What Plants for Cellulosic Biofuel Production?

In an article titled “Feedstocks for Lignocellulosic Biofuels” published in Science, Chris Somerville of the University of California, Berkeley, and Deputy Director Steve Long of the University of Illinois at Urbana-Champaign with bioenergy analysts Caroline Taylor, Heather Youngs and Sarah Davis at the Energy Biosciences Institute suggest that a diversity of plant species, adaptable to the climate and soil conditions of specific regions of the world, can be used to develop “agroecosystem” for fuel production that are compatible with contemporary environmental goals.

Well, press release and research notes aside, they mean that there can be a set of plant species that could provide substantial amounts of biomass grown widely across the planet without an impact on food and feed production.  The troubled firm BP, well before the Gulf well crisis, funded the study.

The study authors discuss the sustainability of current and future crops that could be used to produce advanced biofuels with emerging technologies that use non-edible parts of plants. Such crops include perennial grasses like Miscanthus grown in the rain-fed areas of the U.S. Midwest, East and South; sugarcane in Brazil and other tropical regions, including the southeastern U.S.; Agave in semiarid regions such as Mexico and the U.S. Southwest; and woody biomass from various sources.

The team takes some assumptive license by making some simplifying assumptions: that technology will become available for converting most of the structural polysaccharides that comprise the bodies of plants to sugars, that all the sugars can be used for fuel production, and that the process energy required for the conversion of the sugars to fuels will be obtained from combustion of the other components of the biomass, mostly the lignin.  That way a sugar-to-ethanol bioconversion process using current technology, a metric ton (MT) of switchgrass or poplar, for example, would be expected to yield about 310 liters of ethanol.

The author’s base is founded on the comparative soil impacts.  Maize or corn plants used completely remove much more soil fertility than a perennial plant.  Perennial plants that use C4 photosynthesis, such as sugarcane, energy cane, elephant grass, switchgrass, and Miscanthus, have intrinsically high light, water, and nitrogen use efficiency as compared with that of C3 species as seen in corn.  Moreover reduced tillage and perennial root systems add carbon to the soil and protect against erosion.

While the team reports that tropical Napier Grass in El Salvador natural stands of Echinochloa polystachya on the Amazon floodplain can respectively reach production of 88 and 100 MT/ha/year, temperate Miscanthus x giganteus produced in England at 52°N a peak biomass of 30 MT/ha/year and harvestable biomass of 20 MT/ha/year. (ha is hectare, 2.47 ha per U.S. acre) Miscanthus also offers an important soil protection effect, seasonality leads to an annual cycle of senescence, in which perennial grasses such as Miscanthus mobilize mineral nutrients from the stem and leaves to the roots at the end of the growing season. Thus, harvest of biomass during the winter results in relatively low rates of removal of minerals.

That could account for the observation that stands grown at Rothamsted, UK showed no response to added nitrogen during a 14-year period during which all biomass was removed each year.  In side-by-side trials in central Illinois, unfertilized M. x giganteus produced 60% more biomass than a well-fertilized highly productive maize crop, and across the state, winter-harvestable yields averaged 30 MT/ha/year.

Miscanthis US Growing Area Map. .

The author’s note in an observation that if Miscanthus were used as the only feedstock, less than half of the 14.2 Mha currently set aside for the U.S. Conservation Reserve Program  (CRP) would be required to deliver the ethanol mandate of the Energy Independence and Security Act of 2007.  Contrary to that readers should be informed that a great chunk of the CRP land area is tiny little headlands, terraces, protective filters along watercourses and the like.  But there are vast amounts of highly erodeable land that could better serve the economy than being used for corn or soybean production.

Its worthwhile to note that as the authors seem to overlook some details they turned up others.  The Global Potential of Bioenergy on Abandoned Agriculture Lands published in 2008 reveals that more than 600 Mha of land worldwide has fallen out of agricultural production, mostly in the last 100 years.

Most readers will know that for tropical production sugarcane isn’t beaten yet and won’t most likely.  Harvested cane arrives with the sugar in liquid form ready for fermentation and the plant remnants can be burned for distillation with power left over for the electric grid.  Many other regions of the world beyond Brazil are also well suited to sugarcane production or formerly produced sugarcane on land that has been abandoned. Thus, “the total amount of fuel that may be produced from sugarcane worldwide could eventually be a very substantial proportion of global transportation fuels.” As the authors seem to be aware – the potential in sugarcane defies calculation in responsible numbers for now.

Approximately 18% of the earth’s surface is semi-arid and prone to drought.  The authors suggest various Agave species that thrive under arid and semi-arid conditions with high efficiencies of water use and drought resistance hold a potential opportunity for production of biomass for fuels.  Agave species that thrive under arid and semi-arid conditions by using a type of photosynthesis called Crassulacean acid metabolism (CAM) that strongly reduces the amount of water transpired by absorbing CO2 during the cold desert night and then internally assimilating this into sugars through photosynthesis during the warmer days.  By opening their stomata at night, they lose far less water than they would during the day.  Much of the land noted in the Global Potential of Bioenergy on Abandoned Agriculture Lands that has fallen out of agricultural production worldwide is semi-arid, and it appears that the amount of land that may be available for cultivation of Agave species is vast.

The research paper points out that about 89 to 107 Mha of land that were formerly in agriculture globally are now in forests and urban areas.  The authors bravely note the biomass that is harvested annually in the Northern Hemisphere for wood products has an energy content equivalent to approximately 107% of the liquid fuel consumption in the United States.  Wood resources provide regionally specific opportunities for sustainably harvested biomass feedstocks.  That explains the Chevron and Weyerhaeuser deal for biomass.

For this summary its important to note one more point the authors took the time to briefly discuss.  It is inevitable that some mineral soil nutrients will be removed when biomass is harvested, it will be essential to recycle mineral nutrients, which are not consumed in the production of biofuels, from biomass-processing facilities back onto the land. That is virtually all of the minerals.  It needs to be a built in cost before soils are degraded further by any new biomass effort.

This writer’s summary leaves a lot out from the published study including the references, the supporting documentation and the available links.  For this article Science has free registration, an opportunity cost well worth the small effort.

The authors did a good job here, but left a lot out.  There are lots more plants to consider, but the local weather and soils are going to decide what farming can accomplish and the profit for production will in the end decide.  This writers main concern is that highly profitable biomass could displace prime food and feedstock land and force food and feedstock production onto the less optimal soils.  Some oversight, as oppressive as it is – is going to be needed to balance the demands with the conditions – something competition isn’t going to get done.


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