- Email: [email protected]
Use of bioethanol in a gas turbine combustor
Applied Thermal Engineering 61 (2013) 481e490
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Use of bioethanol in a gas turbine combustor J.A. Alfaro-Ayala a, A. Gallegos-Muñoz b, *, A.R. Uribe-Ramírez a, J.M. Belman-Flores b a
University of Guanajuato, Chemistry Engineering Department, Guanajuato, Gto., Mexico University of Guanajuato, Mechanical Engineering Department, Carr. Salamanca-Valle de Santiago km 3.5þ1.8, Com. de Palo Blanco, Salamanca, Guanajuato, Mexico b
h i g h l i g h t s We apply numerical and thermodynamic analysis to study the effect of the biofuel in gas turbine combustor. We apply different air distribution to check experimental measurements of NOx emissions using conventional fuels and biofuel. Two different conditions to study the power output, efficiency and Turbine Inlet Temperature (TIT), were applied.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 April 2013 Accepted 20 August 2013 Available online 29 August 2013
A study of a gas turbine combustor that considers two conventional fuels and one biofuel is presented. The kind of fuel supplied to the combustor can impact in the Turbine Inlet Temperature (TIT) provoking significant changes in the power output and efficiency. Moreover, it can cause some damage in the initial steps of the gas turbine due to the migration of the hot streak. Natural gas, Diesel and Bioethanol are considered in the combustor in order to compare the performance of the power plant. The use of biofuel in a gas turbine combustor presents some benefits; a) better behavior in the distribution of the TIT, b) slightly higher power output and c) less impact of NOx and CO2 emissions. The analysis was based in the Computational Fluid Dynamics (CFD) and thermodynamics. The results indicate that it is necessary to increase the mass flow rate of bioethanol to maintain the power output of the turbine, due to a significant reduction of the TIT was observed. On the other hand, the use of bioethanol permits an important reduction of NOx emissions when they are compared with the conventional fuels (natural gas or diesel). Also, a noble benefit is obtained due to the biofuel comes from biomass-derived material, resulting in a reduction of CO2 global warming. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Gas turbine Combustor Emissions Bioethanol
1. Introduction The evolution of the gas turbines have been oriented towards the continuous desire to obtain higher temperatures, greater power and minimal emissions to the environment, specially limiting CO2 and NOx emissions in power generation. However, just little information about experimental research has been done in gas turbines in order to study the performance and emissions using biofuels. Recently, in the year 2008, M. Moliere et al. [1] made experimental tests using bioethanol in a gas turbine Frame 6B located at Goa, India. They characterized the combustion on a wide range of naphtha/bioethanol blends (up to 95% of bio-ethanol). Giving as main results, that it is needed an increment of biofuel and the CO
* Corresponding author. Tel.: þ52 (464) 647 9940x2361; fax: þ52 (464) 647 9940x2311. E-mail addresses: [email protected] (J.A. Alfaro-Ayala), [email protected] (A. Gallegos-Muñoz). 1359-4311/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2013.08.025
and UHC (Unburned Hydrocarbons) emissions were to close zero. Also, supplying 100% bioethanol, the NOx was reduced to the half when they are compared with 100% naphtha fuel (principal fuel used in the India). In the years 2006 and 2007, M. Moliere et al. [2] made their experiments in a Gas turbine Frame 6B and Frame 7EA respectively, they supplied biodiesel in both gas turbine models and they found that it is possible to use this kind of biofuel due to there were neither any human accident nor thermo-mechanic fails on the components. In Brazil, the use of bioethanol for the combustion process has been achieved [3]. One of the two gas turbine GE LM6000 (43.5 MW) was adapted to work with bioethanol. The conversion included the replacement of injection nozzle, as well as installation of peripheral equipment such as receiving system, reservoirs, pumps, filters, etc. Each of them allowed receiving, storing and supplying bioethanol to the gas turbine. During the tests, the plant demonstrated significantly lower CO2 emissions and demonstrated
482
J.A. Alfaro-Ayala et al. / Applied Thermal Engineering 61 (2013) 481e490
Fig. 1. Gas turbine scheme.
a 30% reduction of NOx emissions within 150 h of power generation with bioethanol. A comparison between bioethanol, natural gas and diesel revealed that the use of bioethanol fuel releases lower levels of NOx without decreasing its power. When it is trying to change the fuel used in a gas turbine combustor to another different of the original one, it is necessary to put attention in all the equipment downstream. Simone Colantoni et al. [4] showed in their investigation the impact of burning syngas obtained from biomass, by means of pyrolysis, in terms of the performance of the machine, aerodynamic behavior and secondary flows in order to determine the reduction life of components caused by overheat generated due to the combustion of syngas. The analysis was carried out in a GE10-2 model where the results were compared with natural gas. They concluded that the increment of syngas flow impacts in the useful life of the components, mainly in the first step of the gas turbine, computed by the Larson & Miller methodology [5], it also provokes an increment of temperature of the cooling flow and an increment of the temperature exhaust, it means an increment of temperature of combustion gases through the vanes. In this work, the determination of the TIT, considering different fuels, provides important information in order to evaluate the overheating of components due to flow gases downstream. This last topic is usually studied in blades of the gas turbines in which they estimate the thermal-structural failures, Zdzislaw Mazur et al. [6] and A. Fields et al. [7]. Although this work does not focus on economic issues, it is important to mention that the operational economics of supplying some fuel or biofuel is complicated. Anthony Campell et al. [8] say that the economics of providing a fuel is stated by a complex coupling between customer’s economic, environmental concerns and uncertainties of global regulatory policies, and also, the alternative gas or liquid fuel should be profitable without sacrificing power output of the plant and emission characteristics. Goodger [9] says that among the biofuels, the bioethanol is one of the best for the use in gas turbines due to it is very easy to transport and storage. Also, it is important to mention that many works which involve bioethanol issues have been focused in the Life-cycle assessment [10e12], even in our country [13], in order to estimate greenhouse gas emissions (GHG). Applications in the automotive industry of gasolineebioethanol blends, is considered to see the advantage of
higher octane number, emissions and the supplied regardless of the fossil fuel [14e16]. Production process of bioethanol [17] and bioethanol reformer systems to produce hydrogen with sufficient quality to feed a PEM fuel cell system [18,19], have been studied. 2. Gas turbine description This study considers combustor, transition piece and ring connection, these components belong to a gas turbine of 50 MW power output, rotates at 3600 rpm, it has 17 compressor stages and 4 gas turbine stages. The compressor pressure ratio is 7 and airflow of 216 kg/s. This gas turbine can be operated with natural gas and diesel. This type of gas turbine has only one combustor (6.4 m high, 3 m diameter and it weighs about 34 tons) located vertically between the compressor and turbine, the transition piece has a shape like “U” and it joins the combustor with the ring connection of the turbine, this last component is located in the first step of vanes. The Fig. 1 shows a schematic arrangement of the gas turbine, the air supplied for the compressor is taken throw the annulus of the transition piece to the combustor chamber, the combustor chamber has three sections where the air is distributed, this air sections are called primary air, secondary air and dilution or tertiary air. The primary airflow through the diffuser generates a swirl, the secondary air passes through four inner rings, this air is used to avoid high temperature in the combustor wall and the dilution air comes into the combustor for six holes located around the combustor to decrease the high temperatures of the combustion gases, these gases are conveyed to the first step of the turbine blades. As it is well known, the combustion gases always have high temperature but they must not exceed 1273.15 K (1000 C) due to it can cause damage in the turbine components. A protection system, consisting of PLCs model PLC5-20, is installed in the control room of the power plant so that they can determine the TIT. This temperature is estimated according to the measurement of two parameters: the first is the Exhaust Gas Temperature (EGT) and the second is the Compressor Discharge Pressure (CDP). These two parameters are used in the next expression:
TITð CÞ ¼ 1:4079*EGTð CÞ þ 1:529*CDPðpsiÞ þ 90:4ð CÞ In the case of the TIT computed is higher than 1273.15 K (1000 C), the PLC sets off an alarm (Alarm 85-03), this indicates
J.A. Alfaro-Ayala et al. / Applied Thermal Engineering 61 (2013) 481e490
483
Fig. 2. Mesh for the combustor, transition piece and ring connection.
that the gas turbine is holding some damage due to the TIT is exceeding the “Alarm Temperature” (Talarm ¼ 1273.15 K). This alarm temperature will be considered as parameter of reference in the next results. Also, in the case of the TIT exceeds 1338.15 K (1065 C), the PLC sets off another alarm (Alarm 85-04), this indicate that the power plant is taken out of work to protect the components of the gas turbine and prevent further damage.
