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 SYSTEMSwww.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
2 2
2 2
2 2
3 32 2
3 3
2 2
12 22.4 22.4
1.867 1.867
C O co
C O CO
M C M O M CO
kgC m O m CO
m mcC c O c CO
kg kg
2 2 2
2 2 2
2 2 2
3 32 2 2
3 3
2 2 2
2 2
3 3
2 2
1
21
21
2 22.4 22.42
5.6 11.2
0.7 0.7
H O H O
H O H O
M H M O M H O
kgH m O m H O
m mhH h O h H O
kg kg
S O SO
m msS s O s SO
kg kg
Fuels and Combustion
Air, Excess of air
2
2
2 2
3
,
2 2
3
, ,
,
,
, ,
1.867 5.6 0.7 0.7
: 0.21 0.79
1 11.867 5.6 0.7 0.7
0.21 0.21
1
0.21 1 1
O m
Air m O m
Air Air m
Air Air m
O Air m O m
mV c h s o
kg
Air O and N by volume
mV V c h s o
kg
V V
V V
V V V
3
, ,
2 2 2 2
3
, , ,
3
, , ,
2 2,max, , ,
2
1.867 11.2 1.244 0.7 0.8 0.79
1.867 0.7 0.8 0.79
1
1.867 1.867 1.867
1
11.2
CP m Air m
CP m dry Air m
CP CP m Air CP m Air m
CP CP m Air m CP m
mV c h w s n V
kg
CO H O SO N
mV c s n V
kg
mV V V V V
kg
c c cCO CO
V V V V
hH O
2 max, , ,
, ,2
, ,
1.244 11.2 1.244
1
0.21 1 0.21 1
1
CP m Air m CP m
Air m Air m
CP CP m Air m
w h wH O
V V V
V VO
V V V
Fuels and Combustion Combustion products
Combustion Control,
Measured are O2 and CO2
, ,2
, ,
, ,
2
2 2,max, , ,
, ,
, 2,max
2 2
1 0.21 1
1
0.21
0.21
1.876 1.876 1.876,
1
1.867
Air m Air m
CP CP m Air m
CP m Air m
CP CP m Air m CP m
CP m Air m
CP m
V VO
V V V
If V V
O
c c cCO CO
V V V V
If V V
cV CO
CO CO
Fuels and Combustion
Ostwald triangle and Bunte diagram
Example 63
, , ,
2,max, ,
max, ,
'2 max
, ,
1.876 0.7 0.8 0.79
1.876
1.876
0.9335
0.9335 1
2
CP m dry Air m
CP m dry
CP m dry
CP m dry
mV c s n V
kg
cCO
V
cCO
V c
cO CO
V
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
Comp a kJ/kmolK 103 b kJ/kmolK2 106c kJ/kmolK3 M kg/kmol
AIR 26.719 7.372 -1.1113 28.964 N2 27.016 5.811 -0.2887 28.01 O2 25.593 13.251 -4.205 32. H2O 29.857 11.046 0.192 18.02 SO2 31.163 33.394 -10.752 64.02 CO2 27.286 38.469 -11.262 44.05
CO 26.568 7.577 -1.119 28.01 NO 26.945 11.255 -1.76 30.01 OH 29.754 -0.881 1.7547 17.01 H2 29.062 -0.82 1.99 2.016 CH4 13.405 77.027 -18.744 16.04
Specific Heat: Table of Coefficients
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-> H2OS+O2->SO2
Incomplete combustion due to dissociation
Formation Heat T0=288KReaction H0 kJ/kmol lnKp0
CO2 <-> CO+1/2O2 283,197 -103,010H2O<-> H2+1/2O2 241,710 -91,870H2O<-> OH+1/2H2 284,030 -106,510NO <-> 1/2N2+1/2O2 90,624 -34,925
Combustion Kinetics
aA+bB->cC+dD, w=kCIi w1=k1CA
aCBb w2=k2CC
cCDd
w1/w2= k1CAaCB
b /k2CCcCD
d=1
K= k1/k2= CCcCD
d/CAaCB
b Kp= pCc pD
d/pAa pB
b
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 andchemical 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 heaterEvaporator SuperheaterAir preheater
History:
Early 1800 quality fuel, low efficiencylow 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
Q - heat into boiler, kW (MW)Q1- energy used in the boiler, kWD - boiler production of steam, kg/s (t/h)B - consumption of fuel, kg/sHl - fuel calorific value, kJ/kghs - enthalpy of superheated steam, kJ/kghs - enthalpy of feed water, kJ/kgb – boiler efficiency
Mass and energy balance of a steam boiler
