Time‐dependent climate impact and energy efficiency of combined heat and power production from short‐rotation coppice willow using pyrolysis or direct combustion

Abstract

A life cycle assessment of a Swedish short-rotation coppice willow bioenergy system generating electricity and heat was performed to investigate how the energy efficiency and time-dependent climate impact were affected when the feedstock was converted into bio-oil and char before generating electricity and heat, compared with being combusted directly. The study also investigated how the climate impact was affected when part of the char was applied to soil as biochar to act as a carbon sequestration agent and potential soil improver. The energy efficiencies were calculated separately for electricity and heat as the energy ratios between the amount of energy service delivered by the system compared to the amount of external energy inputs used in each scenario after having allocated the primary energy related to the inputs between the two energy services. The energy in the feedstock was not included in the external energy inputs. Direct combustion had the highest energy efficiency. It had energy ratios of 10 and 36 for electricity and heat, respectively. The least energy-efficient scenario was the pyrolysis scenario where biochar was applied to soils. It had energy ratios of 4 and 12 for electricity and heat, respectively. The results showed that pyrolysis with carbon sequestration might be an option to counteract the current trend in global warming. The pyrolysis system with soil application of the biochar removed the largest amount of CO 2 from the atmosphere. However, compared with the direct combustion scenario, the climate change mitigation potential depended on the energy system to which the bioenergy system delivered its energy services. A system expansion showed that direct combustion had the highest climate change mitigation potential when coal or natural gas were used as external energy sources to compensate for the lower energy efficiency of the pyrolysis scenario.

Potentials, Limitations, Co-Benefits, and Trade-Offs of Biochar Applications to Soils for Climate Change Mitigation

“…For example, Clare and colleagues [236] showed that using straw residues for gasification or coal briquettes leads to higher climate change mitigation potentials than biochar in China due to larger offsets of fossil energy. Other studies [69,151,253] reach similar conclusion with standard bioenergy systems achieving more climate benefit compared to biochar sequestration. As a general interpretation, larger climate benefits from using residues for biochar production are achieved in regions with low carbon intensity energy systems, whereas in regions with high carbon intensive energy systems, the use of residues for bioenergy is a better option as it can bring the largest emission savings.…”

“…Some studies do not provide the required disaggregation of the data accross the different life-cycle stages, and in this case, only the total score is included in Figure 3 [236][237][238][239][240][241][242][243][244][245][246][247][248]. Some LCA studies define their functional unit as "per tonne of crop" or "per hectare cultivated" and did not provide conversion factors to transform their results into "per tonne of feedstock" as presented in Figure 3 [249][250][251][252][253][254][255][256][257][258][259][260]. As such, results from these studies are qualitatively discussed but not included in Figure 3.…”

Soil Carbon Modelling in Salix Biomass Plantations: Variety Determines Carbon Sequestration and Climate Impacts

“…The most common climate impact metric used in LCA is global warming potential (GWP 100 ), which is based on radiative forcing and captures the integrated impacts over a single time horizon of 100 years [31]. It does not capture the effect of timing and persistence of GHG fluxes and temporal changes in SOC [32]. It does not represent the actual impacts on ecosystems such as temperature change, sea level change or biodiversity loss.…”

“…Using a time-dependent method can counter this by expressing the climate metric as a function of time. Several studies have developed such alternative methods and applied them in LCA to capture the emissions and fluxes of carbon flows between the atmosphere, biomass and soil [17,[32][33][34]]. An absolute time-dependent climate metric such as the absolute global temperature change potential (∆Ts) developed by Ericsson et al [35] represents the impact on global mean surface temperature from emission or removal of a GHG at a particular point in time.…”

“…This can aid in better understanding of climate impacts of bioenergy as biomass systems capture and emit carbon at different points in time. Several LCA studies have assessed Salix cultivation for bioenergy utilization [17,32,33,[36][37][38][39][40]. However, studies looking at the magnitude of impact of differences between Salix varieties on the overall bioenergy system are rare.…”