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Suspension (PC) Combustion
Chungen Yin
CHY@iet.auc.dk
Institute of Energy Technology
Aalborg University
CHY
Agenda
Topic 1. What Is Pulverized Coal (PC) Combustion?
Topic 2. What Is Tangentially-Fired PC (TFPC)?
Topic 3. Gas Temperature Deviation In TFPC Boilers
Topic 4. NOx Emissions & Controlling In TFPC Boilers
Topic 5. Deposits In TFPC Boilers
Topic 6. PC Combustion Development Prospects
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Topic 1: What Is PC Combustion?
ISSUES:
1. Development of Combustion Mode
2. Stages of Coal Particle Combustion
3. Industrial PC Combustion Manners
4. Processes associated with PC Combustion
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1. Development of Combustion Mode
¾ Stoker Combustion
¾ Suspension Combustion
Popular in recent decades, in large coal fired scale boilers
Solid fuels is milled to a very fine powder
Typically, coal particle: maximum < 300µm; 70%~75% by mass < 75µm
Pass through a burner in suspension.
Namely PC combustion, when coal is the fuel.
¾ Fluidized Bed Combustion (FBC)
Get attention most recently, for smaller scale combustion
Typically, bubbling and circulating fluidized bed (BFB & CFB)
Introduced in another lecture
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2. Stages of Coal Particle Combustion
As the wet coal particle heats up, water is driven off
Drying continues and devolatilization begins
Drying & devolatilization continue; volatiles ignite
Drying complete, devolatilization continues; and
volatiles combustion continues
Devolatilization & volatiles combustion complete; and
residual char combusts
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3. Industrial PC Combustion Manners
¾ Two manners. Choice depends on
Cost factor;
Operating experience;
Emission level;
Manufacture; and so on.
¾ Wall-Fired
(Left)
¾ Tangentially-Fired (Corner Fired )
(Right)
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4. Processes Associated With PC Combustion
ESP- Electrostatic precipitator
FGD- Flue gas desulfurizer
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¾ Equipment ¾ Sub-Process
Coal mills Fuel distribution
Burner design/arrays Ignition/flame stability characteristics
Furnace/Radiant section General flame - combustion characteristics;
Flame to water wall radiation heat transfer;
Heat conduction from outer to inner surface;
Slagging potential
Pendant super-heaters Heat conduction; Slagging potential
Convection banks Heat conduction; Fouling potential
Electro-static precipitators Filtering of fly ash
Flue gas desulphurizers Flue gas desulphurization
NOx control Combustion modification technology.
Flue Gas deNOxing.
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Topic 2: What Is TFPC Combustion?
SYSTEMS:
1. Fuel Preparation & Supply
2. Combustion Air Supply
3. Concentric Firing System
4. Different Combustion Zones
5. Air / Flue Gas Path
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Tangentially-fired PC (TFPC) boiler
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Features Include:
Flame attachment coal
nozzle tips
Concentric Firing
System (CFS) nozzles
Close-coupled OFA
Multilevel separated OFA
Dynamic classifiers
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1. Fuel Preparation & Supply
Particle residence time: several seconds, typically 2~5s, in combustion zone
Coal must be pulverized enough for complete combustion during the time
Equipment: Coal mills
Characterization techniques: Particle size distribution
Rules:
• Mass fraction of PC residue on sieve with 76 µm hole sizes does not exceed volatiles in dry coal;
• Currently, typically with maximum size < 300 µm; more than 70% in mass < 75 µm.
Fuel Supply: Transported through burners into furnace by primary air
Fuel Ignition: Using oil or gas flames
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2. Combustion Air Supply
¾ Burners Arrangement
Burners at each of the 4 corners in a 609MW boiler (Right)
SA: Secondary Air; PA: Primary Air; OFA: Over-Fire Air
¾ Primary Air (PA)
Functions
• Transport PC particles into combustion chamber;
• Provide air in initial combustion stage.
Parameters
• Flow rate: Usually, ~ 1kg PA/ 1kg fuel, independent of fuel;
• Injection velocity: ~ 20m/s, also independent of fuel;
• Temperature: Limited to about 100°C, considering:
High temperature is useful for fuel ignition;
Low temperature is helpful for safe transportation of PC
• Amount: ~15% of total air supply
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¾ Secondary Air (SA)
Function
• Provide combustion air, separately injected from any fuel
Parameters
• Flow rate: Depend on fuel and boiler capacity;
• Injection velocity: Higher than PA, probably ~ 50m/s, depend on boiler
• Temperature: Can be more strongly preheated, possibly > 350°C;
Assist the ignition and burnout of coal particles.
