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Combined Production of PHA Bio-based Polymers and Biomass Energy

The practice of extracting energy and petrochemicals from fossil carbon is unsustainable, currently in light of the contributions to atmospheric carbon dioxide that necessarily accompany it, and over the longer term in light of the finite nature of these resources. It is now generally recognized that atmospheric carbon dioxide levels are being notably increased by human activity, with significant climate changes expected, although not easily predicted in their details. While the production of chemicals and polymers employs only a small portion of the carbon processed for energy, the very large scale of fossil carbon processing involved in supporting energy needs provides economies that the petrochemical industry enjoys, but could not enjoy if petrochemical processing were divorced from energy production.

In addition, the U.S. is no longer self-sufficient in the cleaner forms of fossil carbon such as gas and oil. While there is ample domestic coal, the mining of coal is environmentally offensive, and the burning of coal contributes to the buildup of atmospheric carbon dioxide to the greatest extent of any of the fossil carbon sources. The increasing gap between U.S. demand for fossil carbon for both key materials and energy and domestic supply has led to greater reliance on overseas sources of fossil carbon, in particular to Middle Eastern sources. This reliance for vital raw materials on capacity more and more concentrated in regions of political instability whose political inclinations are not well aligned with those of the West has had profound implications for U.S. foreign policy. The co-production of energy and PHAs using biomass crops will contribute to the overall amount of domestically sourced (i.e., from U.S. agricultural sources) carbon flowing through the U.S. economy and will reduce our dependence on imported carbon.

A sustainable and environmentally benign approach to supplying society's needs for energy and materials is to harness the energy of the sun. Green plants represent highly evolved systems for doing just that: they absorb solar energy and convert atmospheric carbon dioxide into more reduced forms of carbon. The carbon dioxide captured in this plant matter can be recovered to supply current needs for energy and materials. Much as the development of transition metal catalysts has enabled the remarkable expansion in our ability to transform fossil carbon into useful energy and materials during the 20th century, so the current rapid development of the tools of biotechnology will now enable the use of biological systems to efficiently transform atmospheric carbon into useful energy and materials.

Analogously to the practice of extracting both energy and petrochemicals from fossil carbon, Metabolix is developing and perfecting the technology to harness both energy and materials from plants on a sustainable basis, and has recently been awarded a DOE grant to develop PHA production directly in switchgrass. Switchgrass is an attractive host plant for PHA production because it is a leading candidate for bio-based energy production in North America - it grows in high yield, can be grown on land of marginal use for other crops, and fixes about 2 kg carbon dioxide in its root system for every kg of biomass above the ground. Yields average about 5 tons per acre in the U.S. today, and experimental plots have demonstrated yields of about 10 tons per acre over a six year period, with yields as high as 15 tons per acre observed. It can be expected that application of the tools of modern biotechnology will enhance these yields further. Other plant targets, such as sugarcane, which yields about 30 tons per acre, will also be attractive sources of PHAs and energy.

A key question in any biomass based energy scheme is whether meaningful amounts of energy can actually be generated agriculturally. The answer to this question is a very plausible yes. For example, switchgrass has an energy content of about 15.5 Mbtu/dry ton. Assuming that yields of 10 to 15 tons per acre can be reached (and there is no reason to believe that they will not be), then 80 to 120 million acres of switchgrass will provide the energy content equivalent to all of the oil currently imported into the U.S. (i.e., 9 million barrels per day, or 3.3 billion barrels per year). To put this land use into perspective, the U.S. has about 430 million acres designated as cropland today, and the Federal government pays farmers not to cultivate about 40 million acres. A major problem for agriculture in the developed world today is that farmers now produce far more than is needed of traditional crops, and are subsidized in all countries through one mechanism or another. For example, an important issue in the recent negotiations that have led to initiation of a new world trade round was France's insistence on continued export subsidies for its farmers, who are competing with farmers in developing countries where agriculture is a value adding activity appropriate to their pre-industrial stage of development. A new, value-generating crop is needed.

Should switchgrass be adopted as an energy producing biomass crop, only a small portion of the output would be required to support significant PHA usage. For example, if PHAs are accumulated in switchgrass to a level of 20% of dry cell weight and 75% of that is recovered (i.e., 15%), and if switchgrass yields are 10 to 15 tons per acre, then each acre will yield 1.5 to 2.25 tons of PHA bio-based polymers or derived chemicals, and 1 million acres will yield 3.3 to 5 billion pounds of PHAs. Additionally, accumulation of PHAs in energy crops such as switchgrass increases the energy content of the crop, due to the more reduced stated of PHAs versus cellulose. For example, switchgrass containing 20% PHA will have an energy content about 5% greater than ordinary switchgrass, and will have enhanced value as an energy crop.