There are multiple ways to construct a supply chain to convert post-harvest forest residuals (slash) into chemical products like biojet fuel. A supply chain, for instance, could consist of a large centralized biorefinery that would handle all of the processing steps, or could contain smaller depots specializing in a portion of the process. One type of depot considered is a liquids depot. A liquids depot would accept woody biomass like slash to generate a sugar syrup as the primary product. The syrup could then be sold to a fermentation facility for conversion into alcohols, as envisioned in the NARA supply chain to produce biojet fuel, or could be sold for other uses.
Efficiency and yield, translated into cost savings, are often driving forces that determine how the supply chain is structured. Environmental impact is also considered. In a peer-reviewed paper funded by the USDA-NIFA, through NARA, researchers evaluate the environmental impact of a liquids depot located in the Pacific Northwest.
Read Environmental assessments of mild bisulfite pretreatment of forest residues into fermentable sugars for biofuel production.
Model assumptions
To perform the assessment, the authors modeled a liquids depot supply chain using data generated from NARA research and from a techno-economic assessment plus plant designs developed by the National Renewable Energy Laboratory (NREL). The assessment considers seven steps associated with a simulated liquids depot: 1) biomass pre-processing, 2) pretreatment, 3) enzyme production, 4) enzymatic hydrolysis, 5) separation, 6) wastewater treatment, and 7) boiler energy production.
The model assumes that 845,000 green tons of post harvest forest residuals (slash) are used annually with 770,000 tons processed in the liquids depot after loss from grinding rejects. The rejects are used in this scenario for heat and energy production. The slash (35% moisture content, ponderosa pine and Douglas-fir mix) is transported from a primary landing site to a secondary site where the material is chipped, loaded into chip vans, and transported an average of 75 miles to a liquids depot.
The Mild Bisulfite (MBS) pretreatment protocol is used in the model. MBS was developed by NARA researchers and is considered efficient and highly adaptable to pulp mill infrastructure. The model assumes that the MBS pretreatment and enzymatic hydrolysis will provide an 80% yield of fermentable sugars.
The model also assumes that most of the wastewater is reused by the liquids depot and that the biogas, sludge, fines and lignin cake, produced as byproducts, are burned to generate electricity.
Model results
This study reports on greenhouse gas emissions (carbon dioxide, CO2), eutrophication (nitrogen), smog, acidification, ozone depletion, and ecotoxicity. The assessment shows that the pre-processing and MBS pretreatment steps contribute the highest carbon dioxide emissions. The use of diesel fuels to grind and process the slash during pre-processing contributes to the high carbon dioxide levels emitted. The energy used to produce high-pressure steam used in the MBS pretreatment also contributes relatively large CO2 amounts.
A global warming potential (GWP) value (measured in CO2 discharge) was calculated for the sugar product produced by the liquid depot supply chain. The GWP value was less for the MBS-based conversion process than for other pretreatment options evaluated in previous studies and was similar to values assigned to the super critical water pretreatment process.
When measuring eutrophication (a process by which a body of water acquires a high concentration of nutrients), the pre-processing and the MBS pretreatment steps were also high contributors. During pre-processing, the associated burdens of using diesel fuels contributed to water pollution. The water used to wash the biomass and to cool the heat exchanges contributed greatly to the MBS impact. The overall eutrophication impacts were less than those recorded for sugar beet and sugarcane processing.
Measurements for smog, acidification, ozone depletion, and ecotoxicity are also presented in the publication.
Conclusions
This study provides a framework and data that can be used to compare the environmental impact of an MBS-based approach to other pretreatment alternatives. It also quantifies the relative environmental impact of varied stages in a supply chain that uses slash to make a sugar syrup. Identifying key hot spots within that supply chain that most affect global warming and water purity will allow industry to prioritize efforts to reduce the overall environmental impact of the supply chain.
Much of the analysis in this report relied on data generated through NARA on feedstock logistics, supply chain analyses, and pretreatment optimization. The intent is to use this primary data to address economic and environmental questions covering multiple proposed supply chains including a complete life cycle assessment (LCA) of a supply chain that runs from slash removal to the use of the biojet fuel.