Abstract: This paper briefly describes the characteristics and utilization value of calcium carbide furnace gas. From the fact that calcium carbide furnace gas is mainly composed of CO and H2 (about 90%), it can be seen that calcium carbide furnace gas can be used as fuel and chemical raw materials. However, simply using calcium carbide furnace gas as fuel can not maximize its value, and it is more meaningful to develop high value-added chemical products with calcium carbide furnace gas as raw material. Therefore, this paper focuses on the process of producing carbon-1 chemical products from the tail gas of closed calcium carbide furnace, and compares it with the coal gasification process in coal chemical industry. It is found that the process using calcium carbide furnace gas as raw material can not only make full use of the effective gas components in calcium carbide furnace gas, reduce carbon emissions and environmental pollution, but also effectively reduce the production cost and construction investment of carbon-1 chemical products.
[Keywords:] calcium carbide furnace gas; Circular economy; Guyi
Calcium carbide is the basic raw material of organic chemical industry, and acetylene made from it can produce a series of thousands of organic products such as acetic acid, vinyl acetate, polyvinyl chloride, polyvinyl alcohol and acetylene black. In the domestic calcium carbide industry, whether it is a fully enclosed calcium carbide furnace, an internal combustion semi-enclosed calcium carbide furnace or an open calcium carbide furnace, the tail gas has not been well utilized, and some even burned directly, resulting in a great waste of resources.
Gas characteristics and utilization value of 1 calcium carbide furnace
1. 1 gas characteristics of calcium carbide furnace
When producing calcium carbide in a closed furnace, the amount of by-product gas per ton of calcium carbide is about 400Nm3[ 1-2], and its typical composition and physical properties are shown in table 1. As can be seen from the table 1, the furnace gas contains a lot of dust, which is sticky, light and fine and difficult to catch. The furnace gas contains a small amount of tar, which is gaseous when the temperature is higher than 225℃, and easy to precipitate when it is lower than 225℃, which will cause the dust removal bag to stick. The furnace gas itself has a high temperature and contains a lot of dust that is difficult to remove, so it is difficult to control and needs to be fully purified before use.
1.2 Utilization value of calcium carbide furnace gas
It can be seen from the composition of calcium carbide furnace gas that the furnace gas contains a lot of CO and H2, which is a good fuel and chemical raw material. Making good use of this gas can produce great economic and social benefits. Based on the national calcium carbide output of 25 million tons in 20 17, the total by-product calcium carbide furnace gas reaches about 1000 billion Nm3. If all of them can be recovered, about 7.5 billion Nm3CO and 750 million Nm3H2 can be obtained. Therefore, the purification and utilization of furnace gas is of great significance to realize energy recycling, reduce production costs and improve economic benefits.
2. Utilization and economic analysis of calcium carbide furnace gas in chemical industry
At present, calcium carbide furnace gas in some enterprises is only used as fuel to burn lime and boilers after simple treatment, and the value of furnace gas has not been maximized. The main components of calcium carbide furnace gas are carbon monoxide and H2. After deep purification, CO and H2 can be used to develop subsequent high value-added chemical products, and can be used to produce high value-added chemical products such as synthetic ammonia, methanol, ethylene glycol, dimethyl ether and sodium formate [4]. At present, domestic plants for producing sodium formate, synthetic ammonia and ethylene glycol have been successfully built. See Table 2 for details.
2. 1 synthetic ammonia and methanol
[3] According to feed gas analysis and material balance calculation, the volume fraction of nitrogen in calcium carbide tail gas is about 5%. For example, when methanol is produced per unit area, 5% nitrogen will be discharged as invalid gas, which increases the invalid work of the compressor; For example, if the unit output of synthetic ammonia, it is necessary to add nitrogen to the system, add new nitrogen-making devices and increase investment. Considering comprehensively, if methanol production and ammonia synthesis are connected in series, and ammonia synthesis is carried out by using N2 and H2 in furnace gas, the loss of a large number of effective gases caused by inert gas emission in methanol synthesis process can be avoided. The co-production process of alcohol and ammonia not only makes maximum use of calcium carbide furnace gas, reduces emissions, but also creates economic benefits. See 1 for the process flow chart of alcohol-ammonia co-production. Taking calcium carbide furnace gas as raw material, the co-production process of alcohol and ammonia has the following advantages: (1) the gas components in calcium carbide furnace gas are fully utilized, and the utilization rate of furnace gas is high; (2) Using the by-product liquid ammonia in the tail gas after methanol synthesis not only makes maximum use of calcium carbide furnace gas, but also creates economic benefits; (3) Low energy consumption, obviously lower than domestic coal-fired methanol production process; (4) The cost is low, which is obviously lower than the domestic coal-fired methanol production process.