• Amount: ~85% of total air supply
¾ Over-Fire Air (OFA)
Function
• Provide further combustion air over the primary combustion zone
• Make sure the fuel is burnout as much as possible before leaving furnace
Parameters
• Served as SA, injection velocity and angle are similar with those of SA
• Amount: 10~30% of total combustion air
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3. Concentric Firing System
Concept of concentric firing system
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¾ Formation
PA at 4 corners is injected into furnace at a set “firing ” angle to form an
imaginary circle in boiler center.
Part of SA is offset from imaginary circle at a different (usually larger)
firing angle, creating a second imaginary circle. (Not necessarily)
¾ Advantages
PC particles sweep around furnace volume, with longer residence time.
• Improved coal particle burnout;
Fuel-rich zone in inner region, surrounded by O2-rich SA outer
• Low NOx: By detaching high T & high O2, two necessary conditions for NOx;
• Oxidizing atmosphere along furnace water-wall (reducing one is very corrosive)
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4. Different Combustion Zones
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¾ Primary Combustion Zone
Location: In lower part of the furnace
Feature: Reduced firing rate
Formation: By operating under substoichiometric condition; (staging air)
By separating “Fuel-rich” & “Fuel-lean” zones; (offset air)
By staging fuel supply from different PA inlets; (staging fuel)
Benefit: Low NOx production
¾ Reburning Zone (not necessary)
Location: Above the primary combustion zone
Feature: Slightly fuel rich
Formation: By injecting more tiny fuel or gas (10-30% of total heat input)
• Assure a good burnout within shorter residence time;
Benefit: Further consume the un-burnt PC in primary zone;
Further lower NOx- those produced before is reduced to N2
¾ Burnout Zone
Location: Before furnace exit
Feature: Normal excess air
Formation: By using OFA
Benefit: Burnout the remaining char; while not producing much NOx.
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5. Air / Flue Gas Path
Fuel transportation PA burners
(Served as PA)
PA & SA blowers Pre-heaters
(Before boiler exit)
Further pre-heated SA burners
(Served as SA, incl. OFA)
REAR PASS:
Super-heaters; CROSSOVER PASS: Platen
Economizers; Super- and/or Re- Super-heaters FURNACE
Air pre-heaters heaters in the pass In upper furnace
(in turn)
Electro-Static Flue gas Selective catalytic
Stack
Precipitators (ESP) desulphurizer reduction (SCR) units
(Remove dust) (Remove SO2) (Remove NOx) (Into air)
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Topic 3. Gas Deviation In TFPC Boilers
ISSUES:
1. Temperature Deviation & Its Effect
2. Possible Causes Analysis
3. Possible Solution
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1. Gas Temperature Deviation & Its Effect
Inherent feature
Increase with capacity
Typically,
100~150K in 200MW
150~200K in 300MW
200~250K in 600MW
Negative effect:
Tube overheating &
explosion of super-
heaters and/or re-
heaters
(No. 1 operational accidents)
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2. Possible Causes Analysis
¾ Residual Gas Swirling at Furnace Exit
Moments M1, M2, M3
M1: Residual airflow
swirling in upper
furnace;
M3: By induction fan,
const. along width
of crossover pass;
M2: Composite moment
of M1 & M3
Uneven gas velocity
Residual swirling leads
to uneven gas velocity
distribution in upper
furnace & in crossover
pass.
High in right; low in left
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Flow path of the gas in the left
(1) Goes firstly backward to the front-wall (i.e., in –Y direction);
(2) Then turns into the gap between the front wall and SH1;
(3) Flows toward the right with a high speed;
(4) Goes forward into the crossover pass.
The resulted gas temperature distribution
(1) Cooled by the front wall (inner media is low) & the SH1 (inner media T is also low);
(2) Mixed with the gas in the right coming directly from the combustion section;
(3) High gas temperature in the left & low in the right.
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¾ Existence of Platen Super-Heaters
They attenuate residual airflow swirling at furnace exit to some extent;
Thus, reduce the velocity deviation between two side-walls
They alleviate, while not aggravate, gas temperature; this effect is limited
¾ Effect of Particle Trajectories & their Combustion Histories
They can cause gas temperature deviation in upper furnace;
However, they are just a minor factor and their effect is not important.
