| total dollar amount on your power bill by the number of kWh indicated, you will have the price you pay per kWh. In Alabama, this is usually between 6 and 8 cents. It is also important to know that one million Btu is equal to approximately 293 kWh.
Energy Content of Fossil fuels
Since cost of energy is usually expressed in terms of MMBtu, the energy content of fuels is usually expressed in terms of Btu or MMBtu per unit weight (lb or ton) for solid fuels, and Btu per unit volume for liquids (gallons or barrels) and gases (thousand cubic feet, or Mcf). For solid fuels it is also very important to determine whether the energy content is being reported on a completely dry basis (zero moisture content), or on an as-is or as delivered basis, which will include at least some moisture, but in many cases, a considerable amount of moisture.
Coal: There is a considerable variation in the energy content of different types of coal. In general, bituminous coal contains 13,000 to 15,000 Btu per dry pound, or 26 – 30
MMBtu/dry ton, while sub-bituminous coal is about 8,000 – 10,000 Btu per pound, or 16,000 to 20,000 Btu per ton. However, it must be recognized that some coals can have a fairly high moisture content.
Crude Oil: 138,095 Btu/gal or 5.8 MMBtu/barrel (1 barrel = 42 gallons).
Gasoline: 124,071 Btu/gal or 5.2
MMBtu/barrel.
Diesel: 139,000 Btu/gal or 5.8
MMBtu/barrel.
Natural
Gas: 1,026 Btu/cubic ft, or approximately 1 MMBtu/ thousand cubic ft
(Mcf).
Propane: 90,000 Btu/gal
Energy Content of
Biomass Fuels
Surprisingly, biomass fuels differ very little in energy content when this is expressed on a dry weight basis: most contain about 8,000 Btu per pound or about 16 MMBtu per dry ton. However, the moisture content of biomass fuels differs substantially, as does their bulk density, or weight per unit volume. Therefore, their energy content on an as delivered basis or per unit volume basis will also vary substantially. The main difference in this regard is between herbaceous materials like grasses and crop residues that are usually dried down to about 12 % moisture, and wood, that is usually about 45% moisture. Therefore, energy content of these two categories is provided accordingly.
Biomass is also higher in what is called “volatiles” than coal. This results in it being more easily ignited. Finally, the ash content of solid fuels also strongly influences the energy content, because ash is inert and contains no energy. Of the biomass fuels, wood usually has the lowest ash content (less than 2%), grass a little more (4-5%) and broiler litter among the highest (about 20%). Soil is constituted mostly of ash, so contamination of biomass with soil will reduce the energy content.
Herbaceous material
such as grasses and crop residues that are air dried and baled, with a moisture content of 12% contains 7,040 Btu/lb or 14.1 MMBtu/ton as delivered.
Wood chips with a moisture content of 45%: 4,400 Btu/lb or 8.8 MMBtu/ton as delivered.
Poultry litter is another potential biomass fuel that is plentiful in the Southeast. However, it typically contains about 20% ash (which is inert material that contains no energy) and 20% moisture. Therefore, on an as delivered basis it is typically also low in energy: about 5,500 Btu/lb and 11.0
MMBtu/ton.
Bio-ethanol: 85,000 Btu/gal
Cost of Fuels for Energy
The table at bottom left of the page shows the cost of selected fuels in traditional units and per
MMBtu.
Clearly, the implication of these figures is that biomass is highly competitive with diesel, natural gas and propane for heating purposes and for generation of electrical power.
Energy Relationships
Having established the basics, it is interesting to use some of this information to answer questions that may come to mind.
a) How much biomass would be needed to power an average house for a year?
An average house uses about 1,500 kWh/month, or 18,000 kWh/year. A million Btu is equal to 293 kWh, and with traditional technology, energy conversion is only 30% efficient. Therefore, the amount of energy required in the solid fuel needed to power a house for a year is 18,000/293/0.3 = 204.8 MMBtu. Since there are 14 MMBtu/ton of biomass (12% moisture), this amounts to 14.6 tons. At a yield of 7 tons/acre, which is attainable with switchgrass in Alabama, a house could be powered with grass from a little over 2 acres.
b) How much forest land would be needed to power an average house for a year?
Forests usually produce about 3.0 tons of dry matter/acre/year. Therefore, over 10 years they will accumulate 30 tons per acre, or 30 x 16 = 480 MMBtu. Since a house needs 204.8 MMBtu/yr, the amount of land needed for one year would be 204.8/480 = 0.43 acres. However, because this area cannot be harvested again for another 10 years, the total amount of land needed to power a house continuously from forest would be 0.43 acres x 10 years = 4.3 acres, with one tenth of this being harvested each year.
c) How much biomass would be needed to power an average house for a day?
14.6 tons x 2,000 lb/365 days = 80 lb/day, or two small square bales. At a price of $50/ton, this is a fuel cost of only $2.00/day. However, at the current retail price of $0.08/kWh, and 1,500 kWh/month or 1,500/30 = 50 kWh/day, the current total cost of electricity for an average size home is $4.00/day.
d) What is the fuel cost of power produced from biomass?
There are 14 MMBtu/ton of biomass at 12% moisture. At $50/ton, this amounts to $3.57/MMBtu. Since there are 293 kWh/MMBtu and conversion efficiency is 30%, the fuel cost is $3.57/293/0.3 = $0.041/kWh.
e) What is the fuel cost of power produced from coal?
For bituminous coal (26 MMBtu/ton) at a price of $60/ton, the price would be
$60/26/293/0.3 = $0.026/kWh. Since the retail price of electricity is about $0.08/kWh, the fuel cost for coal amounts to only 25% of sale price. |
