Effect of pressure on sulfur capture and NO{sub x}/N{sub 2} emissions during pressurized fluidized bed combustion of coal

Pressurized fluidized bed combustion has particular advantages compared with conventional firing systems, e.g., in-situ pollutant removal (SO{sub 2}), high thermal efficiencies even in intermediate power ranges. The low combustion temperature of about 1,050--1,200 K results in low NO emissions without any secondary flue gas treatment but on the other hand yields considerable N{sub 2}O emissions. To know in which way the operating pressure influences the gaseous emissions of PFBC, especially SO{sub 2}, NO and N{sub 2}O were studied. Furthermore, examinations were made as to what extent the experiences from AFBC, e.g., influence of temperature, air ratio and additives on pollutant formation, can be transferred to pressurized conditions. At the IVD's PFBC test facility (50 kW thermal power) various experiments were carried out with two different coals (high volatile bituminous coal and subbituminous coal) at defined operating temperatures in the range of 1,073--1,213 K and pressures from 1.5 to 13 bar. Dolomite and calcinated lime were used as additives for in-situ sulfur capture at different molar ratios in each case and its influence on NO and N{sub 2}O emissions were studied. It was found, that NO emissions decreased with increasing pressure and increasing Ca/S molar ratio. At pressures above 4 barmore » and with more sorbent addition, NO emissions decreased with increasing temperature. N{sub 2}O emissions showed no significant decrease at elevated pressures and were not influenced by dolomite or CaO added, but were found to depend on CO emissions or, rather, carbon conversion. At higher system pressures better carbon conversion rates could be achieved (higher O{sub 2} partial pressure), which results in lower CO concentrations in the fluidized bed and the freeboard, respectively. Therefore, the reduction of NO to N{sub 2}O decreases so that the overall N{sub 2}O emissions in a PFBC are lower compared with atmospheric operation mode.« less