What Is Synthesis Gas?

Syngas uses carbon monoxide and hydrogen as main components and is used as a raw material gas for chemical raw materials. Syngas has a wide range of raw materials. It can be produced by gasification of solid fuels such as coal or coke. It can also be produced from light hydrocarbons such as natural gas and naphtha. It can also be produced from heavy oil by partial oxidation.

A raw material gas with hydrogen and carbon monoxide as main components for chemical synthesis. From carbon-containing minerals such as coal, petroleum, natural gas and coke oven gas, refinery gas, sludge and biomass. Biomass and sludge also generate a large amount of synthesis gas during pyrolysis or gasification. They are distinguished from the gas components formed. According to the different sources, compositions and uses of synthesis gas, they can also be called coal gas,

The production and application of syngas has an extremely important position in the chemical industry. As early as 1913, ammonia production from syngas has begun, and now ammonia has become the largest tonnage of chemical products. Methanol produced from synthesis gas is also an important large-tonnage organic chemical product. In 1939, the acetylene hydrocarboxylation process developed in Germany was once produced

Syngas can produce a range of chemicals

Before World War II, synthesis gas was mainly produced from coal; after the war, liquid hydrocarbons (petroleum processing fractions) or gaseous hydrocarbons (natural gas) with higher hydrogen content were mainly used as raw materials. Since the 1970s, coal gasification has been valued again. New technologies and various new large-scale installations have appeared one after another, showing that the proportion of coal in syngas raw materials may increase in the future. There are mainly steam conversion and partial oxidation.

Syngas steam reforming

This method uses natural gas or light oil as a raw material and reacts with water vapor to produce syngas. In 1915, A. Mitas and C. Schneider used steam and natural gas, mainly methane, to react with nickel catalysts to obtain hydrogen. In 1928, American Standard Oil Company first designed a small steam reformer to produce hydrogen. During the Second World War, this method began to produce synthetic ammonia feed gas.

Syngas natural gas steam reforming

The main reactions are:

Syngas

The main process parameters are temperature, pressure and water vapor ratio. Because this reaction is a strong endothermic reaction, increasing the temperature can increase the equilibrium constant and the reaction tends to be complete. Increased pressure reduces equilibrium conversion. But because natural gas is under pressure, the synthesis gas also needs a certain pressure in the post-treatment and synthesis reaction. It is economically more advantageous to pressurize the natural gas before the conversion than the post-conversion pressurization. Therefore, the pressurization operation is generally used while increasing the amount of water vapor To increase methane conversion. High water vapor usage also prevents carbon buildup on the catalyst. In addition to the above main reactions, the following reactions occur:

Both reactions are exothermic.

At a temperature of 800 to 820 ° C, a pressure of 2.5 to 3.5 MPa, and a H ₂ O / C molar ratio of 3.5, the composition (volume%) of the reformed gas is: CH ₄ 10, CO10, CO ₂ 10, H ₂ 69, N ₂ 1.

Syngas

In order to realize the natural gas steam conversion reaction in industry, two methods of continuous conversion and batch conversion can be adopted.

Continuous steam reforming process This is the main production method of existing syngas (Figure 1). When natural gas is mixed with hydrogen of 0.25% to 0.5% and heated to 380 to 400 ° C, it enters a desulfurization tank filled with a cobalt-molybdenum hydrogenation catalyst and a zinc oxide desulfurizing agent to remove hydrogen sulfide and organic sulfur to make the total sulfur content Reduced to below 0.5ppm. The raw material gas is mixed with water vapor and enters the convection section of the reformer at 400 ° C. It is further preheated to 500-520 ° C, and then enters each of the conversion tubes equipped with nickel catalyst from top to bottom. The tubes are continuously heated to perform the conversion reaction To generate syngas. The conversion tube is placed in the conversion furnace, and the natural gas is supplied by a burner mounted on the top or side of the furnace for heating (see Natural Gas Steam Conversion Furnace). The conversion tube is subject to high temperatures and pressures, so a centrifugally cast high alloy stainless steel tube containing 25% chromium and 20% nickel is required. Although the continuous conversion method requires such an expensive conversion tube, the total energy consumption is low, and it is a technically and economically superior method for producing syngas.

Syngas

Syngas

The intermittent steam reforming process is also called regenerative steam reforming. Periodic intermittent heating is used to supplement the reaction heat required for the natural gas conversion process (Figure 2). The process can be divided into two stages: the first is the blowing (heating, heat storage) stage: a part of the natural gas is first used as a fuel to perform a complete oxidation reaction with the excess air in the combustion furnace, generating high-temperature flue gas at about 1300 ° C, The regenerative furnace enters the conversion furnace and passes through the catalyst layer from top to bottom, so that the catalyst absorbs a part of the heat. At the same time, the oxidation reaction between the residual oxygen in the flue gas and the metal nickel in the catalyst releases a lot of heat, which further increases the bed temperature. When the flue gas comes out of the bottom of the reforming furnace, it is about 850 , and it is evacuated after recovering heat. Then comes the gas-making stage: the natural gas and water vapor as raw materials (plus air if synthetic ammonia is produced) are preheated to about 950 ° C by a regenerator and enter the catalyst bed for steam conversion reaction. The temperature of the gas coming out of the catalyst bed is about 850 ° C. After the heat is recovered, it is stored in the syngas cabinet. China has used batch steam reformers and built a number of small ammonia plants. These plants do not

