Formulation and Preparation of CWSF

CWSF formulation and preparation have progressed from bench-scale (pound quantities) to pilot-scale (tons/h) and utility-scale production levels. Significant accomplishments include:

• Through programs with the U.S. Departments of Energy and Defense, and the Commonwealth of Pennsylvania, highly loaded and stable CWSFs have been prepared from deeply-cleaned coals for fuel oil substitution in industrial boilers;
• Through activities with Pennsylvania Electric Company, Tennessee Valley Authority, and the Homer City Coal Processing Corporation, low-solids CWSFs have been prepared from fines from active cleaning plants and abandoned impoundments to cofire with pulverized coal in utility boilers;
• Formulation procedures have been documented, and potential problems and solutions in the preparation and handling of CWSF have been identified; and
• It has been found that oxidation of impounded coal fines had not occurred to an extent that affected CWSF formulation, and the same formulation, preparation, and utilization procedures could be used as for coal fines from active cleaning plants.

Fundamental Studies

Early studies at Penn State focused on increasing the combustion rate of CWSFs so that acceptable burnout could be achieved in the available residence time in retrofitted boilers. Significant accomplishments include:

• Oxygen enrichment of combustion air was found to improve the combustion efficiency by increasing the flame temperature and hence the char combustion rate. NO_x emissions increased while SO₂ emissions were relatively unaffected;
• Secondary atomization of CWSF droplets due to explosive boiling required a heat flux in excess of that associated with conventional boilers, regardless of the additives used in the formulation;
• Combustion enhancement by imposing a high-intensity acoustic field to generate large and rapidly fluctuating gas velocities (relative to the CWSF droplets) was attributed to increased convective heat and mass transfer rates;
• Atomization quality, mineral matter size, and occurrence of mineral matter in coal particles affected the resulting ash particle size; and
• An erosion-corrosion model, based on accelerated erosion tests, indicated that significant boiler tube erosion does not occur if the flue gas velocity in the convective section is below the value required to keep the erosion in the oxide scale regime.

Cofiring CWSF and Coal in Utility Boilers

Interest in cofiring CWSF and pulverized coal stems mainly from its potential as a low cost NO_x control technique. As a consequence, the Energy Institute has been active in the development of this technology and has been working with the Pennsylvania Electric Company, Genral Public Utilities (GPU), Tennessee Valley Authority, Electric Power Research Institute's Upgraded Coal Interest Group, Central Illinois Public Service Company, and Southern Indiana Gas and Electric Company.

One example of Energy Institute's involvement with the cofiring programs is testing that was conducted under the Energy Institute's direction using a low solids, low viscosity CWSF formulated and produced from impounded bituminous coal fines and burned with pulverized coal at the Pennsylvania Electric Company (Penelec) Seward Station, located near Seward, Pennsylvania. The boiler is a Babcock & Wilcox (B&W), front-wall fired, pulverized coal boiler (34 MWe). Two B&W pulverizers feed coal to six burners (two burner levels each containing three low-NOx burners). Approximately 20% of the thermal input was provided by CWSF, the balance by pulverized coal.

There was a significant reduction of NO_x emissions when cofiring CWSF and pulverized coal as compared to firing 100% pulverized coal. The level of reduction was dependent upon the cofiring configuration (i.e., cofiring in the upper three, lower three, or all six burners), with NO_x emissions being reduced by as much as 26.5%. The reduction in NO_x emissions was not due to the tempering effect of the water in the CWSF, because a greater reduction in NO_x occurred when cofiring CWSF than when injecting the same quantity of water at the same boiler firing rate. The burner configuration increased the concentration, and changed the spatial distriubtion, of hydrocarbon radicals within the combustor with the net effect of reburning the NO_x produced in the pulverized coal flames to N₂.