Conversion pulverized coal boiler to burn 100% biomass 4

[gierszi]

Helo,

I have a question about conversion pulverized coal boiler to burn 100% of biomass. Its main parameters: steam generation 650 t/h 535/130 bar.

In future we want to burn 100% of wood chips in that boiler.

Have anybody some experience with such conversion?
What are possible steam parameters to achieve?
What are the main problems to do that?

 

[siretb]

I have no idea about other problems, but one I know is that Biomass may generate lots of Alcali (Na / K) in the combustion gases.
This changes the problem data for deposits, fouling and catalysts poisoning if SCR is used. Carefully evaluate the impact on the gas cleaning train.

 

[willard3]

Biomass, wood chips in particular, are approx 8000 btu/lb depending upon moisture content, mixed coal is 12,000 btu/lb depending upon moisture content just for a start.

 

[danberry]

A PC boiler is designed suspension burning while biomass is usually burned on a grate or in a fluidised bed. You are looking at a major conversion and a sizable expenditure but there is no reason it can't be considered.

I would investigate two avenues:

1) thoroughly dry (with the flue gases) and grind the biomass before burning it in suspension in lieu of the coal. You would be significantly modifying the back end of the boiler but this would probably allow for maintaining the same steam production and conditions. This is great if you are driving an existing steam turbine generator.

2) Cut off the bottom the furnace and install a grate (traveling or otherwise). This would leave the boiler relatively intact but you would loose on steam production and conditions.

In any case, the boiler manufacturer should get involved.

Nice project but make sure you can actually get the fuel. An awefull lot of biomass will be needed to satisfy that boiler's apetite.

Good luck

 

[FOURe]

By switching fuels, oxidant (air) requirements will change resulting in different fuel and oxidant (air) mass flow rates, combustion products composition on both weight and volume (mole) basis and combustion products flame temperature.

Most likely, the existing boiler with desired fuels will not be able to generate as much steam as presently being rated to do so -- steam conditions and mass flow rate.

Here are a few plots indicating combustion parameters and trends for three typical fuels such as : coal, oil and gas when combustion takes place at stoichiometric conditions:

​

http://www.engineering-4e.com

 

[gierszi]

The existing boiler is quite old (1980) and oversized. It is russian construction and we don't have contact with constructor of it. I estimated in thermoflow that we can expected something about of 170 MWe instead of 225 MWe on coal. I know that the thermoflow maybe is not appropiate software to do it.

Supply of biomass is solved. We have enough wood logs but the problem is that after it chipped it is quite wet (moisture of 40%). We will need some kind of big dryer. Do you know what is the power needs of such dryer of that size?

What type of grinder will be good for dry wood chips, hammer mill or roller mill? I ask because in hammer mills is quite big risk of dust explosion.

I know that the best idea is to build fluidized boiler but we have to investigate conversion PC boiler.

 

[FOURe]

In general, for each unit amount of biomass, you have 0.4 [kg] of moisture.

Therefore, to convert moisture into steam, the unit amount of heat required is moisture/water h final - h initial at ~ atmospheric conditions -- moisture/water evaporation needs. The numeric value is ~ 2,500 [kJ/kg].

Heat flux requirements for the provided biomass fuel mass flow rate are:

Amount of Heat Flux = Fuel Mass Flow Rate * 0.4 * 2,500 [kW]

The above is valid for an ideal case -- heat transfer losses need to be taken into consideration for more accurate calculations ...

Hopefully, other Eng-Tips.com members will be able to provide you with additional technical input that you need to get in order to put your puzzle together ...

http://www.engineering-4e.com

 

[IRstuff]

I would seriously consider not doing a 100% conversion, if at all possible. With many other biofuels, initial cost estimates turned out to be completely bogus, as initial surpluses were consumed and new supplies needed to be acquired. Once suppliers perceive a demand for what was once undesirable waste, they will jack up the prices to be commensurate with YOUR demand. This would potentially shift the economic advantage away from what you currently perceive it to be.

If at all possible, you should consider a dual-use approach, using wood when the prices and supplies are in your favor, and reverting to coal when they're not.

