*2.1. Microalgae Culturing*

For this study, *Chlorella sorokiniana* CCAP 211/8 K (UTEX Culture Collection) was cultured in axenic conditions. This strain was chosen because Chlorella sp. is amongs<sup>t</sup> the most commonly used for wastewater treatment, possesses high growth rates, high light to biomass conversion, ability to grow under phototrophic, photomixotrophic and heterotrophic conditions, high protein amount, essential amino acids, and fatty acids [24,25]. On the other hand, as compared with *C. vulgaris*, the thermal decomposition of *C. sorokiniana* has been less studied in the literature.

The inoculum of *C. sorikiniana* was cultivated in 250 mL Erlenmeyer flasks in the standard culture medium Mann and Myers [26], which is composed of (per litre of distilled water): 1.2 g MgSO4·7H2O, 1.0 g NaNO3, 0.3 CaCl2, 0.1 g K2HPO4, 3.0 × 10−<sup>2</sup> g Na2EDTA, 6.0 × 10−<sup>3</sup> g H3BO3, 2.0 × 10−<sup>3</sup> g FeSO4·7H2O, 1.4 × 10−<sup>3</sup> g MnCl2, 3.3 × 10−<sup>4</sup> g ZnSO4·7H2O, 7.0 × 10−<sup>6</sup> g Co(NO3)2·6H2O, 2.0 × 10−<sup>6</sup> g CuSO4·5H2O. This inoculum was grown under constant temperature (25 ± 1 ◦C), irradiance (175 <sup>μ</sup>E/m2·s), photoperiod (12:12), and shaking (250 rpm) until reaching a biomass concentration of 0.1 g/L. Afterwards, the culture was grown in a 10 L-PBR after acclimatization to synthetic wastewater in 1 L-bubbling column photobioreactors (PBRs) and growing up to 0.1 g/<sup>L</sup> of biomass concentration. Each litre of synthetic wastewater was prepared by dissolving in distilled water: peptone, 160 mg; meat extract, 110 mg; urea, 30 mg; anhydrous dipotassium hydrogen phosphate (K2HPO4), 28 mg; sodium chloride (NaCl), 7 mg; calcium chloride dehydrate (CaCl2·2H2O), 4 mg; and, magnesium sulphate heptahydrate (Mg2SO4·7H2O), 2 mg. This synthetic wastewater [27] gives a mean DOC concentration of about 100 mg/L. The culture conditions and medium used for the inoculum growth were maintained for microalgae growth in the PBRs, except for the irradiance, which was 650 <sup>μ</sup>E/m2·s, as supplied by eight fluorescent lamps (58 W, 2150 lumen, Philips, France). Furthermore, the PBRs were aerated with filtered air (0.22 μm sterile air-venting filter, MillexFG50-Millipore), at a rate of 0.3 v/v/min., enriched with CO2 at 7% v/v, which was injected on demand to keep a constant pH (pH = 7.5 ± 0.5), as controlled by a pH sensor. At the end of the culture, the 10 L-PBR was dismantled, and microalgae biomass was harvested by 5 min centrifugation of the cellular suspension at 6461 g in a SIGMA 2-16P centrifuge. Microalgae biomass (MB) was then washed twice with distilled water, oven dried during 24 h at 378 K, homogenized, and stored until use at 277 K.

### *2.2. Materials and Characterization*

MB and a bituminous coal (BC) coming from the north coalfield of León (Spain) and commonly exploited in thermal power stations were used in this work. Before thermal analysis, MB and BC were grinded and sieved to have a 0.105 mm < particle diameter < 0.210 mm, which is within the size range that is commonly used in circulating fluidized boilers, allows for minimization of di fferences in heat of combustion values, and is large enough to ensure homogeneous ignition. Subsequently, proximate analyses of MB and BC were carried out following the procedures from ASTM D3172 to D3175 [28–31]. Elemental analysis was performed in a LECO CHNS-932, according to standard procedures, namely ASTM D5373 [32] and ASTM D4239 [33].

The high heating value (HHV) of MB and BC at a constant volume was determined by means of an isoperibol oxygen bomb calorimeter LECO AC-600 and following the procedure UNE-EN 14918:2011 [34]. Additionally, and for comparison purposes, HHV was estimated by the correlations that are listed in Table 1, together with the corresponding assumptions.




**Table 1.** *Cont*.

HHV: high heating value; [C]: carbon content; [H]: hydrogen content; [N]: nitrogen content; [S]: sulphur content; [O]: oxygen content; [FC]: fixed-carbon content; [VM]: volatile matter content; [Ash]: ash content; note: all values are expressed in wt% on dry basis.
