Fuel and Combustion Flashcards
Higher efficiency in the combustion of solid fuel can not be achieved by
keeping the flue gas exhaust temperature very high.
A coal containing very high percentage of durain is called______coal
splint
During combustion of gaseous fuels, deficiency of air
lengthens the flame.
Gross and net calorific value of a fuel will be the same
if its hydrogen/hydrogen compound content is zero.
Which is not used as a binder in coal briquetting
Line
Size of blast furnace grade coke is______mm.
25-80
Presence of______in a dry gaseous fuel does not contribute to its calorific value.
Oxygen
The internal energy of the combustion products is______compared to that of reactants.
less
Junker’s calorimeter is used to determine the calorific value of
gaseous fuels
Combustion reaction of fuels is a/an______reaction.
exothermic
Improper storage condition results in the weathering of coal and spontaneous combustion, which increases its
friability & oxygen content.
Which constituents of coal is the most important in the production of coke?
Carbon
High temperature in gasification of coal favours
low production of CO2. / high production of CO.
Stack heat losses can be minimised by
controlling the excess air.
Laboratory gas is obtained by the cracking of
kerosene
Ratio of primary air to secondary air increases with increase in the rank of coal, because the
ratio of fixed carbon to volatile matter increases.
Carbon Content by weight in air dried wood may be about______percent.
50%
Natural draught produced by a chimney depends upon the
density of the chimney gases/ height of the chimney.
Gross & net calorific value is the same for
L.D. converter gas
Combustion of pulverised coal as compared to that of lump coal
can be done with less excess air.
‘Wobbe index’ is a characteristic o
gaseous fuels
Presence of free moisture in coal during its high temperature carbonisation
protects the volatile products from pyrolysis (cracking) in the presence of hot coke and hot oven walls.
Gray King Assay and Swelling Index of a coal is a measure of its
swelling characteristics.
Explosion limit of blast furnace gas is 37 to 71% gas in gas-air mixture. It means that the blast furnace gas will explode when burnt in a confined space, if its concentration in the gas-air mixture is______ percent.
in between 37 & 71%
Coke oven gas consists mainly of
H2, & CH4
Fuel gases containing hydrocarbons (ie.g. coke oven gas) are not preheated before burning, mainly because
the hydrocarbons crack thereby choking and fouling the heat transfer surface by carbon soot.
Which is the most reactive (as regards the formation of CO + H2 from C + H2O)
Low temperature coke
Calorific value of coal middling generated in coal washers during washing of coal may be around ______ Kcal/kg.
4000 Kcal/kg
Flue gas discharge velocity through chimney of a big thermal power plant may be around ______ m/sec.
10
Which of the following is used for making the explosive ‘TNT’?
Toluene
Adiabatic flame temperature of a fuel is dependent on the initial temperature of
Air and Fuel
Which gaseous fuels has the lowest calorific value ?
Blast furnace gas
Which is not a by-product recovered in a high temperature coal carbonization plant ?
Ethylene
Pensky-Marten apparatus is used for those oils, whose flash points are ______ °F.
> 120
In high temperature carbonization of coal compared to low temperature carbonization
H2 content in the coke oven gas is more.
When incomplete combustion loss is high, the flue gas analysis shows large amount of
CO
The temperature at which plastic layer formation takes place during carbonization of coal varies from ______ °C
350 to 450
Yield of coke oven gas in low temperature carbonization of coal is about______Nm3 /ton of dry coal.
160
Temperature of preheated air used for the transportation of pulverized coal through pipes to the burner of a boiler furnace is restricted to a maximum limit of about 300° C to avoid the
risk of explosion.
Blast furnace coke is made from coal by
→high temperature carbonization.
Yield of blast furnace gas is about______Nm3 /ton of pig iron.
adsorption on bog iron bed.
In Lurgi coal gasifier,
→larger quantity of coal can be processed.
