Fossil-Fueled Power Plants Practice Flashcards

Overview of Fossil-Fueled Power Plants

• Fossil-fueled electric power plants consume $55.5\%$ of the global annual supply of fossil fuel. More than $80\%$ of this fuel is in the form of coal.

• They are major contributors to anthropogenic emissions, including $CO_2$, $SO_2$, $NO_x$, products of incomplete combustion (PIC), and particulate matter (PM).

• Economies of scale typically result in large, centralized units delivering in the range of $500$–$1000\,MW$ of electric power.

• Efficiency improvement and environmental control are primary focus areas for reducing global pollutant emissions and conserving fossil fuel reserves.

• Conversion process: Fossil fuel chemical energy is converted to mechanical energy (via steam or gas turbines) and then into electrical energy.

• Fuel cells are an exception, converting chemical energy directly to electricity, though few such plants exist today.

Thermodynamic Cycles and Efficiencies

• Rankine Steam Cycle: Used in most large-scale plants. Steam produced in a boiler drives a steam turbine, which drives a generator. These typically provide the base load for regional grids, often alongside nuclear plants.

• Brayton Cycle: Used by gas turbine plants, typically for peak loads. Natural gas is burned, and combustion products directly drive the turbine.

• Efficiency Statistics:

  • Best steam cycle plants: Above $40\%$

  • U.S. average (steam): $36\%$

  • Worldwide average (steam): $33\%$

  • Gas turbine plants: $25$–$30\%$

  • Combined cycle plants (Brayton + Rankine): $45\%$

• Reasons for Low Efficiency:

  • Second Law of Thermodynamics: Residual fuel heat must be rejected to a cold reservoir (surface water or atmosphere via cooling tower).

  • Parasitic Losses: Heat loss through walls/pipes, friction, and residual heat escaping with flue gas.

  • Approximately $50$–$75\%$ of input chemical energy is wasted.

Fossil-Fueled Power Plant Components

• Major components include: Fuel storage and preparation, burner, boiler, steam turbine, gas turbine, condenser, cooling tower, generator, and emission control systems.

Fuel Storage and Preparation

• Coal Delivery: Arrives by rail (unit trains of $100$ wagons, $100$ tons each) or water (ship/barge).

• Storage: A $1000\,MW$ plant at $35\%$ efficiency consumes approximately $1 \times 10^4$ metric tons of coal daily. Mounds may contain up to $3 \times 10^5$ metric tons (several weeks’ supply).

• Washing: Performed at the mine to remove mineral content and pyritic sulfur, improving heating value per unit mass. Coal is crushed to less than one centimeter (nut or slack).

• Pulverization: Raw coal is reduced to particles smaller than $1\,mm$ by rotating ring, hammer, or ball mills. It is سپس stored in silos and blown pneumatically into burners.

• Oil: Stored in tank farms; plants typically keep a $30$-day supply (over $1 \times 10^5$ metric tons for a $1000\,MW$ plant).

• Natural Gas: Delivered via pipeline at high pressure (CNG) or as Liquefied Natural Gas (LNG) at $-164\,^{\circ}C$ in refrigerated tankers up to $1.25 \times 10^5\,m^3$.

Burner Systems

• Function: Ensures thorough mixing of fuel and air for complete combustion. Ignition is initially achieved by a spark-ignited light oil jet.

• Ash Formation: Mineral matter forms fly ash (>90\%—blown out) and bottom ash ($\approx 10\%$—falls to the bottom). Fly ash deposited on water pipes forms slag, which hinders heat transfer and must be removed by steam jets or scraping.

• Excess Air: Necessary for complete carbon burn-out.

  • Pulverized coal: $15$–$20\%$ excess air.

  • Oil and gas: $5$–$10\%$ excess air.

• Burner Configuration: Often arranged tangentially along boiler walls to create a single turbulent flame. A boiler may have up to $6$ rows of burners, totaling $24$ units.

• Cyclone Furnaces: Used for poor grade/high-ash coal. Combustion occurs in a water-cooled horizontal cylinder. Most mineral matter forms molten slag. However, high temperatures lead to high $NO_x$ formation, requiring denitrification devices.

• Stoker Firing: Older/smaller boilers use a traveling grate. It is less efficient than pulverized coal due to poorer mixing.

Boiler Systems

• Water Wall Boiler: Developed by Babcock and Wilcox in 1867. Walls are constructed of vertical tubes carrying water or steam.

• Economizer: Preheats feed water to $\approx 315\,^{\circ}C$.

