Analysis of kiln technical problems!
With the development of ceramic industry production and the increase of kiln construction and repair, higher requirements will be put forward for kiln construction in technology and effect, which will promote the improvement and improvement of construction technology. Of course, the development of construction technology depends on the progress of kiln design, refractories and construction equipment.
1. From the primary stage in the near future, it can reduce the mortar joint, increase the overall strength, and improve the efficiency of foot building by appropriately making the bricks large; Gradually increase the application of bulk refractories (such as refractory concrete), which can realize mechanization instead of manual work, speed up the construction progress, and expand its application scope with the progress of production technology and quality of bulk refractories; According to the actual situation, we should popularize and apply the construction methods and tools of other departments, such as conveyor belts for transporting materials, pumps for transporting fire mud, diamond cutting machines for cutting bricks, etc., in order to reduce labor intensity and improve work efficiency.
2. From the long-term advanced stage: with the development of refractories and advanced thermal insulation materials, kiln masonry is also replaced by prefabricated components produced in batches in the construction industry; Like building a skyscraper to achieve rapid completion, the masonry of kilns (especially tunnel kilns) is changed into prefabricated construction method, which makes the masonry of kilns mechanized and commercialized, thus greatly reducing costs and improving quality. Therefore, this requires the kiln body to gradually realize standardization and serialization.
Selection of fuel for modern ceramic kiln
Due to the need of environmental protection, modern ceramic kilns should focus on using clean fuel in fuel selection.
Clean fuels include: diesel, kerosene, natural gas and liquefied petroleum gas. Coke oven gas, water gas and producer cold gas.
1. Diesel oil and kerosene
A. Diesel oil used as clean fuel has the advantages of simple transportation and storage, low equipment investment, high and stable calorific value, low viscosity and easy atomization, and its heating temperature is as high as 2080~2100℃.
Disadvantages are: diesel oil with high price and high freezing point needs to be heated when used in cold environment. When choosing diesel oil, we should pay attention to its sulfur content. If the sulfur content is too high, it will affect the quality of ceramic products, corrode equipment and pipelines and pollute the environment.
B, kerosene as a clean fuel has the advantages that its low calorific value is slightly higher than that of light diesel oil by about 42.8~43.5MJ/kg. It is the top grade of clean fuel, does not solidify, has smaller viscosity than light diesel oil and is easier to atomize.
The disadvantage is that the flash point is not lower than 40℃, otherwise it is unsafe to use and the price is too high.
2. Natural gas and liquefied petroleum gas
A, natural gas as fuel, its combustion characteristics are as follows:
(1) Because its main component is methane, its combustion characteristics depend on methane. The ignition concentration range of methane and air mixture is very narrow, ranging from 5% to 15%. Therefore, it is very sensitive to hypoxia in the combustion process, and at the same time reduces the risk of tempering.
(2) The flame propagation speed of methane is very small. The maximum visible flame propagation speed is less than 1.0m/s at normal temperature and pressure. Therefore, the combustion is slower. Natural gas belongs to the gas with low flame propagation speed. Dehydration is more likely to occur.
(3) The heating temperature of natural gas is about 2000~2040℃, which can almost meet the required firing temperature for all kinds of ceramic products, that is, normal temperature air is used.
(4) When natural gas is well mixed with air, its flame blackness is very small, which is half that of liquefied petroleum gas and much lower than that of liquid fuel. However, for modern ceramic kilns, flame radiation heat transfer does not occupy an important position. Generally, high-speed burners are used, which are flameless combustion with almost no flame.
(5) The theoretical air volume of natural gas is large, about 7.8~11.2Nm3/Nm3, so the mixing performance of burner is required to be high, that is to say, 1Nm3, natural gas should be well mixed with more than 7.8~11.2Nm3 air.
(6) The carbon/hydrogen mass ratio of natural gas (3.0~3.2) is much lower than that of liquid fuel (6.0~7.4) or solid fuel (10~30). Therefore, the combustion products contain more H2O.
