Acetylene is flammable and explosive, with low safety factor, energy consumption in the production process, environmental pollution and high production cost. There are many defects and hidden dangers in production, storage, transportation, use, environmental protection and price, and its development is greatly limited. Therefore, many countries are developing new safer and more energy-saving industrial gases. At the national acetylene production conference, the relevant departments of our country explicitly refused to approve the new construction and expansion of calcium carbide plant and acetylene plant. Existing manufacturers are facing the situation of changing production. As early as the Eighth Five-Year Plan period, China vigorously promoted alkane gas to replace acetylene gas, such as propane gas, propylene gas and natural gas, so as to gradually replace acetylene gas.
In China, 70% of industrial gas is acetylene gas. In the past, acetylene gas was mainly produced in acetylene generator, but it was not allowed to be used (including pipeline type) because of high pollution and insecurity. Bottled acetylene gas is used for industrial cutting. Acetylene is active, explosive and extremely dangerous. When it is mixed with copper, silver and other metals, air and pure oxygen, even if the container has a large diameter, it will cause an explosion. When cutting carbon steel with acetylene gas, it is easy to cause the phenomena of melting the upper edge of the incision, too much slag hanging and local hardening of the cutting surface, which makes the cutting process unsatisfactory. It needs polishing when welding, which increases the production cost. Shipyards in coastal areas have banned the use of acetylene on shipbuilding platforms and switched to other new cutting gases. For many years, people have been trying to replace acetylene with other fuels as cutting gas. However, because the combustion temperature of natural gas, liquefied petroleum gas, propane gas, propylene gas, artificial gas, dimethyl ether and other fuels in oxygen is lower than 2500℃, it is not ideal to use them directly as cutting gas, so it is necessary to add combustion-supporting additives to catalyze, crack and support the mother gas. Change the combustion mode of gas, so as to increase the flame temperature, and make the flame temperature burning in oxygen reach or exceed 3 100℃ of acetylene, thus achieving the purpose of replacing acetylene.
Acetylene has long been an irreplaceable fuel for industrial cutting, welding, flame spraying and other processes. Acetylene plays an irreplaceable role in special cutting, such as the cutting of ductile iron, molybdenum steel, stainless steel and other workpieces. Acetylene has more special advantages than other fuels in the welding process, with simple operation and strong applicability. Flame spraying wire drawing is favored by enterprises because of its fast speed and good quality.
However, with the development of productivity and social progress, human beings pay more and more attention to environmental protection, energy saving, safety and efficiency, and have a clearer understanding of the disadvantages and defects exposed by acetylene gas. In the 1970s, developed countries in Europe, America and Japan began to phase out acetylene gas and replaced it with industrial gases mainly composed of propane, propylene, natural gas, gasoline, coke oven gas and hydrogen.
In order to achieve the use effect of acetylene, we must understand the physical and chemical properties of acetylene, and then we can take corresponding technical measures to realize the substitutability of alkane gas.
The molecular formula of acetylene is C2H2, and the structural formula is HC ≡ CH. According to the hybrid orbital theory, carbon atoms in acetylene molecules participate in bonding through sp hybridization, and two carbon atoms overlap each other with an sp hybrid orbital to form a carbon-carbon σ bond, and each carbon atom overlaps with a hydrogen atom's 1s orbital with an sp orbital to form a hydrocarbon σ bond. In addition, two carbon atoms each have two non-hybridized 2p orbitals perpendicular to each other, and their symmetry axes are parallel to each other, overlapping side by side to form two π bonds perpendicular to each other, thus forming a carbon-carbon triple bond. Two π-bond electron clouds are symmetrically distributed around the carbon-carbon σ bond, which is cylindrical.