Q=BHl
Q1=D(hs -hs)=Qb=BHlb B=D(hs -hs)/(Hlb)
b= Q1/Q
Q - heat into boiler, kW (MW)Q1- energy used in the boiler, kWLoses
Efficiency coefficient of a steam boilerb= Q1/Q=1-ui
Gasification loses u1-u3 because of unburned fuelu1- drop through grid u2- unburned in flying ashesu3- unburned in laying ashes
Furnace loses u1-u6 because combustion products did not receive heatu4- 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
Heat exchanged mainly by radiationQe- heat exchanged in the evaporator, kWHF0 – theoretical enthalpy in the furnace, kJ/kgHF2 – enthalpy of CP at the end of the furnace, kJ/kgh” - enthalpy of saturated steam at boiler pressure, kJ/kgh’ - enthalpy of water at boiler pressure, kJ/kg
Mass and energy balance of an evaporator (furnace)
Qe=D(h” –h’)=B(HF0 –HF2), kW
Heat Transfer in Furnaces
Dominated by radiation2
0.62
30.6 10.62 0.6
1 11 11
1
F F F F
F F
T Bo M MBo
T M Bo
M
1 10 5 21
0.6 6 3 3Since
0 20 0 24 3
0 20 0 0 2
03
0 0 2
21003
3 220
1.7610 11
F FCP CP F F F
R F Fo F o F F F
o F F F
Fo
FF F
B H HBV c T T TQBo
Q T TA T A T T T
Q
A T T T
TA
M TMT
82 4
5.7610W
Sincem K
Example 10
Heat exchanged mainly by convectionQs- heat exchanged in the superheater, kWks- heat transfer coefficient in the superheater, kW/m2Kh1- convection heat transfer coefficient for combustion products, kW/m2Kh2- convection heat transfer coefficient for steam, kW/m2K - conduction heat transfer for the pipe, kW/mK– pipe and fouling thickness, mtlog, th, tl – logarithmic and higher and lower
temperature differences
Mass and energy balance of a superheater
Qs=D(hs-h” )=B(HF2 –Hg1)=As ks tlog, kW1/ks =1/h1+ +1/h2
tlog=(th- tl)/ln th/ tl
Heat exchanged mainly by convectionQa- heat exchanged in the water heater, kWka- heat transfer coefficient in the water heater, kW/m2Kh1- convection heat transfer coefficient for combustion products, kW/m2Kh2- convection heat transfer coefficient for steam, kW/m2K - conduction heat transfer for the pipe, kW/mK– pipe and fouling thickness, mtlog– logarithmic temperature difference
Mass and energy balance of a water heater
Qa=D(h’-ha )=B(Hg1 –Hg2)=Aa ka tlog, kW1/ka =1/h1+ +1/h2
Q1=D(hs -hs)= D(hs-h” )+D(h” –h’)+ D(h’-ha )
Heat exchanged mainly by convectionQz- heat exchanged in the air preheater, kWkz- heat transfer coefficient in the air preheater, kW/m2Kh1- convection heat transfer coefficient for combustion products, kW/m2Kh2- convection heat transfer coefficient for steam, kW/m2K - conduction heat transfer for the pipe, kW/mK– pipe and fouling thickness, mtlog– logarithmic temperature difference
Mass and energy balance of an air preheater
Qz=B(HL-Hl )=B(Hg1 –Hg2)=Az kz tlog, kW1/kz =1/h1+ +1/h2
Q-t (Lentz) DiagramGives 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 efficiencyExample 8
Low-Polluting Combustion
Particles, CO, SO2, CmHn, NOx
Staged CombustionFluidized Bed GasificationFuel Cells‘Zero’ Pollution
Reduce CO2 means to increase user efficiency, Cogeneration
Staged Combustion
Initially rich mixture, shortage of air orRecirculation
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