OBJECTIVE

To determine the Calorific Value (C.V) of natural gas by using Boys Non Recording

Calorimeter.

INTRODUCTION

The calorific value (C.V) also known as the heating value of a fuel is the quantity of heat that is released from the complete combustion of a specific amount of the fuel under isothermal conditions and constant temperature and at specified reference temperature of unit

quantity of the fuel. It is measured in units of energy per unit of the substance, usually kcal/kg, kJ/kg, J/mol, Btu/m³. Heating value is commonly determined by use of a bomb calorimeter. Fuel gas which contains hydrogen or hydrocarbon possess two CV the superior (gross) and the inferior (net). The heat of combustion for fuels is expressed as the HHV, LHV, or GHV.

The gross calorific value of a gas (relative to the volume of dry gas) is defined as the amount of heat released by the complete combustion of the gas, with air at constant pressure of 1.01325bar and a constant temperature of a specific volume under specific conditions, all

the water being formed during combustion being condensed at the temperature (tH)

While the net calorific value of a gas (relative to the volume of dry gas) is defined as the

amount of heat given out by the complete combustion of the gas with air at constant pressure of 1.01325bar and a constant temperature of a specific volume under specific conditions, all the water being formed during combustion remaining in the gaseous phase at

the temperature (tH)

The gross CV provides the basis on which charges are made by the gas industry to consumer.

The calorific value, CV of natural gas is measured in this experiment by the calorimeter (Boys

Calorimeter. This was achieved by complete combustion of the natural gas at atmospheric pressure

and the CV was measured by the calorimeter at constant pressure.

APPARATUS

The apparatus used is the Boys Non-Recording Calorimeter

Boys Calorimeter

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This is fitted with appropriate burners in the base, and a precision, Hyde type gas meter with a capacity of 2.0litres/rev. It consists of a specially designed measuring drum housed in a

gas-tight casing sealed with water.

Figure: Boys Calorimeter

The heated products of combustion from the burner B impinge on a metal box H, through which water is circulating, and then pass downwards and outwards through a spiral cooler

which reduces them practically to the atmospheric temperature. A steady stream of water enters the apparatus by the inflow thermometer O.

EXPERIMENTAL PROCEDURES

The experiment was carried out by following the procedures below

- The gas was turned on and ignited after all the apparatus was set accordingly.

- The water was turned on and the calorimeter placed on its base

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- Flow of gas to the calorimeter was adjusted in such a way that the time for one revolution of the meter hand falls within the limits calculated from the formulae

below:

tmin = 3 .CV100 , tmax =

3 .CV100 + 5

where:

CV = expected calorific value in BTU ft-3

t = time in seconds.

- Adjusted the water flow rate through the calorimeter in such a way that the amount collected during four revolutions of the meter was within 50 ml of the numerical value of the expected CV (Btu/ft3). Water was poured in through one of the effluent

gas holes in the wooden top until it began to run off through the condensate outlet pipe

- The calorimeter was then allowed to settle down until the temperatures indicated by the inlet and outlet water thermometers become steady. Some observations were made

over the test period of four revolutions of the meter indicator.- When the meter hand was at 3 0’clock the inlet thermometer was read. When the

meter hand got to 12 0’clock the chance over funnel is moved so as to direct the outlet water into the container.- When the meter hand was at 3 0’clock again, the first reading of the outlet

thermometer was made and this thermometer was also read at every succeeding quarter turn until fourteen readings was taken. The meter hand then was then kept at 6

0’clock position.- The further reading of this thermometer was made when the meter hand was between 12 0’clock and 3 0’clock giving four inlet thermometer readings.

- The funnel was moved so as to direct the outlet water to waste at the instant when the meter hand got to 12 0’ clock.

- Took readings off the barometer, the meter temperature and the pressure of the gas at the inlet.

- Measured the water collected in the graduated cylinder and the readings recorded in

kilograms.

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Fig 1 Photograph of Laboratory set up of Boys non-recording calorimeter

EXPERIMENTAL RESULTS

Data Given

Barometric Pressure = 752mmHg

= 752 x 1.01325

760

Air Pressure = 1.0026bar = 1002.6mbar

Gas pressure = 20.3 mbar

Air Temp = 23 + 273 = 296K

Gas Temp = 23 + 273 = 296K

Prod. Temp = 20.4 + 273 = 293.4K

Water collected = 978ml = 0.978kgs

Total pressure = 1002.6 + 20.3 = 1022.9

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Table of Experimental Values

Water Temperature (0C)

No. of Meter Revolutions Outlet Inlet

1 34.90 15.40

34.90

34.90

34.90

2 34.90 15.40

34.90

34.90

34.90

3 34.90 15.40

34.90

34.90

34.90

4 34.90 15.39

34.90

34.90

34.90

Ave. T = 34.90 Ave. T = 15.400C

Calculations

CV = T x W x S x F

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V

Where

T = Rise of temperature of water (0C)

W = Weight of water collected (kg)

S = Specific heat of water (1)

F = Conversion factor to convert CV into Btu/ft3 or MJ/m3

V = Volume of gas burnt (ft3) during the test (corrected to MSC)

So, converting CV in Btu/ft3, the conversion factor F has the value,

F = 3.968 + 0.009 = 49.72

0.08

Therefore

CV = 49.72 x W x T

= 49.72 x 0.978 x (34.90 -15.40)

= 948.21 Btu/ft3

Conversion of C.V in MJ/m3, the conversion factor F has the value

F = 4.1965 x 10−3 = 1.89

0.00222

CV = 1.89 x 0.978 x (34.90 -15.40)

= 36.04 MJ/m3

Summary of Results

Btu/ft3 MJ/m3

Calorific Value 948.21 36.04

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DISCUSSION

The calorific value of a gas is an important distinguishing factor of gases and requires a very

stable environment for its measurement, thus determining it should be done with utmost accuracy. It is used to determine billings of the gas sold to customers. In this experiment we recorded 36.04 MJ/m3 as against the theoretical accepted standard of 37.97 MJ/m3. Also, the error in the experiment is minima and almost negligible because the value deviated from British specification with an error of 2.6% which can be attributed to be error due to

measurement.This deviation from theoretical standard may be as a result of

- The presence of other gases (i.e not completely 100% methane)- Parallax error while taking readings off measuring instruments- Heat lost due to radiation

- Laboratory condition

CONCLUSION

In this experiment the calorific value was found to be 36.04 MJ/m3. This shows some deviation from theoretical standard as a result of possible sources of error list above. Being

the potential heat energy possessed by a gas, accuracy in determining it is very important in order to assign heat value and consequently economic value to the gas. Also this experiment

has shown that the Boys calorimeter is the preferred instrument for measuring calorific value because the error obtained was minima and even negligible.

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