3. Numerical procedure The study was carried out in a steady state three-dimensional model of a vertical combustor, transition piece and the ring connection which fits to the first step of vanes, using FLUENTÒ software. The ke3 turbulence model and chemical equilibrium for the combustion process are applied [20]. The pressureevelocity coupling was solved using a SIMPLE approach and all equations by first-order upwind scheme. The standard model of turbulence is a semi-empirical model proposed by Launder and Spalding [21], based on the model of the transport equations for the turbulent kinetic energy (k) and the dissipation rate (3 ). This model is appropriate to study the turbulence in practical engineering flow due to its reasonable accuracy for a wide range of turbulent flows. The chemical equilibrium is based in the power of the mixture fraction modeling approach, where the chemistry is reduced to one or two conserved mixture fractions. Under the assumption, all thermo chemical scalars (species fractions, density, and temperature) are uniquely related to the mixture fraction (f ). It is generally acceptable for turbulent flows where turbulent convection overwhelms molecular diffusion. In addition to solve the problem, the equation for the mixture fraction variance ðf ’2 Þ is required. The mixture fraction variance is used in the closure model describing turbulence-chemistry interactions [22]. Equations for individual species are not solved. Instead, species concentrations are derived from the predicted mixture fraction fields. The thermochemistry calculations are preprocessed and then tabulated for look-up in ANSYS FLUENTÒ. Interaction of turbulence and chemistry is accounted for with an assumed-shape Probability Density Function (PDF). The specific heat, enthalpy and entropy of all species of the analysis can be expressed in polynomial form [23]. The polynomial coefficients are set into the database of FLUENTÒ using the convenient format. The NOx emissions are precursors of smog, contributes to the acid rain and cause damage to the ozone layer. The prediction of NOx emissions is solved by mean of the transport equation for the concentration. In order to generate a numerical model, it is necessary to represent the geometry in a computational domain. The geometry
Fig. 3. TIT vs million cells.
of the model and mesh were created using GAMBITÒ and the governing equations were transformed to algebraic equations inside computational domain [24]. Fig. 2 shows the structured grid used in combustor chambertransition piece and unstructured grid in the ring connection; this last was used due to the complex geometry. The uses of quadrilateral cells reduce the amount of finite volume elements needed for the grid independence (if these were compared with triangular cells). The quadrilateral cells provide stability during the numerical solution and reduce the solution time. The final mesh is about 6 millions of cells. 3.1. Mesh analysis Several models with different amount of cells were made in order to define the independent value of the variables: TIT, maximum temperature inside the combustor (Tmax) and NOx emissions. The Figs. 3 and 4 show the variable vs the amount of cell used in the model, from 1 million to 6 million of cells were analyzed, it can be seen that variables of TIT, Tmax and NOx tend to be uniform with the increment of the cells (independent of the amount of cells), the lower amount of cells have the higher value of the variable computed while the higher amount of cells have lower value of the variable computed numerically and the change of slope is in about 2 million of cells. It means that the tendency of the variable is a decrement with the increment of the cells until to be independent of the amount of cells. Particularly, the Fig. 3 shows the Talarm as reference to see the effect of the mesh in this variable; a coarse grid can led us to a wrong prediction of the TIT showing that the TIT is higher than the Talarm.
Fig. 4. Tmax and NOx vs million cells.
484
J.A. Alfaro-Ayala et al. / Applied Thermal Engineering 61 (2013) 481e490
3.2. Computational domain
Table 1 Operating conditions for modeling combustor-transition piece.
Fig. 5 shows the scheme of the combustor, including the input of the flows; primary, secondary (cooling 1e4), dilution air and the input of the fuel flow. It also includes the vertical combustor chamber, transition piece and ring connection. The primary air enters at the top of the combustor chamber with a swirl provoking the recirculation zone; it allows mixing the air with the fuel. The four cooling flows enter through the annulus to reduce the temperature on the wall of the super-alloy of the combustor and avoid thermo-mechanical damage. The dilution air is used to decrease the temperature of the hot gases generated for the combustion. The transition piece “U” leads the hot gases toward the first step of the gas turbine by mean of the ring connection. The chemistry (reactive and products) is modeled with the use of the chemical equilibrium model proposed by the code guide [20] and it considers 15 main species to model the different fuels according to the following: 3.2.1. Natural gas CH4, N2, O2, H2O, CO2, CO, H2, OH, H, O, HO2, H2O2, HONO, CH3OH, HCO. 3.2.2. Diesel C10H22, N2, O2, H2O, CO2, CO, H2, OH, H, O, HO2, H2O2, HONO, HCO, CHO. 3.2.3. Bioethanol C2H5OH, N2, O2, H2O, CO2, CO, H2, OH, H, O, HO2, H2O2, HONO, O3, HNO3.
Mass flows
Quantity
Units
Fuel Primary air Cooling 1 Cooling 2 Cooling 3 Cooling 4 Dilution air Total mass flow
3.6 116.52 19.57 19.57 19.57 19.57 21.2 219.6
[kg/s] [kg/s] [kg/s] [kg/s] [kg/s] [kg/s] [kg/s] [kg/s]
Pressures
Quantity
Units
Inlet pressure Outlet pressure Drop pressure DP
736,000 694,500 41,500
[Pa] [Pa] [Pa]
Temperatures
Quantity
Units
Air inlet temperature Fuel inlet temperature
550 353
[K] [K]
3.3. Boundary conditions Table 1 shows the airflow distribution; these were set to the computational model as boundary conditions. The modeling of the three different fuels was carried out as follow: It was considered that the total amount of air supplied for the compressor was kept in all the cases studied, the distribution of the air mass flow (primary, cooling and dilution air) was similar for all cases, the inlet temperature of the air and fuel (natural gas, diesel, bioethanol) were the same for all cases and the operational pressure was the same for all cases.
Fig. 5. Air flow distribution of the combustor chamber.
J.A. Alfaro-Ayala et al. / Applied Thermal Engineering 61 (2013) 481e490
4. Thermodynamic procedure
485
Table 3 Comparison of adiabatic flame temperature.
It was planned to have three cases for each conventional fuel and biofuel with the aim of determining the adiabatic flame temperature (Taf) by mean of the stoichiometric reactions. Determining the Turbine Inlet Temperature TIT by mean of the equal amount of fuel (equal mass flow rate of fuel at the inlet) and the TIT by mean of the equal amount of energy (equal amount of energy at the inlet). It was considered that the natural gas consists only of methane CH4, diesel only of decane C10H22 and bioethanol only alcohol C2H5OH. Also, the stoichiometric reactions considers a completed combustion with just CO2 and H2O as species of products and the N2 is obtained as inert. And the equal amount of fuel (EAF) and equal amount of energy (EAE) reactions consider that the CO is close to zero, it means that the entire CO is transformed to CO2 and the O2 specie is considered in the products due to the large amount of excessive air required for the combustion process. Details of the thermodynamics procedure can be found in Ref. [25]. 4.1. Stoichiometric Table 2 shows resume of the reaction considering stoichiometric, EAF and EAE. In this table can be seen the global stoichiometric reaction and its corresponding AireFuel ratio (AF) of the considered fuels. Also, the table shows that the stoichiometric AF ratio is different for each fuel, the highest stoichiometric AF is to the natural gas and it is followed for diesel and bioethanol. 4.2. Equal amount of fuel (EAF) The global chemical reactions with the equal amount of mass flow rate of fuel are shown in Table 2. The same amount of mass flow provokes to have different excessive air for each fuel. The lower is for natural gas 250% of excessive air, for the diesel 300% of excessive air and for the bioethanol 570% of excessive air. 4.3. Equal amount of energy (EAE) Table 2 shows the global chemical reactions with the equal amount of energy for the three fuels considered and its corresponding actual AF ratio. The highest actual AF is to the natural gas 60 and it is followed for diesel 52.7 and then bioethanol 31.42. 5. Results The adiabatic flame temperature (Taf) for each fuel was obtained using the stoichiometric reactions considered in the thermodynamic procedure using the sensible and formation enthalpies of the
Fuel
Taf [K]
Glaude et al. [26], Taf [K]
% Difference
Natural gas Diesel Bioethanol
2330 2399 2295
2227 2292 2240
4.42 4.46 2.39
