Gasification syngas

Syngas is a feed gas consisting primarily of carbon monoxide and hydrogen, used as a raw material in the chemical industry. The range of feedstocks for syngas is extensive; it can be produced by the gasification of solid fuels such as coal or coke, or by the conversion of light hydrocarbons such as natural gas and naphtha, and can also be produced from heavy oil via the partial oxidation process. Syngas requires a variety of different raw materials and by-products, such as coal, biomass, petroleum or biofuels, which are converted into usable syngas through a partial oxidation process.

In today’s ongoing search for low-cost alternative fuel sources, gasification is a technology that is steadily gaining widespread adoption.

There are various methods for producing syngas

Prior to the Second World War, syngas was primarily produced from coal; after the war, liquid hydrocarbons (petroleum processing fractions) or gaseous hydrocarbons (natural gas) with higher hydrogen content were mainly used as feedstocks. Since the 1970s, coal gasification has regained prominence; the emergence of new technologies and various large-scale plants suggests that the proportion of coal in syngas feedstocks is likely to increase in the future. However, syngas is primarily produced from hydrocarbons, with the two main methods being steam reforming and partial oxidation.

Steam reforming

This method utilises natural gas or light oil as feedstock, which reacts with water vapour to produce synthesis gas. In 1915, A. Mitas and C. Schneider produced hydrogen by reacting steam with methane-rich natural gas over a nickel catalyst. In 1928, the Standard Oil Company of America was the first to design a small-scale steam reformer to produce hydrogen. During the Second World War, this method began to be used to produce feed gas for synthetic ammonia.

Natural Gas Steam Reforming

The main reaction is:

The key process parameters are temperature, pressure and the steam-to-feed ratio. As this is a strongly endothermic reaction, increasing the temperature increases the equilibrium constant, bringing the reaction closer to completion. An increase in pressure reduces the equilibrium conversion rate. However, as natural gas is already under pressure, and syngas requires a certain pressure during post-treatment and the synthesis reaction, pressurising the natural gas prior to conversion is more economically advantageous than doing so afterwards. Consequently, pressurised operation is widely adopted, whilst the amount of steam is increased to improve the methane conversion rate. A high steam-to-carbon ratio also prevents coking on the catalyst.

At a temperature of 800–820 °C, a pressure of 2.5–3.5 MPa, and a molar ratio of H₂O/C of 3.5, the composition of the conversion gas (by volume %) is: CH₄ 10, CO 10, CO₂ 10, H₂ 69, N₂ 1.

To implement the steam reforming of natural gas on an industrial scale, two methods may be employed: continuous reforming and batch reforming.

① Continuous steam reforming process: This is the primary method currently used for syngas production. Natural gas, blended with 0.25 per cent to 0.5 per cent hydrogen, is heated to 380–400 °C before entering a desulphurisation vessel packed with a cobalt-molybdenum hydrogenation catalyst and a zinc oxide desulphurisation agent, where hydrogen sulphide and organic sulphur are removed, reducing the total sulphur content to below 0.5 ppm. After steam is introduced into the feed gas, it enters the convection section of the reformer at 400 °C, where it is further preheated to 500–520 °C. It then flows from top to bottom through the various reforming tubes, each containing a nickel catalyst, where it is further heated and undergoes the reforming reaction to produce synthesis gas. The conversion tubes are housed within the conversion furnace and heated by natural gas burned in burners fitted to the furnace roof or side walls (see Natural Gas Steam Reforming Furnace). As the conversion tubes must withstand high temperatures and pressures, centrifugally cast high-alloy stainless steel tubes containing 25 per cent chromium and 20 per cent nickel are required. Although the continuous conversion process necessitates the use of these expensive conversion tubes, its overall energy consumption is lower, making it a technically and economically superior method for producing synthesis gas.

Syngas

② The batch steam reforming process is also known as the regenerative steam reforming method. It utilises periodic, intermittent heating to supplement the reaction heat required for the natural gas reforming process. The process can be divided into two stages: firstly, the preheating (temperature rise and heat storage) stage, in which a portion of the natural gas is first used as fuel to undergo complete oxidation with an excess of air in a combustion furnace, producing high-temperature flue gas at around 1,300 °C. This flue gas passes through the first and second heat recovery furnaces before entering the reformer, where it flows from top to bottom through the catalyst bed, allowing the catalyst to absorb some of the heat. Simultaneously, the residual oxygen in the flue gas reacts with the nickel metal in the catalyst, releasing a large amount of heat and further raising the bed temperature. The flue gas exits the reformer at approximately 850 °C and is vented after heat recovery. This is followed by the gas production stage: natural gas and water vapour (with air added if synthetic ammonia is being produced) are preheated to approximately 950 °C in the regenerative heaters before entering the catalyst bed for the steam reforming reaction. The gas emerging from the catalyst bed has a temperature of approximately 850 °C; after heat recovery, it is stored in the syngas holder. China previously utilised batch steam reforming furnaces to construct a number of small-scale synthetic ammonia plants. These plants did not require expensive alloy steel reforming tubes; their main equipment consisted of cylindrical reforming furnaces lined with refractory materials, which were simple in structure and inexpensive to construct. The disadvantages were that they operated at atmospheric pressure, the equipment was bulky, they required a large footprint, and operating costs were relatively high. Internationally, this method is also used for the production of town gas.

