Utilizing CO2 for Production of CO Rich Synthesis Gas at Low Steam to Carbon Ratio

Introduction

Synthesis gas production is one of the largest industries in the world with its development stemming back from 1930s. Important bulk chemicals as hydrogen, ammonia, and methanol are produced on the basis of this. Haldor Topsoe A/S has many years of experience in reforming, with the first start-up of a steam reformer designed by Haldor Topsoe A/S dating back to 1956.

Reforming of methane with mixtures of steam and carbon dioxide (in the following referred to as CO₂-reforming) is an environmentally interesting process as it in theory offers a way of using CO₂, which in many industries is considered as a waste product and in parallel is a polluting greenhouse gas. Overall, CO₂-reforming is one of few chemical processes which can be designed with an overall negative CO₂-emission, as the process can be designed to utilize more CO₂ than what is produced from the process. The process could therefore play an important role in combination with CO₂ capture technologies, which are receiving much attention currently.

When performing CO₂-reforming with a large import of CO₂, CO will make up a significant fraction of the produced synthesis gas and H₂/CO ratios in the range from 0.5-3 can be produced more or less directly from the process. The large amounts of CO can be used within the chemical industry for production of higher alcohol, synthetic fuels, or acetic acid, among others, but especially the polymer industry has an increasing demand for CO which is an essential component in production of polycarbonates and polyurethane, among others.

One of the primary tasks in the development of CO₂-reforming is to find operating conditions in combination with a suitable catalyst to avoid carbon formation. Carbon formation in a steam reforming plant is dictated by thermodynamics, and in a typical design of a reformer, it is a requirement that there is no affinity for carbon formation anywhere in the catalyst bed. This means that the process gas will have to be balanced with water in order to circumvent the carbon formation area. In the case of the classical fired reformer, the process gas enters the reformer at 400–500°C while leaving the reformer at maybe 950°C (not experiencing temperatures above 1000°C). Thus, when designing a reformer, there must not be an affinity for carbon formation of the equilibrated gas anywhere between 400–1000°C. This criterion can be used to evaluate the carbon limit of the reformer.

Nickel, cobalt, and noble metal catalysts have been tested as catalyst for CO₂-reforming, where nickel has been the most investigated system, as this is a relatively cheap catalyst in comparison to noble metals. Noble metal catalysts typically have a lower affinity for carbon formation and a higher activity for steam methane reforming, however, the markedly higher raw material prices of these material makes them unattractive for extensive use in industrial scale. Consequently, nickel based catalyst are established as the preferred catalyst for industrial use, especially for the conventional steam methane reforming (SMR) reaction, as a Pareto optimality between catalyst price and performance. A large incentive towards continued use of nickel as reforming catalyst therefore also exists.

New high temperature reactor for CO production

In the current work, a new reactor configuration is presented where hot CO₂ is added directly downstream a reformer and equilibrated in an adiabatic reformer, a so-called Adiabatic POst Convertor (APOC). This concept utilizes the high temperature of the principle reformers product gas and detailed understanding of the underlying thermodynamic and kinetic mechanisms to circumvent the carbon formation area. This allow for a method to tailor the product toward practically any H₂/CO ratio without the risk of carbon formation, while still using a cost-efficient nickel based catalyst.

By placing the APOC directly downstream a reformer allows for operating the reformer at its absolute minimum steam to carbon ratio, because no CO₂ will be added to this. This reformer can be practically any known reforming technology including the well-known tubular reformer or the autothermal reformer. Additionally, the introduction of the APOC can be flexibly introduced for both revamps of existing reforming plants as well as new build plants.

Bench scale experiments were done to evidence the operation and the catalyst performance of the APOC. These experiments showed that operation at what would correspond to a steam to carbon ratio of 1 is possible, without the risk of carbon formation. These results were included in process studies, which showed that easy integration of the APOC in a reforming plant can be achieved, where the highlights of the technology is:

• The size of the primary reformer can be reduced by up to ca. 25% by including the APOC. Or the capacity of an existing plant can be boosted accordingly.
• The carbon for the CO product to a larger extent stems from the CO₂ feed and the plant consequently has a large netto use of CO₂.
• No potential for carbon formation is found inside the reactor.
• Practically any H₂/CO ratio can be supplied in the product synthesis gas.

Conclusion

In conclusion, the new high temperature CO production reactor (APOC) is found as a promising technology for producing synthesis gas with a high content of CO at a low practical steam to carbon ratio. The technology can be used to retro-fit an existing production towards more CO production or for boost in capacity, but can also be included in new projects directly and is an excellent match in cases where excess CO₂ is available.