3.1. Pyrolysis Product Yields
shows an area plot of the pyrolysis product yield distribution as a function of the reactor temperature. This area plot is drawn using the discrete values of the different yields obtained from different experimental conditions given in Table 2
. Tars include the condensable species in the volatile matter and the initial moisture of the wood. At low temperatures (below 600 °C), tar and solid together represent more than 50 wt % of the products. A temperature of 500 °C maximizes the oil yield, which reaches 62.4 wt %, while the char yield is close to 14 wt %. Several investigations have mentioned 500–550 °C as the optimal temperature for maximizing the bio-oil yield during biomass pyrolysis [3
]. The water content of the pyrolysis oil (tars) varied between 30% and 35% at 500 and 600 °C, respectively, which includes both moisture water and pyrolysis reaction water. The water content of the oils obtained at higher temperatures was not evaluated.
The production of gases increases with temperature, while that of tar and char decreases. For instance, at 800 °C, the gas yield is close to 78 wt %, while the char yield is 7%. Furthermore, at high temperatures, the formation of soot was observed, starting from 1000 °C. A deep analysis of the operating conditions’ effect on the pyrolysis product distribution can be found in previous investigations [23
]. The main focus of the present study was to identify the effect of the operating conditions on the char characteristics.
The evolution of the char yield with the pyrolysis reactor temperature indicates two separate ranges of linear decrease: low temperatures (500–600 °C) and high temperatures (600–1400 °C). The char yield decreased from 14% to 8% between 500 °C and 600 °C (0.05%/°C, R2 = 0.991), while it decreased from 8% to 4% between 600 °C and 1400 °C (0.005%/°C, R2 = 0.990). The relative uncertainties in the char yields are estimated to be lower than 10%. The evolution of the char yield would be thus significant.
The lower char yield at high temperatures may be explained by several factors. Higher heating rates are known to induce lower char yields [18
]. Moreover, volatile and char formation are two parallel reactions, and the latter is probably favored at higher temperatures and higher heating rates. Also, above 1000 °C, char gasification by H2
O and CO2
is likely to consume part of the formed solid.
3.2. Microscopic Analysis of the Biochar Surfaces
In order to monitor the textural properties and the behavior of minerals on the different chars upon different pyrolysis treatment, we performed SEM analysis coupled with EDX characterization at different enlargements.
Five samples were chosen to observe the effect of the pyrolysis temperature on the surface and the properties of the biomass particles: raw biomass, and samples pyrolysed at low (500, 600 °C) and high (1200, 1400 °C) temperatures.
presents the surface of a particle of beech wood at different enlargements. The cell walls of the raw biomass present a homogeneous surface without any holes; the EDX analysis shows the presence of very small amounts of Ca and K, which are the main minerals present in the biomass. The elemental cartography of the surface shows that K and Ca are homogeneously dispersed inside the biomass.
and Figure 4
show the surface of the samples pyrolysed at 500 and 600 °C respectively. Different areas of the particles were observed in order to be sure that the description of these materials was representative of the materials.
The samples Char-500 and Char-600 present a smooth surface, comparable to the one of the raw biomass. They don’t present any macropores (pores with diameter higher than 50 nm) and the shape of the particles remains identical to the raw biomass particles. The only difference is the concentrations of Ca and K, which increase with pyrolysis temperature (EDX spectra). The intensities of Ca and K peaks increase greatly with the pyrolysis temperature, bearing comparison to the principal Au peak (close to 2 keV). This phenomenon is due to the loss of volatile compounds from the raw biomass and then to the concentration of Ca and K. It also indicates that Ca and K are not completely emitted in the gas phase during the pyrolysis process when performed at 500 and 600 °C. Small amounts of Mg and Mn were observed in both samples.
