The rapid depletion of conventional fossil fuels and day-by-day growth of environmental pollution due to use of extensive use of fossil fuels have raised concerns over the use of the fossil fuels; and thus search for alternate renewable and sustainable sources for fuels has started in the last few decades. In this context biomass derived fuels seems to be the promising path; and various routes are available for the biomass processing such as pyrolysis, transesterification, hydrothermal liquefaction, steam reforming, etc.; and the hydrothermal liquefaction (HTL) of wet biomass seems to be the promising route. Therefore, this article briefly enlightened a few concepts of HTL such as the elemental composition of bio-crude obtained by HTL, different types of feedstock adopted for HTL, mechanism of HTL processes, possible process flow diagrams for HTL of both wet and dry biomass and energy efficiency of the process. In addition, this article also enlisted possible future research scope for concerned researchers and a few of them are setting up HTL plant suitable for both wet and dry biomass feedstock; analysing influence of parameters such as temperature, pressure, residence time, catalytic effects, etc.; deriving optimized pathways for better conversion; and development of theoretical models representing the process to the best possible accuracy depending on nature of feedstock.

A review on hydrothermal liquefaction of biomass

https://eprints.whiterose.ac.uk/82741/1/1-s2.0-S0960852414013911-main.pdf

Hydrothermal liquefaction of biomass: Developments from batch to continuous process

Among the various types of biomass, microalgae have the potential of becoming a significant energy source for biofuel production in the coming years. Currently, research is mainly focusing on optimization of the cultivation methods and the conversion of just a single microalgae fraction (lipids for biodiesel production). Hydrothermal liquefaction is a method for thermochemical conversion of wet microalgae, producing a liquid energy carrier called ‘bio-oil’ or ‘biocrude’, next to gaseous, aqueous and solid by-products. A review of the available literature is presented here, analyzing the influence of parameters such as temperature, holding time and catalyst dosage on the yield and properties of the different product fractions. Also, the strain selection and the status of the technology for hydrothermal processes are analyzed. Finally, based on the findings obtained from the literature review, directions for future research are suggested.

Hydrothermal liquefaction (HTL) of microalgae for biofuel production: State of the art review and future prospects

Hydrothermal processing is an important thermochemical conversion process that is used to convert biomass into valuable products or biofuel. The process is usually performed in water at 250–374 °C under pressures of 4–22 MPa. The biomass is degraded into small components in water. Based on the target products, i.e., bio-oil, bio-gas or bio-carbon, the process conditions (temperature, pressure and time) are chosen. There has been significant effort in evaluating various biomass resources for hydrothermal processing because the process is suitable for any type of biomass including the co-utilization of biomass with waste materials. Additionally, because most biomass resources have high moisture contents, the most efficient way to process them is through hydrothermal processing. To understand hydrothermal biomass processing and the degradation pathway of biomass, it is necessary to understand the properties of water under hydrothermal conditions (i.e., subcritical and supercritical). In this respect, the physicochemical properties of water under subcritical and supercritical conditions and the interactions of water with biomass are discussed in the present paper. This review focuses on the hydrothermal processing of biomass and identifies the characteristics of various types of hydrothermal processing products. Additionally, this review provides an overview of the available biomass, the use of biomass as an energy source and related conversion technologies.

A review of hydrothermal biomass processing

The anaerobic digestion of microalgae is a prospective environmentally feasible option for creating a renewable source of energy for industrial and domestic needs. Microalgae anaerobic digestion is a key unit process that integrates efficiency and beneficially into the production of microalgae derived biofuels. Anaerobic digestion culminating in methane fermentation improves the economic viability of microalgae liquid biofuel production and presents an opportunity for power generation from wastewater derived microalgae. However the anaerobic digestion of microalgae biomass is not straight forward due to several technical restraints including low concentration of digestible biodegradable substrate, recalcitrant substrate constituents, cell wall degradability, low carbon to nitrogen ratio, ammonia toxicity and effects from salinity and associated metal ions. Current production methods for liquid biofuel production from microalgae produce approximately 60–70% residual biomass that is currently a byproduct. Anaerobic digestion provides biogas, but it can also provide essential nutrient recovery from lipid extracted microalgae biomass. The biogas produced from the anaerobic digestion process can be used to generate onsite electrical power or thermal heat to offset biomass processing and extraction processes. When both of these processes are integrated and operated simultaneously, the benefits to microalgae biofuel production and wastewater treatment derived energy production are increased significantly. To consider the integration of anaerobic digestion into a commercial-scale integrated microalgae production and biofuel refinery facility or wastewater treatment plant we present a review of the literature, the current state of the art and future directions for research.

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Anaerobic digestion of algae biomass: A review

Two major considerations of the emerging algae biofuel industry are the energy intensive dewatering of the algae slurry and nutrient management. The proposed closed loop process which involves nutrient recycling of the aqueous phase from the hydrothermal liquefaction of microalgae offers a solution to both aspects. Hydrothermal liquefaction has been shown to be a low energy process for bio-crude production from microalgae. For the purpose of this research, microalgae strains of Chlorella vulgaris , Scenedesmus dimorphus and the cyanobacteria Spirulina platensis and Chlorogloeopsis fritschii were processed in batch reactors at 300 °C and 350 °C. Following liquefaction the product phases were separated and the water phase recovered. The bio-crude yields ranged from 27 to 47 wt.%. The bio-crudes were of low O and N content and high heating value making them suitable for further processing. The water phase was analysed for all major nutrients, TOC and TN to determine the suitability of the recycled aqueous phase for algae cultivation. Growth trials were performed for each algae strain in a standard growth medium and compared to the growth rates in a series of dilutions of the recycled process water phase. Growth was determined by cell count and chlorophyll a absorbance. Growth occurred in heavy dilutions where the amount of growth inhibitors was not too high. The results show that the closed loop system using the recovered aqueous phase offers a promising route for sustainable oil production and nutrient management for microalgae.

C. Hall

Papers

Nutrient recycling of aqueous phase for microalgae cultivation from the hydrothermal liquefaction process

Catalytic Hydrothermal Processing of Microalgae with Integrated Nutrient Recycling