Mineral matter content (M) and ash content (A) in coal are approximately related as
M = 1.1A
When steam is passed over coal resulting in the endothermic reaction represent by, C + H2O = CO + H2, it is called the ______ of coal.
gasification
Dry air requirement for burning 1 Nm3 of CO to CO2 may be around______Nm3
2.4
Degree of carbonization of coal during coke making can be roughly judged by the ______ of the coke produced.
volatile matter
Ammonia content in raw coke oven gas is about______gm/Nm3.
15
Coal is heated in absence of air for 7 minutes at a temperature of about ______ °C, during its volatile matter determination for proximate analysis.
950
Quantity of coke oven gas produced by high temperature carbonization of one ton of dry coal may be around______Nm3 .
300
Coking coals are invariably
bituminous coals
‘Fuel’ can be defined as a substance which produces heat by
→combustion / nuclear fission / nuclear fusion.
The calorific value of a gas of composition CO2 = 20%, H2 = 50%, CH4 = 30%, is y. If the composition of gas is changed to H2 = 50%, CH4 = 30%, CO2 = 10%, N2 = 10%, then the calorifice value will be
y
For which pair of the fuel gases, calorific value (C.V.) of one fuel is almost double that of the other on volume basis (i.e., kcal/Nm3 ), while the C.V. is same on weight basis (i.e., kcal/kg) ?
Propane and acetylene
If CO2 is not fully absorbed by the KOH solution meant for its absorption in the orsat apparatus, it will appear as
SO2
Deficiency of combustion air during combustion of a gaseous fuel
lengthens the flame / causes heat loss of fuel by its partial combustion.
64-132) rank coal (ASTM) means a coal with 64%
fixed carbon and having a heat value of 13, 200 BTU/1b.
The sequence of absorption in flue gas analysis by Orsat’s apparatus is respectively
CO2, O2, CO
Which of the following fertilisers is produced in the by-products recovery (from coke oven gas) plant attached to an integrated steel plant ?
Ammonium sulphate
Function of secondary air in pulverised coal firing is to
ensure efficient burning of coal
Calorific value of gobar gas (containing CH4 = 60% , H2 = 10% ; CO2 = 30%) may be about ______ Kcal/Nm3.
5400
Calorific value of dry wood may be around ______ Kcal/kg.
5
Ultimate analysis of coal determines its ______ content.
carbon, hydrogen, nitrogen & sulphur.
L.D. converter gas (produced in steel plant) comprises mainly of
CO (upto 65%) & CO2
Stack (chimney) height in a big thermal power plant is dictated by the
draught to be created.
Which coal gasification processes will produce gas having maximum methane content ?
Lurgi process
The type of liquor circulation system to be ‘ employed in evaporators (viz. natural or forced circulation) is determined mainly by the __________ of the liquid.
viscosity
In case of surface condensers, a straight line is obtained on plotting 1/U vs __________ on an ordinary graph paper.
1/V^-0.8
Thermal diffusivity is the most important in heat transfer by
conduction
A dephlegmator is a
partial condenser
A tank painted with which of the following coloured paints, would heat up maximum by radiation from sun ?
Black paint
The separation of liquid droplets from the vapor is done by a/an __________ , in the evaporators.
entrainment separator
Fouling factor for a heat exchanger is given by (where, U1 = heat transfer co-efficient of dirty surface U2 = heat transfer co-efficient of clean surface)
1/u1 - 1/u2
If a single tube pass heat exchanger is converted to two pass, then for the same flow rate, the pressure drop per unit length in tube side will
increase by 216
A steel sphere of radius 0.1 m at 400°K is immersed in an oil at 300°K. If the centre of the sphere reaches 350°K in 20 minutes, how long will it take for a 0.05 m radius steel sphere to reach the same temperature (at the centre) under identical conditions ? Assume that the conductive heat transfer co-efficient is infinitely large.
5 minutes
Evaporator tubes are generally
vertical
For a counter current heat exchanger with Tih = 80°C, T°c = 60°C, T°h = 50°C and Tic = 30°C, and the temperature difference between the two streams being the same everywhere along Z, the direction of flow of hot fluid. The temperature profile should satisfy
d^2T / dZ^2 = 0
In a boiling curve, the peak heat flux is called the __________ point.
burnout
Prandtl number is the ratio of
momentum diffusivity to thermal diffusivity.
h.D/K is called the __________ number.
