Selective Conversion of Fuel-bound Nitrogen to N2 with Iron Nanoparticles.

Abstract

The highlight of this review article is a novel method to remove fuel-bound nitrogen as N2 during coal pyrolysis in an inert atmosphere. When a Cl-free iron catalyst is precipitated on Australian brown coal from FeCl 3 solution using Ca(OH) 2 , N2 yield increases with increasing pyrolysis temperature and reaches 50% at 900°C, whereas it is < 5% without the iron. A low loading (1 wt% Fe) is sufficient for the remarkable formation of N2, and the use of H2 instead of inert He is unnecessary. The iron catalyst decreases the partitioning to not only volatile nitrogen (tar, HCN, and NH3) but also char nitrogen. A smaller catalytic effect of the precipitated iron is observed with bituminous coal. The iron catalyst after pyrolysis exists as fine particles with a smaller size of 20-30 nm for the brown coal char, Fe3C and graphitized carbon being also present. It is likely that N2 originates mainly from solid phase reactions involving the formation and subsequent decomposition of iron nitrides. In the pyrolysis of different coals at 1000°C, N2 yields for low-rank coals from China and Germany reach 50-60% despite the absence of catalyst. Demineralization with HCI washing removes mainly Fe and Ca elements from these coals and drastically decreases N2 yield with a corresponding increase in nitrogen retention in the chars. The Ca ion-exchanged with the demineralized coals is inactive for N2 formation. Iron nanoparticles, derived from Fe-containing minerals, catalyze predominantly solid phase reactions to extract N2 from char and/or precursors.