vbi contrariam defendit, qucm im«

Carbon di oxide produced = (0.399 – 0.0188) x 3.666 = 1.3915 kg Moisture produced = (0.0248 x 9 ) = 0.2232 kg.

Moisture in fuel = 0.08 kg.

Moisture in air = 0.013 x 6.212 = 0.0807 kg.

Total moisture in flue gas = 0.3839 kg Sulphur di oxide produced = 0.0038 x 2 = 0.0076 kg.

Nitrogen in air = 6.212 x 0.7685 = 4.7739 kg.

Nitrogen in fuel = 0.0067 kg.

Total nitrogen in the fuel = 4.7739 + 0.0067 = 4.7806 kg.

Excess oxygen in gas = (6.212 – 5.176)x0.2315 = 0.2398 kg.

Total Flue gas produced

Per kg of fuel = 1.391 + 0.3839 + 0.0076 + 4.7806 + 0.2398 = 6.803 kg.

Ex.02 Find the weight of water present in atmospheric air at 60% relative humidity and temperature 40°C.

For 40°C, the saturation pressure of water is = 0.075226 atm (from steam tables) At 60% RH the partial pressure of water vapour is 0.6 x 0.075226

=0.045135 atm

Weight of moisture present in air = 0.622 x Pw/(1.035 –Pw) = 0.622 x 0.045135 (1.035 – 0.045135) = 0.02836 kg/kg.

Ex03. Estimate the efficiency of a boiler firing with coal as a fuel having GCV of 3200 kcal/kg. Furnace is Fluidized bed boiler. Apply ASME PTC 4.1 indirect method to calculate the efficiency. Flue gas temperature leaving the boiler is140°C and ambient air temperature is 40°C. Ash content of the fuel is 42.3% and 20% of total ash is collected in bed and 80% ash is carried in fly ash. As per lab report the loss on ignition of ash samples collected in bed zone and fly ash zone is 0.1% by weight and 4.4%by weight. The boiler is operating at 20% Excess air and the dry kg/kg of gas produced =5.91 and dry kg/kg of air required = 5.696. The moisture and hydrogen present in the fuel is 6% and 2.7% respectively.

Basically following are the losses present in boiler, 1.0 Unburnt carbon loss

2.0 Sensible heat loss through ash 3.0 Moisture loss due to air

4.0 Moisture and combustion of hydrogen in fuel 5.0 Dry flue gas loss

6.0 Radiation loss.

Unburnt Carbon loss =4%

Sensible heat loss in ash,

Flyash = %Flyash x% of ash qty x sp.heat (Tgo – Tamb) x100/GCV

= 0.8 x 0.423 x0.22(140-40) 100/3200

=0.233%

Bed ash

= 0.2x0.423x0.22(900-40)100/3200

=0.5%

Sensible heat loss due to ash = 0.233+ 0.5 =0.733%

Heat loss due to moisture in air

= kg/kg of moist in air x kg/kg of dry air( Enthalpy of steam at Tgo in 0.013ata – Enthalpy of steam at Tamb in 0.013 ata)

= 0.013 x 5.696 x( 660.33–615.25)100/3200

=0.1043%

Note: The above implies that the water vapour at ambient temperature at partial pressure exists in steam form and gets superheated at 140°C

Heat loss due to moisture in fuel and combustion of hydrogen,

=(%of moisture in fuel + % of hydrogen x8.94)(Enthalpy of steam –Tamb)100/3200

= (0.06 + 0.027x8.94)(658.37 –40)100/3200

= 5.824%

Note: The above implies that the water moisture present in fuel is in liquid form, during combustion it will absorb latent heat and superheat from combustion. The hydrogen present in the fuel react with oxygen to form water. From combustion equation of hydrogen it is found that 1 kg of hydrogen form 8.94 kg of water.

Dry flue gas loss,

= kg/kg of dry flue gas x (Enthalpy of gas at Tgo –Air enthalpy at Tamb)x100/3200

=Kg/kg of dry flue gas x Spheat (Tgo –Tamb)100/3200

=5.91 x 0.24 x(140 –40)100/3200 = 4.433%

Radiation loss,

From ABMA Chart the loss is estimated as =0.5%

Note: In the indirect method Blow down losses will not be considered into account. It is assumed the boiler is operated under zero present blow down.

