Development of Calcium Carbide Furnace Technology and Solution of Calcium Carbide Furnace Exhanust Gas

1. Development of the Carbide Industry and Carbide Furnace Technology

As an important chemical raw material, calcium carbide is widely used in the chemical industry, metallurgy, medicine, and many other fields. Calcium carbide is an intermediate product of the coal chemical industry. With the continuous growth of the national economy, especially the rapid increase of PVC demand, the demand for calcium carbide is also expanding. China is rich in coal resources, which provide sufficient raw materials for carbide production. Especially in recent years, the rapid rise of oil prices has led to the rising production costs of the petrochemical industry. Compared with the petrochemical industry, the coal chemical industry has been showing its own advantages, making the rapid development of the carbide industry.

The main equipment for calcium carbide production is the calcium carbide furnace. Calcium carbide furnace belongs to submerged arc furnace (or submerged arc furnace) in the classification of electric furnace types, which belongs to the same category as some ferroalloy electric furnaces.

China is a large coal country and a large carbide producer. There are 50 to 60 large-scale carbide production backbone enterprises in China, and hundreds of medium and small-scale carbide enterprises. The calcium carbide production equipment is mainly small and medium-sized open calcium carbide furnaces, and the largely closed calcium carbide furnaces occupy a relatively small amount. In the future development of the calcium carbide industry, large-scale energy-saving and environment-friendly closed calcium carbide furnaces will be the main type.

In the past, the development of large-scale calcium carbide furnaces was restricted due to the restriction of large-scale electrode equipment technology and process technology. At present, the large-scale electrode with a diameter of 1400mm has been applied to large-scale closed submerged arc furnace in China, and has received good use results, which proves that China has matured in mastering the core technology of large-scale electric furnace, creating favorable conditions for us to design and manufacture new-type 40MVA and above super large-scale calcium carbide furnace.

As the large calcium carbide furnace has the advantages of less labor quota, easy high-level automatic control, less relative investment, and easy comprehensive environmental treatment, it is recommended to adopt a 30 ~ 40MVA large closed calcium carbide furnace for the new calcium carbide furnace in the future.

2. Classification of Calcium Carbide Furnace

A calcium carbide furnace is the main equipment for calcium carbide production. The electrochemical reaction generated by calcium carbide is completed in the furnace. Calcium carbide furnace can be divided into small calcium carbide furnace (general capacity is below 10000kva), medium calcium carbide furnace (general capacity is below 10000 ~ 25000kVA), large calcium carbide furnace (general capacity is below 25000 ~ 40000kva) and super large calcium carbide furnace (general capacity is above 40000kva); Calcium carbide furnace can be divided into open calcium carbide furnace, semi-open calcium carbide furnace and closed calcium carbide furnace according to its structure.

3. What Is a Closed Calcium Carbide Furnace?

A closed calcium carbide furnace means that the electrochemical reaction of calcium carbide production is carried out in closed containers (including furnace body, furnace cover, etc.), and the external air cannot enter the reaction container. Such a calcium carbide furnace is called a closed calcium carbide furnace. Due to the isolation of air in the closed calcium carbide furnace, the CO gas generated by the reaction will not undergo a combustion reaction in the furnace.

Characteristics of Closed Calcium Carbide Furnace:

1) The CO gas produced by the reaction can be used as gas fuel and raw material for the organic synthesis industry after being extracted and treated by cooling and dedusting purification, so that energy can be comprehensively utilized and the cost of calcium carbide production can be reduced;

2) The electrochemical reaction of the closed calcium carbide furnace is carried out under the closed furnace cover. There is no flame and dust around the calcium carbide furnace, which greatly improves the working environment;

3) As the CO gas generated by the reaction is extracted, the combustion reaction will no longer occur on the furnace surface, and the gas temperature in the furnace will be reduced, extending the service life of the equipment;

4) As the combustion reaction no longer occurs on the furnace surface of the closed calcium carbide furnace, the working environment of the furnace has been improved, the power of the furnace can be increased, and the calcium carbide furnace can develop to a large scale. At present, the capacity of the largely closed calcium carbide furnace in the world has reached 100000kva;

5) The closed calcium carbide furnace is fed continuously and automatically, eliminating the manual feeding on-site;

6) The electrode lifting and pressing are completed automatically in the control room, which can reduce the labor quota and reduce the labor intensity of workers;

7) The closed calcium carbide furnace can control the calcium carbide furnace according to various instruments, and use the instruments for safety detection, which improves the safety of calcium carbide furnace operation and provides the basis for automatic electric furnace operation.

