(91c) 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.