Fuel oil ile çalışan endüstriyel tip skoç kazanların doğalgaza dönüşülmesinin bilgisayar simülasyonu

Abstract

Bu çalışmada, fuel oil ile çalışan endüstriyel tipte skoç buhar kazanlarının dogalgaza dönüştürülmesi incelenmiş tir. Sadece yakme ünetesini değiştirmek sureti ile kazanın dogalgaza dönüştürülmesi halinde sistemin bu dizayn dışı (off-design) durumu sayısal olarak modellenmiştir. Bu amaç la BASIC dilinde bir bilgisayar simulasyon programı olan ENKAD hazırlanmıştır. ENKAD simulasyon programı iki ana altprogramdan oluşmuş tur. YANMA altprogramında dogalgazm sabit basınç ve sıcaklıkta, hava ile yakılması sonucu ortaya çıkacak ürün miktarları hesaplanmaktadır. Termokimyasal denge haline ulaşıldı- ğında, duman gazının 12 bileşenden oluşacağı gözönüne alınmıştır. Kompleks termokimyasal denge hesaplarında Gibbs Serbest Enerjisinin mimimizasyonu yöntemi kullanılmıştır. Denklem çözümleri Newton-Raphson iterasyon tekniği ile gerçekleş tirilmiştir. Program ikinci bölümde, konstrüktif boyutları tamamen bilinen buhar kazanının termik hesapları yapılmıştır. Hesaplar, kazanı altı bölüme ayırmak suretiyle yapılmaktadır. Kazan bölümlerinde ısıl denge haline ulaşılıncaya kadar rad yasyonla (alev, gaz radyasyonu) ve konveksiyonla olan ısı transferi miktarları hesaplanmaktadır. Sonuçta her bölümde gerçekleşen ısı transferi miktarı, bölüm sonu gaz sıcaklık ları, kazan verimi gibi performans karakteristikleri elde edilmiştir. ENKAD simulasyon programı ile farklı işletme şartların da kazan performansının ne şekilde etkileneceği incelenebilmektedir. Böylece, dizayn dışı çalışma durumunun getireceği istenmeyen özelliklerin ne tipte önlemlerle ortadan kaldırılabileceğine karar verilmiştir. Çalışmada, sözkonusu önlemlerin maliyet açısından da incelenmesi gözönüne alınmıştır. Böylece, mevcut kazanın dogalgaza güvenli ve ekonomik dönüşü mü sağlanabilecektir.

Natural gas finds an expensive field of application in industry, thanks to its superior combustion properties. The fact that it gives very slight harm to environment as com pared to other fuels increases its chance of preference. Although our country has just been introduced to the natural gas in tne very near past, transformation of the industrial utilities into those using natural gas with necessary modifica tions is under rapid realization. It is a predictable fact, in a near future, the natural gas will have big share in the total energy requirement, as in the other western countries. At present, natural gas trading has been awarded a distin guished position in the world trade, with the achievement of 12.5 % share. On the other hand, Turkey will be purchasing natural gas in the year.,1992 at 5-6 billions m /year from USSR and 1,5 billions m /year from Algeria and Libya provided that the scheduled programme be realized, thus will reach 0,4% share in the World's gas consumption. Up to now, fuel-oil has formed the major portion of the fos sil fuelused fpr combustion in the steam boilers in our country However, with the" delivery of natural gas to our country, the existing boilers fired by. fuel-oil have started tobe converted operating with natural gas through necessary modifications or replaced by a new natural gas-fired boiler. The subject of this studyis do research on how off-design parameters would vary with the conversion of the existingf uel- oil-fired boilers into natural gas-operated ones. A computer simulation program (ENKAD) in the BASIC language has been prepared at this study. Consequently, digital examination has been made possible for the optimum operating conditions as v/ell as combustion and thermal phenomena to occur under these conditions. Substantial differences v/hich exist between fuel-oil and naturals gas with respect to the general fuel characteristics effect the thermal performance of boiler to a great extent. The more number of hydrogen atoms containedin natural gas than that in fuel oil result in increase of the water vapor content of combustion gas. v/hich means more latent heat for : IX evaporation of water, the product of combustion, in case of natural gas. In other words, the difference between HHV (higher Heating Value) and LHV (Lower Heating Value) of the natural gas is caused to increase. On the other hand, due to more carbon atom constituent of fuel oil as compared to natural gas, natural gas flame appears to be pale and in blue colour, while that of fuel-oil becomes lively and bright. This situation has effect on heat transfer by radiation in the furnace. Natural gas contains no sulphur, while sulphur content of fuel-oil is substantiolly high. For this reason, in case of burning natural gas instead of fuel-oil, no So radicals are detected in flue gas. SO compounds produce sulphuric acid through chemical process with water vapour, thus causiag acid rains in atmosphere with high environmental pollution. Since no SO. e.nissison will occur due to combus tion of natural gas, we 'can call natural gas as a clean fuel. Moreover, in case of fuel-oil fired boilers, the flue gas temperature connot be lovered belowa certain minimum value reguired to prevent sulphuric acid formation, because sulphur ic acid causes corrosion. However, in the case of natural gas use no such danger will exist, enabling utilization heat energy of the flue gas to the extent of lower temperatures. In practise, it can be stated that boiler efficiency increases by 1 % for each 20°C drop in the flue gas temperature. A steam boiler or steam generating unit can be defined a heat exchanger, with the combustion process occur ing in its furnace. For this, it is possible to examine a steam boiler in two stages. In the first part, the