3 replies to this topic

[LiveFree]

Process units' energy consumption is sometimes reported on a MMBTU/bbl or tons of equivalent oil per ton basis and compared to similar process units to see how energy consumption compares across technologies.  A more accurate way of analyzing a process unit is to look at the utilities consumed individually, eg. steam, cooling water, electricity, etc.

 

For a particular customer, I am trying to calculate tons of equivalent oil per ton of product for a specific process unit that has steam and cooling water (pretty straightforward here) and a centrifugal compressor (and this is where I'm having trouble).  How is a process units' energy consumption reported on strictly an energy basis when power also needs to be taken into account?  Is there an industry accepted conversion of energy to power?  

 

I have considered looking at steam required to run a turbine to power the compressor; however, there are many types of turbines that will have different ranges of inefficiencies.  I'm at a loss on how to perform this calculation in a way that allows comparison of process units and technologies without having to make major assumptions that could be incorrect.

 

Thanks you in advance,

 

Jason

Edited by LiveFree, 12 March 2018 - 12:07 PM.

[Bobby Strain]

Energy = Power X Time. It's not a standard, but a fact by definition.

 

Bobby

[LiveFree]

Thanks Bobby.  Maybe I did not explain this correctly.  I was hoping to get energy demand per barrel of product/feed.  In China they use Metric Ton of Standard Oil Equivalent - TOE.  This requires some sort of efficiency assumption in power generation.  

 

I found out that in China, they assume 0.087 kgOE/kWh at 100% efficiency.  Then they assume an industry standard efficiency of 34%.  So I did find what I needed.  Thank you for the reply.  

[Saml]

It is usual to convert power to fuel at about 30-35% efficiency based on lower heating value. In many electricity grids this represents the marginal efficiency, that is normally given by open cycle modern gas turbines or coal power plants.

 

So basically you are saying that if you use 1 unit of electricity you will have to marginally burn (somewhere in the grid) 1/0.35 units of natural gas or coal.

 

It becomes a little trickier to determine the marginal energy usage when you use steam from a header on an chemical complex, since the marginal cost of power can vary from near zero (if you are otherwise letting down that steam) to the full steam cycle cost (if you have to increase the firing rate in your boiler plus condense the extra exhaust steam)

 

It is not always the same. Take for example the book "Sustainable Energy Without the Hot Air" where the author states

 

"In some summaries of energy production and consumption, all the different forms of energy are put into the same units, but multipliers are introduced, rating electrical energy from hydroelectricity for example as being worth 2.5 times more than the chemical energy in oil. This bumping up of electricity’s effective energy value can be justified by saying, “well, 1 kWh of electricity is equivalent to 2.5 kWh of oil, because if we put that much oil into a standard power station it would deliver 40% of 2.5 kWh, which is 1 kWh of electricity.” In this book, however, I will usually use a one-to-one conversion rate when comparing different forms of energy. It is not the case that 2.5 kWh of oil is inescapably equivalent to 1 kWh of electricity; that just happens to be the perceived exchange rate in a world- view where oil is used to make electricity. Yes, conversion of chemical energy to electrical energy is done with this particular inefficient exchange rate. But electrical energy can also be converted to chemical energy. In an alternative world (perhaps not far-off) with relatively plentiful electricity and little oil, we might use electricity to make liquid fuels; in that world we would surely not use the same exchange rate – each kWh of gasoline would then cost us something like 3 kWh of electricity! I think the timeless and scientific way to summarize and compare energies is to hold 1 kWh of chemical energy equivalent to 1 kWh of electricity"

 

 

The best you can do is to use some sound approach and document it together with the conclusions for your customer.