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Evaluation of microalgae-based biorefinery alternatives

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Abstract

Microalgae-based biorefineries for the production of renewable biofuels like biodiesel, upgraded bio-oil, biochar, biogas and other high-value chemicals have received great attention in recent decades as potential major sources of energy for the future. Microalgae are a suitable species to produce biodiesel and other high energy density by-products; however, it is questionable whether a net energy gain can be realized or not considering the whole processing chain. In the present study, the energy balances of different algae-based biofuel and bioenergy production technologies are investigated in detail and compared to each other corresponding to a cradle-to-grave overall energetic analysis. The study includes cultivation, harvesting, cell pretreatments (cell disruption, drying, grinding), lipid extraction, transesterification, gasification and hydrothermal liquefaction with bio-oil stabilization and hydroprocessing. The energy consumption and energy gain are estimated for each operational step to determine the net energy ratio (NER, energy output over energy input) for the overall technologies studied. Our detailed investigation enables to detect the most energy consuming unit operation, that is, the bottleneck point(s) of the microalgae-based technologies which should be still improved in the future for the sake of more efficient algae-based biorefineries. The investigation makes also possible to evaluate and compare the different large scale alternatives for biomass transformation. Positive energy balances with a NER value of 1.109 and 1.137 are found in two already existing processes: open raceway ponds and closed photobioreactors, respectively. Our work gives also a detailed algorithm that can be followed at the evaluation of other microalgae-based biorefineries.

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Abbreviations

DAP:

Diammonium phosphate

HHV:

Higher heating value

HTL:

Hydrothermal liquefaction

NER:

Net energy ratio

MEA:

Monoethanolamine

ORP:

Open raceway pond

tPBR:

Tubular photobioreactor

A :

Roughness (−)

A ORP :

Surface of a raceway pond (m2)

c v :

Specific heat of water (kJ kg−1 °C−1)

c hex :

Specific heat of hexane (kJ kg−1 °C−1)

c w :

Latent heat of evaporation (kJ kg−1)

d :

Pipe diameter (m)

d pond :

Depth of a raceway pond (m)

E dryer :

Energy need of drying (MJ)

E gasif :

Energy need of gasification (MJ)

E hex :

Energy required to regenerate hexane (MJ)

E purif :

Energy need of flue gas purification (MJ)

E p :

Total energy demand of paddlewheels (kW)

E rec :

Energy need of water recycling (MJ)

E recirc,tPBR :

Total energy demand of photobioreactors medium recirculation (kW)

E s :

Energy need of steam production (MJ kg−1)

E v :

Usage of electricity (kW)

f :

Friction factor (Blasius) (−)

g :

Gravitational constant (m2 s−1)

h :

Differential head (m)

k β :

Correction factor (−)

L algae :

Microalgae mass lost during drying process (%)

l e :

Equivalent pipe length (m)

m ac :

Mass of algae cake (kg)

m algae :

Mass of algae in the photobioreactor (kg)

\(m_{{{\text{CO}}_{2} }}\) :

Mass of CO2 (kg)

m harv :

Mass of harvested microalgae (kg)

m s :

Mass of steam (kg)

m w :

Mass of water (kg)

N d :

Number of days (−)

N ORP :

Number of raceway ponds (−)

N tPBR :

Number of photobioreactor units (−)

P p :

Energy demand of a paddlewheel (kW)

\(P_{\text{recirc,tPBR}}\) :

Recirculation energy demand of a photobioreactor (kW)

r m :

Curve radius (m)

r harv :

Rate of harvesting (−)

Re :

Reynolds number (−)

V :

Flow velocity (m s−1)

\(V_{{ {\text{centr}}}}\) :

Recovered water through centrifugation (m3)

V flocc :

Recovered water through flocculation (m3)

V holdup :

Holdup in the photobioreactor (%)

V ORP :

Total volume of raceway ponds (m3)

V tPBR :

Total volume of tubular photobioreactors (m3)

w :

Mass flow of hexane (kg h−1)

γ :

Efficiency of pumping (−)

η dryer :

Efficiency of drying (−)

η gasif :

Efficiency of gasification (−)

μ :

Viscosity (kg m−1 s−1)

ξ :

Elbows minor loss coefficient (−)

ρ :

Density (kg m−3)

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Acknowledgments

The financial support of the OTKA 112699, Hungarian Sciences Research Fund is gratefully acknowledged.

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Authors and Affiliations

  1. Department of Chemical and Environmental Process Engineering, The Faculty of Chemical and Biochemical Engineering, Budapest University of Technology and Economics, Budapest, Hungary

    Daniel Fozer, Nora Valentinyi, Laszlo Racz & Peter Mizsey

Authors
  1. Daniel Fozer
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  2. Nora Valentinyi
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  3. Laszlo Racz
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  4. Peter Mizsey
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Corresponding author

Correspondence to Daniel Fozer.

Appendices

Appendix 1

See Table 4.

Table 4 Major information concerning cultivations
Full size table

Appendix 2

See Table 5.

Table 5 Major information concerning cultivations
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Appendix 3

See Table 6.

Table 6 HHV values
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Fozer, D., Valentinyi, N., Racz, L. et al. Evaluation of microalgae-based biorefinery alternatives. Clean Techn Environ Policy 19, 501–515 (2017). https://doi.org/10.1007/s10098-016-1242-8

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  • DOI: https://doi.org/10.1007/s10098-016-1242-8

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