Advanced Energy Systems and
Heat and Mass Transfer
Professor Nikola Stosic
(CM308, Ext 8925)
Professor Ian K Smith
(CM308, Ext 8114)
Dr Russel Lockett
ADVANCED ENERGY SYSTEMS
www.staff.city.ac.uk/~sj376/energy.htm
Low-Pollution Combustion
Fuel and Combustion
Boilers and Furnaces,
Renewables
Energy Management
Calculation examples and problems
Coursework
Low-pollution Combustion:
Fuels and Combustion
General on fuels and combustion
Theoretic relations, Excess of air, Combustion
products
Callorific value, H-t Diagram, Combustion
temperatures
Monitoring of combustion
Fuel reserves, Environmental impacts
Low Pollution Combustion
Boilers and Furnaces
General on furnaces and boilers, Boiler types,
balance, Coefficient of utilization
Heat-Temperature chart
Monitoring of boiler processes,
Radiation in furnaces, chambers and channels,
Combined heat transfer
‘Zero emission’ combustion
Fuel cells
Energy Management
Plant lifetime costs
Fuel switching
Storage Systems, thermal and mechanical
Building management
Industrial refrigeration
Low-pollution Combustion:
Fuels and Combustion
General on fuels and combustion
Solid fuel, coal, brown coal
Liquid fuel, oil, oil derivatives
Gaseous fuel, natural gas
Fuels and Combustion Example 1
Theoretic relations, Fuel components
c - Carbon, h – Hydrogen, S – Sulphur
o – Oxigen
n – Nitrogen
w – Water
a - ashes
c+h+s+n+o+w=1
Fuels and Combustion Example 2
Theoretic relations, Air, Excess of air,
Combustion products
C  O2  CO2
M C C  M O2 O2  M co2 CO2
12kgC  22.4m O2  22.4m CO2
3
3
3
3
m
m
cC  1.867c
O2  1.867c
CO2
kg
kg
1
H 2  O2  H 2O
2
1
M H 2 H 2  M O2 O2  M H 2O H 2O
2
1
2kgH 2  22.4m3O2  22.4m3 H 2O
2
m3
m3
hH 2  5.6h
O2  11.2h
H 2O
kg
kg
S  O2  SO2
3
3
m
m
sS  0.7 s
O2  0.7 s
SO2
kg
kg
Fuels and Combustion
Air, Excess of air
m3
VO2 ,m  1.867c  5.6h  0.7 s  0.7o
kg
Air : 0.21 O2 and 0.79 N 2 by volume
1
1
m3
VAir ,m 
VO2 ,m 
1.867c  5.6h  0.7 s  0.7o 
0.21
0.21
kg
VAir  VAir ,m
VAir     1 VAir ,m
VO2  0.21   1 VAir ,m     1 VO2 ,m
Fuels and Combustion Combustion products
VCP ,m  1.867c  11.2h  1.244 w  0.7 s  0.8n  0.79VAir ,m
CO2
H 2O
SO2
N2
VCP ,m,dry  1.867c  0.7 s  0.8n  0.79VAir ,m
VCP  VCP ,m  VAir  VCP ,m     1 VAir ,m
1.867c
1.867c
CO2 

VCP
VCP ,m     1 VAir ,m
11.2h  1.244 w
H 2O 
VCP ,m     1 VAir ,m
O2 
0.21   1 VAir ,m
VCP

H 2Omax
m3
kg
m3
kg
CO2,max
1.867c

VCP ,m
11.2h  1.244 w

VCP ,m
0.21   1 VAir ,m
VCP ,m     1 VAir ,m
m3
kg
Combustion Control,
Measured are O2 and CO2
O2
  1 VAir ,m


If
VCP ,m  VAir ,m

0.21
0.21  O2
VCP

0.21   1 VAir , m
VCP ,m     1 VAir ,m
1.876c
1.876c
1.876c
CO2 

, CO2,max 
VCP
VCP ,m     1 VAir ,m
VCP ,m
If

VCP ,m  VAir ,m
1.867c
VCP ,m
CO2

CO2,max
CO2
Fuels and Combustion
Ostwald triangle and Bunte diagram
Example 6
VCP ,m,dry  1.876c  0.7 s  0.8n  0.79VAir ,m
CO2,max
COmax
1.876c

