New Coal fired 150 MW, boiler selection

[wayuu1981]

In December 2012, our company must have a new 150 MW coal fired plant in operation or we'll be penalized by government. So far, we are defining some bid-terms to select the contractors for this project.

The next week we have a meeting with the consultants to define the right kind boiler that we need and I feel they are pushing us to use a fluidized bed boiler.

Where can I find some detailed information about selecting the right boiler for a particular capacity and coal type? I'm asking this because I feel there's other kind of boilers we can select with less initial investment and simpler operation an maintenance, as the plant where I'm working now, a 20 years old 150 MW pulverized coal fired plant with tangential burners, economizer and reheater (the coal its quite similar to the coal we're going to use in the new plant).

I've been reading some papers over internet and looking at the COMBUSTION and STEAM books, but I still need more and better information in order to evaluate the consultants recommendations, and finally, select what is better for our company.

Javier Guevara E.
Projects, Mechanical Engineer

 

[alehman]

This is a very large decision to rely on self-study. I would strongly suggest hiring consulting engineering firm experienced in power plant design to help you with this process. What is "best" depends on many, many things. Selection cannot be made in isolation, but must consider outside factors such as the turbine/generator selection, fuel supply, water supply, environmental laws, maintainability, etc, etc.

 

[metengr]

The next week we have a meeting with the consultants to define the right kind boiler that we need and I feel they are pushing us to use a fluidized bed boiler.

The consultants don't need to push you into anything. Stand firm with your indecision and tell them you need to evaluate all options - technical and financial. The consultants should provide for you a list of boiler design options based on fuel, emissions comparisons between fuels and boiler design, O&M cost comparisons, capital cost comparisons, manpower requirements, etc. If they cannot provide this to you, it is time to hire more reputable consultants in the Power Generation industry.

 

[danberry]

Maybe your consultant has good reason to push for a fluidized bed. It mostly depends on the coal you plan on using and the emissions limits you must meet. A PC boiler with FGD and SCR can be economically less attractive than a CFB system.

Nobody in this forum can answer your question without seeing the design criteria. Its a very site specific issue.

 

[wayuu1981]

So far, the main design criteria for the boiler is keeping the SO2 emissions below 2000 mg/Nm3, using >9500 Btu/lb and <1% sulfur coal, the second problem is a relatively low ash fusion temperature, near 2200 F for initial deformation, 2350 F hemispherical (reducing).

What I think is that selecting a FB boiler is the easiest decision for a consultant, but it wont be the most economical approach.

We've been analyzing other options such as a gas recirculation for the ash problem, and coal pre-cleaning to reduce its relatively low sulfur content. There are many other options as cyclone burners for ash and sorbent injection for sulfur, but, the plant location its near the coal mine but far away from sorbent sources like lime, limestone, seawater or ammonia.

About your comments:

Alehan:
about hiring a consultant engineering firm with experience, this is exactly what we did, but we can't just close our eyes and give money relying completely in what consultants says without analyzing it before, finally, we are who will operate and maintain the plant in future.

Danberry:
I'm not asking people in forum to select our boiler, I'm just asking for information sources where we can look for technologies that maybe we are not taking into account, and to re-check the technical and financial aspects in which consultants have based their conceptual design.

Thanks all for you comments, and I'm sorry about mi writing, my English is not as good as it should.


Javier Guevara E.
Projects, Mechanical Engineer

 

[davefitz]

150 MWe is a good size range for a CFB boiler, and a CFB can also be designed to burn biomass . There are many qulified bidders for this techology at the 150 MWe range.

Coal prices are escalating without bound, while there is increased political pressure to lower one's carbon footprint. So it might be prudent to consider a CFB that can also operate on whatever biomass or biofuel or waste fuel is available locally in your market area.

 

[BlrGuy]

Javier,
The first thing boiler manufacturers take into account for the design of a particular plant is your fuel source. A boiler is designed around the fuel it burns. CFB's are very flexible in this regard but you must give the boiler manufacturer all possible coals you might burn so it can be accounted for. For instance, if you sign up for a CFB and your design coal was 8% ash and 9000 Btu/lb, if down the road you want to use 7500 Btu/lb and 15% ash because it is cheaper then your CFB might not be able to handle it as you may know one of the main principles of a CFB is that it uses ash from the coal for its make-up of bed material. Ash content is one of the most important aspects for a CFB. Other things also come into play also such as ash handling system sizing for a new coal and things such as superheater, reheater & economizer surface areas which need to change due to a change due to a change in fuel and also the quantity of limestone injected into the furnace for SO2 control. Again, you must be honest with the manufacturer as to what you may burn so it can be designed properly from the outset. I have started up and operated several CFB's and PC Units and the client often wants to burn something cheaper to them but guarantees are based off of what was specified in the contract...

This is all just my two bits... There are so many different designs of boilers, traditional & Fi-circ CFB's, and different variations of PC's. One last thing, you mentioned that you are a significant distance from limestone for SO2 control. One of the reasons they may be preaching a CFB to you is because CFB's get more bang for the buck when it comes to limestone. The nature of that design allows the limestone to be used more fully thus reducing the consumption necessary as compared to a PC Unit. In a 150 MW Pulverized Coal Unit, a scrubber (FGD) is usually not cost effective and thus they may inject limestone in the furnace where the temperature is conducive for the proper reaction (1500 deg F which is near the operating bed temperatures of a CFB which is one reason it also is more effective in a CFB). The biggest problem with that is that the limestone's residence time in the furnace is then so small that a good quantity of it isn't able to react whereas in a CFB it gets entrained into the circulating bed material and not as much is wasted.

CFB's are also maintenance beasts with a lot of extra equipment you might not be used to. Ash systems, bed removal & addition systems, furnace nozzles, furnace erosion, refractory maintenance, fluidizing air blowers & fluidizing bed heat exchangers just to mention a few. Also you must take into account that operating one is must different than a PC, so be sure to buy extensive operational training if you go the CFB route.

Again, just a few of my thoughts, hope it gives you something to think about...