Transformation of Liquid Digestate from the Solid-Separated Biogas Digestion Reactor Effluent into a Solid NH₄HCO₃ Fertilizer: Sustainable Process Engineering and Life Cycle Assessment

Abstract

The growing interest in biogas production to obtain renewable electricity has led to the increasing availability of liquid digestate byproducts containing major nutrients, such as nitrogen, and the need for sustainable engineering developments toward its utilization. Currently, digestate return to the fields has been most popular but suffers from many problems, such as potent greenhouse gas emissions, including N2O, during storage, transport, and application. This work describes a newly designed process for the production of solid nitrogen fertilizers from liquid biogas production waste that circumvents many of the problems associated with handling and applying liquid digestate. In particular, solid ammonium bicarbonate (NH4HCO3) is engineered using solid separated biogas digestion reactor effluent to yield sustainable nitrogenous fertilizers. NH4HCO3 is considered a marketable fertilizer with a N content of 18% that represents an added value to the biogas producing facilities. The process design was performed to obtain an optimized recovery with virtually no nitrogen losses. The process developed relies on digestate distillation at 3.3 bar with the condenser operating at 49 °C and using cooling water. Solid crystals are obtained in a crystallizer at 12 °C and recovered via drying. For comparison, an open-loop air stripping process was developed to obtain ammonium sulfate ((NH4)2SO4) solid fertilizer. The resulting economics of both processes show that the capital cost associated with the NH4HCO3 process is much lower together with the consumption of the utilities. A life cycle assessment approach was used to evaluate the environmental impacts of the new NH4HCO3 process using distillation and the (NH4)2SO4 process using air stripping technology, compared to the base case with liquid digestate applied directly onto the fields. The two primary impact categories of concern in this technical area are global warming potential (GWP) and eutrophication potential (EP). In particular, NH4HCO3 and (NH4)2SO4 processes have ∼25% lower GWP impact because of the reduced land application which is negated because of the utility use. EP was reduced by ∼50 and 20%. Notable was the negative and sizeable effect of both scenarios on ecotoxicity which stemmed from the need to use defoaming agents to address any potential transport problems across the vapor/liquid boundary.

Circularity of Bioenergy Residues: Acidification of Anaerobic Digestate Prior to Addition of Wood Ash

“…It is noteworthy to highlight that two of these equilibria (Equations ( 2) and ( 3)) are connected by the precipitation of (NH 4 ) 2 CO 3 and NH 4 HCO 3 , which naturally increases the pH of anaerobic digestates [94]. Drapanauskaite et al [95] designed a process for the manufacturing of the NH 4 HCO 3 via processing the liquid fraction of the digestate. Unlike the open-loop strategy of the ammonia stripping, which requires commercial grade H 2 SO 4 for the manufacturing of the (NH 4 ) 2 SO 4 , the reagents for the preparation of the (NH 4 ) 2 CO 3 and NH 4 HCO 3 are components of the anaerobic digestate [95].…”

“…Drapanauskaite et al [95] designed a process for the manufacturing of the NH 4 HCO 3 via processing the liquid fraction of the digestate. Unlike the open-loop strategy of the ammonia stripping, which requires commercial grade H 2 SO 4 for the manufacturing of the (NH 4 ) 2 SO 4 , the reagents for the preparation of the (NH 4 ) 2 CO 3 and NH 4 HCO 3 are components of the anaerobic digestate [95]. The marketable solid NH 4 HCO 3 fertilizer contains 18% of N. In addition to calcium, magnesium, and phosphorus, the wood ash is a source of carbonates [85], which is necessary for the manufacturing of NH 4 HCO 3 and (NH 4 ) 2 CO 3 .…”

“…The marketable solid NH 4 HCO 3 fertilizer contains 18% of N. In addition to calcium, magnesium, and phosphorus, the wood ash is a source of carbonates [85], which is necessary for the manufacturing of NH 4 HCO 3 and (NH 4 ) 2 CO 3 . It is important to mention that the distillation process, as described by Drapanauskaite et al [95], might be hindered by the alkaline pH that the wood ashes provide to the anaerobic digestates, as this prevents the release of CO 2 . According to the specifications of the process of Drapanauskaite et al [95], the pH of the anaerobic digestate should be 7.85 (40 • C) at the entrance of the stripping column.…”

“…It is important to mention that the distillation process, as described by Drapanauskaite et al [95], might be hindered by the alkaline pH that the wood ashes provide to the anaerobic digestates, as this prevents the release of CO 2 . According to the specifications of the process of Drapanauskaite et al [95], the pH of the anaerobic digestate should be 7.85 (40 • C) at the entrance of the stripping column. Limoli et al [64] evaluated some of the difficulties associated with a wood ash-like material (i.e., CaO) during the operation of a turbulent mixer with the liquid fraction of the anaerobic digestate.…”

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

“…Process Modeling Considerations. The basic process model, described previously, 20 consists of a distillation column equipped with a partial condenser to preconcentrate the NH 4 + ion before crystallization. As revealed by process modeling, 2.76 MW of energy in the form of low-pressure steam at 160 °C was needed, contributing the largest portion to the overall operating cost.…”