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Biomass Plus Carbon Capture Yields Negative-Emission Energy


As we saw in yesterday's National Academies of Science (NAS) report, carbon capture and storage may not be ready for prime time, but the process is clearly worth considering whether it should be developed, both technologically and economically. And perfectly timed for the release of the NAS report, Nature Climate Change has published a look at the possible impacts of its use with what's already a carbon-neutral energy source: biomass. The report finds that it could be used to generate as much as 10GW of power while helping to reduce emissions in the Western US and Canada by up to 145 percent compared to 1990 levels.

The report is based on scenarios run with a model called SWITCH, which tracks the area covered by the Western Electricity Coordinating Council (the WECC covers the Western US, small parts of Mexico, and two Canadian provinces). SWITCH is a combined energy/economic model; it allows users to put constraints on things like power generation or carbon emissions and then finds the most economical means of operating within those constraints.

In this case, the authors used SWITCH to test scenarios with various reductions in carbon emissions by 2050, ranging from 86 percent to 145 percent down compared to 1990 levels. The scenarios allowed nuclear plants to continue operations, but they didn't include any new construction. Beyond that, utilities were largely allowed to use any form of generation that kept them within the emissions constraints. The key exception: biomass was allowed in some scenarios but prohibited in control versions.

The Western region contains many areas with excellent wind and solar resources, and these were used extensively to reach the emissions limits dictated by the authors. Carbon capture and storage was used to keep coal in the generating mix in the less stringent emissions scenarios but is gone from the grid in the 145 percent reduction one. As emissions restrictions get tighter, carbon capture starts getting applied to natural gas power plants. Gas retains a place on the grid under all scenarios, as it is able to provide both baseline power and rapidly dispatched power as conditions demand.

The authors estimate that it would be economically viable to put up to 10GW of biomass powered plants onto the grid, depending on the level of emissions limits; that corresponds to a bit under 10 percent of the expected 2050 demand for electricity. The generating plants would be supplied with roughly 2,000 PetaJoules of energy in the form of biomass, primarily from waste and residue from agriculture, supplemented by municipal and forestry waste. In all low-emissions scenarios, over 90 percent of the available biomass supply ended up being used for electricity generation.

Dedicated bioenergy crops are more expensive than simply capturing current waste, and they therefore account for only about seven percent of the biomass used, which helpfully ensures that the transition to biomass would come with minimal land-use changes.

Because carbon capture allows biomass-fueled plants to have negative emissions, it largely serves to increase the amount of fossil fuel generation allowed on the grid. In fact, the authors found that if the electricity generated by biomass were simply dumped instead of used, it would only increase costs by six percent.

It's also not an especially efficient means of extracting energy. The authors calculate that burning biomass directly only allows us to harvest about 30 to 40 percent of the energy that we'd get by converting it to ethanol. However, because electric vehicles are so energy-efficient, we'd still get about 40 percent more miles traveled out of burning biomass for electricity. Provided that electric vehicles are widespread by 2050, ethanol production would lose out.

What wouldn't lose out is simply sequestering the carbon directly through a technology like biochar, which, if it develops as expected, could cost less than burning biomass. But biomass has an infrastructure advantage: it's possible to build plants that switch between burning coal and biomass, which could allow a managed transition that's less subject to the biomass supply fluctuations that will undoubtedly occur as the market develops.

Whether the market develops or not, however, is an entirely separate question. The authors simply looked for the most economical means to a given emissions target; if those targets are never set, then none of this is likely to happen. Still, studies like this are helpful for informing policy makers of what can be achieved for different costs. By identifying major holdups, they can help determine the sorts of technologies we target with our research funding.

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