3. Results and Discussion
3.3 Yields of the pyrolysis products
Pinewood, olive bagasse, wheat straw and rice husk were pyrolysed at the conditions described in section 3.1. Figure 3.1 shows the yields of the products obtained from their pyrolysis. The reaction temperature was considered to be the “outlet temperature” achieved when the pyrolysis reaction was taking place, which gave approximately ≈ 500 ºC for all the experiments.
The highest bio-oil yield was obtained with pinewood (51 ± 0.5 wt.%) and the lowest with olive bagasse (31 ± 1.8 wt.%). Pinewood has shown the highest bio-oil yield and the lowest char yield with numbers comparable to similar screw reactor studies, see Thangalazhy et al. [63] and Ingram et al.
[54]
.
Its relative good performance is related to its low ash content (0.2 wt.%) [35-37], and to its usual major cellulose content, as refereed in Oasmaa et al. [35], from which most part is converted in bio-oil [43]. The agro-biomasses (olive bagasse, wheat straw and rice husk) presented lower yields of bio-oil than that of pinewood as a consequence of their higher ash content (see Table 3.1). Figure 3.2 shows a correlation of the feedstock ash content to the bio-oil yields obtained from pyrolysis. As the ash content of the feedstock increases the bio-oil yield decreases with a linear behaviour comparable with the results of Fahmi et al. [36] and Oasmaa et al. [35], which also pyrolysed agro-biomasses with substantial ash content in a fluidized-bed reactor.After to pinewood, rice husk have shown the second best bio-oil yield with 40 ± 1.5 wt.%, comparable to that obtained by Di Blasi et al. [69]in a packed bed reactor. The considerable ash content (10.5 wt.%) of rice husk promoted the char and gas formation, which affected the bio-oil yield, see Fahmi et al. [36]
.
24 34 38 34
51 31 33 40
25 34 29 26
0 20 40 60 80 100
Pinewood Olive bagasse Wheat straw Rice husk wt. %
Feedstock
Gas Bio-oil Char
Figure 3.1 – Yields of the products obtained from the pyrolysis of the biomasses.
Olive bagasse showed the lowest bio-oil yield due to its relative high ash content (13.1 wt.%), and to its typical high lignin content, as referred in Panopoulos et al. [99], which yield substantial amounts of char and gas. Şensöz et al. [64] obtained comparable yield values for a fixed-bed reactor at 500 ºC. According to Coulson [100], the alkali metals present in the olive bagasse ashes, as potassium (K2O) in high percentage (18.7 wt.%), catalysed pyrolysis reactions to yield extra water and gas and decrease the bio-oil yield. As result, olive bagasse produced the greatest gas yield (34 ± 0.04 wt.%). The yields of wheat straw are comparable with those of olive bagasse, although, it has generated more char than gas. The reason may also be linked to a lower heating rate [64].
The yields obtained for the products are comparable with those obtained in other studies involving screw reactors and fixed-bed reactors [54,63,64], however, previous fast pyrolysis works that pyrolysed pinewood, wheat straw and rice husk in fluidized-bed reactors at 500 ºC have shown larger yields of bio-oil (60 %), see [65-71]. Such discrepancy is due to the own nature of the screw reactor where a very short residence time and high heating rates comparable to those of fluidized-beds are difficult to achieve. According to Bridgwater [13], the “hot vapour residence time can range from 5 to 30 s depending on the design and size of the screw reactor”. The lower bio-oil yields obtained are an indicative of an inherent larger residence time in the screw reactor that is obviously superior to the estimated residence time (2.8 s) with the nitrogen flow rate of 526 mL/min. The larger residence time in the reaction zone tended to increase thermal and catalytic cracking of pyrolytic vapours into gaseous products, which increased gas yield. Moreover, the significant and higher yields of char obtained are an evidence of lower heating rates accomplished within the screw reactor, a common occurrence in these reactors already refereed by Bridgwater [13,18]. The own geometry of the reactor and screw may have compromised an efficient heat transfer between the hot wall and the biomass particles in the reaction zone, which resulted in a deficient heating rate not high enough. Such low heating rate led to an appreciable repolymerisation of char, increasing its production (see Table 1.2 in section 1.2.1). As overall consequence of cracking and repolymerisation, the bio-oil yields obtained
Pinewood
Olive bagasse Wheat straw Rice husk
0 10 20 30 40 50 60
0 2 4 6 8 10 12 14 16 18
Bio-oil yield, wt.%
Ash, wt.%
Figure 3.2 – Correlation of the feedstock ash content to the bio- oil yields obtained from pyrolysis.
