Evaluation of the reliability of solid /sup 13/C NMR spectroscopy for the quantitative analysis of coals: study of whole coals and maceral concentrates

Journal Article · · Energy Fuel; (United States)

OSTI ID:6572566

Botto, R E; Wilson, R; Winans, R E

A detailed investigation has been carried out on a series of coals varying in rank and a set of well-characterized maceral concentrates. Relaxation times describing the cross-polarization dynamics and fraction carbon aromaticity were determined by fitting signal areas in CP/MAS spectra to an expression that describes the evolution of carbon magnetization with the experimental contact time. Macerals were found to exhibit unique carbon polarization profiles with the variation in contact time, and intensity maxima for the aromatic carbons generally occurred at longer contact times than those of aliphatic carbons. These observations were confirmed by an examination of the relaxation times. Carbon aromaticities determined from a fit of the contact time data appeared to be more reliable than corresponding values obtained from a single spectrum recorded with the contact time producing maximum signal. Carbon-spin-counting experiments performed with an internal standard as an intensity reference have established that the percentage of observable carbons in coals and macerals can vary greatly. For macerals, the decrease in total signal intensity was seen to be proportional to the free-radical spin concentration. In some instances the fraction of invisible carbons with CP was found to be considerable, approaching 75% in samples with a high free-radical and low H/C content. The effects due to dipolar interactions of paramagnetic centers or to inefficient carbon polarization have been assessed on the basis of comparing data obtained in Bloch-decay and CP experiments. It is concluded that CP experiments largely discriminate against aromatic carbons in coals; hence, they provide minimum values for the carbon aromaticity. 50 references, 4 figures, 6 tables.

• Research Organization: Argonne National Lab., IL
• DOE Contract Number: W-31109-ENG-38
• OSTI ID: 6572566
• Journal Information: Energy Fuel; (United States), Journal Name: Energy Fuel; (United States) Vol. 1:2; ISSN ENFUE
• Country of Publication: United States
• Language: English

Related Subjects

01 COAL, LIGNITE, AND PEAT
010600* -- Coal
Lignite
& Peat-- Properties & Composition
AROMATICS
BITUMINOUS COAL
BLACK COAL
BROWN COAL
CARBON 13
CARBON ISOTOPES
CARBONACEOUS MATERIALS
CHEMICAL ANALYSIS
COAL
ENERGY SOURCES
EVEN-ODD NUCLEI
FOSSIL FUELS
FUELS
ISOTOPES
LIGHT NUCLEI
LIGNITE
MACERALS
MAGNETIC RESONANCE
MAGNETISM
MATERIALS
NUCLEAR MAGNETIC RESONANCE
NUCLEI
ORGANIC COMPOUNDS
PARAMAGNETISM
PERFORMANCE
RELAXATION TIME
RESONANCE
STABLE ISOTOPES