An exergy based assessment of the production and conversion of switchgrass, equine waste and forest residue to bio-oil using fast pyrolysis

Authors: KEEDY, JOSEPH - Swarthmore College; PRYMAK, EUGENE - Swarthmore College; MACKEN, NELSON - Swarthmore College; POURHASHEM, GHASIDEH - Drexel University; SPATARI, SABRINA - Drexel University; Mullen, Charles; Boateng, Akwasi

Submitted to: Journal of Industrial and Engineering Chemical Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/15/2014
Publication Date: 12/15/2014
Publication URL: https://handle.nal.usda.gov/10113/60196
Citation: Keedy, J., Prymak, E., Macken, N., Pourhashem, G., Spatari, S., Mullen, C.A., Boateng, A.A. 2014. An exergy based assessment of the production and conversion of switchgrass, equine waste and forest residue to bio-oil using fast pyrolysis. Journal of Industrial and Engineering Chemical Research. 54:529-539.

Interpretive Summary: Introduction of biomass derived biofuels is considered a potentially more sustainable alternative to the continued nearly exclusive use of fossil derived liquid fuels for transportation. However, some questions about the real energetic, resource and environmental costs to biofuels production remain unanswered. One technological platform being studied for conversion of biomass to biofuels is pyrolysis, the heating of a material in the absence of oxygen. When applied to biomass, pyrolysis produces a liquid called “bio-oil” that can be refined to “green” gasoline, jet or diesel fuels. In this study, we compared the sustainability of bio-oil production from three feedstocks, switchgrass, forest thinnings (residual woody materials from forest maintenance operations) and horse manure using various pyrolysis processing scenarios. We used a thermodynamic measure of energy in the form of work availability also called exergy, to evaluate the said feedstock production, preparation and the pyrolysis process employed in making said bio-oil. Exergy is an advanced measurement which is related to energy, but measures the “real world” energy in terms of available work which can be done by a material or a process thereby allowing the measurement of sustainability in terms of resource depletion. Since exergy is a measurement of available work, it represents a more nuanced and complete indicator of resource use than energy. Specifically, exergy results under this study were incorporated into a system where biomass is grown, converted to fuel and then consumed as fuel and then used the outcome to determine the sustainability of the entire bio-oil production system. We found that using feedstocks which are by-products of another operation (e.g. forest thinning or horse manure) are much more sustainable, from a resource utilization point of view than a feedstock which is purposely grown as a biofuel feedstock (e.g. switchgrass). We also found that water represents the largest resource consumed by the bio-oil production process. This information will be useful to those developing and commercializing biofuel products via pyrolysis and those studying the sustainability of biofuels development for business or for policy making decisions in a true thermodynamic point of view.

Technical Abstract: The resource efficiency of biofuel production via biomass pyrolysis is evaluated using exergy as an assessment metric. Three feedstocks, important to various sectors of US agriculture, switchgrass, forest residue and equine waste are considered for conversion to bio-oil (pyrolysis oil) via fast pyrolysis, a process that has been identified as adaptable to on- or near-farm application. Biomass and biofuel production pathways are defined, material flows determined and exergy in- and outflows associated with biomass conversion computed, including the depletion of exergy from its natural state to the input of the process (cumulative exergy demand, CExD). Sources of exergy depletion are quantified and categorized by energy carriers, e.g., electricity and diesel fuel, and materials, e.g., fertilizer, as well as renewable and non-renewable resources. Yields for biomass to bio-oil conversion by fast pyrolysis were determined experimentally. Breeding factors, a measure of exergy production, (the ratio of biomass or bio-oil exergy output to process exergy inputs) are determined for the production of biomass and bio-oil. The quantification of exergy depletion for process pathways enables the possible identification of more sustainable (resource efficient) pathways for biomass and bio-oil production. It is shown, for example, that feedstocks grown primarily for biomass such as switchgrass are possibly less sustainable using the exergy measurement compared to use of residue (forest thinnings) or waste biomass (equine waste). With regard to the pyrolysis process, there is substantial reduction in exergy depletion when the co-products bio-char and non-condensable gases are recycled and utilized as a source of heat. The sustainability of biomass production and conversion, as measured by exergy depletion, is strongly influenced by energy carriers. The study reveals that exergy depletion is greater if electricity, required for post-field processing, is obtained from the grid compared to local generation from diesel-powered generators. A large portion of exergy depletion is attributable to fossil fuel and water usage. Exergy breeding factors are considerably lower for biomass and bio-oil exergy production when cumulative exergy depletion is considered, suggesting the need to consider a global assessment when evaluating sustainability.