with the reactor of the present work tended to be somewhat lower than those of fluidized-beds reactors found in literature.
The possibility of such low heating rates are related with a possible over feeding of biomass in the reactor [101] is discarded, since the biomass feed rates (Table 2.1) are lower than those used by Ingram et al. [54] (1kg/h) in a screw reactor with comparable dimensions and slight higher yields.
Therefore, the own geometry of the screw/reactor is pointed out as the main reason for the low heating rates developed in the reactor.
The weaker pre-heating of nitrogen up to 65 ºC before its entrance into the reactor when compared to other works that pre-heated nitrogen up to 350 ºC in fluidized-beds [65,68,71] is also a possible reason to obtain distinguished lower bio-oil yields. A lower pre-heating of the fluidizing gas represents bigger heat losses that could affect the own pyrolysis reaction.
After the pyrolysis reaction it was observed a deficient separation of products instead of a rapid removal of char, which decrease bio-oil formation [13]. For all the experiments a visible portion of hot vapour was trapped in the char flask and kept in direct contact with char before its follow to the condensers. Figure 3.3 shows the contact of the pyrolytic hot vapours with char in the char’s flask. The significant time in contact generated substantial extra-repolymerisation of char and catalytic cracking.
In technical terms, a not achievable residence time of ~ 2 s with a nitrogen flow rate of 526 mL/min, an evident low heating rate in the reaction, and an unable separation of the products while using the screw reactor configuration were the main experimental justifications for the low bio-oil yields. The weak pre-heating of nitrogen may also have had significant influence.
Attending the woody biomass reference yields for fast pyrolysis (see Table 1.1 in section 1.2.1) and to other related works in fluidized-beds with superior bio-oil conversion yields (> 55%), is
Figure 3.3 – Contact of the pyrolytic hot vapours with char in the char’s flask.
not possible or either correct to associate such covered experimental conditions to those of fast pyrolysis. When comparing the yield values of pinewood with reference yields for woody biomass, one concludes that the experimental conditions reached a regime of fast pyrolysis so-called intermediate pyrolysis [13,16]. Figure 3.4 indicates the product distribution obtained from different modes of pyrolysis.
Intermediate pyrolysis is a middle way regime of fast pyrolysis with longer hot vapour residence times (5 – 30 s) and consequent lower bio-oil conversions yields (for woody biomass: bio- oil: 50 wt.%, gas: 25 wt.%, char: 25 wt.%). The clear signs of a longer residence time obtained in the screw reactor when compared to other fluidized-bed studies and the consequent yields obtained for pinewood, which are strictly close to the referred values, let one conclude that the covered experimental conditions led to an intermediate pyrolysis.
The bio-oil yields obtained with the other agro-biomasses are even lower than these woody references due to the own nature of feedstock (ash content and chemical structure). It is interesting to note that their difference (high as 20 wt.%) from the yield of pinewood bio-oil equals the difference obtained with other agro-biomasses from the yield of pinewood in other comparative studies [99]
Nevertheless the oil yields seem to be reasonable while considering other fast pyrolysis studies that reported bio-oil yields low as 17 wt.% [102] for rapeseed or 14 wt.% for rice straw, sugarcane bagasse and coconut shell [103].
