# LCA Practices of Plastics and Their Recycling: A Critical Review
## 2 The Methodological Foundations and Advances in Life Cycle Assessment
### Types of LCA
#### 2.2.3 Dynamic LCA
A prime example of how the timing of emissions and the background system can strongly determine the sustainability of a product can be found in [44], where the burning of wood pellets is shown to be much worse in terms of greenhouse gas (GHG) emissions than using coal fired power in the short-term. The goal of their study was to assess to what degree burning wood lowers CO2 emissions compared to coal, and emphasized that such a question requires dynamic analysis. Figure 2 provides a snapshot of their findings and particularly the time it takes to pay back the initial carbon debt that is incurred when burning wood pellets. The scenarios 1 through 5 consider the following:  
1. Scenario 1: Burning wood pellets has the same combustion, processing, and supply chain efficiency as coal. Interestingly, 25% of biomass harvested from the land occurred through thinning;
2. Scenario 2: Same as Scenario 1 but now with the actual efficiencies and supply chain emissions for wood;
3. Scenario 3: Scenario 2 but with 95% of biomass harvested (clear cut);
4. Scenario 4: Clear cut, no regrowth of harvested wood, and no carbon emissions from soil stocks;
5. Scenario 5: Scenario 4 but with carbon emissions from soil stocks. Even in highly unrealistic scenarios (S1 through S3) that are favorable for the biomass configuration, it can take up to 80 years to pay back the carbon debt. These scenarios assume optimistic wood cutting and forest management practices, no ecological disturbances such as fires, erosion, disease, and supply chain emissions of wood equal to that of coal per end-use energy. Scenarios that are not artificially favorable (S4 and beyond) show that burning wood in fact increases CO2 emissions relative to coal. Remarkably, another set of authors who published their LCA study around the same time found that it only takes a few years to pay back the carbon debt under various assumptions regarding conversion efficiency, biomass decay half-life period, and emission factors (Figure 3). Even though there are some differences in estimating supply chain emissions, both articles agree that up until the point of combustion, burning wood pellets emits more CO2 than coal. Yet their outcomes suggest different conclusions concerning the sustainability of burning wood pellets. The differences exist mainly due to the fact that [45] (Figure 3), while including a dynamic component in their study, is the only CO2 emissions model that pertains to the decay of biomass in the counterfactual scenario--i.e., the wood pellets cause emissions due to decay if they are not combusted. Moreover, [44], in addition to modelling decay, included a more detailed and dynamic background of forest growth, capturing non-linearities and feedback effects in the carbon cycle of biomass which depend on more factors than are typically covered by conventional LCA methods. One of the reasons for these differences in modelling is that in the LCA of [45], the wood pellets used in combustion are considered to be sourced from wood waste and forest residues instead of dedicated harvest, and are thus generated independently of bioenergy demand. Still, explicit modelling of the background system (as was the case in [44]) is a more comprehensive and consistent way of yielding an integrated assessment under various assumptions. This specific comparison serves as an example to show that the emissions profile and dynamics of background systems (in this case forest growth and forest management) play an important role in impact assessment, and are typically excluded in conventional (static) LCAs. In addition, the lessons learned from this example may also be relevant for biopolymer system analyses, as they too rely on the premise of fossil fuel substitution through feedstock obtained from biomass. In other work, [46] applied both dynamic process inventory and dynamic characterization in their study. They developed characterization factors for global warming dynamically and refuted the claim that corn-based ethanol has a better GHG impact than gasoline, which they based on a static LCA.