May 2, 2012
Seed cases rival coal as energy source
Although biofuels could reduce dependence on fossil fuels, there are concerns about their effect on food prices and carbon emissions through indirect land use change. Now a team from the US has proposed that high-lignin crop waste such as coconut shells, which cannot be used as fertiliser or to feed animals or people, could be ideal for producing decentralized bioelectricity.
"A quarter of the world's population has energy needs that are not being met and renewable energy sources that avoid the use of fossil fuels hold promise," Seth DeBolt of the University of Kentucky, Lexington told environmentalresearchweb. "When sustainability constraints are placed on a target feedstock there are not many options. High-lignin endocarp waste from various horticulture crops could provide a source feedstock to improve renewable energy availability."
Drupes, a type of fruit, contain a hard, inedible endocarp layer that acts as a seed case. The layer has a very high lignin content, up to 50% by weight; lignin has double the energy density of cellulose. Indeed, drupe endocarp tissue has a energy content of 16.2–22.8 MJ per kg, making it comparable to coal and better, on average, than dedicated bioenergy crops such as switchgrass, hybrid poplar and reed canarygrass.
DeBolt and colleagues from the University of Kentucky, University of California, Merced and University of Massachusetts, Amherst found that, if used in small-scale gasification systems with an efficiency of 15–40%, drupe endocarp biomass could provide up to 30% of the total energy consumption of Sri Lanka, up to one-quarter of the Philippines' current usage, up to 13% for Indonesia and up to 3% for India. Italy and the US, in contrast, could obtain just 0.16–0.51% and 0.02–0.05% of their electricity consumption from the material.
The team analysed 12 commodities – coconut, mango, olive, walnut, pistachio, cherry, sour cherry, peach, two species of plum, apricot and almond – to examine "whether feedstocks exist that meet sustainability metrics and are growing in areas where energy poverty is evident".
In order to avoid compromising food security, the biomass must not be suitable for consumption by humans or animals, explained DeBolt. What's more, sustainability concerns mean that it cannot be reincorporated into soils to maintain agricultural productivity.
Around 24–31 million tons of drupe endocarp biomass are available each year. In 2000, coconut produced the largest yield of endocarp at just over 13 million tons followed by mango with nearly 4 million tons. Together the pair made up 72% of global drupe endocarp biomass.
The team found that the highest density of drupe endocarp production was in developing countries throughout South Asia. India was top of the table, with 17.3% of the total, followed by Indonesia (17%) and the Philippines (15.4%).
According to the World Health Organization and United Nations Development Program, roughly 1.5 billion people, about one quarter of the world's population, lack basic access to electricity.
"There are many renewable sources of energy that will develop over the coming decade and we will continue to examine sustainable plant-based sources," said DeBolt.
The researchers reported their work in PNAS.
About the author
Liz Kalaugher is editor of environmentalresearchweb.