Fundamental Study On A Novel Technology Of CaC₂ Production From Fine Coke And Fine CaO

CaC2 can be produced by the reaction of coke and calcium oxide, 3C+CaO=CaC2+CO. CaC2 has been an important coal chemical product and platform chemical for production of many organic chemicals, including acetylene and acetylene derived products. It was once regarded as the mother of organic synthesis industry. The CaC2 production technology used today did not evolve significantly in the past and is characterized by moving bed furnace, granular feed and electric arc heating. Because of poor contact between the large feed particles (5-30 mm), the reaction of coke and CaO is severely controlled by solid-solid mass and heat transfer, which inevitably led to high operating temperature (2000-2200℃), long reaction time (1-2 h), as well as low productivity (less than 70 kt/a). All of these resulted in high energy consumption, about 3250 kW-h per ton of CaC2 with a purity of 80% corresponding to a thermal efficiency of about 50%. Nevertheless, CaC2 plays an important role in economic development of China, with a rapid increase in production in past decade and an annual production of over 15 Mt in 2009. It is obvious that sustainable development of China calls for significant reduction in energy consumption in CaC2 production, because an increase in thermal efficiency to 80% would lead to a saving of 1200 kWh/t, corresponding to 5 Mt coal per year at the production level of 2009. To achieve such a large energy saving, novel method for CaC2 production has to be developed.Inspired by the trend of technological advancements in coal combustion and gasification in past decades, i.e. from moving bed technologies with granular feed to entrained flow technologies with pulverized feed to intensify the rates of transport and reaction, this dissertation studies fundamental phenomena of a new CaC2 production method characterized by pulverized feed and oxygen-fuel heating. The researches include influence of particle size of coke and CaO on CaC2 formation, chemical reactions, the phase changes and diffusion behavior involved in CaC2 production, effects of coke property and minerals inherent in coke on CaC2 formation, and process simulation. Important conclusions obtained are:1. The initial CaC2 formation temperature decreases with a decrease in feed particle size. For example, a decrease in the feed particle size from 5.00 mm (the lower limit required by the electric arc method) to 0.022 mm (similar to that for pulverized coal gasification) leads to a decrease in the initial CaC2 formation temperature from about 1927℃ to 1480℃. This behavior indicates not only remarkable energy saving but also a possibility of autothermal (oxygen-fuel) production of CaC2.2. The initial CaC2 formation temperature decreases linearly with an increase in contact points between the feed. This indicates that the initial formation of CaC2 is controlled by mass transfer between the solid coke and CaO, and the smaller the feed particle the more the contact points between them, and the faster of the mass transfer rate. A coke of lower graphitization also leads to a decrease in the initial CaC2 formation temperature, indicating suitability of using fine biochars for CaC2 production with the new method.3. At temperatures higher than 1480℃CaC2 formes at the interface of C and CaO. At temperatures higher than 1695℃eutectics consisting of CaC2 and CaO may be formed, which penetrates into or even coats over the coke particles. This may indicate formation of CaC2 via Kirkendall effect of CaO because 1480℃is higher than the Tammann temperature of CaO (1150℃). This also suggests that the two-step mechanism involving CaO+C=Ca+CO and Ca+2C=CaC2 for CaC2 production reported in literatures is hypothetical. This agrees which the literature that showed impossibility of the reaction CaO+C=Ca+CO at temperatures lower than 2000℃through thermodynamic calculation. The formation of the eutectics may increase mass transfer rate of CaO into the coke due to increased mobility of CaO and increase in contact surface between the coke and CaO. This change in mass transfer mode is supported by the decrease in activation energy of CaC2 formation at temperatures higher than 1695℃.4. The eutectics formed is chemically unstable due to a reaction of CaC2+2CaO=3Ca+2CO. However, this reaction is not observable under conditions with excess amounts of C. This suggests the involvement of Ca diffusion in CaC2 production and high reactivity of Ca with C. The latter is important for keeping Ca from loss through evaporation.5. CaC2 decomposes to yield Ca and C and the rate is controlled by surface evaporation of Ca. This disagrees with the zero-order mechanism proposed in the literatures.6. The main minerals in the feed, such as SiO2 and Al2O3, influence CaC2 production due to formation of Ca-Si-Al-O, which is less reactive with C than CaO.7. The autothermal CaC2 production with fine feeds proposed in this dissertation is superior to the prevailing electric-arc method due to low temperature operation (about 400℃lower) and the possibility of using entrained flow reactors. Computer simulation using Aspen Plus shows a thermal efficiency of higher than 80% for the new process compared with about 50% for the electric-arc process, corresponding to an energy saving of more than 37%.