11.08.2015   BIOFUELS FROM ALGAE

MICROALGAE CULTIVATION IN CLOSED AND OPEN PBRs FOR BIOFUEL PRODUCTION

Production of sustainable biofuels from microalgae is a high-potential option for develop­ing renewable energy. Unfortunately, the production cost of microalgae-based biofuels is still too high, which prevents them from becoming commercially feasible. One of the major obsta­cles that impedes the commercialization of microalgal biofuels is the high cost of photobioreactors and the high demand of auxiliary systems or intensive energy input re­quired during the cultivation of microalgae. Basic conceptual designs for a photobioreactor for the autotrophic cultivation of microalgae are to provide efficient mixing, appropriate light intensity, and rapid gas transport (Singh and Sharma, 2012).

In light of these demands, photobioreactor designs can be generally classified as open sys­tems and closed systems (Table 2.1). Open systems can be divided into natural waters (lakes, lagoons, ponds) and artificial ponds or containers, which are presented in very different ways. Apparently, open systems are potentially subject to contamination resulting from the free gas exchange from the environment to the cultivation system. The cultivation

TABLE 2.1 Advantages and Disadvantages of Open and Closed Algal Cultivation Plants (Pulz, 2001).

Parameter

Open Ponds (Raceway Ponds)

Closed Systems (PBR Systems)

Contamination risk

Extremely high

Low

Space required

High

Low

Water losses

Extremely high

Almost none

CO2 losses

High

Almost none

Biomass quality

Not susceptible

Susceptible

Variability as to cultivatable species

Not given; cultivation possibilities are restricted to a few algal varieties

High; nearly all microalgal varieties

Flexibility of production

Change of production between the possible varieties nearly impossible

Change of production without any problems

Reproducibility of production parameters

Not given; dependent on exterior conditions

Possible within certain tolerances

Process control

Not given

Given

Standardization

Not possible

Possible

Weather dependence

Absolute; production impossible during rain

Insignificant because closed configurations allow production during bad weather

Period until net production is reached after start or interruption

Long; approx. 6-8 weeks

Relatively short; approx. 2-4 weeks

Biomass concentration during production

Low, approx. 0.1-0.2 g/L

High; approx. 2-8 g/L

Efficiency of treatment process

Low; time-consuming, large — volume flows due to low concentrations

High; short-term, relatively small — volume flows

conditions of open systems are usually poorly controlled, and the estimated growth rate of microalgae will be mostly lower than that in closed systems.

In terms of technical complexity, open systems are more widespread than closed systems. From the aspect of operation, closed systems are more suitable for the cultivation of algae for the production of high-value products. In closed systems, the productivity of desired prod­ucts can be enhanced by controlling the microalgae cultivation under optimal operating con­ditions. The design of closed photobioreactors must be carefully optimized for each individual algal species according to its unique physiological and growth characteristics. Providing appropriate light intensity and efficient hydrodynamic mixing are key issues in the success of a productive autotrophic cultivation system (Kumar et al., 2011).

Given the advantages of closed systems over open systems, several different photo­bioreactor designs with closed systems have also been proposed, ranging from lab scale to industry scale. More detailed descriptions of microalgae cultivation in open and closed systems are presented in the following sections.