EFFECT OF CAO SURFACE-AREA ON INTRINSIC CHAR OXIDATION RATES FOR BEULAH ZAP CHARS

This work examines the effect of char burnout level and calcium content on the intrinsic char oxidation rates and physical properties of three series of chars. Three starting chars were prepared by devolatilizing a 63-74-mu m fraction of North Dakota Zap lignite in a flat-flame burner; then, a portion of this char was washed in HCl to remove mineral matter; finally, a portion of the acid-washed char was reloaded with calcium. The three starting chars (ND (untreated), NDW (acid-washed), and NCa (Ca reloaded)) were then oxidized to various levels of conversion (6-92%) in a heated-wall drop-tube reactor (DTR) at high temperature (1523 K) in a 5-7% O-2 environment. Low-temperature intrinsic oxidation rates were determined for each resulting sample using isothermal TGA (648-748 K, 10% O-2) Other measured properties include burnout, NB BET and CO2 DP surface areas, and CaO surface area. The latter was determined using a selective CO2 chemisorption technique. Intrinsic oxidation rate decreased as burnout increased for the calcium containing chars (ND and NCa). For the NDW char (69% Ca removed), the intrinsic rate was independent of burnout. Increased burnout produced general decreases in N-2 BET and CO2 DP surface areas for all three chars, but they did not correlate well with rate. Increased burnout also produced decreases in CaO surface areas for the ND and NCa chars, but not for the NDW. The decreases in CaO surface area paralleled the decreases in intrinsic rates. This led to good correlation of CaO surface area with rate. Furthermore, the normalized values of rate per CaO surface area were essentially independent of burnout for the Ca-containing char series. This result suggests that catalysis by Ca is very significant during low-temperature oxidation. These results also indicate that the reason for decreasing rate with burnout is due to sintering of CaO with increased time of exposure to the high-temperature environment. Activation energies for the chars in the three series were found to be independent of burnout level and calcium content. Average values were 32.9 +/- 1.4, 34.7 +/- 3.3, and 33.6 +/- 1.3 kcal/mol (uncertainties expressed as 95% confidence intervals) for ND, NDW, and NCa, respectively.