Rates of load change for ultra supercritical boilers 1

[athomas236]

I am involved in an ultra supercritical power project in for which the equipment will be supplied by a Chinese EPC contractor.

The Project will consist of 2x700MW coal fired units with boiler steam conditions of 260 bar / 605C / 603C with a main steam flow of 2000tonnes/hour.

At the moment, we are discussing the maximum rates of load change (up and down) and the contractor is offering the following:

1.5%MCR/minute in load range between minimum continuous load without oil firing(approx 35%MCR) to 50%MCR

2.0%MCR/minute in load range from 50%MCR to 100%MCR.

These rates appear to be very slow and appear to contradict one of the claimed advantages of supercritical boilers over drum type, namely, higher ramp rates.

Based on published literature and experience on two other supercritical projects (420MW, 250bar / 570C / 570C) and 660MW / 250bar 570C / 570C), I would have expected ramp rates of 5%MCR/minute in the load range above approximately 40-50%MCR.

I would appreciate any comments or advice that can be offered on maximum ramp rates for ultra supercritical boilers.

Regards,

athomas236

 

[davefitz]

athomas236:

I agree- published claims for supercritical units in Japan and EU are closer to 5% MCR/min, PROVIDED the unit has thermal stress monitoring thermocouples wired to a BSA boiler stress analyzer and uses some form of "advanced" or "predictor" control logic to limit upsets in enthalpy at (a) the furnace waterwall outlet (b) the FSHO and (c) the RHO. 2 stages of superheaer spray are typically used, and there is even proposed to be used a gamma ray densitometer at the waterwall outlet to moitor and limit the upset in enthalpy at that location during load changes.

USC ultrasupercritical units have a couple of other aspects that may cause the boiler mfr to reduce claimed ramp rate. The enthalpy at the furnace waterwall outlet is higher than at conventional SC units, which means the heat capacity at this location is small, so upsets in enthalpy lead to large upsets in fluid and metal temperature==> this can mean that tube-to-tube temperature unbalances can be large during load changes==> this can lead to excessive thermal stress in the waterwall panels and eventual "alligator cracking". Supposedly, this upset can be reduced by using the low mass flux vertical rifled tube design without waterwall inlet orifices, but if the design you bought uses waterwall inlet orifices, then it has a "negative thermalhydraulic sensitivity characteristic" and other stability issues that may worsen the waterewall tube to tube temperature unbalance during fast load changes.

Another issue is that the higher final steam temp implies that fatigue stresses are worsened by creep to a much greater extent than units at a conventional temp of 540C. So the allowable transient upset in final steam temp of +-15C with a conventional unit might need to be reduced to +_10C for a USC unit.

To this date, US boiler mfr's have not yet offered "advanced predictor logic" , which has been offered by japanese and german mfr's since 1990. Likewise, only half the vendors have a license to use the KWU Siemens low mass flux vertical rifled tube waterwall design. So, my guess is that your getting a good deal with the steel, but the design details and the soft product ( ie control logic) is not 'state of the art".

 

[davefitz]

By the way, if the unit is located in an elect dist system that inlcudes a large amount of wind power, here is also great value in having the ability to provide a step change increase in STG output ( to offset the sudden loss in wind generated electricity when the wind turbine farm disconnects as the wind speed increases from 50 mph to 50.000001 mph). The +10% increase in STG output is demonstrated at some german units by fast isolation of steam extraction to feedwater heaters- but this requires use of "header type" feedwater heaters and a detailed review by boiler and STG vendor.

 

[athomas236]

davefitz,

Thanks for your response.

The boilers do have vertical rifled tubes, but I cannot find any information to indicate we have a stress analyser. I will check further with our C&I guys.

Regards,

athomas236

 

[racookpe1978]

The boiler manufactor "might" be downrating his units some to allow for poor material availability, or to increase the units' blowout reserve from fatigue failure.

Chinese manufacturing don't always have poor suppliers and poor construction techniques/QA processes, but many do.