4. Introduction of Energy-saving and Environmentally Friendly Closed Calcium Carbide Furnace

According to the requirements of the production of calcium carbide to meet the needs of sustainable development, and the advanced nature of the equipment, production safety, environmental protection standard discharge, energy-saving, and economic scale requirements, the current new calcium carbide furnace should be large or super large in capacity. The structure should use a closed calcium carbide furnace. Below, the current domestic advanced energy-saving, environmentally friendly closed calcium carbide furnace situation will be introduced.

4.1 Furnace Form of Closed Calcium Carbide Furnace

The furnace body of the calcium carbide furnace consists of a furnace shell and a furnace lining. In the furnace body of the calcium carbide furnace, the charge is melted and reacted to form calcium carbide through its resistance heat and the high temperature emitted by part of the arc. Since the reaction temperature is as high as 2000°C or more, such a high temperature is generally unbearable for refractory materials, so the volume of the furnace body must be larger than the reaction space, that is to say, a layer of charge is left between the reaction zone and the furnace lining to protect the furnace lining. 

There are many shapes of the furnace body, from the shape of the furnace mouth, there are round, oval, square and rectangular. From a thermodynamic point of view, a circular shape is more favorable. In fact, the choice of the furnace shape is determined by the arrangement of the electrode locations and the installation location of the carbon monoxide extraction equipment. It can be said that most of today's calcium carbide furnaces are circular furnaces.

The size of the reaction space in the furnace is determined by the size, distance, and arc action range of the electrodes. The distance between the electrodes is proportional to their diameter, which is a function of the furnace capacity. The electrode diameter is in turn determined by the current density it allows. The current of the electrode is determined by the capacity of the transformer. The final conclusion is that the size of the furnace body is determined by the capacity of its transformer.

Requirements for the furnace shell:

①The strength of the furnace body should be able to meet the severe expansion caused by the heating of the furnace lining, and meet the requirements of thermal expansion and contraction of the furnace lining;

②In the case of meeting the strength requirements, we should strive to save materials and reduce weight;

③Easy to manufacture, the possibility of packaging and transportation should be considered if necessary.

At present, there are two structural forms of furnace shells used in China: one is a cylindrical shell, which is adopted by most factories. The other is a conical furnace shell with a cone angle of 7°. This form of furnace shell has been used in large-capacity closed calcium carbide furnaces in my country before.

4.2 The form of the Furnace Cover of the Closed Calcium Carbide Furnace

The function of the furnace cover is to cover the furnace body to seal the calcium carbide furnace, and the sealing between the furnace cover and the furnace body is sand sealing. The sealing of the furnace body and the furnace cover is to recover all the furnace gas produced in the production of calcium carbide.

The furnace cover is a water-cooled welded structure with refractory knotted on it. There are inspection holes, safety valves, thermometer holes, furnace gas discharge ports, feeding ports, and three-electrode insertion ports on the furnace cover.

4.3 The Structure of the Electrode Device of the Closed Calcium Carbide Furnace

The electrode device of the calcium carbide furnace is an important part of the calcium carbide furnace equipment. The quality of the electrode device directly affects whether the calcium carbide furnace can be produced smoothly. Due to the low production cost and reliable use of the self-baking electrode system, the current calcium carbide furnace electrode system basically adopts the self-baking electrode system. At present, the control of the electrode device of a large calcium carbide furnace mainly adopts the hydraulic system for automatic control. 

Electrode devices are mainly divided into combined gripper-type electrode devices, hydraulic brake-type electrode devices, and steel-belt-holding-press-release electrode devices. The combined handle type electrode device is a new type of electrode device improved on the basis of the introduction of foreign technology in recent years. The electrode device of this structure has been well used in some domestic manufacturers, which fully proves that it is suitable for the production of large calcium carbide furnaces. characteristic. The electrode device with this structure is different from the structure form of the electrode device. In the electrode device of this structure, a plurality of ribs are welded on the electrode shell, and the pressing and conducting of the electrodes are completed by clamping the ribs. It is divided into upper and lower parts.

According to the long-term use, the combination of the handle-type electrode device and other electrode devices has the following remarkable features:

a) The structure is simple, it simplifies the handle and the pressing and releasing mechanism, the use is reliable, and the weight is light;

b) Good interchangeability, the contact device, and the pressing device can be applied to self-baking electrodes of various diameters;

c) The contact resistance of the combined controller is small, the electrical efficiency is high, and it is not easy to catch fire with the motor shell;

d) The electrode shell will not be deformed again;

e) The electrode will no longer lose control during sliding, which increases the safe electrode sliding rate for the smelting process with high power consumption;

f) The electrode firing position is raised due to reduced electrode cooling;

g) Reduced electrode breakage accidents;

h) The service life is long. The service life of the common copper tile type controller is generally short. The block-type holding brake is heavy and easy to wear. The service life of the combined electrode column contact element and the pressure release device can be more than 5 ~ 8 times that of the traditional electrode column.

i) In the electrode conduction system, the contact conductive element and the electrode shell are clamped by springs. When the electrode is pressed and released, the load of the electric furnace does not need to be reduced, but the electrode is pressed and released under the state of the full load of the electric furnace, which greatly improves the calcium carbide furnace. power utilization factor.

