5McCOYGIL-LATESTWoody-Biomass-Drying-and-Dewatering-IDEA-06-2014.pdf

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Outline of Presentation

• Why biomass drying is important

• Drying technologies• Conveyor/Belt

• Rotary Drum

• Other Dryers

• Selection & heat recovery

• Air emissions


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Biomass Fuels

• Hog fuel

– Biomass fuel that has been prepared byprocessing through a "hog" - a mechanicalshredder or grinder

• Bark

• Sawdust (usually dry)

• Clean urban wood waste


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Why is Fuel Drying Important?

Not required for direct combustion, but:

• Drying significantly improves the efficiency of the boiler

system when flue gas is used for drying energy• For boiler:

– (+)5% to 15% improvement in efficiency

– (+)50% to 60% more steam production• Improves combustion efficiency and control

• Reduces air emissions

• Reduces feedstock (fuel) costs

• Reduces ancillary power requirements


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Drying from 60% to 10% Moisture

Content


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Wet Wood Energy BalancesMoisture Content, % by Weight

10% 30% 50% 60%

Potential RecoverableEnergy, Btu/lb, HHV

7,920 6,160 4,400 3,520

Dry Gas Loss, Btu/lb (509) (396) (283) (226)

Hydrogen Loss, Btu/lb (557) (433) (309) (247)

Moisture Loss, Btu/lb (115) (344) (573) (687)

Available Heat, Btu/lb 6,740 4,988 3,235 2,359

% of PotentialRecoverable Energy

85.1 81.0 73.5 67.0

Tons per 100MMBtu/hr Net Input

7.4 10.0 15.5 21.2

Basis: 8,800 Btu/lb (oven dry), 250⁰F Flue Gas ‐ Comb Air, 7% Excess O2

USDA “How

to

Estimate

Recoverable

Heat

Energy

in

Wood

or

Bark

Fuels”,

1979


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Potentially Recoverable versus

Available Heat


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Stack Losses and Combustion Efficiency

From: U.S. DOE Steam System Assessment Tool


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Drawbacks of Drying Fuel• Flame temperature can approach the ash

fusion temperature• Must accommodate dryer downtime (provide

backup fossil fuel boiler or dried fuel storage)

• High flame temperatures can increase NOxemissions

• Expensive dryer materials are required if fluegases are cooled below the dew point


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Most Common Types of

Hog Fuel Dryers• Conveyor/Belt dryer

– Flue gas or air passed through material on a belt

• Rotary Drum dryers – traditional, most common

– Direct-fired• Flue gas or heated air passed directly through biomass

– Indirect-fired• Steam, flue gas or heated air passed through heat exchanger

inside dryer

• Others types: flash and cascade dryers,superheated steam dryers, bed/grate dryers


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Inlet Temperature Comparison

for Drying Hog Fuel

• Rotary Drum dryers – Require at least 500oF for hog fuel

– More optimally operate around 800oF

• Conveyor dryers – Typically operate between 200oF and 400oF


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Conveyor/Belt Dryers

• Have long, proven history in many industries

• Suitable for drying many types of materials

– But fines tend to fall through belt perforations. – Can have tar/fines buildup issues


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Advantages of Conveyor/Belt

Dryers over Rotary Drum Dryers

• Operate at lower temperature

– greater efficiency

– reduced fire hazard

– reduced emission VOCs

– greater opportunity to recover waste heat

• Do not agitate biomass undergoing drying

– Reduced particulates in emissions – Doesn’t ball up sticky or high clay biomass


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Disadvantages of Conveyor Dryers

• Fines that would filter through belt must beseparated out and added in later

• Can take up more floor space if beltsaren’t stacked. Stacking adds complexity

• O&M costs are higher than for direct orindirect-fired rotary dryers


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Footprint Comparison

• If unstacked, conveyor dryer footprint islarger than rotary dryer

• Stacking reduces footprint

• On stacked belts, biomass cascades fromone belt down to another


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Stacking of Conveyor Belts

for Smaller Footprint


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First Cost Comparison

• Rotary drum and conveyor dryers have

similar first costs• In new installations, conveyor dryer

projects can have lower total installedcosts because there may be savings in airpollution control equipment requirements


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Rotary Drum Dryers• Most common dryer used in drying hog fuel

• Have long, proven history in many industries• Suitable for drying hog fuel, sawdust, bark

• Can produce 5 to 50 tons/hour of product dried to

10% moisture content• Will ball up high clay sludge.