 

[davefitz]

athomas:

There are 2 types of "vertical rifled tube" furnace waterwalls. One type , which may be termed the Sulzer / Mitsubishi design, uses a high mass flow, inlet orifices, and a negative thermal hydraulic sensitivity chracteristic.The internal ribs are relatively thin and short.

The second type may be called the KWU Siemens low mass flow design, with no inlet orifices, positive thermal hydraulic sensitivity characteristic, and the internal ribs are relatively thick and extend far into the flowing fluid.

 

[boilerone]

As far as I am aware of, the only Chinese boiler manufacturer with vertical rifled tube in USC design is Harbin Boiler under Harbin Power Engineering Company (EPC) and its USC technology is licensed with Mitsubishi. The other boiler companies in China are licensed with either BHK-Hitachi (Dongfang Boiler) or Alstom (Shanghai Boiler and Wuhan Boiler) which all use spiral tube designs. The Harbin Boiler's literature that I have also confirms that they use high mass flow inlet orifices to take care of vertical tube's temperature differential problems thus, like davefits indicates, might cause lack of capability of doing higher ramp rates. The low mass flow rifle tubes without orifices is only heard used on retrofitting (with success) some existing boilers in China.

Nevertheless, since I have never heard of these Chinese companies typically change the original designs or downgrade materials for SC or USC boilers with imported technologies, it might be interesting to find out if the original technology providers, like Mitsubishi, can do better rates and what could be added on to the Chinese-made boilers if they are currently not providing.

 

[athomas236]

Gentlemen,

Thank you for your replies, all the information is appreciated.

The boilers do have vertical rifled tubes but there is no stress analyser.

Thanks again.

athomas236

 

[davefitz]

There is one new 650 MWe USA SC unit being built with the low mass flow verticle rifled tube- Longview LLC, in Maidsville, WV- major partner is Siemens.

 

[athomas236]

Gentlemen,

This is what I have said to the contractor:

QUOTE
We reviewed the explanations provided for the proposed ramp rates of 1.5/2% stated and it is considered that these rates are not consistent with international practice and with the rates offered by Chinese contractors on another project with which we are involved.

To support this view the following information is provided:

1. 420MW supercritical coal-fired plant for which the contract states.

"2.2.8.7 Load Changing Capability
Between 40% to 100% MCR at rated conditions, the unit shall be capable of accepting 5% of MCR / min. with the following limits:

(i) Steam pressure to be within the range of - 1500 kPa and + 1500 kPa of set point.
(ii) Main steam temperature to be maintained within the range -30C to +8C of set point.
(iii) Reheat steam temperature to be maintained within the range -30C to +8C of set point.

A load changing capability of 5% of Name Plate Rating / min. which complies with the National Electricity Code, shall form the basis of the offered plant. These changes shall be controlled by the DCS and without any manual intervention. Dangerous conditions shall not occur in the boiler and stable conditions at 40% MCR boiler firing shall be designed on the above premise."

2. European utility which operates three supercritical coal fired units with outputs in the 250 to 300MW range has stated in a confidential report to us that when operating in the load range 50% to 90%, their experience indicates that ramp rates of 4%MCR/min can be achieved.

3. European utility operating 800MW supercritical coal fired units, states that based on their experience with an 800MW unit the following ramp rates can be achieved.
Between 40% and 60% - 4%/minute
Between 60% and 100% - 5%/minute
These ramp rates are now included in their standard specifications for 800MW supercritical coal fired units.

4. Two offers recently received from Chinese suppliers for 660MW supercritical coal fired units both state the following ramp rates.

Below 30%BMCR - Not less than 2%MCR/min
Between 30% and 50% - Not less than 3%MCR/minute
Between 50% and 100% - Not less than 5%MCR/minute
UNQUOTE

It is interesting that one of the suppliers mentioned in item 4. is the same as the one offering 1.5/2% ramp rates.