Nusselt
Conductance is given by(where, x = thickness, A = heat flow area, K = thermal conductivity.)
KA/x
The wavelength at which the maximum monochromatic emissive power occurs for a black body, is (where, T = absolute temperature of the black body)
α 1/T
When the ratio of the Grashoff number and to the square of Reynolds number is one, the dominant mechanism of heat transfer is
mixed convection (both free and forced)
A multiple effect evaporator as compared to a single effect evaporator of the same capacity has
higher steam economy.
Which is the most suitable for cold viscous feed?
Backward feed
Boiling point elevation for a strong and concentrated solution is found by Duhring’s rule, which states that at the same pressure, the boiling point of a solution is a linear function of the __________ of pure water.
boiling point
Stefen’s block body radiation law can also be derived from __________ law.
Planck’s
Condensing film co-efficient for steam on horizontal tubes ranges from 5000 to 15000 Kcal/hr.m2 .°C. Condensation of vapor is carried out inside the tube in a shell and tube heat exchanger, when the
condensate is corrosive in nature.
The Sieder-Tate correlation for heat transfer in turbulent flow in pipe gives Nu α Re0.8, where, Nu is the Nusselt number and Re is the Reynolds number for the flow. Assuming that this relation is valid, the heat transfer co-efficient varies with the pipe diameter (D) as
(D))-0.2
A long iron rod initially at a temperature of 20°C has one end dipped in boiling water (100°C) at time, t = 0. The curved surface of the rod is insulated so that heat conduction is one dimensional in the axial direction. The temperature at a distance 100 mm from the dipped end becomes 40°C at time, t = 200 s. The same temperature is achieved at a distance of 200 mm from the dipped end at time
t = 800 s
What is the emissivity of a black body ?
1
Steam is routed through the tube in case of a __________ evaporator.
horizontal tube
A single pass air heater is connected to a two pass unit. For the air flow rate and other conditions remaining the same, the film heat transfer co-efficient for air will vary in the ratio of
2^0.8
A concentric double pipe heat exchanger as compared to the shell and tube heat exchanger for the same heat load requires
more space.
The average heat transfer co-efficient over the entire length of the plate (ha) and the local heat transfer co-efficient (hL), in case of heat transfer over a flat plate in laminar zone is related as
ha = 2hL
Wavelength corresponding to the maximum energy is inversely proportional to the absolute temperature. This is __________ law.
Wien’s
hree material A, B and C of equal thick-nes and of thermal conductivity of 20, 40 & 60 kcal/hr. m. °C respectively are joined together. The temperature outside of A and C are 30°C and 100°C respectively. The interface between B and C will be at a temperature of __________ °C.
70
the correct equation.
Nu = (Re) (Pr) (St)
Walls of a cubical oven are of thickness l, and they are made of material of thermal conductivity k. The temperature inside the oven is 100°C and the inside heat transfer co-efficient is 3k/l. If the wall temperature on the outside is held at 25°C, what is the inside wall temperature in degree centigrade ?
81.25
A sphere of radius ‘R1’ is enclosed in a sphere of radius ‘R2’. The view (or shape) factor for radiative heat transfer of the outer sphere with respect to the inner sphere is
R2/(R1+R2)
For a laminar flow of fluid in a circular tube, ‘h1’ is the convective heat transfer co-efficient at velocity ‘V1’. If the velocity is reduced by half and assuming the fluid properties are constant, the new convective heat transfer co-efficient is
0.794 h1
Extremely large or small volumes of fluids are generally best routed through the shell side of a shell and tube heat exchanger, because of the
flexibility possible in the baffle arrangement.
The capacity of double-effect evaporator is less than half of the capacity of two single effects, each of which is operating over same terminal temperature difference, when the
solution has an elevation of boiling point.
Forced circulation evaporators are useful for the concentration of viscous, salting and scale forming liquors. Which of the following is a forced circulation evaporator?
