Numerical Evaluation of Brick Lining Status in Rotary Kilns

Sammanfattning: Rotary kilns are important in a variety of different manufacturing areas for e.g. calcination and sintering of materials. In fact, two of the most produced materials in the world, cement and iron, are likely to start their journey in a rotary kiln.A rotary kiln is a large cylinder-formed furnace which rotates about its axis and where certain chemical and physical reactions take place by the influence of heat. The slope and the rotation make the material inside to move through the kiln from feed to discharge end. Due to high process temperatures the casing of the kiln is insulated by a refractory lining. Service conditions inside the kiln are rough and the lining is continuously degrading, especially pronounced in the hot zone of the kiln. If the lining is significantly deteriorated and can no longer protect the casing from the heat − the production is shut-down − leading to very high production losses.Despite many improvements of rotary kilns in the past decades there is still a gap in the knowledge regarding refractory linings during usage. Many assumptions are based on practical knowledge. One explanation to this could be the difficulty to study and observe the lining due to kiln size and high operating temperatures. Today, computer programs are of a great help for studying various issues without causing production delays or risking failures. However, the field of rotary kilns has stagnated on this matter and very little documentation can be found regarding numerical simulations of the lining, especially of the thermomechanical character.Purpose of this licentiate work is to study the mechanical behaviour of the lining by means of the finite element method (FEM). For this, a simplified model of a kiln was created and various fundamental cases were studied. The commercial FE-software LS-DYNA is used for the FE-calculations. The main work is based on cases of the kiln in cold condition. However, an initial study in warm condition is presented as well.The studied lining was a brick lining used in a kiln of dimensions typical for iron-ore pelletizing. Additionally, this licentiate thesis gives an overview of some of the most fundamental issues encountered in a refractory brick lining of a rotary kiln in general. Some material tests are presented as well.Model’s geometry was based on a section at the position of the support wheels, having a thickness of one brick. Some simplifications, such as choice of the material model and a rigid riding tyre, were done and a three-fold faster computational time was achieved. Response of the created model was partly verified analytically, by available in-house data and data from literature. It was confirmed that the model gives a good response.One of the important findings is that despite variation of conditions in cold state, e.g. rotational speed and relative ovality of the kiln, the induced stresses in the lining remained harmless. This challenges traditional believes which imply that ovality is of considerable importance for stress generation in the lining. On the other hand, by continuously tracing gaps between the bricks and the casing, it was found that integrity of the lining was significantly affected by rotational speed and ovality. Gaps as large as 80 mm could be observed between bricks and casing in a worst case scenario.An initial study on the kiln in hot state was made. Thermal expansion of a perfectly lined and an disordered brick lining were performed. The results indicate that stresses due to thermal expansion are rising slightly but are harmless in both cases. Additionally, expansion of the kiln stabilizes the lining and the effect of rotation compared to rotation in cold state is small.Analytical and numerical calculations were compared, indicating that analytical assumptions are often coarse and misleading from the reality.