You have to input a fixed amount of work through the compressor shaft to compress a known amount of gas from a defined suction pressure and temperature to a known discharge pressure and temperature. I can generate that work with an internal combustion engine, an electric motor, a steam turbine, or a water wheel. In every case the amount of work input is fixed. The cost of that input is quite variable.

If I am running a gas turbine to generate electricity and I'm willing to spend the capital to recover the waste heat, then using steam is very inexpensive while using the electricity directly to run the compressor has a high opportunity cost (the power can be sold if not used). If I have to burn fuel to generate steam then steam turbines can be quite expensive to operate.

There are situations where I can use gas that would otherwise be vented to run an internal combustion engine, in countries with a carbon tax, the government is paying me to use that fuel instead of venting it, very cheap indeed.

Bottom line of the above discussion is that choice of driver for a particular compression operation in a particular location at a particular time is a complex economic analysis that does not lend itself to rules of thumb. You have to do the analysis honestly, without bias to decide what makes the most economic sense. Once you have that number it is quite appropriate to apply personal, corporate, or regulatory biases to add the consideration of intangibles--with an honest cost breakdown you can determine the cost of indulging in the biases to decide if pandering to them is justified. For example, if the NPV of your electric driven compressor is 80% of the NPV of a gas turbine driven compressor, but adding the turbine would increase the emissions output enough to force the entire plant into a non-attainment status (with accompanying potential for fines, capital expenditure required for other equipment, and bad press) then the lower NPV could easily be justified. In the same example, if the NPV of the electric was 10% of the gas turbine even considering fines and required capital expenditures then accepting the bad press might just be a cost of doing business as you install the turbine.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

Thanks, zdas04,

The project I refer to is a new cracker. The proposed design is to drive the main compressors with condensing steam turbines. However in the entire complex there companies that require process steam and so additional boilers beyond the furnaces steam generators are required. Residual fuel from cracker can be used to generate power in combined cycle and/or in integrated gas turbine (exhaust gases as comburent to furnaces). The point is: the entire cycle is more efficient if the large compressors were motorized (motor + LCI) and steam (produced in the furnaces) could be sent to other companies of the complex. I understand that local energy and fuel costs impact on the viability of any solution. What I could not up to now understand is why there is so few crackers with large motor driven compressors except in France.

Regards

fabio vincent
www.figener.com.br

Thanks, zdas04,

The project I refer to is a new cracker within a petrochemical complex. The proposed design is to drive the main compressors with condensing steam turbines. However in the entire complex there are other companies that require process steam and so additional boilers other than the furnaces steam generators are required. Residual fuel from cracker can be used to generate power in combined cycle and/or in integrated gas turbine (exhaust gases as comburent to furnaces). The point is: the entire cycle is more efficient if the large compressors were motorized (motor + LCI) and steam (produced in the furnaces) could be sent to other companies of the complex. I understand that local energy and fuel costs impact on the viability of any solution. What I could not up to now understand is why there is so few crackers with large motor driven compressors except in France. Any idea why?

Regards

fabio vincent
www.figener.com.br

As I said, it is economics. If you have other profitable uses for the steam, the economics could easily favor running the compressor with electricity. If you have more steam generation capacity than loads then the economics will favor steam. I don't believe it is a country thing, I think it is an individual plant thing.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

I am also working now on an ethylene cracker project. We are now on precommissioning stage. The main compressor charge gas compressor and the tertiary refrigerant compressor is using condensing turbine. One reason for this one is that steam turbine are variable speed that the main compresor needs. If we have to use electric motor, then we have to use variable frequency drive motors which are very expensive.

Variable frequency drives and motors are not that much more expensive than fixed frequency and soft start equipment. I was quite surprised at how little variable frequency added to a project I did recently. If the numbers in your analysis were unbiased, and the decision favored steam drive then that's what I'd build. It has to be a case by case analysis.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"