An advantage of Circulating fluidized bed combustors (CFBCs) is that various ranks of coal can be used as fuels at high efficiencies. In addition, limestone injection into CFBCs makes possible sulphur retention, leading to expensive wet desulphurization process. Structural and operational conditions of CFBCs should be optimized to attain decrease in both SO₂ and NOx emission. The objective of the present study is to clarify the local behavior of SO₂ and NOx emission in the CFBCs and the local sulphur retention and to develop the way to depress NOx emission.
Local SO₂ concentration profile in a laboratory-scale CFBC at 850℃ showed that SO₂ was emitted along the wall dominantly although almost uniformly emitted along the axis. Limestone absorbed SO₂ uniformly in the riser, Limestone in the CFBC was deactivated at a smaller utilization of limestone than in the thermogravimetry. Thus, it is difficult to predict desulphurization behavior of limestone in a CFBC by using a result of the thermogravimetric analysis. An experimental method to predict desulphurization behavior in a CFBC should be developed.
We proposed to install another fluidized bed after the exit of CFBCs and limestone was introduced there．It was confirmed that SO₂ was effectively captured with no increase in NOx emission.