Ash particle size and composition evolution during combustion of synthetic coal and inorganic mixtures

Abstract

To develop methods to predict and mitigate ash deposition problems, a firm understanding of the fundamental mechanisms of ash formation and deposition must be obtained. The first step to understanding the mechanisms of ash deposition is to determine how the initial inorganic constituents in the coal are transformed to produce intermediate species. To avoid some of the complexities involved in coal minerals, simplified, precisely formulated model mixtures consisting of a synthetic coal organic matrix and minerals and inorganic species were prepared, t ~ Mineral inclusions in the synthetic coal were limited in order to isolate specific chemical and physical transformations during combustion under controlled conditions in a laminar flow furnace. Char and ash products were analyzed to determine their chemical and physical characteristics. Although these model mixtures were not exhaustive representations of real coal, they provided a system which could be used to determine the effect of gas temperature and coal inorganic composition on the particle size and composition of specific ash types. Two calcium, silica, and sulfur synthetic coal systems were prepared: one system having calcium in the mineral form as 10-/zm calcite (Ca [min.]-Si-S), and the other having calcium associated in a simulated organic association as ironically dispersed calcium acetate (Ca[org.]-Si-S). A third system consisted of sodium, silica, and sulfur, with the sodium being associated as sodium benzoate simulating an organic association (Na[org.]-Si-S). Silica, in all three systems, consisted of 10-/.Lm quartz that was associated within the synthetic coal particles. Sulfur was extraneously added to the model mixtures in an elemental form and was also included in the organic polymer in very small amounts. The calcite particles were added to the organic matrix during polymerization to allow for their incorporation into the synthetic coal particles. The mineral grains associated with coal particles can be described as being included. The organic matrix consisted of a furfuryl alcohol polymer. The purpose of the Ca -S i -S and Na-S i -S systems was to demonstrate the differences in interactions between included quartz and 1) Ca that is mineral bound as included calcite, 2) Ca that is organically bound or dispersed on an ionic level within the organic matrix, and 3) Na that is organically bound. Reactions involving Ca, Si, and S during combustion are of particular interest when burning low-sulfur and calcium-rich western U.S. subbituminous coals, since calcium sulfate and calcium silicate phases play important roles in the formation of convective pass ash deposits. The effect of temperature on the particle size and composition distribution of ash produced from included calcite and quartz and organically associated calcium and included quartz are of specific interest, as are the reactions of sulfur oxides with the available calcium oxide. The Na (org.)-Si-S system adds another dimension to the role of organically associated elements in the formation of ash, especially with regard to the formation of low melting point Na sulfates and Na silicates on the surface of quartz particles. Quartz was chosen instead of clay (aluminosilicate) as the silicate material to test reactions with the calcium and sodium because lower viscosity and lower melting point liquid phases will form alkali// alkaline earth elements reacting with silica. Lower viscosity liquid phases usually lead to more severe fouling deposits.