A Laboratory Investigation on Combined In-Furnace Sorbent Injection and Hot Flue-Gas Filtration to Simultaneously Capture SO2, NOx, HCl, and Particulate Emissions (2000)

The effectiveness of a novel integrated technique for removing pollutants generated during the combustion of fuels in furnaces is investigated in this experimental and theoretical study. Sorbents, containing calcium/magnesium and organic components, are wet- or dry-sprayed into the postcombustion zone of a furnace. The organic components of the sorbents pyrolyze and reduce NOx under oxygen-lean conditions. The calcium based residues calcine and react with sulfur- and/or chlorine-bearing gases (such as SO2, H2S, HCl, Cl2, etc.), forming stable salts of calcium (CaSO4, CaCl2, etc.). The partially reacted sorbent particles are then collected in the high-temperature ceramic honeycomb filter/reactor, where they continue to react for a prolonged period of time, until the filter is regenerated (cleaned). Using this technique, both the likelihood and the duration of contact between the solid sorbent particles and the gaseous pollutants increases, since reaction takes place both upstream of the filter and inside the filter itself. Hence, the sorbent utilization increases drastically. High filtration efficiency wall-flow honeycomb ceramic monoliths retain particulates, including partially burned carbon (such as soot, char, tar, etc.), ash, and spent sorbent. Complete combustion of carbon may occur inside the high-temperature filter. Periodic aerodynamic regeneration (backpulsing) of the filters removes the collected minerals (ash and spent sorbent). Using this technique, most of the major combustion-generated pollutants can be removed from the effluent with certain efficiencies. In this laboratory study, powders of calcium formate (CF), calcium propionate (CP), calcium(magnesium) acetate (CMA), calcium carbonate (CC), and calcium oxide (CX) sorbents were sprayed in an electrically heated, laminar flow drop-tube furnace where toxic gases of SO2, NO, and HCl were also introduced. Gas temperatures were in the range of 1023−1423 K (750−1150 °C) and gas residence times ≈1 s. A honeycomb ceramic filter, placed at the exit of the furnace, was externally heated to 873−1173 K (600−900 °C). Concentration reductions of the pollutants were measured both with and without the presence of the filter. The “pore tree” mathematical model that describes heterogeneous reaction and transport in porous media was modified to (a) account for simultaneous reaction of calcium with SO2 and HCl, and (b) account for time-dependent reaction in an accumulating bed of sorbent in the filter. Experimental results showed that (i) the sorbent can react with both HCl and SO2 and result in high removal efficiencies for both pollutants, (ii) the filter greatly enhances the SO2 removal and captures particulates, and (iii) the sorbents that contain organics reduce NOx at oxygen-lean conditions. For instance, SO2 removal by CF or CMA, which was 40% without the filter, improved to 80% with the filter (kept at 1173 K), at a Ca/S of 1.5. CMA also reduced NOx by 70%. As NOx reduction mostly depends on fast gas-phase reactions, it was only mildly enhanced at the presence of the filter. Sorbent utilizations, however, were drastically increased in the longer residence time allowed by the presence of the filter. The model's predictions of the experimental data for SO2/HCl concentration reduction and calcium utilizations with and without the filter were fairly successful in most cases, especially at high temperatures.