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Hydrothermal carbonisation of microalgae: Feedstock characterisation, model compound decomposition, and assessment of product recovery methods

Jabeen, Sidra (2020) Hydrothermal carbonisation of microalgae: Feedstock characterisation, model compound decomposition, and assessment of product recovery methods. PhD thesis, Murdoch University.

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Australia presents a reasonable contribution to the World’s greenhouse gas emissions because of its heavy reliance on fossil fuels for energy production. Coal is the primary founder of energy in Australia with the secondary sourcing from oil, gas, solar, wind, biomass and hydro. Bioenergy currently offers only 1 % of the total energy produced in Australia with anticipated uplift to 8 % by 2030 that demands active research on biofuel resources and production processes. Australia lavishes with land, sunlight, and brackish water, necessary for the growth of algal biomass, the unique composition (carbohydrates, lipids, proteins, nucleic acid and moisture) of which makes it an excellent feedstock for hydrothermal processing. Hydrothermal treatment of algal biomass for the production of biofuels and value-added products is an attractive route towards increasing the bio-energy contribution in Australia to meet the challenges of growing greenhouse gas emissions.

Hydrothermal treatment of algal biomass at high temperatures (280 – 700 °C) to produce energy-dense liquid and gaseous fuels are extensively researched for the last three decades with the exceptional attributes achieved in the field. However, the low temperature (180 – 260 °C) hydrothermal carbonisation (HTC) of algal biomass, which essentially serves as an intermediate stage of intense hydrothermal processing, has received less attention. Hydrothermal carbonisation produces hydrochar as a primary product along with biocrude and aqueous phase as co-products, all of which possess exceptional traits to be employed directly or indirectly as biofuels and/or as advanced carbon materials. Despite the rationale research on hydrothermal carbonisation of algal biomass, there are still significant research gaps in assessing the feasibility of hydrothermal carbonisation process for microalgae in the context of an Algal Biorefinery, including (i) the lack of standardised conditions for the characterisation (e.g., proximate analysis) of algal biomass; (ii) detailed mechanisms for the evolution of nitrogen-containing species from thermal decomposition of algal biomass; (iii) the impacts product recovery methods on the yields and properties of hydrochar from HTC of algal biomass; and (iv) detailed characterisation of the biocrude and aqueous phase from HTC of algal biomass.

The present study aims to address the aforementioned research gaps, with the following specific objectives: (1) to validate the applicability of the conventional proximate analysis methods, which are developed for solid biofuels, for algal biomass, using Spirulina and Chlorella as model algal samples, and then develop an analytical procedure for the proximate analysis of algal biomass; (2) to study the thermo-kinetics of the most plausible reactions encountered during thermal decomposition of ‘Leucine’ as an algal biomass model compound and establish a robust kinetic model accounting for the emission of nitrogen-containing species; (3) to investigate the impact of the product recovery methods on the yield and properties of hydrochar from HTC of Chlorella under the identical reaction conditions; and (4) to characterise the biocrude and aqueous phase from HTC of Chlorella and evaluate the impact of reaction temperature and holding time on the recovery of metals in biocrude and aqueous phase, which is essential for recycling and re-use of these products. These research objectives have been successfully fulfilled in this study, with the key findings outlined below.

Firstly, the widely employed ASTM E870–82 designed for woody biomass modified by thermogravimetric analysis (TGA) is found to be inapplicable for the proximate analysis of Spirulina and Chlorella, as indicated by the presence of unburnt carbon and quaternary nitrogen in the resulting ashes, although it accurately determines the contents of volatile matter. While Spirulina can be entirely ashed at 600°C in air, complete oxidation of Chlorella requires the aid of hydrogen peroxide (H2O2) under a similar condition. An analytical procedure is developed for the proximate analysis of algal biomass, using Spirulina and Chlorella as a case study. The procedure consists of three steps: (1) application of the TGA–ASTM E870–82 method to algal biomass followed by observing the colour of the ash residue; (2) direct ashing of algal biomass at 600 ºC in the air to get a fully oxidised ash residue and thereby determine ash content; and (3) oxidation of algal biomass in air at 600 °C for 4 h, with the aid of H2O2. The number of steps required to perform a complete proximate analysis depends on the properties of algal biomass.