VCP ,m,dry
1.876c

VCP ,m,dry  0.9335c
0.9335c 1
 COmax
O2 
VCP ,m,dry 2
'
m3
kg
Combustion Products
Specific Heat Example 3
Polynomial expression in function of temperature
Cp = a + bT + cT2
Mean specific heat
Cp = a + 0.5 b(T+To) + 0.333c(T2+TTo+T2o)
Mean specific heats for air,
N2,O2, H2O, SO2, CO2, CO, NO, OH, H2 and CH4
Specific Heat: Table of Coefficients
Comp
AIR
N2
O2
H2O
SO2
CO2
a kJ/kmolK
26.719
27.016
25.593
29.857
31.163
27.286
103 b kJ/kmolK2
7.372
5.811
13.251
11.046 0.192
33.394
38.469
106c kJ/kmolK3
-1.1113
-0.2887
-4.205
18.02
-10.752
-11.262
M kg/kmol
28.964
28.01
32.
CO
NO
OH
H2
CH4
26.568
26.945
29.754
29.062
13.405
7.577
11.255 -1.76
-0.881
-0.82
77.027
-1.119
28.01
64.02
44.05
30.01
1.7547
1.99
-18.744
17.01
2.016
16.04
Combustion Products
Enthalpy
H =SVi hi = TSVi cpi =T[V CO2 cpCO2 +VH2O cpH2O
+V SO2 cpSO2 + VN2 cp N2 +(l -1) VAir,m cpAir ] kJ/kg
where:
V CO2 = 1.867c; V H2O = 11.2h + 1.244w ;
V SO2=0.7 s and VN2 = 0.8 n + 0.79 VAir,m m3 /kg
H-t diagram, Example 4 gives relation between the temperature
and enthalpy where excess of air is parameter. From it, either
enthalpy, temperature or excess of air can be estimated
graphically. Also these can be calculated, Example 5.
Calorific Value
Hl=34,000 c+120,000(h-o/8)+10,900 s-2500w kJ/kg
Hu=34,000 c+142,000(h-o/8)+10,900 s kJ/kg
Incomplete Combustion
Complete combustion:
C+O2->CO2
H2+1/2O2-> H2O
S+O2->SO2
Incomplete combustion due to dissociation
Formation Heat T0=288K
Reaction
H0 kJ/kmol lnKp0
CO2 <-> CO+1/2O2 283,197
-103,010
H2O<-> H2+1/2O2
241,710
-91,870
H2O<-> OH+1/2H2 284,030
-106,510
NO <-> 1/2N2+1/2O2 90,624
-34,925
Combustion Kinetics
aA+bB->cC+dD, w=kPCIi w1=k1CAaCBb w2=k2CCcCDd
w1/w2= k1CAaCBb /k2CCcCDd=1
K= k1/k2= CCcCDd/CAaCBb
Kp= pCc pDd/pAa pBb
d(lnKp)/dT=H/RT2 =[aT+1/2bT2+1/3cT3+C1] /RT2
=[a/T+1/2bT+1/3cT2+C1/T] /R
lnKp=a lnT/R+bT/2R+cT2/6R+C1/RT+C2
R is universal gas constant, 8314 J/kmol, C1 and C2 are
constants determined for T0 Example 7
Combustion:
Kinetic: Premixed fuel and air, slow chemical
reaction determines the combustion speed
Diffusive: Simultaneous mixing and
chemical reaction, slow mixing determines the
speed
Combustion speed:
1/w=1/wm+1/wc
Control combustion: distribution of air or fuel
Steam Boilers
Heat apparatus to produce steam or hot water
Combustion chamber, furnace
Water heater
Evaporator
Superheater
Air preheater
History:
Early 1800 quality fuel, low efficiency
low capacity and low steam pressure
1900 the same principles as today
1930 the same technology as today,
Forging and welding
Today, 2000 MW, 130 m high, big plant
Associate topics in:
Combustion: flow and chemical reaction
Heat transfer: radiation and convection
Fluid dynamics, turbulent flow
Structure and strength of materials
Process control: combustion, water feed,
steam temperature
Mass and energy balance of a steam boiler
Q=BHl
Q1=D(hs -hs)=Qhb=BHlhb
B=D(hs -hs)/(Hlhb)
hb= Q1/Q
Q - heat into boiler, kW (MW)
Q1- energy used in the boiler, kW
D - boiler production of steam, kg/s (t/h)
B - consumption of fuel, kg/s
Hl - fuel calorific value, kJ/kg
hs - enthalpy of superheated steam, kJ/kg
hs - enthalpy of feed water, kJ/kg
hb – boiler efficiency
Efficiency coefficient of a steam boiler
hb= Q1/Q=1-Sui
Q - heat into boiler, kW (MW)
Q1- energy used in the boiler, kW
Loses
Gasification loses u1-u3 because of
unburned fuel
u1- drop through grid
u2- unburned in flying ashes
u3- unburned in laying ashes
Furnace loses u1-u6 because
combustion products did not
receive heat
u4- chemically unburned
u5- heat lost through carbonization
u6- heat lost with laying ashes
Boiler loses u1-u8 because water did
not receive heat
u7- loss with the combustion
products
u8- external cooling
Mass and energy balance of an evaporator (furnace)
Qe=D(h” –h’)=B(HF0 –HF2), kW
Heat exchanged mainly by radiation
Qe- heat exchanged in the evaporator, kW
HF0 – theoretical enthalpy in the furnace, kJ/kg
HF2 – enthalpy of CP at the end of the furnace, kJ/kg
h” - enthalpy of saturated steam at boiler pressure, kJ/kg
h’ - enthalpy of water at boiler pressure, kJ/kg
Heat Transfer in Furnaces
Dominated by radiation

