Technical and Economic Considerations In

Selection of Boiler Size and Thermal Storage CapacityFor

Commercial and Small Industrial Wood-fired Boiler Systems

“Small can be beautiful”

Raymond J Albrecht, P.E.

Technical Consultant to Lopper North America

rayalbrechtpe@gmail.com

www.LopperNorthAmerica.com

Typical Wood Boiler Oversizing Impacts

Oversizing factor of 2 X or even 3 X peak heating load

Increased space requirements within boiler room

Capital cost premium for boiler, fuel feeding components, multi-cyclones and electrofilters, etc.

Efficiency penalty during start-ups for morning heat during Fall/Spring seasons

Excess hours for low load operating conditions (smoldering) with increased PM 2.5 and UHC/CO emissions due to lower average flame temperature

Tool for sizing of wood boilers: ASHRAE Weather DataAmerican Society of Heating, Refrigerating and Air-conditioning Engineers

Thousands of US/Canada and global locations

Averages and maximum/minimum values

Temperatures

Bin hour temperature data

Humidity

Wind Speed

Solar Radiation

Available at www.ashrae.org website

Heating Bin Hours vs. Outdoor Temperature

Typical northern US location

Design Temp = Minus 5 deg F

4800 hours per year at 55 deg For lower

3900 hours occur between 25oF and 55 deg F

80% of heating hours at 50%or less of peak heating load

Wood-fired boiler sized to provide 100% of peak heating load

Output Analysis - 2 MMBTU/hr wood-fired boiler for entire heating load

Temp bin Annual Heating Wood boiler Oil boiler Total heating Wood boiler Oil boiler(deg F) hours Load output in bin output in bin output in bin output in bin output in bin

in bin Btu/hr BTU/hr BTU/hr MMBTU/yr MMBTU/yr MMBTU/yr

-5/-1 6 1917000 1917000 - 12 12 -

0/4 58 1750000 1750000 - 101 101 - 5/9 66 1583000 1583000 - 104 104 -

10/14 125 1417000 1417000 - 177 177 -

15/19 243 1250000 1250000 - 304 304 -

20/24 354 1083000 1083000 - 383 383 -

25/29 511 917000 917000 - 468 468 -

30/34 957 750000 750000 - 718 718 -

35/39 720 583000 583000 - 420 420 -

40/44 636 417000 417000 - 265 265 -

45/49 577 250000 250000 - 144 144 -

50/54 585 83000 83000 - 48 48 -

Totals 4838 hrs 3144 3144

Wood-fired output = 3144/3144 = 100 percent of total system output

Heating oil savings = Approximately 38,000 gallons

Wood boiler meets 100% of peak heating load

Wood boiler provides 100% of annual heating output

Wood-fired boiler sized to provide 75% of peak heating load

Can meet heating load down to about 10 deg F

Supplemental boiler provides output only for portion of heating load in excess of 1.5 MM Btu/hr at temperatures below 10 deg F

Output Analysis - 1.5 MMBTU/hr wood-fired boiler plus 500,000 Btu/hr Oil-fired Boiler

Temp bin Annual Heating Wood boiler Oil boiler Total heating Wood boiler Oil boiler(deg F) hours Load output in bin output in bin output in bin output in bin output in bin

in bin Btu/hr BTU/hr BTU/hr MMBTU/yr MMBTU/yr MMBTU/yr

-5/-1 6 1917000 1500000 417000 12 9 3

0/4 58 1750000 1500000 250000 101 87 14 5/9 66 1583000 1500000 83000 104 99 5

10/14 125 1417000 1417000 - 177 177 -

15/19 243 1250000 1250000 - 304 304 -

20/24 354 1083000 1083000 - 383 383 -

25/29 511 917000 917000 - 468 468 -

30/34 957 750000 750000 - 718 718 -

35/39 720 583000 583000 - 420 420 -

40/44 636 417000 417000 - 265 265 -

45/49 577 250000 250000 - 144 144 -

50/54 585 83000 83000 - 48 48 -

Totals 4838 hrs 3144 3122 22

Wood-fired output = 3122/3144 = 99 percent of total system output

Wood boiler meets 75% of peak heating load

Wood boiler provides 99% of annual heating output

Can meet heating load down to about 20 deg F

Supplemental boiler provides output only for portion of heating load in excess of 1 MM Btu/hr during temperatures below 20 deg F

Wood-fired boiler sized to provide 50% of peak heating load

Output Analysis - 1 MM BTU/hr wood-fired boiler plus 1 MM Btu/hr oil-fired boiler

Temp bin Annual Heating Wood boiler Oil boiler Total heating Wood boiler Oil boiler(deg F) hours Load output in bin output in bin output in bin output in bin output in bin

in bin Btu/hr BTU/hr BTU/hr MMBTU/yr MMBTU/yr MMBTU/yr

-5/-1 6 1917000 1000000 917000 12 6 6

0/4 58 1750000 1000000 750000 101 58 43 5/9 66 1583000 1000000 583000 104 66 38

10/14 125 1417000 1000000 417000 177 125 52

15/19 243 1250000 1000000 250000 304 243 61

20/24 354 1083000 1000000 83000 383 354 29

25/29 511 917000 917000 - 468 468 -

30/34 957 750000 750000 - 718 718 -

35/39 720 583000 583000 - 420 420 -

40/44 636 417000 417000 - 265 265 -

45/49 577 250000 250000 - 144 144 -

50/54 585 83000 83000 - 48 48 -

Totals 4838 hrs 3144 2915 229

Wood-fired output = 2915/3144 = 93 percent of total system output

Wood boiler meets 50% of peak heating load

Wood boiler provides 93% of annual heating output

Heating Output vs Wood and Oil Boiler Capacity

Horizontal Axis: Wood Boiler Capacity – Percent of Peak Heating Load

Vertical Axis: RED Percent of Annual Wood Heating Output (also percent Oil Savings)