reactive and products. It was observed that the higher adiabatic flame temperature is for diesel (2399 K), it is followed for the natural gas (2330 K) and bioethanol (2295 K). These results are similar as those obtained by Glaude et al. [26], they determined the adiabatic flame temperature to different fuels and biofuels by mean of the software THERGAS [27] considering the species like perfect gas and the Benson approach [28] and the contributions of Domalski et al. [29], their results showed that the diesel generally have higher temperature than the group of FAMEs (Fatty Acids Methil Ester, that correspond to the group of biofuels from rapeseed, RME, soybean, SME, sunflower, etc.), followed by the Naphthas and the last for the natural gas. This is attributed to the relation of H/C (hydrogen and carbon), it implies to have high or less combustion enthalpy. For the cases studied here, the H/C is 2.2, 4 and 3, for diesel, natural gas and bioethanol, respectively. Table 3 shows a comparison of the adiabatic flame temperature, some difference should be note, this work considers the species as ideal gas and four species as products while the investigation of Glaude et al. [26] considers the species as perfect gases, another components in the reactive and eleven species as products. However, the difference of Taf between the methods is for natural gas and diesel about 100 K and for bioethanol is about 50 K. Fig. 6 shows a comparison of TIT obtained by the thermodynamic and numerical analysis considering equal amount of fuel and energy. Under the equal amount of fuel (EAF), the TIT is higher for natural gas, followed by diesel and bioethanol. However keeping equal amount of energy (EAE), increasing the amount mass flow of fuel for diesel and bioethanol, the TIT is increased and it reaches TIT very close to the natural gas. The TIT is achieved successfully without exceeding the alarm temperature. This means that the first step of blades in the gas turbine does not have mechanical damage. Fig. 7 shows the TIT, power output and thermal efficiency. For the natural gas considering EAF, the TIT is about 1239.1 K, the power output is 45.87 MW and the efficiency is 27.35% with 250% excessive air. For the diesel, the TIT is about 1177.8 K, the power output is 40.621 MW and the efficiency is 26.74% with 300% excessive air. Finally, for the bioethanol, the TIT is about 937.7 K, the power output is 20.153 MW and the efficiency is 22.09% with 570% excessive air. The higher impact is observed when the biofuel is supplied. The TIT is very low provoking a reduction of the power
Table 2 Stoichiometric, EAF and EAE reactions. Fuel
Stoichiometric reaction
Stoichiometric AF ratio
Natural gas Diesel Bioethanol
CH4 þ 2(O2 þ 3.76N2) / CO2 þ 2H2O þ 7.52N2 C10H22 þ 15.5(O2 þ 3.76N2) / 10CO2 þ 11H2O þ 58.22N2 C2H5OH þ 3(O2 þ 3.76N2) / 2CO2 þ 3H2O þ 11.28N2
17.19 15.03 8.98
Fuel
Equal amount of fuel reactions
Excessive air [%]
Natural gas Diesel Bioethanol
CH4 þ 7(O2 þ 3.76N2) / CO2 þ 2H2O þ 5O2 þ 26.32N2 C10H22 þ 62(O2 þ 3.76N2) / 10CO2 þ 11H2O þ 46.5O2 þ 233.12N2 C2H5OH þ 20.1(O2 þ 3.76N2) / 2CO2 þ 3H2O þ 17.1O2 þ 75.576N2
250 300 570
Fuel
Equal amount of energy reactions
Actual AF ratio
Natural gas Diesel Bioethanol
CH4 þ 7(O2 þ 3.76N2) / CO2 þ 2H2O þ 5O2 þ 26.32N2 C10H22 þ 54.25(O2 þ 3.76N2) / 10CO2 þ 11H2O þ 38.75O2 þ 203.98N2 C2H5OH þ 10.5(O2 þ 3.76N2) / 2CO2 þ 3H2O þ 7.5O2 þ 39.48N2
60 52.7 31.42
486
J.A. Alfaro-Ayala et al. / Applied Thermal Engineering 61 (2013) 481e490
Fig. 6. Turbine Inlet Temperature for equal amount of fuel (EAF) and energy (EAE).
output; it is about of 50% when is compared with the natural gas (original fuel used for the gas turbine). The thermodynamics procedure to obtain the power and efficiency values were developed in previous work [25]. Supply equal amounts of diesel and bioethanol reduces TIT, and also it produces a diminution of power output and thermal efficiency this is due to minor available energy supplied. Then, an increase of fuel to keep equal amount of energy in order to close TIT, power and efficiency to the original operational condition, is considered. For the diesel considering the EAE, the TIT is about 1257.5 K, the power output is 47.648 MW and the efficiency is 27.57% with 250% excessive air. For the bioethanol, the TIT is about 1252.2 K, the power output is 48.527 MW and the efficiency is 27.95% with 250% excessive air. Using equal amount of energy, the TIT, power output and efficiency are even a slightly higher than the original
operational condition using natural gas. This slight increment is attributed to the increment of mass flow that is expanded in the gas turbine. Figs. 8 and 9 show a comparison of averaged molar fractions of CO2, CO and H2O at the inlet of the gas turbine considering EAF and EAE, respectively. It can be observed that the behavior of these species is very similar in the analytical and numerical results. Fig. 8 considers equal amount of fuel EAF thus the behavior of the CO2 is higher for the diesel; it is followed for natural gas and bioethanol. The H2O is higher for the natural gas; it is followed for diesel and bioethanol. It can be seen that the CO2 and the H2O for bioethanol is lower than natural gas and diesel. However, the behavior changes using equal amount of energy EAE (Fig. 9), the CO2 and H2O reach values higher than the natural gas, CO2 reaches values similar of diesel and H2O reaches values higher than diesel. Also the CO is very low, for all cases considering EAF and EAE. Fig. 10 shows the temperature contours considering EAF and EAE. It can be seen that the higher temperature is for the diesel (2382 K); it is followed for natural gas (2270 K) and bioethanol (1978 K) for the cases with EAF. For the cases with EAE the higher temperature is diesel (2362 K); it is followed for natural gas (2270 K) and bioethanol (2062 K). Also, it can be seen that the temperature is almost homogeneous at the inlet of the gas turbine for all cases. Fig. 11 shows the formation of NOx (ppm) for all cases with 14.5% O2, which correspond to standard value in real condition in gas turbine using natural gas. The formation of NOx was considered for the temperature (Thermal NOx). It can be seen that the formation of NOx for diesel is 2853 ppm; it is followed for natural gas with 941 ppm and bioethanol with 19.28 ppm when it is considered EAF. Moreover, the formation of NOx for the cases of EAE for diesel is 2233 ppm; it is followed for natural gas with 941 ppm and then bioethanol with 71.15 ppm. According to local standards, the bioethanol is clearly under the limit. According to results, the distribution of temperature and TIT obtained are acceptable, but the NOx are very high for the conventional fuels (natural gas and diesel). The hard link that exists
Fig. 7. TIT, power output and efficiency.
J.A. Alfaro-Ayala et al. / Applied Thermal Engineering 61 (2013) 481e490
487
Fig. 8. Molar fraction, EAF.
between temperature and NOx provokes a high increment of NOx with a few degrees of increment of temperature. The air distribution used to obtain the Figs. 10 and 11, it is under theoretical conditions to obtain the desired TIT (without passing the Talarm) for the natural gas, see Table 1. The air distribution used to obtain the Figs. 12 and 13, it is under actual operational conditions to compare the NOx obtained experimentally for the natural gas and between the other fuels. The air distributions used for Figs. 12 and 13 were: primary air is 29%, cooling 1e4 is 60% and dilution air is 11% (total air distribution 216 kg/s). Fig. 12 shows the contours of temperature considered the test condition for EAF and EAE. It can be seen that the higher temperature in the contour is for the diesel (2282 K); it is followed for natural gas (2119 K) and bioethanol (1978 K) for the cases with EAF. For the cases with EAE the higher temperature is diesel (2260 K); it is followed for natural gas (2119 K) and bioethanol (2062 K). The TIT using natural gas was 1275.7 K, diesel 1213.1 K and bioethanol 918 K for the cases of EAF. For the case with EAE the TIT using natural gas was 1275.7 K, diesel 1278.3 K and bioethanol 1249.5 K. Also, in the Fig. 12 the numerical results show that the diesel and natural gas revealed a hot streak that is formed and it migrates towards the inlet of the gas turbine. This impact in the nonuniformity of the TIT may cause damage in the components of the GT, the proof of it was found during the inspection of the GT and
the history of the maintenance during the last decade, the recurrent change of the blades of the first and second steps were observed. Moreover, in the in the cases of the Fig. 12 was observed that the TIT exceeds the Talarm. Also, it can be seen that the maximum temperature is not as high as in Fig. 10, thus the reduction of high temperature inside the combustor makes to decrease the NOx emissions (see Fig. 13). A further work will be to optimize some parameters such as the distribution of air by mean of the geometry [30] in order to decrease the high temperature in hot streak and consequently the NOx emissions. However, a similar behavior was found in the simulation of the combustor chamber using diesel while the use of bioethanol in the simulation does not show migration of the hot streak and the maximum temperatures are lower than natural gas or diesel, thus the problem of migration of hot streak is voided. Consequently, the use of bioethanol produces lower temperature in the hot streak thus this provokes that the NOx emissions decreases. The TIT of numerical cases for biofuel is always near the limit of the Talarm. This means that the power output and efficiency will be kept as the original conditions (using natural gas) and lower emissions of NOx. The experimental measurements were realized to the unit 2 of the power plant using natural gas during the year 2009. The results were: the NOx emissions about 188 [email protected]%O2, the excessive air was 271.7% and the CO was 0 ppm, while the numerical results
Fig. 9. Molar fraction, EAE.
488
J.A. Alfaro-Ayala et al. / Applied Thermal Engineering 61 (2013) 481e490
Fig. 10. Contours of temperature [K].
Fig. 11. Contours of NOx @14.5% O2 [ppm].
J.A. Alfaro-Ayala et al. / Applied Thermal Engineering 61 (2013) 481e490
Fig. 12. Contours of temperature [K].
Fig. 13. Contours of [email protected]% O2 [ppm].
