How to achieve ethical energy and environmental sustainability to satisfy future energy demands … The Use Of Algae Adam A Marsh 0404304 MSc Sustainable Energy and Environment
Different Algae Processes Possibilities Limitations
Chlorella sp. Chlorella sorokiniana: Very fast growing - (doubling time of 2.5 h) Tolerant to high temperatures - (optimum 40oC) Tolerant to high CO2 concentrations - (5 - 40%CO2) Found in hot springs Chlorella vulgaris: Fast growing Found all over the world in lakes and ponds Huge amount of literature
Botryococcus braunii High concentration of lipid hydrocarbons (<70%) At 50% concentration (dry weight), HHV ~ 34MJ/kg Can grow in brackish water Slow growing – (doubling time of a few days) Can form bio films
Chlamydomonas reinhardtii Can produce pure Hydrogen gas when deprived of S Rapid growing – (doubling time of 6.4 h achieved) Limited growth during H2 production
Concentration
Temperature
Retention time
Nutrients
Flow rate
Salinity
light Chlorophyll chemical CO2 + H2O + (CH2O) + O2 + energy energy
Wavelength Intensity Photoperiod
Light Wavelength Chlorophyll mainly absorbs light at approximately 450 and 650 nm, perceived as blue and red respectively. (A)
(B)
Intensity Different species have individual characteristics. However, similar tri-region growth curve fits all. 1. Light dependant region; increased light = increased growth rate. 2. Light independent region; constant growth rate between saturation (A) and photo inhibition (B) light levels. 3. Light dependant region; increased light = reduced growth rate.
6.3 W/m2
47.3 W/m2
Botryococcus Braunii
W/m2
Light Photoperiod Photoperiod is the ratio of light and dark, typically measured out of a maximum 24 hour period. If NADPH and ATP compounds are available, the remaining processes of photosynthesis can take place without the necessity of light. It can be seen above that algae does not utilise much more than 12 hours of light in a 24 hour period. The maximum growth rate of a Chlorella sp. can be maintained in 9s of darkness if each cell has been exposed to 20 W.m-2 for 0.5s. 1 hour ~ 3.2 minutes
Botryococcus Braunii
Light
12h
30W/m2
Temperature Temperature affects the rate of photosynthesis by changing the rates of enzyme reactions involved in systems of the photosynthetic complex. The maximum growth rate will occur at the optimum temperature, with declining rates either side of the optimum.
Botryococcus Braunii
The optimum temperature will vary for individual species. Typical values for algae range from 20 - 35oC When utilising species for a by-product, the optimum temperature for growth may not be the same as that for optimum production.
Botryococcus Braunii
CO2 Supply Concentration Current atmospheric CO2 concentrations of dry air are in the order of 0.038% Typical CO2 concentrations emitted from a 1500MWe coal power station will be in the order of 13% Optimum growth rate of Chlorella vulgaris occurs at 2% CO2 concentration Highest CO2 reduction efficiency also occurs at 2% CO2 concentration at 58% Highest CO2 reduction occurs at optimum growth rate. Chlorella Sorokiniana will grow well at concentrations of 13% CO2. Adding further volumes of bioreactors increases the volume sequestration linearly with similar efficiencies.
Chlorella Sorokiniana
CO2 Supply Flow Rate and Retention Time In small reactors, the capacity of CO2 fixation (and O2 evolution) decreases with an increasing gas flow rate as the retention time is dramatically reduced. Increasing the retention time enables more sufficient contact between algae and CO2 resulting in better absorption. However, larger reactors will increase the retention time by increasing the distance for which the gas must pass through the culture, allowing higher gas flow rates to be used. A faster gas flow rate will increase turbulence. Turbulence will improve the mass transfer Chlorella vulgaris coefficient and will induce mixing of the cells, allowing each cell time in the light intense areas of the reactor. Lihai Fan et al., 2007. Optimization of Carbon Dioxide Fixation by Chlorella vulgaris Cultivated in a Membrane Photobioreactor, Chem. Eng. Technol.; Volume 30, Issue 8, pg. 1094-1099
Chlorella sp.
