Q2Power Generation Sources
Question
Q.2. Describe open cycle gas turbine plants.
Answer
An open-cycle gas turbine plant draws fresh atmospheric air, compresses it, burns fuel with it in a combustion chamber, expands the hot gases through a turbine to produce power, and exhausts the spent gases directly to the atmosphere rather than recirculating them.
In an open-cycle gas turbine (OCGT) power plant, atmospheric air is continuously drawn in, compressed, mixed with fuel and burnt, and the resultant hot exhaust gases are expanded through a turbine and then discharged to the atmosphere — the working fluid is used once and not recirculated, unlike a closed-cycle plant.
The main components are: (1) Air compressor (usually axial-flow, multi-stage) that draws in atmospheric air and compresses it to several times atmospheric pressure; (2) Combustion chamber, where compressed air is mixed with fuel (natural gas or light distillate oil) and burnt at roughly constant pressure, raising the gas temperature to 900-1400°C; (3) Gas turbine, where the high-temperature, high-pressure combustion gases expand, doing work on the turbine blades — part of this work drives the compressor (typically 60-70% of turbine output) and the remainder drives the generator; (4) Exhaust, where the spent gases (still at a few hundred degrees Celsius) are released to the atmosphere or ducted to a heat-recovery steam generator in combined-cycle plants.
Open-cycle plants have advantages of low capital cost, compact size, quick start-up (a few minutes, ideal for peak-load and standby duty) and no requirement of cooling water, but suffer from relatively lower thermal efficiency (typically 25-35%) compared to combined-cycle or steam plants because a large fraction of the fuel heat is lost with the hot exhaust gases.