and Figure 6
show the surface of the chars obtained at 1200 and 1400 °C respectively. These photographs clearly show the modification of the particle surface shape at these temperatures due to sintering. At high enlargement (×50,000) small particles at the surface of the chars can be observed (Figure 4
d and Figure 5
d). Analysis of the surface (EDX cartography—not shown) indicated that these particles contain only carbon, and no minerals. For the char obtained at 1200 °C, the amounts of Ca and K increased (with peak intensities comparable to that of Au) significantly, but the surface didn’t present cracks or macropores (Figure 5
d). Small amounts of Mg and Mn were also observed on these samples.
a illustrates the deep modification of the biomass structure after a thermal treatment at 1400 °C. This sample is different from the others, with an important macro/mesoporosity appearing at the surface of the carbon residue (Figure 6
d). This modification is also correlated with a decrease of the K content. As can be observed from the EDX analysis presented in Figure 6
, the relative intensity of the K peak decreased significantly. This behavior can be explained by the sublimation/boiling of potassium during pyrolysis performed at 1400 °C (boiling point 1420 °C, sublimation at 1500 °C), leading to the appearance of large porosity at the surface of the char particle, or again to the char gasification by H2
formed during the pyrolysis stage. This is consistent with other results obtained under similar conditions [23
], which also showed that potassium released from char at 1400 °C was incorporated into soot particles.
3.4. FTIR Spectroscopy
FTIR spectra of the fresh wood and pyrolysis chars are shown in Figure 7
. The wavenumber range is divided into two sub-ranges for the sake of clarity. The FTIR spectra of the unheated wood sample consist mainly of bands that can be attributed to carbohydrates (cellulose and hemicellulose) and lignin. The most prominent carbohydrate bands in the raw wood can be found between 1000 and 1200 cm−1
, while those related to lignins were identified at approximately 1221, 1269, 1326, 1367, 1423, 1464, 1510 and 1596 cm−1
Noticeable changes can be observed in the spectra of the chars compared to the raw wood. The OH band (around 3350 cm−1
) decreased significantly, starting with the char-500. The peak at 1730 cm−1
, related to the presence of carbonyl groups of esters and uronic acids in the xylan of beech wood progressively disappears as temperature increases. These functions belong to the hemicelluloses of beech wood, which have the lowest thermal stability [19
Moreover, the two small peaks at 1506 and 1458 cm−1, which are related to the Guaiacyl and Syringyl units of the beech wood lignin, are much less pronounced in char-500 than in raw beech, and completely vanish for the char-550 and beyond, confirming the high pyrolysis extent for this experiment. The peak at 1369 cm−1 is probably related to the C-H deformation in cellulose and hemicelluloses. This peak is only visible in raw beech, and no more in chars. The signal between 1000 and 1200 cm−1 is probably imputable to C-O vibrations in cellulose/hemicelluloses. It shows a high decrease for the char-500 confirming a high cellulose/hemicelluloses pyrolysis extent for this experiment. The signal vanishes completely for the char-550.
As stated previously, from 550 °C, the pyrolysis is nearly completed, which is reflected in the closeness of the char-550, char-600 and char-800 spectra. Beyond 800 °C, the signal intensity is very low through the infrared region of analysis. This denotes a much lower proportion of functional groups on the char surface, which is in agreement with the results of the elemental analysis.
3.5. Structural Changes of the Biomass Particles as Revealed by Raman Spectroscopy
The Raman spectra of the different chars are shown in Figure 8
. The spectra are normalized according to the G band height. Raman spectra of amorphous biomass chars are known to exhibit two main peaks around 1350–1370 cm−1
and 1580–1600 cm−1
, commonly called the D and G bands.
