EROEI for Energy Production from Coal

Transcription

1 EROEI for Energy Production from Coal Before we start, let s review the units that are used in the energy industry: Ways to measure weight: -pounds (lbs) -US tons (also called a short ton) There are in 1 (short) ton. How would you convert pounds of coal to tons of coal? Let s have a quick review of unit conversions. To convert from one unit to another, you need to multiply or divide your value by a conversion factor. The key to deciding whether to multiply or divide is that your original units (the ones you want to get rid of) should cancel out. Example: if you know there are in 1 ton you can write the conversion factor in one of two ways: 1 ton or 1 ton If you start with 1500 lbs and you want to convert this value to tons, choose the conversion factor with lbs on the bottom of the fraction, this way lbs cancel out and you are left only with tons: 1 ton (1500) *( ton) ( 1500 lbs) tons Ways to measure volume: -cubic feet (ft 3 ) -gallons (gal) -barrels (bbls) There are 42 gallons in 1 barrel. Note this unit is used for fuel only. Ways to measure energy: -British thermal units (Btu) -joules (J) -kilowatt-hours (kwh) pg 1 of 5

2 How much energy does it require to produce electricity from coal? Over half of the electricity used in the US comes from coal-fired power plants. In the near future, that fraction is likely to increase. Today we will determine how much energy is used in the process of creating electricity from burning coal. 1. Mining and Processing Coal Coal can be mined in surface mines or below ground mines. Once the coal is mined it must be processed to remove impurities. Processing often includes crushing and washing the coal. The US Census Bureau reports that in 1997, the following amounts of fuel were used in the underground mining and processing of bituminous coal 1 : Amount used to extract and process coal Units Coal x 10 3 tons Fuel Oil x 10 3 barrels (bbls) Natural gas x 10 6 cubic feet (ft 3 ) Gasoline x 10 6 gallons (gal) There are 42 gallons in every barrel. Convert the fuel oil amount to gallons: ( 800,700 bbls) * gallons conversion factor Fuel Oil (gal) Amount used Different fuels contain different amounts of energy. Fill in the amount used of each fuel and then calculate how much total energy was used (in kwh) in mining and processing bituminous coal from underground mines in Use the following formula to calculate the total energy used in Btus. ( amount of fuel used) *( energy content of that ) ( energy used) fuel Convert the total energy in Btus to energy in kwh. (Note: 1 Btu 293 x 10-6 kwh.) pg 2 of 5

3 Coal (tons) Amount used Energy content 2 20,681,000 Btu/ton Energy used (Btu) amt. used x energy content Energy used (kwh) amt. used x energy content Fuel Oil (gallons) 139,000 Btu/gal Natural gas (cubic feet) 1026 Btu/ft 3 Gasoline (gallons) 124,000 Btu/gal Total According to the US Census Bureau, in 1997 there were x 10 9 bituminous coal sold from underground mining operations 1. pounds of processed Therefore, it took kwh/lb of coal to mine and process the coal. To create this amount of energy, you would need to burn 1.61 x 10-3 lbs of coal. 2. Transportation According to the US Bureau of Transportation Statistics, an average rail car containing processed coal travels 81 miles from the processing plant to the power plant 3. Transportation by rail requires 53.9 x 10-6 kwhs/mile-lb 4. Calculate the energy costs for transporting one pound of coal: It takes kwhs to transport one pound of coal to the power plant. To create this amount of energy, you would need to burn 1.44 x 10-3 lbs of coal. pg 3 of 5

4 3. Storage and preprocessing for combustion Is there an energy requirement for the storage or preprocessing of coal? Steps 1-3: Total energy required to get coal to a power plant where it is burned for energy is kwh/lb. To create the energy needed to get one pound of coal to a power plant, you would need to burn 3.1 x 10-3 lbs of coal. 4. Energy Use The energy contained in 1 lb of coal is kwh. The efficiency of a power plant is 35%. Therefore, for one pound of coal you produce kwh at the power plant. This energy must be transmitted from the power plant to the outlet in your house using power lines. During this process 10% of the electricity is lost. Consequently, for every pound of coal burned kwh of electricity reaches your house. EROEI - Energy Returned On Energy Invested EROEI is defined as total energy produced / total energy invested in the production process. Calculate the EROEI for coal: EROEI energy energy produced invewted to from burn 1 1 lb lb of of coal coal kwh kwh Published EROEI numbers for coal are around 80. What energy consuming processes might be important but were not considered here? References: 1. or pg 4 of 5

5 Published EROEI Numbers for different fuels Sensitivity Analysis How would the EROEI change if some of our data changed? Transportation: The average travel distance for coal is 81 miles. This number could easily change if power plants were built further from coal sources or if the mining locations changed. There is a coal fired power plant in La Grange, TX. A great deal of coal is mined in Wyoming. If the La Grange plant used Wyoming coal, then the travel distance would be about 1200 miles. In this case the calculated EROEI for that plant would be 13.7 (much less than our previous calculation of 103.1). Distance of transport has a large effect on the amount of energy needed to create electricity from coal. Power plant efficiency: Currently coal fired power plants have a maximum efficiency of about 35%. With new technology, this efficiency could increase in the future. A new EROEI was calculated using all of our original numbers, but a new efficiency. With 50% efficiency, the EROEI would be (this is about 40% more than the original EROEI). In a perfect world where there was 100% efficiency and no loss of energy over the power lines, the EROEI would be (over three times our original EROEI). pg 5 of 5