TTFN

FAQ731-376

 

[rmw]

I have had projects on boilers that co-fired wood and coal (and oil and natural gas if it was cheap) but the primary fuel was wood so it had a traveling grate stoker for the wood but burned the coal in suspension. The back end package, dust collectors, precipitators, etc were designed for wood but handled the coal as well.

What you have to find out is the heat release requirements for the fuels that you want to burn. If your furnace is designed for coal, the furnace volume will be that required for coal on a BTU/ft3 basis using English units and the bottom of the furnace available for grate area will dictate that heat release requirement on a btu/ft2 basis. You are limited to the existing furnace volume and (potential) grate area and that will dictate how much wood you can fire. Find out what their heat release requirements are.

If you find that you have enough furnace volume and grate area, then you will have to look at your back end package to see if it will handle wood ash. Wood typically has more silica as sand in it than does coal so the gas velocities in the tube banks will have to be checked to see if there is any danger of accelerated erosion wear. I suspect not, however.

You will have to revise the air ducting to bring preheated air to the area under the grates and to tuyeres cut into (bent tube sections) the furnace tubing low in the boiler right above the grate for overfire air plus openings for the spreader stoker equipment. Then there will be the equipment for handling the wood fuel and metering it into the boiler, conveyors and screw feeders and spreaders.

The more I think about it, the more formidable it sounds. These are just a few thoughts that occur to me.

rmw

 

[davefitz]

best bet is to use " torrefied" wood- use the air-free torrefaction process- will result in less than 3% moisture in the wood chips, about 5-10% of the initial wood energy used in the drying process. Product chips would have a HHV of about 11,000 btu/lb, and the pulverizers would lose about 10% capacity.

typically russian coal boilers use indirect combustion fuel transport - they dry and transport the pulverized coal first to hoppers using hot fluegas , then educt the pulverized coal/wood from hoppers to the burners. This is different than western boilers, that use hot primary air to dry and directly transport the pulverized fuel to the burners- so this system would need to be modified to address the less dense wood chips.

less dense wood chips would eluetriate from the pulverizer table at a larger particle size than does denser coal, so a dynamic rotating classifier might be needed to ensure better fineness.

flame temp, ash fusion temp, burner to burner fuel flow unbalances will differ from the coal case.

100% wood firing would not have the extreme corrosion issues expected with a 50% coal/ 50% wood firing- in that latter case the combination of SO2 (from coal) with hi CO ( from unburned wood fuel in upper furnace) leads to high temp liquid phase coal ash corrosion at superheater tubes, but this can be avoided at 100% wood.

another issue would be the rapidly dissapearing forests, but if the locals live their life on the internet it no longer matters..

 

[davefitz]

best chance is using torrefied dried wood, using the air-free torrefaction process.

 

[macmet]

You will likely have to change your fans. The overfire/underfire ratios with wood and coal will be different. I'm not sure about total combustion gases off hand, but that might require a new ID fan.

I like the idea of cutting out the bottom and putting in a combustor designed for wood. Proper design of the combustion chamber would make the moisture level an easier matter to deal with.

 

[RossWexford]

Don’t waste your time on this project. The design of a coal burning boiler is optimized for the type of coal fired. A biomass boiler burns a very different type of fuel with a considerably lower heating value and high and very variable moisture content.
Biomass can also produces extreme corrosion of boiler tubes and components in boilers built from normal coal fired boiler materials.
If you want a biomass boiler get in a consultant who knows what he is talking about, but the really critical issue is the fuel supply. Over what distance will the wood chip have to be hauled? Do you have a guaranteed supply for the next 15 or so years? What price and moisture content is guaranteed?

 

[pbrod]

davefiz, can I contact you to have some more information about burning/grinding torrefied material?

 

[davefitz]

pbrod;

It may be better to discuss the issues in a public forum, since sending email info seems to increase the number of messages I get related to the minister of Nigerian finance desperately asking for my bank account number..

Which details on torrefaction are of interest?

 

[vpl]

Pbrod, Why not start a new question?

