Fireye, Inc. The Combustion Control Specialists PPC4000 COMBUSTION EFFICIENCY SYSTEM.
Combustion Efficiency
-
Upload
rashm006ranjan -
Category
Documents
-
view
220 -
download
0
Transcript of Combustion Efficiency
-
7/28/2019 Combustion Efficiency
1/43
Combustion ProcessHistorical Introduction
Fire exists from the earliest existence of man on earth
Until 1880, man did not achieve a quantitativeunderstanding of the combustion process
In 1697, G.E.Stahl proposed Phlogiston theory-Phlogiston was a hypothetical mysterious substancewhich combined with a body to render it combustible
In 1774, Joseph Priestly discovered the unique power
of oxygen for supporting combustion In 1781, Henry Cavendish demonstrated the compound
nature of water
-
7/28/2019 Combustion Efficiency
2/43
Combustion Process
Historical Introduction
About the same time, Lavoisier made the precise measurementsand formulated the volume and weight relationship that underline
the modern theory of combustion
In 1811, Amendeo Avagadro established that the number of
molecules in a unit volume under standard conditions is same forall gases
During the same period, John Dalton enumerated the law of
partial pressures
In 1803, John Daltons study of the physical properties of gasesled to formulation of atomic theory including the law of
combining weight
In 1808, Gay Lussac observed that gases always combine in
volumes that bear simple ratios to each other
-
7/28/2019 Combustion Efficiency
3/43
Combustion Efficiency
Combustion efficiencyeffectiveness ofcombustion equipment to convert the internalenergy in fuel to heat energy for use by the process
Any heat loss lowers the efficiency of the process Combustion efficiency = Total energy contained
per unit of fuel - losses (radiation, unburned andflue gas)
Continuous monitoring of Oxygen andcombustibles (CO or H2)-the best way to improvecombustion efficiency
-
7/28/2019 Combustion Efficiency
4/43
Combustion Theory
Three essential components of combustionfuel,oxygen and heat
Chemical elements that react with oxygen torelease heat are Carbon and Hydrogen commonly
known as hydro carbons C + O2 CO2 + 14093 btu/lb (stoichiometric air
150 ft3 of air / lb of fuel)
H2 + O2 H2O + 61000 btu/lb (stoichiometric
air 2.38 ft3 per ft3 of fuel) Stoichiometric combustion just right amount of
oxygen and fuel mixture (without any excess)
-
7/28/2019 Combustion Efficiency
5/43
Why air instead pure oxygen?
Air contains 21% by volume or 23% by
weight of Oxygen and is readily available
Pure oxygen needs processing, the cost of
which outweighs the benefit on combustion
and heat release
-
7/28/2019 Combustion Efficiency
6/43
Why excess air?
Inadequate mixing of air and fuel, fluctuating
operating and ambient conditions, burner
performance and wear and tear To ensure that fuel is burned completely or with
little combustibles, some amount of excess air is
provided
Normal excess air
Gas 5%, Oil 10%, coal 20%
-
7/28/2019 Combustion Efficiency
7/43
Excess Air Solid fuels require the greatest and the gaseous
fuels require the least quantity of excess air At design load,
Gas
Natural gas 5 to 10%Refinery gas 8 to 15%
Blast furnace gas15 to 25%
Coke oven gas 5 to 10%
Solids
Pulverised coal15 to 30%
Coke 20 to 40%
Wood 25 to 50%
Bagasse 25 to 45%
Liquids
Oil 3 to 15%
-
7/28/2019 Combustion Efficiency
8/43
Negative aspects of high excess air
Increase in auxiliary power (FD & ID fan) Increase in furnace temperature and NOx
formation
Increase in loss of sensible heat carried away by
flue gas
Increase in erosion due to increase in flue gasvelocity
Limitation on boiler load due to exhaustion of IDfan capacity
Shift in heat transfer from furnace to convectionpass resulting in heating up of down stream
components
-
7/28/2019 Combustion Efficiency
9/43
-
7/28/2019 Combustion Efficiency
10/43
Impact of lesser air than
