Date of Award

January 2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical Engineering

First Advisor

Gautham Krishnamoorthy

Abstract

Even though greenhouse gas emissions have gained widespread recent attention, they are not the only form of pollution associated with coal. Several trace elements liberated from the coal matrix during combustion represent additional pollutants that must be understood and controlled. These elements have a variety of ways in which they are associated within coal, which impacts how they may be released into the environment. Namely, trace elements are classified as "included," "excluded," or "organically bound." Specific elements-arsenic, antimony, and selenium-are of particular interest due to their semi-volatile nature.

Modeling the partitioning of semi-volatile elements-arsenic, antimony, and selenium-is the focus of this undertaking. Programming was done in C++ with particle-time-temperature inputs from computational fluid dynamics software. The developed program is unique in that it combines previous mathematical approaches in conjunction with only recently available experimentally determined speciation details to determine the release of trace elements from organically bound forms and pyritic family minerals. The distribution of mineral inclusions is achieved using a semi-random combination approach in conjunction with computer controlled scanning electron microscopy data sets. Exclusions are taken directly from the data sets and organically bound elemental distribution was achieved by mass balance.

Temperature profiles were correlated with data from a 19kW down-fired furnace burning a Powder River Basin subbituminous coal using the chemical percolation devolatilization model. Particles used in the model have a range of properties that include pure mineral grains, pure coal particles, coal particles with included mineral grains, and excluded mineral particles. Pyritic family mineral inclusions with larger initial diameters were found to retain a greater fraction of the initial trace elements present than smaller particles. Arsenic and antimony show similar trace element release trends for particles of similar size and temperature profile. Calculations indicate that a larger fraction of the initial selenium contained in pyritic family minerals were released than either arsenic or antimony for both inclusions and exclusions

By rigorously accounting for thermochemical equilibrium, kinetics, and transport experienced by the various associated forms of trace elements inside the coal, this developed model can be used to visualize aspects related to trace element release from pyritic family mineral groups during pulverized coal combustion.

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