489
490
J.A. Alfaro-Ayala et al. / Applied Thermal Engineering 61 (2013) 481e490
were: the NOx emission 170 [email protected]%O2, the excessive air was 250% and the CO was 0 ppm (see Fig. 13). The difference between the experimental measurements and the numerical results are close, 18 ppm of NOx, 21.7% of excessive air and 0 ppm of CO2.
6. Conclusions The results show that the condition equal amount of fuel (EAF) cannot keep the original performance (natural gas) of the power plant for diesel and bioethanol and there is not hot streak migration problem for all cases but it appears NOx emission problems due to the high temperatures. Then in order to have similar power output, TIT and thermal efficiency of the power plant for all fuels considered in this work, an increment of diesel and bioethanol was required, that leads us to the second condition, equal amount of energy (EAE), it was achieved the performance with low NOx but now the hot streak migration problem appears. With these results, we consider that some optimization techniques can be implemented to the analysis in order to have a balance between performance (Power output, TIT and Efficiency) and hot streaks and NOx emissions problems. The three fuels modeled with the EAE condition can satisfy the TIT and the output power but the main difference founded it was in the CO2 emissions. The bioethanol and diesel produce similar amount of CO2 and higher than natural gas (about 19.5% more), however, the main difference is the native material where the fuel was obtained, the biofuel is obtained from biomass-derived material (sugar cane or corn) and the natural gas and diesel are obtained from fossil material (petroleum). This implies that the CO2 that is formed by the combustion of bioethanol, it comes into the life cycle decreasing global warming. Also, the advantage of using biofuel is the less formation of thermal NOx, this is due to the temperature in the combustor chamber is lower with bioethanol than natural gas or diesel. The reduction of NOx using bioethanol is about 65% and 92% when it is compared with natural gas and diesel at 15.7% O2, respectively. Moreover, the biofuel shows less problematic behavior in the sense of the migration of hot streak to the gas turbine, this makes to decrease the problem of the non-uniformity of the TIT and it would probably have less impact in thermo-mechanical damage in the first step of vanes and blades of the gas turbine. References [1] M. Moliere, M. Vierling, M. Aboujaib, P. Patil, A. Eranki, A. Campbell, R. Trivedi, A. Nainani, S. Roy, N. Pandey, Gas Turbine in Alternative Fuel Application: Bioethanol Field Test, ASME Turbo Expo, Orlando, Florida, 2009. [2] M. Moliere, E. Panarotto, M. Aboujaib, J.M. Bisseaud, A. Campbell, J. Citeno, P.A. Maire, L. Ducrest, Gas Turbine in Alternative Fuel Application: Bio-diesel Field Test, ASME Turbo Expo, Montreal, Canada, 2007. [3] Kris Bevill: http://www.ethanolproducer.com/articles/7031/ge-powersturbines-with-ethanol-in-brazil/ and http://www.power-technology.com/ projects/ethanol-power-plant/, 2010. [4] Simone Colantoni, Stefania Della Gatta, Roberto De Prosperis, Alessandro Russo, Francesco Fantozzi, Umberto Desideri, Gas Turbines Fired with Biomass Pyrolysis Syngas: Analysis of the Overheating of Hot Gas Path Components, ASME Turbo Expo, Orlando, Florida, 2009. [5] F.R. Larson, J. Miller, A timeetemperature relationship for rupture and creep stress, Trans. ASME 74 (1952) 765e775.
[6] Z. Mazur, A. Luna-Ramírez, J.A. Juárez-Islas, A. Campos-Amezcua, Failure analysis of a gas turbine blade made of Inconel 738LC alloy, J. Eng. Fail. Anal. 12 (2005) 474e486. [7] Meherwan P. Boyce, Gas Turbine Engineering Handbook, Gulf Professional Publishing, 2002. [8] Anthony Campell, Jeffrey Goldmeer, Tim Healy, Roy Washam, Michel Molière, Joseph Citeno, Heavy duty gas turbines fuel flexibility, in: Proceeding of ASME Turbo Expo Berlin, Germany, 2008. [9] E.M. Goodger, Transport Fuel Technology, Landfall Press, Norwich, UK, 2000. [10] Lin Luo, Ester van der Voet, Gjalt Huppes, Life cycle assessment and life cycle costing of bioethanol from sugarcane in Brazil, Renewable Sustainable Energy Rev. 13 (2009) 1613e1619. [11] K.L. Kadam, Environmental benefits on a life cycle basis of using bagasse derived ethanol as a gasoline oxygenate in India, Energy Policy 30 (2002) 371e384. [12] M.A. Renouf, M.K. Wegener, L.K. Nielsen, An environmental life cycle assessment comparing Australian sugarcane with US corn and UK sugar beet as producers of sugars for fermentation, Biomass Bioenergy 32 (12) (December 2008) 1144e1155. [13] Carlos A. García, Alfredo Fuentes, Anna Hennecke, Enrique Riegelhaupt, Fabio Manzini, Omar Masera, Life-cycle greenhouse gas emissions and energy balances of sugarcane ethanol production in Mexico, Appl. Energy 88 (2011) 2088e2097. [14] M.A. Costagliola, L. De Simio, S. Iannaccone, M.V. Prati, Combustion efficiency and engine out emissions of a SI engine fueled with alcohol/gasoline blends, Renewable Sustainable Energy Rev. 24 (August 2013) 209e222. [15] W.D. Hsieh, R.H. Chen, T.L. Wu, T.H. Lin, Engine performance and pollutant emission of an SI engine using ethanol-gasoline blended fuels, Atmos. Environ. 36 (2002) 403e410. [16] M.A. Costagliola, L. De Simio, S. Iannaccone, M.V. Prati, Combustion efficiency and engine out emissions of a SI engine fueled with alcohol/gasoline blends, Appl. Energy (2013) 1162e1171. http://dx.doi.org/10.1016/j.apenergy.2012. 09.042. [17] Laxmi Prasad Thapa, Sang Jun Lee, Hah Young Yoo, Han Suk Choi, Chulhwan Park, SeungWook Kim, Development of glycerol-utilizing Escherichia coli strain for the production of bioethanol, Enzyme Microb. Technol. (2013) 206e215. http://dx.doi.org/10.1016/j.enzmictec.2013.04.009. [18] Stefan Martin, Antje Wörner, On-board reforming of biodiesel and bioethanol for high temperature PEM fuel cells: comparison of autothermal reforming and steam reforming, J. Power Sources 196 (6) (15 March 2011) 3163e3171. [19] A. Caravaca, A. de Lucas-Consuegra, A.B. Calcerrada, J. Lobato, J.L. Valverde, F. Dorado, From biomass to pure hydrogen: electrochemical reforming of bioethanol in a PEM electrolyser, Appl. Catal. B: Environ. 134e135 (2 May 2013) 302e309. [20] ANSYS-FLUENTÒ 14, Theory Guide, ANSYS Inc, November 2011. Release 14.0. [21] B.E. Launder, D.B. Spalding, Lectures in Mathematical Models of Turbulence, Academic Press, London, England, 1972. [22] J. Arturo Alfaro-Ayala, A. Gallegos-Muñoz, J.M. Riesco-Avila, A. CamposAmezcua, M.P. Flores-López, A. Mani-González, Analysis of the flow in the combustor-transition piece considering the variation in the fuel composition, J Therm. Sci. Eng. Appl. 3 (2) (2011). [23] B.J. McBride, M.J. Zehe, S. Gordon, NASA Glenn Coefficients for Calculating Thermodynamic Properties of Individual Species, NASA Glenn Research Center, Cleveland, OH, 2002. [24] H.K. Versteeg, W. Malalasekera, An Introduction to Computational Fluid Dynamics, the Finite Volume Method, Longman Scientific & Technical, Longman Group Limited, Longman House, Burnt Mill, Harlow, Essex CM20 2JE, England, 1995. [25] J.A. Alfaro-Ayala, A. Gallegos-Muñoz, J.M. Riesco-Àvila, F. Elizalde-Blancas, C. Mendoza-Covarrubias, Análisis de un ciclo de potencia utilizando diferentes combustibles, in: XVIII Congreso Internacional Anual de la SOMIM, A5e13, Salamanca, Gto, México, Septiembre 2012. [26] Pierre-Alexandre Glaude, René Fournet, Roda Bounaceur, Michel Molière, Adiabatic flame temperature from biofuels and fossil fuels and derived effect on NOx emissions, Fuel Process. Technol. 91 (2010) 229e235. [27] C. Muller, V. Michel, G. Scacchi, G.M. Côme, THERGAS: a computer program for the evaluation of thermochemical data of molecules and free radicals in the gas phase, J. Chim. Phys. 92 (1995) 1154e1178. [28] S.W. Benson, Thermochemical Kinetics, John Wiley, New York, 1976. [29] E.S. Domalski, E.D. Hearing, Estimation of the thermodynamic properties of CeHeNeOeSehalogen compounds at 298.15 K, J. Phys. Chem. Ref. Data 22 (1993) 805e1159. [30] A. Gallegos Muñoz, V. Ayala-Ramírez, J.A. Alfaro-Ayala, B.M. Toledo Acosta, Optimization of the transition piece applying genetic algorithms, Appl. Therm. Eng. 31 (2011) 3214e3225.
