Energy Conversion and Management 51 (2010) 498–504

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Energy Conversion and Management

journal homepage: www.elsevier .com/locate /enconman

Burning clean fuel gas improves energetic efficiency

André Fonseca a, Manuel L.C. Tavares b, Luísa A.C.N. Gomes a,*

a Chemical Engineering Department, Instituto Superior de Engenharia do Porto, Rua Dr. Antonio Bernardino de Almeida 431, P4200-072 Porto, Portugalb Galp Energia, Área de Tecnologia, Apartado 3015, 4451-852 Leça da Palmeira, Portugal

a r t i c l e i n f o a b s t r a c t

Article history:Received 13 June 2008Received in revised form 22 March 2009Accepted 22 October 2009Available online 20 November 2009

Keywords:CogenerationFuel gasLinear programmingGas turbinesClean fuels

0196-8904/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.enconman.2009.10.014

* Corresponding author. Tel.: +351 228340500; faxE-mail address: mln@isep.ipp.pt (L.A.C.N. Gomes).

One the most critical problems faced by refineries nowadays is the continuous increasing of legislationurging emissions reductions specifically SO2, NOx, and particles. Therefore there is a substantial needfor refineries to burn fuel gas instead of fuel oil and avoiding, if possible, the use of imported naturalgas. The refinery case study presents a substantial excess of fuel gas resulting from the production ofhydrogen to obtain clean flues with low sulphurs.

The aim of this paper is to optimize the use of the hydrogen excess with the implementation of a gasturbine with heat recovery with a feed near 28–31% of hydrogen.

The cogeneration system was modelled by GateCycle 5.34.0.r. and the results obtained for the simula-tion were considered optimistic. Considering a production of 13 MW of electrical power the overall effi-ciency reached a value of 76% and 22 t/h of vapour (17 MW) from a feed of 3 t/h of fuel gas (39 MW).

These results allow a higher electrical power production and a consequent reduction in the emissionsof SO2 and CO2.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

The topic of global warming inspires debates among worldleaders, industry representatives and environmentalists. Whilethere is a strong consensus in the scientific community that thegreenhouse effect is a real phenomenon, and that the humans playa fundamental role in that, much remains unknown about thelong-term consequences of anthropogenic activity on the climate.Therefore, the need to produce better and cleaner fuels has becomeone the most concerns of the petrochemical industries.

It is well known that in the refinery process the energy costs isthe second largest cost component after crude and intermediateproducts. Among all kinds of consumed energy sources, fuel gas,which is continuously generated during the production process,contributes as a primary energy source to the energy needs ofthe refinery [1].

Zhang and Rong [1] refer that fuel gas can be converted intoother forms of energy, such as steam, electricity and heat. Optimalscheduling of fuel gas system helps the refinery to achieve energycost reduction and cleaner production.

All improvements made to process, their constituent unitoperations, and their interactions to maximize the effective useof energy, water and raw materials define the term ‘‘Process Inte-gration” (PI). The document published by Varennes (QC) ResearchCentre [2] refers to the analysis and optimization of large and

ll rights reserved.

: +351 228321159.

complex industrial processes. Process Integration, combined withtools as process simulation, is a powerful approach that allowsengineers to systematically analyse an industrial process andthe interactions between the various parts [3]. It is an emergingarea, which offers the promise of improved control and manage-ment of operating efficiencies, energy use, environmental im-pacts, capital effectiveness, process design, and operationsmanagement.

Portugal has a serious dependence in imported fuels, especiallyoils, which has seriously hindered its industrial development. Assuch it is essential for the Portuguese industry to make good useof the available updated technologies for the revamping of indus-trial sites that will lead to better energy efficiency.

One of the most important techniques used to identify energyoptimization opportunities is pinch technology that emerged onthe 1980’s decade as an area in Chemical Engineering in the fieldof Process Integration. Pinch technology presents a simple method-ology for systematically analysing chemical processes and the sur-rounding utility. Based in this technology and resorting to linearprogramming Fonseca et al. [4] designed an efficient hydrogen dis-tribution network to Porto Refinery. The conclusions showed asubstantial excess of fuel gas.

