The heat treatment of coal and coke with steam and oxygen is called solid fuel gasification. The combustible gas obtained by gasification is generally called coal gas, which is used as the initial raw material gas for methanol production. The main equipment of gasification is gas producer. According to the movement mode of coal in the furnace, gasification methods can be divided into fixed bed (moving bed) gasification, fluidized bed gasification and entrained flow gasification.
In China, fixed-bed intermittent gasification method is generally used to produce methanol from coal and coke, and UCJ furnace is used as gas furnace.
The ratio of hydrogen to carbon in the crude feed gas made of coal and coke is too low, so after gas desulfurization, excess carbon monoxide is converted into hydrogen and carbon dioxide, and then excess carbon dioxide is removed by decarbonization. The raw gas is compressed, synthesized and refined to produce methanol.
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1, methanol from natural gas
Natural gas is the main raw material for methanol production. The main component of natural gas is methane, but it also contains a small amount of other alkanes, olefins and nitrogen. There are several methods to produce methanol from natural gas, such as steam reforming, catalytic partial oxidation and non-catalytic partial oxidation, among which steam reforming is the most widely used.
It is carried out under normal pressure or normal pressure in a tube furnace. Because the heat absorption of the reaction must be supplied from the outside to maintain the required conversion temperature, it is generally realized by burning part of fuel gas between tubes, and the steam for conversion is directly generated on the device by the heat of flue gas and conversion gas.
In the synthesis gas produced by steam reforming of natural gas, there is excessive hydrogen, but the amounts of carbon monoxide and carbon dioxide are insufficient. One way to solve this problem in industry is to adopt carbon dioxide steam reforming, and carbon dioxide can be supplied from the outside or recovered from the flue gas of the reformer.
The other method is a two-stage conversion method with natural gas as raw material, that is, the first stage carries out steam conversion of natural gas, only reacting about 1/4 methane, and the second stage is partially oxidized. Not only the proportion of synthesis gas obtained is appropriate, but also the amount of residual methane can be reduced because the reaction temperature in the second stage is raised above 800℃, which increases the effective gas components of methanol synthesis.
Natural gas should be purified to remove harmful impurities before entering the steam reforming furnace, and the sulfur content of purified natural gas should be less than 0. 1mL/m3. The converted gas is compressed to the synthesis section to synthesize methanol.
2. Extracting methanol from petroleum
There are mainly two kinds of oils used for methanol production in industry: naphtha and heavy oil.
The fraction below 220℃ obtained by crude oil rectification is called light oil, also known as naphtha. The methods of producing synthesis gas from naphtha include pressurized steam reforming, catalytic partial oxidation, pressurized non-catalytic partial oxidation, intermittent catalytic reforming and so on. The main method of producing methanol feed gas from naphtha is pressurized steam reforming.
Pressurized steam reforming of naphtha needs to be carried out in a reformer with complex structure. The reformer is equipped with a radiation chamber and a convection chamber, and the hydrocarbon steam reforming reaction is carried out at a high temperature in the presence of a catalyst. After steam reforming, the composition of naphtha can just meet the needs of methanol synthesis. There is no need to add carbon dioxide before and after the conversion or to set up two-stage conversion, and there is no need to adjust its composition through transformation and decarbonization.
Heavy oil is a product in the process of petroleum refining. According to different refining methods, it can be divided into atmospheric heavy oil, vacuum heavy oil, cracked heavy oil and their mixtures. There are two methods to produce methanol feed gas from heavy oil: partial oxidation and pyrolysis. Cracking method needs to crack heavy oil in a heat storage furnace at a high temperature above 1400℃. Although oxygen is not needed, the equipment is complicated and the operation is troublesome, resulting in a large amount of carbon black.
Partial oxidation of heavy oil refers to the combustion reaction between heavy hydrocarbons and oxygen, which releases heat, so that some hydrocarbons are thermally cracked, and the cracked products are further oxidized and reformed, and finally synthesis gas with H2 and CO as the main components and a small amount of CO2 and CH4 is obtained, which is used for methanol synthesis.
The synthetic gas produced by partial oxidation of heavy oil has a high hydrocarbon ratio, and the contents of carbon monoxide and carbon dioxide in the synthetic gas exceed the standard. It is necessary to convert part of the synthesis gas, so that carbon monoxide reacts with water vapor to generate hydrogen and carbon dioxide, and then carbon dioxide is removed to achieve the composition required for methanol synthesis.
Synthetic crude methanol needs refining to remove impurities and water, so as to obtain refined methanol.
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