snapshot of the cellulosic energy grasses

Whenever I mention what I work on, nobody has ever heard of Miscanthus. Of course I consider my friends, family, & acquaintances to be pretty well-informed smart people, so this seemed a bit surprising. This lack of awareness about the incredible potential of Miscanthus & other crops as biofuel crops has been a big reason why I started this blog.

In order to simplify and clarify, I decided to make a brief list & descriptor (in no particular order) of the major energy crops, focusing on the perennial grasses. There are also non-grass cellulosic energy crops (such as poplar, willow, Jatropha and Agave--see previous blog post), which I will not go into here. Disclaimer: this is hardly an exhaustive description of all of these grasses, just what I choose to include...

1) Miscanthus (known sometimes as Elephant grass or Amur silver grass, this includes mainly the sterile hybrid Miscanthus x giganteus, but also relatives such as M. sinensis, M. sacchariflorus, M. floridulus). Tropical to temperate. Native to Asia & Africa.


Advantages: No fertilizer (nitrogen) input required, low degree of invasiveness (hybrid M. x giganteus is sterile), cold-tolerant, highly productive (see photo above!!), a carbon neutral source of fuel when life-cycle is considered, lots of natural variation in M. sinensis & M. sacchariflorus, can grow on marginal lands
Disadvantages: high initial planting costs (must plant rhizomes rather than seeds, due to sterility), M.x giganteus plants typically grown are all cuttings from a single genetic clone (greater genetic variation is typically favored), fairly high water needs during growing season--limits use in arid western U.S.

2) Switchgrass (commonly known as panic grass, Panicum virgatum) Native to North America.
Advantages: cold-tolerant, fairly drought-tolerant, relatively low fertilizer inputs needed, a carbon neutral source of fuel when life-cycle is considered, highly diverse, can grow on marginal lands
Disadvantages: roughly half as productive as Miscanthus in most climates, relatively high water needs during warm growing season (spring/summer) which limits use in much of the western U.S.

3) Maize (corn). Native to North America. Annual rather than perennial.
Advantages: well-established as an ethanol crop when using seeds, annual crops have some benefits, can use the stover (leaf tissue) as a byproduct of seed production for cellulosic ethanol production
Disadvantages: very high fertilizer and water input needs--leading to high carbon costs, not as prolific as dedicated cellulosic perennial grasses, growth for ethanol production competes with growth for food

4) Sugarcane (many species of Sachharum) Tropical to warm temperate climates, Native to South Asia. Brazil is the largest grower of sugarcane, where they generate ethanol as a by-product of sugar production. Brazil is self-sufficient in terms of fuel production due to this investment in sugarcane-based ethanol.
Advantages: high sugar content--sugar is directly fermented into ethanol,
Disadvantages: very water intensive, grows in tropical (warm) climates only thereby limiting its growth in the primarily temperate U.S., not typically grown for cellulosic biofuel production

5) Energy cane (sugarcane hybrids produced to make low sugar varieties).
Advantages: hybrids (crop behind person in above photo) often have increased vigour and are highly prolific, the energy cane varieties are bred to be more cold-tolerant than sugarcane which increases the growing range in the U.S., can potentially convert to ethanol in the same way as other cellulosic biofuel crops, can be more productive than sugarcane (producing more ethanol per unit of land)
Disadvantages: unless a "no sugar" variety is developed, separate conversions to ethanol are necessary (e.g., different process to convert sugar to ethanol vs. converting cellulose to ethanol), relatively high water needs, initial planting costs are high/intensive (established from cuttings rather than seeds), not cold-tolerant

6) Sweet sorghum (many varieties of Sorghum with high sugar content). Native to tropical and sub-tropical regions on all continents (except Antarctica)
Advantages: fairly high yield, relatively drought tolerant, direct conversion to ethanol from sugar (not typically cellulose)
Disadvantages: grown primarily for sugar conversion directly to ethanol rather than as a cellulosic form of ethanol production...but could be used for both (like energy cane), annual rather than perennial, high fertilizer needs, susceptible to pests, not cold tolerant


7) Native prairie (many species, primarily Switchgrass [Panicum virgatum], Indiangrass [Sorghastrum nutans], Eastern Gamagrass (Tripsacum dactyloides), Big Bluestem (Andropogon gerardii), Little Bluestem (Schizachyrium scoparium), and others)
Advantages: high biodiversity, excellent habitat for wildlife, renewable, requires no fertilizer or irrigation
Disadvantages: generates only a fraction of the productivity compared with dedicated energy crops

life-cycle analysis of greenhouse gas emissions

We already know that most of our energy comes from fossil fuels AND fossil fuels spew tons of greenhouse gases (GHG) that contribute to global warming, but are biofuels actually much better?? The answer is...it depends.

