(248at) Fate of Rare Earth Elements during Lab-Scale Combustion of Lignite Coal

Authors: 
Roth, E. - Presenter, National Energy Technology Laboratory
Granite, E. J. - Presenter, U.S. Department of Energy, National Energy Technology Laboratory

If rare earth elements are going to be extracted from coal and coal ash, it is important to determine their

fate during the combustion of coal. Coal is typically burned at power plants in twenty percent excess air at

temperatures of around 1370oC. It is assumed that the rare earth elements are concentrated in the coal ash after

the combustion of coal due to the typically high melting temperatures, and low volatility, of rare earth

compounds. However, a couple common rare earth salts do have relatively low melting temperatures compared

to the average combustion temperature reached in a coal-fired power plant. Additionally, it would be

informative to know what form the rare earth elements are in, after combustion at high temperature. This is

important since rare earth oxides have different solubility characteristics than rare earth phosphates or

carbonates.

This first step of this project was to determine the concentration of rare earth elements in lignite coal and

lignite coal ashed at different temperatures in air. The concentration of rare earth elements in a lignite coal and

lignite coal ash was determined using Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The

assumption that the rare earth element compounds are non-volatile in coal was investigated by combusting

lignite coal at 750°C in air, and measuring the concentration of the rare earth elements in the resulting ash.

Increasing the combustion temperature to temperatures closer to typical boiler temperatures was also examined.

It was determined that even with the difficulty of measuring the small concentrations of rare earth elements in

the coal, that all of the rare earth elements are concentrated in the coal ash. Additionally, a mass balance shows

that most of the REE elements in the parent lignite end up in the resulting ash. Thermogravimetric analysis was

also conducted on the coal, as well as some rare earth phosphates, to determine if the rare earth phosphates

would decompose into rare earth oxides during heating at high temperatures ~900°C in air. By examining the

fate of rare earth elements compounds at elevated temperatures, novel separation strategies can be developed.