Examining CO2 Sources for Algae

Algae is of great interest as a sink for carbon dioxide from power, chemical and fermentation projects. Viable sequestration alternatives are essential to meet changing political and environmental tones set by the federal government.
By Sam A. Rushing | October 14, 2009
Algae consume carbon dioxide (CO2) for normal growth during photosynthesis. The appeal of algae is its ability to contribute to greenhouse gas (GHG) reduction while serving as an oil feedstock for biodiesel production. The driver in algae-based CO2 fixation or sequestration has been CO2 sourcing from coal-fired power plants, which yield only half the power produced while creating about 83 percent of the CO2 emitted from all power sources. For each kilowatt-hour of electricity, about 2.1 pounds of CO2, sent out as flue gas from the coal-fired power plants, are produced.

Between 1.5 and 3 pounds of CO2 are required for every pound of algae cultivated. Power plant projects are under great pressure to reduce CO2 emissions today; however, larger fermentation projects are also viable CO2 source targets, as are a number of commercial energy production and chemical manufacturing sources. But the CO2 streams from power and fermentation plants are different. The CO2 content from coal-fired power plants is considered lean compared to that from fermentation plants, and that difference in CO2 content can make for a broad range in capital expenses and production costs, as well as raw gas specifications for nitrogen oxides (NOx) and sulfur oxides (SOx). Defining which algae strains will accept the use of raw flue gas with lots of sulfur and nitrogen compounds, when compared to a cleaner stream from other manufacturing processes, can be problematic. If flue gas is to be a CO2 source, one challenge is selection of viable algae for cultivation assuming little or no gas purification occurs. The application of large volumes of raw gas could also be tricky.

Algae cultivation as a carbon sink is a popular idea among those in power generation. For U.S. DOE or industry-sponsored demonstration projects, the algae project is usually located near or at the CO2 source.

Required capital investment, specific application technologies, and end-results will vary depending on the amount and type of CO2 emitted. It's not always possible or convenient to allow an "across-the-fence" algae production site, partly because adjacent property isn't always available.

Algae-based fuel can represent up to 30-times more energy per acre than a common crop. Oil palm can average one-fifteenth the energy compared to algae. Given algae's high oil yield, it's estimated that about 1 percent of today's 1 billion U.S. farm and grazing acres (as land, pond, or ocean space) could produce enough algae to replace all petroleum diesel fuel consumed in the U.S.

Therefore, the challenges to successful development of algae for biofuels, when considering CO2 applications, include the distance from the CO2 source to the algae production site, the nature of the CO2 source, and how it impacts the cost and feasibility in this application.

CO2 Sources, Applications
Many of the recently evaluated projects, or those currently being tested, are generally coal-fired power plants, which already account for about 40 percent of today's CO2 emissions. If CO2 emissions are reduced from this sector, this would considerably lower global GHG emissions. In the U.S., CO2 is now being recovered from flue gas produced by coal-fired cogeneration plants; the economic model works because a prior energy law fosters the use of cogenerated steam for an amine (MEA) solvent recovery process-a method of concentrating lean CO2content in flue gas. Ethanol plants are in the spotlight because of their significant, concentrated CO2 emissions from fermentation. Effluent from fermentation, anhydrous ammonia, and certain hydrogen reformer processes used in oil refineries would have CO2 raw gas content, often in a water-saturated state, of 97 percent to 99 percent by volume.

Despite these high concentrations, fermentation plants' volume of emissions is only 12 to 15 percent compared to that from coal-fired power plants. Gas-fired turbine exhaust in cogeneration can be less than 3 percent. Heavier hydrocarbons have higher concentrations of CO2. Some concentrate CO2 through traditional processes such as MEA, which could cost three to five times the amount of applying CO2 from a more concentrated source, such as fermentation. Other processes are being tested, including membrane and refrigeration systems.

