Products found in the anodic oxidation of coal

Abstract

Lignite has been used to depolarize the anodic reaction in aqueous electrolysis. The nature and concentration of the anodic products has been investigated. Iron was removed from coal by washing. Slurries were made of lignite, grass and wastes. Electrolytes were H 2SO 4, H 3PO 4 and NaOH. Temperatures were 80–130°C. Sep-pak cartridges were used for separating coal-derived organic liquids from the aqueous electrolyte phase. (Analysis was performed at the University of Texas, Austin, Texas; Radian Corporation, Austin, Texas, and Battelle, Northwest Laboratory, Richland, Washington.) A liquid-liquid extraction procedure was also used (Battelle, Northwest Laboratory). GC/MS was used to identify products (either by use of computer pattern recognition limits or by comparison with mass spectra of standard compounds). Dissolution of washed lignite to ca 0.1 wt% occurs in 5 M H 2SO 4 at 100°C. Cyclic voltammograms (lignite) show peaks at 0.9 and 1.15 V (NHE). The limiting current densities of systems of lignite in contact with the electrode were three times greater than those obtained on a filtrate of the slurry. About 90% of the current in the washed lignite was from the oxidation of organic compounds. Addition of Ce 4+/Ce 3+ caused a substantial increase in current density. PbO 2 as electrode gave qualitatively similar results to those on Pt, but the currents were 100 times less. Biomass and household wastes gave similar results to those for lignite, but the current densities at similar slurry concentrations were ten times less than with coal. The Sep-pak cartridges produced hydrocarbons themselves in solution, and these were monitored and taken into account, so that only compounds found to occur as a result of electrolysis are reported. These were largely C 8–C 18 hydrocarbons ( ca 10 −3 M each, after three hours of electrolysis). Such results were obtained at the University of Texas, Austin, and at the Radian Corporation, Austin, and were confirmed by matching of patterns found against those with known compounds. In corresponding GC/MS investigations (Battelle, Northwest Laboratory), using liquid-liquid extractions, differing results were obtained—a lactone and some hydrocarbons were found. Reasons are given for accepting the results from the University of Texas and Radian Corporation. A comparison of the charge passed with the concentration of organics obtained is consistent with the results from the University of Texas and Radian Corporation. Monolayer dissolution from porous lignite accounts for the material present. Peaks are due to organic reactions. A Kolbe-type process as the basis of the hydrocarbons is discussed. The efficacy of contact between coal and the electrode is due to abrasion. The observed limiting currents are consistent with the organics present (University of Texas and Radian Corporation). The results suggest that increase of temperature to 150°C with enhanced stirring should give ca 750 mA cm −2 limiting current density. Heavy oil may then be extractable from the anolyte; if this is attainable, the credits taken would make pure electrolytic hydrogen comparable in cost with natural gas.