HEAT PROCESSES. HP10. Combustion and burners.
Combustion and burners
Combustion and burners (pulverized coal, biofuels, oil and gas burners, NOx reduction, CFD analysis of gas burner). Properties of fuels, reaction enthalpy, combustion heat. Enthalpy balances, adiabatic flame temperature. Heat transfer by radiation, emissivity and absorptivity of flue gases. Hottel’s diagram.
Rudolf Žitný, Ústav procesní a zpracovatelské techniky ČVUT FS 2010
Energy, Volume 30, Issue 8, June 2005, Pages 1429-1438
Moving horizontal grate with screw feeder for wood chips combustion with maximum power 240 kW. Two pass heat exchanger for heating water.
Optimisation of primary and secondary air streams, with the aim to minimise NOx emissions.
See also workshop Biomass combustion modelling, Sevilla, 2000 (interesting presentations TNO Netherlands,…, TU Graz - next slide)
CFD analysisof stoker fired furnace (temperature and composition of flue gases)
THERMAL BIOMASS UTILISATION
Technical University Graz
Optimisation of Low-NOx BiomassGrate Furnaces with CFD Modelling
Retrofittravelling grate stoker boilerALSTOM Power Inc.
Biomass Boiler Retrofit to Increase Capacity by 25% and Decrease Ash Carryover by 60%
Wood particle tracks colored by residence time for the retrofit case
Iso-surfaces of turbulence levels for the original and retrofit firing conditions
Arrangement of the Stoker (left) and computational mesh Fluent (right)
Concept of „cigar burner“was devoped in Denmark for combustion of straw bales.
Bales are fed into the furnace with a hydraulic piston, and only the forehead (top) of the bale combusts in the furnace. Bales are being dried while entering the furnace, and after that devolatilisation occurs, while char combustion is done on a movable water-cooled grate. Furnace temperature should not exceed 900C, and water-cooled furnace walls or flue gas re-circulation are necessary.
R. Mladenovic et al. : The boiler concept for combustion of large soya straw bales. Energy 34 (2009) 715–723
Fluidized beds suspend solid fuels on upward-blowing jets of air during the combustion process. The result is a turbulent mixing of gas and solids. The tumbling action, much like a bubbling fluid, provides more effective chemical reactions and heat transfer. FBC plants are more flexible than conventional plants in that they can be fired on coal and biomass, among other fuels.
Combustion systems for solid fuels FBC reduces the amount of sulfur emitted in the form of SOx emissions. Limestone is used to precipitate out sulfate during combustion, which also allows more efficient heat transfer from the boiler to the apparatus used to capture the heat energy (usually water tubes). The heated precipitate coming in direct contact with the tubes(heating by conduction) increases the efficiency. Since this allows coal plants to burn at cooler temperatures, less NOx is also emitted. However, burning at low temperatures also causes increased polycyclic aromatic hydrocarbon emissions. FBC boilers can burn fuels other than coal, and the lower temperatures of combustion (800 °C / 1500 °F ) have other added benefits as well.
Fluidised bed types
Example: Babcock&Wilcox bubbled fluidised bed boiler
Rotational furnace with gas burner
Upright tubular boilers
CFD simulation of a 180 MW coal fired boiler Video Youtube (Fluent)
Pulverised coal boiler
Example: Babcock&Wilcoxspiral wound universal pressure (SWUP™) boiler
Control of secondary air
Control of secondary air swirling
Vortex chamber nozzle
A nice video: Boilers and Their Operation 1956 US Navy Instructional Film
Element composition(C-carbon, atomic mass AC=12,01), (O-oxygen, AO=16), (H-hydrogen, AH=1,008), (N-nitrogen, AN=14,01), (S-sulphur, AS=32,06) and free water explicitly (W-water, MW=18,015 kgkmol-1, moisture is determined by drying of sample at 1050C) andash (A-ash, minerals).
Composition is expressed in mass fractions C (kg carbon in kg of fuel), O, … and these values enable to estimate LHVassuming prevailing chemical reactions
Enthalpy of evaporation
Jigisha Parikh, S.A. Channiwala, G.K. Ghosal:
A correlation for calculating HHV from proximate analysis of solid fuels. Fuel, Volume 84, Issue 5, March 2005, Pages 487-494.
Consumption of oxygen necessary for combustion of 1 kg of fuel with known elemental composition (expressed as volume Nm3/kg)
12kg C requires 1 kmol of O2 (C+O2CO2)
4kg of H requires 1kmol of O2 (2H2+O22H2O)
Volume of 1 kmol of gas at normal conditions (0,1013 MPa and 00C) in m3
Consumption of pure oxygen can be easily recalculated to consumption of humid air ( is relative mumidity, p” pressure of saturated steam)
<1 lean fuel combustion =1 stoichiometric combustion >1 rich fuel combustion
In the same way production of flue gases can be expressed
Example: Combustion chamber f-fuel, o-oxidiser, fg-flue gas streams
Mass flowrate [kg/s]. Streams are composed of O2,N2,CO2,CO,CH4,H2O
Mass balance of mixture
Mass balances of individual components (chemical compounds)
Mass balances of elements (C,H,O,N - four equations)
Enthalpy balance of a combustion chamber
Boiler RUN video
mass flowrate of flue gas [kg/s]
relative flowrate of flue gas [dimensionless]
It would be heating value of fuel if the temperatures Tf,Tair,Tfg will be the same
Relative consumption of air
Relative production of flue gases
So that it could be possible to express enthalpies by temperatures it is necessary to modify the previous equation formally as
This term is heating value qn for Tf=Tair=Tfg=T0
qn is the low heating value as soon as the reference temperature T0 is above the temperature of condensationof water in flue gases
Pierre-Alexandre Glaude, René Fournet, Roda Bounaceur, Michel Molière: Adiabatic flame temperature from biofuels and fossil fuels and derived effect on NOx emissions. Fuel Processing Technology, Volume 91, Issue 2, February 2010, Pages 229-235.
