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En Wobbe Index Control System in Gas Industry ~-4$ EN ! FROI05409 . ..+.. .“+_____________ WOBBE INDEX CONTROL SYSTEM IN GAS INDUSTRY PROCESSES SYSTEME DE CONTROLE DE L’INDEX DE WOBBE DU GAZ NATUREL DANS LES PROCESSUS INDUSTRIALS M. Cassibbaand M. Bertani SNAM, ltaly ABSTRACT Natural gassupplied toindustry forprocess utilisations originates from different sources and that can cause fluctuations in gas composition. Changing gas composition may lead to production problems in industry with sensitive thermal processes (particularly glass industry and thermal metal treatments), such as efficiency and product quality. An equipment suitable to control and adjust such variations has been developed. Experimental tests in laboratory were carried out in order to investigate the control system accuracy and reliability. In particular five different settings were tested: at a preset thermal input by adjusting the natural gas flow rate in respect to Wobbe Index variations; at a set furnace temperature and stack oxygen level with variable thermal input by monitoring the Wobbe Index value; at constant Wobbe Index value by adding air to natural gas; at constant thermal input and prefixed Wobbe Index value by adding air to natural gas and varying the air and gas mixture flow rate; gross calorific value control by adding air or LPG to natural gas. All the tested settings gave good results. This report illustrates these results and the main features of the control system. The control and regulation system was installed in two glass factories for field tests. RESUME Le gaz naturel utilise clans [es processus industrials provient de gisements a caracteristiques differences qui peuvent dormer lieu a des fluctuations clans la composition du gaz. Ces variations peuvent entralner des problemes clans certaines processus industrials tels que, en particulier, ceux du verre et du traitement des metaux. Pour resoudre ces problemes on a developpe un systeme de contr61e en mesure de regler Ies variations de la qualite du gaz et des essais experimentaux ont ete effectues en Iaboratoire pour en determiner la precision et la fiabilite. En particulier, on a experiment cinq configurations differences de reglage : maintien d’une valeur constante du debit thermique par Ie reglage du debit du gaz, en fonction de la variation de I’index de Wobbe ; maintien de valeurs prefixees de la temperature du four et de la concentration d’oxygene clans Ies fumees en fonction de la variation du debit thermique et de I’index de Wobbe ; maintien d’une valeur basse et constante de I’index de Wobbe par I’addition d’air au debit du gaz ; maintien d’une valeur constante du debit thermique, a une valeur de I’index de Wobbe predetermine, par I’addition d’air au debit du gaz et simultanement reglage du debit du melange air/gaz ; maintien d’une valeur constante du pouvoir calorifique par I’addition d’air ou de gaz propane Iiquide au debit du gaz. Toutes Ies configurations testees ont donne d’excellents resultats. Ce memoire illustre Ies resultats de I’experimentation et Ies caracteristiques principals du systeme de contr61e. Le systeme de contr61e et reglage a ete installe clans deux industries du verre pour essai. DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best avaiiable original document. I 1. INTRODUCTION The natural gas used in industrial processes comes from different sources, which may mean that there are variations in its composition. This is not important where most uses of gas are concerned or is, in any case, compensated for by regulation of the gas/air ratio. However, certain combustion processes (such as melting glass, heat treatment of metals) are very sensitive to variations in the quality of natural gas, since such variations can cause problems in process management and affect the quality of the final product. Therefore the problem of the interchangeability of natural gas is of considerable importance in industry. Two gases are perfectly interchangeable if, at the moment when they are used, they behave in a completely identical manner. Experience has shown that one gas can be considered as interchangeable with another when proper functioning of appliances and systems is assured and high product quality is also maintained. In terms of thermal power one parameter which illustrates interchangeability very well is the Wobbe Index. The experiment aimed to develop and validate an automatic processing, control and regulation system in order to make it available on the Italian market. The system would be capable of compensating in real time for variations in natural gas supplied to industrial furnaces and/or combustion processes in general by means of continuous monitoring of the Wobbe Index. A setting which provides for control of gross calorific value was also developed and validated. The following settings were tested: 