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Use of bioethanol in a gas turbine combustor J.A. Alfaro-Ayala a, A. Gallegos-Muñoz b, *, A.R. Uribe-Ramírez a, J.M. Belman-Flores b a
University of Guanajuato, Chemistry Engineering Department, Guanajuato, Gto., Mexico University of Guanajuato, Mechanical Engineering Department, Carr. Salamanca-Valle de Santiago km 3.5þ1.8, Com. de Palo Blanco, Salamanca, Guanajuato, Mexico b
h i g h l i g h t s We apply numerical and thermodynamic analysis to study the effect of the biofuel in gas turbine combustor. We apply different air distribution to check experimental measurements of NOx emissions using conventional fuels and biofuel. Two different conditions to study the power output, efficiency and Turbine Inlet Temperature (TIT), were applied.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 April 2013 Accepted 20 August 2013 Available online 29 August 2013
A study of a gas turbine combustor that considers two conventional fuels and one biofuel is presented. The kind of fuel supplied to the combustor can impact in the Turbine Inlet Temperature (TIT) provoking significant changes in the power output and efficiency. Moreover, it can cause some damage in the initial steps of the gas turbine due to the migration of the hot streak. Natural gas, Diesel and Bioethanol are considered in the combustor in order to compare the performance of the power plant. The use of biofuel in a gas turbine combustor presents some benefits; a) better behavior in the distribution of the TIT, b) slightly higher power output and c) less impact of NOx and CO2 emissions. The analysis was based in the Computational Fluid Dynamics (CFD) and thermodynamics. The results indicate that it is necessary to increase the mass flow rate of bioethanol to maintain the power output of the turbine, due to a significant reduction of the TIT was observed. On the other hand, the use of bioethanol permits an important reduction of NOx emissions when they are compared with the conventional fuels (natural gas or diesel). Also, a noble benefit is obtained due to the biofuel comes from biomass-derived material, resulting in a reduction of CO2 global warming. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Gas turbine Combustor Emissions Bioethanol
1. Introduction The evolution of the gas turbines have been oriented towards the continuous desire to obtain higher temperatures, greater power and minimal emissions to the environment, specially limiting CO2 and NOx emissions in power generation. However, just little information about experimental research has been done in gas turbines in order to study the performance and emissions using biofuels. Recently, in the year 2008, M. Moliere et al. [1] made experimental tests using bioethanol in a gas turbine Frame 6B located at Goa, India. They characterized the combustion on a wide range of naphtha/bioethanol blends (up to 95% of bio-ethanol). Giving as main results, that it is needed an increment of biofuel and the CO
* Corresponding author. Tel.: þ52 (464) 647 9940x2361; fax: þ52 (464) 647 9940x2311. E-mail addresses: [email protected] (J.A. Alfaro-Ayala), [email protected] (A. Gallegos-Muñoz). 1359-4311/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2013.08.025
and UHC (Unburned Hydrocarbons) emissions were to close zero. Also, supplying 100% bioethanol, the NOx was reduced to the half when they are compared with 100% naphtha fuel (principal fuel used in the India). In the years 2006 and 2007, M. Moliere et al. [2] made their experiments in a Gas turbine Frame 6B and Frame 7EA respectively, they supplied biodiesel in both gas turbine models and they found that it is possible to use this kind of biofuel due to there were neither any human accident nor thermo-mechanic fails on the components. In Brazil, the use of bioethanol for the combustion process has been achieved [3]. One of the two gas turbine GE LM6000 (43.5 MW) was adapted to work with bioethanol. The conversion included the replacement of injection nozzle, as well as installation of peripheral equipment such as receiving system, reservoirs, pumps, filters, etc. Each of them allowed receiving, storing and supplying bioethanol to the gas turbine. During the tests, the plant demonstrated significantly lower CO2 emissions and demonstrated
482
J.A. Alfaro-Ayala et al. / Applied Thermal Engineering 61 (2013) 481e490
Fig. 1. Gas turbine scheme.
a 30% reduction of NOx emissions within 150 h of power generation with bioethanol. A comparison between bioethanol, natural gas and diesel revealed that the use of bioethanol fuel releases lower levels of NOx without decreasing its power. When it is trying to change the fuel used in a gas turbine combustor to another different of the original one, it is necessary to put attention in all the equipment downstream. Simone Colantoni et al. [4] showed in their investigation the impact of burning syngas obtained from biomass, by means of pyrolysis, in terms of the performance of the machine, aerodynamic behavior and secondary flows in order to determine the reduction life of components caused by overheat generated due to the combustion of syngas. The analysis was carried out in a GE10-2 model where the results were compared with natural gas. They concluded that the increment of syngas flow impacts in the useful life of the components, mainly in the first step of the gas turbine, computed by the Larson & Miller methodology [5], it also provokes an increment of temperature of the cooling flow and an increment of the temperature exhaust, it means an increment of temperature of combustion gases through the vanes. In this work, the determination of the TIT, considering different fuels, provides important information in order to evaluate the overheating of components due to flow gases downstream. This last topic is usually studied in blades of the gas turbines in which they estimate the thermal-structural failures, Zdzislaw Mazur et al. [6] and A. Fields et al. [7]. Although this work does not focus on economic issues, it is important to mention that the operational economics of supplying some fuel or biofuel is complicated. Anthony Campell et al. [8] say that the economics of providing a fuel is stated by a complex coupling between customer’s economic, environmental concerns and uncertainties of global regulatory policies, and also, the alternative gas or liquid fuel should be profitable without sacrificing power output of the plant and emission characteristics. Goodger [9] says that among the biofuels, the bioethanol is one of the best for the use in gas turbines due to it is very easy to transport and storage. Also, it is important to mention that many works which involve bioethanol issues have been focused in the Life-cycle assessment [10e12], even in our country [13], in order to estimate greenhouse gas emissions (GHG). Applications in the automotive industry of gasolineebioethanol blends, is considered to see the advantage of
higher octane number, emissions and the supplied regardless of the fossil fuel [14e16]. Production process of bioethanol [17] and bioethanol reformer systems to produce hydrogen with sufficient quality to feed a PEM fuel cell system [18,19], have been studied. 2. Gas turbine description This study considers combustor, transition piece and ring connection, these components belong to a gas turbine of 50 MW power output, rotates at 3600 rpm, it has 17 compressor stages and 4 gas turbine stages. The compressor pressure ratio is 7 and airflow of 216 kg/s. This gas turbine can be operated with natural gas and diesel. This type of gas turbine has only one combustor (6.4 m high, 3 m diameter and it weighs about 34 tons) located vertically between the compressor and turbine, the transition piece has a shape like “U” and it joins the combustor with the ring connection of the turbine, this last component is located in the first step of vanes. The Fig. 1 shows a schematic arrangement of the gas turbine, the air supplied for the compressor is taken throw the annulus of the transition piece to the combustor chamber, the combustor chamber has three sections where the air is distributed, this air sections are called primary air, secondary air and dilution or tertiary air. The primary airflow through the diffuser generates a swirl, the secondary air passes through four inner rings, this air is used to avoid high temperature in the combustor wall and the dilution air comes into the combustor for six holes located around the combustor to decrease the high temperatures of the combustion gases, these gases are conveyed to the first step of the turbine blades. As it is well known, the combustion gases always have high temperature but they must not exceed 1273.15 K (1000 C) due to it can cause damage in the turbine components. A protection system, consisting of PLCs model PLC5-20, is installed in the control room of the power plant so that they can determine the TIT. This temperature is estimated according to the measurement of two parameters: the first is the Exhaust Gas Temperature (EGT) and the second is the Compressor Discharge Pressure (CDP). These two parameters are used in the next expression:
TITð CÞ ¼ 1:4079*EGTð CÞ þ 1:529*CDPðpsiÞ þ 90:4ð CÞ In the case of the TIT computed is higher than 1273.15 K (1000 C), the PLC sets off an alarm (Alarm 85-03), this indicates
J.A. Alfaro-Ayala et al. / Applied Thermal Engineering 61 (2013) 481e490
483
Fig. 2. Mesh for the combustor, transition piece and ring connection.
that the gas turbine is holding some damage due to the TIT is exceeding the “Alarm Temperature” (Talarm ¼ 1273.15 K). This alarm temperature will be considered as parameter of reference in the next results. Also, in the case of the TIT exceeds 1338.15 K (1065 C), the PLC sets off another alarm (Alarm 85-04), this indicate that the power plant is taken out of work to protect the components of the gas turbine and prevent further damage.
