Session 8 fossil energy systems


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Session 8 fossil energy systems

  1. 1. Session 8 – Fossil Energy Systems • • • • Exploration, Discovery and Extraction Transportation and Storage Fossil Fuel Conversion Fossil Fuel Combustion 1
  2. 2. Exploration, Discovery and Extraction • Coal: extraction and transportation • Petroleum: quest for deposits; improved extraction – Offshore: few feet in 70s to a mile or more 2
  3. 3. Storage and Transport Natural Gas • • • • Low density – less efficient to transport Stored in caverns, depleted reservoirs (UG) Mercaptan added as odorant; yellow pipe Stored as LNG (above ground) – – – – Liquefaction at -163 °C; maintained at < 83°C Cryogenic vessels and fixed tanks 1/614th the volume of gaseous form Not explosive in liquid state 3
  4. 4. Natural Gas Transmission 4
  5. 5. Natural Gas Transmission 5
  6. 6. Fuel Conversion • “Conversion”: Improving quality of fuel • Quality improved for equipment compatibility • Methods: Chemical, thermal, physical • Coal: size reduction, washing, removals • Nat Gas: removal of H2S, others • Coal gasification • Refining 6
  7. 7. Fossil Fuel Conversion Systems Energy Flows Stack Heat Electrical Energy Output Fuel Input Cooling 7
  8. 8. Fossil Fuel Conversion Systems Energy Flows for a 400 MWe Unit •At 40% thermal efficiency, the input energy is 1GWth •For 90% efficient boiler, 100 MW goes up stack •The remaining waste heat requiring removal is 500 MW Stack Heat 100 MW 400 MWe 1 GW Fuel Input Boiler Turbine/ Env Generator 500 MW Cooling Note: Configuration of subsystems varies for gas-fired unit8
  9. 9. Fossil Fuel Conversion Systems Energy Flows for a 400 MWe Unit Daily Operation at 1 GW requires 8.19 E10 Btu: Stack Heat 100 MW Natural Gas 81,900 MCF/day or Coal 3000 MT/day or Oil 400 MWe 14,600 barrels per day Boiler Turbine/ Env Generator 500 MW Cooling Note: Configuration of subsystems varies for gas-fired unit9 Adapted from Krenz, Energy Conversion and Utilization, Allyn and Bacon, 1976
  10. 10. Fossil Fuel Conversion Systems Energy Flows for a 400 MW Unit Stack Heat Electrical Energy Output Fuel Input Turbine/ Generator Cooling – 500 MW Cooling Rate Required: 500 MW = 1.7 E9 Btu/h = 4.1 E10 Btu/day = 1.0 E13 cal/day waste heat 10
  11. 11. Fossil Fuel Conversion Systems Cooling Choices Direct Condensing (Conduction) Evaporative (Cooling Tower) Natural Convection Diagram of a typical water-cooled surface condenser Requires nearby river or lake If limit on ΔT= 10°C, water req’d = (1E13 cal/day)/10°C =1E9 kg water/day = 1E6 m3/day = 264 million gallons/day = 410 cubic feet per second (The St. Louis River at Scanlon has a 100-yr mean flow of 1060 cfs). Forced Convection For latent heat of evaporation of water of 540 cal/g, and Assuming 1E13 cal/day of cooling, need 1.85 E10 grams of water/day or ~ 5 million gallons/day Direct – 264 E6 gal/day – water conserved Evaporative – 5 E6 gal/day – water lost 11
  12. 12. Minnesota Power’s Boswell Energy Center Cohasset, MN 12
  13. 13. Synthetic Fuels (Syn Fuels or Alternative Fuels) • Alternative to Petroleum-derived fuels – Interest proportional to fear of import disruption – Attractive due to lower sulfur, carbon mgmt. • Input: Coal, biomass, oil shale • Output: Methane and other compounds “Coal to gas” 13
  14. 14. Syn Fuel Process Coal Pyrolysis Gas (H2, CH4, CO2, CO) COAL Heat w/o O2 Liquids (Tar, Light Oils, Liquor) Char (a solid, also called coke) SynGas is the mixture of H2 and CO in different proportions, and serves as a building block for other fuels, such as substitute diesel, gasoline and hydrogen. 14
