comb gasi processes advantages disadvantages

8/3/2019 Comb Gasi Processes Advantages Disadvantages

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Coal Combustion:

Last Class

Isaac Hunsaker

Laurie Marcotte


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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


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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


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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


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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 besuitable for very large operations (>30MW)

Wall fired entrained flow boilers are most efficient for highlyreactive (lower rank, high volume volatiles) coals, whereastangentially-fired entrained flow boilers are better suited for lessreactive coals.

S.J Goidich, SUPERCRITICAL BOILER OPTIONS TO MATCH FUEL COMBUSTION CHARACTERISTICS


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3. Figure 1.10: Entrained flow furnace

256 MW subcriticaldrum-style boiler forburning subbituminous

coal Tangentially fired jet

burners at five differentheights in the fourcorners

Entrained flowcombustion system

Dry-ash furnace

Tangentially

fired furnace:

Flow Pattern


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Figure 1.12-13 NOx control strategy

Distributed mixing concept Reduce oxygen concentration

in fuel NOx formation zones

Reduce flame temperature information 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


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Figure 1.18: Entrained flow gasifier

Combustion engineeringIGCC Repowering Project Dry feed, air-blown, two-

stage, entrained flow

gasifier Limestone injection,

moving bed, hot gascleanup

Gasses coming off arecombusted

Some combusted gasthen sent back to gasifier

Rest sent to steamgenerator and thenturbine to generateelectricity


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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


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Figure 1.22: Fluidized bed combustion

PFBC combined cycle Pressurized fluidized bed combustion

Air supplied by gas turbinecompressor

Coal fed to bed ofdolomite/limestone and ash

Combustion gases go through cycloneto remove 98% of particles Run through turbine

Expend rest of heat by running throughheat exchanger to heat the boilerfeedwater

Gasses are clean in ESP before

discharge Feedwater converted to steam by

boiler to pass through the steamturbine to generate electricity


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Figure 1.26-1.27: Spreader-stokers

Traveling grate spreaderstoker used in fixed bedcombustion Works for wide range of coals

High operating efficiency High fly-ash carry over and

heat loss

Traveling grate overfeedspreader-stoker Coal depth is adjustable by a

gate Low fly-ash carry over

Have problems with highcoking coals

Slower response time


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Figures 1.29: Lurgi gasfier

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


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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


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Figure 1.31: fixed bed gasification

Air blown, Integratedgasification combined cycle

Coal is gasified in apressurized system

Product gas runs throughhot gas cleanup

Then combusted togenerate electricity in gasturbine

Run through a steamgenerator to recover heat

Steam turns turbine togenerate electricity


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4. Please discuss ash disposal.

There are 3 primary destinations for ash

Landfill: viable option for every kind of coal, including highC ash, highly corrosive ash, and high radiation ash.However, it is the least economical, and is becoming moreexpensive as landfill space is limited.

Cement: Requires low C ash. Economically viable (canproduce a small amount of money)

Road fill: Requires low radiation ash. Economically

neutral. (Shipping costs are barely offset by the sale of theash)


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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

Cant mix biomasses due to different combustion properties

Combustion conditions not as homogeneous as fluidized beds

High capital costs

comb gasi processes advantages disadvantages

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