The economics behind which type of CO2 source is used is driven by the raw CO2 content in the gas, and the impurities in the CO2 stream. If the source is relatively clean and well-concentrated, direct application for CO2 fixation by certain algae strains is feasible. The economics differ wildly between using flue gas and a highly concentrated source, however, if these projects are DOE-sponsored or within the forthcoming GHG laws, and CO2 regulations call for economic considerations, concentrating or refining gas streams become more viable.

Select strains of algae may endure harsher environments when applied directly to flue gas from a power plant. It's been found that a broad spectrum of algae cannot bear the NOx and SOx in raw power plant emissions, but algae strains specifically defined as NANNO2 grew in concentrations of NOx up to 300 parts per million. Other strains, specifically NANNP2 and PHAEO2 algae, also proved successful with direct application of untreated power plant emissions.

Some of the above findings have proven well in raceway ponds for algae cultivation when diffusing power plant flue gas, compared to using a refined and/or liquefied CO2. Another consideration beyond algae type and growth tolerance in the direct flue gas setting would be the availability of physical space for algae cultivation, which precipitates the question of transporting the CO2 source to the algae cultivation site.

CO2 Transportation, Algae Cultivation Sites
CO2 is typically piped, trucked or railed in liquid form and purified for merchant markets, but not so for enhanced oil recovery (EOR). Liquid CO2 contains much more product versus trying to transport gaseous, dilute power plant CO2. The construction of a liquid CO2 pipeline can easily cost $1 million per mile. EOR sites receive liquid CO2 piped from more than 100 miles, requiring compression on the front end and compression sub-stations en route. When considering algae fixation as a means of sequestering CO2, and a further means of producing feedstock for biodiesel, it's technically feasible to transport CO2 via pipeline.

Consideration has been given to projects using high pressure from enriched sources of CO2, such as fermentation for various destinations (e.g., EOR). This concept could be applied to biodiesel as algae fixation with CO2, but transporting raw flue gas long distances may be new for such projects. Capital cost considerations are necessary for compression and pipeline as the basic infrastructure. Quantities of CO2 from fossil fuel combustion in the power sector can amount to 20 million tons daily globally, out of a total amount of 75 million tons of CO2 daily from all sources. Thus, all means of containing, sequestering, or fixing CO2 via algae is an extraordinary opportunity.

The greatest level of CO2 content is found among select byproduct streams in chemical manufacturing, and the large-scale plants will probably be targeted in planned legislation and EPA directives. The first 25,000 tons per year are exempt from any cap and trade, or other mechanism proposed by the U.S. House or EPA. Other mechanisms beyond cap and trade, however, may take place with new CO2-related directives.

Therefore, the focus for GHG reduction as CO2 alone will apply to larger industrial projects such as power plants, chemical manufacturing, oil refining and cement plants. If the source , such as fermentation, is enriched, then a higher-quality stream of CO2 is available up to 99 percent by volume with lower impurity levels. If this stream is flue gas from power plants, the CO2 content would probably not exceed 12 to 15 percent by volume. In either case, it's a raw gas stream. If the CO2 is liquefied and/or purified, then further investment is required. The transportation mode for this raw gas would likely be a pipeline, but minimizing the distance between the source and the algae fixation site would make most sense. Long distance transportation is possible at a price. The concepts surrounding the application of various forms of raw CO2feedstock for algae projects are entirely possible. The more complex the raw stream treatment, and the farther away an algae site is from the source, the less economically feasible the project becomes. Since a large focus on fossil-fuel-based power plants is now underway, and since this is the largest single source type for global CO2 emissions, the payback against the investment for the infrastructure surrounding CO2 treatment and transportation, in the form of revenues from the sale of algae for biodiesel, may well outweigh the challenges. This form of sequestering CO2 is unique, since it represents carbon fixation in plant life, and it also is an ingredient essential for the growth of an energy-rich product for biodiesel production.

Sam A. Rushing is president of Advanced Cryogenics Ltd. Reach him at (305) 852-2597 or [email protected].
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