312 Combustion

Three alternate combustor design configurations — silo-type, can-annular, and annular — are employed in power GTs. The can-annular design drew on the early experimental combustor can experience of the jet turbine, even though jets eventually selected an annular combustor. Can-annular design can be shipped integral to the unit, allowing for internal temperature profiles. GTs have the capability of burning a variety of fuels including natural gas, blast furnace gas, coal gas, distillate fuel, residual fuel, etc. However, burning fuels that have potentially corrosive elements requires that fuel contaminant specifications be set. In addition, more stringent emissions regulations have limited the application of these alternative fuels. Water and steam injection approaches provided an interim solution to reducing emissions, paving the way for dry, low NOx approaches. Some manufacturers are active in developing advanced dry, low emission systems for their turbines utilizing one or more of three combustion design approaches: pre-mixed/lean combustion, rich-lean, or catalytic combustion.

The main components of a typical combustion system are the torch ignition system, a dual-fuel injection system, and an annular combustor. Ignition within the main combustor is achieved with a jet of hot gas produced by an auxiliary torch that uses turbine air and fuel. This torch is capable of operation on either liquid or gaseous fuel and uses a low-energy spark plug. When liquid fuel is used, it is atomized with high-pressure air from an external source. The high temperature of the exit gas stream from the torch provides a source of very high energy for the main combustor light-up and thus provides reliable ignition over the range of ambient conditions in which the turbine operates.

Combustion efficiency is affected by evaporation rates for liquids and chemical reaction rates for both gaseous and liquid fuels. Most industrial GTs achieve high reaction rate and evaporation rate efficiencies. A stable, turbulent flow pattern in the primary zone is also required for high combustion efficiency. Important parameters that determine combustion efficiency are combustor volume, combustor operating conditions (mass flow rate, pressure, and primary zone temperature), fuel spray dispersion by droplet size, and fuel evaporation rates.

The temperature distribution at the combustor exit is the combustion system's most important characteristic and has to be developed to optimize the life of the downstream nozzle guide vanes and turbine blades. Because the vanes are fixed relative to the combustor, they must be designed to accept the hottest localized temperature that will be experienced at the combustor exit; however, the turbine blades feel the circumferentially-averaged temperature at any radius. Therefore, the design of the stationary vanes represents the most severe cooling challenge in the turbine, and it is essential to be able to define a non-dimensional parameter which represents the peak measured exhaust temperature.

In a well-developed, practical combustion system, the pattern factor is invariably reduced by approximately 12% during operation using No. 2 diesel when compared to natural gas. However, the radial temperature distribution factor remains essentially the same when either of the two fuels is used. Pattern factor optimization is carried out using natural gas fuel because there is no deterioration of the parameter when the GT is switched to liquid fuel operation.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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