High precision calorimetry to determine the enthalpy of combustion of methane

doi:10.1016/S0040-6031(01)00735-3

Thermochimica Acta

Volume 382, Issues 1–2, 31 January 2002, Pages 47-54

https://doi.org/10.1016/S0040-6031(01)00735-3 Get rights and content

Abstract

The enthalpy of combustion of methane is the most important property used in the determination of the calorific value of natural gas. Only two sets of values with high accuracy and precision and measured under appropriate conditions have been published since it was first determined in 1848. These studies were done by Rossini, at the National Bureau of Standards in the USA in 1931, and Pittam and Pilcher, at the University of Manchester in 1972.

This report details the design and operation of a high precision constant-pressure gas burning calorimeter, based on the design of those used in the previous studies, to measure the superior enthalpy of combustion of ultra-high purity methane at 25 °C.

The use of modern equipment and automatic data collection leads to a value, traceable to national standards, of 890.61 kJ mol⁻¹ with a combined standard uncertainty of 0.21 kJ mol⁻¹. This is in full accord with the value of 890.63 kJ mol⁻¹ calculated from the average of Rossini’s and Pittam and Pilcher’s work (with a random uncertainty based on 1 S.D. of 0.53 kJ mol⁻¹).

Introduction

The “reference calorimeter” at the Technical Directorate of the Office of Gas and Electricity Markets (OFGEM) was designed to be the primary standard for determining the heat of combustion of natural gas samples. The instrument is based on the one used by Pittam and Pilcher, at the University of Manchester in the late 1960s, to study the heat of combustion of methane and other hydrocarbons [1], [2]. This instrument was, in turn, based on the one built by Rossini, at the National Bureau of Standards in the USA in the early 1930s, to study the heat of formation of water [3] and the heats of combustion of methane and carbon monoxide [4].

There have been three major changes from the designs of the previous workers: (1) we directly weigh the sample of gas burnt; (2) we control the experiment and collect data automatically by computer and (3) we take measurements at a faster rate.

The experiment produces a superior heat of combustion in kJ g⁻¹, at a constant pressure, for combustion at 25 °C. For a single component gas, such as methane, the result can be given in kJ mol⁻¹.

The experimental procedure has been analysed to calculate systematic and random uncertainties according to the method described in the latest ISO guide on uncertainty determinations [5]. The method is based on weighing the uncertainties of each individual term in the calculation. The weights are derived from the partial derivative of the equations used in the calculations with respect to each term. The heat of combustion for several sets of determinations have been compared with the uncertainty analysis and the values agree.

The reference calorimeter was constructed by Lythall. The author, working under the direction of Aucott and Sayer, took over the operation of the instrument in February 1993, making minor improvements and calculating the uncertainties. This paper describes the reference calorimeter; shows how it is different to previous calorimeters; gives the uncertainty on results; and gives the heat of combustion of methane.

Section snippets

Calorimeter theory

The objective of calorimetry is to measure the quantity of energy involved in a particular reaction. In the case of the reference calorimeter, the reaction is the complete combustion of a hydrocarbon fuel gas. This is achieved by allowing the energy liberated in the reaction to be given to a well-stirred liquid, in a calorimeter, and measuring its temperature rise. Multiplying this temperature rise by the energy equivalent of the calorimeter gives the amount of energy liberated in the reaction.

Basic structure

The reference calorimeter is shown in Fig. 2. It consists of two nested cans with an air gap between them. The inner can is filled with distilled water and contains a glass reaction vessel with heat exchanger, a calibration heater, a constant speed stirrer and a platinum resistance thermometer (Tinsley). A recess is included for the insertion of a cold finger to bring the calorimeter to its starting temperature. The cold finger is removed and the recess plugged when the calorimeter is in use.

Differences with previous experimenters

There are several differences between the reference calorimeter and those used by Rossini and Pittam and Pilcher. These differences are given in Table 2.

Heat of combustion of methane

Two sets of determinations of the heat of combustion of methane have been carried out using the reference calorimeter (Lythall and Dale). These results along with those of Rossini and Pittam and Pilcher are given in Table 3. Rossini’s results have been reworked by Armstrong and Jobe to bring them into line with modern values of energy, temperature and molar mass [6].

Rossini ignored his first result to give a mean of 890.31 kJ mol⁻¹. The 1983 edition of ISO6976 [7] gives a value of 890.36 kJ mol⁻¹.

Uncertainty analysis

The method given in the ISO publication “Guide to the Expression of Uncertainty in Measurement” was used to evaluate the uncertainties present in the reference calorimeter. The method analyses the equations relating the input quantities, x_i, to the output quantity, f. The input quantities fall into three categories:

1.
physical constants taken from reference books;
2.
values from calibration certificates and
3.
measurements taken during the experiment.

Each input quantity is analysed to calculate its

Conclusion

The reference calorimeter is an accurate and precise instrument for measuring the superior heat of combustion of methane at 25 °C. The reference calorimeter gives a value of 890.61 kJ mol⁻¹ with an uncertainty of 0.21 kJ mol⁻¹. This compares well with the value of 890.63 kJ mol⁻¹ (with a random uncertainty based on 1 S.D. of 0.53 kJ mol⁻¹) in ISO6976: 1995.

References (8)

D.A. Pittam et al.
J. Chem. Soc., Faraday Trans. I
(1972)
D.A. Pittam, The measurement of heats of combustion by flame calorimetry, M.Sc. Thesis, University of Manchester,...
F.D. Rossini
The heat of formation of water
J. Res. Natl. Bureau Standards
(1931)
F.D. Rossini
The heats of combustion of methane and carbon monoxide
J. Res. Natl. Bureau Standards
(1931)

There are more references available in the full text version of this article.

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