Life Cycle Assessment of Drop-in Bio-jet Fuel and Acetic Acid from the Bioconversion of Poplar Biomass

Abstract

In this dissertation, research to evaluate the environmental impacts resulting fromthe commercial scale production of bio-jet fuel and bio-acetic acid produced from short rotation coppice harvest poplar tree feedstock is presented. The poplar feedstock is chipped during harvest and transported to a biorefinery where enzymatic hydrolysis and fermentation steps are used to convert the lignocellulosic biomass to acetic acid. The acetic-acid can be pulled out through a distillation recovery process and sold as a standalone commodity. Or, the acetic acid can go through additional chemical conversion steps, including hydrogenation, to produce a hydrocarbon jet fuel. In either case, the bio-jet fuel and bio-acetic acid are identical to the petroleum based versions of the products they are intended to replace. Life cycle assessment is used to analyze the potential environmental impacts of producing bio-jet fuel and bio-acetic acid at commercial scale. The research in this dissertation consists of three separate, but related studies. The first study is an LCA of bio-jet fuel production. It looks at the system as a whole, without defining a location for the poplar bioenergy farm or the biorefinery, and focuses on the ‘cradle to wake’ global warming potential and fossil fuel use of bio-jet fuel production and use. The second study is a regional poplar feedstock ‘cradle to farm gate’ assessment that uses spatial analysis to locate lands in the western U.S. that could be used to grow poplar trees for regionally located biorefineries. The final study is a LCA ‘cradle to biorefinery exit gate’ analysis of bio-acetic acid production and assesses the global warming potential and fossil fuel use of two different acetic acid recovery methods. Below follows abstracts for the three studies: Hydrocarbon bio-jet fuel from bioconversion of poplar biomass: life cycle assessment Bio-jet fuels compatible with current aviation infrastructure are needed as an alternative to petroleum based jet fuel to lower greenhouse gas emissions and reduce dependence on fossil fuels. Cradle to grave life cycle analysis is used to investigate the global warming potential and fossil fuel use of converting poplar biomass to drop-in bio-jet fuel via a novel bioconversion platform. Unique to the biorefinery designs in this research is an acetogen fermentation step. Following dilute acid pretreatment and enzymatic hydrolysis, poplar biomass is fermented to acetic acid and then distilled, hydroprocessed, and oligomerized to jet fuel. Natural gas steam reforming and lignin gasification are proposed to meet hydrogen demands at the biorefineries. Separate well to wake simulations are performed using the hydrogen production processes to obtain life cycle data. Both biorefinery designs are assessed using natural gas and hog fuel to meet excess heat demands. Global warming potential of the natural gas steam reforming and lignin gasification bio-jet fuel scenarios range from CO2 equivalences of 60 g MJ-1 to 66 g MJ-1 and 32 g MJ-1 to 73 g MJ-1, respectively. Fossil fuel usage of the natural gas steam reforming and lignin gasification bio-jet fuel scenarios range from 0.78 MJ MJ-1 to 0.84 MJ MJ-1 and 0.71 MJ MJ-1 to 1.0 MJ MJ-1, respectively. Lower values for each impact category result from using hog fuel to meet excess heat/steam demands. Higher values result from using natural gas to meet the excess heat demands. Bio-jet fuels produced from the bioconversion of poplar biomass reduce the global warming potential and fossil fuel use compared to petroleum based jet fuel. Production of hydrogen is identified as a major source of greenhouse gas emissions and fossil fuel use in both the natural gas steam reforming and lignin gasification bio-jet simulations. Using hog fuel instead of natural gas to meet heat demands can help lower the global warming potential and fossil fuel use at the biorefineries. Hydrocarbon bio-jet fuel from bioconversion of poplar biomass: life cycle assessment of site specific impacts Hydrocarbon drop-in bio-jet fuels could help to reduce greenhouse gas emissions within the aviation sector. The commercial scale production of these bio-jet fuels will require investments into new bioconversion facilities to convert biomass to hydrocarbon drop-in jet fuel. These conversion facilities will need dependable year-round supply of biomass feedstock. Large tracts of land will be required to grow this feedstock and the change in management of these lands could have significant environmental impacts. This research investigates potential environmental impacts associated with converting land to grow poplar trees for conversion to drop-in bio-jet fuel. Spatial analysis and life cycle methodologies are used to evaluate changes to land use and management in 4 different regions