The State of Iron-Redox Sulfur Plant Technology New Developments to a Long-Establishesd Process Technology

By Douglas L. Heguy and Gary J. Nagl(Merichem)
846 Algonquin Road Suite A100
Schaumburg, IL 60173

Key Words: Iron-redox, LO-CAT®, hydrogen sulfide, chelates, chelated iron, sulfur


The iron-redox process has enjoyed commercial success for over 25 years, generally in applications requiring sulfur removal capacity below 20 tons per day. Key process benefits include high H2S conversion efficiency, significant turndown flexibility, and ability to treat a wide range of gas compositions, and environmentally innocuous process and products. The process has also been know to consume expensive chemicals, produce “low-value” sulfur, and plug. This paper reviews the status of the technology, explains how the operating issues are being addressed in commercial practice, and provides a glimpse of improvements that are in the final stages of development.


The family of liquid redox processes that has been developed since the 1920’s is best represented, currently, by the “iron-redox process” or “chelated-iron” process. This technology has served its clients well for more than 25 years. Units typically achieve 99.9+% H2S removal efficiency, treat a wide variety of gas types over a wide variety of operating conditions, have substantial turndown capability on H2S concentration and gas flow and produce innocuous products and by-products. No wonder more than 200 such units have been licensed around the world – a technical and commercial success by almost any definition!

The iron-redox technology is typically applied to gas streams requiring less than 20 tons per day sulfur removal capacity, unless operating conditions limit use of other sulfur plant technologies, such as Claus. In such cases, Iron-redox may still be the best sulfur removal technology. Highly variable gas and low H2S concentration are examples. Iron-redox plants as large as 80 tons per day are in commercial use.

Also well known are the operating issues that have been associated with this process, such as high chemical cost, chemical degradation, plugging, foaming, production of “low-value” sulfur, inability to treat high-pressure applications.

How are the leading developers of this technology addressing these operating issues? What improvements are on the horizon? With 25 years of commercial application, is iron-redox technology still competitive in today’s commercial environment?

The answers are clear. The iron-redox process technology has been improved continuously over the last 25+ plus years. Numerous testimonials confirm that the technologies/solutions described in this paper are being successfully applied in commercial applications. As further evidence, there continues to be significant commercial activity for this well-developed, well-proven technology in applications for natural gas and associated gas processing, geothermal plants, refinery fuel gas, municipal odor control, landfill gas, and recently, municipal waste gasification, as well as a host of others. Furthermore, improvements in the final stages of development will benefit the users of this process technology in the near-term future, and ensure its long-term commercial viability.

This paper will discuss the benefits of H2S removal, provide a basic description of iron-redox technology, a description of the solutions being commercially employed to successfully address past and current operating issues, and, finally, explore the innovations that are on the horizon.