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3. Possible Solutions
¾ Reasonable Boiler Design
Design of concentric firing system in furnace;
Design of distance between the upmost burners to the furnace exit;
Counter-offsetting part of the air-injections; and so on.
All the measures are aimed to reduce residual swirling at furnace exit.
¾ New Methods (according to my opinion)
A new nose might be added on the front wall near furnace exit;
Re-arranged SH panels to distribute uniformly gas between two sidewalls.
These two new methods are specially aimed to reduce the high-speed gas flow
from one side-wall to the opposite side-wall in the gap between the front wall
and the platen super-heaters.
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Topic 4. NOx Emission & Control
ISSUES:
1. NOx Formation
2. NOx Control Mechanism
3. Practical Ways to Control NOx
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1. NOx Formation
¾ NOx from Coal Combustion
About 95% NO, ~5% NO2, and < 1% of N2O,
Largely depend on combustion intensity
Uncontrolled NOx level of 175~3200ppm
¾ NOx Sources
Thermal NOx
• Formed by attack of O atom on N2 in combustion air
• About 20% of total NOx emission from PC burners
• Mainly affected by flame T & O2 concentration, with the former most important.
Fuel NOx
• Formed by pyrolysis & oxidation of N compounds in coals
• About 80% of total NOx emission from PC burners
• Fuel N vs. fuel NOx relation is complex and unclear:
- Coals with high N do not necessarily produce more NOx;
- Increased N in coal will lead to a decreased conversion rate;
- N in char and in volatiles have different conversion ways to NOx
• O2 concentration plays an important role in fuel N conversion into NOx
• Flame temperature has relatively small influence on Fuel NOx formation
Prompt NOx
• Formed by capture of N2 by hydrocarbon radicals. Too few in coal combustion.
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2. NOx Control Mechanism
¾ Higher NOx result from
Intense combustion (high temperature),
High oxygen availability (high O2 concentration)
The two factors act to minimize differences that exist between coals.
¾ NOx Control Measures (in principle)
Minimize O2 concentration;
Decrease maximum flame temperature;
Minimize residence time in zones where maximum T occurs.
¾ Problems Induced by above Low-NOx Measures
Low combustion efficiency;
• Efficient combustion requires: High T, high O2, and high residence time
Increased corrosion potential
• Reducing atmosphere is very corrosive
• Sub-stoichiometric related problems: slagging, water-wall wastage, etc.
¾ Compromises
Detach the three necessary conditions for NOx formation
Reduce maximum T to some extent while keeping efficient combustion
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3. Practical Ways to Control NOx
¾ General Primary Measures (in furnace)
Reduce peak flame temperature to some extent
• NOx formation is limited
Produce fuel-rich/fuel-lean sequencing
• Favorable for the conversion of fuel-N to N2
NOx re-burning
• Convert NOx formed earlier into N2 by reduction with hydrocarbon radicals
¾ General Secondary Measures (Flue Gas Treatment before into stack)
Selective catalytic reduction (SCR)
Selective non-catalytic reduction (SNCR) at extra costs
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¾ Fuel Staging
Burner out of service
• Shut off fuel flow from one burner or more to create fuel-rich / fuel-lean zones
• Achieving some NOx emission control (10%)
Fuel biasing
• Divert fuel from upper-level to lower burners (or from center to side)
• Create fuel-rich lower (or central) zone and a fuel-lean upper (or side) Create fuel-
rich lower (or central) zone and a fuel-lean upper (or side)
• To lower flame T & improve balance of O2 in furnace
• NOx may be reduced by up to 30% using it.
Reburing
• Use gas, atomized oil or micronized coal as secondary fuel
• Amount: 10~30% total heat input
• Location: between primary combustion zone and burnout zone
• Form fuel-rich O2-deficient reducing zone, decompose NOx formed in primary
combustion zone into N2
• Capable of achieving relatively high NOx reduction (up to 70%)
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¾ Boiler Design
Larger furnace (at a given energy input): lower heat release rate to lower NOx
¾ Low NOx Burners
Control fuel / air mixing at burner
Biasing fuel
Create larger and more branched flames
• Peak flame T is reduced
• Reduce the amount of oxygen available in the hottest part of the flame
¾ Air Staging
Globally, normal excess air supplied to guarantee a good overall combustion effi.
Locally, sub-stoichiometric combustion condition is used.
