °C. It agreed with results reported in the literature for bio-oils derived from other biomasses (Encinar et al., 2000; Kim et al., 2013). The maximum yield of bio-oil would be due to the competition between primary formation of volatiles, taking predominantly place at relatively low temperatures, and secondary degradation of the condensable vapors, that should be favored at higher temperatures (Jand and Foscolo, 2005). Consequently, increase in gas yield at temperatures higher than 500 °C would result from contributions of bio-char degradation and secondary reactions of the condensable vapors. The pronounced decrease in bio-oil yield accompanied by the steep rise of the gas fraction at 700 °C suggests that secondary reactions were predominant.
Chemical characteristics and HHV for the bio-char produced at the different pyrolysis temperatures are reported in Table 4. As expected, the results in Table 4 show that increasing the temperature led to a bio-char with higher fixed carbon content, slight increase in ash, and gradual decrease in volatiles. The increase in ash content should result from progressive concentration of minerals and destructive volatilization of lignocellulosic matter as temperature increased (Zhang et al., 2015). Likewise, enhanced %C and reductions in %H and %O, as a consequence of volatiles release, may be noticed. The temperature had only a weak effect on the HHV of the resulting bio-char. Its influence on both chemical characteristics and HHV was similar to trends reported for bio-char samples derived from other biomass resources (Claoston et al., 2014).
Table 4. Effect of the temperature on bio-char features
a Estimated by difference.
Concerning the potential use of the bio-char as biofuel and the manufacture of briquettes, international standards establish contents of fixed carbon higher than 76 wt% to obtain high quality briquettes (Encinar et al., 2000; Gonzalez et al., 2005; Basso et al., 2005; Cukierman et al., 2012). Accordingly, the bio-char obtained at the highest temperature, possessing a fixed carbon content of ~ 77wt%, fulfils the requirement for this purpose. Another criterion adopted as an index of good quality takes into account the volatile matter content of the char together with the intended application. For domestic use, volatile matter contents ranging between 20% and 30 % are acceptable, whereas these contents are restricted to 10 - 15% for use in the steel industry. Since the biochars derived at 400 - 700 °C have volatile matter contents ranging between ~ 12 and 19 wt % (Table 4), they could be employed for domestic use.
Apart from its use as biofuel, other possible application increasingly reported in the last years, is incorporation of the biochar into the soil in order to increase the long-term storage of carbon, simultaneously providing soil amendment benefits and reduction of greenhouse gases (Lehmann et al., 2006; Woolf et al., 2010; Zimmerman, 2010; Bruun et al., 2012; Creamer et al., 2014; Windeatt et al., 2014; Schimmelpfennig et al., 2014). Although the mechanisms by which the biochar favors soil quality are still not fully understood, it is reported that it positively impacts on soil organic carbon, water holding capacity, cation exchange capacity, pH, and soil microbial ecology (Crombie and Masek, 2014). The stability in soil has been pointed out as a paradigmatic attribute of the biochar when selecting its application in environmental or agricultural applications, and potential indicators for stability prediction have been proposed (Fabbri et al., 2012). Among them, an indicator of the bio-char stability to store atmospheric carbon in soil considers the molar oxygen to carbon ratio (O:C) of the samples. In general, a ratio of O:C lower than 0.2 appears to provide a very prolonged biochar half-life, since it is linked with slower biochar mineralization rates (Spokas, 2010).
Accordingly, based on this indicator, present values of the molar O:C ratios for the biochar obtained at the different temperatures, varying within the range 1 - 1.5 x 10-3, point to highly stable samples.