combustion phenomenon on the basic of general properties of fuel can be examined while examination at the second step covers' heat- transfer from hot combustion gases to the furnace walls. In line with this concept, _2.JKAD simulation program is com posed.0f two subroutines. In the sub-routine named YANMA, calculations have been performed for the purpose of finding out contituents quanti- tes of combustion gas released as a result of burning one mole of natural gas at any excess air coeficient as well as at constant pressure and temperature. Upon investigation of the actual composition of na+-"ral gas supplied from USSR, methane gas of the natural gas has to be 98%. Due to the very small fractions of the other gaseous constituents as compared to methane, they, can be considered negligible and properties of methane gas can be used as the properties natural gas. Nevertheless, in the subroutine YANMA, effects of the other gaseous constituents have been taken into considera tion as well as methane and natural gas has been expressed in terms of the hydrocarbon with its closed formüle of C0.996 H3.976 In the sub-routine YANMA calculation of the contituents quantities at thermochemical is performed through the Gibbs Free Energy minimization method. In this method, constituent quantities at constant pressure and temparature are so selec ted as enable Gibbs Energy become minimized In the program, it has been assumed that 12 products of combustion shall be formed due to burning natural gas with air. These radicals have been selected as N_,H_0 C0_ 0?,CO,H?,H,OH,O,NO,NO and N. Gibbs Free Energy of each radical has been expressed in a formula with 7 coefficients, which has been obtained from the tables of JANAF. if there are i number of reagent constituents in the system, (i=12 as assumed) where equilibrium calculation is to made, it means that there exist i number of unknowns. If the number of elements is j, ( j =4), Jnumber of equations can be written down based on conservation of the number of atoms which means that it is necessary to write down i-j=r (r=8) independent equilibrium reactions. 3y evaluations of the output results of the sub-routine YANMA it is possible to make choice of the optimum extimum excess air coefficent. At an excess air of 5%,cu content in the produces of combustion decreases below 200 ppm. As it is obviously seen, it can be possible to achieve complete combustion with very small quantity of excess air, in case of natural gas use. Upon evaluation on the basis of water vapor; water vapor ratio. contained in flue gas is found to be 18% having an import-ant share, in case of burning natural gas. This reduces LHV of the fuel. Natural gas is also awarded satisfactory grade from the viewpoint of NO derivatives which cause air pollution. When compared to x other fuels, NO emission from flue gas is seen to be less in the natural gas case. In fact, there is no SO emission from flue gas, since the natural gas has no sulphur in its composition. Therefore, it is obvious that the natural gas will bring in advantages for prevention of air pollution. The second part of the simulation program, ENKAD, consists of thermal computations of the boiler. Thermal computations have been carried out by dividing the boiler in to six sections, namely furnace, fire-tube, fire-box, II. nd pass smoke tubes, smoke box and 1 1 1. rd pass smoke tubes. Heat balance calculation is made separately for each section. Heat transfer in the fur'nace section is expected to be realized by flame radiation, while for the other sec tions by gas radiation and convection. XI A sub-routine is avaiable for computation of furnace as well as heat transfer amount through flame radiation. Since there will be heat transfer by radiation from flame to furnace surfa ce, the temperature of the furnace will be lower than adiaba- tic flame temparature. If it were possible for heat energy of the fuel to be transfered completely to the combustion gases, then adiabatic flame temperature could be reached in the furnace. But, in none of the practical applications, it is impossible to reach adiabatic flame temperature. An itera tive method has been followed for the computation of furnace temperature, because furnace temperature and radiant-heat trans fer quantity are function of each other. It is assumed that transfer to be realized in the other five sections boiler shall be by gas radiation and convec tion. Gas radiation. occurs due to emission of electromagnetic waves from the water vapor and C0? gas constituents of the combustion gas, by their turbulance and vibrational motions. In case natural gas usage, gas radiation, spacially fire-tubes and fire-box, is of great importance, due to the increased water vapor content of combustion gas. The partial pressures of water vapor and C0", which are necessary for gas radiation can be obtained from output results of the sub-program YANMA Because, partial pressure of the gas constituent at atmosp heric pressure is directly equal to its mole fractions. For this reason, output results of the sub-program YANMA are important for accurate thermal calculations. In particular, heat transfer in smoke-tubes is realized by convection to a great extent. Convectional heat transfer gets improved with increase in gas velocity. Due to lc^ar excess air coefficients5 required in case of natural gas-fired boilers, flow rate of the combustion gas is. naturally smaller as compared to fuel-oilln order to overcome this unfavovrable situation, velocity of gas can be increased by mounting of so called turbulator inside of the smoke tubes, which are nothing but spiral strips. ENKAD simulation