Gas Compression: Engine Driver vs Electric Motor

[doglip]

Has anyone had a recent experience with using an electric motor for a 5000hp gas compression application? We currently have units utilizing a JGZ compressor and are considering expanding with additional units that will be electric driven. I am not sure how cost effective this typically is, if any. I show my concerns for this electric consideration due to the circumstances. We are not regulated due to emissions, and the gas consumption cost for the original units are included in the rates, so we are not taking a hit on fuel. Adding an electric driven package will more than likely double our MCC size and I have a hard time understanding what legitimate reason would make sense to consider electric. With added softstarts, gear reducers (etc), I assume we are better off sticking with our original design and use the natural gas engine. Can you guys share your thought on this issue? Other than engine maintenance, what are we saving? Thanks for your replies.

 

[ivymike]

is it going to be relocated at some point to a highly-regulated area?

 

[doglip]

No, they will all be there for the life of the facility.

 

[ivymike]

I've got nothing for ya. I've been working w/piston engines for nearly my whole career, though, so I'm not realy up on electric drives.

 

[nickelkid]

Is this a constant speed application? Is it acceptable to run the compressor @ 720 or 900 rpm?

If yes, you may find the electric motor cost effective. As mentioned maintenance will be less, also no oil leaks!

If you desire a variable speed electric motor your initial purchasing and engineering costs will higher than a constant speed application. Either way you should not need a gearbox betwwen the motor and compressor.

 

[dcasto]

Installed many IR-HOS siemans 5500 HP 1200 rpm units with VFD's or without VFD's. Set them beside the same compressor with CAT 3616's 1000 rpm 4885 HP.

The Electric motor drive are cheaper to buy and install, about 30% cheaper. The operating costs of the electric are about 30% of the engine. But with VFD's and harmonic transformers, maybe a 10% savings

The units with VFD's didn't work well at all. The VFD's sign wave was pulsed and the little pulses cause the compressor to pulse and every discharge bottle failed. The units had to be run under 800 RPM. The units with no VFD, did just fine. Solution, get a VFD with 30 steps per cycle, not 12.

Downtime on the electrics was about the same as the engines, but its up to your electric provider. With big machines, you are a low priority in getting service back.

In the end, the costs are the same or favor the gas engine drives. Electricity is so expensive (in 1996 we had $.028/kW-hr).

 

[zdas04]

I've done three comprehensive gas vs. electric studies in the last few years and the place where electric clerly wins is noise mitigitation. The higher frequencies of a motor tend to be a lot easier to mitigate than the engine noise.

Other than that, the capital costs tend to be less with a motor, and the power costs can be significantly less (but that is subject to change as electric demand increases).

The answer for each facility needs to be developed for that facility. Often times one site will be able to come off an existing substation and other times your project will have to shell out $6-10 million for a substation.

David

 

[rmw]

In my experience the engines were replaced for either emissions issues (non attainment areas) or noise (civilization moved out around the station.)

But, that said, you wouldn't have to worry about gas in the crankcase or water in the lube oil would you?

rmw

 

[zdas04]

I don't expect it to be long before the emissions shell game is a thing of the past. When you factor relative effeciencies of electric generation vs. gas-fired engines and then deduct line losses any reduction in carbon footprint between gas and electric is an illusion. I'm guessing that as soon as the EPA starts assesing penalties for CO2 to utilities, they are going to pass those carbon taxes on and the price of power will go way up.

There are some real maintenance savings, but never enough to scrap an existing IC engine and install a motor.

David

 

[ivymike]

it would be interesting to see (in advance) what the eventual effect of carbon taxation would be on all sorts of things. For example, would it make sense to run lean-burn engines with higher fuel efficiency, then reduce NOx with aftertreatment? ...even if that required a small amount of a reagent like ammonia or urea that required gas or coal to produce?

 

[dcasto]

Go here to see the effect of CO2

http://www.climateregistry.org/resources/docs/protocols/grp/GRP_V3_April2008_FINAL.pdf

The taxes on CO2 for your electric compressor is double that on a gas engine.

 

[rmw]

The emissions issue that I was familiar with was where the station was located in a non attainment area and either had to convert to electric or do something to reduce nox. I can't remember what drove the overall decision, but I do remember that nox was an issue. Stations up and down the line away from the major metropolitan area that was labeled 'non attainment' continued to be engine driven.

rmw

 

[HPMach]

5000HP Motor most likely needs to Synchronus motor for recip application, though speed is in the order of 900rpm. Adding such a big motor would definitely push you starting current issues unless you add significant additional generation capacity. As discussed in several responses above, unless it is driven by something else it may be worth sticking back to Gas engine. By the way what standard you are using....API11P or ISO13631. Recent issue of API618 does not refer to API11P. Instead it refers to ISO13631. I guess industry yet to gear up on this.