Agitated film evaporator
If average heat transfer co-efficient is ha and the local coefficient at the end of the plate is hl then in case of heat transfer to a fluid flowing over a flat plate, heated over its entire length
ha = 2hl
200 kg of solids (on dry basis) is subjected to a drying process for a period of 5000 seconds. The drying occurs in the constant rate period with the drying rate as, Nc = 0.5 x 10-3 kg/m2.s. The initial moisture content of the solid is 0.2 kg moisture/kg dry solid. The interfacial area available for drying is 4 m2/1000 kg of dry solid. The moisture content at the end of the drying period is (in kg moisture/kg dry solid)
0.1
In case of a vertical tube evaporator, with increase in the liquor level, the
capacity of the evaporator is decreased.
Which of the following forced convection heat transfer equation accounts for the liquid viscosity effect for viscous liquids ?
Sieder-Tate equation
A multiple effect evaporator has a capacity to process 4000 kg of solid caustic soda per day, when it is concentrating from 10% to 25% solids. The water evaporated in kg per day is
24000
Heat transfer co-efficient (h1) for liquids increases with
increasing temperature.
A fluid is flowing inside the inner tube of a double pipe heat exchanger with diameter ‘d’. For a fixed mass flow rate, the tube side heat transfer co-efficient for turbulent flow conditions is proportional to
d^-0.2
Bulk of the convective heat transfer resistance from a hot tube surface to the fluid flowing in it, is
mainly confined to a thin film of fluid near the surface.
The Nusselt number for fully developed (both thermally and hydrodynamically) laminar flow through a circular pipe, where the wall heat flux is constant, is
4.36
Minimum recommended baffle spacing in a shell and tube heat exchanger is about (where, D = shell diameter)
0.2 D
The equation, , corresponds to __________ analogy.
Prandtl
The heat transfer by radiation from a mild steel surface is to be reduced by reducing the emissivity of the surface. This can be best achieved by
painting the surface white (with aluminium paint).
A composite flat wall of a furnace is made of two materials ‘A’ and ‘B’. The thermal conductivity of ‘A’ is twice of that of material ‘B’, while the thickness of layer of ‘A’ is half that of B. If the temperature at the two sides of the wall are 400 and 1200°K, then the temperature drop (in °K) across the layer of material ‘A’ is
160
Forced circulation evaporators are useful for the concentration of viscous, salting and scale forming liquors. Which of the following is a forced circulation evaporator?
Agitated film evaporator
The equation, (NSt x N2/3Pr) = f/2, is the __________ analogy.
Colburn
Three solid objects of the same material and of equal mass-a sphere, a cylinder (length = diameter) and a cube are at 500°C initially. These are dropped in a quenching bath containing a large volume of cooling oil each attaining the bath temperature eventually. The time required for 90% change in temperature is the smallest for
cube
__________ heat exchanger is used for chilling oil to be dewaxed.
Double pipe
Fresh orange juice contains 12% (by weight) solids and the rest water 90% of the fresh juice is sent to an evaporator to remove water and subsequently mixed with the remaining 10% of fresh juice. The resultant product contains 40% solids. The kg of water removed from 1 kg fresh juice is
0.7
At constant temperature, the thermal conductivities of gases __________ with rise in pressure.
decrease
Heat exchanger tubes are never made of
lead
A hot liquid is kept in a big room. The logarithm of the numerical value of the temperature difference between the liquid and the room is plotted against time. The plot will be very nearly a/an
straight line
In thermal radiation for a black body (where, ε is emissivity and α is absorptivity)
α = 1; ε = 1
Electro-magnetic spectrum range, which is important for radiation varies from __________ microns.
0.5 to 50
__________ paint has the minimum absorption co-efficient.
White lead
1000 Kg of liquid at 30°C in a well stirred vessel has to be heated to 120°C, using immersed coils carrying condensing steam at 150°C. The area of the steam coils is 1.2 m2 and the overall heat transfer co-efficient to the liquid is 1500 W/m2.°C. Assuming negligible heat loss to the surrounding and specific heat capacity of the liquid to be 4 kJ/kg.°C, the time taken for the liquid to reach desired temperature will be
51 min
What is the emissivity of a black body ?
