High Performance Industrial Furnace (4H5924)
Furnace and Combustion Efficiency
Blasiak Wlodzimierz, Prof.
Weihong Yang Associate Prof.
Royal Institute of Technology
School of Industrial Engineering and Management
Division of Energy and Furnace Technology
Stockholm, Sweden
weihong@mse.kth.se
1
Objectives
To equip the student with enough knowledge about the
importance and need of the energy conservation in industry
especially in furnaces and getting knowledge in the fields:
– Evaluating the thermal performance of furnaces
– Energy conservation measures in furnaces
2
Lecture Contents
1.
2.
3.
4.
Introduction
Energy Balance and Efficiency
Measuring Method
Methods of Efficiency increase (Energy Saving
Measures)
– Combustion Efficiency Improvement
– Waste Heat Recovery
– Refractory and Insulation
3
Introduction
¾
Extensive experience work with furnace users has shown that
operating cost savings of 10 – 30 % can often be achieved
With little or no capital outlay.
Top management willingness.
Proper management system.
Applying effective management technique
¾
The enviromental issues and legislation are already haveing a
significant impact on furnace operators and this likely to
increase CO2, CO,SO2, NOX.
¾
Save Energy → Increase Profitability→ Protect the environment
4
Lecture Contents
1.
2.
3.
4.
5.
Introduction
Furnace and combustion efficiency
Factors determining furnace efficiency
Measuring efficiency
Methods of Efficiency increase (Energy Saving
Measures)
– Combustion Efficiency Improvement
– Waste Heat Recovery
– Refractory and Insulation
5
Energy
Balance
•
•
Energy balance is an analyses of a process in which all energy states and flows
(inputs and outputs) through a predefined envelope (system boundary) are
quantified. It is a tool to evaluate the furnace performance and efficiency.
1st Law:
⎛
⎞ dE sys
⎛
⎞
Vi 2
Vo2
&
+ gZ i ⎟⎟ =
+ ∑ m& ⎜⎜ ho +
+ gZ o ⎟⎟ + W& sys
Q sys + ∑ m& i ⎜⎜ hi +
dt
2
2
⎝
⎠
⎝
⎠
Qexhaust
Assume steady state conditions in a furnace
Neglect kinetic and potential energy
For furnace, Qsys is Qsur (surface losses)
QFGR
∑ m& (h ) = ∑ m& (h ) + Q&
i
•
i
o
o
Qloss
sur
The continuity equation is
in
FGR
stock
loss
Q +Q
=Q
+ Q + Qexhaust
Qin
∑ m& = ∑ m&
i
o
Qstock
6
Energy Balance
A energy balance should been obeyed . i.e.
Qin=Qout
Q1 , Sensible heat of wast gases (upto 60-80%)
Q2, Heat losses from the furnace surface,etc
Q3, Incomplete combustion of the fuel
Q4, Heat required by chemical re
taking place in the charge
Qin
Q5 Efficieny heat
Sankey diagram
Qout=Q1+Q2+Q3+Q4+Q5
7
Efficiencies Definition
•
Thermal efficiency is 100 % minus the summation of
all losses
η te =
Q stock
⋅100
Q in
or
η te =
Q in − Q loss
⋅100
Q in
Available Heat is thought of as the
total energy contained per kg (or m3)
of fuel minus the energy carried
away by the hot flue gasses exiting
through the stack, expressed as a
percentage.
η ah =
Q exhaust
⋅100
Q in
8
Lecture Contents
1.
2.
3.
4.