BLUE Percent of Annual Oil Heating Output

Percent PercentOf Peak Of AnnualHeating Heating Output/Load Oil Savings

25% 63%

50% 93%

75% 99%

100% 100%

125% 100%

150% 100%

175% 100%

200% 100%

Eliminating the Smoldering Mode

300 kW wood-fired boiler plus 150 kW wood-fired boiler plus 150 kW oil-fired boiler for Peak and Low Loads

Use oil -fired boiler until heating load reaches at least 20-30 percent of capacity of smaller boiler (i.e., about 50 kW) depending on thermal storage

“On” temperature for wood-fired boiler about 10 deg C

Wood-fired boiler still provides 95% of total annual output

Thermal Storage

1 BTU = 1 lb Water X 1 deg F delta T

Typical practical delta T = Approximately 40 to 60 deg F

One gallon Water = 8.33 lbs

100 gallons Water X delta T of 40 deg F = 33,320 BTU of thermal storage

500 gallons Water X delta T of 60 deg F = 250,000 BTU of thermal storage

20,000 gallons Water X delta T of 40 deg F = 6,650,000 BTU of thermal storage

20,000 gallons Water = 2666 ft3 = Tank dimensions of 18 ft L X 18 ft W x 8 ft H

Case Study: Large Institutional Logwood-fired Boiler System

Abbey of St Benoit du Lac

St Benoit du Lac, QuebecCanada

Twin Commercial Size Logwood-fired Boilers

Ultra-low PM 2.5 emissions (0.01 to 0.02 lb/MMBtu )

Less than 100 ppm NOx

82 – 84 percent efficiency (HHV)

Lambda (air/fuel ratio) control with independent primary and secondary air flow control

Vertical extension to primary gasification chamber to double wood loading capacity up to 16 hours full-capacity burn time

Decoupling of wood loading capacity from firing capacity important factor in meeting user interest in reduced frequency of reloading

Each boiler has 1.1 million BTU/hr (320 kW) thermal capacity

Total thermal capacity of 2.2 million BTU/hr (640 kW)

Peak heating load is about 2.5 million BTU/hr (730 kW)

Approximately 150,000 square feet of heated space

Storage Tank

Benefits include smoother boiler loads plus pre-charging to handle peak heating loads

Approximately 6000 gallons

6000 gal X 8.33 lb/gal X 40 deg F delta T =

2 million Btu storage capacity

Can boost system capacity by 500,000 Btu/hr for 4 hrs during peak morning loads

Storage barns for logwood

350 cords per year

One year drying time

Steel racks for logwood bundles

Boilers can handle up to 48 inch log lengths

Approximately 1/3 cord wood per bundle

One logwood bundle provides for 8 hours of boiler operation under full-load conditions

Boilers can handle two bundles maximum wood loading

Tractor loading of logwood bundles

Tractor with front end loader carries logwood bundles from storage barn to boiler house

System includes small overhead crane

Bundle is lowered into top loading door

Chains are then released and removed with bracket assembly

Economics of Logwood Boiler Installation

Purchase cost of approximately $50,000 per boiler

Installation cost of approximately $50,000 including labor and accessories

Total system installed cost of approximately $150,000

Heating oil savings = 60,000 gallons per year

Heating oil cost savings = about $220,000

Logwood consumption = 350 cords per year

Total logwood fuel cost = $35,000 per year

Labor/equipment cost = $50,000 per year (estimate)

Total cost savings = $130,000 per year

Payback period = Just over one year

Payback period would have been more than two years if boiler oversized by 2 or 3 X factor especially if larger building required

Payback period would be about 9 months if only one boiler had been installed

Abbey of St Benoit du Lac

St Benoit du Lac, QuebecCanada

Craftsbury Outdoor CenterCraftsbury, Vermont

Photos courtesy of WR Boilers of Fairfield, Vermont and Lopper North America

Integrated energy system of industrial-grade logwood-fired boiler (240,000 Btu/hr) with 150 kW biodiesel-fired CHP and solar thermal plus 20,000 gallons thermal storage system

Conclusions

25 % undersizing only 1 % loss of total annual wood heating output and heating oil savings

50 % undersizing only 7 % loss of total annual wood heating output and heating oil savings

Reducing boiler size by 50 % can save 20 to 30 % in capital cost andlength of payback period

Using oil or propane-fired boiler for low load conditions can preventsmoldering conditions with just small increase in oil/propane use

Shorter payback periods More customersHigher efficiency Lower Fuel Costs

Lower emissions Happier neighbors

“Small can be beautiful”

Thank You!