The double D kiln is a parallel flow regenerative lime kiln. When one of the kiln chambers is burning, the exhaust gas generated enters the other kiln chamber through the connecting channel to preheat the limestone, and then is discharged through the dust collector. The weight of the refractory materials facing the kiln body is supported by the refractory materials of the connecting channel. Once the refractory materials on the outer edge of the connecting channel are damaged, the refractory materials facing the wall of the kiln will peel off, and then the refractory materials of other parts of the kiln will peel off. If production is continued at this time, the kiln shell steel plate will deform or even melt.
Main problems before the improvement of the refractory materials of the double D kiln
(1) Peeling of refractory materials in the connecting channel. During kiln ignition, the channel refractory material will peel off due to the rapid temperature change, that is, when the channel temperature reaches above 500℃, small pieces of refractory material will peel off; during kiln production, the dust accumulation in the connecting channel is cleaned every 15 days or so, which takes about 30-40 minutes. As the ignition port is opened, the channel temperature drops, which has an adverse effect on the refractory material; a long period of shutdown caused by equipment failure will also have an impact on the refractory material in the kiln. The above factors lead to the peeling of broken bricks in the connecting channel refractory material after 1.5-2 years of kiln production, the peeling of standard magnesia refractory bricks after 2-2.5 years, and the peeling of magnesium bricks in the connecting channel after 3 years. The kiln cannot continue to maintain production due to the unloading of refractory materials on the kiln wall due to the peeling of magnesium bricks in the connecting channel.
(2) The calcium silicon board is close to the steel structure of the kiln, with a thickness of 75mm. Since the double D-shaped arc surface has a certain curvature, a large gap is left at the arc surface.
(3) The design requires that the insulation ramming material at a height of 158mm above the connecting channel be backfilled one month after ignition to prevent the gap from changing due to thermal expansion of the refractory material during kiln ignition. However, in actual operation, since the insulation ramming material cannot be added with water and there are special magnesium refractory bricks below, it can only enter through a 300mm×300mm square hole. It is difficult to fill the original gap with the ramming material, resulting in a high temperature of the steel plate above the connecting channel, with the highest point temperature exceeding 260℃, which increases the heat loss of the kiln.
(4) There are two thermocouples in the center of the connecting channel, which are the main temperature control points of kiln production. The refractory material at the thermocouple sleeve requires on-site drilling, which can easily lead to the loosening of the surrounding refractory materials and affect the service life of the refractory materials in the connecting channel.
Improvement measures
4.1 Countermeasures for spalling of refractory materials in connecting channels
For the problem of early spalling of refractory materials in connecting channels, it is considered that the reason is that the expansion coefficient of special magnesia bricks is large. When the kiln is heated and expanded after ignition, the stress generated by the refractory materials cannot be released, and the refractory materials squeeze each other and cause them to break. When the kiln stops production or the temperature drops under other circumstances, the refractory materials shrink and spall, and the most likely part to spall is the edge of the upper arch of the channel. As the spalling intensifies, the standard magnesia bricks in the working layer above the channel also spall, which makes the thermal insulation performance of the kiln body worse, and it is ultimately difficult to maintain normal production. Without changing the size of the refractory materials, change the type of refractory materials in the connecting channel, that is, replace the special magnesia bricks with high-alumina refractory bricks of better quality.
4.2 Improvement of calcium silicon board
Improve the size of calcium silicon board. Change the original 1000mm×500mm×75mm calcium silicon board to 600mm×300mm×50mm, and increase the number appropriately. After the improvement, the gap between the calcium silicate board and the kiln shell becomes smaller, which is beneficial to the insulation of the kiln shell.
4.3 Backfilling of thermal insulation ramming material above the channel
During the overhaul of the kiln, after the clay bricks above the channel arch are laid, the thermal insulation ramming material is backfilled to 2/3 of the total gap, and a small amount is backfilled one month after ignition according to the actual situation.
4.4 Treatment of refractory materials at the thermocouple connection channel
Before laying the high alumina refractory bricks on the channel arch, the thermocouple sleeve is welded first. The thermocouple around the sleeve is laid into a circle according to the actual situation. A small amount of bricks can be cut to ensure the circle, and the sleeve is surrounded by ceramic fiber.
Through the improvement of refractory materials, the overall life of the double D kiln is improved, considerable economic benefits are achieved, and the quality of lime is improved.
