CO WHEN COMBUSTION TURBINE LOAD CHANGES

[bigdog50]

Can anybody explain in detail CO in relation to turbine loads on a 501 F or any gas turbine, natural gas unit. Is it because to much air flow vs gas flow or unburned fuel left behind. When a unit is producing higher Co on load changes or is it because IGV's are shifting with load going closed.

alocklear
Florida,USA

 

[btrueblood]

It's been awhile since I dealt with cycle analysis for GT's. But a simple rule is that CO is produced when a combustor is running "rich", and decreases/disappears when running "lean".

So, if a GT is running near stoichiometric, and the load suddenly drops, the engine will accelerate. The increase in airflow will tend to drive the combustor mixture lean, decreasing CO (but possibly sending NOx levels up). The opposite case, a sudden increase in load, should cause the engine to momentarily decelerate, decreasing air flow, and drive combustors rich (driving up CO).

Modern engines (esp. fixed power generation turbines) run a "lean premix" combustion system, with preswirlers and other nifty technology. I wouldn't be surprised if, under some conditions, a reversal of the above "rule" could occur, due to aero effects of sudden changes in the complex burner flow path. (Fancy way of saying I could be wrong!)

- BT

 

[rmw]

CO can be produced in a GT combustor can when the flame is quenched (for turbine inlet temp control, or low NOX purposes) before the combustion is complete. You can have beau coup excess air, but if you don't have enough in the flame zone to complete the combustion, you can have CO.

rmw

 

[gtsim]

I suppose we are discussing conventional (diffusion flame) combustion systems. At maximum load the equivalence ratio is maintained at about 0.8 (i.e. slightly lean). Emissions such as CO and UHC occur due to combustion inefficiencies. Decreasing load decreases the equivalent ratio thus combustion temperature. The load decrease also decreases the combustion pressure. The decrease in these two factors decreases the combustion efficiency and shifts the reaction from CO2 towards CO, thus increases the CO emissions. But the decrease in combustion pressure and temperature reduces the NOx emissions. Therefore, as the load decreases CO and UHC increase while NOx decreases.

The closure of the VIGV with decreasing load endeavours to maintain the equivalence ratio and thus combustion temperature by maintaining the turbine exhaust temperature (EGT). Although the combustion pressure decreases the higher combustion temperature helps reduce the increasing CO emissions at low loads. This is indeed the principle of DLE combustion where the equivalence ratio is now maintained at about 0.6 with load where the NOx and CO emissions are small.

We have developed some gas turbine simulators, which illustrate all these issues. If you wish, you can download them for evaluation. The website is

http://www.gpal.co.uk.

Regards