Increasing the pyrolysis temperature strongly affects the char structure. When increasing the temperature, Raman bands of pyrolysis residues appear as two overlapping but distinguishable peaks at the positions of approximately 1350 and 1600 cm−1
, which correspond to the in-plane vibrations of sp2
-bonded carbon with structural defects D band and the in-plane vibrations of the sp2
-bonded graphitic carbon structures G band, respectively [26
]. If a high proportion of amorphous carbon structures is present—which is the case for biomass chars—these two bands overlap. This overlapping is associated with hydrogen- and oxygen-rich amorphous carbon structures in the samples. This region (between 1400 and 1550 cm−1
) is called the valley region “V” [15
Structural parameters such as the band intensity ratios , or are indicators of the char structure. IV represents the valley intensity (taken as the minimum signal intensity between the D and G bands).
These structural parameters are summarized in Table 4
. The D band position shifts to lower wavenumbers as pyrolysis temperature increases (from 1363 cm−1
at 500 °C to 1327 cm−1
at 1400 °C), while the G band position shifts to higher wavenumbers when increasing the pyrolysis temperature (from 1576 cm−1
at 500 °C to 1596 cm−1
at 1400 °C). This tendency was also observed previously during the characterization of chars prepared by cellulose slow pyrolysis at different temperatures [28
]. This “red shift” in the D band peak position is more pronounced for low temperature chars, which have the highest contents of oxygenated defects structures [29
Furthermore, the intensity in the wavenumber ranges of 800 to 1100 cm−1
and 1700 to 1900 cm−1
strongly decreases with increasing pyrolysis temperature. This is related to the decrease of the highly reactive structures such as cycloheptane- and cyclooctane-centered ring systems, defective cyclic clusters and aromatic rings with pyrene sizes in the region of 800 to 1100 cm−1
, and to the pyrolysis of carbonyl bearing structures in the region of 1700 to 1900 cm−1
]. The decrease of the Raman signal in these regions (together with the valley region which is related to the amorphous carbon structures) is reflected in a decrease of the TRA with temperature, which was also denoted in [16
] for cane trash chars, in [30
] for mallee wood chars, as well as in [31
] for miscanthus chars.
The Raman analysis also shows that the IV/IG ratio decreases sharply between 500 °C and 600 °C (from 0.82 to 0.65), reaches a plateau between 600 °C and 1200 °C, and then decreases sharply between 1200 °C and 1400 °C to reach 0.4. At 1400 °C, the thermal treatment must have been very severe to induce such a brutal change, indicating a probable graphitization in such conditions. McDonald-Wharry et al. [15
] observed that the IV/IG height ratios level out at values approaching 0.4 for heat treatment between 700 and 1000 °C. However, the authors used different heat treatment conditions, with a much lower heating rate (7–30 °C/min) and a dwell/hold time of 20 min. Afterwards, the char was allowed to cool in an inert atmosphere to the ambient temperature. The authors think that this value obtained for IV/IG height ratios likely represents the overlap of broad D and G bands in the valley, and may not represent any amorphous carbon content, as might be the case for severe heat treatment.
decreases from 1.08 for the char-500 to 0.27 for the char-1400 which an ordering of the char structure. Moreover, the ID/IG ratio significantly increases with the pyrolysis temperature (from 0.76 at 500 °C to 1.44 at 1400 °C), indicating a higher proportion of condensed aromatic ring structures with defects. These D structures would be formed by the condensation of small aromatic amorphous carbon structures (valley region which intensity highly decreased with temperature). These results are in accordance with the dynamic molecular structure diagram established by Keiluweit et al. [10
Altogether, an increasing level of order can be noticed in the structure of the char as the pyrolysis temperature increases, with a clearly distinguishable evolution of the Raman spectra of the different chars, especially for the 1400-char sample, the structure of which seems to be highly modified and ordered.
3.6. Char Reactivity towards O2
The reactivities of chars towards O2
were examined using thermogravimetric analysis. The obtained results are shown in Figure 9
. Plots of the normalized mass (
) and negative mass loss rate (−
) are presented for raw beech wood and the different prepared chars.