0.6
TF 2
Bo

TF 2 M  F 0.6  Bo0.6
Since
Bo 
F
1 
  1 


M




1 10 5
2

  1
0.6 6 3
3
1
0.6


MF  MF 

3 

1
1
 1   1 



B  HF0  HF2 
Q BVCP cCPTF 0  TF 0  TF 2 
Bo 

 
4 
QR
 AoTF 0  TF 0  TF 2   AoTF 03 TF 0  TF 2 

Q0
 AoTF 03 TF 0  TF 2 
10
1.76 10
Ao 
 F MTF 03
3
1  TF 0 
 1
2 
M  TF 2 
Since   5.76 108
W
m2 K 4
2
Example 10
2
Mass and energy balance of a superheater
Qs=D(hs-h” )=B(HF2 –Hg1)=As ks tlog, kW
1/ks =1/h1+ / +1/h2
tlog=(th- tl)/ln th/ tl
Heat exchanged mainly by convection
Qs- heat exchanged in the superheater, kW
ks- heat transfer coefficient in the superheater, kW/m2K
h1- convection heat transfer coefficient for
combustion products, kW/m2K
h2- convection heat transfer coefficient for steam, kW/m2K
 - conduction heat transfer for the pipe, kW/mK
– pipe and fouling thickness, m
tlog, th, tl – logarithmic and higher and lower
temperature differences
Mass and energy balance of a water heater
Qa=D(h’-ha )=B(Hg1 –Hg2)=Aa ka tlog, kW
1/ka =1/h1+ / +1/h2
Heat exchanged mainly by convection
Qa- heat exchanged in the water heater, kW
ka- heat transfer coefficient in the water heater, kW/m2K
h1- convection heat transfer coefficient for
combustion products, kW/m2K
h2- convection heat transfer coefficient for steam, kW/m2K
 - conduction heat transfer for the pipe, kW/mK
– pipe and fouling thickness, m
tlog– logarithmic temperature difference
Q1=D(hs -hs)= D(hs-h” )+D(h” –h’)+ D(h’-ha )
Mass and energy balance of an air preheater
Qz=B(HL-Hl )=B(Hg1 –Hg2)=Az kz tlog, kW
1/kz =1/h1+ / +1/h2
Heat exchanged mainly by convection
Qz- heat exchanged in the air preheater, kW
kz- heat transfer coefficient in the air preheater, kW/m2K
h1- convection heat transfer coefficient for
combustion products, kW/m2K
h2- convection heat transfer coefficient for steam, kW/m2K
 - conduction heat transfer for the pipe, kW/mK
– pipe and fouling thickness, m
tlog– logarithmic temperature difference
Q-t (Lentz) Diagram
Gives a graphical presentation of
heat transfer in a steam boiler
Abscissa: Temperature
Ordinate: Heat transferred
A ka=Q/t
Area in the Q:1/ t diagram represents
a measure of a heat transfer efficiency
Example 8
Low-Polluting Combustion
Particles, CO, SO2, CmHn, NOx
Staged Combustion
Fluidized Bed
Gasification
Fuel Cells
‘Zero’ Pollution
Reduce CO2 means to increase
user efficiency, Cogeneration
Staged Combustion
Initially rich mixture, shortage of air or
Recirculation
Low combustion temperature, heat transfer
Later add air, still low temperature
Low temperature for formation of SO2 and NOx
Add limestone, helps retention of SO2
Fluidized Bed
Air velocity:
Stationary layer, Fluidized bed,
Particle flight
Good mixing, no excess of air
Good heat transfer, low combustion
temperature
Nice concept, but
Intensive pipe abrasion
Pressurized fluidized bed, no success
Gasification
Rich mixture, lack of air
Low combustion temperature, no
formation of SO2 and NOx
CP used in gas turbine
Nice concept, but
Particle removal still a problem, no
success
Fuel Cells
Direct conversion of chemical into electrical
energy, efficient if temperatures are low and
pressures are high
Hydrogen or hydrocarbons
Nice concept, but
Low efficiency of electrical to mechanical
conversion
Fuel storage problems
‘Zero’ Pollution
Combustion of hydrocarbons in pure oxigen
Condensation of water vapour, CO2 used as by-product
in extraction of mineral oil
Nice concept, but
a‘cheating’ technology, CO2 returned to environment
Renewables:
Hydro energy and Nuclear energy
Hydro a real potential, but expensive and
irreversible
Nuclear, the only long-term choice, since fission
material is not in demand any more, still
expensive
Renewables:
Wind energy, solar energy, wave energy,
biomass, biogas
Large and ugly units ‘stealing’ from
environment
Very expensive, need a buy-product
Usually extremely favourable legislation
Rational use of existing power sources
Fuel switching, accumulation,
investment/operational cost trade-off
Topping and bottoming cycles, cogeneration
Passive solar, appropriate architecture, energy
management, heat and cool at the same time