1
A cube, sphere & a thin circular plate (all having same mass and made of same material) are all heated to 300°C and allowed to cool in natural air. Which one will cool the slowest
Sphere
A dilute aqueous solution is to be concentrated in an evaporator system. High pressure steam is available. Multiple effect evaporator system is employed, because
total amount of vapor produced per Kg of feed steam in a multiple effect system is much higher than in a single effect.
A composite flat wall of a furnace is made of two materials ‘A’ and ‘B’. The thermal conductivity of ‘A’ is twice of that of material ‘B’, while the thickness of layer of ‘A’ is half that of B. If the temperature at the two sides of the wall are 400 and 1200°K, then the temperature drop (in °K) across the layer of material ‘A’ is
160
Grashoff number, which is defined as g . β(Ts - T∞) ρ2. L3/μ2, is proportional to the ratio of buoyancy force to __________ force.
viscous
If the thermal conductivity of a wall material is independent of temperature, the steady state temperature distribution in the very large thin plane wall having steady, uniform surface temperature follows __________ law.
linear
For a perfectly transparent surface (like gases), the
transmissivity = 1
An evaporator while concentrating an aqueous solution from 10 to 40% solids evaporates 30000 kg of water. The amount of solids handled by the system in kg is
4000
Three solid objects of the same material and of equal mass-a sphere, a cylinder (length = diameter) and a cube are at 500°C initially. These are dropped in a quenching bath containing a large volume of cooling oil each attaining the bath temperature eventually. The time required for 90% change in temperature is the smallest for
cube
Overall heat transfer co-efficient for cooling of hydrocarbons by water is about
50 -100 BTU/hr. ft.2°F
A metal ball of radius 0.1 m at a uniform temperature of 90°C is left in air at 30°C. The density and the specific heat of the metal are 3000 kg/m3 and 0.4 kJ/kg.K respectively. The heat transfer co-efficient is 50 W/m2.K Neglecting the temperature gradients inside the ball, the time taken (in hours) for the ball to cool to 60°C is
0.15
Mode of heat transfer in which the fluid moves under the influence of changes in fluid pressure produced by external work is called
forced convection
1000 kg of wet solids are to be dried from 60% to 20% moisture (by weight). The mass of moisture removed in kg is
400
In a heat exchanger with steam outside the tubes, a liquid gets heated to 45°C, when its flow velocity in the tubes is 2 m/s. If the flow velocity is reduced to 1 m/s, other things remaining the same, the temperature of the exit liquid will be
more than 45°C
In Biot number, the characteristic length used is the ratio of the __________ of the solid.
volume to surface area
Heat transfer co-efficient equation for forced convection, Nu = 0.023 Re0.8 . Prn, is not valid, if the value of
Reynolds number for the flow involved is < 2100.
The overall heat transfer co-efficient for a shell and tube heat exchanger for clean surfaces is U0 = 400 W/m2.K. The fouling factor after one year of operation is found to be hd0 = 2000 W/m2.K. The overall heat transfer co-efficient at this time is
333W/m2.K
In a laboratory test run, the rate of drying was found to be 0.5 x 10-3 kg/m2.s, when the moisture content reduced from 0.4 to 0.1 on dry basis. The critical moisture content of the material is 0.08 on a dry basis. A tray dryer is used to dry 100 kg (dry basis) of the same material under identical conditions. The surface area of the material is 0.04 m2/kg of dry solid. The time required (in seconds) to reduce the moisture content of the solids from 0.3 to 0.2 (dry basis) is
5000
The left face of a one dimensional slab of thickness 0.2 m is maintained at 80°C and the right face is exposed to air at 30°C. The thermal conductivity of the slab is 1.2 W/m.K and the heat transfer co-efficient from the right face is 10 W/m2.K. At steady state, the temperature of the right face in °C is
48.7
The radiation heat flux from a heating element at a temperature of 800°C, in a furnace maintained at 300°C is 8 kW/m2. The flux, when the element temperature is increased to 1000°C for the same furnace temperature is
16.5 kW/m2
Which of the following has the lowest overall heat transfer co-efficient ?
Air
