How To Construction Of Castable For Circulating Fluidized Bed (CFB) Boiler Lining
Most of circulating fluidized bed boilers（CFB）boiler use 0~10mm coal particles. The coal particles have a long residence time in the fluidized bed and tumbling up and down in the furnace, repeatedly impacting the castable lining and wearing the castable lining. Therefore, the quality of the castable refractory must reach to small porosity, large quantity,high density and smooth surface,so as to increase the wear resistance and prolong the service life of the castable of the working layer.
1. Construction Way
The lining castable refractory of circulating fluidized bed boiler is constructed.In order to solve the thermal expansion of castables and the removal of water vapor during the oven, the “small module” method is adopted to construct the lining castable into a size of about 0.8m*0.8m,with Z type construction joints between the modules and the construction joints with 2mm ceramic fibers. However,the structure of each part of the boiler is complex, and the “small module” template is complex and difficult to set up. The quality requirement of the template is high ,and the castable construction progress is slow.
2.Castable refractory experiment
2.1 Block Test
Make a castable refractory test block 200mm*200mm before construction. The water consumption, initial setting time, pore size, porosity and surface finish were tested.It is found that the main problems are large and more porosity,poor surface finish, which can not be solved by repeated test.
2.2Furnace construction test
According to the small module size of 800mm*800mm on the external bed, the mixing time, the amount of water, the transportation time, the vibration,the initial setting time, the porosity and the surface finish are tested. The result of the test is the same as the test of block. The main problem is that the surface pore is larger and more, the porosity is large, poor surface finish, which can’t meet the quality requirement.(Pic 2)
(PIC 1) (PIC 2)
3.Analysis and research
3.1. The cause of pore formation
We study all aspects of castable refractory test one by one. It is found that the weighing of water,mixing time and construction procedure are not strictly controlled in the test, and there are bubble overflow on the surface of the castable refractory during the vibration. Through the above analysis,the main reasons for the formation of pores in the construction of castable are：the amount of water, more water more pores. The water quality of the used water can not react with various substances in the castable refractory. And the various substances in the castable refractory react and produce gas in the mixing and vibration when water adding.
3.2Water amount and quality
During the test,the weighing of table and the measuring cup are accurately weighed, and the amount of water added is strictly controlled.Firstly, using water and pure water, based on the theoretical water addition amount of the castable, the test is carried out according to the increase of each time and reduction of 1% of the water volume. The test is carried out from 1% to 15%, and the water addition is changed. Therefore, through test, it is basically determined that the porosity of the castable refractory has nothing to do with water quantity and water quality, that is, water quantity and water quality are not the main reasons for the porosity of the castable refractory.
After castable refractory is poured into the module, the vibration rod is used to vibrate, and the vibration time is extended without segregation. The vibration time starts from 1 min, and the test is performed every 1 min increment,and 10 groups of tests are done from 1 to 10min. In the test,it was found that the size of pores, number of pores and the porosity of the castable refractory increase with the time of vibration, and the surface of the castable refractory can not see the obvious pores.It is determined by test that the vibration time, and it’s the main reasons for the formation of more pores in the castables refractory construction. (Pic 3)
4.Castable refractory construction
4.1 Castable refractory mixing
The castable refractory requires strict construction temperature, and the construction temperature is below 30℃. In the summer of July to Sep, the local ambient temperature is mostly above 30℃. In order to ensure the quality of the castable refractory, the castable refractory is stored indoors, the mixing station is placed in the lower part of the boiler structure to avoid sun exposure, and the temperature of the mixture is reduced by adding ice in the construction water temperature to 20℃.
4.2Castable refractory transportation
The initial setting time of the castable is 30 min. When the initial setting time is shorter, the mixed material must be transported to the construction site within 15min, and the transportation time is shortened as far as possible. The hoist,the winch and gantry are used as vertical transportation equipment.
4.3 Castables refractory into the mould
The castable refractory should be uniformly distributed in the mould, and should not be poured continuously in one position. For the special shaped castable mould chamber at the interface of the combustion chamber and each part, the castable refractory into the mould is only in a small range,and it should be planed with a small spade while laying the castable. When casting, keep the poured castable surface clean and smooth.
4.4Vibration of castable
The vibrating rod should be dragged slowly to eliminate the bubbles evenly,but the vibration time should not be too long to avoid segregation of aggregate and powder. When casting, a special person is arranged to check the support of the template and prevent expansion and leakage.
4.5 Stripping and curing of castable refractory
Castable can be demoulding after initial condensation of 12h,and the strength of castable during mold release should ensure that the surface and edges of the castable are not damaged. Generally, it should be reversed in the order of template support, that is, the final removal of the last set up, and the final removal of the first support. Do not force the template and castable refractory to prevent damage to the castable.For each hole and channel template,the strength of castable should reach 75% of final strength, and the template can be removed. The castables refractory should be naturally cured after demolishing the template.
5.Construction process and difficulties
Castable construction process advantages: smooth and bright surface, low porosity, high density, good casting quality, the quality of qualified rate of more than 95%.
Construction process difficulties: the template must use high light and clean template, small module are divided into large number of blocks, difficulty,construction process, labor time consuming,castable mixing, transportation, pouring, vibration quality requirements strict. The construction period is relatively long and the construction cost is relatively high.
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