Oxy-fuel combustion characteristics of pulverized-coal in a drop tube furnace

Experiments were conducted in a drop tube furnace (DTF) for a bituminous coal, two coal particle size distributions (unsieved coal with a mass mean diameter of 76 μm and sieved coal with a mass mean diameter of 29 μm) and three oxy-fuel atmospheres (21% O2/79% CO2, 26% O2/74% CO2 and 31% O2/69% CO2) at a furnace temperature of 1100 °C. For comparison purposes, tests were also performed under air firing conditions. The data reported includes gas temperatures, major gas species concentration and particle burnout measured along the DTF. In addition, a number of selected final char samples were also morphologically and chemically characterized. Under oxy-fuel conditions: (i) the near burner region (NBR) temperature increases and the HC and CO concentrations decrease as the O2 concentration in the oxidizer increases, regardless of the coal particle size; (ii) the NOx concentration levels along the DTF increase as the O2 concentration in the oxidizer increases, regardless of the coal particle size, but, compared with air combustion, the results show that oxy-coal combustion lowers the conversion of fuel-N to NO; and (iii) the NBR burnout levels increase as the O2 concentration in the oxidizer increases, regardless of the coal particle size. The combustion of the sieved coal yields higher NBR temperatures, lower NOx concentration and higher burnout levels along the DTF than the unsieved coal, regardless of the oxidizer composition. Carbon and nitrogen are released at about the same rate as total coal mass release, while hydrogen is released more rapidly regardless of the oxidizer composition and coal particle size. In the combustion of both unsieved and sieved coals, under oxy-fuel conditions, the final char structure becomes more porous as the O2 concentration in the oxidizer increases. The final char elemental composition is essentially independent of the oxidizer composition, but the combustion of the unsieved coal leads to chars with higher percentages of S and lower percentages of Cl than those from the sieved coal combustion.