The fate of alkali species in advanced coal conversion systems. Final report

Abstract

ACKNOWLEDGMENTSThe project team included the following staff members who made major contributions to this research program: Gregory P. Smith -Laser-based diagnostic techniques Robert D. Brittain -Thermodynamic equilibrium calculations Robert H. Lamoreaux -Thermodynamic equilibrium calculations Robert M. Platz -Particle collection devices Eldon P. Farley -X-ray diffraction measurements Victor Wong -Combustion experiments In addition, Dr. Thomar, P. Milne of the Solar Research Institute served as a consultant and contributed significant and helpful suggestions on the design of the high-pressure, molecular-beam sampling mass spectrometer. Two successive DOE-METC technical program managers, Dr. Mark Williams and Dr. Norman Holcombe, maintained close liaison with the SRI project leader over the duration of the program, and generously provided help and guidance in both technical and administrative matters. EXECUTIVE SUMMARY The fate of alkali species during coal combustion and gasification was determined expcximentally in a fluidized bed reactor. A molecular-beam sampling mass spectrometer was used to identify and measure the concentration of vapor phase sodium species in the high temperature environment. Concurrent collection and analysis of the ash established the distribution of sodium species between gas-entrained and residual ash fractions. Two coals, Beulah Zap lignite and Illinois No. 6 bituminous, were used under combustion and gasification conditions at atmospheric pressure. Steady-state bed temperatures were in the range 800-950°C. An extensive calibration procedure ensured that the mass spectrometer was capable of detecting sodium-containing vapor species at concentrations as low as 50 ppb. In the temperature range 800°to 950°C, the concentrations of vapor phase sodium species (Na, Na20, NAC1,and Na2SO4) arc less than 0.05 ppm under combustion condition: with excess air. However, under gasification conditions with Beulah Zap lignite, sodium vapor species arc present at about 14 ppm at a temperature of 820°C. Of this amount, NaC1 vapor constitutes about 5 ppm and the rest is very likely NaOH. Sodium in the form of NaC1 in coal enhances the vaporization of sodium species during combustion, Vapor phase concenwation of both NaC1 and Na2SO4 increased when NaCI was added to the Beulah Zap lignite.Ash particles account for nearly 100% of the sodium in the coal during combustion in the investigated temperature range. The fmc fly-ash particles (<10 pm) are enriched in sodium, mainly in the form of sodium sulfate. The amount of sodium species in this ash fraction may be as high as 30 wt% of the total sodium. Sodium in the coarse ash particle phase retained in the bed is mainly in amorphous forms.Thermodynamic equilibrium calculations predict concentrations of vapor phase sodium species higher than those observed by about an order of magnitude. However, these calculations neglect any role of ash particles as adsorbents for vapor species and make simplied assumptions to compensate for the unavailability of accurate flte...