The work mentioned above gave support to the continuity ofthe study. Thus, the enterprise suggested the use of the fuel gasto implement the actual cogeneration, achieving the demand elec-trical energy needed to replace a turbo group (steam turbine)which has a low efficiency, and with the increasing of the steamproduction.

Nomenclature

DG duty from the burning of fuel gasDO duty from the burning of fuel oilDT minimum temperature differenceFG fuel gas flowrateFO fuel oil flowrate

O objective functionSO2,G SO2 emitted from burning fuel gasSO2,O SO2 emitted from burning fuel oil

A. Fonseca et al. / Energy Conversion and Management 51 (2010) 498–504 499

As fuel gas is continuously generated during the productionprocess, it is one of the most important energy sources and it is alsoreadily available. The use of fuel gas presents a cheaper alternativeto natural gas and provides more flexibility to the system. The useof both gases simultaneous may respond to higher plant demandsaccording to its needs, since most of the gas turbines operate withboth fuels. Moreover the refinery may achieve greater refinery pro-cess efficiencies, as a result of greater and more flexible availabilityof high-pressure steam from the cogeneration facility.

Cogeneration is the simultaneous generation of heat and power(usually electricity) in a single process. It takes in a range of tech-nologies, but will always include a power generator and a heatrecovery system. Cogeneration is also known as a combined heatand power (CHP) – a proven and reliable technology with a historyof more than 100 years, which was utilized mainly in large-scalecentralized power plants and industrial applications.

Cogeneration usually displaces boiler plant and electricity-onlyplant using a range of fuels and technologies and typically [5,6]achieves a 25–35% reduction in primary energy usage comparedwith electricity-only generation and heat-only boilers. Traditionalgas turbine cogeneration systems reach 65–87% global efficiencyfrom which 25–42% is obtained in electrical power whereas com-bined systems reach as high as 90% global efficiency [5].

Cogeneration systems are designed to respond to thermal and/or electromechanical needs. To respond to a thermal need the sys-tem is designed to produce as requires for each period of time,ensuring that the heat is the main product and therefore electricalpower comes as a sub-product of the system and vice versa. Aneconomical system should produce electrical power during thehigh consumption periods where the electrical charges are highand sell the excess production to generate an income. Thereforethe system must have auxiliary equipment in order to satisfy partor total heat demand, depending on the operation conditions of thecogeneration system [5,6].

A major strength of cogeneration is its environmental benefits,through the highly efficient use of energy [6]. The consequent lowemissions per unit output can be used to help Europe meet itsemissions targets, as proposed within the Kyoto Protocol. Theobjectives for the European Member States drive to decrease by8% the greenhouse gas emissions in 2010 compared to the 1990 le-vel [7]. The future of cogeneration is based on environmental ben-efits of increased cogeneration avoiding for CO2 emissions and thepotential benefit to Europe in achieving emissions reduction tar-gets. In long term, the potential exists for a significant impact onboth energy efficiency and environmental protection.

The Portuguese Government, in an effort to enables the expo-nential growth in greenhouse gas emissions due to rising electric-ity consumption and transportation, has been aggressivelypromoting renewable energies and energy efficiency policies [8].The Government’s Climate Change Programme under the KyotoProtocol indicates the need to develop an extra 800 MW of electri-cal capacity by 2010, thus reaching a total of 1800 MWe, inPortugal.

Improvement efforts focus on reducing emissions, improvingefficiency and lowering costs without sacrificing reliability like re-

fers Richards et al. [9]. These authors considered that the advancesare occurring on all three fronts, but this is usually achieved on asingle composition fuel, and then there is a need to consider fuelcomposition and fuel property variables on power generationsystems.