stoichiometric requirement Incomplete combustion leading to
Reduction in energy release
Increase in unburned hydro carbons (Co &
CnHm) in flue gas
Increase in unburned carbon level in fly and
bottom ashesSlagging in boiler furnaces
-
7/28/2019 Combustion Efficiency
11/43
-
7/28/2019 Combustion Efficiency
12/43
-
7/28/2019 Combustion Efficiency
13/43
Reference curves for Optimum % Oxygen at
Economiser outlet for minimum heat rate
-
7/28/2019 Combustion Efficiency
14/43
Curve to estimate % excess air based % Oxygen
-
7/28/2019 Combustion Efficiency
15/43
Basis for controlling excess air
By monitoring oxygen and combustibles in
flue gas at Eco outlet by installing on-line
analysers
Monitoring unburned carbon level in fly
and bottom ashes
-
7/28/2019 Combustion Efficiency
16/43
Method of evaluating air leakage in furnace
-
7/28/2019 Combustion Efficiency
17/43
b i ffi i
-
7/28/2019 Combustion Efficiency
18/43
Combustion Efficiency
C b i Effi i
-
7/28/2019 Combustion Efficiency
19/43
Combustion Efficiency
C b i Effi i
-
7/28/2019 Combustion Efficiency
20/43
Combustion Efficiency
F ti l R i t f
-
7/28/2019 Combustion Efficiency
21/43
Functional Requirement for
combustion Equipment
Easy ignition and reliable flame scanning Maximum Heat release (at desired rate)
Optimum turn down
Efficient combustion (Minimum un-burned)
Optimum temperature
Minimum Excess air
Minimum emission
Minimum slag formation Desired flame shape
Heat release profile matching furnace heat absorptionneed
-
7/28/2019 Combustion Efficiency
22/43
-
7/28/2019 Combustion Efficiency
23/43
Fuel aspects
Organic aspectsPetrography
Heat release rate
Inorganic aspectsCCSEM, Ash formation, TMA,
Physical aspects
Grindabity, Specific gravity, particle sizedistribution
-
7/28/2019 Combustion Efficiency
24/43
Heat Value of Fuels
High Heat Value (HHV)
= 8080C+34500(H2-(O2/8))+2220S Kcal/Kg (Dulongs
Formula)
Where C,H2,O2 and S represent weight in kg of Carbon,
Hydrogen, Oxygen and Sulphur per Kg of fuel
Low Heat Value (LHV)
=HHV-Latent heat of steam formed
The amount of steam formed during combustion=9H2
where H2 is the weight of Hydrogen per kg of fuel Latent heat of 1 Kg of steam at 760 mmHg and 100
DegC is 538.9 Kcals/Kg
-
7/28/2019 Combustion Efficiency
25/43
Coal
Coals having FC / VM ratio closer to 1 will have
better flame stability
VM less than 13% is not preferable for PC firing Residence time
110 Mw 1.75 sec, 210 Mw-2.2 sec, 500 Mw-3.5 sec
Crossing point temperature -175 to 250 Deg.C
Flammability temperature400 to 600 Deg.C
-
7/28/2019 Combustion Efficiency
26/43
Coal Quality Vs Air
regimes
Low ash, high volatile, high moisture, high
CV (imported coal)Flame propagation affected more by moisturethan by ash
Priority for drying coal. Hence PA can not be
reduced below a particular levelNecessary to incorporate split coal nozzle or
diverters
-
7/28/2019 Combustion Efficiency
27/43
Coal Quality Vs Air
regimes
Low moisture, low volatile, high ash
Flame propagation affected by high ashReduce primary air to minimum extent possible
Increase OFA
-
7/28/2019 Combustion Efficiency
28/43
-
7/28/2019 Combustion Efficiency
29/43
Arrangement of Coal and
Air Ports in the Wind Box
of a Typical TangentialFired 500 MW Boiler
Furnace
-
7/28/2019 Combustion Efficiency
30/43
Primary Air
P.A / Coal ratio 1.5 to 2.5 (2 for better combustion
efficiency)-lower the P.A better the flame stability
P.A normally 1/4th (20-25%) of total air
Variable P.A control gives better scope to improve burner
performance
Primary air velocity 25 m/sec (to be > 20 m/sec to avoid
settling in coal pipe. To be > 15 m/sec to avoid flash back)
Minimum P.A temperature 57 Deg. C to avoid condensation
Maximum P.A temperature tested 127 Deg.C to avoid millfire and softening & sticking of coal in coal pipe
Normal P.A mix temperature is around 80 Deg.C
-
7/28/2019 Combustion Efficiency
31/43
Functions Of Primary Air
