1. Introduction
The history of cement can be traced back to the mixture of lime and pozzolan used by ancient Romans in buildings, which is very similar to modern lime-pozzolan cement. After hardening, the concrete made of crushed stone cemented with it not only has high strength, but also can resist the erosion of fresh water or salt water. For a long time, as an important cementing material, it has been widely used in construction projects. However, cement is a product with high pollution, high energy consumption and high emission. Therefore, people have been trying to improve its firing system to meet the requirements of cement production and minimize energy consumption and environmental pollution. Since 1824, 10, 2 1, J. Aspdin obtained British Patent No.5022 (Portland cement), the development of cement kiln has experienced the development process of vertical kiln-rotary kiln-suspended preheater kiln-fluidized bed calcination. In these development processes, the cement sintering system has become more and more optimized, which has made great contributions to the development of society.
2. Development course of cement kiln
2. 1 silo kiln
Warehouse kiln: 1824, the calcining equipment of portland cement is bottle kiln. Forty-eight years later, based on the bottle kiln, the silo kiln specially used for cement firing was invented, which became the first generation of cement kiln and initiated the silo kiln era of cement production.
2.2 vertical kiln
Vertical kiln: 1884 Dietzsch invented the vertical kiln and obtained the patent right. The biggest difference with silo kiln is that it changes the firing process from horizontal movement to vertical direction.
There are two kinds of shaft kilns currently used in China: ordinary shaft kilns and mechanical shaft kilns. China has experienced the development process from ordinary shaft kiln with manual intermittent operation to mechanical shaft kiln with continuous production, which has brought about changes in labor intensity reduction, output improvement and quality improvement.
Ordinary vertical kiln is manual feeding and manual unloading or mechanical feeding and manual unloading; Mechanical vertical kiln is mechanical feeding and mechanical unloading. The output, quality and labor productivity of mechanical shaft kiln are higher than those of ordinary shaft kiln. According to the technical policy requirements of building materials, small cement plants gradually replace ordinary vertical kilns with mechanized vertical kilns.
2.3 dry rotary kiln
Starting from dry hollow rotary kiln, waste heat boiler kiln, dry long kiln and vertical kiln have been developed. Dry method makes raw meal into raw meal dry powder, and the water content is generally less than 65438 0%, so the heat required for evaporation of water is reduced compared with wet method. Due to the high exhaust temperature of hollow kiln, the heat consumption is not low. Dry production, raw meal is made into dry powder, and its fluidity is worse than mud. Therefore, the raw materials are not well mixed and the ingredients are uneven.
Hollow rotary kiln: Clayton, England obtained the British patent at 1877; 1895 Americans Hurley and Seaman successfully obtained calcined cement and obtained patents;
Waste heat boiler kiln: German I.A.Bachman invented it in 1897, which solved the problems of high exhaust temperature and low thermal efficiency at the tail of dry hollow rotary kiln. This kiln has long cycle time, but low thermal efficiency, so it is not a common cement sintering equipment.
Dry long kiln: it was adopted in 1930s and 1940s, but its thermal efficiency was poor, and the dust at the kiln end was large, so it was not widely popularized.
Lipper Kiln: 1928, which was manufactured by Lelep Company and Polysius Company in Germany, was named "Leport" and was semi-dry production. It was once popular all over the world, and it was not replaced by a new kiln type until 1960s.
2.4 Wet rotary kiln
19 12 years, F.L.Smith Company of Denmark first made a successful calcination test, and later it was widely popularized as a popular kiln type. Wet production is to make raw meal into slurry with water content of 32%~40%. Due to the preparation of slurry with fluidity, the raw materials are evenly mixed, the composition of raw materials is uniform and the quality of sintered clinker is high, which is the main advantage of wet production.
This kind of kiln includes ordinary wet kiln, slurry evaporator wet kiln, wet long kiln and so on.
