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Advanced Energy Systems and Heat and Mass Transfer

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 Fu el and Combustion

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Advanced Energy Systems and Heat and Mass Transfer

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

  2. 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

  3. 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

  4. 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

  5. Energy Management Plant lifetime costs Fuel switching Storage Systems, thermal and mechanical Building management Industrial refrigeration

  6. 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

  7. Fuels and CombustionExample 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

  8. Fuels and CombustionExample 2 Theoretic relations, Air, Excess of air,Combustion products

  9. Fuels and Combustion Air, Excess of air

  10. Fuels and CombustionCombustion products

  11. Combustion Control, Measured are O2 and CO2

  12. Fuels and Combustion Ostwald triangle and Bunte diagram Example 6

  13. Combustion Products Specific Heat Example 3 Polynomial expression in function of temperature Cp = a + bT + cT2 Mean specificheat 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

  14. Specific Heat: Table of Coefficients 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

  15. Combustion Products Enthalpy H =SVi hi = TSVicpi =T[V CO2cpCO2 +VH2OcpH2O +V SO2cpSO2 + VN2cp N2 +(l -1) VAir,mcpAir] 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,mm3 /kg H-t diagram,Example 4gives 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.

  16. 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

  17. Incomplete Combustion Complete 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 283,197 -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

  18. 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

  19. 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

  20. Steam Boilers Heat apparatus to produce steam or hot water Combustion chamber, furnace Water heater Evaporator Superheater Air preheater

  21. 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

  22. 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

  23. 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

  24. 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- unburnedin 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

  25. 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

  26. Heat Transfer in Furnaces Dominated by radiation Example 10

  27. 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

  28. 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 )

  29. 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

  30. 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

  31. 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

  32. 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

  33. 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

  34. 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

  35. 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

  36. ‘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

  37. 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

  38. 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

  39. 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

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