arippa technical symposium august 2008 neil raskin, senior project manager, services foster wheeler...

ARIPPA Technical SymposiumAugust 2008

Neil Raskin, Senior Project Manager, Services

Foster Wheeler North America Corp.Clinton, NJ

Combustion/Cofiring of Biomass within Circulating Fluidized Bed

(CFB) Boilers

ARIPPA Technical Symposium – August 2008

Combustion/Cofiring of Biomass within CFB Boilers

Why Combust/Cofire Biomass (Biofuel)

Biofuel are the “Politically Correct” fuel to combust/cofire because they are considered “Sustainable”, “Renewable”, and “CO2 neutral”.

“Green Power” produced from Biofuel can be sold at a premium in many states.

Cofiring Biofuel will reduce SO2 and Metals emissions.



Biofuel categories:

Sawmill wastes and by-products of lumber production

Pulp and paper mill waste

Agricultural waste

Forest residue



Biofuel characteristics:

High volatile (70 – 85 wt%)

Low ignition temperature (~400oF)

Varying moisture (20 – 60 wt %)

Low ash and sulfur

Potential high chlorine (corrosion) and alkali (fouling) content.



Standard Design

CFB Boiler Fuel Design ChallengesF

ue

l H

igh

er

He

ati

ng

Va

lue

FUEL RANK

Peat

Bark

Multiple Challenges Some Challenges No Challenge

Various Types of Wood

Demolition Wood

Deinking Sludge

Chip Board

Plywood

Bituminous Coals

Brown Coals, Lignite

Bio & FiberSludge

Paper

Petroleum Coke



Peat

Agricultural Waste• Rice Husks• Oat Hulls• Straw• Bagasse• Tree Prunings• Forest Clearing• Vine Trimmings• Nut Shells

Pulp & Paper Industry• Bark• Sludge

Recycled Wood• Wood Waste• Demolition

Wood• Furniture

Making



The following design parameters are to be considered when combusting or cofiring Biofuel within a Circulating Fluidized Bed (CFB) boiler



Biofuel such as Animal Manure, Chicken Litter, and High Alkali Biofuel (Na values >9% by wgt ash), Agro Wastes, Short Rotation Wood, and Energy Crops are not recommended to be combusted alone in a CFB boiler.

However, they can be cofired in small percentages when mixed with other “safe” fuels.



Property Range

Moisture 20 - 60 %

Ash 0.30 - 10.0 % wgt dry

Fixed Carbon 11 - 30 % wgt. dry

Volatiles 70 - 85 % wgt. dry

Sulfur trace - 0.50 % wgt dry

Chlorine trace - 0.60 % wgt dry

HHV 6,500 - 10,000 Btu/lb

Bulk Density 5 – 25, (avg. 18 lbs/ft3)



SO2: Generally not a concern. For the same emission level limestone usage will be reduced when cofiring Biofuel with coal.

NOx: High moisture content lowers combustion temperatures resulting in less thermal NOx being produced. High volatile content causes the Biofuel to combust in the upper furnace, where the volatiles react with already generated NOx reducing it to N2.



CO: Dry fine Biofuel combust in the upper furnace and cyclone, resulting in higher CO emissions. Larger wet Biofuel, such as wood chips combust in the lower furnace resulting in lower CO emissions.

VOC: Expected to as be low as other fuels.

Methane: Potentially higher than other fuels due to larger portion of the Biofuel’s volatile content is given off as methane.



Biofuel sizing is determined by the requirements for stable Biofuel feed system operation and to prevent after burning. The following is the recommended particle feed size:

98% < ~4.00”

50% < ~0.60”

80% > ~0.12”

Non-Biofuel sizing is determined by fuel volatility, ash content, and friability.



Biofuel to be cofired should be received, prepared, stored, and metered (weighed) separately from the CFB boiler’s main fuel.

However, Biofuel can be fed to the boiler by adding it to the boiler’s main fuel feed system after each fuel is individually weighed.

The above recommendations are based upon the following:



Independent Biofuel storage prevents a reduction in the boiler’s main fuel silo’s storage capacity due to Biofuel’s lower bulk density.

