A Review on Life Cycle Assessments of Prioritized Renewable Chemicals and Their GHG Reduction Potential

Abstract:

The estimated biomass production in the world is 100 petagrams (100 Gt) of carbon per year, about half in the ocean and half on land.¹  Biomass has always been a major source of energy for humankind and is presently estimated to contribute on the order of 10–14% of the world's energy supply.² In today's fossil fuel-driven industrial economy, production of bio-based chemicals and fuels may have numerous benefits but should also aim to decrease life cycle environmental impacts (such as GHG emissions) to be considered as favorable alternatives compared to their conventional counterparts. The RFS2 (renewable fuel standard) program administered by the USEPA regulates minimum content of renewable fuels in regular blend and sets a threshold of greenhouse gas emissions reduction for renewable fuels of least 20% for corn ethanol and a 50-60% reduction for fuels from non-corn feedstocks, cellulosic and agricultural wastes, and  biodiesel.³  While fuels have been a focus of public policy, nearly 50 million tons of bio-based chemicals are also produced annually worldwide, but there is no commensurate threshold for GHG emissions reduction of bio-based chemicals in order to define what can be considered as a renewable chemical. This presentation will review available bio-based chemical pathways and associated non-renewable energy use and life cycle GHG emissions reductions.

The US Department of Energy has conducted two separate screening analyses ⁴ to identify priority bio-based chemicals based on available technologies and the market demands. These chemicals can be produced from both sugar and non-sugar components of biomass resources. In this study, we conducted a thorough literature review on life cycle assessments of 11 sugar-based and 12 lignin-based building blocks reported by USDOE. 11 other bio-based chemicals that are currently in active research and development but not listed by USDOE, are also studied. This additional group can either be produced from sugar or non-sugar components of biomass. Selected studies are comprised of cultivation, preparation, intermediate and final production processes for each chemical; environmental burdens from the use and end of life phases are excluded from this study and system boundary is set to be cradle to gate. Various sources of biomass (e.g, seeds, agricultural and woody waste, pulp and paper waste streams, and algae); technology pathways (e.g., fermentation, catalytic conversion, pyrolysis, and gasification); national or regional differences; and finally life cycle assessment allocation methods (e.g., economic and mass allocation and system expansion) were considered. Tables 1a-c show the chemicals under consideration.

Table 1. a) Sugar-based and b) lignin-based building blocks from DOE reports, and c) other bio-based building blocks of interest

Life cycle GHG emission results from each of these studies are collected and compared against common petrochemical counterparts. For the comparative LCAs, GHG changes were extracted from the articles but in cases where GHG emissions of the petrochemical counterpart was not reported, Eco-Invent unit processes were used for the estimation. Other results including non-renewable energy use was also included for most of the studied scenarios.

Figure 1. Non-renewable energy use versus GHG emission for sugar-based and lignin-based chemicals

Figure 1 shows the trend of non-renewable energy consumption versus greenhouse gas emissions in kg CO₂ equivalent for a functional unit of 1 kg of final product. As expected, there is a linear relationship between these parameters, as an increase in process energy consumption supplied by fossil sources will increase the emissions.

This study shows the gaps in LCA of bio-based chemicals that require more investigation, especially for lignin-based chemicals due to the complexity in primary structure of lignin and treatment methods; aspartic acid, glucaric acid, sorbitol and arabinitol are examples of sugar-based chemicals while styrene, biphenyl and cresols are examples of lignin-based chemicals included in the list but have not yet been studied on life cycle basis. Moreover, current results are dependent on processing pathways and display significant variability. The GHG results vary from >100% increase for acetic acid production from corn starch to >100% decrease for succinic acid production from corn compared to their counterparts. Even for a single chemical such as succinic acid, GHG emissions reductions range from 49% up to 100% depending on the choice of primary source, conversion and allocation methods. Bio-based chemicals that show reductions in GHG emissions also show lower fossil energy use in most cases, as shown in Figure 1.

Applying RFS2 requirements of at least 50% reduction in GHG emissions, succinic acid, PHB (polyhydroxybutyrate), xylitol, PLA (polylactic acid), polyethylene, propylene glycol and butanol show lower average values for sugar-based chemicals while methanol, PLA, PDO (propanediol), and butanol are preferable lignin-based products. Figure 2 shows these results. Average values are indicated with the full range of reported results also indicated.  Blue dots show the average values while red triangles represent chemicals with no LCA studies found, indicating opportunities for further research.

Results of this study can be useful for policy makers in defining a threshold for environmental benefits should be met by renewable chemicals. There are limited data available right now to conduct a harmonization study, though. As shown in Figure 2-b, for most of the lignin-based chemicals one study was found, represented as a single point without error bars, and for the rest there are few life cycle assessment studies. On the other hand, above results are just GHG emission change, a full life cycle assessment accounting for other environmental impacts will assist finding overall benefits of renewable products. Some of the collected studies looked into other impact categories as well, but since most of the regulations are based on GHG emissions, this is the main impact category studied so far. Development of a consistent and comprehensive database for prioritized chemicals, including different pathways, sources and environmental burdens, is required for appropriate decision making.

Figure 2. a) Percent change in life cycle GHG emissions of sugar-based chemicals and b) Percent change in life cycle GHG emissions of lignin-based chemicals

References:

1.            Field, C. B.; Behrenfeld, M. J.; Randerson, J. T.; Falkowski, P., Primary production of the biosphere: integrating terrestrial and oceanic components. Science 1998, 281 (5374), 237-240.

2.            McKendry, P., Energy production from biomass (part 1): overview of biomass. Bioresource Technology 2002, 83 (1), 37-46.

3.            Schnepf, R.; Yacobucci, B. D. In Renewable fuel standard (RFS): overview and issues, CRS Report for Congress, 2010.

4.            Werpy, T.; Petersen, G.; Aden, A.; Bozell, J.; Holladay, J.; White, J.; Manheim, A.; Eliot, D.; Lasure, L.; Jones, S. Top value added chemicals from biomass. Volume 1-Results of screening for potential candidates from sugars and synthesis gas; DTIC Document: 2004;

5.          Holladay, J.; Bozell, J.; White, J.; Johnson, D., Top value-added chemicals from biomass. Volume II–Results of Screening for Potential Candidates from Biorefinery Lignin, Report prepared by members of NREL, PNNL and University of Tennessee 2007.