Patricia Lougheed

******

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

 

[pbrod]

I started a new discussion on:
torrefied biomass combustion
in the industrial category
thanks for the help

 

[boilerone]

It is almost NOT possible to directly burn 100% biomass with the same pulverized fuel burning method in a large PC boiler and wish for continuous operation. Below are the major problems you are going to run into:

1. huge slaggings and foulings
2. quick corrosions and erosions
3. gas flow-induced vibrations and damages
4. fouling and poisoning of SCR

All these problems are because PC boiler design is not really suitable for biomass combustion (except for low ratio co-firing) because of high alkali metal and chlorine contents in the fuel and more flue gas generated from combustion. The only current good technology is for utility scale 100% biomass is fluidized bed, either CFB or BFB. Why fluidized bed is more suitable is because it has much lower combustion temperature thus has much lower chance for ash-melting and slag-building plus chlorine reacting with alkali metal to cause severe corrosions.

BFB is especially good for converting existing PC boilers. Rather than changing the bottom of the furnace into a grate type (also you don't see a grate type for a 200 MW class boiler), you change it into a bubbling fluidized bed. Fluidized bed makes fuel ignition and combustion much easier and more stable. There has been many examples of these conversions and tons of dedicated biomass-burning BFB boilers have been in good operations for many years.

For CFB, even though much better than BFB on boiler size and efficiency, it is not recommended for your application because:
1. too many modifications, you will almost have a new boiler
2. 100% biomass combustion does not generate enough ash for circulation needed for CFB operation
3. system is more complicated and harder on the operation and maintenance aspects, more investment, too

Another big reason why 100% biomass conversion is not possible is because of loosing of partial (almost 40%) steam output capacity. This is because biomass has much lower heating value and density. You can imagine burning charcoal balls vs. dry hay or tree branches in a camp fire or stove. If want to maintain the original MW by forcing to burn more biomass in a furnace, large volume of flue gas generated will make the gas (which carries fly ash) velocity too high and cause quick erosions of heating surfaces. To solve the problem you will have to reduce the spacings of tubes in the heating surfaces but this means also loosing steam production. The problem is definitely smaller for torrefied biomass like Davefitz suggests.

One more note, like many mentioned, make sure you have enough and stable biomass supply. I guess in this part the Europe is better than America both on the wood source and dedicated machinery availability sides but, still, I have to mention an example: a planned UK 300MW biomass CFB in the recent news has to ship fuel from all different countries in Europe plus US and South America. Your fuel yard also has to be about 10 times bigger than what you have right now. So please do not overlook the problems.

 

[davefitz]

I agree with boilerone's comments as they apply to firing raw biomass in the existing, modified , boiler .

This link provides some history of the issues faced firing wood in european boilers over the period 1980-2003:

<

http://www.fwc.com/publications/tech_papers/files/TP_CFB_03_01.pdf>

Pay particular attention to the issues associated with high potassium levels in the fuel , and its affect on max permitted superheater steam temperature for surfaces in the fluegas pass- higher steam temperatures can be obtained by additional superheating in "external heat exchangers" in those CFB's that have such devices. Once these issues have been proven to be effectively addressed, a 95% boiler avaialblity can be guaranteed ( such guarantees may be required for some overseas boilers)

additional information on firing biomass in legacy PC boilers can be found on the webpages for <

http://www.ieabioenergy.com>

 

[boilerone]

Thanks Davefitz for mentioning possible steam temperature issue when converting to fluidized beds. It is because their combustion and flue gas temperatures are much lower than that of PC's so the superheaters and reheaters won't have enough temperature differential or need excessive heating surface additions to be able to heat up the steam especially for those subcritical and supercritical parameter boilers.

Common solution is to, like Davefitz says, put heating surfaces in the hot ash. Alstom CFB has external heat exchangers which take partial ash returns from the cyclone separators to pass through them before going back to the furnace. Foster Wheeler does the same except to have the heat exchangers directly attached to outsides of furnace walls. Metso simply uses the loopseals (the ash return devices under the cyclones) as heat exchangers. Each has certain advantages and disadvantages but at least they all use tube coils buried inside these heat exchangers to directly absorb heat from contacting with the hot ash and you can imagine it is very efficient doing this way. Since these are all just bubbling beds (they maintain the fluidizing intensity low enough just as needed for the ash to be able to move through), the ash velocity is low enough and won't create big erosion problems on the superheater or reheater 'inbed' tubes.

For BFB including a PC converted into BFB, you already have a low velocity bubbling bed inside the furnace so it is even more convenient to put steam tubes inside the bed without having to have any separate heat exchanger.