HIGH-TEMPERATURE INJECTION OF SORBENT-COAL BLENDS UPSTREAM OF A CERAMIC FILTER FOR SO2, NOx, AND PARTICULATE POLLUTANT REDUCTIONS

An integrated method is evaluated for simultaneously reducing NO_x, SO₂, and particulate emissions from power plants. The method combines dry-sorbent injection for SO₂ concentration reduction, coal injection for NO_x emission reduction, in conjunction with a ceramic honeycomb filter for particulate capture. The filter is mounted in an elevated temperature region where it retains sorbent particles for prolonged periods of time and facilitates their utilization until it is regenerated (cleaned). The performance of low-cost commercial sorbents, such as calcium carbonate, CaCO₃; calcium hydroxide, Ca(OH)₂; calcium oxide, CaO; and sodium bicarbonate, NaHCO₃, was evaluated in the laboratory. The sorbent powders were blended with three types of pulverized coal to achieve NO_x reduction (bituminous, subbituminous, and lignite coal) and results were contrasted with the performance of the higher-cost porous sorbent calcium formate, Ca(COOH)₂. The sorbents were injected in a simulated effluent gas containing SO₂ and NO at a gas temperature of 1150°C (1423 K) upstream of the ceramic filter, which was kept either at 600°C (873 K) or at 800°C (1073 K). The molar Ca/S ratio was in the range of 0.5-5, and the fuel-to-air equivalence ratio, , was in the neighborhood of 2 for all tests. The results were compared with numerical simulations using the "pore tree" mathematical model, which has been modified to simulate the formation of a sorbent bed in the filter and to incorporate both high- and moderate-temperature kinetics. Results show that injection of calcium formate achieved 80% SO₂ reduction at Ca/S ratio of 2, that is, a 40% calcium utilization. The less porous and less costly sorbents achieved ≈40% SO₂ reduction at the same Ca/S ratio, that is, utilizations up to 20%. Sodium bicarbonate achieved a 50% SO₂ reduction at an Na/S ratio of 2, a corresponding sodium utilization as high as 50% when injected alone. Sodium bicarbonate performed even better when mixed with the coal, achieving more than 70% SO₂ reduction. Higher SO₂ reduction efficiencies were achieved at higher Ca/S and Na/S ratios, but the sorbent utilization did not improve. NO_x removal efficiencies of 45-55% were achieved at  ≈ 2. Particulate removal efficiencies by the filter have been measured to be in the range of 97-99%. The mathematical modeling of the sulfation reaction was reasonably successful, because the experimentally observed trends and magnitude of the results were predicted.