Microalgae for biodiesel production and other applications: A review

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Abstract

Sustainable production of renewable energy is being hotly debated globally since it is increasingly understood that first generation biofuels, primarily produced from food crops and mostly oil seeds are limited in their ability to achieve targets for biofuel production, climate change mitigation and economic growth. These concerns have increased the interest in developing second generation biofuels produced from non-food feedstocks such as microalgae, which potentially offer greatest opportunities in the longer term. This paper reviews the current status of microalgae use for biodiesel production, including their cultivation, harvesting, and processing. The microalgae species most used for biodiesel production are presented and their main advantages described in comparison with other available biodiesel feedstocks. The various aspects associated with the design of microalgae production units are described, giving an overview of the current state of development of algae cultivation systems (photo-bioreactors and open ponds). Other potential applications and products from microalgae are also presented such as for biological sequestration of CO2, wastewater treatment, in human health, as food additive, and for aquaculture.

Introduction

The transportation and energy sectors are the major anthropogenic sources, responsible in European Union (EU) for more than 20% and 60% of greenhouse gas (GHG) emissions, respectively [1]. Agriculture is the third largest anthropogenic source, representing about 9% of GHG emissions, where the most important gases are nitrous oxide (N2O) and methane (CH4) [2]. It is expected that with the development of new growing economies, such as India and China, the global consumption of energy will raise and lead to more environmental damage [3].

GHG contributes not only to global warming (GW) but also to other impacts on the environment and human life. Oceans absorb approximately one-third of the CO2 emitted each year by human activities and as its levels increase in the atmosphere, the amount dissolved in oceans will also increase turning the water pH gradually to more acidic. This pH decrease may cause the quick loss of coral reefs and of marine ecosystem biodiversity with huge implications in ocean life and consequently in earth life [4].

As GW is a problem affecting different aspects of human life and the global environment, not only a single but a host of solutions is needed to address it. One side of the problem concerns the reduction of crude oil reserves and difficulties in their extraction and processing, leading to an increase of its cost [5]. This situation is particularly acute in the transportation sector, where currently there are no relevant alternatives to fossil fuels.

To find clean and renewable energy sources ranks as one of the most challenging problems facing mankind in the medium to long term. The associated issues are intimately connected with economic development and prosperity, quality of life, global stability, and require from all stakeholders tough decisions and long term strategies. For example, many countries and regions around the world established targets for CO2 reduction in order to meet the sustainability goals agreed under the Kyoto Protocol.

Presently many options are being studied and implemented in practice, with different degrees of success, and in different phases of study and implementation. Examples include solar energy, either thermal or photovoltaic, hydroelectric, geothermal, wind, biofuels, and carbon sequestration, among others [6], [7]. Each one has its own advantages and problems and, depending on the area of application, different options will be better suited. One important goal is to take measures for transportation emissions reduction, such as the gradual replacement of fossil fuels by renewable energy sources, where biofuels are seen as real contributors to reach those goals, particularly in the short term.

Biofuels production is expected to offer new opportunities to diversify income and fuel supply sources, to promote employment in rural areas, to develop long term replacement of fossil fuels, and to reduce GHG emissions, boosting the decarbonisation of transportation fuels and increasing the security of energy supply.

The most common biofuels are biodiesel and bio-ethanol, which can replace diesel and gasoline, respectively, in today cars with little or none modifications of vehicle engines. They are mainly produced from biomass or renewable energy sources and contribute to lower combustion emissions than fossil fuels per equivalent power output. They can be produced using existing technologies and be distributed through the available distribution system. For this reason biofuels are currently pursued as a fuel alternative that can be easily applied until other options harder to implement, such as hydrogen, are available.

Although biofuels are still more expensive than fossil fuels their production is increasing in countries around the world. Encouraged by policy measures and biofuels targets for transport, its global production is estimated to be over 35 billion liters [8].

The main alternative to diesel fuel in EU is biodiesel, representing 82% of total biofuels production [9] and is still growing in Europe, Brazil, and United States, based on political and economic objectives.