Cogeneration has long been deployed in energy intensive indus-tries that have large concurrent heat and power demands. Themost commonly used system for these applications was tradition-ally the steam power generating cycle, using steam turbines whichallowed exhausted steam to be used for process heating.

In recent years cogeneration has become an attractive and prac-tical technology for a wide range of applications. These include theprocess industries and both commercial and public sector build-ings. New micro-scale cogeneration technologies are emerging thatstand [8] to revolutionise the European power sector for bringinglow-environmental impact generation to the level of individualhouseholds. An interesting work of Wu and Wang [10] make a re-view about combined, cooling, heating and power (CCHP). In thiswork a worldwide status quo of CCHP development is presented.The authors concluded that, within decades, promising CCHP tech-nologies can flourish with the cooperative efforts of governments,energy-related enterprises and professional associations.

Cogeneration also provides new opportunities for co-operationleveraging advantages to improve competitiveness. In order to im-prove the competitiveness of CHP systems in liberalised electricmarkets, efficient tools for optimization of the unit commitmentand dispatch for CHP systems in such markets have to be devel-oped. The operating strategy will be conceived for reducing theoverall energy cost while meeting well-balanced heat and electricenergy requirements of the industry. A simple example would in-volve two adjacent industrial facilities, one a heavy power user,the other heavy steam user. Using a joint cogeneration plant bothneeds are met, dramatically reduce their respective utility costs.

Zhang and Rong [1] make a state of art of the work on the opti-mal scheduling of the fuel gas system in refinery. Few works havebeen reported in this theme. Among these works Frangopouloset al. [11] presented a method for the economic operation optimi-zation of a refinery combined-cycle cogeneration system. Using thevarious energy sources these authors formulated an energy systemmodel. In other work a mixed integer linear programming (MILP)model for multi-period operation was proposed [1]. This modelconsiders the storage ability of fuel gas system and the schedulingof fuel gas both in cogeneration system and production system.

The aim of this paper is to maximize the use of hydrogen excesswith the implementation of a cogeneration system, using the Sol-ver Add-in in Microsoft Excel

�. Thus, it is possible to increase elec-

trical power, as well as steam production with reduced overall SO2

emissions in the furnaces.

2. Cogeneration options

A range of technologies can be applied to cogenerate electricityand heat. All cogeneration schemes will always include an electric-ity generator and a system to recover the heat. The following tech-

Fig. 2. Gas turbine with heat recovery boiler (adapted from CHP Technologies:http://www.epa.gov/chp/basic/catalog.html, access in 2009 March 16th).

500 A. Fonseca et al. / Energy Conversion and Management 51 (2010) 498–504

nologies are currently in widespread use: steam turbines, gas tur-bines, combined cycle (gas and steam turbines), diesel and Ottoengines.

The basic elements of a cogeneration plant comprise one ormore prime movers (a reciprocating engine, gas turbine, or steamturbine) driving electrical generators, or other machinery, wherethe steam or hot water generated in the process is used via suitableheat recovery equipment.

This paper presents a gas turbine system with heat recovery forproduction of both electrical power, and heat recovery to use in thecase study process.

Gas turbines either in a simple cycle or in a combined cycle [12–14] are the most frequently used technology in recent cogenerationsystems of medium to high power. Their electric power outputranges from a few hundred kilowatts to several hundred mega-watts. On the other side of the spectrum, recent research anddevelopment aims at the construction of micro turbines, whichhave a power output of a few kilowatts.

Most of the currently gas turbine systems in any sector of appli-cations operate on the open Brayton process (or Joule cycle) that isa compact power generation system such as those in the reviewpapers of Najjar [12] and Pollikkas [13]. The main parts of this sys-tem, as shown in Fig. 1, are the compressor, combustion chamberand expander (turbine). In the compressor, the air is compressed.Fuel burns with compressed air in the combustion chamber. Thehigh temperature combustion gas/air mixture expands in the tur-bine, which drives the generator. The gas/air mixture leaves theturbine at a high temperature (350–600 �C).