3. Numerical procedure The study was carried out in a steady state three-dimensional model of a vertical combustor, transition piece and the ring connection which fits to the first step of vanes, using FLUENTÒ software. The ke3 turbulence model and chemical equilibrium for the combustion process are applied [20]. The pressureevelocity coupling was solved using a SIMPLE approach and all equations by first-order upwind scheme. The standard model of turbulence is a semi-empirical model proposed by Launder and Spalding [21], based on the model of the transport equations for the turbulent kinetic energy (k) and the dissipation rate (3 ). This model is appropriate to study the turbulence in practical engineering flow due to its reasonable accuracy for a wide range of turbulent flows. The chemical equilibrium is based in the power of the mixture fraction modeling approach, where the chemistry is reduced to one or two conserved mixture fractions. Under the assumption, all thermo chemical scalars (species fractions, density, and temperature) are uniquely related to the mixture fraction (f ). It is generally acceptable for turbulent flows where turbulent convection overwhelms molecular diffusion. In addition to solve the problem, the equation for the mixture fraction variance ðf ’2 Þ is required. The mixture fraction variance is used in the closure model describing turbulence-chemistry interactions [22]. Equations for individual species are not solved. Instead, species concentrations are derived from the predicted mixture fraction fields. The thermochemistry calculations are preprocessed and then tabulated for look-up in ANSYS FLUENTÒ. Interaction of turbulence and chemistry is accounted for with an assumed-shape Probability Density Function (PDF). The specific heat, enthalpy and entropy of all species of the analysis can be expressed in polynomial form [23]. The polynomial coefficients are set into the database of FLUENTÒ using the convenient format. The NOx emissions are precursors of smog, contributes to the acid rain and cause damage to the ozone layer. The prediction of NOx emissions is solved by mean of the transport equation for the concentration. In order to generate a numerical model, it is necessary to represent the geometry in a computational domain. The geometry
Fig. 3. TIT vs million cells.
of the model and mesh were created using GAMBITÒ and the governing equations were transformed to algebraic equations inside computational domain [24]. Fig. 2 shows the structured grid used in combustor chambertransition piece and unstructured grid in the ring connection; this last was used due to the complex geometry. The uses of quadrilateral cells reduce the amount of finite volume elements needed for the grid independence (if these were compared with triangular cells). The quadrilateral cells provide stability during the numerical solution and reduce the solution time. The final mesh is about 6 millions of cells. 3.1. Mesh analysis Several models with different amount of cells were made in order to define the independent value of the variables: TIT, maximum temperature inside the combustor (Tmax) and NOx emissions. The Figs. 3 and 4 show the variable vs the amount of cell used in the model, from 1 million to 6 million of cells were analyzed, it can be seen that variables of TIT, Tmax and NOx tend to be uniform with the increment of the cells (independent of the amount of cells), the lower amount of cells have the higher value of the variable computed while the higher amount of cells have lower value of the variable computed numerically and the change of slope is in about 2 million of cells. It means that the tendency of the variable is a decrement with the increment of the cells until to be independent of the amount of cells. Particularly, the Fig. 3 shows the Talarm as reference to see the effect of the mesh in this variable; a coarse grid can led us to a wrong prediction of the TIT showing that the TIT is higher than the Talarm.
Fig. 4. Tmax and NOx vs million cells.
484
J.A. Alfaro-Ayala et al. / Applied Thermal Engineering 61 (2013) 481e490
3.2. Computational domain
Table 1 Operating conditions for modeling combustor-transition piece.
Fig. 5 shows the scheme of the combustor, including the input of the flows; primary, secondary (cooling 1e4), dilution air and the input of the fuel flow. It also includes the vertical combustor chamber, transition piece and ring connection. The primary air enters at the top of the combustor chamber with a swirl provoking the recirculation zone; it allows mixing the air with the fuel. The four cooling flows enter through the annulus to reduce the temperature on the wall of the super-alloy of the combustor and avoid thermo-mechanical damage. The dilution air is used to decrease the temperature of the hot gases generated for the combustion. The transition piece “U” leads the hot gases toward the first step of the gas turbine by mean of the ring connection. The chemistry (reactive and products) is modeled with the use of the chemical equilibrium model proposed by the code guide [20] and it considers 15 main species to model the different fuels according to the following: 3.2.1. Natural gas CH4, N2, O2, H2O, CO2, CO, H2, OH, H, O, HO2, H2O2, HONO, CH3OH, HCO. 3.2.2. Diesel C10H22, N2, O2, H2O, CO2, CO, H2, OH, H, O, HO2, H2O2, HONO, HCO, CHO. 3.2.3. Bioethanol C2H5OH, N2, O2, H2O, CO2, CO, H2, OH, H, O, HO2, H2O2, HONO, O3, HNO3.
Mass flows
Quantity
Units
Fuel Primary air Cooling 1 Cooling 2 Cooling 3 Cooling 4 Dilution air Total mass flow
3.6 116.52 19.57 19.57 19.57 19.57 21.2 219.6
[kg/s] [kg/s] [kg/s] [kg/s] [kg/s] [kg/s] [kg/s] [kg/s]
Pressures
Quantity
Units
Inlet pressure Outlet pressure Drop pressure DP
736,000 694,500 41,500
[Pa] [Pa] [Pa]
Temperatures
Quantity
Units
Air inlet temperature Fuel inlet temperature
550 353
[K] [K]
3.3. Boundary conditions Table 1 shows the airflow distribution; these were set to the computational model as boundary conditions. The modeling of the three different fuels was carried out as follow: It was considered that the total amount of air supplied for the compressor was kept in all the cases studied, the distribution of the air mass flow (primary, cooling and dilution air) was similar for all cases, the inlet temperature of the air and fuel (natural gas, diesel, bioethanol) were the same for all cases and the operational pressure was the same for all cases.
Fig. 5. Air flow distribution of the combustor chamber.
J.A. Alfaro-Ayala et al. / Applied Thermal Engineering 61 (2013) 481e490
4. Thermodynamic procedure
485
Table 3 Comparison of adiabatic flame temperature.
It was planned to have three cases for each conventional fuel and biofuel with the aim of determining the adiabatic flame temperature (Taf) by mean of the stoichiometric reactions. Determining the Turbine Inlet Temperature TIT by mean of the equal amount of fuel (equal mass flow rate of fuel at the inlet) and the TIT by mean of the equal amount of energy (equal amount of energy at the inlet). It was considered that the natural gas consists only of methane CH4, diesel only of decane C10H22 and bioethanol only alcohol C2H5OH. Also, the stoichiometric reactions considers a completed combustion with just CO2 and H2O as species of products and the N2 is obtained as inert. And the equal amount of fuel (EAF) and equal amount of energy (EAE) reactions consider that the CO is close to zero, it means that the entire CO is transformed to CO2 and the O2 specie is considered in the products due to the large amount of excessive air required for the combustion process. Details of the thermodynamics procedure can be found in Ref. [25]. 4.1. Stoichiometric Table 2 shows resume of the reaction considering stoichiometric, EAF and EAE. In this table can be seen the global stoichiometric reaction and its corresponding AireFuel ratio (AF) of the considered fuels. Also, the table shows that the stoichiometric AF ratio is different for each fuel, the highest stoichiometric AF is to the natural gas and it is followed for diesel and bioethanol. 4.2. Equal amount of fuel (EAF) The global chemical reactions with the equal amount of mass flow rate of fuel are shown in Table 2. The same amount of mass flow provokes to have different excessive air for each fuel. The lower is for natural gas 250% of excessive air, for the diesel 300% of excessive air and for the bioethanol 570% of excessive air. 4.3. Equal amount of energy (EAE) Table 2 shows the global chemical reactions with the equal amount of energy for the three fuels considered and its corresponding actual AF ratio. The highest actual AF is to the natural gas 60 and it is followed for diesel 52.7 and then bioethanol 31.42. 5. Results The adiabatic flame temperature (Taf) for each fuel was obtained using the stoichiometric reactions considered in the thermodynamic procedure using the sensible and formation enthalpies of the
Fuel
Taf [K]
Glaude et al. [26], Taf [K]
% Difference
Natural gas Diesel Bioethanol
2330 2399 2295
2227 2292 2240
4.42 4.46 2.39
