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Teaching COGAS Design with Mathcad Abstract

Combined Gas and Steam (COGAS) cycles show great promise for electrical power generation and ship propulsion. In these cycles, fuel is fed to a conventional gas turbine that is usually connected to an electrical generator. The exhaust gas from the gas turbine contains a large amount of usable thermal energy. In a COGAS system, this thermal energy is used to produce steam, and the steam is used to generate additional electrical energy. The thermal efficiency of COGAS systems can approach 60%, which makes them economically attractive.

Students at Webb Institute are learning to design and optimize these cycles, with a particular emphasis on ship propulsion. The course begins with a study of the thermodynamics of COGAS, including optimization analysis. Later in the course, design of the steam turbine is considered. Finally, students design one of the heat exchangers that transfer thermal energy from the gas turbine exhaust to the steam system.

The paper describes the use of Mathcad software to perform the design tasks mentioned above. Use of this tool eliminates the tedium of repetitive calculations that is often associated with complex optimization studies. The course provides an extensive design experience for a system that is on the cutting edge of electrical power generation with particular emphasis on the propulsion of liquefied natural gas (LNG) ships.

Introduction

LNG ships have all been powered by mechanical-drive steam turbines until very recently. The major driving factor has been the ease with which the natural gas that vaporizes from the tanks can be burned in the boilers. Rising fuel prices have motivated the industry to consider propulsion systems with higher cycle efficiencies. The major contenders at present are medium- speed diesel-electric systems and slow-speed diesel-mechanical systems. LNG ships with each of these propulsion systems are already at sea or under construction. With either approach, the boil off gas (BOG) from the tanks can be burned in the engines or reliquified and returned to the tanks.

A third alternative to a steam plant is a gas turbine with or without a waste heat boiler to extract heat energy from the turbine exhaust. The cycle efficiency of a simple gas turbine is rather low, so it is not an economically attractive alternative. However, when the thermal energy of the turbine exhaust is used to make steam, the cycle efficiency can approach 60%. Such systems have been in use at shore-side electrical generating plants for well over a decade.

Gas turbines require high quality fuel, because the combustion products pass directly through the turbine blades. The presence of ash or other abrasive materials in the exhaust will cause rapid wear of the blades. Because natural gas contains no such substances, it is the ideal fuel for a gas turbine. In addition, natural gas fuel causes little or no fouling of heat transfer surfaces in the waste heat recovery system.

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Wiggins, E. (2008, June), Teaching Cogas Design With Mathcad Paper presented at 2008 Annual Conference & Exposition, Pittsburgh, Pennsylvania. 10.18260/1-2--3134