Dr. Boyce was one of the pioneers of the Externally Fired Steam Injected Gas Turbine. A 500 kW turbine was developed by Dr. Boyce under a Department of Energy Contract in 1979. The concept was to develop a turbine that could operate on any type of fuel without reducing the life of the hot section of the gas turbine. The project was developed to burn coal slurry or wood chips which could be combusted in a specially constructed gas turbine combustor.
The combustor used for an external-combustion gas turbine is similar to a direct-fired air combustor. The goal in any combustor is to achieve high temperatures with a minimum pressure decrease of the compressed air, and minimal pollution. The combustor consisted of a rectangular box with a narrow convection section at the top where steam was produced from the exhaust gas. The outer casings of the combustor consist of carbon steel lined with lightweight blanket material for insulation and heat re-radiation.
The inside of the combustor consists of wicket-type coils (inverted "U") supported from a larger-diameter inlet pipe, and a return header running along the two lengths of the combustor. The combustor can have a number of passes for air. The one shown in the Figure has four passes. Each pass consists of 11 wickets, giving a total of 44 wickets. The wickets are made of different materials, since the temperature increases from about 300oF to 1,700oF. Thus the wickets can range from 304 stainless steel to RA330 at the high-temperature ends. The advantage of the wicket design is that the smooth transition of "U" tubes minimizes pressure drops. The U-shaped tubes also allow the wicket to freely expand with thermal stress. This feature eliminates the need for stress relief joints and expansion joints. The wickets are mounted on a rollaway section to facilitate cleaning, repairs, or coil replacement after a long period of use.
A horizontally fired burner is located at one end of the combustor. The flame extends along the central longitudinal axis of the combustor. In this way the wickets are exposed to the open flame and can be subjected to a maximum rate of radiant heat transfer. The tubes should be sufficiently far away from the flame to prevent hot spots or flame impingement.
The air from the gas turbine compressor enters the inlet manifold and is distributed through the first wicket set. A baffle in the inlet prevents the air flow from continuing beyond that wicket set. The air is then transferred to the return header and proceeds further until it encounters a second baffle. This arrangement yields various passes and helps to minimize the pressure drop due to friction. The air is finally returned to the end section of the inlet manifold and exits to the first stage nozzles of the gas turbine.
The combustor was designed for handling preheated combustion air. Preheated combustion air is obtained by diverting part of the exhaust from the gas turbine to the combustor. The air from the turbine is clean, hot air. To recover additional heat energy from the exhaust flue gases, a steam coil is placed in the convection section of the combustor. The steam is used for steam injection into the compressor discharge or to drive a steam turbine. The flue gas temperature exiting from the combustor should be around 600oF, and out of the exhaust after passing the steam coils was about 250ºF.
Internals of an External Fired Gas Turbine Combustor
The initial start up of the externally fired gas turbine is very complex. Unique concepts of start-up for the external gas turbine were developed by Dr. Boyce. The prototype turbine was operated under the DOE contract for over 100 hours.