reactive and products. It was observed that the higher adiabatic flame temperature is for diesel (2399 K), it is followed for the natural gas (2330 K) and bioethanol (2295 K). These results are similar as those obtained by Glaude et al. [26], they determined the adiabatic flame temperature to different fuels and biofuels by mean of the software THERGAS [27] considering the species like perfect gas and the Benson approach [28] and the contributions of Domalski et al. [29], their results showed that the diesel generally have higher temperature than the group of FAMEs (Fatty Acids Methil Ester, that correspond to the group of biofuels from rapeseed, RME, soybean, SME, sunflower, etc.), followed by the Naphthas and the last for the natural gas. This is attributed to the relation of H/C (hydrogen and carbon), it implies to have high or less combustion enthalpy. For the cases studied here, the H/C is 2.2, 4 and 3, for diesel, natural gas and bioethanol, respectively. Table 3 shows a comparison of the adiabatic flame temperature, some difference should be note, this work considers the species as ideal gas and four species as products while the investigation of Glaude et al. [26] considers the species as perfect gases, another components in the reactive and eleven species as products. However, the difference of Taf between the methods is for natural gas and diesel about 100 K and for bioethanol is about 50 K. Fig. 6 shows a comparison of TIT obtained by the thermodynamic and numerical analysis considering equal amount of fuel and energy. Under the equal amount of fuel (EAF), the TIT is higher for natural gas, followed by diesel and bioethanol. However keeping equal amount of energy (EAE), increasing the amount mass flow of fuel for diesel and bioethanol, the TIT is increased and it reaches TIT very close to the natural gas. The TIT is achieved successfully without exceeding the alarm temperature. This means that the first step of blades in the gas turbine does not have mechanical damage. Fig. 7 shows the TIT, power output and thermal efficiency. For the natural gas considering EAF, the TIT is about 1239.1 K, the power output is 45.87 MW and the efficiency is 27.35% with 250% excessive air. For the diesel, the TIT is about 1177.8 K, the power output is 40.621 MW and the efficiency is 26.74% with 300% excessive air. Finally, for the bioethanol, the TIT is about 937.7 K, the power output is 20.153 MW and the efficiency is 22.09% with 570% excessive air. The higher impact is observed when the biofuel is supplied. The TIT is very low provoking a reduction of the power
Table 2 Stoichiometric, EAF and EAE reactions. Fuel
Stoichiometric reaction
Stoichiometric AF ratio
Natural gas Diesel Bioethanol
CH4 þ 2(O2 þ 3.76N2) / CO2 þ 2H2O þ 7.52N2 C10H22 þ 15.5(O2 þ 3.76N2) / 10CO2 þ 11H2O þ 58.22N2 C2H5OH þ 3(O2 þ 3.76N2) / 2CO2 þ 3H2O þ 11.28N2
17.19 15.03 8.98
Fuel
Equal amount of fuel reactions
Excessive air [%]
Natural gas Diesel Bioethanol
CH4 þ 7(O2 þ 3.76N2) / CO2 þ 2H2O þ 5O2 þ 26.32N2 C10H22 þ 62(O2 þ 3.76N2) / 10CO2 þ 11H2O þ 46.5O2 þ 233.12N2 C2H5OH þ 20.1(O2 þ 3.76N2) / 2CO2 þ 3H2O þ 17.1O2 þ 75.576N2
250 300 570
Fuel
Equal amount of energy reactions
Actual AF ratio
Natural gas Diesel Bioethanol
CH4 þ 7(O2 þ 3.76N2) / CO2 þ 2H2O þ 5O2 þ 26.32N2 C10H22 þ 54.25(O2 þ 3.76N2) / 10CO2 þ 11H2O þ 38.75O2 þ 203.98N2 C2H5OH þ 10.5(O2 þ 3.76N2) / 2CO2 þ 3H2O þ 7.5O2 þ 39.48N2
60 52.7 31.42
486
J.A. Alfaro-Ayala et al. / Applied Thermal Engineering 61 (2013) 481e490
Fig. 6. Turbine Inlet Temperature for equal amount of fuel (EAF) and energy (EAE).
output; it is about of 50% when is compared with the natural gas (original fuel used for the gas turbine). The thermodynamics procedure to obtain the power and efficiency values were developed in previous work [25]. Supply equal amounts of diesel and bioethanol reduces TIT, and also it produces a diminution of power output and thermal efficiency this is due to minor available energy supplied. Then, an increase of fuel to keep equal amount of energy in order to close TIT, power and efficiency to the original operational condition, is considered. For the diesel considering the EAE, the TIT is about 1257.5 K, the power output is 47.648 MW and the efficiency is 27.57% with 250% excessive air. For the bioethanol, the TIT is about 1252.2 K, the power output is 48.527 MW and the efficiency is 27.95% with 250% excessive air. Using equal amount of energy, the TIT, power output and efficiency are even a slightly higher than the original
operational condition using natural gas. This slight increment is attributed to the increment of mass flow that is expanded in the gas turbine. Figs. 8 and 9 show a comparison of averaged molar fractions of CO2, CO and H2O at the inlet of the gas turbine considering EAF and EAE, respectively. It can be observed that the behavior of these species is very similar in the analytical and numerical results. Fig. 8 considers equal amount of fuel EAF thus the behavior of the CO2 is higher for the diesel; it is followed for natural gas and bioethanol. The H2O is higher for the natural gas; it is followed for diesel and bioethanol. It can be seen that the CO2 and the H2O for bioethanol is lower than natural gas and diesel. However, the behavior changes using equal amount of energy EAE (Fig. 9), the CO2 and H2O reach values higher than the natural gas, CO2 reaches values similar of diesel and H2O reaches values higher than diesel. Also the CO is very low, for all cases considering EAF and EAE. Fig. 10 shows the temperature contours considering EAF and EAE. It can be seen that the higher temperature is for the diesel (2382 K); it is followed for natural gas (2270 K) and bioethanol (1978 K) for the cases with EAF. For the cases with EAE the higher temperature is diesel (2362 K); it is followed for natural gas (2270 K) and bioethanol (2062 K). Also, it can be seen that the temperature is almost homogeneous at the inlet of the gas turbine for all cases. Fig. 11 shows the formation of NOx (ppm) for all cases with 14.5% O2, which correspond to standard value in real condition in gas turbine using natural gas. The formation of NOx was considered for the temperature (Thermal NOx). It can be seen that the formation of NOx for diesel is 2853 ppm; it is followed for natural gas with 941 ppm and bioethanol with 19.28 ppm when it is considered EAF. Moreover, the formation of NOx for the cases of EAE for diesel is 2233 ppm; it is followed for natural gas with 941 ppm and then bioethanol with 71.15 ppm. According to local standards, the bioethanol is clearly under the limit. According to results, the distribution of temperature and TIT obtained are acceptable, but the NOx are very high for the conventional fuels (natural gas and diesel). The hard link that exists
Fig. 7. TIT, power output and efficiency.
J.A. Alfaro-Ayala et al. / Applied Thermal Engineering 61 (2013) 481e490
487
Fig. 8. Molar fraction, EAF.
between temperature and NOx provokes a high increment of NOx with a few degrees of increment of temperature. The air distribution used to obtain the Figs. 10 and 11, it is under theoretical conditions to obtain the desired TIT (without passing the Talarm) for the natural gas, see Table 1. The air distribution used to obtain the Figs. 12 and 13, it is under actual operational conditions to compare the NOx obtained experimentally for the natural gas and between the other fuels. The air distributions used for Figs. 12 and 13 were: primary air is 29%, cooling 1e4 is 60% and dilution air is 11% (total air distribution 216 kg/s). Fig. 12 shows the contours of temperature considered the test condition for EAF and EAE. It can be seen that the higher temperature in the contour is for the diesel (2282 K); it is followed for natural gas (2119 K) and bioethanol (1978 K) for the cases with EAF. For the cases with EAE the higher temperature is diesel (2260 K); it is followed for natural gas (2119 K) and bioethanol (2062 K). The TIT using natural gas was 1275.7 K, diesel 1213.1 K and bioethanol 918 K for the cases of EAF. For the case with EAE the TIT using natural gas was 1275.7 K, diesel 1278.3 K and bioethanol 1249.5 K. Also, in the Fig. 12 the numerical results show that the diesel and natural gas revealed a hot streak that is formed and it migrates towards the inlet of the gas turbine. This impact in the nonuniformity of the TIT may cause damage in the components of the GT, the proof of it was found during the inspection of the GT and
the history of the maintenance during the last decade, the recurrent change of the blades of the first and second steps were observed. Moreover, in the in the cases of the Fig. 12 was observed that the TIT exceeds the Talarm. Also, it can be seen that the maximum temperature is not as high as in Fig. 10, thus the reduction of high temperature inside the combustor makes to decrease the NOx emissions (see Fig. 13). A further work will be to optimize some parameters such as the distribution of air by mean of the geometry [30] in order to decrease the high temperature in hot streak and consequently the NOx emissions. However, a similar behavior was found in the simulation of the combustor chamber using diesel while the use of bioethanol in the simulation does not show migration of the hot streak and the maximum temperatures are lower than natural gas or diesel, thus the problem of migration of hot streak is voided. Consequently, the use of bioethanol produces lower temperature in the hot streak thus this provokes that the NOx emissions decreases. The TIT of numerical cases for biofuel is always near the limit of the Talarm. This means that the power output and efficiency will be kept as the original conditions (using natural gas) and lower emissions of NOx. The experimental measurements were realized to the unit 2 of the power plant using natural gas during the year 2009. The results were: the NOx emissions about 188 [email protected]%O2, the excessive air was 271.7% and the CO was 0 ppm, while the numerical results
Fig. 9. Molar fraction, EAE.
488
J.A. Alfaro-Ayala et al. / Applied Thermal Engineering 61 (2013) 481e490
Fig. 10. Contours of temperature [K].
Fig. 11. Contours of NOx @14.5% O2 [ppm].
J.A. Alfaro-Ayala et al. / Applied Thermal Engineering 61 (2013) 481e490
Fig. 12. Contours of temperature [K].
Fig. 13. Contours of [email protected]% O2 [ppm].
489
490
J.A. Alfaro-Ayala et al. / Applied Thermal Engineering 61 (2013) 481e490
were: the NOx emission 170 [email protected]%O2, the excessive air was 250% and the CO was 0 ppm (see Fig. 13). The difference between the experimental measurements and the numerical results are close, 18 ppm of NOx, 21.7% of excessive air and 0 ppm of CO2.
