Aquatic biomass and carbon dioxide trapping

Abstract

The technology for growing microalgae as a renewable biomass source can be applied to the production of a diesel fuel substitute (biodiesel). Microalgae are of interest because of their high growth rates and tolerance to varying environmental conditions, and because the oils (lipids) they produce can be extracted and converted to substitute petroleum fuels. Projected global climate change provides a second important rationale for this approach. Climate change has been linked to the accumulation of excess carbon dioxide in the atmosphere. The burning of fossil fuels in power plants is the primary contributor to excess carbon dioxide. Inasmuch as the primary nutrient for microalgal growth is carbon dioxide, operation of microalgal biomass farms has emerged as a promising candidate in the search for alternative approaches to ameliorate global climate change. The production of diesel fuel by microalgae requires very large quantities of carbon dioxide as a nutrient. In areas where microalgae fuel farms operate in tandem with fossil fuel plants to scrub carbon dioxide from flue gases, the release of carbon dioxide could be significantly reduced. If the microalgae are used to produce fuel, a mass culture facility reduces by approximately 50% the carbon dioxide emissions from the power plant per million Btu delivered. For example, although coal is ordinarily considered to be the most polluting fossil fuel on the basis of carbon dioxide emitted per amount of energy produced, the incorporation of microalgal ponds with a coal-fired plant would make this fossil fuel less polluting than existing oil- and natural-gas-fired plants. Similar advantages can be achieved for oil- and gas-fired plants. If commodity chemicals are produced from algae instead of fuels, the net carbon dioxide reduction is significantly greater. Commodity chemicals can be used to produce goods with long-term uses such as building materials. Such uses would result in the sequestering of carbon dioxide for long periods. Of the photosynthetic organisms, microalgae are the most productive carbon dioxide users and can fix greater amounts of carbon dioxide per land area than higher plants. Also, maximum productivities of higher plants and trees are restricted to areas with prime soil, water, and climate (primarily the tropics). Plant leaves exist in an aerial environment and are subject to large evaporative moisture losses, which directly inhibit the process of photosynthesis (carbon dioxide uptake). Microalgae in mass culture are not subject to such photosynthetic inhibition because the water content of the culture can be controlled by proper engineering, and saline water can be used if necessary. This difference is the basis for the several-fold higher carbon dioxide absorption capacity of microalgae compared to plants. Initial application of this technology is envisioned for the Desert Southwest of the United States because this area provides high solar radiation and offers flat land that has few competing uses (hence low land costs). Also, there are large saline aquifers with few competing uses in the region. These could provide a suitable, low-cost culture medium for the growth of many species of microalgae.