2.1. Gas turbine with heat recovery boiler

The economical decisions for the implementation of a gas tur-bine in industrial process usually depend on the effective use ofthe exhaust gas energy, which represent 60–70% of the feed en-ergy. The gases from the turbine are usually at a temperature be-tween 350 and 650 �C, depending on the turbine, which then gothrough four stages inside the boiler [15–17].

The global efficiency of the gas turbine with heat recovery sys-tem is the ratio between absolute power produced (electrical ormechanical plus heat) and the power of the fuel, and it is a functionof the amount of the energy recovered by the exhaust gas of theturbine. The most commonly use of this energy is to the generationof vapour in heat recovery boilers as shown in Fig. 2.

2.2. Heat recovery boilers

Heat recovery boilers come in two different types, with or with-out supplemental burn of fuel. The latter is the simplest and it isbasically made up by several heat exchangers to maximize the heattransfer of the exhaust gas. These units can be designed to achieve

Fig. 1. Schematic diagram of gas turbine system.

approximately 95% recovery of the energy released from the tur-bine to use in a steam generation unit.

In the supplemental burning type, the remaining oxygen in theexhaust gas can be used, allowing for an increment of steam pro-duction or for a higher degree of superheated steam in relationto the former type. Further, it presents a higher flexibility in termsof controlling vapour produced and it allows for an independentoperation of the turbine gas associated to it.

2.3. Wobbe Index

Gas turbines in simple-cycle mode have long been used by util-ities for limited peak power generation. In addition, industrialfacilities use gas turbine units for on-site power generation, usuallyin combination with process heat production, such as, hot waterand process steam. The greater availability of fuel resources, thesignificant reduction in capital costs and the introduction of ad-vanced cycles, have been a success factor for the increased use ofgas turbines for base load applications [13].

Gas turbines are designed to operate within certain parameters.The variation in fuel composition may adversely affect engine per-formance. This effect includes misfire, or stumble, as well as engineknock and overheating depending on the engine’s ability to toler-ate or compensate for the variation in fuel composition.

One way to measure the gas quality is the Wobbe Index as pre-sented in the work of Driftmeier [18]. The Wobbe Index is a mea-sure of the fuel interchangeability with respect to its energycontent and metered air/fuel ratio. The Wobbe Index is calculatedfrom the energy content, or higher heating value of the gas, and therelative density of the gas. It is the best indicator of the similaritybetween a specific natural gas and propane–air mixture.

If the flow of the fuel across a burner orifice varies then the en-ergy produced during combustion will also change. The Wobbe In-dex is defined as the amount of energy introduced to the burners. Iftwo different kinds of fuel have the same Wobbe Index, then for acertain pressure in the burner, the energy released will be thesame. They will also produce the same combustion products andwill require the same amount of air for the combustion. Gas tur-bines that are designed to operate at a certain Wobbe Index andburns fuels with different index will result in a decreasedperformance.

The Wobbe Index relates fuel gas heating characteristics in amanner that is useful for blending fuel gases, or to obtain a con-stant heat flow from a gas of varying composition and it is possibleto achieve the same Wobbe Index for different fuel gases.

Fig. 3. Distribution of fuel gas and fuel oil in boilers of the case study.

A. Fonseca et al. / Energy Conversion and Management 51 (2010) 498–504 501

3. Case study

The steam produced and available at Porto Refinery comes fromthe boilers which burn fuel oil and fuel gas. Fig. 3 shows the fueldistribution along the boilers and the amount of steam produced,where the yellow and red lines represent fuel gas and fuel oil,respectively. The steam main collector is located above the boilers.The steam is then routed to the consumers including the turbogroups (steam turbines) to produce electrical energy and low pres-sure steam if necessary. Analysing the overall performance of theturbo groups it is possible to verify that Turbo Group 2 reaches avalue of 30% of the electrical power from the amount of the steamfeed (low effectiveness). Consequently the first aim of the presentwork is to implement a gas turbine with heat recovery boiler in or-der to complement the Turbo Group 2, and still achieve high-pres-sure steam.

