Microalgae Cultivation Systems for Biodiesel Production
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Abstract
Microalgae represent a sustainable biofuel source because of their high biomass productivity and ability to sequester carbon dioxide from the air and remove water born pollutants. This paper reviews the current status of microalgae cultivation systems, including the advantages and disadva...
microalgae cultivation systems for biodiesel production
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Microalgae Cultivation Systems for Biodiesel Production
Bonface G. Mukabane1*, Benson B. Gathitu2, Urbanus Mutwiwa2, Paul Njogu1
and Stephen Ondimu2.
1
The Institute for Energy and Environmental Technology, Jomo Kenyatta University of
Agriculture and Technology (JKUAT), P.O. BOX 62 000-00200. Nairobi-Kenya.
2
Department of Agriculture and Biosystems Engineering
JKUAT,
*Corresponding author: Phone: +254 721 565 335; E-mail: bmukabane@yahoo.com,
bmukabane@gmail.com
Abstract
Microalgae represent a sustainable biofuel source because of their high biomass productivity and
ability to sequester carbon dioxide from the air and remove water born pollutants. This paper
reviews the current status of microalgae cultivation systems, including the advantages and
disadvantages of both open and closed systems. The key barriers to commercial cultivation of
microalgae and the way forward is also discussed.
Key words: Biofuel, Current Status, Growth systems, Microalgae
1. Introduction
Increased interest in biofuels is mainly driven by; the fluctuating oil prices and recognition of the
fact that the global fossil fuel reserves are getting exhausted, concerns about environmental
pollution and resultant environmental change due to fossil fuel emissions and the provision of
alternative outlets for agricultural producers.
Global biofuel production has been increasing rapidly over the last decade, but the expanding
biofuel industry has recently raised pertinent concerns. In particular, the sustainability of many
first-generation biofuels; fuels made from food and feed crops and mainly vegetable oil, has
been increasingly questioned over concerns such as reported displacement of food crops, effects
on the environment and climate change [1]. In general, there is growing consensus that if
significant emissions reductions in the transport sector are to be achieved, biofuel technologies
must become more efficient in terms of net lifecycle greenhouse gas emission reduction while at
the same time be environmentally and socially sustainable. It is increasingly understood that
most first-generation biofuels, except sugarcane ethanol, will likely have a limited role in the
future transport fuel mix [2].
Biodiesel is a mixture of fatty acid alkyl monoesters (FAMEs) derived from vegetable fats and
oils. It can be used as a replacement of petro-diesel because of their structural similarity.
Biodiesel is produced using vegetable oil, plant oil, and animal fat. Biodiesel is an alternative
fuel for diesel and most diesel engines can use 100% biodiesel [1]. The main feedstock currently
used for biodiesel production includes palm oil, sunflower, rapeseed, soybean, and canola seed.
A great challenge of using vegetable oils for biodiesel production is the availability of crop land
1
, for oil production to produce enough biodiesel that significantly replaces the current fossil fuel
consumption [3]. Chisti [3] estimated that it would take 24% of the existing crop land in the US
to grow oil palm that is considered as a high yield oil crop or over three times of the current
cropland in the US to grow soybean to produce enough biodiesel that would replace 50% of the
transportation fuel in the US. Several studies have been conducted on using alternative oils such
as waste oils from restaurants and kitchens and microalgal oils for biodiesel production [1]. Shah
et al. [4] investigated the utilization of restaurant waste oil as a precursor for sophorolipid and
biodiesel production. Zhang et al. [5] evaluated the Biodiesel production from waste cooking oil
including economic analysis. Miao and Wu [6] studied biodiesel production from heterotrophic
microalgal oil. A great advantage of using microalgal oil over vegetable oils for biodiesel
production is that the production of algal oil does not need cropland and has much higher oil
yield per acre of land because the microalgae can be grown in 3 dimensions in photobioreactors
[1]. However, a big challenge of biodiesel production using algal oil is that the cost of algal oil
production is extremely high [1]. The goal of the present paper is to review recent development
in microalgae production systems and identify strategies for further development.
2. Microalgae cultivation systems
Annual oil production from high-oil microalgae can be in the range of 58 700 to 136 900 litres
per hectare [3]. If this microalgal oil is used for biodiesel production, it would take
approximately 1.0 – 2.5% of the current cropland in the US to meet 50% of the US transportation
fuel needs, which is much more feasible than the current oil crops [1]. Commercially growing
microalgae for value-added products is usually conducted in open ponds (raceways) or closed
photobioreactors (PBRs) under autotrophic (making complex organic nutritive compounds from
simple inorganic sources by photosynthesis) or heterotrophic (cannot synthesize its own food)
conditions at relatively warm temperature (20 – 30 0C) [1]. In autotrophic microalgal cultivation,
the microalgae need sunlight (energy source), CO2 (carbon source) and nutrients (P, N and
minerals) for their photosynthesis and generate oxygen. The main difference of growing
heterotrophic microalgae from autotrophic ones is the carbon source. The former requires
organic carbon source such as glucose to support its growth. Normally autotrophic microalgae
are grown for biodiesel production, mainly because they use CO 2 as their carbon source for
growth [1]. Therefore, the whole cycle of growing microalgae for biodiesel production and
combustion of biodiesel as fuel would generate zero net carbon dioxide emission to the
atmosphere. However, sometimes heterotrophically grown microalgae can make much more oil
than autotrophic ones. Miao and Wu [6] reported the heterotrophic growth of Chlorella
protothecoides resulted in a significant increase of oil content of microalgae from 14.5% under
the original autotrophic growth to 55.2% (dry weight).
In a photobioreactor microalgal growth system, pure high-oil microalgae are grown in closed
glass or plastic tubular bioreactors. Nutrient water is circulated in the bioreactors for keeping the
microalgae from settling and for the growth of the microalgae. Natural sunlight is usually the
energy source for microalgal growth [1]. Although artificial illumination to the photobioreactors
is viable, it is much more expensive than natural illumination. Pure microalgal culture can be
maintained in the photobioreactors. Heat exchanger is usually necessary to maintain an adequate
temperature in the photobioreactors. A high concentration of microalgal biomass can be achieved
in photobioreactors. In that case high dissolved oxygen may inhibit the microalgal growth, so
degassing system is usually necessary to release oxygen from the water [1].
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