• Steam Drum: Separates liquid water from steam by gravity ($\approx 30\,m$ length, $5\,m$ diameter).

• Superheater: Raises steam temperature to $565\,^{\circ}C$ at $24\,MPa$. This is above the critical point of water ($T_c = 374\,^{\circ}C$, $p_c = 22\,MPa$).

• Reheater: Heats exhaust from the high-pressure turbine back to $500\,^{\circ}C$ at $3.7\,MPa$ for the low-pressure turbine.

• Air Preheater: Heats combustion air to $250$–$350\,^{\circ}C$ to optimize efficiency.

• Heat Transfer: Radiation dominates near burners; convection dominates elsewhere. Coal/oil flames are luminous due to carbon/ash particles; gas flames are infrared-dominant.

• Carnot Efficiency: $\eta = (T_H - T_L) / T_H$. For $565\,^{\circ}C$ ($838\,K$) to $25\,^{\circ}C$ ($298\,K$), theoretical maximum is $64\%$, though actual Rankine efficiency is lower due to heat addition at intermediate temperatures and parasitic losses.

Steam Turbine

• History: Commercial units appeared in the late 1890s for ships. First electricity generation: $12\,MW$ in Chicago (1909).

• Designers: Gustav deLaval (Impulse turbine, converging-diverging supersonic nozzle) and Charles Parsons (Multistage reaction turbine).

• Impulse Turbine:

  • Symmetrical blades with equal entrance/exit angles ($\approx 20^{\circ}$).

  • Steam velocity reaches $1650\,m/s$. Staging/compounding reduces rotor speed to avoid unsustainable centrifugal stress.

  • Force: $F = \dot{m}(v_s - v_b)$.

  • Power: $P = F v_b$. Maximum power at $v_b = v_s / 2$ is $P_{max} = \frac{\dot{m}v_s^2}{4}$.

• Reaction Turbine:

  • Blades form converging nozzles; pressure drops across moving blades.

  • Optimum velocity: $v_{b,opt} = v_s \cos(\theta)$.

  • Maximum power: $P_{max} = \dot{m}v_{b,opt}^2$.

  • Usually used for intermediate/low pressure due to leakage risks at high pressure.

• Losses: Fluid friction, leakage, and supersaturation (condensation shock, where steam becomes supercooled before reverting to equilibrium). Combined efficiency loss is $10$–$20\%$.

Gas Turbine

• Directly uses hot combustion gases ($1100$–$1200\,^{\circ}C$) to drive blades.

• Brayton Cycle: Features a compressor, combustion chamber, and turbine. Unlike steam, the working fluid is not recycled; it is vented.

• Applications: Peak load production, auxiliary power, pipeline pumping, and airplane propulsion.

Condenser and Cooling Systems

• Condenser: Rejects heat to create a vacuum, increasing turbine power.

  • Direct contact: Cooling water sprays into turbine exhaust; requires high purity.

  • Surface contact: Shell-and-tube exchanger. A $1000\,MW$ plant may have $100,000\,m^2$ of contact area.

• Wet Cooling Tower: Uses natural draft (spool structure) to cool water via sensible heat transfer and evaporation ($1$–$1.3\%$ of circulating water). A $1000\,MW$ plant at $33\%$ efficiency evaporates $0.63\,m^3/s$ in hot climates.

• Dry Cooling Tower: Heat rejection through finned tubes. Higher cost and turbine back pressure but used in arid areas where make-up water is scarce.

Generator

• Converts mechanical shaft power to electricity via conducting coils in a magnetic field.

• Cooling: High conductivity gases like hydrogen or helium prevent overheating.

• Frequency: $60\,Hz$ (USA/Canada) or $50\,Hz$ (elsewhere).

• Transmission: Voltage stepped up to hundreds of kilovolts to minimize resistance losses ($P_{loss} \propto I^2$).

Emission Control Technologies

Particles (Particulate Matter)

• Uncontrolled Impact: A $1000\,MW$ plant ($10\%$ ash, $2\%$ S, $35\%$ efficiency) could emit $3.2 \times 10^5\,t/y$ of PM and $1.3 \times 10^5\,t/y$ of $SO_2$.

• Electrostatic Precipitator (ESP): Charges particles negatively via corona discharge ($20$–$100\,kV$) to collect them on grounded plates.

  • Deutsch Equation: $\eta = 1 - \exp\left(-\frac{wA}{Q}\right)$, where $w$ is migration speed ($w \approx 0.05 d_p\,m/s$ for diameter $d_p$ in $\mu m$), $A$ is plate area, and $Q$ is volumetric flow rate.

