The thermochemical conversion of biomass through pyrolysis allows to convert dry biomass into a liquid, and to perform, at the same time, both the extraction of hydro­carbon-like material and the cracking of high molecular weight compounds (Fig. 11). The main advantages of this approach are: (1) avoiding the use of any solvent, thus increasing the “greenness” of the process and (2) producing useful co-products, like a combustible gas, usable as process energy source, and a biochar, suitable as soil — amending material as well as stable carbon sink. Thus, this strategy allows to exploit in different ways all the biomass constituents, the liquid bio-oil, and the residues (gas and biochar), increasing the net energy/CO2 balance of a hypothetical algal cultivation (Fig. 12).

In the present work, B. braunii pellets were treated through a fixed bed pyrolysis at 500°C following the same experimental procedure described in the literature [46]. The yields of the three major pyrolysis fractions (bio-oil, biochar, and vola­tiles) and their composition are shown in Table 5.

Pyrolysis gas yield is 37%, corresponding to 28% yield after feedstock water subtraction.

Gas composition is evaluated by pyrolysis coupled with dynamic solid-phase micro-extraction (Py-SPME) followed by GC-MS analysis [47]. This technique is capable of giving information on the composition of gaseous and semi-volatile pyrolysis products, useful to obtain an overall picture of the most volatile compounds. The analysis reveals the presence of light hydrocarbons, aromatic hydrocarbons

(mainly toluene), and some nitrogen-bearing aromatics (pyridine, indole) probably originated from pyrolysis of proteins.

Biochar yield is 38%, of which almost half of weight is composed by inorganic compounds; in fact, it retains almost all biomass ashes and for this reason biochar is not suitable as solid fuel, although usable as carbon stock. In order to evaluate the structural conformation of biochar, the material is visually characterized by scan­ning electronic microscopy (SEM) and the obtained pictures are shown in Fig. 13.

SEM image shows that biochar from 500°C pyrolysis is formed by a randomly aggregated sponge-like material. The dimension of particles is about 10-100 pm.

Bio-oil (25% on dry weight basis) is almost composed by a n-hexane soluble material; after hexane evaporation, this liquid fraction results in a black-reddish ash­free liquid (ashes less than 0.1%), relatively low viscous, of which only a very small part (1%) is insoluble in n-hexane. Moreover, the bio-oil is characterized by a negli­gible amount of water. This finding is quite surprising because the presence in the B. braunii feedstock of a significant amount of proteins (7%) and polysaccharides (20%), since the polar and oxygenated nature of these macromolecules, should give an expected larger amount of reaction water and of oil insoluble in n-hexane.

6

Time (min)

Fig. 14 GC-FID chromatogram of B. braunii pyrolysis oil

Reasonably, we can assume that the “vaporization” and hydrocarbon cracking processes are more effective than carbohydrate/proteins pyrolysis, and that the presence of non-lipid materials determines a low yield of pyrolysis products respect to hydrocarbon material. Nevertheless, if we consider the large carbohydrates content (20% of feedstock), usually able to produce a significant amount of organic tar (33% yield from cellulose using the same reactor [48]), such low amount of n-hexane — insoluble matter is noticeable. When a hydrocarbon-rich microalga as B. braunii is submitted to pyrolysis, proteins and polysaccharides form only a minor amount of bio-oil. From a practical point of view, this means that pyrolysis can be seen as a suitable “solvent-less extraction method” highly selective for hydrocarbons.

The chemical characterization by GC-FID and GC-MS of the n-hexane-soluble bio-oil reveals the presence of typical B. braunii linear dienes and trienes (C27H52, C29H56, C29H54 and C31H60) and, in addition, a series of random hydrocarbon fragments consisting in Cjj-C27 alkanes and alkenes (Fig. 14). From a quantitative point of view, almost the whole amount of the pyrolysis oil (around 90%) consists of GC detectable compounds, indicating that heavy cross-linked hydrocarbon polymers are depolymerised to smaller fragments during the pyrolytic treatment.

In order to have information on the volatilization properties of this potential new fuel, thermogravimetric analysis (TGA) was also done (Fig. 15).

TGA of pyrolytic oil shows four weight loss steps characterized by different extent. First, weight loss, probably generated by a gradual volatilization of small random length Cj 1-C27 hydrocarbons, starts gradually from 50°C and becomes important at 200-300°C. Around 300°C, a sharp weight loss derivatives peak, prob­ably deriving from the boiling of the olefin C2 9H56, is observed. In addition, two smaller weight losses are recorded at 380 and 450°C, probably related to C31H60 and residual high molecular weight matter in the oil. In general, TGA observation confirms the results obtained from GC analysis and gives an indication that

Fig. 15 TGA profile of B. braunii pyrolysis oil.

Dotted line: percent weight change; full line: temperature derivate of weight change.

HMW high molecular weight residue (e. g. ether lipids, or other long chain hydrocarbons)

20

10

0

n-hexane-soluble pyrolysis oil obtainable from B. braunii is lighter than B. braunii non-polar lipids and hydrocarbons from which it is produced (see Fig. 3). Moreover, whole n-hexane-soluble fraction boils out almost totally (>90% weight loss) before 400°C. This could be an indication that the liquid obtained could be directly used as fuel, without any further upgrading or modification.