Algae offers biofuel boost
The production of biofuels from biocrops is mired in controversy, but Frank Rogalla of Aqualia sees a third way - using wastewater to support fast-growing algal biomass
How many litres of biodiesel can you produce with one cubic meter of wastewater? As municipal effluents are full of valuable nutrients, why not use them to grow algae biomass: the tiny plant needs only water, the energy of the sun, nutrients and carbon dioxide to produce vegetable oil through photosynthesis.
Even considering that only 30% of the biomass is oil, some algae strains could yield 10 times more vegetable oil per hectare than palm oil, the most productive terrestrial plant (see Table 1) commonly used for biofuels. Certain algae species grow so fast that they double their size three or four times in one day.
That means they could be harvested much more frequently than crops which are only harvestable a few times a year. Compared with the increasingly controversial first-generation biofuels made from food crops, installations to grow algae can be located in areas unsuitable for agriculture – even deserts.
In that sense, algae technology will not compete for the land with food crops, or with feedstocks of other biomass-based fuel technologies. More importantly, many of the algal species can grow in brackish water.
This means that algae technology will not put additional demand on freshwater supplies needed for domestic, industrial and agricultural use. The unique ability of algae to grow in saline water means that it can be grown in areas where saline groundwater supplies prevent other uses of water or land resources.
Algae production could therefore complement the needs of both agriculture and other biomass based energy technologies. This means it might be possible to use algae for absorbing CO2 at the same time as providing nutrient removal and producing renewable energy – as well as other residues that can be converted into pharmaceuticals, food and fertiliser.
It all sounds too good to be true, and the US federal government halted its main algae research programme over a decade ago, in 1996, after almost 20 years of research. The main focus of the Aquatic Species Program (ASP) was the production of biodiesel from high lipid-content algae grown in ponds, utilising waste CO2 from coal-fired power plants (www.nrel.gov/docs/legosti/fy98/24190.pdf).
One of the main conclusions was that even with the most optimistic assumptions about biological productivity, the costs for biodiesel were more than two times higher than current petroleum diesel fuel costs. The last set of cost estimates, developed in 1995, showed that algal biodiesel cost would range from US$0.4 to US$1.2 per litre, depending on the projections for the performance of the technology.
However, it could never compete with the projected cost of petroleum diesel, where raw oil production has a break-even cost between US$17/ barrel in Kuwait and US$40/barrel in Bahrain or Oman, or in a range from 0.1 to 0.25 US$/l. Since then, technology has advanced, oil prices have climbed beyond the limits of US$60 per barrel (or US$0.4/l), and carbon credits, with assumptions of up to US$50 per ton of CO2, are being discussed.
During the past two years this has led to an upsurge in funding from governments, the Pentagon, big oil companies, utilities and venture capital firms for oil from algae initiatives. At the end of 2007, Chevron Corporation and the US Department of Energy’s National Renewable Energy Laboratory (NREL) announced a collaborative research and development agreement to study and advance technology to produce liquid transportation fuels using algae.
Key technical challenges include identifying the strains with the highest oil content and growth rates, and then trying to figure out how to grow enough of the right strains of algae. Then, cost-effective growing and harvesting methods need to be developed.
So what are the limiting factors for microalgae production, and what quantity of biodiesel could be expected, with what effects on the CO2 and nutrient balance?
The amount of incoming solar electromagnetic radiation per unit area, measured on the outer surface of Earth’s atmosphere in a plane perpendicular to the rays, is measured by satellite to be roughly 1,366W/m2 and referred to as the solar constant. Of the energy received, roughly 19% is absorbed by the atmosphere, while clouds on average reflect a further 35% of the total energy arriving from the sun.
The generally accepted standard for peak power is 1020W/m2 at sea level. The average power, which is an important quantity when one is considering using solar energy, is lower.
In North America the average power lies somewhere between 125 and 375W/m2, between 3 and 9kWh/m2/day. In the semi-arid regions around the equators, there is usually over 3,000 hours of sunshine per year, or one-third of the time.
Only light within the wavelength range of 400 to 700nm (photosynthetically active radiation – PAR) can be used by plants, effectively allowing only 45% of total solar energy to be utilised for photosynthesis. Furthermore, fixation of one CO2 molecule during photosynthesis results in a maximum of only about 25% of the PAR absorbed to be converted into carbohydrates.
On the basis of these limitations, the theoretical maximum efficiency of solar energy conversion for CO2 fixation by a photosynthetic system is around 12%. In practice, however, the magnitude of photosynthetic efficiency observed in the field is further decreased by factors such as poor absorption of sunlight due to its reflection, respiration requirements of photosynthesis and the need for optimal solar radiation levels.
The net result is an overall photosynthetic efficiency of between 3 and 6% of total solar radiation. The main hope of the current research lies in understanding the theoretical limits of solar energy conversion.
Recent advances in photosynthetic mechanisms at a molecular level, as well as in genetic engineering tools for plant systems, could contribute to developing new algae variations which suffer less limitations of light saturation and photo-inhibition. Maximum theoretical solar energy available, assuming a hypothetical 3000h sunshine at 500W/m2, throughout the year, would be 1500kwh/m2 per year.
