Biorefinery of microalgae for food and fuel

The freshwater green microalgal strain Haematococcus pluvialis is the richest source for the production of astaxanthin. Astaxanthin is member of the xanthophyll family of carotenoids and constitutes the highest value product derived by microalgae. So far, algal astaxanthin amounts to

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Commercial astaxanthin production derived by green alga Haematococcus pluvialis: A microalgae process model and a techno-economic assessment all through production line

In response to energy crisis, global warming and climate changes, microalgae have received a great deal of interest as a biofuel feedstock. However, the development of microalgal biofuels witnesses an obvious and serious dilemma. The reported literature shows that “fuel only” option is not economically viable due to the overwhelming investments in capital and operation. Hence, it is questionable whether it is too luxurious to produce microalgae only for biofuel application. In addition, there are some voiced concerns related to the impacts of microalgal biofuel production on the conventional applications, such as cosmetics, pharmaceuticals and nutritious feed. It is therefore questionable whether the microalgal biofuel production will affect the original functions. From a sustainability point of view, the author explores the current challenges in microalgal applications and proposes an innovative framework for microalgal biorefinery, which can realize the production of multiple products in the form of high values and biofuels such as biodiesel, bioethanol and biogas. With the aim at maximizing the value derived from different microalgal components, the innovative microalgal biorefinery concept includes four pathways: high-value products–biodiesel–bioethanol–biogas; high-value products–bioethanol–biogas; high-value products–biodiesel–biogas; and high-value products–biogas. Special attention has been paid to the production of high-value products through system integration and engineering, which is expected to promote the economics of microalgal biofuels. Net energy ratio assessment and cost-effectiveness assessment have been highlighted to testify the feasibility of microalgal biorefinery options, and some crucial actions have been suggested to help establish the process.

Biorefinery as a promising approach to promote microalgae industry: An innovative framework

This study introduces and analyzes a novel system for algal biofuel production that synergistically integrates algal wastewater treatment with hydrothermal liquefaction (HTL) of wastewater biosolids and algae into bio-crude oil. This system maximizes the biofuel potential of wastewater inputs by internally capturing and recycling carbon and nutrients – a powerful concept referred to as multi-cycle nutrient reuse, which amplifies waste nutrients into multiple cycles of algal biomass and bioenergy production. We call this system “Environment-Enhancing Energy” (E2-Energy) because it can simultaneously improve conventional wastewater treatment by nutrient removal and production of a large amount of biofuel co-products. Moreover, E2-Energy resolves several key bottlenecks commonly associated with large-scale algal biofuel production including: contamination of target high-oil algal cultures, high nutrient costs/usage, unsustainable fresh water usage, and large energy inputs for dewatering/extraction. A series of algal cultivation and HTL experiments were conducted to confirm the primary steps and performance characteristics of the E2-Energy system. These experiments showed: (1) low-oil, mixed algal–bacterial biomass can be successfully cultured with the recycled HTL aqueous product; (2) both organics and nutrients are removed from wastewater during algal–bacterial biomass production (63–95% reduction); (3) this low-oil, algal–bacterial biomass can be converted into bio-crude oil via HTL with a high yield (∼50%) and a net positive energy balance; and (4) the HTL step re-releases nutrients to an aqueous phase product that can be recycled back to step (1). This repeating loop of steps 1–4 facilitates multi-cycle reuse of nutrients and thus provides biomass amplification. A mathematical model was also developed using STELLA® to simulate mass balances for long-term E2-Energy operations with internal recycling of nutrients and carbon. The model results showed that E2-Energy can amplify the biomass and biofuel production from wastewater by up to 10 times, which gives it the potential to replace all US petroleum imports using only current wastewater feedstocks and carbon dioxide from the atmosphere or point sources. Thus, E2-Energy represents a major paradigm shift—where wastewater treatment systems become optimized biofuel producers with enhanced effluent quality, which provides a viable and advantageous pathway to sustainable, carbon-neutral energy independence.

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A synergistic combination of algal wastewater treatment and hydrothermal biofuel production maximized by nutrient and carbon recycling

The potential of algal biomass as a source of liquid and gaseous biofuels is a highly topical theme, but as yet there is no successful economically viable commercial system producing biofuel. However, the majority of the research has focused on producing fuels from microalgae rather than from macroalgae. This article briefly reviews the methods by which useful energy may be extracted from macroalgae biomass including: direct combustion, pyrolysis, gasification, trans-esterification to biodiesel, hydrothermal liquefaction, fermentation to bioethanol, fermentation to biobutanol and anaerobic digestion, and explores technical and engineering difficulties that remain to be resolved.

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A. Roubaud

Papers

An economic, sustainability, and energetic model of biodiesel production from microalgae

Comparison of various microalgae liquid biofuel production pathways based on energetic, economic and environmental criteria