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Session 8 – Fossil Energy Systems

• Exploration, Discovery and Extraction

• Transportation and Storage

• Fossil Fuel Conversion

• Fossil Fuel Combustion

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Exploration, Discovery and Extraction

• Coal: extraction and transportation

• Petroleum: quest for deposits; improved extraction– Offshore: few feet in 70s to a mile or more

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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

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Natural Gas Transmission

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Natural Gas Transmission

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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

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Fossil Fuel Conversion SystemsEnergy Flows

Fuel Input

Stack Heat

Electrical EnergyOutput

Cooling

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Fossil Fuel Conversion SystemsEnergy Flows for a 400 MWe Unit

Boiler Turbine/Genera-

torEnvFuel Input

Stack Heat

400 MWe

Cooling

•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

100 MW

1 GW

500 MW

Note: Configuration of subsystems varies for gas-fired unit

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Fossil Fuel Conversion SystemsEnergy Flows for a 400 MWe Unit

Boiler Turbine/Genera-

torEnv

Stack Heat

400 MWe

Cooling

100 MW

Daily Operation at 1 GW requires 8.19 E10 Btu:

500 MW

Note: Configuration of subsystems varies for gas-fired unit

Natural Gas

81,900 MCF/day

Coal 3000 MT/day

Oil14,600 barrels per day

Adapted from Krenz, Energy Conversion and Utilization, Allyn and Bacon, 1976

or

or

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Fossil Fuel Conversion SystemsEnergy Flows for a 400 MW Unit

Turbine/Generator

Fuel Input

Stack Heat

Electrical EnergyOutput

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

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Fossil Fuel Conversion SystemsCooling Choices

Diagram of a typical water-cooled surface condenser

Direct Condensing (Conduction) Evaporative (Cooling Tower)

Natural Convection Forced Convection

Requires nearby river or lakeIf 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).

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 conservedEvaporative – 5 E6 gal/day – water lost

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Minnesota Power’s Boswell Energy CenterCohasset, MN

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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”

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Syn Fuel Process

Coal Pyrolysis

Gas (H2, CH4, CO2, CO)

Liquids (Tar, Light Oils, Liquor)

Char (a solid, also called coke)

COALHeat w/oO2

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 andhydrogen.

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Great Plains SynFuels PlantBeulah, 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.

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Methanation: CO and CO2 reacts with H2 to form CH4

Graphic and data from DakotaGas Co. website, www.dakotagas.com

Great Plains SynFuel Plant

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

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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

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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

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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

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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

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Sidebar: Anodes and Cathodes

• Anode: “ACID”: anode current into device

where “current” is always positive charge flow

Fuel Cell

Anode(neg Term)

Cathode(posTerm)

D-cellbattery

(+)

(+) Cathode

(-) Anode

Δ Diode

(+) Anode

“Electrons always flow from anode to cathode outside the device, regardlessof device type”

(-) Cathode

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Fuel CellsSupply H and O to cellBut need to separate gases, while allowing movement of electrons and ions between electrodesPorous partition or membrane used

H2O2

Hydrogen gives up electrons on the left (electrons flow out of cell), making the left electrode the AnodeHydrogen is oxidized at the AnodeOn 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 membraneThe electrolyte conducts charged particles much larger than electrons, and can be a liquid or solid. A “solute” that produces a conducting solution is an “electrolyte” (e.g., acids, sodium chloride)

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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