Light Oil Steam Reforming

This process was developed by the British company BUNMEIN Chemical Industries in the 1950s, with the first plant commissioned in 1959. The main reaction in this process is:

In many respects, it is similar to the steam reforming of natural gas. The C/H ratio is relatively high; moreover, as the feedstock contains not only alkanes but also aromatics and even small amounts of olefins, carbon deposits are prone to form and precipitate, necessitating the use of a catalyst resistant to carbon precipitation. Generally, nickel catalysts are still employed, with potassium oxide as the co-catalyst and magnesium oxide as the support. Light oil generally contains higher sulphur levels than natural gas, and as this catalyst is highly sensitive to sulphur, rigorous desulphurisation and simultaneous hydrogenation are required prior to steam reforming. Desulphurising cracked light oil is extremely difficult, and it is therefore rarely used to produce synthesis gas. Straight-run light oil is used to produce synthesis gas. As light oil is relatively expensive and has the aforementioned drawbacks, the production of synthesis gas from light oil feedstock is only developed in regions where there is a shortage of natural gas supply.

Partial Oxidation The main reaction in the steam reforming of natural gas or light oil is highly endothermic; the heat required for the reaction is supplied by burning natural gas or other fuels outside the reaction tubes. The partial oxidation process, however, integrates the reactions inside and outside the tubes into a single process. This method does not require pre-desulphurisation, and the structural materials for the reactor are less expensive than those used in the steam reforming process. Furthermore, a key advantage is that it is not selective regarding feedstock; it can be applied to almost any liquid or gaseous hydrocarbon, ranging from natural gas to residual oil.

Partial oxidation of natural gas

By adding an insufficient amount of oxygen, some of the methane is combusted to form carbon dioxide and water:

This is a strongly exothermic reaction. At high temperatures and in the presence of water vapour, carbon dioxide and water vapour can undergo an endothermic reaction with the remaining unburned methane:

Consequently, the main products are carbon monoxide and hydrogen, whilst the final combustion product, carbon dioxide, is produced in only small quantities. To prevent carbon precipitation during the reaction, a certain amount of water vapour must be added. This also enhances the reaction between water vapour and methane.

Partial oxidation of natural gas can be carried out either in the presence of a catalyst or without one.

① Non-catalytic partial oxidation: Natural gas, oxygen and water vapour are fed into a conversion furnace lined with refractory material at a pressure of 3.0 MPa or higher for partial combustion at temperatures as high as 1300–1400 °C. The composition of the gas leaving the furnace (by volume %) is approximately: CO 25, CO 42, H₂ 52, CH₄ 0.5. The reactor is of the self-heating, insulated type.

② Catalytic partial oxidation: Desulphurised natural gas reacts with a certain amount of oxygen or oxygen-enriched air, as well as water vapour, in the presence of a nickel catalyst. When the catalyst bed temperature is approximately 900–1000 °C and the operating pressure is 3.0 MPa, the composition of the gas leaving the converter (by volume %) is approximately: CO 27.5, CO 25.5, H₂ 67, CH₄ < 0.5. The reactor also employs an autothermal adiabatic design, which offers high thermal efficiency. The reaction temperature is lower than that of the non-catalytic partial oxidation process.

Partial oxidation of heavy oil

Various types of heavy oil, including atmospheric residue, vacuum residue and fuel oils derived from deep petroleum processing, are commonly used feedstocks in partial oxidation. The representative reaction is as follows:

The reaction products are primarily carbon monoxide and hydrogen. The reaction conditions are: 1200–1370 °C, 3.2–8.37 MPa; no catalyst is required, and approximately 400–500 kg of steam is added per metric tonne of feedstock. The steam acts as a gasifier, whilst also buffering the furnace temperature and suppressing soot formation. The outlet gas from this type of reactor is rapidly cooled directly with water. The disadvantages of this method are: ① it requires oxygen or oxygen-enriched air, which necessitates the installation of a separate air separation unit; ② the resulting gas has a higher carbon monoxide-to-hydrogen ratio than that produced by the steam conversion process; ③ the partial oxidation of heavy oil results in the formation of carbon black, which not only increases consumption but also affects the subsequent processing and utilisation of the synthesis gas. Carbon black is removed using oil absorption, and the carbon black and absorption oil are then recycled back to the gasifier.

New Coal Chemical Industry

The new coal chemical industry primarily utilises advanced, high-efficiency coal gasification technologies to produce synthesis gas. Compared with traditional coal chemical processes, this synthesis gas features higher pressure, lower inert gas content and easier removal of impurities, making it suitable for a wider range of applications.

Coal gasification processes are divided into three main categories: fixed-bed gasification, fluidised-bed gasification and aerated-bed gasification. Each gasification technology has its own advantages and disadvantages, imposes specific requirements on the quality of the feed coal, and varies in terms of process sophistication and technical maturity.

Aerated-bed gasification technology

Pressurised fluidised-bed gasification technology predominantly utilises pure oxygen as the gasifying agent, with the gasification process taking place under high temperature and pressure. The raw syngas has a high content of effective gases (CO + H₂) and a high carbon conversion rate, whilst producing no tar, naphthalene or phenolic water, making it an environmentally friendly gasification technology. Fluidised-bed gasification technology is primarily divided into water-coal slurry gasification and pulverised coal gasification.

Syngas can be used for a variety of applications, such as power generation, chemical manufacturing and even to power turbines. Monitoring the various stages of these processes is vital for ensuring proper combustion, maintaining efficiency and identifying unknown and potentially undesirable by-products. Process mass spectrometers can provide real-time analysis for all types of gasification.

Process mass spectrometers have long been the analytical system of choice for producers of syngas—including ammonia, methanol, ethanol and hydrogen. They can be rapidly cycled between all gasification streams, such as gasifier composition, sulphur removal, CO conversion, CO₂ removal and numerous end-product processes. They are also suitable for laboratory or pilot-scale reactor studies of syngas reactions.