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