Biodiesel is produced from vegetable oils (edible or non-edible) or animal fats. Since vegetable oils may also be used for human consumption, it can lead to an increase in price of food-grade oils, causing the cost of biodiesel to increase and preventing its usage, even if it has advantages comparing with diesel fuel.

The potential market for biodiesel far surpasses the availability of plant oils not designated for other markets. For example, to fulfill a 10% target in EU from domestic production, the actual feedstocks supply is not enough to meet the current demand and the land requirements for biofuels production, would be more than the potential available arable land for bio-energy crops [10]. The extensive plantation and pressure for land use change and increase of cultivated fields may lead to land competition and biodiversity loss, due to the cutting of existing forests and the utilization of ecological importance areas [11]. Biodiesel may also be disadvantageous when replacing crops used for human consumption or if its feedstocks are cultivated in forests and other critical habitats with associated biological diversity.

Current policies at regional and national levels and the expected cost and difficulties in obtaining fossil fuels will necessarily lead to an increase in biodiesel production and of other types of renewable energy. To become a more viable alternative fuel and to survive in the market, biodiesel must compete economically with diesel. The end cost of biodiesel mainly depends on the price of the feedstocks that accounts for 60–75% of the total cost of biodiesel fuel [12].

In order to not compete with edible vegetable oils, the low-cost and profitable biodiesel should be produced from low-cost feedstocks such as non-edible oils, used frying oils, animal fats, soap-stocks, and greases. However the available quantities of waste oils and animal fats are not enough to match the today demands for biodiesel. Thus transition to second generation biofuels, such as microalgae, can also contribute to a reduction in land requirements due to their presumed higher energy yields per hectare as well as to their non-requirement of agricultural land. Additionally, biodiesel needs to have lower environmental impacts and ensure the same level of performance of existing fuels [13].

Albeit the growing interest and fast growth of this area, it is still on its infancy. A large investment in research and development (R&D) and correct policies and strategies are still needed, for all stages of the biofuels value chain, from raw materials production to delivery and final consumption. Among the various possibilities currently being investigated and implemented at pilot scale or even at industrial scale concerning potential feedstocks, the more interesting ones are microalgae. Besides their cultivation is not directly linked to human consumption, they have low space requirements for its production.

This review focuses its attention on microalgae and how they can be used for biodiesel production. Questions associated with production and processing of microalgae are considered in detail, not only those directly related with biofuels production but also the possibilities of combining it with pollution control, in particular with biological sequestration of CO2 emissions and other greenhouse gases, or wastewater treatment. This work starts by describing which microalgae are normally used for the production of biofuels and their main advantages when compared with other available feedstocks. Then, the current status of biodiesel production from microalgae, concerning their growth, harvest, and processing is reviewed. Other potential applications and how to combine them with biodiesel production are also described.

Section snippets

What are microalgae?

Microalgae are prokaryotic or eukaryotic photosynthetic microorganisms that can grow rapidly and live in harsh conditions due to their unicellular or simple multicellular structure. Examples of prokaryotic microorganisms are Cyanobacteria (Cyanophyceae) and eukaryotic microalgae are for example green algae (Chlorophyta) and diatoms (Bacillariophyta) [14], [15]. A more in depth description of microalgae is presented by Richmond [16].

Microalgae are present in all existing earth ecosystems, not

Environmental applications

Production of biodiesel and other bio-products from microalgae can be more environmentally sustainable, cost-effective and profitable, if combined with processes such as wastewater and flue gas treatments. In fact various studies demonstrated the use of microalgae for production of valuable products combined with environmental applications [107], [108], [109], [110], [111].

Conclusions

Current efforts and business investment are driving attention and marketing efforts on the promises of producing algal biodiesel and superior production systems.

A large number of companies are claiming that they are at the forefront of the technology and will be producing algal biodiesel economically within the next few years. However most of these companies have limited technical expertise and few have actually made biodiesel from algae.

Producing algal biodiesel requires large-scale

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