Ex07 Estimate the FD and ID fan flow and power required for a bagasse fired dumping grate boiler, whose bagasse consumption at 100% MCR capacity is 31000 kg/hr and the boiler is operating at 35% excess air. The fuel air requirement is 3.909 kg/kg of fuel and gas generation is 4.873 kg/kg.

FD fan

Total air requirement = 31000 x 3.909 = 121179 kg/hr.

Fan design flow with 15% margin = 121179 x 1.15/(3600 x1.128)

= 34.31 m³/sec FD fan head

Pressure head required for air flow sections like airheater, air ducts and grate are to be calculated. Now in most of the practical applications the pressure drop works out to be 165 mm WC and the same can be assumed for this calculation.

FD fan head with margin = 165 x 1.2 = 200mmWc FD fan power required.

= flow x head/102 x efficiency

= 34. 31 x 200 / (102 x 0.8)

= 84.09 KW

Motor selected = 84.09 x 1.1 = 92.5 KW (next nearest motor standard is 110 KW) ID fan

Total gas produced = 31000 x 4.873 = 151063 kg/hr.

Fan design flow with 25% margin = 151063 x 1.25 x (273 +140)/(3600 x1.295x273)

= 61.27 m³/sec ID fan head

Pressure head required for gas flow sections like Furnace, Bank, Economiser, air heater, gas ducts and dust collectors are to be calculated. Now in most of the practical applications the pressure drop works out to be 230 mm WC and the same can be assumed for this calculation.

Motor selected = 225 x 1.1 = 247.7 KW (next nearest motor standard is 250 KW) Table showing percentage margin on flow and head required for different boiler application.

S.N Description Grate type AFBC CFBC OIL

fired

The design of furnace is considered as the vital part in the boiler. The furnace is the zone experiencing a high temperature in boiler. The performance of the furnace reflects or has an impact over other parts behind it such as super heater, evaporator, and air heaters. For instant, how the furnace design affects super heater can be

illustrated with following. If furnace outlet temperature (FOT) is high, then the next zone is super heater it gets high amount of heat input naturally the metal

temperature is high and the steam temperature also increased, which in turn reflects in the performance and cost of material. On the other hand if the furnace is over sized the FOT will be lesser, to get the required steam temperature the super heater heat transfer area to be increased. If the heat transfer area is increased it calls for larger space and cost wise it becomes uneconomical.

3.2 EFFECT OF FUEL ON FURNACE DESIGN:

The type of fuel, form of fuel, heat content and the properties of the fuel such as ash fusion temperature are also form as constraint over the furnace design. The type of fuel whether solid or liquid or gas and quantity decides how efficiently we can burn.

Whether we can have a burner (for liquid & gases), solids bubbling bed or dumping or travelling grate. When the fuel is some thing like bagasse (fibrous and long strand structure) it can be burnt well in dumping or travelling grate.

A gaseous fuel offers fewer problems since it is clean. Fuel oil brings its own problems like high or low temperature corrosion and additives have to be used. For coal ash fusion is the problem, since ash slag down deposits on the wall hindering heat transfer to steam water mixture. Depends on property of coal, whether it can be crushable to powdered form, pulverized firing or bubbling bed or cyclone furnace can be decided.

When we go for oil or gas firing, we can have higher heat flux in the furnace because of the higher emissivity of oil flame and relative cleanliness of walls compared to coal firing. There by size of furnace will be smaller for oil or gas fired steam generators.

The volume of the furnace for oil fired boilers will be 60 to 65 percentage of pulverized fuel firing. However, if a furnace designed for both coal and oil it is normally designed for coal and performance for oil firing in that furnace will be carried out. When a furnace designed for coal operated with oil, the higher furnace absorption results in a lower furnace outlet temperature. Lower FOT means super heater pick up in super heater will be less and steam outlet temperature will be less.

This is avoided by several techniques out of which, when oil is fired FOT will be increased by gas recirculation, otherwise when coal is fired FOT will be reduced by some means of bed absorption (This is used in FLUIDISED BED COMBUSTION techniques). Furnace size also governed by length of flame in gas or oil fired boiler since the flame should not impinge on the water walls and cause overheating.

