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Pathway to cleaner energy

Pathway to cleaner energy

TA News Bureau

For policy makers grappling with sustainability issues aimed at protecting the global environment and to address climate change issues, research in the application of life cycle assessment (LCA) and material flow analysis (MFA) methods are critical to arrive at appropriate decisions. Here they turn to the research of Dr Sabrina Spatari, an Associate Professor with the Philadelphia-based Drexel University College of Engineering. Besides her groundbreaking work in these fields, she has specific interests in biomass and bioenergy, biofuels, and urban infrastructure. In this interview to Tyre Asia, the well-known industrial ecologist speaks about biomass-based polyisoprene. Her research is guiding those who are seeking carbon abatement through development of biomass-to-bioenergy pathways

Please elaborate your statement that the use of biomass-based polyisoprene contributes to much less to global warming potential compared to conventional polyisoprene?

The main reason for the large difference between bio-isoprene and synthetic isoprene is that biomass resources are used to make the rubber monomer. In the case of synthetic isoprene, the feedstock and process energy used to make the monomer comes from petroleum (fossil fuel) resources. Moreover, the biorefinery that manufactures the bio-isoprene relies on the biomass feedstock for thermal and electrical energy, with the surplus power sold to the electricity grid. When considering the displacement of co-products (jet fuel and electricity) manufactured in this process, this amounts to crediting the isoprene product. Producing a “low carbon” rubber monomer relies on sustainably harvesting biomass feedstocks and this means not depleting soil organic carbon from the agricultural fields.
If sustainably harvested biomass resources, such as corn stover, an agricultural residue, and forest and milling residues are used to generate fermentable sugars, the collection and processing of these residues, while requiring use of fossil energy resources, is only one aspect in the life cycle greenhouse gas (GHG) intensity calculation. The embodied carbon in the feedstock used to make isoprene comes from biogenic carbon as opposed to fossil carbon, and this is the main source for the low GHG intensity of the rubber product, and what makes it “bio-isoprene”.

What are your research findings on the alternative pathway for producing polyisoprene?

Our research investigated the opportunity to convert fermentable sugars derived from biomass to synthesize isoprene, the monomer that can be polymerized to polyisoprene as a substitute for natural or synthetic rubber using life cycle assessment (LCA). LCA is an analytical tool used to systematically evaluate the environmental impacts of products. Our research examined life cycle greenhouse gas (GHG) emissions associated with biomass-derived isoprene (bio-isoprene), a renewable resource that can be polymerized to make polyisoprene (rubber), and compared this new pathway to existing sources of rubber, natural latex from the rubber tree and synthetic rubber from petroleum. LCA models consider all of the input energy and materials (chemicals) used to manufacture polyisoprene from “cradle-to-grave”, therefore, the models we build to estimate the conversion of biomass to polyisoprene consider energy and materials for acquiring the biomass feedstocks, transporting them to the biorefinery, and all unit operations involved in converting the biomass to polyisoprene. When comparing the bio-polyisoprene to petroleum-based rubber, the two materials are considered to be “functionally equivalent”, in other words their service life and retirement are assumed to be identical.
Sugars can be generated from many possible biomass resources, including agricultural and forest residues. Theycan be used as feedstock for renewable fuels (such as ethanol) or chemicals. Biomass (lignocellulose) is comprised of three main fractions, cellulose, hemicellulose, and lignin. In the bio-isoprene process we examined, the biomass is pretreatedwith dilute acid to release hemicellulose sugars and prepare the cellulose for enzymatic hydrolysis, which then releases the sugars bound in cellulose. The lignin is separated from the feedstock and used as a source of energy to run the biorefinery. The sugars are fermented to an alcohol (methyl butane, MBE), which is dehydrated in the presence of a catalyst to isoprene. MBE can also be converted to jet fuel blend to coproduce the two products.
My collaborators at Technology Holding LLC in SLC are developing a novel process to produce bio-polyisoprene. Dr. Mukund Karanjikar is leading that work.

Please also explain your comment that polyisoprene from rubber trees has the most significant land use intensity compared to other feedstock?

Our research examined the life cycle environmental tradeoffs between greenhouse gas emissions and land use over the life cycle of different pathways for making polyisoprene. When it comes to land uses to grow feedstocks for fuel and material markets will require more land than fossil fuel resources such as petroleum, which need almost 0 hectares of land for each kilogram of polyisoprene. What is significant for natural rubber trees is that, although the land used to grow natural latex can be used over and over again over the life of the plantation, if converting new forested lands to plantations, the loss of carbon stock on the land can be extremely high, rendering GHG intensity for marginal production of latex.

When considering expanding plantations for growing rubber trees in Southeast Asia, both the land intensity and the GHG emissions per marginal kilogram of latex are both higher than polyisoprene derived from corn stover and forest residues.

Are there cost advantages to the alternative pathway that you have suggested?

Through support from a U.S. Department of Energy grant, our research group and collaborators at Technology Holding (TLC) in Salt Lake City, Utah, are currently estimating the cost of scaling technology to directly ferment isoprene with jet fuel blend. We are currently exploring conditions in which a biorefinery that produces combinations of the two final products are profitable for an early stage facility.

Photos Credit: Masha Kushnir

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