(155f) Modeling and Optimization of a Modified Claus Process As Part of An Integrated Gasification Combined Cycle (IGCC) Plant with CO2 Capture

Authors: 
Bhattacharyya, D., West Virginia University
Turton, R., West Virginia University
Zitney, S. E., National Energy Technology Laboratory, U.S. Department of Energy


The modified Claus process is one of the most common technologies for sulfur recovery from acid gas streams.  In the case of an Integrated Gasification Combined Cycle (IGCC), the feeds to the Claus process are the off-gas from the sour water stripper (SWS) and the acid gas from the stripper of the acid gas removal (AGR) unit.  Because of the stringent environmental regulations considered in this work, the tailgas from the Claus process is further treated in a hydrogenation unit, compressed from about 1 atm to 50 atm, and recycled back to the AGR unit for recapture.  Important design criteria for the Claus unit, when part of an IGCC power plant, are the ability to destroy ammonia completely and deep sulfur recovery from a relatively low purity acid gas stream without sacrificing flame stability.  Due to these criteria, modifications are often required to the process, resulting in a modified Claus process.  For studies discussed here, these modifications include the use of a 95% pure oxygen stream as the oxidant, a split flow configuration, and preheating of the feeds with the intermediate pressure steam generated in the waste heat boiler (WHB).

In the future, for IGCC plants with CO2 capture, the Claus unit must satisfy the emission standards without sacrificing the plant efficiency in the face of typical disturbances of an IGCC plant such as rapid change in the feed flowrates due to load-following and wide changes in the feed composition because of changes in the coal feed to the gasifier. The Claus unit should be adequately designed and its control design should be appropriately implemented to satisfy these objectives. A representative dynamic model is needed for these studies. Even though the Claus process has been commercialized for decades, most papers treat the key reactions in a Claus process as equilibrium reactions. Such models may be validated by manipulating the temperature approach to equilibrium for a set of steady-state operating data, but are of limited use for dynamic studies.

In a Claus process, especially in the furnace and the WHB, a number of reactions take place along with the widely-considered Claus reaction. In this work, a set of linearly independent reactions has been identified and kinetic models of the flame zone, anoxic zone, WHB, and catalytic reactors have been developed. Various approaches are taken to derive the kinetic rate expressions which are either missing in the open literature or found to be inconsistent. A set of plant data is used for optimal estimation of the kinetic parameters while a set of plant data is used for the model validation. The final model agrees well with the published plant data.

Using the developed kinetics models of the Claus reaction furnace, WHB, and catalytic stages, sensitivity studies are completed to examine whether important process variables could be controlled in the face of the typical process disturbances from the IGCC plant.  Additionally, a study is carried out to examine the optimal operation of a Claus plant considering three, highly interactive process variables in the context of an IGCC power plant with CO2 capture.  The first variable is the single pass conversion of H2S in the Claus unit. Because of a very high operating pressure of the AGR process in this IGCC plant, considerable compression cost is associated with recycling the unconverted tail gas. The second variable is the flow of hydrogen to the AGR unit from the Claus unit. Hydrogen sent to the AGR unit can be converted to electric power more efficiently by the gas turbine than burning it in the Claus furnace and raising steam. The third variable is the consumption of oxygen in the Claus unit. Considerable operating cost is incurred in the air separation unit for producing oxygen. An optimization study examines the tradeoffs of these process variables along with the net power requirement of the Claus unit.