En Wobbe Index Control System in Gas Industry

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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. As there is a linear ratio between the percentage of residual oxygen and the Wobbe Index and this ratio is set in the instrument when it is calibrated, the value of the Wobbe Index is automatically determined by measuring the percentage of oxygen. The main technical specifications of the Wobbe Index meter used are: ● range of measurement 40+55 MJ/Sm3 ● precision + 0.4% Of the measured value ● repeatability t ().1“A of the measured vaiue ● response time c 8 sees (90Y0 of the measured Vi3k.E)

A pump is inserted between the gas intake point and the Wobbe Index meter in order to maintain the pressure at the entrance to the measuring device at 3 bar. Figure 1 shows the graph for the Wobbe Index during switching between two gases of different composition. As maybe seen, for all three curves switching was completed in under 15 minutes.

54- ,

52-

‘~ 5’ 350- — ~ j ~ 49

48 \ 47 , 46 I o 2 4 6 6 10 12 14 time[m”n] Fig. 1- Wobbe Index during switching between two gases of different composition.

This information is used to set the regulation parameters correctly during tests.

2.3. Measurement of the Calorific Value

The calorific value of a gas represents the quantity of heat developed during complete combustion, at constant pressure, of a cubic metre of dry gas, when the products of combustion are brought back to their initial temperature. Depending on whether the heat from condensation of water vapour caused by the combustion is included it is possible to distinguish between gross calorific value (GCV) and net calorific value (NW).

Regulation by the control and regulation system occurs through measurement of the gross calorific value. For the measurement two different instruments are used: a gas chromatography and a gross calorific value analyser (a Wobbe Index meter with included a densimeter). As well as working in different ways the two instruments are also quite different as regards regulation, because their response times are around 15 minutes for the gas chromatography and under 8 seconds for the gross calorific value analyser.

The gas chromatography measures the concentration of the components of the natural gas and from these it calculates the main characteristics, including the calorific value and the Wobbe Index. The way the gas chromatography works is briefly described below. The analyser begins the measurement process by taking a sample of gas of a preset volume. This is led, using a transporter gas (helium in this case) to the separation columns. In the columns the phenomenon of elution (the opposite of dilution) takes place. This leads to the separation of the gas .

into its components: as the molecules move along the columns at different speeds due to their different molecular weights they come to the end of the columns at different times. By measuring the thermal conductivity the instrument can identify the component and its concentration.

The gross calorific value analyser calculates the calorific value of the natural gas by means of the ratio:

~1 = GCV &

where:

WI = Wobbe Index GCV = Gross Calorific Value d, = relative density of the gas compared with air

measuring the Wobbe Index and at the same time measuring the density of the gas using a density sensor.

A pump is inserted between the gas intake point and the meter in order to maintain the pressure at the-entrance to the measuring device at 3 bar.

2.4. Circuit for Mixing Natural Gas with Air

In order to mix the natural gas with air there is a line (shown in Figure 2) which comprises:

● an adsorption drier,, complete with a pre-filter oil remover and a post-filter particIe remover, for treatment of the mixing air; s a flow meter to measure air flow rate; ● a variable section regulating valve; ● a mixing air line feed into the natural gas line.

2.5. Circuit for Mixing Natural Gas with Air and LPG

There is a mixing circuit including a mixing chamber, the gas input lines and the mixture exit line, to enable the natural gas to be mixed with air or with LPG. The mixing chamber uses the kinetic energy generated by the acceleration of the natural gas, which happens as a result of the particular internal shape of the entrance area; this area creates a Venturi-type vacuum inside the chamber which eases the intake of the gas to be mixed. Homogeneous dispersion of the gases in the main chamber is also facilitated, both by use of gas with a pressure slightly above that of natural gas and by the arrangement and type of mixing nozzles, which are positioned both in counterflow and in the direction of flow and are at different angles. The resulting mixture is compressed and sent to a different section of the chamber, which ensures that the gases are completely mixed. The mixer is made of steel and is equipped with safety valve and drainage valve to eliminate any traces of impurities. . .

FUELGASSUPPLY to burner

AIR dSUPPLY

control from regulator

Fig. 2- Mixing air line.

