Once-Through HRSG Duct Burner Flame Stability

[thermmech]

Greetings!

I am working on a project where a large horizontal OTHRSG is envisioned after a gas turbine. OTHRSG is supposed to have supplemental duct burner positioned AFTER first heat exchanger module. This is a case of so-called mid-firing.

We are trying to establish whether:

1. there is a minimum GT exhaust gas temperature required for successful firing? We now think that GT exhaust gas tempererature should be arount 400 - 435 °C [750 - 815°F] at duct burner location.
2. we need to carry out CFD simulation of GT exhaust gas flow in the duct leading to the first heat exhanger module, and before and after duct burner. (Please note that we will have two (2) distribution grids.)

I'll appreciate if you share your thoughts with me.

Regards,

 

[davefitz]

Normal duct burner operation philosophy is to delay use of duct burner until after the CTG has reached full load. Tof fire the duct burner prior to this load is worsen plant heat rate , and places the integrity of teh downstream superheaters in jeapordy.

The duct burner flame has a high radiant heat flux that is imposed on the superheater and reheater tubes adjacent to the duct burner cavity. These tubes rely on the assumption of a high steam flowrate ( consistent with 100 % MCR CTG output) in the tubes to ensure adequate cooling of the tubes to maintian a metal temp below design metal temp. You would have to confirm with the HRSG vendor is the tubes are adequate for duct burner operation at a lower CTG load.

If you ahve already addressed the tube metal temp issue,then you should ask the burner vendors what are their requirments for combustiona air temp.

 

[stanweld]

Heed davefitz's warnings! We have seen a number of overheated and ruptured tubes in both the superheater and hot reheat sections of a number of HRSGs due to firing the duct burners on startup of the CTs or with but limited delay in their firing.

 

[thermmech]

Thank you for you replies! I apologize for not having responded to your comments earlier, but it was a 4-day weekend in Canada.

This OTHRSG is to produce 75% wet steam, so no superheaters are present in HRSG. The OTHRSG vendor will have to take into the account all of the GT load cases for a range of ambient air temperatures. Coil metal temperatures will be calculated on that basis, dependant (of course) on wet BFW/wet steam in-tube flows. We will require vendor to provide us with calculated metal temperatures for each zone, and to state minimum required in-tube flow to ensure sufficient cooling.

a) RE low exhaust gas temperature at the duct burner location and flame instability: we have already spoken to both boiler vendors and burner vendors and there seems to be a little bit of a disagreement re the exact temperature value that promotes flame instability. We will follow up on this with them.

b) RE CFD analysis of the duct/burner system in order combat exhaust gas maldistribution: proper duct/grid design should help ensure good flame geometry and burner operation. Again, there is a little bit of disagreement whether the CFD analysis is REALLY required.

Any further comments / thoughts are appreciated.

 

[davefitz]

I assume your othrsg is the type made of incolnel and designed in Ontario. This type of OTHRSG uses very high inlet pressure drop ( across small orifieces at the inlet to teh tube bundle) to minimize tube to tube flow unbalances. As such , this orifice pressure drop is only effective at fullload feedwater flows- at lower flowrates, the orifice DP drops by the square of the feedwater flowrate, implying worse tube to tube flow unabalances and upsets in outlet tube temperature unabalances.

When firing a duct burner and heating a 75% wet steam tube , there can be local tube overheats caused by "dryout" or exceeding critical heat flux. The corellation for determineing the dryout heat flux is as per Doroschuk ( see papers by Siemens KWU Keifer Kohler Hein et al). This overheat is also related tothe flowrate of fluid flowing thru the tube, and if this flow is much less tahn the average flowrate then the "worst tube" will dry out and overheat. For the incolnel tube,the typcial problem might be excessive tube growth and bowing out of plane and subsequent damage to tube supports.

This type of OTHRSG uses the change in fluegas temperature as indicative of the average outlet enthalpy of the steam leaving the HRSG. You will need to ensure that this contorl logic is properly corrected for the use of duct burner heat input with less tahn full laod CTG exhaust gas flow.

 

[thermmech]

davefitz,

Sorry for not checking for replies for such a long time. My projects has been postponed, but I still want to address this and other issues. The HRSG in question is NOT typical power boiler with exotic alloys, but a relatively new breed of once-through HRSGs made out of CS which produce 75% quality steam for SAGD process, which works with high TDS (upper limit 8,000 - 12,000 ppm) in the BFW. SAGD process is used to extract heavy oil from tar sands in Alberta, Canada. HRSG could be designed / fabricated in Alberta or somewhere in USA or Korea.

This type of boiler uses pass-balancing control valves, controlled by a control system which gets the feedback from the flow measuring devices and 'apparent density' measuring devices installed in each pass. As it is possible to calculate the steam quality from the 'apparent density' at the discharge of each pass, that is how one can control the steam quality in each pass via modulating the flow.

