Novel Hot Gas Cleaning/Heat Recovery System

Abstract:

<p>Nearly all types of coal gasification based advanced power generation systems under development incorporate hot gas cleaning stages to remove particulates and gas phase contaminants prior to the gas turbine. These hot gas cleaning systems offer significant benefits over conventional wet scrubber clean-up systems. However the development of a continuous fully integrated process, in which gas cooling, sulphur/halide removal, using regenerable sorbents would give substantial benefits.</p> <p>Systems of this type have a number of advantages: the use of regenerable sorbents produces less waste and reduces the operating cost associated with disposal of classified waste products; the fuel gas cooler is located in a benign environment and can therefore be used to generate superheated steam at supercritical conditions yielding a further improvement in cycle efficiency. In addition, the removal of gas contaminants early in the hot gas path will directly improve the environment for downstream components, e.g. hot gas filter parts. On the basis of the expected reduction in the corrosivity of the fuel gas, components' lives may be extended by up to ten times. This benefit would apply to all types of gasifier, including conventional oxygen blown IGCC's where the introduction of hot gas cleaning would otherwise happen downstream of the raw gas cooler and the hot gas filter, both of which would have to operate in a highly aggressive environment.</p> <p>This project was targeted at developing such a novel integrated raw gas cooler and sulphur and halide removal process for gasification plant. The desulphurisation process is based on a twin fluidised bed system employing direct solids transfer between adjacent vessels. Halide removal is achieved by means of sorbent injection.</p> <p>The first stage of the project developed a series of mathematical models for the twin-bed desulphurisation concept. Then a 2-D cold model was designed and manufactured to demonstrate the concepts and the validity of the mathematical models produced. After a series of modifications were carried out and their effects assessed, a twin bed unit was designed and manufactured that was capable of being used initially as a 3-D cold model and then being retrofitted to an existing atmospheric pressure gasifier. The 3-D unit functioned as anticipated as a cold model, demonstrating the expected particle flux between the twin beds, and also showed that there were low gas leakage rates between the two beds. After being retrofitted to an existing atmospheric pressure gasifier, the twin bed unit was used to demonstrate the effect of sulphur sorbent on real gasifier derived fuel gases. Limestone, a well known sulphur sorbent in oxidising atmospheres and reducing atmospheres, was selected to test the effectiveness of the twin bed unit in this project. The twin bed was operated with the outlet gases from the gasifier in one side (absorber side) and with air in the regeneration side of the system. Several operating conditions and variables have been studied in the system: gas velocity, bed temperature. The use of the limestone sorbent in the twin-bed reduced the H2S level in the fuel gas stream under all the conditions investigated.</p> <p>The twin bed system seems to be a promising technology for a heat exchanger system, due to the good particle flows between the two fluidised beds, and for the reduction in contaminant emissions. However, further work is required to improve the understanding of the twin-bed hydrodynamics, as well as to develop sorbents with operating temperatures that are compatible with the twin-bed concept. Two options for the twin bed system have been suggested as worth pursuing as viable use of this technology in gasification plant design. The first involve a twin-bed gasification-heat exchange system where gas from a gasifier is fed to one vessel and heat is transferred to a second by means of re-circulating solids. The second option is a triple-bed adsorption-regeneration-heat exchange system, where the gas from the gasifier is fed to a vessel and the H2S is removed. Catalyst/sorbent is transferred to a second bed for regeneration, and solids are transferred to a third vessel where heat is removed.</p> This report is divided into the following sections: <ol> <li>Introduction and Background</li> <li>Aims and Objectives</li> <li>Reactor Design</li> <li>Pilot Scale Desulphurisation Studies</li> <li>Halide Removal Studies</li> <li>Scale-Up Issues</li> <li>Conclusions</li> <li>Recommendations</li> <li>References</li> <li>Meanings of Symbols</li> </ol>

Author(s):

Grasa, G., Wellman, R.G., Kilgallon, P., Simms, N.J. and Oakey, J.E.