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.
Abstract: This work focuses on the production of biodiesel from wet, lipid-rich algal biomass using a two-step process involving hydrothermal carbonization (HTC) and supercritical in situ transesterification (SC-IST). Algal hydrochars produced by HTC were reacted in supercritical ethanol to determine the effects of reaction temperature, time, ethanol loading, water content, and pressure on the yield of fatty acid ethyl esters (FAEE). Reaction temperatures above 275 °C resulted in substantial thermal decomposition of unsaturated FAEE, thereby reducing yields. At 275 °C, time and ethanol loading had a positive impact on FAEE yield while increasing reaction water content and pressure reduced yields. FAEE yields as high as 79% with a 5:1 ethanol:fatty acid (EtOH:FA) molar ratio (150 min) and 89% with a 20:1 EtOH:FA molar ratio (180 min) were achieved. This work demonstrates that nearly all lipids within algal hydrochars can be converted into biodiesel through SC-IST with only a small excess of alcohol.
Pub.: 10 Apr '13, Pinned: 31 Jul '17
Abstract: The need for efficient and clean biomass conversion technologies has propelled Hydrothermal (HT) processing as a promising treatment option for biofuel production. This manuscript discussed its application for pre-treatment of microalgae biomass to solid (biochar), liquid (biocrude and biodiesel) and gaseous (hydrogen and methane) products via Hydrothermal Carbonisation (HTC), Hydrothermal Liquefaction (HTL) and Supercritical Water Gasification (SCWG) as well as the utility of HT water as an extraction medium and HT Hydrotreatment (HDT) of algal biocrude. In addition, the Solar Energy Retained in Fuel (SERF) using HT technologies is calculated and compared with benchmark biofuel. Lastly, the Life Cycle Assessment (LCA) discusses the limitation of the current state of art as well as introduction to new potential input categories to obtain a detailed environmental profile.
Pub.: 31 Oct '15, Pinned: 31 Jul '17
Abstract: Thermal hydrolysis and hydrothermal processing show promise for converting biomass into higher energy density fuels. Both approaches facilitate the extraction of inorganics into the aqueous product. This study compares the behaviour of microalgae, digestate, swine and chicken manure by thermal hydrolysis and hydrothermal processing at increasing process severity. Thermal hydrolysis was performed at 170°C, hydrothermal carbonisation (HTC) was performed at 250°C, hydrothermal liquefaction (HTL) was performed at 350°C and supercritical water gasification (SCWG) was performed at 500°C. The level of nitrogen, phosphorus and potassium in the product streams was measured for each feedstock. Nitrogen is present in the aqueous phase as organic-N and NH3-N. The proportion of organic-N is higher at lower temperatures. Extraction of phosphorus is linked to the presence of inorganics such as Ca, Mg and Fe in the feedstock. Microalgae and chicken manure release phosphorus more easily than other feedstocks.
Pub.: 30 Nov '15, Pinned: 31 Jul '17
Abstract: Nitrogen-doped carbon materials are synthesized via an effective, sustainable, and green one-step route based on the hydrothermal carbonization of microalgae with high nitrogen content (ca. 11 wt %). The addition of the monosaccharide glucose to the reaction mixture is found to be advantageous, enhancing the fixation of nitrogen in the synthesized carbons, resulting in materials possessing nitrogen content in excess of 7 wt %, and leading to promising reaction yields. Increasing the amount of glucose leads to a higher nitrogen retention in the carbons, which suggests co-condensation of the microalgae and glucose-derived degradation/hydrolysis products via Maillard-type cascade reactions, yielding nitrogen-containing aromatic heterocycles (e.g., pyrroles) as confirmed by several analytical techniques. Increasing the HTC processing temperature leads to a further aromatization of the chemical structure of the HTC carbon and the formation of increasingly more condensed nitrogen-containing functional motifs (i.e., pyridinic and quaternary nitrogen).
Pub.: 01 May '12, Pinned: 31 Jul '17
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