Improving Energy Efficiency through Biomass Drying

Improving Energy Efficiency through Biomass Drying

Gilbert McCoy, Senior Energy Systems Engineer Northwest CHP Technical Assistance

Partnership

International District Energy Association Woody Biomass CHP & District Energy Workshop

Seattle, Washington June 11, 2014

Outline of Presentation

• Why biomass drying is important

• Drying technologies

• Conveyor/Belt

• Rotary Drum

• Other Dryers

• Selection & heat recovery

• Air emissions

Biomass Fuels

• Hog fuel

– Biomass fuel that has been prepared by processing through a "hog" - a mechanical shredder or grinder

• Bark

• Sawdust (usually dry)

• Clean urban wood waste

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

Drying from 60% to 10% Moisture

Content

Wet Wood Energy Balances

Moisture Content, % by Weight

10% 30% 50% 60%

Potential Recoverable Energy, 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 Potential Recoverable Energy

85.1 81.0 73.5 67.0

Tons per 100 MMBtu/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

Potentially Recoverable versus

Available Heat

Stack Losses and Combustion Efficiency

From: U.S. DOE Steam System Assessment Tool

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 NOx emissions

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

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

Conveyor/Belt Dryers

Material is laid on a moving

perforated belt or belts.

Rotary Drum Dryers

Inlet Temperature Comparison for Drying Hog Fuel

• Rotary Drum dryers

– Require at least 500^oF for hog fuel

– More optimally operate around 800^oF

• Conveyor dryers

– Typically operate between 200^oF and 400^oF

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

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

Disadvantages of Conveyor Dryers

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

• Can take up more floor space if belts

aren’t stacked. Stacking adds complexity

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

Footprint Comparison

• If unstacked, conveyor dryer footprint is larger than rotary dryer

• Stacking reduces footprint

• On stacked belts, biomass cascades from one belt down to another

Stacking of Conveyor Belts

for Smaller Footprint

First Cost Comparison

• Rotary drum and conveyor dryers have similar first costs

• In new installations, conveyor dryer projects can have lower total installed

costs because there may be savings in air pollution control equipment requirements

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  increased

operation costs

Rotary Drum Dryer Operation

• Operate most efficiently at higher inlet temperatures

– 800^oF inlet temperature and 150^oF exhaust

temperature is typical for hog fuel (exhaust above 220^oF prevents acid and resin condensation)

• Temperature cannot be so high that material is scorched

• Moister biomass requires higher temperatures

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&M costs than indirect-fired rotary dryers

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

Direct-Fired Rotary Dryers

• Retention time of 10 to 30 minutes for larger material

• Disadvantage of greater VOC emissions (may require a regenerative thermal

oxidizer (RTO) for VOC control)

• Greatest fire hazard

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

Rotary Drum Dryer Example

100 tons per day of bark:

• Dryer Size: 6 feet diameter and length of 24 to 30 feet required

• Cost: $400,000 to $500,000 roughly

• Because of small size, this dryer would probably not be cost effective unless it makes good use of waste heat recovery

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

Sizing Considerations

Size the boiler and dryer together:

– Dryer capacity should be well matched with the boiler fuel requirements

– Smaller boiler will be required for a rated

maximum steam production when a dryer is used

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 heat sources

• Recover heat from dryer exhaust

Flue Gas Heat Recovery

• With a rotary drum dryer, flue gas heat recovery is less 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 more cost 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 heat

recovery methods

VOC Emissions

• Volatile organic compounds (VOCs), such as

terpenes and wood oils, are exhausted from hog fuel dryers

• VOC emissions depend on biomass type,

operating temperature, residence time, and final moisture content

– Southern Pine 8 to 9 lbs/dry ton – Hardwoods 1 to 2 lbs/dry ton

• VOC destruction may require a regenerative

thermal oxidizer (RTO). Provides 98% destruction

– BACT trigger is 40 tpy for VOCs

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

temperature

Recommendation: Talk early and often

Operation and Maintenance

• Conveyor dryers have highest O&M costs and 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 all hog fuel dryers

References

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

http://www.northwestchptap.org/NwChpDocs/BiomassDryingAndDe wateringForCleanHeatAndPower.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”, Caddet Energy Efficiency, 1997, http://lib.kier.re.kr/caddet/ee/R273.pdf

• “Biofuel Drying as a Concept to Improve the Energy Efficiency of an Industrial CHP Plant”, doctoral dissertation by Henrik Holberg,

Helsinki University of Technology. April 2007

http://lib.tkk.fi/Diss/2007/isbn9789512286492/isbn9789512286492.p df

Gilbert McCoy, Senior Energy Systems Engineer Northwest CHP Technical Assistance Partnership

Washington State University Extension Energy Program [email protected]

Questions?