(111e) Hydrothermal Liquefaction of the Brown Macroalgae Saccharina Latissima for the Production of Biofuel

Seaweed aquaculture has matured over the last century for use as an energy source to produce biofuels which have a high feedstock requirement and need large amounts of biomass. Compared with land-based biomass, cultivated macroalgae in water presents numerous advantages such as freeing up land for food crops farming, increased carbon absorption, and cleaning of contaminated water. Macroalgae usually has a high-water content, as much as 90 wt. % in some species, and hydrothermal liquefaction (HTL) has gained attention due to its intrinsic advantage to produce biofuels using water in sub-critical region conditions as a reaction medium. This process eliminates the cost associated with setting up a separate drying unit for the feed, since the wet macroalgae can be used directly.

We performed hydrothermal liquefaction of brown macroalgae (Saccharina latissima) in the presence of homogenous and heterogenous catalysts such as KOH, Na₂CO₃, Pd/C, HZSM-5, Ce/HZSM-5, and Ni/SiO₂-Al₂O₃ in hot compressed water, to investigate their effects on product yields (biocrude, water-soluble product, gas, and solid residue) and compounds distribution in the biocrude. The experiments were conducted in a batch reactor at 330^oC and 120-130 bar, for a residence time of 1 hour. To investigate hydrocarbon production, we also used methanol as a co-solvent (methanol-to-water 1/4 vol %) and as the only liquefaction solvent. We conducted further experiments on the macroalgae to investigate the effects of varying temperature, residence time and algae-to-water ratio on the product yields. Only one parameter was varied at a time for these catalytic hydrothermal liquefaction runs: at four different temperatures (240℃, 270℃, 300℃, and 330℃), three residence times (20 minutes, 40 minutes, and 60 minutes), and algae-to-water ratios (1:5, 2:15, 1:10 and 2:25). The final factor we studied was the effect of macroalgae pretreatment using deionized water on the product yield and biocrude composition. We conducted the gas Chromatography – Mass Spectrometry (GC-MS) analysis of the biocrude using Agilent GC 8890 equipped with a quadrupole mass spectrometric (MS) detector 5889, with a semi-polar capillary column. The GC inlet temperature was 300^oC and the oven-programmed temperature was heated from 35^oC to 300^oC, at a heating rate of 10^oC.min^-1. We identified the compounds in the biocrude using the National Institute of Standards and Technology Mass Spectra (NISTMS) library and quantified them by external calibration with analytical standards, to obtain their yields and selectivities. Finally, we explored pyrolysis as a secondary process to upgrade the biocrude obtained from HTL to produce hydrocarbons. We used a pyrolyzer connected to the GC-MS and conducted catalytic fast pyrolysis of the biocrude with HZSM-5 catalyst (1:1 and 1:5 weight-to-weight ratio).

The biocrude we obtained from all hydrothermal liquefaction runs was a very viscous dark-brown liquid with a pungent smell. The biocrude yields from the runs with water ranged from 12.9 wt.% (non-catalytic run) to 23.6 wt.% (with Ce/HZSM-5) and that of solid residue ranged from 5.0 wt.% (with KOH) to 19.1 wt.% (with Pd/C), while gas yields were from 18.6 wt.% (with KOH) to 28.3 wt.% (with Ni/SiO₂-Al₂O₃). The largest fraction of the products (23.8 wt.% - 39.9 wt.%) were dissolved organics in the aqueous phase. The maximum biocrude yield of 25.7 wt.% was obtained using methanol as a liquefaction co-solvent. We noticed no significant difference in the biocrudes yield from using methanol as co-solvent or only solvent, which could be because methanol can effectively extract and dissolve the organic matter from macroalgae regardless of whether it is used as co-solvent or only solvent. We analyzed the biocrude and found it to be a complex mixture of aldehydes, ketones, phenols, hydrocarbons, fatty-acids, esters, and heterocyclic nitrogenous compounds. Each catalyst led to varying selectivities for the classes of compounds identified in the biocrude. In the methanol runs, fatty acids and esters were the most abundant compounds identified in the biocrude with an increasing trend in their quantities for corresponding increase in the volume of methanol used as solvent. The best catalysts that showed relatively higher selectivities to produce hydrocarbons in water medium were HZSM-5 (3.8%), Ce/HZSM-5 (3.6%), and Pd/C (1.8%).

The highest average yield of biocrude (20.9 wt.%) was obtained at 330°C, revealing an upward tendency of yield with increasing reaction temperature. This assessment agrees with previous studies that have found that high temperature facilitates reactions such as repolymerization, hydrolysis and transformation in the hydrothermal liquefaction process. The biocrude yield also followed an upward trend for increasing residence time with 60 minutes leading to the highest yield in the range we studied. We found the effect of algae-to-water ratio on biocrude yield to be insignificant within the range of our study. Without pretreatment, the bio-crude yield was discovered to be lower, with higher yield of water-soluble products, and lower yield of gases and solid residues compared to the pretreated run. The yield of biocrude produced without pretreatment was 17.2 wt.%, compared to 20.9 wt.% after treatment. We hypothesized that sea salts present in the macroalga reduce the amounts of biocrude obtainable during conversion, because they can act as catalyst poisons during the HTL process. The biocrude we obtained without pretreatment, was mostly composed of complex nitrogenous compounds as well as unsaturated ketones and ethers, compared to the pretreated macroalgae under the same operation condition which had a high selectivity for fatty acids & esters. Upgrading of the biocrude through pyrolysis-GCMS led to very high hydrocarbon selectivity of about 90%, majorly composed of aromatics like benzene, toluene, styrene, and ethylbenzene.

These results show promise in assessing the prospect of using S. latissima as a natural source to produce transportation fuels and high-value chemicals. All the catalysts led to increased biocrude yields, and the choice of catalysts and solvent had significant impact on the composition of biocrude produced during the liquefaction process. Temperature was discovered to be the most important factor affecting the biocrude yield, with the amount of biocrude obtained increasing from 11.1 wt.% (at 240^oC) to 20.9 wt.% (at 330^oC). Of the three tested residence times, 60 minutes was most favorable as it allowed enough time for the maximum amount of biocrude (20.9 wt.%) to form without losing components to the water-soluble products or solid residues. The effect of algae-to-water ratio was not very pronounced as the variation in biocrude yield was minimal. Pretreating of the feedstock by washing with deionized water generally helped in eliminating impurities, improving biocrude yield and the distribution of compounds in the obtained biocrude. Pyrolysis appeared to be a viable process for upgrading this biocrude to more closely approximate transportation fuels and obtaining high-value chemicals. The zeolite catalyst (HZSM-5) was most effective in the biomass-to-catalyst ratio, 1:5 to pyrolyze the biocrude obtained from hydrothermal liquefaction to produce 4.1 wt.% hydrocarbons (89.8% selectivity), that was primarily composed of aromatics.