A sustainable option overlooked? Prolific green algae that capture carbon dioxide to make biodiesel. Dr. Mae-Wan Ho
The article, along with others, will form part of ISIS’ 2006 Energy Report, Which Energy to be submitted to UK government’s energy review.
The UK government has invested in a £25 million Carbon Capture and Storage (CCS) demonstration project [1] led by British oil company BP [2] as a way of reducing greenhouse gas emissions. This involves capturing carbon dioxide from the exhausts of power stations and hydrogen or oil and gas production facilities in a relatively pure form and storing it geologically underground, in porous layers of rock, depleted or near-depleted oil and gas fields, deep saline aquifers (porous rock layers containing salty water deep underground), or in coal seams that cannot be mined [3].
UK’s energy sector is responsible for 35 percent of carbon emissions, so CCS seems an effective way of tackling the problem head on. UK’s sector of the North Sea has a large storage potential, estimated at ~20 000 to 260 000 Mtonne CO2. But it is illegal to dump large quantities of CO2 under the North Sea at the moment. Nevertheless, the Norwegian company Statoil has been re-injecting CO2 co-produced with natural gas into a deep aquifer overlying its offshore Sleipner field since 1996, and nothing seems to have leaked from the 6 Mtonne CO2 stored so far [1]. Geologists are continuing to monitor the situation.
Another approach already adopted in the North Sea Enhanced Oil Recovery (EOR) programme is to pump CO2 underground to dissolve in the oil, making it more mobile and easy to extract [3]. The North Sea Oilfields have an estimated storage capacity of ~700 Mtonne CO2. UK’s oil operations are nearing the end of their operation, and EOR could postpone decommissioning and recover more oil.
However, there are problems in capturing carbon dioxide economically and without compromising the efficiency of the power plants [3, 4]. The carbon dioxide captured also has to be transported to the storage sites, and that could involve thousands of kilometres of pipelines.
Moreover, there are major concerns over the integrity of the geological storage, the possibility of leakage during storage and transport, and the potential impacts on the marine ecosystem when CO2 is injected into the deep ocean.
According to the US Department of Energy [4], CO2 capture is the limiting factor economically, and is generally estimated to represent 75 percent of the total cost of CCS.
What no one seems to be aware of is that the humble green algae could offer a cost-effective and environmentally benign way to capture carbon dioxide on-site that does not need transport or storage, and at the same time, provides renewable biodiesel fuel much more effectively and sustainably than energy crops (“Biofuels for oil addict”, this volume).
Isaac Berzin, a rocket scientist at Massachusetts Institute of Technology, is using algae to clean up power-plant exhaust, saving greenhouse gas emissions and satisfying energy needs [5].
The idea occurred to him three years ago, although it is not exactly new (see later). He bolted onto the exhaust stacks of a 20 MW power plant rows of clear tubes with green algae soup inside. The algae grew happily, gobbling up 40 percent of the carbon dioxide for photosynthesis, and as a bonus, 86 percent of the nitrous oxide as well, resulting in a much cleaner exhaust.
The algae is harvested daily and its oil extracted to make biodiesel for transport use, leaving a green dry flake that can be further processed to ethanol, also a transport fuel (but see “Ethanol from cellulose biomass not sustainable nor environmentally benign”, this series).
GreenFuel, the company set up by Berzin in Cambridge Mass., has already attracted £11 million in venture capital funding and is conducting a field trial at 1 000 MW plant owned by a major southwestern power company. GreenFuel expects two to seven more such demo projects, scaling up to a full production system by 2009.
One key to success is to select an alga with a high oil density – about 50 percent by weight. Algae are prolific and can produce 15 000 gallons of biodiesel per acre, compared to just 60 gallons from soybean. Berzin estimates that a 1 000 MW power plant using his system could produce more than 40 million gallons of biodiesel and 50 million gallons of ethanol a year. But that would require a 2 000 acre farm near the power plant.
Greenfuel is not alone in racing to make oil out of algae. Greenshift Corporation, an incubator company based in Mount Arlington New Jersey, licensed a CO2-scrubbing screen-like filter developed by David Bayless, researcher at Ohio University. A prototype is capable of handling 140 cubic metres of flue gas per minute, an amount equivalent to the exhaust from 50 cars or a 3-megawatt power plant.
The US National Renewable Energy Laboratory (NREL) had a research project from1978 to1996 on creating renewable transportation fuel with algae making use of waste CO2 from coal fired power plants [6]. The project, led by NREL scientist John Sheehan, was funded at $25.05 m over the 20-year period, compared to the total spending under the Biofuels Program over the same period of $459 m. It resulted in a collection of 300 species of green algae and diatoms, now housed in the University of Hawaii and still available to researchers. Although some technical and economic problems remained to be solved, it was estimated that just 15 000 square miles (or 3.8 m ha) of desert (the Sonoran desert in California and Arizona is more than 8 times that size) could grow enough algae to replace nearly all of the nation’s current diesel requirements, and algae use far less water than traditional oilseed crops.
Researchers also suggested using algae to clean up Salton Sea in Southern California [7], into which more than 10 000 tons of nitrogen and phosphate fertilizers are discharged annually. The idea was to use some 1 000 ha of pond system to grow algae such as Spirulina with the sea water, harvest the algae biomass and convert that into fuels, while the residual sludge could be recycled to agriculture for its fertilizer value. An estimate suggests that such a process could mitigate several hundred thousand tons of CO2 emissions at below $10/ton CO2 equivalent.
But it is perhaps the algae’s potential for carbon-capture that makes them most attractive, and it is as yet almost untapped.
Article first published 03/03/06
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