Brayton Power
Many conventional gas-turbine power-generation systems employ the so-called Brayton power cycle.
Air is compressed, mixed with fuel and burned in a combustor, and then expanded through a turbine to produce useful work. Almost 2/3rds of the expansion work drives the compressor. Only the remaining 1/3rd is used to generate electric power.
Drawbacks in This Cycle
- The exhaust gas exits at a high temperature. Thus a large amount of energy is wasted.
- To maintain the temperature of the gas entering the turbine at an acceptably low level (1100°C-1500°C), a very large amount of excess air—over and above the amount of oxygen required for combustion–needs to be fed into the process through the compressor. This air absorbs some of the excess heat produced in the reactor. But this extra air has to be compressed and later expanded in the turbine. Some efficiency is lost through this. The most important effect is to greatly increase the size and cost of the compressor to more than half of the total capital cost of the hardware.
- With high excess air, pollutant gases, mainly CO and NOx are often produced at unacceptably high levels.
- The temperature of the gas leaving the combustor is difficult to control accurately. This causes operational problems in controlling the overall system.
- At off-peak loads the temperature of the gas entering the turbine drops, reducing efficiency. This cyclic temperature variation further increases corrosion and maintenance.
- At off-peak electrical loads, the efficiency of the cycle falls off quite rapidly. Since such power generation systems often operate at less than peak load, this has an adverse effect on the average cost of the electricity produced.
How do we improve the performance of such a process? By clever use of water and steam injection at one or more points in the cycle.
