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.

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