An alternative biorefinery approach to address microalgal seasonality: Blending with spent coffee grounds

Andre Prates Pereira, Tao Dong, Eric P. Knoshaug, Nick Nagle, Ryan Spiller, Bonnie Panczak, Christopher J. Chuck, Philip T. Pienkos

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Abstract

An effective method for the production of fuels and chemicals from microalgae is to ferment the carbohydrate fraction, extract the lipids and convert the resulting solids through hydrothermal liquefaction (HTL). In this process, known as Combined Algal Processing (CAP), multiple fuel precursors are produced effectively. However, one of the key challenges associated with a microalgae-based biorefinery is the reduced productivity of algae in the colder seasons. In this investigation, the potential for spent coffee grounds (SCG), a potentially valuable waste stream, to be blended with biomass from the microalgae Scenedesmus acutus (HCSD) to make up for the productivity shortfalls in periods of lower microalgae productivity to maximize the capacity for downstream equipment throughout the year was evaluated. Two different blend ratios were compared to only microalgae biomass or SCG, one representing winter season (40% microalgae and 60% SCG-blend 1) and another representing autumn and early spring (60% microalgae and 40% SCG-blend 2). Pretreatment of the blends showed higher monosaccharide release yields compared to microalgae alone, with an increase in mannose and galactose specifically. In the fermentation of the pretreated slurries, all the monosaccharides were consumed, resulting in ethanol titers of up to 23 g L-1 for the SCG blend, compared to 14 g L-1 ethanol for the algae alone. The lipid extraction from the blends resulted in yields of 95.5-99.7% (which translates to 173.8-193.5 kg per tonne of dry biomass processed in this biorefinery scenario) compared to 92.2% in HCSD (216.2 kg per tonne of dry biomass) and 68.1% in SCG (90.8 kg per tonne of dry biomass) alone. The residual solids left after fermentation and lipid extraction were converted via hydrothermal liquefaction (HTL) to produce bio-crude. The bio-crude yield was higher for microalgae (24.6%) than for the two blend cases (blend 1-17.5% and blend 2-19.7%). Theoretical energy calculations showed that the addition of SCG gave similar yields of fuel (gallon of gasoline equivalents) from the blends when compared to microalgae alone (94.7-96.5% depending on the blend of SCG). This work demonstrates that SCG can be easily incorporated with microalgae into a combined processing methodology and can therefore be used effectively during periods of lower availability of microalgae maintaining maximum operating levels of the conversion process equipment year-round. Moreover, co-processing algae with SCG not only leads to increased ethanol titers in the fermentation but also improves the lipid extraction yields. This journal is

Original languageEnglish
Pages (from-to)3400-3408
Number of pages9
JournalSustainable Energy and Fuels
Volume4
Issue number7
Early online date29 May 2020
DOIs
Publication statusPublished - 31 Jul 2020

Funding

We gratefully acknowledge the University of Bath International Research Funding Schemes for the funding awarded for the six month internship at NREL for Andre Prates Pereira. The authors would like to thank Stefanie van Wychen for her assistance in analytical chemistry. This work was authored in part by the National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding provided by U.S. Department of Energy Office of Energy Efficiency and Renewable Energy. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes.

ASJC Scopus subject areas

  • Renewable Energy, Sustainability and the Environment
  • Fuel Technology
  • Energy Engineering and Power Technology

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