Likewise in coal fired boilers flue gas velocity should be optimized to prevent higher rate of erosion due to carry over particles in flue gas. Normally a flue gas velocity of 6 to 8 meters per sec was allowed for coal fired boilers and 12 to 15 meters per sec was allowed for bagasse fired boilers.

3.3 FORCED OR NATURAL CIRCULATION:

Water wall is receiving radiation from flames and are exposed to high heat flux and there is a possibility of over heating. The boiling is the phenomenon, which governs the rate of heat transfer from combustion to steam water mixture inside the tube. In boiling when bubbles formed at tube wall hinders the heat transfer which cause

tubes over heating and tube failure. This sort of boiling occurs at nucleate boiling stage. Therefore proper circulation must be ensured to cool all tube. Circulation ratio (CR) is the ratio between mass of water circulated inside the boiler to rate of steam generation. Hence CR is also directly related to dryness fraction of steam by the expression CR = 1/x. which implies in one circulation 1/CR quantity of dry steam was produced. Circulation number will be higher when the difference in density between steam and water is more (i.e.) due to higher difference in density; steam water mixture velocity will be more thereby overheating will be prevented. If the proper circulation is not there, circulation in the boiler circuit is effected by means of external agency (normally a circulation pump will be used). This type of circulation is called Forced or controlled circulation.

3.4 HEATFLUX TO FURNACE WALLS:

Boiling phenomenon can be represented by a log-log plot of heat flux Vs surface temp-bulk temperature as shown

Q max.

H E A T F L U X

A B C D

SURFACE TEMP

The different regimes of boiling indicated by the letters A, B, C, D. Absence of bubble formation and the influence of natural convection on the heat transfer process is predominant in the region A (pool boiling). Formation of vapour bubbles at the nuclei with resulting agitation of liquid by the bubble characteristics at the region B (nucleate boiling). The most important perhaps the critical region with respect to the heat flux is C. In this region the unstable film boiling manifests with an eventual transition to a continuous vapour film. In the final region D film boiling becomes stabilized. This phenomenon of stable film boiling is referred as “ LEINDENFROST EFFECT”

In the regime of boiling the maximum wall heat flux is observed in region C. Many experimentalists refer this state of maximum wall heat flux as “BURN OUT FLUX’.

The reason being when the wall is heated electrically, the heating element frequently burn out when the wall heat flux reaches Q maximum. Hence the design engineers should have an idea of average heat flux to the tubes, how they vary around periphery and fin tip temperature in case of membrane wall construction. Calculation of fin temperature was discussed in latter part of this chapter.

3.5 POINTS TO BE NOTED WHILE DESIGNING FURNACE

1.0 Optimal heat transfer area to reduce the gas temperature to a temperature required from the point of super heater.

2.0 Sufficient height to ensure adequate circulation in the water walls

3.0 Fins in the wall to be properly cooled, accordingly the pitch of water wall to be selected.

4.0 Flames should not impinge on water wall

5.0 Proper provision should be there to remove ash generated.

6.0 Optimal furnace outlet temperature.

7.0 Sufficient residence time inside the furnace for complete combustion

3.6 CLASSIFICATION OF FURNACE

i) According to ash removal

a) Dry bottom: It consists of water walls or refractory walls enclosing the flame. Ash shall be removed dry from bottom. The fuel used has low heat flux and high ash fusion temperature.

b) Wet bottom: Ash removed from bottom is of molten form. The fuel having high heat flux low ash fusion temperature is used. The flue gas generated here or clean and free from fly ash and hence erosion, fouling problems are minimized.

ii) According to Type of combustion a)Conventional firing

1) Travelling grate 2) Dumping grate 3) Pulsating grate 4) Step grate 5) Fixed grate

b)Bubbling Fluidized bed combustion c)Circulated Fluidized bed combustion d)Pulverized fuel combustion

e) Cyclone furnace.

iii) According to draft system

a) Balance draft: In balanced draft both Forced draft and Induced draft fans are used so to maintain vacuum or zero pressure in furnace. There is no leakage of combustion product in the atmosphere. In the atmospheric pressure air leaks into furnace. This type of draft system is widely adapted in industries.

b) Forced draft or pressurized draft: Considering economic aspect in oil or gas fired boilers Forced draft fan alone used. The furnace pressure will be of the order of 100 to 150 mm a water column. The furnace has to be designed to without leakage. Otherwise combustion product will leak into atmosphere.

c) Induced draft: Induced draft fan is used for sucking the flue gas generated.

