Effects of Solid Residence Time and Inherent Metal Cations on the Fate of the Nitrogen in Coal during Rapid Pyrolysis

Abstract

Six coals with carbon contents ranging from 69 to 80 wt % dry ash-free (daf) were rapidly pyrolyzed at 1300°C under a stream of high purity He in a free fall-type graphite reactor incorporating a graphite filter to control the residence time of coal particles. In this manner, the effects of both residence time and inherent mineral contents on N 2 formation from char-N following devolatilization were examined. At a residence time of zero, 20−30% of the coal-N was released as volatile-N species, such as tar-N, HCN and NH 3 , while <15% was transitioned to N 2 and the remainder (60−80%) was retained in the char. When the residence time was increased to 120 s, the yields of tar-N, HCN and NH 3 were almost unchanged, irrespective of the coal type, although the N 2 yield increased and the char-N decreased with increasing residence times, with the changes in yields of both species being roughly equal. When an Indonesian char sample already devolatilized at 1000°C was pyrolyzed again by applying a slow heating rate of 10°C/min to 1300°C, the nitrogen in the 1000°C char was converted almost exclusively to N 2 . These observations demonstrate that char-N and/or its precursors are the main source of the N 2 in pyrolysis products. The enhancement in N 2 yield observed on increasing the residence time from zero to 120 s increased with increasing inherent Ca or Fe contents in the range of ≤0.3 wt % (dry), although the data were somewhat scattered. Thus, small amounts of naturally occurring Ca and Fe appear to promote N 2 formation from the devolatilized char-N. It appears likely that Ca-or Fe-containing minerals in the coal are partly transformed into CaO or α-Fe, respectively, under the present rapid pyrolysis conditions, both of which enhance the conversion of char-N to N 2 as well as the transition of amorphous carbon to crystallized carbon with turbostratic structures. A linear relationship was observed between the N 2 yield and the proportion of crystallized carbon formed, which may indicate that the yield of one may be predicted from the yield of the other. The catalysis of N 2 formation from char-N without volatile materials through the presence of mineral-derived CaO or α-Fe is discussed herein in terms of solid−solid reactions of these metal particles with heterocyclic nitrogen species in the char.

Nitrogen Transformation during Pyrolysis of Various N-Containing Biowastes with Participation of Mineral Calcium

“…The addition of exogenous Ca did not alter the N yield in solid char, while it significantly decreased the oil-N yield by 15–50% ( p < 0.05), and increased the gas-N yield by 8–55% ( p < 0.05). It indicated that Ca could promote the volatile N-containing macromolecules to crack into more light gas phase during pyrolysis. ,, Meanwhile, the decrease of oil-N generation might also be attributed to the catalysis of Ca on the thermal cracking of heavy tar, which reduced its block in the inner pores and afforded a richer porosity for biochar. As shown in Table , loading Ca enlarged the pore sizes of biochar, which confirmed the hypothesis.…”

“…In this regard, it could be inferred that although Ca accelerated dehydration in pyrolysis, it simultaneously retained more hydrogen on the char, which could further combine with HCN to form NH 3 . In addition, Ca could react with HCN to produce nitride intermediates, especially CaC x N y , which can immediately be decomposed to generate N 2 because of their inherent thermodynamic instability. , Liu et al also speculated that calcium oxide hindered the cracking of nitrile compounds in oil and further decreased the HCN yield. In summary, N 2 was originated from two pathways: the hydrolysis of oil-N and the decomposition of HCN or NH 3 , , and our study found that all these processes could be reinforced by Ca.…”

“…Substantial alkali metals (K, Na), alkaline earth metals (Ca, Mg), and transition metals (e.g., Fe and Al) exist inherently in biowastes as mineral matters . Some of them catalyze thermal chemical reactions and change the distribution and transformation of N-containing products in the three phases. − Removing inherent minerals from biowastes by diluted acid rising (HCl, H 2 SO 4 , and H 3 PO 4 ) is a pathway to explore the influences of minerals on pyrolysis products, , while the operation would alter the content and forms of N-containing substances in the samples . There were also a few studies probing the influences of minerals on nitrogen conversion by doping exogenous minerals, but until now only explorations regarding the coal processing were carried out, across pyrolysis temperatures of 400–1000 °C.…”

Formation of molecular nitrogen and hydrogen sulfide during high‐temperature pyrolysis of coals

“…In addition, the N/C ratios for the 1200 and 1350 °C chars, normalized to that of the 1000 °C char, were lower at higher temperatures, irrespective of the coal type. These observations demonstrated that the N 2 increase was primarily due to char‐N after devolatilization …”

“…It has also been reported that, in pulverized coal combustion, conversion of coal‐N to NO x increases when the N in the devolatilized solid residue (char) increases upon pyrolysis . Most researchers, however, have not considered the release of nitrogen and sulfur from char following devolatilization, such that only a few papers have been published on this topic to date …”