6. Conclusions The results show that the condition equal amount of fuel (EAF) cannot keep the original performance (natural gas) of the power plant for diesel and bioethanol and there is not hot streak migration problem for all cases but it appears NOx emission problems due to the high temperatures. Then in order to have similar power output, TIT and thermal efficiency of the power plant for all fuels considered in this work, an increment of diesel and bioethanol was required, that leads us to the second condition, equal amount of energy (EAE), it was achieved the performance with low NOx but now the hot streak migration problem appears. With these results, we consider that some optimization techniques can be implemented to the analysis in order to have a balance between performance (Power output, TIT and Efficiency) and hot streaks and NOx emissions problems. The three fuels modeled with the EAE condition can satisfy the TIT and the output power but the main difference founded it was in the CO2 emissions. The bioethanol and diesel produce similar amount of CO2 and higher than natural gas (about 19.5% more), however, the main difference is the native material where the fuel was obtained, the biofuel is obtained from biomass-derived material (sugar cane or corn) and the natural gas and diesel are obtained from fossil material (petroleum). This implies that the CO2 that is formed by the combustion of bioethanol, it comes into the life cycle decreasing global warming. Also, the advantage of using biofuel is the less formation of thermal NOx, this is due to the temperature in the combustor chamber is lower with bioethanol than natural gas or diesel. The reduction of NOx using bioethanol is about 65% and 92% when it is compared with natural gas and diesel at 15.7% O2, respectively. Moreover, the biofuel shows less problematic behavior in the sense of the migration of hot streak to the gas turbine, this makes to decrease the problem of the non-uniformity of the TIT and it would probably have less impact in thermo-mechanical damage in the first step of vanes and blades of the gas turbine. References [1] M. Moliere, M. Vierling, M. Aboujaib, P. Patil, A. Eranki, A. Campbell, R. Trivedi, A. Nainani, S. Roy, N. Pandey, Gas Turbine in Alternative Fuel Application: Bioethanol Field Test, ASME Turbo Expo, Orlando, Florida, 2009. [2] M. Moliere, E. Panarotto, M. Aboujaib, J.M. Bisseaud, A. Campbell, J. Citeno, P.A. Maire, L. Ducrest, Gas Turbine in Alternative Fuel Application: Bio-diesel Field Test, ASME Turbo Expo, Montreal, Canada, 2007. [3] Kris Bevill: http://www.ethanolproducer.com/articles/7031/ge-powersturbines-with-ethanol-in-brazil/ and http://www.power-technology.com/ projects/ethanol-power-plant/, 2010. [4] Simone Colantoni, Stefania Della Gatta, Roberto De Prosperis, Alessandro Russo, Francesco Fantozzi, Umberto Desideri, Gas Turbines Fired with Biomass Pyrolysis Syngas: Analysis of the Overheating of Hot Gas Path Components, ASME Turbo Expo, Orlando, Florida, 2009. [5] F.R. Larson, J. Miller, A timeetemperature relationship for rupture and creep stress, Trans. ASME 74 (1952) 765e775.
[6] Z. Mazur, A. Luna-Ramírez, J.A. Juárez-Islas, A. Campos-Amezcua, Failure analysis of a gas turbine blade made of Inconel 738LC alloy, J. Eng. Fail. Anal. 12 (2005) 474e486. [7] Meherwan P. Boyce, Gas Turbine Engineering Handbook, Gulf Professional Publishing, 2002. [8] Anthony Campell, Jeffrey Goldmeer, Tim Healy, Roy Washam, Michel Molière, Joseph Citeno, Heavy duty gas turbines fuel flexibility, in: Proceeding of ASME Turbo Expo Berlin, Germany, 2008. [9] E.M. Goodger, Transport Fuel Technology, Landfall Press, Norwich, UK, 2000. [10] Lin Luo, Ester van der Voet, Gjalt Huppes, Life cycle assessment and life cycle costing of bioethanol from sugarcane in Brazil, Renewable Sustainable Energy Rev. 13 (2009) 1613e1619. [11] K.L. Kadam, Environmental benefits on a life cycle basis of using bagasse derived ethanol as a gasoline oxygenate in India, Energy Policy 30 (2002) 371e384. [12] M.A. Renouf, M.K. Wegener, L.K. Nielsen, An environmental life cycle assessment comparing Australian sugarcane with US corn and UK sugar beet as producers of sugars for fermentation, Biomass Bioenergy 32 (12) (December 2008) 1144e1155. [13] Carlos A. García, Alfredo Fuentes, Anna Hennecke, Enrique Riegelhaupt, Fabio Manzini, Omar Masera, Life-cycle greenhouse gas emissions and energy balances of sugarcane ethanol production in Mexico, Appl. Energy 88 (2011) 2088e2097. [14] M.A. Costagliola, L. De Simio, S. Iannaccone, M.V. Prati, Combustion efficiency and engine out emissions of a SI engine fueled with alcohol/gasoline blends, Renewable Sustainable Energy Rev. 24 (August 2013) 209e222. [15] W.D. Hsieh, R.H. Chen, T.L. Wu, T.H. Lin, Engine performance and pollutant emission of an SI engine using ethanol-gasoline blended fuels, Atmos. Environ. 36 (2002) 403e410. [16] M.A. Costagliola, L. De Simio, S. Iannaccone, M.V. Prati, Combustion efficiency and engine out emissions of a SI engine fueled with alcohol/gasoline blends, Appl. Energy (2013) 1162e1171. http://dx.doi.org/10.1016/j.apenergy.2012. 09.042. [17] Laxmi Prasad Thapa, Sang Jun Lee, Hah Young Yoo, Han Suk Choi, Chulhwan Park, SeungWook Kim, Development of glycerol-utilizing Escherichia coli strain for the production of bioethanol, Enzyme Microb. Technol. (2013) 206e215. http://dx.doi.org/10.1016/j.enzmictec.2013.04.009. [18] Stefan Martin, Antje Wörner, On-board reforming of biodiesel and bioethanol for high temperature PEM fuel cells: comparison of autothermal reforming and steam reforming, J. Power Sources 196 (6) (15 March 2011) 3163e3171. [19] A. Caravaca, A. de Lucas-Consuegra, A.B. Calcerrada, J. Lobato, J.L. Valverde, F. Dorado, From biomass to pure hydrogen: electrochemical reforming of bioethanol in a PEM electrolyser, Appl. Catal. B: Environ. 134e135 (2 May 2013) 302e309. [20] ANSYS-FLUENTÒ 14, Theory Guide, ANSYS Inc, November 2011. Release 14.0. [21] B.E. Launder, D.B. Spalding, Lectures in Mathematical Models of Turbulence, Academic Press, London, England, 1972. [22] J. Arturo Alfaro-Ayala, A. Gallegos-Muñoz, J.M. Riesco-Avila, A. CamposAmezcua, M.P. Flores-López, A. Mani-González, Analysis of the flow in the combustor-transition piece considering the variation in the fuel composition, J Therm. Sci. Eng. Appl. 3 (2) (2011). [23] B.J. McBride, M.J. Zehe, S. Gordon, NASA Glenn Coefficients for Calculating Thermodynamic Properties of Individual Species, NASA Glenn Research Center, Cleveland, OH, 2002. [24] H.K. Versteeg, W. Malalasekera, An Introduction to Computational Fluid Dynamics, the Finite Volume Method, Longman Scientific & Technical, Longman Group Limited, Longman House, Burnt Mill, Harlow, Essex CM20 2JE, England, 1995. [25] J.A. Alfaro-Ayala, A. Gallegos-Muñoz, J.M. Riesco-Àvila, F. Elizalde-Blancas, C. Mendoza-Covarrubias, Análisis de un ciclo de potencia utilizando diferentes combustibles, in: XVIII Congreso Internacional Anual de la SOMIM, A5e13, Salamanca, Gto, México, Septiembre 2012. [26] Pierre-Alexandre Glaude, René Fournet, Roda Bounaceur, Michel Molière, Adiabatic flame temperature from biofuels and fossil fuels and derived effect on NOx emissions, Fuel Process. Technol. 91 (2010) 229e235. [27] C. Muller, V. Michel, G. Scacchi, G.M. Côme, THERGAS: a computer program for the evaluation of thermochemical data of molecules and free radicals in the gas phase, J. Chim. Phys. 92 (1995) 1154e1178. [28] S.W. Benson, Thermochemical Kinetics, John Wiley, New York, 1976. [29] E.S. Domalski, E.D. Hearing, Estimation of the thermodynamic properties of CeHeNeOeSehalogen compounds at 298.15 K, J. Phys. Chem. Ref. Data 22 (1993) 805e1159. [30] A. Gallegos Muñoz, V. Ayala-Ramírez, J.A. Alfaro-Ayala, B.M. Toledo Acosta, Optimization of the transition piece applying genetic algorithms, Appl. Therm. Eng. 31 (2011) 3214e3225.