Utilization of calcium carbide off-gas purification
Thread Content
One of the long-standing problems in the calcium carbide industry is the purification and utilization of exhaust gases. Leaving open and semi-enclosed systems aside, the purification of exhaust gases from enclosed furnaces is also a matter of debate; it’s unclear whether it is feasible to use the purified exhaust gases from such furnaces to produce chemical products If the purpose of installing a purification device is merely to burn it in order to utilize its calorific value, then the significance of recycling it is limited.
What are the differences in the exhaust gas composition of open, semi-enclosed, and enclosed furnaces? Is the purification process the same?
The main components of the off-gases from calcium carbide furnaces include carbon monoxide (content: 75%–90%), hydrogen (content: 2%–10%), methane (content: 2%–4%), etc. There are also more than a dozen other components such as sulfides, phosphides, carbides, calcium and magnesium oxides, and coal tar. Furthermore, due to its high outlet temperature (400–800°C) and low gas pressure, both transportation and purification are very difficult. Although methods such as cyclone separation, electrostatic dust removal, and multi-stage bag dust removal can reduce the dust content in the off-gases from calcium carbide furnaces to as low as 20 milligrams per cubic meter, the carbon monoxide content can be increased to over 85%. However, to further separate components such as carbon dioxide, methane, and hydrogen from it in order to turn it into a feed gas that can be used to produce chemical products like synthetic ammonia and methanol, not only is the investment large and the operating costs high, but there are also very few mature technologies available for this purpose. A good purification method nowadays is to first remove a large amount of dust from the calcium carbide furnace exhaust gas using dry dust removal, and then eliminate other impurities and residual dust through a wet spraying process. Some companies have adopted this technology to separate and purify the calcium carbide furnace exhaust gas in order to produce products such as sodium formate. However, two problems remain: first, the coal tar after washing, along with carbides and calcium-magnesium oxides, forms sludge that blocks the pipes; therefore, companies have to shut down the operation from time to time to clean the pipes, which affects the continuous operation of the facility over long periods. Secondly, it is not yet known whether the purified carbide furnace exhaust gas can meet the purity requirements for synthetic raw gas in the production of chemical products such as ammonia and methanol; no industrial-scale facilities have been used to verify this yet. Due to the high purity requirements for feed gas in ammonia and methanol production, even trace amounts of hydrogen sulfide and other impurities can cause catalyst poisoning, preventing the plants from operating properly. Therefore, even if the calcium carbide furnace exhaust can meet the requirements for sodium formate production after purification, it cannot be guaranteed that such purification technology will satisfy the needs of ammonia and methanol production regarding the purification of calcium carbide furnace exhaust. In China, several manufacturers have developed mature technologies for using exhaust gas to heat boilers and generate steam; this technology is now quite advanced.
What are the requirements regarding trace impurities such as S and P in the raw material CO for the production of sodium formate? Are there any specific data requirements in this regard? Are there any industry standards?
The requirements for impurities shall be based on the data provided by the catalyst manufacturer. Under the current operating conditions, the synthetic catalyst requires a sulfur level of less than 0.1 ppm and a chlorine level of less than 1 ppb.
Such technology is now mature.,