program has been prepared for the triple pass scotch type boilers generating saturatedsteam. The same program also covers alternative types of the boiler such as plain-ashpit, corrugated-ashpit, wet-back, dry-back types. Provision of the program with its main menu wakes it possible to perrorm easilythe functions of print-out of data and results call, store and replace. Results of the thermal calculations show that thermal performance of boiler will be influenced to a great extent by use of natural gas in the boilei^ instead of fuel-oil. Due to the fact that natural gas flame 'is more opaque compared to fuel-oil and in blue colur, a decrease in the radiant-heat transfer in furnace. occurs. Therefore, in case of natural gas use temperature sub.tantially increases. Hot combustion XLl gases leaving the furnace may cause material deformations in boiler. At smoke-tube joints with header, tube ends might be subjected to the danger of burning. For this reason, smoke tubes must absolutely be welded to header at joint ends and offset parts of tube ends must be protected by ceramic fer rules. ENKAD simulation program has been run at variable ope rating characteristics, for a steam boiler with 100 m hea ting surface area. Performance comparison has been made with that of the same size, capacity, etc, boiler in case of fuel-oil firing based upon the results obtained. At operating conditions of 5% excess air coefficient and full-load, furnace temperature becomes 1600 C, while this temperature is 1200 C, in case of fuel-oil firing. Flue gas outlet temperature at the same conditions is 288 C in case of natural gas against 244 C value of the fuel-oil- fired boiler. A drop of 6% in full-lead is expectable in of natural gas use in the boiler. A systematical method been adapted for comparison of the boiler performances. In the first group, research studies have been carried out. in order to find how boiler performance with be influenced by its operation at variable excess air coefficients at full-load. In such case furnace temperature decreases with increase excess air coef ficient, due to increase in specific quantity of combustion gas. However, due to velocity increase caused by excess air, heat transfer quantity by convection in smoke-tubes is obser ved to increase. It is seen that boiler efficiency will have' decreasing trend and flue-gas outlet temperature will increase both with rise in excess air coefficient. Regarding the second group of studies, the results have been obtained and evaluated, in case the toiler is run at variable loading factors at a fixed excess air coefficient of 5%. Under these conditions, furnace temperature comes out be independent from boiler load factor. But in the other sections of the boiler, it has been observed that section-end boundary temperature decreases in case of operation below nominal (rated) load of the boiler, which in turn results in boiler efficiency increase. However, the boiler shall not be allowed to operate all the time at partial load. Elimination of the undesirable conditions resultting from conversion of the existing boiler in to the one using natural gas in included in the content of this study. Accordingly, some measures are to be taken for the sake of conversion of the boiler into natural gas-fired one.- Such measures can be classified in two groups, namely xm the constructive measures and efficiency-improving measures. Those measures which are taken so as to prevent material defects due to high temperatures, constructive measures. It is expected that a certain amount of efficieny decrease will be caused by conversion of boiler into natural gas-fired one. Equipping the boiler with some additional systems for the purpose of eliminating this deficiency forms the efficiency emprovement measures. It is possible to achieve waste-heat recovery of the flue gas, to a great extent, by use of economizer. Energy ob tained in heating boiler feed water as well as heating the fuel-burning air in the air preheater or in preheating fuel. This study takes into consideration the utilization of heat energy recovered in economizer for heating of the inter nal building. The subject of how enable efficiency impro vement by integration of an economizer to the existing low- pressure hot water-system is examined in this study, and at the same time, advantages to be provided by such a system modification with to its cost are also indicated, together with the economical analysis of the system performed. An economizer choice has been made according to be print out values of the ENKAD program. Conclusin is such that this economizer wi 11 B provide a heat recovery of 190. 000Kcal/h, with the operating at full-load. That energy to be obtained has been foreseen to be utilized in domestic! negating during winter season. Cost analysis of the system integrated with the econo mizer has also been performed on this study. It has been examined as well, namely that in what period of time the energy saving achieved due to heat recovery can pay-back the initial investment cost of the system. For this purpose, an estimate-summary (bill of quantity) of the economizer-entegra- tion system has been derived and its cost has been obtained by calculation on the basis of May, 1990 prices. In conclusion, ENKAD simulation program indicates that fuel-oil-fired boiler can be successfully converted that all necessary precautions be taken. However, it is indispensable to replace the boiler, which are nearly about to complete their economical life, with a new one designed on the natu- ral gas firing, instead of making modifications on them» for conversion into natural gas-fired boiler.