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

Abstract

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.

Integrated Multi-Layered Fabric with Tunable Water Supply to the Photothermal Conversion Layer for an Efficient Solar Water Evaporation

“…The shortage of clean water resources is one of the major crises faced by human society. , The research and development of new methods for cheap, efficient, portable, and reliable access to clean water resources have attracted widespread attention both at industrial and individual levels. − Solar energy is an inexhaustible source of clean energy with zero greenhouse gas emissions. − Using abundant solar energy and seawater or other unpurified water to generate and collect freshwater through desalination and distillation is a practical and green approach. This technique has great potential to provide a solution to the growing challenge of water scarcity. − …”

Dynamic Control of Liquid Biomass Digestate Distillation Combined with an Integrated Solar Concentrator Cycle for Sustainable Nitrogen Fertilizer Production

“…The RPlug reactor model within Aspen Plus was used to simulate solar trough concentrator SCONC-1 with simulated solar flux, which can be adjusted from 15.305 kW/m 2 to zero to simulate day/night regimes, according to a procedure described recently. 19 Briefly, parabolic trough technology was used, whereby the mirrors reflect the sunlight to an absorber tube located at the focal point of the mirror. EG was heated in 68-unit solar trough heaters designed with L = 7.8 m and D = 0.075 m with a heat flux of 15.305 kW/m 2 .…”

“…The process with a capacity of 30,000 kg/h of liquid digestate with ∼2 g/L NH 4 + was utilized to recover solid NH 4 HCO 3 in a downstream crystallizer , as shown in Figure . A total of 1,610 kmol/h multicomponent digestate was mixed with the 67 kmol/h recycle from the crystallization unit, the latter not modeled in this work explicitly but previously described. , The resulting liquid was preheated to 91 °C in economizer HX-1 and fed into the distillation column deNH 3 stage 2. The overhead vapor was condensed against cooling water in the HX-3 condenser operating at 49 °C.…”

“…Vapor–liquid equilibria for NH 3 –CO 2 –H 2 O used to model biomass digestion process byproduct liquid was validated previously. , Briefly, the electrolyte nonrandom two-liquid model and the Redlich–Kwong cubic equation of state were used to calculate the gas–liquid and the electrolyte properties. Henry’s law constant parameters for O 2 , N 2 , and CO 2 with water are retrieved from Aspen Plus Databanks.…”

“…To obtain a solid NH 4 HCO 3 green fertilizer, it utilizes low-temperature heat in the reboiler and can be situated in remote agricultural areas that can provide plenty of land for solar trough installations. Examples of these steady-state designs emerged recently, , where a distillation column was equipped with a partial condenser and a steam powered reboiler operating at 118–141 °C and consuming up to 2.76 MW of low-pressure steam (LPS), well within capacity reported for multiple unit-containing solar trough installations. In this work, we performed a dynamic controllability study of the solar cycle integrated conventional distillation column and described an efficient control structure needed for uninterrupted operation.…”