Requirements of the methanation reaction for CO2

This gas composition can be methanized, but the space velocity of the catalyst needs to be lower; it is recommended that the volumetric space velocity not exceed 10,000.

I work in methanation technology; I’ll try to answer your questions. 1. The CO2 content in the feed gas is very high, reaching 10%; the remainder is hydrogen along with 10% CO. Can such a gas be methanized? What impact does it have on the catalyst? And what is the final conversion rate of CO2 to CO? Theoretically, methanation can take place as long as the hydrogen-to-carbon ratio meets the requirements of the reaction; the molar ratio for the reaction between CO and hydrogen is 3, while it is 4 for CO2 and H2. In this case, the components are 10% CO2, 10% CO, and 80% H2. Even with a small amount of inert gas present, H2 is still in excess, so methanation can occur. Moreover, the carbon conversion rate will be very high. As for the issue of high CO2 content, some coke oven gas projects deliberately add carbon to the methanation section. In coal-to-gas projects, the CO2 concentration is generally below 2% due to the use of a low-temperature methanol washing unit. However, in methanation reactions, it is the total carbon content (CO + CO2) that matters, as it determines the amount of heat released. A total carbon content of 20% is similar to that seen in coal-to-gas methanation processes; the total carbon content in the raw materials used for such processes is usually around 21%–23%, with most of it being CO. As for the respective proportions of CO2 and CO in the total carbon, they have little impact on the total heat release; the reaction of CO occurs more rapidly than that of CO2, resulting in slightly higher heat release. In short, it’s possible to carry out methanation using this raw material composition, and with a slight excess of hydrogen, the carbon conversion rate will be very high, possibly exceeding 99%. 2. If the ratio of CO2 to CO is adjusted according to the reaction equation along with the amount of hydrogen used, what level of purity can the resulting methane achieve? After the reaction ratio is set, the extent of methanation reaction is also influenced by factors such as reaction pressure, reaction temperature, and the amount of water separated. The methane concentration in the methaneation products of coal-to-natural gas production, as they exit the methaneation unit (containing saturated water), is generally >95%; after downstream drying, it can reach 97%–98%. In the bid for a coal-to-gas project last year that utilized the methanation technology we developed, the client required that the hydrogen content be reduced to the lowest possible level. By lowering the reaction temperature and increasing the amount of water used, we managed to reduce the H2 content to around 1%; as a result, the methane concentration was high, at about 97%, and it became even higher after drying. For your reference, feel free to reach out if you have any questions regarding methanation~~ haha

May I ask if the requirement by some clients to keep the hydrogen content at 1% refers to the product gas from the methanation unit?

Let me share my thoughts on this. Based on Topsoe’s process package, as long as the purity of the fresh gas in the syngas is high enough, with low levels of inert gases such as nitrogen and argon, and the methane content reaches 99%, there should be no problems. It refers to the content after methane is synthesized and cooled to remove water; the hydrogen content can be less than 0.5. Therefore, the owner’s requirement for this must have come from hearing it from someone else. It’s possible that it was introduced during discussions with Topso or other technology providers. For specific details, please consult a professional.

Thank you for the introduction. In other words, when methanation occurs, it’s fine if the CO2 level is high. As long as there is enough hydrogen, it doesn’t matter if the CO2 content is three times that of CO; the carbon conversion rate can be ensured, and the process will not differ significantly from conventional coal-to-natural gas processes. Can it be understood this way?

The component mentioned by the original poster can definitely be methanated, with a purity that will not be low

Yes, it can be achieved at the outlet of the methanation unit. This project does indeed have high requirements, but economic considerations must also be taken into account. If there are gas power plant users downstream, the H2 content is required to be less than 3%; however, if it is to be used for city gas, there is no need to reduce the hydrogen content to such a low level, as this would result in a decrease in the volume of the product gas. If payment is based on volume rather than calorific value, it isn’t worth it.

Yes, in fact, in coal-to-natural gas methanization processes, in order to slow down the rate of heat release, a conversion unit is sometimes added before the methanization step, in order to reduce the CO concentration and increase the CO2 concentration. Since CO2 reacts more slowly than CO and releases less heat, this approach is more conducive to controlling the temperature of the entire system. There are examples of foreign companies using this method in domestic projects. It can even be said that a higher proportion of CO2 is actually beneficial for temperature control in the entire system, and it has no impact on the carbon conversion rate. Consider that in the final product gas, the CO2 concentration is less than about 1%, with almost no CO present; all of this carbon ends up as CH4, so the conversion rate is not affected~

Oh, thank you, I see. I was asking how our company manages to keep the hydrogen content in the product gas at 1-2.5%