Thermochemical conversion of microalgal biomass for biofuel production

https://doi.org/10.1016/j.rser.2015.04.186Get rights and content

Abstract

Reliable and sustainable energy supply is critical to effective natural resource management, and it encompasses functioning efficiency of energy resources as well as socio-economic and environmental impact considerations. The complete reliance on fossil fuels is recognized as unsustainable throughout the world, and this is due to, amongst others, the rapid declining of fossil fuel reserves and the emission of significant quantities of greenhouse gases associated with their production and combustion. This has resulted in escalating interest in research activities aiming to develop alternative and somewhat carbon neutral energy sources. Algal biofuels, so called third generation biofuels, appear to be promising in delivering sustainable and complementary energy platforms essential to formulate a major component of the renewable and sustainable energy mix for the future. Algal biomass can be converted into various portfolios of biofuel products, such as bio-hydrogen, biodiesel, bioethanol and biogas, via two different pathways: biochemical and thermochemical pathways. Thermochemical conversion is considered as a viable method to overcome the existing problems related with biochemical conversion such as lengthy reaction time, low conversion efficiency by microbes and enzymes, and high production costs. This paper discusses process technologies for microalgae-to-biofuel production systems, focusing on thermochemical conversion technologies such as gasification, pyrolysis, and liquefaction. The benefits of exploiting upstream microalgal biomass development for bioremediation such as carbon dioxide mitigation and wastewater treatment are also discussed.

Introduction

The supply of sustainable energy at an affordable price is a major global endeavor. Energy is viewed as a baseline for economic and social development as it improves living standards and social strata [1]. However, the number of human activities and extensive use of fossil fuels for transportation, manufacturing and power generation has significantly contributed to the emission of greenhouse gases (GHG) and other harmful pollutants resulting in global undesired climate change [2], [3], [4], [5]. In addition, anthropogenic emission of carbon dioxide is expected to be 2×1010 t/year, most of which is a result of fossil fuels combustion [6]. It has therefore become important to reduce carbon emissions by promoting sustainable alternative resources for energy and implementing policies to reduce the impacts. More renewable and sustainable sources of energy seem to be an auspicious approach to tackle the problem. Notable renewable energy portfolios include solar energy, wind power, nuclear energy, hydropower and biomass [7], [8]. In recent years, biomass has gained significant attention as it is considered as a potential renewable energy source for future sustainable energy mix [9], [10]. Examples of biomass being explored for biofuel production include plants, trees, food materials, bio-wastes and cellular materials from algae and bacteria.

Energy harnessing from algal biomass is not a new idea. Many studies have reported the production of biofuels from algal biomass [11], [12]. Algae are unicellular, microscopic and photosynthetic organisms typically found in fresh water or marine systems. These organisms consume three primary components: sunlight, carbon dioxide and water to produce significant quantities of lipids, proteins, carbohydrates and other bioactive compounds within a short period of time [13], [14].

Algae can be classified into macroalgae (filamentous) and microalgae (phytoplankton) as shown in Table 1. Macroalgae, also known as seaweed, is further divided into three main categories based on pigmentation. These are Phaeophyceae (Brown seaweed), Rhodophyceae (Red seaweed) and Chlorophyceae (Green seaweed). In contrast, microalgae are unicellular organisms divided into four classes. These are Bacillariophyceae (diatom), Chlorophyceae (green algae), Cyanophyceae (blue algae) and Chrysophyceae (golden algae) [15]. Algal biomass generally consists of 9.5–42% lipid, 17–57% carbohydrate and 20–50% protein (dry weight basis) depending on the species and some species possess oil content up to 79.5% [16].

There are numerous advantages of microalgal biomass as a feedstock for biofuel production compared to existing biomass such as Jatropha, grains, sugarcane, oil seeds and corn. These include:

  • 1.

    Microalgae are capable of fixing high CO2 from the environment [7]. Microalgae can utilise CO2 emissions from power plants and other industrial sources for their growth in a CO2 biosequestration process. Typically 1 kg of microalgal biomass synthesis requires about 1.8 kg of CO2 [17], [18].

  • 2.

    Microalgae can grow in different types of environments. They do not require traditional agricultural resources, as they can be cultivated with or without land and in seawater or freshwater [7], [19]. They also require lesser volumes of water for cultivation compared to terrestrial crops.

  • 3.

    The photosynthesis mechanism in microalgae is similar to other plants. However, microalgae can convert more solar energy (at about 4–7.5%) during cellular metabolism compared to 0.5% for land based crops [20].

  • 4.

    Microalgae have a high growth rate within a short duration of time compared to land based crops and it could be harvested throughout the year. They double up in mass by converting carbon dioxide and sunlight into energy within 24 h. Some species require only 3.5 h for biomass production [17], [18].

  • 5.

    Microalgal biomass can be used to generate numerous valuable products such as food, feed for animals and fuels, including jet fuel, aviation gas, biodiesel, gasoline, bioethanol. Residual microalgal biomass may be used as feed or fertilizer.

Section snippets

Microalgae as a source of food

Microalgae are one of the most important biomass sources on the earth containing proteins, carbohydrates, enzymes and vitamins A & C and minerals such as iodine, potassium, iron and calcium. Green microalgae have been used in Asian countries, including China, Japan and Korea, as the source of certain food nutrients for hundreds of years [21]. Commercially used microalgae as food supplements and additives for humans and animals are Chlorella vulgaris, Haematococcu spluvialis, Dunaliella salina

Conversion technologies for microalgal biofuels production

Photosynthetically grown microalgal biomass is widely regarded to produce environmental friendly biofuels such as solid fuel, biohydrogen, biodiesel, bioethanol and syngas via two different pathways: biochemical and thermo-chemical conversion processes as illustrated in Fig. 1. The selection of the right conversion technology is a key step to ensuring that biofuel production is economically viable and environmentally sustainable. Currently, there are no clear established advantages between

CO2 mitigation using microalgae

Biomass derived energy is not only environment friendly and carbon neutral but it also reduces our dependence on fossil fuels, thereby contributing efficiently to energy security and clean climate change mitigations. Kyoto 1997 protocol was aimed to act in order to reduce greenhouse gas emissions, particularly carbon dioxide, from different anthropogenic activities [99]. As microalgae possess photosynthetic efficiency higher than terrestrial plants with efficient carbon biosorption, it has a

Conclusion

The environmental issues associated with anthropogenic activities, such as global warming and climate change, mostly resulting from energy production and utilization cannot be ignored much longer as they affect nature and human lives. In response to the problem, microalgae are considered a sustainable biofuel feedstock and with attractive bioremediation capabilities. Its continuous higher production rate compared to other biofuel biomass will promote the sustenance of biofuels as a key

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