Effects of Pressure and Oxygen Concentration on the Combustion of Different Coals

Abstract

A pyrometric method was used for simultaneous in situ measurement of the temperature and size of individual coal particles in a pressurized entrained flow reactor. Several series of measurements were made in the gas temperature range 1150-1270 K to study the effects of pressure (0.2-1.0 MPa) and oxygen volume fraction (3-30 vol %) on the particle temperature and size distributions and on the mean degree of burnoff at well-defined residence times. The fuels used in the experiments varied strongly in their reactivity: lignite from France (Gardanne), high-volatile bituminous (hvb) coals from Germany (Westerholt, Go ¨ttelborn) and Poland (mixture), and anthracite from Germany (Niederberg). Milled fuel was sieved into nominal-size fractions in the range from 75 to 180 µm. The strongest increase in combustion rate at increased pressure occurred for anthracite, which was the least reactive among the fuels studied. This is shown by increasing the mass-loss rate and increasing particle temperatures. Pressure had no effect on the combustion of the lignite sample, the most reactive fuel studied. Some evidence of swelling of hvb coal particles was observed during the early stages of combustion. The particle size remained roughly unchanged until the degree of burnoff exceeded 90%, whereafter the particle size started to decrease due to fragmentation. Experimental results from particle temperature measurements at various oxygen concentrations are compared with literature data. Results from in situ particle measurements including size and temperature recording have not been presented previously at elevated pressures for such a wide range of coals. Measurements on combustion rates, particle temperatures and particle sizes were analyzed with a single-particle combustion model. The effect of pressure on the surface reaction kinetics for an anthracite was found to be small compared to the oxygen content and temperature.

An overview of coal rank influence on ignition and combustion phenomena at the particle level

“…Indeed, Field [70], Mitchell [28], and Hurt and Mitchell [30] reported that the reactivity of chars increases several fold as the rank decreases from anthracite to lignite. This conclusion has also been supported by Joustenoja et al [25] and more recently by Shaddix et al [68]. Moreover, Hurt and Mitchell [30] presented unified high-temperature char combustion kinetics for a number of coals of various ranks and illustrated that their apparent reactivity decreases with increasing rank of the parent coal; or in other words it correlates inversely with the carbon content of coals.…”

“…Depending on the rank of coal, the geographical origin and the seam as well as the operating conditions, such as the particle size, the heating rate in the furnace, the stoichiometry and the mixing rate, the volatile matter evolution and homogeneous combustion, and the heterogeneous combustion of the solid residue may occur separately or simultaneously [20,21]. In the early stages of combustion, a coal particle reaches sufficiently-high temperatures to release volatile matter, which surrounds the surface of the particle forming tar, soot, light hydrocarbons and hydrogen [22][23][24][25][26][27]. As devolatilizing gases diffuse/transport from the particle interior to its surface they cause swelling in bituminous coals [28; 25, 29, 8] or fragmentation in low rank coals [11; 25, 30].…”

Single particle ignition and combustion of anthracite, semi-anthracite and bituminous coals in air and simulated oxy-fuel conditions

“…In addition, if there is sufficient oxygen, the homogeneous combustion of the gasification-derived CO to CO 2 could provide extra heat. Joutsenoja et al [29] measured temperature snapshots of burning single particles in an entrained flow reactor using a pyrometric method, when the oxygen mole fraction ranged between 3 and 30% and they also reported that the particle temperature increased with oxygen mole fraction.…”

Coal and biomass char reactivities in gasification and combustion environments

“…Many previous studies have been undertaken to characterize the kinetic mechanisms associated with the steam and dry char gasification reactions, C + H 2 O M CO + H 2 and C + CO 2 M 2CO, respectively, as well as the char combustion reactions C + O 2 M CO 2 and C + 1/2O 2 M CO. Of concern in the gasification studies have been the kinetics of key reaction paths at atmospheric [1][2][3][4][5][6][7][8][9][10][11][12][13] and elevated pressures [14][15][16][17][18][19][20][21], the inhibiting effects of H 2 and CO on carbon conversion rates [15,22,23,9,[24][25][26][27][28][29], the reductions in carbon conversion rates due to heat treatment [30], and the catalytic effects by the types of minerals present in coals [31][32][33]. Efforts to characterize the kinetic mechanism associated with the char combustion reaction have also been undertaken, with the temperature dependence of the key reaction pathways for CO and CO 2 formation [34][35][36][37][38][39][40][41]…”