Boiler starting fuel tank capacity 1

[wayuu1981]

Hello,

What could be a minimum recommended storage capacity for the light fuel oil tank of a 300 MWe coal fired CFB boiler, to take it from cold state up to 40% BMCR capacity, fuel oil won't be required for combustion support. Is there any standard suggesting minimum capacities? or is there any other scenarios to be considered such as the capacity required for how many startups or how many hours at maximum oil flow rate (in this case 40%BMCR)?.

For this particular case it has been estimated a 85t requirement of fuel oil per cold start-up (10 hours approx.) and a 24t/h fuel flow with all oil burners at full capacity. what could be an appropriate oil storage tank capacity?

Javier Guevara E.
Projects, Mechanical Engineer

 

[abeltio]

There are no standards regarding "spare capacity" they are completely dependent on local needs, commercial strategy, company policy etc., etc.

Some of the things we consider is:
01. Most important, and most frequently overlooked factor for reliable system operation: settling time at least 24 h, preferred 48/72 h - ALWAYS USE FLOATING SUCTION.

02. Volume loss due to water condensation (bottom drains): typically the bottom 30 in (760 mm) of the tank are unusable. May be higher in high humidity locations (above 70%)

03. Is tank heating needed due to ambient conditions? add another 30 in (760 mm) on top of the water condensation allowance

04. Cycle frequency (start/stops) vs. tank refill opportunity: if the cycle is operated once a week and the fuel is brought in by barge/truck once a month: need fuel enough for 4 starts/stops with some additional capacity.

05. Operational experience: how frequent are the trips? how frequent are failed starts? Need to add to the start frequency

06. Light diesel oil might be needed for shutdown as well to avoid having all the fuel passages blocked after cool down. Need to know the normal shutdown sequence as well

07. Refill supply: how often and how secure is it? Commercial decision: how many days of normal operation must be guaranteed without primary fuel?
08. How critical is the service? are the penalties associated with not running or not being able to start higher than the extra fuel storage capacity? What if there is a problem with the tank (e.g. suction is damaged) what are the project requirements for redundancy of critical equipment?

09. Is there a re-circulation line? might need an external cooling system

10. Are there any space (including spill basin volume) constraints?

11. The consideration of the factors above vs. the budget and customer specifications may dictate the need to supply two smaller tanks instead one big one. The added flexibility, availability, reliability and maintainability (FRAM) will exceed by far the situation with just one tank.
Note: settling time is so important that this factor combined with starting frequency alone may dictate the need for redundant tanks.

Espero que te sirva.


saludos.
a.

 

[wayuu1981]

Thanks for the answer, but it would be great if anybody can provide a reference value of diesel oil tanks capacity installed in existing plants to compare values of storage_capacity/MW others are using, as I said, just for reference.

Javier Guevara E.
Projects, Mechanical Engineer

 

[crshears]

Hello wayuu1981,

No offense, but please read abeltio's first sentence as many times as required...

Abeltio has taken the time to type out a considered response to your inquiry in an effort to tell you that your question is not one with a straightforward answer. Start-up fuel tanks are one of those things [and there are many] that cannot and should not be designed on 'rules of thumb.' I have had bitter experience with a number of the problems pointed out, and I can attest to the validity of the advice provided. I strongly recommend that you start from the basic numbers you have provided and do the hard work of factoring the items suggested into your local situation.

In my view, if you're only looking for a simple answer, you've come to the wrong place. If on the other hand you're looking for a good answer, simple or not, this is the place to find it.

Pardon the brutality, but if you design equipment using 'reference values,' you should be prepared to fail, and your employers would be well served to minimize their expectations.

 

[davefitz]

The startup oil problem at each site needs to be carefully studies at each site, to allow minimization of use of costly oil.

The theoretical minimum warmup energy and time needs to be calculated and plotted, and the effects of typical delays needs to be listed and addressed in some manner. It is not neccesarily true that the best solution is to assume all startup requirements and delays must be addressed by additional oil consumption- other options exist if youo are clever about it.

For a CFB, the theroretical energy needed to bring the equipment from ambient to stable coal combustion temperature must be calculated , for example:
-boiler tubes + headers , from 60 C to 350C
-refractory, bed material ,cycloome ash inventory, from 60 C to 650 C ( a steam cooled cyclone greatly reduces this startup limitation)
- main steam line, STG , from 60 C to 350 C

The time delays needed to avoid fatigue damage to the boiler thick walled parts, boiler refactory , and steam turbine must be observed:
-boiler steam drum or HP main steam outlet header max permtited ramp rate based on a 2000 cycle fatigue life must be observed for a cold startup
-STG OEM has guidelines for rate of heat up and minimum temperature needed for each speed hold point. This time can be minimized if you load and synchronize thru the IP turbine
-thick refractory lined cyclones have a very slow permitted warmup rate- for this reason you should consider the steam cooled cyclone .

Other delays relate to feedwater quality holds, etc.

Clever approaches are used to minimize the startup oil consumption. For example a coal or wood fired aux boiler can supply warmup steam to pre-warm all boiler and STG parts prior to oil ignitors. Steam cooled cyclones avopid the 20 hr warmup delay demanded by refractory cyclones. Hot-slumping the bed ( at 875 C) without purging the bed ( only purging above-bed space after trip) allows very fast restart after a 12 hour outage without any use of startup oil . Eahc of these has a cost that can be offset by saved fuel oil .

"Whom the gods would destroy, they first make mad "