Possibility for a Bagasse Boiler Retrofitted with Pulverized Coal Burner?

Why not try for an actually sustainable fuel first? Coal is a REALLY dirty fuel. Biogas? Switchgrass? Where are you?

I'm from the Philippines.

Fuels like biogas and switchgrass and bamboo would be nice but the trouble with them is that you're never assured of constant supply. Also you just can't fire any type of biomass fuel in your boiler if your boiler is not designed for it. It will lead to many problems and would cause you a lot of plant shutdowns.

I understand the 'enviromental impact' of coal but from a business standpoint you have to consider what fuel is most accessible and relatively cheap to maintain your production. Bagasse fuel for boiler is great for both businessman and the environment. But like I said it doesn't last the entire year to sustain their ethanol/sugar production. So the next best and logical fuel for them is coal when the bagasse runs out.

So that's why I'm asking if it's possible to retrofit a bagasse fired boiler with pulverized coal burners? I would like to hear your experience with this.

Hello,

I am in charge of the Industrial Planning Department in a Sugar Mill in El Salvador, Central America. We have came to the same point as you and are looking for other fuels to feed the boilers during the season we are not producing sugar. I would not recommend to use coal in your boilers, mostly because of the negative impact this could have on your equipment. If you still want to give it a try, take into consideration that:

1. The heat of combustion of coal is way higher than bagasse (bagasse is around 4,000 BTU/lb and wooden chip ~7,000 BTU/lb, I can imagine that coal has a more powerful heat of combustion than wooden chip, although I do not have any measured reference)

2. Your boiler and generators are designed to work in a specific temperature and pressure range, if you do not know how your boiler will react with coal (by calculating an enthalpie balance of your system) and how to set it for the new type of fuel, you can seriously damage your equipment

What we are currently doing is to feed wooden chip on the bagasse return band (about 90% bagasse and 10% chip). These increases the overall effectivness of the boiler, which means it needs less fuel to produce the amount of steam we need. We can therefore produce electricity for a longer period after the crop/harvest time has ended.

The wooden chip comes from trees of regenerative forests and/or from the pruning season of coffee plantations.

I hope that this information is of any help and please contact me if I can be of further assistance.

Thank you sir for that information

Hi sir!

I would like to know what type of trees are you using as fuel for your boiler? Have you tried having a great percentage mix of that fuel wood which you are using? Like 30, 40 or 70 % more? What's the fuel characteristics of the fuel? Cane trash would have been a good option but the Alkali content of it is not good for the metal parts of the boiler. And how long and what's the planted area do you require for those 'fuel wood'

I just think that's a good alternative fuel as well.

Hello Sir,

We mainly use eucalyptus. We have measured a very high heat of combustion (around 7000 BTU/lb) but there are a lot of options and you can seek for the best one, suiting your interests. We do have tried to feed more wood than just the 10% but you have to carefully calculate the speed and amount of wood that you feed into the boilers. The temperature and pressure changes are very high, so you take the risk of not only damaging the metal components of the boiler but rotors, seals and bearings (among others) of your turbines and generator.

The planted area has to be measured according to the expected production of wood that you want to have in one year. The thumb rule indicates that you have to calculate that amount of wood (and surface) times five, meaning that in five years you can regenerate the whole cycle and use again the trees that you cut on the first year.

It is a very interesting field but I recommend you to carefully calculate all the elements in the process and make sure of keeping the balance on the trees you cut and the ones you plant. The environmental issue has to be THE priority.

I hope that this information helps you.

Thank you very much Sir CarlosDG.

The possiblity is there but you have to look at the economics of retrofitting it. Will it be worth it or not in the short term or long term? And why was the boiler designed to 100% bagasse fuel fired (if the study was conducted) when it knows that bagasse supply is not sufficient the whole year?

I am not a boiler expert by any means, but I think you might have problems running pulverized coal through a stoker-type boiler. Pulverized coal is best in a fluid-bed style boiler because of its very fine particle size. Crushed (sized) coal might be a viable alternative, but you should investigate the refractory of your boiler because of the difference in burning temperature and heat value between bagasse and coal. Depending on your existing boiler design you might rupture your pipes. See the attached for additional information.

http://www.epa.gov/ttn/chief/ap42/ch01/bgdocs/b01s08.pdf

I have commissioned a number of Bagasse fired boilers. Manufacturer was Babcock and Wilcock. These boilers were designed for coal firiing. Bagasse burns higher off the grate than coal. By that I mean that the fireball was closer to the superheater pendants. Coal firing has the fireball further away.The result of this was very high steam temperatures. We had to cut out a number of superheater tubes to meet the downstream conditions.

You should have no problem burning coal but if you have a superheater you may have lower final steam temperatures than before. As somebody else has mentioned you would probably be better off with a stoker coal feeder rather than going to the expense of pulverising coal. Using PF could result in hi carbon discharge if the boiler flue is not long enough for the coal particles to be completed. Stoker firing can be very responsive to load changes.

What sort of grate do you have now? Let me know that and I can give you more acurate advise.

Good luck, it is a very viable project.

Thank you very much for this info sir. It pretty much confirmed what the other people were telling me. I still don't know what type of grate it is but I will soon find out. Thank you again.

Hi sir

I'm just wondering though, why did you cut the superheater tubes if the steam temperature was very high? Those would have helped in bringing down the desired temp and press.

The more tubes, the more heat that can be absorbed. Think about the flow through the tubes,