That leftover Thanksgiving turkey on your plate? The waste it produced over the course of its life could be repurposed as vast amounts of energy-dense fuel, according to a new study published in Applied Energy. In fact, poultry litter could be a sustainable replacement for 10% of the coal that’s used for global electricity generation, the study’s Ben-Gurion University researchers show.
To make their discovery, they collected excrement from a poultry farm and examined different methods for converting it into fuel. They found that one particular method–called hydrothermal carbonisation–can convert poultry litter into a form that’s functionally comparable to coal.
Poultry excrement might seem unorthodox as a fuel source, but researchers are increasingly looking for creative ways to handle the estimated 938 megatons of waste that chickens, turkeys, and other poultry generate worldwide each year. These industrial levels of excrement pose a polluting threat to water and soil. But equally, poultry waste holds promise as a more sustainable replacement for traditional biomass, which is produced from specially-grown crops–a process that itself is linked to problems like habitat loss, excessive fertilizer application, and water pollution.
To measure poultry litter’s value as a recycled fuel source, the researchers examined two production methods. The first is slow pyrolysis, which heats organic matter up to 600 °C in the absence of oxygen to make biochar, a solid fuel. The other, hydrothermal carbonisation (HTC), heats matter inside a closed reactor up to 250 °C, in the presence of water, which makes a similar product called hydrochar. The researchers measured the resulting gaseous emissions at different production stages, and the energy density of each final product.
Both processes produced substances that have a similar quality to coal, but hydrochar had a much higher energy density–creating 24% more energy per kilogram of poultry waste. There were some caveats: compared to biochar, hydrochar production generates more nitric oxide and sulphur dioxide emissions at certain stages of production. But importantly, compared to biochar, when burned at optimal temperatures, hydrochar released significantly less methane gas–a highly potent and harmful emission whose global warming potential is up to 25 times higher than carbon dioxide. Since coal production is associated with high methane emissions, litter-based hydrochar might therefore hold special advantages as a fuel substitute.
On balance, hydrochar’s greater energy efficiency, its role in repurposing agricultural waste, and its lower methane emissions, could make it a more sustainable fossil fuel substitute, the researchers reason. And if we repurposed all the poultry-derived litter as hydrochar worldwide, they estimate that we’d create roughly 10 billion gigajoules of renewable energy–enough to supply 10% of the electricity that’s currently provided by coal.
“This could significantly reduce greenhouse gas emissions associated with electricity generation,” the researchers write. “ conversion to renewable energy can offer a solution while concomitantly reducing environmental impact and reliance on fossil fuels.”
Source: Mau et al. “Energy conversion and gas emissions from production and combustion of poultry-litter-derived hydrochar and biochar.” Applied Energy. 2017.