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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Atom-and energy-efficient synthesis of a crystalline calcium urea sulfate ([Ca(urea) 4 ]SO 4 ) cocrystal was explored using thermally controlled mechanochemical methods with calcium sulfate compounds containing various amounts of crystalline water (CaSO 4 •xH 2 O, x = 0, 0.5, 2). Small-scale (200 mg) experiments in a shaker mill were first performed, and the progress was monitored by in situ Raman spectroscopy and in situ synchrotron powder X-ray diffraction. Time-resolved spectroscopy data revealed that the presence of water in the reagents' crystalline structure was essential to the reaction and largely determined the observed reactivity of different calcium sulfate forms. Reactions at elevated temperatures were shown to proceed significantly faster on all synthetic scales, while changes in rheology caused by adding external water hindered the reaction progress. The average yield of a 21 mm horizontal twin-screw extruder experiment was ∼5.5 g/ min of extrusion (∼330 g/h). Energy consumption during the milling reactions required to achieve complete conversion ranged from 7.6 W h/g at 70 °C for a mixer mill to 3.0 W h/g at a 50 g scale and 4.0 W h/g at a 100 g scale for a planetary mill or 4.0 W h/g at both 70 °C and RT for a twin-screw extruder, showing a significant improvement in energy efficiency at large-scale production. The obtained crystalline cocrystal exhibited a significantly lower solubility in aqueous solutions, nearly 20 times lower per molar basis compared to that of urea. Furthermore, reactive nitrogen emissions in air at 90% relative humidity, measured as NH 3 , showed slow and nearly linear nitrogen loss for the cocrystal over 90 days, while the same level of emissions was achieved with urea after 1−2 weeks, showing the potential of this cocrystal material as a large-scale nitrogen-efficient fertilizer.

Population growth is necessitating a significant increase in crop production, while regulations require less use of nitrogen (N) (as fertilizers, such as urea) to minimize its environmental influx. A large fraction of applied N fertilizers is currently lost with significant negative environmental effects. The urea decomposition pathways explored in the literature chiefly concern the gas emissions but provide less mechanistic insights into the urea particle/soil interface after deposition and during their environmental processing (aging). The present work investigated the mechanistic role of relative humidity (RH) at the model humic material (salicylic acid)–urea interface and the resulting surface reactions using dynamic vapor sorption and in situ spatially resolved Raman spectroscopy, combined with ab initio thermodynamic calculations. The formation of a reaction product between urea and salicylic acid, used as a model compound of humic substances, was observed, resulting in the profoundly different response to RH, with the hysteresis due to the bulk urea no longer apparent. Ab initio and resulting NH3 emission measurement experiments suggest a decreased propensity of such reaction products to hydrolyze due to the formation of strong molecular bonds between urea and salicylic acid at the interface. The reaction was facilitated by the formation of a supersaturated layer of aqueous urea on the surface, which is likely the driving force behind the new product formation due to its higher vapor pressure. The results suggest that RH-driven reactions of urea and humic substances of soil could profoundly influence gas-phase emissions and thus affect the global nitrogen cycle. The impact of the stabilizing structure and the properties of the resulting urea reaction products on organic moieties needs to be further studied in the future to better understand the implications toward global N cycle.