Gas Engine (Reciprocating)

[debal]

Our fuel gas calorific value has dropped significantly due to increase in CO2 percentage. Any idea, how to keep operating with low calorific gas? Currently, we are considering two options- a) remove CO2 using filter; b) change the throttle valve design, to enable higher fuel-to-air ratio mixing.

 

[zdas04]

Most industrial engines have kits for this. Between about 600 MMBTU/MCF and 800 MMBTU/MCF you can put a low-BTU kit in the Carb and get back to full horsepower. Between 400 and 600 MMBTU/MCF you need to replace the whole Carb with a "digester carb" to get back to full horsepower. I've been running Waukesha F-18's and H-24's and Cat 3412LE, 3408LE, 3406TAW, and 3306TAW like this on 20+% CO2 for years without any problems at all.

I don't know what kind of "filter" you're looking at, but the ones I looked at a few years ago (mostly in the class of Adsorbtion Filters like activated charcoal) were not up to the task of filtering 20% CO2 down to 8% at the rate of 100 MCF/day. I tried the mole sieve approach to getting rid of the CO2, but it was very expensive, very wasteful of fuel gas, and didn't work all that well at very low pressures (my gathering line pressure was under 40 psig then). "Small" amine units were just outrageously expensive and manpower intensive.

The carb kits were a perfect solution. Quite low cost, very effective, zero care and feeding after installation, and adaptable to changing conditions.

David

 

[ElwoodGreen]

debal, you must provide more information. What model of engine? Turbocharged? Automatic fuel ratio control? etc.

My fuel (landfill gas) changes by the minute. In 3 hours this morning it went from 57.9% to 59.9% methane. It will likely drop that much later this evening, since I increased power output. The night operator will likely lower our output to compensate. If he doesn't, the wellfield will experience too much air intrusion and we will see less than 57% methane. The engines are set up for 58% methane, and we experience much difficulty if methane drops to about 56.5%.

At other sites, they operate with as low as 44% methane, but the fuel system is sized to allow it. The fuel system must flow enough Btu (MJ) for the engine, regardless of calorific value. Recip engine heat rates are generally around 10,000 Btu/kWh on a saturated lower heating value basis. For a 1000 kW engine, you need about 10,000,000 Btu/hr. With 500 Btu/scf fuel, that engine's fuel system needs to flow at least 20,000 scf/hr or 333 scf/min.

You have to identify the component (or components) in the system that prevents you from operating at the lower calorific value for your particular engine. You should contact the engine manufacturer's applications or engineering department to get their opinion and recommendations. We have at times increased our main line fuel presure in order to squeeze a bit more gas through the regulators. We learned how to widen the parameters of our automatic fuel ratio controller. We are contemplating a change for the Impco fuel valve cones to allow more fuel flow for those times when we have low methane. If we were to experience long term reduction in calorific value, I suspect we would need larger regulators to go along with the change in fuel valve cones.

Good luck, and I hope I have written something useful to you.