Syngas

An expensive alloy steel conversion tube is used. Its main equipment is a cylindrical conversion furnace lined with refractory materials. The structure is simple and the construction cost is low. Disadvantages are normal pressure operation, large equipment, large area, and high operating costs. There are also internationally used to produce city gas. ^[1]

Syngas light oil steam reforming

It was developed by the British Bunner Chemical Industry Corporation in the 1950s, and the first factory was completed in 1959. The main reaction of this method is:

Syngas

It is similar in many ways to natural gas steam conversion. The C / H ratio is relatively high, and in addition to alkane, there are aromatic hydrocarbons and even a small amount of olefins, which are easy to form carbon and precipitate. Therefore, a catalyst against carbon evolution must be used. Generally, a nickel catalyst is still used, and potassium oxide is used as a co-catalyst, and magnesium oxide is used as a support. The sulfur content of light oil is generally higher than that of natural gas, and this catalyst is very sensitive to sulfur. Therefore, before steam conversion, it is necessary to strictly desulfurize and hydrogenate at the same time. Desulfurization of cracked light oil is very difficult and is rarely used to make syngas. Used to make syngas is straight-run light oil. Due to the high price of light oil and the disadvantages mentioned above, only in areas where natural gas supply is lacking, the development of synthesis gas based on light oil raw materials is developed.

The main reaction of partially oxidized natural gas or light oil steam conversion is a strong endothermic reaction. The heat required for the reaction is supplied by burning natural gas or other fuel outside the reaction tube, while the partial oxidation method integrates the reaction inside and outside the tube. This method does not require pre-desulfurization, and the reactor structural material is cheaper than the steam conversion method. In addition, the main advantage is that regardless of the raw material, almost any liquid or gaseous hydrocarbon from natural gas to residue can be used.

Syngas natural gas partial oxidation

Add insufficient oxygen to burn part of the methane to carbon dioxide and water:

Syngas

This reaction is a strongly exothermic reaction. In the presence of high temperature and water vapor, carbon dioxide and water vapor can undergo endothermic reactions with other unburned methane:

So the main products are carbon monoxide and hydrogen, while the final product of combustion is not much carbon dioxide. In order to prevent carbon precipitation during the reaction, a certain amount of water vapor needs to be added. This also strengthens the reaction between water vapor and methane.

Partial oxidation of natural gas can be carried out in the presence or absence of a catalyst.

Non-catalytic partially oxidized natural gas, oxygen, and water vapor enter the refractory-lined conversion furnace for partial combustion at a pressure of 3.0 MPa or higher. The temperature is as high as 1300 to 1400 ° C. : CO ₂ 5, CO42, H ₂ 52, CH ₄ 0.5. The reactor is self-heating and adiabatic.

Catalytic partial oxidation uses desulfurized natural gas to react with a certain amount of oxygen or oxygen-enriched air and water vapor under a nickel catalyst. When the temperature of the catalytic bed is about 900-1000 ° C and the operating pressure is 3.0 MPa, the gas composition (volume%) of the converter gas is about: CO ₂ 7.5, CO 25.5, H ₂ 67, and CH ₄ <0.5. The reactor is also self-heating and adiabatic, with high thermal efficiency. The reaction temperature is lower than the non-catalytic partial oxidation method. ^[1]

Syngas heavy oil partial oxidation

Various heavy oils, including atmospheric residues, vacuum residues, and fuel oils obtained from deep processing of petroleum, are all commonly used raw materials in partial oxidation.

Syngas

The sexual response is:

The reaction products are also mainly carbon monoxide and hydrogen. The reaction conditions are: 1200 to 1370 ° C, 3.2 to 8.37 MPa, no catalyst is used, and the amount of water vapor per ton of raw material is about 400 to 500 kg. Water vapor acts as a gasifier, at the same time, it can buffer the furnace temperature and inhibit the formation of char. The outlet gas of this reactor is quenched directly with water. The disadvantages of this method are: oxygen or oxygen-enriched air is required, that is, a separate air separation device is needed; the generated gas has a higher ratio of carbon monoxide to hydrogen than the steam conversion method; This not only increases consumption, but also affects the next step in the processing and use of syngas. A method for removing carbon by oil absorption, and recycling the carbon and absorption oil back to the gasifier.

Syngas new coal chemical industry

The new coal chemical industry mainly uses advanced and efficient coal gasification technology to produce syngas. Compared with traditional coal chemical industry, syngas has high pressure, low inert gas content, and easy removal of impurities.

Coal gasification technology is divided into three categories: fixed-bed gasification technology, fluidized-bed gasification technology, and entrained-bed gasification technology. Each gasification technology has its own advantages and disadvantages. Requirements, the advanced nature of the process, the degree of technological maturity also vary. ^[2]

Syngas entrained bed gasification technology

The entrained gasification technology of the entrained bed uses pure oxygen as the gasifying agent. The gasification process is completed at high temperature and pressure. The crude gas has a high content of effective gas (CO + H ₂ ), a high carbon conversion rate, and does not produce tar, naphthalene and phenol. Water, etc., is an environmentally friendly gasification technology. The entrained bed gasification technology is mainly divided into coal water slurry gasification technology and pulverized coal gasification technology.