# Advanced Energy Systems and Heat and Mass Transfer

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Advanced Energy Systems and Heat and Mass Transfer
Professor Nikola Stosic (CM308, Ext 8925) Professor Ian K Smith (CM308, Ext 8114) Dr Russel Lockett

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

Fuels and Combustion Air, Excess of air

Fuels and Combustion Combustion products

Combustion Control, Measured are O2 and CO2

Fuels and Combustion Ostwald triangle and Bunte diagram Example 6

Combustion Products Specific Heat Example 3
Polynomial expression in function of temperature Cp = a + bT + cT2 Mean specific heat Cp = a b(T+To) c(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 a kJ/kmolK 103 b kJ/kmolK2 106c kJ/kmolK3 M kg/kmol AIR N O H2O SO CO CO NO OH H CH

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 w ; V SO2=0.7 s and VN2 = 0.8 n 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
C+O2->CO2       H2+1/2O2-> H2O S+O2->SO2 Incomplete combustion due to dissociation Formation Heat T0=288K Reaction DH0 kJ/kmol lnKp0 CO2 <-> CO+1/2O2 283, ,010 H2O<-> H2+1/2O2 241, ,870 H2O<-> OH+1/2H2 284, ,510 NO <-> 1/2N2+1/2O2 90, ,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=DH/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

Diffusive: Simultaneous mixing and
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 Q1=D(hs -hs)=Qhb=BHlhb
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 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 Gasification loses u1-u3 because of unburned fuel u1- drop through grid u2- unburned in flying ashes u3- unburned in 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

Mass and energy balance of a superheater
Qs=D(hs-h” )=B(HF2 –Hg1)=As ks Dtlog, kW 1/ks =1/h1+ d/l +1/h2 Dtlog=(Dth- Dtl)/ln Dth/ Dtl 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 l - conduction heat transfer for the pipe, kW/mK – pipe and fouling thickness, m Dtlog, Dth, Dtl – logarithmic and higher and lower temperature differences

Mass and energy balance of a water heater
Qa=D(h’-ha )=B(Hg1 –Hg2)=Aa ka Dtlog, kW 1/ka =1/h1+ d/l +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 l - conduction heat transfer for the pipe, kW/mK – pipe and fouling thickness, m Dtlog– 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 Dtlog, kW 1/kz =1/h1+ d/l +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 l - conduction heat transfer for the pipe, kW/mK – pipe and fouling thickness, m Dtlog– 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/Dt Area in the Q:1/ Dt 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