Improving Energy Efficiency through Biomass Drying

Transcription

1 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

2 Outline of Presentation Why biomass drying is important Drying technologies Conveyor/Belt Rotary Drum Other Dryers Selection & heat recovery Air emissions

3 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

4 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

5 Drying from 60% to 10% Moisture Content

6 Wet Wood Energy Balances Moisture Content, % by Weight 10% 30% 50% 60% Potential Recoverable 7,920 6,160 4,400 3,520 Energy, Btu/lb, HHV 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 Tons per 100 MMBtu/hr Net Input Basis: 8,800 Btu/lb (oven dry), 250⁰F Flue Gas Comb Air, 7% Excess O 2 USDA How to Estimate Recoverable Heat Energy in Wood or Bark Fuels, 1979

7 Potentially Recoverable versus Available Heat

8 Stack Losses and Combustion Efficiency From: U.S. DOE Steam System Assessment Tool

9 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

10 Most Common Types of Conveyor/Belt dryer Hog Fuel Dryers 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

11 Conveyor/Belt Dryers Material is laid on a moving perforated belt or belts.

12 Rotary Drum Dryers

13 Inlet Temperature Comparison for Drying Hog Fuel Rotary Drum dryers Require at least 500 o F for hog fuel More optimally operate around 800 o F Conveyor dryers Typically operate between 200 o F and 400 o F

14 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

15 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

16 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

17 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

18 Stacking of Conveyor Belts for Smaller Footprint

19 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

20 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

21 Rotary Drum Dryer Operation Operate most efficiently at higher inlet temperatures 800 o F inlet temperature and 150 o F exhaust temperature is typical for hog fuel (exhaust above 220 o F prevents acid and resin condensation) Temperature cannot be so high that material is scorched Moister biomass requires higher temperatures

22 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

23 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

24 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

25 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

26 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

27 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

28 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

29 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

30 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

31 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

32 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

33 References Biomass Drying and Dewatering for Combined Heat and Power, Northwest CHP TAP, October 2013, Dr. Carolyn Roos, wateringforcleanheatandpower.pdf Report on Biomass Drying Technology, National Renewable Energy Laboratory, November 1998, Recent advances in biofuel drying, Chemical Engineering and Processing, Issue 38, pp , 1999 Biomass Drying Technology Update, Tappi BioPro Expo, Atlanta, GA, March 14-16, 2011, by Matt Worley of the Harris Group. Drying wood waste with flue gas in a wood fuel dryer, Caddet Energy Efficiency, 1997, 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 df

34 Questions? Gilbert McCoy, Senior Energy Systems Engineer Northwest CHP Technical Assistance Partnership Washington State University Extension Energy Program