Algae: cleaning up biofuels

The role of algae as a component of a successful carbon sequestration program is gaining interest in the biofuels industry.
By Sam A. Rushing | October 06, 2009
New methods of carbon sequestration are essential to meet the changing political and environmental tone set by the U.S. House of Representatives and the U.S. EPA. Algae can be a partial solution for greenhouse gas reduction as CO2 is an important ingredient used by algae for normal growth, during photosynthesis. A range of 1.5 pounds to 3.0 pounds of CO2 is required for every pound of algae cultivated. There is also strong interest in using algae as a source of feedstock material for biodiesel, and perhaps fermentation. Algae cultivation as a carbon sink is fast becoming a popular consideration among those in the power generating business.

The call for a meaningful reduction of atmospheric CO2 content creates a constructive opportunity for carbon-emitting power generators. Power plant projects, with their higher-than-average emissions of CO2, are under the greatest pressure to reduce emissions.

Most of the testing for CO2 fixation by algae has been via the coal-fired power plant, which produces lean CO2 compared to a fermentation project. The difference in CO2 content can make for a broad range in capital expenses and production costs. Also, while particular algae strains will accept the use of a raw flue gas, selecting a viable algae strain to use depends on raw gas specifications for nitrogen oxides and sulfur oxides.

In addition, larger fermentation projects and chemical manufacturing plants are also viable CO2 source targets. The U.S. DOE or industry sponsored demo projects have produced most of the headlines in this area recently. Typically, the algae project is located around or near the power facility, chemical manufacturer, or other projects that have a significant CO2 output.

In algae fuel, this can represent up to 30 times more energy value per acre than a crop such as soybeans. Given the high oil yield from algae, it is estimated that approximately 1 percent of today's 1 billion acres used in the United States for farming and grazing would be sufficient (as land, pond, or ocean space) to produce enough algae to replace all petroleum-based diesel fuel used in the U.S. today. That is a significant number, and algae should be utilized and developed to take advantage of opportunities such as this.

Sources of CO2, Direct Source Application
Coal-fired electricity generating plants account for about 40 percent of current CO2 emissions and reductions in this sector would have a substantial impact on greenhouse gases (GHG). When considering relatively large CO2 emitters, the ethanol industry has been in the spotlight due to a substantial amount of CO2 emitted in a concentrated form as a direct byproduct of fermentation. As to fermentation, anhydrous ammonia and certain hydrogen by-products, CO2 raw gas content generally falls around 97 percent to 99 percent by volume, often in a water-saturated state.

In the United States, CO2 is now being recovered from the flue gas produced from coal-fired cogeneration plants. The economic model works due to a prior energy law which fosters the use of cogenerated steam which is used in an mono-ethanol amine (MEA) solvent recovery processa method of concentrating the CO2 from a lean content in the flue gas.

When comparing with emissions from fossil fuel combustion, CO2 levels range within the 12 percent to 15 percent by volume, though volume increases by orders of magnitude.

Gas-fired turbine exhaust in cogeneration can be below 3 percent CO2 by volume; and heavier hydrocarbons have higher concentrations of CO2 accordingly. Some consider the need to concentrate the CO2 via traditional processes, such as MEA, which is quite expensive. If using MEA, this would represent between three and five times the cost of applying CO2 from a concentrated source, such as those named above. Other novel or test applications are underway with so-called proprietary processes, including membrane and refrigeration systems.

The economics behind what type of CO2 source is used is driven by the raw CO2 content in the gas source type, as well as the impurities found in this CO2 source. If the source is relatively clean, and well-concentrated, direct application for CO2 fixation by certain algae strains is entirely feasible. Separately, when concentrating a flue gas versus using a highly concentrated source (chemical manufacturing byproduct for example), the economics are highly disparate.

On the other hand, if these projects are DOE sponsored, or within the forthcoming GHG laws and CO2 emissions regulations perhaps the need for concentrating or refining is a viable possibility.

CO2 Transportation, Algae Cultivation Sites
Traditionally, CO2 has been transported via pipeline, truck and rail in a liquid formalways purified when used in the merchant markets. The exception to much or any purification has been for enhanced oil recovery (EOR). It is important to remember that liquid CO2 would represent a great deal more CO2 presence versus trying to transport a gaseous, dilute, power plant product. The construction of a liquid carbon dioxide pipeline can easily cost $1 million per mile; and when transported as a liquid via pipeline, this distance can be substantial. Pipelines which transport liquid CO2 to EOR sites are often long distance lines, up to one hundred, and even hundreds of miles, requiring sufficient compression on the front end and compression sub-stations in route.

When considering algae fixation as a means of sequestering CO2, and a further means of producing a substantial raw material for the manufacture of biodiesel, it is technically feasible to transport CO2 via pipeline. Consideration has been given to projects which use high pressure from enriched sources of CO2, such as fermentation for various destinations such as EOR.

In the end, CO2 from fossil fuel combustion in the U.S. power sector can amount to 20 million tons daily on a global scale. Total world output is on the order of 75 million tons of CO2 daily emitted by all sources. When taking this into consideration, all means of containing, sequestering, or fixing CO2 via an environmentally friendly and extremely useful product such as algae is an extraordinary opportunity. The end result is twofoldthe production of an extremely useful and energy-rich value versus grain and other organic matter feedstock materials such as soy and palm oil.

The greatest level of CO2 content would be found among select byproduct streams in the chemical manufacturing industry; and the larger scale plants are probably those to be targeted in the planned new legislation and EPA directives. The first 25,000 tons per year are exempt from any cap and trade, or other mechanism proposed by the House of Representatives or the EPA; however, other mechanisms beyond cap and trade may take place with the new CO2 related directives. Sequestering CO2through algae is unique since it represents carbon fixation in plant life, and is an ingredient essential for the growth of an energy rich product for the biofuels industry. EP

Sam A. Rushing is president of Advanced Cryogenics, Ltd, a carbon dioxide consulting firm based in Tavernier, Fla. Reach him at rushing@terranova.net.