1 / 21

Class 15

Class 15. Aaron Lewis Robert Laycock David Brunner. 1. Please discuss the advantages and disadvantages of making electricity from the following coal-fired processes:.

annot
Télécharger la présentation

Class 15

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Class 15 • Aaron Lewis • Robert Laycock • David Brunner

  2. 1. Please discuss the advantages and disadvantages of making electricity from the following coal-fired processes: •Entrained flow (pulverized) combustion – is by far the most common method of coal use. Particle sizes are near 74 microns. Pulverized coal enters the burner and furnace region with primary air (small part of total air needed for combustion). The secondary air (the greater part of the total air) is heated and introduced through the burner ports in the furnace to ensure complete combustion. Combustion rates are very high for pulverized coal due to the high surface area. Pulverized coal particles are entrained by combustion air and transported in suspension through the furnace. 1-2 second residence time is usually sufficient for complete combustion. Large particles (400-500 micron) don’t always burn out. A lot of ash leaves the furnace entrained as fly-ash and is removed by an electrostatic precipitator. Pulverized coal combustion usually has SO2, NOx, and particulates. High reliability, full automation, adaptability to all coal ranks, excellent capacity for increasing unit size are advantages. High energy consumption to pulverize coal, high particulate emissions, SO2 & NOx emissions are some disadvantages. •Entrained flow (pulverized) gasification –in coal gasification, only about 20% of the O2 needed for stoichiometric combustion is provided, so only a fraction of the available carbon is consumed by combustion. The remaining carbon is consumed in gasification rxns with CO2, steam, and H2. The gasification rxns are endothermic & must rely on the heat from the exothermic combustion rxns. The fate of the devolatilization products depends on the conditions where they are released. If they are released in conditions where there’s excess O2 & the temperature is sufficiently hot, the tar & oils are cracked & oxidized. Main pollutants are H2S & COS since gasification takes place in a reducing atmosphere. Almost no NOx is formed. Nitrogen in coal is converted mainly to NH3 and to a less extent, HCN. Main characteristics of entrained gasification are: high temp, slagging operation, entrainment of some slag in the product gas, high oxidant requirements, large sensible heat in the product gas, and ability to handle all coal ranks regardless of caking characteristics & the amount of fines. Gasification rates are much slower than combustion rates.

  3. Question 1 continued… • •Fluidized bed combustion – technology hasn’t been around as long as the entrained-flow combustors. Consists of a bed of particles set in turbulent motion by combustion air blowing upward through the bed. Coal particles only make up 1% of the bed mass; the rest of the particles made of inert materials like coal ash or sand. At velocities exceeding the free fall particle velocity, the particles become entrained in the air and carried out of the furnace. The entrained particles are separated from combustion gases in a cyclone, and circulated back to the bed (circulating fluidized bed). Bubbling fluidized beds is when the air flows through the bed in bubbles at velocities exceeding the minimum fluidizing velocity. Coal is crushed to less than 3.2 cm (doesn’t have to be as small as entrained combustion). Particle residence time is usually around 1 minute, achieving 80 to 90% burnout. Bed temperature is usually 1120 K (not very hot). Temperature of the bed is uniform; high heat transfer rates in the bed result in compact furances & heat exhangers. Low bed temperature prevents formation of thermal NOx and helps reduce fuel NOx. The low temp also reduces slagging and fouling. Also allows SOx clean-up since limestone particles can be in the bed, & SO2 converts to calcium sulfate. Fluidized bed boilers can burn a variety of coals, including high sulfur coals, cleanly & efficiently. • •Fluidized bed gasification –devolatilization products are released in region of high temp & excess O2 so tars, oils, and HC gases are cracked & don’t accumulate on any surfaces. Moderate O2 & steam requirements, difficulties in handling caking coals, & difficulties in obtaining high carbon conversion for high rank coals.

  4. Question 1 continued… • •Fixed bed combustion –includes grate furnaces and underfeed stokers. Primary air passes through a fixed bed, where drying, devolatilization, and combustion take place. Is the oldest method of coal use & it used to be the most common (very well-established technologies). Recently, stokers have lost their traditional market to fluidized beds. Coal is crushed to 3.2 cm (95%) and 0.6 cm (doesn’t have to get as small as pulverized coal). The coal is then fed-pushed, dropped, or thrown-on a slowly moving bed of coal particles on a grate. The primary air flows through the grate & bed of coal particles. The flow of solids is almost independent from the flow of gases & the increased dependence of the rxn rates on diffusion. Little research or development of this technology has been done in the last 30 years. • •Fixed bed gasification – 89% of the coal gasified in the world is gasified by fixed bed (well-established technology). Is process of choice for mild gasification. Solid residence times in the drying, devolatilization, gasification, & oxidation zones may be on the order of several hours. Tars, oils, and heavier HC’s aren’t cracked or oxidized, but added to the product gas. The main characeristics of this technology are: minimal pretreatment of feed coal; high thermal efficiency, low oxidant requirements, relatively high methane content in the product gas; difficulties in handling coking coals & difficult to handle coal fines.

  5. 2.        Postulate on the advantages and disadvantages of wall-fired entrained flow boilers versus tangentially-fired entrained flow boilers.

