Modern life is intimately linked to the availability of fossil fuels, which continue to meet the world's growing energy needs even though their use drives climate change, exhausts finite reserves and contributes to global political strife. Biofuels made from renewable resources could be a more sustainable alternative, particularly if sourced from organisms, such as algae, that can be farmed without using valuable arable land. Strain development and process engineering are needed to make algal biofuels practical and economically viable.

Exploiting diversity and synthetic biology for the production of algal biofuels

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 carbonization (HTC) of biomass involves contacting raw feedstock with hot, pressurized water. Through a variety of hydrolysis, dehydration, and decarboxylation processes, gaseous and water-soluble products are produced, in addition to water itself and a solid char. In this experimental effort, a 2 L Parr stirred pressure vessel was used to apply the HTC process to a mixed wood feedstock. The effects of the reaction conditions on product compositions and yields were examined by varying temperature over the range of 215−295 °C and varying reaction hold time over the range of 5−60 min. With increasing temperature and time, the amounts of gaseous products and produced water increased, while the amount of HTC char decreased. The energy density of the char increased with reaction severity. At reaction conditions of 255 °C for 30 min, the HTC char had 39% higher energy density than the raw biomass feedstock. Aqueous solutions from HTC experiments at lower temperatures (215−235 °C) contained signific...

Hydrothermal Carbonization (HTC) of Lignocellulosic Biomass

A review on FAME production processes

A critical review of biochemical conversion, sustainability and life cycle assessment of algal biofuels

In an effort to process wet algal biomass directly, eliminate organic solvent use during lipid extraction, and recover nutrients (e.g., N, P, and glycerol) for reuse, we developed a catalyst-free, two-step technique for algal biodiesel production. In the first step, wet algal biomass (ca. 80% moisture) reacts in subcritical water to hydrolyze intracellular lipids, conglomerate cells into an easily filterable solid that retains the lipids, and produce a sterile, nutrient-rich aqueous phase. In the second step, the wet fatty acid-rich solids undergo supercritical in situ transesterification (SC-IST/E) with ethanol to produce biodiesel in the form of fatty acid ethyl esters (FAEEs). Chlorella vulgaris grown sequentially under photo- and heterotrophic conditions served as the lipid-rich feedstock (53.3% lipids as FAEE). The feedstock and process solids were characterized for lipid components using highly automated microscale extraction and derivatization procedures and high-temperature gas chromatography. Hyd...

Biodiesel Production from Wet Algal Biomass through in Situ Lipid Hydrolysis and Supercritical Transesterification

This article reviews the relatively new field of supercritical fluid phase synthesis of biodiesel fuel. We assess the current state of the art and then suggest several directions for new or additional research that would lead to important advances in this field. Biodiesel synthesis at supercritical conditions is technologically feasible and perhaps economically competitive with conventional synthesis routes for low-cost feedstocks such as waste cooking oil. A better understanding of the reaction kinetics (both metal-catalyzed and uncatalyzed) and phase behavior (e.g., location of liquid−liquid−vapor and liquid−vapor regions and critical temperature and pressure as they change during the course of the reaction) is needed. Additionally, there is a need for more detailed analysis of the economics, energy requirements, and environmental impacts of the supercritical process relative to conventional technology.

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Assessment of Noncatalytic Biodiesel Synthesis Using Supercritical Reaction Conditions

We investigated the esterification of oleic acid with excess ethanol at sub- and supercritical reaction conditions (based on ethanol). This reaction can be used to make fatty acid ethyl esters, which are components of biodiesel fuel. The esterification proceeds smoothly with no added catalyst even at subcritical temperatures and pressures, which indicates that the more severe supercritical conditions are not required. Moreover, the reaction does not require a large excess of alcohol, and it can tolerate the presence of modest amounts of water in the feed stream. For the first time, the kinetics of noncatalytic ethyl esterification were determined. We report the forward and reverse rate constants at five different temperatures and the corresponding Arrhenius parameters.

Noncatalytic esterification of oleic acid in ethanol

We determined the kinetics for ethyl oleate hydrolysis in high-temperature water and for the reverse reaction, oleic acid esterification, in near- and supercritical ethanol. Hydrolysis was clearly autocatalytic. The experimental data, from reactions at 150−300 °C, times from 5 to 1440 min, and with different initial concentrations of reactants and products, were used to estimate thermodynamically and thermochemically consistent Arrhenius parameters for the forward and reverse reactions in an autocatalytic reaction model. The model provided a good correlation of the data and also exhibited the ability to make quantitatively accurate predictions within the parameter space investigated. The model also accurately predicted the experimental trends when extrapolated outside the original parameter space. Sensitivity analysis confirmed that data from both fatty acid esterification and fatty acid ester hydrolysis need to be used together if one desires reliable estimates for all of the Arrhenius parameters in the ...

Modeling Hydrolysis and Esterification Kinetics for Biofuel Processes

We have elucidated the mechanism for ethyl oleate hydrolysis in high temperature water and its reverse reaction, oleic acid esterification in near- and supercritical ethanol in the absence of any other added compounds. Both reactions are acid catalyzed. H+ (from dissociation of water and oleic acid) and oleic acid serve as catalysts for hydrolysis and H+ alone is the catalyst for esterification. The rate equation arising from the proposed mechanism provided a good fit of experimental conversion data for both hydrolysis and esterification. The rate equation accurately predicted the influence of pH on hydrolysis for acidic and near-neutral conditions. The mechanistic model exhibits the ability to make quantitatively accurate predictions within and outside the original parameter space, especially for a multicomponent system. Sensitivity analysis shows that the values of the dissociation constant of oleic acid in ethanol, water, and ethanol–water systems strongly influence the predicted conversions. There is ...

Tanawan Pinnarat

Papers

Biodiesel Production from Wet Algal Biomass through in Situ Lipid Hydrolysis and Supercritical Transesterification

Assessment of Noncatalytic Biodiesel Synthesis Using Supercritical Reaction Conditions

Noncatalytic esterification of oleic acid in ethanol

Modeling Hydrolysis and Esterification Kinetics for Biofuel Processes

Mechanistic Modeling of Hydrolysis and Esterification for Biofuel Processes