Figure 3.4 – Product distribution obtained from different modes of pyrolysis for woody biomass. Source: Ref. [13].
3.4. Analysis of the Bio-oils
The bio-oil obtained in the first condensation flask for each biomass was collected and subsequently analysed as described in section 2.5.2. Table 3.2 shows the physical properties of the original feedstock and the bio-oils obtained from the pyrolysis of pinewood, olive bagasse, wheat straw and rice husk.
The pinewood oil analysis have shown a single-phase oil with a specific gravity, water content and heating value of 1.2, 30 wt.% and 16.6 MJ/kg, respectively. Such properties matches the conventional values refereed by Czernik et al. [72]for a typical woody pyrolysis bio-oil. The analysis of the oils obtained from olive bagasse, wheat straw and rice husk have shown non-homogeneous oils (two visible phases) with higher water contents (49-67%) and lower densities, which correlate to their water content.
The heating values ranged from 9.5 MJ/kg to 19.6 MJ/kg for rice husk and for olive bagasse, respectively. Olive bagasse bio-oil has shown the highest heating value even with the worst bio-oil conversion yield (31 ± 1.8 wt.%). According to Butler et al. [15], this result is a clear sign of a large portion of lignin in the olive bagasse, as in accordance with Panopoulos et al. [99]. The large lignin portion was cracked better due to the catalysing effect of alkali metals present in the initial feedstock in significant quantities, such as potassium (K2O - 18.7 wt.%). The better degradation of the lignin portion led to a bio-oil with lower oxygen content and therefore with an energy density higher than the own raw material (17.5 MJ/kg) [44], even with substantial water content (48 wt.%). Its heating value is also superior to that of conventional woody bio-oil as that of pinewood [72].
Table 3.2 – Physical properties of the original feedstock and the bio-oils obtained from pyrolysis of pinewood, olive bagasse, wheat straw and rice husk.
Oil Raw Oil Raw Oil Raw Oil Raw
Density, kg/m3 1249 - 1212 - 1101 - 1050 -
Moisture, wt.% 30 13.6 49 9.4 58 8.9 67 9.4
C (wt.%) 45 46.48 56 43.2 57 58.7 62 40.7
H (wt.%) 7 6.85 8 5.6 7 0.5 7 6
N (wt.%) < 0.5 0.0 3.1 1.9 1.5 1.1 0.9 0.5
O (wt.%) 47.5 32.87 32.9 26.8 34.5 16.1 30.1 32.9
O/C 1.06 0.71 0.59 0.62 0.61 0.27 0.49 0.81
H/C 0.16 0.15 0.14 0.13 0.12 0.01 0.11 0.15
HHV, MJ/kg 16.6 18.1 19.6 17.5 11.7 19 9.5 15.7
LHV, MJ/kg 15.1 16.7 17.9 16.4 10.2 18.8 8.0 14.4
Phases 1 - 2 - 2 - 2 -
Analysis Pinewood Olive bagasse Wheat straw Rice husk
Pinewood has shown the second higher heating value with 16.6 MJ/kg, a value that is below of that obtained by Thangalazhy et al. [63], 19.1 MJ/kg, due to its higher water content of 30 wt.%.
Although, the heating value obtained for pinewood still matches the conventional heating value for woody biomass, according to Czernik et al. [72]
.
Besides the water content, its higher oxygenated composition than that of the raw material with an O/C ratio of 1.06 is another reason for not reaching a higher heating value, maybe similar to that of the raw material (18.1 MJ/kg) [72].At last, wheat straw and rice husk bio-oils have shown poor heating values of 11.7 and 9.5 MJ/kg, respectively, as a direct result of their higher water contents (58 and 67 wt.%, respectively).
Such conclusion are possible in view of other studies that obtained bio-oils from wheat straw and rice husk with low heating values of 16.9 MJ/kg and 17.42 MJ/kg and moisture contents of 19.9 wt.% and 25.2 wt.%, respectively [64,65].