Modern mono-crystalline solar panels, with an efficiency of 15%, would be able to generate about 225kWh/m2/year under these conditions. However, losses during actual operation due to factors such as elevated temperature, reduced insolation intensity and dirty glass, as well as electrical losses caused by the inverter, transformer and electrical resistance also need to be factored in, as does the efficiency of the distribution network. After these considerations, the overall electrical efficiency is considered about 80%, reducing the annual average available electrical energy of a panel to 180kWh/m2/year.
Presently, with an algae producing bioreactor, operating at maximum theoretical capacity and using the best algae strains, 12% of the light would be converted to biomass, of which 50% can be expected to be oil energy. Thus about 90kwh/m2/year will be available as biodiesel, once the algae oil has been converted, or only half of the electrical equivalent of solar panels.
A single kilogram of biodiesel has an energy value of around 12kWh/kg, therefore 7.5kg/m2 of biodiesel per year could be generated. With the oil price at US$100/barrel, and a diesel fuel density of 0.84kg/l, the value produced per m2 would be US$5.6/m2 per year.
The best data available from 1,000m2 pond systems built and tested in Roswell, New Mexico by the ASP program gave maximum single day productivities as high as 50g/m2 of algae per day, which would be the long-term target for future installations. Taking a yield of 40% lipids (oils) by weight, this result is very close to the production of 7.5kg /m2/year of biodiesel assumed in the above theoretical calculation.
That would signify about 90,000l/ha of biodiesel, compared to the more common yields of 190l/ha for corn or 6000l/ha for palm oil. Compared to a daily oil consumption of 5l per person in Western Europe, microalgae could produce the needs of about 50 people/ha/year.
With this optimistic scenario, research suggests that “algae could supply enough fuel to meet all of America’s transportation needs in the form of biodiesel, using a scant 2.5% of the nation’s arable land. In fact, enough algae can be grown to replace all transportation fuels in the US on only 40,000 km2 – roughly the size of Switzerland”.
CO2 uptake and land area
Microalgal biomass contains about 50% carbon by dry weight, all of which is typically derived from carbon dioxide. Producing 100t of algal biomass fixes roughly 183t of carbon dioxide.
Typical coal-fired power plants emit flue gas containing up to 13% CO2. This high concentration of CO2 enhances transfer and uptake of CO2 in algae ponds. The concept of coupling a coal-fired power plant with an algae farm provides an elegant approach to recycle of the CO2 from coal combustion into a useable liquid fuel.
Tests of the ASP in Roswell, New Mexico, proved that outdoor ponds could be run with extremely high efficiency of CO2 utilisation. Careful control of pH and other physical conditions for introducing CO2 into the ponds allowed greater than 90% use of injected CO2.
More recent tests at the Massachusetts Institute of Technology (MIT), confirmed that, once filtered through the algae broth, fumes from a cogeneration plant came out 50-85% lighter on CO2, and contained 85% less of another potent greenhouse gas, nitrogen oxide.
With the above postulated algae productivity of 50g of algae per m2/d, the CO2 uptake would mean 90g of CO2/m2/d, or 334t/ha/year of CO2. As the average emissions per capita of all industrialised nations is about 11t of carbon dioxide per person per year, one hectare of algae culture would mitigate the emissions of only 30 people, compared to the biofuel production of 50 people.
To sequester all carbon from a population of 50 million people, a surface of 1.7M hectares would be necessary, or the surface of half of Belgium. Of course the main difficulty is to have a concentrated stream of emissions, which makes the concept practical only for larger smokestacks.
Globally, power generation emits nearly 10 billion tons of CO2 per year, and is the most easily accessible and most concentrated source of greenhouse gases. The US, with over 8,000 power plants out of the more than 50,000 worldwide, accounts for about 25% of that total, or 2.8B tonnes.
Some of the largest power plants in the US, the so-called dirty dozen, emit about 20Mt CO2 each. For instance, the Gibson Generating Station in Owensville, Indiana, is the third-largest coal power plant in the world and produces up to 3750MW in five units and occupies an area of 16km2. The 20M t CO2 emitted annually would need a pond surface area of 600km2.
As far as economical value goes, the carbon sequestration would only play a minor part. Today’s market price of carbon in Europe is US$13.50/t. Even if carbon certificates were valued at US$68/t CO2, the algae would generate a yearly income of US$16,700/ha, less than a third, compared to the diesel value of up to US$56,000 (at US$100/barrel).
The typical biomass composition of algae would be: 45%C, 10%N, 1%P. A major emphasis of the ASP final report was the potential of microalgae CO2 use during wastewater treatment (WwT). Whereas the energy requirement for nitrogen removal, when most of the nitrate is re-used for carbon oxidation, results in about 3kwh/kg N, microalgae WwT uses less energy, and thus fossil fuels, than conventional treatment processes.
This results in a reduction of greenhouse gas emissions. WwT processes could provide a pathway to developing large-scale microalgae production. Under ideal conditions, nitrogen polishing ponds with microalgae would eliminate 5gN/m2/d, meaning significant surface area.
A plant with an influent flow of 100,000m3/d and 50mg/l of nitrogen, would need a pond surface area of 100ha, or on ha for each 1000m3/d. A first experimental plant has been set up at the Laguna WwT Plant in Santa Rosa, California, to demonstrate algae’s potential to polish wastewater.
Last year, the system set up with Sonoma State University removed about 1g N/m2/d over a four-month period. While it was as much as 30 to 50% more than other local studies have achieved, it is still far from the long term targets. So the sewage treatment plant is now the kind of place to show off to potential investors in green energy, eminent scientists or grant makers with open chequebooks.
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