**7. Economic and environmental feasibility**

In order to be economically feasible, microalgal biodiesel must be cost competitive with petroleum-based fuels. We have investigated the relationship between algal lipid productivity and cost in order to determine the range of productivities that need to be achieved for economic viability. Based on values from Chisti (2007), a model was set up to estimate cost per litre of algal oil as a function of algal biomass productivity and lipid content. Where the cost of producing a litre of algal biodiesel was below the price of a litre of fossil-fuel derived diesel, it was considered economically viable (i.e. no profit margin was introduced). The price of fossil-fuel derived diesel is partly dependent on the price of crude oil, which has varied widely in the last few years, hence several scenarios were evaluated.

Assumptions made in the execution of the model were:


The economic model was run for three prices of crude oil, based on fluctuations over the last few years. These scenarios of 'high' (\$ 130), 'medium' (\$ 90) and 'low' (\$ 50) cost of crude oil per barrel gave the price limits for algal oil of 0.90, 0.62 and 0.35 US\$ per L respectively. The results of the model are shown in Fig. 2a (raceway ponds) and 2b (closed photobioreactors).

feasible for low value products (Mata et al., 2010). The selection of drying technique is dependent on the scale of operation, the speed required and the downstream extraction

Lipid extraction can be done in a number of ways. Solvent extraction techniques are popular, but the cost and toxicity of the solvent (e.g. hexane) is of concern and solvent recovery requires significant energy input. Other methods involve disruption of the cell wall, usually by enzymatic, chemical or physical means (e.g. homogenization, bead milling, sonication (Mata et al., 2010)), allowing the released oil to float to the top of the solution. Ultrasound and microwave assisted extraction methods have been investigated (Cravotto et al., 2008). Supercritical CO2 extraction is an efficient process, but is too expensive and energy intensive for anything but lab-scale production. Direct transesterification (production of biodiesel directly from algal biomass) is also possible. Some of these techniques do not require dry biomass, but the larger the water content of the algal slurry, the greater the

Once the algal oil is extracted, it can be treated as conventional vegetable oil in biodiesel production. Direct pyrolysis, liquefaction or gasification of algal biomass have also been suggested as means of producing fuel molecules. One of the concerns for biodiesel production through transesterification, shared with any biodiesel feedstock, is the quality of the biodiesel produced. Biodiesel must meet certain international regulations, for example, the ASTM international standards or the EN14214 in Europe. It has been calculated that the fatty acid profile of certain microalgal species will produce biodiesel that does not meet these specification, therefore blending or additives may be required

In order to be economically feasible, microalgal biodiesel must be cost competitive with petroleum-based fuels. We have investigated the relationship between algal lipid productivity and cost in order to determine the range of productivities that need to be achieved for economic viability. Based on values from Chisti (2007), a model was set up to estimate cost per litre of algal oil as a function of algal biomass productivity and lipid content. Where the cost of producing a litre of algal biodiesel was below the price of a litre of fossil-fuel derived diesel, it was considered economically viable (i.e. no profit margin was introduced). The price of fossil-fuel derived diesel is partly dependent on the price of crude oil, which has varied widely in the last few years, hence several scenarios were

1. Cost per kg algal biomass: US\$ 0.6 for raceway ponds, and US\$ 0.47 for

2. In order to be economically viable, the cost of algal oil per litre must be less than 6.9 x

The economic model was run for three prices of crude oil, based on fluctuations over the last few years. These scenarios of 'high' (\$ 130), 'medium' (\$ 90) and 'low' (\$ 50) cost of crude oil per barrel gave the price limits for algal oil of 0.90, 0.62 and 0.35 US\$ per L respectively. The results of the model are shown in Fig. 2a (raceway ponds) and 2b (closed

process (Mohn, 1980).

energy and solvent input required.

