De-Rating Vs Turn down the boiler to less than 20% MCR 1

[Smeh]

Hi,

We have foster wheeler water tube boiler operating pressure at 450psig with steam production of 250,000Lb/Hr. This boiler can only operate at a minimum of 20%MCR, i.e. 50,000Lb/Hr per the OEM. I was wondering if someone has converted these huge boilers to operate up to 10% MCR, i.e. 25,000Lb/Hr as we have no steam demand left. I understand bringing the pressure down will create other issues like higher steam velocities and may overwhelm the separators but not sure for such a huge size boiler. Is there anyone who has done this successfully and if yes, what is required to be done by keeping in mind that the circulation ratios are not affected and avoid the carry over as well. We may also have to look into the burner design to make sure we stay compliant from emissions stand point like NOX, CO etc. If anyone has any information or advice, please share.

 

[mk3223]

The de-rating or re-rating of the existing operation system are quite typical, but need a complete engineering study for the process and equipment in the system. The issues as you have mentioned are the items should be included in the study. IMO, the cost may also be one issue to be included in the proposal.

 

[Smeh]

Thanks for the reply.
We are also looking into purchasing the smaller once through boiler and looking at the cost Vs converting the existing boiler to run at lower MCR.

 

[chicopee]

I can state that during a 1986 Cleaver Brooks (C.B.) seminar at their Lebanon facility in PA, a 100 hp fire tube boiler with a rated steaming capacity 0f 3,450 lbs/hr (=28 gph fuel rate) can produce 850 lbs/hr (=7gph fuel rate) with a turn down ratio of four(4) or 25% of the rated value at 100%. The efficiency of fuel to steam at 100% to 25% firing rate dropped around 1%. At the time of the seminar, C.B. was introducing burners with a turn down ratio of ten(10) however, the steaming rate was not revealed, probably because no trials were yet conducted. I suspect though that the steaming rate at 10% will have a greater efficiency loss than the 1% ;by how much, I don't know. As mentioned by one of the responders a trial test would need to be conducted.

 

[Smeh]

We are not looking at this from the efficiency stand point; reason being, we need this boiler to start the combined cycle operation and shut down after the combined cycle is up and self sustaining. So for every start of the combined cycle, this boiler has to be running for a maximum of 4 hours and shut down after that and that may happen only 100 days of the whole year. Right now we have no choice as per the OEM to run at a minimum of 20% MCR, i.e. @ 50,000PPH where our demand to start the combined cycle is only ~10,000PPH and the remaining 40,000PPH of steam is being vented out through the sky vent.
We are going to talk to one of the company in our area and hoping that they have some experience without jeopardizing the circulation ratios on the water side but if any one have converted the huge boiler like that to low turndown and have any thing to share, please do so.
Thanking you all.

 

[LittleInch]

From what you're saying that sounds like a significant waste of energy.

So basically in order to be able to re-start a CC gas turbine power plant? someone has decided to use a boiler sized at 25 times the required capacity??

Why?

I can't see any boiler being able to do that level of turndown.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Smeh]

We had a host to whom we use to provide steam and the host is down now. Boilers were originally sized according to the load conditions but after the host is gone, they are too big for what we need the steam for.