Although the combined electrode column has the above advantages, it must be manufactured accurately and debugged according to its characteristics in order to give full play to its advantages.

5. Introduction to Hollow Electrode System of Calcium Carbide Furnace

In recent years, with the development of production technology, hollow electrode system has been developed and applied well, and good economic benefits have been achieved. The hollow electrode system of the calcium carbide furnace directly adds lime powder, coke and anthracite powder as raw materials from the inside of the hollow electrode into the furnace in proportion. The hollow electrode system has the following characteristics:

1) The hollow electrode system of the calcium carbide furnace directly adds the screened powder (lime, coke, and anthracite powder) from the center of the electrode into the furnace to improve the utilization rate of raw materials. The maximum addition rate of powder can reach 30%; (for calcium carbide furnace without a hollow electrode system, the powder is generally treated as waste)

2) Because the electrode is a hollow structure, the electrode roasting effect is good;

3) Quickly adjust the raw material ratio of the dissolving pool and the furnace condition;

4) Since the fine raw materials are uniformly mixed in the furnace and directly enter the reaction zone, it is more conducive to the reaction and reduces the power consumption by about 5%;

5) In carbide production, more coke can be added without causing the electrode to move up, which can provide more favorable reduction conditions and improve the product quality;

6) The use of a hollow electrode system can reduce the consumption of electrode paste and electrode shell by more than 1/3;

6. Gas Analysis Solution for Calcium Carbide Furnace

Calcium carbide (CaC2) is a basic organic chemical raw material, once known as the mother of organic synthesis. The acetylene gas produced from it can replace petroleum products to produce gas raw materials for a series of thousands of organic products, such as acetic acid, vinyl acetate, polyvinyl chloride, polyvinyl alcohol, acetylene carbon ink, etc. As a branch of the coal chemical industry, the carbide chemical industry is not only a viable industry, but also high energy consumption and high pollution industry. 

In the production of calcium carbide, the flue gas from the calcium carbide furnace is the largest pollution source. In order to strengthen environmental protection and governance, realize energy recovery and utilization, reduce production costs, and improve enterprise and social benefits, according to the opinions of the national development and Reform Commission (2007) No. 70 document access conditions for the calcium carbide industry (2007 Revision). The newly built calcium carbide furnace must meet the following four requirements:

(1) Construction of largely closed calcium carbide furnace;

(2) The capacity of a single furnace is more than 25000kVA;

(3) Initial total capacity 100000kva;

(4) Calcium carbide furnace gas must be comprehensively utilized.

The purification and utilization of calcium carbide furnace gas are difficult problems at home and abroad. In order to make full, safe, and effective use of calcium carbide furnace gas, it is necessary to effectively monitor the tail gas of the calcium carbide furnace. There are three main methods for analyzing the tail gas of calcium carbide furnace: Manual (austenitic gas meter) analysis, chromatography analysis and online analysis.

This solution only briefly analyzes the online monitoring of closed calcium carbide furnace gas, and briefly introduces the design of a calcium carbide furnace tail gas analysis system. It is hoped that the tail gas from the newly-built calcium carbide furnace and the old calcium carbide furnace can be fully utilized, so as to realize environmental protection, labor protection, energy recovery, and utilization, reduce production costs and improve enterprise and social and economic benefits.

7. Characteristics of Furnace Gas

In the closed calcium carbide furnace in the production process, the main reaction formula of the product after electrification is: CaO+3C=CaC2+CO

1) Furnace gas volume: 400Nm3/t calcium carbide;

2) Temperature: Normal 400–800°C, (instantaneous) 1000°C.

3) Composition of furnace gas (general)

Element	CO	CO2	CH4	N2	H2	O2	CN-	Tar
Content(%)	70-90	1-10	0.5	1-8	0-10	0.2-0.6	microscale	microscale

4) Dust content and composition

Dust content: 50-150 g/Nm3

Type of dust	Ca2	C	SiO2	Fe2O3	AI2O3	Others
Proportion	37.2	34.1	15.8	0.96	7.1	4.84

7.1 Furnace gas features

(1) The discharge temperature of calcium carbide furnace tail gas is high and unstable;

(2) The furnace gas volume fluctuates and is unstable. The concentration of carbon monoxide in the tail gas is high, about 74-85%, and the temperature is high. It is a flammable and explosive gas;

(3) Tail gas dust content is large and particle size is small, 5 ~ 10 μ M accounts for about 40% of the dust, more than 40 μ M, and up to 150g / Nm3 of soot makes the recovery of tail gas very unfavorable;

(4) The tail gas has a certain calorific value of about 2300-2600 kcal / Nm3;

(5) Tail gas contains a small amount of tar, which adheres to dust after precipitation, which brings inconvenience to low-temperature dust removal.

