Producing
biofuels from a fast-growing grass delivers vast savings of carbon dioxide
emissions compared with petrol, a large-scale study has suggested.
A team of US researchers also found that switchgrass-derived
ethanol produced 540% more energy than was required to manufacture the fuel.
One acre (0.4 hectares) of the grassland could, on average,
deliver 320 gallons of bioethanol, they added.
Their paper appears in the Proceedings of the National Academy
of Sciences.
The five-year study, involving 10 farms ranging in size from
three to nine hectares, was described as the largest study of its kind by the
paper's authors.
Co-author Ken Vogel of the US Department of Agriculture's (USDA)
Agriculture Research Service, based at the University of Nebraska, Lincoln, said
that all previous energy analyses had been based on data from research plots and
estimated inputs.
Last year, a team of scientists had also examined the energy
gains from ethanol produced from switchgrass, but their model suggested that the
net gain was in the region of 343%, which was considerably less than the USDA
team's findings.
"A lot of their information was based on small plot data and
also estimates of what would be needed in the agronomic production of biofuels,"
Dr Vogel explained.
"We had on-farm trials, so we had all the data from the farmers
on all the inputs needed to produce the crops.
"We were able to take this information and put it into this
model and able to come up with a very real-world estimate."
The energy inputs required to produce the crops included
nitrogen fertiliser, herbicides, diesel and seed production.
However, he added that as there were no large-scale
biorefineries in operation, the team did have to estimate how much bioethanol
such a plant would be able to produce in order to calculate the net energy gain.
"Right now, the Department of Energy is co-funding the
construction of six biorefineries in the US. These plants will be completed
around 2010, and will be above the pilot plant scale."
Although the process to produce ethanol from switchgrass was
more complex than using food crops such as wheat or corn, the so-called "second
generation" biofuel could produce much higher energy yields per tonne because it
utilised the whole plant rather than just the seeds.
Carbon cuts
The team also calculated that the production and consumption of
switchgrass-derived ethanol cut CO2 emissions by about 94% when compared with an
equivalent volume of petrol.
SWITCHGRASS FACTS
Scientific name: Panicum virgatum L
Species is a perennial grass
Distribution: North and South America, parts of
Africa
Grows to heights of 0.5-2.5m
Produces an average of 320 gallons of bioethanol per
acre
(Source: USDA; Cardiff School of Biosciences)
Burning biofuels releases carbon dioxide, but growing the plants
absorbs a comparable amount of the gas from the atmosphere.
However, the energy inputs used during the growing and
processing of the crops means the fuel is rarely "carbon neutral".
"Greenhouse gas (GHG) emissions of ethanol from switchgrass,
using only the displacement method, showed 88% less GHG emissions than
conventional ethanol," the researchers wrote.
"The use of... biomass residue for energy at a... biorefinery is
the main reason why switchgrass and human-made prairies have theoretically lower
GHG emissions than biofuels from annual (food) crops, where processing is
currently derived from fossil fuels."
A number of organisations, including the UN, have expressed
concern that biofuels could do more harm than good.
The criticisms of the technology include taking large areas of
arable land out of food production, inflating crop prices and limited carbon
emission savings.
"In contrast to most European countries, the US has quite a bit
of land that is being held outside of (food) production at the moment," Dr Vogel
told BBC News.
"We are looking at the use of switchgrass on marginal cropland
The intent is to have energy crops being grown on marginal cropland, so it would
not be in competition with food crops on our best land.
He also added that there were other factors within the process
of producing the biofuel that limited its financial and environmental
feasibility.
"Because there is going to be a lot of tonnage of material
shipped to the biorefinery, there is going to be some economics involved."
In order to maximise the carbon reductions, he said: "A
biorefinery will have a feedstock supply radius of about 25 to 50 miles, so the
feedstock of any biorefinery needs to be localised."
As the switchgrass had to be sourced within the local area, Dr
Vogel said it was important that the land delivered a high yield of grass in
order to meet the refinery's demands.
Annual rainfall was a key factor affecting the delivery of the
necessary yields.
