(685f) Industrial Scale CO2-Reforming for Production of CO-Rich Synthesis Gas at Low Steam to Carbon Ratio

1. Introduction

Important bulk chemicals such as hydrogen, ammonia, and methanol are typically produced in a multiple step process. The first step is normally conversion of natural gas, or a similar feedstock, to produce hydrogen or synthesis gas (a mixture consisting of mainly hydrogen and carbon monoxide). This is followed by the actual synthesis and purification. The synthesis gas production step is often carried out by reforming of the feedstock with mixtures of steam and carbon dioxide (henceforth referred to as CO₂-reforming).

CO₂-reforming is an environmentally interesting process as it offers a way of utilizing CO₂, which is a polluting greenhouse gas, and in many industries often is considered as a waste product. CO₂-reforming is a process which can be designed with overall negative CO₂-emissions, or in other words can be designed to utilize more CO₂ than what is produced. It is therefore expected to play an important role in combination with CO₂ capture technologies.

An increase in the amount of CO₂ added to the process will result in an increased fraction of CO in the produced synthesis gas. In principle, synthesis gas with H₂/CO ratios in the range 0,5 – 3 can be produced. These CO-rich gasses are typically utilized in the production of functional chemicals.

2. CO₂-reforming in industry

When talking CO₂-reforming, it is critical to define optimal operating conditions, in combination with a suitable catalyst to avoid carbon formation. Carbon formation in a typical steam reforming plant is dictated by thermodynamics. In the typical reformer design, no affinity for carbon formation is allowed anywhere in the catalyst bed. In Fig. 1 the carbon limits are illustrated for various feed compositions with methane, steam, and CO₂.

In most cases, Nickel-based catalysts are used in steam reformers. In the case of CO₂-reforming over a Nickel-catalyst, the process gas will have to be balanced with large amounts of steam to circumvent the potential for carbon formation. This in turn will lead to larger, inefficient reformers. Alternative technologies exist in the market, such as noble metal based catalysts which have a lower affinity for carbon formation and a higher activity for CO₂-reforming. However, high raw material prices make this alternative unattractive for extensive use on an industrial scale. Therefore, there is incentive to continuously develop new improved concepts which allow for the continued use of Nickel-based catalysts for CO₂-reforming.

3. New high temperature reactor for production of CO-rich synthesis gas

A new reformer technology has been developed, where preheated CO₂ is added directly downstream of a main reformer and then equilibrated in an adiabatic reactor. Supported by a detailed understanding of the underlying thermodynamic and kinetic mechanisms, the technology utilizes the high temperature of the synthesis gas from the main reformer to circumvent carbon formation. This is illustrated in Fig. 2 for an example where the main reformer is operated at steam to carbon ratio of 1.0.

In theory, this technology can be utilized to tailor a synthesis gas to practically any H₂/CO ratio, using a cost-efficient nickel based catalyst, while maintaining a low steam to carbon ratio in the main reformer – all this, without the risk of carbon formation. In principle the adiabatic reactor may be added downstream of any reformer type and can be used for both grassroot and revamp cases.

Bench scale experiments have been carried out to demonstrate carbon free operation with the new reactor technology at significantly lower steam-to-carbon ratios than is possible with stand-alone CO₂-reforming.

A series of economic and process calculations have been carried out to illustrate the benefits of the new technology. Selected results include:

• The size of a steam reformer can be reduced by up to ca. 20%;
• The synthesis gas composition is shifted towards CO with high selectivity relative to the converted CO₂;
• The solution can be tailored to produce synthesis gas covering a large range of H₂/CO ratios.

4. Conclusion

In conclusion, the new high temperature CO production reactor is a promising solution for producing synthesis gas with a high content of CO at a low steam to carbon ratio. The technology can be used to retro-fit an existing unit towards more CO production or included in new projects. It is an excellent match in cases where excess CO₂ is available. In the presentation, the new technology will be described and the benefits will be illustrated with specific examples.