Thank you for comments and the tip re the dryout heat flux correlation. In the meantime, we managed to find min in-tube flow correlation in:

"Two-Phase Flow and Heat Transfer in the Power and Process Industry"

by AE Bergles, JG Collier, JM Delhaye, GF Hewitt, F Mayinger.

I will definitelly also try to look up the papers you referenced. If my company doesn't have those papers in their libraty, I would appreciate if you could give me some pointers on how to obtain them.

Regards,
Sean

 

[davefitz]

s27289:
A good text for 2-phase flow correlations,and other modern boiler technology , is "boilers, evaporators , and condensers" edited by Sadik Kakac' . He presents a summary of the DNB and dryout correlations by Kon'Kov and Doroscuhk , strictly valid for vertical smooth bore tubes. These also form the basis of more compicated correlations for rifled tubes and horizontal tubes , as per KWU authors Keifer, Kohler and Hein, parts of which were published in the Int'l journal of multiphase flow over the years 1981-1996 . ( original basis in Teploenergitika)

It sounds like the type of evaporator you are using is similar to the old Sulzer technology for supplying pure steam while the feedwater has a lot of salt in it. Some other new technololgy that can be alernatively used is:

a) KWU proposes the use of gamma ray densitometers to deterrming outlet 2-phase enthalpy

b) IST uses gas side thermocouples to determine the enthalpy of the 2-phase mixture- if you know the water side flowrate and gas side flowrate, a simple heat and mass balance shows the enthalpy gasin of water is directly proportional to the gas temperature drop times the ratio of mass flows.

c) older boiler technology will simply discharge the outelt mixture to a centrifugal seperator, and adjust the fedwater flow until the seperator blowdown is 5% of feedwater flow.

 

[thermmech]

Thank you for the tips once again!
Regards,
Sean

 

[timbones]

The good old EOR OTSG! Careful with these things, it seems like it's easy to screw them up!

Is there a particular reason why you want to put the duct burner mid boiler somewhere? The recent EOR OTSG design I have seen (on the back of a GE7FA PetroCan plant at Mackay River) has the burner at the front of the boiler in the usual location to most HRSG's.

You might want to consider a physical scale model flow study to check for instabilities. There is a company in St. Catherines Ontario called Nels Consulting that has done quite a few of these on various HRSG's designs with quite reliable results.

 

[thermmech]

The reason is the required steam production (larger than at Mackay River)... when firing midway you let the gases from the turbine give off more heat to the BFW, and then you can fire harder with supplementary duct burners.

And yes, we have considered Nels!

Thanks and regards!

 

[timbones]

If you need that much heat, buy a bigger engine! Of course moving the power out of Fort Mac is a bit of a problem.

Maybe you should consider fresh air fans on the front of the HRSG. It would boost the engine exhaust flow and allow quite a bit more duct firing. Not sure whether it has been tried before, but then this is EOR technology! It's all very new stuff anyway!

The whole idea of mid-HRSG burners and the potential flow problems sounds tricky at best.

Tim

 

[davefitz]

There may be 2 different instabilities to be concerned with. The combustion instability is a burner issue. There can also be steam flow instability or "thermal hydraulic sensitivity" induced by subjecting this once thru circuitry to strong heat absorption unbalance at a load that is lower than full load.

If the OTSG is the canadian version, it has a single pass without intervening headers. The inlet to the OTSG has small diameter orifices for each tube; these are sized to provide adequate stability and equal flow to each tube, but only at full load. At loads less than full load, the presure drop thru these orifices decreases by the square of the flow ( ie, at 30% flow they have 9% of the design pressure drop). A low pressure loss at the circuit's inlet cannot provide the requried stability if the boiler is designed with a single pass without intervening headers, in this particular desing of OTSG.

The instability can become unacceptable if you then impose a strong , unbalanced, heat load at the midpoint of the circuit.

 

[davefitz]

To understand why modern once thru boilers use full mix headers part way thru the circuitry to prevent flow instability and unacceptable flow unabalances and overheat, see the following:

"Flow distribution among parallel heated channels" A R Gruber, S C Hyman, Chem Eng Prog v2No2Jun 56 pp199-205

"Control of flow distribution by mixing headers", S C Hyman, A R Gruber, L Joseph Chem Eng Prog v4 no 1 mar 58 pp 33-36

These papers were originally written in the 1950's to explain why it is not such a good idea to develope a supercritical nuclear reactor with one mother of all passes. Apparently, the researchers involved in the curent program to develope the gen IV SCWR haven't read these papers yet.

 

[thermmech]

Hi, all;
Thanks for your comments.

We are trying to maximize the steam production on the basis of the existing (already large) gas turbine. Tehrefore, we are going for mid-firing as that has been done before on conventional power HRSGs and is, by far, the cheapest option.

We do not propose the orifice plates but control valves in order to balance flow in each parallel. That is a proven approach in conventional OTSGs in Alberta.

Thanks again for referenced literature re instabilities. I will try to look it up. We have already addressed the concern of minimum steam flow required at the turndown in order to:
- prevent uneven distribution of wet steam inside the tubes, and
- ensure proper cooling of tube metal.