Solar Steam Generation Integration into the Ammonium Bicarbonate Recovery from Liquid Biomass Digestate: Process Modeling and Life Cycle Assessment

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Department of Civil, Environmental, and Geospatial Engineering


Current management strategies for utilizing increasing amounts of liquid digestate, the main byproduct of anaerobic agricultural and municipal solid waste digestion, pose significant environmental risks if utilized directly for agricultural purposes as a nutrient-containing soil improver. Instead, efficient removal and precipitation of nitrogen present in the digestate have been recently proposed in the form of ammonium bicarbonate, NH4HCO3, and a new process was designed to produce solid NH4HCO3 fertilizer material from the liquid digestate using distillation. Environmental impacts of this new process can be advantageous over the direct disposal of digestate to the soil. To further understand and improve the underlying economic and environmental implications of this technology, several new scenarios are proposed and evaluated in this work, which examines the influence of key process variables, including (a) process improvement to obtain a portion of the heat necessary for the distillation process using solar concentrators and (b) fate of the post-processed liquid digestate stream, including disposal into the wastewater treatment plant, release into the water body, or direct land application. An optimized solid NH4HCO3 synthesis scenario was designed using a distillation column with 95% nitrogen recovery. Solar steam generation was incorporated to reduce fossil fuel-generated steam consumption in the distillation column reboiler. This resulted in the distillation column operating at 1.5 bar and a low reflux ratio. This allowed column bottoms to operate at 118 °C for 5 h per day utilizing only solar steam. A detailed economic analysis of the overall process was performed and showed that $20/tonne of feed credit was necessary if the product was valued at $0.10/lb in the most realistic base case scenario. The life cycle assessment (LCA) modeling results obtained suggest that the integration of solar heating can provide important benefits in regard to the overall environmental impacts, but for many environmental impact metrics, including greenhouse gas (GHG) emissions and eutrophication potential, the choices of where to dispose of the post-processed digestate stream and the resulting assumptions about N and C mobilization at that stage can exert a larger influence on the overall environmental impact. Further experimental work is needed to provide certainty to the factors that are used to estimate nitrogen and carbon fate within LCA modeling frameworks.

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ACS Sustainable Chemistry and Engineering