within the western United States. The four regions are based around biorefineries proposed to be located near Pilchuck WA, Hayden ID, Jefferson OR, and Clarksburg CA. Each of the four regions would annually supply 125 million tonnes of poplar biomass to a biorefinery producing 380 million liters of bio-jet fuel. The amount of land converted is based off of predicted poplar yields for each region. The type of land converted to growing poplar, as well as the impacts associated with land use change will depend on regionally specific factors. The Clarksburg region is predicted to have the highest poplar yield and least amount of land converted. Of the land converted in the Clarksburg region, the majority of the land would come from croplands. Relative to the other regions, the conversion of intensively managed cropland to less intensively managed poplar production results in a decrease of fertilizer use and small increase in chemical inputs and fuel use. This translates to the lowest annual Global Warming Potential (GWP) for the Clarksburg region relative to the GWPs of Pilchuck, Jefferson, and Hayden. Conversely, the land in the Jefferson region would primarily come from unmanaged rangelands. Bringing this rangeland into managed production results in a regional increase of nitrogen fertilizer use, chemical inputs, and fuel use, as well as the largest increase in GWP, relative to the other regions. The type of land converted isn’t the only predictor for changes in agricultural inputs and GWP; total land converted also plays a significant role as demonstrated by the Hayden region. Poplar yields are predicted to be lower in the Hayden region and more land must be converted to meet the biorefinery feedstock needs. The increased use of land leads to higher fuel use and greater greenhouse gas emissions in the Hayden region. Combining life cycle assessment methodology with spatial analysis can help provide a more detailed view of the shifts in land use and resulting impacts. Feedstock growth and harvesting is a necessary and important process in the production for biofuels. The total contribution of feedstock production to the overall global warming potential likely will not be as significant as downstream conversion and processing of biomass into biofuel, but changes to land use and management could result in changes at the local level that could result in unintended negative environmental consequences. It is important that these impacts to land use, along with greenhouse gas emissions, are modelled and evaluated to better understand the regional and local implications of building a biofuels industry. Production routes to bio-acetic acid: Life cycle assessment Similar to biofuels, numerous chemicals produced from petroleum resources can also be made from biomass. In this research we investigate cradle to biorefinery exit gate life cycle impacts of producing acetic acid from poplar biomass using a bioconversion process. A key step in developing acetic acid for commercial markets is producing a product with 99.8 % purity. This process has been shown to be potentially energy intensive and in this work two distillation and liquid-liquid extraction methods are evaluated to produce glacial bio-acetic acid. Method one uses ethyl acetate for extraction. Method two uses alamine and diisobutyl ketone. Additionally two different options for meeting energy demands at the biorefinery are modeled. Option one involves burning lignin and natural gas onsite to meet heat/steam and electricity demands. Option two uses only natural gas onsite to meet heat/steam demands, purchases electricity from the grid to meet biorefinery needs, and sells lignin from the poplar biomass as a co-product to a coal burning power plant to be co-fired with coal. System expansion is used to account for byproducts and co-products for the main life cycle assessment. Allocation assessments are also performed to compare the life cycle tradeoffs of using system expansion, mass allocation, or economic allocation for bio-acetic acid production. Finally, a sensitivity analysis is conducted to determine potential effects of a decrease in the fermentation of glucose to acetic acid. Global warming potential (GWP) and fossil fuel use (FFU) for ethyl acetate extraction range from 1000 - 2500 kg CO2eq. and 32 - 56 GJ per tonne of acetic acid, respectively. Alamine and diisobutyl ketone extraction method GWP and FFU ranges from -370 - 180 kg CO2eq. and 15 - 25 GJ per tonne of acetic acid, respectively. Overall the alamine/diisobutyl ketone extraction method results in lower GWP and FFU values compared to the ethyl acetate extraction method. Only the alamine/diisobutyl extraction method finds GWP and FFU values lower than those of petroleum based acetic acid. For both extraction methods, exporting lignin as a co-product produced larger GWPs and FFU values compared to burning the lignin at the biorefinery.