Furnace lining materials (such as refractory bricks and refractory castables) absorb significant amounts of moisture during construction. If heated directly without drying, this moisture rapidly evaporates, causing cracks, bulging, and even explosions in the lining. Furnace drying gradually evaporates this moisture by slowly heating it, protecting the refractory materials.
How is Water Removed During the Furnace Drying Process?
After refractory construction is completed, three main types of water exist in the system: free water, physically bound water, and chemically bound water.
During furnace drying, a large amount of free and bound water must be removed. This water evaporates significantly between 80°C and 150°C, after which the drainage rate slows. As the temperature continues to rise, around 300°C, the free and bound water are essentially completely removed. At 500°C to 600°C, the refractory material completes its crystal transformation. At around 1100°C, the sintered strength required for design is reached, and stress is completely released. To fully remove deep-seated free and bound water and complete crystal transformation, constant temperatures must be maintained for a period of time at different temperature ranges.
During furnace drying, the 80°C to 300°C stage is a critical phase. This is primarily because water transforms into water vapor at 110°C. During this phase, the amount of water removed is large and the rate of water removal is rapid. The rate of water removal significantly affects the refractory material. If moisture is removed too quickly, the remaining internal moisture will not diffuse as quickly as the surface evaporation rate. The internal moisture will also heat up and convert to vapor, causing expansion. This can cause cracking in the material, reduce bond strength, and weaken the material. Therefore, the insulation period should be appropriately extended. The ideal drying period is a constant drying rate, where the amount of moisture evaporating from the surface matches the amount of moisture removed from the interior.
When heating from 80°C to 600°C, the temperature should be increased slowly. Too rapid a temperature increase will cause the refractory surface to dry quickly, preventing the large amount of vaporized moisture from evaporating. This will generate destructive vapor pressure and cause cracks or fissures in the refractory. At 300°C, bound water and crystallization water convert to gas and are released through pores. Therefore, sufficient drying time is required to completely remove the crystallization water.
Typical Furnace Curve
The above describes the general principles of furnace curing. Because different refractory materials have different compositions and thicknesses, the furnace curing curve must be determined by the design company in conjunction with the refractory manufacturer. The figure below shows a typical furnace curing curve found online.
- First, slowly increase the temperature (20-30°C/hour) to 140°C and hold it at this temperature for 20 hours.
- Then, slowly increase the temperature (20-30°C/hour) to 350°C and hold it at this temperature for 10 hours.
- Then, slowly increase the temperature (20-30°C/hour) to 420°C and hold it at this temperature for 10 hours.
- Finally, slowly cool the temperature back to ambient temperature.
Furnace Drying in Waste-to-Energy Plants
Furnace drying is part of the furnace construction process. Generally, furnace construction is divided into three phases: construction and cold-state inspection, low- to medium-temperature drying, and high-temperature drying.
Construction and cold-state inspection, in simple terms, involve the construction of the furnace walls and refractory components. High-temperature drying refers to the drying of the furnace lining, which is performed simultaneously with the corrosion-resistant passivation treatment of the boiler’s steam-water system. This improves efficiency, saves fuel, and enhances economic efficiency.
The purpose of low- to medium-temperature drying is to remove moisture that cannot naturally separate from the castings, allowing them to further solidify and ensure the performance and quality of the furnace lining. This process virtually completely removes free water and most crystallized water, bringing the moisture content to less than 2.5%, a satisfactory standard.
Of course, a more important aspect of furnace drying is the inspection of the furnace walls after the relatively mild fire. Deformation, cracks, and collapse should be avoided to ensure they meet the performance requirements of the furnace under normal operating conditions.
The basic requirements for furnace drying are that the furnace is complete and has been naturally dried for seven days. While the temperature of a medium-low temperature furnace drying is not particularly high, it is still considered the first ignition. As a thermal equipment, the boiler must undergo its initial thermal testing. Therefore, requirements such as boiler insulation, drum water level, exhaust and drain piping, water and steam pipe hangers, sealed flue gas and air duct openings, pressurized water pressure control, lighting, fire protection, and safety precautions must be in place.
Next comes the preparation of furnace drying equipment and materials.
Before the advent of high-temperature flue gas furnace drying, the furnace drying process was typically accomplished using a combination of wood and fuel oil. Firewood was ignited at a defined location, and after the temperature reached a certain level, a starter oil burner was ignited to raise and maintain the required furnace temperature. Limited by the burning point of the firewood, the labor intensity involved, the high flame temperature of the starter oil burner, and the concentrated heat load, a slow and uniform furnace heating process was impossible. Structural constraints inevitably created “dead spots” within the boiler that were not accessible for drying, making furnace drying quality difficult to ensure.
Currently, most furnaces use light diesel fuel and utilize a furnace drying machine to generate hot flue gas outside the furnace. This hot flue gas serves as the heat source, and after entering the furnace, the furnace walls primarily absorb heat through convection. This prevents high-temperature radiation from the flame from damaging the furnace walls, ensuring uniform heat absorption.
This allows for a slow heating process for the lining material, adhering to the principle of slow and uniform moisture release from refractory and wear-resistant lining materials. Controlling the drying temperature and temperature rise rate promotes the smooth release of different forms of moisture. Ensure temperature control, avoid dead spots, and ensure even heating of the furnace walls.
During the furnace drying process, ensure that all furnace temperature monitoring data is recorded and archived. Ensure moisture drain holes are properly opened and sealed after the boiler is started. At all times, ensure that heating surface tubes are protected from overheating and overburning. Always inspect the smoke exhaust.
