Post on 16-Dec-2015
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.
ARIPPA Technical Symposium – August 2008
Combustion/Cofiring of Biomass within CFB Boilers
Biofuel categories:
Sawmill wastes and by-products of lumber production
Pulp and paper mill waste
Agricultural waste
Forest residue
ARIPPA Technical Symposium – August 2008
Combustion/Cofiring of Biomass within CFB Boilers
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.
ARIPPA Technical Symposium – August 2008
Combustion/Cofiring of Biomass within CFB Boilers
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
ARIPPA Technical Symposium – August 2008
Combustion/Cofiring of Biomass within CFB Boilers
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
ARIPPA Technical Symposium – August 2008
Combustion/Cofiring of Biomass within CFB Boilers
The following design parameters are to be considered when combusting or cofiring Biofuel within a Circulating Fluidized Bed (CFB) boiler
ARIPPA Technical Symposium – August 2008
Combustion/Cofiring of Biomass within CFB Boilers
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.
ARIPPA Technical Symposium – August 2008
Combustion/Cofiring of Biomass within CFB Boilers
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)
ARIPPA Technical Symposium – August 2008
Combustion/Cofiring of Biomass within CFB Boilers
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.
ARIPPA Technical Symposium – August 2008
Combustion/Cofiring of Biomass within CFB Boilers
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.
ARIPPA Technical Symposium – August 2008
Combustion/Cofiring of Biomass within CFB Boilers
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.
ARIPPA Technical Symposium – August 2008
Combustion/Cofiring of Biomass within CFB Boilers
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:
ARIPPA Technical Symposium – August 2008
Combustion/Cofiring of Biomass within CFB Boilers
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
ARIPPA Technical Symposium – August 2008
Combustion/Cofiring of Biomass within CFB Boilers
Typical front wall Biofuel feed
system
ARIPPA Technical Symposium – August 2008
Combustion/Cofiring of Biomass within CFB Boilers
Typical rear wall (loopseal) Biofuel
feed system
Non-Pressured Feeder
Rotary feeder
Expansion joint
Isolation Valve
Air bustle
Feed Chute
Loopseal connection
ARIPPA Technical Symposium – August 2008
Combustion/Cofiring of Biomass within CFB Boilers
Typical rear wall – duel Biofuel feed system to loopseal including surge silo
ARIPPA Technical Symposium – August 2008
Combustion/Cofiring of Biomass within CFB Boilers
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.
ARIPPA Technical Symposium – August 2008
Combustion/Cofiring of Biomass within CFB Boilers
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.
ARIPPA Technical Symposium – August 2008
Combustion/Cofiring of Biomass within CFB Boilers
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.
ARIPPA Technical Symposium – August 2008
Combustion/Cofiring of Biomass within CFB Boilers
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.
ARIPPA Technical Symposium – August 2008
Combustion/Cofiring of Biomass within CFB Boilers
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
ARIPPA Technical Symposium – August 2008
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.
ARIPPA Technical Symposium – August 2008
Combustion/Cofiring of Biomass within CFB Boilers
Typical Sand Feed from Pneumatic Truck
ARIPPA Technical Symposium – August 2008
Combustion/Cofiring of Biomass within CFB Boilers
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.
ARIPPA Technical Symposium – August 2008
Combustion/Cofiring of Biomass within CFB Boilers
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
ARIPPA Technical Symposium – August 2008
Combustion/Cofiring of Biomass within CFB Boilers
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.
ARIPPA Technical Symposium – August 2008
Combustion/Cofiring of Biomass within CFB Boilers
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.
ARIPPA Technical Symposium – August 2008
Combustion/Cofiring of Biomass within CFB Boilers
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.