Independent coal and Biofuel feed control allows for:

Constant total heat input into the boiler to prevent load swings due to varying Biofuel heating value

Accurate measurement of the Biofuel for correlating heat input to accurately calculate the “Green Power” produced



Typical front wall Biofuel feed

system



Typical rear wall (loopseal) Biofuel

feed system

Non-Pressured Feeder

Rotary feeder

Expansion joint

Isolation Valve

Air bustle

Feed Chute

Loopseal connection



Typical rear wall – duel Biofuel feed system to loopseal including surge silo



Primary vs. secondary air split is 60%/40% for coal and 40%/60% for Biofuel. Therefore cofiring Biofuel with coal could affect the capacity of the primary and secondary air fans.

Biofuel have a higher moisture content than most fuels resulting in an increase in the total volumetric combustion gas flow which will affect ID fan and increasing the potential for erosion due to increased flue gas velocities.



Furnace and heat recovery area (HRA or back pass) velocities may increase when cofiring Biofuel with coal due to the increased volume of the flue gas.

Therefore, due to the higher potential for both corrosion and erosion Biofuel flue gas velocities within the HRA are limited to 25 - 35 ft/sec as compared to coal which is generally higher.



Biofuel bottom and fly ash unburned Carbon (UBC) will be less than coal. Thus cofiring Biofuel should reduce total UBC in the ash.

Ash re-circulation is not recommended when firing Biofuel since this would tend to enrich the alkali within the bed material thus increasing the potential for fouling.



The majority of Biofuel ash will be fly ash unless there is a substantial amount of dirt and rock present, which adds to the bottom ash.

Bottom/fly ash sold to produce concrete could be curtailed due to the present standard for concrete that limits the ash source to coal. This standard is presently being reviewed to allow for the potential use of non-coal based ash.



Bed inventory makeup will depend on the ash content of the fuel cofired with the Biofuel.

Additional bed make-up materials are Natural Sand, Limestone, or PC Ash.

Natural Sand recommended size distribution:

100% < 600 microns

75% < 250 microns

50% < 180 microns

25% < 130 microns


Combustion/Cofiring of Biomass within CFB BoilersNatural sand is preferred to quartz sand for bed inventory make-up because it contains less free quartz which tends to react with alkali to form low melting compounds.

Additionally quartz sand has a tendency to fracture at high temperatures and during thermal cycles resulting in higher bed inventory make-up rates.

Lastly, natural sand is chemically and structurally more stable at high temperatures.



Typical Sand Feed from Pneumatic Truck



Chlorides within Biofuel and limestone (naturally or from transportation) combined with sulfur can promote corrosion of pressure and non-pressure part metal surfaces, producing a combined “corrosion and erosion” affect.

The amount of total chlorine (wgt%, dry) within the fuel and limestone should be limited to <0.10%.

Total chlorine >3.0% (wgt%, dry) are considered to be a high corrosion potential and should be avoided.



Biofuels containing sodium and potassium can cause fouling.

Potential for fouling is based on Total Alkali Note 1 in the Total Ash Note 2:

Low Medium High <4.5 4.5-9.0 >9.0

Note 1: Total Alkali (% by wgt) = Na (% by wgt) + K/1.7 (% by wgt)

Note 2: Total ash (% by wgt) = fuel ash + limestone enerts + calcination and sulfation reaction products + make-up bed material + additives



Silica (SiO2) in the form of dirt, rocks, etc. from Biofuel harvested from fields or forests can increase the potential for fouling when combined with the alkali found in the Biofuel.



Fouling can be mitigated by adding basic compounds, such as CaO, MgO and Al2O3.

These compounds tend to make the bed material less sensitive to the formation of low melting alkali compounds, thus reducing fouling related problems.



Refractory issues: corrosion from Biofuel alkali can weaken refractory binders resulting in erosion of the refractory.

Low cement refractory that are manufactured with bauxite alumina in place of mulite alumina are less susceptible to alkali.

Additionally, man-made alumina have proven to be even more corrosion – erosion resistant.



Questions?

arippa technical symposium august 2008 neil raskin, senior project manager, services foster wheeler...

Documents

arippa technical symposium

cfb boilers biofuel

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lbsft3 slide

wgt ash

high alkali biofuel

wgt dry hhv

wgt dry chlorine trace