Best regards,

athomas236

 

[vpl]

athomas

Maybe the supplier mentioned in item 4 has had failures based on the old quote and is wanting the lower heatup/cooldown rates to avoid further failures?

Patricia Lougheed

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Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[davefitz]

athomas;

The two primary issues addressed in "advanced" predictor type control logic in SC units is

(a) the mismatch in rate of response of the feedwater flow control elements ( FW pump drive turbine or control valve)vs the response rate of the coal pulverizers . The feedwater flow elements can respond perhaps 10 times faster than the coal mills, unless the coal mills utilize dynamic rotating classifiers ( which may lower the difference to a factor of 5 : 1). Classic control logic simply tries to maintain a fixed ratio of ( firing rate/ feedwater flow) in order to hold a fixed outlet MS enthalpy, but that classic method ignores the difference in rate of response and also the thermal inertia of the superheater metals.

and (b) the thermal inertia of the superheater metals must be factored into the "observer" model that controls the transient response of the superheater spray water attemporator. Classic logic typical attempts to hold a spray water flow setpoint of 0.5% of main steam flow, with transient adjustments to hold MS outlet steam temp setpoint at ( 1005 F) . Modern predictor or observer control logic includes a dynamic model of teh superheater metals that accounts for the thermal inertia of the superheater metals. Try a google search using "sulzer observer" for an explanation of one vendor's approach to this issue.

 

[athomas236]

davefitz,

Thanks for the information, I will certainly make a google search as suggested.

For your information, we have got dynamic rotating classifiers but, as far as I can tell, no "observer" model.

I am supposed to be meeting the contractor in Paris this week. Should be interesting.

Regards,

athomas236

 

[davefitz]

The below link describes the sulzer/cci "state controller with observer"- in use since about 1995

<

http://www.ccivalve.com/pdf/512.pdf>

 

[davefitz]

The below link is for a typical commercially available BSA boiler stress analyzer, by Metso

<

http://www.metso.com/automation/ep_prod.nsf/WebWID/WTB-041029-2256F-5900F>

 

[davefitz]

athomas236:

So, what happened at the meeting?

 

[athomas236]

The meeting took place but without the EPC contractor. The purpose of the meeting was just to finalise the technical parts of the bid ready for submission at end of October.

Before the meeting the EPC contractor changed the ramp rates from 1.5% to 2% and from 2% to 3% without any explanantion.

Now the bid submission date has been postponed for a fourth time to the end of January 2010 so there may yet be an opportunity to discuss this matter.

I am now working on another project where the same contractor is offering rates of 2%, 3% and 5% as stated above. There may be an opprtunity to discuss the ramp rates but with the contract completion date as 1 December 09 and the contractors bid being received in two weeks this seems unlikely. Silly tiescales we have to work to these days.

Regards,

athomas236

 

[davefitz]

another recent factor that lowers the allowable rate of response of an ultrasupercritical unit is the use of the new alloys T23 and T24 in the waterwall panels. Apparently the fabricators have not been post weld stress relieving these panels, and the result has been cracking at the end of panels at the fin-to-tube welds.

While I have not read any metallurgical reports per se, it is my understanding that the weld zone has a increased sensitivity to creep-fatigue cracking- if proper heat treatment is not provided , then the fatigue stresses caused by large tube-to-tube temperature differences during fast load changes may cause thses cracks to appear within the first year of operation- not good for the boiler OEM getting a final payment.

 

[davefitz]

perhaps publishing this will void any patent potential, but the problem of overheat at the waterwall outlet during fast load decreases can be prevented by a "negative spray" ==> the waterwall outlet can have a 2 phase fluid dump to condenser sized for 5% MCR at full pressure. The control logic would be to transiently pulse open this ( superheater bypass) when the waterwall outlet enthalpy exceeds some critical value during fastload changes.

 

[rmw]

Ouch!! What do you have against condensers? They love shocks like these / not.

Well, the HRSG dumps in CC plants have taught the condenser mfgr's a lot of new tricks, so they will be able to plan for this one. But make sure they know it is coming at them.

rmw