Introduction
Energy Balance and Efficiency
Measuring Method
Methods of Efficiency increase (Energy Saving
Measures)
– Combustion Efficiency Improvement
– Waste Heat Recovery
– Refractory and Insulation
9
Energy
Balance
... Methods
There are two methods of measuring efficiency:
•
Input-Output method
η te =
•
Q stock
⋅100
Q in
heat loss method
η te =
Q in − Q loss
Q loss
⋅100 = (1 −
).100
Q in
Q in
10
Energy
Balance
•
... Methods
Heat loss method
η te =
Q in − Q loss
Q loss
⋅100 = (1 −
).100
Q in
Q in
The losses measured are:
• heat loss due to unburned carbon in refuse,
• heat loss due to dry flue gas,
• heat loss due to moisture in ”as fired” fuel,
• heat loss due to moisture from burning hydrogen,
• heat loss due to moisture in the air,
• heat loss due to heat in the atomizing medium (steam, air),
• heat loss due to formation of carbon monoxide,
• heat loss due to unburned hydrogen,
• heat loss due to unburned hydrocarbons,
• heat loss due to surface radiation and convection,
• heat losses in ash pit
11
Energy
Balance
•
... Measurement
The calculation of energy balance requires the measurements of Weight of
feedstock used, ms
–
–
–
–
–
–
–
–
–
•
Weight of kiln car and kiln furniture (moving parts through the system), mfur
Amount of energy used (fuel flow, mf and combustion air flow, ma)
Walls temperature, Tw
Ambient temperature, Tamb
Combustion air temperature, Ta
Fuel temperature, Tf
Flue gas temperature, Tg
Fuel composition, molar or mass fractions (C, H, S, O2, N2, ash, moisture...etc)
Flue gas composition (at least O2 or CO2)
Minimum measuring equipment required:
–
–
–
–
–
–
–
Flue gas analyser (CO2 or O2)
Flow meters, (fuel or air)
Immersion temperature probe (flue gas, Combustion Air, fuel)
Surface temperature probe (furnace surface walls, feedstock)
Balance (stock and kiln furniture weights)
Length measaure
Fuel ultimate analysis (weight basis); C, H, S, H2O, ... Etc.
12
Energy Balance
... Combustion Analysis
•
The following parameters should be determined in order to calculate the
flue gases losses and therefore the Energy balance:
1. Theoretical Air (TA): This is the minimum amount of air that supplies sufficient
oxygen for the complete combustion of all the fuel
C + O 2 + 3,76 ⋅ N 2 → CO 2 + 3,76 ⋅ N 2
(kg air)
N O 2 × M O 2 + 3,76 ⋅ N N 2 × M N 2
TA =
= 11 .44
× Min Cgeneral:
The following equation can N
beCused
TA = 11 ,5(%C) + 34 ,5(%H 2 -(%O 2 )/ 8 ) + 4 ,32 (%S)
13
13
Energy Balance
... Combustion Analysis
2.
% Excess air in flue gas (EA): An additional quantity is required to achieve
⎛ CO 2 MAX
⎞
EA = ⎜⎜
- 1 ⎟⎟ × 100 %
⎝ CO 2 Act .
⎠
3.
The actual air fuel ratio (AF): is the actual total air supplied for 1 kg of fuel
(kg air)
4.
⎛ EA
⎞
+ 1⎟
AF = TA × ⎜
⎝ 100
⎠
Total flue gas:
(kg flue gases)
TFG = ( AF + 1)
5.
Act. H2O in combustion air =
6.
if O2 rather than CO2 is measured →
(kg vapor)
SH × AF
O2 % ⎞
⎛
CO 2 % = ⎜ 1 −
⎟ × CO 2 Max
21 ⎠
⎝
14
Energy Balance
... Energy Input to the
system
•
Inputs:
∑ m& (h ) = ∑ m& (h ) + Q&
i
1) Chemical enthalpy in fuel
i
o
o
sur
Q f = m& f ⋅ HHV
Tf
2) Sensible heat (physical
enthalpy) in fuel
3) Sensible heat (physical
enthalpy) in air
Q f , sen = m& f ∫ Cp f ⋅ dt
TR
Ta
Q a , sen = m& a ∫ Cp a ⋅ dt
TR
Ta
4) Sensible heat in moisture
contained in combustion air
Q H 2 O, air = m& f ⋅ AF ⋅ SH ⋅ ∫ Cp H 2 O ⋅ dt
5) Energy contained in stock
Q stock ,i = m& s C (Ts ,in − TR )
TR
15
15
Energy Balance
... Energy Output from the system
•
Outputs:
∑ m& (h ) = ∑ m& (h ) + Q&
i
i
o
o
sur
1) Energy contained in feedstock – useful energy
Q stock,o = m& s C(T s,out − TR )
2) Physical enthalpy in exhaust gas – loses
i.
