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Show ponse, environmental sensitivity and poor accuracy (Wobbemeters intended for industrial use are closely related to the simplified calorimeters). The Wobbemeter nevertheless offers a significant advantage in that it provides, by the measurement of a single characteristic -- the Wobbe number or ratio of the gas's gross calorific value to the square root of its relative density (gcv/Id) -information taking into account both the relative density and the calorific value, which is easy to exploit. (By first approximation, the heat output of a burner supplied at a constant pressure is directly proportional to the value of the gas's Wobbe number). The chromatograph, finally, might appear to be the ideal sensor, since it provides very comprehensi ve information -- the chromatogram, on the basis of which it is possible to automatically and precisely compute the composition and main characteristics of the sampled gas. However, the analysis takes a very long time (so that the data is not continuous but only point-related), maintenance is difficult and the price very high in the range of industrial-purpose instruments. The Wobbe meter being apparently the only practical instrument for industrial use, it was soon evident that in light of the constraints of accurary and response time required today, it would be necessary either to improve its performance or to conceive a new type of sensor and transducer superceding the Wobbemeter in usefulness. It was the latter alternative which was pursued in view of the unsurmountable disadvantages tied to the Wobbemeter' s design fundamentals. This research effort led to the development of a new instrument now marketed under the name "COMBURIMETRE". THEORY OF OPERATION BASIC DIAGRAM - The name "comburimetre", like the term "combustion" is derived from the Latin "comburere" -- to burn up completely -- of which two syllables are carried into the French in such words as "comburant" for "combustion agent". In fact, the device combusts the sample gas with air, the two fluids being mixed continuously under special conditions (Figure 4). Combustion is carried out with excess air and completely. The percentage of residual oxygen in the combustion products is then measured by a zirconium dioxide cell. The measured oxygen content is a function of the air factor in the air-gas mixture having been made, which is itself inversely proportional to the stoichiometric air requirement of the gas. Since the air input is constant and the gas input is inversely proportional to the square root of the gas relative density (as in a real burner), it follows that this oxygen content is directly related to the ratio between the said combustion air requirement of the gas and the square root of its relative density, an index characteristic of the gas and related to the stoichiometric air requirement just as the Wobbe number is related to the calorific value. 207 __. ..L._ c_on stant Q AIR MIXER COMBUSTION constant Q GAS x Vd Fig. 4 : COMBURIMETRE DESIGN PRINCIPLE These two indices are therefore to a certain extent comparable, except that the former is a dimensionless number while the latter is expressed in terms of the unit of calorific value. The new index is called the "combustion air requirement index" and is denoted by the letter "B", by reference to G. Barbier, who first proposed its definition in a paper presented to the French gas technology association (ATG) congress of 1977. [1] . Va B OPERATING FORMULA FOR THE COMBURIMETRE USED TO ANALYZE NATURAL GAS - The issue is to determine the relation obtaining between the oxygen content of the combustion products of the gas sample and the characteristics of the sampled gas. The combustion of 1 cubic meter (n) of gas with a stoichiometric air requirement Va, with an excess air of R - Va, where R denotes the effective air/gas ratio by volume, yields a volume of flue gases equal to the humid flue gas volume (V'fo) augmented by the excess air (R - Va). The oxygen content of the combustion products is thus (1) 0.21 (R - Va) V'fo + (R - Va) This is a general formula. It applies regardless of the type of gas being analyzed. The value of R, which represents the ratio of the air input to the gas input, depends on the mixing device and its adjustement. For the whole set of natural gases, the following approximation can be made : V'fo = 1 + Va Moreover, the COMBURIMETRE' s air/gas mixer provides a constant air flowrate and a gas flowrate which varies as the reciprocal of the square root of its relative density. We can therefore write, based on Equation (1) : |
