【Synthetic Ammonia】How high is the natural gas consumption for producing synthetic ammonia through two-stage conversion?

For two-stage conversion, the natural gas consumption per ton of ammonia produced is likely to be as described. The consumption of feed gas should remain more or less unchanged, while the fuel gas consumption is likely to be relatively high. Traditional single-stage converters are of the external heating type, making it difficult to improve their thermal efficiency; this is why energy savings are a key focus at present. As a result, some process suppliers are inclined to abandon traditional single-stage converters in favor of developing new conversion devices, such as heat-exchange type converters, low-stage and high-stage conversion systems, and so on!

Here, we have a lower conversion rate in the first stage and a higher conversion rate in the second stage; 20% more air is supplied to the second-stage furnace

Reply to 3# shfuchenko: Is it convenient to use?

The gas consumption for two-stage conversion should be around 860 cubic meters. The figure provided by Liu Hua for two-stage conversion using heat exchange technology is around 760 cubic meters; however, this method relies heavily on external steam supply, and its stability is far inferior to that of two-stage conversion.

The two-stage conversion process is quite convenient; the gas consumption should be around 860 cubic meters, which is a roughly accurate figure.

Our factory also has a system that uses two-stage conversion, with gas consumption of less than 900 cubic meters; it is not dependent on external steam supply. If only heat-exchange conversion is used, then dependence on external steam supply becomes inevitable. However, if a separate boiler is installed, that’s a different situation

Our plant has two units with two-stage conversion, and the gas consumption at full load is around 860 cubic meters.

Reply to 1# lxq700918: In the two-stage natural gas conversion process, the natural gas consumption per ton of ammonia produced is between 850 NM3 and 860 NM3. The conversion catalyst has good activity; when the conditions in other processes are favorable, this value can drop to 845–850 NM3. Generally, however, it is not possible to reach the designed value of 830–840 NM3. The energy-saving principle of the two-stage conversion process is to utilize the high-temperature gases from the second-stage furnace as a heat source for the methane steam conversion reaction in the heat-exchange type conversion furnace; thus, no natural gas needs to be burned for the methane conversion in that section ； The reduction in its natural gas consumption represents the amount of natural gas that is not burned, thus representing a savings ； But from another perspective, the gas produced in the second stage is meant to be used in the conversion reactor to generate steam. By using this heat for the methane steam conversion reaction, the amount of steam produced by the conversion reactor will inevitably decrease ； In the traditional stage 1 and stage 2 conversion processes, the steam used for the methane conversion reaction can achieve self-equilibrium, and there is still a need to supply steam to outside the system ； The steam in the two-stage conversion process cannot balance itself; it requires external supply of 3.2 MPa steam to make up for the deficiency in the steam generated internally. Generally, 400–500 Kg of steam is needed per ton of ammonia. From another perspective, the natural gas saved through the two-stage process is actually obtained in exchange for steam; however, at current natural gas prices, it is profitable to use steam in exchange for natural gas. In other words, the total cost per ton of ammonia produced via the two-stage process is still lower compared to the traditional one-stage and two-stage processes.

Reply to 5# liujianguo1116: The conversion process you mentioned, Liu Hua’s process, is likely a full heat-exchange conversion process. This type of process features a preheating furnace and a startup heating furnace. The preheating furnace is used primarily for preheating desulfurized natural gas and the vaporization mixture, as well as for preheating air or oxygen. It also plays an important role in generating steam from the flue gases in the furnace itself (by using additional burners there), which is then supplied to the conversion system. The start-up heater is used for secondary preheating of the vapor-air mixture and air when air or oxygen is introduced, in order to raise the temperature forcibly; during normal production, this heater also operates at a low flame level. The vaporized mixture first enters the converter, and then proceeds to the second-stage furnace. The high-temperature gas emerging from the second-stage furnace goes into the shell of the converter to heat the conversion tubes inside it; after cooling down, it enters the waste gas boiler. With this process, natural gas consumption can be kept below 800 NM3, but it is relatively difficult to operate; if external steam is available, it makes control easier ； A small device weighing tens of thousands of tons can operate normally without external steam supply ； Driving it is very difficult; first, it is necessary to turn on the auxiliary burner to generate steam before any further operations can be carried out. This process is commonly used in methanol plants with capacities of tens of thousands of tons, but I’m not familiar with larger-scale plants; therefore, I don’t know whether it’s possible to reduce costs in such large plants or whether they can operate stably.

It is also related to measurement; generally, the value is within 860. In cases of special operations with unstable conditions, the value may be higher, but it still remains within 900 in most cases.