Sensible heat in flue gases
Q g,sens = m& f ⋅ (TFG) ⋅ ∑
Tg
∫ x Cp
i
i
⋅ dt
TR
ii. Sensible heat due to moisture in air
Tg
Q H 2 O, air = m& f ⋅ AF ⋅ SH ⋅ ∫ Cp H 2 O ⋅ dt
TR
iii. Latent heat due to H2 in fuel
Q g, Latent , H 2 = m& f ⋅ 0 ,09 ⋅ (%H) ⋅ h fg
iv. Latent heat due to water in fuel
Q g , Latent , H 2 O = m& f ⋅ (%H 2O) ⋅ h fg
16
Flue Gases Losses
Propane
70%
60%
•
Can be determined by only
knowing
Tg and CO2 or O2 in flue gases
50%
600oC
40%
30%
400oC
20%
200oC
Flue gases losses increases with
Flue gas temperature Tg
10%
Excess air, O2%
0%
14
CO2 %
•
Energy losses in flue gas (%)
Tg = 800oC
0
2
4
6
8
10
12
Oxygen in flue gas, on dry basis (%)
12
10
8
CO2 Max for
Propane is 13,8%
6
0
2
4
6
8
10
12
O2% in flue gases, dry basis
17
Flue gases losses due to H2
in Fuel.
•
•
•
Heat loss due to burning hydrogen
in fuel can be a high portion in flue
gases
This amount of energy might not
appear in the analysis of energy
balance if the lower heating value
were chosen.
However they exist in flue gases
and can be recovered.
Heat loss due to burning hydrogen in fuel
18
18
Energy Balance
... Heat losses from the system
∑ m& (h ) = ∑ m& (h ) + Q&
i
•
i
o
o
sur
Radiation and convection losses through furnace walls – surface losses
Losses by radiation are calculated as
(
Q& r = σ ⋅ ε ⋅ T sur − T
4
4
amb
Losses by convection can be calculated as
)⋅ A
(Watt)
sur
(Watt)
25
& =Total
Consequently, Q
The
heat −
loss
due)1.to
radiative and convective heat
C ⋅ surface
Asur ⋅ (T
Tamb
c
sur
transfer
(Watt)
Q sur = Q& r + Q& c
19
Energy Balance
Balance Sheet ...
Input
1
2
3
4
Feedstock
Fuel /enthapy of comb
Fuel/sensible
Combustion air
Total
•
•
•
Outputs
T/hr
30
0,5
8
MWH
0,200
5,833
0,009
0,089
%
3,3%
95,1%
0,1%
1,4%
1
2
3
4
5
6,131
Specific energy consumption = 0,0185
(kg fuel oil / kg Product)
Useful energy (η) = 32,4 %
A Balance sheet shows all possible
energy conservation measures
Flue gases heat recovery
Surface losses from furnace walls
Off-spec products
Product
Off-Spec Product
Flue gas/ sensible
Flue gas/ latent
Surface losses
Total
Flue gas/
latent
10%
T/hr
27
3
8,5
0,9
MWH
2,192
0,325
2,361
0,611
0,642
6,131
%
35,7%
5,3%
38,5%
10,0%
10,5%
Surface
losses
11%
Flue gas/
sensible
38%
Product
36%
Off-Spec
Product
5%
20
Lecture Contents
1.
2.
3.
4.
Introduction
Energy Balance and Efficiency
Measuring Method
Methods of Efficiency increase (Energy Saving
Measures)
– Combustion Efficiency Improvement
– Waste Heat Recovery
– Refractory and Insulation
21
Factors determining
Furnace Efficiency
•
•
•
•
Excess air
Air infiltration
Stack loss
Combustion losses
Qexhaust
QFGR
Qloss
Qin
Qstock
22
Increase combustion
efficiency
•
Efficient combustion requires the correct
air/fuel ratio and adequate mixing
High excess air levels result in
–
–
–
–
–
•
Dilution of the flue gases due to increasing of the
total air supplied.
Reduction in flue gas temperature.
Reduction in heat transfer rate.
Increasing in flue gas losses.
Reduction in combustion efficiency.
Low excess air operation can cause
–
–
–
–
–
–
unburned hydrocarbons to discharged
leads to fuel wastage,
reduce throughput,
poor product quality,
excessive emissions and/or
structure damage to the furnace.
23
23
Increase combustion
efficiency
•
Many factors may cause undesirable
deviations:
–
–
–
–
–
•
Measures
–
–
–
•
Burner wear
Hysteresis in control system
Variation in fuel properties
Variation in combustion air temperature
Variation in furnace pressure
Modern automatic control (A/F, Pfur)
Routing eficiency monitoring (CO2%, Tg)
Regular burner and controls maintenance
Pay back could be immediate
Rule of thumb: 10% reduction of EA → 1% increase in eficiency
24
24
Saving Energ from Stock gas
How to recover the energy from waste gas :
Q exhaust = ∑ m& i (hi )
1) Reduce the final exhaust flue gas temperature
•
Recuperation, made of metallic or ceramic elements, heat
recover efficient 10-50%, preheated air temperature
Max.800 K.
