07-14-2009 04:49 AM
11-06-2009 01:11 PM
I have no experience with gas turbines but I have run 399 turbo engines at full load with gas quality as low as 27%.
I can tell you that to run lean you need to compress the airfuel mix very high so the molecules of gas are close together. The air-fuel will not ignite or will not completely burn unless the mixture is highly compressed. The leaner the air-fuel is the more it needs to be compressed. Then to ignite it under high pressures you need a very hot spark and a spark plug that will not leak spark energy through or around it's insulator. The needle electrode (iridium) types are best for firing under high pressures without wear or corrosion. The idea is to initiate an ignition and rapid flame travel while the pressure in the combustion chamber is the highest to enable a complete burn of the mixture. If the ignition source is located in the center of the combustion charge the flame moves out in all directions from the point of ignition instead of from one direction so this in return speeds the flame across the combustion chamber before the pressure drops.
In a piston engine, running under lean mixtures, the flame will sometimes go out before it reaches across the combustion chamber thereby not burning all the air-fuel. This is due to the fuel molecules being far apart in a lean mixture. also, as the piston starts its downward travel on the power stroke, the cylinder pressure starts to drop off so the fuel molecules become even further apart.
Another situation occurs with lean mixtures and igniting them under high pressures. The spark energy will track along the insulator on the combustion side of a spark plug instead of arching across the gap. It may also track on the outside insulator( if the wire boot isn't tight on the plug) and leak though the insulator to the metal shell of the plug. The cylinder will appear to be firing but is not firing clean which will slow down the flame travel. The flame will move too slow to burn across the combustion chamber at the highest cylinder pressures so the flame will go out before it burns all the mixture. The engine appears to run slightly rough although its not really misfiring. Hydrocarbons in exhaust will rise and power levels will drop as a result. Decreasing the spark plug gap will help eliminate this but decreasing the gap will have the same effect on slowing down the flame so the engine will still perform poorly.
To burn very lean the ignition source should be located close to the center of the combustion chamber and the compression pressures need to be very high. The spark energy has to also be very high and the ignition component in the combustion chamber needs to carry the energy across a wide spark gap without energy leakage. Turbulence in the combustion chamber is also very important and has a big effect on burning the fuel quickly and completely. The engine or turbine has to breath very good both on the intake side and the exhaust side. Lean mixtures must be controller very closely because if the air-fuel becomes rich under high compression the turbine or engine will most likely detonate. If to lean, it will just not fire and stop running or won't start.
Natural gas burning engines operate a little different than gasoline engines in that the exhaust and cylinder temps run cooler when leaned out. Nox and Co levels drop off also with extremely lean mixtures. On the other hand, they act the same as gasoline engines in regard to the control of lean mixtures for proper operation. I don't believe a gas turbine can be run with gas as low as 1% ch4 unless this gas is feed into another fuel stream at richer mixtures that the turbine will run on.