Regarding the O/C ratios, pinewood bio-oil is the most oxygenated. Such result may be a consequence of its lower ash content in the initial feedstock. The oxygen present in the mineral matrix of the ashes in the initial agro-biomasses followed to char rather than for bio-oil, which may have resulted in less oxygenated bio-oils even with higher water contents.
The agro-biomasses have shown a residual amount of nitrogen as a direct result of their initial feedstock. It varied from 3.1 wt.% to olive bagasse to 0.9 wt.% to rice husk. It is clear that the amount of nitrogen in the bio-oil correlates to the amount of nitrogen in the initial feedstock.
The higher water contents in the agro-biomass bio-oils are a consequence of their higher ash content in the feedstock that catalysed pyrolysis reactions to yield extra water, as concluded by Coulson et al. [100]. Oasmaa et al. [35]have proven such influent effect of the ashes when pyrolysed agricultural residues in a fluidized-bed reactor at ~ 500 ºC and obtained moisture contents as high as 51.1 wt.%. Tsai et al. [103] also obtained large amounts of water in bio-oils (>65 wt.%) when pyrolysing rice straw, sugarcane bagasse and coconut shell, with poor yields of bio-oil (down to 17 wt.%). The evident larger resident times of the hot vapours and low heating rates in the screw reactor may also increased even more this catalyst effect [18]. Besides the catalyst effect of the ashes, an insufficient drying prior to the tests is also a possible reason for such moisture contents.
It also seems reasonable to question how good was the performance of the condensation stages once in the trial tests prior to conducting the present tests they have shown a compromising behaviour (Appendix E). The analysis on Table 3.3, including the moisture contents, relate to the representative bio-oil samples retained in the first condensation flask at the end of each test (Fig. 2.2 in section 2.2). It was observed throughout the tests that major part of the water steam has condensed into this first flask, as in accordance with Chen et al. [104], and that a substantial portion of bio-oil, heavier and more viscous, was trapped in the channels of the condensers without flowing into the flasks (e.g. for wheat straw 36 wt.% of the whole bio-oil was trapped in condensers). Both combined effects may have resulted in a representative bio-oil sample in the first flask of condensation with
superior water content, in relative terms, when compared to the gross bio-oil obtained. Figure 3.5 shows a viscous portion of bio-oil trapped in a condenser.
For these reasons, would be reasonable to assume that the covered methodology and the own inability of the condensers to deal with viscous bio-oil tented to increase the water content in the representative sample of the bio-oils, which influenced the other properties. For the same reasons, authors such as DeSisto et al [66] analysed the bio-oil collected in the electrostatic precipitator rather than the bio-oil fraction condensed in the condenser.
Figure 3.6 shows the bio-oils of pinewood and olive bagasse. The bio-oil of pinewood presented a homogeneous aspect while the bio-oil of olive bagasse, as the bio-oils from the other agro-biomasses, presented a heterogeneous biphasic aspect as consequence of their superior water content, which caused phase instability [16]. According to Czernik et al. [72]
,
the lower water content of pinewood bio-oil (30 wt.%) enabled the miscibility of water in the whole emulsion that resulted in a single-phase oil. Adversely, for the agro-biomass bio-oils with higher water contents the solubilizing effect of the hydrophilic compounds was not enough to prevent phase separation into two phases [73], a water-soluble (aqueous phase) and a water-insoluble phase (tar).
Figure 3.5 – Viscous portion of bio-oil trapped in the first condenser.
The phases are visually distinguishable in the bio-oils (Fig. 3.4 b)): the hydrophilic aqueous phase (top phase) and the heavier non-soluble phase (tar) that settled at the bottom. Besides the own water content, the usual higher amount of extractive matter (neutral substances) contained in the feedstock of the agro-biomasses may have helped to yield a bigger aqueous phase [99].
These phase-separated oils may be desirable in some applications where fractionation is required [13]. Şensöz et al. [63] and Yanik et al. [64] are examples of studies where these phases were fractionated/separated and the only fraction taken into analysis was the heavier phase (tar), which has the higher carbon content and consequent energy density.