**7. Economic and environmental feasibility** 

Assumptions made in the execution of the model were:

10-3 times the cost of crude oil in US\$ per barrel (Chisti, 2007) 3. Density of algal oil: 0.86 g.cm-3 (Barsanti & Gualtieri, 2007)

photobioreactors (Chisti, 2007)

(Stansell, 2011).

evaluated.

photobioreactors).

Fig. 2. Lipid contents and biomass productivities required for economic feasibility in (a) large-scale, outdoor raceway ponds and (b) large-scale, outdoor photobioreactors. Dark grey region: productivities economically feasible at US\$ 50 per barrel crude oil (cost of algal oil per L lower than cost of regular diesel per L). Additional region for crude oil price US\$ 90 per barrel = mid-grey and US\$ 130 = light grey

Based on this model, the results for raceway ponds show that algal biodiesel will not be economically feasible, either in ponds or photobioreactors, at current costs below a biomass productivity of 1 g.L-1.day-1. Assuming a maximum realistically achievable lipid content of 50% DW, algal biodiesel becomes economically feasible at biomass productivities of 1.5 g.L-1.day-1 (US\$ 130 per barrel crude oil), close to 2 g.L-1.day-1 (US\$ 90), and 2.5 g.L-1.day-1 (US\$ 50) in raceway ponds. At lower lipid contents, higher biomass productivity is required, e.g. at a lipid content of 25% DW, algal biodiesel only becomes cost effective at 2 g.L-1.day-1 for US\$ 130 per barrel. The model for photobioreactors is based on a lower cost per kg algal biomass than raceway ponds, hence economic feasibility is reached at slightly lower biomass productivities and lipid contents, e.g. at a biomass productivity of 2 g.L-1.day-1, a lipid content of only 20% DW is required to be viable at US\$ 130 per barrel crude oil.

Currently reported biomass productivities in outdoor raceway ponds average around 0.17 g.L-1.day-1, with a lipid content of 26% DW (Griffiths and Harrison, 2009), which is far from being economically feasible. Biomass productivities for closed photobioreactors (1.33 g.L-1.day-1) are closer to being within the economically viable range, if they can be maintained in the long term, concurrent with sufficiently high lipid content. As a reflection of this, there are currently no industrial facilities producing biodiesel from microalgae (Lardon, 2009). For cultivation to be economically viable, productivities must be increased, costs lowered, or additional income streams developed. The economics of algal biofuel production could be greatly improved through the production of co-products. For example, high value compounds such as pigments could be produced along with lipid. The residual biomass after lipid extraction could be sold as animal feed, fertilizer or soil conditioner, anaerobically digested to produce biogas, gasified or merely burned to provide some of the heat or electricity required in the process.

In addition to economic feasibility, algal biodiesel must be environmentally desirable. It is critical that the energy embodied in the fuel produced is greater than the energy input required to produce it. Net energy analysis and life cycle analysis (LCA) are tools used to quantify the environmental burdens at every stage of production, from growth of the

Advantages and Challenges of Microalgae as a Source of Oil for Biodiesel 191

The choice of algal strain is a key consideration. The diversity of algal species is much greater than that of land plants (Scott et al., 2010) allowing selection of species best suited to the local environment and goals of the project. Although there have been several screening programs, building on the work of the Aquatic Species Program (Sheehan et al., 1998), the majority of strains remain untested, few species have been studied in depth and the data reported in the literature is often not comparable due to the different experimental procedures used. We conducted a broad literature review of the growth rates and lipid contents of 55 promising microalgal species under both nutrient replete and limited conditions. The original study (Griffiths & Harrison, 2009) has been extended here through the use of two key assumptions to

Lipid productivity is determined by both growth rate and lipid content. Lipid content (P) was typically reported as percentage dry weight (% DW). Data presented in pg lipid.cell-1 was discarded if no cell weight was available for conversion. Growth rates were reported as doubling time (Td) or specific growth rate (µ). These were inter-converted according to

