SOBRE EL FONDO DEL ASUNTO

5.2.1 Repeatability of dry pyrolysis and HTC

As already done for the oak wood in Chapter 4, the carbonization of

Laminaria hyperborea was carried out in triplicate in order to assess its

repeatability. In particular, the variation of the carbonization yield was determined for both dry pyrolysis and for hydrothermal carbonization. Furthermore, for each HTC run, the errors related to the water balance (see Chapter 3) were also calculated.

Around 5 g of raw Laminaria hyperborea were pyrolyzed in a ceramic furnace, and each run was performed under the same conditions (see Chapter 3). As already mentioned in Chapter 4, it was not possible to estimate the repeatability of liquid and gas fractions, as the limited amount of volatiles produced was dragged away to the exhaust by the purge gas before it could condense in the trap.

Standard amounts of raw material (24 g) and water (220 g) were used for the hydrothermal treatment repeats. The mixture was then heated up at 250 °C and held at this temperature for 1 h.

As seen in Table 5-1, the pyrolysis treatment led to higher yield variability than that measured for the HTC repeats. This was indicated by the higher value of standard deviation from the mean (up to 7.33%) calculated for the LH_S800’s yield compared to that obtained for LH_S250 (3.56%). Yet, the repeatability of both carbonization processes was within the maximum tolerable value (i.e. 10% [282]).

The average pyrolysis yield (ca. 27 wt%) was higher than that recorded for the pyrolyzed wood (21.50 wt%, see Chapter 4). This might be due to the catalytic effect of the higher inorganic fractions (especially K) contained by

Laminaria hyperborea (see Table 5-11 and Table 5-12), which are expected to

increase the char production [134, 181, 200, 201]. On the other hand, proximate analyses revealed that the proportion of fixed carbon (FC) measured for pyrolyzed Laminaria hyperborea (27.5 wt%) was much lower than that measured for OW800 (66.1 wt%). This was probably because of the larger amount of ash (A)-free organic matter contained within the raw oak wood, which

was converted to ash-free carbon [296]. This might suggest oak wood as a better candidate for activated carbon production.

Pyrolysis of Laminaria hyperborea was also conducted in a larger scale (stainless steel) rig under the same conditions held when using the ceramic furnace, but loading larger amounts of raw feedstock. The large-scale pyrolysis was carried out in duplicate, yet a different mass of raw material was used for each run (ca. 65 or ca. 140 g). Interestingly, the carbonization runs led to very similar yields (ca. 30.36 wt% and 30.79 wt% respectively), thus showing that the mass of raw feedstock did not affect the final char yield. In addition to that, it is notable that the pyrolysis process conducted in the large-scale rig resulted in a slightly higher char formation compared to the carbonization carried out in the small-scale rig (i.e. ceramic furnace). Accordingly, if the influence of the reactor loading was to be excluded, this finding might be ascribed to a slower pyrolysis (lower heating rate (HR)) occurred in the large-scale rig, which might have facilitated char production [181]. This result was also in line with that found by Ross et al. [134], reporting a larger char formation for slow ramp rate pyrolysis than for flash pyrolysis.

According to the author’s best knowledge, no data related to the carbonization of Laminaria hyperborea under similar conditions used in this study were found in the literature. However, few studies about the pyrolysis of different types of Laminaria have been reported. For instance, Bae et al. [138] pyrolyzed Laminaria japonica at 600 °C for 1h, measuring a product yield of ca. 40 wt%. This was higher than that measured in this study likely because of the lower pyrolysis temperature applied. In addition, Ferrera-Lorenzo et al. [203] pyrolyzed a macroalgae solid waste under similar conditions to those used in this work (i.e. 750 °C for 1 h). A carbonization yield of ca. 30 wt% was reported by these authors, which agrees with that found in the present study, especially when pyrolyzing Laminaria hyperborea in the large scale reactor. Also, Stratford

et al. [202] reported a lower char formation (i.e. 17 wt%) after pyrolysis of Laminaria digitata (oarweed) at 800 °C. However, in this case the product yield

was determined considering the solid residue following pyrolysis and acid washing.

The hydrothermal treatment of Laminaria hyperborea yielded a slightly larger amount of solid residue (ca. 34 wt%) than that measured after pyrolysis. This was in accordance with results already found for oak wood (see Chapter 4), as the hydrothermal synthesis of biomass was carried out at much lower temperature compared to that used for the pyrolytic treatment.

Previous studies on the hydrothermal treatment of macroalgae are also limited. A solid residue of ca. 25 wt% was reported by Anastasakis and Ross [206] who performed HTC of Laminaria hyperborea at 250 °C for 15 min. Furthermore, another type of macroalgae (i.e. Enteromorpha prolifera) was hydrothermally carbonized by Xu et al. [208] at 250 °C for 60 min, obtaining a

solid yield of 15 wt%. HTC of Enteromorpha prolifera was also carried out by Zhou et al. [205], who reported hydrochar yields below 20 wt% when conducting hydrothermal treatment at temperatures of 240 and 260 °C held for 30 min. It is worth noting that the higher yield obtained for Laminaria hyperborea in this work might indicate this feedstock as a more convenient precursor in terms of hydrochar production.

Table 5-1 Repeatability of carbonization processes of Laminaria hyperborea. SD is the standard deviation of the three measurements.

Carbonization Water balance

LH_S80027 _{LH_S250}28

Run _CYdb29 Run CYdb minitial mrecovered mlost

- wt% - wt% mg mg mg 1 24.63 1 34.05 220.00 166.58 53.42 2 28.16 2 35.26 220.00 162.88 57.12 3 27.93 3 32.84 220.00 163.79 56.21 Mean 26.91 Mean 34.05 - 164.42 55.58 SD 1.97 SD 1.21 - 1.57 1.57 SD (% Mean) 7.33 SD (% Mean) 3.56 - 0.96 2.83

Nonetheless, in contrast to results found for traditional pyrolysis, the average yield measured for hydrothermal carbonization of Laminaria

hyperborea (ca. 34 wt%) was substantially lower than that obtained for the

hydrothermally carbonized wood (ca. 58.5 wt%). This finding might signify that the catalytic effect of inorganic matter during hydrothermal treatment was not as important as that during pyrolysis. In fact, some inorganics might have been released into the HTC process water. On the other hand, wood-derived biomass normally features higher levels of lignin, which is believed to favour char production [134, 181, 200, 201].

A very low variability of the water balance was achieved when repeating HTC of Laminaria hyperborea, with standard deviation below 1%. The average amount of process water recovered after hydrothermal carbonization of

27 _{Laminaria hyperborea summer (LH_S) pyrolyzed at 800 °C} 28 _{LH_S hydrothermally carbonized at 250 °C}

Laminaria hyperborea (164.42 mg) was comparable to that found for oak wood-

derived hydrochar (160.90 mg), which seems to suggest that the type of feedstock did not significantly affect the amount of water lost during the hydrothermal process.