Table 1Values of tail gas composition of the purification unit and correspondent WobbeIndex range.

Wobbe Index (MJ/m3) Wobbe Index (MJ/m3)

Lower Upper Lower Upper

41.2 45.2 35.1 38.6H2%V 28.6 H2%V 31.3C1%V 23.4 C1%V 29.7C2%V 22.1 C2%V 18.3C3%V 13.6 C3%V 14.1C4%V 6.1 C4%V 4.9C5%V 4.1 C5%V 1.2

3.1. Data analysis

In order to obtain a constant value of heat power feed to the gasturbine, a single source of fuel gas will be used, to avoid adjust-ments of parameter to provide a good performance of the turbinesuch as the one mentioned, Wobbe Index. This gas is a sub-productof a purification unit (Pressure Swing Adsorption) and it has a stea-dy heat power derived from a 28.6% to 31.3% of hydrogen in themixture, 2.8 t/h. Table 1 shows the Wobbe Index of the gas at thelower and higher purity.

C6+%V 2.1 C6+%V 0.5100.0 100.0

Table 2Revenue values of the gas turbine simulation.

Gas turbine

Electrical power 12.9 MWEfficience 33.2%Generator efficience 97.2%

3.2. GateCycle simulator

A cogeneration system using a gas turbine and a heat recoveryboiler were designed in order to obtain a simulation for the desiredsystem. The simulator used was GateCycle 5.34.0.r of the GE EnterSoftware (GateCycle Software Website, 2004). This software allowsmodelling, simulating and predicting the performance of severalenergy systems, such as the cogeneration systems [3]. Values suchas fuel gas composition, heat power and flowrate (2.8 t/h) wereintroduced in the design model. Siemens SGT-400 design values

were used in the present model in order to obtain a productionof 13.4 MW (approximately the actual production of the TurboGroup 2).

4. Results and discussion

GateCycle simulation results return 33.2% efficiency in the gasturbine as shown in Table 2. The true simulation is in the boilerparameters, such as, water temperature inlet and the minimumtemperature difference achieved by it (the ratio of outlet heat tem-perate and the outlet cold temperature) in order to obtain a varia-tion of produced vapour in function of both parameters.

The results obtained by simulation are shown both in Fig. 4 andFig. 5, where in the first one there is a linear variation of steam pro-

Fig. 4. Simulated values for the steam flowrate production at different values ofminimum temperature difference.

Fig. 5. Simulated values for the steam flowrate production at different values ofwater inlet temperature.

502 A. Fonseca et al. / Energy Conversion and Management 51 (2010) 498–504

duction with the minimum temperature difference. In the secondone an exponential variation is obtained showing an increase ofsteam production along with the water temperature inlet.

Fig. 6. Cogeneration model obt

Through the analysis of the results given by the simulator it isnow possible to represent a cogeneration model. Fig. 6 shows aproduction of 12.90 MW of electrical power and it is designed tooperate at a 450 K water inlet temperature and a minimum tem-perature difference considered by the model.

5. Linear programming (LP)

The application of linear programming in this study, as alreadyused by the authors [4] aims to achieve minimum SO2 emissionsderived from burning fuel oil and fuel gas where fuel gas wasneeded before it was sent to the gas turbine. The linear program-ming will also be a valuable tool because it will also give us theinformation how both fuel gas and fuel oil must now be conductedto the consuming units. Fig. 7 shows how these connections will beobtained. Both, fuel gas and fuel oil are considered sources in thelinear programming system where the duty of the furnaces andthe new gas turbine will be the sinks.

5.1. Sinks restrictions

The duty needed for each furnace must equal the amount ofduty given by both fuel gas and fuel oil feed to that furnace.

