A pinboard by
Sidra Jabeen

PhD student, Murdoch University


Hydrochars produced from hydrothermal carbonisation of microalgae under sub critical water condition

Hydrothermal carbonisation (HTC) of microalgae is one of the bioenergy pathways, targets at producing the solid biofuel ‘hydrochar’ along with the nutrient rich, aqueous phase and gases as co-products. The principal product of microalgae HTC is hydrochar, with the heating value comparable to that of sub-bituminous coal and found applications for energy production in combination with other thermochemical and hydrothermal processes. Other applications of hydrochars are as soil amendment agent, for carbon sequestration, waste water treatment and nutraceuticals. Aqueous co-product is found to be rich in nutrients especially nitrogen and is recycled to algal bioreactors to aid algae growth. The process occurs at the sub critical conditions of water over the temperature range 180 – 260 ºC and pressures 2 – 6 MPa for 5 – 240 min. Over the sub critical temperature range, the properties of water are remarkably different from the ambient water with low density, viscosity, surface tension and dielectric constant, while high ionisation product and diffusivity. So, sub critical water behaves as non-polar solvent, acid-base catalyst and facilitates higher mass transfer rates. Carbohydrates, proteins, lipids and nucleic acids are the major biochemical components of microalgae composing of vast variety of organic and inorganic elements and trace metals, all of which, undergoes the multiple complex chemical reactions under hydrothermal conditions to yield hydrochar (20 – 55 %), aqueous product (30 – 55 %) and gases (2 – 6 %). The yields of HTC products are highly dependent on the reaction temperature and time. The general trend follows the decrease of hydrochar yield and increase of co-products yield with increasing temperature and time because, more solid algae becomes soluble in water at higher reaction temperature and time. However, there are many other factors like reaction pressure, pH, algae particle size and pre-treatment, heating and cooling rates, hydrochar post treatment and reactor configuration, which may affect the yields of the HTC products. Likewise, properties of hydrochar vary depending on the process parameters. Higher reaction temperature enhances the heating values and high holding time increases the structural stability of the hydrochars. Hydrothermal carbonisation is not a developed technology particularly for microalgae and research is focused to optimize the process conditions to produce hydrochar with rationale yield and improved properties.