The United States is highly dependent on fossil fuels to meet its energy needs. As such, a major driving force that continues to dictate American foreign and economic policies relates to efforts to secure domestic and imported sources of petroleum, natural gas and coal9. The fact that global fossil reserves are declining and expected to reach critically low levels over the next century10, clearly demonstrates that our current energy policy is not sustainable. In addition to economic and political considerations, a second consequence of our dependence on fossil fuels is global climate change.
A century of burning fossil fuels has significantly increased the amount of carbon dioxide in the atmosphere11. As such, heat from the sun, which should reflect off earth’s surface and back into space, is absorbed by atmospheric CO2resulting in the temperature elevation of the Earth’s surface12. Scientists predict that unless carbon emissions are mitigated, catastrophic events including rising sea levels, disease pandemics, crop failures and mass extinctions could occur13. It is clear that we need to develop sustainable and environmentally benign alternatives to fossil fuels for energy production.
Biomass to Energy.
The use of biomass feedstocks for domestic energy production offers a renewable and potentially carbon neutral alternative to fossil fuels. The assimilation of atmospheric carbon dioxide through the process of photosynthesis provides a means of generating energy rich highly reduced carbon compounds with high fuel potential. As such plants can be used to produce biomass for direct electricity generation through direct‐firing, co‐firing, anerobic digestion, gasification, or pyrolysis14. Alternatively, energy rich compounds could be extracted from plant derived biomass and converted chemically or biologically into a liquid biofuel.
For example, all plants are capable of producing cellulosic material that could be processed into simple sugars for fermentative production of alcohol fuel. Additionally, a number of species can also produce energy rich lipophilic compounds that can be chemically converted into diesel or gasoline fuel equivalents. These “bio-crude” compounds include triacylglycerides (i.e. oil), terpenoid (i.e. resins, rubber and pigments), and straight chain hydrocarbons (i.e. waxes and alkanes). Seed derived oils are used in the commercial production of biodiesel.
Biomass Dilemma: Food or Fuel?
The recent volatility in petroleum prices and greenhouse gas initiatives has led to the implementation of governmental incentives to expedite biofuel production in the U.S. and other countries15. Since the agricultural, market and political infrastructures are well established for major commodity crops, it should not be too surprising that first generation biofuels are currently derived from major food crops. While corn, cassava, and sugar cane are the primary feedstocks for ethanol, soybean, palm kernel and rapeseed oils are the primary feedstocks for biodiesel16.
Unfortunately, consequences related to food security, environmental issues and economic stability are beginning to be realized from the increased use of food crops for biofuel production17,18. As such policies are currently being discussed to limit the competition between energy‐based and food‐base agriculture for resources including arable land, water and fertilizers17,18,19. To alleviate these problems it has been suggested that an emphasis should be placed on the development of next generation biofuel/biomass crops that can be cultivated on “degraded lands abandoned from agricultural use19.” Such crops would allow for the production of energy crops without impinging on resources needed for food production.
Gumweed (Grindelia squarrosa)
Gumweed is a biennial native plant in Nevada (as well as much of the Intermountain West) that grows commonly along roadsides and other disturbed areas. Studies completed to date indicate that the immature buds and flowers can produce over 30% (dry weight) extractable hydrocarbons. The primary hydrocarbon produced is a bicyclic C20 carboxylic acid, which, following methylation can be mixed with diesel fuel at 5%(B5) to 20% (B20) to give a biodiesel mix that meets ASTM standards as a diesel fuel. Other terpenoid compounds are also present in the extract, most of which have a lower molecular weight. The viscosity of the derivatized extract is sufficiently low that it cannot be used directly as a biofuel.
Grindelic acid is similar to abietic acid, another C20 hydrocarbon acid that is commonly used in rosin products, including a variety of laquers, paints and glue. Thus, grindelic acid may well have a larger market as a value‐added feedstock. Unlike rosin, which is extracted from a variety of pinus species, gumweed extract is dark in color, and would require decolorization to render it useful. We have been moderately successful in decolorization of the extracts and are presently isolating pure grindelic acid.
Gumweed is a biennial, in that it germinates in the spring of the first year and sends roots deep in the soil and establishes a low (approximately 4 cm tall) plant. During the second year of growth, gumweed develops a large plant mass of up to 1 meter tall, and produces large amounts of milky buds that turn into flowers, and, in late September, generate the seed.
One of the attractive features of gumweed is that it grows with normal precipitation (6‐9”) under Nevada conditions. However, the prevalence of gumweed along roadsides indicates that it would do well with limited additional deep water reserves, which are produced as runoff from paved areas. Thus, gumweed has a high potential for biofuels production on non‐agricultural land, and has low water requirements, two of the criteria that are emerging as important qualities for biofuel production.
Gopherweed (Euphorbia lathyris)
Gopherweed is a latex producing plant species that has been investigated for a number of years as a potential feedstock for the production of hydrocarbon‐based biofuels20. Gopherweed produces copious amounts hydrocarbon rich latex which represent as much as 8% of total dry shoot weight21. This latex is rich in terpenoid compounds and has been used as a feedstock for the production of a liquid fuel capable of generating 16,000 BTU/lb 21. While the energy content of the gopherweed derived fuel is less than that reported for gasoline (18,500 BTU/lb), it is significantly better than the ethanol (11,500 BTU/lb). It has been estimated that Gopherweed could produce between 7.5 and 20 barrels of liquid hydrocarbon fuels per hectare per year20,22. With an average annual harvest of 10 tons per acre, scientists21estimate that the 25 million BTU/acre/year could be generated from gopherweed derived hydrocarbons.
In addition to hydrocarbons, gopherweed generates other energy rich co‐products including fermentable sugars, seed oil and lignocellulosic biomass. It has been reported that gopherweed plants can produce 2 tons of fermentable sugars per acre per year, which when converted to ethanol would yield ~20 million BTUs21. Additionally, gopherweed seeds contain greater than 40% triacylglycerol (oil) by weight, making this plant a potential feedstock for biodiesel production. In terms of lignocellulosic material, it has been estimated that from a 10 ton harvest the post‐extraction bagasse would yield approximately 59 million BTU/acre on an annual basis23.
Although gopherweed is not native to the state, it is particularly well suited as a crop for Nevada. The plant can thrive in USDA hardiness Zones 5b through 10a, which encompasses the entire state. Because gopherweed can survive with in areas with annual rainfall total as low as 5 cm24, it can be grown in all but the most arid regions of the state. Additionally, gopherweed is fairly salt tolerant and can grow unimpeded when irrigated with water containing up to 5000 ppm Sodium Cloride22. Studies have also shown that this plant requires low nitrogen fertilizer inputs25and tolerated infertile soils23.
Our goal to develop plant species that are predisposed to thrive in arid environments and marginal soils as biofuel/biomass feedstocks directly addresses this issue.