Formation of π bond and distribution of electron cloud in acetylene molecule. Modern physical methods prove that all atoms in acetylene molecule are in a straight line, and the bond length of carbon-carbon triple bond is 0. 12 nm, which is shorter than that of carbon-carbon double bond. This is because the electron cloud density between two carbon atoms is higher, which makes the two carbon atoms closer than ethylene. The bond energy of three bonds is only 836.8 kJ mol-1,which is less than the sum of bond energies of three σ bonds (345.6 kJ mol-1× 3). This is mainly because the P tracks overlap laterally, and the degree of overlap is small. The boiling point, melting point and relative density of simple alkynes are generally higher than those of alkanes and olefins with the same number of carbon atoms. This is because alkynes are short, small and slender. In liquid and solid state, molecules can get close together, and van der Waals force between molecules is very strong. Due to the special chemical properties of acetylene, in the combustion process, the heat energy release efficiency is high, the chemical reaction speed is fast, the chemical bond is easy to break, and the flame combustion speed is three times that of propane gas. Therefore, combustion-supporting additives need strong help to decompose the molecules of alkane gas, so as to achieve the purpose of rapid combustion and realize instantaneous temperature rise.
Propane is a by-product of petrochemical industry, which is rich in resources, low in price and pollution-free in combustion. It is a feasible substitute for acetylene. Because of the low flame temperature of propane, the preheating time is relatively longer than that of acetylene, which is a major difficulty in popularization and application. Due to the dispersed heat distribution and low temperature of propane flame, the possibility of metal melting caused by flame is small, so the upper edge of the incision is not easy to collapse, the incision is smooth and flat, and the lower edge of the incision has less slag hanging, which is easy to remove.
Both the flame center and the outer flame of propylene have high heat release, and the heat distribution in the flame center is similar to that in acetylene, and the heat in the outer flame is higher than that in acetylene. Therefore, propylene has both the properties of acetylene flame and the high heat content of propane external flame. The flame temperature is about lower than that of acetylene flame, but higher than that of propane flame, so it is a better cutting gas. The cutting characteristics of propylene flame are: higher flame temperature, longer cutting preheating time than acetylene, but faster than propane. Because of the high heat content of the external flame, it is beneficial to cut thick components.
Liquefied petroleum gas comes from refinery gas, wet natural gas or oil field associated gas. The main components of liquefied petroleum gas obtained from natural gas and associated gas are propane (commonly known as residual liquid), butane, butene and a small amount of pentane. The composition of liquefied gas is complex, the flame is not concentrated during combustion, the heat is unbalanced, the flame temperature is low, the cutting preheating time is correspondingly increased, the cutting speed is reduced and the efficiency is poor.
Natural gas is a multi-component mixed gas, the main components are alkanes, of which methane accounts for the vast majority, and a small amount of ethane, propane and butane. In addition, it generally contains hydrogen sulfide, carbon dioxide, nitrogen and water vapor, and a small amount of inert gases such as helium and argon. Under standard conditions, methane to butane exists in gaseous state, and above pentane is liquid state.
Due to the low calorific value of natural gas, slow combustion speed and low flame temperature, the cutting preheating time is correspondingly increased, the consumption of gas and oxygen is large, and the comprehensive cost is high. When cutting thick steel plates, the total heat and gas consumption are high. In order to maintain the cutting speed, thick components need high external flame heat output, which is easy to widen the slit and enlarge the heat affected zone. When preheating the perforation, it is easy to reverse the slurry or it is difficult to penetrate, which affects the metal surface and requires additives to increase the flame temperature.
Catalytic combustion is a complete oxidation reaction of fuel on the surface of catalyst. In the process of catalytic combustion reaction, the reactants form low-energy surface free radicals on the catalyst surface, produce vibration excited state products, and release energy in the form of infrared radiation; When the reaction is completed, the side reactions that produce toxic and harmful substances are effectively suppressed by the selectivity of the catalyst, and basically no or little pollutants such as nitrogen oxides, CO and HC are produced.
After the combustion-supporting additives are combined with natural gas (liquefied petroleum gas) molecules, it is easier to crack and decompose the molecules, thus changing the properties of gas, changing the wavelength, combustion frequency, combustion speed and enhancing heat energy of gas. In the combustion state, the secondary complete combustion is realized, the generation of harmful substances is reduced, the heat loss is reduced, and the purpose of high temperature catalytic combustion is achieved. The combustion-supporting catalyst itself has good combustion chemical energy, so catalytic combustion is one of the widely used forms.