The furnace pressure will be maintained below atmospheric pressure.

d) Natural draft: There is no draft fan will be provided for this system. Natural draft generated due to chimney itself used for the boiler draft. Very small capacity steam generators will be of this type.

3.7 MODES OF HEAT TRANSFER

In general heat transfer from higher temperature to lower temperature is carried out in three modes.

1.0 Conduction 2.0 Convection 3.0 Radiation

Conduction

Conduction refers to the transfer of heat between two bodies or two parts of the same body through molecules, which are more or less stationary. Fourier law of heat conduction states rate of heat flux is linearly proportional to temperature gradient.

Q = --K dt/dx

Where,

Q rate of heat flux watts per sq.meter

K thermal conductivity (property of material)W/m°k dt/dx temperature gradient in x –direction

Negative sign indicates heat flows from high temperature to low temperature.

Heat transfer by conduction in plate and cylinder Plate Q = k.A. (t1- t2) watts

X Cylinder Q =k.(A2

-A1).(t1

-t2) (r2

-r1) ln(A2/A1) where,

A area of plate

A1 outside cylinder surface A2 inside cylinder surface

‘r cylinder radius

‘t temperature of surfaces

Convection

Convection is a process involving mass movement of fluids. When a temperature difference produces a density difference which results in a mass movement.

Newton s law of cooling governs convection. In convection there is always a film immediately adjacent to wall where temperature varies.

- kf A (tf - tw)

Q =

Where,

is film thickness

kf thermal conductivity of film

h = kf / heat transfer coefficient (kcal/ sq.m hr °C or W/sq.m °C)

Radiation

All bodies radiate heat. This phenomenon is identical to emission of light. Radiation requires no medium between two bodies, irrespective of temperature the radiation heat transfer takes place between each other. However the cooler body will receive more heat then hot body. The rate at which energy is radiated by a black body at temperature T( °K) is given by Stefan Boltzmann law.

Q = A T⁴

Q rate of energy radiation in Watts A Surface area radiating heat sq.m

Stefan boltzmann constant = 5.67 x 10^–8 Watt/sq.m K⁴ 4.88 x 10^–8 Kcal/sq.m hr K⁴

3.8 HEAT TRANSFER IN FURNACE

Furnace heat transfer is a complex phenomenon, which can not be calculated by a single formula. It is the combination of above said three modes of heat transfer.

However in a boiler furnace heat transfer is predominantly due to radiation, partly due to luminous part of the flame and partly due to non-luminous gases. Overall heat transfer coefficient in furnace is governed by three T’s temperature, turbulence and time and calculated by two parts.

Hc - heat transfer coefficient by convection Hr - heat transfer coefficient by radiation.

HEAT TRANSFER COEFFICIENT BY CONVECTION (Hc)

Heat transfer by convection may carry out in turbulent or laminar flow of the fluid. In forced convection turbulence or laminar flow depends on mean velocity, characteristic length L, density and viscosity. These variables are grouped together in a dimensionless parameter called Reynolds number. Reynolds number is the ratio between inertia force to viscous force.

Reynolds number = (mass x acceleration)/(shear stress x cross sectional area) Mass = volume x density

Acceleration = velocity / time

Volume = cross sectional area x velocity

Shear stress = dynamic viscosity x velocity gradient(v / l) Re = density x velocity x characteristic length Dynamic viscosity.

When Re > 2100 then flow is turbulence

< 2100 then flow is laminar. In practical case the flow is most often turbulent only.

In free convection turbulence or laminar flow depends on the buoyancy force and temperature difference, coefficient of volume of expansion. These variables are grouped to form dimensionless numbers called Grashoff number and Prandl number.

Laminar or turbulence is identified with product of Grashoff number and prandl number

When, Gr.Pr < 10⁹ flow is laminar

Gr.Pr > 10⁹ flow is turbulent.

DIMENSIONAL ANALYSIS FOR HEAT TRANSFER COEFFICIENT

In document Additiones, notae, resolutiones, ad. 7 partitarum, glossas, et cogita D. Gregorii Lopet II (página 78-98)