The gas input lines are fitted with non-return valves and temperature and pressure gauges. Each line is also equipped with an equal percentage control valve which is normally closed, complete with pneumatic servocontrol and electropneumatic positioner (signal 4-+20 mA). The mixture exit line is also fitted with temperature and pressure gauges.

2.6. Regulation Equipment

The equipment used performs several functions: it calculates the correct gas flow rate, it compensates for variations in the quality of the natural gas and, if required, it also regulates a characteristic process variable and/or a second variable correlated with it. The system is designed particularly to: ● calculate the flow rate using the entrances of differential pressure, relative pressure and line temperature; ● manage the automatic regulation of the process related to the five settings; ● manage the interface with the operator by means of keyboard and display on the machine, allowing the equipment to be programmed, the variables concerned to be seen, operational status, local set points and regulator parameters to be altered; ● manage the interface with the operator by means of a PC connected to the system, by means of which graphical screens and control panels allow regulation parameters, set points” and/or exit values to be altered; ● manage the settings of the equipment (entrances, regulation exit, feed, regulation card etc.).

3. TESTS AND TEST RESULTS

3.1. Test 1

3.1.1. Test 1 Setting

The test involved use of a test furnace fuelled by natural gas of different compositions, in conditions of constant thermal input to the burner and with natural gas flow rate regulated in accordance with variations in the Wobbe Index (See Fig. 3). .,

,.-.---

FUEL GAS SUPPLY 4 to burner >

Wb&. WI analyser ‘x , Q“ ‘------” ‘lyuY;’ v- , / SET -. .-...... ‘ATHERMAL INPUT regulator

Fig. 3- Test 1: setting at constant thermal input.

3.1.2. Test 1 Description

During the first stage the furnace was fuelled with natural gas from the network until it reached a steady state, “represented by constancy over time of all furnace control parameters. In the second stage the natural gas from the network was replaced with natural gas with a low Wobbe Index, supplied by CNG tanker. The behaviour of the furnace was then analysed, both during the change from network gas to low Wobbe Index gas and at steady state. The feed was then changed, from natural gas with a low Wobbe Index to a gas with a high Wobbe Index, also supplied by CNG tanker. In the same way as before, the behaviour of the furnace at changeover and its behaviour at steady state were noted. At this point the control and regulation system was activated and the input gas to the burner was repeatedly varied. The behaviour of the furnace and the control and regulation system were analysed both during the changeovers and at steady state. The aim was to check the control and regulation system capacity to keep the heat input to the burner constant despite variations in the Wobbe Index. In order to keep the set heat input level the regulation system had to vary the gas flow rate according to the variations in the Wobbe Index. In order to evaluate the efficiency of the regulation system the variation in reference temperature of the furnace was analysed during the tests.

3.1.3. Test 1 conclusions

In the different tests carried out the system was capable of keeping the heat input constant for the different test gases within a band of *2O MJ/h (less than 0.5’% of the desired set value). The regulation time varied from 10 to 15 minutes and the furnace reference temperature varied by a maximum of 5 “C. Fig. 4 shows the graphs for heat input and the Wobbe Index during regulation, in the case of switching from a gas with a high Wobbe Index to one with a low Wobbe Index.

— thermal input Wobbe Index [MJlh] [MJ/Sm3]

7350 I ~ 53 ●☛

7250 ...... 7200 band

7150- -47 7100- thermal input set point = 7200 M.I/h 7050 0 1 2 3 4 5 6 7 8 9 10 11

time [rein] Fig. 4- Test 1: regulation results of the setting at constant thermal input.

3.2. Test 2

3.2.1. Test 2 Setting

The test involved use of a test furnace fuelled by natural gas of different compositions, in conditions of variable thermal input to the burner and with natural gas-air flow rate regulated in accordance with variations in the Wobbe Index, maintaining a constant furnace temperature and a constant stack oxygen level (See Fig. 5).

, -----, ----- . . --...---, ,, II analyser ,,

* : temperature FUEL GAS SUPPLY : ------,

‘-o :iii;iiiz+++ controller I ov- FURNACE

COMBUSTION AIR SUPPLY

Fig. 5- Test 2: setting at variable thermal input and variable Wobbe Index. .