2.2 ethylene glycol
Ethylene glycol synthesis gas is high purity H2(99.9%, vol) and CO(99%, vol), and the volume ratio of H2 and CO is about 1.95. If calcium carbide furnace gas is used as ethylene glycol synthesis gas, compared with coal as raw material, the process of making gas from coal is omitted, the consumption is low, and the raw material cost is greatly reduced, which is undoubtedly a raw material route superior to the production of ethylene glycol from coal alone. See Figure 2 for the process flow of ethylene glycol synthesis from calcium carbide furnace gas. It can be seen that calcium carbide furnace gas can be used as the raw material of ethylene glycol to produce hydrogen only after proper conversion and separation, without the process of coal gasification, which can save a lot of investment and has good economic and social benefits. Judging from the utilization of calcium carbide furnace gas in chemical products in China, Xinjiang Tianye Group has achieved success. Xinjiang Tianye started construction of the first phase of the 250,000-ton/year coal-to-ethylene glycol project in Shihezi, Xinjiang in July of 201year, with calcium carbide furnace gas as raw material and an annual output of 50,000 tons of ethylene glycol. The first phase of the project was completed in June 20 13, and excellent ethylene glycol was successfully produced, the purity of which exceeded the national standard for excellent products. On the basis of the success of the first phase, the second phase of the 200,000-ton/year ethylene glycol project started construction in May 20 13 and was completed and put into operation in September 20 15.
2.3 PVC
[4] -PVC produced by calcium carbide method is not only used in C 1 chemical industry, but also has two important chemical reaction processes: (1) hydrogen reacts with chlorine to synthesize hydrogen chloride; (2) Hydrogen chloride and acetylene react to synthesize vinyl chloride. In the process of hydrogen chloride synthesis, in order to avoid acetylene explosion when the content of free chlorine in hydrogen chloride is too high, the hydrogen participating in the reaction is generally about 10%. However, when electrolyzing sodium chloride, the amount of chlorine and hydrogen produced is the same, which requires excessive hydrogen resources. However, chlor-alkali enterprises generally rely on the production of liquid chlorine to balance the shortage of hydrogen, or use water electrolysis hydrogen production or natural gas hydrogen production to supplement hydrogen, which will bring about the problems of increasing investment and production cost. According to the composition of calcium carbide furnace gas, the furnace gas contains about 80% CO and 5%~ 10% H2 (by volume), so the recovered furnace gas can be separated by isothermal transformation, pressure swing adsorption and other processes to synthesize hydrogen chloride, and the remaining CO can continue to be used as the raw material of carbon-1 chemical industry. The process flow of furnace gas recycling is shown in Figure 3. In the case of low profit in chlor-alkali industry, the recovery and utilization of furnace gas not only reduces the production cost of calcium carbide, but also provides hydrogen for PVC production, with remarkable economic benefits.