Considering the experiment done on beech wood as reference, the mass loss below 300 °C decreases as pyrolysis temperature increases, indicating lower proportions of low, thermally stable components (reactive carbonaceous structure, incompletely pyrolysed wood) in the chars. The chars prepared at 500 °C and 550 °C (to a lower extent) show peaks (a maximum of degradation rate) at a relatively low temperature of 325 °C, probably indicating the presence of unconverted wood. The final residual mass increases with pyrolysis temperature, which was foreseeable, given that char obtained at higher temperatures has higher mineral content.
The char oxidation characteristics, defined previously in the materials and methods section, are summarized in Table 5
. Char-600 unexpectedly shows higher thermal stability compared with char-800 and char-1000. In fact, the temperature corresponding to 50% of conversion is slightly higher for char-600 (405 °C) than for char-800 (379 °C) or char-1000 (389.4 °C). The prolonged char residence time in the reactor after pyrolysis has probably contributes to the thermal stability enhancement. Such behavior is confirmed by examination of the Raman spectra of the different samples. In fact, analysis of the Raman Spectra shows that char-600 has a lower intensity in the regions between 800 and 1200 cm−1
and 1700–1900 cm−1
when compared to the samples of char-800 and char-1000, respectively. Raman signal in these regions can be associated with the highly reactive structure in the char [29
As shown in Figure 10
, although the evolution of
is somewhat chaotic between 500 °C and 1000 °C, it increases following a perfect linear relation (R2
= 0.9999) with the char yield for the chars obtained between 1000 °C and 1400 °C. This could be linked to structural ordering and graphitization above 1000 °C.
Moreover, the char mean reactivity significantly decreases as pyrolysis temperature increases, going from 0.198 min−1 for the char-500 to 0.067 min−1 for the char-1400, which represents nearly a threefold decrease.
The reactivity of fast pyrolysis beech bark and beech stick chars obtained between 450 °C and 850 °C were shown to be highly dependent on the pyrolysis temperature [20
]. The authors think that at low pyrolysis temperatures, the high H and O contents of the char are associated with the presence of amorphous carbon structures and active sites that increase the char reactivity; whereas, when raising the pyrolysis temperature, the char reactivity decreases due the formation of more aromatic, less functionalized and less reactive structures.
We looked for possible relationships between the different reactivity parameters and the content of heteroatoms in the char (H and O). As shown in Figure 11
, we found, for instance, that the mean reaction rate
is linearly correlated with the O/C ratio. This linear dependence clearly expresses the influence of the surface functional groups containing O atoms on the char oxidation reaction rate.
indicates that Char-1400 clearly presents the highest thermal stability and the lowest reactivity. In particular, Char-1400 has the lowest values for Rmean
compared to the other samples. This thermal behavior may be linked to a more ordered carbonaceous structure [32
], as revealed by the Raman spectroscopy analysis and reflected in the values of ID/IG and IV/IG, which were the highest and lowest, respectively, among all the samples, as well as representing the lowest
is, remarkably, correlated to the TRA following a linear relation (Figure 12
). This relationship, indicating that
increases when TRA decreases, makes sense, as the decrease of TRA is mainly due to the decrease of the contribution of the most reactive carbon structures to the Raman signal (amorphous carbon forms in the valley region and highly reactive groups on the two sides of the Raman spectra), which are less present in the char when the pyrolysis treatment is more severe. Other researchers found a linear correlation between
and the 2490 cm−1
band width (second order region in the Raman spectra) for cellulose chars treated between 600 and 2600 °C [19
], while others found a linear relationship between
and the area ratio of the G band to the TRA [22
Reactivity index can also be correlated to the IV/ID structural ratio, as shown in Figure 13
. This evolution here again makes sense, as the more reactive and amorphous small aromatic structures char contains, compared to condensed structures, the higher its reactivity.
Altogether, these oxidation reactivity tests show that the more severe the thermal treatment in the EFR is, the lower the reactivity of the char samples towards O2 will be. The char reactivity, defined through several parameters, is also found to be remarkably correlated with many characteristics of the chars related to their structure and chemical composition.