3.2.2. Test 2 Description

During the first stage the furnace was fuelled with natural gas from the network until it reached a steady state, represented by constancy over time of all furnace control parameters. During the second stage the natural gas from the network was replaced with natural gas with a low Wobbe Index and then, again with the same gas, water-cooled pipes were introduced into the furnace, allowing the heat input to be altered suddenly. The behaviour of the furnace was analysed, both during the changes and at steady state, for the two heat input situations. The same was done for the gas with the high Wobbe Index. At this point the control and regulation system was activated and both the feed gas and the heat input were repeatedly varied. The behaviour of the furnace and the regulation and control system were analysed, both during the transitions and at steady state. The aim of the test was to develop a control and regulation system which would be capable of regulating process variables such as the internal furnace temperature and the stack oxygen level despite variations in the quality of the natural fuel gas and the heat input, leaving the combustion ratio unchanged. Because of the difficulties experienced setting up the regulation, which lengthened the time taken for the test, and no longer having available the natural gas supplied by CNG tanker, the test was completed using only network gas. The gas variation was, therefore, simulated by setting different regulation values for the temperature and the oxygen. There is a correspondence between the variation in the quality of natural gas and the variation of the percentage of stack oxygen and the internal furnace temperature. Therefore, if the set points for these two parameters are changed, using the same gas, the control and regulation system must respond in a manner similar, even if not quite identical, to an actual change in natural gas. For the heat input variation the water-cooled pipes were, however, introduced or withdrawn.

3.2.3. Test 2 Conclusions

In the different tests performed the system was capable of maintaining the furnace reference temperature constant for the different test gases, within a band of +1 O “C and the stack oxygen level within a ~0.5’Yo band, with an air/( air+fueI) ratio between ~0.35°Y& Regulation time varied from 20 to 40 minutes. Fig. 6 shows furnace temperature, stack oxygen, combustion air flow rate and gas flow rate during regulation for one of the tests performed.

3.3. Test 3

3.3.1. Test 3 Setting

The test involved use of a test furnace fuelled by natural gas of different compositions mixed with air, in such percentages as to keep the reference Wobbe Index constant (equivalent to a value lower than the minimum Wobbe Index for the gases used in the test) (See Fig. 7).

3.3.2. Test 3Description

During the first stage the furnace was fuelled with natural gas from the network until it reached a steady state, represented by constancy over time of all furnace control parameters. In the second stage the control and regulation system and gas/air mixing were activated, fuelling the furnace alternately with gas with a low Wobbe Index, gas with a high Wobbe index and gas from the pipeline network. In the tests performed the percentage of air in the mixture was varied, according to the set point decided for the Wobbe Index and to the fuel gas supplied to the furnace, from a minimum value of 2,3% to a maximum of g.syo by volume. .-

furnace temperature stack oxygen PC] po]

1230 1 i 16

-14 1220

-12 1210 -10 1200 band f -8 1190 ...... 6 ...u.--m\ 1180 ..dead ...... ~~~Vwe~’~------...... 4 w -- ba~~.~~>!’r~ ...... ~...... -#. -. 1170 -2 J%J”J 1160 0 3 6 9 12 15 18 21 24 27 30 33 36 39 42

time [rein]

stack oxygen set point = 3.2 Y. - - stack oxygen — f urnace temperature furnace terrperature set point= 1200 “C ‘“

combustion air flow rate natural gas flow rate [Sm3/h] [Sm3/h] 1800 190 I

w.- + -- \ 1500- - 150 1400- - 140 1300- - 130

1200- 120

1100 J- 110

1000 ~ —H—t++J 100 O 3 6 9 12 15 18 21 24 27 30 33 36 39 42 time [rein]

Fig. 6- Test 2: regulation results of the setting at variable thermal input and variable Wobbe Index.

During the tests the alterations in the Wobbe index, the behaviour of the furnace and the control and regulation system were all measured and recorded, both during the changeovers and when operating at steady state. The aim was to check that the control and regulation system could compensate for the variations in natural gas quality, maintaining a pre-established constant Wobbe Index value. .-

FUEL GAS SUPPLY to burner > A

y.llc:l

analyser I - I regulator I

~------‘------1 I

flow control meter valve AIR SUPPLY

Fig. 7- Test 3: setting at constant Wobbe Index.