DUTY P FG � DG þ FO � DO ð1Þ

The minimum SO2 (1700 mg/L, limit value of the actual legisla-tion) must be equal or above the one emitted by the both fuelsburned in the furnace

SO2;minimum PXn

i

SO2;GiþXn

i

SO2;Oið2Þ

Note that for the gas turbine only fuel gas will be used and theduty is a know parameter. Also, some furnaces can burn both fuels.

ained from the simulator.

Fig. 7. Representative table of the LP case.

A. Fonseca et al. / Energy Conversion and Management 51 (2010) 498–504 503

5.2. Sources restrictions

The amount of fuel gas available must be equal or above the oneconsumed in the system (furnaces and gas turbine), and the sameconsiderations for fuel oil.

FG PXn

i

FGið3Þ

FO PXn

i

FOið4Þ

5.3. Objective function

It is possible to define several objective functions depending onthe desired final result, whether to minimize the fuel consumptionor to minimize the SO2 emissions. This study was performed toachieve a fuel distribution in order to accomplish the specificationsrequired by the furnaces and the gas turbine and enabling theoperation inside the legislation relative to SO2 emissions. Thereforethe desired objective function, O, will be to minimize the SO2

emissions.

O ¼minXn

i

SO2Emittedið5Þ

Table 3Economical balance of the cogeneration system (prices values presented in thesebalances are referred to the year of 2007).

Fuel oil (€/t) 167.50CO2Emitted (€/t) 15.00Fuel oil Savings (t/h) 0.870CO2Not emitted(t/h) 2.784FuelSavings in the boilers (t/h) 0.32

SavingsFuel oil (€/year) 1, 276, 551.00CO2 (€/year) 365, 817.60

Cogeneration revenueSteamProduced (€/year) 1, 720, 657.04Electrical powerProduced (€/year) 2, 803.20Total savings (€/year) 3, 365, 828.84

Investment (€) 12, 000, 000Return (years) 3.57

Table 4Conversion factors (prices values presented in these balances are referred to the yearof 2007).

Produces (t) Cost (€)

1 ton of fuel oil 3.2 de CO2 –1 kWe – 0.11 ton of steamproduction – 6.4

6. Economic evaluation

Reporting to a refinery study where the investment in a cogen-eration system was of 12 M€, it was possible to reach the followingeconomic evaluation presented in Table 3. Table 4 shows someconversion factors in order to achieve the economical balance.Prices values presented in these balances are referred to the yearof 2007.

7. Conclusions

GateCycle simulator allowed obtaining optimistic values to thecogeneration system required. The gas turbine presented an effi-ciency of 33.2% with a production of 12.9 MW and the boilerachieved a vapour production of 21.6 t/h reaching an overall sys-tem efficiency of 76%.

It was possible to save 0.9 t/h of fuel oil achieving a higher flex-ibility in the fuel distribution network. With this fuel oil consump-tion reduction one can save to 1.2 M€/year and obtain a reductionin CO2 emissions of 2.8 t/h which implies savings of approximately0.365 M€/year. The 21.6 t/h of produced vapour has a value of1.7 M€/year.

The overall emissions in the furnaces can be reduced to1500 mg/L of SO2, a value near to 200 mg/L less than today’slegislation.

Turbo Group 2 of the refinery is kept working at minimum de-signed operations, producing at 4 MW, in order to display flexibil-ity to the vapour system of the refinery.

The implementation of the cogeneration system achieves a an-nual income of 3.3 M€ reaching a revenue in 3.6 years

Acknowledgments

The authors gratefully acknowledged the financial support fromthe Portuguese National Team on Process Integration (Grupo Nac-ional de Integração de Processos – GNIP/IEA). The collaboration ofGALP ENERGIA in the form of industrial information and refinerycase study, and Dr. Raquel Moita are gratefully acknowledged. Thispaper is dedicated to the memory of Luísa Gomes who has passedaway and to those of us left behind who miss her.

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