Ammonia Evolution in Glycine Pyrolysis via Ionic-Pair Reaction Mechanisms

Area of Science:

• Biomass pyrolysis
• Chemical kinetics
• Environmental science

Background:

• Amino acids are significant sources of nitrogenous emissions during biomass pyrolysis.
• The precise reaction mechanisms of amino acid thermal degradation are not fully understood.
• Ammonia (NH3) is a key precursor to harmful nitrogen oxide (NOx) species.

Purpose of the Study:

• To investigate the thermal decomposition pathways of glycine (Gly), the simplest amino acid.
• To elucidate the mechanisms governing ammonia (NH3) formation during Gly pyrolysis.
• To reconcile theoretical predictions with experimental findings on NH3 and water (H2O) evolution.

Main Methods:

• Utilized systematic reaction path search algorithms.
• Employed chemical insight and density functional theory (DFT) simulations.
• Derived a comprehensive reaction network for Gly decomposition.

Main Results:

• NH3 evolution is kinetically unfavorable in the gas phase at moderate temperatures and low pressures.
• Condensed-phase mechanisms, involving many-body ionic-pair proton-exchange-driven polymerization, significantly lower NH3 formation barriers.
• NH3 evolution competes with H2O formation in the condensed phase.

Conclusions:

• The study clarifies the complex mechanisms of glycine thermal decomposition.
• Condensed-phase reactions are crucial for understanding NH3 emissions from biomass pyrolysis.
• The findings reconcile theoretical models with experimental data, improving predictions of nitrogenous emissions.