OCR Text |
Show (2) 0.21 Eg J d: - va] l+RgJ d dg - Rg being the airlgas ratio determined initially with a standard gas of known relative density dg, - and Va and d being combustion air requirement and the relative density respectively of the sample gas. From equation (2) can be derived : Va Rg --=---- Id ffg (3) ~~~~ [~ + ~ J It may be assumed that natural gases, despite their diverse origins, differ little in relative density (relative densities between 0.5 and 0.7) and we can therefore set the term 1 0.21 [ -1- + --Rg- ] in equation (3) appro- Id ffg ximatively equal to a constant independent of d. Consequently, we obtain a linear relation between the stoichiometric combustion air re~uirement index Valid and the oxygen content L O2 ] measured by the instrument. This theoretical curve which expresses the relation between the combustion air requirement index Valid and the O2 content depends on the AIR. 1 bar 800 mbar standard gas (through dg) and the selected setting (through Rg), and the approximation of 1/1d. LABORATORY TESTS BAS IC COMPONENTS OF THE COMBURIMETRE - The COMBURIMETRE's final design configuration is diagrammed in Figure 5. Air and gas flowrates are precisely controlled by flow and pressure controllers. Only a small part of the mixture is used in the analysis, the remainder being diluted in air and evacuated. This considerably speeds the response time of the COMBURIMETRE. The oxygen analyzer is designed to analyze an air-gas premixture. The zirconium dioxide cell, used to analyze the oxygen, is located in a small chamber electrically heated to 812°C. The mixture to be analyzed enters the chamber and flows over a platinum-grid combustion catalyst to achieve a complete combustion. INSTRUMENT CALIBRATION - Adjustment of the air-gas ratio provided by the mixer must be done using a gas of known characteristics. We chose methane, as a pure and readily available gas, as the calibrating gas since it is the essential component of natural gases. The calibration of the combuimetre with 2% oxygen using pure methane is tied to the range of maximum variation of the combustion air requirement of the gases to be analyzed and the characteristics of the oxygen analyzer. On the one hand, the excess air must be sufficient for complete combustion of the highest 15 I/h (tuning control) Check valve GAS ,. 1 bar Filters Pressure control 800 mbar' 301/h ~ Combustion chamber .... , .• : Flowrate ..... control Zirconium dioxide cell / Fig. 5 COMBURIMETRE OPERATING PRINCIPLE (Measurement of Valva) 208 • |