CO2 Reduction This paper claimed the optimum gas flow rate was 1.25 L/min. Membranes were used to enhance the gasliquid mass transfer rate. 8W/m2,
At the rather low luminous intensity of and CO2 concentration of 1%, the CO2 fixation rate was approximately 0.14g/L.h
Chlorella Vulgaris
T = 25oC; Density; 5x107 cells/mL; Irradiance ~ 8 W/m2; CO2 concentration; 1%
Original cell density was 5x107 cells/ml
~8W/m2 Lihai Fan et al., 2007. Optimization of Carbon Dioxide Fixation by Chlorella vulgaris Cultivated in a Membrane Photobioreactor, Chem. Eng. Technol.; Volume 30, Issue 8, pg. 1094-1099
1500MWe Power Station Coal HHV ≈ 30MJ/kg Coal ≈ 143.3kg/s C ≈ 80% O ≈ 13% H ≈ 6% S ≈ 1%
η=35% 1500MWe
4300MWth
CO2 ≈ 13% SO2 ≈ 0.005% O2 ≈ 3.8% N2 ≈ 82.9%
C ≈ 114.6 kg/s
CO2 ≈ 420 kg/s
C + O2 = CO2
nCO2 ≈ 9.6 kmol/s
(12) + 2(16) = (44)
VCO2 ≈ 235 m3/s
1
: 2.67 : 3.67
( @ 25oC )
VT ≈ 1815 m3/s
CO2 Reduction Φ 130mm
120mm
Using Chlorella sorokiniana; 1.25 L/min 13% CO2
1m
0.14g CO2/L.hr
CSA = 0.0133 m2
0% CO2
Length ? 0.14 g/L.hr : = 2.3 g/m3.s
Power plant actually produces 1815 m3/s @ 13% CO2
1.25 L/min @ 13% CO2 : = 2.7x10-6 m3/s CO2 = 4.8x10-3 g/s CO2
= 87.3x106 times more massive
Assuming constant reduction rate, reactor volume required :
11,610 km of pipeline are needed.
4.8x10-3 / 2.3 = 1.77x10-3 m3
4 pipes in each metre width
Reactor length required :
2.9x106m2 horizontal area
1.77x10-3 / 0.0133 = 0.133 m
1.7km2
87.3x106 x 0.133 = 11.61x106 m
≈ 2 km2
Cell Growth Original cell density = 5x107 cells/ml = 0.7kg/m3 Original Total Mass = 0.7 x 154.5x103 = 108x103 kg Doubling time of 2.5 hours = 9.6 doublings a day Mass grown = 9.6 x 108x103 = 1.04x106 kg/day
(dry weight)
= 12 kg/s Chlorella Sorokiniana has a HHV of 15.6MJ/kg This is too low for combustion. However, would provide 187MWth This is about 4% of power station
Anaerobic Digestion Anaerobic digestion is a process carried out by micro organisms that can degrade carbon based matter without the presence of oxygen. The main products are Carbon Dioxide and Methane.
(60% CH4 and 40% CO2)
Biomass is a suitable source for anaerobic digestion.
Feedstock 1.7 kg VS/m3.day
Reactor 35oC 20-30days
CO2
CH4
0.68 kg /m3.day 60% reduction in organic matter
Anaerobic Digestion
Methane has a HHV of 55.5 MJ/kg Kelp can produce biogas at a rate of : 0.4 m3/kg VS We are producing : 1.04x106 kg/day 12 kg/s (dry weight) This equates to : 416x103 4.8 m3/s
m3/day
Therefore : 266.4 MWth could be produced. (~6% of power plant) Bioreactor can take 1.7 kg/m3.day Therefore : 245x103 m3 reactors would be needed = 311, 10m high, 10m diameter reactors 3.2 m3/s CO2
(~1.3% of plant)
Conclusions Algae can serve to be a natural way for removing CO2 emissions. 98% is a realistic reduction value. Huge crops of algae will be produced daily. Using as a fuel for the power plant is not a realistic possibility. Using as a transport fuel will simply re-release the CO2. Alternative uses must be found. Research is being carried out in this area. - Bio-fuels - Bio-Plastics - Foods - Fertiliser
QUESTIONS ?
1500MWe, η=35%
Coal HHV ≈ 30MJ/kg
1500 / 0.35 = 4286
4300 / 30 = 143.3
≈ 4300MWth
Coal ≈ 143.3kg/s
C + O2 = CO2
C ≈ 80% O ≈ 13% H ≈ 6% S ≈ 1%
(12) + 2(16) = (44) 1
: 2.67 : 3.67
CO2 ≈ 13% SO2 ≈ 0.005% O2 ≈ 3.8% N2 ≈ 82.9%
114.6 x 3.67 = 420.6 CO2 ≈ 420 kg/s 420 / 44 = 9.55 kmol/s CO2 ≈ 9.6 kmol/s 9.6x103 x 24.5x10-3 = 236 VCO2 ≈ 236 m3/s ( @ 25oC )
143.3 x 0.8 = 114.6
VT = 236 / 0.13 = 1815.4
C ≈ 114.6 kg/s
VT ≈ 1815 m3/s
Photo-limitation
I ln Io
k C d
Where; I = Light intensity at depth of penetration d Io = Incident light intensity C = Algae concentration (kg.m-3 or g.l-1) d = Depth (m) k = Light extinction coefficient (m2.kg-1) To make enough fuel from algae to run entire plant : Starting density of 11.6kg/m3 is needed