  • Inefficient for submicron particles.

• Fabric Filter (Baghouse): Sucks gas through fabric bags. Effective for submicron particles. Air-to-cloth / filtering ratio is typically $0.5$–$4\,cm/s$.

Sulfur Control ($SO_2$)

• Before Combustion:

  • Coal Washing: Removes up to $50\%$ of pyritic sulfur ($10$–$25\%$ of total sulfur).

  • Coal Gasification: Produces syngas, sulfur removed before combustion.

  • Oil Desulfurization (Claus Process): $RS + H_2 \rightarrow H_2S + R$; then $H_2S + \frac{3}{2}O_2 \rightarrow H_2O + SO_2$ and $2H_2S + SO_2 \rightarrow 2H_2O + 3S$.

• During Combustion:

  • Fluidized Bed Combustion (FBC): Burns coal with limestone ($CaCO_3$) sorbent. Lower temperatures ($800$–$900\,^{\circ}C$) reduce thermal $NO_x$. Sulfur capture is $40$–$60\%$.

• After Combustion (Flue Gas Desulfurization - FGD):

  • Sorbent Injection (SI): Sorbent injected into upper boiler; $50\%$ efficiency.

  • Wet Scrubber: Uses aqueous limestone slurry. Reactions: $CaCO_3 + SO_2 + \frac{1}{2}H_2O \rightarrow CaSO_3 \cdot \frac{1}{2}H_2O + CO_2$; subsequent oxidation forms gypsum ($CaSO_4 \cdot 2H_2O$). Efficiency: $90$–$99\%$.

  • Dry Scrubber: Slurry spray evaporates completely, creating dry powder collected by fabric filter. Efficiency: $70$–$90\%$.

Nitrogen Oxide Control ($NO_x$)

• Fuel NOx: From organic nitrogen in coal/oil.

• Thermal NOx: Recombination of atmospheric $N_2$ and $O_2$ at high temperatures ($N_2 + O_2 \rightleftharpoons 2NO$).

• Low-NOx Burner (LNB): Staged combustion (fuel-rich inner flame, fuel-lean outer flame) reduces $NO_x$ by $30$–$55\%$.

• Selective Catalytic Reduction (SCR): Ammonia injected over titanium/vanadium catalyst. $4NO + 4NH_3 + O_2 \rightarrow 4N_2 + 6H_2O$. Efficiency: $80$–$90\%$. Problem: "Ammonia slip" (unreacted leakage).

• Selective Noncatalytic Reduction (SNCR): Urea injected at $900$–$1000\,^{\circ}C$. $4NO + 4CO(NH_2)_2(aq) + O_2 \rightarrow 4N_2 + 4CO_2 + 2H_2O$. Efficiency: $75$–$90\%$.

Toxic and Waste Management

• Heavy Metals: Arsenic, selenium, cadmium, and mercury emission concerns. Mercury bioaccumulates in the food chain.

• Radioactivity: Radon emissions from trace uranium in coal minerals.

• Waste Disposal: A $1000\,MW$ plant produces $3.2 \times 10^5\,t/y$ of fly ash and $3$–$4 \times 10^5\,t/y$ of wet scrubber sludge. This requires $10$–$20$ acres of landfill ($1\,foot$ deep) annually.

Advanced Cycles

Combined Cycle (GTCC)

• Topping cycle (gas turbine) and bottoming cycle (heat recovery boiler driving steam turbine). Achieves $45\%$ efficiency. No particle or $SO_2$ systems needed if using natural gas.

Integrated Gasification Combined Cycle (IGCC)

• Coal is gasified via steam and pure oxygen: $3C + O_2 + H_2O \rightarrow 3CO + H_2$.

• Methanation increases heating value: $3H_2 + CO \rightarrow CH_4 + H_2O$.

• Efficiency: $40$–$45\%$. Demonstrated by the Coolwater Plant (105 MW) in Barstow, CA (1984–1989).

Cogeneration and District Heating

• Provides both electrical power ($P_{el}$) and useful heat ($Q_{proc}$).

• $Q_{fuel} = P_{el} + Q_{ex}$

• $P_{el} = \eta_{th} Q_{fuel}$

• $Q_{proc} = \eta_{xch} Q_{ex} = \eta_{xch} (1 - \eta_{th}) Q_{fuel}$

• Highest fuel efficiency when heat is used at low temperatures (space heating/drying).

Fuel Cells

• Direct chemical-to-electrical conversion; not a heat engine.

• Theoretical efficiency near $100\%$, but parasitic losses reduce active power efficiency to $45$–$50\%$.