Kubota, N. (2007) Thermochemistry of Combustion, in Propellants and Explosives: Thermochemical Aspects of Combustion, Second Edition, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany. doi: 10.1002/9783527610105.ch2
Adiabatic flame temperature is the temperature of flue gases for the case that the combustor chamber is thermally insulated (Q=0). This maximal temperature follows directly from the previous enthalpy balance
Specific heat capacities of fuel and air (cf,cair) can calculated easily, but the specific heat capacity cfg depends upon temperature and upon unknown composition of flue gases. Fortunately the product of density and specific heat capacity depends upon composition only weakly and can be approximated by linear function of temperature
c0=1300 [J.m-3.K-1] c1=0,175 [J.m-3.K-1]
Substituting this linear relationship results to a quadratic equation for adiabatic flame temperature with the following solution
Actual flame temperature and actual temperature of flue gases cannot be calculated so easily. It is necessary to express the power Q in terms of mean temperature of flame TS and the temperature of wall Tw .
Heat transfer by radiation dominates at high temperatures. In this case the heat flux is proportional to 4th power of thermodynamic temperature and
Irradiated heat transfer surface
Heat flow emitted by hot gas and absorbed by wall
Heat flow emitted by wall and absorbed by molecules of gas
Photons emmited at high temperature TS (short wavelength)
Photons emmited at low temperature Tw (long wavelength)
Photon absorbed by opposite wall – no contribution to Q
Photon absorbed by molecule of water
Photon is not absorbed by oxygen
Most photons emitted by gas are absorbed by wall
Photon absorbed by CO-no net contribution to Q
Wall of combustor chamber is almost “black body”, therefore all photons impacting to wall are absorbed and not bounced off. On the other hand the photon emitted by wall has only limited probability to be absorbed by a heteropolar molecule (H2O, CO2, homeopolar molecules like O2,N2 are almost transparent for photons). The probability of absorption is proportional to density of heteropolar molecules (to their partial pressure) and to the length of ray L. Probability of catching depends also upon the photon energy (wavelength), the greater is energy the lower is probability of absorption.
Let us return back to the expression for resulting power exchanged between the hot gas and the wall of combustion chamber
Absorptivity of gas corresponding to wall temperature Tw
Emissivity of gas corresponding to temperature of gas Ts
According to Kirchhoff’s law Emissivity=Absorptivity (g = Ag ) but this equivalence holds only at the same wavelength (monochromatic radiation). Emissivity and absorptivity of photons depends upon their wavelength (frequency, energy). The first term (g) should be evaluated for high energy photons emitted by hot gas, while the second term (Ag) for photons emitted by colder wall.
Hottel’s diagram for emissivity of CO2 and H2O as a function of temperature and pL (partial pressure pCO2 is calculated from composition of flue gas, and length of ray L=3.5V/S – empirical approximation)
Instead diagrams this approximation can be used
Subtractinq equations (enthalpy balance for real and insulated combustors)
we arrive to the equation for two unknown temperatures Tfg and TS
The flame temperature TS must be somewhere between Tfg and Tfg,max and can be approximated by geometric average of these two temperatures, giving
Quadratic equation for flue gas temperature
The solution of quadratic equation for flue gas temperature can be expressed in terms of Boltzmann criterion (ratio of overall transferred heat to the heat transferred only by radiation)
Remark: this formula is only a rough approximation. Its application will be demonstrated on the following example.
Furnace for steam reforming (reaction proceeds inside a set of vertical tubes) makes use a row of gas burners, consuming natural gas as fuel.
For given mass flowrate of fuel
It is possible to evaluate consumption of air and production of flue gases
For temperature of methane and preheated air TCH4=291 K, Tair=573 K, and for heating value of methane qn=49,9 MJkg-1it is possible to evaluate temperature of adiabatic flame
The relative emisivity g(TS) is calculated for estimated flame temperature TS2000 K
The relative absorptivity Ag=g(Tw) is calculated for estimated temperature of wall Tw1200 K.
Mean path of ray L is estimated from geometry of combustion chamber (rectangular channel of height 10.8 m a width 2.5 m) as L=3,5 V/S=3,5(10,8.2,5)/(2.10,8)=4,4 m
Partial pressures are determined by composition of flue gas composed of H2O, CO2,a N2. This calculation follows from previously evaluated relative volume of air VO2=2,9 (m3 oxygen/kg methane) and stoichiometry of reaction
Corresponding ratio of partial pressures is
and because sum of pressures is atmospheric pressure p=pH2O+pCO2+pN2 the partial pressures of heteropolar gases are
pH2O=0,0192 MPa, pCO2=0,0096 MPa.
Using these values in Hottel’s diagrams (or using mentioned correlation for relative emissivity follows
g(TS)=0,258 and Ag=g(Tw)=0,49, and final result (flue gas temperature)
check Vfg= VH2O+ VCO2+VN2