1. constant thermal input to the burner by regulation of the flow rate of natural gas according to changes in the Wobbe Index; 2. variable thermal input to the burner and regulation of the natural gas-air flow rates according to changes in the W obbe Index, with furnace temperature and stack oxygen level both constant 3. Wobbe Index kept constant by mixing natural gas with air, 4. constant thermal input to the burner and Wobbe Index kept constant by mixing the natural gas with air and regulating the flow rate of the gas mixture; 5. control of gross calorific value by mixing the natural gas with air or with LPG. For each of these settings the effectiveness of the control and regulation system was tested, using natural gas samples with different chemical compositions, different Wobbe Index and different gross calorific value. During all tests all parameters required for development of the control and regulation system were measured and recorded. 2. TESTS 2.1. Test Equipment All tests were carried out at the Centro Sviluppo Materiali (CSM) in Genoa Cornigliano. Tests on the first four settings used a furnace lined with refractory bricks, with a fanned draught flue, a combustion chamber measuring 3 x 3 x 6 m, recovery of combustion air from exhaust gas for balanced draught and maximum installable capacity of around 2.5 MW. The tests were conducted using: . a 1.5+2.0 MW industrial burner, used in steelmaking in furnaces for heating thick slabs and billets; ● a control panel for obtaining and recording all system function parameters; . an analyser for measuring the Wobbe index of the natural gas; ● a circuit for mixing the natural gas with air; ● a control and regulation system specially developed in cooperation with s.d.i .-automazione industrial, a Milanese industrial automation company. The tests employed natural gas from the network but also made use of two other test gases, one with a low Wobbe Index and the other with a high one, each having a different chemical composition. These were supplied by CNG tanker. The air used for mixing with natural gas was purified and dried to remove any oil and/or moisture using a suitable filter system. The air pipe inlet into the natural gas pipe was made at a sufficient distance from the Wobbe Index measurement device to ensure that mixing was complete. During the tests all parameters needed for development of the control and regulation system were measured and recorded (furnace temperature, natural gas flow rate, combustion air flow rate, mixer air flow rate, stack oxygen level, Wobbe Index). The fifth setting was tested using a furnace for testing burners up to 500 kW, with walls of refractory brick, a controlled draught flue, a combustion chamber measuring 1 x 1.2 x 2.2 m and cooling pipes to reduce the heat input. The tests for this setting were conducted using: ● an industrial burner of the type used in steelmaking of around 500 kW; ● a control panel for obtaining and recording all system function parameters; . an analyser for measuring the gross calorific value of the natural gas; ● a gas cromatograph; . a control and regulation system specially developed in cooperation with s.d.i.-automazione industrial, a Milanese industrial automation company, ● a circuit for mixing the natural gas with air or LPG as required. The tests employed natural gas from the network but also made use of two other test gases, one with a low gross calorific value and the other with a high one, each having a different chemical composition. These were supplied by CNG tanker. The air used for mixing with the natural gas, from the CSM internal compressed air line, was purified and dried to remove any oil and/or moisture using a suitable filter system. The LPG used was supplied in bottles. This was commercial propane - a mixture of 88% propane with 127. butane. 2.2. Measurement of the Wobbe Index The heat output of a burner is proportional to the calorific value of the gas per unit of volume and to the gas flow rate to the burner, which is inversely proportional to the square root of its density. The Wobbe Index (higher) is defined by the following ratio: WI.? r where: WI = Wobbe Index GCV = gross calorific value dr = relative density of the gas compared with air Therefore the Wobbe Index is expressed by the same unit of measurement of the calorific value that is in MJ per unit of volume. This means that two gases which have the same Wobbe Index give the same heat output to the burner, for the same supply pressure. A Wobbe Index meter was used to measure continuously the Wobbe Index. The principle on which the meter works is briefly described below. Inside the instrument a fixed quantity of gas, taken in continually, is burnt under prefixed conditions of air excess. A zircon dioxide cell measures the percentage of residual oxygen in the exhaust gas from combustion.
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