B, liquefied petroleum gas as fuel has the following characteristics:
(1) The calorific value is very high, which is the highest among gas fuels.
(2) The theoretical air volume is as high as 24~30Nm3/Nm3, so it is more difficult to mix combustion air with it than natural gas. Liquefied petroleum gas can be diluted with air or flue gas before use. But when diluted with air, it shall not be within the range of ignition concentration to prevent explosion. It is generally stipulated that the volume concentration of liquefied petroleum gas must be higher than 1.5 times the upper limit of the ignition concentration range.
(3) The flame propagation speed is low and the combustion is slow, but it is faster than natural gas.
(4) purity. Generally, it contains less sulfur and is a high-quality fuel for firing high-grade ceramic products.
(5) The density is relatively high, which is about 1.5~2.0 times that of air at the same temperature and pressure. When it leaks, it sinks and is easy to mix with air to reach the ignition concentration range, and it will explode in case of fire.
(6) Generally, the vapor pressure of liquefied petroleum gas is relatively high. It is about 0.9~1.5Mpa at 37.8℃. This is advantageous as a gas fuel. However, in use, the pressure reducing valve of gasification station must be good to ensure safety.
3. Coke oven gas, water gas and producer gas
The artificial gas fuels such as coke oven gas, water gas and producer gas are made from coal gasification, which are also collectively called coal-to-gas
A, coke oven gas
The average composition of coke oven gas is:
H2 46%~61% CO 4.0%~8.5% CH4 21%~30% CmHn 1.5%~3.0%
CO2 1.0%~4.0% N2 3.6%~26% O2 0.3%~1.7%
The low calorific value of dry gas is 13.2~19.2MJ/Nm3, which belongs to medium calorific value gas, and its heating temperature is about 2100~2130℃. Because of its high hydrogen content, the flame propagation speed is higher than that of natural gas and liquefied petroleum gas, and its maximum flame propagation speed is about 1.5m/s at normal temperature and pressure. The ignition range of coke oven gas is also large, with a lower limit of about 6% and an upper limit of about 31%. Coke oven gas can meet the requirements of various firing temperatures of modern ceramic kilns, and it burns quickly and is not easy to dehydrate, but the explosion danger of tempering is greater.
B, water gas
Water gas is the fuel gas generated by the following reactions between water vapor and carbon in red-hot coke.
H2O C=CO H2 2H2O C=CO2 2H2
The generated CO and CO2 generate the following reactions.
CO H2O=CO2 H2 CO2 C=2CO
It can be seen from the reaction formula that the molecular number of CO and H2 generated by the reduction of water vapor by carbon is equal. Therefore, the theoretical composition of water gas is: 50% CO and 50% H2, and the theoretical low calorific value is 11.78MJ/Nm3. Water gas is also a medium calorific value gas, and its heating temperature is as high as 2200℃.
C, producer gas
At the present stage in our country, the cold gas of atmospheric fixed-bed producer is very suitable for ceramic kiln, because this kind of coal-based gas belongs to low calorific value gas, but it can fully meet the general firing temperature of ceramic products.
Atmospheric fixed bed producer gas belongs to low calorific value gas. The calorific value of cold gas is generally in the range of 4.49~7.62MJ/Nm3, and its heating temperature is 1650℃~1750℃. If the air coefficient is 1.10 and the high temperature coefficient is 0.83, the relationship between combustion air temperature and actual combustion temperature can be obtained. For ceramic products with firing temperature of 1280℃, the actual combustion temperature is about higher than that. If the firing temperature is 1300~1350℃, the actual combustion temperature needs to reach about 1400℃, and the combustion air temperature needs to be about 200℃, which is not difficult for ordinary tunnel kiln, roller kiln and intermittent kiln with heat exchanger. For ceramic products with low firing temperature of 1100~1200℃, producer gas can be used.