Energy analysts and economists are now looking at the entire life-cycle of producing fuels (which includes extraction of fuel, or planting to processing, transport, and the burning of fuels as emissions). Gasoline is usually used as the reference, and ideally, other forms of fuels emit LESS greenhouse gases than gasoline when the entire life cycle is taken into account.

In this 2009 life-cycle analysis of different types of fuels (http://www.pnas.org/content/106/6/2077.full), Hill et al. found that corn based ethanol produced as much or even more greenhouse gas emissions (including CO2, N2O, and CH4), than gasoline, a surprising result at first. But the GHG emissions differed in production of corn ethanol depending on the source of heat at the biorefinery (whether it was fueled by natural gas, coal, or corn stover--the left-over "leaf" part of a corn plant after the fruits are harvested).

As a bright point, cellulosic ethanol has significantly reduced levels of GHG emissions relative to either corn ethanol or gasoline, when the life-cycle of the fuel production is accounted for. Of note, however, is that the data used to estimate emissions for cellulosic biofuels is relatively limited as this form of ethanol production has not been done to scale.

Costs of GHG (A) and particulate matter: PM2.5 (B) emissions. Per liter and per gallon estimates are shown alongside total costs arising from production of an additional billion gallons of ethanol or an energy-equivalent volume of gasoline. (C) Combined costs of GHG and PM2.5. From PNAS article by Hill et al. 2009. http://www.pnas.org/content/106/6/2077.full

Why is corn-based ethanol not an improvement over gasoline with respect to fossil fuel emissions? The answer is that corn is a fairly intensive crop, it needs a lot of nitrogen fertilizers , and has higher fossil fuel input, all of which contribute to increased GHG emissions. Cellulosic ethanol production is better, when the life-cycle is accounted for, because it requires little to no fertilizer and lignin combustion from the cellulosic crops provides excess heat and power at the biorefinery, which displaces fossil fuel and electricity consumption.

While cellulosic biofuels are a large improvement over conventional forms of corn-ethanol and gasoline in terms of GHG emissions, there is rarely such thing as a golden ticket. That is, there are trade-offs to GHG reductions in terms of other forms of air pollution. From studies (http://dancingflames.org/dancingflames/EnvSci/Articles/EnvScipdffiles/EthanolPublicHealth.pdf, http://www.afdc.energy.gov/afdc/pdfs/technical_paper_feb09.pdf) looking at E85 blend fuels (85% ethanol blended with 15% gasoline) compared to straight gasoline, emissions are reduced for several greenhouse gases, but emissions INCREASE for other pollutants (such as ethanol, formaldehyde, and acetaldehyde). Both formaldehyde and acetaldehyde are nasty carcinogens, as classified by the U.S. EPA. Unburned ethanol can also oxidize to acetaledehyde, so these air pollutants (while not classified as greenhouse gases) have potential human health risks.

Like most things, there is no simple single solution. And it would be good to see ways to reduce both GHG emissions AND other forms of air pollution if we want to move forward with renewable fuel sources. Sorry, but the grass is not always cleaner.

Carbon-free energy

Globally, 87% of our energy comes from fossil fuels. These are dense sources of energy (see previous post on why oil tastes so good) & oil transports and stores well...but what about greenhouse gas emissions? Reams of evidence have demonstrated that greenhouse gases, such as CO2 (carbon dioxide), in excess, contribute to the overall warming of our planet. And fossil fuels produce A LOT of CO2. Another problem with fossil fuels is that they are a finite resource. Some estimates say that only ~100 years of oil resources remain (Source: World Energy Assessment 2000 & 2004/UNDP ). http://www.undp.org/energy/activities/wea/drafts-frame.html

Renewable energy sources (such as solar, wind, biomass, and geothermal), on the other hand, have very low CO2 output per kilowatt-hour of energy produced (See figure at http://www.sciencemag.org/cgi/reprint/329/5993/786.pdf). But given the amount of energy we currently use, is there enough renewable energy to meet this demand? The single largest potential comes from solar energy, where the total amount of solar energy available on earth's surface is several orders of magnitude greater than what we currently use, as a planet. And there is a good chance we could capture enough of that to generate the amount of power the world now consumes. One of the current challenges with solar, however, is energy storage. Sunlight does not shine in one solar panel 24/7, and we need more inexpensive, more efficient ways to store this energy when it is not being generated. There are many great leads to building better batteries out there..but I am not an engineer and will pass on further comment for now.

Biomass can be thought of as another source of solar energy, as plants convert sunlight into energy that is stored in the plant as sugar (via photosynthesis). Even though plant growth is seasonal, with an abundance of plant material (feedstock) at the end of the summer growing season, the leaves can be harvested and stored until energy is needed. A disadvantage of biomass, however, is that is requires resources such as water and land.

While there is much promise in renewables replacing fossil fuels, most experts agree that the best future outcome will include a number of different renewable energy sources and technologies.