  6. 3.        Please be prepared to discuss at length Figures 1.10, 1.12-1.13, 1.18, 1.20, 1.22, 1.26-1.27, 1.29-1.31. • Fig. 1.10 • Tangentially fired (burners at corners of furnace) • Coal leaves the silo, is pulverized, then sent through the burners • Flue gas is used to preheat air

  7. 3.        Please be prepared to discuss at length Figures 1.10, 1.12-1.13, 1.18, 1.20, 1.22, 1.26-1.27, 1.29-1.31. • Fig. 1.12 • Coal and primary enter as a fuel rich mixture • Avoids formation of NOx • Secondary and tertiary air are added • Allows for combustion of remaining carbon • Fig. 1.13 • Coal, primary air, and secondary air all exit near center of burner • Tertiary air exits away from burner

  8. 3.        Please be prepared to discuss at length Figures 1.10, 1.12-1.13, 1.18, 1.20, 1.22, 1.26-1.27, 1.29-1.31. • Fig. 1.18 • Integrated Gasification Combined Cycle (IGCC) • Coal gasified in entrained glow gasifier • Product gas is cleaned (particulate, Sox) and burned in a gas turbine to generate electricity • Waste heat is recovered and used to generate steam that passes through a steam turbine to generate electricity

  9. 3.        Please be prepared to discuss at length Figures 1.10, 1.12-1.13, 1.18, 1.20, 1.22, 1.26-1.27, 1.29-1.31. • Fig. 1.20 • Bubbling atmospheric fluidized bed combustion • Coal is introduced to a bed of ash and sand particles • Air is blown up from underneath the bed, fluidizing the bed • In-bed heat exchanger maintains relatively low bed temperature • Particles collected in cyclones are recycled back to the bed

  10. 3.        Please be prepared to discuss at length Figures 1.10, 1.12-1.13, 1.18, 1.20, 1.22, 1.26-1.27, 1.29-1.31. • Fig. 1.22 • Pressurized fluidized bed combustion • Boiler heats fead water to produce steam to spin a steam turbine to produce electricity • Clean, hot, pressurized flue gases are expanded through a gas turbine to produce electricity

  11. 3.        Please be prepared to discuss at length Figures 1.10, 1.12-1.13, 1.18, 1.20, 1.22, 1.26-1.27, 1.29-1.31. • Fig. 1.26 • Fixed bed combustion (spreader stoker) • Like the boilers at the BYU heating plant • Coal is thrown over top of a grate and air passes up through the coal • The grate slowly carries coal toward ash hopper, allowing the coal to burn out and dumping ash in the hopper • Fast response times to load changes • Fig. 1.27 • Fixed bed combustion (overfeed stoker) • Much like spreader stoker, but the coal is carried in from the feeder by the grate, rather than being thrown over the grate • Slower response times to load changes

  12. 3.        Please be prepared to discuss at length Figures 1.10, 1.12-1.13, 1.18, 1.20, 1.22, 1.26-1.27, 1.29-1.31. • Fig. 1.29 • Fixed bed gasification • Lurgi gasifier • Countercurrent flow • Coal drops down through top • Steam and oxygen enter through bottom • Results in non-uniform temperatures • Large amounts of steam are used to reduce gas temperature below ash fusion temperature • Steam is generated in the water jacket

  13. 3.        Please be prepared to discuss at length Figures 1.10, 1.12-1.13, 1.18, 1.20, 1.22, 1.26-1.27, 1.29-1.31. • Fig. 1.30 • Slagging gasifier • Works similarly to regular Lurgi gasifier, but at higher temperatures • Fluxing agents are added to reduce slag viscocity • Ash passes through slag tap hole and is quenched then removed through slag lock hopper

  14. 3.        Please be prepared to discuss at length Figures 1.10, 1.12-1.13, 1.18, 1.20, 1.22, 1.26-1.27, 1.29-1.31. • Fig. 1.31 • IGCC using fixed bed gasification • Same as fig. 1.18, but with a fixed bed gasifier

  15. 4. Please discuss ash disposal. • Types of disposal options: • Concretes and mortars, • Asphalt, drywall, • Landfill , dumps or abandoned mines, • Embankments on golf courses, farm soil, • Bowling balls.

  16. 4. Please discuss ash disposal. -http://www.epa.gov/rpdweb00/tenorm/coalandcoalash.html

  17. 4. Please discuss ash disposal. -http://www.triplepundit.com/2010/05/investors-raise-coal-ash-disposal-issues-with-coal-utilities/

  18. 5. The BYU Heating Plant is currently burning coal in a spreader-stoker. Dr. Baxter has tried to convince them to add biomass to cut down on fuel costs. Please discuss the issues involved.

  19. 5. The BYU Heating Plant is currently burning coal in a spreader-stoker. Dr. Baxter has tried to convince them to add biomass to cut down on fuel costs. Please discuss the issues involved. • Spreader-Stoker Issues Involved: • The coal & biofuel will need to be distributed evenly. • A different spreader-stoker may need to be purchased to handle the biofuel separately.

  20. 5. The BYU Heating Plant is currently burning coal in a spreader-stoker. Dr. Baxter has tried to convince them to add biomass to cut down on fuel costs. Please discuss the issues involved. • Ash / Deposits Issues Involved: • Sulfur and chlorine species in the fuel also get released into the gas phase at flame temperatures as sulfur oxides and HCl. These species then react with alkali and alkaline earth metal oxides in the ashes and deposits to form sulphates/sulphites and chlorides. • Slag formation increases on the surfaces of the boiler furnace when co-firing biomass with coal. • Partially fused slag deposits are more receptive to oncoming particles, which causes deposits to grow more rapidly.

  21. 5. The BYU Heating Plant is currently burning coal in a spreader-stoker. Dr. Baxter has tried to convince them to add biomass to cut down on fuel costs. Please discuss the issues involved. • Cost Issues Involved: • Where do you store the biofuel? A large hopper will need to be built along with a transport system to get the biofuel to the reaction zone. • Where do you buy the biofuel? A supplier will need to be found (or you could use waste biomass from the grounds-crew but you would have to store the biomass when it is not being burned). • Transportation issues will need to be resolved.

More Related