3.3.3. Test 3 Conclusions

In the various tests performed the system was able to maintain a constant Wobbe Index for the different test gases, within a limit of +0.375 MJ/Sm3. Regulation time varied between 15 and 20 minutes. Figure 8 shows the trend for the Wobbe Index and the mixer air during regulation, in the case of switching from a gas with a low Wobbe Index to a gas with a high Wobbe Index.

Wobbe Index mixing air flow rate [MJ/Sm3] [Sm3/h]

47 16 I I II — Wobbe Index - - - - tixing air f low rate I I I I 14 ------. t ,------”” 46 12

45 8

dead 44 4 band ...... t i2 \ WobbeIndexset point= 44 fvUISrF I

0 3 6 9 12 15 18 21 time [rein] Fig. 8- Test 3: regulation results of the setting at constant Wobbe Index. .-

3.4. Test 4

3.4.1. Test 4 Setting

The test involved use of a test furnace fuelled by pipeline natural gas in conditions of constant heat input to the burner and mixture of natural gas with air in such a percentage as to keep the reference Wobbe Index constant (equivalent to a value lower than the minimum Wobbe Index for the pipeline gas) (See Fig. 9).

------...... , FUEL GAS SUPPLY ~ FUEL GAS+ AIR SUPPLY to burner

~:.~ [ W#& W[ analyser ‘x (manual ‘------” input) ,,10 ,, Q ,, ‘:”E=H regulator ~ Q flow AIR SUPPLY Q meter .7 ‘W:’

Fig. 9- Test 4: setting at constant Wobbe Index and constant thermal input.

3.4.2. Test 4 Description

During the first stage the furnace was fuelled with natural gas from the network until it reached a steady state, represented by constancy over time of all furnace control parameters. In the second stage the control and regulation system was activated and the gas was mixed with air. As only pipeline gas was available the test was conducted by simulating the switching between different qualities of natural gas by setting the regulator to different set values of Wobbe Index. In order to maintain the Wobbe Index set point and the heat input set point the system had to regulate the mixing air flow rate and the mixed gas flow rate. In the tests performed the percentage of mixing air was varied, according to the set values used, frOm a minimUm Of 3.4% tO a Wtaxh-irn VdUe Of 7’.!Y2o by VOkJme. During the tests data for the Wobbe Index, the behaviour of the furnace and the control and regulation system were recorded and analysed, both for the changeovers and at steady state. The aim was to check that the control and regulation system could maintain constant heat input to the burner, maintaining a pre-established constant Wobbe Index value.

3.4.3. Test 4 Conclusions

In the different tests carried out the system was capable of maintaining the Wobbe Index within a band of ~0.3 MJ/Sms and heat input between f50 MJ/h (less than 17. of the desidered set value). Regulation time varied from 10 to 15 minutes and the furnace reference temperature varied by a maximum of 5 ‘C. Figure 10 shows the trends for heat input, Wobbe Index, mixing air flow rate and mixed gas flow rate during regulation in one of the tests carried out. “t-

hermal input Wobbe Index [MJ/h] [MJ/Sm3]

6700- - ...... iwtir --- =-s -0- .,---- .-*-*-.-,-.% ...*-*. . . . :.W! dead ●* 6600- ● 46 . . . . . band...... 6500- - . n● 8~ 6400 ● 45

6300 ✎✎✎✎✎✎✎✎✎✎✎✎ ..&a& ......

%- 6200 -.-. -.-tia.>*~*------44 6100

6000 43 thermal input set point = 6250 M.I/h . - . - ~obbe ,ndex 5900 — thermal input Wobbe Index set point= 46 hAJ/SrIP 5800 42 0 2 4 6 8 10 12 14 16

time [rein]

mixed gas flow rate mixing air flow rate [Sm3/h] [Sm3/h] .-. 1w 12 *. ------145 10

140 8

“v’=%t 135 ) .--- =_*-- ...... - ...... 6 { ,-**-. .e ------“ .