Secondly, we map out the potential energy surface for a wide array of unimolecular and self-condensation reactions operating in the thermal decomposition of leucine, which was elected as an algal biomass surrogate compound. Decarboxylation, dehydration, and deamination ensue by eliminating CO2, –OH, and NH2, respectively, from α-carbon of leucine. The reaction rate constants indicated comparable branching ratios for decarboxylation and dehydration channels with a minimal contribution from the deamination route. The calculated kinetic parameters served to model a plug-flow reactor that demonstrates species profiles in the gas phase and shows that conversion of leucine attains 100 % in the temperature range 700 – 950 K with a residence time of 10 s. Our kinetics model predicts the formation of isoamylamine (C5H13N), 3-methyl butane-1-imine (C5H11N), 3-methylbutane nitrile (C5H9N), CO2, NH3, H2O, C3H6, and C2H4 as significant products from thermal decomposition of leucine. Kinetic analysis of the plausible channels concluded that the dehydration constitutes the dominant pathway and self-condensation reactions contributed marginally to thermal decomposition of leucine in the gas phase.

Thirdly, the hydrochar produced from HTC of Chlorella with 10.0 wt % of solid loading at 180 – 220 °C for holding time of 15 and 60 min was recovered from the batch reactor by two methods that have been commonly employed in literature. The first method involved direct filtration of the product mixture and subsequent drying of the hydrochar, without the use of solvent, whereas the second method used dichloromethane (DCM) to rinse the reactor contents, which recovered hydrochar from the aqueous phase and biocrude, followed by drying of the hydrochar. These two methods are hereafter termed as “direct filtration” and “DCM-aided filtration”, respectively. We found that direct filtration retains heavy biocrude on the hydrochar surface, which results in high mass and energy yields, high carbon levels, low ash contents, low reactivity and improved higher heating values (HHVs) of the hydrochars, in comparison with their counterparts from DCM-aided filtration. The adsorption of biocrude on hydrochar surface leads to blockage of the pores and gives higher intensities of functional groups on the hydrochar surface, both of which strongly influence the retentions of inorganic elements in the hydrochar.

Finally, we performed the detailed characterisation of the biocrude and aqueous phase from HTC of Chlorella at 180 – 220 °C for holding time of 15 and 60 min. As the reaction proceeds from mild to severe conditions, mass yield and energy recovery of biocrude increase, and molecular weight distribution of constituent compounds shifts to the smaller size. The highest yield of biocrude was found to be 34.6 wt % on a dry basis of algae at 220 ºC and 60 min with a corresponding heating value of 34.0 MJ/kg. The mass yield of aqueous phase first increases, and then slightly reduces, showing decomposition to biocrude and gaseous products. The relative abundance of N-containing heterocyclic compounds (e.g. 3,6-Diisopropylpiperazin-2,5-dione) in the biocrude increase and oxygenates (such as 1-Hexadecen-3-ol,3,7,11,15-tetramethyl) decrease with an increase of reaction temperature and holding time. The recovery of different metals (Na, K, Mg, Ca, Fe, and Zn) in the biocrude appears to be a strong function of reaction temperature and holding time. For instance, iron in the biocrude decreases from 800.23 to 564.45 mg/kg with an increase in temperature from 180 – 220 °C at 15 min, and a substantial reduction is observed at 60 min (i.e. from 1111.87 to 238.71 mg/kg). The aqueous phase contains most of the essential macronutrients e.g. Na+, K+, CH3COO-, H2PO4- and NH4+, and micronutrients such as Mn, Fe, and Zn, which can be recycled as cultivation media for the growth of algal biomass.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Engineering and Energy
United Nations SDGs: Goal 7: Affordable and Clean Energy
Supervisor(s): Gao, Xiangpeng, Dlugogorski, Bogdan and Altarawneh, Mohammednoor
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