1 / 16

Coal Combustion: Last Class

Coal Combustion: Last Class. Isaac Hunsaker Laurie Marcotte. 1. Discuss the advantages and disadvantages of making electricity from the following coal-fired processes: Entrained Flow. Entrained flow (pulverized) combustion Advantages:

soo
Download Presentation

Coal Combustion: Last Class

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. Coal Combustion: Last Class Isaac Hunsaker Laurie Marcotte

  2. 1. Discuss the advantages and disadvantages of making electricity from the following coal-fired processes: Entrained Flow • Entrained flow (pulverized) combustion • Advantages: • Fully automated and highly reliable – less shut downs and start ups = continuous electricity generation • Adaptable to all coal ranks • Capacity for increasing unit size – Grows with energy need of the population • Disadvantages: • High energy consumption – high electricity cost to consumer • High particulate emissions – higher electricity cost due to higher emission control • SOx and NOx emissions – Increased electricity cost due to emission control • Entrained flow (pulverized) gasification • Advantages: • Adaptable to all coal types regardless of caking characteristics and the amount of fines – can choose lowest cost fuel • Little to no NOx formation – lower electricity cost • Disadvantages: • High temperature and pressure – increased operating costs = increased electricity cost • Slagging – maintenance shut down of units • Requires extra feed as steam – operating costs for steam production

  3. 1. Continued: Fluidized Bed • Fluidized bed combustion • Advantages: • SOx are reduced during combustion – Less gas cleanup • NOx also slightly reduced • Requires smaller furnaces and heat exchangers due to high heat transfer rates – less capital costs • Low running temperature means lower amounts of slagging and fouling – less operating costs • Disadvantages: • High solid loading leads to increased corrosion – Increased capital costs down the line • Refractory fails in circulating fluidized beds – Increased capital costs down the line • Large solid waste due to sorbent – disposal costs • Increased emissions of N2O – emissions control • Fluidized bed gasification • Advantages: • High char recycling rate • Uniform and moderate temperature – moderate operating costs • Disadvantages: • Moderate oxygen and steam requirements – increased operating costs • Difficulty in handling caking coals – increased maintenance • Difficult to obtain high conversion rates for high rank coals – increased disposal costs

  4. 1. Continued: Fixed Bed • Fixed bed combustion • Advantages: • Flow of solids is independent of the flow of gas • Simple, oldest design • Disadvantages: • Reaction rates have an increased dependence on diffusion • Fixed bed gasification • Advantages: • Minimal pretreatment of feed coal • High thermal efficiency • Low oxidant requirements • Disadvantages: • High methane content in product gas • Tars, oils, and heavy hydrocarbons in product gas • Difficulties in handling caking coals and fines

  5. 2. Postulate on the advantages and disadvantages of wall-fired entrained flow boilers (WFEFB) versus tangentially-fired entrained flow boilers (TFEFB). • Both methods are amenable to N0x reduction by air staging. • Both have no moving parts in the hot combustion chamber • TFEFB has lower excess oxygen, creating greater efficiency. However, it has high investment costs, so would probably only be suitable for very large operations (>30MW) • Wall fired entrained flow boilers are most efficient for highly reactive (lower rank, high volume volatiles) coals, whereas tangentially-fired entrained flow boilers are better suited for less reactive coals. S.J Goidich, SUPERCRITICAL BOILER OPTIONS TO MATCH FUEL COMBUSTION CHARACTERISTICS

  6. 3. Figure 1.10: Entrained flow furnace • 256 MW subcritical drum-style boiler for burning subbituminous coal • Tangentially fired jet burners at five different heights in the four corners • Entrained flow combustion system • Dry-ash furnace Tangentially fired furnace: Flow Pattern

  7. Figure 1.12-13 NOx control strategy • Distributed mixing concept • Reduce oxygen concentration in fuel NOx formation zones • Reduce flame temperature in formation zones for thermal NOx • Distributed mixing burner • Employs the distributed mixing concept • Coal plus primary air for entrainment • Two secondary air streams • Increase oxygen in close to burner region • Tertiary air stream • Reduce temperature in far from burner region

  8. Figure 1.18: Entrained flow gasifier • Combustion engineering IGCC Repowering Project • Dry feed, air-blown, two-stage, entrained flow gasifier • Limestone injection, moving bed, hot gas cleanup • Gasses coming off are combusted • Some combusted gas then sent back to gasifier • Rest sent to steam generator and then turbine to generate electricity

  9. Figure 1.20: Fluidized bed combustion • Bubbling bed • Burns mix of Illinois #6 (hvbit) and low sulfur (subbit) coals • To meet sulfur limits • Retrofit with bubbling bed to raise the rating to 130 MW keeping low emissions

  10. Figure 1.22: Fluidized bed combustion • PFBC combined cycle • Pressurized fluidized bed combustion • Air supplied by gas turbine compressor • Coal fed to bed of dolomite/limestone and ash • Combustion gases go through cyclone to remove 98% of particles • Run through turbine • Expend rest of heat by running through heat exchanger to heat the boiler feedwater • Gasses are clean in ESP before discharge • Feedwater converted to steam by boiler to pass through the steam turbine to generate electricity

  11. Figure 1.26-1.27: Spreader-stokers • Traveling grate spreader stoker used in fixed bed combustion • Works for wide range of coals • High operating efficiency • High fly-ash carry over and heat loss • Traveling grate overfeed spreader-stoker • Coal depth is adjustable by a gate • Low fly-ash carry over • Have problems with high coking coals • Slower response time

  12. Figures 1.29: Lurgigasfier • Dry-ash, oxygen-blown, fixed bed gasifier • Coal enters the top • Distributed fall on the grate by the rotary distributor • Ash falls through the grate • Steam and oxygen enter through the bottom • Large steam required to reduce Temperature below ash fusion limit • Some steam generated in jacket around the gasifier • Non-uniform temperature distribution due to counter-current flow • Low temperature results in liquid tars, oils, and phenols • Product gas is condense to remove these

  13. Figure 1.30: Slagging Lurgi gasifier • Slagging, oxygen-blown, pressurized, fixed-bed • Operates at high temperatures • Ash melts to form slag • Fluxing agents sometimes added to reduce viscosity • Requires 15% steam of other gasifier

  14. Figure 1.31: fixed bed gasification • Air blown, Integrated gasification combined cycle • Coal is gasified in a pressurized system • Product gas runs through hot gas cleanup • Then combusted to generate electricity in gas turbine • Run through a steam generator to recover heat • Steam turns turbine to generate electricity

  15. 4. Please discuss ash disposal. • There are 3 primary destinations for ash • Landfill: viable option for every kind of coal, including high C ash, highly corrosive ash, and high radiation ash. However, it is the least economical, and is becoming more expensive as landfill space is limited. • Cement: Requires low C ash. Economically viable (can produce a small amount of money) • Road fill: Requires low radiation ash. Economically neutral. (Shipping costs are barely offset by the sale of the ash)

  16. 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. Advantages: • Low fuel costs • Low investment costs for plant materials • Low dust in flue gas Disadvantages: • Decreased efficiency due to increased excess air • Increased NOx removal costs • Can’t mix biomasses due to different combustion properties • Combustion conditions not as homogeneous as fluidized beds • High capital costs

More Related