•
Regeneration, made of honeycomb, heat recover efficient
80-90%, preheat air temperature, 1500 K. (HTAC)
2) Reduce waste gases mass, mwaste
•
Rich oxygen combustion
•
Pure oxygen combustion (OXY-FUEL)
25
Saving Energ from Stock gas
Available heat vs exhaust gas temperature for C3H8 combustion at 2% oxygen
concentration in exhaust gases
26
Oxyfuel Combustion
4132 nm3/h wet
3725 nm3/h dry
C O2
11%
H2 O
10%
O2
4%
576 nm3/h wet
337 nm3/h dry
N2
75%
3.4 MW
N2 O2
9% 4%
CO2
46%
H2O
41%
2.0 MW
AGA
Air/Fuel
Oxygen/Fuel
27
Waste Heat Recovery
For Propane, energy saving by 20°C
recovery from flue gases is
approximately ranging between
0,7% and 1,5% depending on the
level of excess air in the
combustion process
1,4%
1,2%
o
•
Energy savings every 20 C reduction in flue gases
1,6%
Rule of thumb: 20°C reduction of Tg
→ 1% increase in efficiency
1,0%
0,8%
0,6%
0%
50%
100%
150%
Excess air level
28
28
Waste Heat Recovery
9 Up to 50% energy saving is possible, payback (1 –5 years).
• Economy: The retrofit modification is more applicable to large,
continuous furnaces and least applicable to smaller, intermittent
ones.
• The heat sink can be to the furnace itself by raising the
combustion air temperature.
1234-
flue gas recuperation
self recuperative burners
flue gas regeneration
stock recuperation
9 Lower Tg → will reduce problems after chimney
X higher combustion air → higher flame temperature → increase
Nox.
Rule of thumb: 20°C reduction of Tg → 1% increase in efficiency
29
29
Waste Heat Recovery
Recovery heat can also be transferred for
use in other processes:
1234-
•
drying
space heating
process steam
steam for power generation
(waste heat boiler)
Heat available and heat required should
match in
– quantities,
– temperature
– timing
30
30
Waste Heat Recovery
... Flue Gas Recuperators
•
•
!
!
This is the most popular → the characteristics of the
heat sink and source are inevitably match.
A range of heat exchanger are available made of
–
–
–
steel,
high temperature alloys or
ceramic.
Flame temp. and Nox emission will increase.
Large heat recovery system.
31
31
Regenerative Burners
•
Uses a short-term cyclic heat storage device as
the means of achieving waste heat recovery.
•
Efficiency ~ 65 – 95%
•
Combustion air preheated to very high
temperatures, up to 50 degrees below
the furnace operating temperature.
!
NOx emission can be very high.?!
!
Flame temperature is very high.?!
(But can be reduced if the right combustion
technology is used)
Example of the ceramic regenerators
32
Operation of two pairs of compact
regenerative burners
33
Stock Recuperation
•
Incoming feedstock can be
preheated using waste heat
contained in flue gases as a
useful means of heat recovery.
•
Easily applied to continuous
furnaces
•
No problems in matching the
timing
•
Applicable to intermittent
furnaces.
34
34
Refractory and Insulation
•
1.
2.
3.
4.
5.
6.
7.
•
•
Consideration for selecting refractory:
Maximum operating temperature
Efectiveness as insulator: κ ↑
Thermal mass (LTM); Ceramic fibber,
specially for intermittent furnaces
Corrosion resistance
Errosion resistance
Coefficient of expansion
Longevity: replacement is expensive.
They are often made of layers
Monitoring the surface temperature of the
furnace from time to time.
35
35
Improving Furnace
Yield
•
Furnace yield: quantity of usable product per unit feedstock.
Increase furnace yield → Increase throughput and Minimize waste
•
•
There is often a trade off between throughput and product quality
Throughput is limited by
Loading and unloading
Rate of heat transfer
Upstream and downstream processes
•
Measures to increase furnace yield:
Optimise the furnace operation for maximum yield furnace. Consider other
processes when scheduling furnace operation.
Feedstock must be on-spec to prevent wasted firing.
Ensure the correct temperature profile and furnace atmosphere.
Continually monitor the yield by weighing the feedstock and product.
Monitor the key variables that affect throughput and product quality
Use effective Quality Assurance techniques, e.g. hot inspection.
36
36
Thank You For Your Attention !
37