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MEJatNDL (Electrical) (OP)
26 Jul 07 10:50
Hi Folks,

I am evaluating an opportunity for a co-generation project at an industrial site.  This site uses, during peak times 30,000 lbs per hour of steam at 250 psi saturated and has process use for 10,000 lbs hour of lower pressure 50 lbs per hour.  During half of the year steram flow is about 50%Steam is currently produced by #6 fuel oil.

This plant can use all of the electrical power, typically at 600 volts,that can be produced by a co-gen project.     Bio waste and saw mill hog fuel is available for a potential new wood fired boiler.

Looking at new boiler, turbine, generator and possible steam condensing equipment.

Does anyone have experience with this type or size of project in terms of approximate costs, potetial payback, project challanges or similar opportunities?

Regards,
Mike  
rmw (Mechanical)
26 Jul 07 23:02
What determined the 1.5 MW figure in the title to your post?

I am not going to do your math for you, but at that little power production, it is going to take a mighty long time to amortize the capital cost of the amount of equipment it is going to take to get it all done, and for an incrementally small amount, much more electricity can be generated if you have somewhere to put it, such as a local utility (if the fact that the industrial plant only needs 1.5 MW.)

Answer that and then we might get into the challenges, etc.  There are plenty of those to discuss.

rmw
MEJatNDL (Electrical) (OP)
28 Jul 07 9:09
Thanks rmw,
I was Looking at sizing the turbine generator for the potential plant steam load which is about 28,000 lbs per hour at 250 psi during peak periods.  Nominal flow for 6 months of the year is about 50% of this.  Based on a quick calculation and review of another published application it appeared that a condensing tubrine could generate 1.5 MW with all steam condensed. Ideally in this plant higher pressure steam can be produced and lower pressure steam extracted for process use using a back pressure turbine.  The plant itself can use 4MW of electrical power throughout the entire year and nomimaly requires 8MW with peaks at10MW during2 months of the year.

Mike
Helpful Member!(2)  ccfowler (Mechanical)
29 Jul 07 3:29
MEJatNDL,

The economics of cogeneration can be very powerful when everything is right (powerful, phooey! they can be spectacular!), but imbalances can be VERY expensive.  The dominant cost and energy advantages arise from the power that is generated with steam that ultimately goes to a process use.  That cuts out the condenser losses.  This implies that the steam generator must operate at substantially greater temperatures and pressures than the process steam requirement, and all of the related steam generation equipment will be more costly than if it was just to produce steam for your process.  The higher the initial steam temperature and pressure, the higher your cycle efficiency can get, but the capital cost gets greater, too.

In this power range, it is unlikely that cycle efficiencies will be very high, but they should not be shamefully low, either.  Most likely, operation of the system for just power generation will not be economically attractive unless both the fuel costs are very low and the sale price or avoided costs for power are very high.  Your cycle will not get anywhere near comparable to that of a mullti-hundred MW utility base load supercritical steam unit, but by having a "market" for the "waste steam" from your power generation cycle, the economics can become highly attractive if everything balances well.

If your process steam needs mesh nicely with the utility's peak power needs (your plant doesn't need much steam when the utility needs peaking power), you may be able to sell your excess power at a premium rate if it is sufficiently reliable.  Also, it will matter where your plant is located within the the utility's grid.  If your plant is in a weak area during peak utility system loads, then the potential for support of the utility's grid will be valuable.  However, if there is a shortage of transmission capacity to handle the power that your plant can produce, it will be of limited value to the utility.

The main advantages that you can depend upon are the reduction of utility demand charges and peak power costs, but the reliability of your generation must be superb to realize these benefits.  One episode of high demand at a peak power time will cancel these potential savings for a long time.

You may do well to think in terms of two half-size systems for the sake of reliability and dealing with both periodic and emergency outages.  The added costs may be surprisingly modest compared to the risks that would be mitigated.

If your plant's steam needs are more diverse than simply 250 psi and 50 psi, you may do well to extract steam at several different pressures so that the maximum work can be recovered for power generation before the steam goes to "waste" in your process.

Your economic analysis should include proper evaluation of the cost and value of the steam at each pressure level needed by the process.  Also, it will be important to recognize how the extraction pressures will vary with power generation levels.  You may need to allow for extracting steam from different points for each process need for different levels of power generation.

Another consideration will be regenerative feedwater heating.  Your cycle efficiency can be greatly improved by wise use of multiple extraction points.  (Believe it:  All that stuff in moldy old thermodynamics text books really does work!)  Unfortunately, substantial capital costs are involved, too.  "Thermo" texts usually show individual pumps for the condensate from each feedwater heater, but this is almost certainly not necessary for your system.  The condensate from one feedwater heater can be throttled and "flashed" into the next lower pressure feedwater heater with relatively little thermodynamic penalty and great operating, maintenance, and capital savings.

You will need to take a very hard look at capital, staffing, maintenance, ash (and other waste) disposal, stack gas cleaning, and fuel costs vs. probable steam needs, avoided electric power purchase costs, and probable power electric power sales.

At the power levels that you are dealing with, you can expect that turbine and cycle efficiencies may increase markedly with turbine size.

Stack gas cleaning will be a very big issue since you are considering solid fuels.  You can plan on endless grief with the first puff of smoke from your chimney.

Also, you may want to look at other alternatives such as gas turbines or reciprocating engines with steam generated by exhaust heat.
rmw (Mechanical)
29 Jul 07 5:36
I would add to ccfowler's excellent post that you should take a look at your company's experience too.  Do you have experience with handling and burning the solid fuels that you mention?  If not, see if you can visit a plant that is currently doing the same thing (burning bio waste and/or hogged wood waste fuel) and after a thorough walk down, ask yourself 'would my company do this well?'  

Another consideration is the transportation aspect.  Availability of the fuels is one thing, getting them to your plant, storing and handling them is quite an endeavor.

If all that passes muster, then your idea would seem viable to me all other factors considered.

rmw

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