Why is it necessary to heat and increase the pressure in the calcination furnace?

Control schedule for heating the pipes and furnace and starting material feeding: Control stage, Control pressure (MPa), Control time (min). Heating pipes and furnace: 0–0.1, 90 minutes; Heating pipes and furnace: 0.1–0.5, 60 minutes. Pressure increase: 0.5–1.0, 60 minutes; Pressure increase: 1.0–2.0, 30 minutes. Starting material feeding at increased pressure: 2.0–3.2, 30 minutes

The furnace expands when heated and contracts when cooled, which generates corresponding stresses. To prevent the furnace body from being subjected to excessive stress over a short period of time, it is necessary to heat the furnace up.

When starting to increase the steam pressure, the temperature and pressure of equipment such as the heating tubes, steam inlet shafts, and furnace body inside the calcination furnace rise rapidly. The heating surface area of these devices is uneven, and under the force of intense water impact, it is easy for local pressures to become too high, which can cause the furnace to deform. Therefore, the furnace should be warmed up gradually while increasing the pressure, and any remaining water in the heating tubes and steam pipes should be drained step by step.

If the pressure increase is not carried out in accordance with the prescribed curve during furnace warming, it will inevitably shorten the service life of the calcination furnace.

At the beginning of the temperature increase, the moisture in the refractory lining material must be removed; if the temperature rises too rapidly, a large amount of vapor will be generated within the structure of the refractory material, leading to the breakdown of the lining. This is accompanied by crystal structure changes; too rapid a change in the volume of the lining can also lead to fractures, thereby reducing the service life of the furnace. Therefore, the temperature should be increased slowly in accordance with the heating curve.

The furnace body and the heating tubes should be heated evenly to ensure simultaneous temperature rise. The requirement regarding the time taken for pressure to increase is based on equipment maintenance considerations. Similarly, after the calcination furnace is shut down, it must continue to operate at a low speed; the furnace cannot be stopped immediately while it is still hot

Because it is a modular component, its structure is complex and large.

When increasing the steam pressure, the heating surface area of the equipment is uneven, leading to uneven heat exchange; the condensate water flow is unstable, and the expansion of each heat exchange tube varies. As a result, some pipe joints are subjected to excessive local expansion stresses. In cases where the welding quality is poor, cracks can develop, and these cracks may expand further, or stress can build up to a level that exceeds the strength of the steel material or reaches a critical failure point. Tiny cracks can form within the welds, and rapid temperature increases during the warming-up process will inevitably accelerate the spread of these cracks, resulting in unnecessary repairs and problems. Therefore, it is important to increase the pressure slowly during the warming-up process. It is safer to follow the operating procedures.

Our company follows the following procedures for operating the calcination furnace; please refer to this for reference in the workshop: 7.2.7 Check the temperature rise in various areas, and only start increasing the pressure once no issues are detected; the time from starting steam supply to increasing pressure should be no less than 90 minutes. 7.2.8 The pressure increase must be carried out in strict accordance with the specified procedures; the rates and temperature rise curves are as follows: 0–0.05 MPa over 90 minutes, 0.05–0.3 MPa over 90 minutes, 0.3–0.7 MPa over 60 minutes, 0.7–1.3 MPa over 30 minutes, 1.3–1.8 MPa over 30 minutes, and 1.8–2.2 MPa over 30 minutes. 7.2.9 Carefully check the operation of all components, especially to ensure that there are no leaks in the tubes inside the furnace. 7.2.10 If the pressure increase of the heater stops due to a fault, it is necessary to increase the pressure again. 7.3 Putting into production 7.3.1 When the steam pressure reaches 1.0 MPa, about 5 tons of light ash is added to carry out rapid furnace switching; (if a material level was maintained previously, furnace switching is carried out directly at this time). 7.3.2 When the temperature of the alkali returned to the furnace reaches 150°C and the steam output temperature exceeds 108°C, preparations are made to start using heavy alkali.

Since the furnace expands and contracts with temperature changes, generating corresponding stresses, it is necessary to heat the furnace up in order to prevent the furnace body from being subjected to excessive stress over a short period of time. When starting to increase the steam pressure, the temperature and pressure of equipment such as the heating tubes, steam inlet shafts, and furnace body inside the calcination furnace rise rapidly. The heating surface area of these devices is uneven, and under the force of intense water impact, it is easy for local pressures to become too high, which can cause the furnace to deform. Therefore, the furnace should be warmed up gradually while increasing the pressure, and any remaining water in the heating tubes and steam pipes should be drained step by step.