130 4

125 [ 2 II — nixed gas flow rate - - - - nixing ai[ flow rate [ I 120 1 ‘ 0 0 2 4 6 8 10 12 14 16 time [rein] Fig. 10- Test 4: regulation results of the setting at constant thermal input and constant Wobbe Index.

3.5. Test 5

3.5.1. Test 5 Setting

The test involved use of a test furnace fuelled by natural gas of different compositions, with the natural gas being mixed with air when the gross calorific value of the gas was higher than the pre- established upper limit or with LPG when this fell below the pre-established lower limit (See Fig. 11). 4!r-

FUEL GAS SUPPLY to burner *

Gross ~~” C;:;c

analyser , , ...... J regulator ‘ ------

/ SET’ Gcv > SET2 c1 ,, ,, ,, ,, II !------. ------. ,

u~AIR SUPPLY LPG SUPPLY Fig. 11- Test 5: setting at prefixed Gross Calorific Value.

3.5.2. Test 5 Description

During the first stage the furnace was fuelled with natural gas from the network until it reached a steady state, represented by constancy over time of all furnace control parameters. In the second stage the control and regulation system was activated and the gas was mixed with air or with LPG, with the furnace being fired with gas of different composition, supplied from CNG tankers. Data for the Wobbe Index, the behaviour of the furnace and the control and regulation system were then recorded and analysed, both for the changeovers and at steady state. The aim was to show that the control and regulation system was capable of maintaining the gross calorific value within a preset range, despite variation in the composition of the fuel gas. In order to make the natural gas mix more homogeneously with the air or the LPG and make the measurement of the gross calorific value more reliable a suitable mixer was installed on the gas line.

3.5.3. Test 5 Conclusions

In the various tests effected the system was capable of maintaining the gross calorific value of the gas within the preset range. Average regulation times were under 60 minutes for regulation carried out by measurement with the gas chromatography. Regulation times were, on the other hand, much shorter (around 15 minutes) when regulation was carried out by measurement with the gross calorific value analyser (Wobbe Index meter with densimeter). This difference is linked to the different ways these instruments work and, therefore, to their different response times (15 minutes for the gas chromatography and under 8 seconds for the gross calorific value analyser). For the gas chromatography almost two measurement cycles have to be completed before regulation can be said to be finished.

4. FIELD TESTS

The control and regulation system tested was installed in two Italian glass factories. The first installation was in a melting furnace for producing glass for the pharmaceutical industry. In this case the Wobbe Index was controlled by mixing the natural gas with air. *..-

The system manages to keep the preset Wobbe Index constant within a +0.3 MJ/Sm3 band, with a regulation time of under 10 minutes. Variations in the operating temperature of the furnace are limited to around 3-4 ‘C, without the system these variations are over 10 ‘C. Furthermore, the quality of the finished product is unchanged. The second installation was carried out on a float furnace for production of flat glass. In this case the gross calorific value of the fuel gas was controlled by mixing it with air or with LPG, using a gas chromatography to measure the calorific value. The system is capable of compensating for variations in the gross calorific value ranging from 38 to 41 MJ/Sm’, bringing them within a range of variability determined by presetting a lower set point and an upper set point, with a dead band of ~0.2 MJ/Sm’ variation. Average regulation times are around 30+45 minutes.

5. CONCLUSIONS

The control and regulation system tested gave results which were more than satisfactory for all five test settings. The choice of setting best adapted to the user’s needs must be made on an individual basis, depending on the process which the system is to be used for and the parameters which the user decides to keep under control. In general it maybe said, however, that setting 1 is more suitable for single uses with constant heat input, setling 2 for single uses with processes where heat input is variable, setting 3 in cases where a number of uses require constant gas quality, setting 4 for a number of uses where constant gas quality and constant heat input are required. Setting 5 is suitable for use where the most important factor is to keep the calorific value of the natural gas within a certain range, since fluctuations in calorific value which fall outside the optimum value for the process may cause system management and capacity problems, as well as problems with the quality of the finished product. Settings 3 and 5 have been successfully used in two Italian glass factories.