Discussion on Carbon Reduction Technologies for Ammonia Synthesis via the Kellogg Process

Lao Haichuan, I just happened to see a fertilizer factory being transformed into a brewery – it’s a great idea indeed. You’re referring to synthetic ammonia, right? If that’s the case, why not simply mix the flue gas with fresh air over a certain distance?

This post was last edited by Huang Xinhui on 2021-8-17 at 15:41. You’re right; it’s the gas-based ammonia synthesis process. The Caloger process is the most typical method for producing ammonia using natural gas as a raw material. As for why the flue gas is not sent directly into a Section 1 furnace, my opinion is that the reasons are as follows: Burning in a Section 1 furnace requires oxygen, and air is the most convenient option – it has a reliable source, controllable costs, and is technically feasible. Can that smoke be fed in to continue burning? It’s not impossible either, as the oxygen content in flue gas is generally kept around 2%, so there is still some oxygen in the flue gas available for combustion. However, because there is a large amount of nitrogen (nitrogen oxides) in the flue gas, part of it can be returned for recycling as a means to reduce nitrogen oxides; this is scientifically known as \"low-nitrogen combustion and flue gas recirculation\". The purpose is to reduce nitrogen oxide emissions. The carbon emissions from ammonia synthesis using gas as a feedstock are primarily associated with the combustion of fossil fuels for heat extraction outside the reactor. The ways to achieve low-carbon development are either to eliminate a portion of the heat extraction outside the furnace or to recycle the flue gas. Among these, there are mature and industrialized technologies for eliminating or reducing external heat extraction, namely heat-exchange conversion; some sources seem to refer to it as two-stage conversion. As for the recovery and reuse of flue gas, the common practice is to wash, absorb, regenerate, and dry the flue gas, and then use it after pressurization. This is also a mature technology; its process flow, technical equipment are not very different from those of traditional MDEA absorption. My idea is to replace part or all of the flue gas with air and introduce it into the second furnace. One of the purposes of the two-stage conversion is to introduce the nitrogen required for ammonia synthesis, as there is a large amount of nitrogen in the flue gas. At the same time, the flue gas also contains water vapor and carbon dioxide produced by combustion. Water vapor itself is a material required for the synthesis of ammonia as well as for various conversion reactions; at the same time, it enables the reuse of carbon dioxide that would otherwise be emitted, achieving two goals with one action. The main characteristics of flue gas from a furnace are its large volume, low pressure, and relatively complex composition. By comparing the material balance diagrams for ammonia synthesis using gas heads, it was found that it is not possible to achieve equilibrium in terms of ammonia carbon, ammonia nitrogen, and the secondary furnace without making significant adjustments to the existing production processes and material balances. To achieve this, it is necessary to completely redesign and reevaluate the entire conversion system (including processes such as primary conversion, secondary conversion, and transformation). One more thing to add: carbon supplementation (which generally involves two approaches – pre-supplementation and post-supplementation) is quite common in methanol production, and there is a lot of relevant information as well as practical experience available on this topic; those interested can find numerous resources online. Personally, I believe that the prospects for low-carbon development—or rather, a revolutionary transformation—in the ammonia synthesis industry (nitrogen fertilizers/chemical fertilizers) lie in how to creatively rely on green hydrogen technology to develop and utilize green hydrogen resources. At the same time, it’s crucial to also make use of carbon dioxide captured and sequestered by other high-carbon industries, thereby facilitating the transition from a high-carbon to a low-carbon model. More importantly, this approach can serve as an effective means and valuable tool for reducing carbon emissions.

In the primary reformer, steam is added to the feed gas; the purpose of the secondary reforming is, among other things, to introduce the nitrogen required for ammonia synthesis. The main gas exiting the secondary reformer is carbon dioxide. Hydrogen, nitrogen, and various impurity gases are all removed in the conversion and decarburization stages; only hydrogen and nitrogen end up reaching the syngas compressor. Carbon dioxide is sent to the urea production stage

Another thing: has the combustion gas from that furnace been desulfurized?

