Review of Biomethane from Marine Biomass

The objective of this report is to review the history, results, and conclusions of research on marine biomass conducted under the sponsorship of the U.S. Navy, gas industry (American Gas Association and Gas Research Institute), and U.S. Department of Energy. The scope of this program was to determine the technical and economic feasibility of production of substitute natural gas (SNG) from marine biomass using anaerobic digestion as a conversion process. This work began in 1968 and continued until about 1990, ending as a result of low energy prices in the U.S. and reduced emphasis in renewable energy.

The focus of this report is on growth of seaweeds and conversion to methane via anaerobic digestion. Since this program ended in 1990, interested parties met seve ral times to continue discussing this topic and possibilities for obtaining new support its further development. The results of our dialogue at these meetings are summarized, including alternative ideas for marine energy farms and conversion of methane to methanol. Research from other concurrent programs sponsored by the gas industry to produce SNG from biomass and wastes is summarized and compared with those presented for marine biomass.

These programs addressed herbaceous and woody species, water hyacinth and sludge generated from aquatic plant waste treatment systems, and municipal solid waste. For each of these feedstock categories, feedstock growth or collection (in the case of wastes), harvesting, conversion by anaerobic digestion, and systems and economic analysis are addressed. Also discussed is the potential impact of this form of renewable energy on mitigation of carbon dioxide emissions from fossil fuels. In general, marine biomass was the least developed of these systems by this research effort. The greatest uncertainties were related to the technical and economic feasibilityof large-scale growth of macroalgae in the open ocean, especially concerning provision of nutrients. The anaerobic conversion aspect of this system was better developed and is not likely to be significantly different than that developed for other similar feedstocks.

The gas cost estimates for marine biomass systems were 3-6 times those for U.S. fossil fuel gas. Terrestrial biomass systems were developed to a greater extent by this research because of a better prior knowledge of growth and harvest of the feedstocks emphasized. SNG from this category was about 2-3 times that of U.S fossil fuel gas. The lowest cost was associated with SNG from municipal solid waste, reflecting the tipping fee received for treating this waste. However, these costs are not competitive with landfilling.