To dry the moisture in coal and facilitate
better grinding in the mill
Transport the pulverised coal from the mill
to the furnace at a velocity higher thansettling velocity of pulverised particle and
that of flash back
-
7/28/2019 Combustion Efficiency
32/43
Functions Of Fuel Air
Helps to position the flame front (Not too
away with potential for blow off-Not too
close with potential for heating & distorting
nozzle)Considerations
Good Flame Stability
View For Flame Scanner Protection of Nozzle From Distortion
-
7/28/2019 Combustion Efficiency
33/43
Impact of Fuel Air
increasing with Feeder Speed
Primary stream need not be uniform in all
the four corners Fuel air increase may further degenerate
flame where PA/Coal ratio is already high
-
7/28/2019 Combustion Efficiency
34/43
Fuel Air Vs Feeder Speed
Fuel Air can not be increased with feeder speed
With increase in feeder speed, the primary airwould increase
Since Indian coals have more non combustibles (50%
compared to 10-20% in North American coals). Muchmore primary air is required than required for volatilecombustion
Addition of fuel air can affect the flame stability and
unburned carbon level
Feeder speed will increase if fuel CV goes down as well asboiler load increases
-
7/28/2019 Combustion Efficiency
35/43
Secondary Air
75 to 80% of total air distributed at different tiers
Secondary air velocity ~40 m/sec for bettermomentum and mixing
Secondary air temperature ~ 227 deg.c
Air distribution in tiers decide combustionefficiency
Fuel air is provided for the twin purposes ofcooling nozzles and for positioning flame front-
close the damper if flame front is away and open ifflame front is close to nozzle
Other damper openings to be adjusted dependingon the operating tiers
-
7/28/2019 Combustion Efficiency
36/43
Functions Of Auxiliary Air
Ensuring completion of combustion
Enough momentum to penetrate intoprimary stream (expanded flame jet
containing the char of the coal particles) andprovide air to the whole cross section ofchar to be burnt
Stage the air for gradual mixing to reduceNOx
-
7/28/2019 Combustion Efficiency
37/43
0
0
0
0.5 to 0.8mmR
Aux Air
Fuel Air
Aux Air
PA + Coal
Temperature
Flame Front
Typical Mixing and Flame Front In Corner Firing
-
7/28/2019 Combustion Efficiency
38/43
Effect of changing
auxiliary air
Auxiliary air quantum should not be below
the level where the momentum is notadequate to penetrate the primary jet flame
This happens if PA is high and FA is also high
and the total air is limited to 3.5% O2
(Under such conditions the wind box pressure
-
7/28/2019 Combustion Efficiency
39/43
Functions Of Over Fire Air
Primarily indented to reduce unburned
carbon in fuel burnt in the top elevations
Since the air in the bottom elevations are
proportionately reduced it also reducesThermal NOx formation in the lower
elevations
-
7/28/2019 Combustion Efficiency
40/43
Effect of Over Fire Air
OFA intended for reducing unburned and
NOx By closing OFA combustion completion of
lower elevations is advanced, FOT comes
down, Unburned may go up
-
7/28/2019 Combustion Efficiency
41/43
Need for biasing / extent
of biasing AuxAir Biasing is
subjectiveFuel, Mill System, Boiler size
ObjectiveUnburned carbon reduction, Lower/higher
FOT, Reduction of spray, Higher SH steam temperature
Eg. IFFCO/PHULPHUR VU40
Biasing towards bottom reduced unburned levels in
bottom ash NTPC, Ramagundam 500 Mw
Biasing towards top reduced SH Spray
-
7/28/2019 Combustion Efficiency
42/43
IMPROVING THE EFFICIENCY OF THE
EXISTING COAL BASED POWER PLANTS
Boiler Operational Improvements
Tuning Combustion Air Regimes
Prevention of air leakage PA/Coal ratio around 2
Flame front 300 to 500 mm away from nozzle tip
Fuel air at minimum opening
Excess air to reduce carbon loss and slagging
Minimize OFA if furnace is slagging
-
7/28/2019 Combustion Efficiency
43/43