• Not as suited for heat recovery as they require a

higher operating temperature increasedoperation costs


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Rotary Drum Dryer Operation• Operate most efficiently at higher inlet

temperatures – 800oF inlet temperature and 150oF exhaust

temperature is typical for hog fuel (exhaust above

220oF prevents acid and resin condensation)• Temperature cannot be so high that material is

scorched

• Moister biomass requires higher temperatures


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Direct-Fired Rotary Dryers• Flue gas or hot air is passed directly

through the medium to be dried• Exhaust gas recirculation (EGR) improves

heat transfer and reduces fire risk

• Have lower electrical power and O&Mcosts than indirect-fired rotary dryers

• Good energy efficiency: 1,500 to 1,800Btu/lb of water evaporated


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Direct-Fired Rotary Dryers

• Retention time of 10 to 30 minutes for

larger material

• Disadvantage of greater VOC emissions

(may require a regenerative thermaloxidizer (RTO) for VOC control)

• Greatest fire hazard


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Indirect-Fired Rotary Drum Dryers• Steam or flue gas is passed through tubes or

heat exchanger inside the dryer – instead of directly through the material to be

dried as in direct-fired dryers

• Well suited for drying fine and dusty materials• Efficiency of an indirect-fired steam dryer itself is

less than for direct-fired dryers because of the

heat exchanger


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Rotary Drum Dryer Example

100 tons per day of bark:

• Dryer Size: 6 feet diameter and length of24 to 30 feet required

• Cost: $400,000 to $500,000 roughly

• Because of small size, this dryer wouldprobably not be cost effective unless it

makes good use of waste heat recovery


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Considerations in Selecting a

Biomass Dryer

• Heat Recovery

• Energy Efficiency

• Air Emissions

• Sizing Boiler and Dryer Together • Operations and Maintenance

• Feed & Discharge

• Electrical Energy Consumption


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Sizing ConsiderationsSize the boiler and dryer together:

– Dryer capacity should be well matched with theboiler fuel requirements

– Smaller boiler will be required for a rated

maximum steam production when a dryer is used


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The Key is Heat Recovery

Heat recovery is key to a cost-effective

dryer project

• Recover heat from flue gas of power boiler

• Recover heat from other waste heatsources

• Recover heat from dryer exhaust


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Flue Gas Heat Recovery

• With a rotary drum dryer , flue gas heat recovery isless cost effective – A boiler feedwater economizer can recover boiler flue gas

heat more cost effectively than a dryer – Requires higher temperature so exhaust from economizer

is not adequate for drying purposes

• With conveyor dryers, flue gas heat recovery is morecost effective – Lower temperature, so we can recover heat with a

combustion air preheater and a feedwater economizer – Can cascade heat from the air preheater to economizer to

dryer to take full advantage of multiple flue gas heatrecovery methods


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Air Permit Requirements• The local air quality management district has

jurisdiction

• Each project is addressed on its own merits

• Potential Issues:

– Need for an afterburner or RTO – Integrate new equipment with particulate controls

– Plume rise and dispersion with reduced stack

temperatureRecommendation: Talk early and often


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Operation and Maintenance

• Conveyor dryers have highest O&M costsand hence lowest availability

– More parts to maintain. Chain, belt, drive, etc• Steam dryers have greater O&M costs

than flue gas dryers

• Corrosion and erosion is a problem in allhog fuel dryers


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References• “Biomass Drying and Dewatering for Combined Heat and Power”,Northwest CHP TAP, October 2013, Dr. Carolyn Roos,

http://www.northwestchptap.org/NwChpDocs/BiomassDryingAndDewateringForCleanHeatAndPower.pdf

• “Report on Biomass Drying Technology”, National Renewable

Energy Laboratory, November 1998,http://www.nrel.gov/docs/fy99osti/25885.pdf • “Recent advances in biofuel drying”, Chemical Engineering and

Processing, Issue 38, pp. 441-447, 1999• “Biomass Drying Technology Update”, Tappi BioPro Expo, Atlanta,

GA, March 14-16, 2011, by Matt Worley of the Harris Group.http://www.tappi.org/content/events/11biopro/19.2worley.pdf

• “Drying wood waste with flue gas in a wood fuel dryer”, CaddetEnergy Efficiency, 1997, http://lib.kier.re.kr/caddet/ee/R273.pdf

• “Biofuel Drying as a Concept to Improve the Energy Efficiency of anIndustrial CHP Plant”, doctoral dissertation by Henrik Holberg,Helsinki University of Technology. April 2007http://lib.tkk.fi/Diss/2007/isbn9789512286492/isbn9789512286492.pdf


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Gilbert McCoy, Senior Energy Systems Engineer

Northwest CHP Technical Assistance PartnershipWashington State University Extension Energy Program

[email protected]

Questions?

5McCOYGIL-LATESTWoody-Biomass-Drying-and-Dewatering-IDEA-06-2014.pdf

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