Annular shaft kiln for lime burning with kiln gas recirculation

During the operation of certain older types of annular shaft kilns used for lime burning, one may encounter the dilemma of how to operate the kiln at an optimal air excess ratio. Due to excessively high temperatures in the combustion chambers at optimal air excess ratio, the flame must be cooled with a greater amount of relatively cold secondary air. This results in greater heat loss with the exhaust kiln gas (stack losses), greater fuel consumption and lower kiln efficiency. The effects of increasing air excess ratio during the process of lime burning in an annular shaft kiln are specifically analysed. The paper presents a case of a remodelled annular shaft kiln, in which the air excess ratio can be reduced to its optimal level by recirculation of recuperator waste gas and its injection into the combustion chambers. This also reduces stack losses, while the temperatures in the combustion chambers remain within the permissible range, which means that the specific energy consumption for lime burning is reduced.