torrefied biomass combustion

[pbrod]

Hello,
I'm investigating the posibilities to burn torrefied biomass in a classical boxer fired PC boiler. the main aspects we want to investigate are:
- explosion safety/fire in the coal mills when processing torrefied biomass. An assesment of the risk of milling this material.
- combustion behaviour of this material and how it interacts with the furnace (slagging, fouling, corrosion, flame stability,...)

Does someone have experience with burning torrefied biomass in large utility boilers. Are there other cases known?

thanks in advance

 

[boilerone]

To add to Davefitz's recommendations on making sure to have a direct blowing system without intermediate storage bins seen in the thread of 'Conversion pulverized coal boiler to burn 100% biomass', I would add that to use similar explosion and safety precautions while dealing with PRB coal which is dangerous when dried. There are tons of experiences there. If the torrefied biomass really comes much dryer than expected and gives trouble, I would even recommend adding water to it before milling. A little bit better than pulverized coal is that for biomass it has less requirement on milled particle size therefore gives less surface area for explosion happening. Other methods include using cold air instead of hot air (because the fuel is already dry) or mix with flue gas which has low oxygen content.

Slagging, fouling and corrosion are typical problems for burning biomass and torrefied biomass is not an exception. Torrefied woody biomass rather than herbaceous biomass and lower combustion and flue gas temperatures (not very possible with PC, better go with fluidized bed) are recommended. The higher the biomass ratio the worse fouling problem plus harder to control temperatures on existing boilers without major modifications. Nobody has ever been able to do continuous 100% biomass on a large PC boiler without having large problems. 15% is pretty much the current limit from almost all references. 100% biomass combustion is only achieved by converting into fluidized beds with partial loss of total MW output.

Torrefied biomass should be much better than untreated biomass on flame stability because of less and consistent moisture content and it will require less boiler heating surface adjustment because of being closer to that of typical coal with high volatile content. I guess that's why Davefitz recommends it. Contact boiler manufacturers for their references and get better confidence. I think every body is interested in this topic and want to get your business now-a-days.

I guess the disadvantage of torrefied biomass would be to use energy to dry the biomass and the raised cost. This is similar to that of these two companies that are currentlt drying PRB coal to make it into higher grade. One is using steam the other is using flue gas. You might be able to offset the cost by having lower transportation spending plus higher heating values. Most critical is that this might be the only option if want to have a higher co-firing rate on your existing PC boiler.

 

[davefitz]

A few points from memory:

a) this link <

http://www.techtp.com/recent papers/BE2002.htm>

may provide a primer on what torrefied wood is, using teh air-free porcess.

b) in North Carolina, we are required to generate about 15% of our power using renewable resoureces by 2021. Corfiring wood chips or biomass in a PC unit is usually limited to about 5% heat input as biomass- higher than that , then one risks accelerated corrosion ( liquid phase ash) of the superheater due to low temperture ash eutectics formed by the combination of coal ash volatiles, sulphur from coal, potassium from biomass, and reducing environments due to rapid elutriation of unburned biomass to the upper furnace superheaters . To get past this 5% limit with minimal capital investment, torrefied wood TW was investigated.

c) Southern Alternative Fuels was proposing to build 2 torrefaction facilities in NC , nominally to supply torrefied wood chips for export to England. In 2008 a rough estimate of $100/ton for TW was pro-offered at that time, and this was close to the 2008 cost of coal. So , economically, it sort of sounded feasible. Their grindability tests showed that it would cause only a 5% derate in the mills capability, and the very low moisture content plus HHV close to eatern bit coal suggested that we could reach a 15% fuel heat input using TW simply by adding it to the coal pile or conveyor.

d) we have not yet conducted a test burn, but one such test burn was conducted in August of 08 by ( city of Springfield ?) . Their 4 hr test burn ( 30% fuel heat input) showed no decline in mill performance or boiler perfomance, but it sounded like a very informal test- accurate long term test data is required to make a reasoned estimate .

e)More testing by the mill oem on mini-mills to confirm grindability and determination of need for mill inerting or pulverizer reinforcement ( to 50 psig puffs per NFPA) is also needed.

f) another means of meeting the 15% renewable fuel heat input is to install a 600 psig aux boiler, firing 100% biomass, and used in a parrallel powered combined cycle mode( ie, isolate STG steam extractions to the feedwater heaters and use LP steam from the biomass boiler instead).

TW at 2% moisture at $100/ton is not as economimical as wet raw waste wood at $20/ton delivered, even considering the lower boiler HHV efficiency with wet raw wood , but TW may have a place in achieving the 15% renewable input by allowing continued use of legacy PC boilers without modivfication. Regulatory issues are also hindering its use.

 

[davefitz]

correction:

The TW test burn was during August 2009, at city of Springfield James river plant, burning 20% TW for 7 hrs , pulverized in CE raymond bowl mills.