Energy from Plants

Biomass --> converting organic matter into energy. The energy source is stored solar energy. Heating via woodburning is an example of using biomass as an energy source

In general, biomass burning is perceived as being a sensible form of energy generation.

A Case Study in BioMass Cogeneartion

Student prepared summary on Biomass

Okay, let's look at this with a critical eye:

Some common forms of biomass from which energy can be extracted:

• trees and timber waste
• wood chips
• corn
• sugar cane
• grass clippings and leaves
• manure and municipal solid waste

Most biomass is made of Carbon and Hydrogen (e.g. Methane = CH₄)

Half of the World cooks with wood. In developing countries, biomass accounts for about 70% of the energy generated

In the US, there are now wood waste power plants that have a capacity of 80--100 Megawatts --> approximately 10% of the capacity of a coal fired steam plant.

Production Line Strategies for increasing the power yield:

• Grow specialized crops
• short rotation forestry
• improve boiler efficiency (currently 30--40%) --> efficiency translates directly into biomass tonnage

• have 20% of the US energy budget come from biomass by the year 2010--> Is this feasible

Example Calculation:

• One pound of dry plant material --> 7500 BTUs
• Equivalent to 4300 calories per gram
• Annual Corn yield in the US is about 3300 grams per square meter or 15 tons per acre
• Total US harvested land is 350 million acres

• Available Energy is then:

  15 tons/acre yr * 350 x 10⁶ acres * 7500 BTU/lb * 2000 lb/ton =79 x10¹⁵ BTU

  In 1990, the total energy used in the US was 81 x10¹⁵ BTU

  So the potential is there for a substantial fraction of our energy budget to come from biomass burning.

Only 5% of a plant's total biomass is suitable for food. There are two alternatives for the remaining 95%:

• burn it
• turn it into fuel

Conversion of biomass into fuel is somewhat inefficient and costly. At present, its best to just burn the biomass in electrical power plants.

Conversion of corn into fuel:

• stalk and cobs ground up and mixed in water
• cook it to convert starches into sugars
• ferment the sugar into ethanol
• distill to remove ethanol from the rest of the crud
• blend ethanol with another fuel

At the moment the production of ethanol and methanol is subsidized

• Sizeable surplus of corn in the US
• Many states (Midwest mostly) provide fuel tax exemptions to encourage the conversion of this surplus into fuel
• So is gasohol economically viable in a free market?
• Is there a net gain? --> lots of fossil fuels are used in agriculture to grow the corn in the first place

In other countries, the econmics of biomass conversion are more viable:

• Brazil has limited oil reserves --> therefore use ethanol as a major transportation fuel
• 1/3 of their 10 million cars run on hydrated ethanol (192 proof)
• Pertoleum imports would end if 2% of brazilian land area were devoted to growing sugarcane for ethanol

Converting biomass to Methane:

• anaerobic fermentation of municipal waste
• one ton of sludge converts to about 10 million BTUs (32% coal equivalent)
• conversion process is costly but is quite efficient (50--70%)
• economics are favorable if the organic matter used is waste
• currently, it is not competitive to use waste to produce methane instead of relying on existing natural gas supplies

But, BE WARNED, current agriculturual processes have a heavy reliance on fossil fuels and this reliance is often hidden

• 1900 one farmer feed 5 people
• 1974 one farmer feed 50 people
• Factor of 10 increase in yield due to increased use of:
  • machinery (runs on fuel)
  • irrigation
  • pesticides
  • fertilizers
  • better seeds
• All of the above are quite energy intensive and the infrastructure needed to develop all of these products involves a lot of people

• Hence, it is not clear if there is any net energy gain associated with the conversion of agriculturual crops into gaseous and liquid fuels

Current Air Pollution from BioMass Burning:

• CO = 15% that of gasoline powered motor vehicles
• Particulates = 30% of Coal burning facilities
• Sulfur-Oxides = none
• Hyrdocarbons = 10% of gasoline powered motor vehicles
• Nitrous-Oxides = 5% of gasoline powered motor vehicles

• Forest Fires: Dominate particulate emission in biomass burning but have slightly lower releases of CO and slightly higher releases of Hydrocarbobns

Next Page

________________________________________________________________