Three forms of air staging:
• Horizontally staging by creating fuel-rich and lean zones with offset air
• Vertically staging by separating PA and SA burners
• Overfire air (OFA)
NOx formation is discouraged by sub-stoichiometric condition
Sub-stoichiometric combustion reduces flame temperature in primary comb. zone
OFA achieve 10~35% NOx reductions, without any bad effect on boiler efficiency
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¾ Flue Gas Re-circulation
20~30% of the flue gas (350~400°C) is re-circulated into furnace or burner
To decrease flame temperatures and availability of oxygen
Mainly aimed to reduce thermal NOx
It alone in PC boiler achieves a low NOx reduction efficiency (<20%)
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Topic 5. Slagging & Fouling
ISSUES:
1. What Are Slagging & Fouling?
2. Locations of Deposits
3. Summary of Causes & Effects
4. Possible Solutions
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1. What Are Slagging & Fouling?
Two main types of deposits in boilers.
¾ Slagging
Deposits within furnace, in areas directly exposed to flame radiation
such as furnace walls and some widely spaced pendant super-heaters.
Take place in the hottest parts of boiler
¾ Fouling
Deposits in areas NOT directly exposed to flame radiation
such as the more closely spaced tubes in convection sections of boiler
Take place as flue gas & suspended fly ash cool down
¾ Effects of deposition on boiler performance
Reduction of heat transfer from combustion gas to water-steam
• Lead to an increase in gas temperature
• Lead to a further increase in deposition rate
• Result in continually changing conditions in boiler
Formation of sticky surfaces which then collect other particles
Formation of huge clinkers on heat transfer tubes, possibly dozens of tons
Explosion caused by falling of huge slag during operation
• About 40 dead by serious slagging-related explosion in a CE-600MW boiler (1992)
Increased corrosion & erosion, directly due to deposition or due to sootblowing
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2. Locations of Deposits
1— Ash hopper (bridging)
2— Ash slope (mechanical damage)
3— Burners (eyebrows)
4— Water-wall slag
5— Division wall slag
6— Platen super-heaters (bird-nesting)
7— Convection bank (bonded deposits)
8— Economizer (bonded deposits)
9— Air pre-heaters (gas inlet fouling)
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¾ 1- Ash hopper (bridging)
Usually caused by
- slag running down the boiler walls and solidifying;
- large sintered deposits falling off super-heater platens and falling into hopper;
It is largely unpredictable, but coals with
- high iron content & low ash fusion temperature are particularly susceptible;
- high heat content resulting in high flame temperature also have an effect.
May be removed
- by thermal shock (from a load reduction or water lancing);
- by mechanical prodding, or ultimately during a shut-down.
- In severe cases, the bridge may have to be removed with help of explosives.
¾ 2- Ash slope (mechanical damage)
May caused by
- other accumulations higher up in the boiler coming loose & dropping down
May lead to
- damaging the tubes
- slide down & bridge over the hopper exit, if an ash slab there breaks loose
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¾ 3- Burners (eyebrows)
Eyebrows formed above or below the burner mouth
Bad effect:
- distort the flow pattern from the burner;
- cause quarl damages and flow blockage (in severe cases);
- develop into large lumps of slag hanging onto the burner tip;
- difficult to diagnose and correct.
¾ 4- Water wall slag
Possible causes
- burn coals with a low ash fusion temperature and/or high heating values;
- interaction between burner type and boiler dimensions can be critical;
- swirl degree on short flame turbulent burners is critical;
- local reducing conditions due to lack of a sufficient air supply;
- Particle size is important: a coarse grind may result in local slagging.
¾ 5- Division wall slag
In certain designs, slag can form on internal division walls within furnace.
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¾ 6- Platen super-heaters (bird-nesting)
Birdsnesting in tube platens is due to deposits of sintered/fused ash
- build-up firstly on the bottom of the platens, may be removed fairly easily;
- larger accumulations resist on-load cleaning, become very harder with age;
- eventually bridge across tube bank, cause a major distortion of flows patterns;
- result in erosion & increased pressure drop, and may cause tube distortion;
- larger lumps can fall off, possibly damaging burners & main boiler hopper.
¾ 7- Convection bank (bonded deposits)
Often the result of condensing alkali metal sulfates
- Hard thick deposits can form as other particles stick to the surface;
- initial deposits may be difficult to remove, and can cause tube corrosion;
- part deposits may be removed by differential expansion during load variations.