2.4 "Economic Analysis of Furnace Gas Recovery"
Using coal and calcium carbide furnace gas as raw materials to produce ethylene glycol syngas, the main cost difference is reflected in the preparation of syngas raw gas, so the cost of coal-based syngas and calcium carbide furnace gas is mainly compared as the treatment cost of syngas. 2.4. 1 comparison premise (1) produce the same specification and the same amount of synthesis gas (H2+ carbon monoxide); (2) Coal-to-natural gas is regarded as the gasification process of coal-water slurry; (3) Calcium carbide furnace gas is supplied outside the project, and is priced according to 0.3 yuan /Nm3; (4) The raw gas is compared to synthesis gas, that is, the subsequent conversion and separation are not considered. 2.4.2 Comparison of consumption Based on the comparison premise of 2.4. 1, the process of producing synthesis gas from gas and calcium carbide furnace gas is calculated and analyzed, and the raw materials and utilities consumption of the two processes are obtained, as shown in Table 3. It can be clearly seen from Table 3 that the raw material is calcium carbide furnace gas, which is directly supplied by the calcium carbide plant, and the consumption has been included in the production of calcium carbide, so the raw material gas has been prepared for the chemical plant; However, if coal gasification is used, the consumption of utilities and raw coal will be increased in the process of gas production, which requires ensuring the coal resources where the project is located and building utilities for coal gasification. 2.4.3 Cost comparison According to the consumption in Table 3, it can be seen that the calcium carbide furnace gas is the raw material gas, and only the cost of the raw material calcium carbide furnace gas needs to be calculated. In fact, calcium carbide furnace gas is a by-product of calcium carbide production, so its cost can be considered as zero. This comparison shows that calcium carbide furnace gas is purchased from the calcium carbide factory outside the project and should be included in the cost of raw gas according to the purchase price. The cost comparison results are shown in Table 4. From the comparison results in Table 4, if calcium carbide furnace gas is used as raw material, the cost of synthesis gas per 1000Nm3 can be reduced by more than 200 yuan, and the cost of ethylene glycol per ton can be reduced by about 500 yuan, which is very significant. If calcium carbide furnace gas can be supplied internally, the cost of calcium carbide furnace gas can be ignored, and the cost of synthesis gas can be reduced more significantly. 2.4.4 Investment comparison Due to different sources of syngas, the investment difference will be relatively large. The investment difference between gas and calcium carbide furnace gas is compared and estimated, and the comparison results are shown in Table 5. As can be seen from Table 5, if the investment reference value of coal-to-gas is 0, and the scale of synthetic gas is 93,750 nm3/h (which can meet 300,000 tons/year of ethylene glycol production), the one-time investment with calcium carbide furnace gas as raw material can be reduced by about 950 million.
2.5 The influence of calcium carbide scaling
Although calcium carbide furnace gas, as the raw material of synthetic gas, is much lower in investment and operation cost than coal-to-gas route, it still depends on the scale of calcium carbide plant to use calcium carbide furnace gas well. For example, if calcium carbide furnace gas is used to produce sodium formate, 654.38+ 10,000 tons/year of calcium carbide can be matched with 70,000 tons/year of sodium formate device; However, using calcium carbide furnace gas to produce high value-added chemical products such as synthetic ammonia, methanol and ethylene glycol, 65,438+10,000 tons/year of calcium carbide can only support 36,000 tons/year of methanol or synthetic ammonia plant. It is difficult for such a small chemical plant to produce economic benefits, which is equivalent to using the resources of calcium carbide furnace gas, but it consumes more energy in the supporting chemical plants, greatly reducing the social, economic, environmental and energy-saving benefits of calcium carbide furnace gas recycling. Therefore, when using calcium carbide furnace gas, the scale of calcium carbide plant must be considered. At present, the scale of calcium carbide enterprises in Xinjiang, Inner Mongolia and other places is generally above 600,000 tons/year. For example, the calcium carbide production capacity of Xinjiang Tianye has reached more than 2 million tons/year, which can provide enough raw gas for chemical production. Therefore, if the newly-built carbon-1 chemical project does not have enough calcium carbide furnace gas, we can consider cooperating with the surrounding large calcium carbide enterprises to provide calcium carbide furnace gas to chemical enterprises to realize resource complementation and recycling.
3 Conclusion
Based on the above analysis, using calcium carbide furnace gas to produce chemical products has obvious advantages in both one-time investment and product cost. More and more attention has been paid to the development and utilization of calcium carbide furnace gas, especially in the chemical field. The application of calcium carbide furnace gas in carbon-1 chemical products has been successful in industry at present. If it is gradually extended to more chemical products, it will greatly reduce the cost and investment of related products, save energy and reduce emissions, and improve economic benefits, which is a bright spot in developing circular economy. In addition, the metallurgical industry produces ferroalloy, industrial silicon, yellow phosphorus, corundum and other processes, in addition to producing calcium carbide, it also produces furnace gas mainly composed of carbon monoxide, and its furnace gas emission is about twice that of calcium carbide furnace gas. If the successful application of calcium carbide furnace gas in chemical products is extended to the whole metallurgical industry, it will make great contributions to the energy saving, resource utilization and environmental protection of the whole national economy.
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