| dc.description.abstract | The Netherlands imports wood pellets (a type of solid biofuel) from the south-eastern US to co-fire them in power plants, with the aim of mitigating climate change and achieving European renewable energy targets. The use of wood pellets to generate electricity results in extraction of carbon from the environment and greenhouse gas emissions to the atmosphere. Carbon payback times reflect the time interval during which using wood pellets causes larger net emissions than the (fossil) reference system; hence carbon payback times are highly relevant to judge whether using wood pellets mitigates climate change within policy relevant timescales. This study compared carbon payback times of bioelectricity that is generated using wood pellets from four different feedstock types (low-quality roundwood, thinnings, harvest residues and mill residues). Furthermore, two differen tree types (softwood and hardwood), and two temporal perspectives on the referene amount of carbon in the system were studied (just before harvesting vs. when trees start growth). Both carbon debt payback times (comparing wood pellet bioelectricity to the fossil refrence system only) and carbon parity payback times (comparing wood pellet bioelectricity to a counterfactual scenario) were determined. The choice of wood pellet feedstock does not substantially affect carbon debt payback times of wood pellet bioelectricity, except when thinnings are used as feedstock– resulting in shorter carbon debt payback times. When reference carbon stocks are determined at the moment trees start growth (rather than just before harvesting), carbon debt payback times of wood pellet bioelectricity are zero years. Carbon parity payback times vary substantially. Wood pellet feedstock itself hardly affects carbon parity payback times, but does determine what counterfactual scenarios are applicable. The counterfactual does strongly influence carbon parity payback times: when feedstock is left to decompose or used in traditional forest products carbon parity payback times are relatively short, when feedstock is not harvested at all carbon parity payback times are relatively long. Using feedstock material to provide heat locally results in carbon parity never being reached, whereas burning feedstock materials without energy capture results in instant carbon parity. The selection of counterfactuals used in this study is larger than previous studies and may better reflect reality. Results on feedstock and counterfactuals showed considerable uncertainty, but thinnings seem the best wood pellet feedstock in terms reducing the carbon impact of wood pellet based bioelectricity. In most cases carbon payback times of wood pellet bioelectricity are shorter when softwood pellet feedstocks are used. In almost all cases the temporal perspective on reference system carbon stocks does not affect carbon parity payback times of bioelectricity generated in the Netherlands using wood pellets from the south-eastern US. This result suggests that temporal perspective should not substantially influence the outcome of the wood pellet bioelectricty’s carbon balance and resulting policy decisions, as long as counterfactuals are properly accounted for. To improve the climate mitigation effect of using wood pellets from the US South-East to provide Dutch power, the next should be to get an even better understanding of what exact (mix of) counterfactuals is relevant for a feedstock. Using this information, feedstock selection could be optimised for climate benefits – within the boundaries posed by costs of feedstock and technical possibility to process different feedstocks at the various pellet mills in the US South-East. | |