2.5 New dry rotary kiln
Suspended preheater kiln appeared in 1950s, and precalciner kiln developed 20 years later.
Suspended preheater kiln: 1932 Czech patent for four-stage cyclone suspended preheater was first put forward, which was first applied to cement production by Humboldt Company of Germany in the early 1950s.
Composition: preheater+calciner and other kiln tail systems+rotary kiln+cooler+kiln head burner, etc.
Suspended preheater kiln and precalciner kiln are collectively called new dry cement production.
Flow chart of cyclone suspension preheater kiln
Suspended preheater is called SP for short, and rotary kiln with suspended preheater is called SP kiln.
Production flow chart of precalciner kiln
Preheaters are divided into cyclone preheater and vertical tube preheater. The cyclone preheater kilns mainly include Humboidt, Smidth and Dopel vertical preheater kilns, including klapp, ZAB and Prerov. The flow and heat transfer characteristics of different preheaters include downstream swirl, countercurrent swirl, jet-swirl, spouting and swirl-swirl.
Pre-decomposition kiln: first created by Ishikawa Island Heavy Industry Co., Ltd. in 197 1. The difference between it and SP kiln is that a precalciner kiln is added between the kiln end and preheater, which is called NSP kiln for short.
2.6 fluidized bed cement kiln
Rotary kiln is a reliable cement clinker calcining equipment, but its fatal weakness is low thermal efficiency, large rotating power and huge volume, which has always been the object that people want to "revolution". Therefore, since 1950s, the United States, Japanese, China, Russian, Indian and other countries have successively studied the rotary kiln-free boiling sintering process. Due to the limitation of science and technology at that time, it is difficult for cement clinker to self-granulate without sticking to the furnace wall at high temperature (65,438+0,300℃-65,438+0,400℃), and it is difficult to agglomerate and maintain normal fluidized operation. Before the 1990s, it was not completely successful and could not meet the requirements of industrialization. In the cement industry, fluidization technology has been successfully applied to the preheating and pre-decomposition of cement raw materials, which fundamentally changed the heat exchange process of material and air flow during the preheating and pre-decomposition of raw materials and shortened the preheating and pre-decomposition time of raw materials to tens of seconds, thus doubling the kiln output and greatly reducing the fuel consumption. It can be said that the successful application of fluidization technology in preheating and pre-decomposition of cement raw meal is a major change in the history of cement development. Based on the above advantages of fluidization technology, whether the sintering process of cement clinker can also be placed in fluidization state has always been one of the topics studied by cement workers all over the world. However, due to the complexity of high-temperature gas-solid reaction, the immaturity of large particle fluidization technology and the influence of various factors on the amplification of experimental equipment, the research work of this technology is only in the stage of theoretical research and semi-industrial experimental research. The characteristics of fluidized bed cement kiln are: 1, which greatly expands the selection range of coal types. Bituminous coal, anthracite coal or low-quality coal can be selected. 2. Good energy-saving indicators. Heat consumption can be reduced by 10-25%. 3. High heat recovery efficiency. Through the combination of granulating device and sintering device, the heat recovery rate is more than 80%, which is more than 20% higher than that of the existing grate cooler. 4. Good environmental protection performance. Carbon dioxide emissions are reduced by 10-25%, and nitrogen oxide emissions are reduced by more than 40%. 5. Save the construction cost and reduce the operation cost. Compared with the rotary kiln of the same specification, the equipment investment is saved by 20% and the operating cost is reduced by 25%. Because of its good working index and small area, it is suitable for large-scale and large-scale vertical kiln transformation in China.
5. Conclusion
The cement clinker burning system has become more and more perfect in the long-term development process, which has brought great contributions to human modernization, but there are still many problems to be solved, such as China's cement energy consumption exceeding that of developed countries. Cement is still a product with high energy consumption, high emission and high pollution, and we still have many problems to solve. These problems are our unshirkable responsibility as cement workers, and the road is long, and I will go up and down.