USING STA COMBINED WITH FTIR TO COMPARE AMMONIA GAS RELEASE AND THERMAL PROPERTIES OF COMMON FERTILIZERS

Ammonia (NH₃(g)) release to the atmosphere can lead to environmental and human health issues. In recent decades, global NH₃(g) emissions have increased, and the use of N-based fertilizers have been cited as a significant source. In this work, simultaneous thermal analysis (STA) combined with Fourier transform infrared spectroscopy (FTIR) for evolved gas analysis (EGA) was performed to investigate the thermal stability and decomposition of ammonium-bearing fertilizer compounds. Ammonium chloride (NH₄Cl), ammonium sulfate ((NH₄)₂SO₄), ammonium bicarbonate (NH₄HCO₃), diammonium hydrogen phosphate ((NH₄)₂HPO₄), struvite (MgNH₄PO₄·6H₂O), urea (CH₄N₂O) and commercial fertilizers (CF-A, B, C) were heated dynamically from 25-500 ºC in a dry nitrogen gas atmosphere. The thermogravimetric (TG) trace and its derivative (DTG), which indicate mass loss and rate of mass loss, revealed only CF-B decomposed completely but NH₄Cl, (NH₄)₂SO₄, and NH₄HCO₃ had < 10% of their mass remaining. The overall mass loss trend was: CF-B > NH4Cl ≈ (NH₄)₂SO₄ > NH₄HCO₃ > urea > struvite > CF-C > CF-A ≈ (NH₄)₂HPO₄, with 36% mass loss for CF-A and (NH₄)₂HPO₄. DSC curves indicated all simple compounds, with the exception of urea, produced endothermic reactions while urea and CFs produced both exothermic and endothermic reactions. Maxima in the DSC curves corresponded to DTG peaks and confirmed that heat absorbance was directly proportional to mass loss. FTIR analyses confirmed the decomposition reaction pathways by identification of gaseous products. A good correlation was found between Gram Schmidt (GS) peaks and DTG. Along with NH3(g) other identified gases included HCl, H2O, CO2, CO, and SOx. The temperature of initial NH3(g) release ranged from 29-360 oC with NH4HCO3 < (NH4)2HPO4 < CF-A <struvite < urea < CF-B < (NH₄)₂SO₄ < CF-C < NH₄Cl. Though NH4Cl is the most stable, it has the highest wt% NH₃(s) at 31.8%, and may therefore release more NH₃(g) when decomposition occurs. NH₄HCO₃ is the least stable, and has 21.5wt% NH₃(s). Response to thermal stress may be a good predictor of relative susceptibility to release of NH₃(g), and other gases of concern, for both simple and more complex fertilizer compounds. This work demonstrates that STA combined with FTIR for EGA is a fast yet thorough method for comparing thermal properties of common fertilizers.