Posted by Stella Kin
Microalgae are sunlight-driven cell factories that convert CO2 to potential biofuels, foods and feeds.
Biodiesel production
The parent oil consists of triglycerides. It is reacted with methanol (transesterification) producing methyl esters of fatty acids, that are biodiesel, and glycerol.
Yields of methyl esters exceed 98%. The biodiesel is recovered by repeated washing with water to remove glycerol and methanol.
Biomass production
Photosynthetic growth requires light, carbon dioxide, water and inorganic salts (N, P, Si).
Temperature must remain generally within 20 to 30 °C.
To minimize expense, biodiesel production must rely on freely available sunlight, despite daily and seasonal variations in light levels.
Raceway ponds
Closed loop recirculation channel
0.3m deep
Mixing and circulation by paddlewheel
Built in concrete or compacted earth, may be lined with white plastic
During daylight, the culture is fed continuously in front of the paddlewheel where the flow begins Broth is harvested behind the paddlewheel, on completion of the circulation loop. The paddlewheel operates all the time to prevent sedimentation.
The largest raceway-based biomass production facility occupies an area of 440,000 m2 (Earthrise Nutritionals).
Disadvantages
Temperature fluctuates within a diurnal cycle and seasonally.
Evaporative water loss can be significant.
Because of significant losses to atmosphere, raceways use carbon dioxide much less efficiently than photobioreactors.
Productivity is affected by contamination with unwanted algae and microorganisms that feed on algae.
The biomass concentration remains low because raceways are poorly mixed and cannot sustain an optically dark zone.
Raceways are perceived to be less expensive than photobioreactors, because they cost less to build and operate. Although raceways are low-cost, they have a low biomass productivity compared with photobioreactors.
Photobioreactors
Unlike open raceways, photobioreactors permit essentially single-species culture of microalgae for prolonged durations.
A tubular photobioreactor consists of an array of straight transparent tubes that are usually made of plastic or glass. This tubular array, or the solar collector, is where the sunlight is captured.
Light
Solar collector tubes 0.1m or less in diameter. Tube diameter is limited because light does not penetrate too deeply in the dense culture broth that is necessary for ensuring a high biomass productivity of the photobioreactor.
Microalgal broth is circulated from a reservoir (i.e. the degassing column) to the solar collector. to increase reflectance, and back to the reservoir.
The solar collector is oriented to maximize sunlight capture. In a typical arrangement, the solar tubes are placed parallel to each other and flat above the ground.
Instead of being laid horizontally on the ground, the tubes may be made of flexible plastic and coiled around a supporting frame to form a helical coil tubular photobioreactors.
Flow
CO2
The culture must periodically return to a degassing zone that is bubbled with air to strip out the accumulated oxygen. Typically, a continuous tube run should not exceed 80 m
Carbon dioxide is fed in the degassing zone in response to a pH controller. Additional carbon dioxide injection points may be necessary at intervals
Temperature
Outdoor tubular photobioreactors are effectively and inexpensively cooled using heat exchangers. Large tubular photobioreactors have been placed within temperature controlled greenhouses (Pulz, 2001), but doing so is prohibitively expensive for producing biodiesel.
Comparison of raceways and tubular photobioreactors
Variable | Photobioreactor facility | Raceway |
Annual biomass production (kg) | 100000 | 100000 |
Volumetric productivity (kg m-3 d-1) | 1.54 | 0.12 |
Areal productivity | 0.048 a 0.072 c | 0.045 b |
Biomass concentration in broth (kg m-3) | 4 | 0.14 |
Dilution rate (d-1) | 0.38 | 0.25 |
Area needed (m2) | 5681 | 7828 |
Oil yield (w3 ha-1) | 136.9 d 58.7 e | 99.4 d 42.6 e |
Annual CO2 consumption (kg) | 183333 | 183333 |
System geometry m2/pond | 132 parallel tubes / unit 80m long tubes 0.06 m tube diameter | 978 12 m wide 82 m long 0.3 m deep |
Number of units | 6 | 8 |
a Based on facility area.
b Based on actual pond area.
c Based on projected area of photobioreactor tubes.
d Based on 70% by wt oil in biomass.
e Based on 30% by wt oil in biomass
Conclusion
Photobioreactors provide much greater oil yield per hectare compared with raceway ponds. This is because the volumetric biomass productivity of photobioreactors is more than 13-fold greater in comparison with raceway ponds
Harvesting
Recovery of microalgal biomass from the broth is necessary for extracting the oil. Biomass is easily recovered from the broth by filtration, centrifugation, and other means. Cost of biomass recovery can be significant. Biomass recovery from photobioreactor cultured broth costs only a fraction of the recovery cost for broth produced in raceways. This is because the typical biomass concentration that is produced in photobioreactors is nearly 30 times the biomass concentration that is generally obtained in raceways (see table). Thus, in comparison with raceway broth, much smaller volume of the photobioreactor broth needs to be processed to obtain a given quantity of biomass.
Economics
Recovery of oil from microalgal biomass and conversion of oil to biodiesel are not affected by whether the biomass is produced in raceways or photobioreactors.
Hence, the cost of producing the biomass is the only relevant factor for a comparative assessment of photobioreactors and raceways for producing microalgal biodiesel.
From table, estimated cost of producing microalgal biomass per kg is:
100 tonnes 10,000 tonnes
photobioreactors $2.95 $0.47
raceways $3.80 $0.60
economies of scale
and assuming CO2 at zero cost.
Assuming biomass contains 30% oil by weight, cost of biomass for providing a litre of oil:
photobioreactors $1.40
raceways $1.81
Oil recovered from lower-cost biomass produced in bioreactors is estimated to cost $2.80/L. This assumes that recovery process contributes 50% to the cost of the final recovered oil.
A reasonable target price for microalgal oil is $0.48/L for algal diesel to be cost competitive with petrodiesel (in the US).
Improving economies using biorefineries
Enhancing algal biology
increase photosynthetic efficiency to enable increased biomass yield on light;
enhance biomass growth rate;
increase oil content in biomass;
improve temperature tolerance to reduce the expense of cooling;
eliminate the light saturation phenomenon so that growth continues to increase in response to increasing light level;
reduce photoinhibition that actually reduces growth rate at midday light intensities that occur in temperate and tropical zones; and
reduce susceptibility to photooxidation that damages cells.
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