8. Closed Calcium Carbide Furnace Exhaust Gas Analysis System

The general analysis system can be divided into three parts in terms of structure, namely, the sampling system, pretreatment system, and analysis system. According to the above characteristics of furnace gas, after years of experience accumulation, Tianyu intelligent control has divided the analysis system from the function and designed and manufactured the complete set of the exhaust gas analysis system for closed calcium carbide furnace, which has achieved good application results in the new large-scale closed calcium carbide furnace (50000 T) and the transformation of old calcium carbide furnace. The following briefly introduces the design of the analysis system.

8.1 Selection of sampling points

The selection of sampling points shall meet the following requirements: it can truly and timely reflect the furnace condition parameters concerned by users; The temperature, pressure, and flow at the sampling point are relatively stable; The amount of dust and tar shall be as small as possible. If the sampling point is improperly selected, it will bring an additional burden to the follow-up system, and even can not truly reflect the required furnace condition parameters.

8.2 Design of sampling system

The sampling system is the first gateway of the whole analysis system. Its success will directly lead to the success or failure of the whole analysis system. The temperature of the tail gas sampling point of calcium carbide furnace is about 200-300 ℃, and the relative pressure is between micro positive pressure and micro negative pressure. Due to the particularity of furnace gas, the sampling system must have the ability to preliminarily physically separate the sample components and separate the solid particles around the sampling probe from the gaseous fluid. Therefore, the sampling system should not only have a probe (sampling pipe), but also a filtering system and a probe cleaning system. In addition, It shall also have a structure convenient for maintenance. The sampling probe independently developed by Tianyu intelligent control can be greater than 0.3 μ 99% of dust m is filtered out. This task can be undertaken by using a special sampling probe and a program-controlled automatic purging device.

8.3 Design of pretreatment system

The pretreatment system mainly completes the safe, timely, and effective treatment of the gas samples transmitted from the sampling system to meet the service conditions of the instrument analysis system. Firstly, the design of the transmission pipeline must meet the requirements of aerodynamics to make the sample flow path unobstructed; Next, the sample gas is subject to a series of continuous treatments, such as dust removal, tar removal, and pressure regulation, water removal, flow stabilization, etc. Dry and clean the treated sample gas so that the key components in the instrument analysis system can be accurately analyzed. So as to provide a scientific basis for the production safety of calcium carbide and the effective utilization of furnace gas.

The dust removal system includes filtration, cleaning and re filtration to prevent pipeline blockage; The decoking and oil removal system includes temperature control, washing out, adsorption, etc., which is conducive to smooth pipeline and protect the analysis equipment; The water removal system includes steam water separation, condensation, fog capture, gas water separation, drying and filtration, etc., to protect the analyzer and ensure the accuracy of the test; The pressure adjustment system includes pump, regulating valve, fast circuit, etc. to ensure the normal working conditions of the instrument. Programmable control (PLC) system, mainly including PLC, program software, actuator, man-machine interface, etc., is mainly used to realize automatic control of sampling pretreatment system and alarm in case of abnormal conditions.

8.4 Instrument analysis system

The system includes a flow regulation device, abnormal water inflow prevention device, calibration device, analytical instrument, etc. The analyzer is configured according to the calcium carbide production process and the use of tail gas. The O2, H2, CO, and other gases in the exhaust gas of the generally closed calcium carbide furnace are mandatory items. The content of O2 and H2 is the safety index, and CO is the core of tail gas utilization.

9. Application of a Complete Set of Analysis Systems for Calcium Carbide Furnace Exhaust Gas

The closed calcium carbide furnace is suitable for large-capacity furnace types. After the furnace gas is led out from the furnace, comprehensive utilization can be realized, which not only reduces the smelting energy consumption, but also reduces the huge waste of energy. At the same time, it greatly reduces the direct emission of flue gas, which is conducive to environmental protection. In line with the relevant national industrial policies, it is bound to achieve rapid development. At the same time, there will be higher and higher requirements for its exhaust gas online analysis system. Therefore, the calcium carbide furnace tail gas analysis system should put forward higher requirements in terms of progressiveness, applicability, reliability, stability and safety.

If you are interested in Calcium Carbide Furnace, please mail at [email protected] or [email protected].