  15. 15. Great Plains SynFuels Plant Beulah, North Dakota • Lignite coal input (6 million tons annually) • 54 billion cubic feet of Natural Gas annually (U.S. production= 19 trillion cf/yr) • Subsidiary of Basic Electric Power Coop. 15
  16. 16. Great Plains SynFuel Plant Methanation: CO and CO2 reacts with H2 to form CH4 Byproducts Annually •Anhydrous Ammonia 4E5 tons •Acids for Manuf. 33E6 lbs •Krypton, Xenon 3E6 liters •Liquid Nitrogen 24E6 gal •Naptha 7E6 gal •Phenol 33E6 lbs •CO2 for oil recovery 200E6 SCF/day Graphic and data from Dakota Gas Co. website, 16
  17. 17. More Notes on Combustion • Heat released used to combust more fuel • Combustion dominates fossil fuel conversion • Combustion dominates anthropogenic CO2 emissions to atmosphere • Combustion creates diverse pollutants • Stationary technology well developed, controlled • Revolutionary advances unlikely • Combustion requires: – – – – Contact between fuel and oxidant molecules Reactants must be heated to be able to react “fast enough” Reaction must last “long enough” to allow complete reaction Three T’s: turbulence, temperature, time 17
  18. 18. Fuel Cells • Hydrogen + Oxygen = electricity + water + heat • Oxidation occurs, but not as fast as in combustion • Why not directly convert chemical energy to electrical energy (with up to 75% efficiency), than be limited by thermodynamic conversion efficiencies of 35 to 40%?) • Today’s technology most promising for vehicles 18
  19. 19. Fuel Cells • Zero Emission Vehicles (ZEVs) relied primarily on batteries prior to 2003; then, litigation in California shifted work to fuel cells; 5 of 6 automakers abandoned batteries temporarily • Today: fuel cells 20 times more costly than IC engine; last three years, hydrogen storage problems, no stations • Carmakers propose making 2500-5000 fc vehicles by 2014 19
  20. 20. Fuel Cells Basic fuel cell: 2H2(gas) + O2(gas)→ 2H2O Separate into half reactions at each electrode: H2 + 2OH- → 2H2O + 2e-Anode O2 + 2H2O + 4e- → 4OH- Cathode 20
  21. 21. Sidebar: Anodes and Cathodes • Anode: “ACID”: anode current into device where “current” is always positive charge flow Cathode (pos Term) Anode (neg Term) Fuel Cell (+) Cathode ( + ) D-cell battery (-) Cathode Δ Diode (+) Anode (-) Anode “Electrons always flow from anode to cathode outside the device, regardless of device type” 21
  22. 22. Fuel Cells H2 O2 Supply H and O to cell But need to separate gases, while allowing movement of electrons and ions between electrodes Porous partition or membrane used Hydrogen gives up electrons on the left (electrons flow out of cell), making the left electrode the Anode Hydrogen is oxidized at the Anode On the right, the negatively charge ions (anions) that result from the reduction of oxygen flow to the left (cathode to anode) through the electrolyte and the membrane The electrolyte conducts charged particles much larger than electrons, and can be a liquid or solid. A “solute” that produces a conducting 22 solution is an “electrolyte” (e.g., acids, sodium chloride)
  23. 23. Fuel Cells Advantages • Low maintenance, high reliability if stationary • Low noise level, only emission is water • Can stack and parallel cells for V and I Disadvantages • Poor voltage regulation (drops under load) • Best for applications with steady loads • High cost, durability in changing environments, weight • Cost effectively supplying the hydrogen • Matching rate of hydrogen supply to cell load 23