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¾ 8- Economizer (bonded deposits)
Vulnerable to build-up of bonded dust, especially finned tube economizers
- high calcium content in ash can exacerbate this problem (for bituminous type ash)
- new problem resulting from the sootblowing on it, since
most of the resulting debris is carried forward with flue gas;
much of this is collected in economizer hopper located at bottom of back pass;
difficulties in clearing this material are sometimes encountered.
¾ 9- Air pre-heaters (gas inlet fouling)
May due to
- large particles dislodged by sootblowing bypassing earlier collection hopper;
So-called popcorn ash sometimes accumulates on air pre-heaters
- low density;
- may be deposited temporarily, then removed by sootblowing or by load changes;
- It then moves further along with flue gas into ESP and stack.
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3. Summary of Causes & Effects
¾ The causes for increased deposits may be summarized as,
Coal quality
Improper coal fineness
Combustion problems and poor flame stability
Low excess O2 or O2 imbalance
Inadequate soot-blowing
High furnace exit gas temperature
¾ Effects on overall performance of power station may be summarized as,
Reduction in heat transfer in boiler
Increased maintenance cost
A reduction in boiler efficiency, and hence in the amount of fuel needed
and in the amount of CO2 formed
Increased possibility of unplanned shut-downs
Increased capital cost (for new plant)
For existing plant, there is a trade-off between cost of a particular coal supply, and
its effects on operating costs of the plant
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4. Possible Solutions
¾ Possible measures on gas-side to remedy deposits potential:
Increase in boiler size (and hence cost) for coals containing particular extraneous
components, or combinations of components;
Increase excess air level to maintain oxidizing conditions
- at least in the zones slagging more probably occurs
- It will result in loss of efficiency, and increase NOx levels
Use offset air to create “air-on-wall” or “air wrapping coal particles” combustion
manner to reduce the chance of collision of PC particles to walls.
Install more sootblowers;
Closer monitoring & testing conditions in a boiler to check factors such as uniform
distribution of fuel to different burners.
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Topic 6. Development Prospects
ISSUES:
1. Improve Thermal Efficiency
2. Meet Higher Environmental Requirement
3. Combustion Control to Improve Boiler Performance
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1. Improve Overall Thermal Efficiency of Power Plant
Increasing thermal efficiency can save energy resources, and can also have
the potential for reducing pollutants emissions per MWe.
¾ By Increasing Main-Steam Pressure & Temperature
Average efficiency is in range of 35-36%
- large existing plants with sub-critical units firing higher quality coals
Increasing steam pressure/temperature from 25MPa/540°C to 30MPa/600°C
can increase efficiency by nearly 2%.
New plants with supercritical units: overall thermal efficiencies in 43-45%
- Denmark is one of the few countries with operating experience of such plants
¾ Using A Second Reheat Stage
Can add about 1% to thermal efficiency
¾ Reducing Excess Air Ratio
Reducing excess air ratio from 25% to 15% can bring a small increase.
¾ Reducing Stack Gas Exit Temperature
Reducing stack gas temperature by 10K can bring about a similar increase.
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Improve Thermal Efficiency
Thermal Efficiency(%) HHV
60
AGMCFC
50 IGMCFC
USC
Supercritical Boiler TC
IGCC IGHTA
40
Ranking Barrier
PFBC
30
AGMCFC -Advanced gasification-molten carbonate fuel cell
IGMCFC -Integrated gasification-molten carbonate fuel cell
20 TC -Topping cycle
Pulverized Coal
USC -Ultra super-critical (DENMARK in leading position)
IGHTA -Integrated gasification-humid-air turbine
10 IGCC -Integrated gasification-combined cycle
PFBC -Pressured fluidized-bed combustion
First Station
0 Years
1880 1900 1920 1940 1960 1990 2000 2020
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2. Meet Higher Environmental Requirement
Environmental problems have attracted widespread attention.
Technologies need to be developed to control emissions, including
¾ NOx, SO2, particulate matter (Traditional)
¾ CO, other PIC (Products of In-complete Combustion) (Traditional)
¾ Trace elements, volatile organic compounds (VOC) (Recently)
¾ CO2 (More recently)
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3. Combustion Control to Improve Boiler Performance
To improve boiler performance, every stage and course of the combustion
process needs to be well controlled, for example
¾ Coal flow and distribution control to ensure an improved distribution
¾ Burner exit temperature
¾ Initial combustion process control
¾ Precise furnace stoichiometry history control
¾ Furnace exit gas temperature
¾ Boiler exit temperature
¾ Predict effects of coal quality on emissions & combustion performance
¾ Boiler slagging, fouling and corrosion control
