T. Ferguson, University of Minnesota, Duluth. 2008

Session 12 - Biomass

• Definition• Photosynthesis Again• Examples

– Heat and Power from

Wood and Wood Waste– Biogas– Ethanol

• Environmental Impacts• R&D

T. Ferguson, University of Minnesota, Duluth. 2008

Definition of Biomass

• Living Plant Matter and Organic Wastes– Trees– Grasses1

– Animal Dung– Sewage– Garbage– Wood Construction Residues

• Biomass second only to hydro

1This includes corn, other grains

T. Ferguson, University of Minnesota, Duluth. 2008

Photosynthesis

6 CO2 + 6 H2O → C6H12O6 + 6 O2

ΔH = +2800 kJ = 668 kcal

• Electromagnetic to Chemical Energy• Of the solar radiation not reflected:

– Light penetration of canopy is 80 % efficient– Only visible light, or 43 % of total light, is useful– Glucose formation from photons is 28.6 % efficient– Respiration reduces energy by 33% (67 % efficiency factor)– All told, of absorbed light energy, about 7 % is stored chemically1

• Plants are a great energy storage medium!

1From upei.ca/~physics/p261/Content/Sources_Conversion/Photo-_synthesis/photo-_synthesis

T. Ferguson, University of Minnesota, Duluth. 2008

Photosynthesis

Rate of production of Energy = Rate of production of dry biomass/land area

X Total growing area available

X Unit energy content of dry biomass

X Efficiency of conversion

My backyard: • 1 acre yields 500 mature poplar trees/20 years or 25 tons/yr/acre• At 7000 Btu/lb, or 14 E 6 Btu/ton, annual energy is 350 E6 Btu• Conversion efficiency = 20% (crude genset)• Rate of Energy Production = 70 E6 Btu/yr or 2.3 kW continuously• At 8 c/kWh, I could purchase this electrical energy for $1640.

T. Ferguson, University of Minnesota, Duluth. 2008

Biomass Properties

• Heating value similar to lignite (7000-9000 Btu/lb)

• H/C (1.2-2) better than coal, less than CH4

• Low sulfur, nitrogen oxide emissions• Drawbacks

– Water content– Non-organic content– Non-beneficial oxygen adds to bulk– Expensive to haul, low production density, competition

• Supplies about 1.4% of US generating capacity

T. Ferguson, University of Minnesota, Duluth. 2008

Biomass: For Heat or Power?

Biomass3.1 Quads

For Heat/Steam:2.5 Quads

For Electrical Generation:0.6 Quads (input)

Figures are for the US for 2005. Source: EIA (renewables/electricity net gen)

T. Ferguson, University of Minnesota, Duluth. 2008

Biomass PropertiesHeat Contents in MMBtu/ton

• Wood/wood waste 9.961

• Utility Poles 12.500

• Tires 26.865

• Peat 8.000

• Paper Pellets 13.029

• Municipal Solid Waste 9.945

• Agricultural Byproducts 8.248

Source: EIA, 2008

T. Ferguson, University of Minnesota, Duluth. 2008

Heat and Power from Wood

• 2007 Electrical Generation: 55 million MWh from Biomass – 39 million from wood

• Pulp and paper use vast majority for steam

• Minnesota Power (based in Duluth):– Three wood-fired plants (including co-firing):

• Hibbard (Duluth) 48 MW• Cloquet Energy Center 23 MW• Rapids Energy Center 30 MW

T. Ferguson, University of Minnesota, Duluth. 2008

Virginia (MN) Public Utilities Biomass Generating Station• Co-generation: municipal steam (~1500 homes) + electrical generation• Fuel: 75% PRB coal + 25% wood (reverse in future)• Annually: 175,000 tons of wood – 75 economic radius for wood• Daily: 20 truck loads @ 25 tons (4-5 trucks/hour); 15 hour inventory• Rankine Cycle: 800 °F turbine inlet, 40-50 °F lake temp• Generators: 18.5 MW, 6.5 MW• Issues: Fugitive dust, boiler performance, attaining capacity factor (86% target),

lake mist

T. Ferguson, University of Minnesota, Duluth. 2008

Biogas from Wet Biomass

• Anaerobic Digestion of Wet Biomass• Produces ~50/50 mix of CH4 and CO2

• With pure CH4, get ~ 1000 Btu/cubic foot• Biogas, then, yields ~ 500 Btu/SCF• Largest producers in US are landfills• Why pursue?

– Environmental hazards of dairy farms, feedlots (coliform bacteria, methane)

– Methane is a significant GHG (GWP of 56 times CO2 over 30 years)

– Heat and power may be secondary benefits

T. Ferguson, University of Minnesota, Duluth. 2008

Biogas Production

• Bacteria digest biomass in absence of oxygen, in a digester

• Operates between 68 °F and 150 °F• Higher temp accelerates production• Farm Scale: 500 dairy cows: 8000 gallons of

manure/day = 1070 ft3 = 30 m3.• Landfill Scale: >40 ft deep and > 1 million tons of waste

(Landfill is fastest growing biomass electricity source: 4%/yr)

• Production: 30 m3/day*1.0m3gas/day/m3 = 30 m3 of biogas/day = 1070 ft3, or 500 SCF of CH4 or 0.5 million Btu. Across 24 hours at 35% conversion efficiency, this represents 6.1 kW.

Source: Oregon.gov/Biomass Energy Home Page; text p. 438

T. Ferguson, University of Minnesota, Duluth. 2008

Ethanol from Corn

• Biomass to ethyl alcohol• From Text, page 240, for corn fermentation:

– Input for 1 gallon of ethanol:• Corn Production 55,300 Btu• Fermentation/Distillation 74,300

» Total 129,600 Btu

– Output in combustion: 76,000 Btu

• Gasoline: Output to input ~ 7• Sustainability Rule of Thumb: Ratio > 4• Cellulosic Ethanol:

T. Ferguson, University of Minnesota, Duluth. 2008

Biomass R&D

Focus is on:– Production yields– Harvesting costs– Conversion efficiencies