> ln 2 *Td*

Standard units of g.L-1.day-1 were chosen for biomass productivity. Specific growth rate (µ, in units of day-1) can be converted to volumetric biomass productivity (QV, in g.L-1.day-1) where the biomass concentration (X, in g.L-1) is known (Equation 2). Biomass productivity is often reported on the basis of surface area (QA), in units of g.m-2.day-1. This can be converted to QV using Equation 3 where the depth (D, in m) of the culture vessel can be calculated

> *Q X <sup>V</sup>*

Lipid productivity (QP) was infrequently reported in the literature, and was generally reported in g.L-1.day-1 or mg.L-1.day-1. This parameter could be calculated from volumetric biomass productivity (QV, in g.L-1.day-1) and lipid content (P in % DW) where appropriate

The calculation of lipid productivity for the majority of species necessitated two

1. Conversion of areal productivities (in g.m-2.day-1) to volumetric productivities

2. Conversion of specific growth rate to biomass productivity using an average biomass

The average literature values for the 55 species are shown in Table 4. Among the species with the highest reported lipid productivity were *Neochloris oleoabundans*, *Navicula pelliculosa*, *Amphora*, *Cylindrotheca* and *Chlorella sorokiniana* (Fig. 3). Other findings were that green algae (Chlorophyta) generally showed an increase in lipid content when nitrogen deficient, whereas

(g.L-1.day-1), using an average depth of 0.1 m, based on best fit of the data

concentration of 0.15 g.L-1, based on typical experimental results.

*V*

1000 *A*

(1)

(2)

*<sup>Q</sup> <sup>Q</sup> D* (3)

*QQP P V* (4)

convert data into common units of biomass and lipid productivity.

Equation 1.

from the reactor geometry.

data were available (Equation 4).

assumptions:

biomass to combustion of the fuel. Lardon et al. (2009) conducted a life-cycle analysis of a hypothetical algal biodiesel production facility. Two different culture conditions: fertilizer feeding and nitrogen starvation, as well as two different extraction options: dry or wet, were investigated. The study confirmed the potential of microalgae as an energy source, but highlighted the necessity of decreasing energy and fertilizer consumption. Energy inputs, such as the energy required for mixing and pumping, the embodied energy in the materials used and the energy cost of harvesting and processing must be minimized. Recycling of material and energy from waste streams is also important wherever feasible (Scott et al., 2010). The use of nitrogen stress, as well as the optimization of wet extraction were indicated as desirable options. The anaerobic digestion of residual biomass was also suggested as a way of reducing external energy usage and recycling of nutrients.

We conducted a LCA on a hypothetical algal biodiesel process. Biomass production in three different reactor types (open ponds and two types of closed reactor: horizontal tubular and vertical tubular) was evaluated. In all cases, harvesting was modeled as an initial settling step followed by centrifugation. Hexane extraction was used to recover the oil, with the residual biomass sent for anaerobic digestion and the resulting energy from biogas production recycled to the process. The hexane was recovered and the oil converted to biodiesel using an enzymatic process. The basis chosen was production of 1000 kg of biodiesel from *Phaeodactylum tricornutum*. The net energy return (the energy embodied in the biodiesel produced divided by the energy input required) was positive (1.5) for the open pond, neutral (0.97) for the horizontal tubular reactor and negative (0.12) for the vertical tubular reactor. In this model, open ponds were the most energetically favorable reactor type, yielding 50% more energy than was put in. Horizontal tubular reactors required an energy input equivalent to the output, and vertical tubular reactors were the most unfavorable, requiring several times the energy input as that in the product, where system optimization was not conducted.

The overriding energy input in the process was found to be that required to run the reactor. Reactor energy was by far the most dominant determinant of the overall process energy requirement. This was largest in the vertical tubular reactor as these were continually mixed by gas sparging. Energy required for pumping between unit processes was also significant, particularly at lower biomass concentrations due to the larger volume of culture to be processed. The major energy inputs in downstream processing were that embodied in the lime used as a flocculation agent, and the energy required for solvent recovery. Lipid productivity and species choice had a significant impact on the energy balance.