In Table 5, characteristics of the bio-oil generated at the different temperatures are reported. Bio-oils are multi-component mixtures of different size molecules possessing appreciable proportions of water, usually ~ 15 - 30 wt%, derived both from the original moisture in the biomass feedstock and as a product of dehydration with the pyrolysis course and storage. The distribution of chemicals in bio-oils in terms of polarity is quite complex including the presence of highly non-polar to intermediate polarity substituents, such as normal and branched aliphatics, alicyclics, olefins, and aromatics. Polar and highly polar chemicals mainly include oxygenates like carboxylates, carbonyls, aryl ethers, phenols, and alcohols (Kanaujia et al., 2014). Physico-chemical and energy properties of bio-oil differ significantly from petrol distillate fuels (Qi et al., 2007; Guo et al., 2015). As shown in Table 5, the organic fraction of the bio-oils showed carbon contents ranging between 61% and 67%. Instead, %C for conventional fuels is comprised between 83% and 89% (Ghazi, 2013). Values of the HHV of the YMT-derived bio-oils were relatively higher than those reported for bio-oils generated from other biomasses because the former were determined for the organic fraction. However, if water content of the bio-oils is considered, lower values, comparable to the reported ones (Mohan et al., 2006; Kim et al., 2013), are obtained. Density of the bio-oils as produced, i.e., including both the aqueous and organic fractions, was rather higher (~ 1 kg dm-3) than that for conventional hydrocarbon fuels, attributable to their higher oxygen content and presence of water. The results also indicated that the temperature did not affect drastically the characteristics of the resulting bio-oils. The derived crude bio-oils could be directly burnt by standard atomization techniques in industrial scale combustion systems, using a proper burner to account for bio-oils particular characteristics. Atomization and combustion of bio-oils in engines are favored by their content of water since it reduces viscosity and enhances fluidity (Kanauija et al., 2014). However, upgrading of the bio-oils should be necessary in order to employ them as a petrol distillate fuel alternative. The upgrading of bio-oils has been subject of intense research in the last years with the aim of reducing their moisture content and acidity, as well as of improving the heating value and storage ability. It may include hydrogenation, hydrodeoxygenation, esterification, catalytic cracking, molecular distillation, supercritical fluidization, emulsification, steam reforming, and blending, among other methods. For instance, moderate upgrading has been applied to substitute for heavy fuel oils to power static appliances including boilers, furnaces, engines, and electric generators. Nevertheless, large-scale application of bio-oils as transportation fuels requires sustainable upgrading techniques that have not been still achieved. Hydrodeoxygenation, which is a high pressure process involving H2 to exclude oxygen from the bio-oil to yield a high grade oil product equivalent to crude oil, appears to have the greatest potential, even though several issues involved in this process still have to be solved (Mortensen et al., 2011). Other possible applications of bio-oils concern their potentialities to obtain valuable chemicals, preservatives, lubricants, binders, paints, thickeners, stabilizers (Qi et al., 2007; Garcia Perez et al., 2008; Butler et al., 2011; Bridgwater, 2012).
On the other hand, main gaseous species evolved with pyrolysis course comprise CO2, CO, CH4, H2 and, to a lesser extent, C2H6, and C2H4. Typical concentration-time profiles for the main species are illustrated in Figure 3 (a-d) for the different temperatures investigated. Furthermore, Figure 4 shows the molar fraction of the gaseous species produced from YMT pyrolysis and the HHV of the gases mixture, as a function of the pyrolysis temperature. HHV was calculated considering the amount of moles produced per mass unit of biomass (Gi) and the heat of combustion for each of the species, according to the following equation:
GCH4, GH2, and GCO in Equation (7) are the total amount of each of the gaseous species produced per mass unit of biomass at each temperature.
Figure 3. Effect of the temperature on concentration profiles of the main gaseous species produced at: a) T = 400 °C, b) T = 500 °C, c) T = 600 °C, d) T = 700 °C.
Table 5. Effect of the temperature on bio-oil characteristics
Estimated by difference; corresponding to the organic fraction.
As may be appreciated in Figure 3, composition of the gaseous mixtures depends on the pyrolysis temperature. For all the temperatures investigated, the mixtures were mostly composed of CO2, followed by CO, and in less proportion, by CH4 and H2. Seemingly, CO2 results mainly from reforming and cracking of carbonyl and carboxyl functional groups, whereas CO is generated from the rupture of C-O-C and C=O. In turn, CH4 should be produced from decomposition of methoxyl-O-CH3. Cracking and deformation of C=C and C-H groups could explain H2 generation (Qu et al., 2011; Park et al., 2012; Cukierman et al., 2012).
Proportions of CO2 in the mixtures progressively decreased with temperature rise, whereas those corresponding mainly to CO and H2 increased, thus leading to HHV enhancement. As seen in Figure 4, HHV of the generated gas attained values from 1 to 4 MJ kg-1 of biomass in the range of investigated temperatures, namely from 5 to 11 MJ m-3, indicating that pyrolysis of the twigs yields low to medium heating value-gases. It agrees with some data reported in the literature for gases arising from the pyrolysis of other biomassic feedstocks for similar conditions (Encinar et al., 2009). The gas produced could contribute to provide heat, which may help to the sustainability of the pyrolysis process (Strezov et al. 2008; Bridgwater, 2012).
Figure 4. Effect of the temperature on the composition and HHV of the gaseous mixtures produced from pyrolysis of the yerba mate twigs.
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