The yields of the products already discussed together with such water content values (“half water, half organics”) are important evidences from which one can conclude that the pyrolysis process carried with the described experimental conditions was in fact an intermediate pyrolysis process, according to Bridgwater [13].
a) b)
Figure 3.6 – Bio-oil of a) pinewood and b) olive bagasse
3.5. Analysis of Chars
The char obtained in the char’s flask for each biomass was collected and subsequently analysed as described in section 2.5.6. The characteristics of chars obtained from pyrolysis are depended on the pyrolysis conditions such as temperature and heating rate as well as the composition of the biomass [64,105]. Once the biomasses were pyrolysed under the same conditions, the properties of the chars are just related to the biomass initial composition.
Table 3.3 shows the physical properties of the char obtained from the pyrolysis of pinewood, olive bagasse, wheat straw and rice husk.
The ash analysis has shown that the ash content in the char correlates to that in the initial biomass. Pinewood with the lowest ash content in its initial composition (0.2 wt.%) had de lowest ash content in its char (11 wt.%) while the agro-biomasses with ash contents as high as (14.7 wt.% for straw) in their initial composition had ash contents as high as 43 wt.% for straw.
The ash contents in the chars agro-biomasses are a prove that the oxygen settled in the mineral matrix of initial feedstock followed to char rather than for bio-oil, which may have resulted in less oxygenated bio-oils but higher oxygenated chars for the agro-biomasses. The high O/C ratios are evidences of such oxygenation. As result, pinewood with the lowest ash content had the highest carbon content and the lowest O/C ratio.
Due to the higher content of carbon (75 wt.%), the char obtained from pinewood had the higher heating value of 27.2 MJ/kg. This value is in agreement with Thangalazhy et al. [63] and
Table 3.3 – Physical properties of the char obtained from pyrolysis of pinewood, olive bagasse, wheat straw and rice husk.
Ash, wt.% 11 36 43 37
Moisture, wt.% 3.1 1.3 2.3 3.9
C (wt.%) 75 60 47 53
H (wt.%) 3.5 2.9 2 2.6
N (wt.%) < 0.5 2.2 < 0.5 < 0.5
S (wt.%) < 2 < 2 < 2 < 2
O (wt.%) 19 32.9 48.5 41.9
O/C 0.25 0.55 1.03 0.79
H/C 0.09 0.05 0.04 0.05
HHV, MJ/kg 27.2 21.6 17.7 21.3
LHV, MJ/kg 26.4 21.0 17.3 20.7
Wheat straw
char Rice husk char
Analysis Olive bagasse
Pinewood char char
DeSisto et al [66], which obtained a heating value for the char from pinewood of 28.1 MJ/kg and 28.5 MJ/kg at 500 ºC, respectively.
The char from wheat straw had the worst heating value of 17.7 MJ/kg with the worst carbon content (47 wt.) and, consequently, the higher oxygenation (O/C ratio of 1.03). Such fact is in agreement with its highest ash content in the initial feedstock that turns the char into a more oxygenated product. Yanik et al. [64]also obtained a char from the pyrolysis of wheat straw at 500 ºC with a heating value of 19 MJ/kg and an ash content of 38.3 wt.%.
The olive bagasse pyrolysis resulted in a char with a heating value of 21.6 MJ.kg, a slightly lower than that reported by Şensöz et al. [63]of 24.8 MJ/kg. Such fact may be related to the lower heating rate imposed in the pyrolysis of olive bagasse, which result in a char with higher carbon content (73.1 wt.%).
The char from rice husk presented a heating value of 21.3 MJ/kg, higher than that obtained by Di Blasi et al. [69], which also pyrolysed rice husk at 580 ºC in a packed bed and obtained a char with a heating value of 18.7 MJ/kg with less carbon content (51.5 wt.%)