Feasibility: From the perspective of energy utilization, the flue gas temperature in a single-stage reformer is relatively high, generally ranging from 700 to 1,000°C. This portion of high-temperature flue gas contains a large amount of thermal energy; sending it into the secondary reformer allows for the effective utilization of this heat. In a two-stage converter, the steam conversion reaction of methane is an endothermic reaction; the high-temperature flue gas can provide the heat required for this reaction, reducing the need for additional fuel combustion and thereby lowering production costs to some extent. For example, in traditional processes, two-stage converters need to burn fuels such as natural gas to maintain the reaction temperature. If the heat from a portion of the flue gas is utilized, this amount of fuel can be saved. From a chemical reaction perspective, the flue gas from the primary furnace contains a certain amount of reducing gases such as hydrogen and carbon monoxide. Upon entering the two-stage converter, these gases can participate in the methane conversion reaction as well as subsequent shift reactions (), thereby improving the utilization rate of the feedstock. For example, hydrogen can react with oxygen in the two-stage furnace to produce water vapor, and the water vapor can then react with methane to facilitate the formation of the raw gas used for ammonia synthesis. From the perspective of process integration, considering the entire ammonia synthesis process, feeding the flue gas from the first furnace into the second furnace can simplify the process flow. It reduces the pressure on the flue gas emission treatment process, and enhances the integration of heat and materials within the entire system, which helps to improve the overall efficiency of the system. There is a problem related to the complex composition of the gases: in the flue gas from a certain type of furnace, in addition to useful components such as hydrogen and carbon monoxide, there are also large amounts of carbon dioxide, nitrogen, water vapor, and other components. The presence of carbon dioxide may have an adverse effect on the catalysts in the two-stage converter, such as causing catalyst poisoning or a decrease in their activity. For example, some ammonia synthesis catalysts have limited tolerance to carbon dioxide; high concentrations of carbon dioxide can cover the active sites of the catalyst, thereby affecting the reaction rate and conversion efficiency. The large influx of nitrogen will change the gas composition inside the two-stage furnace, which may affect the equilibrium and selectivity of the reaction. Because in the ammonia synthesis reaction, the ratio of nitrogen to hydrogen must be strictly controlled; too much nitrogen will interfere with the reaction equilibrium from shifting in the direction of ammonia production. Pressure matching issue: The pressure of the flue gas from the first stage is usually not matched with the operating pressure of the second-stage converter. The flue gas pressure in the primary reformer is relatively low; whereas, to ensure the efficient progression of the reaction, the secondary reformer generally operates under higher pressure. To feed flue gas from one stage of the furnace to another, it is necessary to increase its pressure; this requires additional pressure-raising equipment and also consumes extra energy. For example, the pressurization process requires equipment such as compressors, whose investment and operation/maintenance costs are high, and the operation of compressors also consumes a large amount of electrical energy. The challenge of flow control: The flow rate of flue gas in a certain section of the furnace varies depending on the operating conditions of that section (such as the flow rate, temperature, pressure, etc., of the feed gas). It is quite difficult to steadily feed it into the secondary furnace and control the flow rate. If the flow rate is too high, it may cause issues such as excessive temperature in the secondary furnace and pressure fluctuations, affecting the stability of the reaction as well as the safety of the equipment ； If the flow rate is too low, the desired effects of energy savings and improved raw material utilization cannot be achieved. Safety risks increase as flue gases from certain furnaces contain combustible gases such as hydrogen and carbon monoxide. If a leak occurs during transportation and entry into the second furnace, there is a risk of explosion. Moreover, high-temperature flue gas itself poses safety risks such as burns, requiring higher standards for the safety protection of operators and equipment.

In terms of feasibility, heat supply: Flue gas from a furnace has a high temperature and contains a large amount of thermal energy. Feeding it into the second-stage furnace can provide additional heat for the reactions taking place there, helping to maintain the reaction temperature in that furnace. This reduces the amount of air that needs to be added to reach the desired reaction temperature, and thus lowers the energy consumption associated with air compression. Utilization of hydrogen: The flue gases from the first-stage furnace usually contain a certain amount of combustible gases such as hydrogen. Feeding these gases into the second-stage furnace allows them to participate in the reactions there, further improving the efficiency of using the raw materials. It also reduces, to some extent, the amount of air required for burning the hydrogen through the introduction of air. Streamlining the process: From the perspective of system integration, by directly using the flue gas from the first furnace in the second furnace, it is possible to reduce equipment and processes related to air preheating, thereby making the entire ammonia synthesis process more compact and simplified, and reducing equipment investment and operating costs. Challenges and limitations: Complex gas composition: In addition to hydrogen, the flue gas from the first furnace also contains carbon dioxide, water vapor, nitrogen, as well as small amounts of other impurity gases such as carbon monoxide. These impurity gases, once they enter the second-stage furnace, can have an adverse effect on the reactions taking place there; for example, they can alter the equilibrium conditions of these reactions, reduce the activity and selectivity of the catalysts, and thus affect the yield and quality of ammonia synthesis. Pressure matching issue: The pressure of the flue gas from the first-stage furnace is usually low, while the operating pressure in the second-stage furnace is relatively high. Pressurizing the flue gas from the first-stage furnace and sending it to the second-stage furnace requires additional energy to overcome the pressure difference; moreover, the investment and operating costs of the pressurization equipment must also be taken into account. If the pressure is too high, it may also cause certain components in the flue gas to condense or undergo other physicochemical changes, further affecting the stability and performance of the system. Difficulty in flow control: Accurately controlling the flow rate of flue gas from one furnace section to another is key to achieving stable operation. Excessive flow rates may cause overly intense reactions in the two-stage furnace, exceeding the design parameters and affecting the equipment’s lifespan and safety ； If the flow rate is too low, it will not be possible to achieve the desired reduction in air consumption and lower power usage. Additionally, since the volume and composition of the first-stage furnace flue gas are influenced by various factors—such as the composition of the feed gas and the operating conditions of the first-stage furnace—it is rather difficult to achieve precise flow control. Risk of catalyst poisoning: Certain impurity components in the flue gas may poison the catalyst in the secondary furnace, thereby reducing its activity and lifespan. For example, impurities such as sulfides may undergo chemical reactions with the active components in the catalyst, rendering them inactive and thereby affecting the progress of the ammonia synthesis reaction. Therefore, before sending a stream of flue gas from the first furnace to the second furnace, it is necessary to thoroughly purify the gas in order to remove harmful impurities from it; however, this increases the complexity and cost of the system.

Both of the above responses are generated by intelligent AI and have not been altered in any way. For your reference. I have always believed that, from the perspective of carbon reduction, this approach is worth in-depth study and exploration.